CLC of coal

Hindawi Publishing CorporationISRN Chemical EngineeringVolume 2013, Article ID 565471, 11 pageshttp://dx.doi.org/10.1155/2013/565471

Research ArticleGasication ouled hemical ooing omustion o oalA Thermodynamic Process Design Study

Sonali A. Borkhade, Preksha A. Shriwas, and Ganesh R. KaleChemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411008, India

Correspondence should be addressed to Ganesh R. Kale; [email protected]

Received 28 October 2012; Accepted 18 November 2012

Academic Editors: V. Baglio, N. Kakuta, M.-H. Li, and K. Okumura

Copyright 2013 Sonali A. Borkhade et al. is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

A thermodynamic investigation of gasication coupled chemical looping combustion (CLC) of carbon (coal) is presented in thispaper. Both steam and CO2 are used for gasication within the temperature range of 5001200C. Chemical equilibrium modelwas considered for the gasier and CLC fuel reactor. e trends in product compositions and energy requirements of the gasier,fuel reactor, and air reactor were determined. Coal (carbon) gasication using 1.5mol H2O and 1.5mol CO2 per mole carbon at 1bar pressure and 650C delivered maximum energy (390.157 kJ) from the process. Such detailed thermodynamic studies can beuseful to design chemical looping combustion processes using dierent fuels.

1. Introduction

Coal is the most abundantly available cheap fossil fuelworldwide and its reserves are estimated to outlast oil andnatural gas reserves [1]. Coal is mainly used for energygeneration: in coal red power plants to produce electricity[2], hydrogen [35], and syngas production for FT synthesisor fuel cells [6, 7]. Combustion of coal or coal derived syngasin air results in generation of product gas mixture containingCO2, N2, and NO [8]. e separation of CO2 from suchgaseous streams is extremely dicult and expensive. Hencethese product gases are directly vented to the atmospherewithout CO2 separation. is environmental pollution is amajor drawback of energy generation from coal. CO2 emis-sions from such processes are mainly responsible for globalwarming and climate change phenomenon [9].e 2010CO2emissions have increased to 389.0 ppm and burning of fossilfuels is one of the main causes as reported by the WorldMeteorological Organization [10]. Such tragic scenarios wereforeseen and therefore research in clean energy generationusing coal had already started globally. Chemical loopingcombustion (CLC) technology is a result of such researcheorts. CLC uses a solid oxygen carrier (OC) to oxidize thecarbon and hydrogen present in the fuel to CO2 and H2O,

respectively, in an endothermic fuel reactor. Oxides such asNiO, CuO, and Fe2O3 and sulphates such as CaSO4 havebeen widely used as oxygen carriers in chemical loopingprocesses. e reduced OC is regenerated by air oxidationin an exothermic air reactor [11, 12]. Both the reactorsare interconnected and operate simultaneously. e energyreleased in the CLC system is of similar magnitude asdirect combustion, but with a crucial advantage of havingCO2 and nitrogen streams completely separated [13, 14].e pure CO2 steam can be captured/sequestered easilythereby reducing CO2 emissions to the atmosphere [15].Coal mainly contains carbon which is a highly stable solidspecies and hence it requires high temperature and highenergy input for chemical reactions. Further, the coal-OCsystem is a solid-solid complicated reaction system that has itsintrinsic problems like slow reaction kinetics, low reactivityand conversions, need of gaseous medium, and so forth [16].ese problems in coal CLC system prompted researchers tosearch for better options. A popular option is coal gasicationto syngas (CO + H2) and CLC of the generated syngas. Someresearch studies have been published with this theme [1721]. Coal gasication can be done using steam or CO2 orboth [2226]. Coal gasication using steam (SG) is alreadya developed technology [27]. e generated syngas is more

2 ISRN Chemical Engineering

Air

GSGS

GS

NiO, Ni

CO2 + H2O

Ni,NiO,

III

NiO, Ni

N2 (+CO2)

III

CO,H2,H2O,CO2,CH4

CO2

Coal

CC

H2O

I: Gasification reactor,

II: Fuel reactor,

III: Air reactor,

GS: Gas solid separator.

