BİTÜMLÜ KÖMÜR VE BİYOKÜTLENİN BİRLİKTE PİROLİZİ

7/30/2019 BTML KMR VE BYOKTLENN BRLKTE PROLZ

1/8

Product distributions from isothermal co-pyrolysis of coal and biomass

Nathan T. Weiland a,b,, Nicholas C. Means a,c, Bryan D. Morreale a

a National Energy Technology Laboratory, Pittsburgh, PA, United Statesb Mechanical & Aerospace Engineering Dept., West Virginia Univ., Morgantown, WV, United Statesc URS Corporation, Pittsburgh, PA, United States

a r t i c l e i n f o

Article history:

Received 3 June 2011

Received in revised form 5 October 2011

Accepted 20 October 2011

Available online 7 November 2011

Keywords:

Pyrolysis

Biomass

Switchgrass

Isothermal

Products

Coal

a b s t r a c t

Co-gasification and co-pyrolysis of coal and biomass are being studied as a means to reduce the carbon

footprint of an IGCC plant. Co-feeding creates many challenges in the thermochemical conversion of coal/

biomass such as the variable nature of biomass feedstocks, potential nonlinear reaction rate effects dur-

ing conversion and the varying composition of the products. An experimental study on isothermal co-

pyrolysis of Illinois #6 coal and switchgrass was done in a drop reactor at 900 C to investigate the effects

of co-feeding on pyrolysis product distributions under conditions relevant to transport gasifiers. Coal/bio-

mass mixtures were fed to the reactor in feed ratios of 100/0, 85/15, 70/30, 50/50, and 0/100, while pri-

mary gaseous products (CO, CO2, CH4, H2 and H2O) were monitored and analyzed online. Ultimate

analysis of solid and liquid products is used to track the distribution of the feedstocks elements and

energy content into its pyrolysis products, while GCMS andash elemental analyses areprovided to more

fully characterize these products. Experimental results show that under the conditions studied, product

distributions do not display any non-linear effects, and can be estimated as a mass-weighted sum of the

product distributions of the pure feedstocks. This result is likely due to the higher temperatures used in

this study, though it is inherently useful in the development of higher-temperature gasification systems.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

The thermochemical conversion of coal is a complex reaction

process which takes advantage of an abundant natural resource.

Recoverable coal reserves in the United States have been estimated

at 260 billion short tons. This is approximately 27% of the total

world recoverable coal reserves [1]. Coal can be burned to produce

heat, coupled to gas turbines or fuel cells to produce electricity or

converted into synthesis gas (syngas) to produce liquid fuels and

other chemicals. The use of carbonneutral biomass is one of the

most attractive ways of reducing CO2 emissions from coal, since

the carbon in biomass is generated from atmospheric CO2 by pho-

tosynthesis. Combined thermochemical conversion of coal and bio-

mass for power may become a carbon-negative process with the

use of carbon capture and sequestration (CCS) technologies. Bio-

mass is the third largest energy resource in the world behind coal

and oil [2,3]. Plant derived biomass material is commonly available

from forestry and lumber waste, algae, crop residue and industrial

waste [4,5]. Unfortunately, the majority of the potential energy

from these biomass sources is not realized and is lost to natural

decomposition [6].

The addition of biomass to coal during thermal conversion may

have various effects. The reaction of coal and biomass to syngas

may be impacted due to synergistic interactions and may show

variations in thermal reactivity or in the chemical or physical prop-

erties of the solid, liquid and gaseous products [4].

An overview of the thermochemical conversion process is

shown in Fig. 1 [7]. During thermochemical conversion, a solid car-

bonaceous material first undergoes thermal decomposition (pyro-

lysis) to products such as syngas, tars, phenols and char. The

volatile products can then take part in gas phase reactions such

as cracking, reforming, combustion and shift. Finally, the solid char

can react to produce syngas compounds in gasification, or CO2 and

H2O in combustion.

In many coal utilization processes, the pyrolysis reaction occurs

separately from gasification or combustion. Because of this, under-

standing the pyrolysis reaction is important when analyzing ther-

mochemical conversion technologies. The focus of this paper is on

the pyrolysis reaction as a precursor to gasification, though the re-

sults apply equally well to combustion processes. Primary pyroly-

sis is a devolatilization reaction which involves the breakdown of

the macromolecular structure of the solid fuel material and the re-

lease of volatile products. A secondary devolatilization occurs at

higher temperatures resulting in the release of secondary volatile

gases such as CH4, C2H4, and HCN [8].

