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Energy & Fuels 1996, 10, 348-356
Laboratory Studies on Devolatilization and Char Oxidation under PFBC Conditions. 1. Volatile Release and Char Reactivity Yong Lu Department of Energy Engineering, Helsinki University of Technology, Espoo, FIN-02150, Finland Received June 19, 1995X
Characteristics of solid fuels during devolatilization and char oxidation under pressurized FBC conditions have been recently studied in a laboratory-scale PFBC batch reactor. The reactor tube has an inside diameter of 60 mm and height of 1.2 m and can be controlled at well-defined operating conditions. Combustion behaviors, including devolatilization rate, volatile-C amount, devolatilization time, char burnout time, and char reactivity, were investigated under the base testing conditions (Tb ) 850 °C, p ) 11 bar, O2 ) 3% v/v, Vf ) 0.24 m/s) with 10 different fuels, from high-volatile peat to relatively low-volatile coke. A special focus of this study was the effect of pressure. It was examined in two parts: oxygen partial pressure (with changing either oxygen concentration in the fluidizing gas or system pressure) and total pressure (with changing system pressure and oxygen concentration simultaneously at fixed oxygen partial pressure). Results showed that both oxygen partial pressure and total pressure affected the processes of devolatilization and char oxidation. They affected devolatilization rate in the same direction but char reactivity in the opposite direction. The influences of other parameters, such as batch particle size, bed temperature, and fluidizing velocity, on the combustion characteristics were also studied in detail for three fuels: lignite, Illinois No. 6 coal, and Kiveton Park coal. Overall, devolatilization was controlled by chemical kinetics and char burning rate mainly by oxygen transfer and diffusion under the examined PFBC conditions.
Introduction As one of the attractive technologies for clean coal utilization, pressurized fluidized bed combustion (PFBC) promises low environmental impact and a high efficiency of power generation by means of the combined cycle process. With the recent development of commercial scale PFBC units, fundamental information about the influence of pressure on combustion characteristics is required to assess the implications for startup, overall combustion efficiency, sulfur release and capture, and nitrogen oxide formation and destruction. For example, elevated pressure affects the partial pressure of oxygen in the inlet gas mixture and diffusion coefficients of gaseous components, changes the chemical kinetics during devolatilization and char oxidation, and influences the composition of volatile products, gaseous emissions, and particle fragmentation and attrition. For combustion studies in detail, the influence of pressure can further be divided into many related parts, such as total pressure and the partial pressure of gaseous components, such as oxygen and carbon dioxide. The combustion of solid fuel, e.g., coal, occurs in two consecutive stages: the release of volatiles and their subsequent burning out (devolatilization), followed by the oxidation of the remaining char. These two processes are very different; i.e., gas-phase reactions are dominant during devolatilization and heterogeneous reactions are mainly involved in char oxidation. The devolatilization time is negligible compared to char X
Abstract published in Advance ACS Abstracts, February 1, 1996.
0887-0624/96/2510-0348$12.00/0
combustion time, but the devolatilization period has an essential role in determining char properties, such as specific surface area, porous structure, and size distribution. In addition, devolatilization changes the gasphase composition in the combustor, and the gaseous products react further to build other compounds, which are important in creating pollutants and in heat transfer. Regarding the estimation of char burnout, therefore, a proper understanding of the kinetics of devolatilization, and knowledge of devolatilization conditions and the products in gas-phase and solid residues, are required. Combustion characteristics of solid fuel under FBC conditions have been given increasing attention since a rapid development of the commercial scale FBC units during last decade. However, most of the investigations have concentrated on atmospheric FBC, and little experimental evidence is available concerning the basic nature of combustion characteristics under pressurized FBC conditions. Literature data from previous researchers indicate that the combustion of millimeter-sized coal particles at the temperature range typically encountered in FBC is controlled by both chemical kinetics and diffusional transfer. Devolatilization time is close to being largely controlled by chemical reactions and heat transfer between gas and particle and also in the particles.1,2 On the other hand, char burning rate of non-fragmenting particles is mainly determined by (1) Agarwal, P. K.; Genetti, W. E.; Lee, Y. Y. Fuel 1984, 63, 17481752. (2) Zhang, J. Q.; Becker, H. A.; Code, R. K. Can. J. Chem. Eng. 1990, 68 (6), 1010-1017.
