Aspen PlusModel 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.com Revision History Version Description V7.2 First version V7.3 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 V7.3.2 V8.2 Update the model to V8.2 V8.4 Update the model to V8.4 Revision History 1 ........... 15 5 Simulation Approach ................................................................................................................................................................................................................9 4............................................................................................................................2 Reaction kinetics..............................9 4................ 20 5.........................2 Reaction kinetics...........................1......................................2 Introduction ..............................................................................28 7 Conclusions ........................................................................................... 13 4................................. 26 5......................................... 20 5........ 13 4.........1 Contents......................................................................... 26 5..............1 Reactions...................................... 14 4........1.................32 References .........................................................................3 1 Components ..............................................................3 Char gasification...................................................................................................2 Volatile combustion ....................3 Calculator Blocks ................9 4...............................................1...................................................................4 2 Process Description..................................2.......2.3.....................................................................................2 Streams ...................................2 Amount of each pyrolysis product ........... 20 5............................................................ 14 4...... 20 5........................................................................................3 Char gasification ..............................................1 Coal pyrolysis.........................................................................................1 Coal pyrolysis ............................ 27 6 Simulation Results ..Contents Revision History ...........................................................................................................1 Unit Operations ......................................33 2 Contents .........................................................................................................................................................................................................................9 4...........................................................................4 Convergence ....5 3 Physical Properties........................................................7 4 Reactions ......................................19 5........2 Volatile combustion..................................1 Reactions.......1................. 13 4...................................3..................1...1 Reactions............................. e. The temperature inside the coal particle is assumed to be uniform.f o USRPRES. The gas phase is assumed to be instantaneously and perfectly mixed with the solid phase.f o USRSUB. coal pyrolysis. The reaction kinetics for char gasification is considered. The pressure drop in the gasifier is neglected.Introduction This file describes an Aspen Plus kinetics-based model for Texaco down-flow entrained flow gasifiers. and char gasification. Coal particles are assumed to be spherical and of uniform size. The model accounts for major physical and chemical processes occurring in the gasifier. i. 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. The ash layer formed remains on the particle during the reactions based on the unreacted-core shrinking model[2].dll o USRSUB.pdf o USRKIN.pdf USRSUB. a compound file containing these six files: o Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier. The model follows the modeling approach proposed by Wen and Chaung[1].bkp o Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier. The hydrodynamics to calculate solid residence time is taken into account. volatile combustion.opt Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier.bkp Aspen_Plus_Model_for_Entrained_Flow_Coal_Gasifier. The model includes the following features: The model is a steady-state model.apwz.dll USRSUB.opt Introduction 3 . 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 -----. ------ *: C6H6 represents tar. CHAR1 represents the solid phase after coal pyrolysis at 1atm. 4 1 Components . CHAR2 represents the solid phase after pressure correction from 1atm to system pressure. ------ CHAR1* NC -----.1 Components The following table represents the chemical species present in the process: Table 1. ------ CHAR2* NC -----. ------ ASH NC -----. 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. as shown in Fig. In this section. The slag and 2 Process Description 5 .2 Process Description The Texaco gasifier is a typical entrained flow gasifier. The operating pressure is usually at 20-50atm and the temperature is typically higher than 1000ºC[4]. Figure 1. 