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Development of a Reactivity Test for Coal-BlendCombustion: The Laboratory-Scale Suspension-Firing

Reactor

D. Peralta, N. P. Paterson,* D. R. Dugwell, and R. Kandiyoti

Department of Chemical Engineering and Chemical Technology,Imperial College (University of London), London SW7 2BY, U.K.

Received June 14, 2001. Revised Manuscript Received October 27, 2001

A laboratory-scale technique has been developed for assessing changes in the combustionreactivity of coal blends with blend composition. The test consists of the incomplete combustionof a batch of coal particles in a novel suspension-firing reactor. Residual chars are analyzed forcomposition and reactivity by nonisothermal thermogravimetry. Results from the bench-scalecombustor were compared with observations from a full-sized power station and a single-burnerpilot installation. Using the same sets of coals, the suspension-firing reactor gave the same orderof reactivities as that observed in larger-scale equipment. Systematic preferential combustion ofthe higher-reactivity coal in blends has been identified; the extent of preferential combustionhas been quantified from the thermogravimetric profiles of the residual chars. Our findingsindicate that the reactivity of residual coal-blend chars is determined by the extent of enrichmentof the lower-reactivity component in the blend. The coupled use of the new suspension-firingreactor and nonisothermal thermogravimetric analysis provides a powerful tool in comparingthe combustion performance of different coal blends. Initial indications are that this simple systemcan be used as a predictive tool.

Introduction

Coal-blend combustion is increasingly being seen asan attractive route for alleviating problems of fuelselection for pulverized-fuel (pf) power stations. Blend-ing introduces greater fuel flexibility and provides a wayof minimizing costs, e.g., by the use of several lower-grade coals to achieve desirable average blend proper-ties. Coal blending may also provide a useful approachto controlling pollutant emissions, e.g., by mixing high-sulfur and low-sulfur coals to limit SO2 emissions toacceptable levels.1

Although some utility companies have already gainedoperating experience with coal blends in pulverized-fuelcombustors, the underlying mechanisms are still poorlyunderstood. Problems have been reported in plant trials,including high levels of unburned carbon in fly ash,flame instability, increased slagging and fouling, COemissions, and plume opacity. The success of coal-blendfiring seems to be strongly dependent on individualplant operators’ experience1. To date, there has been noaccepted test method for assessing or predicting thepotential performance of blends, other than in large-scale combustion trials.

The outcomes of coal-blending trials are not straight-forward to predict. When higher- and lower-reactivitycoals are mixed, higher local temperatures and moreintense radiative heat transfer would result from the

more rapid combustion of the higher-reactivity compo-nents. One likely outcome would be the assisted ignitionand the more complete combustion of the less reactivecoals/chars. However, it is also possible to speculate thatfaster combustion of higher-reactivity components mightinduce partial oxygen starvation of the lower-reactivitycoals/chars. Outcomes of plant-based coal-blend com-bustion trials suggest that the latter may be thepredominant effect; declining burnout rates appear tobe particularly pronounced when low-NOx technologiesare employed.2

Three types of laboratory-scale devices have previ-ously been used to study the combustion of coal blends:thermogravimetric analyzers (TGA), drop-tube furnaces,and a bomb-calorimeter-based test. TGA-based coal-blend combustion studies have shown additive3 as wellas synergistic4 (i.e., non additive) effects. In the case ofbinary blends, additivity of behavior may be identifiedby the presence of two independent peaks in the TGAsignal. Possible synergistic effects were indicated by the“nonadditivity” of certain characteristics, such as igni-tion temperature and burnout time. It has also beenclaimed that the initial ignition temperature of anonisothermal TGA method could be correlated with thecombustion efficiency of a research boiler. The resultscould be used as an empirical indicator of the relative

* Corresponding author. E-mail: [email protected].(1) Carpenter, A. M. Coal Blending for Power Stations; IEA Coal

Research: London, 1995; pp 11-14.

