MINE ENVIRONMENTAL ENGINEERING

Department of Mining Engineering

National Institute of Technology, Rourkela

“Role of bacteria, pyrite, relative humidity, heat of melting, surface area,

adsorption, chemisorption, peroxy-radical, threshold temperature,

activation energy and frequency factor on coal combustion with examples

from India and abroad”

Submitted By:

Abhijeet Dutta

Roll No.: 711MN1172

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CONTENTS

Sl. No. TOPIC Page No.

1. Role of Bacteria 6

2. Role of Pyrite 9

3. Role of Relative Humidity 11

4. Role of Heat of Melting 18

5. Role of Surface Area 20

6. Role of Adsorption 27

7. Role of Chemisorption 30

8. Role of Peroxy-Radical 36

9. Role of Threshold temperature 36

10. Role of Activation energy 38

11. Role of Frequency Factor 46

12. References 49

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Q. Discuss the role of bacteria, pyrite, relative humidity, heat of melting, surface area, adsorption,

chemisorption, peroxy-radical, threshold temperature, activation energy and frequency factor on

coal combustion with examples from India and abroad.

Ans. Mineral matter has always been the nemesis of coal-burning industries. Since the dawn of the

industrial revolution, the “impurities” in coal have had a major effect on the design of boiler furnaces and

how they are operated. An estimated 100 million tons of mineral matter, converted to “coal ash,” will

pass through the pulverized-coal-fired boiler furnaces in the United States in 1984. The worldwide figure

is less certain. This huge mass of troublesome material will lead to some of the most serious operational

problems facing the utility industry, and it will cost the industry tremendously in capital investments and

in availability because of ash limited generation. In industrial applications, too, clinker formation In

fixed-fuel beds, as in stokers, will continue to limit burning rates, just as it has done for more than a

hundred years. In the huge utility-operated, pulverized-coal-fired units, slagging will still be a major

determinant in fixing furnace size, and hence relative cost, for a given output of steam, just as it has done

since 1920. Fouling in these same steam generators will dictate the spacing and the location of convective

tube banks for superheating and reheating steam. In short, the mineral matter in coal continues, as it

always has, to be a dominating factor in deciding on the dimensions of boiler furnaces for a given steam

output; in proportioning the heat-receiving surfaces for design steam production and properties; in setting

limits for flue-gas velocity and flow patterns to minimize metal loss by erosion; and in influencing the

physical and chemical properties of the fly ash leaving the stack to ensure its capture by emission-control

systems. It is the mineral matter in coal that plays the dominant role in fuel selection, in setting the design

and size of the furnace, and in establishing how that boiler furnace will be operated. In the 1960s coal

was losing ground as the principal primary fuel for electricity generation. Oil was plentiful and cheap,

and nuclear power appeared to be poised for rapid growth. By the mid-l970s the fuel situation had

undergone a drastic change. The price of liquid fuel had increased sharply and, with the exception of the

oil-exporting countries, hardly any new oil-fired power stations have been built anywhere else since that

date. It was inevitable, therefore, that there should have been renewed interest in utilizing coal for power

generation. A number of countries are currently planning to exploit new coal fields for their own use and

for export. Some countries where there are no significant deposits of indigenous coal, notably Japan and

the Scandinavian countries, are in the process of building up a substantial electricity-generating capacity

based on imported coals. Future coal prospects on a worldwide basis have been assessed in the Report of

World Coal Study, edited by Greene and Gailagher (1980), and by Ion (1980). More extensive use of

solid fuel for power generation will accentuate the boiler operation problems associated with mineral

impurities in coal, and there is a need for a comprehensive source of information on various difficulties.

First, an account is given of changes in the design of coal-fired utility boilers from the small, 1- to 2-MW

capacity, stoker-fired boilers to the present-day large pulverized-fuel-fired units of 500 to 1300-MW

output. The need for more efficient utilization of fossil fuel has always been the driving force for the

large and more economical power plant. However, difficulties caused by coal mineral impurities, in

particular those of ash deposits on heat-exchange surfaces, have influenced the changes in boiler design.

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In spite of extensive research and the wealth of practical experience there remain some enigmatic aspects

in the subject of boiler slagging. For example, the massive build-up of ash deposits can take design and

boiler operation engineers by surprise. The nature of different mineral species of solid fuels, bituminous

and non-bituminous coals, and lignite is evaluated. The difficulties of coal grinding and the wear on fuel

handling and milling plants are discussed in relation to the abrasive minerals in coal. This is followed by

a synopsis of the physical changes and chemical reactions of different mineral species in coal flame;

changes and reactions of great importance, not only to formation of boiler deposits but also in relation to

other salient properties of ash. These properties are the corrosion propensity of ash, the abrasiveness of

ash related to erosion wear of boiler tubes, the electrical resistivity of ash (which is relevant to the efficient

working of the electrical precipitators), and the pozzolanic (cemenhitious) properties of ash when it is

used in concretes and in grouts as a cement extender. The chapters on the flame reactions are followed

by discussions on the viscosity as a rate-controlling parameter in ash sintering and fusion, an assessment

of the slagging propensity of ashes, the formation and adhesion of deposits ni boiler plant, and the effects

of boiler deposits on heat transfer. Under the heading of counter measures to combat boiler slagging, the

topics discussed are design considerations, combustion control, and conventional and unconventional

methods of boiler cleaning. A separate chapter is devoted for discussion on the specific ash-related

problems with U.S. low- and high-rank coals. A wide variety of solid fuels is utilized for power

generation in the United States, and the deposit-forming characteristics of the lignite and sub-bituminous

coal ashes can be markedly different from those of bituminous coal ashes. The literature on low-

temperature additives used to improve the ash capture performance of the electrical precipitators is

extensive. In Contrast, less information is available on the use of high-temperature additives to combat

boiler slagging. The manifestation of boiler slagging can be immediate, and large quantities of ash

deposits can form in the combustion chamber within a few hours, whereas boiler tube corrosion is usually

a long-term event determined largely by the “quality” rather than by the quantity of the fuel impurity

deposit. The mode of formation of potentially corrosive deposits of molten alkali-metal sulfates is

discussed, followed by an assessment of the corrosion propensities of different coals. High-temperature

slagging and corrosion by ash deposits cease to be serious problems in the middle section of boiler plant,

but the economizers and the hot sections of air heaters can be plugged by the high-temperature deposit

debris and ash compacts. There is another problem that occurs in this boiler section, namely, the tube

erosion wear by ash impaction when the impact velocity exceeds that compatible with the abrasive

property of ash. In this book the abrasive property of ash in different coals will be discussed with the

intention of helping design engineers to arrive at an optimum velocity for the ash-laden flue gas in boiler

ducts without causing significant erosion wear. Condensation of sulfuric acid in air-heaters and in

chimneys of pulverized-coal-fired boilers constitutes a lesser problem than it does in residual oil-fired

boilers. The design of combustion systems has undergone marked changes since the time coal was first

used for electricity generation. The earliest utility plants were based on the principle of burning coal in

fixed and moving grates, and Mayers (1945) commented when discussing combustion in fuel beds that

it was not an exaggeration to say that the great (U.S.) coal industry existed primarily for the purpose of

supplying the nation’s (fuel beds, in electricity utility and industrial boilers, and in coke ovens. However,

since that time there has been an extensive changeover to pulverized fuel firing for large electricity utility

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boilers. The mode of combustion where there is a hot bed of burning coal was undesirable when

considered from the point of view of the behavior of coal mineral species at high temperatures. Extensive

volatization of the mineral matter can occur in the reducing atmosphere either in the elemental state, or

in the form of sulfides and chlorides. Subsequent deposition, oxidation, and sulfation of the condensable

material on boiler tubes can result in a formation of strongly bonded deposits. In particular, coals of high

chlorine content and phosphorous-rich fuels are difficult to burn successfully in stoker-fired boilers. From

the point of view of reducing the rate of buildup of boiler deposits, the inorganic material in coal should

remain at high temperatures for the shortest possible time, and the hot atmosphere should be consistently

oxidizing. The development of a pulverized coal-fired system where coal is burned in a cloud rather than

in a thick bed was a major step towards meeting these two requirements. The residence time of ash in

this mode of combustion is cut to a few seconds, and there is less chance for the mineral species to

undergo reduction reactions in the flame when oxygen is present in excess of that required to burn the

flame-borne coal particles. Introduction of the system of pulverized coal combustion paved the way for

a rapid increase in the capacity of boiler units, and for raising the temperature and pressure of superheated

steam. Thus, there has been a significant increase in the generating efficiency of electricity utility plants.

With the advent of pulverized-coal firing, the design and combustion engineers thought that they had a

panacea for all fouling and slagging problems. They were delighted to find that high-ash coals could be

burned successfully, and it was thought that the nature of mineral species in coal did not matter to any

significant degree when the fuel was burned in the form of a flame-borne cloud. However, as the

combustion intensity was increased, resulting in higher rates of heat release and temperatures, large

amounts of running slag formed on furnace walls with some coals. The large amounts of running slag led

to the next logical step in the development of boiler design, that is, construction of slag-tap and cyclone-

fired boilers. Systems were designed where 50 to 85 percent of the total ash was removed as a continuous

stream of molten slag. The design was taken up particularly in the United States and in Germany, where

large numbers of slag discharge boilers were built in Britain. However, only a few small slag discharge

boilers were built in the l950s and the idea was soon abandoned, It was claimed that a successful operation

of a slag tap or cyclone boiler could not have been guaranteed with British coals. The fuel for a power

station is usually supplied from a number of pits, and therefore the variable slag flow characteristics are

not consistent with a trouble-free slag discharge. British boiler design engineers therefore retained the

original concept of pulverized coal, “dry-bottom” or “dry-ash” boilers, and utility undertakings in other

countries are returning to these dry-ash boilers for their 500-, 600-, 900-, and 1200-MW units. The

experience with slag discharge boilers has shown that these units are less tolerant than the dry-ash

discharge boilers of the inevitable changes in the quantity and quality of mineral matter in coal. For

example, a 900-MW unit requires between 300 and 400 t coal per hour. Assuming that the unit will

generate electricity for 25 years and that it will be in service on average 6000 hours a year, the unit will

consume about 50 million t coal in its lifetime. There are few coal mines in the world that could provide

such amounts of coal of a consistent quality.

For the next few decades pulverized-coal-fired boilers are likely to remain the principal generating units

for converting coal to electricity. The chief advantage of the system is its versatility, and boilers can be

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designed to burn all types of coals, as discussed by Bennett and Bannister (1981). The quality of solid

fuels covers a wide spectrum from low-volatile content anthracite, through the range of bituminous and

sub-bituminous coals to lignite and peat. The mineral matter in each type of solid fuel brings to bear its

own particular boiler plant operation problems. The surveys on boiler plant failures carried out in the

United States and reported by Koppe and Olson (1979) and by Armor et al. (1981) have shown that the

development from small to large units has significantly reduced the availability of the generating plant.