3]. and then the slurry together with oxygen is simultaneously introduced into the top section. 1. Coal pyrolysis. The lower section is a quench vessel. volatile combustion and char gasification reactions take place subsequently to produce the syngas. The total gasifier is divided internally into two sections[1. a special refractory material is lined to withstand the severe operating environment. A reservoir of water is maintained at the bottom of the gasifier by continuous injection of cooling water. The syngas is saturated with water and removed from the gas space above the water. 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. 6 2 Process Description . carbon. and organic sulfur.3 Physical Properties In this model. sulfate. and oxygen. sulfur. The proximate analysis gives the weight contents of moisture. it requires only these two component attributes: ULTANAL and SULFANAL. HCOALGEN and DCOALIGT models are used to calculate the enthalpy and density of non-conventional components. 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. hydrogen. ultimate analysis results (denoted as ULTANAL in Aspen Plus). The results of proximate. chlorine. respectively. The sulfur analysis gives the weight fractions of sulfur divided into pyritic. Table 2 shows the component attributes of coal used in our model. 5]. For the DCOALIGT model. The ultimate analysis gives the weight composition of coal in terms of ash. ultimate. and ash. The HCOALGEN model requires these three component attributes for non- conventional components: proximate analysis results (denoted as PROXANAL in Aspen Plus). the property method RK-SOAVE is used to calculate the physical properties of mixed conventional components and CISOLID components. nitrogen. Based on these analysis results. the enthalpy and density of coal are calculated. 3 Physical Properties 7 . the same methodology as the coal is applied and the same models are used to calculate their enthalpy and density. fixed carbon. which are from the literatures of Wen and Chuang[1. and sulfur analysis results (denoted as SULFANAL in Aspen Plus). respectively. For the characterization of char and ash generated in coal conversion. volatile matter. 32 Ash 15.05 Pyritic 0.%) basis) basis) Moisture 0.37 (dry basis) S 1.%.2 C 74. dry (wt.53 8 3 Physical Properties .01 H 6.46 N 0. Component Attributes of Coal Used in the Model Proximate analysis Ultimate analysis Sulfur analysis Value Value Value Element Element (wt.%.59 (wet basis) Fixed carbon 58.59 (dry basis) Volatile matter 26. [1.77 O 1.53 Cl 0.71 Organic 0.5] Table 2. dry Element (wt.59 (dry basis) Ash 15.25 Sulfate 0. 2 Amount of each pyrolysis product In our model. CH4. the amount of each pyrolysis product at 1atm is from the work of Wen and Chuang[1. When coal is fed into the gasifier.[6] describe how to get the results of coal pyrolysis at 1atm in experiment.1 Coal pyrolysis 4. H2. In our model. H2O. 4 Reactions 9 . the temperature is typically higher than 1000ºC[4]. adjusting them from 1atm to system pressure. it first undergoes the pyrolysis process to decompose to volatile matter and char.1 Amount of each pyrolysis product at 1atm Suuberg et al. these reactions take place in sequence: coal pyrolysis.4 Reactions When the coal. which is made at 1atm. to yield the amount of each product at the operating conditions of the real gasifier. 4. 5]. 4. Their corresponding results are summarized here. 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. H2S. the amount of each pyrolysis product is determined based on the results of the pyrolysis experiment. C6H6 is used to represent the tar. as shown in Eq.1. (1). It should be noted that in our model. CO2. volatile combustion and char gasification. N2 and C6H6. A pressure correction is applied to the results of this experiment.2. volatile matter includes CO. oxygen and steam are simultaneously introduced into the gasifier.1.1.1 Reactions In gasifier. Coal Char CO H 2 H 2O CO2 CH 4 H 2 S N 2 C6 H 6 (1) 4. The first trap is operated at room temperature with the Porapak Q chromatographic packing. Figure 2. Flow chart of experiment Fig. Then. the yield of each product is analyzed. and collects 10 4 Reactions . 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 74µm average diameter are held in a folded strip of stainless steel screen. Any condensation on non-lined reactor surfaces is collected by washing with methylene-chloride-soaked filter paper. electricity is used to heat the coal particles under 1atm helium or vacuum to produce the pyrolysis products. The apparatus consists of five components: Reactor. The second type of products is collected at the conclusion of a run by purging the reactor vapors through two lipophilic traps.’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. The apparatus for the pyrolysis experiment in Suuberg et al. 2 is a schematic diagram for the apparatus of the pyrolysis experiment. After collecting the products. all of Suuberg et al. V2 = total yield of volatiles at the pressure of the real gasifier.2 Amount of each pyrolysis product at system pressure Most pyrolysis experiments are carried out at 1atm. i. The total yield of volatiles at 1atm is 27. The second trap is also packed with Porapak Q but operated at -196ºC in a dewar of liquid nitrogen. and only the total yield of volatiles is corrected. This trap collects all fixed gases produced by pyrolysis. (2) and the relative composition of volatile products in the gas phase obtained from pyrolysis experimental results at 1atm. (2). In the pressure correction. Take the calculation of CO yield as an example. 