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Thermochemical Process Study on a Jet-Fluidized-Bed Gasifier Reaction System by an Equivalent Chemical Reactor Network Jie Feng,* Xuecheng Hou, Xiaohui Chen, Yalong Jia, and Wenying Li* Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, People’s Republic of China ABSTRACT: To study the effects of operational parameters on gasifying characteristics in the jet-fluidized-bed reactor and illustrate the scaling relationships of jet-fluidized-bed coal gasifier in a scale-up process, we established an equivalent reactor network model for the gasifier using CHEMKIN 4.1 software. The model is based on hydrodynamic properties in the gasifier and knowledge of the burning zone and gasifying zone of the reactor. It involves two-phase flow between materials, heat and mass transfer, as well as the coal gasification process in the presence of mixed gases, most notably H2O, O2, N2, etc. The model was validated by comparing it to the experimental data of outlet gas compositions. To optimize characteristics of the jet-fluidized-bed coal gasifier, the influence of the oxygen feed rate into the center nozzle and the coal feed rate on the gasification process and gas composition were studied by the equivalent established reactor network. Within the calculation range, with the increase of the oxygen feed rate into the center nozzle, the jet region nearly doubled, the temperature increased by 306 K, carbon conversion efficiency rose from 68 to 97%; however, the temperature of the jetting region should be controlled within the coal ash-softening temperature, and in generated gases, the CO and H2 contents had clearly changed. With the increase of the coal processing capacity, the jet region temperature decreased by 252 K, the gasifier overall temperature decreased, and carbon conversion reduced from 98 to 74%.
1. INTRODUCTION Fluidized-bed coal gasification is regarded as one of the most effective means to use coal, especially high ash and high ashmelting point coals.1 It has been developed in pilot scale known as the U-gas process, the Westinghouse (now KRW) process in the U.S.A., and the institute of coal conversion (ICC) process in China.2 To understand and, therefore, to be able to optimize and scale up/ down the process, substantial experimental research has been undertaken in the related fields, for example, the reaction kinetics of coal gasification, the hydrodynamics in fluidized beds, pollutant emission, sulfur removal, etc., which have enriched the wealth of knowledge of the fluidized-bed coal gasification technology.3 However, on the basis of works with lab-/pilot-scale reactors, there is not a detailed understanding of the scaling relationships to be used in the design of larger/commercialized scale reactors. There, some problems exist, such as scaling effects and uncertain factors affecting the normal operation and running of reactors. To illustrate the key factors affecting the gasification reaction in the jet-fluidized bed and to help solve the problems of engineering scale-up design and configuration optimization, it is necessary to study the thermochemical properties of the key region that has a strong influence on the composition of the produced gases. Currently, literature on the scale-up of jetting bed reactors4,5 has mainly concentrated on the discussion of cold-state flow behavior changes in the scaling process.1,6 Because of the efficient mixing of gases and particles in the gasifier, it is very difficult to test either the temperature profile of the specific region or the jetting zone range at the same time during coal gasification. Researchers generally make measurements and predictions on the temperature profile by means of thermocouple measurements and numerical simulation [computational fluid dynamics (CFD) models], for example, bubble r 2011 American Chemical Society
fluidized bed,1,7 one-dimensional hydrodynamics,8 district,911 particle track,12 and more dimensional model.13 However, these methods make it difficult to record the jetting bed temperature of any spot and are unable to demonstrate the impact of temperature changes on a specific region. Meanwhile, the pilot jetting bed reactor experiment results14 have not reflected the variations of the regional gasification characteristics in the reactor under different operational conditions. In this paper, on the basis of jetting bed hydrodynamic properties, mass balance, energy balance, and chemical equilibrium and from the perspective of gasification kinetics,15 we establish an equivalent jet-fluidized-bed reactor network model reflecting the coal reaction process and analyze the variations of gasification reaction process and gas compositions when the gasification parameter changes, to investigate how the operating parameters influence gasification characteristics of the jetfluidized-bed coal gasification system and to illustrate the scaling relationships of the jetting fluidized-bed coal gasifier in the scale-up processing.14,16
