Investigation of Coal Conversion under Conditions Simulating the

Energy & Fuels 2003, 17, 489-497

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Investigation of Coal Conversion under Conditions Simulating the Raceway of a Blast Furnace Using a Pulsed Air Injection, Wire-Mesh Reactor S. Pipatmanomai, N. 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 August 12, 2002. Revised Manuscript Received January 16, 2003

Operating difficulties are encountered when high coal injection rates are used into the blast furnace raceway. An insight into the problems has been gained using a wire mesh reactor, which has been modified to inject short (5-500 ms) pulses of air or O2-enriched air through the sample holder, once the particles have reached peak experimental temperature. By manipulating the test procedure, it has been possible to measure the extents of successive pyrolysis, char combustion, and CO2-gasification that occur under simulated raceway conditions. In the wire-mesh reactor, the release of volatiles was completed within the heat-up period (∼300 ms, at a heating rate of 5000 °C s-1). For 20 ms air pulse times, complete consumption of the inlet O2 occurred (with concentrations in the range 21-75%) and the extent of combustion was limited by the amount of O2 supplied (i.e., it was diffusion-limited). Extents of CO2-gasification were measured in the temperature range 800-1500 °C, and the results suggest than many seconds are needed to achieve a reasonable extent of gasification. Overall, the results indicate that the extents of combustion and gasification of the char in the raceway (residence time < 50 ms) are likely to be low. However, a significant proportion of the char will subsequently react by gasification within the blast-furnace bed. Unreacted char and soot may either be trapped in the coke bed or entrained in the gas stream, giving rise to the problems observed at high coal injection rates.

Introduction The cost of the metallurgical coke used in blast furnaces is high, and this has led to the use of coal to replace a portion of the coke feed. The coal is added through a lance into the preheated air (or enriched air) blast as it passes, at high velocity, from the tuyere to the raceway at the base of the furnace shaft.1 The raceway is a high-temperature reaction zone that forms around the circumference of the furnace at the point where the air blast meets the base of the coke bed. However, it has been found that operational problems are encountered when a high coal injection rate is used (i.e., when it exceeds approximately 200 kg of coal/ton of pig iron2). These include unacceptable increases in the level of fines production, which are blown out from the furnace top and poor drainage of molten slag and iron downward through the bed. These problems are related to the extent of reaction that occurs in the raceway region of the furnace. Although earlier studies2-5 of factors influencing the extents of reaction occurring in this region have been attempted, they have not resulted in the development of a thorough understand* Corresponding author. E-mail: [email protected]. (1) Biswas, A. K. Principles of Blast Furnace Iron Making: Theory and Practice; Cootha Publishing House: Brisbane, Australia, 1981. (2) Yamakuchi, K.; Ueno, H.; Tamura, K. ISIJ Int. 1992, 32 (6), 716. (3) Atkinson, C. J.; Willmers, R. R. Fuel Process. Technol. 24, 1990, 99, 107. (4) Maki, A.; Sakai, A.; Takagaki, N.; Mori, K.; Ariyama, T.; Sato, M.; Murai, R. ISIJ Int. 1996, 36 (6), 650. (5) Chung, J. K.; Hur, N. S. ISIJ Int. 1997, 37 (2), 119.

ing of the complex physical and chemical processes that govern the extent of reaction in this region. This is needed so that the efficiency of coal utilization can be improved and coal injection rates maximized. The temperature in the raceway is high (1500-2000 °C) and residence time is short (20-50 ms). It is under these extreme conditions that a high level of coal conversion needs to be achieved. The reactions that can occur include the release of the coal volatiles, the combustion of the products of pyrolysis, partial combustion of the residual char, and the gasification of residual char by CO2. The use of wire mesh reactors was first reported in the mid 1960s, and since then they have been used for a range of coal related studies.6,7 The technique is now developed in a variety of forms that have enabled studies to be done under a wide range of conditions that are representative of those in pilot and commercial scale processes. The development and application of the technique have been covered by several reviews.8,9 In this study, a high-temperature wire mesh reactor (WMR) has been used to investigate the extent of these reactions occurring under conditions simulating those (6) Loison, R.; Chauvin, R. Chim. Ind. (Paris) 1964, 91, 269. (7) Juntgen, H.; van Heek, K. H. Fuel 1968, 47, 103. (8) Howard, J. B. Chemistry of Coal Utilisation; Elliot, M M., Ed.; Wiley: New York, 1981; Supplementary Volume. (9) Zhuo, Y.; Herod, A. A.; Kandiyoti, R. The Thermochemical Reactions of Middle Rank Coals in Natural and Laboratory-Simulated Thermal Geochemical Processes; Ikan, R., Ed.; Kluwer Academic Publishers: Dordrecht (to appear 2003).

