Coal Gasification Characteristic in a Pressurized Fluidized Bed

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Energy & Fuels 2003, 17, 1474-1479

Coal Gasification Characteristic in a Pressurized Fluidized Bed Jiejie Huang,* Yitian Fang, Hanshi Chen, and Yang Wang Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China Received March 5, 2003. Revised Manuscript Received July 29, 2003

Pressurized coal gasification is not only the key part of the Integrated Gasification Combined Cycle (IGCC) but also the better approach to increase the gasification reaction rate and capacity of the unit gasifier. In this work, the gasification characteristics for a sub-bituminous coal with an air-steam mixture at the gage pressure of 0.5-1.4 MPa and the temperature of 880-980 °C in a 200 mm inside diameter pressurized fluidized bed gasifier have been investigated. Results show that the gasification process is dominant in the transition zones between chemical reaction (or pore diffusion) and gas film diffusion at the pressure of 1.0 MPa. For the sub-bituminous coal tested, the increase of the capacity of gasifier is almost in linearity with pressure up to 1.2 MPa, and then it slows down because of transferring to gas diffusion of gasification reaction. In addition, the effect of pressure on the gas composition and the size distribution of bed holdup and fly ash are discussed in detail.

1. Introduction Coal as a kind of fossil fuel will still play an important role in the world energy supplies with the global economic growth. However, there has also been increasing environmental concern related to the coal utilization, for example, pollutant emissions and CO2 emissions. Clean coal technology is one of the ways to solve this conflict. Coal gasification can convert coal into clean gaseous fuel to generate electricity or synthesize chemicals, which can also increase the total coal utilization efficiency and reduce the CO2 emission. Integrated Gasification Combined Cycle (IGCC) with precombustion removal of CO2 can compete with Natural Gas Combined Cycle (NGCC) with post combustion CO2 removal.1 Fluidized bed coal gasification with dry ash discharging, which was first industrialized with the Winkler process, has been further developed with ash agglomerating gasifiers (U-gas, KRW, ICC-China). The feedstock has been enlarged from higher reactive lignite to lower reactive bituminous coal and anthracite. Compared with the entrained flow gasifier, the fluidized bed gasifier has the advantages of more flexible feedstock for a wide range of ash content or fusion temperature, lower cost of gasifier construction and maintenance, and lower oxygen consumption; however, it has the disadvantages of difficult scale-up, relative lower capacity, and carbon conversion. For improving the fluidized bed process it is necessary to get an in-depth understanding of coal gasification under pressure that could increase the unit gasifier capacity and carbon conversion. The pressurized coal gasification was regarded as one of the * Corresponding author. Tel.: +86-351-2021137. Fax: +86-3514041153. E-mail: [email protected]. (1) Holt, N. A. H. Coal Gasification Research, Development and DemonstrationsNeeds and Opportunities. Presented at the Gasification Technologies Conference, CA, Oct. 10, 2001.

breakthroughs of coal gasification in the 20th century.2 In this work, the gasification results for a sub-bituminous coal are reported, which was operated at the gage pressure range of 0.5-1.4 MPa for air-steam blowing in a 200 mm i.d. gasifier. 2. Experimental Section Figure 1 shows the schematic diagram of the coal gasifier. The gasifier has a 200 mm i.d. at the bottom part and 300 mm at the upper freeboard, and it can operate at pressures of 0.5-1.4 MPa. The gasification system consists of coal preparation, air compressors, gasifier, cyclones, waste heat boiler, and instrumental controller, etc. For the tests, the coal was crushed to the size of less than 3 mm and then dried in a rotary dryer to the moisture of coal of less than 5%. The dried coal was stored in a coal tank. In the test, the dried coal was elevated to the lock coal hoppers by a bucket elevator from the coal tank and entered the gasifier by air-blowing controlled by a screw feeder. The gasifying agents entered the gasifier through a reverse conical gas distributor and bottom ash discharging duct in which bigger and heavier particles dropped down to the ash hopper controlled by the gas velocity. The fly ash (fine char) carried by hot gas out of the gasifier was separated by two cyclones in series and collected in the hoppers. The gas was cooled in a waste heat boiler and a heat exchanger between cold air and hot gas to 200 °C, then depressurized and cooled again in a water scrubber. The gas was sampled at an interval of half an hour and analyzed by gas chromatogram.

