aspen plus model for entrained flow coal gasifiernsw/chbe446/aspen_plus_model... · 2017-04-24 ·...
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Model for Entrained Flow Coal Gasifier
Aspen Plus
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Revision History 1
Revision History
Version Description
V7.2 First version
V7.3 Update the model to V7.3 and add a paragraph in Introduction sectionto describe what files are released.
V7.3.2 Update the model to V7.3.2
V8.2 Update the model to V8.2
V8.4 Update the model to V8.4
V8.6 Update the model to V8.6
2 Contents
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.............................................................................................94.1.1 Reactions......................................................................................94.1.2 Amount of each pyrolysis product ....................................................9
4.2 Volatile combustion................................................................................... 134.2.1 Reactions.................................................................................... 134.2.2 Reaction kinetics.......................................................................... 13
4.3 Char gasification....................................................................................... 144.3.1 Reactions.................................................................................... 144.3.2 Reaction kinetics.......................................................................... 15
5 Simulation Approach ..........................................................................................19
5.1 Unit Operations ........................................................................................ 205.1.1 Coal pyrolysis .............................................................................. 205.1.2 Volatile combustion...................................................................... 205.1.3 Char gasification .......................................................................... 20
5.2 Streams .................................................................................................. 265.3 Calculator Blocks ...................................................................................... 265.4 Convergence............................................................................................ 27
6 Simulation Results .............................................................................................28
7 Conclusions ........................................................................................................32
References ............................................................................................................33
Introduction 3
Introduction
This file describes an Aspen Plus kinetics-based model for Texaco down-flowentrained flow gasifiers. The model follows the modeling approach proposedby 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 occurringin the gasifier, i.e. coal pyrolysis, volatile combustion, and chargasification.
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 withthe 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 basedon 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 theGUI\Examples\Entrained Flow Coal Gasifier folder of the Aspen Plusinstallation:
Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.apwz, a compoundfile containing these six files:
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
4 1 Components
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 pyrolysisat 1atm. CHAR2 represents the solid phase after pressure correction from1atm to system pressure.
2 Process Description 5
2 Process Description
The Texaco gasifier is a typical entrained flow gasifier, as shown in Fig. 1. Thetotal 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 typicallyless than 500µm[4] is mixed with water to form the coal-water slurry, andthen the slurry together with oxygen is simultaneously introduced into the topsection. Coal pyrolysis, volatile combustion and char gasification reactionstake place subsequently to produce the syngas. In this section, a specialrefractory material is lined to withstand the severe operating environment.The operating pressure is usually at 20-50atm and the temperature istypically higher than 1000ºC[4].
The lower section is a quench vessel. A reservoir of water is maintained at thebottom of the gasifier by continuous injection of cooling water. The slag and
6 2 Process Description
syngas leaving the top section of gasifier pass through a water-cooled diptube into the water reservoir. The slag remains in the water and then isremoved. The syngas is saturated with water and removed from the gasspace above the water.
3 Physical Properties 7
3 Physical Properties
In this model, the property method RK-SOAVE is used to calculate thephysical properties of mixed conventional components and CISOLIDcomponents. HCOALGEN and DCOALIGT models are used to calculate theenthalpy and density of non-conventional components, respectively.
The HCOALGEN model requires these three component attributes for non-conventional components: proximate analysis results (denoted as PROXANALin Aspen Plus), ultimate analysis results (denoted as ULTANAL in Aspen Plus),and sulfur analysis results (denoted as SULFANAL in Aspen Plus). Theproximate analysis gives the weight contents of moisture, fixed carbon,volatile matter, and ash. The ultimate analysis gives the weight compositionof coal in terms of ash, carbon, hydrogen, nitrogen, chlorine, sulfur, andoxygen. The sulfur analysis gives the weight fractions of sulfur divided intopyritic, sulfate, and organic sulfur. For the DCOALIGT model, it requires onlythese two component attributes: ULTANAL and SULFANAL. Table 2 shows thecomponent attributes of coal used in our model, which are from theliteratures of Wen and Chuang[1, 5]. Based on these analysis results, theenthalpy and density of coal are calculated, respectively.
For the characterization of char and ash generated in coal conversion, thesame methodology as the coal is applied and the same models are used tocalculate their enthalpy and density. The results of proximate, ultimate, andsulfur analyses for the char and ash are determined from the analysis data oforiginal coal and the amount of gasified gaseous product in terms of massbalance.