© 1996 American Chemical Society
Devolatilization and Char Oxidation. 1
oxygen transfer to the particles and oxygen diffusion in the particle.3,4 Therefore, the batch combustion behavior under such circumstances will be sensitive to variations in coal particle size, oxygen concentration, pressure, and bed temperature. The data and observations in the literature are insufficient to quantify the effect of pressure on combustion characteristics. The effect of pressure on devolatilization, as summarized in a review by Saxena,5 seems to be the contribution to the secondary reactions of certain reactive species such as tar. When pressure increases, in general, the residence time of volatiles within the particles increases, resulting in more extensive secondary reactions and producing less volatiles and more char and hydrocarbons. Significant effect of pressure on fuel reactivity has been reported by Aho et al.,6 who have recently investigated the influence of pressure in detail at a pressurized entrained flow reactor, including total pressure and partial pressure of oxygen and carbon dioxide. Compared to the entrained flow mode, the fluidized bed mode exhibits a big difference in particle size and in fluid dynamics, especially within the bubbling bed. Both of them have been proved to be very important factors in carbon conversion and combustion kinetics. In the fluidized bed mode, fragmentation and attrition of the particles are also important mechanisms. The rates of devolatilization and char oxidation under simulated PFBC conditions have been measured by Shiao et al.7-9 However, their data obtained in fixed beds do not confirm the changes from fixed bed mode to fluidized bed mode in mass and heat transfer, namely, the effect on the combustion behavior and the influence of pressure on devolatilization and char burnout. A few investigators have attempted to determine the coal combustion characteristics under pressurized FBC conditions and the effect of pressure. Horvath et al.10 found that the burning rate was enhanced at higher pressure in a PFBC batch reactor, and the effect of pressure on the devolatilization rate for bituminous coal differs from peat. Turnbull et al.11 reported the measurement of burnout time for small coke and char particles (0.15-1.7 mm diameter) in an air-fluidized bed of sand at bed temperature of 750 and 900 °C at pressure up to 17 bar absolute. They pointed out that pressure reduced the burnout time by the increased rate to the higher oxygen partial pressure accelerating the chemical reactions, thereby reducing the influence of chemical rate control on the overall combustion rate. However, Wallman et al.12 investigated the combustion rate at a bench-scale PFB reactor with pressures of 2, 10, and 20 bar and concluded that pressure had no (3) La Nauze, R. D. Chem. Eng. Res. Des. 1985, 63, 3-33. (4) Turnbull, E.; Davidson, J. F. AIChE J. 1984, 30, 881-889. (5) Saxena, S. C. Prog. Energy Combust. Sci. 1990, 16 (1), 55-94. (6) Aho, et al. LIEKKI-2 Combustion Research Program: Report L95-1, 1995, Naantali, Finland. (7) Shiao, S-Y.; Warchol, J.; Perna, M.; Chandran, R. 10th Int. Conf. Fluidized Bed Combust. San Francisco 1989, 23-29. (8) Shiao, S-Y.; Warchol, J.; Botros, P. 11th Int. Conf. Fluidized Bed Combust. Montreal, Canada 1991, 1183-1190. (9) Shiao, S-Y.; Perna, M.; Sutherland, D.; Rowley, D.; Daw, C. 12th Int. Conf. Fluidized Bed Combust. San Diego, CA 1993, 661-670. (10) Horvath, A., Jahkola, A.; Hippinen, I. Finnish-Swedish Flame Days 1990, Turku, Finland. (11) Turnbull, E.; Kossakowksi, E. R.; Davidson, J. F.; Hopes, R. B.; Blackshaw, H. W.; Goodyear, P. T. Chem. Eng. Res. Des. 1984, 62 (4), 223-234. (12) Wallman, P.; Carlsson, R. 11th Int. Conf. Fluidized Bed Combustion, Montreal, Canada, 1991, 1517-1522.
Energy & Fuels, Vol. 10, No. 2, 1996 349
significant effect on the combustion rate for a mediumvolatile bituminous coal over a wide range of practical FBC conditions. The particle temperature measured in their tests was relatively low and also independent of pressure. As a conclusion from the available results in the literature, considerable work remains to be done to understand the related details under PFBC conditions. Background and Objectives Research on PFBC has been carried out at the Helsinki University of Technology since the mid-1980s. The work, done at the Otaniemi PFBC test rig, has been concentrated on the combustion and emission behavior using various solid fuels. In addition, for detailed investigation on fundamental phenomena a laboratoryscale PFBC/G batch reactor was introduced to perform the batch combustion under well-defined operating conditions, which are usually limited at the PFBC test rig in continuous operation. In the previous work in the PFBC batch reactor a Polish bituminous coal and a Finnish peat were used to study the combustion behavior over the pressure range of 2-16 bar.10 The results showed that the increase of char burning rate at higher pressure led to an apparent decrease of the burnout time. However, the effect of pressure on devolatilization was not remarkable for bituminous coal, and the burning rate of volatiles decreased with increasing pressure for peat. It was also observed that the O2 depletion plays an important role in batch combustion especially during devolatilization.13 The behavior of O2 depletion differs for different fuels. The effect of pressure on batch combustion can be largely attributed to its influence on O2 depletion and the profile of O2 concentration during the burning process. In addition, when the fluidizing gas was replaced with pure nitrogen under pyrolysis conditions, pressure showed more pronounced influence for the fuels of high volatiles.13 This work is a part of the LIEKKI-2 project, “Partial gasification of fuel and combustion of residual char in pressurized fluidized-bed”, supported by the Finnish national combustion and gasification research program. It aims at the experimental studies on the fuel reactivity and the formation of nitrogen oxides under pressurized FBC conditions. The combustion characteristics and fuel nitrogen (fuel-N) conversion are compared during devolatilization and char oxidation using 10 different fuels, from high-volatile peat to relatively low-volatile coke. A special focus on pressure effect, involving both oxygen partial pressure and total pressure, was pursued for three fuels. Oxygen partial pressure was set with changing either O2 concentration in the fluidizing gas or system pressure, and the total pressure with changing O2 concentration and system pressure simultaneously in order to keep oxygen partial pressure in constant. The effects of other parameters, such as bed temperature, fluidizing velocity, and particle size, on the behavior of combustion and fuel-N conversion are also investigated in detail. The results of the work are divided in two parts. Part 1, focusing on volatile release and char reactivity, is presented in this paper, and part 2, fuel-N conversion to nitrogen oxides, such as NO, (13) Lu, Y.; Hippinen, I.; Jalovaara, J.; Jahkola, A. Report 55, Otaniemi, 1993, Helsinki University of Technology, 90p.