2. MODEL SETUP The reactor network model is established on the basis of the pressured ash-agglomerating fluidized-bed coal gasifier, with a 300 mm diameter (which is 300 mm inner diameter) and 1.5 MPa design pressure. The schematic diagram of the gasifier is shown in Figure 1a. The bottom region of the gasifier is assumed to be divided into two parts, the jet region with a high temperature and the annulus dense-phase region, in which the gas and solid are well-mixed, using the perfectly stirred reactor Received: November 28, 2010 Revised: July 29, 2011 Published: August 01, 2011 4063
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Figure 1. Schematic diagram of the gasifier and its equivalent reactor network. (PSR) in CHEMKIN 4.1 software as a substitute, and in the freeboard zone of the gasifier, the mixing of material is in a plug flow state, using the plug flow reactor (PFR) as a substitute. Heat exchange exists between the jet region and the dense-phase region, including convective heat transfer and radiative heat transfer from the jet region to the annulus region, gas heat exchange between the jet region and the annulus region, etc. Considering materials transfer and heat transfer in different phase regions, the reactor network model of the jet-fluidized-bed coal gasifier is built, as shown in Figure 1b. 2.1. Model Parameters. (1) Determination of the nozzle diameter: The ratio of the inlet nozzle diameter to gasifier diameter (dor/Dt) is a significant factor for the hydrodynamic characteristics of gassolid mixing. When dor/Dt increases from 0.08 to 0.12, the jetting diameter shows no obvious modification;4 therefore, in the present study, we take dor/Dt = 0.11, namely, dor = 33 mm. (2) The volume of each region is accordance with the following formula. Jetting diameter:17,18 !0:3 dj fi Fr dor 0:2 ¼ 1:56 pffiffiffi ð1Þ dor Dt k tan jr Jetting depth:19 0:1754 Lj u ¼ 30:4 Fr 0:293 Re0:1138 dp umf
Scheme 1
The volume of the jetting region: π Vj ¼ L j d j 2 4 The volume of annulus dense-phase region: Va ¼
umf ¼
Fdp u Ff uor 2 1 sin jr , k¼ , Re ¼ , ð1 εmf ÞFp dp g 1 þ sin jr μ
π Lj ðDt 2 dj 2 Þ 4
ð4Þ
The initial diameter of the dilute-phase zone is 30 cm, and then it is extended to 40 cm. The total height of the gasifier is 250 cm. (3) The computation process of the temperature and heat loss of regions in the gasifier are shown in Scheme 1. The temperature of the jetting zone can be obtained by Hess’s law Z T1 I I mi ΔHf0, i, 298 þ mi CP, T, i dT
∑i
∑i
ð2Þ ¼
Among them Fr ¼
ð3Þ
J
∑j
mj ΔHf0, j, 298 þ
298
Z
J
∑j
T1
mj
CP, T, j dT þ Q1
ð5Þ
298
According to Hess’s law, the coal enthalpy of formation can be expressed20 ΔHf0, coal, 298 ¼ HHV ð327:86Car þ 1418:79Har
dp 2 ðFp Ff Þg
þ 158:67Mar Þ
1650μ 4064
ð6Þ
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Figure 2. Calculated and experimental gas composition of JC anthracite. The energy conservation of the annular zone is calculated as follows: Z T1 J0 J0 m0 j ΔHf0, j, 298 þ m0 j CP, T, j dT
∑j
∑j
¼
K
298
K
∑k mk ΔHf0, k, 298 þ ∑k mk
Z
T2
CP, T, k dT þ Q2
ð7Þ
T1
The average temperature of the annular zone can be calculated according to the steady state. The energy conservation of the dilute-phase zone is calculated as follows: Z T2 K K mk ΔHf0, k, 298 þ mk CP, T, k dT
∑k
∑k
¼
L
∑l
ml ΔHf0, l, 298 þ
T1
Z
L
∑l
T3
ml
CP, T, l dT þ Q3
ð8Þ
T2
2.2. Calculation Statement. Because the gasifier temperature distribution is the key factor affecting the gasification characteristics,2,13,2126 we investigate the two most effective operational parameters determining the gasifier temperature distribution. Those are the oxygen feed rate into the nozzle and the coal feed rate. The effect of these two parameters on the jet, dense-phase, and dilutephase regions in the gasifier was also analyzed. In the ash-agglomerating pilot-plant experiments,27 the JinCheng (JC) anthracite [with 94.32% C, 2.83% H, 1.31% O, 1.20% N, and 0.34% S in a dry and ash-free (daf) basis] was chosen as the sample. The range of the oxygen feed rate into the nozzle was 19.5435.07 N m3 h1; in the calculation process, it ranged from 18.55 to 37.09 N m3 h1, and other operational parameters were unchanged. At the same time, the designed feed rate of coal was 2.4 tons day1, which was 100 kg h1, while 75.87123.30 kg h1 is in the simulation of the process.