F 1: rocess diagram for gasication coupled chemical looping combustion of coal.

reactive gaseous species than solid coal. is syngas has highreactivity with OC which can help commercialization of thisCLC pathway. CLC of syngas has been successfully studiedby researchers with many OCs [28, 29]. NiO is a very popularOC in chemical looping processes and it is used in thisstudy. Coal gasication using CO2 (dry gasication: G)results in CO rich gas, while SG provides syngas (H2+CO).CO2 is a relatively inert gas, not available in pure formfreely, and its reaction with coal is relatively slower enablingcomplete conversion only at high temperatures. On theother hand, water is cheaply available, but the conversionof water to steam requires huge energy due to high latentheat of water. Hence a right combination of CO2 and steammight be useful for coal gasication to produce syngas forits further use in CLC. Coal gasication using steam/CO2without air is a highly endothermic process. A comprehen-sive theoretical research study is needed to understand theoptimum conditions for the process development involvingcombined gasication coupled CLC of coal. Such a detailedstudy has not yet been published in literature. ermody-namic studies are key starting points for chemical processdesign [30, 31]. inetics of coal gasication rely on theinorganic metal contents of coal, but thermodynamic studiesthat determine the maximum possible conversions underparticular conditions of temperature, pressure, and feedcomponents are not aected by these components/catalysts.ermodynamic studies based on chemical equilibrium havebeen done for coal gasication systems [3236] and alsofor CLC of coal [3739]. e systematic thermodynamic

study of gasication coupled CLC is presented in this paperto understand the product distribution trends at dierent(gasifying agent) feed, temperature, and pressure conditionsto identify the best quantity of gasication agents leading toprocess temperatures and pressures for maximizing desiredproducts and minimize the undesired products at maximumextractable energy from the overall process. Such a theoreticalstudy is vital to understand the overall process aspects andreduce the time and eorts on experimentation studies whichcan further help fast track commercialization.

1.1. Process Description. Figure 1 shows the conceptual pro-cess design for gasication coupled CLC of coal. e processscheme consists of a gasier, CLC fuel reactor, and CLC airreactor. Initially, preheated coal, CO2, and H2O were fed tothe gasication reactor in calculated quantities to producesyngas includingminor amounts of CH4, CO2, and H2O.egasication products were assumed to be in thermodynamicequilibrium at the exit of the gasier. Complete conversionof coal and maximum syngas production are targeted inthe gasier. e products obtained from the gasier at hightemperature are directly fed to the fuel reactor with calculatedquantity of heated OC (NiO) recirculating from the air reac-tor. It is assumed that the NiO oxidizes CO, H2, and CH4 toCO2 andH2O in theCLC fuel reactor.emoles ofOC (NiO)for fuel reactor are varied depending on the input moles ofCO, H2, and CH4 from the gasier. Complete conversion ofsyngas and CH4 to CO2 and H2O is desired; however theconversion is limited by thermodynamic equilibrium. Coke

ISRN Chemical Engineering 3

T 1: Gasication conditions.

Feed Conditions Input moles of carbon Input moles of CO2 Input moles of H2OGasifying agent to carbon ratio(GaCR) ((H2O + CO2)/C)

A1 1 0 1 1A2 1 0.25 0.75 1A3 1 0.5 0.5 1A4 1 0.75 0.25 1A5 1 1 0 1B1 1 0 2 2B2 1 0.5 1.5 2B3 1 1 1 2B4 1 1.5 0.5 2B5 1 2 0 2C1 1 0 3 3C2 1 1 2 3C3 1 1.5 1.5 3C4 1 2 1 3C5 1 3 0 3

formation can also take place in the CLC fuel reactor. eOC (NiO) gets reduced (to Ni) in the CLC fuel reactor.e products of the CLC fuel reactor pass through a gas-solid separator and the gaseous products mainly containingCO2 and H2O may be partly recycled to the gasier (ifpure) and the rest can be cooled for CO2 separation andsequestration with heat recovery. e solid products of thefuel reactor mainly containing OC (and coke) are transferredto the air reactor, in which preheated air is added forcomplete oxidation of coke to CO2 and regeneration ofreduced OC. It is assumed that these oxidation reactions goto complete conversion. e regenerated NiO is separatedfrom the air reactor product stream by a gas-solid separatorand is recycled back to the fuel reactor. e air reactor is themajor source of heat in the entire process. It is assumed thatthe three reactors (gasier, fuel, and air reactor) operate atsame temperature and pressure. is study assumes energyfor chemical reactions (including heating/cooling) only andother energies such as OC transport energy, coal feederenergy, gas-solid separator energy, and heat losses are notconsidered in this basic theoretical study.