Some decrease in the availability of a large boiler plant is not entirely unexpected, since it takes just a

single failure of a high-pressure water or steam tube to take a 50-MW or 500-MW unit out of service,

and chances of tube failures in large units are correspondingly higher. It is therefore imperative that the

design and operation engineers should have the support they need in their task of maintaining the

electricity generating units in an efficient operation. It appears that the system of fluidized-bed

combustion is the strongest candidate for the next stage in the development of converting coal to

electricity. From the point of view of problems associated with coal mineral matter that would be a logical

step ahead. The main advantage would be a much-reduced combustion temperature, 1100 to 1200 K,

compared with the peak temperature of 1750 to 2000 K of the pulverized coal flame. The low temperature

would ensure that a great deal of potentially reactive mineral species remains dormant in the ash. As a

result, boiler corrosion and fouling should be markedly reduced, but some ash sintering may occur when

the deposit temperature exceed 1100 K. Also, there may be some erosion and abrasion wear damage of

the heat exchange tubes caused by the coarse mineral particles in the fluidized bed. It is therefore

inevitable that there will be always some problems with ash, whatever the system of coal combustion.

The major use of coal is for the generation of power by combustion on a large scale. This means that

most trace elements will be released and redistributed into bottom ash, flyash, fine flyash and the gaseous

phase. Up to about 20% of the original mineral matter is found in the bottom ash and up to about 80% in

the flyash. The bottom ash remains in the combustion area in the furnace, whereas the flyash is conveyed

through the system where most (more than 99%) is removed by electrostatic precipitation or fabric filters.

However, a small proportion, mostly fine particles of less than about 10 μm in diameter, is emitted to the

atmosphere with the stack gases. Very few elements are in the vapour state, probably only variable

proportions of the total amount of Hg and the halogens, but even these may be associated with the surfaces

of very fine flyash particles. The environmental interest is in the trace elements in the bottom ash and

flyash (removed by the particle attenuation devices) that are stored in ash disposal ponds, and in the stack

emissions. The main concern with the ash disposal ponds is the possibility of the release of leachates that

could transport trace elements into nearby underground and surface waters. However, efforts are usually

made to prevent or restrict such losses by having the ponds in clay-rich soil and by using clay liners and

plastic sheeting. The dispersion of trace elements in the stack gases means that varying amounts of trace

elements are deposited in the environs of power stations.

EXAMPLES:

Measurements of trace elements in deposition collected at various locations at different distances and

aspects were carried out at Wallerawang power station over a 4-year period 3, 20 and 21. Wallerawang

is about 120 km northwest from Sydney, New South Wales, Australia. The installed capacity is 1240

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MW, flyash attenuation is by electrostatic precipitation and the bituminous feed coal is from the Lithgow

seam, the ash yield being 21.4±4% and the total sulfur being 0.6±0.1% S. The method of collection of

deposition from the atmosphere was flat fine-mesh envelopes containing 2 g of prepared Sphagnum moss.

The envelopes were replaced at 3-monthly intervals and the moss was analysed for up to 39 trace elements

using optical emission spectrography, atomic absorption spectrometry and instrumental neutron

activation analysis. It was found, as expected, that there was a decrease in the amounts of trace elements

deposited with distance from Wallerawang. Variations in amounts at different aspects depended on

changes in wind strength and direction. An important finding was the variation in amounts of deposition

with time of sampling, the ratio of maximum to minimum values being about 5:1. This stresses the

importance of time of sampling at any location in order to obtain significant results. Using the marked

difference in Ge contents of local soil and fine flyash emitted to the atmosphere (a ratio of 1:50), the

proportions of flyash and soil in samples of deposition were calculated. This showed for example, that at

1.8 km from the power station, the deposition contained 7–80% with a mean of 40% flyash, while at 5.3

km, there was <1–5% with a mean of 2.5% flyash. The amounts of trace elements in the depositions were

low compared with those in surface soils and mostly less than the amounts from weathering and litter

decay. Although this study has established that trace elements from the combustion of a specific coal

under the conditions of operation at Wallerawang power station have not been detrimental to the

environment, it must be emphasised that these results can at best be only a guide to other situations. Real

answers for other places can only be obtained by carrying out similar experiments there.

1. Bacteria Effects:

Differential thermogravimetry (DTG) was used to obtain information about the effect of biological

desulphurisation on the entire coal combustion process. Two coals of different rank, a semianthracite

(HVLl) and a high volatile bituminous coal (HMl), were inoculated with autochthonous bacterial cultures

isolated from natural mine sludge and from bacteria inherent in the coal. From the combustion profiles

and the reactivity in air at 500°C of the chars produced after pyrolysis, several characteristic parameters

were determined. The results indicated that the influence of the biological treatment on combustion

performance and char reactivity is more significant in the higher rank coal. The overall process seems to

cause a slight beneficial effect on the ignition properties, evaluated from the burning profiles, of the

semianthracite treated samples.

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Table1: Characteristic parameters from the derivative thermogravimetric curves (burning profiles) for

the parent and biologically treated coals. HVLl and HMl: parent coals; HVL2 and HM2: non-

inoculated; HVL3 and HM3: inoculated with an autochtonous bacterial culture; HM4: inoculated with a

culture of bacteria inherent in the coal

After ten days of treatment the sulphur content was reduced by 22% and the pyritic sulphur was reduced

by 41% for the non-inoculated HVL2 sample. Forty-two percent total sulphur and 71.2% pyritic sulphur

reductions were achieved for the HVL3 sample inoculated with an autochthonous bacterial culture. The

HVLl parent coal and the coal samples obtained after its treatment have been denominated as HVL coal

series. The peak temperature or temperature of maximum reaction rate for the HVL coal series remained

fairly steady at about 585-588°C for the three samples and is in good agreement with the values normally

found for high rank coals. This parameter is generally considered to be the most important feature of the

DTG curve. Coals with lower peak temperatures can generally be ignited and burned more easily. This

is the case of the HM coal series which present a medium peak temperature of 523°C characteristic of

high volatile bituminous coals 1131 and lower values of the temperature of combustion onset. However,

the burnout times for the HVL coal series are smaller than those of the HM coals. This might be due to

the higher mineral matter content of the HVL coals, resulting in a lower content of the combustible matter,

and the catalytic effect that some mineral species can exert, mainly after the devolatilisation stage.

Regarding the effect that the biological treatment produces on the burning profiles of the coals studied,

one of the most characteristic parameters, the peak temperature T remains constant in the HVL coal

series. However, the temperature TV for the HVLl coal is higher than the other two coals of the series.

This can be due to the higher oxygen content of HVL2 and HVL3 coals which have experimented some

degree of oxidation during the biodesulphurisation process with an increase of more than 40% in the

oxygen content. This is a matter of considerable importance, since even a small amount of oxidation may

affect the value of coal for different utilisation purposes. When considering the HM coal series the

parameter TV remains practically constant. In this case the maximum oxygen change occurs in the HM4

sample with an increase of only 6.6% and for a coal that already has a relatively high oxygen content.

The peak temperature of the HM coal series seems to be very sensitive to the biological treatment,

probably due to their high volatile matter and oxygen contents which can originate some distortion in the

shape of the thermogravimetric curves.

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A reactivity test was performed to evaluate the relative reactivities of the chars. The technique involves

two steps:

a. Coal devolatilisation at a heating rate of 15 K/min to a maximum temperature of 850°C under a

nitrogen atmosphere, and

b. Reactivity measurement in an air stream of the chars produced.

The volatile-release profiles of the HVL coal series were qualitatively different. Two peaks were

observed for the original coal HVL1, whilst the second peak that occurred at a higher temperature,

disappeared in the HVL2 blank sample and the inoculated HVL3 coal sample. This indicates that some

changes have occurred in the coals structure as a consequence of the biological treatment. A decrease in

the volatile matter content of the treated coals when compared with the untreated HVL1 parent coal was

observed. This effect could be attributed to a high content in minerals, i.e. carbonates, of the parent

sample which decompose after the treatment in acid medium and that are not present in the products

HVL2 and HVL3. The evolution of the volatile matter in high rank coals, as semianthracite HVL1, takes

place at higher temperatures than in lower rank coals due to a more stable coal structure with a lower

volatile matter content. Thermogravimetric analysis offered a means to make a preliminary evaluation of

possible changes in the combustion behaviour of biologically treated coal samples. The most important

differences produced in the coal samples by effect of the biological desulphurisation were observed in

the higher rank coal. This is particularly remarkable in the volatile-release profiles. The burning profiles

of the parent and treated coals have shown some differences that can be related to structural changes

originated as a consequence of the biological treatment. The temperatures of combustion onset, TV, of

the HVL2 and HVL3 treated samples were smaller than that of the HVLl original sample probably due

to the oxidation caused by the overall biological treatment. A modification in the volatile release profiles

of the HVL semianthracite coal series was also observed, with a disappearance of the volatiles evolved

at higher temperatures. A slight decrease in the reactivity in air at 500°C of the chars produced from the

HVL treated coals was obtained, this effect being mainly attributed to the changes in mineral matter

content. No very remarkable differences in the combustion behaviour of the high volatile bituminous HM

coal series were attained. Nevertheless, a slight decrease in the combustibility of this series, indicated by

an increase in both the bum-off time and the end of combustion temperature, was observed.

Role of SuperSaver® Coal Enhancer reduce calorie loss and prevent spontaneous combustion in

stockpiles:

Such phenomena explained above are created by anaerobic methanobacteriaceae bacteria and sulphate-

reducing bacteria within the coal that normally lay dormant in its natural state. Once the coal is unearthed,

these bacteria begin feeding off the coal (creating calorie loss) and breeding at an exponential rate,

creating increased heat that eventually leads to spontaneous combustion. SuperSaver® inhibits the

growth of these bacteria. This is done by adding oxygen molecules to the coals molecule structures, where

such bacteria perish when it feeds on oxygen.

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1.1. Principles of desulferisation:

The biological enzymes in SuperSaver® Coal Enhancer can crack the bonds between carbon to carbon

(c-c) and carbon to sulphur (c-s) and produce oxygen, carbon, hydrogen compounds and sulphate by-

products etc. The desulfurization process is achieved by the catalytic process between bacteria. The

biological enzyme catalysts do not form any toxic byproducts or side reactions.

Thermophilic bacteria have potential for application in the bioleaching of minerals as they exhibit higher

mineral sulphide oxidation rates than mesophilic bacteria (Clark and Norris, 1996a; Stott et al., 2000b;

Witne and Phillips, 2001). In heap-bioleaching, which is increasingly of commercial interest for lower

value metals such as copper, thermophiles therefore have potential for increasing the rate of leaching if

the heap is able to reach the required temperature. Temperature, and other conditions such as aeration,

within heaps are variable (Lizama, 2001; Brierley, 2001) so microbial strains which are able to survive

for reasonable periods under unfavourable conditions would be an advantage. During heating, heaps pass

through moderately thermophilic temperatures during which time the moderate thermophiles would be

expected to be most active.

