adeyemi2014

Chapter 49Detailed Kinetics-Based Entrained FlowGasification Modeling of Utah BituminousCoal and Waste Construction Wood UsingAspen Plus

Idowu A. Adeyemi and Isam Janajreh

Abstract This study seeks to develop a kinetic-based ASPEN Plus model for theBrigham Young University (BYU) gasifier, an atmospheric oxygen-blownentrained flow gasifier. The model consists of 11 components, 3 in-built FORTRANcalculations and a FORTRAN subroutine. The in-built FORTRAN calculationswere used for the estimation of the drying process, the separation of char intoconstituents and the estimation of the gasifier residence time while the FORTRANsubroutine was used to determine the char gasification kinetics based on theunreacted shrinking core model of Wen and Chaung [1]. The model takes intoaccount the passive heating through moisture release, pyrolysis, volatile combus-tion and char gasification. The model has been validated with the experimentalwork of Brown et al. [2] with Utah bituminous coal, which was used as a baselinefor the analysis of wood waste. In addition, the effect of operating parameters hadbeen studied to determine the influence of fuel type and gasifier diameter on theprocess metrics like the gasification efficiency, species distribution along the cen-terline etc. Based on the available knowledge, this is the first detailed non-empiricalASPEN Plus kinetic model for entrained flow gasification (EFG) studies in theoxygen-blown atmospheric Brigham Young University laboratory gasifier set-up.

Keywords Pyrolysis Entrained flow gasification Kinetics Char Speciesdistribution Gasification efficiency

I. A. Adeyemi I. Janajreh (&)Masdar Institute, Abu Dhabi, United Arab Emiratese-mail: [email protected]

M. O. Hamdan et al. (eds.), ICREGA14 - Renewable Energy: Generationand Applications, Springer Proceedings in Energy, DOI: 10.1007/978-3-319-05708-8_49, Springer International Publishing Switzerland 2014

607

49.1 Introduction

Millions of tons of solid wastes are generated annually that continue to pose seriousenvironmental and ecological threats to our planet. In 2009 alone, the total amountof solid waste in the Emirate of Abu Dhabi, United Arab Emirates, was 5,756thousand tons according to the estimates of the Center of Waste Management-AbuDhabi, with the construction sector contributing 61 % of the total waste due to theconstruction boom taking place in the Emirate [3]. In addition, nearly 30 % of the240 million tons generated waste in the US is recycled while the rest is primarilydestined to land filling. Landfill gas (LFG), which is mainly composed of carbondioxide and methane, is widely recognized as one of the largest sources of methaneemission to the atmosphere and a central contributor to greenhouse gases (GHG).Methane, however, is 21-folds more potent than carbon dioxide by weight, and it issecond most abundant GHG after carbon dioxide. The estimate of global methaneemission from solid waste disposal sites ranges from 20 to 70 Tg/year, or about520 % of the total estimated methane emission of 375 Tg/year from anthropogenicsources. Therefore, alternative waste to energy systems which are less harmful toour environment and avoids other problems associated with landfilling like landavailability, health issues, etc., should be sought. Which alternative technologyhas the capability to resolve these problems? Gasification is one of the technologiesthat have been sought for its lower emission and higher efficiency. Besides its abilityto resolve most of the landfilling problems, it helps so solve other issues too.Gasification does not compete with food supplies as against fermentation, and itdoes not produce noxious pollutants like incineration. Furthermore, gasificationhelps to take over from recycling after a product has been recycled several times. Inaddition, the syngas produced during gasification can be used as fuel in differentkind of power plant such as gas turbine cycle, steam cycle, combined cycle, internaland external combustion engine and Solid Oxide Fuel Cell (SOFC) [4].

Although there are some equilibrium-based ASPEN Plus models for EFG [510],there are very few studies on the detailed kinetics-based ASPEN Plus model devel-opment [11, 12] and centerline experimental studies for an EFG process [2, 13, 14].Kong et al. [5] developed a three stage equilibrium model for the gasification of coalin the Texaco type coal gasifiers using ASPEN Plus to calculate the composition ofthe product gas, carbon conversion, and gasification temperature. Gartner et al. [11]developed a kinetic based entrained flow gasifier model in ASPEN Plus for thesimulation of the gasification of fuel blends (dried lignite, extraction residue andchar) in the CSIRO air-blown pressurized entrained flow reactor (PEFR). Theirstudy was validated with the gasification studies on three Australian coals (CRC252,CRC274, and CRC299) investigated in this reactor [13]. Lee et al. [12] investigatedthe effects of burner type on a bench-scale entrained flow gasifier in ASPEN Plusand validated their model with the experimental data from the 1-ton-per-day oxy-gen-nitrogen blown Korea Electronic Power Corporation (KEPCO) ResearchInstitute gasifier. Brown et al. [2] conducted experiments on the gasification of fourcoal types in an oxygen-blown atmospheric entrained flow gasifier at the Brigham

