CES2016_Kuo

Thermodynamic analysis of a combined heat and power systemwith CO2utilization based on co-gasification of biomass and coalPo-Chih Kuo,Wei Wu nDepartment of Chemical Engineering,National Cheng Kung University,Tainan70101,TaiwanH I G H L I G H T SA CHP system based on co-gasification of biomass and coal is investigated.Identifying the CBP through the specific optimization algorithm.The performance of the CHP system can be effectively improved under CO2addition.Co-gasification of torrefied biomass and coal blends is able to suppress the CO2specific emissions.a r t i c l e i n f oArticle history:Received12June2015Received in revised form31October2015Accepted23November2015Available online11December2015Keywords:Co-gasificationTorrefied biomassCO2utilizationCarbon boundary pointCombined heat and powerOptimizationa b s t r a c tIn this article,a co-gasification system blending coal and biomass is examined in the Aspen Plusenvironment in terms of energy conversion efficiency(ECE)and exergy efficiency(EE).Although thepercentage of raw wood(RW),torrefied wood(TW)and coal in steam co-gasification is one of the mostimportant parameters that affect the gasification process,the addition of CO2could effectively improveECE and EE while the steam-to-carbon ratio(S/C)is adjusted at the carbon boundary point(CBP).Theoptimum operating conditions such as S/C and CO2supply ratio,are determined by solving a series ofconstrained optimization algorithms for maximizing ECE and EE.A combined heat and power(CHP)system using the maximum waste heat recovery and Rankine cycle is illustrated to assess performance interms of power generation and CO2utilization.The results show that the total power generation byfeeding the TW-based fuel blend of40wt%TW and60wt%coal is increased by8.43%,as compared tothat of the RW-based fuel blend of40wt%RW and60wt%pared with100wt%coal fuel,theTW-based fuel can significantly reduce CO2specific emission by38.23%.&2015Elsevier Ltd.All rights reserved.1.IntroductionGasification is a promising thermo-chemical conversion pro-cess for producing syngas(i.e.H2þCO)(Goransson et al.,2011;Woolcock and Brown,2013).Syngas has been widely applied tothe synthesis of chemicals,such as methanol,dimethyl ether(DME),and methyl tert-butyl ether(MTBE),as well as some liquidtransportation fuels via the Fischer–Tropsch synthesis methodbased on various H2/CO ratios(Goransson et al.,2011;Tijmensenet al.,2002;Woolcock and Brown,2013).Moreover,both heat andpower can be generated in an integrated gasification combinedcycle(IGCC)power plant(Sadhukhan et al.,2010).Typically,syngas is converted from carbonaceous fuels,such ascoal,biomass,plastic and waste,of which coal currently plays animportant role in energy and electricity production(Goransson etal.,2011;Woolcock and Brown,2013).However,coal gasificationcombined-cycle power plants release a large amount of green-house gas(GHG)emissions,mainly carbon dioxide(CO2),nitro-gen-(N)and sulfur-(S)based gases,into the atmosphere,thuscausing environmental pollution(Taba et al.,2012).It is thusnecessary to reduce the GHG emissions associated with thistechnology.Biomass is one of the most important renewable and sustain-able energy sources to produce syngas and electricity,and thus apromising alternative to the use of fossil fuels(Abdollahi et al.,2010;Taba et al.,2012).Biomass is an abundant resource that isenvironmental friendly because of its low nitrogen and sulfurcontents.It is also seen as a carbon-neutral fuel,which producesessentially zero net CO2emissions to the environment(Abdollahiet al.,2010;Taba et al.,2013).However,when raw biomass isdeveloped for bioenergy purposes the utilization efficiency isrelatively low,due to the disadvantageous intrinsic properties ofContents lists available at ScienceDirectjournal homepage:/locate/cesChemical Engineering Science/10.1016/j.ces.2015.11.0300009-2509/&2015Elsevier Ltd.All rightsreserved.n Corresponding author.Tel.:þ88662757575x62689;fax:þ88662344496.E-mail address:weiwu@.tw(W.Wu).Chemical Engineering Science142(2016)201–214this material compared to those of coal.For example,the major properties of raw biomass include(Chen and Kuo,2011;Sarvar-amini et al.,2013;Taba et al.,2013;Thanapal et al.,2014):(1)high moisture content,(2)low calorific value,(3)low energy density,(4)high hydrogen-to-carbon(H/C)ratio and