Aspen Plus
Model for Entrained Flow Coal Gasifier
Copyright (c) 2010-2013 by Aspen Technology, Inc. All rights reserved. Aspen Plus, the aspen leaf logo and Plantelligence and Enterprise Optimization are trademarks or registered trademarks of Aspen Technology, Inc., Burlington, MA. All other brand and product names are trademarks or registered trademarks of their respective companies. This software includes NIST Standard Reference Database 103b: NIST Thermodata Engine Version 7.1 This document is intended as a guide to using AspenTech's software. This documentation contains AspenTech proprietary and confidential information and may not be disclosed, used, or copied without the prior consent of AspenTech or as set forth in the applicable license agreement. Users are solely responsible for the proper use of the software and the application of the results obtained. Although AspenTech has tested the software and reviewed the documentation, the sole warranty for the software may be found in the applicable license agreement between AspenTech and the user. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS DOCUMENTATION, ITS QUALITY, PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE. Aspen Technology, Inc. 200 Wheeler Road Burlington, MA 01803-5501 USA Phone: (1) (781) 221-6400 Toll Free: (1) (888) 996-7100 URL: http://www.aspentech http://www.aspentech.com .com
Copyright (c) 2010-2013 by Aspen Technology, Inc. All rights reserved. Aspen Plus, the aspen leaf logo and Plantelligence and Enterprise Optimization are trademarks or registered trademarks of Aspen Technology, Inc., Burlington, MA. All other brand and product names are trademarks or registered trademarks of their respective companies. This software includes NIST Standard Reference Database 103b: NIST Thermodata Engine Version 7.1 This document is intended as a guide to using AspenTech's software. This documentation contains AspenTech proprietary and confidential information and may not be disclosed, used, or copied without the prior consent of AspenTech or as set forth in the applicable license agreement. Users are solely responsible for the proper use of the software and the application of the results obtained. Although AspenTech has tested the software and reviewed the documentation, the sole warranty for the software may be found in the applicable license agreement between AspenTech and the user. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS DOCUMENTATION, ITS QUALITY, PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE. Aspen Technology, Inc. 200 Wheeler Road Burlington, MA 01803-5501 USA Phone: (1) (781) 221-6400 Toll Free: (1) (888) 996-7100 URL: http://www.aspentech http://www.aspentech.com .com
Revision History Vers Versio ion n
Des Descrip cripti tion on
V7.2
First version
V7.3 V7.3
Upda Update te the the mode modell to V7.3 V7.3 and and add add a para paragr grap aph h in Intr Introd oduc uctio tion n sect sectio ion n to describe what files are released.
V7.3 V7.3.2 .2
Upda Update te the the mode modell to V7.3 V7.3.2 .2
V8.2
Update the model to V8.2
V8.4
Update the model to V8.4
Revision History
1
Contents Revision History ......................................................................................................1 Contents..................................................................................................................2 Introduction............................................................................................................3 1 Components .........................................................................................................4 2 Process Description..............................................................................................5 3 Physical Properties...............................................................................................7 4 Reactions .............................................................................................................9 4.1 Coal pyrolysis.............................................................................................9 4.1.1 Reactions......................................................................................9 4.1.2 Amount of each pyrolysis product ....................................................9 4.2 Volatile combustion................................................................................... 13 4.2.1 Reactions....................................................................................13 4.2.2 Reaction kinetics.......................................................................... 13 4.3 Char gasification....................................................................................... 14 4.3.1 Reactions....................................................................................14 4.3.2 Reaction kinetics.......................................................................... 15 5 Simulation Approach ..........................................................................................19 5.1 Unit Operations ........................................................................................ 20 5.1.1 Coal pyrolysis .............................................................................. 20 5.1.2 Volatile combustion ...................................................................... 20 5.1.3 Char gasification ..........................................................................20 5.2 Streams .................................................................................................. 26 5.3 Calculator Blocks ...................................................................................... 26 5.4 Convergence ............................................................................................ 27 6 Simulation Results .............................................................................................28 7 Conclusions ........................................................................................................32 References ............................................................................................................33
2
Contents
Introduction
This file describes an Aspen Plus kinetics-based model for Texaco down-flow entrained flow gasifiers. The model follows the modeling approach proposed by Wen and Chaung[1]. The model includes the following features:
The model is a steady-state model. The model accounts for major physical and chemical processes occurring in the gasifier, i.e. coal pyrolysis, volatile combustion, and char gasification. The reaction kinetics for char gasification is considered. The hydrodynamics to calculate solid residence time is taken into account. The gas phase is assumed to be instantaneously and perfectly mixed with the solid phase. The pressure drop in the gasifier is neglected. Coal particles are assumed to be spherical and of uniform size. The ash layer formed remains on the particle during the reactions based on the unreacted-core shrinking model [2]. The temperature inside the coal particle is assumed to be uniform.
The following files related to this example can be found in the GUI\Examples\Entrained Flow Coal Gasifier folder of the Aspen Plus installation:
Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.apwz, a compound file containing these six files:
Introduction
o
Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.bkp
o
Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.pdf
o
USRKIN.f
o
USRPRES.f
o
USRSUB.dll
o
USRSUB.opt
Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.bkp Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.pdf USRSUB.dll USRSUB.opt
3
1 Components
The following table represents the chemical species present in the process:
Table 1. Components Used in the Model ID
Type
Name
Formula
O2
CONV
OXYGEN
O2
CO
CONV
CARBON-MONOXIDE
CO
H2
CONV
HYDROGEN
H2
CO2
CONV
CARBON-DIOXIDE
CO2
H2O
CONV
WATER
H2O
H2S
CONV
HYDROGEN-SULFIDE
H2S
N2
CONV
NITROGEN
N2
CH4
CONV
METHANE
CH4
C6H6*
CONV
BENZENE
C6H6
C
SOLID
CARBON-GRAPHITE
C
S
SOLID
SULFUR
S
COAL
NC
------
------
CHAR1*
NC
------
------
CHAR2*
NC
------
------
ASH
NC
------
------
*: C6H6 represents tar. CHAR1 represents the solid phase after coal pyrolysis at 1atm. CHAR2 represents the solid phase after pressure correction from 1atm to system pressure.
