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Energy & Fuels 2002, 16, 404-411
Development of a Reactivity Test for Coal-Blend Combustion: 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 combustion reactivity of coal blends with blend composition. The test consists of the incomplete combustion of a batch of coal particles in a novel suspension-firing reactor. Residual chars are analyzed for composition and reactivity by nonisothermal thermogravimetry. Results from the bench-scale combustor were compared with observations from a full-sized power station and a single-burner pilot installation. Using the same sets of coals, the suspension-firing reactor gave the same order of reactivities as that observed in larger-scale equipment. Systematic preferential combustion of the higher-reactivity coal in blends has been identified; the extent of preferential combustion has been quantified from the thermogravimetric profiles of the residual chars. Our findings indicate that the reactivity of residual coal-blend chars is determined by the extent of enrichment of the lower-reactivity component in the blend. The coupled use of the new suspension-firing reactor and nonisothermal thermogravimetric analysis provides a powerful tool in comparing the combustion performance of different coal blends. Initial indications are that this simple system can be used as a predictive tool.
Introduction Coal-blend combustion is increasingly being seen as an attractive route for alleviating problems of fuel selection for pulverized-fuel (pf) power stations. Blending introduces greater fuel flexibility and provides a way of minimizing costs, e.g., by the use of several lowergrade coals to achieve desirable average blend properties. Coal blending may also provide a useful approach to controlling pollutant emissions, e.g., by mixing highsulfur and low-sulfur coals to limit SO2 emissions to acceptable levels.1 Although some utility companies have already gained operating experience with coal blends in pulverized-fuel combustors, the underlying mechanisms are still poorly understood. Problems have been reported in plant trials, including high levels of unburned carbon in fly ash, flame instability, increased slagging and fouling, CO emissions, and plume opacity. The success of coal-blend firing seems to be strongly dependent on individual plant operators’ experience1. To date, there has been no accepted test method for assessing or predicting the potential performance of blends, other than in largescale combustion trials. The outcomes of coal-blending trials are not straightforward to predict. When higher- and lower-reactivity coals are mixed, higher local temperatures and more intense radiative heat transfer would result from the * Corresponding author. E-mail:
[email protected]. (1) Carpenter, A. M. Coal Blending for Power Stations; IEA Coal Research: London, 1995; pp 11-14.
more rapid combustion of the higher-reactivity components. One likely outcome would be the assisted ignition and the more complete combustion of the less reactive coals/chars. However, it is also possible to speculate that faster combustion of higher-reactivity components might induce partial oxygen starvation of the lower-reactivity coals/chars. Outcomes of plant-based coal-blend combustion trials suggest that the latter may be the predominant effect; declining burnout rates appear to be particularly pronounced when low-NOx technologies are employed.2 Three types of laboratory-scale devices have previously been used to study the combustion of coal blends: thermogravimetric analyzers (TGA), drop-tube furnaces, and a bomb-calorimeter-based test. TGA-based coalblend combustion studies have shown additive3 as well as synergistic4 (i.e., non additive) effects. In the case of binary blends, additivity of behavior may be identified by the presence of two independent peaks in the TGA signal. Possible synergistic effects were indicated by the “nonadditivity” of certain characteristics, such as ignition temperature and burnout time. It has also been claimed that the initial ignition temperature of a nonisothermal TGA method could be correlated with the combustion efficiency of a research boiler. The results could be used as an empirical indicator of the relative (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.; Shanxi Science & Technology Press: Taiyuan, China, 1999; Vol. 1, pp 531534.
