analysis of cumulative energy consumption in an oxy-fuel combustion power plant integrated with a...

Energy Conversion and Management xxx (2014) xxx–xxx

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Energy Conversion and Management

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Analysis of cumulative energy consumption in an oxy-fuel combustionpower plant integrated with a CO2 processing unit

http://dx.doi.org/10.1016/j.enconman.2014.02.0480196-8904/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +48 32 237 1049; fax: +48 32 237 2872.E-mail addresses: [email protected] (A. Ziebik), [email protected]

(P. Gładysz).1 Tel.: +48 32 237 2962; fax: +48 32 237 2872.

Please cite this article in press as: Ziebik A, Gładysz P. Analysis of cumulative energy consumption in an oxy-fuel combustion power plant integratedCO2 processing unit. Energy Convers Manage (2014), http://dx.doi.org/10.1016/j.enconman.2014.02.048

Andrzej Ziebik ⇑, Paweł Gładysz 1

Silesian University of Technology, Institute of Thermal Technology, Konarskiego 22, 44-100 Gliwice, Poland

a r t i c l e i n f o a b s t r a c t

Article history:Available online xxxx

Keywords:Oxy-fuel combustionSystem analysisCumulative energy consumptionMathematical modelingInput–output analysis

A balance of direct energy consumption is not a sufficient tool for an energy analysis of an oxy-fuel com-bustion power plant because of the indirect consumption of energy in preceding processes in the energy-technological set of interconnections. The sum of direct and indirect consumption expresses cumulativeenergy consumption. Based on the ‘‘input–output’’ model of direct energy consumption the mathematicalmodel of cumulative energy consumption concerning an integrated oxy-fuel combustion power plant hasbeen developed. Three groups of energy carriers or materials are to be distinguished, viz. main products,by-products and external supplies not supplementing the main production. The mathematical model ofthe balance of cumulative energy consumption based on the assumption that the indices of cumulativeenergy consumption of external supplies (mainly fuels and raw materials) are known a’priori. It resultsfrom weak connections between domestic economy and an integrated oxy-fuel combustion power plant.The paper presents both examples of the balances of direct and cumulative energy consumption. Theresults of calculations of indices of cumulative energy consumption concerning main products are pre-sented. A comparison of direct and cumulative energy effects between three variants has been workedout. Calculations of the indices of cumulative energy consumption were also subjected to sensitive anal-ysis. The influence of the indices of cumulative energy consumption of external supplies (input data), aswell as the assumption concerning the utilization of solid by-products of the combustion process havebeen investigated.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction boiler island, steam cycle, cooling water system, air separation unit

An oxy-fuel combustion (OFC) power plant integrated with aCO2 processing unit is one of clean coal technologies. It is basedon the utilization of high-purity oxygen in the combustion process.Therefore, flue gases contain a high concentration of CO2 [1]. Dueto a limited adiabatic temperature of combustion some part ofCO2 must be recycled to the boiler in order to maintain a propertemperature of the flame, as stated in [2] and also in [3]. By recy-cling flue gases, a gas consisting mainly of CO2 and H2O is gener-ated, which is ready for sequestration without stripping of theCO2 from the gas stream [4]. Although exact requirements of thequality of the CO2 product for different ways of storing have notbeen clarified yet [5], the high concentration of CO2 in flue gasesprovides a great advantage to the OFC technology. An oxy-fuelcombustion power plant integrated with a CO2 processing unitconsists of the following modules of the technological scheme:

(ASU), flue gas quality control (FGQC) module, CO2 processing unit(CPU) and a module of water treatment. The CPU unit is dividedinto a CO2 purification module and a set of compressors. Thus,the integrated oxy-fuel combustion power plant constitutes a com-plex energy system, the analysis of which requires a system ap-proach including the analysis of cumulative energy consumption.

The analysis of the direct consumption of energy does not in-clude all the energy required for the production of any given usefulenergy carrier (or any other product). Other energy carriers usedfor its production (e.g. fuels) also require the consumption ofenergy in intermediate processes of production and transport.Thus, the energy carrier (or any other product) is produced notonly as a result of direct but also indirect energy consumption innumerous preceding processes in the energy and technologicalset of interconnections, as for instance [6]:

� extracting non-renewable primary energy and rawmaterials,

� producing materials and semi-products,� processing primary energy into final energy carriers,

with a

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Nomenclature

Main symbolsA matrix of the coefficients of the consumption of energy

carriers and materialsaij coefficient of consumption of energy carriers and mate-

rialsD vector of external suppliese� vector of indices of the cumulative energy consumptione� index of the cumulative energy consumptionF matrix of the coefficients of the by-productionfij coefficient of by-production of energy carriers or mate-

rialsG column vector of the main productionI unit matrixK column vector of the final production

Subscripts and superscriptsD external supplyDD external supply not supplementing the main productionDG external supply supplementing the main productionF by-product

FG by-product supplementing the main productionG main product

AbbreviationsASU air separation unitCCS carbon capture and storageCPU CO2 processing unitDEA deaeratorESP electrostatic precipitatorFGD flue gas desulphurizationFGQC flue gas quality controlG generatorGWP global warming potentialHP high pressureIP intermediate pressureLHV lower heating valueLP low pressureMAC main air compressorOFC oxy-fuel combustionUSC ultra super-critical

2 A. Ziebik, P. Gładysz / Energy Conversion and Management xxx (2014) xxx–xxx

� transporting raw materials, semi-products, materials andenergy carriers,

� constructing production plants and installations in whichthe given product (energy carrier) is manufactured.