5]. V2 27. the relative composition of gas components is assumed to be constant.16% .28% . the pressure is usually much higher than 1atm. typically 20-50atm[4]. For example. 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. (2)[1]: V2 V1 1 a ln Pt (2) Where V1 = total yield of volatiles at 1atm. respectively. with the exception of hydrogen which is determined by direct vapor phase sampling with a precision syringe.28%. a = constant. intermediate weight oils such as benzene. Combining the total yield of volatiles calculated in Eq. Pt = pressure in real gasifier. This indicates that the pressure effect on yield of each product needs to be considered.56% 2. V1 27.1.28% 1 0. The third type of product. In our model.e.e. The gasifier in our model is operated at 24atm. The analysis results are summarized in Fig. Based on Eq. the yield of each volatile product at the operating conditions of real gasifier is calculated. 3. The products of the second type collected in the first and second traps are first warmed to 240ºC and 100ºC. char. The relative composition of CO in gas phase is 0. Table 3 shows the yield of coal pyrolysis products at 1atm used in our model. remains on the screen and is determined gravimetrically.59% / 27.56% . and then fed into the gas chromatography for analysis. Analysis of pyrolysis products The different types of products collected are analyzed by different methods.’s results[6] shown in Fig. which is from the work of Wen and Chaung[1. Then.47% .066 ln 24 21. a = 0. 4. atm. toluene and xylene.28% 2. 4 Reactions 11 .16% 0. the yield of CO at system pressure is 21. i.2.066. However. in real coal gasifiers. Pt 24atm . 3 are obtained at 1atm. The first type of products is measured gravimetrically. The total yield of volatiles is corrected by Eq. ()CO. Pressure=1atm (helium). carbon monoxide. and carbon dioxide from lignite pyrolysis to different peak temperatures [(Δ)H2O. (a) (b) (c) (d) [6] Figure 3. 12 4 Reactions . tar. and CO2. and hydrogen from lignite pyrolysis to different peak temperatures [(Δ)CH4. Coal pyrolysis results of Suuberg et al. (points inside Δ) 270 to 470ºC/s. (c) Yields of water. (d) Elemental compositions of chars from lignite pyrolysis to different peak temperatures [(*)C. (b) Yields of methane. (Δ)tar and other hydrocarbons (HC). heating rate=1000ºC/s]. but two-step heating. ethylene. (*)C2H4. HC. Pressure=1atm (helium). and H2O. CO2. heating rate=1000ºC/s]. (º)H. CO. Heating rate: (single points) 1000ºC/s. Pressure=1atm (helium).’s work : (a) Pyrolysis product distributions from lignite heated to different peak temperatures [()tar. HC. heating rate=1000ºC/s]. HC. (×)CO2. (*)tar. and CO. (º)H2. (º)tar. (points inside □) 1000ºC/s. ()S.e. (points inside º) 7100 to 10000ºC/s. i. (×)N. CO. (T)total. (Δ)O. Pressure=1atm (helium). H2O. (1).7272 Total 1 4. the reaction kinetics of the volatile combustion process are neglected in the model. 5] Components Yield (mass basis on original coal) CO 0.0035 CH4 0.2 Reaction kinetics Since the reaction rate of gaseous combustion is generally fast and the combustible gases will be consumed up in a short time. CO. and C6H6 are combustible gases. CO2.0094 N2 0.2. 4 Reactions 13 . CH4. CH4.0079 H2S 0. N2.1 Reactions From Eq. the volatile matter is composed of CO. as shown in reactions (3-6). H2.0059 H2 0. Table 3. The conversions of C6H6. these combustible gases will react with oxygen fed into the gasifier.003 H2O 0.2. C 6 H 6 7. and CH4 are assumed to be 100%.071 Char 0.2 Volatile combustion 4.0084 CO2 0. and C6H6. CO.1637 C6H6 0.5O2 CO2 (5) CH 4 2O2 CO2 2 H 2O (6) 4.5O2 H 2O (4) CO 0. After getting the yield of each volatile product. So after the coal pyrolysis. Among these gases. the yield of char is found by subtracting the yield of all volatile products from unity.5O2 6CO2 3H 2O (3) H 2 0. Yield of Coal Pyrolysis Products at 1atm Used in the Model[1. H2. H2S. H2. At d p 0. decreases with the increase in d p . is constant at d p 0.1cm . This process may include reactions (4-6) above.0.005 Z CO 0.095 Z 2 6249 2500 e T >0.005 d p 0.3. At 0. T is temperature. as well as reactions (7-13): 1 1 2 C O2 21 CO 1CO2 (7) C H 2O CO H 2 (8) C CO2 2CO (9) C 2 H 2 CH 4 (10) S H 2 H 2S (11) CH 4 H 2O CO 3H 2 (12) CO H 2 O CO2 H 2 (13) In reaction (7). is independent of temperature and has the value of 1.005cm and d p 0. For a given temperature. Fig.005 Z 2 Z d p 0.1 Reactions After the volatile combustion process.1cm . Table 4.1cm . 4 shows the calculated relationship between and d p at various temperatures. 4. 14 4 Reactions .005-0.0 Note: CO and CO 2 are concentrations of CO and CO2. the char from coal pyrolysis is further gasified by the reaction with gases in the gas phase. For a given d p . 