10.1021/ef010127p CCC: $22.00 © 2002 American Chemical Society Published on Web 01/17/2002
Reactivity Test for Coal-Blend Combustion
combustion characteristics of coals and coal blends when larger tests were difficult to do.5 In drop-tube furnaces, the combustion performance of coal blends is additive.3,6 This result is expected, as normal drop-tube operation does not allow high enough particle densities. In sparse particle density environments, particles appear to burn independently, in configurations where volatile clouds of different particles do not necessarily overlap. In a more recent investigation, a standard bomb calorimeter has been used to investigate the combustion of coal blends.7 Coals and coal blends were partially combusted in the bomb, using lower oxygen pressures (than the usual 30 bar), where volatiles and char particles can interact in the reaction zone. Two sets of coal blends previously combusted in a single-burner pilot plant (U.K.) and in a power station (Chile) were tested in the bomb calorimeter. Relative orders of reactivities observed in the bomb calorimeter were found to reproduce trends observed in the larger-scale trials: the degree of burnout of blends followed the trends previously obtained in both the pilot plant and power station trials. The preferential combustion of the higherreactivity coal in blends was identified. However, the method did not perform as expected when testing highswelling coals; in the face of partial melting of the sample, uniform distribution of oxygen could not be maintained throughout the calorimeter crucible. As in the case of the crucible test for volatile matter determinations, the method does not attempt to reproduce the hydrodynamic conditions of a pf burner. Nevertheless, it provides a readilysand commerciallysavailable bench-scale test for estimating relative reactivities of coal blends, which plant operators could use for planning and preparing their feedstocks. The present paper describes the more recent development of a novel laboratory-scale method based on a suspension-firing reactor, for analyzing the combustion of coal blends under conditions relevant to pulverizedfuel combustion. The reactor provides some of the operating conditions typical of pf burners that are relevant to blend combustion: fast heating rates in a well-defined reaction zone, where coal particles (suspended in the oxidizing gas) interact with evolving volatiles during combustion. The design has evolved from an earlier apparatus constructed to investigate the decomposition of limestone and fuel burnout in precalciners of cement works.8 A similar reactor configuration is currently being used to investigate toxic trace-element releases during cofiring of coal and biomass.9 Experimental Section Description of the Suspension-Firing Reactor. Figure 1 presents a schematic diagram of the suspension-firing reactor. The reactor is made of quartz (5 cm ID, 115 cm long). (5) Pisupati, S. V.; Scaroni, A. W. In Proceedings of the 9th International 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.
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Figure 1. Suspension-firing reactor. The top and bottom chambers are electrically heated using two independent coil heaters (1 and 2 kW, respectively) to a maximum of 1000 °C. The bottom chamber of the reactor serves to preheat incoming air, prior to delivery to the (top) combustion chamber through a constriction. The narrow entry serves to increase the inlet velocity of air into the upper chamber and prevents sample coal particles from falling down, out of the reaction chamber. During an experiment, a batch of coal particles (fed from the top) is suspended in preheated air and partially combusted in a cloud of released volatiles (∼1000 °C, atmospheric pressure). Visualization tests at room temperature, with the air flow scaled up to simulate the conditions at high temperature, have been carried out to ensure that coal particles are in suspension during the experiments. These tests were done using an air flow equal to 1.5 times that used in the high-temperature experiments so as to provide the same particle terminal velocity at both temperatures. Nitrogen is used to transport fuel particles through a slightly pressurized plug valve (0.1 bar above atmospheric pressure) and a water-cooled probe into the reactor. The probe is cooled to avoid the premature oxidation of coal. A sintered disk holds the thermocouple holder and the cooled-probe in place; it also serves as a screen, preventing particles from being pushed out of the reactor from the top. The experiment itself is based on determining extents of char survival for different coal blends, under conditions of incomplete combustion. This is achieved by reversing the direction of flow and flooding the reactor with gaseous nitrogen, 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 through valve 1, sweeping the particles out of the heated zone. Eventually, particles are trapped in the “ash collector”; the nitrogen stream leaves the reactor through valve 2.