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Energy & Fuels, Vol. 17, No. 2, 2003

in the raceway. This type of reactor10 enables precise and variable control of the reaction conditions (time, temperature, heating rate, pressure, and gas composition), using a monolayer of fuel particles. A continuous stream of sweep gas minimizes the secondary reactions of tars and the interaction between pyrolyzing char and evolved volatiles. Extensive investigations employing these reactors, under pyrolysis conditions (in He and H2) and gasification conditions (in CO2 and steam), using coal and biomass fuels, over a range of temperatures and pressures have already been published.11-17 Combustion experiments have been limited to a study in a mildly oxidizing atmosphere (3.5% O2 in He), to simulate the gaseous conditions in the post-flame region of the pulverized-fuel combustors.18 Modifications have been made to the WMR during the present study to enable the time scales of the actual coal injection process in the blast furnace to be simulated. The modifications have enabled the pyrolysis stage during heat-up to be followed successively by a short pulse (5-500 ms) of air, or enriched-air (to simulate combustion in the raceway) and a CO2-gasification stage, at temperatures up to 1500 °C. The short pulse injection system has enabled the char-O2 reaction to be studied under conditions simulating the raceway of the blast furnace. However, it has not been possible to study the combustion of the volatile products of pyrolysis with this apparatus. In this paper, the details of the modifications and experimental procedures for short-pulse reactive gas injection experiments are presented. The results of tests, using this equipment, to study the extents of pyrolysis, combustion, and gasification (by CO2), under conditions relevant to the blast furnace raceway are then described. Experimental Section The High-Temperature Wire-Mesh Reactor. A schematic diagram of the high-temperature, atmospheric pressure WMR is shown in Figure 1. Detailed descriptions of the equipment and the experimental procedures have been presented elsewhere.12-14 Briefly, about 5-6 mg of sample is evenly distributed in the folded wire-mesh sample-holder, which also serves as the resistance heater. The mesh is held between a pair of water-cooled electrodes. The particle size range used is 125-150 µm and this is determined by the size of the mesh used to hold the sample. The brass support is lined with a 2 mm thick ceramic sheet to prevent electrical contact between the sample-holding mesh and support plate. A quartz tar trap, tightly packed with stainless steel wire-mesh, is placed firmly on the top of the sample holder to collect evolved tars. A continuous flow of gas is swept through the sample during a test and this carries the evolving volatiles away from the sample holder and minimizes interactions with the char. By separately weighing the sample plus mesh and the tar trap (10) Pipatmanomai, S. Ph.D. Thesis, University of London, 2002. (11) Cai, H.-Y.; Kandiyoti, R. Energy Fuels 1995, 9, 956. (12) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (13) Gibbins, J. R.; King, R. A. V.; Wood, R. J.; Kandiyoti, R. Rev. Sci. Instrum. 1989, 60 (6), 1129. (14) Cai, H.-Y.; Dugwell, D. R.; Kandiyoti, R. 3rd Eur. Conf. Ind. Furnaces Boilers 1995, 503. (15) Gibbins, J. R.; Gonenc, Z. S.; Kandiyoti, R. Fuel 1991, 70, 621. (16) Gu¨ell, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943. (17) Messenbo¨ch, R. C.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1999, 13, 122. (18) Cai, H.-Y.; Dugwell, D. R.; Kandiyoti, R. 3rd Eur. Conf. Ind. Furnaces Boilers 1995, 591.

Pipatmanomai et al.

Figure 1. The wire-mesh reactor: [1] gas-exit; [2] tar trap; [3] liquid-N2; [4] Pyrex bell; [5] wire-mesh sample holder; [6] electrode clamps; [7] ceramic sheet; [8] brass support plate; [9] water-cooled spring; [10] power supply; [11] wound corrugated tube; [12] flow distributing unit; [13] sinter disks; [14] water-cooled support plate stands; [15] copper seals; [16] gas inlet; [17] O-ring seals; [18] clamps; [19] base plate. before and after an experiment, the total volatile and tar yields were determined with the repeatability of (1% for the total volatile yield and (2% for the tar yield. Stainless steel wiremesh and K-type thermocouples were used for experiments carried out at or below 1000 °C in inert atmospheres or in CO2. For higher temperatures, molybdenum wire-mesh and S-type thermocouples were used. The heating rate and temperature were controlled using a PC with Turbo Pascal 6.01 software. Two pairs of thermocouples attached to the wire-mesh measured the temperature inside the reaction zone. The output power was adjusted by PID feed-forward control on the amplified temperature readings from the thermocouples, with 100 Hz sampling frequency. Details of the temperature control system have been presented elsewhere.19,20 Modification of the WMR for Short-Pulse Combustion Tests with Air (or O2-Enriched Air). A set of valves and a computer-controlled switching sequence was incorporated into the gas inlet line to the WMR to enable tests to be done with a short duration air pulse. It was important that the exposure time of the sample on the mesh to air (or enriched air) could be varied in the range experienced by particles in the raceway of the blast furnace (generally 20-50 ms). It was also necessary that the reaction of the sample with the short pulse of air occurred as soon as the peak temperature was reached. To achieve these requirements, two computer-controlled, 3-way solenoid valves were used. Air-Injection Unit. A schematic diagram of the short-pulse gas injection unit is shown in Figure 2. The gas injection system was constructed using a pair of 3-way solenoid valves (supplied by the Lee Company, LFHA series) and was installed immediately below the WMR. These are miniaturized valves, with low dead volume and fast response time. With the use of a high-speed driven circuit, i.e., 3 times rated voltage, the (19) Xu, B.; Dix, M.; Kandiyoti, R. Rev. Sci. Instrum. 1995, 66, 3966. (20) Cai, H.-Y.; Megaritis, A.; Messenbock, R.; Dix, M.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 1273.