3. RESULTS AND DISCUSSION 3.1. Model Validation. Calculations were carried out to simulate the pilot pressurized gasifier using the equivalent reactor network model. The operational capacity of the pilot gasifier is 94.5 kg h1 JC anthracite; the operational pressure is 0.53 MPa; the oxygen capacity and steam quantity of distributor are 107.94 N m3 h1 and 97.92 kg h1, respectively; and the oxygen of the nozzle is 30.91 N m3 h1. The combustion heat Q of JC
anthracite is 25.10 MJ kg1; the coal density is 657 kg m3; and the mean particle diameter is 0.56 mm. The coal ash mass fraction contains 43.09% SiO2, 29.83% Al2O3, 15.99% Fe2O3, 5.28% CaO, 0.80% MgO, 1.24% TiO2, 2.75% O3, 0.78% K2O, 0.08% Na2O, and 0.09% P2O5. The results of the calculated axial gas composition change compared to experimental data are shown in Figure 2. The comparison of the calculated results of JC anthracite and the experimental results show that the difference in the CH4 content is large. Considering that the CH4 content of generated gas is generally less than 5%, the difference has little effect on the whole generated gas composition.26 The results did not detect the O2 content, but considering that trace O2 exists in the outlet gas of the industrial gasifier,28 it is logical that there is trace O2 in the calculated results. Therefore, it is demonstrated that the calculation results are consistent with the experimental results and that the reactor network model can basically reflect the variation of the internal gas production. 3.2. Influence of the Oxygen Feed Rate into the Nozzle on the Jetting Region. The model has been confirmed as above in the ash-agglomerating pilot-plant simulation of the process. The effect of varying the oxygen feed rate into the nozzle has been investigated (using flows of 18.55, 30.91, and 37.09 N m3 h1), and the predicted influences are shown in Table 1. With the increase of the oxygen feed rate into the nozzle, the volume of the high-temperature jet region increases from 708 to 1999 cm3, the combustion reaction intensifies, and the hightemperature region in the jet rises from 1453 to 1759 K in the gasifier. As known, the operational temperature of the jetting bed coal gasifier should be controlled close to the coal ash-softening temperature. If the temperature is too high, far exceeding the ashmelting temperature, there will be large pieces of slag that can break gas flow and result in operational difficulties. The ash agglomeration temperature of JC anthracite is 1724 K. As shown in Table 1, the minimum temperature in the jet region is 1453 K and the average bed temperature tends to be low, as well as the low degree of coal gasification. The jetting region temperature is near the ash-melting temperature of 1645 K under the normal operating condition. With a further increase in the oxygen feed rate, the temperature of the jetting region rises to 1759 K, beyond the ash-melting temperature of JC coal; therefore, this is an unacceptable operating condition. Also, the maximum temperature point of the axial distance rises from 26.82 to 32.95 cm because of the increasing jet penetration depth. The gas outlet temperature rises from 958 to 1180 K, as shown in Figure 3a. It is well-known that, in a jetting bed gasifier, the ash mainly melts in the high-temperature jetting region; therefore, the jetting region temperature control is the key factor ensuring that the operation proceeds normally.29 In other words, it is not reasonable when the velocity of the nozzle is 12 m s1. As shown in Figure 3b, with the increase of the oxygen feed rate into the nozzle, the carbon conversion increases from the initial 68.16 to 97.38% and the coal has almost been completely converted. However, from the results discussed above, we know that the ash easily slags under 37.09 N m3 h1 oxygen feed rate, which cannot ensure stable and continuous operation; therefore, the operational condition is inappropriate. When the oxygen feed rate into the nozzle is 18.55 N m3 h1, although oxygen consumption is less, the carbon conversion of coal is only 68.16% and the gasification degree is very low; therefore, it is not suitable for large-scale operations. In view of the above, the effect of the oxygen feed rate into the nozzle on the carbon 4065