1.2. Process Methodology. HSC Chemistry soware version5.11 is used to generate the thermodynamic equilibriumdata for gasier and CLC fuel reactor in this process designstudy [40, 41]. ermodynamic equilibrium calculations inthe Gibbs routine of HSC Chemistry are done using theGibbs free energy minimization method.e Gibbs programnds the best combination of most stable species where theGibbs free energy of the system achieves its minimum ata xed mass balance (a constraint minimization problem),constant pressure, and temperature. Hence chemical reactionequations are not required in the input. e carbon content(in weight) in coal is variable, generally varying from 3585%depending on its origin (American, Australian, Chinese,etc.). Coal also contains minor amounts of hydrogen (5%),oxygen (7%), sulfur (


0

20

40

60

80

100

120

Carbon conversion (%)

Process feed conditions

C conversion (1 bar)



A1 A2 A3 A4 A5 B1 B2 B3 B4 C1 C2 C3B5 C4 C5

(a)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Syngas yield (moles)


Syngas moles (1 bar)



A1 A2 A3 A4 A5 B1 B2 B3 B4 C1 C2 C3B5 C4 C5

(b)

F 2: (a) Carbon conversion in gasier at 600C. (b) Syngas yield in gasier at 600C.

the air reactor is highly exothermic. It is assumed that the airsupplied to the air reactor is in stoichiometric amount givenby the reactions

Ni + 0.5O2 = NiO (1)

C +O2 = CO2 (2)

e equilibrium compositions of the coal gasier and CLCfuel reactor were used to calculate the energy involvedin those reactors. For example, the equilibrium productcomposition for case B2 in the gasier was [0.51CO2 (g),0.49H2O (g), 0.99CO (g), 1.01H2 (g)] at 800C. is productcomposition was used to formulate the following reaction:

C + 1.5H2O g + 0.5CO2 g = 0.51CO2 g + 0.49H2O g

+ 0.99CO g + 1.01H2 g(3)

is reaction was fed to the Reaction Equation moduleof HSC Chemistry soware and the reaction enthalpy of = 15.4 kJ at 800C was obtained. Similar strategy wasfollowed for CLC fuel reactor material and energy balances.e calculations for the air reactor were done using reactionsequations only as complete conversion (no equilibrium) wasassumed. e general calculations required to determineenergy requirements of heating and cooling of chemicalspecies were done manually using standard data [42].

2. Results and Discussions2.1. Eect of Pressure. Pressure is an important processparameter.e eect of pressure on gasication coupled CLCof coal was investigated for 1, 5, and 10 bars. Initially, theeect of pressure on carbon conversion and syngas yield in

the gasier was studied. It was observed that the theoreticalcarbon conversion reached amaximum value (approximately100%) at higher temperatures (>600C). Hence the eect ofpressure was studied at 600C only. Figure 2(a) shows thecarbon conversion at 600C for the 15 feed input condi-tions (A1C5). It was observed that the carbon conversiongenerally decreased with increase in pressure for all cases.It was also observed that the carbon conversion in CG atconstant pressure generally decreased with increase in feedCO2 moles at constant GaCR for all pressures. It was alsoseen that the carbon conversion increased as the feed steamto carbon ratio (SCR) increased from 1 to 3 for all pressuresfor SG cases (A1, B1) and sometimes saturated at 100%(B1, C1). Similar observation was also noted for DG cases(A5, B5, C5) with increase in the feed CO2 to carbon ratio(CCR) from 1 to 3. Figure 2(b) shows the syngas yield forthe dierent feed conditions at 600C. It was observed thatthe syngas yield decreased with increase in pressure for allcases. It was also observed that the syngas yield in CG atconstant pressure decreased with increase in feed CO2 molesat constant GaCR for all pressures. It was also seen that thesyngas yield increased as the respective feed GaCR increasedfrom 1 to 3 for all pressures for SG cases (A1, B1, C1) and forDG cases (A5, B5, C5).

Considering the negative eect of pressure on carbonconversion and syngas yield in the gasier, it was notedthat the maximum carbon conversion with maximum syngasyield can be achieved at low pressure (1 bar). Hence 1bar pressure was selected for further analysis of gasicationcoupled CLC process. e carbon conversion in the gasierfor the dierent feed conditions at 1 bar pressure is shownin Table 2. It was observed that the carbon conversionreached its maximum (100%) as the gasication temperatureincreased from 5001200C. It was generally observed thathigher GaCR required relatively lower temperatures for 100%


T 2: Carbon conversion in gasier.