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Table 1. Proximate and Ultimate Analyses of Coal Samples volatile matter (%, daf) ash (%, db) moisture (%, ad) carbon (%, daf) hydrogen (%, daf) nitrogen (%, daf) sulfur (%, daf)
Rheinbraun
Taff Merthyr
A
B
C
E
F
G
54.9 3.3 9.5 66.5 4.8 0.7 0.3
13.5 5.3 0.5 91.5 3.9 1.5 0.4
9.5 17.1 0.1 88.7 2.8 1.2 1.7
11.8 21.3 0.2 86.8 3.2 1.3 1.0
20.6 15.1 0.1 84.2 4.0 1.4 1.5
49.1 2.1 5.4 70.0 5.5 1.0 0.1
45.7 11.1 1.1 81.1 5.7 1.5 2.0
44.1 11.6 1.1 81.0 5.5 1.6 1.5
In these experiments, the flows of air and fuel-conveying nitrogen were 1.8 and 0.6 L/min respectively, resulting in an oxygen concentration of 16% in the reaction zone. Variable residence times of particles in the reaction zone from 2 to 5 s have been used. All experiments were done in duplicate. Coal Samples. The experimental method has been developed using blends of Rheinbraun lignite (Germany) and Taff Merthyr semianthracite (UK). These coals were selected because the wide differences between their combustion characteristics were expected to facilitate the detection of possible synergistic effects in the performance of their blends. In this context, synergistic effects are understood as parameters of combustion performance that cannot be predicted from the performance and composition of the individual coals in the blend by simple additivity. Sets of coal samples and blends were provided by a Chilebased and a U.K.-based power generating company. Three coal samples (E, F, G) were received from GENER (Chile). These coals and their blends have been tested in one of the 135-MW units of the Tocopilla power station. The performance of two sets of blended mixtures E-F and E-G, both containing coal E at 25, 50 and 75 wt %, have been compared between plant and bench-scale level. Ashes collected from the same section of the electrostatic-precipitator system during each test were also supplied. The UK set consisted of a suite of 3 coals (A, B, C), of higher rank than the Chilean set. The 0.5-MW single-burner pilot plant tests by the utility company had actually been carried out with blends of four coals. In this study, the fourth coal was not used for testing because it had not been found to affect blend performance. Information on the carbon-in-ash concentrations 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 then stored under nitrogen in a refrigerator. Proximate and ultimate analyses of the coals are shown in Table 1. Proximate analyses of coals were carried out by TGA. The properties of the blends were calculated from these analyses and the fractions of the individual components in the blends. Blending Method. Crushed coals were blended by rotating the sample holder on the rollers of a ball-mill for 30 min (at ∼150 rmp). Calculation of the Degree of Burnout. Char particles collected after each test were analyzed by nonisothermal thermogravimetry. The amount of residual combustible matter in the char was determined and the degree of burnout calculated using the ash-tracer method:
(
burnout (%) ) 1 -
AdCa (100 - Ca)(100 - Ad)
)
100
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 operating conditions of the test, it does not make sense to calculate an absolute value of excess oxygen since coal particles are injected as a batch whereas the flow of air is constant. However, different amounts of sample were injected for different coals so 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; its combustible content per unit mass was the lowest of all samples used. Calculations were carried out on the basis of 130 mg of Rheinbraun. The amount of combustible material in samples was corrected to a dry, ash-free basis. The following reactions were considered to be taking place:
C + O2 f CO2 2H2 + O2 f 2H2O S + O2 f SO2 It was assumed that the fuel oxygen took part in the combustion, which for purposes of this calculation proceeded to completion. The mass of oxygen supplied into the reaction zone at switching times of 2, 3, 4, and 5 s represents 25%, 29%, 33%, and 37% of the required oxygen for complete combustion of the fuel charge. It has been found that, under the current operating conditions, a significant fraction of volatile matter leaves the reaction zone prior to combustion and therefore a major portion of oxygen is consumed by the residual char particles. Thus, the degree of burnout, calculated from the mass loss of combustible material, represents the complete devolatilisation of coal plus partial combustion of char. Density of particles suspended in the reaction zone is typical of those found in the primary stream of commercial pf combustors (1.5-2.0 kg of air per kg of coal). Nonisothermal Thermogravimetric Analysis of Residual Chars. A nonisothermal thermogravimetric method10 was used to analyze the residual char samples. In ref 10, the authors claimed that the use of pure air as the oxidation medium could promote the uncontrolled self-heating of the sample. To reduce this effect, the oxygen concentration was reduced to 7% by mixing air with nitrogen. In our work, however, this uncontrolled self-heating was not observed, even though we used air throughout all the runs. This might be due to the small amount of sample analyzed in the present set of experiments (1-2 mg). A Perkin-Elmer TGA 7 (Series 1020) was used, with the air flow set at 40 mL min-1. Samples were held in the furnace at 30 °C for 3 min and then heated to 400 °C at 40 °C min-1. The heating rate was then changed to 15 °C min-1 and the temperature raised to 900 °C. Data analysis was performed between 400 and 900 °C, where the major combustion process took place. Finally, the temperature was kept constant at 900 °C for 4 min to ensure that all combustible matter was consumed, to determine the amount of unburned carbon in the char.