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Lu Table 1. Properties of the Fuels
fuel
moisture (wt %)
volatile (wt % db)
ash (wt % db)
C (wt % db)
H (wt % db)
N (wt % db)
S (wt % db)
O(diff) (wt % db)
peat B peat A lignite Iowa coal Dawmill coal Illinois No. 6 coal Kiveton Park coal Harworth coal Polish coal coke
14.3 17.9 11.5 17.8 3.5 3.8 4.3 1.8 2.6 0.2
72.5 69.6 53.2 43.2 34.3 33.9 30.1 29 28.4 1.7
2.5 3.6 4.6 7.5 9.8 10.6 21.3 21.8 17.4 11.1
52.3 54.6 64.6 69.8 73.2 68.7 63.7 65.1 66.3 86.8
5.7 5.7 4.6 4.5 4.7 4.4 4.2 4.4 4.0 0.2
0.9 1.4 0.7 0.9 1.4 1.3 1.4 1.5 1.1 1
0.1 0.2 0.4 0.5 1.6 2.4 2.0 2.1 1.2 1.0
38.5 34.5 25.1 16.8 9.3 12.6 3.4 5.1 10 0.1
NO2, and N2O, can be found elsewhere.14 The results presented are enable of an increased understanding of the mechanisms for the fuel batch combustion and the development of kinetic modeling. Experimental Section Fuel Selection. Ten different fuels were selected in a wide fuel-rank range, including two relatively high-volatile Finnish peats, one Germany lignite, one US subbituminous coal (Iowa coal), five bituminous coals (Dawmill, Kiveton Park, and Harworth from UK, Illinois No. 6 from US and Polish coal), and one extremely low volatile Norway coke. Table 1 presents the properties of these fuels. Three of them, lignite, Illinois No. 6, and Kiveton Park coal, were selected for studying the effects of particle size and other parameters on combustion characteristics. The particle size of the analyzed fuels in Table 1 fell within the sieve range of 1.0-1.25 mm (except for coke particle size, 0.71-1.0 mm), which is the size used for comparison of the effects of fuel type and operating parameters. As also analyzed, the content of C and ash for different particle size (from 0.55 to 2.19 mm in average) change somewhat but irregularly, and contents of H and N are almost constant in different batch particle size. Test Facility. The test facility includes a separate pressure vessel around an electrically heated reactor tube, which is a stainless-steel cylinder with an inside diameter of 60 mm and height of 1.2 m above the perforated steel distributor. There are four pairs of electrical heating elements, surrounding the inside reactor tube along the tube height, to preheat the fluidizing gas and to keep the well-defined temperatures in the bed and freeboard area. The heating elements are covered by a thick layer of thermal insulation material, located inside of the pressure vessel. The reactor has a maximum operation pressure of 2.0 MPa and a maximum bed temperature of 950 °C. The flow diagram of the reactor and its technical data are shown in Figure 1. Test Operation. Test operation is generally divided into two steps. The first step is to set combustion conditions, i.e., to heat the precharged bed to the desired bed temperature and then to pressurize the reactor to the desired pressure. The fluidizing gas (a mixture of oxygen and nitrogen), after passing through the rotameters, is mixed and heated before entering the reactor. The preheated gas is passed through the perforated steel air distributor to the bed to warm up the bed material. The thermocouples along the reactor, both inside and outside the reactor tube, are used to control the desired bed temperature. In this study, a silica sand with a sieved size of 0.25-0.5 mm was used as bed material; the static bed height was approximately 250 mm with sand of 1.5 kg. After passing through the freeboard, the hot gas is cooled by a heat exchanger before the pressure let-down valve. The second step is the batch fuel feeding and the procedural measurements during combustion. When the reactor is set to the desired conditions, a small charge of fuel, i.e., fuel batch, is dropped onto the bed by means of a pressurized simple (14) Lu, Y. Energy Fuels 1996, 10, 357-363.
