Low-carbon power generation cycles: the feasibility of CO capture

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Journal of Power Technologies 91 (1) (2011) 6–13journal homepage:papers.itc.pw.edu.pl

Low-carbon power generation cycles: the feasibility of CO2 capture andopportunities for integration

Wojciech M. Budzianowski∗

Wrocław University of Technology,Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland

Abstract

Low-carbon power generation is receiving increasing interest due to climate warming concerns. The presentarticle analyzes three low-carbon power cycles. The focus is on the feasibility of CO2 capture and opportuni-ties for energy and mass integration. The first power cycle is a zero-carbon solid biomass fuelled multi-stepgasification gas turbine power cycle involving multi-step solid biomass conversion, which is a more reversibleprocess than one-step biomass combustion. The second zero-carbon coal-fired oxy-gasification steam chem-ical looping combustion gas turbine cycle benefits from: (i) improved cycle efficiency due to the increasedreversibility of the chemical looping combustion process, (ii) cycle mass and energy integration due to the sev-eral recirculation loops involved, and (iii) extremely high CO2 capture rate due to the purity of the CO2/H2Omixture achieved at the outlet of a syngas reactor. The last power cycle - a biogas fuelled oxy-reforming fuelcell cycle - is superior in terms of the feasibility of CO2 capture, i.e. CO2 is captured from CO2-enrichedstreams, and due to the utilization of renewable biogas, negative net CO2 atmospheric emissions are achieved.It is concluded that high CO2 capture rates are feasible from pressurized CO2-enriched streams compris-ing either water or hydrogen, thus necessitating oxy-fuel power cycles. Opportunities for mass and energyintegration are found to be greater in systems involving closed mass and energy recirculation loops. Thediscussions also emphasize that low-carbon power cycles could achieve minimized exergy losses by applyingmore reversible energy conversion processes.

Keywords: Low-Carbon Economy, power generation, CO2 capture, integration, reversibility.

1. Introduction

A Low-Carbon Economy (LCE) is an economywhich has a minimal output of greenhouse gasesinto the biosphere. The means for achieving LCEare: renewable energy, nuclear energy, energy effi-ciency and carbon capture and sequestration (CCS).

∗Corresponding authorEmail address:

[email protected] (Wojciech M.Budzianowski )

The end product of most power generation technolo-gies involving carbonaceous fuels is CO2, which isreleased to the atmosphere. This power generation-derived anthropogenic CO2 accumulates in the atmo-sphere in a process which is practically irreversiblein the modern era since photosynthesis-based naturalprocesses are very slow and due to industrializationand deforestation plants’ potential for atmosphericCO2 binding is now substantially lower than in pre-vious centuries. Consequently, anthropogenic CO2

emissions affect the equilibria of natural carbon cy-

Journal of Power Technologies 91 (1) (2011) 6–13

cles and CO2 concentration in the atmosphere is in-creasing at a rate of around 2 ppm year-1 currentlyreaching around 390 ppm. At this rate the Copen-hagen Accord target of 450 ppm CO2/+2 °C will bereached before 2040. This clear need to cut green-house gas emissions to the atmosphere is driving re-search and investment in low-carbon power genera-tion technologies. The CO2 generation intensity ofrenewable and nuclear electricities ranges from 10to 50 kg CO2 MWh−1. In contrast, fossil fuel-firedpower generators are characterized by substantiallylarger CO2 emission of around 400 kg CO2 MWh−1