Biological cleaning involves using bacteria that literally "eat" the sulfur out of the coal. Scientists are

trying to improve the sulfur-removing characteristics of the bacteria through experimentation. Other

scientists are using fungi, while still others are trying to find a way to duplicate the enzyme, or chemical,

inside of the bacteria that eat the sulfur. They can then inject the enzyme directly into the coal to speed

the cleaning process.

So biological cleaning is a process of generating clean coal technology and it has varied effects on

combustion of coal as discussed above.

2. Pyrite Effects:

Pyrite is probably the most environmentally interesting mineral, because of its propensity to oxidise

during weathering producing sulfuric acid. The raspberry-like texture of framboidal pyrite has discrete,

spheroidal aggregates of microcrystallites of about 1 μm in diameter that give ready access to air and

water, thereby increasing the rate of oxidation markedly. The initial oxidation yields ferrous iron (and

sulfuric acid) that is further oxidised to ferric iron that can react directly with pyrite to produce ferrous

iron and sulfuric acid. The second-stage oxidation to ferric iron is a slow reaction unless it is catalysed

by iron-oxidising bacteria, for example, Thiobacillus ferrooxidans. This important finding of the role of

the bacterial catalyst is relevant to the oxidation of ferrous iron in natural acidic environments. The

bacterium acts best at low pH, less than about 4, so that the sulfuric acid produced favours further rapid

oxidation of pyrite. The rate of oxidation of pyrite depends on several factors, namely, the concentration

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of oxygen, surface saturation with water, pH, temperature, microbiological factors, surface area of pyrite

and the presence of other minerals. The association of As with pyrite increases the rate of oxidation of

pyrite. The most important factors are air, water and iron-oxidising bacteria. The presence of other

minerals, notably carbonate minerals, modifies the acidic conditions by reacting with the sulfuric acid to

form sulfates.

The presence of pyrite in coal mine and beneficiation wastes and in many metal mine wastes is a major

environmental concern. The weathering of pyrite produces acid conditions that may leach trace elements

and release trace elements associated with pyrite. Sulfur is a vital element for sustenance of plant life,

and it is therefore not surprising to find a relative abundance of sulfur compounds in coal deposits and in

associated mineral matter strata. The coal deposits laid down in brackish marine areas are usually rich in

ash, sulfur, and nitrogen (Williams and Keith, 1963). Periodic flooding in these areas reduced the

originally high acidity of plant deposits by dilution, resulting in an enhanced environment for bacterial

sulfur fixation. Neavel (1966) considered that iron sulfide (pyrites. FeS2) found in peat and coal deposits

had been largely formed as the result of activity of anaerobic bacteria. Sulfur originated from plant and

animal protein (organic sulfur), or it was brought in by semi water in the form of sulfates. Iron originated

from weathered silicate minerals: consequently. synthetic pyrites deposits appear frequently in clay

bearing sediment. Coals deposited in calcium-rich swamps also show a high degree of sulfidation.

Calcareous basements or Influx of calcium-rich waters effectively increased the pH of the aqueous

environment, thus creating favorable conditions for a high rate of sulfur fixation. An extreme example of

this type of coal is that found in Istria district of Greece, which contains 11 percent by weight of sulfur

(Petrascheck, 1950).

By contrast, the peat deposits and subsequently formed coal deposits laid down in raised bogs of inland

areas are usually low both in asti (clay minerals) and in sulfur. In the absence of flooding, the acidity of

peat water had remained high with a pH value between 3 and 5, and such an acidic medium did not sustain

a high degree of sulfidation bacterial activity; the bulk of sulfur therefore was returned to the atmosphere

in the form of H2S and SO2. Coals laid down in those areas have a low sulfur content, around 0.5 percent

by weight. Currently, such coals are in great demand for electricity generation because of the low level

of SO2 emission and the low-sulfur fuels. However, on a worldwide basis, occurrences of low-sulfur

coals are comparatively rare, and the vast reserves of bituminous coals have a sulfur content between 1

.0 and 4.5 percent. For example, the average sulfur content of British coals is 1.6 percent (CEGB, 1977),

which may be taken as a typical figure for coals of medium sulfur content. Wandless (1959) concluded

after his extensive study of the mode of distribution of sulfur in British coals that on average the coals

contained 0.8 percent by weight organically compounded sulfur, and the remainder was present as pyrites

(FeS2). The amounts of sulfatic sulfur in British coals, as in the niajority of all bituminous coals, were

egligible. Neavel (1966) arrived at a similar conclusion, that there was a relationship between ¡lie relative

abundance of pyritic sulfur and that of organic sulfur. Gluskoter and Simon (1968) reported a mean value

of 1.56 for the ratio of pyritic to organic sulfur in 473 face-channel samples of Illinois coal. Arguably the

most thoroughly investigated high sulfur coals are those mined in Illinois, where the sulfide species

ColiSlitLIte about 25 percent by weight of the total mineral matter, as discussed by Rao and Gluskoter

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(1973), Kuhn et al. (1973), and Shimp et al. (1975). Pyrite and marcasite, both of an approximate

chemical formula of FeS2 , are the principal pyritic minerals found in coal. Pyrrhotte (FeS), chalcopyrite

(CuFeS2), mispickel (FeS2FcFS2), galena (PbS), and sphalerite (ZnS) are other sulfur-containing

mineral species that have been identified in coal (Gumz et al., 1958: Watt, 1968). Microstructural

dispersion of the pyritic mineral species in coal seams and clay sediment-strata has been studied in the

scanning electron microscope, as discussed by Greer (1977), Finklernan and Stanton (1978), and Moza

et al. (1979). Freshly mined coals usually do not contain any significant quantity of sulfates, but a variety

of iron sulfates will form when the sulfides are oxidized in air. Rao and Gluskoter (1973) and Gluskoter

(1977) have identified a number of hydrated ferrous and ferric sulfates in weathered coals, but the amount

of sulfates in coals is usually insufficient to make it worthwhile considering their behavior in coal-

cleaning processes or in the boiler flame.

3. Relative Humidity Effects:

A spontaneous increase in coal temperature with a possible transition into fires represents a direct hazard

to coal storage and transportation. The effect of relative humidity of the gas supply on the self-heating

rates of coal is studied. It is shown that an increase in the gas supply relative humidity has a marked effect

on the self-heating rates of the coal. In this work the sub-bituminous coal sample undergo oxidation most

rapidly when the relative humidity of the gas supply is about 70%. The effect of relative humidity

decreases as oxygen concentration in the gas supply increases. A practical consequence of this finding is

that improved fire safety measures should be considered during sub- bituminous coal related operations

in hot and humid regions. High-moisture low rank coals have been found to be more suitable for domestic

use due to low heat output and self-heating problems upon long term storage and transportation. Some

major damage related to spontaneous combustion of coal is summarized. Recent increase in utilization

of high-moisture low rank coals following oil price rise has necessitated understanding fire safety aspects

of coal storage and transport especially in humid and high ambient temperature conditions. In general,

the spontaneous combustion of a coal pile are significantly affected by many factors including coal rank,

the oxygen content of the coal, the flow rate of the air, particle size, the moisture content of the coal and

the humidity of the air. Moisture plays an important role on behavior of coals in stockpiles. The complex

processes of self-heating in the existence of water have been investigated by many workers. The amount

of water contained by coal for a given value of relative humidity is described by its adsorption and

desorption isotherm. The interaction between water and coal can be exothermic or endothermic

depending on whether the water condenses or evaporates. In sufficient quantities, water suppresses self-

heating by blocking access of oxygen to active sites and by taking up the heat released by oxidation as it

occurs. However, if the moisture content of the coal is lowered, it may be expected that these moderating

effects will become less effective and a significantly greater level of self-heating will occur. In addition,

it has been reported that low rank coals undergo the highest heating rate when their moisture content is

reduced to about one-third of the original as-received moisture content. On the contrary, the liability of

spontaneous of a low-moisture Turkish lignite was increased by increasing moisture content of the coal

12

above its as-received value. A number of laboratory methods have been developed to study the propensity

of coal to ignite spontaneously. The testing conditions of the suggested methods were for the most parts

quit different from those actually encountered by stockpiles. For example, the suggested test methods

and the correspondent oven temperature range are (i) basket method, including Frank-Kamanetskii and

the crossing-point temperature (CPT) methods (100 to 1800C), and (ii) crossing point method, CPT (130

to 2000C). Studies on the storage of bagasse showed conclusively that the usual high temperature small

sample basket ignition tests were not capable of being extrapolated to the practical low temperature-

occurs in the large size case. Adiabatic oxidation test consists of measuring the temperature rise in a coal

sample reacting with oxygen under adiabatic conditions. The latter are commonly achieved using a

temperature controlled oil bath and adiabatic oven, within which the sample is enclosed, and increasing

the temperature of the oven to match that recorded by a thermocouple within the sample.

Examples:

Coal self-heating has been the subject of many investigations and extensive research has been carried out

to better understand the relationship between coal quality and corresponding self-heating indices. R70

testing has been the benchmark in this field for the Australian and New Zealand coal mining industries

and remains a key instrument in determining the propensity of coal to self-heat due to the extensive

database of results. The R70 test however only accounts for the intrinsic coal reactivity, and because of

this, developments have been made to the testing method to closer replicate site specific parameters.

Extrinsic effects such as the mine ambient temperature, relative humidity of the air and the as-mined

moisture content of the coal are now incorporated into an adiabatic simulation test (SponComSIM™).

Gas evolution testing (SponComGAS™) can also be used to determine specific gas trends for a given

coal sample in an as-mined condition for all the stages of a self-heating event. The gas trends recorded

provide a unique signature that can be used on site to set appropriate trigger levels or alarm points for

taking appropriate actions and implementing timely responses to mitigate against a spontaneous

combustion event.

R70 testing, first reported by Humphreys, Rowlands and Cudmore[15], has been the benchmark in this

field. An extensive database of R70 values allows comparison between previous studies and current

testing to be made, providing a relative indication of the intrinsic propensity of the coal to spontaneously

combust. In addition, spontaneous combustion indices can be derived from the coal quality data to further

understand the intrinsic propensity of the coal.

The SponComSIM™ test incorporates both the intrinsic and extrinsic effects on coals propensity to self-

heat and provides a definitive assessment of the mines spontaneous combustion risk while quantifying

the various extrinsic factors that are unique to each mine site. The data is used to obtain a minimum time

for spontaneous combustion events to be initiated in loose pile conditions (worst case scenario).

The SponComGAS™ test provides an assessment of gas evolution during a coal heating event. It must

be noted that no single gas indicator obtained should be used in isolation for the development of TARPs,

as it is the combination of these parameters that fully characterises the stages a heating has reached. In

13

this instance carbon monoxide, Graham's ratio and ethylene provide a good indication of the stages of

heating but these indicators can vary between mines.