oxygen-to-carbon(O/C)ratio,(5)hygroscopic nature,and(6)a low bulk density,making both transportation and storage costs higher.These properties mean that raw biomass gasification has a lower conversion effi-ciency than that seen with coal gasification.In recent years various studies have reported that using the combination of coal and biomass as a feedstock,in a process known as co-gasification,is as promising technology for the production of syngas.This process has the advantages of being both economical and environmentally friendly(García et al.,2013;Taba et al.,2012,2013),and a number of researchers have examining blending various types of biomass with coal.For instance,Saw and Pang(2013)examined the use of lignite and wood as a feedstock at various blending mass ratios for co-gasification.They reported that the H2/CO ratio can be enhanced during co-gasification,and that a H2/CO ratio of2(for Fischer–Tropsch synthesis)can be obtained by blending60%wood with40%lignite.Aigner et al.(2011)reported an increase in hydrogen production and a decrease in CO production with a ris-ing coal ratio.Furthermore,the concentrations of impurities such as NH3and H2S in the gas product have also been found to decrease during the co-gasification process.Howaniec and Smo-linski(2013)investigated the influence of synergetic effects on the steam co-gasification process.They reported that the reactivity of fuel blends increased at a gasification temperature of900°C,due to the alkali content of the biomass,and especially the presence of potassium,which acts as a catalyst to aid in the process of char gasification.Compared to traditional approaches to the production of raw biomass energy,the application of torrefied biomass is attracting increasing attention in recent years,as it offers several advantages. Torrefied biomass is produced by thermally pretreating the raw biomass.This process is known as torrefaction,which is a mild pyrolysis process carried out in the temperature range of200–300°C under an inert or nitrogen atmosphere.Torrefaction enhances the physical and chemical properties of raw biomass. Torrefied biomass has the following improved properties com-pared with the untreated material(Chen and Kuo,2011;Sarvar-amini et al.,2013;Thanapal et al.,2014;Kuo et al.,2014):(1)lower moisture content(hydrophobicity),(2)higher energy density, (3)greater ignitability,reactivity and grindability,(5)a lower oxygen-to-carbon(O/C)ratio,and(6)lower costs of transportation and storage.Torrefied biomass is thus seen as a more valuable and economic fuel than raw biomass.In the recent literature Deng et al.(2009)reported that a combination of torrefaction and co-gasification is a promising process for the production of syngas. Chen et al.(2011)gasified torrefied sawdust in a bench-scale laminar entrained-flow gasifier,and found that the cold gas effi-ciency increased compared to that of raw sawdust,especially for sawdust torrefied at250°C.Weiland et al.(2014)observed that carbon conversion was improved by using torrefied wood residue as a feedstock in an entrainedflow gasifier.Furthermore,and more recently,Dudyński et al.(2015)used torrefied woody biomass for industrial-scale gasification experiments,and found that syngas with a higher calorific value was obtained by using torrefied pel-lets as a feedstock.These earlier studies show that torrefaction in association with gasification is an effective and promising method to enhance the performance of gasification.Although numerous works have investigated the gasification characteristics of torrefied biomass, little information is available on the co-utilization of coal and torrefied biomass during a steam co-gasification process.On the other hand,the integrated gasification combined cycle(IGCC)power generation is an important and potential clean coal tech-nology(CCT)that is able to reduce emissions of CO2,the primary greenhouse gas that is responsible global warming.A review of the past literature suggests that most of the related studies focus on the type of oxygen-blown(Emun et al.,2010;Zhang et al.,2013), air-blown(Klimantos et al.,2009;Giuffrida et al.,2013),and a mixture of steam/oxygen(Lee et al.,2014;Asif et al.,2015)IGCC systems,while relatively little research has been carried out using a mixture of CO2/O2(Oki et al.,2011;Prabu,2015).If CO2is used as a