Although much work has been done on the thermochemical

conversion of coal, it is not well understood how co-feeding coal

0016-2361/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.fuel.2011.10.046

Corresponding author at: Mechanical & Aerospace Engineering Dept., West

Virginia Univ., Morgantown, WV, United States. Tel.: +1 412 386 4649.

E-mail address: [email protected] (N.T. Weiland).

Fuel 94 (2012) 563570

Contents lists available at SciVerse ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l
http://dx.doi.org/10.1016/j.fuel.2011.10.046mailto:[email protected]://dx.doi.org/10.1016/j.fuel.2011.10.046http://www.sciencedirect.com/science/journal/00162361http://www.elsevier.com/locate/fuelhttp://www.elsevier.com/locate/fuelhttp://www.sciencedirect.com/science/journal/00162361http://dx.doi.org/10.1016/j.fuel.2011.10.046mailto:[email protected]://dx.doi.org/10.1016/j.fuel.2011.10.046


2/8

and biomass will influence the reaction kinetics, overall conversion

and product distributions during gasification and pyrolysis. There

is some debate in the gasification literature over whether or not

there are nonlinear kinetic and product distribution effects when

biomass is added to coal. Before computational models can reason-

ably predict the effects of coal-biomass pyrolysis and gasification/

combustion, detailed experimental investigation is required. Theo-

ries for the nonlinear dependence of co-feeding include increased

reactivity of coals with biomass volatiles, catalytic effects of bio-

mass ash components on coal gasification kinetics, and differences

in surface reactivity between coal and biomass constituents.

The pyrolysis of coal may be influencedby the presence of prod-

uct gases (CO, CO2, H2, CH4, H2O, etc.) which are rapidly evolved

from the biomass at high temperature. These pyrolysis gases may

take part in gascoal interactions resulting in variations in reac-

tion kinetics, conversion and product distributions [4,9,10]. Bio-

mass is composed of a much higher quantity of oxygen

containing species compared to coal, which may be a major con-

tributor to the gascoal interactions. The production of syngas

fromcoal is impeded by a low H:C ratio. Because of this, it has been

suggested that syngas yields may be improved by adding H2 from

another source, such as biomass [10]. The increased quantity of

hydrogen produced from biomass devolatilization may react with

evolving species from coal, thus preventing the recombinationreaction. The recombination reaction impairs the conversion of

coal because free radicals produced from coal devolatilization can

react, or recombine, with char to produce a low reactivity char,

which is undesirable. The increased presence of hydrogen may pre-

vent the recombination reaction and promote an increase in coal

conversion [11].

The thermochemical conversion of coal may also be influenced

by charcoal interactions due to the presence of alkali and alka-

line earth metal species (Ca, K, Mg and Na) which are in greater

abundance in biomass relative to coal [5,12]. Previous studies have

shown that the presence of these alkali and alkaline earth metals

can have a catalytic effect on the reactivity and volatile product

distribution during pyrolysis [1316]. Similarly, other studies have

demonstrated that inorganic species can influence the gasificationof coal in a variety of gasifying environments including H2O, CO2and O2 [11,14,1719]. The results showed a positive effect on the

reactivity of coal pyrolysis [14,15] and gasification [11,14,1719].

The influence of metal species on coal pyrolysis varies between

coal types. The activity of the inorganic matter can be described

as K$ Ca > Na for bituminous coal and Ca> K > Na for lignite

[20]. Currently, it is uncertain whether alkali and alkaline earth

metals in biomass will contribute catalytic activity to coal during

fast, high temperature pyrolysis. The proximity of these metal-con-

taining functional groups in biomass relative to coal will be of great

importance. It is also a known fact that at elevated temperatures,

volatilization of alkali and alkaline earth metals from the biomass

material will occur. These volatilized species may contribute cata-

lytic activity to coal pyrolysis as well as gas phase reactions [12]. Inthis study, mixtures of Illinois #6 coal and switchgrass will be co-

fed into a drop-tube reactor at 900 C under an inert atmosphere of

argon. Coal and biomass species pairing was done based on geo-

graphical proximity. Illinois #6 coal is ranked as a high volatile C

bituminous coal. Switchgrass (Panicum virgatum L.) is a fast grow-

ing crop native to North America and is commonly used as a bio-

mass source for ethanol, electricity and heat production. The

effect of the wt% of biomass fed into the reactor will be investi-

gated for variations and distributions of solid, liquid and gaseous

products.