The sum of direct and indirect consumption of energy has beencalled cumulative energy consumption. This system approach hasbeen applied in the case of an oxy-fuel combustion power plantintegrated with a CO2 processing unit.

The calculus of cumulative energy consumption may concernboth primary energy and final energy (e.g. electricity). Practically,the cumulative consumption of primary energy is applied in sys-tem analysis. The indices of cumulative primary energy consump-tion are determined, first of all, on the level of the economy of thewhole country [7]. The most effective and accurate method ofcalculating the mean indices concerning the country is the‘‘input–output’’ method [8] of the balances of cumulative energyconsumption. This ‘‘input–output’’ method has been applied inthe case of an oxy-fuel combustion power plant integrated witha CO2 processing unit. The base of the ‘‘input–output’’ algorithmconcerning analysis of the cumulative energy consumption is amathematical model of the balance of direct consumption ofenergy in the integrated oxy-fuel combustion power plant [9].

The input quantities in the balance of cumulative energy con-sumption concerning integrated oxy-fuel power plants are indicesof cumulative energy consumption connected with the externalsupplies of energy carriers and materials. Their values are deter-mined basing on the analysis of cumulative energy consumptionconcerning the economy of the whole country as proposed in [7]and later also in [6]. Thus, all the interconnections in the networkof technological and energy processes and transport, comprising allthe sectors of the economy of the country, are taken into account.Analyzing the outlay of an integrated power plant with oxy-fuelcombustion from the point of view of cumulative energy consump-tion we assume that its effect on the entire economy of the countryis only rather weak. Such an assumption permits to apply in thecalculations average values of the indices of cumulative energyconsumption affecting the processes of getting and supplying fuels,raw materials, as well as materials utilized in an integrated oxy-fuel combustion power plant. The indices of cumulative energy

Please cite this article in press as: Ziebik A, Gładysz P. Analysis of cumulative enCO2 processing unit. Energy Convers Manage (2014), http://dx.doi.org/10.1016

consumption concerning the by-production are determined basingon the principle of omitted energy outlays in replaced processes.The principle of avoided outlay of fuels results from the theory ofcogeneration processes [6].

In the case of cogeneration (e.g. production of heating steam onseveral levels of thermal parameters) additionally the exergy meth-od is applied [6], permitting to divide the consumption of fuel affect-ing the production of heat between the fluxes of steam with variousthermal parameters. The exergy method may also be applied in thecase of producing technical gases in the air separation unit.

2. Oxy-fuel combustion power plant

The OFC technology belongs to the group of CCS technologies(beside Post Combustion and Pre Combustion) dedicated to meetthe goal of reducing CO2 emissions in the electricity generationbased on fossil fuels. All of them are predicted to be transitionaltechnologies on the way to the renewable power generation onthe global scale. Basically the CCS technology consists of threemain parts: CO2 capture, transport and storage (or utilization).Although the last two parts are important and indispensable com-ponents of CCS, this article will discuss only OFC, as CO2 capturetechnology. It is still not clear which one of those three carbon cap-ture and storage technologies (post-combustion, pre-combustionor oxy-fuel combustion) will be globally adapted. It has been men-tioned in many publications presented by the U.S. Department ofEnergy (e.g. [10]) and the Global Carbon Capture and Storage Insti-tute (e.g. [11]) that the OFC technology is a very promising solutionfor CO2 capture. It can be implemented in already existing powerplant, new build ones, and also can be implemented in other CO2

emission intense process (like steel or cement production) [11].The higher cost of producing electricity, caused by implementingthe CO2 capture process, requires the application of process inte-gration. One of the main ways of integration is the utilization ofheat from compressor interstage cooling systems concerning theair separation and CO2 processing units with the steam cycle.

In recent years interest has grown in OFC technology, wheremany projects and studies were performed in order to estimateits full potential. Most of the published papers deal with the

ergy consumption in an oxy-fuel combustion power plant integrated with a/j.enconman.2014.02.048


A. Ziebik, P. Gładysz / Energy Conversion and Management xxx (2014) xxx–xxx 3

process analysis of an OFC power plants, both for newly built onesand retrofit of existing coal fired power plants. The most compre-hensive process analyses were presented by the U.S. Departmentof Energy [12] and the International Energy Agency [13]. Thenew, advanced concepts of the OFC technology are also analyzed[14], including pressurized oxy-fuel combustion power cycle[15]. As different carbon capture technologies are considered forcoal-fired power plants, the comparative process analyses are alsoperformed [16]. Various softwares are used (e.g. Aspen Plus [17] orEngineering Equation Solver [18]) not only to analyze, but also tooptimize the whole process, for example by implementation of dif-ferent integration schemes [19]. Also the use of by-products of anASU is considered to improve the net efficiency of a whole powerplant [20]. The results of those studies show the drop of the overallnet plant efficiency (compared with the non-CCS power plants) byabout 6–12%, which still cannot be accepted by the potential inves-tors. Several Second Law analyses can be found in literature usingexergy [21] or basing on exergy indicators, like for example exer-goenvironmental index [22] to evaluate the performance of anOFC power plant. LCA analyses can also be found in literature,where the OFC technology results in a net reduction of the GWPof the power plant through its life cycle amounting to between76% and 97% [23]. A comparison of the different carbon capturetechnologies, from the LCA point of view, is also presented in[24] as well as [25], where the oxy-fuel combustion capture tech-nologies are shown to be slightly better than post-combustion [26].The thermodynamical and environmental analyses are supple-mented by the economic evaluations of the OFC capture method,some of which are strictly connected with the exergy analysis[27]. One of the economic analysis is focusing on different config-urations of the CPU module [28] and their impact on the cost ofelectricity. Different configurations of the heat integration are eval-uated from the economic point of view [29] along with investiga-tions of the impact of the price of CO2 emission allowance.