Figure 1. Schematic diagram of the PFBC batch reactor. lockhopper (Figure 1). At an elevated pressure in the lockhopper the batch sample can be mixed with sand bed and heated up very rapidly within the bed, which means a relatively high heating rate for the fuel batch. The flue gas sample is extracted from the top of freeboard. The gaseous components, such as O2, CO, CO2, CH4, NOx and SO2, are measured with conventional analyzers. The analyzed results are collected to be reconstructed in situ as combustion profiles on the monitor and saved every 2 s by means of a data logging system. Process of Devolatilization and Char Oxidation. During the combustion of the fuel batch, devolatilization and char oxidation happens in series, and it should be possible to separate the combustion of volatiles and char from the timeresolved measurement in the concentrations of the reaction products. Figure 2 shows the release of carbon products as a function of burning time in burning Illinois No. 6 coal. In the early stage of devolatilization, carbon release increases rapidly to reach the peak and then decreases rapidly until a turning point is reached. The decrease in carbon release is more gradual beyond this turning point where char oxidation predominates. The turning point can be used to differentiate between devolatilization and char oxidation, and thus volatile-C amount, devolatilization rate, char burnout time, and char reactivity can be derived as individual particle combus-
Devolatilization and Char Oxidation. 1
Energy & Fuels, Vol. 10, No. 2, 1996 351 Table 2. Parameters Investigated and Their Ranges parameter
range 650, 750, 850,a 900 1, 3,a 6, 10
bed temperature, Tb, °C oxygen in fluidizing gas, O2, %v/v system pressure, p, bar total. pressure, P, (p, bar; O2, % v/v) fluidizing velocity, Vf, m/s particle size, dp, mm a
5, 8, 11a 5 (5, 6.6), 8 (8, 4.13), 11 (11, 3)a 0.14, 0.24,a 0.34 0.5-0.59, 0.71-1.0, 1.0-1.25,a 1.25-1.6, 2.0-2.38
The fixed value when the other parameters are changed.
∫ w{[CO ] + [CO] + [CH ]} dt t
carbon mass balance
Figure 2. Typical combustion profiles of C release and O2 concentration with Illinois No. 6 coal.
Figure 3. Temperature profiles vs burning time at base testing conditions with Illinois No. 6 coal. tion. The corresponding O2 concentration in the off-gas is also shown in Figure 2, which indicates another method to determine the turning point between devolatilization and char oxidation. The oxygen minimum occurs during the rapid initial burn of the volatiles and is dependent on the fuel properties and operating conditions. Test Conditions and Calculation. Combustion characteristics for 10 fuels were compared at a pressure of 11 bar and bed temperature of 850 °C. The fluidizing gas is a mixture of 3% v/v O2 and 97% v/v N2 at a velocity of 0.24 m/s, which corresponds to a residence time of 5 s in the reactor before entering the gas sampling probe. These operation settings are called the base testing conditions. Figure 3 shows the typical temperature profiles in burning Illinois No. 6 coal under the base testing conditions and indicates that the reactor temperatures are kept in constant very well in the whole process except a slight increase during devolatilization. The effects of other parameters, such as O2 concentration, pressure, bed temperature, gas velocity, and batch particle size, on carbon conversion and char reactivity were investigated in detail. Table 2 lists the parameters and their operating ranges. The tests for every fuel were performed three times under the same conditions, and most of them were reproducible. The data presented below are the average values of replicate tests. The carbon mass balance for the combustor is calculated from (1) the carbon release as CO2, CO, and CH4 in the offgas, and (2) the carbon content in fuel batch. The values are determined as
0
2
4
mcxc/Mc
,%
where w is gas flow rate in mol/s, [CO2], [CO], and [CH4] are the concentrations of the carbon species in the off-gas, mc is the weight of the fuel batch in g, xc is the weight fraction of carbon in the fuel in wt %, and Mc is the atomic weight of carbon in g/mol. A relatively good carbon mass balance, within 100 ( 15%, was achieved in most of the tests and all the results presented in this paper are selected from these tests. The error sources of carbon mass balance are mainly caused by (1) the difference of the carbon content between the analysis samples and the tested batches, (2) the missed carbon in other hydrocarbons, such as C2H2 and C2H4, particularly during devolatilization, and (3) the error from the gas analyzer. In this paper, devolatilization time is defined as tv (Figure 2) and devolatilization rate Rv as carbon release rate in mol C/s, and volatile-C is the fraction of carbon released during devolatilization in the form of CO2, CO, and CH4. Char burnout time τ is defined as the time from the turning point to 96% of the total released carbon is obtained. Char reactivity Rc is defined as the average char burning rate until 96% of the total carbon released. The burning rate (R) is expressed as
R)
1 dw , g/(g s) Wc dt
where WC is the carbon weight remaining in the fuel at time t. dW is the amount of fuel carbon release during dt. The relative influences of parameters are compared to a power correlation between the burnout time τ and the parameter. It is
τ (τ0,96) ) aXj-n, s where Xj is the influencing factor, such as oxygen concentration (O2), oxygen partial pressure (pO2), and bed temperature (Tb) etc., a is a regression constant, and n is the power of the correlation.