for natural gas, 850 kg CO2 MWh−1 for oil and 900kg CO2 MWh−1 for coal. Thus, the decarboniza-tion of fossil fuel-based power generation necessi-tates CO2 capture and sequestration (CCS). CO2 cap-ture can be realized by using a variety of processessuch as reactive absorption or membranes. CO2 se-questration can be achieved through, for example,geological storage or through some much more in-teresting options such as CO2 utilization in chemicalindustries, fuel synthesis, algae cultivation, enhancedgas and oil recovery or enhanced methane coal-bedrecovery. Sustainable and renewable energy sourcessuch as hydro, wind, solar and geothermal can offerlow-carbon power generation, but they have limitedenergy capacities and produce relatively costly elec-tricity. Fossil fuels, on the other hand, are availablein large quantities and are used to produce relativelycheap electricity. The main problem, however, arethe atmospheric CO2 emissions caused by the com-bustion of carbonaceous fossil fuels. It is thus desir-able to use the primary energy of fossil fuels, suchas coal or hydrocarbons, but with limited CO2 emis-sions. Alternatively, power can be produced fromrenewable biomass-derived fuels. However, in thiscase additional technical measures are needed to de-crease the costs of electricity from biomass-basedpower cycles. Those two important topics are thusaddressed in this paper. The main idea behind pre-combustion CO2 capture techniques [1] in relationto carbonaceous fuels is to oxidize carbon to CO2

while transferring all of the fuel’s chemical energyinto H2, which can be further oxidized without anyatmospheric release of CO2. This article analyzes thefeasibility of CO2 capture and opportunities for massand energy integration of low-carbon power genera-

tion cycles. In the first power cycle CO2 is not sepa-rated due to the use of biomass fuel and the focus ison energy integration. The second and the third cy-cles involve pre-combustion CO2 capture and discus-sions relate to the feasibility of cycle decarbonizationand to opportunities for mass and energy integration.

2. Low-carbon power cycles

The investigated power cycles utilize various fu-els such as solid biomass, coal and biogas and avariety of power technologies such as gas turbines,steam turbines and fuel cells. Moreover, novel pro-cesses suitable for low-carbon power generation areincluded, i.e. torrefaction, oxy-gasification, chemi-cal looping combustion and oxy-reforming.

2.1. Solid biomass fuelled multi-step gasificationgas turbine (MSG-GT)

Biomass is an abundant, untapped and environ-mentally friendly renewable energy carrier. Sus-tainable solid biomass resources available for en-ergy use include: (i) agricultural residues (straw,grasses), (ii) forest residues (from e.g. sawmills),(iii) biomass production on surplus degraded land,(iv) organic wastes and (v) energy crops. Cur-rently, most large-scale biomass-based power tech-nologies are direct combustion (via e.g. CFB boil-ers) and co-combustion with coal. However, directbiomass combustion suffers from low energy conver-sion efficiencies (around 20 %) due to high moisturecontent and thus low flame temperatures achieved.This problem is overcome in co-combustion butthis means biomass must be transported to central-ized coal-fired power plants over long distances,which substantially increases overall costs. There-fore, novel, more efficient biomass-based power cy-cles are needed.

2.1.1. Promising technologies for solid biomass fu-elled power plants

Solid biomass can be converted to gaseous fu-els by heating in a gasification agent such as oxy-gen/steam, air or CO2. In a gasifier solid biomassis pyrolyzed to volatile tars and to char. The char isthen simultaneously oxidized and reformed to pro-duce syngas. Biomass gasification is well suited for

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biomass IGCC (BIGCC) with combustion of syn-gas in a gas turbine (as a topping cycle) while hotgases from the gas turbine are used for steam gen-eration in a heat recovery steam generator (HRSG)to run a steam turbine (a bottoming cycle). Tor-refaction is a mild pyrolysis process that improvesthe fuel properties of solid biomass. Biomass tor-refaction is realized at temperatures below 300 °C inthe absence of oxygen. The main products formedare torrefied biomass and volatilities - torrefactiongas. The torrefied solid biomass is characterized bylow moisture content, high energy density and smallvolume. In addition to improvements in the easeof grinding/pelletization, long-term storage poten-tial and reduced long-distance transportation costsare achieved [2]. The torrefied biomass is suitablefor direct combustion, co-combustion and gasifica-tion. Pre-treatment of biomass via torrefaction priorto gasification enables the use of a pressurized en-trained flow gasifier and thus higher gasification tem-peratures and higher gasification thermodynamic ef-ficiency can be achieved.