Temperature (C) 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200Feed Conditions Carbon conversion %A1 46 48 54 62 73 84 92 96 98 99 100 100 100 100 100A2 29 33 40 51 65 79 88 94 97 98 99 100 100 100 100A3 16 21 29 41 57 73 85 93 96 98 99 99 100 100 100A4 8 13 21 34 51 69 83 91 96 98 99 99 100 100 100A5 3 7 15 28 46 65 81 90 95 97 99 99 100 100 100B1 91 97 100 100 100 100 100 100 100 100 100 100 100 100 100B2 57 66 80 100 100 100 100 100 100 100 100 100 100 100 100B3 32 42 58 83 100 100 100 100 100 100 100 100 100 100 100B4 16 25 42 67 100 100 100 100 100 100 100 100 100 100 100B5 7 15 30 55 91 100 100 100 100 100 100 100 100 100 100C1 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100C2 72 86 100 100 100 100 100 100 100 100 100 100 100 100 100C3 49 63 88 100 100 100 100 100 100 100 100 100 100 100 100C4 31 45 70 100 100 100 100 100 100 100 100 100 100 100 100C5 10 22 45 83 100 100 100 100 100 100 100 100 100 100 100

carbon conversion and the 100% carbon conversion in SGoccurred at relatively lower temperatures than analogousDG cases. e minimum temperatures for 100% carbonconversion (HCCT: Hundred percent Carbon ConversionTemperature) were important and sucient for this processdesign. Hence these HCCTs (Bold in Table 2) were selectedfor further process analysis for the respective feed conditions.us the pressure and temperature for the process reactorswere xed for further process design. e productenergyanalysis of the individual reactors depended solely on the feedcomposition conditions and is discussed in the next sections.

2.2. ts the ase. e product composition of thegasier obtained for the various (A1 to C5) feed conditionsat 1 bar pressure and respective HCCTs was analyzed andplotted in Figure 3.

2.2.1. Syngas. Syngas is the most desired product of the coalgasier. As seen in Figure 3, the syngas yield in CG generallyincreased with increase in feed CO2 moles at constant GaCRand reached a maximum for almost all cases. e maximumsyngas yield was found to be 2.00mol (cases B5 and C5),while the minimum yield was observed to be 0.95mol (caseC1). It was observed that the syngas yield for the SG cases(A1, B1, C1) decreased with increase in SCR; but the syngasyield was almost constant (approximately 2.00mol) for DGcases (A5, B5, C5), while the syngas yield slightly decreasedfor equimolar input moles of H2O and CO2 with increase inGaCR from 1 to 3 (cases A3, B3, C3). Similarly, the trendsof the individual H2 and CO gases were also analysed andpresented in Figure 3.

2.2.2. Methane. Methane is not a desirable product of gasi-er, but is inevitably formed in the gasication process(where steam is in the input). As depicted from Figure 3,the methane yield in CG generally decreased with increase

0

0.5

1

1.5

2

2.5

Products of gasifier (moles)


CO

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 C3 C4 C5

H2

CH4Syngas (CO + H2)

F 3: roducts of gasier at dierent feed conditions.

in feed CO2 moles at constant GaCR (except for A1A5conditions where it was almost zero). e methane yieldslightly increased for equimolar inputmoles of H2O and CO2with increase in GaCR from 1 to 3 (cases A3, B3, and C3). Asa general observation, the methane yield was insignicant inthe gasication product gas at the conditions (1 bar processpressure) chosen in the process design study.

2.. negy nayss ase. Coal Gasier has two maincontinuous energy demands for consideration: preheatingenergy (energy to preheat the gasication reactor rawmateri-als: coal, water, andCO2 to the gasier temperature) and gasi-er enthalpy (energy required for endothermic gasicationreactions). e trends in the gasication reactor enthalpy,gasier feed preheating requirements, and net energy of thegasier were studied at 1 bar pressure and respective HCCTsfor the dierent feed conditions (A1 to C5) based on the


20

70

120

170

220

270

Energy an

alysis of gasifier (kJ)


Total gasifier energy

Gasification enthalpyFeed preheating energy


F 4: Energies in gasication process.

equilibrium compositions obtained in the earlier section andare shown in Figure 4.

2.3.. aication Enthaly. Carbon gasication in absenceof air is an endothermic process. It was observed fromFigure 4 that the gasication reaction endothermicity generally increased with increase in feed CO2 moles at constantGaCR for all CG cases. e maximum gasier enthalpywas found to be 170.86 kJ (case C5), while the minimumreaction enthalpy was observed to be 52.36 kJ (case C1). egasication enthalpy for the SG cases (A1, B1, C1) decreasedwith increase in SCR, while the reaction enthalpy for DGcases (A5, B5, C5) slightly increased with increase in CCR.It was also observed that the gasication enthalpy decreasedfor equimolar input of H2O and CO2 with increase in GaCRfrom 1 to 3 (cases A3, B3, and C3).