Results Method Development Using Rheinbraun Lignite and Taff Merthyr Coal. Rheinbraun lignite, Taff Merthyr semianthracite, and their blends, containing 25, 50, and 75 wt % of Rheinbraun, were combusted in the suspension-firing reactor. The mass of sample injected 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.
Reactivity Test for Coal-Blend Combustion Table 2. Mass of Coal Injected into the Reactor (Rheinbraun-Taff Merthyr) Rheinbraun (wt %)
mass of sample (mg)
0 25 50 75 100
84 92 102 114 130
of fuel) is shown in Table 2; the amount of sample increased with increasing proportions of Rheinbraun as this lignite contained less combustible material per unit mass. This set of samples was combusted using “residence times” of 2, 3, 4, and 5 s (Figure 2). The dashed lines in Figure 2 represent theoretical burnout levels calculated assuming linear additivity of burnout from individual pure blend components. The degree of burnout of individual coals displayed the expected trend. Rheinbraun (RB) lignite was markedly more reactive than Taff Merthyr (TM) semianthracite. The performance of all blends was found to be additive at short residence times. However, at longer residence times, the level of burnout fell below the linear additivity line, particularly for the 50% and 75% blends. By contrast, the blend containing 25% of Rheinbraun did not show significant deviation from additivity with increasing residence time. Thermogravimetric profiles of chars collected from the suspension-firing reactor have been plotted as the rate of weight loss of combustible material versus reciprocal temperature in the range 400-900 °C (Figure 3). Significant differences were found between profiles of individual mixtures. The RB-derived char exhibited the highest rate of weight loss at lower temperatures. The coal-blend char originally containing 75% of RB displayed two practically independent peaks, corresponding to the sequential combustion of Rheinbraun and Taff Merthyr residues in the char. By contrast, the
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samples originally containing 25% and 50% of Rheinbraun showed small contributions from Rheinbraun residue. These findings clearly indicated that RBlignite, the higher-reactivity coal in these blends, combusted preferentially under the experimental conditions of the suspension-firing reactor. The concentration of RB-derived carbon in the coalblend chars was calculated from data shown in Figure 3. These calculations were done assuming simple additivity of the individual Rheinbraun and Taff Merthyr char profiles. The proportion of RB-derived carbon best matching the experimental blend profiles represented the composition of the blend char. A more accurate calculation would have involved measuring the area under the curve in Figure 3 and calculating the proportion of RB-derived char accordingly. However, this method would not distinguish individual components when extensive overlapping existed between peaks (e.g., for more similar coals); the former method was therefore employed in developing the technique. The results of these calculations are shown in Table 3. For a given blend, the concentration of RB-derived carbon decreased with increasing residence time. Similarly, for a fixed residence time, the concentration of RB-derived char increased with increasing proportions of RB-lignite in the original blends. Thus, the preferential combustion of Rheinbraun, the higher-reactivity coal, was clearly demonstrated and quantified. Tests with GENER Samples. Coals E, F, and G and the corresponding sets of blends E-F and E-G, each containing 25, 50 and 75 wt % of coal E, were also partially combusted in the suspension-firing reactor. The performance of these blends in one of the 135-MW units of the Tocopilla power station was evaluated by determining loss-on-ignition of residual fly ash collected in the electrostatic precipitators. On the basis of these values, the degree of burnout was calculated assuming that all the mineral matter in coal, ultimately oxidized
Figure 2. Burnout of Rheinbraun-Taff Merthyr samples at different residence times.