2.1.2. Thermodynamic considerations for efficientdesign of solid biomass fuelled power plants

Practical design principles for efficient energyconversion in solid biomass fuelled power plants canbe derived directly from the First and Second Law ofThermodynamics. Namely, the First Law of Ther-modynamics (the conservation of energy) necessi-tates that a power system under design should mini-mize energy losses to the environment. This can beachieved by thermal integration measures such as (i)minimizing radiation losses (via e.g. heat recoveryor insulation techniques) and (ii) minimizing lossesin output streams (via e.g. heat recirculation [3], fluegas recirculation [4] or fuel pre-drying. More in-teresting power plant design insights arise from theSecond Law of Thermodynamics (the quality of en-ergy). Namely, in order to minimize exergy lossesand thus entropy generation rates a solid biomasspower plant should involve biomass conversion op-erations characterized by decreased irreversibility. Incombustion, most exergy destruction occurs duringthermal energy exchange between hot reaction prod-uct molecules and cold reactant molecules [5]. Thefuel oxidation itself is relatively efficient. This ef-

Figure 1: The flowsheet of the solid biomass fuelled powerplant utilizing multi-step gasification gas turbine (MSG-GT)power cycle

fect arises from the fact that in combustion processesfresh cold substrates are in direct contact with hotproducts, which is very inefficient, since the maximaltemperature is reduced and thus less work can be de-livered. Further, if an energy conversion process cantake place with sufficient degrees of freedom, the op-timal process follows a trajectory of optimal thermo-dynamic states, a so-called highway in state space,which is characterized by constant local entropy pro-duction [6]. This power plant design principle knownas the equipartition of entropy production is satisfiedin many biological systems such as human lungs (see[6] in relation to fuel cell design), and thus it can in-spire highly efficient design of power plants.

2.1.3. Multi-step gasification gas turbine (MSG-GT)power cycle

In light of the aforementioned first- and second-law thermodynamic considerations one-step solidbiomass combustion seems to be less efficient so-lution than multi-step solid biomass gasification.Namely, the multi-step solid biomass gasificationtechnology can assure more reversible gradual fueloxidation in multiple sub-units thus limiting thermalenergy exchange between hot reaction products andcold reactants as well as enabling advanced energyintegration.

Figure 2 presents a flowsheet of a power plant uti-lizing multi-step solid biomass gasification technol-ogy. Accordingly, the proposed multiple biomass

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conversion steps involve biomass pre-treatment pro-cesses such as drying and torrefaction followed bygasification and final syngas combustion in a com-bined cycle gas turbine. Gasification itself can be re-alized in a high-temperature air-blown entrained flowgasifier involving chemical quench via the injectionof torrefaction gases, thus offering improved charand tar conversion even at low oxygen consumptionas well as improved ash to slag fusion. Moreover, theproposed flowsheet is in accordance with the princi-ple of equipartition of entropy production, i.e. en-tropy is produced more uniformly in all sub-units ofthe proposed power plant. The biomass conversionroute according to the flowsheet from Figure 2 canbe more energy efficient, cleaner and optionally itcan enable to manufacture a new valuable renewableproduct - the torrefied biomass.

2.2. Coal fuelled oxy-gasification steam chemicallooping combustion gas turbine (OG-SCLC-GT)

Chemical looping combustion can potentially of-fer increased energy conversion efficiency. The gainin efficiency is possible due to the enhanced re-versibility of the two redox reactions. In conven-tional direct combustion, the release of a fuel’s chem-ical energy occurs in a highly irreversible manner- departing considerably from equilibrium [5]. Incontrast, in CLC, both redox reactions can occur al-most reversibly and at relatively low temperatures.This can allow a power plant using CLC to approachthe ideal work output for an internal combustion en-gine without exposing its components to excessiveoperating temperatures. Moreover, CLC facilitatesCO2 capture because the two redox reactions gener-ate two intrinsically separated flue gas streams withone consisting entirely of CO2 and H2O. This givesCLC clear benefits when compared with competingcarbon capture technologies, as the latter generallyinvolve a significant energy penalty associated witheither post combustion scrubbing systems [7] or thework input required for air separation plants. There-fore, CLC-based technologies can achieve high ef-ficiency power generation with low energy penaltycarbon capture. A steam chemical looping combus-tion (SCLC) method consists of oxidation and reduc-tion processes, which are undertaken in two separate

reactors. In the reduction step, the syngas is reactedwith an oxygen carrier such as iron oxide to formcarbon dioxide and water. After condensing the wa-ter vapor, the captured carbon dioxide stream can besent to the sequestration sites. The reduced form ofiron is re-oxidized in an oxidation reactor to its origi-nal form using steam and recycled back to the reduc-tion reactor. In the fuel reactor the syngas is oxidizedwith iron oxide (magnetite) according to the follow-ing reactions:

Fe3O4 + 4CO→ 3Fe + 4CO2 (1)

Fe3O4 + 4H2 → 3Fe + 4H2O (2)

In the steam reactor the reduced form of the oxy-gen carrier (iron) is oxidized back by using steam toregenerate the iron oxide and to produce hydrogenaccording to the reaction:

Fe + 4H2O→ Fe3O4 + 4H2 (3)

Hydrogen is then sent to a combined cycle gasturbine. Figure 2 presents the flowsheet of an oxy-gasification steam chemical looping combustion gasturbine (OG-SCLC-GT) power system.

A main advantage of the OG-SCLC-GT is that itcan offer a CO2 capture rate exceeding 99 % whilemaintaining efficiency comparable to other powergenerating/CCS technologies. The consumption ofoxygen can be minimized in optimized gasifier de-signs [8]. The water recirculation loop (i.e. gasi-fier/CLC/condenser) minimizes water consumption,thus mass and energy integration is ensured. Coalpre-drying enables energy to be extracted from fluegases.

2.3. Biogas fuelled oxy-reforming fuel cell (OR-FC)Biomass includes carbon entirely assimilated from

atmospheric CO2 during its growth via photosyn-thesis with the contribution of solar energy, sinceplants are unable to assimilate carbon from any othersource. Hence, bioenergy processes that involve theutilization of biomass-derived biogas with simulta-neous CO2 capture are characterized by ’negativenet CO2 emissions’ to the atmosphere [9]. De-carbonization of biogas is a much more attractiveoption than CO2 separation from fossil fuel power

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Figure 2: The flowsheet of the coal fuelled oxy-gasificationsteam chemical looping combustion gas turbine (OG-SCLC-GT) power cycle

generation-derived flue gases. This arises from thefact that biogas contains less impurities such as par-ticulate matter, NOx, and SO2. Further, biogas andbiogas-derived post-processing gases are more en-riched in CO2 than air-fuel combustion-derived fluegases. This CO2-enrichment provides an opportunityfor low cost CO2 capture due to the increased drivingforces in CO2 separation processes. Furthermore, byusing a WGS reactor the syngas is shifted to a CO2-H2 mixture from which the separation of CO2 is tech-nologically simpler than that from e.g. CO2-H2 orCO2-CH4 mixtures. The aim of the investigated de-carbonized oxy-reforming fuel cell (OR-FC) powercycle is to generate fully decarbonized power via asyngas route from renewable biogas. This conceptrelies on oxy-fuel CH4 splitting into CO and H2. COis then oxidized to CO2 providing energy for the en-dothermic CH4 as well as H2O splitting. H2 is fur-ther separated and used as a fuel for electrically ef-ficient fuel cells [10]. Since steam methane reform-ing is thermodynamically an equilibrium reaction aresulting unreacted CO/H2O mixture can be eitherrecycled back into an oxy-reforming reactor [11] orshifted to CO2/H2 by a selective catalytic water-gasshift (WGS) reaction. While the present study ana-lyzes only the latter option, a combination of the twoapproaches is also feasible, as well as WGS equi-libria can be shifted by separating one of the twoproducts of the WGS reaction, i.e. H2 or CO2. Bio-gas includes completely oxidized carbon in the formof CO2 and unoxidized carbon in the form of CH4.The objective of the current OR-FC process is to splitCH4 by means of oxy-reforming into free H2 and si-multaneously completely oxidize the remaining COto CO2. In order to avoid any dilution of reactionproducts with nitrogen, pure oxygen is supplied asan oxidizer. Therefore, the fully decarbonized OR-FC process seeks improved conditions for separatinga CO2-H2 mixture at the expense of using oxygen asan oxidizer. Oxygen generation can be beneficiallyintegrated in a closed mass and energy recirculationloop with a fuel cell unit and three reactors of theOR-FC cycle. Further, the addition of H2O is benefi-cially another source of H2 as well as oxygen thus re-ducing the consumption of pure oxygen and increas-ing the yield of H2 in product gases. By ensuringstrict molar C/O ratio in substrates equal to 0.5 the