3. Results and Discussion 3.1. Coal Analysis. The analysis of coal used is shown in Tables 1 and 2. A Chinese Fugu coal tested belongs to a weak-caking, long-flame coal with the character of low sulfur, low ash, and low ash melting point. (2) Tang, H.-q. J. Fuel Chem. Technol. (Chinese) 2001, 29 (1), 1-5.

10.1021/ef030052k CCC: $25.00 © 2003 American Chemical Society Published on Web 09/12/2003

Coal Gasification in a Pressurized Fluidized Bed

Energy & Fuels, Vol. 17, No. 6, 2003 1475

Figure 1. Schematic diagram of coal gasifier. Table 1. Proximate and Ultimate Analysis of Fugu Coal (air-dry base, wt %) proximate analysis moisture ash volatiles Fugu coal

4.32

9.22

29.32

ultimate analysis C

H

St

O

N

63.52 4.93 0.40 16.69 0.92

3.2. Fluidization Velocity under Pressure. Under the experimental conditions of the gasifier [with a pressure of 0.5-1.4 MPa (gage pressure) and a temperature of 880-980 °C], the superficial gas velocity of the dense bed of the gasifier was operated in the range of 0.8-1.0 m/s. Figure 2 shows the calculated minimum fluidization velocity (Umf) as a function of operating pressure based on the equations in the literature3 and real operating velocity. The average particle size of bed holdup is about 0.86 mm, and the maximum particle size is 3 mm. It can be noted that for the particle size of 3 mm, the effect of pressure on Umf is great; however, for the smaller size particle (dp ) 0.86 mm) the effect of pressure is little. It is found that the operating gas velocity, which is much larger than the Umf of the average particle size of bed holdup and also larger than Umf of maximum particle size, could make good fluidization without local slagging in the bed and other problems. 3.3. Effect of Pressure on the Coal Gasification Reaction. The effect of pressure on the coal gasification reaction has been widely investigated. Groenevold et al.4 studied the C-H2O-CO2 reaction. An empirical equation was derived:

-RA ) kCs(CCO2 + CH2O)0.7

concentration of gasifying agent CO2 and H2O to the power 0.7. Another kinetic model is the LangmuirHinshelwood expression:5

(1)

It is based on the volumetric reaction model and shows that the reaction rate is in direct proportion to the

RH2O )

k1PH2O 1 + KH2OPH2O + KH2PH2

(2)

It can be seen that the reaction rate would become a first-order reaction with respect to PH2O if the partial pressure of H2O and H2 in the reactor are very low, but the reaction would become a zero-order with respect to PH2O if the PH2O is very large and PH2 is very low in the reactor. Equations 1 and 2 indicate that the reaction rate will become less dependent on the reactant partial pressure at higher pressure. The reaction rate constant k in eq 1 is a function of temperature usually expressed by Arrhenius form. If the reactant concentration or partial pressure is the same at different temperatures and does not change with time, eq 1 can be simplified as the following form:

dx ) k′e-Ea/RT dt(1 - x)

(3)

where k′ is a combined constant containing concentration items; x is the carbon conversion. The integral of eq 3 will produce

ln(-ln(1 - x)) ) ln k′t -

Ea RT

(4)

For a set of data of x-T with same reaction time t, the activation energy Ea can be obtained from eq 4. Figure 3 shows the effect of temperature on the gasifi(3) Yates, J. G. Chem. Eng. Sci. 1996, 51, 167-205.

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Huang et al. Table 2. Fugu Coal Ash Analysis

ash fusion temperature (°C)

ash composition (wt %)

DT

ST

FT

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3

others

1090

1100

1140

41.29

15.64

17.40

14.34

1.94

0.77

5.60

3.02

Figure 2. Umf (920 °C) and operating gas velocity Uf (912965 °C) at different pressures.