2. Experimental details

2.1. Feedstocks

Illinois #6 bituminous coal and switchgrass were used as the

feedstocks in this study. The coal was obtained from the Penn State

Coal Sample Bank and Database. The switchgrass is a Forestburg

cultivar from South Dakota that was baled in mid-September and

later pelletized. This feedstock was reground on site, producing a

lower aspect ratio feedstock than raw switchgrass that is more

desirable for its feeding qualities. The coal and biomass samples

were ground and sieved to 18 50 mesh (2971000 lm) and

16 50 mesh (2971190 lm), respectively. The feedstock mate-rials are fed into the reactor as:

100% coal.

50 wt% coal, 50 wt% biomass.



100% biomass.

The mixtures are weighted towards lower biomass composi-

tions, which are more likely to be used in commercial gasifier oper-

ation. Likewise, samples were not dried prior to testing, in order to

better approximate feed conditions in actual gasifier operation.

Consol Energy supplied proximate and ultimate analyses for the

coal and switchgrass, which is shown in Table 1 for both feed-stocks. Volatile matter makes up about 75% of switchgrass by mass.

This is nearly twice the volatile matter present in Illinois #6 coal.

Ultimate analysis shows that sulfur is a dominant species

($5 wt%) present in Illinois #6 coal, however the amount of sulfur

in switchgrass is only 0.12 wt%. This decrease in sulfur containing

compounds is verified in qualitative product gas analysis and may

help to reduce the impact of sulfur on downstream processes.

Other major differences in material composition include over three

times more oxygen in switchgrass than Illinois #6 coal, and coal

containing more ash than in switchgrass. Typically, the ash content

in late-harvest switchgrass is less than half of the reported value of

9.08% [16,21,22], however, much of this difference can be attrib-

uted to soil picked up by the switchgrass baler, as ash analysis re-

veals unusually high Si content (>50%) in the switchgrass ash[23,24].

SolidCarbonaceousMaterial Pyrolysis

Pyrolysis Gases(CO, H2, CH4, H2O, etc.)

Tar, Oil, Naphtha

Oxygenated Compounds(Phenols)

Char

CO, H2, CH4, CO2,H2Oand crackingproducts

CO, H2, CH4, CO2,

H2OChar-gas reactions(Gasification, Combustion, CO Shift)

Gas phase reactions

(Cracking, Reforming,Combustion, CO Shift)

Fig. 1. Coal/biomass thermochemical reaction sequence. (reproduced from [7]).

564 N.T. Weiland et al./ Fuel 94 (2012) 563570


3/8

2.2. Experimental setup and procedure

A schematic of the experimental setup used in this study is

shown in Fig. 2. The reactor is made from a 38.1 mm OD Inconel

800H containment tube with a wall thickness of 2.77 mm and a

length of about 0.91 m, housed inside a set of three resistance heat-

ers, each 0.152 m in height. This containment vessel is lined with a

quartz inner tube with a 27.0 mm ID, 1.5 mm wall thickness, and

0.705 m in length. The reactor inlet consists of a coal/biomass hop-

per separated from the reactor by a ball valve, which is opened to

drop the sample onto a porous quartz frit of about 3 mm thickness,

located in the center of the 45.6 cm hot zone defined by the height

of the resistance heaters.

Isothermal co-pyrolysis experiments are carried out at atmo-

spheric pressure, and an argon sweep gas flow rate of 2000 sccm.

Approximately 1.0 g of coal and/or biomass material was loaded

into the hopper valve, above the hot zone, prior to testing. The

top two heaters of the reactor are heated to 900 C under purge

gas flow and maintained at this temperature for approximately

one hour prior to introduction of the solid feed material. Afterthe reactor has thermally reached a steady state, the purge gas by-

pass valve is closed and the hopper valve is opened, allowing the

coal and/or biomass material to fall onto the hot frit and pyrolyze

in the center of the hot zone. After dropping the sample onto the

frit, pyrolysis products are transported out the bottom of the reac-

tor by the sweep gas.

The bottom ofthe quartztuberests ona seriesof three 8 lmash-

less filter papers supported by a metal screen. The exit of thereactor

is maintained at a temperature of 150 C, and mass that condenses

on these components, as determined by pre- and post-test weight

difference, is denoted as heavy tar. Heavy tars collected during

one of the triplicate tests are extracted from the filter papers using

a 2:1 vol% mixture of toluene and methanol for qualitative analysis

in a separate GCMS. Tars from the other two triplicate tests are ex-tracted with acetone for later ultimate analysis.