Coal Conversion in the Raceway of a Blast Furnace

Figure 2. Schematic diagram of the wire-mesh reactor equipped with the air injection system: [1], [2] solenoid valves; [3] wire-mesh reactor; [4] gas supply; [5], on/off valve; [6] vacuum pump; [7] relay box; [8] computer; [9] watchdog; [10] transformer; [11], [12], [13] metering-valves; [14], [15], [16] pressure gauges. response time of the valves was estimated to be ∼2 ms. The wire-mesh control program was modified for additional control of the valve actuation. A wound-corrugated tube, previously used in steam gasification experiments,17 was installed vertically beneath the mesh to help distribute the inlet gas flow and to minimize turbulence. The solenoid valves are labeled 1 and 2 in Figure 2. At the start of each test using the air pulse, valve 1 was in its normally open position, so that N2 was flowing to the WMR during the heat-up period. After a time-delay, valve 1 was switched to its normally closed position, which allowed air to flow toward the WMR. Next, valve 2 was switched to its normally closed position, which stopped the air flow and admitted N2. This swept the air pulse forward and through the WMR. The valve switching times were preset, so as to allow pulses of air to be admitted, which had durations that were similar to that in the raceway of the blast furnace. The timing of the valve sequence was controlled by a relay box, which was calibrated in the range of 5-500 ms, using an oscilloscope. Performance of the Air/Enriched Air Injection System. To check that the air pulse contained the correct mass of O2, the whole of the gas flowing through the mesh during a test at ambient temperature was collected in a gas sample bag. The average O2 concentration in this sample was then measured with a paramagnetic O2 analyzer and the volume was measured with a dry gas meter. The values measured for valve switching intervals of 50, 100, and 200 ms were in good agreement with values calculated from the air injection pulse times and the gas flow rates through each valve. The agreement for a valve-switching interval of 20 ms could not be checked as the concentration of O2 in the gas sample was below the detectable limit for the gas analyzer. Optimization of the Time for the Initiation of Valve Switching. The initiation of the valve switching sequence was adjusted, so that the pulse of air reached the sample on the mesh, while the mesh was at its peak temperature. This was determined by varying the time interval from the start of the test, when the valve switching was initiated and measuring the weight gain of an empty mesh. Under this condition, the only reaction to occur was the oxidation of the Mo mesh and this occurred at the highest rate at the peak temperature. The maximum weight gain (Figure 3) occurred when the air pulse was released 0.5 s after the heating was triggered, which suggested that the pulse of air had reached the mesh at the peak temperature. At longer valve initiation times, the extent of mesh oxidation declined, as the air pulse will have reached the mesh after the 1 s hold time at peak temperature, and the mesh will have been cooling. The experimentally estimated air travelling time (calculated as the total heating time minus

Energy & Fuels, Vol. 17, No. 2, 2003 491

Figure 3. Weight gain due to oxidation of the molybdenum mesh as a function of the shifting time. Table 1. Analysis of the Coal Samples % (db) sample

VM

ash

C

H

N

1 2

17.4 13.2

8.7 6.5

82.7 85.9

4.3 4.3

1.1 1.6

the minimum valve switching time from the start of the test to achieve maximum weight gain of the mesh) was in good agreement with that calculated from the pipe dimensions and gas flow rates. Experimental Procedure. A weighed amount of sample (typically 5 mg) was placed on the wire mesh and the equipment was assembled. The samples were generally heated at a rate of 1000 °C s-1 in N2 to 1500 °C. The air pulse was initiated at the preset time within the heating period to ensure that the air reached the mesh as soon as the peak temperature was reached. During the experimental program, the air pulse time was varied between 20 and 220 ms and the O2 concentration between 21 and 75%. A hold time of 1 s was allowed at the peak temperature after the termination of the air pulse to ensure that all of the pulse has been purged through the mesh. Investigation of the Gasification Reactivity of Combustion-Derived Chars. It is possible that prior oxidation can influence the subsequent gasification reactivity of the chars. Therefore, in this work an experimental protocol was used that simulated the particle history in the raceway before measuring the gasification reactivity of the char. This was achieved using the equipment described above for the air pulse experiments, but switching to a CO2/N2 mixture after the air pulse, rather than N2. The conditions of the exposure to the CO2/N2 mixture (temperature and time) could be varied to enable reaction data to be obtained. Thermogravimetric Analysis. A standard Perkin-Elmer thermogravimetric analyzer TGA7 was used to determine the relative combustion reactivity of the char samples produced in the WMR. An isothermal method was used in which the samples were exposed to air at 500 °C: the details of basic features and procedure have already been described elsewhere.11,14 The time to achieve 50% burnout (t1/2) was used as the index of char reactivity. Coal Samples. The tests were done using two coals that have been used in blast furnaces. Coal 1 was a 50/50 blend of two low-mid volatile coals (one of which had modest fluid properties) and Coal 2 was a low-volatile noncoking coal. They were supplied by Corus (UK). The proximate and elemental analyses of the two samples are given in Table 1. Samples were sieved and dried in a vacuum oven at 35 °C for 18 h and stored under nitrogen until required. The particle size range used in

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this study was 125-150 µm, which is governed by the size of the apertures in the wire mesh. It is slightly coarser than a typical pf (normally 70% less than 75 µm, with only 2% greater than 300 µm).

