energy effectiveness of heat and electricity cogeneration ... · may be used for preheating the...

maj 2015 strona 331www.energetyka.eu

NomenclatureMain symbolsĖ – flux of energye – unit consumption of driving energyN – powerṖ – flux of fuel Q͘ – flux of heatq – specific heatα – coefficient of the share of cogenerationΔ – increaseε – relative internal consumptionη – efficiency σ – power to heat ratio

Subscriptsb ‒ boilerc – by consumersch – chemicalcog ‒ cogenerationcom ‒ compressorsE – energyel – electricet ‒ electricity transmission ex ‒ extraction steamfg – flue gasesFP – fans and pumpsG – grossh – heathp ‒ heating plantht ‒ heat transportint – integratedloss ‒ lossesme – electro-mechanicalN – netpp – power plantrec – recovery ref – reference reg – regenerationsb ‒ steam boilersys ‒ systemwh – water heater

1) E-mail addresses: [email protected], [email protected], [email protected]

AbbreviationsBHE – basic heat exchangerCHP – combined heat and powerCOP – coefficient of performanceCPU – CO2 processing unitDHS – district heating systemEUF – energy utilization factorFPC – fans, pumps, compressorsLHV – lower heating valuePHE – peak heat exchangerPWH – peak water heater

The adequate measure of energy effectiveness of heat and electricity cogeneration is the savings of the chemical energy of fuels in comparison with the separate production of heat and electricity. This is confirmed by the cogeneration Directive of EU [1] which conditions the admission of high-efficiency cogenera-tion on the value of the index of Primary Energy Savings [1,2]. The energy effectiveness of the cogeneration of heat and elec-tricity depends on the ratio of power to heat. The higher value of this ratio leads to a more effective realization of the cogenera-tion. Although, the cogeneration of electricity and heat influences some reduction of CO2 emissions, it does not solve entirely the problem of reducing the emission of CO2 as requires by EU. In order to meet these requirements a CHP plant ought to be equ-ipped with a CO2 processing unit (CPU) comprising installations of CO2 capture and CO2 compressors. Post-combustion CO2 processing units based on amine chemical absorption are no-wadays available [3] although the investigations concerning the decrease of heat demand for the regeneration of the solvent are continuously carried out [4]. The application of this technology le-ads to a radical increase of internal heat consumption supplying the installation of regeneration of the solvent and also the inter-nal consumption of electricity driving the compressors of CO2 as well as fans and pumps in a CO2 processing unit. The additional demand for heat effects an increase of the consumption of the chemical energy of fuels but on the other hand it leads to an increase of the gross production of electricity and also incre-ases savings of the chemical energy of fuels if compared with the separate production of electricity. Therefore, we have to do with

Andrzej Ziębik, Michał Budnik, Marcin Liszka1)

Silesian University of Technology, Institute of Thermal Technology,

Energy effectiveness of heat and electricity cogeneration integrated with amine CO2 processing unit

Efektywność energetyczna skojarzonej gospodarki cieplno-elektrycznej zintegrowanej

z aminową instalacją usuwania CO2

strona 332 maj 2015www.energetyka.eu

a partial compensation of a larger fuel outlay in a cogeneration plant integrated with a CO2 processing unit. This is possible only in the case of cogeneration realized in CHP plants.

Process steam is used to supply a module of regeneration of the solvent. The amine solvent should be regenerated in the temperature range of 110-120°C. During the desorption of CO2 the CO2+H2O mixture comes into being. This mixture is supplied to the steam separator and the enthalpy of H2O condensation may be used for preheating the network water in the district he-ating system. Additionally, waste heat from interstage cooling of CO2 compressors may be also used to preheat the district he-ating network water.

