Model for Entrained Flow Coal Gasifier

Aspen Plus

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Revision History 1

Revision HistoryVersion Description

V7.2 First versionV7.3 Update the model to V7.3 and add a paragraph in Introduction section

to describe what files are released.V7.3.2 Update the model to V7.3.2V8.2 Update the model to V8.2V8.4 Update the model to V8.4

2 Contents

ContentsRevision 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 occurring

in 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 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 theGUI\Examples\Entrained Flow Coal Gasifier folder of the Aspen Plusinstallation: Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.apwz, a compound

file containing these six files:o Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.bkpo Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.pdfo USRKIN.fo USRPRES.fo USRSUB.dllo 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 ModelID Type Name Formula

O2 CONV OXYGEN O2CO CONV CARBON-MONOXIDE COH2 CONV HYDROGEN H2CO2 CONV CARBON-DIOXIDE CO2H2O CONV WATER H2OH2S CONV HYDROGEN-SULFIDE H2SN2 CONV NITROGEN N2CH4 CONV METHANE CH4C6H6* CONV BENZENE C6H6C SOLID CARBON-GRAPHITE CS SOLID SULFUR SCOAL 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 500m[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 1000C[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

Element Value(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 1000C[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 1atmSuuberg 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 experimentFig. 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 time-

temperature history Time-temperature monitoring system Product collection system Product analysis system

A thin layer of coal particles with 74m 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 productsThe 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 -196C 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 productsThe 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 240Cand 100C, 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 pressureMost 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]. Thetotal yield of volatiles at 1atm is 27.28%, i.e. %28.271 V . The relativecomposition of CO in gas phase is %16.2%28.27/%59.0 . The gasifier in ourmodel is operated at 24atm, i.e. atmPt 24 . Based on Eq. (2),

%56.2124ln066.01%28.272 V . Then, the yield of CO at systempressure 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;()tar and other hydrocarbons (HC); (*)tar, HC, and CO; ()tar, HC, CO, and CO2; (T)total, i.e.tar, HC, CO, CO2, and H2O. Pressure=1atm (helium). Heating rate: (single points) 1000C/s;(points inside ) 7100 to 10000C/s; (points inside ) 270 to 470C/s; (points inside ) 1000C/s, but two-step heating;(b) Yields of methane, ethylene, and hydrogen from lignite pyrolysis to different peaktemperatures [()CH4; (*)C2H4; ()H2. Pressure=1atm (helium); heating rate=1000C/s].(c) Yields of water, carbon monoxide, and carbon dioxide from lignite pyrolysis to different peaktemperatures [()H2O; ()CO2; ()CO. Pressure=1atm (helium); heating rate=1000C/s].(d) Elemental compositions of chars from lignite pyrolysis to different peak temperatures [(*)C;()H; ()N; ()S; ()O. Pressure=1atm (helium); heating rate=1000C/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 0.0059H2 0.0084CO2 0.003H2O 0.0079H2S 0.0094N2 0.0035CH4 0.1637C6H6 0.071Char 0.7272Total 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 121121 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 showsthe calculated relationship between and pd at various temperatures. For agiven 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 CoalParticle[1]

pd (cm) Comment

0.1 0.1

Note: CO and 2CO are concentrations of CO and CO2, respectively. T istemperature, 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 surface

reactions because of high operating temperature (typically above1000C).

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):

*

2 11111

1ii

dashsdiff

iC PP

YkYkk

R

(14)

Where

diffk = gas film diffusion constant, g/cm2atms.

16 4 Reactions

sk = surface reaction constant, g/cm2atms.

dashk = ash film diffusion constant, g/cm2atms.n

diffdash kk , where isvoidage in the ash layer; n is a constant ranging from 2 to 3. In the model,

75.0 and 5.2n .

31

11

fx

rr

Yp

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 atany time after pyrolysis is completed, based on original d.m.m.f. coal; and fis 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 andChaung[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 notavailable 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 infour 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

SHeq P

PK , where SHP 2 and 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 diffk sk *ii PP Comment Source

(7)

pt dP

TT

75.1

180026.4292.0

Te17967

8710

2OP ------ [1]

(8)

pt dP

T 75.042000

1010

Te21060

247

eq

COHOH K

PPP 2

2 T

eq eK 8.130260644.17

[1]

(9)

pt dP

T 75.042000

1045.7

Te21060

247

2COP ------ [1]

(10)

pt dP

T 75.032000

1033.1

Te17921

12.0

eq

CHH K

PP 4

2 Teq eK 8.1

18400

34173175.0

[1]

(11)

pt dP

T 75.032000

1033.1

Te17921

12.0

eq

SHH K

PP 2

2 T

eq eK7225.185570657.5

[1]*

Note: T=temperature, K; tP =total pressure, atm; pd =diameter of coalparticle, cm; is calculated according to the relations in Table 4.