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. respectively. Expressions of for Different Size of Coal Particle[1] dp (cm) Comment 2Z 2 <0.1 2 Z 2 CO 2 0. K.1cm . shows a slight change with the temperature at d p 0.3 Char gasification 4.1 1 . The overall rate is expressed as Eq. Relationship between and dp at various temperatures 4. Since the solid loading in the gasifier is usually very small. In the real gasifier. the char-gas reactions can be considered as surface reactions because of high operating temperature (typically above 1000ºC). The unreacted-core shrinking model[2] is used to describe their kinetics. gas film diffusion and chemical reaction are considered. This is attributed to the following two points.2 Reaction kinetics Reactions (7-11) are caused by the reaction of char with gaseous components in the gas phase.3. 4 Reactions 15 . Figure 4. g/cm2·atm·s. the particle collision is infrequent and then the ash layer formed can be assumed to remain on the particle during reactions. the effects of ash layer diffusion. (14): RC i 1 1 1 1 1 P P i i * (14) 1 Y 2 k diff k sY k dash Where k diff = gas film diffusion constant. In this model. and Pi Pi are given in Table 5. k dash k diff n . Pi Pi * = effective partial pressure of i-component taking account of the reverse reaction. coal.m. Transform the equation fitted in the third step to generate the relationship between K eq and T. 2. coal. rp is the rp 1 f radius of the whole particle including the ash layer.m. atm. 1 r 1 x 3 Y c . k dash = ash film diffusion constant. The corresponding k diff . * In the derivation of Pi Pi * . 0. The k diff . PH 2 3.m. Use a single RGibbs block to produce the equilibrium composition of reaction (11) at various temperatures. based on original d. g/cm2·atm·s. n is a constant ranging from 2 to 3. x is coal conversion at any time after pyrolysis is completed. g of carbon/(cm2 of coal particle surface area)·s. based on original d. because the kinetics model is not available now.75 and n 2. Make a linear fit with ln K eq as Y-axis and 1/T as X-axis. atm. k s = surface reaction constant. 4. k s and Pi Pi * of reactions (7-10) from the work of Wen and Chaung[1] are listed in Table 5. where PH 2 S and PH 2 are partial pressures.5 . the relationship between K eq and T is found in four steps: 1. and f is coal conversion when pyrolysis is completed.m. 16 4 Reactions . In the model. k s . g/cm2·atm·s. Calculate K eq at various temperatures based on the equation PH 2 S K eq .f. RC i = reaction rate. where rc is the radius of the unreacted core. For the kinetics of reaction (11). where is voidage in the ash layer. we adopt the expression similar to that of reaction (10).f. 12e T K eq K eq e T [1]* Pt d p Note: T=temperature. In the K eq expression of reactions (8) and (10). The difference is that the calculation of K eq from the CCO C H3 2 equilibrium composition is based on the equation K eq . atm. Pt =total pressure. In deriving its relationship between K eq and T. where the reaction rate of the reverse reaction is not considered. PCH 4 and PH 2 S =partial pressures.644 21060 (8) 2000 247 e T Keq K eq e 1 .26 T 0. In 30000 their work. the kinetics for the CH4-H2O reaction are rewritten as the expression in Table 6. [1] Pt d p 0. PH 2 .0657 17921 (11) 2000 0. PH 2O . PCO2 .33 10 PH 2 S 18557 . Parameters for Kinetics of Reactions (7-11) Reactions k diff ks Pi Pi * Comment Source 1. mol/m3. K. the reaction rate of reaction (12) is described as 312e 1. PCO . C H 2 . atm.45 10 21060 (9) 2000 T PCO2 -----. where CCH 4 C H 2O CCO . However. the reaction of CH4 and H2O is generally reversible. The kinetics of reactions (4-6) and (12-13) are shown in Table 6. So.75 3 T 1.987T CCH 4 . 4 Reactions 17 .33 10 17921 PCH4 0.75 T 10 10 4 PH2 PCO 30260 PH2O 17 .8 before T is caused by the unit conversion from Rankine degrees to Kelvin. is calculated according to the relations in Table 4.8T 18400 (10) 2000 0. the coefficient 1. [1] 247 e Pt d p 0 .75 4 T 7.12 e T PH2 K eq e [1] Keq 34173 Pt d p 0. PO2 . The kinetics of reaction (12) are modified according to the work of Wen and Chaung[1]. Table 5. d p =diameter of coal particle. the steps are similar to those adopted for reaction (11).8 T [1] Pt d p 0. CCH 4 .75 4.175 1.292 17967 (7) T 1800 8710 e T PO2 -----.7225 PH 2 5. [1]* means that the source is from reference [1] and some changes are made for this model. and C H 2O are concentrations.75 3 T 1. cm. Table 6. C H 2 . 9 e C CO C O 2 9 . mol/m3·s [7] 3 . Pt =total pressure.987 T 33 . 315 T C H 2 CO2 9 .315 before T stands for the universal gas constant in J/mol·K.304 10 5 (6) -----.1371 (12) 312 e K eq e mol/m3·s [1]* T 4 K eq C H 2O PCO xCO Pt e 27760 Fw 2.77 10 xCO x 5 * CO 1. 18 4 Reactions . CCO .987 T 1 PCO2 PH 2 mol/[s·(g (13) P 5553 * x CO of ash)] [1] 0. In the reaction rate expressions of reactions (4-6). 315 T -----. 976 10 4 (4) -----. K. PCO2 . In the K eq expression of reaction (13).552 10 e 11 8 .6893 K eq e 1 . and PH 2O =partial pressures. which represents the relative catalytic reactivity of ash to that of iron-base catalyst.2 .0499 1 . atm.91 Pt K eq PH 2O Pt 250 e T 7234 3 . mol/m3·s [7] 30 . In the model. mol/m3. and C H 2O =concentrations. the coefficient 1.987 before T means the universal gas constant in cal/mol·K. Kinetics of Reactions (4-6) and (12-13) Reaction Reaction rate Comment Unit Source 9 . atm. In reaction rate expressions of reactions (12-13). Fw 0 . 976 10 4 (5) 8 .8 before T is caused by the unit conversion from Rankine degrees to Kelvin. CCH 4 . 