8 3 Physical Properties
Table 2. Component Attributes of Coal Used in the Model[1,5]
Proximate analysis Ultimate analysis Sulfur analysis
ElementValue(wt.%)
ElementValue(wt.%, drybasis)
ElementValue(wt.%, drybasis)
Moisture
(wet basis)0.2 C 74.05 Pyritic 0.59
Fixed carbon
(dry basis)58.01 H 6.25 Sulfate 0.59
Volatile matter
(dry basis)26.46 N 0.71 Organic 0.59
Ash
(dry basis)15.53 Cl 0.37
S 1.77
O 1.32
Ash 15.53
4 Reactions 9
4 Reactions
When the coal, oxygen and steam are simultaneously introduced into thegasifier, these reactions take place in sequence: coal pyrolysis, volatilecombustion and char gasification.
4.1 Coal pyrolysis
4.1.1 ReactionsIn gasifier, the temperature is typically higher than 1000ºC[4]. When coal isfed into the gasifier, it first undergoes the pyrolysis process to decompose tovolatile matter and char, as shown in Eq. (1). In our model, volatile matterincludes CO, H2, H2O, CO2, CH4, H2S, N2 and C6H6. C6H6 is used to representthe tar.
66224222 HCNSHCHCOOHHCOCharCoal
(1)
4.1.2 Amount of each pyrolysis productIn our model, the amount of each pyrolysis product is determined based onthe results of the pyrolysis experiment, which is made at 1atm. A pressurecorrection is applied to the results of this experiment, adjusting them from1atm to system pressure, to yield the amount of each product at theoperating conditions of the real gasifier. The next two sections describe howto get the pyrolysis results at 1atm from the experiment and how to make apressure 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 inexperiment. Their corresponding results are summarized here. It should benoted that in our model, the amount of each pyrolysis product at 1atm is fromthe work of Wen and Chuang[1, 5].
10 4 Reactions
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 gaseousenvironment of known pressure and composition
Electrical system, which is used to expose the sample to a controlled time-temperature 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 foldedstrip of stainless steel screen. Then, electricity is used to heat the coalparticles 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.’swork[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 reactorand on a paper filter at the exit of the reactor. Any condensation on non-linedreactor surfaces is collected by washing with methylene-chloride-soaked filterpaper.
The second type of products is collected at the conclusion of a run by purgingthe reactor vapors through two lipophilic traps. The first trap is operated atroom temperature with the Porapak Q chromatographic packing, and collects
4 Reactions 11
intermediate weight oils such as benzene, toluene and xylene. The secondtrap is also packed with Porapak Q but operated at -196ºC in a dewar of liquidnitrogen. This trap collects all fixed gases produced by pyrolysis, with theexception of hydrogen which is determined by direct vapor phase samplingwith a precision syringe.
The third type of product, char, remains on the screen and is determinedgravimetrically.
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 thesecond type collected in the first and second traps are first warmed to 240ºCand 100ºC, respectively, and then fed into the gas chromatography foranalysis. The third type of product is measured gravimetrically and itselemental analysis is analyzed by the ASTM (American Society for Testing andMaterials) standard method. The analysis results are summarized in Fig. 3.
4.1.2.2 Amount of each pyrolysis product atsystem pressure
Most pyrolysis experiments are carried out at 1atm. For example, all ofSuuberg et al.’s results[6] shown in Fig. 3 are obtained at 1atm. However, inreal coal gasifiers, the pressure is usually much higher than 1atm, typically20-50atm[4]. This indicates that the pressure effect on yield of each productneeds to be considered. In the pressure correction, the relative composition ofgas components is assumed to be constant, and only the total yield ofvolatiles is corrected. The total yield of volatiles is corrected by Eq. (2)[1]:
tPaVV ln112 (2)
Where
1V = total yield of volatiles at 1atm.
2V = total yield of volatiles at the pressure of the real gasifier.
tP = 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 relativecomposition of volatile products in the gas phase obtained from pyrolysisexperimental results at 1atm, the yield of each volatile product at theoperating conditions of real gasifier is calculated. Take the calculation of COyield as an example. Table 3 shows the yield of coal pyrolysis products at1atm 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. %28.271 V . The relative
composition of CO in gas phase is %16.2%28.27/%59.0 . The gasifier in our
model is operated at 24atm, i.e. atmPt 24 . Based on Eq. (2),
%56.2124ln066.01%28.272 V . Then, the yield of CO at system
pressure is %47.0%16.2%56.21 .