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Figure 3. Rate thermograms of Rheinbraun-Taff Merthyr char samples at 3 s residence time. Table 3. Concentration of Rheinbraun-derived Carbon in the Coal-Blend Chars coal-blend chars RB (wt %) original blend RB (wt %)
2s
3s
4s
5s
25 50 75
0 5 16
0 3 20
0 2 8
0 2 11
to form ash, had been recovered as bottom ash and fly ash. Plant operators consider it safe to assume that the bottom ash accounted for 20 wt % of the total, containing ∼2 wt % of uncombusted organic material. Figure 4 shows the order of combustion reactivity of the individual coals as
E>G>F For the set of E-F blends, burnout was found to be linearly additive. By contrast, burnout levels of E-G blends were measurably lower than could have been expected from linearly additive behavior. These power plant derived results may be compared with levels of burnout, observed in the suspensionreactor. 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 of increasing residence times (2, 3, and 4 s). The degree of burnout of individual coals was E > F > G, which was different from that found in the power station where coal G was more reactive than coal F. The combustion performance of the set of blends E-F (Figure 5) was additive at all residence times, except for the blend containing 25% of Coal E that showed somewhat less
burnout than predicted. The overall trend of linear additivity in these data reflects closely the trends in burnout levels found with the power station ashes. Figure 6 shows that the set of blends E-G exhibited consistently lower burnout than predicted from linear additivity for all residence times. The low burnout was particularly significant for the blends containing 25% and 50% of Coal E. Interestingly, this result also mirrored that found in the power station ashes, where deviation from additivity was more pronounced for the set of blends E-G (see Figures 4 and 6). TGA thermograms of the set of chars E-F at 3 s residence time are presented in Figure 7. Compared to the Rheinbraun-Taff Merthyr chars (Figure 3), this set of coal-blend chars showed significant overlap between individual char components. This result was a consequence of the relative similarities in rank, of Coals E and F compared to RB-lignite and TM-semi anthracite (see Table 1). Clearly, differences in combustion reactivity between chars are reduced when using more similar coals. The corresponding profiles of the set of chars E-G (not shown) were qualitatively similar to those in Figure 7; coals F and G have very similar properties. The proportion of coal E in the coal-blend chars was calculated from these rate thermograms (Table 4). For a given blend, the proportion of coal E decreased with increasing residence time. For a given residence time, the proportion of coal E increased with increasing concentration of coal E in the original blends. Preferential combustion of coal E was observed for the blends initially containing 25% and 50% of coal E, but for the 75% coal E blend, differences were small and did not amount to significant preferential combustion.
Figure 4. Burnout of GENER samples in the 135-MW power station (Tocopilla, Chile).
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Figure 5. Burnout of E-F set samples at different residence times.
Figure 6. Burnout of E-G set samples at different residence times.
Tests with UK Power Utility Samples. Coals A, B, and C were mixed in the same proportions as used in the blends tested by the utility company. The compositions of the utility company blends and of those produced in this laboratory are shown in Table 5. The performance of the blends in the single-burner pilot plant was expressed in terms of loss-on-ignition of fly ash, with low loss-on-ignition values indicating less carbon-in-ash and better performance by a particular blend. The pilot-plant data are presented in Table 6, showing that blend 2 exhibited greater combustion reactivity (i.e., lower loss-on-ignition) than blend 1. The three individual coals (A, B, and C) and the two blends (blends 1 and 2) were partially combusted at 2 s residence time in the suspension-firing reactor. The degree of burnout of these samples is illustrated in Table 7; the order of increasing burnout for individual coals was A < B < C. Blend 1, containing a higher proportion of coal A, was less reactive than blend 2. Once again, the result from the suspension-firing reactor was found to reproduce the trend observed in the larger-scale, pilot-plant, tests (Table 6). The chars collected from the suspension-firing-reactor tests were also analyzed by thermogravimetry. In this case, the rate thermograms of all coals and blends exhibited complete overlapping and no differences between them could be observed. Considering the similar ranks of these coals (Table 1), the result was not surprising. Thus, the thermogravimetric profiles of coal-
