Simulation of Blast-Furnace Tuyere and Raceway Conditions in a Wire

Energy & Fuels 2007, 21, 2325-2334

2325

Simulation of Blast-Furnace Tuyere and Raceway Conditions in a Wire Mesh Reactor: Extents of Combustion and Gasification Long Wu, N. Paterson,* D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK ReceiVed December 12, 2006. ReVised Manuscript ReceiVed February 26, 2007

A wire mesh reactor has been modified to investigate reactions of coal particles in the tuyeres and raceways of blast furnaces. At temperatures above 1000 °C, pyrolysis reactions are completed within 1 s. The release of organic volatiles is probably completed by 1500 °C, but the volatile yield shows a small increase up to 2000 °C. The additional weight loss at the higher temperature may be due to weight loss from inorganic material. The residence time in the raceway is typically 20 ms, so it is likely that pyrolysis of the coal will continue throughout the passage along the raceway and into the base of the furnace shaft. Combustion reactions were investigated using a trapped air injection system, which admitted a short pulse of air into the wire mesh reactor sweep gas stream. In these experiments, the temperature and partial pressure of O2 were limited by the oxidation of the molybdenum mesh. However, the tests have provided valid insight into the extent of this reaction at conditions close to those experienced in the raceway. Extents of combustion of the char were low (mostly, less than 5%, daf basis). The work indicates that the extent of this reaction is limited in the raceway by the low residence time and by the effect of released volatiles, which scavenge the O2 and prevent access to the char. CO2 gasification has also been studied and high conversions achieved within a residence time of 5-10 s. The latter residence time is far longer than that in the raceway and more typical of small particles travelling upward in the furnace shaft. The results indicate that this reaction is capable of destroying most of the char. However, the extent of the gasification reaction appears limited by the decrease in temperature as the material moves up through the furnace.

Introduction When the coal injection rate into a blast furnace exceeds about 200 kg per ton hot metal, operational difficulties are found, which include blockage of the coke bed by fines in the vicinity of the raceway and carbon carry-over from the furnace top. These problems are directly related to the incomplete utilization of coal inside the furnace, particularly in the tuyere and raceway. After leaving the coal lance, injectant coal particles are heated rapidly and release volatiles: these form a plume around the particles. This is followed by the combustion of volatiles and partial combustion of chars. It is reasonable to assume that volatiles in the plume combust preferentially and deplete O2 in the gas that reaches char surfaces. In the raceway, the temperature normally reaches above 1700 °C and the pressure is in the range of 0.3∼0.6 MPa. The average blast velocity approaches 200 m s-1, which means that the residence time of a particle travelling in the oxidizing environment is only about 20 ms. The oxygen in the raceway region is depleted very quickly, and the utilization of the remaining uncombusted chars must depend on CO2 gasification at the end of the raceway and in the furnace shaft. A high-pressure wire-mesh reactor equipped with a newly developed short oxygen pulse injection system has been used to simulate the conditions in the tuyere and raceway regions of the blast furnace. The system uses three solenoid valves, which were activated simultaneously during a combustion test. The gas flow rate could be controlled precisely and continuously. The pressure range covered the full operating pressure range in * Corresponding author. Phone: 44-207-594-5581. Fax: 44-207-5945604. E-mail: [email protected].

a commercial blast furnace. The residence time of the diluted oxygen pulse could be varied, and the oxygen concentration in the mixed gas could be adjusted accurately. The development, testing, and preliminary data obtained with the new reactor configuration has been described in a previous paper.1 The reactor allows precise and variable control of the reaction conditions, such as oxygen residence time, oxygen concentration, heating rate, and peak temperature and pressure. Single particle behavior has been simulated, by using a monolayer of fuel particles and a continuous stream of sweep gas, to minimize the secondary cracking of tars and interactions between the char and evolving volatiles. In this paper, detailed experimental data obtained using the new experimental procedure are described and implications of the results for blast furnace operation are discussed. Experimental Details Equipment. Figure 1 presents a schematic diagram of the highpressure wire-mesh reactor (WMR). Detailed descriptions of the equipment and the experimental procedure have been presented elsewhere.2-4 Briefly, 5-6 mg sample is evenly distributed in the folded wire-mesh sample holder, which also serves as the resistance heater. The mesh is held between water-cooled electrodes. The particle size range used is 125-150 µm, determined by the size of the holes in the mesh. This is at the coarse end of the particle (1) Wu, L.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2006, 20, 2527. (2) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (3) Gu¨ell, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943. (4) Cai, H. Y.; Kandiyoti, R. Energy Fuels 1995, 9, 956. (5) Messenbo¨ck, R.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1999, 13, 122.

10.1021/ef060632s CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007

2326 Energy & Fuels, Vol. 21, No. 4, 2007

Wu et al. Table 1. Analysis of the Coal Samples proximate analysis

ultimate analysis (%,db)

coal name

VM

ash

C

H

N

S

O

BS2 EC2287LV EC2288HV S/B 3

13.2 21.6 36.6 17.4

6.5 5.6 5.3 8.7

85.9 83.0 80.8 82.7

4.3 5.9 8.1 4.3

1.6 1.8 1.5 1.1

0.5 0.9 1.1 n/a

1.2 2.8 4.0 n/a

Table 2. Analysis of the Fine Char

CY

Figure 1. High-pressure wire-mesh reactor:5 [1] gas exit; [2] quartz bell; [3] electrode clamps; [4] mesh support plate; [5] current supply; [6] sinter disc; [7] support plate stands, hollow to allow water flow; [8] copper seal; [9] gas inlet; [10] base plate; [11] throw over sealing ring; [12] flow smoothing cell; [13] spring, hollow to allow water flow; [14] wound corrugated tube; [15] pressure bell; [16] mesh; [17] line to pressure gauge.