2OP , OHP 2 ,

2HP , COP , 2COP , 4CHP and SHP 2 =partial pressures, atm. In the eqK expressionof 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

987.130000

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 itsrelationship between eqK and T, the steps are similar to those adopted forreaction (11). The difference is that the calculation of eqK from the

equilibrium composition is based on the equationOHCH

HCOeq CC

CCK

24

2

3

, where

COC , 2HC , 4CHC , and OHC 2 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

315.810976.9

51083.8 OHT CCe

------ mol/m3s [7]

(5)2

4

315.810976.9

9.30 OCOT CCe

------ mol/m3s [7]

(6)24

5

315.810304.9

1110552.3 OCHT CCe

------ mol/m3s [7]

(12)

OHeq

HCOCH

T

CKCC

Ce2

2

4

3987.1

30000

312 Teq eK

0499.250141371.33 mol/m

3s [1]*

(13)

TP

t

TCOCOw

eP

exxFt 555391.8

2505.0

987.127760

*51077.2

t

COCO P

Px

OHeq

HCO

tCO PK

PPP

x2

221*

Teq eK 8.1

72346893.3

mol/[s(gof ash)] [1]

Note: T=temperature, K;2H

C ,2O

C , COC , 4CHC , and OHC 2 =concentrations,mol/m3; COP , 2COP , 2HP , and OHP 2 =partial pressures, atm; tP =total pressure,atm; wF =adjustable parameter, which represents the relative catalyticreactivity of ash to that of iron-base catalyst. In the model, 2.0wF . In thereaction rate expressions of reactions (4-6), the coefficient 8.315 before Tstands for the universal gas constant in J/molK. In reaction rate expressionsof reactions (12-13), the coefficient 1.987 before T means the universal gasconstant in cal/molK. In the eqK expression of reaction (13), the coefficient1.8 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 BlockBlock Model Function

PYROLYS RYield Simulate 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 RStoic Decompose 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 Calculator Determine 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, thePYROLYS and PRESCORR blocks. The first RYield reactor, PYROLYS, is used tosimulate 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 kineticsFrom the kinetics models in section 4.3.2, most kinetics are so complex thatthey cant 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/ms. 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/ms =(kgmole/m3s)(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/ms.

2. Calculate the total reaction rate of each component according to thestoichiometry of reactions.

Unit conversion for rate of each reactionThe 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

21

344

p

piCiC

r

rRR

Step 2: 6

312

1012/10

iCiC RR

Step 3: bediCiC VDRR 14223

Combining above three steps gives the following total conversion expression:

22 5 Simulation Approach

bedp

iCiC VrDRR

1

1016 32

3 (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 coalcoalparticle

24

, where coalF is coal flow rate; t

is residence time of coal in the gasifier; coal is coal density; and h isgasifier 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 (2HS

R ) is g of sulfur/(cm2 of coal particlesurface 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:

bedp

HSHS VrDRR

1

101283

3

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 VDRR 4

102

32 (17)

For reaction (13), the rate of reaction ( OHCOR 2 ) is in the unit of mol/[s(g ofash)]. 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 1221

Step 2: coalOHCOOHCO RR 12

22

Step 3: 6

323

1010

22

OHCOOHCO RR

Step 4: bedOHCOOHCO VDRR 14234

22

Combining above four steps yields the total conversion expression:

bedcoalashmoistureOHCOOHCO VDYYRR

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 componentsAfter 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

Reactions Stoichiometriccoefficient of H2Rate of reaction(kgmole/ms)

(4) -12

22 OHR

(8) 13

2OHCR

(10) -23

2HCR

(11) -13

2HSR

(12) 32

24 OHCHR

(13) 1 42OHCO

R

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 solidIn the model, the char gasification process is simulated by an RPlug reactor.In RPlug, the residence time is calculated by Eq. (20):

R

VdV

Vt R 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-sectionalarea 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 solidIn the entrained-flow gasifier, the coal particle size is very small, typically lessthan 500m[4]. Using the coal parameter of 500m and the input conditions in