12 4 Reactions
(a) (b)
(c) (d)
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 CO2; (T)total, i.e.tar, HC, CO, CO2, and H2O. 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 peaktemperatures [(Δ)CH4; (*)C2H4; (º)H2. Pressure=1atm (helium); heating rate=1000ºC/s].(c) Yields of water, carbon monoxide, and carbon dioxide from lignite pyrolysis to different peaktemperatures [(Δ)H2O; (×)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].
4 Reactions 13
After getting the yield of each volatile product, the yield of char is found bysubtracting the yield of all volatile products from unity.
Table 3. Yield of Coal Pyrolysis Products at 1atm Used inthe Model[1, 5]
Components Yield (mass basis on original coal)
CO 0.0059
H2 0.0084
CO2 0.003
H2O 0.0079
H2S 0.0094
N2 0.0035
CH4 0.1637
C6H6 0.071
Char 0.7272
Total 1
4.2 Volatile combustion
4.2.1 ReactionsFrom Eq. (1), the volatile matter is composed of CO, H2, CO2, H2O, H2S, N2,CH4, and C6H6. Among these gases, CO, H2, CH4, and C6H6 are combustiblegases. So after the coal pyrolysis, these combustible gases will react withoxygen fed into the gasifier, as shown in reactions (3-6).
OHCOOHC 22266 365.7 (3)
OHOH 222 5.0 (4)
225.0 COOCO (5)
OHCOOCH 2224 22 (6)
4.2.2 Reaction kineticsSince the reaction rate of gaseous combustion is generally fast and thecombustible gases will be consumed up in a short time, the reaction kineticsof the volatile combustion process are neglected in the model. Theconversions of C6H6, H2, CO, and CH4 are assumed to be 100%.
14 4 Reactions
4.3 Char gasification
4.3.1 ReactionsAfter the volatile combustion process, the char from coal pyrolysis is furthergasified by the reaction with gases in the gas phase. This process may includereactions (4-6) above, as well as reactions (7-13):
22 121
121
COCOOC
(7)
22 HCOOHC (8)
COCOC 22 (9)
422 CHHC (10)
SHHS 22 (11)
224 3HCOOHCH (12)
222 HCOOHCO (13)
In reaction (7), is a coefficient which depends on the diameter of the coal
particle ( pd ) and can be calculated by the relations[1] in Table 4. Fig. 4 shows
the calculated relationship between and pd at various temperatures. For a
given temperature, is constant at cmd p 005.0 and cmd p 1.0 . At
cmd p 1.0005.0 , decreases with the increase in pd . For a given pd ,
shows a slight change with the temperature at cmd p 1.0 . At cmd p 1.0 ,
is independent of temperature and has the value of 1.0.
Table 4. Expressions of for Different Size of Coal
Particle[1]
pd (cm) Comment
<0.0052
22
Z
Z
Te
CO
COZ
6249
2
2500
0.005-0.1
2095.0
005.022
Z
dZZ
p
>0.1 0.1
Note: CO and 2CO are concentrations of CO and CO2, respectively. T is
temperature, K.
4 Reactions 15
Figure 4. Relationship between and pd at various temperatures
4.3.2 Reaction kineticsReactions (7-11) are caused by the reaction of char with gaseous componentsin the gas phase. The unreacted-core shrinking model[2] is used to describetheir kinetics. This is attributed to the following two points.
In the real gasifier, the char-gas reactions can be considered as surfacereactions because of high operating temperature (typically above1000ºC).
Since the solid loading in the gasifier is usually very small, the particlecollision is infrequent and then the ash layer formed can be assumed toremain on the particle during reactions.
In this model, the effects of ash layer diffusion, gas film diffusion andchemical reaction are considered. The overall rate is expressed as Eq. (14):
*
21
1111
1ii
dashsdiff
iC PP
YkYkk
R
(14)
Where
diffk = gas film diffusion constant, g/cm2·atm·s.
16 4 Reactions
sk = surface reaction constant, g/cm2·atm·s.
dashk = ash film diffusion constant, g/cm2·atm·s.n
diffdash kk , where is
voidage in the ash layer; n is a constant ranging from 2 to 3. In the model,
75.0 and 5.2n .
3
1
1
1
f
x
r
rY
p
c, where cr is the radius of the unreacted core; pr is the
radius of the whole particle including the ash layer; x 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.
*ii PP = effective partial pressure of i-component taking account of the
reverse reaction, atm.
iCR = reaction rate, g of carbon/(cm2 of coal particle surface area)·s.
The diffk , sk and*
ii PP 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 theexpression similar to that of reaction (10), because the kinetics model is not
available now. The corresponding diffk , sk , and*
ii PP are given in Table 5.