Consistent detection of ethylene and any subsequent increase in ethylene concentration can in this case

be used as a high level trigger in the TARP. Ethane evolution at higher temperatures appears to be

sympathetic with ethylene and an increasing trend for both gases would confirm an advanced stage of

heating. The results of gas evolution testing can be used in combination with pre-established baseline

mine gas compositions to identify and assess the development of underground self-heating events. This

information can be used to develop reactive strategies for intervention, reducing the likelihood of

spontaneous combustion events. A spontaneous increase in coal temperature with a possible transition

into fires represents a direct hazard to coal storage and transportation. This paper evaluates the self-

heating characteristics of a sub-bituminous coal under adiabatic oxidation conditions. This paper assesses

the effect of relative humidity of the gas supply on the self-heating rates of coal. It is shown that an

increase in the gas supply relative humidity has a marked effect on the self-heating rates of the coal. In

this work the sub-bituminous coal sample undergo oxidation most rapidly when the relative humidity of

the gas supply is about 70%. The effect of relative humidity decreases as oxygen concentration in the gas

supply increases. A practical consequence of this finding is that improved fire safety measures should be

considered during sub- bituminous coal related operations in hot and humid regions. High-moisture low

rank coals have been found to be more suitable for domestic use due to low heat output and self-heating

problems upon long term storage and transportation. Some major damage related to spontaneous

combustion of coal is summarized. Recent increase in utilization of high-moisture low rank coals

following oil price rise has necessitated understanding fire safety aspects of coal storage and transport

especially in humid and high ambient temperature conditions. In general, the spontaneous combustion of

a coal pile are significantly affected by many factors including coal rank, the oxygen content of the coal,

the flow rate of the air, particle size, the moisture content of the coal and the humidity of the air. Moisture

plays an important role on behavior of coals in stockpiles. The complex processes of self-heating in the

existence of water have been investigated by many workers. The amount of water contained by coal for

a given value of relative humidity is described by its adsorption and desorption isotherm. The interaction

between water and coal can be exothermic or endothermic depending on whether the water condenses or

evaporates. In sufficient quantities, water suppresses self-heating by blocking access of oxygen to active

sites and by taking up the heat released by oxidation as it occurs. However, if the moisture content of the

coal is lowered, it may be expected that these moderating effects will become less effective and a

significantly greater level of self-heating will occur. In addition, it has been reported that low rank coals

undergo the highest heating rate when their moisture content is reduced to about one-third of the original

as-received moisture content. On the contrary, the liability of spontaneous of a low-moisture Turkish

lignite was increased by increasing moisture content of the coal above its as-received value. A number

of laboratory methods have been developed to study the propensity of coal to ignite spontaneously. The

testing conditions of the suggested methods were for the most parts quite different from those actually

encountered by stockpiles.

14

The self-heating rate becomes measurable at approximately 40–50% of the moisture holding capacity of

the coal. Above this critical level of moisture content, the heat produced by oxidation is dissipated by

moisture evaporation and coal self-heating is significantly delayed. More recently, using similar method

it has been reported that an Indonesian high-moisture sub-bituminous coal was found to undergo

oxidation most rapidly when its moisture content was reduced to about 25% of the original as-received

value. In tropical countries, the fire safety aspect of coal related operations should also consider the

effects of high relative humidity and ambient temperature. Since all coal and storage piles are exposed to

air with some relative humidity, a quantitative knowledge of the influence of humid environment to self-

heating rate is important. Using the Crossing Point Method (CPT), Küçük, A., et al. reported that the

liability of spontaneous combustion of a low-moisture lignite was increased with decreasing humidity of

the air. However, B.F. Gray et al. showed that increasing relative humidity outside the pile can cause

ignition of a pile in a stable steady state already attained by exposure to a lower external relative humidity.

In addition, some theoretical work in this problem has been carried out by McIntosh et al., where it was

shown that if a lump of reactive material is initially dry and sub-critical, a slight change in the atmospheric

humidity can have a marked effect on the ignition characteristics of the material. In the present study, the

adiabatic oxidation method was used to evaluate the effect of relative humidity in gas supply on the self-

heating characteristics of a high-moisture sub-bituminous coal from Indonesia.

Table 2: Summary of measured R70 and Time to 1270C values.

15

Fig. 1: Self-heating curves showing the effect of oxygen concentration in gas supply at dry conditions.

Fig. 2: Self-heating curves showing the effect of oxygen concentration in gas supply at a constant

relative humidity of 70%.

16

Fig. 3: Self-heating curves showing the effect of oxygen concentration in gas supply at a constant

relative humidity of 90%.

The effect of relative humidity on the self-heating curves at various oxygen concentrations is given in

Figures. For all oxygen concentrations applied, the effect of humidity to the self-heating rates increased

as the relative humidity of the supply gas moved from dry to 70%. If a dry coal is exposed to an

atmosphere of fixed relative humidity, it will gain moisture until equilibrium is reached. Increasing the

relative humidity of the surrounding will then cause the coal to gain more moisture to reach another

equilibrium state. On the other hand, an increase in the humidity causes an uptake of moisture in coal

which leads heat generation. This heat is in addition to that heat generated from oxidation. Therefore it

was clear that oxidation of dried-coal in a humid atmosphere released more heat as represented by higher

R70 values in Figs. 8 and 9. However, at a higher value of 90% relative humidity, the self-heating rates

slightly lessened compared to 70%. As shown in the 3rd and 4th columns of Table 2, there is an optimum

trend on the effect of relative humidity of gas supply on the R70 and t127 values. The optimum trend

obtained might correlate with the amount of moisture gained by the dry coal. In order to estimate the rate

of hydration for the dry sample, the experimental data and predictions obtained by Monazam et al. [14]

was considered. Their experimental data was for a dry Wyoming char at 25oC and relative humidity (RH)

of 30% and 80%. Within the period of two hours, the moisture content of the dry char increased from

about 1% to 2% (at 30% RH) and 5% (at 80% RH), respectively. On the other hand, the author’s previous

work [10] using a slightly higher moisture content sub-bituminous coal showed that the sample’s undergo

oxidation most rapidly when its moisture content was about 5% or one-forth of the original as-received

value. We speculate that the humid gas supply having 70% relative humidity may increase the moisture

content of the dry coal to an optimum value that accelerates the self-heating rates. Since the moisture

uptake was higher in a more humid atmosphere (at 90% RH), as expected the R70 values were lessen

due to more heat were needed to overcome the additional moisture.

17

In addition, the influence of moisture in gas supplied on R70 and t127 values decreased as oxygen

concentration increased. This finding has a practical significance in risk assessments of self-heating

using adiabatic oxidation approach. The use of gas flow with oxygen well above 21% can minimize the

effect of moisture content in the gas supply.

Fig. 4: Self-heating curves showing the effect of relative humidity (RH) in gas supply at a constant

oxygen concentration of 44%.

Fig. 5: Self-heating curves showing the effect of relative humidity (RH) in gas supply at a constant

oxygen concentration of 71%.

18

Fig. 6: Self-heating curves showing the effect of relative humidity (RH) in gas supply at a constant

oxygen concentration of 96%.

The adiabatic oxidation method provides important information on the process of self-heating under

conditions close to nature. The results of this study show that the sub-bituminous coal tested is extremely

reactive to oxygen at various concentrations. This study revealed that humidity of the atmosphere or gas

supply plays a key role on the self-heating rates of the coal tested. It is shown that an increase in relative

humidity of the oxygen has a marked effect on the self-heating rates of the coal. In this work the

subbituminous coal sample undergo oxidation most rapidly when the relative humidity of the gas supply

is about 70%. The effect of relative humidity is decreases as oxygen concentration in the gas supply

increases. A practical consequence of this finding is that improved fire safety measures should be taken

into account especially during stockpiling and long distance transport of sub-bituminous coals in hot and

humid regions.

4. Heat of Melting Effects:

Heat of melting is also known as the heat of fusion. It is defined as heat released when the substance

undergoes change of state from solid to liquid form. Coal also undergoes such a stage.

4.1. Clinker formation:

Clinker is a mass of rough, hard, slag-like material formed during combustion of coal due to low fusion

temperature of ash present in coal. Presence of silica, calcium oxide, magnesium oxides etc. in ash lead

to a low fusion temperature. Typically Indian coals contain ash fusion temperature as low as 11000C.

Once clinker is formed, it has a tendency to grow. Clinker will stick to a hot surface rather than a cold

one and to a rough surface rather than a smooth one.

19

Based on observations made at the Eastman Chemical Company, slag viscosity should be considered

along with the ash melting temperature because coals with the same ash fusion temperature have different

slag viscosities and, therefore, behave differently in the slag gasifier. Chemical Company data. The slag

of coal #2 would be very viscous even at 1399 °C (2550 F). The high-temperature slag would wear the

gasifier’s refractory and reduce total gasification efficiency. Therefore, slag viscosity measurement is

important in the gasifier along with ash slagging temperature. The chemical composition of coal ash is

an important factor in slagging gasifiers because it affects ash fusibility, slag viscosity, and refractory

life. Silicon dioxide, aluminum oxide, ferric oxide, titanium oxide, phosphorus pentoxide, calcium oxide,

magnesium oxide, sodium oxide, potassium oxide, and sulfur trioxide are the major components of coal

ash. These components mainly contribute to the melting characteristics of the ash. All these components,

specifically the calcium and iron contents are believed to be indicators for ash fusion properties. CaO in

particular is an important factor in the viscous properties of slag. As the CaO content increases, the

viscosity of slag increases. Trace components, such as mercury, chlorine, fluorine, etc., contribute greatly

to the environmental issues associated with coal usage. However, in comparing ash from a laboratory

muffle furnace with slag from an industrial gasifier, it was found that the major oxide content and trace

compositions in ash are higher than those in slag. Certain chemical components of coal ash (i.ecan attack

the refractory and cause cracks. In addition, residence time affects the amount and composition of ash

formed in a high-temperature gasifier. Ash fusion temperature is an important index for all gasifiers. It

can strongly influence the formation of slag. The ASTM D-1857 ash fusibility test (AFT) is designed to

simulate the behavior of coal ash when it is heated in either a reducing or an oxidizing atmosphere. The

test is the most accepted method of assessing the propensity of coal ash to slag and gives an average flow

property. It measures approximate temperatures at which the ash cone will sinter (i.e., the solid ash

particles will weld together without melting), melt, and flow. Four temperatures are reported the initial

deformation temperature (IDT), the softening temperature (ST), the hemispherical temperature (HT), and

the flow temperature (FT). For blended coal ash, thermo-mechanical analysis (TMA) was used to

characterize ash fusibility because TMA temperatures changed with blended proportions of coals while

AFT did not. The fusion temperatures of coal ash decrease with increasing CaO, Fe2O3, and MgO

contents then increase after reaching a minimum; for example, when CaO is higher than 35% the fusion

temperature increases rapidly. In general, if both iron and calcium are high in coal, the softening and

melting temperatures will be reduced. Conversely, fusion temperatures increase as SiO2/Al2O3 ratios

increase. The acid components of Al2O3, SiO2, and TiO2 all increase the ash flow temperature, with

aluminum having the strongest effect. The increase in rate of furnace slagging and convective pass

plugging rate caused by increasing the PRB content in the fuel blend is due to ash interactions and the

formation of low melting temperature compounds in the furnace. It is widely known that a furnace

designed for bituminous coal will be more prone to slagging when a PRB/bituminous blend is fired.

