Further Development of a Suspension Fired Reactor To Assess the

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Further Development of a Suspension Fired Reactor To Assess the Relative Performance and Synergistic Effects during the Combustion of Coal Blends A. A. Majid, N. Paterson,* G. P. Reed, D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, Prince Consort Road, London SW7 2BY, United Kingdom Received July 6, 2004. Revised Manuscript Received January 5, 2005

An improved suspension-firing reactor has been constructed and a new experimental procedure developed that is able to assess the relative performances of coals and coal blends. The method measures the total combustion time of the samples by measuring the exit gas analysis from the reactor on a continuous basis. The new equipment and procedure overcome some deficiencies that were identified in an earlier suspension-firing method that was based on the extent of burnout of the sample at the end of an arrested combustion test. The presence of a synergistic effect on blending is indicated by a difference between the actual performance of the blend and that predicted by additivity from the performance of the individual coals and the composition of the blend. Three sets of binary blends and their parent coals, which have also been used in utility pf furnaces, have been tested in the suspension-firing reactor. The results have been compared with the performance at the commercial scale, as indicated by the C content of the electrostatic precipitator fly ashes. In all three cases, the performance as predicted by the suspension-firing reactor was consistent with that observed at the commercial scale. This suggests that this relatively straightforward small scale test may provide a useful route for the assessment of coals prior to purchase and for the optimization of blend performance.

Introduction Coal blending is used to increase the range of coals that can be used in pulverized fuel (pf) power stations. It provides increased fuel flexibility and can improve power station economics, e.g., by the use of several lower-grade coals to achieve desired blend properties. Coal blending may also provide a useful approach to controlling pollutant emissions, e.g., SO2 emissions may be controlled at an acceptable level by blending highand low-sulfur-content coals.1 Although utility companies have already gained much operating experience with coal blends in pulverized fuel combustors, the underlying mechanisms are still poorly understood. Problems that have been reported in plant trials include 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' experience.2 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 higherand lower-reactivity coals are mixed, higher local temperatures and more-intense radiative heat transfer can * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Takeshita, M. Air Pollution Control Costs for Coal Fired Power Stations, IEAPER/17; IEA Coal Research: UK, 1995. (2) Carpenter, A. M. Coal Blending for Power Stations, IEA CR/81; IEA Coal Research; UK, 1995.

result from the more rapid combustion of the higherreactivity components. One possible outcome is 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, and hence lower conversion, of the lower-reactivity coals/ chars. 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.3 Attempts have been made to predict the performance of coal blends using laboratory-scale tests. Thermogravimetric analyzers (TGA), drop-tube furnaces, and a bomb-calorimeter-based test have been used with limited success. TGA-based coal-blend combustion studies have shown additive,4 as well as synergistic5 (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 trace. 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 (3) Maier, H.; Splietoff, H.; Kicherer, A.; Fingerle, A.; Hein, K. R. G. Effect of Blending and Particle Size on NOx Emission and Burnout. Fuel 1994, 73, 1447-1452 (4) Artos V., Scaroni A. W., TGA reactivities and burning profiles of coal blends. 9th Annual Pittsburgh Coal Conference (USA) 1992, 659-665. (5) Li, B. Q., Liu, Z. Y., Eds. Prospects for Coal Science in the 21st Century, Shanxi Science and technology Press: China, 1999; Vol. 1, 531-534.

10.1021/ef049844k CCC: $30.25 © 2005 American Chemical Society Published on Web 04/28/2005