size range for pulverized fuel (pf) injected into blast furnaces and at the fine end of the range if granular coal is injected. The brass support plate under the mesh is lined with a 2 mm thick alumina sheet to prevent electrical contact between the sample holder and the support plate. A stream of gas is used to sweep the sample holding part of the mesh, carrying evolving volatiles away from the sample holder; this helps to minimize the interactions between volatiles, the char, and the mesh. During pyrolysis experiments, total volatile yields can be determined with a repeatability of (1%, by separately weighing the sample plus the mesh before and after an experiment. This value was determined from the results of a suite of 100 separate tests done under the same conditions. In these experiments, Mo mesh has been used both as sample holder and as a resistance heater. This is the only metal that, at least in part, appears to tolerate the extreme conditions of the test and is available as a woven mesh of appropriate aperture size. However, Mo does oxidize to a certain extent in O2 concentrations above 3% by volume, temperatures above ∼1600 °C, and at pressures above 0.5 MPa. At lower pressures and lower temperatures, the effect of higher O2 concentrations can be meaningfully studied. These limitations were described in a previous paper1 together with a discussion of how the experimental procedure was developed and verified. These constraints do not apply to tests under pyrolysis or CO2 gasification conditions. Determination of Extents of Combustion. The experiments were carried out under severe constraints. Extents of combustion were determined under conditions where the impact of mesh oxidation could be controlled and quantified. At the same time, oxygen availability needed to be sufficiently high, so as not to limit the extent of combustion. A series of tests were done to enable the extent of combustion to be determined for set of operating conditions. Briefly: (1) A test with an empty mesh is done, using the conditions for the experiment with coal. This causes an increase in the mesh weight and enables the extent of mesh oxidation to be calculated. (2) The test is repeated with a sample of coal on the mesh, using N2 as the sweep gas at the same temperature and pressure as for the intended combustion test: this enables the determination of the weight loss by pyrolysis.

moisture (%)

ash (% db)

VM (% db)

char (vol % mmf)

coke (vol % mmf)

1.3

40.9

0.2

94.0

6.0

(3) Then, a test with a limited exposure to O2 is done using the trapped air injection system. After exposure to air, the N2 is introduced for a short period before the end of the test. In this test, a weight increase occurs due to mesh oxidation, together with weight losses due to pyrolysis and combustion. Test 1 is used for determining the amount of O2 that remains after reaction with Mo to react with the coal sample. After each test with coal, the char is carefully removed from the mesh for weighing. The char weight is subtracted from the initial coal weight to give the weight converted during each test. The volatile yields are expressed as the weight converted as a percentage of the initial coal weight (dry ash free basis). The extent of combustion is calculated as follows: volatile yield during the combustion test (test 3) minus weight loss during the pyrolysis test (test 2). Samples. The coal samples (bituminous) used in this work were supplied by Corus (UK) Ltd., from among those used in commercial scale blast furnaces (Table 1). BS2 and EC2287LV were low volatile coals, whereas EC2288HV was a high volatile coal. Char Sample for Gasification Tests. The fine char sample, used for the gasification tests in CO2, is a carryover dust from the single tuyere rig at Teesside Laboratory (Corus UK Ltd).6 The coal used in the single tuyere rig was S/B 3, a blend of two coals. The analysis of S/B 3 is also shown in Table 1. The dust sample was collected from the cyclone (the final dust removal from the exhaust gas, fed from both conditioning towers) during tests in 1999. The proximate and ash analyses of this sample are given in Table 2. It is evident that the volatile matter content is low, which is understandable in terms of having undergone pyrolysis and partial combustion in the furnace. The dust contains a high portion of injectant coal derived char (94.0% char plus 6.0% coke; this has been determined by microscopy/point counting) and was chosen as a representative postoxidation char for use in this study. Thermogravimetric Analysis. A Perkin-Elmer TGA7 thermogravimetric balance was used to measure the relative combustion reactivity of the chars formed in the WMR. A non-isothermal procedure was used. The char sample (about 2 mg) was placed on the balance pan and heated to 50 °C under N2, and that temperature was maintained for 5 min. The furnace was then heated at 50 °C min-1 to 400 °C, under N2, and again kept at this temperature for 5 min. Air was then passed through the balance chamber to initiate combustion, and the furnace was heated at 15 °C min-1 to 900 °C. The sample was kept at this temperature for 5 min to ensure complete combustion. The indicator of reactivity was the time elapsed for 50% sample weight loss (dry ash free basis).

Results and Discussion Coal Pyrolysis Behavior. A study of the effects of pyrolysis conditions on the total volatile yield was carried out on three coal samples using the high-pressure wire-mesh reactor. The (6) Willmers, R. R.; Fellows, P. M. Effect of blast furnace coal injection upon bosh coke properties, coke combustion and furnace permeability, Ironmaking Conference Proceedings-AIME, Chicago, IL, 1989; pp 352402.