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Table 1. Influences of the Oxygen Feed Rate into the Nozzle on the Jet Region
a
oxygen feed rate
velocity of the
into the nozzle (N m3 h1)
nozzle (m s1)
Lja (cm)
djb (cm)
volume of the
temperature of the
jetting zone (cm3)
jetting zone (K)
18.55
6
53.64
4.10
708
1453
30.91
10
62.43
5.57
1522
1645
37.09
12
65.90
6.22
1999
1759
The jetting depth. b The jetting diameter.
Figure 3. Influence of the oxygen feed rate into the nozzle in the jetting region.
conversion of the gasifier is significant. The oxygen feed rate into the nozzle should be controlled in a reasonable range to make sure that there is a higher carbon conversion, the produced gas must also be adequate for the intended purpose, the jetting temperature is not too high, and the operation is stable. When the oxygen feed rate into the nozzle is 18.55, 30.91, and 37.09 N m3 h1, the changes of gas composition with the axial distance are shown in Figure 4. The increase of the oxygen feed rate into the nozzle has a significant effect on the dense-phase region outlet gas composition, especially on the CO and H2O contents. The CO content increases along with the increase of the oxygen feed rate into the nozzle to 9.34, 17.62, and 20.64%, respectively, while the H2O content decreases at 50.40, 40.70, and 38.49%, respectively. Meanwhile, with the increase of the oxygen feed rate into the nozzle, there are also significant changes in the compositions of outlet gases as follows: the H2 content decreases to 30.54, 29.67, and 27.10%, respectively; the CO content increases to 12.46, 21.96, and 24.95%, respectively; the CO2 content decreases to 21.71, 19.57, and 18.31%, respectively; the CH4 content decreases to 3.08, 2.75, and 2.66%, respectively. The reason is that, because of the increase in the oxygen feed rate into the nozzle, combustion intensifies, resulting in the increase of the CO2 content. However, the increase in the gasifier temperature promotes the rate of the Boudouard reaction of char with CO2, as well as the char and watergas shift reaction and the CH4 reforming reaction. Meanwhile, when the temperature is increased, the equilibrium of the CO shift reaction is driven toward the left. All of the above results show a decrease of
the H2O content, CO2 content, H2 content, and CH4 content to varying degrees and an increase in the CO content. The increase of the oxygen feed rate into the nozzle has little influence on the outlet gas composition of the dilute-phase region (between the outlet of the dense-phase region and outlet of the gasifier in Figure 4). In the dilute-phase region, the changes in the compositions of gases are as follows: the H2O content decreases intensely; the H2 content increases; the CO content at first increases and then has a slight decrease; the CO2 content is almost unchanged; and the CH4 and O2 contents are very low (CH4 less than 3.5% and O2 less than 2%) and decrease slightly. The increase of oxygen into the gasifier raises the overall temperature, which promotes the endothermic reaction. Without a doubt, these are the balances because of changes in the extents of all of the different reactions (combustion, shift reaction, gasification, reforming reaction, etc.). As described above, with the increase of the oxygen feed rate into the nozzle, the changes of the gasifier temperature, gas composition, and carbon conversion are appreciable. The increase of the oxygen feed rate into the nozzle also has a great effect on the gas composition changes. The jetting depth temperature region is a critical parameter that affects the gasification process in the jetting fluidized-bed gasifier and also supplies heat to the gasification reaction. Its temperature and region changes are the key factors on the gas composition changes. Thus, in the jetting fluidized-bed design and optimization process, the nozzle design should be considered significant. 3.3. Influence of the Coal Feed Rate on the Jetting Region and Overall Temperature in the Gasifier. When the coal feed 4066
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Figure 4. Gas composition changes with the oxygen feed rate into the nozzle.