608 I. A. Adeyemi and I. Janajreh

Young University with the objective of investigating the temperature and syngascomposition along the centerline of the gasifier. Harris et al. [14] examined thebehavior of fifteen Australian coals and one petroleum coke at 2.0 MPa pressure andat temperatures between 1,373 and 1,773 K under conditions allowing for coalgasification behavior to be investigated under well-controlled entrained flow con-ditions. The effects of O:C ratio, residence time and coal type on conversion levelsand product gas composition were studied.

The main objective of this work is to develop a detailed kinetics-based non-empirical ASPEN Plus model for the oxygen-blown BYU entrained flow gasifier(Fig. 49.1) and determine the effects of fuel type (Utah Bituminous Coal andConstruction Waste Wood) on the gas composition and gasification efficiency.This predictive kinetic-based ASPEN Plus model with low computational cost,takes into consideration four processes: moisture release, pyrolysis, volatilecombustion and char gasification. A plug flow reactor was used to simulate thechar gasification process in order to eliminate the assumptions of constant tem-perature in equilibrium-based models. This model also helps in gaining insightabout the processes that occur inside the gasifier and hence, help in the optimi-zation of a gasification system.

49.2 Model Assumptions

The following assumptions were made in the development of the entrained flowgasification model:

The gasification model is in steady state The gas phase is assumed to be instantaneously and perfectly mixed with the

solid phase

185cm

=2.85cm

Primary StreamSecondary Stream

Presure Outlet

=1.3cmFig. 49.1 The schematicdiagram of the BYU gasifier

49 Detailed Kinetics-Based Entrained Flow Gasification Modeling 609

The pressure drop in the gasifier is neglected The 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 The temperature inside the particle is assumed to be uniform.

49.3 Fuel Characterization and Gasification Conditions

49.3.1 Proximate Analysis

This analysis is done with the thermo-gravimetric analyzer (TGA) in order tofragment the feedstock into moisture, volatile, fixed carbon and ash. Based on thisanalysis, we can determine the quality of the fuel to be used for gasification. Afeedstock with low moisture and ash content is a good candidate for gasification.The detailed result for both feedstocks is presented in Table 49.1.

49.3.2 Ultimate Analysis

The ultimate analysis (Table 49.2) is based on the examination of the elementalcomposition of the fuel. The elemental composition of any carbonaceous materialin terms of the mass percentages of C, H, O, N, S components can be determinedusing FLASH Elemental Analyzer. Determination of the elemental content

Table 49.1 Proximate analysis of fuels

Proximate analysis (wt%) Utah bituminous coal [2] Construction waste wood

Moisture 2.40 8.95Volatile 45.60 68.89Fixed carbon 43.70 21.88Ash 8.30 0.28

Table 49.2 Ultimate analysis of fuels

Ultimate analysis (wt%) Utah bituminous coal [2] Construction wood waste

Carbon 71.00 49.45Hydrogen 6.00 6.26Nitrogen 1.30 0.39Oxygen 12.70 43.60Sulfur 0.70 0.02Ash 8.30 0.28


composition is very important in gasification as that helps to determine theequivalence ratio of the fuel. Any fuel with high oxygen does not require largeamount of oxygen for gasification.

49.3.3 Bomb Calorimetry

The bomb calorimetry helps to determine the heating value of the feedstocks to begasified. The equipment used for this analysis was the Bomb calorimeter (Parr6100). The heating value obtained for Utah bituminous coal and constructionwaste wood are 29.8 and 18.7 MJ/kg respectively.

49.3.4 Boundary Conditions

The boundary condition in Table 49.3 was applied to the developed model for boththe Utah bituminous coal and the construction waste wood.

49.4 Model Descriptions

The ASPEN Plus model consists of the moisture release through passive heating,devolatilization, volatile combustion and char gasification (Fig. 49.2).