gasifying agent it can not only be converted to carbon monoxide via the Boudouard reaction in gasification,but also reduce the level of CO2emission into the atmosphere(Prabu,2015).Fur-thermore,a number of studies have examined the use of steam and CO2as a gasifying agent.Castaldi and Dooher(2007)adopted the Aspen Plus simulator to investigate coal steam gasification with a CO2recycle stream to the reformer.They found that the addition of CO2could lead to decreased energy consumption in the reformer,and thus this is a suitable technique for reducing the energy requirements of such systems.Butterman and Castaldi (2007)also studied biomass steam gasification by introducing CO2 via thermogravimetric analysis-gas chromatography(TGA-GC), and found that the existence of CO2significantly promotes the production of CO when the operating temperature was beyond 700°C,while the production of H2is reduced.Prabowo et al. (2014)compared the gasification performance under the atmo-spheres of steam,CO2,and steam–CO2mixtures.They found that the gasification thermal efficiency increased along with the CO2 mixing ratio at high gasification temperatures.Jayaraman and Gokalp(2015)carried out char gasification to investigate the kinetic behaviors of gasification under a steam–CO2environment in the temperature range of850–950°C.They found that a higher temperature reduced the time required for50%char conversion.Despite the potential advantages of using steam–CO2mixtures as a gasifying agent,as detailed above,the process of co-gasification under a steam–CO2environment for a class of power generation system has not yet been investigated.The present study thus aims to design a combined heat and power(CHP) system based on the steam co-gasification of biomass and coal with the addition of CO2.Particular emphasis is placed on the comparison of co-gasification performance between raw/torrefied biomass and coal blends.Two important parameters of the steam-to-carbon ratio(S/C)and CO2supply ratio(y CO2)are adjusted to evaluate their influences on system performance.Finally,the optimum operating conditions are obtained by solving a series of constrained optimization algorithms2.Process simulationCoal and two types of biomass,namely raw wood(RW)and torrefied wood(TW),are selected as the feedstock for gasification and co-gasification in this study(Park et al.,2012).Their proper-ties,such as proximate analysis,elemental analysis,and higher heating values(HHV),are presented in Table1.The simulation model is established using a commercial heat and mass balance analysis package,Aspen Plus V8.4.The following main assump-tions are made in this study for developing the process model: (1)the process is steady-state;(2)the feedstock is at normal conditions(i.e.25°C and1atm);(3)the solid and gaseous phases are in a state of thermodynamic equilibrium;(4)the following gaseous species are considered in the whole system:H2O,H2,O2, N2,CO,CO2,CH4,H2S,COS,SO2,HCN,NO,NO2and NH3;and (5)char only contains carbon and ash,and tar formation is neglected(Kannan et al.,2013;Shen et al.,2008;Song et al.,2013). In the simulation,the global thermodynamic property method used in this model is the Peng-Robinson equation of state with theP.-C.Kuo,W.Wu/Chemical Engineering Science142(2016)201–214 202Boston-Mathias alpha function (PR-BM).The fuels are de fined as an unconventional component,based on the experimental results of elemental analysis and proximate analysis.Due to the non-conventional stream for the fuels,the HCOALGEN model,including a number of empirical correlations for heat of combustion,heat of formation,and heat capacity,is used in the simulation.The density of nonconventional fuels is established by the DCOALIGT model (Ramzan et al.,2011).The simpli fied block diagram of the whole system is shown in Fig.1,which consists of a gasi fication unit,gas cooling unit,gas cleaning unit,gas burning unit,and power generation unit.2.1.Co-gasi fication systemFig.2a shows a schematic of the co-gasi fication process,which is composed of a number of blocks.The total mass flow rate of fuel into the system is kept constant at 100kg/h throughout the simulations.When the “fuel ”(stream 1)is fed into the system,the first stage is the drying of the fuel.An RStoic reactor is used to simulate the drying process and the heat produced by the reaction is simulated