In the derivation of *ii PP , the relationship between eqK and T is found in
four steps:
1. Use a single RGibbs block to produce the equilibrium composition ofreaction (11) at various temperatures.
2. Calculate eqK at various temperatures based on the equation
2
2
H
SH
eqP
PK , where SHP
2and
2HP are partial pressures, atm.
3. Make a linear fit with eqKln as Y-axis and 1/T as X-axis.
4. Transform the equation fitted in the third step to generate the
relationship between eqK and T.
4 Reactions 17
Table 5. Parameters for Kinetics of Reactions (7-11)
Reactions diffksk *
ii PP Comment Source
(7)
pt dP
T
T
75.1
1800
26.4292.0
Te17967
8710
2OP ------ [1]
(8)
pt dP
T75.0
4
20001010
Te21060
247
eq
COH
OHK
PPP 2
2 T
eq eK 8.1
30260644.17
[1]
(9)
pt dP
T75.0
4
20001045.7
Te21060
247
2COP ------ [1]
(10)
pt dP
T75.0
3
20001033.1
Te17921
12.0
eq
CH
HK
PP 4
2 T
eq eK 8.1
18400
34173
175.0 [1]
(11)
pt dP
T75.0
3
20001033.1
Te17921
12.0
eq
SH
HK
PP 2
2 T
eq eK7225.18557
0657.5
[1]*
Note: T=temperature, K; tP =total pressure, atm; pd =diameter of coal
particle, cm; is calculated according to the relations in Table 4.2OP , OHP
2,
2HP , COP ,2COP ,
4CHP and SHP2
=partial pressures, atm. In the eqK expression
of reactions (8) and (10), the coefficient 1.8 before T is caused by the unitconversion from Rankine degrees to Kelvin. [1]* means that the source isfrom 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 kineticsof reaction (12) are modified according to the work of Wen and Chaung[1]. In
their work, the reaction rate of reaction (12) is described as4
987.1
30000
312 CHT Ce
,
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 theCH4-H2O reaction are rewritten as the expression in Table 6. In deriving its
relationship between eqK and T, the steps are similar to those adopted for
reaction (11). The difference is that the calculation of eqK from the
equilibrium composition is based on the equation
OHCH
HCO
eqCC
CCK
24
2
3
, where
COC ,2HC ,
4CHC , and OHC2
are concentrations, mol/m3.
18 4 Reactions
Table 6. Kinetics of Reactions (4-6) and (12-13)
Reaction Reaction rate Comment Unit Source
(4)22
4
315.8
10976.951083.8 OH
T CCe
------ mol/m3·s [7]
(5)2
4
315.8
10976.9
9.30 OCOT CCe
------ mol/m3·s [7]
(6)
24
5
315.8
10304.91110552.3 OCH
T CCe
------ mol/m3·s [7]
(12)
OHeq
HCO
CHT
CK
CCCe
2
2
4
3
987.1
30000
312 Teq eK
0499.250141371.33
mol/m3·s [1]*
(13)
T
P
t
TCOCOw
eP
exxF
t 555391.8
2505.0
987.1
27760*51077.2
t
COCO
P
Px
OHeq
HCO
t
COPK
PP
Px
2
221*
Teq eK 8.1
72346893.3
mol/[s·(gof ash)]
[1]
Note: T=temperature, K;2HC ,
2OC , COC ,4CHC , and OHC
2=concentrations,
mol/m3; COP ,2COP ,
2HP , and OHP2
=partial pressures, atm; tP =total pressure,
atm; wF =adjustable parameter, which represents the relative catalytic
reactivity of ash to that of iron-base catalyst. In the model, 2.0wF . In the
reaction rate expressions of reactions (4-6), the coefficient 8.315 before Tstands for the universal gas constant in J/mol·K. In reaction rate expressionsof reactions (12-13), the coefficient 1.987 before T means the universal gas
constant in cal/mol·K. In the eqK 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 madefor this model.
5 Simulation Approach 19
5 Simulation Approach
Fig. 5 shows the flowsheet for the whole coal gasification process. The quenchsection for cooling the hot gas from the gasification section is not simulated inthis model. The function of each block is shown in Table 7. PYROLYS andPRESCORR blocks are used to simulate the coal pyrolysis process. TheCOMBUST block is used to model the volatile combustion process. TheGASIFIER block is for the char gasification process. Other blocks are used forhelping these four blocks to simulate the above three processes.