Information on ash fusion temperature for the individual coals and for a blend are given in the enclosed

table. The ash softening temperature cannot be simply calculated like the other coal properties. Ash fusion

data from other power stations have shown that the ash softening temperature is approximately 2,400°F

for a blend containing 60 percent PRB and 40 percent Eastern bituminous coals, which is approximately

350°F lower than for 100 percent Eastern coal. Flue gas temperature measurements taken by our group

20

indicate that, typically, the furnace exit gas temperature (FEGT) increases as the percentage of PRB coal

in the blend increases. This increase is in part due to the more reflective nature of the PRB ash that

deposits on the waterwalls. The combination of higher FEGTs and ash with lower fusion temperatures

increases the risk of plastic ash formation and slagging on the close-to-the-furnace heat transfer surfaces,

such as superheater surfaces. Ash deposition phenomena are influenced by coal type (ash compositions,

melting temperature, and distribution of mineral matter), reaction atmosphere, particle temperature, the

surface temperature of heat exchanger tubes and tube materials, flow dynamics, and so forth. Several

reviews relating to the ash deposition characteristics have already been reported. For instance, Raask

elucidated the deposit initiation, and Walsh et al. and Baxter studied the deposition characteristics and

growth. Beer et al. attempted to develop theories of ash behavior. Benson et al. summarized the behavior

of ash formation and deposition during coal combustion. Naruse et al. evaluated the ash deposition

characteristics under hightemperature conditions. Vuthaluru et al. evaluated the ash formation of brown

coal. Li et al. investigated coal char-slag transition under oxidation conditions. Bai et al. studied the

characterization of low-temperature coal ash at high temperatures under a reducing atmosphere. Abbott

et al. investigated the adhesion force between slag and oxidized steel. Harb et al. predicted ash behavior

using a chemical equilibrium calculation. Hansen et al. quantified ash fusibility using differential

scanning calorimetry, and Ichikawa et al. measured the liquid phase ratio of ash using differential thermal

analysis. Song et al. investigated the effect of coal ash composition on ash fusion temperatures. Even for

those references, however, precise and quantitative knowledge of the deposition of coal ash with a low-

melting temperature during coal combustion has been insufficient. Our previous studies have proven that

the molten slag fraction in ash obtained by chemical equilibrium calculations is one useful index with

which to predict the coal blending method to reduce the deposition fraction of ash.

5. Surface Area Effects:

In the oxidation of highly porous carbons, the internal surface area can increase as a function of

conversion due to pore growth and the opening up of sealed internal pores or cavities. Consequently, rate

expressions for carbon oxidation are more accurately described in terms of the intrinsic reactivity, where

differences in surface area and porosity are accounted for. The oxidative reactivity of coal chars is

complicated by a number of different factors which are explored in this paper. These include (i) the

development of the pore structure during devolatilisation of the coal, (ii) the ash content ant its

distribution in the carbon matrix, (iii) the H and N functional groups present on the solid matrix, and the

interrelation with volatile species present, (iv) the graphitic nature of the carbon surface and, the active

surface area available for reaction.

The oxidation of coal will occur on any available coal surface, including both external and internal pore

surfaces, with oxygen present. The reaction rate of spontaneous heating of coal in coal stockpiles was

found to be related to the external surface area for nonporous coal particles with small pore diameters,

and weakly related or not related to particle size for small porous coal particles with larger pore diameters.

Nugroho et al. showed that particle size has considerable influence on the self-heating character of coal.

21

While a smaller particle reduces the critical ambient temperature for spontaneous ignition to occur, the

product of the exothermicity and the pre-exponential factor QA, and the activation energy of the coals

increase with decreasing particle size. Nugroho also reported that change of the critical ambient

temperature with particle size is almost negligible for porous coals, but significant for harder, nonporous

coals. The effect of coal surface area on the spontaneous heating was examined by using different surface-

to-volume ratios, S/V, for No 80-1 coal shows the maximum temperature histories for simulations using

three surface-to-volume ratios: 36, 12 and 2 / m. This is equivalent to an average coal particle diameter

of 10, 30, and 180 cm, respectively. With S/V = 2, the average particle diameter is 180 cm. In reality,

this does not indicate a volume with coal particles with a diameter of 180 cm. Instead, it represents a

volume with small coal particles coexisting with other non-coal rock particles. When the S/V was reduced

to 12/m, not only was the induction time increased to about 17 days, but also the temperature rise in the

second stage became less steep, and after about 5 days changed into another slow temperature rise stage,

probably because of less heat released in a unit volume. After the S/V was further reduced to 2, maximum

temperature rise was only a few degrees in 35 days, and there was no thermal runaway. This indicates

that decreasing the coal reaction surface area will greatly re-duce the spontaneous combustion fire hazard.

Fig. 7: Temperature–time histories in three areas for No. 80-1 coal.

The effects of changes in pore surface area can be significant for combustion in regimes II and I, but

especially so in the latter as the rate is directly proportional to the surface area of the pores (Eq. (16)).

The well-known fact that rates can change with burn-off under regime I conditions, adapted from the

study of graphite oxidation by Tyler et al. 117. The specific rate of oxidation (per unit weight of carbon

burning) increased until about 25% burn-off, and then started to decline. That specific surface areas can

pass through a maximum during combustion using data on the oxidation of anthracite. The numbers on

the line denote percent burn-off and the data show the linear variation of rate with the development of

22

surface area during reaction. Some data have been published on the development of pore surface area

during the combustion of chars. The case of semi-anthracite particles areas reduced steeply with

increasing burnoff. It was shown, however, that the reduction could well be due to annealing away of the

fine pores, as the data at high levels of burn-off were produced at high combustion temperatures (-2000

K).

Fig. 8: Variation of specific surface area with burn-off

In summary, where combustion is limited to the outer surface of the particle or to a shallow zone below

the outer surface, particles will burn with constant density but with a steady reduction in size. When

oxygen penetrates completely within a particle's pore structure, combustion occurs at constant size but

with decreasing density. For pf-sized particles burning in regime II conditions the penetration depth of

oxygen is of the order of particle radius, consequently these particles burn with reduction in both size and

density. Thin-walled cenospheres burn with reducing density but at constant size whether oxygen

penetrates the pores or not. In regime II conditions, as pore diffusion plays a major role, it is important

to have data on the opening of pores and changes in pore surface. In regime I it is important to understand

the development of pore surface area during combustion.

23

Fig. 9: Relationship between burning rate and pore surface area u9 (numbers denote % burn-off).

24

Table 3: Data for coal chars

Surface area measurements were made in the PTGA at room temperature and a pressure of 10 atm using

CO2 as the adsorption gas. The approach due to Brunauer, Emmett, and Teller [3] was used in the analysis

of the adsorption data to yield specific surface areas. During selected oxidation tests, in situ surface area

measurements were made throughout the course of oxidation by abruptly interrupting the test at a selected

extent of conversion by switching to a nitrogen environment and cooling to 298 K, obtaining CO2

adsorption data at 10 atm and 298 K, and reheating to the oxidation test temperature in a nitrogen

environment before restarting the oxygen flow and continuing the oxidation test. Results of a typical test

are shown in Fig. 8; the least squares fit to the data yielded a structural parameter of ϕ =7 for this particular

char. Based on the analysis of the results of our investigations of the heterogeneous reaction mechanism,

the following conclusions can be drawn:

• The migration of chemisorbed oxide complexes on char surfaces influences the distribution of

desorption energies, especially at temperatures above 1000 K.

• For adsorbed surface species that have activation energy-based distributions, a 4-site approximation, at

minimum, is needed to capture the behavior of the full distribution. Using fewer sites to represent the

25

distribution is likely to lead to results that misrepresent the true behavior of the adsorbed oxygen

complexes.

• Failure to dynamically link surface area evolution to changes in surface species concentration can lead

to significant errors in modeling the char conversion process. These errors are significant with

microporous chars, especially at high extents of conversion.

Fig 10: Weal Zone and Strong Zone Burning profiles

26

Fig. 11: Trend of maximum rate of combustion (R) versus surface area (CO,-adsorbate) of untreated

samples of coals of different rank

27

Fig. 12: Trend of maximum rate of combustion (R) versus modified surface area

6. Adsorption Effects:

Mercury adsorption on activated carbon Activated carbon (AC) has been extensively tested in lab-scale

and full-scale systems and has shown capacity to capture both elemental and oxidized Hg in coal

combustion flue gas. A relatively large amount of activated carbon injection is required for the control of

Hg from subbituminous-coal- or lignite-combustion flue gas. Depending on the system conditions, an

activated carbon-to-mercury mass ratio of at least 3000–20,000 (C/Hg) can be necessary to achieve 90%

Hg removal (Padak and Wilcox, 2009). Currently, the design of more effective Hg capture technology is

28

limited by incomplete understanding of the mechanism(s) of Hg oxidation and adsorption (Hower et al.,

2010 and Padak and Wilcox, 2009).

The complicated nature of Hg adsorption on activated carbon arises from the multiple mechanisms that

play a role in Hg uptake by activated carbon. When considering the removal of Hg from a gas stream

using activated carbon, both physisorption and chemisorption must be considered. In general,

physisorption has only a small impact on Hg removal, while chemisorption, which involves the

heterogeneous oxidation of Hg to Hg2 +, is dominant. Furthermore, other species present in the gas

stream (including HCl, Cl2, SOx, NOx, H2O and O2) often play a role in the capture, having either

competing or beneficial effects on the oxidation that tend to vary with temperature and preparation of the

sorbents. It is generally accepted that acidic sites on the surface are responsible for elemental Hg capture

on activated carbon (Mibeck et al., 2009 and Olson et al., 2004). In its atomic state, Hg acts as a base in

that it has the propensity to oxidize (i.e., donates electrons to a surface or another gas-phase molecule);

therefore, Hg will readily interact with acidic sites on the carbon surface. However, once oxidized, and

thus acidic in nature, Hg species are thought to compete with acidic gases for the basic sites available on

the carbon surface. In general, experimental and theoretical investigations of Hg adsorption on activated

carbon sorbents reveal no evidence of Hg adsorbed on the surface, but support chemisorption mechanisms

(Huggins et al., 2003, Hutson et al., 2007 and Padak and Wilcox, 2009). To gain further insight into the

effect of halogen and acid gas presence on the adsorption of Hg on AC, each will be discussed next.