Fig. 1. Block-diagram of an oxy-f


The aim of this study, as a part of the Polish National StrategicProject co-realized by the authors, called ‘‘Advanced Technologiesfor Energy Generation. Project no. 2: Oxy-combustion technologyfor PC and FBC boilers with CO2 capture’’, is not only to performa process analysis and optimization of an OFC power plants, butalso to perform the system analysis [30] which includes analysisof direct and cumulative energy and exergy consumption, the ther-moecological cost and LCA [3]. The results of those studies can behelpful for understanding the global (in the scale of a single coun-try or EU) impact of this technology on the whole electroenergysystem. They can also help to evaluate the various of proposed ap-proaches to the production of oxygen, CO2 removal or heat integra-tion options.

Fig. 1 presents the block diagram of an oxy-fuel combustionpower plant in which seven technological modules are shown cor-responding to seven main products (Table 1).

Fig. 1 also illustrates the system of the main energy-materialinterconnections between the respective technological modules.The steam boiler is fired with coal and an oxidizer which is a mix-ture of oxygen and recycled CO2-rich flue gases. The boiler is sup-plied with feeding water preheated in the steam cycle module.Other energy carriers are electricity and low-temperature processheat used to preheat the recycled stream of CO2-rich flue gases.The main product of the boiler is HP & IP process steam passedto the steam cycle module. By-products are flue gases containingmainly CO2 (about 66%), fly ash and bottom ash. The main productof the steam cycle module is electricity. The by-products are LPprocess steam and LT process heat. Besides the main driving steam(HP & IP process steam) the module is fed with interstage coolingheat (LT, MT and HT process heat) from ASU and CPU, as well ascooling duty and make-up water.

The cooling water system is closely connected with the steamcycle module and also with ASU and CPU. The main product, viz.cooling duty, is first of all applied in the steam condenser and

uel combustion power plant.



Table 1Technological modules and the corresponding main products.

No. Module Main product

1� Boiler island High-pressure process heat2� Steam cycle Electricity3� Cooling water system Cooling duty4� Flue gas quality control system CO2-rich stream5� Water treatment system Make-up water6� Air separation unit Oxygen7� CO2 processing unit CO2 product


the interstage cooling system of compressors in ASU and CPU. Inthis module electricity and make-up water are consumed. Thewater treatment system is a module strictly connected with thesteam cycle, cooling water system and flue gas quality control(FGQC) system, to which lead outputs from this module. The inputpart of this module comprises raw water, waste water andelectricity.

The ASU module is based on the cryogenic technology of sepa-rating oxygen from air. The fundamental part of input energy iselectricity driving the air compressors. The recovery of heat fromthe interstage cooling system is the source of interstage coolingheat which is passed to the low-pressure part of regeneration inthe steam cycle module. Fig. 2 presents variants of the realizationof interstage cooling of the compressors. The ASU module is alsofed with LP process steam in the form of steam from the steam cy-cle module and cooling duty from the cooling water system. Thedominating consumer of oxygen is the boiler island. A smallamount of oxygen is also consumed by the FGQC system.

The aim of the FGQC module is the conditioning of flue gasesfrom the boiler island. These flue gases comprise CO2 (about66%), H2O (about 20%), N2 (about 8%), Ar (about 3%), O2 (about2%), SO2 (about 0.3%) and fly ash. In this module the flue gasesare dedusted in electrofilters, desulphurized and dehumidized. Inresult CO2-rich steam is obtained which is the input to the CPUand a large part is recycled to the boiler (about 70%). The moduleFGQC system is supplied with limestone, electricity, make-upwater and oxygen. Besides the main product, CO2-rich stream, use-ful effects of FGQC module operation are by-products, namely gyp-sum and fly ash.

The module CO2 processing unit prepares the stream of sepa-rated CO2 to be transported to the place of storing. In the CPU mod-ule CO2 is compressed and dried. The CO2 product must satisfy thepreset criteria of purity, namely raw product produced using 95%oxygen, desulphurized (98% removal efficiency) and dehydratedto 0.015% (by volume) H2O [12]. Besides the main input, theCO2-rich stream, the module is fed with electricity (mainly in orderto drive the compressors), cooling duty and LP process steam.A by-product is interstage cooling heat (low-, medium- and

Fig. 2. Variants of realization of inte


high-temperature process heat), which is passed to the steam cyclemodule and utilized in it, depending on the temperature level, inthe low- and high-pressure part of heat regeneration.