Figure 5. Flowsheet for coal gasification
20 5 Simulation Approach
Table 7. Function of Each Block
Block Model Function
PYROLYS RYieldSimulate the coal pyrolysis based on the results of thepyrolysis experiment at 1atm
PRESCORR RYieldMake pressure correction for the yield of each product fromthe pressure in the pyrolysis experiment (i.e. 1atm) to thepressure in the real gasifier
SEPSG Sep2 Separate the gas and solid char
COMBUST RStoic Model the volatile combustion
SEPELEM RStoicDecompose char into C, H2, O2, N2, S, and ash in order toeasily 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 CalculatorDetermine the stoichiometric coefficients of C, H2, 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 pyrolysisIn 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 pyrolysisexperiment. The second RYield reactor, PRESCORR, is used to make apressure correction for the yield of each component generated in thePYROLYS block. The correction method is described in section 4.1.2.2. In themodel, the correction is automatically done by a user-subroutine calledUSRPRES.
5.1.2 Volatile combustionSince the reaction rate of volatile combustion is generally fast and thecombustible gases can be considered to be consumed up in a short time, thekinetics of volatile combustion process is neglected in the model. An RStoicreactor, the COMBUST block, is used to simulate the volatile combustionprocess. The fractional conversions of combustible gases are all set as 1.0.
5.1.3 Char gasificationIn the model, the char gasification process is model with an RPlug reactor, theGASIFIER block. In this process, the reaction kinetics and residence time of
5 Simulation Approach 21
char are the two main factors affecting the remaining carbon conversion andproduct composition.
5.1.3.1 Treatment of reaction kinetics
From the kinetics models in section 4.3.2, most kinetics are so complex thatthey can’t be treated by the built-in kinetics expression template in AspenPlus. So these reactions' kinetics are provided in a user subroutine calledUSRKIN. In the user kinetics subroutine of RPlug, the output is the reactionrate of each component taking part in the reactions. The reaction rate of eachconventional component must be provided in the unit of kgmole/m·s. Thisunit is derived by the relation that rates per unit volume are multiplied by thecross-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 eachcomponent, 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 thestoichiometry of reactions.
Unit conversion for rate of each reaction
The rates of reactions (7-10) are in the unit of g of carbon/(cm2 of coalparticle surface area)·s. The conversion of this unit follows the steps shown inFig. 6. In the conversion, the coal particle is assumed to be spherical.
Figure 6. Schematic diagram for unit conversion of reactions (7-10)
Step 1:3
2
1
3
4
4
p
p
iCiC
r
rRR
Step 2: 6
312
10
12/10
iCiC RR
Step 3: bediCiC VDRR 14
223
Combining above three steps gives the following total conversion expression:
22 5 Simulation Approach
bed
p
iCiC Vr
DRR
11016 3
23
(15)
Where
pr = radius of coal particle, cm.
D = diameter of gasifier, m.
bedV = void fraction in gasifier. particlebed VV 1 , where particleV is particle
fraction in gasifier. hD
tFV coalcoal
particle
24
, where coalF is coal flow rate; t
is residence time of coal in the gasifier; coal is coal density; and h is
gasifier length. bedV is first calculated in a calculator block called GASIFCAL,
then transferred back to the user kinetics subroutine.
The unit of reaction (11)’s rate (2HSR ) is g of sulfur/(cm2 of coal particle
surface area)·s, which is very similar to the unit of reactions (7-10). Thedifference is that we just change the molecular weight of carbon to that ofsulfur. The final relation is:
bed
p
HSHS Vr
DRR
110128
33
23
22
(16)
The units for rates of the four gaseous reactions (4-6) and (12) aremol/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 1:31 10 ii RR
Step 2: bedii VDRR 212
4
Based on the above two steps, the total conversion expression is:
bedii VD
RR
410
232
(17)
For reaction (13), the rate of reaction ( OHCOR2 ) is in the unit of mol/[s·(g of
ash)]. The unit conversion takes the steps shown in Fig. 8.