Blast-Furnace Tuyere and Raceway Conditions

parameters examined were the peak temperature, pressure, and heating rate. The effect of holding time at the peak temperature has previously been investigated by Pipatmanomai,7 who showed that volatile release was completed within 1 s at 1000 °C. At 1500 °C, volatile release appeared to have been completed by the time peak temperature had been reached. Therefore, a 1 s hold time was used for each test in the current study, to ensure complete volatile release over the whole temperature range. (A) Effect of Heating Rate. Li et al.8 observed an increase of pyrolysis yield when coals were heated to 700 °C at 1000 °C s-1, compared to heating at 1 °C s-1 (also see ref 2). They also reported that the greater volatile evolution observed at higher heating rates consisted mostly of additional tar release. One likely explanation is that greater tar survival should be possible due to the more rapid expulsion of tars and tar precursors from the particles; slow heating would allow more time for the intra-particle repolymerization of tar precursors (also cf. the work of Gray9). More recently, Kandiyoti et al.10 (also cf. the work of Fukuda et al.11) have indicated that many noncoking coals are marginally deficient in donatable hydrogen and that during rapid heatup more of the reactive free radicals might be stabilized (quenched) by locally available hydrogen at high heating rates, giving rise to larger tar yields. Working with particles in the 106-152 µm diameter range, Cai12 reported that tar and total volatile yields during pyrolysis were not sensitive to a heating rate above 1000 °C s-1. Howard and co-workers13,14 also found that tar yields remained constant above 1000 °C s-1 for particles in the 106-125 µm diameter range. However, for 63-75 µm coal particles, they reported small increases in tar yields when heating rates were raised above 1000 °C s-1. These results are important in underlining the interrelationships between tar yields, heating rates, and the effects of intraparticle recombination reactions of tars. They are also consistent with vacuum pyrolysis experiments showing approximately 5% more tar yield, i.e., 5% intraparticle tar loss at atmospheric pressure for particles in the 106-152 µm size range.15 Pipatmanomai7 studied the effect of varying the heating rate between 1000 and 5000 °C s-1 at the peak temperature of 1000 °C with zero holding time. The work was done with particles in the 125-150 µm diameter range. She noted little difference in either total volatile or tar yield over this range of heating rates. In the present work, tests were done with particles in a similar size range, at a peak temperature of 1500 °C, at 0.3 Mpa, and with heating rates of 1000 and 5000 °C s-1. Using the 125-150 µm particle size range, the increase in the total (7) Pipatmanomai, S. Investigation of Coal Behavior under Conditions Simulating Injection into Blast Furnaces, Ph.D. Imperial College London, South Kensington Campus, Thesis, University of London, 2002. (8) Li, C. Z.; Madrali, E. S.; Wu, F.; Xu, B.; Cai, H. Y.; Guell, A. J.; Kandiyoti, R. Fuel 1994, 73, 851. (9) Gray, V. R. Fuel 1988, 67, 1298. (10) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and HeaVy Hydrocarbon Liquids: Thermal Characterization and Analysis. Elsevier Science Pub.: Amsterdam, Oxford, London, and New York, 2006; ISBN: 0-08-044486-5, cf. Chapter 6. (11) Fukuda, K.; Dugwell, D. R.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2004, 18, 1140. (12) Cai, H. Y. Fast Pyrolysis of Coals and Char Characterisation in Relation to Pulverised Coal Combustion, Ph.D. Thesis, Imperial College London, South Kensington Campus, University of London, 1995. (13) Griffin, T. P.; Howard, J. B.; Peters, W. A. Energy Fuels 1993, 7, 297. (14) Howard, J. B.; Peters, W. A.; Derivakis, G. S. Energy Fuels 1994, 8, 1024. (15) Li, C-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 1459-1468.

Energy & Fuels, Vol. 21, No. 4, 2007 2327 Table 3. Effect of the Heating Rate on the Pyrolysis Yieldsa sample

peak temp (°C)

heating rate (°C s-1)

pressure (MPa)

total volatile yield (% daf)

BS2

1500

1000

0.3

BS2

1500

5000

0.3

18.5 17.7 17.4 18.2

a

Hold time ) 1 s, in N2. Table 4. Effect of Heating Rate on Char Reactivitiesa heating rate (°C s-1)

t1/2 (min)

1000 5000

23.0 22.5

a The pyrolysis chars were prepared from BS2 coal, with a peak temperature 1500 °C, with 1 s of holding time, and at 0.3 MPa.

volatile yields at heating rates above 1000 °C s-1 appears to be within the experimental scatter (Table 3). In the blast furnace tuyere and raceway, the heating rate of the injectant coal is in the range 104 to above 105 °C s-1, depending on the velocity of the air blast and the rate of coal injection. However, in the WMR, the high heat capacity of the molybdenum mesh causes a lag in the temperature control; the highest heating rate that can be controlled successfully is 7500 °C s-1. Tests were done with heating rates of 1000 and 5000 °C s-1 to assess the impact of this parameter on the reactivity of chars formed in the WMR. The reactivity was assessed using a non-isothermal test in a TGA, and the results are shown in Table 4. The data shows a negligible difference in the reactivity of the char at two heating rates. These tests were done at 1500 °C, and the annealing of the char structure must have been sufficiently rapid to remove any effect of the heating rate. Therefore, it is assumed that the difference in heating rates between the WMR tests and in the raceway would only have had a marginal impact. (B) Effect of Peak Temperature. There is some variation in the temperatures quoted for the raceway in blast furnaces, and they do seem to depend on the method used: although, all show that temperatures are likely to be in excess of 1800 °C. For many blast furnaces, the real adiabatic flame temperature (a calculated value) would be in the range 2000-2300 °C. Measured gas temperatures (using two-color pyrometry) show values in the range 1850-2150 °C, while coke graphitization measurements indicate solids temperatures in the raceway around 1800 °C. In the present work, a series of experiments were carried out on BS2 coal to investigate the effect of the peak temperature under a pressure of 0.3 MPa. Total volatile yields as a function of peak temperature between 1000 and 2000 °C are shown in Figure 2. Pipatmanomai7 studied the pyrolysis behavior of some blast furnace coals over the temperature range between 500 and 1500 °C, using an atmospheric-pressure wire-mesh. She found a rapid increase in total volatile yield between 500 and 900 °C, depending on coal type; above 900 °C, the yield increase was much smaller. Also in this laboratory, Peralta et al.16 used a modified high-pressure wire-mesh reactor to study coal pyrolysis up to 2000 °C and observed a further volatile yield increase at higher temperatures. Normally, the pyrolysis of the organic volatiles in coal is complete within 1 s at temperatures above 1000 °C. The continued increase at temperatures between 1500 and 2000 °C suggests that other mechanisms also contributed (16) Peralta, D.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2005, 19, 532.


Wu et al. Table 5. Effect of Coal Rank and Volatile Matter Content on the Pyrolysis Yieldsa coal sample

VM (% db)

C (% db)

BS2 EC2287LV EC2288HV

13.2 21.6 36.6

85.9 83.0 80.0

pyrolysis yields (% daf) 17.7 25.9 49.3

18.5 26.7 49.7

a Experiments carried out at 1500 °C, under a pressure of 0.3 MPa in N2, at a heating rate of 1000 °C s-1.

Figure 2. Effect of peak temperature on the pyrolysis yields. Experiments were carried out using BS2 coal at 0.3 MPa, 1 s holding time in N2, and a heating rate of 1000 °C s-1.