by a heat stream (Q drying ).The process of drying is simulated by the following chemical reaction (Chen et al.,2013):Fuel -0:0555H 2O þfuel dryð1ÞAfter fuel drying,a yield reactor is used to simulate the decomposition of fuel.In the yield reactor,the unconventional components of the fuel (stream 2)are decomposed into conven-tional constituents (stream 3),including N 2,O 2,H 2,S,H 2O,carbonand ash.The product yield distributions in RYield are calculated by a calculator block which is controlled by the FORTRAN statement in accordance with the component characteristics of the feedstock (Table 1).The heat of the reaction from the decomposition is simulated by a heat stream (Q decomp ).For the gasi fication model,an RGibbs reactor is used to model reactions according to chemical equilibrium.In this block,the chemical and phase equilibrium calculations are carried out by minimizing the Gibbs free energy.The mathematical model of the RGibbs reactor is described in a previous study (Kuo et al.,2014),and this reactor is operated at 900°C (Howaniec and Smolinski,2013;Jayaraman and Gokalp,2015;Prabowo et al.,2014).The heat of the reaction from the endothermic gasi fication reactions is simulated by a heat stream (Q gasi fier ).The detailed operating con-ditions of the co-gasi fication process are presented in Table 2,and the major chemical reactions that take place during the gasi fica-tion are listed below (Prabowo et al.,2014;Taba et al.,2012,,2013):Water gas reaction C þH 2O -CO þH 2;ΔH 0¼þ131:4kJmol À1ð2ÞWater gas shift reaction CO þH 2O 2CO 2þH 2;ΔH 0¼À42kJmol À1ð3ÞBoudouard reaction C þCO 2-2CO ;ΔH 0¼þ172:6kJmol À1ð4ÞMethanation reaction C þ2H 22CH 4;ΔH 0¼À75kJmol À1ð5ÞCH 4þH 2O 2CO þ3H 2;ΔH 0¼À206kJmol À1ð6ÞRegarding the gasifying mediums (stream 4),steam and CO 2,are mixed and preheated by a heater (H1)before being fed into the RGibbs reactor.Notably,a heater (H1)with a heat stream (Q gasifying agents )is used to adjust the inlet temperature from 25°C (stream 4)to 200°C (stream 5).The outlet stream (stream 6)resulting from the RGibbs reactor is divided into two streams,product gas (stream 7)and char (stream 8)through an SSPLIT block.The product gas is cooled down by a cooler (C1),which reduces the temperature to 150°C (Emun et al.,2010).The steam and acid gases produced from the co-gasi fication process are then removed from the product gas in a separator block,where a heat stream (Q sep )is simulated to the energy demand of the separator.The clean gas (stream 9)is then obtained and combusted with excess air (stream 11)in order to provide complete combustion in theTable 1Proximate and elemental analyses of the feedstock used in the simulation (Park et al.,2012).FeedstocksRaw wood Torre fied wood CoalProximate analysis (wt%)Moisture3.81 3.19 6.67Volatile matter (VM)88.7281.1927.25Fixed carbon (FC)7.4215.5154.5Ash0.050.1111.58Elemental analysis (wt%)C 46.7352.2274.12H 6.46 5.18 4.22N 0.410.55 1.91O a 46.3541.94 6.93S000.41Higher heating value (MJ/kg)18.5120.5626.82aBy difference.Co-gasificationRaw gasGasifying agents (Steam/CO 2)Clean gasGas BurningHeat Recovery Steam Generator (HRSG) and Rankine cycleFlue gasFuels(RW, TW, Coal)PowerGas cooling and cleaningFig.1.Block diagrams for the CHP system.P.-C.Kuo,W.Wu /Chemical Engineering Science 142(2016)201–214203burner to form flue gas (stream 12),which can be used in the power generation system.To validate the present model,simula-tions have been performed for gasi fication of rubber wood in a fixed bed gasi fier operated at atmospheric pressure (1atm)(Mahishi and Goswami,2007)and the gasi fication temperature is 900°C.The validation has been examined in a previous study (Kuo et al.,2014),where the predicted results are in good agreement with the experimental values.2.2.Power generation systemThe schematic process of power generation is shown in Fig.2(b)where the power generation system major consists of a steam generator and Rankine cycle (Chen and Wu,2014).After burning the product gas from the co-gasi fication process,the high tem-perature flue gas is obtained and sent to the heat recovery steam generator (HRSG)and steam turbine (ST)process.A series of heat exchangers is used,including a radiant water-wall evaporator,radiant superheater,and economizer,in which steam is generated in conjunction with the heat transfer from hot product