4
1 Components
2 Process Description
The Texaco gasifier is a typical entrained flow gasifier, as shown in Fig. 1. The total gasifier is divided internally into two sections[1, 3].
Figure 1. Schematic diagram of Texaco down-flow entrained flow gasifier [1]
The top section is for coal gasification. The pulverized coal with size typically less than 500µm[4] is mixed with water to form the coal-water slurry, and then the slurry together with oxygen is simultaneously introduced into the top section. Coal pyrolysis, volatile combustion and char gasification reactions take place subsequently to produce the syngas. In this section, a special refractory material is lined to withstand the severe operating environment. The operating pressure is usually at 20-50atm and the temperature is typically higher than 1000ºC[4]. The lower section is a quench vessel. A reservoir of water is maintained at the bottom of the gasifier by continuous injection of cooling water. The slag and
2 Process Description
5
syngas leaving the top section of gasifier pass through a water-cooled dip tube into the water reservoir. The slag remains in the water and then is removed. The syngas is saturated with water and removed from the gas space above the water.
6
2 Process Description
3 Physical Properties
In this model, the property method RK-SOAVE is used to calculate the physical properties of mixed conventional components and CISOLID components. HCOALGEN and DCOALIGT models are used to calculate the enthalpy and density of non-conventional components, respectively. The HCOALGEN model requires these three component attributes for nonconventional components: proximate analysis results (denoted as PROXANAL in Aspen Plus), ultimate analysis results (denoted as ULTANAL in Aspen Plus), and sulfur analysis results (denoted as SULFANAL in Aspen Plus). The proximate analysis gives the weight contents of moisture, fixed carbon, volatile matter, and ash. The ultimate analysis gives the weight composition of coal in terms of ash, carbon, hydrogen, nitrogen, chlorine, sulfur, and oxygen. The sulfur analysis gives the weight fractions of sulfur divided into pyritic, sulfate, and organic sulfur. For the DCOALIGT model, it requires only these two component attributes: ULTANAL and SULFANAL. Table 2 shows the component attributes of coal used in our model, which are from the literatures of Wen and Chuang [1, 5]. Based on these analysis results, the enthalpy and density of coal are calculated, respectively. For the characterization of char and ash generated in coal conversion, the same methodology as the coal is applied and the same models are used to calculate their enthalpy and density. The results of proximate, ultimate, and sulfur analyses for the char and ash are determined from the analysis data of original coal and the amount of gasified gaseous product in terms of mass balance.
3 Physical Properties
7
Table 2. Component Attributes of Coal Used in the Model Proximate analysis
Element
Moisture (wet basis) Fixed carbon (dry basis) Volatile matter (dry basis) Ash (dry basis)
8
Ultimate analysis
[1,5]
Sulfur analysis
Element
Value (wt.%, dry basis)
Element
Value (wt.%, dry basis)
0.2
C
74.05
Pyritic
0.59
58.01
H
6.25
Sulfate
0.59
26.46
N
0.71
Organic
0.59
15.53
Cl
0.37
S
1.77
O
1.32
Ash
15.53
Value (wt.%)
3 Physical Properties
4 Reactions
When the coal, oxygen and steam are simultaneously introduced into the gasifier, these reactions take place in sequence: coal pyrolysis, volatile combustion and char gasification.
4.1 Coal pyrolysis 4.1.1 Reactions In gasifier, the temperature is typically higher than 1000ºC[4]. When coal is fed into the gasifier, it first undergoes the pyrolysis process to decompose to volatile matter and char, as shown in Eq. (1). In our model, volatile matter includes CO, H2, H2O, CO2, CH4, H2S, N2 and C6H6. C6H6 is used to represent the tar.
Coal Char CO H 2 H 2O CO2 CH 4 H 2 S N 2 C 6 H 6 (1)
4.1.2 Amount of each pyrolysis product In our model, the amount of each pyrolysis product is determined based on the results of the pyrolysis experiment, which is made at 1atm. A pressure correction is applied to the results of this experiment, adjusting them from 1atm to system pressure, to yield the amount of each product at the operating conditions of the real gasifier. The next two sections describe how to get the pyrolysis results at 1atm from the experiment and how to make a pressure correction for the amount of each product.
4.1.2.1 Amount of each pyrolysis product at 1atm Suuberg et al.[6] describe how to get the results of coal pyrolysis at 1atm in experiment. Their corresponding results are summarized here. It should be noted that in our model, the amount of each pyrolysis product at 1atm is from the work of Wen and Chuang [1, 5].
4 Reactions
9
Flow chart of experiment Fig. 2 is a schematic diagram for the apparatus of the pyrolysis experiment. The apparatus consists of five components:
Reactor, which is designed to contain a coal sample in a gaseous environment of known pressure and composition
Electrical system, which is used to expose the sample to a controlled timetemperature history
Time-temperature monitoring system Product collection system Product analysis system
A thin layer of coal particles with 74µm average diameter are held in a folded strip of stainless steel screen. Then, electricity is used to heat the coal particles under 1atm helium or vacuum to produce the pyrolysis products. After collecting the products, the yield of each product is analyzed.