Figure 3. Effect of pressure on the pyrolysis yields: (∆) EC2288HV; (O) BS2. Experiments were carried out using BS2 coal, 1 s holding time, 1500 °C in N2, and a heating rate of 1000 °C s-1.

to the weight loss in tests at very high temperatures. There are two possible explanations: (1) further devolatilization of the organic structure at the higher temperatures and/or (2) decomposition of part of the inorganic mineral material. Although the data does not allow a clear differentiation between the two effects, the weight losses observed suggested that the loss of mineral matter is more likely. (C) Effect of Pressure. The pressure inside the blast furnace raceway is about 0.3∼0.6 MPa.7 Figure 3 presents the pyrolysis yields as a function of pressure at 1500 °C in nitrogen. The trend is for the total volatile yield to decrease with pressure over the range from 0.15 to 0.5 MPa at 1500 °C. This trend is consistent with that observed by Gu¨ell and Kandiyoti3 and Messenbo¨ck et al.5 who used previous versions of the same wiremesh reactor to investigate the effect of pressure from atmospheric pressure to 15 MPa. In these studies, an initial sharp reduction of volatile yields was reported for small pressure increases (below 1.0 MPa). This can be explained by the effect of the greater resistance exerted by the sweeping gas on the escaping volatiles at the higher pressure. This causes an increase in the residence time within the particles, where repolymerization reactions occur and carbon is reincorporated into the forming char.16 The data shown in Figure 3 also suggest that

elevated pressures had more impact on EC2288HV coal with higher volatile matter content (36.6% db (dry basis)) than BS2 coal (volatile matter: 13.2% db). Compared to the previous work cited above, however, variations with pressure at these far higher temperatures appear less pronounced. (D) Effect of Coal Rank. Three coal samples were used to investigate the effect of the coal rank and coal volatile matter content on the pyrolysis behavior. Experiments were carried out at a heating rate of 1000 °C s-1. The pyrolysis yields were measured at 1500 °C under 0.3 MPa with a 1 s holding time. The results are shown in Table 5, along with the volatile matter and carbon contents of each coal sample. As might be expected, under the same experimental conditions, total volatile yields increased with decreasing carbon content and increasing volatile matter content. These results are in good agreement with those observed in a previous study.17 Study of Char Combustion Behavior. An earlier study1 has shown that Mo mesh reacts with O2 during experiments in the wire-mesh reactor. The nature of the Mo-O2 reaction is sensitive to the amount of O2 in the sweep gas, with the products ranging from mainly solid MoO2 at relatively low exposures, to the formation of vapor phase Mo oxides at relatively high exposures. At intermediate exposures, a mixture of the solid and vapor oxides are formed. In this part of the study, the conditions were chosen to limit the mesh reaction to the formation of mainly solid MoO2. In this way, it was possible to calculate the amount of O2 reaching the sample and to check that the extent of combustion of the char on the mesh was not being limited by the supply of O2. In order to avoid uncertainties caused by the formation of low amounts of volatile Mo oxides on the weight change, the char was removed from the mesh to determine sample weight loss during the experiments. The effects of peak temperature, pressure, and O2 concentration on the extents of combustion of two blast furnace coals (BS2 and EC2287LV) were investigated. The results are described below, together with a comparison of the trends observed with results from other workers. (A) Extents of Combustion as a Function of Peak Temperature. Tests were done in the temperature range from 1200 to 1600 °C, at a pressure of 0.3 MPa, an air exposure time of 105 ms, and with an inlet reactive gas containing 5% O2 (by volume). However, because of O2 reaction with the bottom layer of the mesh, the concentration of O2 reaching the sample was 3%: the derivation of this value was explained in a previous paper.1 Figure 4 shows the extents of combustion as a function of peak temperature for two of the sample coals. The extents of combustion for both coals were only ∼3-4% at 1600 °C. If this experiment may be considered to successfully simulate the conditions inside the blast furnace raceway, the results suggest that char combustion is limited, in the center of the coal plume where oxygen concentrations are low. The data also indicate that the extents of combustion increase with temperature up to (17) Wang, B. M.; Li, X. Y.; Xu, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2005, 19 (5), 2006.


Figure 4. Effect of peak temperature on the extents of combustion: (heating rate) 1000 K s-1; (holding time) 1 s; (pressure) 0.3 MPa; (residence time) 105 ms O2; (O2 concentration) 3% (by volume).

1400 °C but begin to level off at higher temperatures, suggesting the onset of external diffusion control over the combustion process. The results are broadly consistent with data obtained by other researchers. Using an entrained flow reactor to calculate the char burning rates, Mitchell18 concluded that the transition toward zone III (gas-phase diffusion limited) conditions appears to occur at temperatures as low as 1450 °C. This is a similar temperature to the change of slope noted in Figure 4. Young et al.19 measured the combustion rates of an Australian brown coal char over the temperature range 670-1150 °C and calculated the apparent reaction order to be 0.4, which means that the char particles were burning under zone II (pore diffusion control) conditions. At slightly higher temperatures, the present data (using less reactive coals) show a significant increase in the extent of reaction with temperature, which is consistent with pore diffusion control. Wells et al.20 investigated char combustion in a drop tube furnace, in air at temperatures of approximately 1030, 1230, and 1430 °C. Their observation suggested an increase in external and pore diffusional resistances with increasing temperature, i.e., a shift toward zone III combustion. In Figure 4, the extents of combustion of EC2287LV coal at two temperatures (1300 and 1500 °C) are also shown. The values are almost identical with those of BS2 coal, and the deviations are within the experimental error. If the chars were combusted under pore diffusion control (zone II), then a variation in behavior might be expected. The similarity of the results obtained at 1300 °C infers that the chars had a similar pore structuresor the onset of external diffusion control. At the higher temperature, a shift to fully external diffusion control has been suggested by the change in slope of the graph in Figure 4; in this situation, the extents of reaction would be expected to be similar. (B) Extents of Combustion as a Function of Total Pressure. The effect of total pressure on the extents of (18) Mitchell, R. E.; Mclean, W. J. On the Temperature and Reaction of Burning Pulverized Fuels, Nineteenth Symposium (International) on Combustion/The Combustion Institute (Pittsburgh, Pennsylvania), The Technion-Israel Institute of Technology, Haifa, Israel, August 8-13, 1982; 1113. (19) Young, B. C.; Smith, I. W. Combust. Flame 1989, 76, 29. (20) Wells, W. F.; Kramer, S. K.; Smoot, L. D. Reactivity and Combustion of Coal Chars, Twentieth Symposium (International) on Combustion/The Combustion Institute (Pittsburgh, Pennsylvania), The University of Michigan, Ann Arbor, Michigan, August 12-17, 1984; 1539.