Results and Discussion The primary result from tests with the WMR is the overall weight change. This is a composite of the sequence of weight changes that occur during the experiments, caused by changing the gas flowing through the wire mesh reactor during the different stages of the test. During the heatup period, in N2, the coal will loose weight as volatile matter is released and swept out of the reactor. The char residue from this pyrolytic stage then partially reacts with the O2 in the short air/ enriched air pulse, causing a further weight decrease. A complicating effect noted during the initial work was that the mesh itself also partially oxidized through exposure to air. This oxidation caused a weight gain. During tests to measure the extent of combustion, N2 was re-admitted after the air (or enriched air) pulse and no further reaction will occur during this final stage of the test. The weight loss by pyrolysis and the weight gain by mesh oxidation were estimated during independent tests under relevant conditions and the values obtained used to correct the overall sample weight change (during the short-pulse air injection tests) for these changes. In other experiments, the extent of gasification following the combustion step was examined. This was done by switching the sweep gas from air (or enriched air) to a CO2/N2 mixture (rather than N2). The extent of gasification was derived by measuring the extents of pyrolysis, combustion, and mesh oxidation during separate tests and using the values obtained to remove their effects from the composite weight to reveal the extent of gasification. At the high temperatures used in this study, even weakly oxidizing gas mixtures appear to attack the molybdenum mesh. It is noted that, for short pulse times, experimental uncertainties were significant, which was primarily due to the low intensity reactions of the mesh. Apart from elemental tungsten, which seems to have its own difficulties, molybdenum is the only other metal sold as a mesh and as such available for work under these conditions. Nevertheless, the reactor system has enabled much insight to be gained into the extent of several reactions occurring in the raceway; this has not been possible with other types of laboratory scale reactors. Assessment of the Weight Change Caused by Mesh Oxidation. Initial work with the modified WMR showed that the molybdenum mesh was oxidizing slightly when exposed to the short air pulse at high temperature and this caused it to increase in weight. The effect has been compensated for in this work by measuring the increase in weight in a separate test, without a fuel sample on the mesh, but under otherwise identical experimental conditions. The only weight change, which could occur under these conditions, was caused by mesh oxidation and the result obtained has been used to compensate for the effect during tests with the sample of fuel on the mesh. Some typical weight changes caused by oxidation of the mesh are shown in Table 2. The data show that the weight increase in the

Pipatmanomai et al. Table 2. Typical Weight Changes of the Mo Mesh Caused by Oxidation O2 pulse length (ms) weight increase (mg)

20

59

112

220

0.15

0.31

0.51

0.69

Table 3. The Effect of the Heating Rate on the Total Volatile Yield from Coal 2a

a

heating rate (°C s-1)

total volatile yield (%, daf)

1000 5000

14.5 15.0

Experiments were done at 1000 °C with 0 s holding time.

mesh did not increase in proportion to the increase in the pulse time. This is consistent with the formation of an initial oxide layer at short pulse times. At longer times, the oxide layer progressively reduced the rate of diffusion of O2 to the fresh Mo surface, which reduced the rate of increase in weight. With a 20 ms pulse time, the correction for mesh oxidation was equivalent to approximately 3% of the coal sample weight (daf basis), i.e., the weight increase of the mesh by oxidation as a proportion of the coal sample weight. With a 220 ms pulse time, the correction increased to approximately 15% of the sample weight. These are an indication of the values that have been used to adjust the total weight losses for mesh oxidation during tests with coal. In the analysis of the data contained in this paper, it is assumed that the extent of mesh oxidation as measured with an empty mesh, applies to the tests with samples on the mesh. In reality, the extent of mesh oxidation with a sample present could be lower than that measured without a sample, because the presence of the char produced by pyrolysis of the coal sample on the mesh could insulate the mesh from reaction with O2. The impact of this assumption would be to over-correct for mesh oxidation and this would cause an underestimate of the extent of combustion. Unfortunately, there is no way of determining the extent of mesh oxidation in the presence of the sample. The magnitude of the mesh oxidation correction figures and the uncertainty of the extent of mesh oxidation in the presence of a fuel sample must limit the accuracy of the estimation of the extent of combustion. Investigation of the Pyrolysis of the Coal Samples. The pyrolysis of the samples was studied during tests without the air pulse system in operation. Figure 4 shows the effect of peak temperature (in the range 500-1500 °C) on the total volatile yields, in N2, for Coals 1 and 2. The heating rate to the peak temperature was 1000 °C s-1 and the test was terminated as soon as the peak temperature was reached. The yields increased linearly up to a peak temperature of approximately 1000 °C and then tailed off. The data suggest that, at a heating rate of 1000 °C s-1, the volatile release from the coals was virtually complete by the time the temperature reached 1500 °C. Tests were also done with heating rates in the range 1000 to 5000 K s-1 at a peak temperature of 1500 °C, and data for Coal 2 are shown in Table 3. These data show that the total volatile yield did not increase by much as the heating rate was raised from 1000 to 5000 °C s-1. Investigation of the Extent of Char Combustion at Short Exposure Times. The extent of combustion is the overall weight loss measured during the experi-