5 Simulation Approach 25

Tables 2-3 and 11-12, the viscosity (), velocity (u) and density () ofproduct gases at the outlet of gasifier are calculated by this Aspen Plusmodel. = 5.7310-5Pas. u = 0.03m/s. = 3.03kg/m3. Then, the Reynoldsnumber of particles is calculated to be 0.79 based on the equation

ud pp Re , where pd is the coal particle diameter, 510-4m. In the whole

gasifier, the temperature at the outlet is the lowest, indicating the and 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, wecan assume that the pRe in the whole gasifier is less than 0.79. Consideringthe valid regime of Stokes law, i.e. 2Re p [8], we can conclude that Stokeslaw is applicable for the solid flow in this system. According to Newtonssecond law and Stokes law, Eq. (21) is derived for downward velocity of solid( sv )[1].

bttgbtiss evvevv 1, (21)Where

2

18

psdb

.

tv = terminal settling velocity of particle in a static fluid.

18

2 gdv pgst

,

where isv , is initial velocity of solid; gv is velocity of gas phase; is gasviscosity; s is density of solid; g is density of gas; pd is diameter of solidparticles.

Calculation of solid residence timeIntegrating Eq. (21) gives the relationship between gasifier length (h) andsolid residence time (t):

betvve

bv

dtvhbt

tgbtist

s11,

0(22)

Based on Eq. (22), the solid residence time is calculated by Newtons method.In the calculation, g , , and gv use the values at the inlet of the GASIFIERblock; s takes the average value in the gasifier based on the harmonious

square root, i.e.2,

2,

2,

2,2

osis

osiss

, 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 thegasifier 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 GASIFIERThrough 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 ModelName 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 BlockItems Parameters Setup

Integrationparameters

Integration convergence tolerance 0.0001Initial step size of integration variable 1E-8Maximum step size of integration variable 0.001Maximum number of integration steps 1E6

CorrectorConvergence method NewtonError tolerance ratio 0.1

Integration errorError scaling method DynamicMinimum 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/sTemperature 505.22 KPressure 24 atmDiameter of particle 350 mVelocity entering into gasifier 3 m/s

OxygenRatio of oxygen to coal flow rates 0.866 dimensionlessTemperature 298 KPressure 24 atm

SteamRatio of steam to coal flow rates 0.241 dimensionlessTemperature 696.67 KPressure 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 inthe 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 atmLength 3.25 mDiameter 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 Chaungs work[1] are also shown in thetable. From the table, it can be seen that Wen and Chaungs 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 Chaungs 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 Chaungsmodel[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 CO2flow 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 Chaungs work.

30 6 Simulation Results

Table 13. Comparison of Experimental and ModelingResults

Parameters

Experimental[1]Wen andChaungsmodel[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 57.41H2 6.01 39.13 6.23 39.84 5.96 38.23 5.99 38.71CO2 9.985 2.95 10.04 2.92 6.462 1.90 10.24 3.03CH4 0.15 0.12 0.20 0.16 0.13 0.10 0.24 0.20H2S 0.133 0.06 0.726 0.27 1.405 0.53 1.04 0.40N2 0.53 0.12 0.454 0.208 0.54 0.25 0.54 0.25Carbon conv. (%) 98.64 98.88 99.95 98.69Temp. (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 Chaungs 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 Chaungsmodel. 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 andChaungs 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

VOL.

%(W

etba

sis)

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. The

component 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 (), D.-L. Zhang (), Y.-Q. Ren (), Large-scalecoal gasification technology (), Beijing: ChemicalIndustry 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 (), M.-J. Ni (), Z.-Y. Luo (), J.-H. Yan (), Y. Chi (), M.-X. Fang (), X.-T. Li (), L.-M. Cheng (), Theory, design and operation of circulating fluidized bed boilers (), Beijing: Chinese Electric Power Press,1998.

[8] H. Masuda, K. Higashitani, H. Yoshida, Powder technology handbook(3rd edition), CRC Press, 2006.

Introduction1 Components2 Process Description3 Physical Properties4 Reactions4.1 Coal pyrolysis4.1.1 Reactions4.1.2 Amount of each pyrolysis product

4.2 Volatile combustion4.2.1 Reactions4.2.2 Reaction kinetics

4.3 Char gasification4.3.1 Reactions4.3.2 Reaction kinetics

5 Simulation Approach5.1 Unit Operations5.1.1 Coal pyrolysis5.1.2 Volatile combustion5.1.3 Char gasification

5.2 Streams5.3 Calculator Blocks5.4 Convergence

6 Simulation Results7 ConclusionsReferences