3. Mathematical model of the balance of direct energyconsumption of an integrated oxy-fuel combustion power plant[30]

A power plant operating in oxy-fuel combustion technologyintegrated with ASU and CPU is a system with a considerably en-larged number of interconnections between its respective modulesif compared with a traditional power plant. The modules ASU andCPU are, in addition to the main production, sources of waste heat,the utilization of which improves the energy effectivity of thepower plants. The heat resulting from interstage cooling of thecompressors of air and CO2 can be usefully applied in the systemof preheating the condensate in the steam cycle. Nitrogen, a by-product of ASU, may be used to dry the coal [20]. If, however, theoxy-fuel combustion technology is realized at elevated pressure,the nitrogen is expanded previous to its application for drying coalin the recovery turbine producing additional electricity.

Thus, in an integrated power plant operating in the oxy-fuelcombustion technology besides the main products, for instanceelectricity or oxygen, by-products do exist. By-products of onebranch may supplement the main products of other branches(e.g. electricity obtained in the process of utilizing high pressurenitrogen supplements the main production of electricity in thesteam cycle). Mostly, however, by-products of an integrated oxy-fuel combustion power plant do not supplement the main produc-tion. They are useful products utilized either inside the integratedpower plant (e.g. LP process steam) or sold as final product (e.g.gypsum).

The main products of an integrated oxy-fuel combustion powerplant are the first group of energy carriers constituting the set of‘‘input–output’’ balance equations including the following items:

� the vector of main production – G,� the vector of by-production supplementing the main pro-

duction – FGG,� the vector of external supplies supplementing the main

production – DG.

All of them are input part of the balance. The output part con-sists of:

� the vector of the consumption of main products – AGG,� the vector of final production of main products – KG.

Expressed in the form of a matrix, the set of balance equationsof main products is as follows:

rstage cooling of compressors.



Fig. 3. Cumulative energy balance of the j-th branch; e�i – average-weighted indexof cumulative energy consumption concerning the i-th energy carrier; e�Gj – index ofcumulative energy consumption concerning the main production of the j-th branch;e�DDp – index of cumulative energy consumption concerning the p-th external supplyof the energy carrier or material; e�Fl – index of cumulative energy consumptionconcerning the by-production of the l-th energy carrier not supplementing the mainproduction; e�Fi – index of cumulative energy consumption concerning the by-production of the i-th energy carrier supplementing the main production.


Gþ FGGþ DG ¼ AGGþ KG ð1Þ

where FG – matrix of the coefficients concerning by-production sup-plementing the main production, AG – matrix of the coefficients ofthe consumption of energy carriers concerning the main produc-tion.The second group of balance equations comprises by-productswhich do not supplement the main products. The input part of theseequations is in this case the vector of by-products not supplement-ing the main production – FG. Practically, there are no supplement-ing external supplies. The output part in the balance of this group ofenergy carriers comprises:� the vector of the consumption of by-products – AFG,� the vector of final production of by-products – KF.

In matrix notation we have:

FG ¼ AFGþ KF ð2Þ

where F – matrix of the coefficients concerning by-production notsupplementing the main production, AF – matrix of the coefficientsof the consumption of energy carriers belonging to by-productionnot supplementing the main production.

The third group of energy carriers and materials comprisesmerely external supplies (not supplementing the own production).To this group belong mainly fuels, raw water and limestone. The in-put part is in this case the vector of merely external supplies – D,whereas the output part is the vector of the consumption of merelyexternal supplies – ADG. Expressed in the form of a matrix, the set ofbalance equations of merely external supplies is as follows:

D ¼ ADG ð3Þ

where AD denotes the matrix of the coefficients of the consumptionof merely external supplies.

Eqs. (1)–(3) constitute the mathematical model of the balanceof direct energy consumption. This is the base of elaborating themathematical model of the balance of cumulative energy con-sumption. In comparison with the publication in [30] the pre-sented paper contains algorithms and examples concerningcumulative energy consumption.

4. Mathematical model of the balance of cumulative energyconsumption of an integrated oxy-fuel combustion power plant

The indices of cumulative energy consumption are mostly cal-culated as average indices concerning the given country. In sucha case energy-technological interconnections concern the entireeconomy of the country, for which the set of ‘‘input–output’’ equa-tions of the cumulative energy balance has been set up. The aver-age indices of cumulative energy consumption burdening thefundamental fuels, raw materials and semi-products may be ap-plied for the purpose of calculating the indices of cumulative en-ergy consumption concerning the respective technologies (e.g.oxy-fuel combustion power plant) because the interconnectionsof any given technology with the entire economy of the countryare rather weak connections. Then the average indices of cumula-tive energy consumption of basic domestic or imported productsare treated as assumed values. Such an approach has been appliedin the technology of oxy-fuel combustion in a power plant.

Fig. 3 presents the block diagram of the j-th branch of an oxy-fuel combustion power plant.

The average-weighted index of the cumulative energy con-sumption of an energy carrier is defined as follows:

e�i ¼ rGie�Gi þ rFGie�FGi þ rDGie�DGi ð4Þ

where rGi, rFGi, rDGi denote the share of main production,by-production supplementing the main production and externalsupplies supplementing the main production in the input of the


i-th energy carrier and e�FGi; e�DGi denote indices of cumulative energy

consumption concerning the i-th by-production supplementing themain production and external supply supplementing the mainproduction.