Figure 4. The effect of pyrolysis peak temperature on total volatile yield for Coals 1 and 2 (heating rate: 1000 °C s-1; hold time: 0 s). Table 4. The Repeatability of the Measurement of the Extent of Combustiona pulse time, ms

20 ms

extent of 5.0 combustion 3.7 (%, daf) 2.6 average a

59 ms 6.8 5.4 5.4

3.8 ((1.2) 5.9 ((0.8)

112 ms 9.8 10.2 8.4

220 ms 13.3 14.5

9.5 ((0.4) 13.9 ((0.8)

Values in brackets are the standard deviation.

ments using the short pulse air injection system, corrected for the weight loss by pyrolysis and the weight gain by mesh oxidation. The extent of mesh oxidation was determined for each experimental condition. Pyrolysis weight losses in the WMR were not sensitive to the experimental conditions within the range of conditions studied and weight losses (daf basis) of 30.0% for Coal 1 and 20.3% for Coal 2 have been used throughout this work. The tests were done with a peak temperature of 1500 °C and a heating rate of 1000 °C s-1, as the earlier pyrolysis tests had shown that virtually all of the volatiles had been released by the time the peak temperature was reached under these conditions. The extent of combustion of freshly formed char was then measured in the combustion stage of the test. Repeatability of the Measurements. The repeatability of the measurement of the extent of combustion is shown in Table 4. The data show that the repeatability of the tests improved as the pulse time was increased. This is the expected trend as the effect of any errors in the measurement of the extent of pyrolysis and mesh oxidation would have a larger impact on the lower values of the extent of combustion (i.e., those obtained at the shorter pulse times). This is reflected in the standard deviations shown in Table 4. Clearly, the repeatability values do not include any systematic effects, such as could be introduced by the use of the sample free mesh oxidation corrections in the estimation of the extent of combustion. The Effect of the Air Pulse Time. Experiments were performed with air pulse lengths of 20, 50, 100, and 200 ms. Results are shown in Figure 5 for Coals 1 and 2. This also shows the maximum possible extent of combustion that could be achieved assuming that all of the O2 in the pulse reacted with the sample, after correcting for the amount that reacted with the mesh.


Figure 5. The effect of the air injection length on the extent of combustion of Coals 1 and 2: comparison between the experimental and the maximum conversions (heating rate: 1000 °C s-1; hold time: 1 s; peak temperature: 1500 °C).

With air pulse lengths of 20 and 50 ms, the extents of combustion for both coals were similar to each other and were close to the calculated limits for the conditions used. The differences are considered to be within the limits of the errors associated with the measurements. This infers that, at short pulse times, the extents of combustion were limited by the supply of O2 to the samples. These pulse times are similar to the residence times of the coal in the raceway of the blast furnace. The extents of combustion were approximately 3% and 6% of the original sample (daf basis) for air pulse lengths of 20 and 50 ms, respectively. This suggests only relatively low extents of char combustion may be expected at the short residence time of the raceway. An illustration of the possible influence of the extent of mesh oxidation is given for the 20 ms pulse time data. A correction of 3% (of the coal sample weight, daf basis) has been applied in the calculation of the extent of combustion, which is the maximum correction, based on a test with a clean mesh. If the presence of the sample reduced the extent of mesh oxidation, then higher values of the extent of combustion would be estimated. If no mesh oxidation occurred (which seems unlikely) then, the extent of combustion would be increased to 6%. The measured extent of combustion would still be similar to the calculated value as the same values for mesh oxidation are used in the both sets of calculations. With air pulse times of 125 and 220 ms, the measured values increasingly fell below the calculated maximum conversions for both coals. This is consistent with the early development (i.e., in about 50 ms) of a diffusional resistance, which limited the extent of reaction. This could be caused by the diffusion of product CO2 (also possibly CO) away from particle surfaces, which limited the access of O2 to the particles. Rates of CO2 diffusion are slower than those of O2 at all temperatures.21 The results show a lower extent of combustion with Char 2 at longer pulse times than with Char 1, i.e. the difference between the calculated maximum and measured values were greater for Char 2. This was probably caused by different particle surface morphologies, which could influence the rates of external diffusion. Char 1 was formed from a coal with moderate coking properties (21) Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; Wiley: New York, 1954; p 579.