gas and then sent to three steam turbines,namely a high pressure (HP),intermediate pressure (IP),and low pressure (LP)one,for power generation.The detailed operating conditions of the steam cycle are shown in Table 2.First of all,0.279kg/s of the feedwater (stream 13)with a temperature of 45°C goes through a water pump in which it is pressurized from 1to 240atm,resulting in energy consumption (W Pump ).The high pressure feedwater (stream 14)is then fed into shell side of the multi-stream heat exchangers (feedwater regenerative heaters)and this water gets heated by the exhaust steam entering in the tube side,so that the steam is heated to 250°C (stream 17)and sent to the economizer.The steam (stream 17)is first heated by the outlet stream of the hot flue gas to the temperature of 550°C after leaving the boiler (stream 19).To achieve the supercritical state,the temperature of the steam is then raised from 550to 600°C through superheater 1,and it subsequently feeds into a high pressure turbine (HPT)(stream 20)to expand and produce electricity (W HPT ).The pres-sure of the steam exhausted from HPT is 49atm (stream 21).The Fsplitter (F1)is then speci fied as 0.7and used to split the steam stream;that is,a 0.195kg/s of steam stream (stream 22)flows to superheater 2to reheat the temperature to 620°C (stream 23),while a 0.0837kg/s of steam stream (stream 28)is sent to the feedwater regenerative heaters to reheat the feedwater.Similarly,after reheating to 620°C (stream 23)in superheater 2,the steam is also delivered into an intermediate-pressure turbine (IPT)and exhausted to a low-pressure turbine (LPT)(stream 25)to produce electricity (W IPT and W LPT ).After removing the heat from the flue gas,the temperature of exhausted gas drops to 50°C (stream 38)(Sorgenfrei and Tsatsaronis,2014),and the exhaust steam (stream 37)is then cooled down to 45°C at 0.1atm as saturatedwaterFig.2.Process flow chart of (a)the co-gasi fication system,and (b)power generation system.P.-C.Kuo,W.Wu /Chemical Engineering Science 142(2016)201–214204(stream13)by the cooling water in an external water-cooled condenser(C2).2.3.Process parametersThe steam-to-carbon(massflow rate)ratio(S/C ratio)and CO2supply ratio(y CO2)are two significant parameters in this process.They are expressed asS=C¼F steamy c F fuelð7Þy CO2¼F CO2steamþF CO2ð8Þwhere y c is the carbon content in the fed fuel,and y CO2is definedby the weight ratio of the CO2contained in the total gasifyingagents from both steam and CO2.That is,when y CO2is equal to0,itmeans that steam is the only gasifying agent fed to the gasifier.In addition,four biomass blending ratios(BR)of raw/torrefied biomass and the coal are considered for the co-gasification pro-cess,and these are20,40,60,and80wt%,which are expressed as BRðwt%Þ¼F biomassbiomassþF coalÂ100%ð9Þ2.4.Energy analysisThe energy conversion efficiency(ECE)and net energy effi-ciency(NEE)are two important indexes with regard to the per-formance of the system,and thus they are evaluated in this study.The ECE is defined as the ratio of energy output of the co-gasification system to the energy input,whereas NEE is the ratio of net power generation to energy input(Mahishi and Goswami, 2007;Speidel and Worner,2015).The energy input is calculated as the sum of the energy content of the fuel,heat of reaction,and energy requirement of the preheating gasifying agents and separator.The energy output is the lower heating value(LHV,kJ/ Nm3)of the product gas for the co-gasification system(Lv et al., 2004;Kuo et al.,2014)or net power output(W n et,kW)for the power generation system.These are defined as follows:ECE%ðÞ¼F productgas UG P U LHV productgasF fuel U LHV fuelþQ HÂ100%ð10ÞQ H¼Q d ryingþQ d ecompþQ g asifierþQ g asifying agentsþQ s epð11ÞLHV product gas¼30:0x COþ25:7x H2þ85:4x CH4ÀÁÂ4:2ð12ÞNEEð%Þ¼W n etF fuel U LHV fuelþQ HÂ100%ð13ÞW n et¼W H PTþW I PTþW L PTÀW P umpð14Þwhere F productgas and F fuel are the massflow rates of the product gas and fuel(kg/s),respectively;G P is the volume of product gas from the gasification per unit weight of fuel(Nm3/kg-fuel);Q H is the heat required for the co-gasification process(kW);x stands for the mole fraction of gas species in the product gas(dry basis); W n et is the net power of the whole system(kW);W P ump is the energy requirement of the water pump(kW),and W H PT,W I PT,and W L PT