Table 49.3 Boundaryconditions [2]

Conditions Value

Primary streamFlow rate (kg/s) 0.0073Component mole fractionOxygen 0.85Argon 0.126Steam 0.024Stream temperature (K) 360Secondary streamFlow rate (kg/s) 0.00184Component mole fractionSteam 1Stream temperature (K) 450Particle loading 0.910


49.4.1 Moisture Release

The first stage of the gasification process is the moisture release, particularly whenwet fuels are being injected. The moisture release process was modeled in ASPENPlus with the RStoic block labeled as DRYER and the Flash2 block labeled asFLASH-A. With the help of the RStoic block and an in-built FORTRAN code, theamount of moisture to be released from the wet fuel was estimated as 95 % of themoisture content in the proxanal attribute. Subsequently, the Flash2 separator wasused to remove the vapor from other components of the fuel. Because the reactor isclosed and there is no means of releasing the lost vapor into the atmosphere, thelost vapor was re-introduced via the stream L-H2O-B. The moisture release pro-cess can be represented as shown in Eq. (49.1).

CxHyOzNmSnAshpMoisq ! CxHyOzNmSnAshpMoisr q r Mois 49:1

49.4.2 Pyrolysis or Devolatilization

After most of the moisture content of the fuel had been dried off, the volatiles inthe dry feedstock are released. The volatiles that are considered in this model arecarbon monoxide, vapor, hydrogen, carbon dioxide, methane, hydrogen sulfide,nitrogen and benzene. The tar produced during the pyrolysis process, which isusually very minute for entrained flow gasification due to their high temperature,was taken as benzene for this model [15]. Besides the volatiles that are evolved,solid char with intrinsic ash is left behind. Pyrolysis of feedstocks usually begins

Fig. 49.2 The ASPEN plus EFG model


between 400 and 600 C and ends in few to several milliseconds [2]. Hence, thedevolatilization process can be assumed to be instantaneous and can be modeled inASPEN Plus with an RYield reactor labeled as YIELD-A, a mixer labeled as MIX,a separator labeled as SEP-B and an RGibbs reactor labeled as GIBBS. TheYIELD-A reactor breaks down the fuel into char and elements consisting ofcarbon, hydrogen, nitrogen, oxygen and sulfur. The elements, char, argon, andreleased vapor (during the moisture release stage) are mixed together using themixer-MIX from where they were sent to a separator-SEP-B. The SEP-B separatesthe char from other components which consist of the volatile elements and sendsthe volatile components into the GIBBS reactor. The RGIBBS reactor in ASPENPlus utilizes the Gibbs minimization method to find the equilibrium composition ofthe volatiles which were identified as argon, carbon monoxide, hydrogen, carbondioxide, vapor, hydrogen sulfide, nitrogen, methane and benzene. The expressionfor the pyrolysis of the dry fuel is as shown in the Eq. (49.2).

CxHyOzNmSnAshpMoisr !Cx1 Ashp x2CO x3H2 x4H2O x5CO2 x6CH4 x7H2S x8N2 x9C6H6

49:249.4.3 Volatile Combustion

Immediately the volatiles are evolved, they start reacting with the oxidant intro-duced, which is usually oxygen. Only four of the released volatile components,namely carbon monoxide, hydrogen, methane and benzene, can undergo com-bustion. Hence, four reactions were modeled in ASPEN Plus with an RStoicreactor labeled COMBUST as shown in Eqs. (49.349.6).

C6H6 7:5O2 ! 6CO2 3H2O 49:3

H2 0:5O2 ! H2O 49:4

CO 0:5O2 ! CO2 49:5

CH4 2O2 ! CO2 2H2O 49:6

The rates of gas phase combustions are generally much faster than those ofsolidgas reactions. The calculated rate of combustion of carbon monoxide basedon the correlation of Hottel et al. was found to be so high that it can be consideredinstantaneous [16]. The conversion of carbon monoxide, hydrogen and benzeneare thus assumed to be 100 %.


49.4.4 Char Gasification and Homogeneous Reactions

The char gasification process and the subsequent homogeneous reactions weremodeled in ASPEN Plus with the RPlug reactor labeled as PLUGFLW. The RPlugreactor eliminates the assumption of constant temperature in equilibrium-basedmodels. In addition, this reactor block allows for the observation of the axialtemperature and concentration of the product gases and hence, it helps in theestimation of the optimal length and diameter for entrained flow gasification. Thefollowing reactions in addition to the Eqs. (49.749.13) were considered:

C 1/

O2 ! 2 1 1/

CO 2/ 1

CO2 49:7

C H2O ! CO H2 49:8

C CO2 ! 2CO 49:9

C 2H2 ! CH4 49:10

S H2 ! H2S 49:11

CO H2O ! CO2 H2 49:12

CH4 H2O ! CO 3H2 49:13

where / = the mechanism factor based on the stoichiometric relation of CO andCO2 and can be obtained from the work of Wen and Chaung [1].

