Catalytic Coal Gasification Process Simulation with Alkaline Organic Wastewater in a Fluidized Bed Reactor Using Aspen Plus
Abstract
:1. Introduction
2. Method Description
2.1. Process Assumptions
- Gasification temperature remains stable.
- The raw material particles are mixed with the gasifying agents in the furnace quickly and uniformly.
- H, O, N, and S in the coal all changed into the gas phase, and C was assumed to be incompletely transformed according to the reaction.
- The char contains only fixed carbon and ash.
- Ash in coal is inert and does not participate in chemical reactions.
- The syngas consists of H2, CO, CO2, CH4, H2O, N2, and C6H6.
- The fluidized bed was divided into dense and dilute phases.
- The composition of the gas phase under the simulation conditions is regarded as an ideal gas, and it is applicable to the physical properties, methods, and models in the software.
- The catalytic effect was introduced into the gasification model of the fluidized bed using a correction factor.
2.2. Reactions
2.3. Process Modeling
2.3.1. Component Definitions and Method
2.3.2. Process Description
- (1)
- Drying: A stream of coal, with a mass flow, was fed into the DRYER reactor in which the physical moisture bound was released completely into the gas phase. The amount of vaporized water was determined by the proximate analysis of the coal. We assumed that coal drying was instantaneous compared with that of the coal gasification process. Water content for the specific coal we used was set to x%, according to the proximate analysis. As a result the yield of dried coal was set to 1 − x%. The output was separated by a gas–solids separator called SEP-1.
- (2)
- Pyrolysis: The output of the drying process was called DRY-COAL, whereafter it entered the PYROLYS block. Pyrolysis is a complex thermochemical process that occurs during coal gasification which is difficult to simulate accurately using the database of Aspen Plus. Two ways are available to simulate coal pyrolysis. One is based on experiments and the other is to use functional group models based on a theoretical method [19]. Using experimental results will be simpler compared with using a functional group. Thus, the pyrolysis experimental data for a specific coal species was used to specify every component of the PYROLYS block. In this block, dried coal was broken down into CO, H2, CH4, CO2, N2, H2O, C6H6, and char. The gas–solids separator SEP-2 block separated the upstream product into two streams, namely CHAR and PYRO-GAS. This reaction was also considered instantaneous.
- (3)
- Decomposition: CHAR was downstream of the CHAR-DEC block. CHAR, as a NC component, must be transformed to participate in the gasification reaction. In this study, block RStoic was chosen to simulate the decomposition process. CHAR was decomposed into C, H2, O2, N2, S, and ash for the solid–gas reactions. The stoichiometric coefficients of the elements mentioned above were determined automatically according to char’s ultimate analysis in the CALCULATOR block. The output was separated by the gas–solids separator SEP-3 block into two streams of CHAR-GAS (including H2, O2, and N2) and CHAR-SLD.
- (4)
- Gasification: The air, steam and the stream CHAR-GAS gaseous products, were mixed in MIX-GASIN. Then, the stream CHAR-SLD together with MIX-GASIN and the stream PYRO-GAS from the upper block PYROLYS was reacted in the RCSTR reactor, named as block GASIF-1/2. We used two RCSTR reactors to simulate the dense and the dilute regions of the fluidized bed. The uncatalyzed kinetic parameters are shown in Table 1, which were written in an external FORTRAN subroutine and was linked to the simulation. The gaseous outputs were mainly CO2, CO, H2, CH4, and C6H6.
- (5)
- Catalytic effects: In fact, catalysts, such as alkali metals, will respectively increase the reaction rates of the chemical Equations (2)–(7), mentioned in Section 2.2, during the gasification process. Adding catalytic effects into the gasification model requires correcting of the kinetic parameters compared with the uncatalyzed simulation. We assumed that alkali metals merely increase the rate of the carbon-steam reaction (3), which is the step-determining step in the steam/air gasification process after the char is decomposed [31]. A correction factor is introduced based on the kinetics of catalytic coal gasification with alkaline organic wastewater in the fluidized bed. The correction factor was obtained via the following method: it equaled the ratio of kexp to kn, where kexp is the reaction rate constant from the catalytic gasification experiment, and kn is the reaction rate constant without catalytic effects calculated from Table 1 as used in Equation (8).