Combustion of Coal Blends

initial ignition temperature measured by a non-isothermal 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 combustion characteristics of coals and coal blends when larger tests were difficult to do.6 The combustion performance of coal blends has been found to be additive in drop-tube furnaces.7,8 This result is expected, as normal drop-tube operation does not allow high enough particle densities. In low-particle-density environments, particles appear to burn independently and volatile clouds of different particles do not interact. In a more recent investigation, a standard bomb calorimeter has been used to investigate the combustion performance of coal blends.9 Coals and coal blends were partially combusted in the bomb, using lower oxygen pressures than the usual 30 bar used in the determination of the calorific value, such that the sample was only partially combusted. The advantage of the technique is that 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 higher-reactivity coal in the blends was identified. However, the method did not perform as expected when testing high-swelling coals; partial melting of the sample occurred with high swelling coals, and this prevented the maintenance of a uniform distribution of oxygen throughout the calorimeter crucible. A method based upon a suspension-firing reactor was developed next to increase the range of coals that could be tested.10 The reactor provided 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 was based on an earlier apparatus constructed to investigate the decomposition of limestone and fuel burnout in precalciners of cement works.11,12 A method was developed that was based upon the measurement of the extent of combustion that occurred during a standard test time. The time was set so that complete combustion was only approached by (6) Pisupati, S. V.; Scaroni, A. W. 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 11511154. (7) Artos, V.; Scaroni, A. W. TGA and drop-tube reactor studies of the combustion of coal blends. Fuel 1993, 72, 927-933. (8) Peralta, D. Ph.D. Thesis, University of London, London, 2001. (9) Peralta, D.; Paterson, N. P.; Dugwell, D. R.; Kandiyoti, R. Coal blend performance during pulverized-fuel combustion: estimation of relative reactivities by a bomb-calorimeter test. Fuel 2001, 80, 16231634. (10) Peralta, D.; Paterson, N. P.; Dugwell, D. R.; Kandiyoti, R. Development of a reactivity test for coal-blend combustion: the laboratory-scale suspension-firing reactor. Energy Fuels 2002, 16, 404411, (11) Khraisha, Y. H.; Dugwell, D. R. Chem. Eng. Res. Des. 1988, 67, 52-57. (12) Khraisha, Y. H.; Dugwell, D. R. Coal combustion and limestone calcinations in a suspension reactor. Chem. Eng. Sci. 1992, 47, 9931006,

Energy & Fuels, Vol. 19, No. 3, 2005 969

the most reactive test coals. The method was able to show the differences between the performances of a suite of coals and blends. The trends in the performance in the laboratory-scale test were the same as those found in larger-scale trials in a burner test rig and at a commercial power station. However, it was realized that the method was limited in its accuracy, as errors occurred in the measurement of the unconverted C. This was due to difficulties in removing all of the residual char from the reactor after the test, as it tended to adhere to the walls of the reactor. To remove the impact of this, the ash tracer method was used to estimate the combustibles content of the char. However, this method assumes that the composition of the input mineral matter is the same as the output ash, and this is not strictly correct. In addition, some volatiles were thought to escape from the reactor without combustion, and this will have influenced the chemical environment within the reactor. The effect is thought to have been caused by the use of N2 as the coal transport gas, which resulted in a small, but finite, time being required for mixing the sample with O2 prior to combustion. A new suspension-firing reactor and operating procedure have now been developed that overcome the deficiencies of the earlier method. The new method measures the time taken for complete combustion of the sample under a standard set of operating conditions. It represents a significant improvement in the accuracy of the assessment since the combustion time is estimated from the gas analysis, which is measured continuously and with good precision and accuracy. The development, validation, and use of this method are described in this paper. Experimental Section The Higher-Temperature Suspension-Firing Reactor. The higher-temperature suspension reactor (HTSR) has been developed from an earlier lower-temperature suspension-firing reactor (LTSR). This has been described in detail elsewhere.10 Figure 1 shows a schematic diagram of the HTSR, which is designed to achieve temperatures up to 1300 °C at atmospheric pressure. The quartz reactor (5 cm i.d., 95 cm long) is divided into two zones, the air-preheat zone and the combustion zone (38 and 25 cm long, respectively). The temperature is measured using a Pt-Rh thermocouple placed inside the reactor with its tip projecting 45 mm into the combustion zone. The reactor is heated electrically using 10 silicon carbide rods placed in parallel on either side of the quartz reactor. The bottom chamber serves to preheat incoming air prior to delivery to the top chamber through a constriction. Air flowing into the upper chamber suspends the particles and maintains the particles in circulation within the reaction zone. A sintered disk allows flue gas to exit but prevents loss of particles from the reactor. Coal-blend samples are fed into the combustion zone in batches from a slightly pressurized hopper (0.1 bar above atmospheric pressure). It is fed through a ball valve using a transport gas, which transfers the sample through a watercooled probe into the reactor. The water-cooled jacket of the probe prevents premature combustion of the coal-blend samples during its transport into the reactor. The transport gas is comprised of 6% O2 in N2. The presence of O2 in the transport gas minimizes O2-diffusion limitations and allows the volatiles to combust as soon as they are released on entering the reaction zone. This overcame a deficiency of the earlier version of the reactor, where loss of uncombusted volatiles is known to have occurred. In the experiments, the flows of air and