(2) Smart, J. P.; Nakamura, T. J. Inst. Energy. 1993, 66, 99-105.(3) Artos, V.; Scaroni, A. W. Fuel 1993, 72, 927-933.(4) Rubiera, F.; Fuente, E.; Arenillas, A.; Pis, J. J. In Prospects for

Coal Science in the 21st Century; Li, B. Q., Liu, Z. Y., Eds.; ShanxiScience & Technology Press: Taiyuan, China, 1999; Vol. 1, pp 531-534.

404 Energy & Fuels 2002, 16, 404-411

10.1021/ef010127p CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 01/17/2002

combustion characteristics of coals and coal blends whenlarger tests were difficult to do.5

In drop-tube furnaces, the combustion performanceof coal blends is additive.3,6 This result is expected, asnormal drop-tube operation does not allow high enoughparticle densities. In sparse particle density environ-ments, particles appear to burn independently, inconfigurations where volatile clouds of different particlesdo not necessarily overlap.

In a more recent investigation, a standard bombcalorimeter has been used to investigate the combustionof coal blends.7 Coals and coal blends were partiallycombusted in the bomb, using lower oxygen pressures(than the usual 30 bar), where volatiles and charparticles can interact in the reaction zone. Two sets ofcoal blends previously combusted in a single-burnerpilot plant (U.K.) and in a power station (Chile) weretested in the bomb calorimeter. Relative orders ofreactivities observed in the bomb calorimeter were foundto reproduce trends observed in the larger-scale trials:the degree of burnout of blends followed the trendspreviously obtained in both the pilot plant and powerstation trials. The preferential combustion of the higher-reactivity coal in blends was identified. However, themethod did not perform as expected when testing high-swelling coals; in the face of partial melting of thesample, uniform distribution of oxygen could not bemaintained throughout the calorimeter crucible. As inthe case of the crucible test for volatile matter deter-minations, the method does not attempt to reproducethe hydrodynamic conditions of a pf burner. Neverthe-less, it provides a readilysand commerciallysavailablebench-scale test for estimating relative reactivities ofcoal blends, which plant operators could use for plan-ning and preparing their feedstocks.

The present paper describes the more recent develop-ment of a novel laboratory-scale method based on asuspension-firing reactor, for analyzing the combustionof coal blends under conditions relevant to pulverized-fuel combustion. The reactor provides some of theoperating conditions typical of pf burners that arerelevant to blend combustion: fast heating rates in awell-defined reaction zone, where coal particles (sus-pended in the oxidizing gas) interact with evolvingvolatiles during combustion. The design has evolvedfrom an earlier apparatus constructed to investigate thedecomposition of limestone and fuel burnout in precal-ciners of cement works.8 A similar reactor configurationis currently being used to investigate toxic trace-elementreleases during cofiring of coal and biomass.9

Experimental Section

Description of the Suspension-Firing Reactor. Figure1 presents a schematic diagram of the suspension-firingreactor. The reactor is made of quartz (5 cm ID, 115 cm long).

The top and bottom chambers are electrically heated using twoindependent coil heaters (1 and 2 kW, respectively) to amaximum of 1000 °C. The bottom chamber of the reactorserves to preheat incoming air, prior to delivery to the (top)combustion chamber through a constriction. The narrow entryserves to increase the inlet velocity of air into the upperchamber and prevents sample coal particles from falling down,out of the reaction chamber. During an experiment, a batchof coal particles (fed from the top) is suspended in preheatedair and partially combusted in a cloud of released volatiles(∼1000 °C, atmospheric pressure). Visualization tests at roomtemperature, with the air flow scaled up to simulate theconditions at high temperature, have been carried out toensure that coal particles are in suspension during theexperiments. These tests were done using an air flow equal to1.5 times that used in the high-temperature experiments soas to provide the same particle terminal velocity at bothtemperatures.

Nitrogen is used to transport fuel particles through aslightly pressurized plug valve (0.1 bar above atmosphericpressure) and a water-cooled probe into the reactor. The probeis cooled to avoid the premature oxidation of coal. A sintereddisk holds the thermocouple holder and the cooled-probe inplace; it also serves as a screen, preventing particles from beingpushed out of the reactor from the top.