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overall reaction of the OR-FC process can proceedaccording to the following overall reaction:

CH4 +1

1.8CO2 + nO2 + 2(1 − n)H2O↔ (4)

2.81.8

CO2 + 2(2 − n)H2

Autothermal operation of the OR-FC process canbe achieved when n = 0.3412 [11] and hence eq. (4)becomes:

CH4 + 0.5556CO2 + 0.3412O2 + 1.3176H2O↔1.5556CO2 + 3.3176H2

4H298 = 0kJ/mol (5)

From eq. (5) it can be calculated that from bio-gas comprising e.g. 64.29 %v CH4 and 35.71 %vCO2 (i.e. CH4:CO2 = 1.8) with the addition of oxy-gen and H2O in quantities satisfying C/O = 0.5 andC/H = 0.2344 one can obtain by autothermal conver-sion a mixture comprising 68.1 %v H2 and 31.9 %vCO2. Consequently, by utilizing the OR-FC processH2 can be autothermally produced from renewablebiogas with a yield of 3.3176 H2:CH4 (molar ba-sis) and the remaining gas beneficially includes al-most solely CO2, i.e. is ready for compression andsequestration. The separation of this CO2-H2 mix-ture is technologically simpler than the separation ofCO2-N2 or CO2-CH4 mixtures. However, the over-all reaction given in eq. (5) is reversible and hencethe product gases from the OR-FC process includesome CO and H2O, since process equilibria are dom-inated by WGS chemistry. In order to force shiftingof CO/H2O to CO2/H2 two selective WGS reactorsare involved. Accordingly, a high temperature WGS(HT-WGS) reactor and a low temperature WGS (LT-WGS) reactor are sequentially set up after the oxy-reforming reactor.

The flowsheet of the OR-FC cycle is presented inFigure 3. Accordingly, after the oxy-reforming re-actor the reaction mixture still includes some quan-tities of CO and H2O which need to be shifted toCO2 and H2. This is accomplished by the inclu-sion of two WGS reactors. The OR-FC process in-volves two elementary reversible chemical reactions,

Figure 3: The flowsheet of the OR-FC power cycle. Notation:FC - flow controller, TC - temperature controller

i.e. steam methane reforming (SMR) and WGS.The SMR reaction is strongly endothermic and pro-duces moles thus, according to Le Chatelier’s princi-ple it is favored at high temperatures and low pres-sures. High temperatures are thus practiced in in-dustrial conditions. However, in terms of pressure itmust be noted that the SMR reaction requires cata-lysts, which are better utilized under high-pressureconditions. Therefore, the oxy-reforming reactionsare usually conducted under high-pressure [12]. Incontrast, the WGS reaction is thermodynamically fa-vored by low temperatures. However, to prevent areduction in the reaction rate, temperatures must de-crease gradually, i.e. by passing process gases fromthe oxy-reforming reactor through the HT-WGS re-actor to the LT-WGS reactor. Further, according tothe OR-FC process, the separated H2 is sent to afuel cell unit. The oxy-reforming and WGS reac-tors can operate under high-pressure, which is bene-ficial for H2/CO2 separations by increasing their sep-aration driving forces. Besides, high-pressure oper-ation enables improvements in the utilization of cat-alysts by increasing the amount of fuel gas flow perunit amount of catalysts per unit time. Finally, thefuel cell unit transforms H2 into electricity with highfuel-to-electricity efficiency. An O2 generation unit(OGU) can utilize either membrane air separation orH2O electrolysis operated on electricity generated inthe fuel cell stack. In the latter case, H2 generated byelectrolysis is fed to the fuel cell. The whole OR-FCprocess can thus be beneficially integrated by meansof energy and mass recirculation loops. The think-ing underlying the proposed concept of biogas con-version via the OR-FC cycle with negative net CO2

emissions can also be useful in other similar biogas— 11 —


conversion cycles.