Figure 5. Influence of the coal feed rate inside the gasifier.
rate is 75.9 kg h1, far below the design feed rate, the temperature of the jetting region is 1718 K, very close to the ash-melting temperature. When the coal feed rate is 94.5 kg h1, according to the design feed rate, the temperature of the jetting region is 1645 K. When the coal feed rate is much greater than the design feed rate, the temperature of the jetting region is 1566 K. The influence of the coal feed rate on the total temperature in the gasifier is shown in Figure 5.
Coal combustion is the main reaction in the jetting region and provides the energy for the gasification process and maintaining the gasifier temperature.2933 When the oxygen feed rate into the nozzle is certain, heat from the pulverized-coal combustion is certain. With the increase of the coal feed rate, the total gasifier temperature decreases. The reason is that, with the increase of cold material in the dense-phase annulus zone inside the gasifier, the overall jet and annulus region convective heat transfer and the 4067
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Figure 6. Gas composition changes with the coal feed rate.
radiant heat transfer from the jet region to the annulus region intensify, resulting in the reduction of the temperature of the jetting region. When the coal feed rate increases and the coal combustion amount is certain, the gasification amount increases accordingly and the heat absorbed by the gasification reaction increases, which is one of main reasons for the resulting reduction of the total gasifier temperature. As shown in Figure 5a, when the coal feed rate is 75.9 kg h1, the gas production and heat absorbed by the gasification reaction are lower and the gasifier temperature decreases gently. When the coal feed rate is 123.3 kg h1, the gas production is larger, heat absorbed by the gasification reaction accordingly increases, and the gasifier temperature decreases greatly. The higher the coal feed rate, the greater the decrease in the gasifier temperature. As shown in Figure 5b, with the increase of the coal feed rate, the carbon conversion reduces sharply. When the coal feed rate is 75.9 kg h1, a share of pulverized coal combustion is greater, the temperature of the gasifier is 1714 K, which is conducive to the gasification reaction, and the carbon conversion is 98%, almost a complete conversion. When the coal feed rate is 123.3 kg h1, the carbon conversion is only 74.4%. Thus, the coal feed rate should be strictly controlled in an actual operation. When the coal feed rate is lower, the temperature of the jetting region easily exceeds the ash-melting temperature, resulting in gasifier slag and affecting the stability of the operation. When the coal feed rate is larger, the gasification level is low and not helpful to carbon conversion. When the coal feed rate is 75.9, 94.5, and 123.3 kg h1, the changes of gasifier gas composition are shown in Figure 6. We know from the figures above that the increase of the coal feed rate
results in significant changes to H2O and H2 contents in the gas composition of the dense-phase region. Under the three conditions, the H2O content reduces to 44.11, 40.70, and 39.07%, respectively, and the H2 percentage increases to 16.72, 19.18, and 20.88%, respectively. The main reason is that a sufficient reaction of watergas leads to a large H2O consumption and an increase of the H2 content because of the increase of the coal feed rate. The CO percentage is 16.57, 17.62, and 17.01%, respectively. The CO content does not change significantly because of the coal and CO2 Boudouard reaction, which raises the CO content. While the watergas shift reaction consumes CO, the two reactions co-act to make the CO content almost unchanged. In outlet gas composition, the H2 content changes are greater to 24.35, 29.77, and 32.69%, respectively, which shows that, with the increase of the coal feed rate, the H2 content in gas composition rises for the same reason as previously described. The CH4 content rises slightly to 2.37, 2.75, and 3.40%, respectively, because of the increase of the CH4 content generated from coal pyrolysis along with the increase of the coal feed rate. The CO content almost remains unchanged at 20.05, 21.90, and 21.23%, respectively, which shows that the influence of the coal feed rate on the CO content in the outlet gas composition is not obvious. In comparison to the three kinds of coal feed rates, the variation of effective gas components in the dilute-phase region is similar but different in extent. The H2 percentage rose by 7.63, 10.59, and 11.81%, respectively. The CO2 content rose slowly, with the percentage rising by 0.95, 1.69, and 2.32%, respectively. The CO percentage rose by 3.48, 4.28, and 4.23%, respectively. The CH4 content was very low, less than 4%, and also decreased 4068
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Energy & Fuels by a small amount. It shows that the dilute-phase region has a great effect on gas composition and the increment of the coal feed rate has an obvious effect on the H2 content variation and a limited effect on CO, CO2, and CH4 content variations.