Figure 3. Effect of temperature on the carbon conversion at pressurized gasification (Pg, 1.0 MPa).

cation reaction at the pressure of 1.0 MPa. It can be noted that the carbon conversion increases with increasing temperature, but the apparent activation energy, which results from the regression of ln(-ln(1 - x)) with 1/T as shown in Figure 4, is just 26.12 kJ/mol; that is much less than the data from the experiments using a pressurized thermogravimetric analyzer conducted by Roberts et al.6 who investigated the effect of pressure on intrinsic reaction kinetics of char gasification with H2O and reported that the apparent activation energy of a high-volatile bituminous is 227 kJ/mol and 231 kJ/ mol at pressures of 0.1 MPa and 1.0 MPa, respectively. According to the three characteristics ranges of ratetemperature dependence in the reactions of gases with porous solids, if the diffusion through the pores of the solid influences the rate of reaction, the apparent activation energy is equal to half the true activation energy. If the reaction rate is determined by the diffusion rate through a boundary layer, the apparent activation energy is near zero. Therefore it can be (4) Groenevold, M. J.; van Swaaij, W. P. M. Chem. Eng. Sci. 1980, 35, 307-313. (5) Junten, H. Carbon 1981, 19, 167-173. (6) Roberts, D. G.; Harris, D. J. Energy Fuels 2000, 14, 483-489.

Figure 4. Linear relationship of ln(-ln(1 - x)) with 1/T.

Figure 5. Effect of pressure on the gasification capacity of gasifier.

concluded that the gasification reaction of sub-bituminous coal at temperatures of 900-1000 °C and pressure of 1.0 MPa in the fluidized bed gasifier is in the transition zones between chemical reaction (or pore diffusion) and gas film diffusion. Figure 5 shows the change of the gasification capacity of gasifier with pressure. It can be noted that the amount of carbon gasified per hour is in direct proportion to the pressure to the power ∼ 0.76 up to 1.2 MPa and then to the power 0.55 for the pressure range of 1.2-1.4 MPa. As mentioned above, the process has been influenced strongly at the pressure of 1.0 MPa, and with the further increasing of pressure the reaction rate would be dependent on the gas diffusion rate. For gas diffusion, the diffusion flux is in direct proportion to the concentration gradient and the diffusion coefficient that can be calculated by the Chapman-Enskog formula. The diffusion coefficient is in inverse proportion to pressure to the power one. However, the reactant partial pressure or concentration gradient increases with elevating system pressure. When the process is controlled by external gas film diffusion and the reactant concentration on the particle surface is near zero, the diffusion rate would be in inverse proportion to pressure to the



Figure 7. Effect of air-coal ratio on gas heating value. Figure 6. Carbon content in discharging ash versus gasification pressure.

power 0.5. It indicates that the gasification reaction over 1.2 MPa would become being basically controlled by gas diffusion. Ash is the byproduct of coal gasification. The rate of ash discharging is depended on the coal feed rate and ash content in the coal used. To keep a suitable bed height and ash concentration, the rate of ash discharging from bed would increase with the growth of coal feed rate or operating pressure. Because the gasifier was designed to have an ability to selectively remove the ash from the bed, the ash produced as free state with certain size could be withdrawn from the bed. It can be seen that the carbon content in discharged ash is reduced with pressure as shown in Figure 6, which indicates that increasing pressure could decrease the carbon loss by the ash discharging. 3.4. Product Gas under Pressure. The gasifier was operated in the auto-thermal mode and the temperature is not an independent variable that is related to the feeding air, steam, and coal flow rate. At a constant temperature, as the pressure rises, the gasification rate would increase and superficial gas velocity and bed holdup would decrease. To reach a normal fluidization status in the gasifier it is necessary to increase the coal feed rate and air and steam flow rate. But the ratio of air to coal or steam to coal can be varied for the same gasification temperature, and the composition and yield of gas would be affected. In the experiments it is observed that gas yield increases with air-coal ratio, while the gas heating value decreases as shown in Figure 7. For air-steam blown gasification, an increasing air-coal ratio corresponds to increasing nitrogen content in the product gas, and hence a decrease in gas heating value. In addition, increasing the air-coal ratio the combustion of volatile or produced gas would be enhanced, which would reduce the combustible gas content. Table 3 shows the effect of pressure on the product gas composition. It can be noted that the combustible gas content is a little lower and the content of N2 and CO2 is a little higher at the pressure of 0.53 MPa than that with higher pressures. However, there is no obvious difference of gas composition with the pressures from 0.75 MPa to 1.4 MPa. As we know, the gas composition from the gasifier is a result of coal simultaneous devolatilization, combustion, and gasification. When the