2.3.2. Preheating Energy. As seen in Figure 4, the preheatingenergy generally decreased with increase in feed CO2 molesat constant GaCR for all CG cases. e maximum preheatingenergy was found to be 190.08 kJ for case C1 (highest steaminput), while the minimum preheating energy was observedto be 75.22 kJ for case A5. e preheating energy for the SG(cases A1, B1, and C1) increased with increase in SCR andsimilarly the preheating energy forDG (cases A5, B5, andC5)also increased with increase in CCR.is was due to the hugedierence ofHCCTof feed conditions.epreheating energyrequirement increased for equimolar input of H2O and CO2with increase in GaCR from 1 to 3 (cases A3, B3, and C3),that is, it increased from 90.90 to 154.65 kJ (from A3 to C3).

2.3.3. otal aication Energy. e total energy required forgasication is the sum of gasication enthalpy and preheatingenergy. It was seen that SG requires higher preheating energy,while DG requires higher gasication enthalpy. As seenin Figure 4, the total gasication energy for CG generallyincreased up to A3, B3, and C3 with increase in feed CO2moles (with simultaneous decrease in feed H2O moles) at

constant GaCR and aerwards followed their individualtrends. e maximum total energy requirement for gasication was found to be 286.50 kJ (case C3), while the minimumtotal gasication energy was observed to be 219.99 kJ (caseB1). e total energy for the SG (A1, B1, C1) rst decreased(due to huge dierence of HCCTs) and then increased withincrease in SCR, while the total energy for DG cases (A5, B5,C5) only increased with increase in CCR.e total energy forgasication increased for equimolar input of H2O and CO2with increase in GaCR from 1 to 3 (cases A3, B3, and C3).

2.4. OC Requirement of CLC Fuel Reactor. e gasierproduct gas contains H2, CO, and CH4 which are reactivespecies for CLC fuel reactor.e OC requirement in the CLCfuel reactor depends on the input quantities of these gaseswhich in turn depend on the feed input variations to thegasier.e product distribution trend of the gasier productgases has already been discussed in the earlier sections. estoichiometric requirement ofOC (S) for dierent feed inputswas calculated according to (4), (5), and (6) reactions:

H2 + NiO = H2O +Ni (4)

CO + NiO = CO2 +Ni (5)

CH4 + 4NiO = CO2 + 2H2O + 4Ni (6)

Although these main reactions occur, some side reactionsalso take place and hence the conversions in the fuel reactorare limited by thermodynamic equilibrium constraints. Itwas therefore necessary to study the equilibrium productcomposition of the CLC fuel reactor using stoichiometricamount of OC (Case S) at 1 bar pressure and respectiveHCCTs. However the OC is generally used in excess of thestoichiometric requirement in CLC processes to enhancethe syngas and CH4 conversion in the fuel reactor. Hencetwo more cases (1.5S and 2S) using higher amounts (1.5times and 2.0 times the stoichiometric requirement) of OCwere also investigated. Figure 5 shows the syngas and CH4compositions in product gases of the CLC fuel reactor fordierent inputs of OC at various process feed inputs at 1 barpressure and respective HCCTs. It was observed that the H2,CO, and CH4 emissions from the fuel reactor decreased withincrease in the amount of OC. It was also observed that the(CH4 + CO + H2) moles generally increased with increasein feed CO2 moles at constant GaCR except for some caseslike B4 and C4. e maximum amount of (CH4 + CO +H2) exit moles were found to be 0.226mol (A5), while theminimum quantity of (CH4 + CO + H2) exit moles wereobserved to be 0.127mol (B1). Higher reactor temperatures(A1A5) produced relatively higher (CH4 + CO + H2) molesfor stoichiometric OC usage. e decrease in (CH4 + CO +H2) emissions from the CLC fuel reactor was very signicantbetween S and 1.5S conditions, while these emissions wereof almost similar magnitude for the use of 1.5S and 2S OCinputs. e complete conversion of syngas and methane toCO2 and H2O is highly benecial in the CLC fuel reactor.Hence the amount of OC to be used in the process was xedto 1.5S (1.5 times the stoichiometric OC requirement) forfurther process calculations.



A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 C3 C4

(CH

4+CO

+H

2) moles

S-NiO

1.5S-NiO

2S-NiO

0.05

0.1

0.15

0.2

0.25

0

F 5: (CH4 + CO + H2) moles emitted from the fuel reactor.