83 10 e 5 8 .8 T Note: T=temperature. PH 2 . mol/m3·s [7] 8 . the coefficient 1. Fw =adjustable parameter. [1]* means that the source is from reference (1) and some changes are made for this model.315 T C CH 4 C O2 30000 C CO C H3 2 C CH 25014 .5 t 8. the coefficient 8. PCO . C O2 . Flowsheet for coal gasification 5 Simulation Approach 19 . The function of each block is shown in Table 7. The quench section for cooling the hot gas from the gasification section is not simulated in this model. PYROLYS and PRESCORR blocks are used to simulate the coal pyrolysis process. Other blocks are used for helping these four blocks to simulate the above three processes. 5 shows the flowsheet for the whole coal gasification process.5 Simulation Approach Fig. The COMBUST block is used to model the volatile combustion process. The GASIFIER block is for the char gasification process. Figure 5. S. Function of Each Block Block Model Function Simulate the coal pyrolysis based on the results of the PYROLYS RYield pyrolysis experiment at 1atm Make pressure correction for the yield of each product from PRESCORR RYield the pressure in the pyrolysis experiment (i. SPELMCAL Calculator and ash in reaction of the SEPELEM block GASIFCAL Calculator Correct the solid residence time in the GASIFIER block 5.3 Char gasification In the model. In this process.1. the reaction kinetics and residence time of 20 5 Simulation Approach .e. the COMBUST block. The correction method is described in section 4. O2. is used to make a pressure correction for the yield of each component generated in the PYROLYS block.1. The fractional conversions of combustible gases are all set as 1. N2. N2.2. The first RYield reactor. The second RYield reactor. H2. the GASIFIER block. 5. 1atm) to the pressure in the real gasifier SEPSG Sep2 Separate the gas and solid char COMBUST RStoic Model the volatile combustion Decompose char into C. the char gasification process is model with an RPlug reactor. O2. Table 7. An RStoic reactor.1 Coal pyrolysis In the model.0.1 Unit Operations 5. is used to simulate the coal pyrolysis at 1atm based on the results of the pyrolysis experiment. In the model. the correction is automatically done by a user-subroutine called USRPRES. the PYROLYS and PRESCORR blocks. is used to simulate the volatile combustion process.1. S. and ash in order to SEPELEM RStoic 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 Determine the stoichiometric coefficients of C. PRESCORR.1. H2.2. coal pyrolysis process is simulated with two RYield reactors. PYROLYS.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. 5. the kinetics of volatile combustion process is neglected in the model. the coal particle is assumed to be spherical. In the user kinetics subroutine of RPlug. char are the two main factors affecting the remaining carbon conversion and product composition.2 to kgmole/m·s. Figure 6. most kinetics are so complex that they can’t be treated by the built-in kinetics expression template in Aspen Plus. So in order to get the required reaction rate of each component. The reaction rate of each conventional component must be provided in the unit of kgmole/m·s.1 Treatment of reaction kinetics From the kinetics models in section 4. Schematic diagram for unit conversion of reactions (7-10) 4rp2 Step 1: R 1 C i RC i 4 3 r p 3 10 3 / 12 Step 2: RC2 i RC1 i 10 6 2 Step 3: RC3 i RC2 i D 1 Vbed 4 Combining above three steps gives the following total conversion expression: 5 Simulation Approach 21 . kgmole/m·s = (kgmole/m3·s)·(m2).3. This unit is derived by the relation that rates per unit volume are multiplied by the cross-sectional area covered by the reacting phase. Convert the unit of each reaction rate in section 4.3.3. i. the output is the reaction rate of each component taking part in the reactions. Unit conversion for rate of each reaction The rates of reactions (7-10) are in the unit of g of carbon/(cm2 of coal particle surface area)·s. 6. Calculate the total reaction rate of each component according to the stoichiometry of reactions.1.2. 5. 1. So these reactions' kinetics are provided in a user subroutine called USRKIN. 2. In the conversion. The conversion of this unit follows the steps shown in Fig.e. the following two steps are taken. cm. coal is coal density. Vbed 1 V particle . The final relation is: 3D 2 R S3 H 2 RS H 2 1 Vbed (16) 128 10 3 rp 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. and h is gasifier length. where V particle is particle V particle Fcoal t coal fraction in gasifier. The unit conversion takes the steps shown in Fig. D = diameter of gasifier. 7. then transferred back to the user kinetics subroutine. Schematic diagram for unit conversion of reactions (4-6) and (12) Step 1: Ri1 Ri 10 3 2 Step 2: Ri2 Ri1 D Vbed 4 Based on the above two steps. Vbed is first calculated in a calculator block called GASIFCAL. . the total conversion expression is: D 2 Ri2 Ri 10 3 Vbed (17) 4 For reaction (13). the rate of reaction ( RCO H 2O ) is in the unit of mol/[s·(g of ash)]. 22 5 Simulation Approach . The difference is that we just change the molecular weight of carbon to that of sulfur. D 2 RC3 i RC i 1 Vbed (15) 16 10 3 rp Where rp = radius of coal particle. t 4 D 2 h is residence time of coal in the gasifier. Figure 7. The unit of reaction (11)’s rate ( R S H 2 ) is g of sulfur/(cm2 of coal particle surface area)·s. m. where Fcoal is coal flow rate. which is very similar to the unit of reactions (7-10). 8. Vbed = void fraction in gasifier. 