Table 3. Effect of Pressure on the Gas Composition (gasifying temperature, 930 °C) pressure (gage), MPa coal feed rate, kg/h air flow rate, Nm3/h steam flow rate, kg/h dry gas composition, vol % H2 N2 CO CH4 CO2

0.53 61.5 137 37

0.75 106 170 64

1.00 108 227 110

1.40 135 231 89

14.57 52.70 13.88 2.91 15.90

18.04 51.89 13.97 2.93 13.17

18.56 51.44 12.54 2.73 14.74

18.08 51.17 14.30 2.55 13.89

Table 4. Effect of Temperature on the Gas Composition (gasifying pressure Pg, 1.0 MPa) dry gas composition, vol %

904 °C

930 °C

972 °C

H2 N2 CO CH4 CO2

17.57 53.72 8.02 3.59 17.10

18.56 51.44 12.54 2.73 14.74

16.63 50.20 15.14 2.63 15.40

gasifier is operated at higher temperature, there must be a heat loss of equipment to the atmosphere that will increase the consumption of air and produce more N2 and CO2. The ratio of the heat loss of the equipment to the input heat of coal gasified decreases with pressure, and then the effect of heat loss on the gas composition would become insignificant. Table 4 shows the changing of gas composition with temperature at the pressure of 1.0 MPa. It indicates that, with increasing temperature, the N2 content in gas goes down and CO content increases, which can be attributed to the increase of the char-steam gasification rate with temperature that results in the growth of the yield of CO and H2. Methane is of a higher heating value and can contribute a lot for gas heating value. However, it can be found that the content of CH4 in gas does not increase but decreases somewhat with pressure, as shown in Table 3. The content of CH4 ranges from 2.5 to 3.0%. According to the equilibrium of the methanation reaction (CO + H2 ) CO2 + CH4), the equilibrium concentration of CH4 in the air-blown gas should be less than 0.2% at a temperature of 930 °C. That implies that the CH4 would be mainly formed from the volatile cracking. Figure 8 shows the error of the experimental value of K (K ) PH2PCO2/PCOPH2O) to the equilibrium constant (Keq) of the water-gas shift reaction (CO + H2O ) H2 + CO2) at different gasification temperatures. It can be found that the errors for most cases are positive, i.e., K > Keq, and just a few values of the errors are negative.

1478 Energy & Fuels, Vol. 17, No. 6, 2003

Huang et al.

Figure 10. Effect of pressure on the size distribution of holdup material in the bed. Figure 8. The errors of K to Keq versus temperature.

Figure 11. Effect of pressure on the size distribution of fly ash. Figure 9. Size distribution for the coal feed, bed holdup, and fly ash (gasification pressure (gage), 0.75 MPa; temperature, 930 °C). Table 5. Fly Ash Rate Carried by the Gas pressure (gage), MPa coal feed rate, kg/h fly ash rate, kg/kg

0.53 61.5 0.23

0.75 106 0.23

1.00 108 0.22

1.40 135 0.20

Generally, the simulation of the coal gasification process has a big problem in determining the outlet gas composition. Many researchers assume that the gas composition is in equilibrium or with a correcting factor. From Figure 8, it can be seen that the errors of K to Keq have no certain relationship with temperature, and range from 0 to 0.6 for most cases. 3.5. Size Distribution of Particles in the Gasifier and Fly Ash Carry-over. The size distributions of the coal feed, bed holdup, and fly ash for gasification at the pressure of 0.75 MPa are shown in Figure 9. According to the particle terminal velocity estimation, for the particle size of 0.35 mm the Ut is about 0.45 m/s, a value that is almost equal to the gas velocity of upper freeboard of the gasifier. From the data in Figure 9, it can be seen that 77% of fly ash is less than 0.35 mm in size and 93% of bed holdup is within the size from 0.32 mm to 2.5 mm. Table 5 lists the fly ash rates at different operating pressures. It indicates that the fly ash rate ranges from 0.20 kg to 0.23 kg per kilogram of coal feed without fines recycling. The fly ash can be produced in three ways that include coal fines in the feedstock, attrition, and chemical reaction. It is known that the