6.1. The effect of halogens

The presence of halogens (i.e., bromine, chlorine, and iodine) promotes the oxidation of Hg on carbon

surfaces (Ghorishi and Gullett, 1998 and Olson et al., 2004). Subsequently, AC demonstrates higher Hg

removal performance in the flue gas of coals with greater chlorine content, since the combustion of such

coal results in a higher concentration of HCl in the flue gas. Hutson et al. (2007) exposed brominated and

chlorinated AC to Hg-laden simulated flue gas and characterized the sorbents using X-ray absorption

spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Within this work no evidence was

found for homogeneous oxidation of Hg, and no Hg was on the AC surface; however, oxidized Hg was

found on the surface, present as a chlorinated or brominated species. It is important to note that, due to

the low coverage of Hg on the carbon, the speciation of Hg was not determined. Given the results, the

authors proposed that Hg capture on chlorinated and brominated carbons occurs via surface oxidation of

Hg with subsequent adsorption on the carbon surface. Bromine is thought to have a stronger promotional

effect on Hg oxidation/adsorption, but the reason for the difference between bromine and chlorine is not

well understood. Homogeneous oxidation studies indicate that the higher polarizability of bromine may

be responsible for its enhanced ability to oxidize Hg (Wilcox and Okano, 2011). Lee et al. (2006)

examined the effects of pyrolyzed pulverized carbon and noted that Hg capture by AC is not linked to

surface area, but instead due to chemisorption, which is in line with Hg oxidation on the carbon surface.

Recently, the oxidation of Hg was demonstrated on a coal-derived Cl-promoted AC in both N2 and flue

gas (Hu et al., 2010). The adsorption of Hg on AC was shown to be a complete chemisorption process,

where all Hg was oxidized to Hg2+ on the surface due to chlorine promotion. While chlorine was

29

consumed, Hg2+ was still noted as being present in the outlet gas, indicating that the AC was still capable

of catalyzing Hg with flue gas components.

6.2. The effect of sulfur species

Experimental work has shown that sulfur can have either a positive or negative impact in oxidizing and

capturing Hg on AC, depending on the species and presence either on the surface or in the gas phase.

Lopez-Anton et al. (2002) investigated sulfur-treated AC versus untreated AC for both combustion flue

gas and gasification fuel gas applications. The capture of Hg was more extensive on the sulfur-treated

AC in both gas environments, but with only minor differences between the two. The sulfur-treated AC

demonstrated an absence of temperature dependence, while the untreated AC proved to be ineffective at

Hg capture above 120 °C. Again, it was concluded that the difference in capture capacity was due to

differences in sorption mechanisms, with both physisorption and chemisorption taking place on the

sulfur-treated AC and only physisorption taking place on the untreated AC. Olson et al. (2005) found that

HCl and S4+ compete for the basic binding site, suggesting that the decrease in AC performance with

increasing sulfur is due to competition between Hg2+ and S4+ for the basic carbon site. Presto and Granite

(2007) indicated that Hg capture by activated carbon is independent of SO2 but the presence of SO3 can

reduce or completely eliminate Hg capture. Further investigation showed that SO3 significantly impacted

Hg capture on activated carbons due to the adsorption of SO3 and Hg at the same surface sites (Presto et

al., 2007). Uddin et al. (2008) noted the complex relationship between Hg capture and SO2 using both

treated and untreated commercial ACs. Mercury capture was more effective at lower temperatures, which

is indicative of physisorption. Sulfur dioxide, O2 and H2O were shown to be required for capture on

unpromoted AC, while AC pretreated with either SO2 or H2SO4 removed Hg in the absence of SO2.

However, the presence of SO2 and sulfur-treated AC resulted in a suppression of Hg removal. In coal-

derived fuel gas, the oxidation of Hg was noted on commercial AC due to the presence of HCl. The

presence of H2S decreased Hg removal even in the presence of HCl, while the Hg species captured on

the AC was less stable when H2S was present. The Hg species stability was similar to that of HgClx

species, indicating oxidation even in a reductive environment (Uddin et al., 2009). A recent study on two

commercial ACs in an inert atmosphere indicated that the presence of SO2 increases Hg adsorption while

HCl and O2 increase Hg adsorption due to heterogeneous oxidation (Diamantopoulou et al., 2010).

6.3. Effect of other acidic gases

Increasingly, experiments involve simulated flue gases to investigate the effects of various combinations

of halogens, NOx, SOx, and H2O. The oxidant for Hg oxidation may be provided from the interaction

between the carbon surface and acid gas-phase components such as HCl and NO2. For the subsequent

adsorption of oxidized Hg, other acidic gas components, especially SO2 and H2O, may also interact with

the carbon surface and affect the oxidized Hg adsorption mechanism. For instance, investigations of the

effect of acid gases on Hg adsorption support a competition model: SO2 oxidation (by NO2) and

subsequent hydrolysis (on the carbon surface) lead to the formation of H2SO4, which is thought to

displace surface-bound Hg2+, resulting in release of HgCl2 (Carey et al., 1998, Laumb et al., 2004 and

Olson et al., 2005). Granite et al. (2000) discuss a variety of activated carbon sorbents, comparing

30

commercially available activated carbons with sulfur-, iodine-, chlorine- and nitric acid-promoted

carbons. The promoted carbons outperformed the unpromoted carbons, due to chemisorption and the

oxidation of Hg on the surface, contrasted with simple physisorption on the unpromoted carbon surface.

Wang et al. (2009) studied Hg transformation and removal in five coal-fired-boiler flue gases. It was

observed that Hg oxidation is promoted by NOX, SO2, HCl and Cl2 and the presence of unburned carbon

on fly ash impacted the capture and removal of Hg from the gas stream.

Fig. 13: Hg 4f core level XPS spectra for activated carbon sorbents at various indicated conditions; (a)–

(c) brominated AC sorbents and (d) virgin AC sorbent

7. Chemisorption Effects:

7.1. Decomposition under a nitrogen atmosphere

A typical thermogravimetric heating curve for coal in nitrogen is shown in figure 14. The zone labeled I,

around 373 K, was due to loss of inherent moisture, and the major decomposition zone labeled II, at about

731 K, was due to the volatiles released. Further decomposition occured at a relatively constant mass-

loss rate until the final set temperature of 1173 K was reached. Table 1 shows the effect of calcium

chloride and calcium acetate on a number of parameters related to zone II: the temperature at which the

31

maximum rate of mass-loss occured; R, the reactivity at T(mi) max; t the time at which 40% (dry and

free) of the sample had decomposed; and To+,, the temperature at t, T(II) did not vary with the amount

of calcium chloride added, not did R,, vary significantly. However, from Fig. 14 it is clear that the

presence of additional chloride reduced the volatile matter evolved at these low heating rates (200 C).

The cumulative yield of volatile material produced in each case is summarized in Table 3.

Fig. 14: Effect of CaCl2 on the decomposition of coal in nitrogen (105 N/m2)

Table 4:

32

Table 5:

Table 6:

Table 5 shows the mass loss results obtained when the furnace temperature was set at 1173 K, which

provided an average sample heating rate of 2100°C min1. In this case the overall sample decomposition

rate was decreased in the presence of either calcium acetate or calcium chloride. However, the effect of

calcium acetate was somewhat less than that of calcium chloride.

Table 7:

7.2. DSC runs in air

33

Figure 3 shows a number of DSC heating curves for coal. The results obtained when coal was reacted in

air agree with those obtained using the TGA, namely calcium acetate shifted zone II to a lower

temperature whereas calcium chloride produced a shift to higher temperatures. The DSC curves,

however, illustrated more clearly the promoting effect of both additives in zone III and provided

somewhat greater details in the temperature interval 673—773 K. showing the presence of another

reaction zone. Combustion of char in air (Fig. 4) was also enhanced as a result of treating the parent coal

with calcium chloride. Only one exothermic zone was clearly formed (Fig. 5). The char was believed to

be mostly fixed carbon.

The effect of chemisorption dynamics on thermogravimetric determination of carbon reactivity is studied,

based on the validity of the pseudo-steady state assumption. It is shown that in the typical conditions of

thermogravimetric studies, the effect of chemisorption dynamics is always negligible for gasification of

carbon in carbon dioxide while it is generally important for gasification in oxygen unless very low oxygen

pressure is used (< 0.1 atm). Subsequently a new approach is proposed for obtaining the activation energy

distributions for chemisorption of oxygen on carbon, without the normal assumption of a known

distribution function and a constant pre-exponential factor. The activation energy distribution function is

found to consist of two discrete flat distributions, suggesting two groups of active sites. Following the

chemisorption studies, the energetics of another important reaction, i.e. the reaction of CO desorption

from graphite edge site is calculated using the ab initio molecular orbital theory. The energy of the

reaction is found to be sensitive to the carbon structure and the coverage of active sites.

The new approach is also used to obtain the activation energy distribution of thermal annealing of a

bituminous coal in an entrained flow reactor. The distribution function again appears to consist of two

discrete functions. The characteristic time of thermal deactivation is found to be comparable to that of

combustion, indicating the importance of thermal deactivation in coal combustion. The variation of

structure of an anthracite as well as its reactivity with heat treatment time at various temperatures is then

studied. It is demonstrated that the reactivity can be correlated with the fraction of organized carbon in

char, suggesting the importance of the structural ordering in thermal deactivation. Iron is found to

catalyze the process of structural ordering, evidenced by the observation of nearly perfect structure

around the iron particles.

The pore structure as well as the crystallite structure during gasification of coal chars heat treated at

different temperatures is then studied to improve the understanding of the gasification process. The effect

of heat treatment temperature on the development of the pore structure is different for different coals.

The results also show that air gasification is different from CO2 gasification in the development of the

small micropores (< l0Å). The surface area and volume of the small micropores increases quickly initially

and remains almost constant during air gasification while it keeps rising sharply during CO2 gasification.

The difference is attributed to the different rates of opening of the closed pores. It is also shown that the

variation of the crystallite structure can be different for two coal chars when that of the pore structure is

similar.

34

Percolative fragmentation of char particles has been confirmed to occur during gasification of several

coal chars. The electrical resistivity of the coal chars is measured and found to increase sharply after a

certain conversion, indicative of percolative fragmentation. Two percolation models are applied

successfully to fit the variation of the electrical resistivity and the percolation thresholds are obtained.

The partially gasified char particles are observed under an optical microscope and number of fragments

are found to increase with the increase of conversion, confirming the occurrence of fragmentation.

Chemisorption of oxygen on carbonized cellulose is a precursor leading to gasification. Still, there exists

a main disparate view on whether gasification and chemisorption are two distinct processes or an

interrelated on in the modelling of char combustion 1,2. Typically, the relation of the two processes is

studied via experiments with chars prepared from the doped samples with additives 3,4. Their effect on

pyrolysis kinetics and chemisorption kinetics of the doped samples hitherto provides the required

arguments to support or counteract the proposition. In all such studies, the need to understand the relation

between the two processes is motivated by the better understanding that could enhance the modelling and

the control of the smouldering combustion of cellulosic materials.