Energy & Fuels, Vol. 21, No. 4, 2007 2329

Figure 5. Effect of total pressure on the extents of combustion of BS2: (O2 concentration (by volume)) 0, 3%; O, 6%; (temperature) 1300 °C; (O2 residence time) 105 ms.

combustion of char formed from BS2 coal was investigated at two oxygen concentrations: 3% and 6%. The temperature was maintained at 1300 °C, and the reaction time was 105 ms. The results are presented in Figure 5. They show a slight increase in the extents of combustion with pressures up to 0.5 MPa. There is general agreement in the literature on the main effects of pressure on char combustion. Previous studies21-23 at fixed gas compositions showed two different trends. The first was observed in kinetics-limited combustion: an increase in total pressure produced a rise in oxygen partial pressure, thereby accelerating the chemical rate. The second concerned diffusionlimited combustion in which no effect of pressure was observed. This has been explained by Essenhigh,24 who calculated the diffusion rates, suggesting that the influence of total pressure on the diffusion rate was dependent on the product of gas density and oxygen diffusion coefficient only. Since the oxygen diffusion coefficient is inversely proportional to pressure and density is proportional to pressure, the product is then (nearly) independent of total pressure. However, Macneil et al.25 investigated the effect of pressure on char combustion in a pressurized circulating fluidized bed boiler and found that, at 850 °C, the char reaction rate increased with pressure up to 0.5 MPa and a further increase in the pressure led to a decrease in the reaction rate. This trend was also observed by Monson et al.26 at higher temperatures. In addition, both studies noted that the initial rise was more pronounced under higher oxygen partial pressures. By studying the two-step gas-solid kinetics including the adsorption and desorption rates, Essenhigh24 indicated that the reaction rate has a small dependence on pressure which he attributed to the adsorption term being affected by a so-called reaction penetra(21) Horvath, A.; Jakhola, A.; Hippinen, I. Pressurized fluidised combustion of peak and coal, Finnish-Swedish Flame days 1990, The International Flame Research Foundation, Turku, Finland, September 4-5, 1990. (22) Monson, C. R.; Germane, G. J.; Blackham, A. U.; Smoot, L. D. Combust. Flame 1995, 100, 669. (23) Turnbull, E.; Kossakowski, E. R.; Davidson, J. F.; Hopes, R. B.; Blackshaw, H. W.; Goodyear, P. T. R. Chem. Eng. Res. Des. 1984, 62, 223. (24) Essenhigh, R. H. Influence of Pressure on the Combustion Rate of Carbon, Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute (Pittsburgh, Pennsylvania), The University Frederico II, Napoli, italy, July 28-August 2, 1996; 3085. (25) Macneil, S.; Basu, P. Fuel 1998, 77, 269. (26) Monson, C. R.; Germane, G. J.; Blackham, A. U.; Smoot, L. D. Combust. Flame 1995, 100, 669.


Wu et al. Table 6. Reactivity of the Coal A (BS2) Chars Produced in WMR at 0.3 and 0.5 MPaa thermogravimetric half-life (min) O2 concentration (%)

0.3 MPa

0.5 MPa

0 (pyrolysis only) 3 20

18.0 17.5 15.8

20.0 19.1 nd

a

Temperature ) 1300 °C, 105 ms of air exposure time.

and widening of the char pores. This is consistent with a previous study by Pipatmanomai,7 who found that the char had a higher burnout and became more reactive when it was exposed to longer air pulses.

Figure 6. Effect of oxygen concentration on the extents of BS2 combustion: (temperature) 1300 °C; (pressure) 0.3 MPa; (oxygen residence time) 105 ms.

tion factor, but the dependence is second-order. He also found that with increasing temperature, the influence of pressure became more marked but remained a secondary effect. (C) Effect of the Partial Pressure of Oxygen. A set of combustion tests has been conducted with O2 concentrations between 3% and 20% (by volume), at a pressure of 0.3 MPa, at 1300 °C. The extents of combustion of BS2 coal as a function of O2 concentration, with an injection pulse length of 105 ms, are shown in Figure 6. The data show that the extent of combustion increased nearly in proportion to the increase in the O2 concentration. Theoretically, the char combustion rate is proportional to the nth power of O2 partial pressure,24 where n stands for the reaction order. At 1300 °C, char is considered to burn under pore diffusion control (zone II), in which the combustion rate is largely dependent upon the diffusivity of O2. Slattery et al.27 developed a model to calculate the diffusivities of some binary gas mixtures at a particular temperature and pressure. This indicated that the diffusivity increased with the partial pressure of the reactive gas. In another study, Field28 measured the char burnoff in a laminar flow reactor. The experiments were carried out using two oxygen concentrations: 5% and 10%, respectively, at temperatures above 1100 °C. The measured burnoff was 3∼4 times higher at 10% oxygen concentration than at 5%. Pipatmanomai7 studied the effects of O2 partial pressure on the extents of combustion at 1500 °C and observed that the char burnoff was about 3 times higher at 75% O2 than at 21%. (D) Reactivity of the Residual Chars. The half-lives derived from the thermogravimetric analysis (TGA) weight/time profiles for the chars produced in the WMR during tests at 0.3 and 0.5 MPa are shown in Table 6. These data show that combustionderived chars are more reactive than the pyrolysis-derived chars prepared in nitrogen, under otherwise similar conditions. It was also observed that char half-lives decreased with increasing O2 concentrations, i.e., the char became more reactive. It has already been shown that the char achieves a higher burnout at higher oxygen concentrations. This would have two effects: (1) more intense burnoff of the secondary (less reactive) carbon deposited from tars on the char surface by pyrolytic processes would occur and (2) the available surface area would increase by the creation (27) Slattery, J. C.; Bird, R. B. AIChE J. 1958, 4, 137. (28) Field, M. A. Combust. Flame 1969, 13, 237.