Figure 2. The apparatus for the pyrolysis experiment in Suuberg et al.’s work [6]
Collection of pyrolysis products The pyrolysis products can be divided into three types:
Products condensing at room temperature, such as tars Products in the vapor phase at room temperature Char The first type of products is collected primarily on foil liners within the reactor and on a paper filter at the exit of the reactor. Any condensation on non-lined reactor surfaces is collected by washing with methylene-chloride-soaked filter paper. The second type of products is collected at the conclusion of a run by purging the reactor vapors through two lipophilic traps. The first trap is operated at room temperature with the Porapak Q chromatographic packing, and collects
10
4 Reactions
intermediate weight oils such as benzene, toluene and xylene. The second trap is also packed with Porapak Q but operated at -196ºC in a dewar of liquid nitrogen. This trap collects all fixed gases produced by pyrolysis, with the exception of hydrogen which is determined by direct vapor phase sampling with a precision syringe. The third type of product, char, remains on the screen and is determined gravimetrically.
Analysis of pyrolysis products The different types of products collected are analyzed by different methods. The first type of products is measured gravimetrically. The products of the second type collected in the first and second traps are first warmed to 240ºC and 100ºC, respectively, and then fed into the gas chromatography for analysis. The third type of product is measured gravimetrically and its elemental analysis is analyzed by the ASTM (American Society for Testing and Materials) standard method. The analysis results are summarized in Fig. 3.
4.1.2.2 Amount of each pyrolysis product at system pressure Most pyrolysis experiments are carried out at 1atm. For example, all of Suuberg et al.’s results [6] shown in Fig. 3 are obtained at 1atm. However, in real coal gasifiers, the pressure is usually much higher than 1atm, typically 20-50atm[4]. This indicates that the pressure effect on yield of each product needs to be considered. In the pressure correction, the relative composition of gas components is assumed to be constant, and only the total yield of volatiles is corrected. The total yield of volatiles is corrected by Eq. (2) [1]:
V 2 V 1 1 a ln P t
(2)
Where
V 1 = total yield of volatiles at 1atm. V 2 = total yield of volatiles at the pressure of the real gasifier. P t = pressure in real gasifier, atm. a = constant. In our model, a = 0.066.
Combining the total yield of volatiles calculated in Eq. (2) and the relative composition of volatile products in the gas phase obtained from pyrolysis experimental results at 1atm, the yield of each volatile product at the operating conditions of real gasifier is calculated. Take the calculation of CO yield as an example. Table 3 shows the yield of coal pyrolysis products at 1atm used in our model, which is from the work of Wen and Chaung [1, 5]. The total yield of volatiles at 1atm is 27.28%, i.e. V 1 27.28% . The relative
composition of CO in gas phase is 0.59% / 27.28% 2.16% . The gasifier in our model is operated at 24atm, i.e. P t 24atm . Based on Eq. (2),
V 2 27.28% 1 0.066 ln 24 21.56% . Then, the yield of CO at system pressure is 21.56% 2.16% 0.47% .
4 Reactions
11
(a)
(b)
(d) (c) Figure 3. Coal pyrolysis results of Suuberg et al.’s work [6 : (a) Pyrolysis product distributions from lignite heated to different peak temperatures [( )tar; (Δ)tar and other hydrocarbons (HC); (*)tar, HC, and CO; (º)tar, HC, CO, and CO 2 ; (T)total, i.e. tar, HC, CO, CO2 , and H 2O. Pressure=1atm (helium). Heating rate: (single points) 1000ºC/s; (points inside º) 7100 to 10000ºC/s; (points inside Δ) 270 to 470ºC/s; (points inside □) 1000ºC/s, but two-step heating; (b) Yields of methane, ethylene, and hydrogen from lignite pyrolysis to different peak temperatures [(Δ)CH 4 ; (*)C 2H 4 ; (º)H 2. Pressure=1atm (helium); heating rate=1000ºC/s]. (c) Yields of water, carbon monoxide, and carbon dioxide from lignite pyrolysis to different peak temperatures [(Δ)H 2O; (×)CO2 ; ( )CO. Pressure=1atm (helium); heating rate=1000ºC/s]. (d) Elemental compositions of chars from lignite pyrolysis to different peak temperatures [(*)C; (º)H; (×)N; ( )S; (Δ)O. Pressure=1atm (helium); heating rate=1000ºC/s].
12
4 Reactions
After getting the yield of each volatile product, the yield of char is found by subtracting the yield of all volatile products from unity.
Table 3. Yield of Coal Pyrolysis Products at 1atm Used in the Model[1, 5] Components
Yield (mass basis on original coal)
CO
0.0059
H2
CO2
0.0084
0.003
H2O
0.0079
H2S
0.0094
N2 CH4 C6H6
0.0035
0.1637
0.071
Char
0.7272
Total
1
4.2 Volatile combustion 4.2.1 Reactions From Eq. (1), the volatile matter is composed of CO, H 2, CO2, H2O, H2S, N2, CH4, and C6H6. Among these gases, CO, H 2, CH4, and C6H6 are combustible gases. So after the coal pyrolysis, these combustible gases will react with oxygen fed into the gasifier, as shown in reactions (3-6).
C 6 H 6 7.5O2 6CO2 3 H 2O
(3)
H 2 0.5O2 H 2 O
(4)
CO 0.5O2 CO2
(5)
CH 4 2O2 CO2 2 H 2O
(6)
4.2.2 Reaction kinetics Since the reaction rate of gaseous combustion is generally fast and the combustible gases will be consumed up in a short time, the reaction kinetics of the volatile combustion process are neglected in the model. The conversions of C6H6, H2, CO, and CH4 are assumed to be 100%.