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Pipatmanomai et al. Table 5. Effect of Air Injection Pulse Length on the Relative Combustion Reactivity of Char t1/2 (min)

air injection pulse (ms)

Char 1

Char 2

no air injection 20 59 112 220

1790 844 465 384 202

946 591 nd 351 267

Table 6. Effect of O2 Partial Pressure on the Post Combustion Reactivities of Chars at 20 and 112 ms Injection Pulse Time

Figure 6. The effect of O2 partial pressure on the extent of combustion of Coal 1: comparison between the experimental (exp) and the maximum conversions (th) (heating rate: 1000 °C s-1; hold time: 1 s; peak temperature: 1500 °C).

and the surface would be more rounded and smoother, whereas char 2 was formed from a non coking coal and the char would be more angular and rough. Also, the chemical structure of the char, e.g., the H content, is known to play an important role in determining the char combustion reactivity.22,23 The Effect of the Partial Pressure of O2. These tests were done to identify whether raising the partial pressure of O2 raised the extent of combustion of the char. Coal 1 was exposed to short (i.e., 20 ms) pulses of O2enriched air, which contained O2 in the range 21-75% (by volume) in N2. Figure 6 shows that the extent of combustion increased with increasing O2 partial pressure. The experimental values agreed well with the calculated maximum conversions at all partial pressures. This clearly shows that rate of supply of O2 to the char surface played the dominant role in controlling the reaction at short pulse times, even with substantially higher O2 concentrations than are present in the raceway of a blast furnace. At longer pulse times (112 ms), the experimental value for the extent of combustion was below the maximum value at all of the O2 partial pressures that were studied. This is the same behavior as has already been noted with air and is thought to indicate the early development of a diffusion resistance to the inward movement of O2 to the char and to possible changes in the physical properties of the char. The Post Combustion Reactivity of the Chars. Tables 5 and 6 show the relative combustion reactivities of residual chars from the pyrolysis-combustion experiments, determined using the isothermal test in the TGA at 500 °C. Under the conditions used, the rate of char oxidation should be controlled by chemical reaction. Results are presented as t1/2, which is the time taken for 50% of the initial mass of sample to react, i.e., lower t1/2 values show that the char was more reactive. The reactivity of a pyrolysis-derived char prepared from Coal 2 (Char 2) was higher than that prepared from Coal 1 (22) Khan, M. R. Fuel 1987, 66, 1626. (23) Cai, H.-Y. Ph.D. Thesis, University of London, 1995.

t1/2 (min)

O2 partial pressure (%)

20 ms

112 ms

21 25 50 75

844 752 458 566

384 474 314 458

(Char 1). This may reflect the higher extent of microporosity in the higher rank coal, together with differences in the chemical structure and composition. The reactivities of the chars exposed to the pulses of air were considerably higher than those prepared under pyrolysis conditions alone and their reactivity increased (i.e., t1/2 decreased) with increasing air pulse duration. The extent of the increase in reactivity was greater for Char 1, such that with the 112 and 220 ms air pulse times, the subsequent reactivities of Chars 1 and 2 at each pulse time were similar. The effect of the air pulse on the char reactivity is an unexpected effect. The WMR data suggests that the reaction of the sample with the O2 in the air pulse is supply limited and therefore the reaction should have occurred in the outer char layer. It is difficult to see how this could have affected the subsequent reactivity of the whole char. It is suggested that the exposure to O2 makes the outer char layer more reactive, by opening the pores and exposing active char sites for subsequent reaction, and by the removal of secondary C formed by pyrolysis. These outer parts of the char are then able to react more rapidly when exposed to O2 in the TGA test. Data obtained with increased partial pressures of O2 in the pulse gas are shown in Table 6 for pulse times of 20 and 112 ms. After a prior exposure to a 20 ms pulse of air/enriched air, the t1/2 values exhibit an increase in reactivity (apart from the 75% O2 data). This is consistent with the effect on subsequent char reactivity when the air pulse time was varied. After a 112 ms pulse of gas, there was no clear increase in the subsequent char reactivity with increasing concentration of O2; however, all of the values are higher than those measured with the highest partial pressure of O2 and a 20 ms pulse time. This suggests that there may be a limit to the extent of increase in the reactivity caused by prior exposure to O2. The limit is reached after an exposure to air of approximately 200 ms duration and 100 ms with higher partial pressures of O2. Investigation of the Extent of Char Gasification of the Combustion-Derived Chars. The extent of char gasification was studied using an 18% CO2/N2 mixture, which simulates the gaseous environment in the post combustion region of the raceway and in the lower part of the blast furnace shaft. The effects of



Table 7. The Extent of Gasification temperature (°C)

extent of gasification by CO2 (%, daf)