5 Simulation Approach 23
Figure 8. Schematic diagram for unit conversion of reaction (13)
Step 1: ashmoistureOHCOOHCO YYRR 122
1
Step 2: coalOHCOOHCO RR 12
22
Step 3: 6
323
10
1022
OHCOOHCO RR
Step 4: bedOHCOOHCO VDRR 14
234
22
Combining above four steps yields the total conversion expression:
bedcoalashmoistureOHCOOHCO VD
YYRR
1
1041
3
24
22
(18)
Where
moistureY = moisture fraction in original coal, wet basis.
ashY = 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 thetotal reaction rate of each component according to the stoichiometry ofreactions. Take the total reaction rate of H2 as an example. From sections4.2.1 and 4.3.1, there are six reactions involving H2, which are reactions (4),(8), and (10-13). The stoichiometric coefficient of H2 in each reaction is listedin Table 8. Meanwhile, the abbreviation for the rate of each reaction is alsolisted in Table 8. Based on Table 8, the total reaction rate of H2 is:
4223332 22422222
32 OHCOOHCHOHHSHCOHC RRRRRRHR (19)
24 5 Simulation Approach
Table 8. Stoichiometric Coefficient of H2 in Each Reactionand Abbreviation of Each Reaction Rate
ReactionsStoichiometriccoefficient of H2
Rate of reaction(kgmole/m·s)
(4) -12
22 OHR
(8) 13
2OHCR
(10) -23
2HCR
(11) -13
2HSR
(12) 32
24 OHCHR
(13) 14
2OHCOR
Note: If H2 is a reactant, the coefficient is negative. If H2 is a product, thecoefficient 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):
R
V
dVV
tR
0
1(20)
Where
RV = 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 RPlugmainly depends on the velocity of the gas phase. However, the solidresidence time is closely related to the velocity of the solid phase. So to getthe correct residence time of solids in the GASIFIER block, an external FortranCalculator block called GASIFCAL block is used to calculate the solid residencetime before executing the GASIFER block. This Calculator block includes threemain 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 lessthan 500µm[4]. Using the coal parameter of 500µm and the input conditions in
5 Simulation Approach 25
Tables 2-3 and 11-12, the viscosity (µ), velocity (u) and density (ρ) ofproduct gases at the outlet of gasifier are calculated by this Aspen Plusmodel. µ = 5.73×10-5Pa·s. u = 0.03m/s. ρ = 3.03kg/m3. Then, the Reynoldsnumber of particles is calculated to be 0.79 based on the equation
ud p
p Re , where pd 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 ofgases 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, andthen the corresponding u of gases is the largest in the whole gasifier. So, we
can assume that the pRe in the whole gasifier is less than 0.79. Considering
the valid regime of Stokes’ law, i.e. 2Re p[8], we can conclude that Stokes’
law is applicable for the solid flow in this system. According to Newton’ssecond law and Stokes’ law, Eq. (21) is derived for downward velocity of solid
( sv )[1].
bttg
btiss evvevv 1, (21)
Where
2
18
psdb
.
tv = terminal settling velocity of particle in a static fluid.
18
2 gdv
pgs
t
,
where isv , is initial velocity of solid; gv is velocity of gas phase; is gas
viscosity; s is density of solid; g is density of gas; pd is diameter of solid
particles.
Calculation of solid residence time
Integrating Eq. (21) gives the relationship between gasifier length (h) andsolid residence time (t):
b
etvve
b
vdtvh
bt
tgbtist
s
11,
0(22)
Based on Eq. (22), the solid residence time is calculated by Newton’s method.
In the calculation, g , , and gv 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.2,
2,
2,
2,2
osis
osis
s
, where
is , and os, are the solid densities
at the inlet and outlet of GASIFIER block, respectively. Because theconversion of useful components in coal is generally close to 100% inpractical application, we assume that the solid density at the outlet of the
gasifier is equal to the density of ash, i.e. ashmoisturecoalos YY 1, , where
26 5 Simulation Approach
coal is inlet coal density; moistureY is moisture content in coal, wet basis; and
ashY 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 hasbeen approximately calculated in the GASIFCAL block. The next step is totransfer the results from the GASIFCAL block to the GASIFIER block, so thatthe solid residence time in the GASIFIER block can be correctedcorrespondingly. However, in the GASIFIER block, the residence time is usedas an output variable, not an input variable. This means that the solidresidence time cannot be transferred directly, so another route is taken. Thediameter of the gasifier, which is an input variable in the GASIFIER block, isused as the transferred variable.
The residence time calculated in GASIFCAL block is first transformed to thegasifier diameter (D) based on Eq. (23):
hDtVg 2
4
(23)
Where
gV = 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:
h
tVD
g
4
(24)
After getting the gasifier diameter based on Eq. (24) and transferring it to theGASIFIER block, the solid residence time in the GASIFIER block is correctedcorrespondingly.
5.2 StreamsStreams represent the material and energy flows in and out of the process.This model includes two types of streams, material and heat streams, asshown in Fig. 5. The streams with solid lines represent material streams. Thestreams with dashed lines represent heat streams.