4 Reactions
13
4.3 Char gasification 4.3.1 Reactions After the volatile combustion process, the char from coal pyrolysis is further gasified by the reaction with gases in the gas phase. This process may include reactions (4-6) above, as well as reactions (7-13):
C
1 2 O2 21 CO 1CO2 1
(7)
C H 2O CO H 2
(8)
C CO2 2CO
(9)
C 2 H 2 CH 4
(10)
S H 2 H 2 S
(11)
CH 4 H 2O CO 3 H 2
(12)
CO H 2 O CO2 H 2
(13)
In reaction (7), is a coefficient which depends on the diameter of the coal particle ( d p ) and can be calculated by the relations[1] in Table 4. Fig. 4 shows the calculated relationship between and d p at various temperatures. For a given temperature, is constant at d p
0.005cm
and d p
0.1cm . At
0.005 d p 0.1cm , decreases with the increase in d p . For a given d p , shows a slight change with the temperature at d p
0.1cm . At d p 0.1cm ,
is independent of temperature and has the value of 1.0.
Table 4. Expressions of for Different Size of Coal Particle[1] d p (cm) <0.005
0.005-0.1
>0.1
Comment
2 Z 2
Z 2
2 Z 2
Z d p 0.005
0.095 Z 2
Z
CO CO 2
2500 e
6249 T
1.0
Note: CO and CO 2 are concentrations of CO and CO2, respectively. T is temperature, K.
14
4 Reactions
Figure 4. Relationship between
and d p at various temperatures
4.3.2 Reaction kinetics Reactions (7-11) are caused by the reaction of char with gaseous components in the gas phase. The unreacted-core shrinking model [2] is used to describe their kinetics. This is attributed to the following two points.
In the real gasifier, the char-gas reactions can be considered as surface reactions because of high operating temperature (typically above 1000ºC).
Since the solid loading in the gasifier is usually very small, the particle collision is infrequent and then the ash layer formed can be assumed to remain on the particle during reactions.
In this model, the effects of ash layer diffusion, gas film diffusion and chemical reaction are considered. The overall rate is expressed as Eq. (14):
RC i
1 1
k diff
1 1 1 k sY 2 k dash Y 1
P P *
i
i
(14)
Where
k diff = gas film diffusion constant, g/cm2·atm·s.
4 Reactions
15
k s = surface reaction constant, g/cm2·atm·s. n k dash = ash film diffusion constant, g/cm2·atm·s. k dash k diff , where is
voidage in the ash layer; n is a constant ranging from 2 to 3. In the model, 0.75 and n 2.5 . 1
Y
1 x 3 , where r c r p 1 f r c
is the radius of the unreacted core; r p is the
radius of the whole particle including the ash layer; is coal conversion at any time after pyrolysis is completed, based on original d.m.m.f. coal; and f is coal conversion when pyrolysis is completed, based on original d.m.m.f. coal.
P i P i * = effective partial pressure of i -component taking account of the reverse reaction, atm.
RC i = reaction rate, g of carbon/(cm2 of coal particle surface area)·s. The k diff , k s and P i P i of reactions (7-10) from the work of Wen and *
Chaung[1] are listed in Table 5. For the kinetics of reaction (11), we adopt the expression similar to that of reaction (10), because the kinetics model is not available now. The corresponding k diff , k s , and P i P i are given in Table 5. *
In the derivation of P i P i , the relationship between K eq and T is found in *
four steps: 1. Use a single RGibbs block to produce the equilibrium composition of reaction (11) at various temperatures. 2. Calculate K eq at various temperatures based on the equation
K eq
P H 2S P H 2
, where P H 2 S and P H 2 are partial pressures, atm.
3. Make a linear fit with ln K eq
as Y-axis and 1/T as X-axis.
4. Transform the equation fitted in the third step to generate the relationship between K eq and T .
16
4 Reactions
Table 5. Parameters for Kinetics of Reactions (7-11) Reactions
(7)
k diff
k s
4.26 T 0.292 T 1800
P i P i *
Comment
Source
P O2
------
[1]
1.75
17967
8710 e
T
21060
21060
P t d p
T 10 10 2000
0.75
4
(8)
T
247 e
P t d p
T 7.45 10 2000
K eq e
P CO2
------
17 .644
30260
[1]
1.8T
0.75
4
(9)
P H 2 P CO P H 2O K eq
247 e
T
[1]
P t d p
T 1.33 10 2000
0.75
3
(10)
0.12 e
17921
T
P H 2
P t d p
T 1 .33 10 2000 P t d p
K eq
K eq
18400
0.175 34173
e 1.8T
[1]
0.75
3
(11)
P CH 4
0.12e
17921 T
P H 2
P H 2 S K eq
K eq e
5.0657
18557.7225
[1]*
T
Note: T =temperature, K; P t =total pressure, atm; d p =diameter of coal particle, cm; is calculated according to the relations in Table 4. P O2 , P H 2O ,
P H 2 , P CO , P CO2 , P CH 4 and P H 2 S =partial pressures, atm. In the K eq expression of reactions (8) and (10), the coefficient 1.8 before T is caused by the unit conversion from Rankine degrees to Kelvin. [1]* means that the source is from reference [1] and some changes are made for this model. The kinetics of reactions (4-6) and (12-13) are shown in Table 6. The kinetics of reaction (12) are modified according to the work of Wen and Chaung [1]. In
their work, the reaction rate of reaction (12) is described as 312e
30000 1.987T
C CH
4
,
where the reaction rate of the reverse reaction is not considered. However, the reaction of CH4 and H2O is generally reversible. So, the kinetics for the CH4-H2O reaction are rewritten as the expression in Table 6. In deriving its relationship between K eq and T , the steps are similar to those adopted for reaction (11). The difference is that the calculation of K eq from the equilibrium composition is based on the equation K eq
3 C CO C H 2
C CH 4 C H 2O
, where
C CO , C H 2 , C CH 4 , and C H 2O are concentrations, mol/m 3.