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Majid et al. Carbocol-Kangra coal blends received from Mitsui Babcock. Where full-scale tests had been performed, fly ashes collected from the electrostatic precipitators were supplied together with the raw coals. This enabled a comparison to be made of the trends produced in the HTSR with that of the power station burners. The bulk samples were sieved to provide test samples in the particle-size range 106-150 µm. Prepared samples were dried at 50 °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. The analyses of the blends were calculated from these values and the weight fractions of the component coals in the blends. At the commercial scale, it is usual to blend the coals prior to pulverization, which can cause preferential grinding of the softer particles.13 This can potentially lead to effects on the combustion performance caused by the presence of larger hard particles and softer small particles of coal. To eliminate this possibility in the current study, the coals were crushed separately, sieved to isolate the required size fraction, and then blended in the laboratory. This was to ensure that each component coal in the blend had the same particle-size distribution. The samples were blended mechanically by rotating the sample holder on the roller of a ball-mill for 30 min at 150 rpm. The samples were shaken by hand at 5 min intervals during this period. In this study, therefore, the suspension-firing reactor is able to investigate effects on the combustion performance caused by the coal itself, in isolation from effects caused by variations in particle size.

Results and Discussion Figure 1. High-temperature suspension-firing reactor. transport gas were 1.8 and 0.6 L/min respectively, resulting in an initial oxygen concentration in the reactor of 17%. For enriched oxygen experiments, the air stream was replaced by gas mixture containing 50% O2 in N2, which, after dilution by the transport gas, produced a mixture containing 39% O2 in the combustion zone. During experiments, devolatilization, volatile combustion, and complete char oxidation take place inside the combustion zone. The concentrations of O2, CO2, and CO in the flue gas were analyzed continuously using a Servomex Xentra gas analyzer, which was connected to the outlet pipe from the reactor. The analyzer was calibrated daily using a certified gas mixture supplied by Air Products, Ltd. The flue gas was dried by passage through magnesium perchlorate prior to analysis. The measurements were logged using a PC at intervals of between 1 and 3 s, until complete combustion of the samples was achieved. This was indicated by the return of the O2 concentration to its initial level. Between runs, the reactor was purged with nitrogen to remove any ash sticking to the walls of the reactor. All experiments were done in duplicate. Coal Samples. The experimental method was developed using blends of Rheinbraun lignite (Germany) and Taff Merthyr semi-anthracite (UK). These coals were chosen because of the wide difference in their ranks and therefore combustion characteristics. The different properties were expected to assist in detecting any synergistic effects in the combustion performance of the blends. Synergistic effects are described as performance characteristics of the blends that cannot be linearly predicted from the performance characteristics and composition of the constituent coals. Four different sets of blends were then tested in the HTSR. These were, E-G blends previously tested in a 135 MW power station operated by GENER in Chile (and also used in the previous study), Yan Zhou-Klein Kopke and Poduff-Petroluem Coke blends tested in a 285 MW boiler in Denmark and

Comparison of the Performance of the Lowerand Higher-Temperature Suspension-Firing Reactors. Burnout data, obtained at a temperature of 1000 °C, for Rheinbraun-Taff Merthyr blends in the HTSR have been compared with results from previous runs using the LTSR (Table 2). The tests were done with combustion times varying between 2 and 5 s. For all runs, the % burnout of the blends in the HTSR was consistently slightly lower than the burnout in the LTSR. The differences in the levels of burnout, however, decreased at the longer combustion times. The difference in behavior between the two reactors is thought to be due to an effect of the longer, water-cooled feed tube, designed to cope with the higher temperatures in the HTSR. The particles took longer to enter the combustion zone of the HTSR, and this caused a slight reduction in the residence time at reaction temperature, which resulted in a lower burnout value. However, despite the slight difference in the burnout values, the trends in performance, with increasing residence time, observed in the LTSR were reproduced by the HTSR. Development of a New Test Method for the Higher-Temperature Suspension-Firing Reactor. A new test procedure was developed for the HTSR to enable operation at higher temperatures, as it was not thought practicable to conduct comparative tests under conditions of partial burnout at temperatures in excess of 1000 °C. The rates of reaction were considerably higher, which meant that test times would be unacceptably short to achieve partial levels of burnout. In addition, it was desirable to avoid the use of the ash tracer technique to derive the % burnout, as this (13) Conroy, A. P.; Juniper, L. A.; Phong-Anant, D. The impact of coal pulverising characteristics on power plant performance. 5th International Conference on Coal Science (Japan) 1989, 1003-1006.