The experiment itself is based on determining extents ofchar survival for different coal blends, under conditions ofincomplete combustion. This is achieved by reversing thedirection of flow and flooding the reactor with gaseousnitrogen, after a preset period (2-5 s) following fuel injection.Two three-way solenoid valves (valves 1 and 2 in Figure 1)switch automatically and nitrogen floods the reactor throughvalve 1, sweeping the particles out of the heated zone.Eventually, particles are trapped in the “ash collector”; thenitrogen stream leaves the reactor through valve 2.

(5) Pisupati, S. V.; Scaroni, A. W. In Proceedings of the 9thInternational Conference on Coal Science; Ziegler, A., van Heek, K.H., Klein, J., Wanzl, W., Eds.; DGMK: Essen, Germany, 1997; Vol. 2,pp 1151-1154.

(6) Peralta, D. Ph.D. Thesis, University of London, London, 2001.(7) Peralta, D.; Paterson, N. P.; Dugwell, D. R.; Kandiyoti, R. Fuel

2001, 80, 1623-1634.(8) Khraisha, Y. H.; Dugwell, D. R. Chem. Eng. Res. Des. 1988, 67,

52-57.(9) Miller, B. B.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels,

submitted for publication.

Figure 1. Suspension-firing reactor.

Reactivity Test for Coal-Blend Combustion Energy & Fuels, Vol. 16, No. 2, 2002 405

In these experiments, the flows of air and fuel-conveyingnitrogen were 1.8 and 0.6 L/min respectively, resulting in anoxygen concentration of 16% in the reaction zone. Variableresidence times of particles in the reaction zone from 2 to 5 shave been used. All experiments were done in duplicate.

Coal Samples. The experimental method has been devel-oped using blends of Rheinbraun lignite (Germany) and TaffMerthyr semianthracite (UK). These coals were selectedbecause the wide differences between their combustion char-acteristics were expected to facilitate the detection of possiblesynergistic effects in the performance of their blends. In thiscontext, synergistic effects are understood as parameters ofcombustion performance that cannot be predicted from theperformance and composition of the individual coals in theblend by simple additivity.

Sets of coal samples and blends were provided by a Chile-based and a U.K.-based power generating company. Three coalsamples (E, F, G) were received from GENER (Chile). Thesecoals and their blends have been tested in one of the 135-MWunits of the Tocopilla power station. The performance of twosets of blended mixtures E-F and E-G, both containing coalE at 25, 50 and 75 wt %, have been compared between plantand bench-scale level. Ashes collected from the same sectionof the electrostatic-precipitator system during each test werealso supplied.

The UK set consisted of a suite of 3 coals (A, B, C), of higherrank than the Chilean set. The 0.5-MW single-burner pilotplant tests by the utility company had actually been carriedout with blends of four coals. In this study, the fourth coalwas not used for testing because it had not been found to affectblend performance. Information on the carbon-in-ash concen-trations from the pilot tests was also supplied.

Samples in the particle-size range 106-150 µm were used.Coals were dried at 40 °C under vacuum for 16 h and thenstored under nitrogen in a refrigerator. Proximate and ulti-mate analyses of the coals are shown in Table 1. Proximateanalyses of coals were carried out by TGA. The properties ofthe blends were calculated from these analyses and thefractions of the individual components in the blends.

Blending Method. Crushed coals were blended by rotatingthe sample holder on the rollers of a ball-mill for 30 min (at∼150 rmp).

Calculation of the Degree of Burnout. Char particlescollected after each test were analyzed by nonisothermalthermogravimetry. The amount of residual combustible matterin the char was determined and the degree of burnoutcalculated using the ash-tracer method:

where Ad is the ash content of raw coal (%, dry basis) and Ca

is the percentage of combustible matter in the collected char(wt %).