5.3 Calculator BlocksThis model includes two Calculator blocks, as shown in Table 9.
5 Simulation Approach 27
Table 9. Calculators Used in the Model
Name Function
SPELMCAL Determine the stoichiometric coefficients of C, H2, O2, N2, S, and ashin reaction of the SEPELEM block
GASIFCAL Correct the solid residence time in the GASIFIER block
5.4 ConvergenceThe convergence method impacts simulation performance greatly.Inappropriate convergence methods may result in the failure of convergenceor long running time. In this model, the choice of convergence method for theRPlug reactor called GASIFIER is very important. The convergence parametersfor 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 Parameters Setup
Integrationparameters
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
CorrectorConvergence method Newton
Error tolerance ratio 0.1
Integration errorError scaling method Dynamic
Minimum scale factor 1E-10
28 6 Simulation Results
6 Simulation Results
In this model, the input conditions for the simulation and the correspondingexperimental results are from the open literature[1, 5]. The input conditions forthe simulation are summarized in Tables 2, 3, 11, and 12. Table 2 gives thecomponent attributes of coal including the results of proximate, ultimate, andsulfur analyses. Table 3 shows the yield of each pyrolysis product obtainedfrom the coal pyrolysis experiment at 1atm. Table 11 summarizes the feedconditions of coal, steam, and oxygen streams. For the coal stream, itincludes the flow rate of coal, inlet temperature and pressure, diameter ofcoal particle, and velocity of coal particle entering into the gasifier. For thesteam stream, it contains the ratio of steam to coal flow rates, inlettemperature, and pressure. At our feed conditions of 696.67K and 24atm, thesteam enters the gasifier in a superheated state. For the oxygen stream, itincludes the ratio of oxygen to coal flow rates, inlet temperature, andpressure.
Table 11. Feedstock Conditions for Simulation[1, 5]
Feedstock Parameter Value Unit
Coal
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
Oxygen
Ratio of oxygen to coal flow rates 0.866 dimensionless
Temperature 298 K
Pressure 24 atm
Steam
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 thegasifier, which are the operating pressure, gasifier length, and diameter. Inthe work of Wen and Chaung[5], they assume that the coal pyrolysis andvolatile combustion processes account for 4.6% of the length of wholegasifier. So in our model, the length for char gasification process simulated in
the GASIFIER block is set as cm310%)6.41(325 .
6 Simulation Results 29
Table 12. Operating Condition and Configuration ofGasifier[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 thegasifier, as shown in Table 13 as Aspen Plus model a. For comparison, thecorresponding results of Wen and Chaung’s work[1] are also shown in thetable. From the table, it can be seen that Wen and Chaung’s results[1] are in abetter agreement with experimental data. In our simulation results, the COand CO2 flow rates and carbon conversion are somewhat different from theexperimental data. The CO flow rate and carbon conversion are greater thanthe experimental data. The CO2 flow rate is less than the experimental data.
The difference in CO and CO2 flow rates and carbon conversion may beattributed 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 increasein temperature of gasifier speeds up the reaction rate of solid carbon andgases, i.e. rate of reactions (7-10), and then makes the carbon conversion inour simulation greater than the experimental data. The difference in flowrates of CO and CO2 depends on reaction (13), an exothermic reaction.Increasing the temperature will make the reaction shift in the backwarddirection, i.e. increasing the amount of CO and decreasing the amount of CO2.So, increasing the temperature decreases the simulated CO2 flow rate andincreases the simulated CO flow rate.
Why does our simulation generate the higher temperature in the gasifier? Thismay 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 built-in method (Boie correlation) in Aspen Plus due to lack of accurateexperimental data. However, we believe the HCOMB has a significant effecton the enthalpy of coal. So, the inaccurate HCOMB may make the incorrectenthalpy of coal and then result in the large departure of gasifiertemperature. The second point is the amount of heat loss in the gasifier. Inour work, the model is simulated in an adiabatic mode. In Wen and Chaung’smodel[1], the heat loss to the environment is considered. Therefore,combining these two points may cause the higher temperature of the gasifierin our model.
In order to further validate our explanation that the difference in CO and CO2
flow rates and carbon conversion is caused by the higher temperature in ourmodel, we manually input the HCOMB of coal to match our outlet temperaturewith Wen and Chaung’s work.