Below the heavy tar filters, the pyrolysis products are sampled

at 1 Hz by a quadrupole mass spectrometer (QMS) until the reac-

tion ceases (about 15 min). All connections above this point are

maintained at 150 C by external heat tape to prevent condensa-

tion of pyrolysis products out of the product stream. The QMS is

calibrated to quantitatively measure the primary gas constituents,

including: argon, CO, CO2, CH4, H2, and H2O. The QMS calibration is

checked daily with a NIST-traceable calibration gas bottle contain-

ing 50% Ar, 25% CO, 5% CO2, 10% CH4, and 10% H2, the quantitative

gas species measurements are corrected accordingly and inte-

grated over the course of the reaction to yield the total gas and

water products.

Following the QMS sampling, the product gases pass throughcopper coils and quartz wool in an ice bath to remove some of the

condensabletar andwater fromthe pyrolysis products. Tarcollected

onthe quartz wool(light tar) is extracted with acetone for lateranal-

ysis. The remainder of the condensable products is captured by an-other set of filter papers and a column of Drierite (anhydrous

calcium sulfate) at room temperature. Mass accumulating in the

cold trap and its following filters and Drierite is determined by

weightdifference, the measured water fractionfrom the QMSis sub-

tracted from this number, and the result is denoted as light tar.

Lastly, just prior to the connection to the lab vent line is a valve

for capturing the product gas evolved during the first $20 s of the

reaction. Although not representative of the total gas production

over the $15 min reaction, the bag sample is useful for later qual-

itative analysis of its minor gas constituents using a PerkinElmer

Clarus 500 GCMS. A 30 m long and 0.32 mm i.d. capillary column

with a silica plot coating was used in the GCMS with a heliumcar-

rier gas flow set at 1.3 mL/min. The temperature profile for this

analysis was an initial hold at 35 C for 10min, 10 C/min to200 C and a final hold at 200 C for 5 min.

Table 1

Proximate and ultimate analysis of coal and biomass used in this study.

Sample Illinois #6 coal Switchgrass

Proximate analysis (wt%, dry basis)

Moisture 3.55 6.29

Volatile 42.06 74.98

Ash 11.41 9.08

Fixed carbon 46.53 15.94

Ultimate analysis (wt%, dry basis)

Carbon 63.87 44.70

Hydrogen 4.78 5.78

Nitrogen 1.20 0.57

Oxygen 13.87 39.76

Sulfur 4.87 0.12

Higher heating value (calculated, dry basis)

HHV (MJ/kg) 24.09 18.31

Fig. 2. Schematic of co-pyrolysis drop tube reactor.

N.T. Weiland et al./ Fuel 94 (2012) 563570 565


4/8

Overall mass balances for each test are calculated, and only

those with mass closures between 90% and 105% are used in sub-

sequent analyses. The largest source of error in the total mass mea-

surements is the QMS gas measurements, which are integrated

over about 1200 data points, and depend on the accuracy of the ar-

gon mass flow controller for quantitative measurements.

Following the reactor tests, the heavy and light tars extracted

with acetone, are added, a little at a time, to platinum foil sample

holders, and the acetone allowed to evaporate at room tempera-

ture, until enough tar residues remain to perform an ultimate anal-

ysis per ASTM D5373. Char residues also undergo ultimate

analyses, and the chars and raw feedstocks are all analyzed per

ASTM standard D6349-98 for their ash content by Inductively Cou-

pled Plasma Optical Emission Spectrometry (ICP OES).

The energy content of the product gas is calculated directly

from the Higher Heating Value (HHV) of its measured major fuel

constituents. This number is likely to be lower than its actual en-

ergy content, however, as it does not account for contributions

from unmeasured energy-containing species such as H2S, and C2

C5 hydrocarbons, which appear in the product gas. The HHV (dry

basis) of the feedstocks and their chars and tars are calculated from

their ultimate analyses according to the formula developed by

Channiwala and Parikh [25]:

HHV 0:3491C 1:1783H 0:1005S 0:1034O 0:0151N

0:0211A 1

where HHV is in units of MJ/kg, and C, H, S, O, N, and A are the ulti-

mate analysis percentages of carbon, hydrogen, sulfur, oxygen,

nitrogen and ash on a dry basis.

Prior to pyrolysis testing, the operation of the reactor was char-

acterized with a series of tests. Varying the sweep gas flow rate

showed that a minimum argon flow rate of 1500 sccm is required

to avoid tar cracking and secondary char formation during the

pyrolysis reaction. In addition, the QMS calibration gas bottle

was made to flow through the reactor as the reactor temperaturewas heated from ambient temperature to 900 C. Measurements

of these gas species changed by less than 3% of their original com-

positions, confirming that homogeneous gas phase reactions gas

are negligible in the $4 s traverse time between the reaction loca-

tion and the gas sampling location. This time delay, while neces-

sary to maximize product collection efficiency, is unfortunately

sufficient to preclude the use of the QMS gas species data for eval-

uating the kinetics of the co-pyrolysis reaction. Kinetics of the co-

pyrolysis of coal and switchgrass is the subject of ongoing work.