300

250

200

150

100

0

50

Energy analysis of fuel reactor (kJ)

Net fuel reactor energyFuel reactor enthalpyEnergy from cooling products



50

F 6: Energies of the CLC fuel reactor.

2.5. Energy Analysis of Fuel Reactor. At a steady state processoperation, the fuel reactor receives hot OC from air reactorand hot syngas rich stream from the gasier. Due to equalreactor temperature assumption, it does not have any feedpreheating requirements.eonly energy calculations are thefuel reactor enthalpy and energy recoverable from productgases. ese fuel reactor energies and the resulting net fuelreactor energies at 1 bar pressure and respective HCCTs werecalculated for the dierent feed conditions (A1 to C5) usingthe equilibrium compositions obtained in the earlier sectionsand are plotted in Figure 6.

2.5.1. Fuel Reactor Enthalpy. e fuel reactor enthalpydepends mainly on the syngas and methane content of thegasier product gas. As seen in Figure 6, the exothermicityof the fuel reactor generally increased with increase in feedCO2 moles at constant GaCR for all cases. e maximum

exothermicity was found to be 93.195 kJ (case B5), whilethe minimum reactor enthalpy was observed to be 27.44 kJ(case C1). e exothermicity for the SG cases (A1, B1, C1)decreased with increase in SCR, while in DG cases (A5, B5,C5), the fuel reactor exothermicity was almost constant asthe CCR was increased from 1 to 3. It was seen that, the fuelreactor exothermicity decreased for equimolar input of H2Oand CO2 with increase in GaCR from 1 to 3 (cases A3, B3,and C3).

2.5.2. Extractable Energy by Cooling Fuel Reactor Products.e main gaseous products of the fuel reactor are CO2 andH2O at high fuel reactor temperature (except cases A5, B5,and C5 where only CO2 is emitted by the fuel reactor whichcan be directly recycled to the gasier aer purging andmakeup). For other cases, it was found that minor amountof H2, CO, and CH4 are also present in the gaseous streamsand the CO2/H2O ratio of the fuel reactor product gas wasnot near to the gasier CO2/H2O ratio requirement. All the15 streams were considered for heat recovery by cooling to25C. It was observed that higher (H2 + CH4) gasier streamproduced higher steam in fuel reactor which on coolinggave higher extractable energy. As depicted in Figure 6, theextractable energy by cooling products generally decreasedwith increase in feed CO2 moles at constant GaCR for allCG cases. e maximum extractable energy by cooling theproducts was found to be 203.83 kJ (case C1, max. steaminput to process), while the minimum extractable energywas seen for case B5 (104.55 kJzero steam input). It wasalso observed that the extractable energy for the SG cases(A1, B1, C1) increased with increase in SCR (higher steaminput gave higher energy on cooling), while in DG cases,that is, A5, B5 and C5, the extractable energy from coolingproduct gases rst slightly decreased from 108.027 kJ to104.55 kJ (A5 and B5) and then increased to 128.708 kJ(C5) with gradual increase in CCR. It was also observed thatthe extractable energy increased for cases of equimolar inputof H2O and CO2 with increase in GaCR from 1 to 3 (casesA3, B3, and C3).

2.5.3. Net Energy of Fuel Reactor. It was observed fromFigure6 that the net energy of the fuel reactor was in the exothermicregion for all cases (due to syngas feed). e net energyexothermicity of the fuel reactor in CG cases increased withincrease inCO2 moles till A3, B3, andC3 for respectiveGaCRand then followed individual trends as shown in the gure. Itwas also observed that the range of net energy increased withincrease in GaCR from 1 to 3. e maximum exothermicitywas found to be 228.06 kJ (case C3), while the minimumexothermicity was observed to be 158.34 kJ (case B1).

2.6. Material Balance for Air Reactor. e air reactor contin-uously receives hot depleted OC and carbon from the fuelreactor unconverted carbon from the gasier and preheatedair to oxidize them to NiO and CO2 completely. In this study,the unconverted carbon from the gasier and carbon formedin the fuel reactor were negligible (due to choice of processconditions). e air supply was in exact stoichiometric


0.10.4

0.9

1.4

1.9

2.4

2.9

3.4

3.9

4.4

4.9

5.4

5.9

Material balan

ce of air reactor (m

oles)



O2-N2 output

I1-air input

I2-input moles Ni

I3-NiO input

O1-NiO output

F 7: Input and output species of air reactor.