Calculation of the Amount of Fuel. Under the operatingconditions of the test, it does not make sense to calculate anabsolute value of excess oxygen since coal particles are injectedas a batch whereas the flow of air is constant. However,different amounts of sample were injected for different coalsso as to feed the same absolute amount of combustible material(on a mass basis)sand maintain comparability between ex-

periments. Rheinbraun lignite was used as reference coal; itscombustible content per unit mass was the lowest of allsamples used. Calculations were carried out on the basis of130 mg of Rheinbraun. The amount of combustible materialin samples was corrected to a dry, ash-free basis. The followingreactions were considered to be taking place:

It was assumed that the fuel oxygen took part in the combus-tion, which for purposes of this calculation proceeded tocompletion.

The mass of oxygen supplied into the reaction zone atswitching times of 2, 3, 4, and 5 s represents 25%, 29%, 33%,and 37% of the required oxygen for complete combustion ofthe fuel charge. It has been found that, under the currentoperating conditions, a significant fraction of volatile matterleaves the reaction zone prior to combustion and therefore amajor portion of oxygen is consumed by the residual charparticles. Thus, the degree of burnout, calculated from themass loss of combustible material, represents the completedevolatilisation of coal plus partial combustion of char.

Density of particles suspended in the reaction zone is typicalof those found in the primary stream of commercial pfcombustors (1.5-2.0 kg of air per kg of coal).

Nonisothermal Thermogravimetric Analysis of Re-sidual Chars. A nonisothermal thermogravimetric method10

was used to analyze the residual char samples. In ref 10, theauthors claimed that the use of pure air as the oxidationmedium could promote the uncontrolled self-heating of thesample. To reduce this effect, the oxygen concentration wasreduced to 7% by mixing air with nitrogen. In our work,however, this uncontrolled self-heating was not observed, eventhough we used air throughout all the runs. This might bedue to the small amount of sample analyzed in the presentset of experiments (1-2 mg). A Perkin-Elmer TGA 7 (Series1020) was used, with the air flow set at 40 mL min-1. Sampleswere held in the furnace at 30 °C for 3 min and then heatedto 400 °C at 40 °C min-1. The heating rate was then changedto 15 °C min-1 and the temperature raised to 900 °C. Dataanalysis was performed between 400 and 900 °C, where themajor combustion process took place. Finally, the temperaturewas kept constant at 900 °C for 4 min to ensure that allcombustible matter was consumed, to determine the amountof unburned carbon in the char.

Results

Method Development Using Rheinbraun Ligniteand Taff Merthyr Coal. Rheinbraun lignite, TaffMerthyr semianthracite, and their blends, containing25, 50, and 75 wt % of Rheinbraun, were combusted inthe suspension-firing reactor. The mass of sampleinjected into the reactor (to maintain a constant amount

(10) Russell, N. V.; Beeley, T. J.; Man, C. K.; Gibbins, J. R.;Williamson, J. Fuel Process. Technol. 1998, 57, 113-130.

Table 1. Proximate and Ultimate Analyses of Coal Samples

Rheinbraun Taff Merthyr A B C E F G

volatile matter (%, daf) 54.9 13.5 9.5 11.8 20.6 49.1 45.7 44.1ash (%, db) 3.3 5.3 17.1 21.3 15.1 2.1 11.1 11.6moisture (%, ad) 9.5 0.5 0.1 0.2 0.1 5.4 1.1 1.1carbon (%, daf) 66.5 91.5 88.7 86.8 84.2 70.0 81.1 81.0hydrogen (%, daf) 4.8 3.9 2.8 3.2 4.0 5.5 5.7 5.5nitrogen (%, daf) 0.7 1.5 1.2 1.3 1.4 1.0 1.5 1.6sulfur (%, daf) 0.3 0.4 1.7 1.0 1.5 0.1 2.0 1.5

burnout (%) ) (1 -AdCa

(100 - Ca)(100 - Ad))100

C + O2 f CO2

2H2 + O2 f 2H2O

S + O2 f SO2

406 Energy & Fuels, Vol. 16, No. 2, 2002 Peralta et al.

of fuel) is shown in Table 2; the amount of sampleincreased with increasing proportions of Rheinbraun asthis lignite contained less combustible material per unitmass.

This set of samples was combusted using “residencetimes” of 2, 3, 4, and 5 s (Figure 2). The dashed lines inFigure 2 represent theoretical burnout levels calculatedassuming linear additivity of burnout from individualpure blend components.