Energy & Fuels, Vol. 19, No. 3, 2005 971 Table 1. Proximate and Ultimate Analyses of Coal Samples

Rhein braun

Taff Merthyr

E

G

Yan Zhou

Klein Kopke

Poduff

Pet Coke

Carbocol

Kangra

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

49.1 2.1 5.4 70.0 5.5 1.0 0.1

44.1 11.6 1.1 81.0 5.5 1.6 1.5

43.5 8.5 0.8 85.8 5.8 0.9 0.7

34.0 11.9 0.7 80.3 4.8 1.4 1.0

49.1 4.1 0.4 78.0 5.3 1.5 1.1

27.9 0.5 0.3 83.8 4.1 1.3 5.7

44.7 6.0 0.77 78.7 5.7 1.1 0.6

38.2 9.4 0.92 79.4 5.1 1.4 1.4

volatile matter (%, daf) ash (%, db) moisture (%, ad) carbon (%, daf) hydrogen (%, daf) nitrogen (%, daf) sulfur (%, daf)

Table 2. Burnout of Rheinbraun Lignite and Taff Merthyr Anthracite in the LTSR and HTSR combustion time, s Rheinbraun Taff Merthyr

burnout, % 2 3 2 3 4 5

LTSR

HTSR

86.9 90.5 49.0 54.7 51.8 54.2

85.0 89.1 41.3 48.5 50.2 50.7

introduced some errors in the final data. The principle of the new method was to allow combustion to proceed to completion and to measure the time required for this to occur. The output gas analysis was measured on a continuous basis during each test to monitor the progress of the combustion. Differences in performance would be indicated by differences in the total combustion times. It is considered that the new method has resulted in a considerable improvement in the reliability of the test. Whereas the previous method was based upon incomplete combustion in an arrested experiment, the new method employs a purpose-designed gas analysis method, which is dynamic and proceeds to the completion of combustion and, as such, is more reliable. To enable the comparison of the performance of different individual coals and blends, it was important that the same level of excess oxygen was supplied to each sample. As the level and flow rate of O2 were fixed, the mass of sample injected was therefore varied between different types of sample. Rheinbraun lignite was chosen as the baseline fuel because its combustible content per unit mass was the lowest of all samples used. The mass of Rheinbraun lignite to be used was determined during a series of tests with increasing amounts of fuel. Too much fuel resulted in incomplete combustion of volatiles, as indicated by the accumulation of tar on the Kaowool filter placed on top of the reactor, while too little fuel produced reduced the repeatability of the results. After several tests, 90 mg of Rheinbraun lignite was chosen as the baseline sample weight. The amount of the other coals and blends were calculated so that the sample contained the same amount of combustible material as in the 90 mg of the baseline lignite. The following reactions of C, H, and S with O2 were included in the calculation. Fuel oxygen was assumed to form gaseous oxygen and subsequently to take part in the oxidation reactions below.

C + O2 f CO2 2H2 + O2 f 2H2O S + O2 f SO2 The mass of each coal and blend injected into the

Table 3. Mass of Coals Injected into the Suspension-Firing Reactor coal

mass injected (mg)

Rheinbraun lignite (reference) Taff Merthyr semi-anthracite coal E coal G Yan Zhou Klein Kopke Poduff Petroleum Coke Carbocol Kangra

90 57 77 67 61 70 65 58 65 68

suspension-firing reactor is given in Table 3.Calculation of Oxygen Consumption and Time for Complete Combustion. A mass balance can be performed on the reactor from the O2 concentration-time curve obtained for each run. The integral or area enclosed between the curve and its value at the start of the test gives the amount of O2 consumed by each charge of sample. The amount of O2 consumed by the reaction should be the same for each coal and blend because the same amount of combustible matter was supplied. For the complete combustion of both volatiles and char, a total of 148 mg of O2 would have to be consumed. The standard deviation of the O2 concentration-time curve gives a measure of the dispersion of the distribution and provides an indication of the relative time for complete combustion (i.e., the time for the O2 concentration to return to its original value). The standard deviation of a curve is defined as