500

400

300

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100

0

100

200

Energy an

alysis of air reactor (kJ)

Preheating energy of air

Air reactor enthalpy

Energy from cooling N2Air reactor net energy



F 8: Energies of CLC air reactor.

requirement for the Ni oxidation for its complete conversionto NiO in this study. Figure 7 shows the inputs and outputsof the air reactor at dierent feed conditions at 1 bar pressureand HCCTs. e input streams to the air reactor are I1-air,I2-Ni, and I3-NiO, while the output streams of the air reactorare O1-NiO andO2-nitrogen. It was observed that there wereonly slight variations in each stream for the 15 feed conditionsconsidered in this study and hence detailed analysis was notdone for this part of study.

2.7. Energy Analysis of Air Reactor. e material balances ofthe air reactor (discussed in the earlier section) were usedto calculate the energy of the air reactor streams. Figure 8shows the trends in air preheating energy requirement, airreactor enthalpy, energy extracted by cooling of nitrogen,and resulting net energy of air reactor at 1 bar pressure andHCCT for the dierent feed conditions (A1 to C5). It wasseen that the air requirements for the individual 15 cases were

almost similar and hence the HCCTs dominated the energycalculations.

2.7.1. Preheating Energy Requirement. e air reactor rec-eives preheated air for OC regeneration. As seen in Figure8, the preheating energy generally increased with increasein feed CO2 moles at constant GaCR for all CG cases. Itwas also observed that the air preheating energy requirementgenerally decreased with increase in combined GaCR dueto relatively lower HCCTs. e preheating energy of air forthe SG cases (A1, B1, C1) decreased with increase in SCRand similar observation was noted for DG cases (A5, B5,C5). e preheating energy of air decreased for equimolarinput of H2O and CO2 with increase in GaCR from 1to 3 (cases A3, B3, and C3). e maximum preheatingenergy of air was found to be 155.41 kJ (A3) at 1100C andminimum preheating energy requirement of air was found tobe 67.016 kJ (C1) at 500C.

2.7.2. Air Reactor Enthalpy. Energy generation in the airreactor is the most important aspect for the process as theair reactor supplies energy for the entire process. It wasobserved from Figure 8 that the exothermicity of the airreactor slightly varied for all subcases of constantGaCR and itslightly increased with increase in GaCR from 1 to 2 but wasalmost constant for GaCR 2 and 3 due to nearby individualcase HCCTs. e exothermicity of air reactor increased forequimolar input of H2O and CO2 with increase in GaCRfrom 1 to 3 (cases A3, B3, and C3), that is, it increased from453.56 kJ (A3) to 466.69 kJ (B3) till 468.25 kJ (C3). emaximum exothermicity of the air reactor was found to be468.88 kJ (C1) at 500C, while the minimum exothermicitywas observed for case A1 (449.72 kJ) at 1100C.

2.7.3. Energy Extracted by Cooling Nitrogen. Oxygen from airis used up in the air reactor for regeneration of depleted OCand carbon oxidation.e le-over hot nitrogen gas evolvingout of the air reactor can be cooled to 25C and energy canbe extracted for use in the process. It was seen in Figure 8that this extracted energy generally increased with increasein feed CO2 moles at constant GaCR except for case A3, A4,and A5 where it remained almost constant. It was also seenthat the exothermicity for SG cases (A1, B1, C1) and also forDG cases (A5, B5, C5) decreased with increase in respectiveGaCR due to relatively lower HCCTs. It was also observedthat the exothermicity decreased for equimolar input of H2OandCO2 with increase inGaCR from1 to 3 (cases A3, B3, andC3). e maximum extractable energy by cooling nitrogenfrom the air reactor was found to be 120.39 kJ (A3 and A41100C) while minimum was found to be 52.075 kJ (C1500C).

2.7.4. Net Energy of Air Reactor. e net energy of air reactorwas found by summing the energies of air preheating, airreactor enthalpy, and nitrogen stream cooling energy atrespective conditions and is shown in Figure 8. It wasobserved that the trend of net energy of air reactorwas similarto that of air reactor enthalpy already discussed in the earlier


T 3: Comparison of net energy obtainable from direct combustion and gasication coupled CLC of coal.