blend chars did not show the separate contribution from different components (as previously observed for the Rheinbraun-Taff Merthyr and GENER samples in Figures 3 and 7) due to the similarity of the components in the blends. Discussion Figure 8 presents elemental carbon contents of individual coals used in this study, plotted against burnout levels in the suspension-firing reactor, clearly showing decreasing levels of burnout with increasing rank. In this sense, the reactor is shown to reproduce a wellestablished trend. Furthermore, preferential combustion of the higher-reactivity coal in coal-blends has been identified and, where possible, quantified by nonisothermal thermogravimetry. The extent of enrichment of the lower-reactivity component in the coal-blend chars increases with increasing differences in reactivity between the individual components in the blend. For example, for the Rheinbraun-Taff Merthyr blends (two samples at opposite ends of the coal rank range), the coal-blend chars were increasingly concentrated with Taff Merthyr-derived char (Table 3). When the difference between the reactivities of the individual coals decreased (sets E-F and E-G), a lesser extent of enrichment of the lower-reactivity component in the blends could be observed (Table 4). This implies that the greater the difference in combustion reactivities
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Figure 7. Rate thermograms of E-F char samples at 3 s residence time. Table 4. Concentration of Coal E-derived Carbon in the Coal-Blend Chars E-F and E-G coal-blend chars Coal E (wt %) original blend coal E (wt %)
2s
3s
4s
25 50 75
Set E-F 14 20 65
9 25 70
5 17 57
25 50 75
Set E-G 10 41 73
10 38 75
4 18 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 20% B 13% C 27% D
10% A 47% B 13% C 30% D
55% A 27% B 18% C
14% A 67% B 19% C
Table 6. Loss-on-Ignition (%) of UK Power Utility Samples in the Pilot Plant excess oxygen (%)
blend 1
blend 2
2 3 4
17.5 17.5 10.0
12.0 8.8 6.0
Table 7. Burnout of U.K. Power Utility Samples in the Suspension-Firing Reactor sample
burnout (%, daf)
A B C blend 1 blend 2
38.0 45.2 53.1 44.3 54.3
between the blend components, the larger the extent of enrichment of the lower-reactivity component in the coal-blend chars. The observed lower burnout levels compared to linear additivity suggest that in coal-blend combustion, partial oxygen-starvation of lower-reactivity components may be taking place. At present, fluid dynamics modeling is being carried out to estimate possible oxygen starvation on combustion in the suspension-firing reactor. The incomplete combustion of coal blends in the suspension-firing reactor has thus displayed both ad-
Figure 8. Burnout of single coals at different residence times in the suspension-firing reactor.
ditive and nonadditive effects. Additive effects were more pronounced at shorter residence times, while nonadditivity increased with increasing residence times. It is likely that this shift with time corresponds to different rates of thermal deactivation within the reactor,11 in line with different rates of combustion for individual components causing changes in the blend composition. At shorter residence times, the higherreactivity coal will react faster as has been observed (11) Hurt, R.; Sun, J. K.; Lunden, M. Combust. Flame 1998, 113, 181-197.
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experimentally. However, uncombusted organic material will be progressively deactivated, and at longer residence times, this deactivation process will have a negative effect on burnout. Conclusions A laboratory-scale suspension-firing reactor has been developed for investigating coal-blend combustion under pf-firing conditions. “Incomplete-combustion” tests in this reactor have allowed establishing correlations with results from a full-sized power station and a singleburner pilot installation. Using similar samples, the suspension-firing reactor gave the same hierarchy of reactivities as observed in larger-scale equipment. For all blends, systematic preferential combustion of the higher-reactivity coals has been identified; the extent of preferential combustion has been quantified using thermogravimetric profiles of the residual chars. Our findings indicate that the reactivity of residual coal-
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blend chars is determined by the extent of enrichment of the lower-reactivity component in the blend. The coupled use of the new suspension-firing reactor and nonisothermal thermogravimetric analysis provides a powerful tool in comparing the combustion performance of different coal blends. Initial indications are that this simple system can be used as a predictive tool, which could be applied to coal selection and optimization of blend composition. Acknowledgment. Funding for this project by the European Union under ECSC Contract No. 7220/ED/ 094 is gratefully acknowledged. The authors would also like to thank GENER of Chile for supplying samples and the Consejo Nacional de Ciencia y Tecnologia de Mexico (CONACyT - Mexico) for the award of a fellowship to David Peralta. EF010127P