Results and Discussion Volatile Release and Char Reactivity. Figure 4a shows the burnout curves of various fuels, and their corresponding O2 profiles in the off-gas are compared in Figure 4b. Table 3 presents the main results for all fuels under the base testing conditions. The results indicate that devolatilization rate and burnout time are mainly a function of fuel rank for the selection of fuels in this study. Roughly, both volatile and char burning rates increase for the fuels containing higher volatiles, leading to shorter burnout time. The shortest burnout time was observed with peat B (63 s) and the longest one with coke (317 s). The power (n) of the correlation between burnout time and volatile content in the fuel
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Figure 4. Combustion profiles with various fuels: (a) batch burnout vs burning time and (b) profiles of O2 concentration in off-gas as a function of carbon conversion. Table 3. Main Results of Combustion Characteristics at Base Testing Conditionsa fuel
Rv (mol C/s)
Rc (g/(g s))
tv (s)
τ (s)
Cv (%)
CCO2 (%)
CCO
CCH4 (%)
peat B peat A lignite Iowa coal Illinois No. 6 coal Dawmill coal Kiveton Park coal Harworth coal Polish coal coke
0.14 0.13 0.11 0.11 0.08 0.10 0.08 0.09 0.07
0.061 0.045 0.025 0.029 0.022 0.014 0.013 0.012 0.016 0.010
17 17 17 15 19 18 19 19 18
62 75 127 128 157 234 241 298 213 316
66 57 35 36 27 29 30 30 25 0
70 71 87 84 92 88 90 88 91 92
12 11 5 6 5 4 4 6 5 8
20 19 8 10 4 8 6 6 4 0
a R , R ) devolatilization rate and char reactivity, t , τ ) devolatilization time and burnout time, C ) volatile-C, C v c v v CO2, CCO, CCH4 ) CO2, CO, and CH4 contribution in carbon release. Tb ) 850 °C, p ) 11 bar, O2 ) 3% v/v, and Vf ) 0.24 m/s, Gf ) 5.28 mol/min.
Figure 5. Devolatilization rate and char reactivity as function of volatiles in the fuels.
gives 0.874. No clear dependence of burnout time on volatiles was observed for five bituminous coals, in the volatile range of 28-34%. In the case of the combustion of these coals, the highest devolatilization rate was obtained with Dawmill coal and the highest char burning rate with Illinois No. 6 coal. Devolatilization rate, Rv, and char reactivity, Rc, as a function of volatile content in the fuels are shown in Figure 5. The chars, which are produced from higher volatile parent fuels, have higher reactivity compared with the chars from lower volatile fuels. This observation is assumed to be
due to low content of mineral materials and high porosity in the remaining char from higher volatile fuels. One exception was observed for Iowa coal, which contains lower volatiles but shows greater reactivity than lignite. This phenomenon is probably attributed to a higher moisture content and different ash composition in Iowa coal compared with those in lignite (Table 1). Higher moisture may produce more reacting surface for further oxidation after devolatilization and result in the increase of burning rate. As shown in the Table 3, the process of devolatilization completes around 15-19 s for all tested fuels. The shortest time was obtained with Iowa coal. The minor difference could be attributed to the different primary fragmentation, which is affected by fuel type.15 This conclusion that devolatilization time is independent of fuel type agrees well with the literature data.2,16 Volatile-C amount also exhibits difference markedly for various fuels, and increases with volatile matter in the fuel. It was also noted that the fraction of carbon released from volatiles was lower than the volatile content (wt %) in proximate analysis (Table 1), especially for the low-rank fuels. It implies that char-C content is somewhat higher than its parent fuel-C content under typical PFBC conditions. The major carbon compound in the off-gas was carbon dioxide, the total amount of which increased for higher rank fuels. The amount of carbon monoxide and hydrocarbons, such (15) Chirone, R.; Massimilla, L. Powder Technol. 1989, 57, 197. (16) Stubington J. F., Chui, T. Y. S.; Saisithidej, S. Fuel Sci. Technol. Int. 1992, 10 (3), 397-419.
Devolatilization and Char Oxidation. 1
Figure 6. Profiles of burning rate as a function of fractional carbon conversion at different oxygen partial pressures in changing O2 concentration in the fluidizing gas.