Because entrained flow gasification occurs in the reaction zone III where thetemperature is high, most char-gas reactions can be considered as surface reac-tions. Furthermore, the solid loading in entrained flow gasifiers is very small thatthe particle collisions are likely to be infrequent and the ash layer formed can beassumed to remain on the fuel particle during reactions. Hence, the unreacted-coreshrinking model [1] can be reasonably applied to estimate the heterogeneous solid-gas reaction rates.

The overall rate, according to this model, can be expressed as:

Rci 11kdiff

1ksY2 1kdash 1Y1 Pi Pi 49:14

where

kdash kdiff n 49:15


Y rcrp 1 x

1 f 1

3

49:16

where ks is the surface reaction constant, kdash is the ash film diffusion constant,kdiff is the gas film diffusion constant, e is the voidage in the ash layer, n is a constantbetween 2 and 3, rc is the radius of the unreacted core, rp is the radius of the wholeparticle including the ash layer, x is coal conversion at any time after pyrolysis iscomplete, f is the coal conversion when pyrolysis is complete, Pi - Pi* is theeffective partial pressure of the components and Rc-i is the reaction rate.

The expressions for the kinetic constants kdiff, ks, kdash, and (Pi - Pi*) for each

char-gas reactions can be obtained from the work of Wen and Chaung [1].

49.5 Results and Discussions

49.5.1 Model Validation

The developed kinetic ASPEN Plus model was validated with the work of Brownet al. [2] based on the oxygen-blown atmospheric BYU experimental gasifier set-up with Utah bituminous coal (Figs. 49.349.6). It is evident that this modelpredicts the gas composition along the centerline of the gasifier reasonably wellconsidering the fact that the RPlug reactor used in ASPEN Plus is a 1-D reactorwithout turbulence effects being considered. This validated model could be used instudies, which are cumbersome to be performed in experiments, to obtain themetrics of a gasifier set-up.

49.5.2 Effect of Gasifier Diameter

One of the important parameters to be considered in the design of an optimalgasifier is the size. Hence, the effect of the gasifier diameter on the gas compo-sition along the centerline was investigated in order to determine the best designsize for a gasifier (Figs. 49.749.10). The diameter of the gasifier was variedbetween 0.1 and 0.3 m. While a rise in the diameter size leads to an increase in themole fraction of the carbon monoxide, carbon dioxide and hydrogen throughoutthe length of the gasifier, an opposite trend was observed for the vapor compo-sition. In addition, the effect of the diameter was more pronounced between 0.1and 0.2 m than between 0.2 and 0.3 m. This implies that there is a limit to the sizeof a gasifier for optimization above which there will be no effect and the extradiameter will be redundant.


49.5.3 Effect of Fuel Type on Gas Composition

Another important concern in the combustion community is the viability of thegasification of other feedstocks besides coal. We have therefore decided toinvestigate the effect of gasifying construction waste wood on the gas compositionalong the centerline of the gasifier (Figs. 49.1149.14). Based on this sensitivity

Fig. 49.3 Mole fraction ofCO along the centerline

Fig. 49.4 Mole fraction ofH2 along the centerline


study, it was observed that the mole fraction of the carbon monoxide, carbondioxide and hydrogen was lower for the construction waste wood. Furthermore, thevapor mole fraction was higher for the wood. This is because the wood has highermoisture content (proximate analysis), higher oxygen content (ultimate analysis)and lower carbon content (ultimate analysis). One interesting phenomenon was that

Fig. 49.5 Mole fraction ofCO2 along the centerline

Fig. 49.6 Mole fraction ofH2O along the centerline


at the inlet of the plug flow reactor, the carbon dioxide was more for the woodwaste, but it was soon approached and advanced by the coal. This is because of thelow char content (proximate analysis) in the wood waste.




49.6 Conclusions

A comprehensive kinetics-based ASPEN Plus model has been developed for theoxygen-blown atmospheric BYU gasifier. The model predicts reasonably well thegas composition along the axis of the gasifier and can be used to optimizethe gasifier. An increasing diameter of the gasifier gives more CO, CO2 and H2 butless H2O along the centerline. Wood waste yielded a gasification efficiency of38.25 % as compared to 57.18 % for coal. Based on this study, the gasificationefficiency of wood waste can be further increased by drying the wood beforegasification and lowering the mass flow rate of the oxidant.

Acknowledgments The authors highly appreciate the support and sponsorship of MasdarInstitute and the members of the Waste-2-Energy group.