The results are also in good agreement with those of Aarna and Suuberg,29 who examined the changes in reactive surface area and porosity during char oxidation and found that the surface area usually passed through a maximum at some intermediate burnoff. After this point, the rate of formation of the new area was offset by the rate of destruction of the old area. They also found that the development of surface area was independent of whether reaction occurred in zone I or II. Banin et al.30 used a shock tube reactor to measure the char reactivities over a broad range of particle temperatures (1227-2727 °C) and partial oxygen pressures (0-100% of O2 at 0.7-0.9 MPa). They indicated that in the first stage of burnout the reaction rate increased because of an increase in internal surface area and the maximum reaction rate was reached when the pores started to overlap. Kinetic Studies. The combustion model for single particles, developed by Field,31 has been used to derive kinetic constants, for comparison with results of other workers, using different experimental techniques. The overall reaction rate coefficient (k) of char combustion is defined as the rate of removal of carbon per unit external surface area, per unit partial pressure of oxygen in the gas, and is expressed by the equation below. (29) Aarna, I.; Suuberg, E. M. Changes in Reactive Surface Area and Porosity during Char Oxidation, Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute (Pittsburgh, Pennsylvania), The University of Colorado at Boulder, CO, August 2-7, 1998; 2933. (30) Banin, V.; Moors, R.; Veefkind, B. Fuel 1997, 76, 945. (31) Field, M. A.; Gill, D. W.; Morgan, B. B; Hawkley, P. D. W. Combustion of pulverised coal, Leatherhead, British Coal Utilisation Research Association, 1967. (32) Lawn, C. J. Principles of Combustion Engineering for Boilers, Academic Press Limited: London, 1987. (33) Smith, L. W.; Tyler, R. J. Fuel 1972, 51, 312. (34) Field, M. A. Combust. Flame 1969, 13, 237. (35) Mitchell, R. E. Experimentally Determined Overall Burning Rates of Pulverized-coal Chars in Specific O2 and CO2 Environments, twentyFirst Symposium (International) on Combustion/The Combustion Institute (Pittsburgh, PA), The Technical University of Munich, Germany, August 3-8, 1986; 173. (36) Miura, K.; Hashimoto, K.; Silveston, P. L. Fuel 1989, 68, 1461. (37) Valix, M. G.; Harris, D. J.; Smith, I. W.; Trimm, D. L. The Intrinsic Combustion Reactivity of Pulverised Coal Chars: The Use of Experimental Pore Diffusion Coefficient, Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute (Pittsburgh, Pennsylvania), The University of Sydney, Australia, July, 5-10, 1992; 1217. (38) Croiset, E.; Mallet, C.; Rouan, J. P.; Richard, J. R. The Influence of Pressure on char combustion Kinetics, twenty-Sixth Symposium (International) on Combustion/the Combustion Institute (Pittsburgh, Pennsylvania), The University of Frederico II, Napoli, Italy, July 28-August 2, 1996; 3095. (39) Hargrave, G.; Pourkashanian, M.; Williams, A. The Combustion and Gasification of Coke and Coal Chars, Twenty-First Symposium (International) on Combustion/The Combustion Institute (Pittsburgh, Pennsylvania), The Technical University of Munich, Germany, August 3-8, 1986; 221. (40) Wall, T. F.; Gururajan, V. S. Combust. Flame 1986, 66, 152.



dm ) -kPo2(πDp2) dt Where, 1/k ) (1/kc) + (1/kd) and can be expressed as k ) A exp(-E/RTp); dm/dt ) the rate of mass loss (kg s-1); k ) overall reaction rate coefficient (kg m-2 s-1 (atm O2)-1); kc ) chemical reaction rate coefficient (kg m-2 s-1 (atm O2)-1); kd ) mass transfer coefficient (kg m-2 s-1 (atm O2)-1); Dp ) particle diameter (m); PO2 ) O2 partial pressure (atm); Tp ) surface particle temperature (K); A ) pre-exponential factor (kg m-2 s-1 (atm O2)-1); E ) activation energy (kJ mol-1); R ) gas constant ) 8.314 (kJ kgmol-1 K-1). Several assumptions have been made to enable these calculations. (1) It was assumed that combustion was conducted in an ideal gas environment. (2) The release of the volatile products was assumed complete; these, therefore, were not included in the calculation. (3) The char particles were assumed to be spherical, and their density, constant. (4) The reaction was assumed to be first order to enable the comparison of rate coefficients obtained by different workers. Figure 7 shows the calculated char combustion rates derived from the experiments in the wire-mesh reactor. Data obtained by other workers, using mainly entrained flow and laminar flow reactors, over a range of temperatures, has also been shown. The data shown in the figure exhibits a significant variation, for a given temperature, between the results of different workers. This might be explained by either the different methods for particle surface temperature measurements, the different methods for sample burnoff determination, or the variation of particle temperature during combustion. The rate coefficients obtained during the wire-mesh combustion experiments are within the range of the data suite. This gives confidence that the experimental protocol developed for the trapped air tests in the WMR is yielding credible data. The overall picture suggests, however, that the experimental community has not succeeded in “controlling” all relevant parameters in comparable ways. Under the conditions used in this study (temperature above 1200 °C, pressure 0.3 MPa, and O2 concentration 3%), the extent of combustion was limited by both the rates of chemical reaction and mass transfer. The rate coefficients, under zone I conditions (chemical rate control only), were also determined using the wire-mesh reactor at low temperatures (below 700 °C). The char was burnt in the temperature range from 500 to 700 °C, in air. The holding time was dependent upon the reactivity of the char at each temperature; for example, it was set at 60 s for 500 °C and 10 s for 700 °C. The reaction was assumed to occur isothermally. The results were compared with those obtained using an isothermal TGA method, with pyrolyzed char produced in the wire-mesh reactor (using BS2 coal, at 1300 °C, 0.3 MPa, holding time 1s). In the TGA, experiments were done in the range 500 to 575 °C. Dry air was fed into the balance crucible, which contained the pyrolyzed char sample. Figure 8 shows the Arrhenius diagram of the rate coefficients determined by the two techniques. For comparison, some combustion test data obtained using the wire-mesh reactor at higher temperatures (from 1100 to 1300 °C, under ∼20% O2) are also shown. Values of the activation energy E and pre-exponential factor A may be estimated from the slope and intercept of the Arrhenius plot. The different slopes of the graphs shown on the figure indicate the different combustion regimes. At low temperature (500700 °C), the slope is steep and this is characteristic of zone I (chemical control). Between 700 and 1100 °C the slope decreases, which indicates the transition from zone I to II

Figure 7. Comparison of simple first-order char oxidation rate coefficients from different investigators: (s) results obtained from WMR; (- - - -) results obtained from entrained flow or laminar flow reactor;32-37 (- - -) results obtained from fixed-bed reactors;38 (- -) results obtained from TGA;39 (- -) results obtained from the ignition method.40

Figure 8. Use of the wire-mesh reactor and an isothermal TGA method to determine the Arrhenius form of combustion rate coefficients. The char was burnt under ∼21% O2.