as methane, increased with decreasing fuel rank, particularly released from the devolatilization. Effect of Pressure. Oxygen Partial Pressure. When the system pressure is fixed, increasing O2 concentration in the inlet gas mixture is one of the ways of increasing O2 partial pressure in the reactor. Figure 6 shows the profiles of burning rate as function of fractional carbon conversion at O2 concentration of 1, 3, 6, and 10% v/v. The main results of the influences of the oxygen partial pressure and total pressure are summarized in Table 4. O2 concentration in the inlet gas mixture has been proved to be a very dominant parameter in both processes of devolatilization and char oxidation since oxygen partial pressure is the major driving force for O2 transfer from bulk gas to particle surface and the diffusion in the particles. The increases of devolatilization rate and char burning rate at higher O2 concentration, seen in Figure 6, lead to considerable decrease of burnout time. The orders (n) of the correlation between burnout time and O2 concentration give 0.874 for lignite, 0.859 for Illinois No. 6 coal, and 0.743 for Kiveton Park coal. The orders, close to 1, indicate that the burning rate is mainly controlled by oxygen transfer and diffusion rather than chemical kinetics at the examined conditions.3 It also indicates that the chemical reactions play a more significant role in affecting the overall combustion rate for lower volatile fuel (Kiveton Park coal) compared to higher volatile one (lignite). Increase of volatile-C amount was also observed at higher O2 concentration, seen in Table 4. The result could be attributed to higher peak temperatures usually achieved by increasing O2 concentration in devolatilization,17 which minimizes the secondary reactions of certain reactive species and therefore increases volatile-C amount. A similar result was obtained at atmospheric pressure by Hayhurst and Lawernce,18 who found that the fraction of fuel-C evolved with volatiles increased roughly 10% when air was used instead of N2 as fluidizing gas. The volatile-C amount at 10% v/v O2 might be lower than the actual value because the transition from devolatilization to char oxidation, turn(17) Ross, I. B., Patel, M. S.; Davidson, J. F. Trans. Inst. Chem. Eng. 1981, 59, 83-88.
Energy & Fuels, Vol. 10, No. 2, 1996 353
ing point, was not clear enough from the profiles of offgas concentration (Figure 6). Another interesting observation was that the effect of O2 concentration on char oxidation was more significant than on volatile release. There is no marked difference of the peak devolatilization rate for various O2 concentration, except a little bit lower one at O2 ) 1% v/v. The secondary way to estimate the effect of oxygen partial pressure in this study was to change system pressure at a constant O2 concentration of 3% v/v. Figure 7 shows burning rate versus the fraction of carbon conversion at the pressure of 5, 8, and 11 bar absolute. Like the case in changing O2 concentration, the rates of volatile release and char oxidation increased with increasing oxygen partial pressure, leading to faster coal burnout at higher pressures. However, there was difference between the influences of pressure and O2 concentration on combustion behavior. In comparison to O2 concentration, the effect of pressure was much more pronounced at increasing the devolatilization rate but was weaker at char oxidation rate. Higher system pressure increases not only the oxygen partial pressure, like O2 concentration, and also the partial pressures of the gaseous products from volatiles. Both of them accelerate the chemical reactions in volatile combustion, leading to higher devolatilization rate, and thereby the influence of system pressure is pronounced. Regarding the influence on char oxidation, however, higher system pressure increases only oxygen partial pressure, accelerating oxidation reactions as O2 concentration does, but unlike O2 concentration, pressure has virtually no influence on the driving force for oxygen transfer to particles and diffusion in particle pores because the higher oxygen partial pressure is offset by a proportional lowering of the oxygen diffusion coefficient.4 In addition, elevated pressure increases the resistance of gaseous products diffusion from particles, which has the negative effect on char combustion in more or less, and thereby the improvement of batch burning is weakened in increasing the pressure comparing to the O2 concentration in the inlet gas mixture. The orders (n) of the correlation between char burnout time and oxygen partial pressure in changing system pressure are 0.394 for lignite, 0.474 for Illinois No. 6 coal, and 0.327 for Kiveton Park coal, which are less than half of those in changing O2 concentration. It implies that the increase of oxygen transfer and diffusion is more remarkable than the acceleration of chemical reactions; i.e., oxygen transfer and diffusion predominate in char burning process at the examined conditions. It differs from the control mechanisms in volatile combustion, which is a chemical rate controlled process and can significantly be improved by system pressure contributing to the increase of partial pressure of oxygen and gaseous volatile products. This assumption has been proved by changing system pressure and O2 concentration together to keep constant oxygen partial pressure. The results are showed below. Like the increase of O2 concentration, greater volatile-C amount was obtained at higher pressure. It differs from the observation from other investigators,5 who reported that with increasing pressure the residence time of volatiles within the particles increased and volatiles amount decreased. One explanation for this phenomenon is that higher system pressure leads
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Table 4. Main Results of Pressure Effect on Combustion Characteristicsa fuel O2 (% v/v) Rv (mol C/s) Rc (g/(g s) tv (s) τ (s) Cv (%) p (bar) Rv (mol C/s) Rc (g/(g s) tv (s) τ (s) Cv (%) P (O2, % v/v) Rv (mol C/s) Rc (g/(g s)) tv (s) τ (s) Cv (%) a
lignite 1 0.06 0.010 20 326 26 5 0.07 0.020 20 167 30 5(6.6) 0.09 0.044 17 88 35
3 0.11 0.025 17 128 35 8 0.09 0.024 18 139 35 8(4.1) 0.10 0.034 19 107 35
6 0.12 0.059 15 66 39 11 0.11 0.025 17 128 35 11(3.0) 0.11 0.025 17 128 35
Illinois No. 6 coal 10 0.14 0.101 14 47 41
1 0.05 0.007 23 428 23 5 0.05 0.015 22 232 21 5(6.6) 0.07 0.033 18 113 26
3 0.08 0.022 19 158 27 8 0.06 0.019 21 187 23 8(4.1) 0.07 0.025 19 142 25
6 0.10 0.044 16 88 29 11 0.08 0.022 19 158 27 11(3.0) 0.08 0.022 19 158 27
Kiveton Park coal 10 0.11 0.075 15 63 30
1 0.06 0.006 23 516 26 5 0.06 0.013 21 310 23 5(6.6) 0.08 0.023 19 150 29
3 0.08 0.013 19 242 30 8 0.07 0.012 21 271 28 8(4.1) 0.08 0.021 19 170 29
6 0.09 0.023 19 148 33 11 0.08 0.013 19 242 30 11(3.0) 0.08 0.013 19 242 30
10 0.10 0.036 17 100 35
Rv, Rc ) devolatilization rate and char reactivity, tv, τ ) devolatilization time and burnout time, Cv ) volatile-C.