4. CONCLUSION An equivalent reactor network model has been set up and made valid by the experiment. From this model, the thermochemical process in the jet-fluidized-bed gasification system was successfully analyzed. The high-temperature jetting region is the key that provides heat for the gasification reaction, maintains the gasifier temperature, and moreover, has a critical effect on gasification products and a stable operation of the gasifier. The temperature distribution of the gasifier is one of the most critical factors affecting gas compositions, while the oxygen feed rate and the coal feed rate result in obvious changes of the temperature distributions of the gasifier. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: +86-351-6018957. Fax: +86-351-6018453. E-mail:
[email protected] (J.Feng);
[email protected] (W.Li).
’ ACKNOWLEDGMENT This work is financially supported by the National Basic Research Program of China (973 Project 2005CB22120102), National Natural Science Foundation (NSFC) of China (21076136), and Shanxi Provincial NSFC (2011011005-1). ’ REFERENCES (1) Gao, K.; Wu, J. H.; Wang, Y.; Zhang, D. K. Bubble dynamics and its effect on the performance of a jet fluidized bed gasifier simulated using CFD. Fuel 2006, 85, 1221–1231. (2) Fang, Y. T.; Huang, J. J.; Wang, Y.; Zhang, B. J. Experiment and mathematical modeling of a bench-scale circulating fluidized bed gasifier. Fuel Process. Technol. 2001, 69 (1), 29–44. (3) Zhao, J. T.; Huang, J. J.; Zhang, J. M.; Wang, Y. Influence of fly ash on high temperature desulfurization using iron oxide sorbent. Energy Fuels 2002, 16, 1585–1590. (4) Zhao, J.; Lim, C. J.; Grace, J. R. Flow regimes and combustion behavior in coal-burning spouted and spout-fluid beds. Chem. Eng. Sci. 1987, 42 (12), 2865–2875. (5) Matsen, J. M. Scale-up of fluidized bed processes: Principle and practice. Powder Technol. 1996, 88 (3), 237–244. (6) Guenther, C.; Shahnam, M.; Syamlal, M.; Longanbach, J.; Cicero, D.; Smith, P. V. CFD modeling of a transport gasifier. Proceedings of the 19th Annual Pittsburgh Coal Conference; Pittsburgh, PA, Sept 2426, 2002. (7) Liu, H.; Luo, C. H.; Toyota, M.; Uemiya, S.; Kojima, T. Kinetics of CO2/char gasification at elevated temperatures. Part II: Clarification of mechanism through modelling and char characterization. Fuel Process. Technol. 2006, 87 (9), 769–774. (8) Pannala, S.; Syamlal, M.; O’Brien, T. J. Computational GasSolids Flows and Reacting Systems: Theory, Methods and Practice; IGI Global: Hershey, PA, 2010; pp 178202. (9) Arena, U.; Malandrino, A.; Massimilla, L. Modelling of circulating fluidized bed combustion of a char. Can. J. Chem. Eng. 1991, 69 (4), 860–868. (10) Wang, X. S.; Gibbs, B. M.; Rhodes, M. J. Modelling of circulating fluidized bed combustion of coal. Fuel 1994, 73 (7), 1120–1127.
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