The degree of burnout of individual coals displayedthe expected trend. Rheinbraun (RB) lignite was mark-edly more reactive than Taff Merthyr (TM) semi-anthracite. The performance of all blends was found tobe additive at short residence times. However, at longerresidence times, the level of burnout fell below the linearadditivity line, particularly for the 50% and 75% blends.By contrast, the blend containing 25% of Rheinbraundid not show significant deviation from additivity withincreasing residence time.

Thermogravimetric profiles of chars collected from thesuspension-firing reactor have been plotted as the rateof weight loss of combustible material versus reciprocaltemperature in the range 400-900 °C (Figure 3).

Significant differences were found between profilesof individual mixtures. The RB-derived char exhibitedthe highest rate of weight loss at lower temperatures.The coal-blend char originally containing 75% of RBdisplayed two practically independent peaks, corre-sponding to the sequential combustion of Rheinbraunand Taff Merthyr residues in the char. By contrast, the

samples originally containing 25% and 50% of Rhein-braun showed small contributions from Rheinbraunresidue. These findings clearly indicated that RB-lignite, the higher-reactivity coal in these blends, com-busted preferentially under the experimental conditionsof the suspension-firing reactor.

The concentration of RB-derived carbon in the coal-blend chars was calculated from data shown in Figure3. These calculations were done assuming simple addi-tivity of the individual Rheinbraun and Taff Merthyrchar profiles. The proportion of RB-derived carbon bestmatching the experimental blend profiles representedthe composition of the blend char. A more accuratecalculation would have involved measuring the areaunder the curve in Figure 3 and calculating the propor-tion of RB-derived char accordingly. However, thismethod would not distinguish individual componentswhen extensive overlapping existed between peaks (e.g.,for more similar coals); the former method was thereforeemployed in developing the technique. The results ofthese calculations are shown in Table 3. For a givenblend, the concentration of RB-derived carbon decreasedwith increasing residence time. Similarly, for a fixedresidence time, the concentration of RB-derived charincreased with increasing proportions of RB-lignite inthe original blends. Thus, the preferential combustionof Rheinbraun, the higher-reactivity coal, was clearlydemonstrated and quantified.

Tests with GENER Samples. Coals E, F, and G andthe corresponding sets of blends E-F and E-G, eachcontaining 25, 50 and 75 wt % of coal E, were alsopartially combusted in the suspension-firing reactor.The performance of these blends in one of the 135-MWunits of the Tocopilla power station was evaluated bydetermining loss-on-ignition of residual fly ash collectedin the electrostatic precipitators. On the basis of thesevalues, the degree of burnout was calculated assumingthat all the mineral matter in coal, ultimately oxidized

Table 2. Mass of Coal Injected into the Reactor(Rheinbraun-Taff Merthyr)

Rheinbraun (wt %) mass of sample (mg)

0 8425 9250 10275 114

100 130

Figure 2. Burnout of Rheinbraun-Taff Merthyr samples at different residence times.


to form ash, had been recovered as bottom ash and flyash. Plant operators consider it safe to assume that thebottom ash accounted for 20 wt % of the total, containing∼2 wt % of uncombusted organic material. Figure 4shows the order of combustion reactivity of the indi-vidual coals as

For the set of E-F blends, burnout was found to belinearly additive. By contrast, burnout levels of E-Gblends were measurably lower than could have beenexpected from linearly additive behavior.

These power plant derived results may be comparedwith levels of burnout, observed in the suspension-reactor. Figures 5 and 6 present “% burnout” versus“coal E content” of E-F and E-G blends, respectively.In both sets of blends, results are given as a function ofincreasing residence times (2, 3, and 4 s). The degree ofburnout of individual coals was E > F > G, which wasdifferent from that found in the power station wherecoal G was more reactive than coal F. The combustionperformance of the set of blends E-F (Figure 5) wasadditive at all residence times, except for the blendcontaining 25% of Coal E that showed somewhat less

burnout than predicted. The overall trend of linearadditivity in these data reflects closely the trends inburnout levels found with the power station ashes.