30 6 Simulation Results
Table 13. Comparison of Experimental and ModelingResults
Parameters
Experimental[1]Wen andChaung’smodel[1]
Aspen Plusmodel a
Aspen Plusmodel b
Flowrate(g/s)
Molefraction(%, drybasis)
Flowrate(g/s)
Molefraction(%, drybasis)
Flowrate(g/s)
Molefraction(%, drybasis)
Flowrate(g/s)
Molefraction(%, drybasis)
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 modelb, HCOMB of coal is manually input as 13416Btu/lb.
When the HCOMB of coal is input as 13416Btu/lb (compared with the HCOMBof coal calculated as 14080.83Btu/lb by the Boie correlation), the outlettemperature 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 andcarbon 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 correspondingprofile of main product gases (CO, H2, H2O, and CO2) in char gasificationprocess. In Fig. 9b, Wen and Chaung show the profile of product gas in thewhole gasifier and the corresponding profile in the char gasification process ismarked on the figure. Our model shows a similar result to Wen and Chaung’smodel. As the residence time increases, H2O and CO2 contents in the gasphase decrease, and CO and H2 contents increase.
Based on these phenomena, it can be determined that the product gascomposition and carbon conversion are strongly dependent on thetemperature in the gasifier, and HCOMB of coal and heat loss are two keyparameters in determining the temperature in the gasifier.
6 Simulation Results 31
(a)
(b)Figure 9. Profile of product gas composition: (a) in char gasification processbased on Aspen Plus model and (b) in whole gasifier based on Wen andChaung’s model[1] (solid residence time in whole gasifier is 9.5s).
0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7 8 9 10
VO
L.%
(Wetbasis
)
Residence time (s)
CO
H2
H2O
CO2
32 7 Conclusions
7 Conclusions
A Texaco down-flow entrained flow gasifier model is developed with the AspenPlus simulator. The model follows the modeling approach suggested by Wenand Chaung[1]. In the model, the kinetics of char gasification and thehydrodynamics for calculating solid particle residence time are considered.Reasonable simulation results were obtained compared with the experimentalresults. The Aspen Plus model provides a useful modeling framework forfuture refinements as new knowledge is gained with the entrained flowgasifier. To use this model, the following data should be provided:
Component attributes and higher heat of combustion of coal. Thecomponent attributes of coal include the data of proximate, ultimate, andsulfur analyses.
Yield of coal pyrolysis products from coal pyrolysis experiment at1atm.
Feed conditions of coal, oxygen and steam streams, which includethe flow rate, temperature and pressure. The coal stream also includesthe diameter of coal particle and velocity fed into the gasifier.
Configuration parameters and operational conditions of thegasifier, which include gasifier height, gasifier diameter, operatingpressure, and heat loss. The heat loss can also be in-situ calculated byproviding heat transfer coefficient and environmental temperature.
Model parameters, which include porosity of ash layer and reactivity ofash for the reaction of CO and H2O.
From the model, the following information can be obtained:
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
References 33
References
[1] C.-Y. Wen, T.-Z. Chaung, “Entrainment coal gasification modeling”, Ind.Eng. Chem. Process Des. Dev., 18: 684-695, 1979.
[2] C.-Y. Wen, “Noncatalytic heterogeneous solid fluid reaction models”,Ind. Eng. Chem., 60: 34-54, 1968.
[3] R. Govind, J. Shan, “Modeling and simulation of an entrained flow coalgasifier”, AIChE J., 30: 79-92, 1984.
[4] S.-S. Xu (许世森), D.-L. Zhang (张东亮), Y.-Q. Ren (任永强), “Large-scale
coal gasification technology (大规模煤气化技术)”, Beijing: Chemical
Industry Press, 2006.
[5] C.-Y. Wen, T.-Z. Chaung, “Entrained-bed coal gasification modeling”,Report submitted to Department of energy, Contract E(49-18)274,1978.
[6] E.M. Suuberg, W.A. Peters, J.B. Howard, “Product composition andkinetics of lignite pyrolysis”, Ind. Eng. Chem. Process Des. Dev., 17:37-46, 1978.
[7] K.-F. Cen (岑可法), M.-J. Ni (倪明江), Z.-Y. Luo (骆仲泱), J.-H. Yan (严建
华), Y. Chi (池涌), M.-X. Fang (方梦祥), X.-T. Li (李绚天), L.-M. Cheng (程
乐鸣), “Theory, design and operation of circulating fluidized bed boilers (
循环流化床锅炉理论设计与运行)”, Beijing: Chinese Electric Power Press,
1998.
[8] H. Masuda, K. Higashitani, H. Yoshida, “Powder technology handbook(3rd edition)”, CRC Press, 2006.