Case C3 C1 C5 B5 B3 B2 C2 B1 C4 B4HCCT (C) 650 500 700 750 700 650 600 600 650 700Air required (moles) 3.76 3.76 3.76 3.76 3.76 3.76 3.76 3.76 3.76 3.76Preheating of coal (kJ) 10.06 7.09 11.11 12.19 11.11 10.06 9.04 9.04 10.06 11.11Energy to heat air (kJ) 90.20 67.69 97.80 105.45 97.80 90.20 82.65 82.65 90.25 97.80Reaction enthalpy from directcombustion (kJ) 394.44 394.11 394.54 394.65 394.54 394.44 394.33 394.33 394.44 394.54

Energy to cool N2 (kJ) 69.95 52.60 75.81 81.71 75.81 69.95 64.13 64.13 69.95 75.81Energy to cool CO2 (kJ) 29.56 21.78 32.22 34.91 32.22 29.56 26.93 26.93 29.56 32.22Net energy from directcombustion (kJ) 393.68 393.71 393.66 393.63 393.66 393.68 393.69 393.69 393.68 393.66

Net energy from gasicationcoupled CLC process (kJ) 390.16 389.07 388.46 388.43 388.35 388.34 388.24 388.17 388.16 388.02

Energy Dierence(kJ) 3.52 4.64 5.20 5.20 5.31 5.34 5.46 5.53 5.52 5.64

500

400

300

200

100

0

100

200

300

Energy an

alysis of process (kJ)

Gasifier total energy Air reactor total energy

Net process energy



Fuel reactor total energy

F 9: Trend of net process energy.

section.emaximumexothermicity of air reactorwas foundto be 455.128 kJ (case C1) at 500C, while the minimumwas observed to be 417.22 kJ (case A5). It was seen thatthe exothermicity of air reactor for DG cases (A5, B5, C5)increased with increase in CCR while the exothermicity ofair reactor increased with increase in GaCR from 1 to 3 (casesA3, B3, and C3) for equimolar input of H2O and CO2.

2.8. Net Process Energy. Net energy of the process is the sumof individual energies of process components (gasier, fuel,and air reactor). e net energy obtainable from the processat 1 bar pressure and respective HCCTs were calculated forthe dierent feed conditions (A1 to C5) and are shown inFigure 9. e maximum exothermicity for the whole processwas found to be 390.157 kJ for case C3 (650C), while theminimum exothermicity was found to be 372.82 kJ for caseA1 at 1000C. e net energy obtainable from the processwas always low for GaCR = 1 cases (A1A5) due to relativehigh HCCTs. e preferred conditions for process operationranged as follows: C3 (390.157 kJ), C1 (389.07 kJ),

C5 (388.457 kJ), B5 (388.43 kJ), B3 (388.349 kJ), B2(388.341 kJ), C2 (388.24 kJ), B1 (388.165 kJ), C4(388.16 kJ), and B4 (388.02 kJ). e dierence in energieslooks small as the calculations are based on 1mol coal feedbut these dierences will become huge for a pilot plant oper-ation. A detailed comparison of process energy calculationsof gasication coupled CLC and direct air oxidation of coalwas also done and is presented in Table 3. e methodologyfor the direct oxidation calculations was the same, that is,preheating carbon and air to reaction temperature (HCCT),combustion enthalpy, energy obtainable by cooling productCO2 and N2, and net process energy. It was observed thatthe energy obtainable in direct coal combustion was slightlyhigher (36 kJ) than the corresponding cases of gasicationcoupled CLC.us themagnitude of net energy obtainable ingasication coupled CLC process and direct coal combustionprocess was found to be of similar nature. But the gasicationcoupled CLC process delivered an almost pure CO2 streamfor sequestration making it clean energy process.

3. Conclusionsis theoretical systematic process study was done to under-stand the material and energy balances of the new processdesign of gasication coupled CLC of coal. e comprehen-sive study considered the eect of temperature, pressure andcombined gasication using steam and CO2 on the process insteps. It was concluded that coal (carbon) gasication using1.5mol H2O and 1.5mol CO2 per mole carbon at 1 barpressure and 650Cdeliveredmaximum energy from the pro-cess. e other process conditions yielding near maximumenergies are also identied and can be used as necessary.e results obtained in this detailed study can be used forscale up of the process. Experimental evaluation will behelpful to further enhance the technology commercializationprospect. se of gasication step in this study can also helpdesign similar processes using other fuels such as natural gas,which can be converted to syngas (via steam/dry reforming)and then used in CLC processes. e CLC processes mayultimately become limited to syngas-CLC, which will help


the OC development and reactor design aspects and ensurefast track commercialization. is process scheme has anexothermic syngas fuel reactor which may help solve manyheat transfer problems in the CLC systems. Further studiesusing this process design and actual conditions like coalcompositions, incomplete coal conversion in the gasier,process heat losses, dierent reactor operating temperaturesand pressures, operating energies of intermediate processequipments, for example, gas solid separators, and so forthcan also be evaluated to generate data to help design practicalcoal CLC systems.

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