Chemisorption of oxygen by carbonaceous materials has been well researched. However, the char

samples investigated were exposed to high heat treatment temperature in the range of 400°C to 800°C 1.

The basis to draw inferences on the reactivity and kinetics onto a lower heat treated coal chars is

questionable. The nature of coal chars changes by the rate and the final heat treatment temperature it is

pyrolysed. Add to these variables is the duration for which the isothermal heating is being maintained.

One could get a glimpse of the enormity of the issue by looking at the combination of these variables.

7.3. Example:

This study is interested in chemisorption and combustion behaviour of low temperature chars created at

140°C and 150°C exposed to long-term heating in air. Since maximum chemisorption kinetics is reported

for nitrogen pyrolysed chars created at HTT of 550°C 5, it would be of interest to examine the

chemisorption kinetics of the same inert chars that are created at a much moderate temperature of 300°C.

The temperatures at which these categories of coal char - both oxidatively and non-oxidatively pyrolysed,

more closely resemble the cases encountered in built environment.

7.3.1. Chemisorption for Samples Preheated in Nitrogen

Oxygen chemisorption (21% oxygen in air) on Kapur char that has been pyrolysed at heat treatment

temperature (HTT) of 300°C for 1.5 minutes in TGA which were then subjected to chemisorption runs

at a series of chemisorption temperatures (CST). Because of the low heat treatment temperature, the coal

chars were invariably very reactive. Indeed, for chemisorption temperatures above 140°C, gasification

set in early. For the 207°C run, gasification was dominant and this data has to be excluded. The

chemisorption data for 168°C and 185°C were truncated before gasification set in. On the other hand,

coal chars tested at low chemisorption temperatures equal and below 140°C did not show any weight

loss, henceforth, these coal chars data could be interpreted for the full run for chemisorption. Since

gasification products could not be detected below 140°C threshold for coal chars 3, and desorption of

35

surface oxides are negligible below1000°C, it permits these chemisorption runs to be treated as a single

oxygen adsorption process.

Fig. 15: Oxygen chemisorption of lignocellulosic char in air in isothermal heating under atmospheric

pressure

The observed chemisorption behaviour of gases on heterogeneous surfaces of coal chars above can be

described by the Elovich equation as:

The impact of a lower ignition temperature on the pyrolysis kinetics on coal chars however remains

unknown. There is a question whether lower ignition temperature means a change in the pyrolysis kinetics

of coal chars, and hence an alteration in the pyrolysis pathways. Alternatively, it is also a question if the

different ignition temperatures between subsequent extension of preheating duration and/or change in

heat treatment temperature are kinetically significant, thereby providing a clue as to how chemisorption

has affected the combustion behaviour of solid. Low temperature coal chars possess altogether a different

type of active sites responsible for chemisorption activities, which are remarkably different from the more

36

carbonized and mature graphitic carbon polymers. Other than inert-heated coal chars, chemisorption also

occurs in long-term air-preheated coal chars. Lower ignition temperatures have subsequently been noted

for preheated coal chars. The experimental findings show that air-preheated coal chars would also

undergo chemisorption if continued heating removes the surface oxides and reactivates the surface active

sites. Despite the fact that chemisorption leads to a lower ignition temperature, the observed range of

lowered ignition temperatures is not shown to be kinetically significant on coal pyrolysis, neither does

the change in ignition temperature suggests a switch in pyrolysis pathway in coal decomposition.

8. Peroxy-Radical Effects:

The decomposition reaction of CaO2 and its catalytic effect on combustion efficiency of different ranks

of pulverized coal were investigated by employing the thermal analysis, and the mechanism of catalytic

combustion was proposed. The results indicated that the temperature region of CaO2 decomposition

reaction approached coal pyrolysis, and the active oxygen prepared by the decomposition of CaO2 can

improve oxidation reaction of pyrolysis gas and combustion reaction of fixed carbon, which resulted in

enhancement of heat release of three pulverized coals. The kinetic study was carried out and the results

were presented. Activation energy of samples will decrease with CaO2 addition. By analyzing the results,

the synergy between the decomposition temperature of CaO2 and the pyrolysis temperature, as well as

the ignition mode, are very important to the catalytic effect of coal samples.

9. Threshold Temperature Effects:

Ignition experiments on high-volatile bituminous coal showed that the ignition temperature is strongly

dependent on the mode of preheating the sample to the ignition temperature point, particle size, air flow

rates, and sample compaction. When using air alone for preheating, it was found to be impossible to attain

a uniform sample temperature just before ignition. Accordingly, a technique was evolved to first raise

the temperature of the sample close to the ignition temperature by passing a stream of hot nitrogen or

carbon dioxide through the sample. Once the constant temperature of the sample was obtained, the stream

of gas was switched to air. A similar inert preheating procedure has been used by Hardman et.al. to

determine the spontaneous ignition temperature of activated carbon. Ignition experiments on high-

volatile bituminous coals The ignition temperatures of three, high-volatile bituminous coals from

Elkhorn, Ohio and Pittsburgh seam are studied. These three coals were chosen because of their large

differences in their petrographic composition, especially in their vitrinite and exinite content. It has been

suggested that there is a close relationship between the coal maceral type and spontaneous oxidation

potential. Chamberlain and Hall demonstrated this kind of role when they found that exinites oxidized

much more readily than vitrinites and inertinites. If such is the case then one will expect Ohio coal to

show the lowest ignition temperature. The char prepared from one of the cobalt—exchanged samples

was initially so reactive that the rate of gasification exceeded the detection limits of the detector, I.e., all

of the oxygen in the combustion air supply was consumed by combustion of product gases. In this case

37

the maximum rate of gasification was not observable, and the extent of gasification was therefore

indicated as being “greater than” the value of the integrated detector response. The change in sample

weight is indicated to show the total extent of reaction. The reactivities of the chars containing cobalt

catalyst are clearly less dependent on HTT than are those of the chars containing calcium and potassium.

The reactivities of thef latter chars toward gasification at 800°C increase by at least a factor of two as the

HIT is reduced from 10000 to 800°C. This behavior Is typical of trends shown by other Investigators

who have studied the effects of HTT on catalyzed gasification of lignite chars (6—8). By contrast, chars

prepared from cobalt-exchanged coal at 800° and 1000°C are gasified to a similar extent at 800°C, When

HTT and gasification temperature are reduced to 600°C, the char prepared from cobalt-treated coal is

completely gasified, whereas the chars prepared from calcium and potassium—exchange coal are

completely unreactive at this temperature. When cobalt exchanged coal was charred and gasified at

4000C, no gasification occurred.

Table 8:

Sample HTT Char Yield (% d.a.f.) Percent Gasified

800 9.2 16

700 10.9 66

600 12.9 70

500 17.2 3

38

10. Role of Activation Energy:

10.1. Activation energy and heat of reaction:

Fig. 16: Effect of CaCl2 on the decomposition of char in air (105 N/m2): Char+ means from CaCl2

treated coal

Both calcium Chloride and Calcium Acetate reduced the activation energy of coal and increased the

overall heat of reaction.

Although the effect was less pronounced in the case of char (Table 8), the trend was still true. It is this

relationship between E and H for coal, observed in an earlier study, that prompted us to suggest that

this apparent activation energy is a measure of the net heat resulting from a number of reactions within a

given temperature interval.

39

10.2. Example:

Fig. 17: DSC heating curves for char in air (105 N/m2): Char+ means from CaCl2 treated coal

Table 8:

The elemental analysis of the coal, while Figure 1 presents its proximate analysis obtained by TG is

presented. The high ash and sulfur contents typical of the raw south brazilian coals are observed. X-ray

diffraction analyses indicated that sulfur is present in the compounds CaSO4.2H2O and (NH4)3Fe(SO4)3.

Contrary to the expected, the analyses did not detect any presence of pyrite/marcasite (FeS2). Those

compounds are generally present in coals in variable amounts. For instance, it represents in average up

to about 1.7 % of the total sulfur present in north american coals, and up to 72 % of the sulfur in Polish

coals47. The X-ray difraction results were confirmed in repeated experiments.

40

Fig. 18: TG Proximate analysis of coal (I-Moisture; II-Volatiles; III-Fixed Carbon; IV-Ashes)

Figure 19 presents TG results on pyrolysis and combustion which were used for determining the ignition

temperature. The ignition temperature is assumed to be the average temperature in the last time interval

where both curves coincide7. The determination of ignition temperature through TG showed to be

dependent on the mass of the sample. However, a variation of 100 % on the sample mass (from 10 to 20

mg) caused a variation of only 3 % on the temperature of ignition (from 482 to 468 ºC).

41

Fig. 20: TG results for pyrolysis and combustion of the coal used for determining the ignition

temperature

Figure 21 shows DTA results of combustion which allow to evaluate about the type of ignition. Flatter

curves indicate homogeneous combustion, while sharp pics are indicative of heterogeneous

combustion48. The figure shows a very sharp pic in the exothermic event, indicating a predominant

heterogeneous combustion.

Fig. 21: DTA results for combustion of the coal

Figure 22 shows results of TG experiments carried out for determining the reaction rate coefficient, i.e.

the pre-exponential factor and the activation energy. Figure 5 presents DTG curves obtained by time

derivation of the TG curves from Figure 4. Two main reactive events take place which are identified as

primary and secondary combustion8. Those events are well characterized through the observed changes

of behavior of the DTG curves from ignition, which happened around 470 ºC for all the heating rates.

Results showed that at higher heating rates both primary and secondary combustion are more pronounced.

Also, the higher the heating rate the higher the temperature where the maximum reaction rate takes place.

42

Fig: 22: TG results on combustion of the coal for the chemical kinetics analysis

For the primary combustion step it follows that where wr is the residual weight of the sample as primary

combustion is finished, and k1 is the reaction rate coefficient for the primary combustion step.

Fig. 23: DTG curves obtained from TG curves of Figure 22

where wf is the residual weight of the sample as secondary combustion is finished, and k2 is the reaction

rate coefficient for the secondary combustion step.

43

Fig. 24: Arrhenius plot for TG Combustion of the coal for both primary and secondary step

From the TG and DTG results for the various heating rates considered and are obtained as a function of

1/T, for both primary and secondary combustion steps. Those are plotted in Figure 6. The Arrhenius

analysis of the primary and secondary combustion stages were made excluding transition regions, namely

from drying to primary combustion, from primary to secondary combustion, and from secondary

combustion to burnout.

Fig. 25: Arrhenius plot for TG combustion of the coal for a unique combustion step.

44

By applying least squares regression to both the primary and the secondary combustion regions, pre-

exponential factors and activation energies are obtained. It can be observed from the plots that the higher

the heating rate the lower the resulting activation energy in both combustion stages. Accounting for all

the data for all the heating rates, the Arrhenius fits for the primary and the secondary combustion steps

result, respectively.