Figure 6 shows that the set of blends E-G exhibitedconsistently lower burnout than predicted from linearadditivity for all residence times. The low burnout wasparticularly significant for the blends containing 25%and 50% of Coal E. Interestingly, this result alsomirrored that found in the power station ashes, wheredeviation from additivity was more pronounced for theset of blends E-G (see Figures 4 and 6).

TGA thermograms of the set of chars E-F at 3 sresidence time are presented in Figure 7. Compared tothe Rheinbraun-Taff Merthyr chars (Figure 3), this setof coal-blend chars showed significant overlap betweenindividual char components. This result was a conse-quence of the relative similarities in rank, of Coals Eand F compared to RB-lignite and TM-semi anthracite(see Table 1). Clearly, differences in combustion reactiv-ity between chars are reduced when using more similarcoals. The corresponding profiles of the set of chars E-G(not shown) were qualitatively similar to those in Figure7; coals F and G have very similar properties.

The proportion of coal E in the coal-blend chars wascalculated from these rate thermograms (Table 4). Fora given blend, the proportion of coal E decreased withincreasing residence time. For a given residence time,the proportion of coal E increased with increasingconcentration of coal E in the original blends. Prefer-ential combustion of coal E was observed for the blendsinitially containing 25% and 50% of coal E, but for the75% coal E blend, differences were small and did notamount to significant preferential combustion.

Figure 3. Rate thermograms of Rheinbraun-Taff Merthyr char samples at 3 s residence time.

Table 3. Concentration of Rheinbraun-derived Carbonin the Coal-Blend Chars

coal-blend chars RB (wt %)

original blendRB (wt %) 2 s 3 s 4 s 5 s

25 0 0 0 050 5 3 2 275 16 20 8 11

Figure 4. Burnout of GENER samples in the 135-MW power station (Tocopilla, Chile).

E > G > F


Tests with UK Power Utility Samples. Coals A,B, and C were mixed in the same proportions as usedin the blends tested by the utility company. Thecompositions of the utility company blends and of thoseproduced in this laboratory are shown in Table 5. Theperformance of the blends in the single-burner pilotplant was expressed in terms of loss-on-ignition of flyash, with low loss-on-ignition values indicating lesscarbon-in-ash and better performance by a particularblend. The pilot-plant data are presented in Table 6,showing that blend 2 exhibited greater combustionreactivity (i.e., lower loss-on-ignition) than blend 1.

The three individual coals (A, B, and C) and the twoblends (blends 1 and 2) were partially combusted at 2 sresidence time in the suspension-firing reactor. Thedegree of burnout of these samples is illustrated inTable 7; the order of increasing burnout for individualcoals was A < B < C. Blend 1, containing a higherproportion of coal A, was less reactive than blend 2.Once again, the result from the suspension-firing reac-tor was found to reproduce the trend observed in thelarger-scale, pilot-plant, tests (Table 6).

The chars collected from the suspension-firing-reactortests were also analyzed by thermogravimetry. In thiscase, the rate thermograms of all coals and blendsexhibited complete overlapping and no differences be-tween them could be observed. Considering the similarranks of these coals (Table 1), the result was notsurprising. Thus, the thermogravimetric profiles of coal-

blend chars did not show the separate contribution fromdifferent components (as previously observed for theRheinbraun-Taff Merthyr and GENER samples inFigures 3 and 7) due to the similarity of the componentsin the blends.

Discussion

Figure 8 presents elemental carbon contents of indi-vidual coals used in this study, plotted against burnoutlevels in the suspension-firing reactor, clearly showingdecreasing levels of burnout with increasing rank. Inthis sense, the reactor is shown to reproduce a well-established trend. Furthermore, preferential combustionof the higher-reactivity coal in coal-blends has beenidentified and, where possible, quantified by noniso-thermal thermogravimetry. The extent of enrichmentof the lower-reactivity component in the coal-blend charsincreases with increasing differences in reactivity be-tween the individual components in the blend. Forexample, for the Rheinbraun-Taff Merthyr blends (twosamples at opposite ends of the coal rank range), thecoal-blend chars were increasingly concentrated withTaff Merthyr-derived char (Table 3). When the differ-ence between the reactivities of the individual coalsdecreased (sets E-F and E-G), a lesser extent ofenrichment of the lower-reactivity component in theblends could be observed (Table 4). This implies thatthe greater the difference in combustion reactivities

Figure 5. Burnout of E-F set samples at different residencetimes.