4 Reactions
17
Table 6. Kinetics of Reactions (4-6) and (12-13) Reaction
(4)
(5)
Reaction rate
8 . 83 10 5 e
9 . 976 10
9 .976 10
30 . 9 e
Unit
Source
------
mol/m3·s
[7]
------
mol/m3·s
[7]
------
mol/m3·s
[7]
mol/m3·s
[1]*
mol/[s·(g of ash)]
[1]
4
8 . 315 T
C H 2 C O 2
4
8 . 315 T
C CO C O 2
9.304 10
5
(6)
3 .552 10 11 e
(12)
3 C CO C H 1 .987 T 312 e C CH K eq C H O
Comment
8.315 T
C CH 4 C O2
30000
2
4
K eq e
33 .1371
25014 .0499
T
2
xCO (13)
* F w 2.77 10 5 xCO xCO e 0.5
P t
P t 250
e
8.91
5553 T
27760 1.987 T * x CO
P CO P t 1 P CO2 P H 2
P t K eq P H 2O
K eq e
3 .6893
7234 1.8T
Note: T =temperature, K; C H 2 , C O2 , C CO , C CH 4 , and C H 2O =concentrations, mol/m3; P CO , P CO2 , P H 2 , and P H 2O =partial pressures, atm; P t =total pressure, atm; F w =adjustable parameter, which represents the relative catalytic reactivity of ash to that of iron-base catalyst. In the model, F w 0 .2 . In the reaction rate expressions of reactions (4-6), the coefficient 8.315 before T stands for the universal gas constant in J/mol·K. In reaction rate expressions of reactions (12-13), the coefficient 1.987 before T means the universal gas constant in cal/mol·K. In the K eq expression of reaction (13), the coefficient 1.8 before T is caused by the unit conversion from Rankine degrees to Kelvin. [1]* means that the source is from reference (1) and some changes are made for this model.
18
4 Reactions
5 Simulation Approach
Fig. 5 shows the flowsheet for the whole coal gasification process. The quench section for cooling the hot gas from the gasification section is not simulated in this model. The function of each block is shown in Table 7. PYROLYS and PRESCORR blocks are used to simulate the coal pyrolysis process. The COMBUST block is used to model the volatile combustion process. The GASIFIER block is for the char gasification process. Other blocks are used for helping these four blocks to simulate the above three processes.
Figure 5. Flowsheet for coal gasification
5 Simulation Approach
19
Table 7. Function of Each Block Block
Model
Function
PYROLYS
RYield
Simulate the coal pyrolysis based on the results of the pyrolysis experiment at 1atm
PRESCORR
RYield
Make pressure correction for the yield of each product from the pressure in the pyrolysis experiment (i.e. 1atm) to the pressure in the real gasifier
SEPSG
Sep2
Separate the gas and solid char
COMBUST
RStoic
Model the volatile combustion
SEPELEM
RStoic
Decompose char into C, H2, O2, N2, S, and ash in order to easily deal with the solid reactions in GASIFIER block
MIXER
Mixer
Mix the feedstock for the GASIFIER block
GASIFIER
RPlug
Model the char gasification process
SPELMCAL
Calculator
Determine the stoichiometric coefficients of C, H 2, O2, N2, S, and ash in reaction of the SEPELEM block
GASIFCAL
Calculator
Correct the solid residence time in the GASIFIER block
5.1 Unit Operations 5.1.1 Coal pyrolysis In the model, coal pyrolysis process is simulated with two RYield reactors, the PYROLYS and PRESCORR blocks . The first RYield reactor, PYROLYS, is used to simulate the coal pyrolysis at 1atm based on the results of the pyrolysis experiment. The second RYield reactor, PRESCORR, is used to make a pressure correction for the yield of each component generated in the PYROLYS block. The correction method is described in section 4.1.2.2. In the model, the correction is automatically done by a user-subroutine called USRPRES.
5.1.2 Volatile combustion Since the reaction rate of volatile combustion is generally fast and the combustible gases can be considered to be consumed up in a short time, the kinetics of volatile combustion process is neglected in the model. An RStoic reactor, the COMBUST block, is used to simulate the volatile combustion process. The fractional conversions of combustible gases are all set as 1.0.
5.1.3 Char gasification In the model, the char gasification process is model with an RPlug reactor, the GASIFIER block. In this process, the reaction kinetics and residence time of
20
5 Simulation Approach
char are the two main factors affecting the remaining carbon conversion and product composition.
5.1.3.1 Treatment of reaction kinetics From the kinetics models in section 4.3.2, most kinetics are so complex that they can’t be treated by the built-in kinetics expression template in Aspen Plus. So these reactions' kinetics are provided in a user subroutine called USRKIN. In the user kinetics subroutine of RPlug, the output is the reaction rate of each component taking part in the reactions. The reaction rate of each conventional component must be provided in the unit of kgmole/m·s. This unit is derived by the relation that rates per unit volume are multiplied by the cross-sectional area covered by the reacting phase, i.e. kgmole/m·s = (kgmole/m3·s)·(m2). So in order to get the required reaction rate of each component, the following two steps are taken. 1. Convert the unit of each reaction rate in section 4.3.2 to kgmole/m·s. 2. Calculate the total reaction rate of each component according to the stoichiometry of reactions.
Unit conversion for rate of each reaction The rates of reactions (7-10) are in the unit of g of carbon/(cm 2 of coal particle surface area)·s. The conversion of this unit follows the steps shown in Fig. 6. In the conversion, the coal particle is assumed to be spherical.