Table 9: Some literature data of activation energy on combustion of coals and chars considering nth

order (power law) Arrhenius Kinetics

Hakvoort et al.49 observes that the stage of reaction where volatiles combustion take place (primary

combustion) is characterized by lower activation energies as compared to fixed carbon combustion alone

(secondary combustion). Contrary to that, in the present work the activation energy was lower for the

combustion of fixed carbon alone. Such is a consequence of the transient nature of the non-isothermal

experiments that have been performed, in which the physical structure of the coal is continuously

changing during reaction. At higher temperatures, where secondary combustion takes place, the reaction

tends to be controlled by intra-particle diffusion. As combustion proceeds carbon is continuously

removed from the particles thereby opening pores and reducing diffusion resistances. A progressive

conversion mode of combustion may also become dominant, thereby enlarging the fraction of a particle

actually available to reaction. Those effects enhance reaction rate and contribute to a decreasing

activation energy. Alongside with decreasing diffusion resistances, BET surface area is supposed to be

45

reduced by both changing of micro and mesopores into macropores, and sintering. Considering the above

trends, and in view of the presently observed reduction of activation energy in the secondary combustion

stage, it appears that the effect of reducing diffusion resistances predominates over the opposite effect of

loosing BET surface area. It could be argued that the higher temperatures and consequent decreased

kinetic resistances would be the only reason for enhanced reaction and decreased activation energy in the

secondary combustion stage. However, the proposition of a decreasing diffusion resistance suits better

the results of the present work. The TG curves of Figure 4 show that the reaction rates, as primary

combustion ends, quickly change to higher levels and keep mostly constant throughout the whole range

of increasing temperatures as secondary combustion develops. It seems that the activation energy depends

very much on the evolving particle structure, so that a correlation better than Arrhenius' should be sought

if coal combustion kinetics is to be more accurately addressed.

Table 9 presents some literature data of activation energy on combustion of coals and chars in comparison

to the present result for a brazilian coal. It is seen that the present result falls inside the common range of

activation energies reported in literature, which was expected since the activation energy is supposed to

be independent of coal type and properties17. Ilic et al. studied the kinetics of combustion of a high ash

brown coal (21.7% ash, 32.5% volatiles, 36.9% fixed carbon) and found an activation energy of 93.3

kJ/mol. This figure is very close to the present result of 104.2 kJ/mol found for a high ash brazilian

bituminous coal (44.50% ash, 19.25% volatiles, 35.44%), in spite of the differences on the proximate

analysis of the two coals. Also, the present result is inside the range reported by Cumming10 for

bituminous coals (72 - 114 kJ/mol).

Regarding the pre-exponential factor, comparisons to literature data are quite difficult since the parameter

varies widely even for the same coal in different reaction stages. Besides, it is quite dependant on the

order of reaction regarding reactant gases. Even very slight changes on the activation energy cause huge

changes on the pre-exponential factor. In the present experiments the pre-exponential factor resulted

65271 s-1 for the primary combustion step, and 8.3951 s-1 for the secondary combustion step. Those

numbers differ to each other by 4 orders of magnitude. Therefore, besides the dependence on coal type,

the pre-exponential factor shows to be quite sensitive to the reaction stage as well.

With the increase of the mixing ratio of sludge, the apparent activation energy and the frequency factor

of the mixture decline (Lou and Wang, 2011). However, the reducing trend gradually slows down, which

is due to the coal (or other fuels) regarded as the composition of the activation energy distributed from

low to high. While the sludge mixing ratio goes up, the ignition temperature elevates. The combustion

reaction moves to the low temperature area. The material of high activation energy can be reacted at low

temperature. At the same time, the apparent activation energy increases during the low-temperature

period, which leads to a downward trend. The frequency factor reflects the activity of material internal

molecules at certain temperatures. The activity of the sludge is lower than that of coal. The frequency

factor decreases with the growth of the sludge ratio.

The decline of the coal particle size is observed with a minor reduction apparent in activation energy,

while frequency factor rises. The results reveal that smaller particle size of coal has bigger specific surface

46

area. The contacting and collision with oxygen promote the frequency factor. For the same kind of coal,

with the decline in particle size, the content of volatilizing and fixed carbon increases. Relatively, the

content of ash decreases. And this is an external macroscopic expression when the structure of coal

microcosmic granule is affected by various particle sizes. With the decrease of the particle size of coal,

the heat reactivity is more rapid and the heat easily releases from the combustion products. Furthermore,

the combustion performance improves. From performance of the macro effect, It can be seen that when

absorbing less heat, the burning process keeps on going and the activation energy reduces.

11. Role of Frequency Factor:

The frequency factor, A, depends on how often molecules collide when all concentrations are 1 mol/L

and on whether the molecules are properly oriented when they collide.

Combustion kinetic parameters (i.e., activation energy and frequency factor) of coal have been proven to

relate closely to coal properties; however, the quantitative relationship between them still requires further

study. This paper adopts a support vector regression machine (SVR) to generate the models of the non-

linear relationship between combustion kinetic parameters and coal quality. Kinetic analyses on the

thermo-gravimetry (TG) data of 80 coal samples were performed to prepare training data and testing data

for the SVR. The models developed were used in the estimation of the combustion kinetic parameters of

ten testing samples. The predicted results showed that the root mean square errors (RMSEs) were 2.571

for the activation energy and 0.565 for the frequency factor in logarithmic form, respectively. TG curves

defined by predicted kinetic parameters were fitted to the experimental data with a high degree of

precision.

A method of Achar-Brindley-Sharp-Wendworth (ABSW) was applied to a simultaneous calculation of

the kinetic parameters (including the apparent activation energy, the reaction order and the frequency

factor). Meanwhile, this study also revealed that both the burning performance and the characteristic

parameters improved when sludge mixing ratio was smaller (10 wt.%). The ignition temperature

advanced with an increase of the sludge proportion, while the combustion characteristic index dropped.

As the sludge mixture ratio rose to 70 wt.%, the DTG curve reached three peaks at 293 °C, 580 °C and

748 °C Decreasing the coal particle size led to the advancement of the devolatilization, fixed carbon

burning stage and maximum weight loss rate, and the reduction of the corresponding temperature.

Additionally, the apparent activation energy and frequency factor of the mixture reduced when the

proportion of the sludge mixing ratio went up. Also, the experiment results indicated that with the decline

of the particle size distribution of coal, the apparent activation energy followed a downward trend, while

the frequency factor increased.

47

Example:

Table 11: Dynamic parameters under different sludge mixture proportions

Mixing ratio T (°C) n E (kJ kg-1) A (s-1) r

0 wt.% 503-735 1.1 104.82 96054 0.990

10 wt.% 497-716 1.2 88.06 54782 0.991

20 wt.% 481-739 0.9 76.68 8348 0.987

30 wt.% 478-739 1.1 68.35 2415 0.986

50 wt.% 470-739 1 65.80 1852 0.983

70 wt.% 416-741 1.3 63.08 1678 0.984

100 wt.% 227-743 3.5 35.07 730 0.981

Table 12: Values of A and E for varied mesh size

Particle size (mesh) T (°C) n E (kJ kg-1) A (s-1) r

80-100 475-768 1 68.57 1048 0.986

100-120 469-758 1 67.63 1537 0.983

120-150 465-747 1 65.59 1875 0.984

150-180 460-735 1 59.58 2004 0.992

180-200 450-699 1 54.57 2107 0.996

The co-combustion characteristics of sludge and coal are the results of their joint action. In the

cocombustion process, both of them keep their original combustion characteristics separately.

When the mixing ratio of sludge is small (10 wt%), they have strong synergy and acceleration,

which promotes the co-combustion performance and burning process. If the proportion of sludge

is bigger, it is beneficial to stable combustion and harmful to the ignition.

With the decrease of the particle size of coal, the weight loss of mixture increases from 48% to

53%, which accelerates the separation of volatile. Meanwhile, the combustion process becomes

fiercer. Additionally the ignition temperature, the maximum of weight loss rate, and the burnout

temperature reduce, while the combustible index and the comprehensive combustion

characteristic index increase. Thus, the comprehensive burning level improves.

With the increase in the mixing ratio of sludge, the apparent activation energy reduces from

104.82 kJ kg-1 to 35.07 kJ kg-1, and the frequency factor falls from 96054 s-1 to 730 s-1.

48

Moreover if the particle size of coal decreases, the relative specific surface area gradually

increases, and the resistance of the combustion reduces. Specifically the apparent activation

energy reduces to 54.57 kJ kg-1 by 68.57 kJ kg-1, and the frequency factor increases from 1048

s-1 to 2107 s-1.

TG non-isothermal chemical kinetic study was undertaken as in the above discussions for the combustion

of a brazilian raw mineral coal. DTA figures showed a very sharp pick of temperature in the exothermic

event, indicating heterogeneous combustion. X-ray difraction analyses of the coal indicated the presence

of sulfur in the compounds CaSO4.2H2O and (NH4)3.Fe(SO4)3. No pyrite (FeS2) was found. comparisons

to literature data are quite difficult since the parameter varies widely even for the same coal in different

reaction stages. Besides, it is quite dependant on the order of reaction regarding reactant gases. Even very

slight changes on the activation energy cause huge changes on the pre-exponential factor. In the present

experiments the pre-exponential factor resulted 65271 s-1 for the primary combustion step, and 8.3951

s-1 for the secondary combustion step. Those numbers differ to each other by 4 orders of magnitude.

TG results showed that at higher heating rates both primary and secondary combustion are more

pronounced. The higher the heating rate the higher the temperature where the maximum reaction rate

takes place. Also, higher heating rates produced lower activation energies. Contrary to some literature

data, the activation energy resulted lower during the secondary combustion stage as compared to the

primary combustion step. It was concluded that such shall be a consequence of decreased intra-particle

diffusion resistances owing to the changing structure of the coal particles as temperature is raised. A

decreased kinetic resistance was ruled out as a cause for the enhanced reactivity in the secondary

combustion step since reaction rate keeps mostly constant throughout the whole range of increasing

temperatures as reaction develops.

As well known, non-isothermal studies are relevant for reactive processes where only truly kinetic effects

take place. In high ash high volatile coal combustion, however, other effects happen which must be

accounted for. Those are mainly related to intra-particle mass transfer and structural changes affecting

the availabilitity of carbon active sites for chemical reaction. The unknown evolution of porosity and

BET surface area during reaction is a major difficulty to overcome. The kinetic parameters derived in

this work come from reaction rates characterized only by temperature, irrespective of mass transfer and

structural transient variations. In fact, at each temperature of the process there were very instantaneous

and well defined as well as unknown conditions of porosity and BET surface area. The kinetic parameters

presented in this work must be approached with care since they were determined in a process under

changing conditions other than temperature. Notwithstanding, the activation energies found in this work

are inside the common range for coals and chars reported in literature.

49

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