2.4. Characteristics of Feed Coal and Operation Parameters
2.5. Calculation of CCG, CGE, and LHV Values
3. Results and Discussion
3.1. Model Validation
3.1.1. Validation of Fluidized Bed Model without Catalytic Effect
3.1.2. Validation of the Fluidized Bed Model with a Catalytic Effect
3.1.3. Calculation of Root Mean Square Error (RMSE)
3.2. Influence of Operating Conditions on Gasification Performance
3.2.1. Effect of Gasification Temperature
3.2.2. Effect of the Steam to Coal (S/C) Ratio
3.2.3. Effect of the ER Ratio
3.3. Energy Analysis
4. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Nomenclature
Mad | moisture of the coal sample under air dry basis (%) |
Vd | volatiles of the coal sample under dry basis (%) |
Ad | ash of the coal sample under dry basis (%) |
T | reaction temperature (K) |
mass transfer coefficient for gas film diffusion (gm-mole/cm3·sec·atm) | |
mass transfer coefficient for ash diffusion (gm-mole/cm3·sec·atm) | |
porosity of ash | |
average radius of shrinking unreacted coal particles (cm) | |
average radius of feed coal (cm) | |
[C] | concentration of char |
C | mole concentration of the subscripted substance (mol/m3) |
P | partial pressure of the subscripted substance (atm) |
P* | back reaction equilibrium pressure of the subscripted substance (atm) |
mole fraction of the subscripted substance (%) | |
ash correction factor | |
equilibrium constant of the water–gas shift reaction |
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Reaction | Reaction Rate | Remarks |
---|---|---|
2 | ||
3 | ||
4 | ||
5 | ||
6 | ||
7 | - |
Block Name | Block | Description |
---|---|---|
DRYER | RYield | Simulate coal drying based on the water content value in proximate analysis of coal |
PYROLYS | RYield | Simulate coal pyrolysis based on pyrolysis experiment |
CHAR-DEC | RStoic | Decompose char into C, H2, O2, N2, S, and ash |
GASIFI-1/2 | RCSTR | Simulate char gasification |
MX | MIXER | Mix the feed-in streams |
SEP | SEP | Separate the gas and solid |
Proximate Analysis | Ultimate Analysis (d) | ||||||
---|---|---|---|---|---|---|---|
Mad | Ad | Vd | C | H | N | S | O a |
5.69 | 72.04 | 17.88 | 75.86 | 1.26 | 1.33 | 1.78 | 0.77 |
Types of Coal | Yongding Fujian Anthracite |
---|---|
Coal-feeding flow rate | 0.6 kg/h |
Steam flow rate | Steam/coal = 0 to 5 |
Air flow rate | ER = 0 to 1 |
Gasification temperature | 550 to 900 °C |
Pressure | 0.1 MPa |
Fluidized bed volume | 0.001 + 0.001 |
Void fraction | 0.25 to 0.65 (dense phase and dilute phase) |
Temp. (K) | f | |
---|---|---|
1023 | 0.0216 | 1593 |
1073 | 0.0266 | 699 |
1123 | 0.037 | 380 |
1173 | 0.0416 | 180 |
Composition | CO | H2 | CO2 | CH4 | N2 |
---|---|---|---|---|---|
RMSE | 0.2714 | 0.1667 | 0.1526 | 0.2883 | 0.0728 |
T (°C) | S/C | ER | CO (%) | CO2 (%) | H2 (%) | CH4 (%) | Q (W) | LHV (MJ/Nm3) | CCE (%) | CGE (%) |
---|---|---|---|---|---|---|---|---|---|---|
650 | 0.5 | 0.2 | 17.5 | 10.6 | 16.0 | 3.9 | 11.7 | 5.4 | 50.8 | 35.7 |
650 | 0.5 | 0.4 | 15.9 | 8.4 | 6.9 | 2.5 | 345.4 | 3.6 | 67.8 | 44.0 |
650 | 1 | 0.4 | 15.2 | 8.7 | 8.8 | 2.4 | 156.9 | 3.7 | 69.1 | 56.7 |
700 | 0.5 | 0.4 | 16.8 | 8.1 | 7.3 | 2.4 | 271.4 | 3.8 | 70.2 | 47.6 |
700 | 1.0 | 0.4 | 15.7 | 8.9 | 9.7 | 2.3 | 75.9 | 3.9 | 72.1 | 63.8 |
750 | 0.5 | 0.4 | 17.0 | 8.4 | 7.5 | 2.4 | 240.1 | 3.8 | 71.8 | 49.1 |
750 | 1 | 0.4 | 15.6 | 9.6 | 10.2 | 2.3 | 48.3 | 3.9 | 74.2 | 67.3 |
800 | 0.5 | 0.4 | 16.7 | 9.1 | 10.2 | 2.4 | 235.9 | 3.8 | 72.7 | 62.1 |
800 | 1.0 | 0.4 | 15.0 | 10.6 | 10.6 | 2.3 | 50.6 | 3.9 | 75.5 | 69.6 |
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Xiao, X.; Wang, X.; Zheng, Z.; Qin, W.; Zhou, Y. Catalytic Coal Gasification Process Simulation with Alkaline Organic Wastewater in a Fluidized Bed Reactor Using Aspen Plus. Energies 2019, 12, 1367. https://doi.org/10.3390/en12071367
Xiao X, Wang X, Zheng Z, Qin W, Zhou Y. Catalytic Coal Gasification Process Simulation with Alkaline Organic Wastewater in a Fluidized Bed Reactor Using Aspen Plus. Energies. 2019; 12(7):1367. https://doi.org/10.3390/en12071367
Chicago/Turabian StyleXiao, Xianbin, Xueying Wang, Zongming Zheng, Wu Qin, and Yumengqiu Zhou. 2019. "Catalytic Coal Gasification Process Simulation with Alkaline Organic Wastewater in a Fluidized Bed Reactor Using Aspen Plus" Energies 12, no. 7: 1367. https://doi.org/10.3390/en12071367