Thus, the indices of cumulative energy consumption concerningenergy carriers produced in an integrated power plant operating incompliance with oxy-fuel combustion and passed to the j-th mod-ule are average-weighted values of indices concerning the mainproduction, by-production and supplementing external supply.The input values preset in the balance of cumulative energy con-sumption are indices of cumulative energy consumption burden-ing the external supply of energy carriers and materials. Theindices of cumulative energy consumption concerning the by-pro-duction are determined basing on the principle of avoided outlay ofenergy in replaced processes.

Expressed in the form of a matrix, the set of balance equationsof cumulative energy consumption is as follows:

ATGe� þ AT

De�DD ¼ e�G þ FTGe�FG þ FT � AT

F

� �e�F ð5Þ

where e* – vector of average-weighted indices of cumulative energyconsumption, e�DD – vector of indices of cumulative energy con-sumption of external supply not supplementing the main produc-tion, e�G – vector of the indices of cumulative energy consumptionof main products, e�FG – vector of indices of cumulative energy con-sumption of by-production supplementing the main production, e�F– vector of indices of cumulative energy consumption of by-produc-tion not supplementing the main production.

The vector of the average-weighted indices of cumulative en-ergy consumption is defined as follows:

e� ¼ rdGe�G þ rd

FGe�FG þ rdDGe�DG ð6Þ

where rdG – diagonal matrix of the shares of main production in the

global production of the system, rdFG diagonal matrix of the shares of

by-production supplementing the main production in the globalproduction of the system, rd

DG – diagonal matrix of the shares ofthe external supply supplementing the main production in the glo-bal production of the system, e�DG – vector of the indices of cumula-tive energy consumption of the external supply supplementing themain production.

The matrix Eqs. (5) and (6) constitute the algorithm of calculat-ing the indices of cumulative energy consumption in an integratedpower plant operating as oxy-fuel combustion.



Table 2Case descriptions [14].

Table 3List of energy carriers and materials.


5. Examples of balances of direct and cumulative energyconsumption

The examples presented in this paper are based on [14], whereseveral advanced OFC technologies for bituminous coal powerplants are analyzed. Three cases have been chosen, mainly the basecase (current technology), the ultra-supercritical (USC) case (ad-vanced boiler materials) and the advanced CO2 compression case(shock wave compression). The characteristic parameters for allthree cases are listed in Table 2.

The analyzed cases include a supercritical (or ultra-supercriti-cal) pulverized coal OFC power plant with a wet FGD unit and a bag-house to remove particles. The PC boiler design is based on abituminous coal-fired unit, where the theoretical adiabatic flametemperature of the boiler is controlled by varying the amount offlue gases recycled to the boiler. The oxidant is supplied by conven-tional cryogenic ASU technology which produces 95% pure oxygen.The recycled flue gases (wet recycle is realized) are superheated by9 K (where the condensate or LP steam from the steam cycle areused) before entering the primary and induced draft fans in orderto ensure that the primary and secondary streams do not producea condensate in the ducts or enter the fans in saturated conditions.In all cases the CO2 product is compressed to 15.3 MPa and meetsthe requirements concerning the purity to be sequestered in a sal-ine formation [14]. The compression is accomplished in eight stagesof centrifugal compression with intercooling between each stage(in the basic and ultra-supercritical case) or in the advanced CO2

compression case the CO2 compression system utilizes advancedshock wave compression technology with a higher stage compres-sion efficiency than in the basic case. The supersonic shock wavecompression technology is similar in its concept to an aircraft ram-jet engine, characterized by a rotating disk that operates at highperipheral speeds to generate shock waves that compress the CO2

[10]. When shock wave compression is realized, the interstage com-pression heat is recovered in the boiler feedwater system, which re-duces the amount of steam extracted from the steam cycle and canincrease the power output of the steam cycle. Among the men-tioned higher compression efficiency and opportunity of waste heatrecovery, the advanced shock wave compression technology offersan additional potential advantage like high, single-stage compres-sion ratios, which leads to a lower capital cost in comparison withthe conventional CO2 compression technology [14]. In a conven-tional system (basic case) the intercooling of each stage is realizedby cooling water without useful heat recovery. The main reason fornot recovering the heat in a conventional system is that the temper-ature is too low and recovering is economically unprofitable.

Those three cases are analyzed in order to compare the resultsof direct and cumulative energy consumption. All the data con-cerning matrices of the balance of direct and cumulative energyconsumption are presented for the current technology case basedon the process model presented in [14].


5.1. ‘‘Input–output’’ model of the balance of direct energy consumption

In the analyzed integrated OFC power plant certain groups ofenergy carriers can be distinguished. The first and second groupsconsist of energy carriers and materials being the main productsand by-products of the respective modules of an integrated oxy-fuel combustion power plant. The third group consist of energycarriers and materials supplied from outside. In Table 3 thesegroups have been presented.

The energy carriers and materials numbered 1–7 comprise themain production in the respective module (Table 1), number 8–20concern the by-production and number 21–24 the external supplies.

Table 3 presents a universal structure of matrices concerningthe main products, by-products and external supplies. In the con-sidered variants of oxy-fuel combustion technology there do notexist by-products 12 and 18 nor an external supply of naturalgas. Thus, in the respective matrices the value of the coefficientsof production and consumption assumed zero values. The men-tioned by-products and external supply are in the case of mem-brane air separation unit.