Figure 6. Burnout of E-G set samples at different residencetimes.


between the blend components, the larger the extent ofenrichment of the lower-reactivity component in thecoal-blend chars.

The observed lower burnout levels compared to linearadditivity suggest that in coal-blend combustion, partialoxygen-starvation of lower-reactivity components maybe taking place. At present, fluid dynamics modeling isbeing carried out to estimate possible oxygen starvationon combustion in the suspension-firing reactor.

The incomplete combustion of coal blends in thesuspension-firing reactor has thus displayed both ad-

ditive and nonadditive effects. Additive effects weremore pronounced at shorter residence times, whilenonadditivity increased with increasing residence times.It is likely that this shift with time corresponds todifferent rates of thermal deactivation within the reac-tor,11 in line with different rates of combustion forindividual components causing changes in the blendcomposition. At shorter residence times, the higher-reactivity coal will react faster as has been observed

(11) Hurt, R.; Sun, J. K.; Lunden, M. Combust. Flame 1998, 113,181-197.

Figure 7. Rate thermograms of E-F char samples at 3 s residence time.

Table 4. Concentration of Coal E-derived Carbon in theCoal-Blend Chars E-F and E-G

coal-blend chars Coal E (wt %)

original blendcoal E (wt %) 2 s 3 s 4 s

Set E-F25 14 9 550 20 25 1775 65 70 57

Set E-G25 10 10 450 41 38 1875 73 75 72

Table 5. Composition of U.K. Power Utility Blends

U.K. power utility this laboratory

blend 1 blend 2 blend 1 blend 2

40% A 10% A 55% A 14% A20% B 47% B 27% B 67% B13% C 13% C 18% C 19% C27% D 30% D

Table 6. Loss-on-Ignition (%) of UK Power UtilitySamples in the Pilot Plant

excess oxygen (%) blend 1 blend 2

2 17.5 12.03 17.5 8.84 10.0 6.0

Table 7. Burnout of U.K. Power Utility Samples in theSuspension-Firing Reactor

sample burnout (%, daf)

A 38.0B 45.2C 53.1blend 1 44.3blend 2 54.3

Figure 8. Burnout of single coals at different residence timesin the suspension-firing reactor.


experimentally. However, uncombusted organic mate-rial will be progressively deactivated, and at longerresidence times, this deactivation process will have anegative effect on burnout.

Conclusions

A laboratory-scale suspension-firing reactor has beendeveloped for investigating coal-blend combustion underpf-firing conditions. “Incomplete-combustion” tests inthis reactor have allowed establishing correlations withresults from a full-sized power station and a single-burner pilot installation. Using similar samples, thesuspension-firing reactor gave the same hierarchy ofreactivities as observed in larger-scale equipment. Forall blends, systematic preferential combustion of thehigher-reactivity coals has been identified; the extentof preferential combustion has been quantified usingthermogravimetric profiles of the residual chars. Ourfindings indicate that the reactivity of residual coal-

blend chars is determined by the extent of enrichmentof the lower-reactivity component in the blend. Thecoupled use of the new suspension-firing reactor andnonisothermal thermogravimetric analysis provides apowerful tool in comparing the combustion performanceof different coal blends. Initial indications are that thissimple system can be used as a predictive tool, whichcould be applied to coal selection and optimization ofblend composition.

Acknowledgment. Funding for this project by theEuropean Union under ECSC Contract No. 7220/ED/094 is gratefully acknowledged. The authors would alsolike to thank GENER of Chile for supplying samplesand the Consejo Nacional de Ciencia y Tecnologia deMexico (CONACyT - Mexico) for the award of afellowship to David Peralta.

EF010127P