Figure 6. Schematic diagram for unit conversion of reactions (7-10)
Step 1:
R
1 C i
RC i
4 r p2 4 3
2 C i
R
1 C i
Step 2:
R
Step 3:
RC 3 i RC 2 i
r p3
10 3 / 12 10 6
4
D 2 1 V bed
Combining above three steps gives the following total conversion expression:
5 Simulation Approach
21
3 C i
R
RC i
D 2 16 10 3 r p
1 V bed
(15)
Where
r p = radius of coal particle, cm. D = diameter of gasifier, m. V bed = void fraction in gasifier. V bed 1 V particle , where V particle is particle fraction in gasifier. V particle
F coal t coal , where F coal 4 D 2 h
is coal flow rate; t
is residence time of coal in the gasifier; coal is coal density; and
h is
gasifier length. V bed is first calculated in a calculator block called GASIFCAL, then transferred back to the user kinetics subroutine. The unit of reaction (11)’s rate ( R S H 2 ) is g of sulfur/(cm2 of coal particle surface area)·s, which is very similar to the unit of reactions (7-10). The difference is that we just change the molecular weight of carbon to that of sulfur. The final relation is:
R
3 S H 2
RS H 2
3 D 2 128 10 3 r p
1 V bed
(16)
The units for rates of the four gaseous reactions (4-6) and (12) are mol/m3(gas phase)·s. The steps for this unit conversion are shown in Fig. 7.
Figure 7. Schematic diagram for unit conversion of reactions (4-6) and (12)
Step 1:
Ri1 Ri 10 3
Step 2:
Ri Ri 2
1
4
D V bed 2
Based on the above two steps, the total conversion expression is:
R Ri 10 2 i
3
D 2 4
V bed
(17)
For reaction (13), the rate of reaction ( RCO H 2O ) is in the unit of mol/[s·(g of ash)]. The unit conversion takes the steps shown in Fig. 8.
22
5 Simulation Approach
Figure 8. Schematic diagram for unit conversion of reaction (13)
Step 1:
1 RCO H 2O RCO H 2O 1 Y moisture Y ash
Step 2:
2 1 RCO H 2O RCO H 2O coal
3 CO H 2 O
R
2 CO H 2O
Step 3:
R
Step 4:
RCO H 2O RCO H 2O 4
3
10 3 10 6
4
D 1 V bed 2
Combining above four steps yields the total conversion expression:
R
4 CO H 2 O
RCO H O 1 Y moisture Y ash coal 2
D 2 4 10
3
1 V bed
(18)
Where
Y moisture = moisture fraction in original coal, wet basis. Y ash = ash fraction in original coal, dry basis.
Total reaction rate of components After making the unit conversion for the rate of each reaction, we can get the total reaction rate of each component according to the stoichiometry of reactions. Take the total reaction rate of H 2 as an example. From sections 4.2.1 and 4.3.1, there are six reactions involving H2, which are reactions (4), (8), and (10-13). The stoichiometric coefficient of H 2 in each reaction is listed in Table 8. Meanwhile, the abbreviation for the rate of each reaction is also listed in Table 8. Based on Table 8, the total reaction rate of H2 is: 2 4 3 RCO R H 2 RC 3 H 2O RC 3 H 2 2 RS 3 H 2 R H 2 2 O2 RCH H 2O 4 H 2 O
5 Simulation Approach
(19)
23
Table 8. Stoichiometric Coefficient of H2 in Each Reaction and Abbreviation of Each Reaction Rate Stoichiometric coefficient of H2
Rate of reaction (kgmole/m·s)
(4)
-1
2 R H 2 O2
(8)
1
RC 3 H 2O
(10)
-2
RC 3 H 2
(11)
-1
R S 3 H 2
(12)
3
2 RCH 4 H 2 O
(13)
1
4 RCO H 2 O
Reactions
Note: If H2 is a reactant, the coefficient is negative. If H2 is a product, the coefficient is positive.
5.1.3.2 Residence time of solid In the model, the char gasification process is simulated by an RPlug reactor. In RPlug, the residence time is calculated by Eq. (20):
t
V R
0
1 V
dV R
(20)
Where
V R = reactor volume.
V = volumetric flow rate of gases. In Eq. (20), V is the product of gas velocity multiplied by cross-sectional area of the reactor. This means that the calculation of residence time in RPlug mainly depends on the velocity of the gas phase. However, the solid residence time is closely related to the velocity of the solid phase. So to get the correct residence time of solids in the GASIFIER block, an external Fortran Calculator block called GASIFCAL block is used to calculate the solid residence time before executing the GASIFER block. This Calculator block includes three main parts: 1. Selection of model of downward velocity of solid; 2. Calculation of solid residence time; 3. Return of results from the GASIFCAL block to the GASIFIER block.
Selection of model of downward velocity of solid In the entrained-flow gasifier, the coal particle size is very small, typically less than 500µm[4]. Using the coal parameter of 500µm and the input conditions in
24
5 Simulation Approach
Tables 2-3 and 11-12, the viscosity ( µ), velocity (u) and density ( ρ) of product gases at the outlet of gasifier are calculated by this Aspen Plus model. µ = 5.73×10-5Pa·s. u = 0.03m/s. ρ = 3.03kg/m3. Then, the Reynolds number of particles is calculated to be 0.79 based on the equation
Re p
d p u
, where d p is the coal particle diameter, 5×10-4m. In the whole
gasifier, the temperature at the outlet is the lowest, indicating the µ and ρ of gases at the outlet are the lowest and the largest, respectively. Meanwhile, the amount of product gases is the largest at the outlet of the gasifier, and then the corresponding u of gases is the largest in the whole gasifier. So, we can assume that the Re p in the whole gasifier is less than 0.79. Considering
2 [8], we can conclude that Stokes’
the valid regime of Stokes’ law, i.e. Re p
law is applicable for the solid flow in this system. According to Newton’s second law and Stokes’ law, Eq. (21) is derived for downward velocity of solid ( v s )[1].
v s v s ,i e
bt
v g vt 1 e bt
(21)
Where
b
18 s d p2
.
vt = terminal settling velocity of particle in a static fluid. vt
s
g d p2 g 18
,
where v s ,i is initial velocity of solid; v g is velocity of gas phase; is gas viscosity; s is density of solid; g is density of gas; d p is diameter of solid particles.