800 1300 1500

7.1 11.0 32.0

temperature, hold time, and prior exposure to O2 have been studied. The Effect of Gasification Temperature. The extent of gasification has been measured at 1500, 1300, and 800 °C. These temperatures reflect those in the raceway and the lower temperatures in the furnace shaft. The hold time was 10 s for each test. Gas residence times in the blast furnace vary between about 4 and 40 s, depending on the extent of obstruction to the gas flow by the coke charge. Prior to exposure to the gasification atmosphere, the sample of coal was heated to 1500 °C in N2 (at a rate of 1000 °C s-1) and then reacted with a 20 ms pulse of air. The temperature program was then set to either enable the gasification test to be done at 1500 °C or at one of the lower temperatures; the latter were reached by reducing the temperature of the mesh and sample, without cooling to ambient and reheating. Other studies have shown that cooling to ambient and reheating affects the subsequent reactivity of the char.24,25 The extents of CO2-gasification (Table 7) have been estimated from the overall weight change during the experiment as described earlier. The data shows that the extent of gasification increases with temperature. At the highest temperature, with a 10 s reaction time, a weight of char equivalent to 32% of the original sample on a daf basis was gasified. If the data is expressed as a percent of the char present at the start of the gasification test, then at 1500 °C, the extent of gasification was ∼50%. This shows that high char conversions by gasification are possible under the conditions in the blast furnace. However, the residence time in the raceway is too short for appreciable reaction there and the gasification must be occurring during the longer residence time in the furnace itselfsat lower temperatures. The above data is shown in Figure 7, plotted as log rate vs the reciprocal of the absolute temperature. Despite the experimental scatter, some features are discernible. The rate increases over the whole temperature range studied and the quasi-linearity of the Arrhenius plot suggests that even at the high temperature of 1500 °C, the reaction is under chemical control. Not surprisingly, the gasification rates were much lower than those for combustion, where data was consistent with diffusion control. The differing rates of gasification and combustion are shown by the substantially different reaction times for these reactions. During the combustion tests (with a 200 ms pulse time), approximately 14% (daf basis) of the sample reacted, whereas 10 s was needed to react 32% of the sample during subsequent gasification. In other words, a 50-fold increase in reaction time only gave an approximate 2-fold increase in extent of reaction. The Effect of Hold Time. The effect of hold time has been studied over the range 1 to 10 s. In each test, the (24) Messenbo¨ch, R C. Ph.D. Thesis, University of London, 1998. (25) Peng, F. F.; Lee, I. C.; Yang, R. Y. K. Fuel Process. Technol. 1995, 41, 233.

Figure 7. Arrhenius plot of the logarithmic rates of gasification versus reciprocal absolute temperatures.

Figure 8. Total weight losses vs rate of gasification of Coal 1 at 1500 °C, at different hold times.

sample was heated (in N2, at 1000 ks-1) to 1500 °C and then exposed to a 20 ms pulse of air; this was immediately followed by the gasification period of the test (in an 18% CO2/N2 mixture at 1500 °C). The extent of gasification (%, daf basis) over the whole of each test period and the extent of gasification/second (calculated from the total extent of gasification and the test time) are shown as a function of the hold time in Figure 8. These data show that the total extent of gasification increased over the range studied, although the increase tailed-off at the longest hold time studied. This effect is reflected in the decreasing extent of gasification/s shown on the figure. The decrease in the observed extent of reaction/second with increased hold time is probably due to a combination of factors. These include a decrease in the amount of carbon present and increased thermal annealing of the remaining char (deactivation). Comparison of the Gasification Behavior of PyrolysisDerived and Combustion-Derived Chars. The relative combustion reactivity (as determined in the isothermal TGA test) of the combustion-derived chars has been shown to increase with exposure to a 20 ms air pulse. At longer pulse times, the subsequent reactivity (as measured by the TGA test) seems to have reached a steady value and did not change with increasing partial pressure of O2. The residence time in the raceway is typically 20 ms and it is possible that activation of the sample by the short exposure to O2, may influence the

496


Figure 9. Total weight losses of Coal 1 after 20 ms combustion and gasification in 18% CO2/N2 mixture at 1500 °C: (2) after combustion only; (0) after combustion followed by 1 s gasification; (9) after combustion followed by 2 s gasification.

Figure 10. Extent of gasification of Coal 1 as a function of air injection pulse length during combustion: (0) after 1 s gasification; (9) after 2 s gasification.

subsequent rate of gasification. Tests in the WMR have therefore been done to assess whether the apparent increased reactivity seen in the TGA test was also apparent in the gasification reactivity in the WMR. This has been done by comparing the extents of gasification of chars formed under pyrolysis/combustion conditions with chars formed under pyrolysis-only conditions (i.e., the air pulse system was not activated). Tests were done with 1 and 2 s hold times with an initial heating rate of 1000 °C s-1 and a gasification temperature of 1500 °C. The total weight losses (i.e., the combined effect of pyrolysis, mesh oxidation, combustion, and gasification) measured during these tests are shown in Figure 9. The “0 ms” data are those obtained for chars from the pyrolysis/gasification test, whereas the remaining data were obtained using chars from pyrolysis/combustion/ gasification tests. The 2 s hold time data is consistently higher than the 1 s hold time data and this is due to the increased extents of gasification. The increase in the total weight loss that is apparent with increasing pulse time reflects the increasing extent of combustion. Figure 10 shows the “extent of gasification” for the individual tests. They were estimated from results shown in Figure 9 by removing the weight changes caused by pyrolysis, combustion, and mesh oxidation. These data do not show a progressive increase in the extent of gasification. When the repeatability of the measurement is taken into account (i.e., (3% of the average value), the extent

Pipatmanomai et al.