Based on the ‘‘input–output’’ table the matrices AG, F, AF and AD

have been segregated, concerning respectively:

� coefficients of the consumption of energy carriers and materialsmanufactured as main products:

� coefficients of the by-production of energy carriers andmaterials:




� coefficients of the consumption of energy carriers and materialsmanufactured as by-products:

� coefficients of the consumption of external supplies:

As we see, in the matrix of the main-production interbranch theflows are to be found in the case of the first six energy carriers (ormaterials). From among them, electricity is consumed in all the se-ven modules (branches). For instance, the coefficient aG

2;3 denotesthe consumption of electricity for the production of cooling water(cooling duty) and the coefficient aG

3;2 the consumption of coolingwater for the production of electricity. Both these elements, situ-ated on either side of the main diagonal indicate a connection offeedback character. The main production is accompanied by thir-teen by-products, e.g. the coefficient f8,2 denotes the amount oflow-pressure process steam which is the by-product of the steamcycle and is usefully applied in other modules. This LP processsteam is used in ASU (coefficient aF

8;6) for regeneration of the frontend separation of the ASU and in CPU (coefficient aF

8;7) for dryingCO2-rich steam in the CO2 drier [12]. The analyzed system maybe fed by four external supplies. The supply of coal feeding the boi-ler is defined by the coefficient aD

21;1.For the purpose of system analysis, first of all, the inverse ma-

trix to the ‘‘input–output’’ matrix of main production is used. Afterchanging of the coefficients of direct consumption and the forma-tion of a new matrix A0G a new balance of the main production isobtained, which leads to a change in the vector of external suppliesof energy carriers and materials. If the process change also involvesa change of the coefficients of the consumption of external sup-plies, a new matrix A0D must be used, and we get:

DD ¼ A0DðI� A0GÞ�1K0G � ADðI� AGÞ�1KG ð7Þ

where A0D – matrix of coefficients of the consumption of externalsupplies after a process change, A0G – matrix of coefficients of theconsumption of main products after a process change, K0G – vectorof final products after a process change.

Table 4 presents the results of the balance of the integrated OFCpower plant concerning the base case.

As an example the system effects were simulated and comparedwith the advanced CO2 compression case. As the authors of [14]say, this leads to a considerable change in energy consumptionfor CO2 compression and allows to use the heat from interstagecooling of CO2 compressors to preheat the feedwater for the boiler.

DD ¼

DD20

DD21

DD22

DD23

26664

37775 ¼

�683;559;5540

�433;444�3;195

26664

37775


The vector DD expresses the results of a change of external sup-plies to the OFC power unit due to the implementation of the ad-vanced CO2 compression installation. The analysis was performedbasing on the assumption that the net electric power is constant(550 MWel) [14].

5.2. ‘‘Input–output’’ model of the balance of cumulative energyconsumption

Basing on the ‘‘input–output’’ model of the balance of direct en-ergy consumption, the cumulative energy consumption can beanalyzed. In order to perform such an analysis the indices of cumu-lative energy consumption of external supplies (e.g. coal [6,31] orraw water [32]) and the indices of cumulative energy consumptionof the by-products (e.g. gypsum [7]) none of them supplementingthe main production, have to be provided. All the indices are ex-pressed by MJ units of energy per unit of energy carrier or material(Table 2).

Based on the literature review the presented vectors of indicesof cumulative energy consumption have been distinguished for theanalyzed basic case:

� vector of indices of cumulative energy consumption ofby-production not supplementing the main production:

� vector of indices of cumulative energy consumption of externalsupply not supplementing the main production:

As we see, in the presented vector concerning by-production,only eight energy carriers (or materials) have different values than0. This means that only those energy carriers (or materials) are use-ful by-products, which can replace other energy carriers or materialsin the analyzed case (avoided outlay of energy in replaced pro-cesses). As far as the similar parameters of the by-products 8, 9,10, 11 are concerned, the indices of cumulative energy consumptionare the same. Index of cumulative energy consumption e�F16 describethe amount of energy required for the production of gypsum in analternative process. Thus the by-production of gypsum in the ana-lyzed integrated OFC power plant will replace the production inthe alternative process, and because of that, it will affect the totalcumulative energy demand for manufacturing the main products.In the case of other useful products of coal combustion the analysisof cumulative energy consumption was performed with certainassumptions concerning the substitution of other products [33]:

� bottom ash substitutes the aggregate (60% of useful product,rest is solid waste),



Table 4Vectors of global and final main production as well as external supplies (with capacity factor of 0.85 [14]).

No. Energy carrier or material Unit Global production Final production External supplies

1� HP process heat (TJ) 44842.45 0 –2� Electricity (TJ) 21197.87 14743.62 –3� Cooling duty (TJ) 30232.99 0 –4� CO2-rich steam (Mg) 17,292,930 0 –5� Make-up water (Mg) 16,336,812 0 –6� Oxygen (Mg) 4,019,340 0 –7� CO2 product (Mg) 5,028,947 5,028,947 –

. . .

21� Coal (LHV) (TJ) – – 48585.3122� Natural gas (LHV) (TJ) – – 023� Raw water (Mg) – – 16,336,81224� Limestone (Mg) – – 187,614

Fig. 4. Relative changes of the indices of cumulative energy consumption.


� fly ash substitutes cement, aggregate, sand and gypsum (15% ofeach as useful product, rest is solid waste).