Calculation of solid residence time Integrating Eq. (21) gives the relationship between gasifier length (h) and solid residence time (t ):
1 e bt 1 e v g vt t h v s dt 0 b b t
v s ,i
bt
(22)
Based on Eq. (22), the solid residence time is calculated by Newton’s method. In the calculation, g , , and v g use the values at the inlet of the GASIFIER block; s takes the average value in the gasifier based on the harmonious square root, i.e. s
2 s2,i s2,o
s2,i s2,o
, where s ,i and s,o are the solid densities
at the inlet and outlet of GASIFIER block, respectively. Because the conversion of useful components in coal is generally close to 100% in practical application, we assume that the solid density at the outlet of the gasifier is equal to the density of ash, i.e. s ,o coal 1 Y moisture Y ash , where
5 Simulation Approach
25
coal is inlet coal density; Y moisture is moisture content in coal, wet basis; and Y ash is ash content in coal, dry basis.
Return of results from GASIFCAL to GASIFIER Through the above calculation, the solid residence time in the gasifier has been approximately calculated in the GASIFCAL block. The next step is to transfer the results from the GASIFCAL block to the GASIFIER block, so that the solid residence time in the GASIFIER block can be corrected correspondingly. However, in the GASIFIER block, the residence time is used as an output variable, not an input variable. This means that the solid residence time cannot be transferred directly, so another route is taken. The diameter of the gasifier, which is an input variable in the GASIFIER block, is used as the transferred variable. The residence time calculated in GASIFCAL block is first transformed to the gasifier diameter (D) based on Eq. (23):
V g t
4
D 2 h
(23) Where
V g = volumetric flow rate of gas phase at the inlet of GASIFIER block. t = residence time.
h = gasifier length. Transforming Eq. (23) gives the expression for D:
D
V g t 4
h
(24)
After getting the gasifier diameter based on Eq. (24) and transferring it to the GASIFIER block, the solid residence time in the GASIFIER block is corrected correspondingly.
5.2 Streams Streams represent the material and energy flows in and out of the process. This model includes two types of streams, material and heat streams, as shown in Fig. 5. The streams with solid lines represent material streams. The streams with dashed lines represent heat streams.
5.3 Calculator Blocks This model includes two Calculator blocks, as shown in Table 9.
26
5 Simulation Approach
Table 9. Calculators Used in the Model Name
Function
SPELMCAL
Determine the stoichiometric coefficients of C, H2, O2, N2, S, and ash in reaction of the SEPELEM block
GASIFCAL
Correct the solid residence time in the GASIFIER block
5.4 Convergence The convergence method impacts simulation performance greatly. Inappropriate convergence methods may result in the failure of convergence or long running time. In this model, the choice of convergence method for the RPlug reactor called GASIFIER is very important. The convergence parameters for the GASIFIER block in the example model are summarized in Table 10. These are specified on the sheet Blocks | GASIFIER | Convergence | Integration Loop.
Table 10. Convergence Parameters for GASIFIER Block Items
Integration parameters
Corrector Integration error
5 Simulation Approach
Parameters
Setup
Integration convergence tolerance
0.0001
Initial step size of integration variable
1E-8
Maximum step size of integration variable
0.001
Maximum number of integration steps
1E6
Convergence method
Newton
Error tolerance ratio
0.1
Error scaling method
Dynamic
Minimum scale factor
1E-10
27
6 Simulation Results
In this model, the input conditions for the simulation and the corresponding experimental results are from the open literature [1, 5]. The input conditions for the simulation are summarized in Tables 2, 3, 11, and 12. Table 2 gives the component attributes of coal including the results of proximate, ultimate, and sulfur analyses. Table 3 shows the yield of each pyrolysis product obtained from the coal pyrolysis experiment at 1atm. Table 11 summarizes the feed conditions of coal, steam, and oxygen streams. For the coal stream, it includes the flow rate of coal, inlet temperature and pressure, diameter of coal particle, and velocity of coal particle entering into the gasifier. For the steam stream, it contains the ratio of steam to coal flow rates, inlet temperature, and pressure. At our feed conditions of 696.67K and 24atm, the steam enters the gasifier in a superheated state. For the oxygen stream, it includes the ratio of oxygen to coal flow rates, inlet temperature, and pressure.
Table 11. Feedstock Conditions for Simulation[1, 5] Feedstock
Coal
Oxygen
Steam
Parameter
Value
Unit
Flow rate
76.66
g/s
Temperature
505.22
K
Pressure
24
atm
Diameter of particle
350
µm
Velocity entering into gasifier
3
m/s
Ratio of oxygen to coal flow rates
0.866
dimensionless
Temperature
298
K
Pressure
24
atm
Ratio of steam to coal flow rates
0.241
dimensionless
Temperature
696.67
K
Pressure
24
atm
Table 12 gives the operating conditions and configuration parameters of the gasifier, which are the operating pressure, gasifier length, and diameter. In the work of Wen and Chaung [5], they assume that the coal pyrolysis and volatile combustion processes account for 4.6% of the length of whole gasifier. So in our model, the length for char gasification process simulated in the GASIFIER block is set as 325 (1 4.6%) 310cm .