are the energy outputs(kW)from individual steam turbines, corresponding to the high-pressure turbine,intermediate-pressure turbine,and low-pressure turbine,respectively.2.5.Exergy analysisIn addition to the energy analysis,the exergy efficiency(EE)of the co-gasification system is also evaluated.For a steady state process,the overall exergy balance between the inlet and outlet streams can be written as(Kaska,2014;Prins et al.,2003;Zhang et al.,2012,2013):XinE x i¼XoutE x jð15ÞXinE x i¼E x fuelþE x gasifying agentsþE x heatð16ÞXoutE x j¼E x product gasþE x charþE x wþE x lossð17ÞwherePinE x i andPoutE x j are the exergy rates of the input and outputstreams,respectively.E x fuel is the input exergy rate of fuel(kW), E x gasifyingagents is the exergy rate of the gasifying agents(kW),E x heat is the exergy rate of heat required by the gasification system(kW), E x product gas is the output exergy rate of the product gas(kW),E x char is the exergy rate of char(kW),E x w is the exergy rate of work (kW),and E x loss is exergy loss rate from the system(kW).Therefore,the total exergy of the material streams includes physical exergy and chemical exergy,and these are defined in the following equations:E x total¼E x phþE x chð18Þwhere E x total represents the total exergy of the material streams (kW),E x ph is the physical exergy of the material streams(kW),and E x ch is the chemical exergy of the material streams(kW).Table2Operating parameters of co-gasification for power generation systems. Parameter ValueFeedstock CH1.68O0.752N0.008(raw wood)CH1.04O0.486N0.006(torrefied wood)CH0.683O0.070N0.022S0.002(Coal)Fuelflow rate(kg/h)100Blending ratio(wt%)20–80Gasifier Temperature(°C)900Pressure(atm)1 Burner Airflow rate(kg/h)1000 Sensitivity analysis Steam to carbon ratio(S/C)0.1–2CO2content in gasifyingagents0.01–2Heat recovery steam generator (HRSG)and Rankine cycle Feedwater(kg/s)0.279HP steam turbine inletpressure(atm)240HP steam turbine inlettemperature(°C)600Fsplitter(F1)0.7Steam massflow rate(kg/s)0.195(stream22)0.084(stream28)IP steam turbine inletpressure(atm)49IP steam turbine inlet tem-perature(°C)350Fsplitter(F2)0.9Steam massflow rate(kg/s)0.186(stream25)0.009(stream32)LP steam turbine inletpressure(atm)5Fsplitter(F3)0.9Steam massflow rate(kg/s)0.176(stream27)0.009(stream35)Condenser pressure(atm)0.1P.-C.Kuo,W.Wu/Chemical Engineering Science142(2016)201–214205In the foregoing equation,the E x ph for each species in the product gas can be described by E x ph ¼ðh Àh 0ÞÀT 0ðs Às 0Þð19Þwhere h and s are the speci fic enthalpy and entropy of the gasspecies at a given state,while h 0and s 0are the speci fic enthalpy and entropy of the gas species at the environment state T 0¼25°C and P 0¼1atm,respectively.The E x ch for each species in the gas mixture can be evaluated as follows:E x ch ¼X i n i Ex ch ;i þRT 0ln n iP n ið20Þwhere n i is the mole flow rate of species i in the product gas (kmol/s),Ex ch ;i is the standard chemical exergy of species i in the product gas,and R is the universal gas constant (kJ kmol À1K À1).The exergy of heat streams is de fined as follows (Kaska,2014):E x heat ¼X 1ÀT 0Q H ð21Þwhere T is the operating temperature for the system,and Q H is the heat requirement of the system (kW).For coal,the speci fic chemical exergy can be expressed by (Zhang et al.,2013)E x coal ¼Q L Â1:0438þ0:0013H =C þ0:1083O =C þ0:0549N =CÀÁþ6:710S O =C r 0:666ÀÁð22Þwhere Q L is the heating value of the coal (kJ/kg),and C ,H ,O ,N ,S are the mass fractions of carbon,hydrogen,oxygen,nitrogen,and sulfur,respectively.However,for biomass,the speci fic chemical exergy can be obtained by (Prins et al.,2003;Zhang et al.,2012)E x biomass ¼βF biomass LHV biomassð23Þβ¼1:044þ0:0160H =C À0:3493O =C ð1þ0:0531H =C Þþ0:0493N =C1À0:4124O =CðO =C r 2Þð24Þwhere LHV biomass is the heating value of the biomass (kJ/kg),and C ,H ,O ,N are the mass fractions of carbon,hydrogen,oxygen,and nitrogen,respectively.As a result,the exergy ef ficiency (EE)of the product gas in the co-gasi fication system is de fined by (Prins et al.,2003;Zhang et al.,2012)EE ð%Þ¼E x product gasE x fuel gasifying agents heatÂ100%ð25ÞFig.3.Distributions of (a)H 2flow rate,(b)CO flow rate,(c)CO 2flow rate in the product gas,and (d)char flow rate from steam gasi fication of three fuels.P.-C.Kuo,W.Wu /Chemical Engineering Science 142(2016)201–214206。