Basing on the literature review the average share of the usage ofthose by-products has been estimated on the level of 60% for bothEurope [33] and Poland [34]. The indices of cumulative energy con-sumption of aggregate, cement and sand were taken over from theliterature, mainly [7,31].In the case of the vector of external supply(not supplementing the main production) the indices of cumula-tive energy consumption represent the amount of energy requiredfor the gaining, transport and processing. For example, the index ofcumulative energy consumption of coal (e�DD21) expresses theamount of energy required for the gaining and transport of coal.

Eqs. (8)–(14) describe the balance equations of cumulative en-ergy consumption in all the seven branches (Table 1), in compli-ance with the matrix equation (Eq. (5)) concerning the basic case.

aG2;1e�2 þ aG

4;1e�4 þ aG6;1e�6 þ aF

9;1e�F9 þ aD21;1e�DD21

¼ e�G1 þ f13;1e�F13 þ f14;1e�F14 ð8Þ

aG1;2e�1 þ aG

2;2e�2 þ aG3;2e�3 þ aG

5;2e�5 ¼ e�G2 þ f8;2e�F8 þ f9;2e�F9 ð9Þ

aG2;3e�2 þ aG

5;3e�5 ¼ e�G3 ð10Þ

aG2;4e�2 þ aG

5;4e�5 þ aG6;4e�6 þ aF

13;4e�F13 þ aD24;4e�DD24

¼ e�G4 þ f15;4e�F15 þ f16;4e�F16 ð11Þ

aG2;5e�2 þ aF

19;5e�F19 þ aD23;5e�DD23 ¼ e�G5 þ f20;5e�F20 ð12Þ

aG2;6e�2 þ aG

3;6e�3 þ aF8;6e�F8 ¼ e�G6 þ f17;6e�F17 ð13Þ

aG2;7e�2 þ aG

3;7e�3 þ aG4;7e�4 þ aF

8;7e�F8 ¼ e�G7 þ f19;7e�F19 ð14Þ

In the example Eq. (9) describes the balance equation of cumu-lative energy consumption for the second branch (steam cycle),where aG

1;2e�1, aG2;2e�2 and aG

3;2e�3 define the cumulative energy con-sumption of the main products, i.e. HP & IP process steam, electric-ity and cooling duty, respectively. The index of cumulative energyconsumption of electricity (e�G2) as well as the cumulative energyconsumption of the by-production of LP process steam (f8;2e�F8)and LT process heat (f9;2e�F9) are outputs of the second branch.

The indices of cumulative energy consumption of main prod-ucts e�G are equal to the average-weighted indices of cumulative en-ergy consumption e� due to the lack of by-productions or externalsupplies supplementing the main productions in the consideredcase (Eq. (6)).

In result of the ‘‘input–output’’ analysis based on the model ofthe balance of cumulative energy consumption, the vector of the


indices of cumulative energy consumption of the main productsof an integrated OFC power plant in the basic case is as follows:

In the analyzed integrated OFC power plants there are two mainproducts characterized by the final production – electricity andCO2 product. For example, e�2 expresses the cumulative energy con-sumption charging the electricity production in an integrated OFCpower plant and e�7 denote the same concerning CO2 product.

In this paper the basic, ultra-supercritical and advanced CO2

compression cases have been compared [14]. The results have beengathered in Fig. 4, where the relative changes calculated in relationto the basic case are presented. They concern the indices ofcumulative energy consumption of both not conventional casesmentioned above, viz. USC and advanced CO2 compression.

When USC steam parameters are applied, the indices of cumu-lative energy consumption charging the HP & IP process steam,electricity, cooling duty, CO2-rich stream, oxygen and CO2 productare lower, due to the general higher efficiency of the cycle itself. Inthe case of cumulative energy consumption concerning electricityproduction it drops by about 8% (in comparison with the basecase), which later on influences other modules, where electricityis consumed.

When advanced CO2 compression is applied, the indices ofcumulative energy consumption charging the electricity are higher(from 3.127 MJ/MJ to 3.231 MJ/MJ), but the indices of cumulativeenergy consumption of CO2 product drop from 1468 MJ/Mg to



Fig. 5. Direct and cumulative net energy efficiencies.

Fig. 6. Results of sensitivity analysis concerning external supplies.

Table 5Results of sensitivity analysis concerning by-products of coal combustion.

The index of the cumulative energy consumption of:

Unit

Useful by-products taken into account: bottom ash

fly ash gypsum

fly ash gypsum

bottom ash gypsum gypsum

HP process steam [MJ/MJ] 1.445 1.445 1.446 1.446 Electricity [MJ/MJ] 3.127 3.127 3.13 3.13

Cooling duty [MJ/MJ] 0.062 0.062 0.062 0.062 CO2-rich steam [MJ/Mg] 179.9 179.9 180.2 180.2 Make-up water [MJ/Mg] 41.48 41.48 41.49 41.49

Oxygen [MJ/Mg] 2738 2738 2741 2741 CO2 product [MJ/Mg] 1468 1468 1470 1470


1035 MJ/Mg (almost 30% lower). It is due to the higher direct en-ergy consumption for CO2 compression (aG

2;7) when advanced CO2

compression is applied, although it allows to use the heat (ontwo temperature levels) from interstage cooling (f10,7 and f11,7)which decreases the cumulative energy consumption chargingthe CO2 product.