1 and 4. From sections 4. we can get the total reaction rate of each component according to the stoichiometry of reactions. there are six reactions involving H2. (8). The stoichiometric coefficient of H2 in each reaction is listed in Table 8.2. wet basis. dry basis. Schematic diagram for unit conversion of reaction (13) H 2O RCO H 2O 1 Ymoisture Yash 1 Step 1: RCO H 2O RCO H 2 O coal 2 1 Step 2: RCO 10 3 H 2 O RCO H 2O 3 2 Step 3: RCO 10 6 2 Step 4: 4 RCO H 2 O RCO H 2O 3 D 1 Vbed 4 Combining above four steps yields the total conversion expression: D 2 H 2 O RCO H 2 O 1 Ymoisture Yash coal 1 Vbed 4 RCO (18) 4 10 3 Where Ymoisture = moisture fraction in original coal. Based on Table 8. Take the total reaction rate of H2 as an example.1. Yash = ash fraction in original coal. which are reactions (4). Figure 8.3. and (10-13). Total reaction rate of components After making the unit conversion for the rate of each reaction. the abbreviation for the rate of each reaction is also listed in Table 8. the total reaction rate of H2 is: R H 2 RC3 H 2O RC3 H 2 2 RS3 H 2 RH2 2 O2 RCH 2 4 H 2O 3 RCO 4 H 2O (19) 5 Simulation Approach 23 . Meanwhile. 3. However. the coefficient is negative. V = volumetric flow rate of gases. typically less than 500µm[4]. Stoichiometric Coefficient of H2 in Each Reaction and Abbreviation of Each Reaction Rate Stoichiometric Rate of reaction Reactions coefficient of H2 (kgmole/m·s) (4) -1 R H2 2 O2 (8) 1 RC3 H 2O (10) -2 RC3 H 2 (11) -1 R S3 H 2 2 (12) 3 RCH 4 H 2O 4 (13) 1 RCO H 2O Note: If H2 is a reactant. the coefficient is positive. 3. In RPlug. In Eq. the solid residence time is closely related to the velocity of the solid phase.1. 5. Using the coal parameter of 500µm and the input conditions in 24 5 Simulation Approach . (20): VR 1 t dVR (20) 0 V Where VR = reactor volume. V is the product of gas velocity multiplied by cross-sectional area of the reactor. the coal particle size is very small. Selection of model of downward velocity of solid. (20). This Calculator block includes three main parts: 1. the residence time is calculated by Eq. This means that the calculation of residence time in RPlug mainly depends on the velocity of the gas phase. Table 8. So to get the correct residence time of solids in the GASIFIER block. If H2 is a product.2 Residence time of solid In the model. Selection of model of downward velocity of solid In the entrained-flow gasifier. Return of results from the GASIFCAL block to the GASIFIER block. 2. the char gasification process is simulated by an RPlug reactor. Calculation of solid residence time. an external Fortran Calculator block called GASIFCAL block is used to calculate the solid residence time before executing the GASIFER block. (22). g is density of gas.79 based on the equation d p u Re p . Because the conversion of useful components in coal is generally close to 100% in practical application. (21) gives the relationship between gasifier length (h) and solid residence time (t): 1 e bt t h vs dt vs . s .e.o are the solid densities s2. where s . s d p2 vt = terminal settling velocity of particle in a static fluid. . where 5 Simulation Approach 25 . 18 where v s . s is density of solid. the viscosity (µ). s takes the average value in the gasifier based on the harmonious 2 s2.i s2. the Reynolds number of particles is calculated to be 0. ρ = 3. In the calculation. Considering the valid regime of Stokes’ law. is gas viscosity. i. the amount of product gases is the largest at the outlet of the gasifier. we assume that the solid density at the outlet of the gasifier is equal to the density of ash. where d p is the coal particle diameter.e.73×10-5Pa·s. Then. we can assume that the Re p in the whole gasifier is less than 0. we can conclude that Stokes’ law is applicable for the solid flow in this system. velocity (u) and density (ρ) of product gases at the outlet of gasifier are calculated by this Aspen Plus model. the temperature at the outlet is the lowest.i s2.79. d p is diameter of solid particles.o square root. vt s g d p2 g . i. v g is velocity of gas phase.e. Calculation of solid residence time Integrating Eq. s . the solid residence time is calculated by Newton’s method. u = 0.i bt 1 e v g vt t (22) 0 b b Based on Eq. (21) is derived for downward velocity of solid ( vs )[1]. g . According to Newton’s second law and Stokes’ law. Eq. respectively.i and s. So.03kg/m3. and then the corresponding u of gases is the largest in the whole gasifier. Tables 2-3 and 11-12.o coal 1 Ymoisture Yash . 5×10-4m. vs vs .o at the inlet and outlet of GASIFIER block. i.i is initial velocity of solid. µ = 5. respectively. indicating the µ and ρ of gases at the outlet are the lowest and the largest. and v g use the values at the inlet of the GASIFIER block. Re p 2 [8].03m/s. In the whole gasifier.i e bt v g vt 1 e bt (21) Where 18 b . Meanwhile. Ymoisture is moisture content in coal. wet basis. is used as the transferred variable. h = gasifier length. the residence time is used as an output variable. (23): 2 Vg t D h 4 (23) Where Vg = volumetric flow rate of gas phase at the inlet of GASIFIER block. Transforming Eq. The next step is to transfer the results from the GASIFCAL block to the GASIFIER block. material and heat streams. the solid residence time in the gasifier has been approximately calculated in the GASIFCAL block. so another route is taken. so that the solid residence time in the GASIFIER block can be corrected correspondingly. as shown in Table 9. 