weight of feeding-coal-size less than 0.5 mm accounts for 29.7%, which means that about 0.19 kg of fly char fines could be produced from a kilogram of coal feed. Huang and Watkinson7 found that particle size reduction is very little when the carbon conversion is less than 60%, but it will become severe when the carbon conversion is over 60%, and that for the average char size of 1.38 mm the char size decreases by more than 45%; however, for a less reactive char with the average size of 0.75 mm the particle size reduction is just about 17% up to a carbon conversion of 90%. That suggests that the particle size reduction only occurs at higher char conversion and becomes difficult for smaller size of char. Thus, it can be seen that most of the fly ash comes from the fines in the feedstock, and the fines produced by char gasification just make up a small part. The effect of pressure on the size distribution of bed holdup and fly ash can be noted from Figures 10 and 11. With an increase of pressure, the peaks of size distribution for bed holdup and fly ash moves toward the large size, which indicates that the char with the particle size of 0.3-0.5 mm is transferred into the fly ash as the pressure increases from 0.75 MPa to 1.4 MPa. That could be a result of the decreasing of particle terminal velocity with pressure and the forming of more cenospherical char during coal pyrolysis at elevated pressures8 that has lower density and smaller terminal velocity. (7) Huang, J.; Watkinson, A. P. Fuel 1996, 75, 1617-1624.


4. Conclusions The gasification experiments with air-steam blowing were performed with a Chinese sub-bituminous coal at the total gage pressure from 0.5 to 1.4 MPa in a fluidized bed. The following conclusions are drawn from the study: (1) The coal gasification reaction at the pressure of 1.0 MPa in the fluidized bed has an apparent activation energy of 26.12 kJ/mol, which infers that the gasification process is greatly influenced by the mass transfer. For the sub-bituminous coal tested, the increase of the capacity of gasifier is almost in linearity with pressure up to 1.2 MPa and then slows down because of transferring to gas diffusion of gasification reaction. (2) In the pressure ranging from 0.5 to 1.4 MPa, the gas composition varies obviously with temperature but little with pressure. The content of CH4 in the product gas also changes little with pressure and mainly depends on the volatile cracking. The gas yield increases with air-coal ratio, while the gas heating value decreases. The water gas shift reaction does not reach equilibrium, and the errors of K to Keq range from 0 to 0.6 for most cases. (3) The distribution of particle size for bed holdup and fly ash varies with pressure. At the operating superficial (8) Wall, T. F.; Lui, G.-s.; et al. Prog. Energy Combust. Sci. 2002, 28, 405-433.


velocity of 0.8-1.0 m/s, some chars with the particle size of 0.3-0.5 mm are transferred into the fly ash as the pressure increases from 0.75 MPa to 1.4 MPa. Acknowledgment. This work is subsidized by the Chinese Academy of Sciences and the Special Funds for Major (China) State Basic Research ProjectssBasic Research in Coal Pyrolysis, Gasification and Hot Gas Cleaning (G19990221). Nomenclature Glossary C dp k K p Pi Pg RA, RH2O R t T Umf Uf x xi

reactant concentration particle diameter, mm apparent rate constant chemical reaction or adsorption equilibrium constant size distribution, xi/∆dp, mm-1 partial pressure, i ) H2; H2O gage pressure reaction rate gas constant, 8.314 J/mol‚K reaction time temperature, K minimum fluidization velocity, m/s superficial gas velocity, m/s carbon conversion weight fraction for ith particle size EF030052K

Coal Gasification Characteristic in a Pressurized Fluidized Bed

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