of gasification appears insensitive to any effect of prior combustion. It is apparent that the characteristics of the combustion-derived char (formed in the WMR) that led to the observed increase in the relative combustion reactivity (TGA test) for the 20 ms chars have not had a similar impact on the gasification reaction (in CO2). This may be attributed to the further thermal deactivation of the char at 1500 °C during the relatively long gasification stage of the test in the WMR. This effect would not apply to the relative combustion reactivity test in the TGA, as this is done at a low temperature and the char structure would be stable. Relevance of Results to Reactions in Blast Furnace Raceways. A wire mesh reactor has been used to study the extent of the pyrolysis, combustion, and gasification reactions that occur under conditions that simulate those in the raceway of the blast furnace. The weight loss that occurs during pyrolysis has been studied at temperatures between 500 and 1500 °C and at heating rates between 1000 and 5000 °C s-1. The data shows that the weight loss for Coal 1 increased from 4 to 33% as the temperature was raised and from 1 to 22% for Coal 2 over the same temperature range. At 1500 °C, the weight loss seems to be close to its highest value, with a heating rate of 1000 °C s-1 or above, as the measured values have reached a plateau. This means that under the raceway conditions, the volatiles emission will approach its maximum value, provided this can be achieved within the limited residence time. It is probable that secondary carbon will form as part of the sequence of reactions involved in the progressive decomposition of the original volatile matter. This may be deposited within the coal char structure or carried forward with the gas stream to become trapped in the coke structure of the furnace or emitted from the top of the furnace, where it will cause emission problems. Some of the emitted volatiles will react with the O2 in the air (or enriched air) blast and raise the temperature of the gas stream and entrained char particles. The emitted volatiles will also have the effect of partially insulating the char particles from O2, which will also tend to limit the extent of char combustion. The extent of char combustion has been studied using the modified WMR, which enables samples of char to be reacted with short pulses of air, that are similar to the particle residence time in the raceway of the blast furnace (less than 50 ms). A heating rate of 1000 °C s-1, and a peak temperature of 1500 °C were used, as the emission of volatiles would be completed during the heating stage under these conditions. The effect of air pulse time and partial pressure of O2 have been investigated. It has been found that at very short pulse times (20-50 ms), the extent of combustion is close to the limit calculated from the O2 input with input gases containing between 21 and 75% O2. At longer pulse times, the measured extent of combustion progressive fell below the maximum value calculated from the O2 inlet. This is thought to be a result of the development of a diffusion resistance to the inward flow of O2 to the char particles, caused by the outward flow of product CO2. However, at residence times that are relevant to the raceway (2050 ms), our data show that the extent of combustion is limited by the supply of O2 that diffuses to the particles. The extent of combustion at short pulse times was low.


The theoretical maximum value (based on the O2 input from a 220 ms pulse of air) should have been approximately 25%, whereas the experimentally determined values were 14% (daf basis) or less. The total pressure in the WMR was limited to 1.5 bara, which is lower than the pressure in the raceway (typically 3 bara). To simulate the effect of total pressure in our study, the concentration of O2 was raised to 75%, which has the effect of raising the mass flow of inlet O2 to values that would be present at higher pressures. The results obtained with the higher inlet partial pressure of O2 show the same effects with pulse time as were observed with air. At pulse times that were representative of the residence time in the raceway, the extent of combustion was limited by the O2 supply rate, but the values did not exceed 9%, with 75% O2 in the inlet gas. These results suggest that the extent of char combustion that can occur under the conditions relevant to the raceway will be low. The extent of gasification (in CO2) that occurs under the simulated raceway conditions has also been studied using the WMR, by switching to a CO2/N2 mixture after the air pulse. The rate of gasification is high at 1500 °C, but the data suggests that a residence time of at least 10 s is needed to achieve a reasonable extent of char gasification. Consequently, the extent of gasification occurring in the raceway will be very low. However, our results suggest that the char will be gasified at the longer residence time, but lower overall temperature in the coke bed of the furnace.


tered in the raceway. In our work the heating time was 330 ms (at a heating rate of 5000 ks-1) and the maximum volatile emission occurred within this time. In the raceway, the residence time is less than 50 ms, hence it cannot be proved that pyrolysis will be completed in the raceway, but a high extent of reaction is thought to be likely. The extent of char combustion was measured at different pulse times and with different partial pressures of O2. Comparison between the experimental values and the maximum conversions shows that, at short injection pulse times (∼50 ms or less), the reaction was limited by the supply of O2 (i.e., complete utilization of the O2 occurred). This was observed for O2 partial pressures between 21 and 75%. At longer pulse times, the experimental and calculated extents of reaction increasingly diverged. This is thought to show the development of a diffusion resistance to the inward flow of O2 to the particles, caused by the outward diffusion of product CO2. The data suggests that this effect should not become significant at the short residence time in the raceway. However, the results suggest that only low extents of combustion of the char are likely to be achieved under the conditions in the raceway. The rates of gasification by CO2 were measured in the temperature range 800-1500 °C and the results suggest that the extent of gasification of the char in the raceway itself (residence time

Investigation of Coal Conversion under Conditions Simulating the

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