Figure 7. Profiles of burning rate as a function of fractional carbon conversion at different oxygen partial pressures in changing system pressure.
Figure 8. Profiles of burning rate as a function of fractional carbon conversion at different total pressures.
to an increase of peak temperature, and the increase of volatile-C by minimizing secondary reactions at higher peak temperature is more pronounced than the decrease by longer residence time within the particles. Total Pressure. O2 concentration in the fluidizing gas and system pressure were changed simultaneously in order to find the effect of total pressure at a constant oxygen partial pressure (pO2 ) 0.33 bar). Therefore, when the system pressure was changed from 5 to 8 to 11 bar absolute the O2 concentration corresponded from 6.6 to 4.13 to 3% v/v. The main results in changing total pressure are also presented in Table 4. Figure 8 shows the profiles of burning rate as function of fraction of carbon release with Illinois No. 6 coal. It is interesting to note that the effect of total pressure differs from oxygen partial pressure on coal combustion. Unlike the influence of oxygen partial pressure, the volatile-C amount and devolatilization time did not change remarkably with increasing total pressure. With increasing total pressure, seen in Figure 8, the burning rate during the devolatilization increases as oxygen partial pressure increased; however, the burning rate during char oxidation decreases in contrast to the trend at higher oxygen partial pressure, leading to longer burnout time.
These results agree well with the above results with respect to change in O2 concentration in the fluidizing gas and pressure separately, i.e., comparing to pressure, O2 concentration has much stronger influence on char reactivity, but weaker on devolatilization rate. At the fixed oxygen partial pressure, the orders (n) of correlation between burnout time and O2 concentration give 0.5 for lignite, 0.45 for Illinois No. 6 coal and 0.58 for Kiveton Park coal, which are considerably smaller compared to an order of 0.85 in changing only O2 concentration. This phenomenon, the influence of O2 concentration on coal combustion weakening in changing total pressure, was attributed to the lowering of driving force of oxygen transfer and chemical reactions of char burning at constant oxygen partial pressure. Effects of Other Parameters. Temperature. Figure 9 shows burning rate as a function of carbon conversion in burning lignite at a bed temperature of 650, 750, 850, and 900 °C. Bed temperature clearly affected the process of volatile release. Higher devolatilization rate was achieved with increasing temperature, which can be explained by that combustion of volatiles involves a group of gas-phase reactions that are accelerated at higher temperatures; thereby the devolatilization rate since the process is controlled by chemical kinetics. The enhancement of heat transfer
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Figure 9. Profiles of burning rate as a function of fractional carbon conversion at different bed temperatures.
Figure 10. Profiles of burning rate as a function of fractional carbon conversion at different gas velocities.