Standard Deviation, s )

x

∑i fi(Xi - Xh )2 ∑i fi

where fi is the frequency (O2 concentration) that occurs at Xi (time) and X h is the mean time. For moderately skewed curves, 99.7% of the distribution lies between X h - 3s and X h + 3s. Applying this to the curves, the combustion time for any sample lies within an interval of 6 times the standard deviation of the O2 concentration-time distribution. Method Development Using Rheinbraun-Taff Merthyr and E-G blends. The method was initially developed using a temperature of 1000 °C, so that the data could be compared with that obtained in the LTSR and the same fuel combinations. This was done to ensure that the new method was able to detect similar trends to those seen with the previous method. Rheinbraun lignite, Taff Merthyr semi-anthracite and blends containing 25, 50, and 75% (by wt) of Rheinbraun were injected into the suspension-firing reactor and were

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Figure 2. O2 concentration-time profile for Rheinbraun-Taff Merthyr blends at 1000 °C.

allowed to combust to completion. The concentration of oxygen at the outlet of the reactor during the tests was recorded and is shown in Figure 2. The O2 consumption was calculated from the area between the actual O2 concentration curve and the initial concentration of O2. As the same amount of combustible matter was fed at the start of each test, the same mass of oxygen should have been required for complete combustion. The mass of O2 consumed by the samples was found to be between 145 and 155 mg. This agreed, within the errors of the experiments, with theoretical calculations based on the elemental composition of the samples, which showed that a total of 148 mg of O2 was required for complete combustion. This was taken as confirmation that complete combustion was occurring under the conditions used at 1000 °C. The data (Figure 2) show that the magnitude of the initial fall in the O2 concentration increased and the combustion time decreased as the proportion of lignite in the blend was increased. The repeatability of the test method was determined by conducting a series of three tests under the same conditions. It was found that the combustion time could be repeated within the limits of (1.5 s of the average value. All of the data presented in this paper are the average of three determinations at each set of conditions and similar repeatability limits apply. The same behavior was observed for E-G blends (data not shown), where the profiles were dependent on the amount of E, the higher-reactivity coal. However, for blends E-G, the O2 concentration-time profile of the component coals and blends were more similar because the blends were made up of component coals with more similar properties. Figure 3a shows the combustion times of the blends plotted against the proportion of Rheinbraun in the blend. It is evident from these results that the time for complete combustion of the blends were generally longer than predicted from the combustion times of individual coals and composition of the blend (i.e., by additivity). E-G blends also displayed the same effect (Figure 3b). This negative synergistic effect (i.e., the performance of the blend was lower than predicted assuming additivity) suggests a possible drop in the overall reactivity of the blended fuels during combustion in a commercialscale furnace. The trends in combustion times above were compared with the burnout data available from a previous blend combustion study in the LTSR.10 For both RheinbraunTaff Merthyr and E-G blends, the % burnout of the blends was lower than predicted from additivity. Fur-

Figure 3. Combustion times of (a) Rheinbraun-Taff Merthyr and (b) E-G blends at 1000 °C.

thermore, the LTSR burnout results for E-G blends mirrored that found in power station ashes, where a drop in the % burnout of the blends (as indicated by the composition of the electrostatic precipitator ashes) was observed. The drop in the % burnout of the blends, both in the LTSR and power station, is consistent with the negative synergy displayed in the total combustion time results from the HTSR. A plausible mechanism for the effect involves the preferential reaction of the more reactive components of the blends (Rheinbraun lignite and component E), which starved the less reactive components (Taff Merthyr and G) of O2. This provided a short, but significant, time for the less reactive coal to deactivate, so that when O2 became available again, the less reactive component reacted at a lower rate than would be expected from the performance of the neat coal. A further insight into this was obtained by measuring the TGA burning profiles of the blend chars from the partial burnout tests in the LTSR. This confirmed the enrichment of the lower-reactivity coal char in the blend char, which showed that preferential reaction of the more reactive component had occurred in the LTSR. The similarity of the data trends obtained in both versions of the suspension reactor, at 1000 °C, served as a validation of the new test procedure. Effect of Temperature and O2 Concentration. The effect of reactor temperature and inlet O2 concentration were investigated using the new HTSR test procedure. Figure 4a and b show the O2 concentrationtime profile of Rheinbraun-Taff Merthyr blends at 1100 and 1200 °C. The initial period of each test (about 25 s), during which there was no apparent change in the profile, represents the response time of the analysis equipment. Thereafter, the large drop in the O2 concentration is due to the rapid release and combustion of the volatiles from the fuel. It is assumed that this occurred immediately during the rapid heating of the injected sample. The lignite contained more volatile matter than the Taff Merthyr coal, and therefore, the



Figure 5. Combustion times for Rheinbraun-Taff Merthyr at (a) 1100 and (b) 1200 °C.