3. Results and discussion

3.1. Overall product distribution

Variations in gaseous, solid (char) and condensable products

generated for various feed blend ratios during pyrolysis at 900 C

are shown in Fig. 3. The general production trend shows that as

the quantity of switchgrass in the solid feed increases, the amount

of gaseous products and light tars increases linearly, while the

char, water, and heavy tar products decrease linearly. This is lar-

gely consistent, qualitatively and quantitatively, with the results

of Zhang et al. [10], who studied co-pyrolysis of legume straw

and lignite in a drop tube reactor from 500 to 700 C, and noted

a trend towards linear product distributions at higher tempera-

tures. The large difference in char and gas pyrolysis products is ex-

pected based on the proximate analysis reported in Table 1, which

shows that switchgrass contains about twice as much volatile mat-ter as Illinois #6 coal, and less than half as much fixed carbon.

3.2. Elemental product distributions

Ultimate analysis of the chars and tars produced in the pyrolysis

experiments allows for detailed element balances to be performed.Element mass closures are shown in Fig. 4 for carbon, hydrogen,

nitrogen and oxygen. The amount of heavy and light tars produced

was not sufficient to obtain measurements of its sulfur content,

thus its oxygen content could not be determined by difference,

and this contribution to the element balance for oxygen is ne-

glected. Overall, carbon and hydrogen balances are very good,

and range from 75% to 108%. Nitrogen balances are lower, as gas-

eous N-containing species are not measured by the QMS. Oxygen

balances are often well over 100%, even when excluding the oxy-

gen content in the tars. This is likely due, in part, to the fact that

oxygen is determined by difference, and is thus a function of the

inaccuracies in the measurements of ash from the proximate anal-

ysis, and C, H, N, and S from the ultimate analyses. In particular, a

measure of the mineral matter in the feedstocks and chars(unavailable for the small quantities used in this study) would be

0%

10%

20%

30%

40%

50%

60%

0 0.2 0.4 0.6 0.8 1

Wt%ofFeed

stock

Wt% Biomass in Feedstock

Char Heavy Tar

Gas Light Tar

Water

Fig. 3. Product yields frompyrolysis of switchgrass andIllinois #6 coal at 900C for

varying feed blend ratios.

50%

75%

100%

125%

150%

175%

0% 20% 40% 60% 80% 100%

E

lementMassClosure


Carbon

Hydrogen

Nitrogen

Oxygen

Fig. 4. Element mass balances.



5/8

more accurate for this purpose than the proximate analysis ash

measurement, which includes unknown mineral transformations

(e.g., oxide and sulfide formation, release of carbonates, etc.) that

occur during the ashing process [26].

Distribution of the C, H and O components of the feedstock to

the various pyrolysis products are shown in Fig. 5. Carbon from

coal is shown to more likely remain in the char following pyrolysis,

while the carbon in switchgrass typically forms gaseous and light

tar compounds. The low carbon content of the coal char, which af-fects the energy analyses in Section 3.3 below, is likely due to an

erroneous result in the ultimate analysis, and contributes to the

low C mass closure at this condition in Fig. 4. Hydrogen from both

coal and switchgrass tends to form water and gases, with slightly

more H from switchgrass also forming light tars. Oxygen from coal

is more likely to form water or remain in the char, while O fromswitchgrass more typically yields gaseous products.

(a)

(b)

(c)

0%

10%

20%

30%

40%

50%

60%

Wt%Cfrom

Feedstock


Char

Heavy Tar

Light Tar

Gas

0%

10%

20%

30%

40%

50%

Wt%HfromFeedstock


Char

Heavy Tar

Light Tar

Water

Gas

-20%

0%

20%

40%

60%

80%

100%

0% 20% 40% 60% 80% 100%

0% 20% 40% 60% 80% 100%

0% 20% 40% 60% 80% 100%

Wt%OfromFeedstock


Char

Water

Gas

Fig. 5. Distribution of feedstock major elements in pyrolysis products for: (a)

carbon, (b) hydrogen, and (c) oxygen.