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6 Simulation Results
Table 12. Operating Condition and Configuration of Gasifier[1, 5] Parameter
Value
Unit
Pressure
24
atm
Length
3.25
m
Diameter
1.5
m
Based on above input conditions, we get the results at the outlet of the gasifier, as shown in Table 13 as Aspen Plus model a. For comparison, the corresponding results of Wen and Chaung’s work [1] are also shown in the table. From the table, it can be seen that Wen and Chaung’s results [1] are in a better agreement with experimental data. In our simulation results, the CO and CO2 flow rates and carbon conversion are somewhat different from the experimental data. The CO flow rate and carbon conversion are greater than the experimental data. The CO2 flow rate is less than the experimental data. The difference in CO and CO2 flow rates and carbon conversion may be attributed to the higher temperature in the gasifier calculated by our model. In our model, the temperature at the outlet of gasifier is 1771.2K. However, the outlet temperature is 1421.9K in Wen and Chaung’s model. The increase in temperature of gasifier speeds up the reaction rate of solid carbon and gases, i.e. rate of reactions (7-10), and then makes the carbon conversion in our simulation greater than the experimental data. The difference in flow rates of CO and CO2 depends on reaction (13), an exothermic reaction. Increasing the temperature will make the reaction shift in the backward direction, i.e. increasing the amount of CO and decreasing the amount of CO 2. So, increasing the temperature decreases the simulated CO2 flow rate and increases the simulated CO flow rate. Why does our simulation generate the higher temperature in the gasifier? This may be caused by two points. The first point is the heat of combustion (HCOMB) of coal. In our model, the HCOMB of coal is calculated by the builtin method (Boie correlation) in Aspen Plus due to lack of accurate experimental data. However, we believe the HCOMB has a significant effect on the enthalpy of coal. So, the inaccurate HCOMB may make the incorrect enthalpy of coal and then result in the large departure of gasifier temperature. The second point is the amount of heat loss in the gasifier. In our work, the model is simulated in an adiabatic mode. In Wen and Chaung’s model[1], the heat loss to the environment is considered. Therefore, combining these two points may cause the higher temperature of the gasifier in our model. In order to further validate our explanation that the difference in CO and CO 2 flow rates and carbon conversion is caused by the higher temperature in our model, we manually input the HCOMB of coal to match our outlet temperature with Wen and Chaung’s work.
6 Simulation Results
29
Table 13. Comparison of Experimental and Modeling Results Experimental Parameters
[1]
Wen and Chaung’s model[1]
Aspen Plus model a
Aspen Plus model b
Flow rate (g/s)
Mole fraction (%, dry basis)
Flow rate (g/s)
Mole fraction (%, dry basis)
Flow rate (g/s)
Mole fraction (%, dry basis)
Flow rate (g/s)
Mole fraction (%, dry basis)
CO
123.77
57.57
123.94
56.60
127.71
58.98
123.44
57.41
H2
6.01
39.13
6.23
39.84
5.96
38.23
5.99
38.71
CO2
9.985
2.95
10.04
2.92
6.462
1.90
10.24
3.03
CH4
0.15
0.12
0.20
0.16
0.13
0.10
0.24
0.20
H2S
0.133
0.06
0.726
0.27
1.405
0.53
1.04
0.40
N2
0.53
0.12
0.454
0.208
0.54
0.25
0.54
0.25
Carbon conv. (%)
98.64
98.88
99.95
98.69
Temp. (K)
------
1421.9
1771.2
1423.2
Note: In model a, HCOMB of coal is calculated by Boie correlation; in model b, HCOMB of coal is manually input as 13416Btu/lb. When the HCOMB of coal is input as 13416Btu/lb (compared with the HCOMB of coal calculated as 14080.83Btu/lb by the Boie correlation), the outlet temperature is equal to 1423.2K, which matches the temperature (1421.9K) in Wen and Chaung’s work. At the same time, the CO and CO2 flow rates and carbon conversion are also in a good agreement with the experimental data, as shown in Table 13 as Aspen Plus model b. Fig. 9 shows the corresponding profile of main product gases (CO, H 2, H2O, and CO2) in char gasification process. In Fig. 9b, Wen and Chaung show the profile of product gas in the whole gasifier and the corresponding profile in the char gasification process is marked on the figure. Our model shows a similar result to Wen and Chaung’s model. As the residence time increases, H 2O and CO2 contents in the gas phase decrease, and CO and H 2 contents increase. Based on these phenomena, it can be determined that the product gas composition and carbon conversion are strongly dependent on the temperature in the gasifier, and HCOMB of coal and heat loss are two key parameters in determining the temperature in the gasifier.
30
6 Simulation Results
70
70
60
60
50
50
) s i s a b 40 t e W ( % . 30 L O V
40
30 CO H2
20
20 H2O CO2
10
10
0
0 0
1
2
3
4
5
6
7
8
9
10
Residence time (s)
(a)
(b) Figure 9. Profile of product gas composition: (a) in char gasification process based on Aspen Plus model and (b) in whole gasifier based on Wen and Chaung’s model [1] (solid residence time in whole gasifier is 9.5s).
6 Simulation Results
31
7 Conclusions
A Texaco down-flow entrained flow gasifier model is developed with the Aspen Plus simulator. The model follows the modeling approach suggested by Wen and Chaung[1]. In the model, the kinetics of char gasification and the hydrodynamics for calculating solid particle residence time are considered. Reasonable simulation results were obtained compared with the experimental results. The Aspen Plus model provides a useful modeling framework for future refinements as new knowledge is gained with the entrained flow gasifier. To use this model, the following data should be provided:
Component attributes and higher heat of combustion of coal. The component attributes of coal include the data of proximate, ultimate, and sulfur analyses.
Yield of coal pyrolysis products from coal pyrolysis experiment at 1atm.
Feed conditions of coal, oxygen and steam streams, which include the flow rate, temperature and pressure. The coal stream also includes the diameter of coal particle and velocity fed into the gasifier.
Configuration parameters and operational conditions of the gasifier, which include gasifier height, gasifier diameter, operating pressure, and heat loss. The heat loss can also be in-situ calculated by providing heat transfer coefficient and environmental temperature.
Model parameters, which include porosity of ash layer and reactivity of ash for the reaction of CO and H 2O.
From the model, the following information can be obtained:
32
Profile of flow rate of products Profile of carbon conversion Profile of temperature Pressure of exit gas and solid Solid residence time in the gasifier
7 Conclusions