Figure 4. O2 concentration-time profiles for RheinbraunTaff Merthyr blends at (a) 1100 and (b) 1200 °C.

magnitude of this initial drop increased with the proportion of lignite present. In comparison with Figure 2 (1000 °C), the maximum depth of the drop observed for the 100% Rheinbraun lignite has increased between 1000 and 1100 °C but has not changed further between 1100 and 1200 °C. This must be indicating that the rate of volatile release has achieved its maximum release rate by 1100 °C. With 100% Taff Merthyr coal, a small increase in the initial depth of the drop in the O2 concentration can be seen over the whole temperature range. The difference in the behavior of the neat coals reflects the differing volatile matter contents and reactivities. The behavior of the blends was intermediate between the two extremes. After the initial drop in the O2 concentration, the rate of return of the O2 trace to its starting value is a function of the overall reactivity of the mixed char plus the effect of any remaining volatile material. The same trend was also observed for E-G blends, although the data lines were closer together because of the greater similarity between the constituent coals. The total combustion times, at 1100 and 1200 °C, for both blends plotted against the proportion of the more reactive coal showed shorter blend combustion times than predicted from additivity. Data obtained with the Rheinbraun-Taff Merthyr blends are shown in Figure 5. This is the opposite effect to that observed at 1000 °C. It is thought that the effect has been caused by the more rapid release of volatiles from the blends at the higher temperatures. Their release must have caused a momentary oxygen deficiency in the reactor, and uncombusted volatiles must have been swept away. However, this also meant that the char had a lower residence time before it was combusted (because the O2 scavenging effect of the volatiles was over more quickly).

Figure 6. Ratio of CO to CO2 release from Rheinbraun-Taff Merthyr blends.

Therefore, the char was more reactive, as there was a slightly lower time for deactivation of the evolving char, which resulted in a shorter total combustion time. The deficiency of O2 inside the reactor was confirmed by the increase in the measured CO emission with increasing temperature and increasing concentration of the more reactive coal. There was only a low release of CO from the reactor at 1000 °C with any of the fuel combinations. However, the data obtained with Rheinbraun-Taff Merthyr at 1100 and 1200 °C (Figure 6) show that the CO emission (as indicated by the CO/CO2) increased with the proportion of the more reactive coal and with temperature. The devolatilization of the coal and the reaction of CO2 on the surface of the char particles produced the CO, which could not be combusted due to a momentary deficiency of O2. The hypothesis was tested by enriching the O2 content of the inlet gas without changing the velocity profile inside the suspension-firing reactor. This was done by using 50% O2 in N2 instead of the zero-grade air used in the earlier tests. Experiments at 1200 °C were repeated under this enriched O2 condition (i.e., overall 39% O2 after dilution by the sample injection gas). In this test, there must have been sufficient O2 to react with the CO from the fuel, as no CO was detected in

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Figure 7. Combustion times of Rheinbraun-Taff Merthyr blends at 1200 °C under excess O2 conditions.

the flue gas. A plot of combustion time of RheinbraunTaff Merthyr blends against the mass fraction Rheinbraun at a temperature of 1200 °C, using the higher inlet concentration of O2, is shown in Figure 7. The results show that the trend of negative synergy in the total combustion time observed at 1000 °C could also be observed at higher temperatures. However, at 1200 °C and with a higher O2 concentration, the differences in the combustion times of the component coals and their blends were smaller and this made it more difficult to discriminate between the fuels. In view of this, it was decided to compare coals and blends on the basis of their total combustion time and to adopt a test temperature of 1000 °C as standard. Tests with Coals and Blends Supplied by Utility Companies. The combustion performance of four sets of commercial blends, E-G, Yan Zhou-Klein Kopke, Poduff-Petroleum Coke, and Carbocol-Kangra, have been tested in the HTSR. The E-G coals and blends had been previously tested in Topocopilla power station in Chile, and the Yan Zhou-Klein Kopke and PoduffPetroleum Coke blends had been tested in a power station in Denmark. Fly ashes collected from the electrostatic precipitators of these power stations were provided with the fuel samples and have been used to calculate the burnout of the pure coals and their blends at the commercial scale. These values have been compared with the combustion time results obtained from the HTSR. A similarity in the trends at both commercial and laboratory test scales would give credibility to the HTSR as a laboratory-scale technique for assessing the likely behavior of coal blends at the commercial scale. In making this comparison, it is assumed that the limited amount of sample (several grams) removed from the ESP at the power station is representative of that produced from the coal samples supplied for testing in the HTSR. E-G Blends. The results obtained with the E and G coals and their blends have already been discussed (Figure 3b). The E-G blend performance was poorer than that predicted by additivity in the HTSR test, and this was in agreement with the burnout data obtained with the precipitator fly ashes collected from the power plant (for instance, the burnout for a 50:50 mixture of coals E and G was measured as 99.1, whereas a value of 99.6%, was indicated by additivity 10). Yan Zhou-Klein Kopke Blend. The combustion times of Yan Zhou-Klein Kopke coals and blend in the HTSR