Fig. 5 illustrates the change of the direct and cumulative net en-ergy efficiency (LHV) of the OFC power plants in all three cases. Thecumulative energy efficiency is calculated by means of theequation:

g�EelN ¼K2

D21 � e�21 þ D23 � e�23 þ D24 � e�24ð15Þ

where K2 – final production of electricity, D21 – external supply ofbituminous coal, e�21 – index of cumulative energy consumptionconcerning coal, D23 – external supply of raw water, e�23 – index ofcumulative energy consumption concerning raw water,D24 – external supply of limestone, e�24 – index of cumulative energyconsumption concerning limestone.

Lower values of cumulative net energy efficiencies result fromtaken into consideration also raw materials (limestone and rawwater) as components of input energy as well as its charging thecumulative energy consumption.

6. Sensitivity analysis of calculating the cumulative energyconsumption

In the analyses of the cumulative energy consumption certaininput values have to be preset in the balance of cumulative energyconsumption. Those are the indices of cumulative energy con-sumption burdening the external supplies of energy carriers andmaterials and the indices of cumulative energy consumption con-cerning the by-products. For the same energy carrier or materialsvarious values can be found in literature. Thus, the sensitivity anal-ysis has been performed for the base case. The influence of changesof the indices of cumulative energy consumption of external sup-plied on the index of cumulative energy consumption of electricityhas been investigated.

In Fig. 6 the influence of a change of the indices of cumulativeenergy consumption of external supplies has been presented,where minimum and maximum values quoted in literature havebeen used. Based on the presented results, the greatest influencecan be observed when the index of cumulative energy consump-tion of coal changes. In the case of the other two external suppliesa very slight influence is to be observed.

A sensitivity analysis has been also performed for the cumula-tive energy consumption of by-products that can be assessed


basing on the principle of the avoided outlay of energy in replacedprocesses. Besides the base case, three other options have beenanalyzed in order to estimate the influence of various variants ofapplying fly ash, bottom ash and gypsum as useful products havebeen taken into account. In each of the three options few of theby-products has been assumed to be waste (the index of cumula-tive energy consumption is then equal to 0). The results have beenpresented in Table 5, where all the indices of the cumulative en-ergy consumption of main products have been presented.

Based on the sensitivity analysis performed for the by-productswe see that the utilization of fly ash as a useful by-products has thegreatest influence on the indices of the cumulative energy con-sumption. Gypsum was always treated as a useful product, becauseit is very unlikely to be treated as solid waste.

7. Conclusions

An analysis of direct energy consumption is not a sufficient toolfor the assessment of the entire consumption of energy. This re-sults from the fact that energy carriers and materials supplied tothe given process are already charged by the energy consumptionin previous processes (e.g. gaining primary fuels and raw materials,transport and processing primary energy into final energy).

The mathematical model of calculating the indices of cumula-tive energy consumption dealt with in the paper and based on ‘‘in-put–output’’ analysis has been verified by examples concerning anintegrated oxy-fuel combustion power plant. The obtained valuesof the indices of cumulative energy consumption of the mainproducts (e.g. electricity, oxygen) are similar to those quoted in lit-erature [6].

This model was applied in a comparative energy analysis of aconventional (base case) and advanced technologies in an inte-grated oxy-fuel combustion power plant. The recovery of waste




heat from interstage cooling of the compressors for heating up thecondensate in the case of an advanced unit leads to a decrease ofthe index of cumulative energy consumption of the CO2 productby about 30%. The cumulative energy efficiencies increase by about0.4 percentage points. The ultra-supercritical steam parameterslead to a significant decrease of cumulative energy consumptionin the production of electricity by more than 8%, which correspondto the increase of the net cumulative energy efficiency by 3percentage points.

The sensitivity analysis proved that the index of cumulativeenergy consumption is most affected by the index of cumulativeenergy consumption of coal. This analysis also indicates that theutilization of fly ash exerts the highest influence on the indicesof the cumulative energy consumption of the main products.

The oxy-fuel combustion is a promising technology for CO2 cap-ture and storage, both in new and in retrofitted power plants. Fromthe point of view, when the whole electroenergy system is con-cerned, based on the presented results (the obtained values ofthe cumulative energy consumption for electricity production),the OFC technology is a competitive option for carbon capture.The obtained values mentioned above are lower than those quotedin literature for Polish conditions, which is consistent with the newtechnologies that are applied in the analyzed cases. Unfortunately,both of those advanced technologies are not mature enough to beapplied in commercial power plants. One of the first OFC demon-stration power unit is the pilot plant in Schwarze Pumpe, devel-oped by Vattenfall in 2008 [35]. Since that time, a growinginterest in OFC technology can be observed, which leads to the con-clusion that in the course of the next 5–10 years bring the firstcommercial-scale power plant with oxy-fuel technology may beconstructed. The most promising project which is being developedis the White Rose CCS Project in UK [36].

Acknowledgments

This scientific work was supported by the National Centre forResearch and Development, within the confines of Research andDevelopment Strategic Program ‘‘Advanced Technologies for En-ergy Generation’’ project no. 2 ‘‘Oxy-combustion technology forPC and FBC boilers with CO2 capture’’. Agreement no. SP/E/2/66420/10. The support is gratefully acknowledged.

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