(a)

(b)

0%

10%

20%

30%

40%

50%

60%

PercentofFee

dstockEnergy


Char Heavy Tar

Gas Light Tar

0

5

10

15

20

25

30

35

40

0% 20% 40% 60% 80% 100%

0% 20% 40% 60% 80% 100%

HHVofProduct(MJ/kg

)


Char Heavy Tar

Gas Light Tar

Feedstock

Fig. 6. Energy content in the pyrolysis products as a function of: (a) energy content

of original feedstock and (b) weight of pyrolysis product.

0%

10%

20%

30%

40%

0 0.2 0.4 0.6 0.8 1

Wt%ofFeedstock


CO2 CO

CH4 H2

Fig. 7. Distribution of gaseous pyrolysis products vs. biomass in feedstock.



6/8

It is interesting to note that as much hydrogen is present in gas

from coal pyrolysis as from switchgrass pyrolysis, since only

slightly more H is available in the switchgrass feedstock (see Ta-

ble 1). The theory behind non-linear gascoal interactions sup-

poses that an increase in hydrogen content surrounding the coal

particle results from the use of biomass, which then prevents

recombination reactions that decrease the reactivity of the coal

char [11]. If the amount of hydrogen liberated from coal increases

to the level of that of biomass at the higher temperatures investi-

gated here, this may explain the lack of any non-linear or synergis-

tic effects occurring in the product distribution trends as a result of

this gascoal mechanism.

3.3. Energy content distributions

Based on the ultimate analyses of the raw feedstocks and their

pyrolysis tars and chars, the energy content (HHV) of each of the

parent and product components can be evaluated per Eq. (1).

Fig. 6a shows the distribution of the feedstock energy content into

the various pyrolysis products. Closure of the energy balances

averages 87%, with the remaining 13% likely residing in either

unmeasured light (C2C5) hydrocarbons that appear in the product

gas, or in highly volatile light tar species that vaporize at room

temperature. Pyrolysis product energy contents are largely linear

with respect to wt% of switchgrass in the feedstock. Coal mixtures

mostly retain their energy in the char after pyrolysis, while bio-

mass energy content is mostly transferred to gas and light tar

products.

The specific energy content of each of the products is shown in

Fig. 6b, where the HHV of the char and light tar products do not

vary considerably with feedstock content. Light tars are more en-

ergy dense than the chars due to higher hydrogen concentrations

and the absence of ash, although the water content that is normally

part of this condensable fraction is not included in this calculation.

Heavy tars appear to be the most energy-dense product, though

due to the small quantity of heavy tars available for ultimate anal-

ysis, particularly for high switchgrass mixtures, the results of theultimate analysis and the subsequent energy calculation are fairly

uncertain. Gas energy density is the most variable, and is largely

due to the presence of more CO2 with increasing switchgrass blend

ratio, as seen in the following section.

3.4. Evaluation of gaseous products

The gaseous products generated by co-pyrolysis of coal and

switchgrass at 900 C are presented in Fig. 7, where biomass is

shown to produce about four times as much CO as coal. At roughly

1/3 of the weight of the original feedstock, CO represents the larg-

est single product from the isothermal pyrolysis of switchgrass.

Carbon dioxide and methane likewise increase with switchgrass

content, while hydrogen decreases slightly. These results are lar-

gely linear with respect to biomass blend ratio, and again follow

closely the results of Zhang et al. [10]. Primary differences in the

results of this work include higher methane content and lower

hydrogen content than the results of Zhang et al. This may indicate

an enhancement of the methanation reaction at the higher temper-

atures and closer particle spacing used in the present work, though

this would conflict with results from equilibrium gasification prod-

uct studies.

Secondary volatile components are qualitatively analyzed by

GCMS, as shown in Fig. 8, where major differences in identified

species are observed. Gaseous products from pyrolysis of Illinois

#6 coal and its blends contained sulfur species such as carbonyl

sulfide (COS), hydrogen sulfide (H2S), carbon disulfide (CS2), and

sulfur dioxide (SO2) that were not present in the pure switchgrass

product gas. This result agrees with expected species present based

on the high quantity of sulfur in Illinois #6 coal compared to

switchgrass as seen in the ultimate analysis and Table 1. Light

hydrocarbons also appear in these gas bag samples, particularly

for the high switchgrass blend ratios. As discussed above, these

species are not quantitatively measured but carry significant en-

ergy content, and thus constitute a portion of the unaccounted en-

ergy from pyrolysis of the feedstocks.