3. Conclusions

The paper analyzed low-carbon power generationconcepts. Three promising low-carbon power cycleswere examined with an emphasis put on the feasi-bility of CO2 capture and integration opportunities.It was shown that high CO2 capture rates were fea-sible from pressurized CO2-enriched streams com-prising either water or hydrogen, thus necessitatingoxy-fuel power cycles. Opportunities for mass andenergy integration were found to be increased in sys-tems involving closed mass and energy recirculationloops. The discussions also emphasized that low-carbon power cycles could achieve minimized ex-ergy losses by applying more reversible energy con-version processes. The first low-carbon power cycleunder study, the MSG-GT cycle, offered the possibil-ity of avoiding problems encountered in the two cur-rent large-scale biomass-based power technologies:(i) one-step direct combustion (low efficiency) and(ii) co-combustion with coal usually in centralizedpower plants (need to transport biomass over longdistances). The MSG-GT cycle investigated involvedbiomass pre-treatment via drying and torrefactionfollowed by biomass gasification to syngas and highefficiency syngas combustion. It was emphasizedthat such a multi-step cycle led to more reversiblefuel processing than achieved in one-step combus-tion. Therefore, in accordance with the principle ofentropy equipartition, entropy was generated moreuniformly in all sub-units, which means less ex-ergy destruction and thus increased cycle efficiency.The second low-carbon cycle investigated, the OG-SCLC-GT cycle, benefited from both improved cycleefficiency and the feasibility of extremely high CO2

capture rate. In the OG-SCLC-GT cycle coal wasdried, gasified in the oxygen-blown gasifier and theresulting syngas was fed to the syngas CLC reactorthat produced a CO2/H2O mixture. Further, the re-duced oxygen carrier was used in the steam reactor toproduce hydrogen, which was sent to the combinedcycle gas turbine. The OG-SCLC-GT cycle was su-perior in the use of advanced mass and energy inte-gration via the inclusion of several closed recircula-tion loops and due to the extremely large CO2 capture

rate. The third decarbonized power cycle, the OR-FC cycle, offered to produce hydrogen from renew-able biogas with a maximal yield of ca. 3.3 H2:CH4

on a molar basis in autothermal conditions. Throughcapturing CO2 the OR-FC power cycle achieved neg-ative net CO2 emissions. The OR-FC power cycleinvolved the oxidative reforming of biogas to syngasfollowed by a high-temperature shift reaction and fi-nally a low-temperature shift reaction was performedto produce H2. The OR-FC process was integratedwith CO2 capture, the fuel cell and the oxygen gener-ator. The main advantage of the OR-FC process overother existing decarbonized processes included H2

separation from highly concentrated CO2-H2 mix-tures while the remaining CO2 could be compressedand sequestered. Pure hydrogen was converted intoelectricity in the efficient fuel cell. Also energy andmass integration was assured by involving closed re-circulation loops.

AcknowledgementsThe author (WMB) gratefully acknowledges the

financial support from Wrocław University of Tech-nology under grant no. 344069 Z0311.

NomenclatureCCS carbon capture and sequestrationC/H C to H ratio in a mixture (molar basis)CLC chemical looping combustionC/O C to O ratio in a mixture (molar basis)GT gas turbineHT-WGS high-temperature water gas shiftIGCC integrated gasification combined cycleLCE Low-Carbon EconomyLT-WGS low-temperature water gas shiftMSG-GT multi-step gasification gas turbinen stoichiometric coefficient in eq. (4)OG-SC- oxy-gasification steam chemical-LC-GT looping combustion gas turbineOR-FC oxy-reforming fuel cellSCLC steam chemical looping combustionSMR steam methane reformingWGS water gas shift

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Low-carbon power generation cycles: the feasibility of CO capture

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