(chemical to pore diffusion control). Between 1100 and 1300 °C, the slope appears fairly constant, so this may still be within zone II. Above this temperature and not indicated here, will be the transition to zone III (external diffusion control). The results also show that at lower temperatures (below 700 °C), the Arrhenius expressions derived from the wire-mesh reactor and TGA have the same activation energy (the slope of the line) but different pre-exponential factors (the intercepts). The activation energy, essentially, affects the temperature sensitivity of the reaction rate, whereas the pre-exponential factor is related more to the material structure. Rubak et al.41 found that the pre-exponential factor reflects the active sites available for reaction on the internal and external particle surfaces. The present study suggests that, under the same conditions, different reactors may influence the value of the pre-exponential factors. The Gasification of Char in CO2 under Simulated BlastFurnace Conditions. In tuyeres and raceways, the pyrolysis of injected coal particles and the combustion of resultant chars occur rapidly. When all injected oxygen is depleted, the main char consuming reaction is gasification by CO2. Mostly, this is thought to take place at the end of the raceway and in the furnace shaft. To examine gasification rates under simulated furnace (41) Rubak, W.; Karcz, H.; Zembrzuski, M. Fuel 1984, 63, 488.


Figure 9. Extents of gasification as a function of peak temperature. Experiments were carried out in 18% CO2, at a pressure of 0.3 MPa, and with a holding time of 1 s.

conditions, a fine char formed under representative conditions, in a single tuyere rig (operated by Corus (UK) Ltd) has been used. Gasification tests were done using an 18% CO2/82% N2 mixture, in the WMR, over a range of temperatures, pressures, and holding times. The sample contained low (but non-zero) residual volatile matter, so pyrolysis studies were also conducted with this sample. This enabled the impact of these reactions to be removed from the results of the gasification tests. The pyrolysis study42 shows that the char sample lost 9-10% of its weight at 1500 °C and 0.3 MPa in helium. This is probably due to the loss of inorganic components. (A) Effect of CO2 Gasification Temperature. Figure 9 shows the extent of gasification as a function of temperature in the 1000-1500 °C range. The experiments were carried out at 0.3 MPa, which is the lower end of the pressure range for a raceway and close to the pressure in the furnace shaft. The extent of reaction increased over this temperature range. The results are in a good agreement with those obtained by Pipatmanomai,7 who examined the effect of gasification temperature, under atmospheric pressure. (B) Effect of Holding Time. Figure 10 shows the extent of gasification as a function of holding time. The sample was gasified at 1500 °C, in 18% CO2, at 0.3 MPa. Experiments were performed at three different holding times: 1, 4, and 10 s. The data shows that the extent of gasification increased over the range of holding times studied and, by 10 s, complete conversion had been achieved. (C) Effect of CO2 Gasification Pressure. The extent of gasification as a function of pressure, at 1500 °C with a hold time of 1 s, is shown in Figure 11. The CO2 concentration was 18% (by volume). The data indicate that below 0.7 MPa, the effect of total pressure on the extent of gasification is negligible. This may be explained by the enhanced reactivity of CO2 with pressure being compensated by the increased secondary char deposition during the initial pyrolysis stage; deposited char would be less reactive.43 In a previous study using the same reactor at lower temperatures, Messenbo¨ck et al.43,44 found that the gasification weight loss went through a minimum as the (42) Wu, L. Ph.D. Thesis A Study of the Behavior of Coal Injected into the Blast Furnace, Imperial College London, South Kensington Campus, University of London, 2006. (43) Messenbo¨ck, R.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 781. (44) Zhuo, Y.; Messenbo¨ck, R.; Collot, A.-G.; Paterson, N.; Dugwell D. R.; Kandiyoti, R. Fuel 2000, 79, 793.

Wu et al.

Figure 10. Extents of gasification as a function of holding time. Experiments were carried out in 18% CO2, at 1500 °C, and at 0.3 MPa.

Figure 11. Extent of gasification as a function of gasification pressure. Experiments were carried out in 18% CO2, at 1500 °C, and at 1 s of holding time.

pressure reached 1.0 MPa (peak temperature 1000 °C and holding time 10 s). It was shown that the minimum is observed more clearly when reaction conditions lead to a relatively slower reaction rate, i.e., at lower tempresures, lower pressures, and/ or less reactive gases. (D) Effect of CO2 Concentration. The sample was exposed to CO2 concentrations in the range between 0% and 18%. The peak temperature was 1500 °C, with a 1 s holding time. The operating pressure was maintained at 0.3 MPa. The results are shown in Figure 12. The extent of gasification increased proportionally with the CO2 concentration, which suggests that, under these conditions, the gasification is probably a first-order reaction. Implications of the Results on Conversions in Blast Furnaces. The experiments described in this paper have pushed the WMR to the limits of its operation. However, despite the complexities involved in gathering valid data, it is considered that the tests have provided a credible insight into the suite of reactions that occur during the short residence time, at high temperature, in the tuyere and raceway. It is accepted that there are differences between the conditions of our simulation and those in the raceway proper. The O2 exposure time in the WMR was longer than that in the raceway; therefore, the tests would tend to overestimate the extent of combustion. The heating rate was lower, but this is thought to only have a minor effect,


Figure 12. Extent of gasification as a function of CO2 concentration. Experiments were carried out at 1500 °C, with 1 s of holding time, and at 0.3 MPa.