3.5. Evaluation of liquid and solid products

Qualitative GCMS analysis of the heavy tars from co-pyrolysis

of Illinois #6 coal and switchgrass are shown in Fig. 9, where aro-

matic hydrocarbons are shown to be present in all of the tars. The

primary differences in the analyses are the presence of benzenedi-

ols, which occur mostly in the switchgrass tar, and sulfur-contain-

ing dibenzothiophene, which only appears in the coal-derived tars.

Identification of these and other heavy tar species may be impor-

tant in determining the influence of tar on downstream processes

in a gasification system.

Ash elemental analysis was performed via ICP OES on the raw

feedstocks as well as the chars produced from pyrolysis at

900 C. As shown in Fig. 10, raw switchgrass contains significantly

higher quantities of alkali and alkaline earth metals such as Ca, K

and Mg, while Illinois #6 coal contains greater amounts of transi-

tion metals such as Al, Fe and Ti. The presence of alkali and alkaline

earth metals in switchgrass is significant due to known catalytic

0 2 4 6 8 10 12 14 16

Response time (min)

C2

H6

CO2 C2H4Ar, CO, CH4

COS

C2H2

H2S

C3H6

CS2 SO2

C4H8100% SG

30% SG/70% Coal

15% SG/85% Coal

100% Coal

Fig. 8. Chromatograms of switchgrass, Illinois #6 coal and biomass/coal blends from GCMS analysis of collected gas bag samples from pyrolysis at 900 C.



7/8

activity of these metals during the thermochemical conversion of

coal. Comparison of the char ash contents with the raw feedstocks

shows that some alkali and alkaline earth ash components are

more likely to volatilize during pyrolysis, namely Na and Ba, and

to a lesser extent, K and Mg. Removal of these catalysts from the

solid char at 900 C may explain, in part, the lack of non-linear

product distribution trends that might otherwise result fromcharcoal interactions.

4. Conclusion

The thermochemical conversion of coal and biomass mixtures

by isothermal pyrolysis at 900 C has been investigated for differ-

ences in solid, liquid and gaseous product distributions. These

studies include primary gas analysis, ultimate analysis of tar and

char products, ICP OES elemental analysis of chars, and qualitativeGC/MS analysis of trace gases and heavy tars. Contrary to some of

Solvent

Solvent

Benzaldehyde

Phenol

MethylPhenol

Naphthalenol

Benzenediol

Hydroquinone

MethylBenzenediol

PropoxyPhenol

Pyrene

DiethylPhthalate

MethylPyrene

Fluorene

Anthracene

MethylAnthracene

Triphenylene

Biphenylene

0 10 20 30 40 50

Time [min]

100% Coal

100% SG

15% SG/

85% Coal

30% SG/70% Coal

Fig. 9. Chromatograms of switchgrass and Illinois #6 coal heavy tar from GCMS analysis from pyrolysis at 900 C.

0.01

0.1

1

10

100

Al Ba Ca Fe K Mg Mn Na Sr Ti

Wt%inAsh

Illinois#6 Coal - Raw

Illinois#6 Coal - Char

85% ILL - 15% SG

70% ILL - 30% SG50% ILL - 50% SG

Switchgrass - Char

Switchgrass - Raw

Fig. 10. ICP OES ash analysis of feedstocks and char material produced from pyrolysis of switchgrass and Illinois #6 coal at 900 C.



8/8

the existing literature on coal/biomass co-pyrolysis, product distri-

butions appear to be linear with respect to the wt% of switchgrass

in the feedstock. This linear relationship extends to individual ma-

jor product gas species, heavy and light tar fractions, water produc-

tion, and energy content in the pyrolysis products. This result may

be due to the nature of fast pyrolysis experiments. As expected,

pyrolysis of coal tends to yield more char and heavy tar compo-

nents than pyrolysis of switchgrass, while switchgrass yields more

gas and light tar products. As previously discussed, one source of

coal-biomass interaction is gascoal reactions. These interac-

tions may be more pronounced in a slow pyrolysis reaction

typically observed during thermogravimetric experiments. Alter-

nately, catalytic interactions may have greater influence on co-gas-

ification and do not appear to have a major impact on the product

distributions during fast co-pyrolysis.

Qualitative GC/MS analysis of the product gases show that iso-

thermal coal pyrolysis yields sulfur containing species, such as H2S,

COS, which do not appear in the low-sulfur switchgrass product

gas, while low molecular weight aliphatic hydrocarbons (C2C4species) were more prevalent in the biomass product gas. In the

heavy tar analysis, the coal tar contained more aromatic hydrocar-

bons, whereas the switchgrass tar contained some chlorine-con-

taining compounds.