5. in the GASIFIER block. 26 5 Simulation Approach . The diameter of the gasifier. (24) and transferring it to the GASIFIER block. and Yash is ash content in coal.2 Streams Streams represent the material and energy flows in and out of the process. t = residence time. The residence time calculated in GASIFCAL block is first transformed to the gasifier diameter (D) based on Eq. This model includes two types of streams.3 Calculator Blocks This model includes two Calculator blocks. Return of results from GASIFCAL to GASIFIER Through the above calculation. as shown in Fig. not an input variable. 5. which is an input variable in the GASIFIER block. dry basis. However. The streams with solid lines represent material streams. coal is inlet coal density. the solid residence time in the GASIFIER block is corrected correspondingly. 5. (23) gives the expression for D: Vg t D (24) h 4 After getting the gasifier diameter based on Eq. The streams with dashed lines represent heat streams. This means that the solid residence time cannot be transferred directly. Table 10. Inappropriate convergence methods may result in the failure of convergence or long running time.0001 Integration Initial step size of integration variable 1E-8 parameters Maximum step size of integration variable 0.001 Maximum number of integration steps 1E6 Convergence method Newton Corrector Error tolerance ratio 0.4 Convergence The convergence method impacts simulation performance greatly. N2. the choice of convergence method for the RPlug reactor called GASIFIER is very important. and ash in reaction of the SEPELEM block GASIFCAL Correct the solid residence time in the GASIFIER block 5. Convergence Parameters for GASIFIER Block Items Parameters Setup Integration convergence tolerance 0. These are specified on the sheet Blocks | GASIFIER | Convergence | Integration Loop. S. The convergence parameters for the GASIFIER block in the example model are summarized in Table 10. Calculators Used in the Model Name Function SPELMCAL Determine the stoichiometric coefficients of C. In this model. H2.1 Error scaling method Dynamic Integration error Minimum scale factor 1E-10 5 Simulation Approach 27 . Table 9. O2. 67K and 24atm. and velocity of coal particle entering into the gasifier. The input conditions for the simulation are summarized in Tables 2. and 12. and oxygen streams. gasifier length. Feedstock Conditions for Simulation[1. and pressure. steam.6% of the length of whole gasifier.241 dimensionless Steam Temperature 696. the length for char gasification process simulated in the GASIFIER block is set as 325 (1 4. it includes the ratio of oxygen to coal flow rates. the steam enters the gasifier in a superheated state. For the steam stream. At our feed conditions of 696. In the work of Wen and Chaung[5]. and sulfur analyses. Table 2 gives the component attributes of coal including the results of proximate. Table 3 shows the yield of each pyrolysis product obtained from the coal pyrolysis experiment at 1atm. inlet temperature. and pressure. and diameter.6 Simulation Results In this model. the input conditions for the simulation and the corresponding experimental results are from the open literature[1. which are the operating pressure. 3.66 g/s Temperature 505.22 K Coal Pressure 24 atm Diameter of particle 350 µm Velocity entering into gasifier 3 m/s Ratio of oxygen to coal flow rates 0. inlet temperature and pressure. it contains the ratio of steam to coal flow rates.6%) 310cm . For the coal stream.67 K Pressure 24 atm Table 12 gives the operating conditions and configuration parameters of the gasifier. For the oxygen stream. 5] Feedstock Parameter Value Unit Flow rate 76. Table 11 summarizes the feed conditions of coal. 5]. So in our model. it includes the flow rate of coal. they assume that the coal pyrolysis and volatile combustion processes account for 4. ultimate. Table 11. diameter of coal particle. 28 6 Simulation Results .866 dimensionless Oxygen Temperature 298 K Pressure 24 atm Ratio of steam to coal flow rates 0. inlet temperature. 11. 9K in Wen and Chaung’s model. increasing the temperature decreases the simulated CO2 flow rate and increases the simulated CO flow rate. Increasing the temperature will make the reaction shift in the backward direction. From the table. In our model. However.25 m Diameter 1. and then makes the carbon conversion in our simulation greater than the experimental data. the temperature at the outlet of gasifier is 1771. The first point is the heat of combustion (HCOMB) of coal. the heat loss to the environment is considered. the model is simulated in an adiabatic mode. 6 Simulation Results 29 . we get the results at the outlet of the gasifier. an exothermic reaction.2K. the HCOMB of coal is calculated by the built- in method (Boie correlation) in Aspen Plus due to lack of accurate experimental data. The second point is the amount of heat loss in the gasifier. The increase in temperature of gasifier speeds up the reaction rate of solid carbon and gases. However. Table 12. the outlet temperature is 1421. increasing the amount of CO and decreasing the amount of CO2. 