Majid et al.

Figure 8. Combustion times of Yan Zhou coal, Klein Kopke coal, and a blend. Table 4. Burnout of Yan Zhou-Klein Kopke Blends at the Commercial Scale blend composition % more-reactive coal

burnout, as indicated by fly ash composition%

0 50 100

98.4 98.9 98.9

are given in Figure 8. The combustion time for a 50:50 blend was close to that predicted assuming additivity (it was the same within the error limits of the measurement), which is a different effect to that seen with the Rheinbraun-Taff Merthyr and E-G blends, under the same experimental conditions, where a drop in the reactivity of the blended fuels was noted. The observation is consistent with the closer C contents of this pair of coals (85.8% and 80.3%, daf basis, for the Yan Zhou and Klein Kopke, respectively), in contrast to the greater differences in the C contents of the Rheinbraun ligniteTaff Merthyr and E-G blends (see Table 1). These data suggest that the performance of this coal blend at the commercial scale should be similar to that predicted by additivity. The burnouts of this blend and the parent coals at the commercial scale have been determined from the residual carbon content of the power-station fly ashes, and the results are presented in Table 4. This shows that the blend burnout was 98.9%, which is within 0.25% (actual) of that predicted by additivity. This difference is within the error limit of the analytical measurements, and hence, the result can be said to be predicted by additivity. This is in accordance with the prediction made using the results of the HTSR. Poduff-Petroleum Coke Blend. The combustion times for the Poduff coal, the petroleum coke, and blends containing a low proportion of the coke are given in Figure 9. The results show that the petroleum coke was less reactive in the HTSR than the Poduff coal. The data for the blends, containing 89 and 92% of Poduff, suggest a slight reduction in the combustion time compared with that predicted by additivity, although the difference is within the limits of the experimental errors. This suggests that the addition of a relatively small amount of the less-reactive petroleum coke to Poduff coal does not cause a decline in the overall reactivity, compared with that predicted by additivity. It could even be argued that the performance in this test series was maintained at the level expected for the neat Poduff coal, despite the addition of a less-reactive component. This effect is more difficult to explain. The petroleum coke should be a more temperature stable material than



Figure 9. Combustion times for Poduff coal, Petroleum Coke, and their blends.

Figure 10. Combustion times for Carbocol-Kangra blends. Table 5. Burnout of Poduff-Petroleum Coke Blends at the Commercial Scale blend composition % more-reactive coal

burnout, as indicated by fly ash composition%

89 92 100

99.4 99.5 99.6

coal, as it has already been heat-treated and may, therefore, not become less reactive with time at temperature, as is found with coal chars. Therefore, the negative effects seen with the Rheinbraun lignite and E-G blends would not be expected. It is plausible that the presence of the less-reactive coke enabled a higher proportion of the more reactive Poduff coal to react during the early part of the test before it had deactivated. When this effect is combined with a no-change effect in the reactivity of the coke, then an overall enhancement in performance might be expected. These data suggest that it may be possible to maintain or even improve blend reactivity slightly by adding small proportions of relatively unreactive, but stable, coke to a coal. Due to its low reactivity, the petroleum coke used in this study was not combusted on its own in the power station. However, the burnouts at the commercial scale have been estimated for the pure Poduff coal and the blends containing a low proportion of the coke. The results (Table 5) show that the burnout has not been affected by the addition of the coke (within the limits of error of the measurements). This is consistent with the results obtained with the HTSR. Carbocol-Kangra Blend. The combustion times for Carbocol-Kangra blends are presented in Figure 10. The results for the blends are close to those predicted assuming additivity. This is consistent with their similar C contents (78.7% and 79.4% for Carbocol and

Figure 11. Comparison of the performance of the different coals and blends in the HTSR.