This work demonstrates that the product distributions from iso-

thermal co-pyrolysis of a coal/biomass mixture can be estimated as

a mass-weighted sumof the product distributions of the pure feed-

stocks. Such a result is inherently useful in the development of co-

pyrolysis and co-gasification processes, where knowledge of the

char, tar, and gas compositions are critical in the development of

downstream processing and gas cleaning systems. This result also

holds value in the determination of co-pyrolysis kinetics, where

product distributions must also be determined. These results

may not hold, however, under slower heating rate conditions, at

lower temperatures, or under reactive (gasification) atmospheres.

The effects of these conditions are the subject of future work in this

field.

Acknowledgements

This technical effort was performed in support of the National

Energy Technology Laboratorys ongoing research in Advanced

Gasification Technologies under the RES Contract DE-FE0004000.

References

[1] Energy Information Administration, US coal reserves; 1997 Update.

[2] Werther J et al. Combustion of agricultural residues. Prog Energy Combust Sci

2000;26:127.

[3] Kirubakaran V et al. A review on gasification of biomass. Ren Sustain Energy

Rev 2009;13:17986.

[4] Haykiri-Acma H, Yaman S. Interaction between biomass and different rank

coals during co-pyrolysis. Ren Energy 2010;35:28892.

[5] Vassilev SV et al. An overview of the chemical composition of biomass. Fuel2010;89:91333.

[6] Wihersaari M. Evaluation of greenhouse gas emission risks from storage of

wood residue. Biomass Bioenergy 2005;28:44453.

[7] Higman C. Gasification. Boston: Elsevier/Gulf Professional Pub; 2003.

[8] Sadhukhan AK et al. Modeling of pyrolysis of coal-biomass blends using

thermogravimetric analysis. Bioresour Technol 2008;99:80226.

[9] Sonobe T et al. Synergies in co-pyrolysis of Thai lignite and corncob. Fuel

Process Technol 2008;89:13718.

[10] Zhang L et al. Co-pyrolysis of biomass and coal in a free fall reactor. Fuel

2007;86:3539.

[11] Sjstrm K et al. Promoted reactivity of char in co-gasification of biomass and

coal: synergies in the thermochemical process. Fuel 1999;78:118994.

[12] Keown DM et al. Effects of volatilechar interactions on the volatilisation of

alkali and alkaline earth metallic species during the pyrolysis of biomass. Fuel

2008;87:118794.

[13] Yang J-b, Cai N-s. A TG-FTIR study on catalytic pyrolysis of coal. J Fuel Chem

Technol 2006;34:6504.

[14] Quyn DM et al. Fuel 2003;82:58793.

[15] Liu Q et al. Effect of inorganic matter on reactivity and kinetics of coalpyrolysis. Fuel 2004;83:7138.

[16] Fahmi R et al. Theeffectof alkalimetals on combustionand pyrolysis of Lolium

and Festuca grasses, switchgrass and willow. Fuel 2007;86:15609.

[17] Zhu W et al. Catalytic gasification of char from co-pyrolysis of coal and

biomass. Fuel Process Technol 2008;89:8906.

[18] Veraa MJ, Bell AT. Effect of alkali metal catalysts on gasification of coal char.

Fuel 1978;57:194200.

[19] Kajitani S et al. Co-gasification reactivity of coal and woody biomass in high-

temperature gasification. Energy Fuels 2010;24:14551.

[20] Lemaignen L et al. Factors governing reactivity in low temperature coal

gasification. Part II. An attempt to correlate conversions with inorganic and

mineral constituents. Fuel 2002;81:31526.

[21] Yan J et al. Chemical compositions of four switchgrass populations. Biomass

Bioenergy 2010;34:4853.

[22] McLaughlin SB,Kszos LA.Developmentof switchgrass (Panicum virgatum)as a

bioenergy feedstock in the United States. Biomass Bioenergy 2005;28:51535.

[23] Dien BS et al. Chemical composition and response to dilute-acid pretreatment

and enzymatic saccharification of alfalfa, reed canarygrass, and switchgrass.

Biomass Bioenergy 2006;30:88091.

[24] El-Nashaar HM et al. Genotypic variability in mineral composition of

switchgrass. Bioresour Technol 2009;100:180914.

[25] Channiwala SA, Parikh PP. A unified correlation for estimating HHV of solid,

liquid and gaseous fuels. Fuel 2002;81:105163.

[26] Speight James G. Handbook of coal analysis. New Jersey: Wiley; 2005. p. 51.


Documents

BİTÜMLÜ KÖMÜR VE BİYOKÜTLENİN BİRLİKTE PİROLİZİ