References

1. C.Y. Wen, T.Z. Chaung, Entrainment coal gasification modeling. Ind. Eng. Chem. ProcessDes. Dev. 18, 684695 (1979)

2. B.W. Brown, L.D. Smoot, P.J. Smith, P.O. Hedman, Measurement and prediction ofentrained-flow gasification processes. AIChE J. 34, 435446 (1988)

3. Statistics Center Abu Dhabi, Waste Statistics in the Emirate of Abu Dhabi 2009 (2011),http://www.scad.ae/SCADDocuments/Waste%20Statistics%20in%20the%20Emirate%20of%20Abu%20Dhabi%202009.pdf. Cited12 Dec 2013

4. F. Bellomare, M. Rokni, Integration of a municipal solid waste gasification plant with solidoxide fuel cell and gas turbine. Renew. Energy 55, 490500 (2013)

5. X. Kong, W. Zhong, W. Du, F. Qian, Three stage equilibrium model for coal gasification inentrained flow gasifiers based on Aspen Plus. Chin. J. Chem. Eng. 21, 7984 (2013)



6. C.H. Frey, N. Akunuri, Development of optimal design capability for coal gasification systems:performance, emissions and cost of Texaco gasifier-based systems using ASPEN. In: Technicalreport, U.S. Department of Energy, National Energy Technology Laboratory (2001), http://www.cmu.edu/epp/iecm/rubin/PDF%20files/2001/2001rc%20Frey%20et%20al,%20Aspen%20IGCC%20Tech.pdf. Cited 14 Dec 2013

7. S.V. Nathe, R.D. Kirkpatrick, B.R. Young, The gasification of New Zealand coals: acomparative simulation study. Energy Fuels 22(4), 26872692 (2008)

8. E. Biagini, A. Bardi, G. Pannocchia, L. Tognotti, Development of an entrained flow gasifiermodel for process optimization study. Ind. Eng. Chem. Res. 48, 9028 (2009)

9. M. Perez-Fortes, A.D. Bojarski, E. Velo, J.M. Nougues, L. Puigjaner, Conceptual model andevaluation of generated power and emissions in an IGCC plant. Energy 34, 17211732(2009)

10. A.S. Valmundsson, I. Janajreh, Plasma gasification process modeling and energy recoveryfrom solid waste. Paper presented at the 5th international conference on energy sustainability,Washington, USA, August 710 2011

11. L.E. Gartner, M. Grabner, D. Messig, W. Heschel, B. Meyer, Kinetic entrained flow gasifiermodeling in Aspen Plusa simulation study on fuel blends. Paper presented at DBFZWorkshop zur Fliebbildsimulation in der Energietechnik, Leipzig, 2012

12. J. Lee, S. Park, H. Seo, M. Kim, S. Kim, J. Chi, K. Kim, Effects of burner type on a bench-scale entrained flow gasifier and conceptual modeling of the system with Aspen Plus. KoreanJ. Chem. Eng. 29, 574582 (2012)

13. S. Hla, D.J. Harris, D.G. Roberts, Gasification conversion modelPEFR. In: ResearchReport 80, Pullenva (2007), http://trove.nla.gov.au/work/34165934?q&versionId=46623731.Cited 15 Dec 2013

14. D.J. Harris, D.G. Roberts, D.G. Henderson, Gasification behavior of Australian coals at hightemperature and pressure. Paper presented at the 21st annual international Pittsburgh coalconference, Osaka, Japan, 2004

15. H.J. Park, S.H. Park, J.M. Sohn, J. Park, J.K. Jeon, S.S. Kim, Y.K. Park, Steam reforming ofbiomass gasification tar using benzene as a model compound over various Ni supported metaloxide catalysts. Bioresour. Technol. 101, 101103 (2010)

16. H.C. Hottel, G.C. Williams, N.M. Nerheim, G.R. Schneider, Kinetic studies in stirredreactors: combustion of carbon monoxide and propane. Paper presented at the 10thinternational symposium on combustion, University of Cambridge, Cambridge, England,1721 August 1964


49 Detailed Kinetics-Based Entrained Flow Gasification Modeling of Utah Bituminous Coal and Waste Construction Wood Using Aspen PlusAbstract49.1Introduction49.2Model Assumptions49.3Fuel Characterization and Gasification Conditions49.3.1 Proximate Analysis49.3.2 Ultimate Analysis49.3.3 Bomb Calorimetry49.3.4 Boundary Conditions

49.4Model Descriptions49.4.1 Moisture Release49.4.2 Pyrolysis or Devolatilization49.4.3 Volatile Combustion49.4.4 Char Gasification and Homogeneous Reactions

49.5Results and Discussions49.5.1 Model Validation49.5.2 Effect of Gasifier Diameter49.5.3 Effect of Fuel Type on Gas Composition

49.6ConclusionsAcknowledgmentsReferences