Kangra, respectively). These blends were not tested at the commercial scale. However, the performance measured in the HTSR suggests that the performance should be similar to that predicted assuming additivity. Results for All Blends Combusted in the HTSR. The combustion times of five sets of blends have been measured in the HTSR. The blends ranged from those made up of component coals that were very different in terms of rank, such as Rheinbraun lignite and Taff Merthyr semi-anthracite, to blends consisting of coals with very similar properties, e.g., Carbocol-Kangra blends. The experimental conditions prevailing inside the HTSR ensured the complete combustion of volatiles and char inside the reactor. The suspended nature of the particles enhanced the volatile-volatile and volatilechar interactions of the different coal particles within the combustion zone of the reactor. The combustion time of the samples provided a measure of the reactivity of the coal or blends combusting in the reactor. Figure 11 shows the combustion time data for the five sets of coals and blends plotted on a single graph as a function of the amount of more-reactive coal in each blend. The ability of the HTSR method to show the differences in the reactivities of the individual coals is demonstrated by the values shown on the left- and righthand sides of the figure. Differences in the performances of the blends tested are also clearly shown. Synergistic effects are indicated by the extent of the difference between a straight line drawn between the values for the individual parent coals and the actual measured values for the blends, i.e., the extent of curvature of the line (shown on the figure) indicates the magnitude of the effect. Figure 12 gives the combustion times of all coals and blends combusted in the HTSR, plotted against their carbon content (%, daf basis). It is observed that there is a trend of increasing combustion time with increasing carbon content of the coal or blend. This demonstrates the ability of the HTSR to rank the coal or blend based on their combustion time in the reactor. It is important to note that the suspension-firing reactor was not designed to reproduce exactly the conditions that would be experienced by coal particles burning inside a full-scale power station. The operating temperature for testing the blends, 1000 °C, is considerably lower than the typical peak temperatures for pulverized-coal burners of between 1500 and 1800 °C. However, where most other blend testing methods have failed, the suspension-firing reactor has allowed the

976


Figure 12. combustion times of pure coals (0) and blends (O) combusted in the HTSR.

interactions of different coal particles within a welldefined reaction zone. In the case of three sets of blends tested in the HTSR, which had also been fired in commercial scale furnaces, the same effects of blending on the performance were observed at both scales of testing. Conclusions The suspension-firing technique, which investigates the performance of coal blends, has been further developed to overcome a potential source of error that was apparent in the earlier method. The new method measures the time for complete combustion (as indicated by the exit gas analysis from the reactor) under a standard set of conditions. The method was developed using blends produced from a lignite and a semianthracite. Appreciable reductions in the combustion times were measured as the proportion of the morereactive lignite in the blends was increased. However, a negative synergistic effect was also apparent in that the combustion times of the blends were longer than

Majid et al.

would be predicted by additivity of behavior (i.e., from the performance of the parent coals and the composition of the blend). This is consistent with the starvation of the less-reactive component (the semi-anthracite) of O2 during the early stages of combustion. This caused the less-reactive coal char to become more thermally annealed before it had the opportunity to combust, and therefore, it reacted more slowly. Four sets of blends and their parent bituminous coals, supplied by utility companies, were then tested. One of the blend sets exhibited a negative synergistic effect compared with that expected from additivity, although the effect was smaller than exhibited by the lignite/semi-anthracite blend. The performance of the other three sets of blends was close to that predicted by additivity (within the error limits of the measurements). Samples of electrostatic precipitator fly ash from the commercial operations were supplied with three of the sets of coals and blends. The power station burnout has been estimated from the residual C content of these ashes. The performance of the blends compared with that of the parent coals in the power station has been assessed from these data. The same effects were found in the results obtained with the HTSR and with the commercial trials, i.e., one set exhibited a negative synergistic effect, with the performance of the other two sets being very similar to that predicted by additivity. This comparison of performance at the laboratory and commercial scales of operation gives some credibility to the HTSR technique as a method for testing simple coal blends. Acknowledgment. The authors thank the ECSC for providing the funding for this project under contract no. 7220-PR-071. EF049844K

Further Development of a Suspension Fired Reactor To Assess the

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