because of the high char annealing rates at the high temperature of the test. The effect of particle size has not been assessed, although it is noted that the particle size range used is at the upper end of the range used for pulverized coal injection, and the impact should not be significant. Under inert gas in the WMR, pyrolysis was complete within a hold time of 1 s, at 1500 °C. Above 1500 °C, increases in volatile yields were relatively small and likely to be caused by the volatilization of inorganic species. The nature of the released inorganic volatiles has not been investigated, but it is possible that in the blast furnace they would be condensed as they rise up the shaft of the furnace and that gases are progressively reduced in temperature. Previous data16 have shown that the release of organic volatiles is complete at 1500 °C and a hold time of 1 s. However, the residence time at this level of temperature is only about 20 ms in the tuyere and raceway, and our experiment does not have a sufficiently short time discrimination to prove that pyrolysis will be completed in this short time. In fact, it is probable that pyrolysis will occur throughout the passage of the devolatilizing coal through the raceway and be completed in the lower part of the furnace shaft. In the tuyere and raceway, the released volatiles are thought to form a plume that will accompany the particles as they stream toward the furnace shaft. They will react with the O2 and minimize the amount that reaches the char surface. The extents of combustion have been measured with O2 concentrations in the range 3-5%, at 1600 °C. Higher concentrations have been used at lower temperatures. These experimental limits have been imposed by the reaction of the Mo mesh with O2. It is thought that this low O2 concentration may not be dissimilar to the concentration seen by the char particles in the raceway, because of O2 scavenging by the combusting cloud of volatile matter. The data obtained shows that only low extents of char combustion are attained at an exposure time of 100 ms, which is about 5 times longer than the actual residence time in the raceway. This strongly suggests that only low extents of char combustion actually occur in blast-furnace tuyeres and raceways. The limit is likely imposed by the scavenging effect of the volatiles and by the short residence time. The impact of the temperature difference between our tests and the raceway may not be significant, as the kinetic study suggests that, at the higher temperatures, reaction is occurring in a diffusioncontrolled regime and the effect of higher temperatures has less impact.


The extents of gasification by CO2 have also been studied under conditions that represent those in the raceway and furnace shaft. The data shows that complete char gasification can be achieved with a contact time of 10 s, at 1500 °C. This reaction is therefore capable of completely destroying the char formed during coal injection into the raceway, but the residence time at such high temperatures is far too short. With current designs of furnaces, the major proportion of the char reaction is thought to occur at decreasing temperatures in the furnace shaft. The presence of char in the collected (albeit small amount of) fines shows that the reaction conditions have not been sufficiently intense to allow complete reaction. However, there does seem to be potential to engineer the conditions to optimize the extent of this reaction. One way to accomplish this would be to increase the residence time in the raceway, and the incorporation of an entrained flow gasification section into the tuyere/raceway can be envisaged. Somewhat longer residence times would be required. It would probably not be economic to consider such a change to an existing furnace, but it would be worth considering for new furnaces. An increase in the residence time at high temperature would also ensure complete release of the volatiles and destruction of tars, before the gases enter the coke bed in the furnace shaft. This would assist in minimizing the potential for blocking the pores of the coke. Other factors, such as increasing the CO2 content in the blast gas would also assist in raising the char conversion. This last option is already under consideration as a means of improving furnace performance, by reducing the thermal inefficiency caused by the presence of the N2 from the air and also producing an exit CO2 rich stream for sequestration. However, to maintain the temperature, more oxygen would need to be introduced to combust more coal and provide the energy requirement for increased gasification. Conclusions A high-pressure wire-mesh reactor equipped with a newly developed short oxygen pulse injection system has been used to simulate the conditions in the tuyere and raceway regions of blast furnaces. Under an inert atmosphere, peak temperatures up to 2000 °C could be achieved, which is close to the temperatures inside the blast-furnace raceway. Short air-pulse char combustion data were collected under conditions constrained by the molybdenum mesh oxidation, i.e., up to 1600 °C with 3% oxygen concentration at 0.3 MPa. These are thought to represent conditions near the center of the coal plume in the raceway, although the temperature there may be several hundred degrees higher than that achievable in the WMR. It was found that, during pyrolysis, the total volatile yields reached their maximum value after a 1 s hold time in the WMR and the volatile release increased with increasing temperature. Above 1500 °C, at least some of the increase appears to be due to the vaporization of inorganic materials, as the release of organic volatiles is thought to be largely completed by the time the sample reaches this temperature. Below 1500 °C, total volatile yields increased with decreasing coal carbon content and increasing volatile matter content of the coal sample. Within the pressure range studied (0.15-0.5 MPa), increasing pressure shows a negative effect on the pyrolysis yields: an effect observed in other studies and caused by the increased resistance to the release of volatiles from the fuel particles. The extent of combustion of pyrolysis-derived chars in 3% O2/97% N2, at 1600 °C, was low, for exposure times as high as 105 ms, being typically less than 5% (daf). At 1300 °C, when the O2 concentration reached 20%, the extent of combustion


within the same residence time was also lower than 5% (daf). The char burning rate was dependent upon the combustion temperature. Generally, the extent of combustion increased with an increase of temperature. However, the kinetic study suggested that above 1400 °C, the char is burnt in zone III or transitions from zone II to zone III. Therefore, further increasing the temperature would not increase the extent of combustion significantly. As might be expected, at 1300 °C, increasing O2 concentration, at a fixed total pressure, increased the extent of combustion. It was also found that increasing the total pressure at a fixed O2 concentration enhanced the extent of combustion slightly. Also, the relative reactivity of the post-combustion residues was higher when the char had been exposed to higher O2 concentrations. The CO2-gasification experiments showed that the extent of gasification, after 10 s, in 18% CO2, at 1500 °C, and 0.3 MPa, was close to 100% (daf basis). This suggests that char will be completely consumed within the blast furnace, providing

Wu et al.

itresides at high temperature for approximately 10 s. The gasification reactivity increased significantly with temperatures up to 1500 °C, especially between 1300 and 1500 °C. It was also found that the extent of gasification increased with an increase of CO2 concentration. Results showed that below 0.7 MPa, the effect of pressure at a fixed CO2 concentration was negligible. The extent of gasification at 1.0 MPa was only 4-5% (daf) higher than that obtained at 0.7 MPa. Thus, the modest pressures of commercial furnaces (up to about 0.5 MPa) are not likely to produce significant increases in conversion. Acknowledgment. The authors gratefully acknowledge the grant received from BCURA Ltd (Grant Number B67) for this work. We also thank Dr. Colin Atkinson (Corus UK, Ltd) for technical advice and discussion and for providing some of the samples used in this work. EF060632S

Simulation of Blast-Furnace Tuyere and Raceway Conditions in a Wire

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