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 our model. In our model. In our simulation results. i. combining these two points may cause the higher temperature of the gasifier in our model. For comparison. it can be seen that Wen and Chaung’s results[1] are in a better agreement with experimental data.e. 5] Parameter Value Unit Pressure 24 atm Length 3. The difference in flow rates of CO and CO2 depends on reaction (13). The CO flow rate and carbon conversion are greater than the experimental data. So. In our work. the CO and CO2 flow rates and carbon conversion are somewhat different from the experimental data. the inaccurate HCOMB may make the incorrect enthalpy of coal and then result in the large departure of gasifier temperature. we believe the HCOMB has a significant effect on the enthalpy of coal. rate of reactions (7-10). we manually input the HCOMB of coal to match our outlet temperature with Wen and Chaung’s work. So. as shown in Table 13 as Aspen Plus model a. In Wen and Chaung’s model[1]. The CO2 flow rate is less than the experimental data. Therefore. Why does our simulation generate the higher temperature in the gasifier? This may be caused by two points. Operating Condition and Configuration of Gasifier[1. the corresponding results of Wen and Chaung’s work[1] are also shown in the table.5 m Based on above input conditions.e. 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. i. H2O.20 H2S 0.462 1.94 56.92 6. As the residence time increases. in model b.23 39. At the same time.53 1.84 5.12 0.90 10.95 10.16 0.405 0.83Btu/lb by the Boie correlation). HCOMB of coal is calculated by Boie correlation.15 0.2K. When the HCOMB of coal is input as 13416Btu/lb (compared with the HCOMB of coal calculated as 14080. 9b.64 98. it can be determined that the product gas composition and carbon conversion are strongly dependent on the temperature in the gasifier.133 0. Table 13. dry (%. Our model shows a similar result to Wen and Chaung’s model.99 38.98 123.03 CH4 0.88 99. In Fig.60 127.24 3.2 Note: In model a.96 38.25 0. (K) -----.985 2.13 0.44 57.53 0.04 2.40 N2 0. (%) 98. the outlet temperature is equal to 1423.25 Carbon conv.77 57. dry (g/s) (g/s) (g/s) (g/s) basis) basis) basis) basis) CO 123.71 58. dry (%. H2.57 123.54 0.208 0.20 0.27 1. 30 6 Simulation Results .95 98.69 Temp. the CO and CO2 flow rates and carbon conversion are also in a good agreement with the experimental data.41 H2 6. and CO2) in char gasification process. which matches the temperature (1421.2 1423. H2O and CO2 contents in the gas phase decrease.23 5. Comparison of Experimental and Modeling Results Wen and Aspen Plus Aspen Plus Experimental[1] Chaung’s model a model b model[1] Parameters Mole Mole Mole Mole Flow Flow Flow Flow fraction fraction fraction fraction rate rate rate rate (%.04 0.9 1771. 9 shows the corresponding profile of main product gases (CO. HCOMB of coal is manually input as 13416Btu/lb.454 0. dry (%. and CO and H2 contents increase.71 CO2 9.24 0. and HCOMB of coal and heat loss are two key parameters in determining the temperature in the gasifier.726 0. Fig.06 0. 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.54 0.13 6. as shown in Table 13 as Aspen Plus model b.9K) in Wen and Chaung’s work.12 0. 1421.10 0.01 39. Based on these phenomena. 70 70 60 60 VOL. 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 . %(Wet basis) 50 50 40 40 30 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. Feed conditions of coal. The coal stream also includes the diameter of coal particle and velocity fed into the gasifier. 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 32 7 Conclusions . In the model. which include porosity of ash layer and reactivity of ash for the reaction of CO and H2O. and heat loss. From the model. oxygen and steam streams. To use this model. temperature and pressure. The model follows the modeling approach suggested by Wen and Chaung[1]. the kinetics of char gasification and the hydrodynamics for calculating solid particle residence time are considered. The heat loss can also be in-situ calculated by providing heat transfer coefficient and environmental temperature.7 Conclusions A Texaco down-flow entrained flow gasifier model is developed with the Aspen Plus simulator. which include the flow rate. operating pressure. Yield of coal pyrolysis products from coal pyrolysis experiment at 1atm. Model parameters. which include gasifier height. the following data should be provided: Component attributes and higher heat of combustion of coal. ultimate. Configuration parameters and operational conditions of the gasifier. gasifier diameter. and sulfur analyses. The component attributes of coal include the data of proximate. The Aspen Plus model provides a useful modeling framework for future refinements as new knowledge is gained with the entrained flow gasifier. Reasonable simulation results were obtained compared with the experimental results. L.-Z. D. Y. M. Wen.-M. Shan. 1998. Chem. Process Des. 1978.-H. “Noncatalytic heterogeneous solid fluid reaction models”.M. 2006. Wen. Higashitani.-Z..-S. Fang (方梦祥). Yoshida. J. “Modeling and simulation of an entrained flow coal gasifier”. 1984. Govind.. 17: 37-46. K. Ren (任永强). design and operation of circulating fluidized bed boilers ( 循环流化床锅炉理论设计与运行)”. Eng. Yan (严建 华). [4] S. Masuda. T. Ind. J. Howard. [3] R. Peters. CRC Press. [5] C. AIChE J. Ind. Dev. X. Cheng (程 乐鸣). Zhang (张东亮). Ind. Beijing: Chinese Electric Power Press. 60: 34-54. 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