at higher temperature may also attribute to the increase of volatile release because heat transfer was found to be an important mechanism in the process.1,2 An increase of volatile-C amount was obtained with increasing bed temperature, which can be assumed that the devolatilization rate increased at higher bed temperature, resulting in higher heating rate and also peak temperature, and hence volatile-C amount since volatile yield is a function of heating rate and peak temperature. However, a minor change in char burnout time and burning rate was observed at different bed temperatures, which further indicates that char oxidation at PFBC conditions is mainly controlled by mass transfer rather than chemical kinetics. These findings were also reported by Hayhurst and Lawernce when the bed temperature increased from 750 to 900 °C.18 In this study, the results of bed temperature affecting char reactivity and burnout time might be weakened by the simultaneous decrease of the amount of gas flow with increasing bed temperature to keep the constant fluidizing velocity. Gas flow amount had a great contribution to combustion characteristics, as did in changing system pressure and fluidizing velocity. Higher gas flow inputs more oxygen to the reactor, leading to higher combustion rate. The results showed that the burnout time decreases significantly with increasing gas flow (the details of the effect of gas velocity will be discussed below). At various bed temperatures the combustion profiles were very similar, but the distribution of carbon species in the off-gas is different. Considerably higher concentrations of hydrocarbons and carbon monoxide were observed with decreasing bed temperature due to the lowering of oxidizing reactions, particularly in devolatilization. Fluidizing Velocity. Figure 10 shows the burning rate profiles versus fractional carbon conversion at three fluidizing velocities of 0.34, 0.24, and 0.14 m/s. An apparent increase of devolatilization rate and char reactivity, resulting in shorter burnout time, was achieved at higher fluidizing velocity. It can be explained by the enhancement of both mass transfer and heat transfer, including the transfer between particles
and between particles and bubbles based on the twophase theory in fluidizing bed. At higher velocity the increase of volatile release rate is mainly attributed to the enhanced heat transfer, and hence the increase of batch heating rate and peak temperature. The improvement of char burning is attributed to the enhanced mass transfer, including oxygen reaching the fuel particles and oxygen diffusion in particle pores and the products leaving the particles. Also, the increase of volatile-C amount was obtained at higher fluidizing velocity. The effect of gas velocity on volatile-C amount might contribute to the influence on batch heating rate and peak temperature, by the higher burning rate of volatiles and thereby the secondary reactions of volatile release. Higher CO concentration was obtained at higher velocity. This phenomenon indicates that the further oxidation of CO is occurring at the freeboard and dependent on the residence time. The orders (n) of the correlation between burnout time and fluidizing velocity give 0.31 for lignite, 0.41 for Illinois No. 6 coal, and 0.7 for Kiveton Park coal. It suggests that the influence of gas velocity on char reactivity is considerable and more pronounced for lower volatile coal. Particle Size. Figure 11 shows the burning rate profiles as a function of carbon conversion with five particle sizes of 2.0-2.5, 1.25-1.6, 1.0-1.25, 0.71-1.0, and 0.5-0.59 mm. As expected, the larger particle burned considerably slower than the smaller particle one. It was noted that both devolatilization time and char burnout time decreased for smaller particles, and the effect of particle size on char oxidation was more significant than on volatile release (Figure 11). These observations are in good agreement with the above conclusion that chemical rate control is dominant in devolatilization and mass transfer in char burning rate. It means that the devolatilization time should be proportional to the particle diameter (∝d) and the char burnout time to the square of the particle diameter (∝d2).3 Under the examined conditions the power of the correlation was only 0.4 between devolatilization time and particle size and ∼1.15 between burnout time and particle size for all tested fuels. This phenomena could mainly be attributed to the development of particle
(18) Hayhurst, A. N.; Lawrence, A. D. Combust. Flame 1995, 100, 591-604.
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Figure 11. Profiles of burning rate as a function of fractional carbon conversion of different particle sizes.
cracks, fragmentation, and attrition in the fluidized bed combustor. On the other hand, this order also indicates that particle size is the greatest factor affecting the char burning rate, compared with the orders of correlation between burnout time and other parameters. It was also observed that batch size does not affect the volatile-C amount and the contribution of carbon oxides in the off-gas. A considerably lower concentration of hydrocarbons was only observed for the biggest batch, probably attributed to the change of combustion behavior for different particle sizes. A low carbon mass balance (65-76%) was observed for the smallest batch and could be attributed to the elutriation of char particles; i.e., the gas velocity was too high for the smallest batch to complete combustion in the reactor. Further work is currently being undertaken to ascertain the influence of the pressure and other operating parameters, such as bed temperature and batch size, on the PFB combustion of various chars, including gasification residues.
Concerning the volatile release and char reactivity, the following conclusions can be made under the examined pressurized fluidized bed combustion conditions in the laboratory batch reactor: It was successful to study the combustion behavior during devolatilization and char oxidation separately by means of a batch reactor. It has been proved that under the examined PFBC conditions, devolatilization is mainly controlled by chemical kinetics and char burnout by mass transfer. Devolatilization rate and burnout time were strongly dependent on the nature of fuel. However, devolatilization time was independent of the fuel type. High char reactivity was obtained for the chars produced from the parent fuels containing high volatiles. Combustion of batch fuel can be improved by increasing oxygen partial pressure, i.e., increasing either O2 concentration in the fluidizing gas or system pressure. Pressure, affecting the O2 driving force and products diffusion, showed more complicated influence on combustion behavior than O2 concentration. In the tested ranges of pressure and O2 concentration, pressure showed a stronger effect on devolatilization than did O2 concentration, but a weaker effect on char burnout. Total pressure showed a minor influence on combustion characteristics compared to oxygen partial pressure. Increasing total pressure increased the devolatilization rate but decreased the char reactivity, resulting in a significantly long burnout time. Acknowledgment. The research work presented in this paper is a part of the LIEKKI-2 national combustion research program. The work was financed by the Ministry of Industry and Trade of Finland, A. Ahlstrom Oy, Imatran Voima Oy, and Helsinki University of Technology. The author gratefully acknowledges the personnel of the author’s laboratory. EF9501152