The main problem concerning power plants and CHP plants integrated with amine CO2 processing unit (CPU) is connected with a reduction of heat required for regeneration of solvent. Re-port [5] presents reduction of unit consumption of heat for re-generation from 4.07 to 2.76 MJ/kg of removed CO2. Mitsubishi Heavy Industries [6] presents results of experiment concerning commercial plants with CPU characterizing by unit consumption of heat regeneration about 2.9 MJ/kg of removed CO2. CASTOR project [7] shows possibility of reduction this unit consumption from 4 to 3.2 MJ/kg of removed CO2. In this paper the unit con-sumption of heat for regenerating the solvent is a parameter in analysis of energy effectiveness of heat and electricity cogenera-tion. It has been decided to analyse three values of this parame-ter, viz. 4.0, 3.4, 3.15. The first one has often been used in hither-to realized investigations. The second case may be considered as a value possible to achieve today and the third is the value which is the most probably one in the nearest future.

The Authors [8] present interesting results of analysis con-cerning vision of converting domestic energy system of Denmark after adding of CO2 processing unit including an underground CO2 storage. In this paper the Authors deals with among others analysis of CHP plant after adding the amine CO2 processing unit. Denmark belongs to the leading countries from viewpoint of the implementation of sustainable energy systems, particularly cogeneration technology. The analysis of partial compensation of additional internal consumption of heat for regenerating the solvent in CPU installation thanks to cogeneration presented in this paper should be interesting in Denmark because of 50% pro-duction of electricity is realized in CHP plants. The added CO2 processing unit and installation of waste heat recovery leads to the integration of the CHP plant. It requires some modification of thermodynamic indices describing the energy effects of cogene-ration as presented in [9] and [10]. In literature only few papers have been devoted so far to the modification of the algorithms describing the cogeneration thermodynamic indices concerning CHP plants integrated with a CO2 processing unit. There are no such fundamental book as [11] concerning the fundamentals of classical cogeneration (without integration with a CO2 processing units). Exceptions to this rule are [12, 13, 14, 15]. The Authors of [12] have proposed an exergy method for the analysis of coge-neration indices. They also stressed the possibility of effective integration of CHP plant with installation of waste heat recovery in the CO2 chemical absorption process and interstage cooling of CO2 compressors. Integrated industrial CHP plants equipped with post-combustion CO2 capture installation presented in [13] confirm that this kind of CO2 processing unit is hitherto the most reasonable solution. The paper [14] is devoted to the analysis of

energy and ecological indices concerning CHP plants of com-plex buildings. In the paper [15] energy and ecological analysis is devoted to large-scale CHP plants fired with biomass and in-tegrated with CO2 processing unit. The Authors of [14, 15] used the conception of partial energy efficiencies in cogeneration but the presented formulae correspond to so-called “arithmetic” par-tial energy efficiencies of heat and electricity production which possess neither energetic nor economic interpretation [16, 17]. They can be useful only in the evaluation of the index PES (Pri-mary Energy Savings) according to EU Directive on the promo-tion of cogeneration [1,2].

The base of the presented theoretical analysis are the results of simulation calculations carried out by the professio-nal computer code Thermoflex [18]. Besides the analysis of energy saving concerning classical CHP plants also analysis of the effects of the partial compensation of the increased con-sumption of fuels caused by the integration of the cogenera-tion unit with the CO2 processing unit have been carried out. All algorithms and further results of calculations are obligatory for the nominal loads.

Savings of the chemical energy of fuels in the cogeneration of heat and electricity

Savings of the chemical energy of fuels are determined ba-sing on the assumption that the demand for heat and electricity by the consumers are the same in the case of cogeneration and separate production of heat and electricity replaced by the coge-neration process:

The condition of constancy of the flux of heat is obliga-tory for all the considered variants, whereas the second con-dition Nel cog = const depends on the value of the production of heat in cogeneration must be satisfied for each variants se-parately according to the power rating characteristic for the given variant.

Figs. 1 and 2 present separate production of heat and elec-tricity (replaced heating plant and power plant) and the classical CHP plant with a back pressure turbine. These diagrams are the base for the formulation of Eqs. (2),(3), (4) describing consump-tion of the chemical energy of fuel respectively in the replaced heating plant and power plant, as well as in the CHP plant.

Savings of the chemical energy of fuel due to cogeneration are calculated as a difference between the consumption in se-parate production of heat and electricity and consumption in the cogeneration process:

(1)

where:Ėch hp ‒ flux of the chemical energy of fuels consumed in the he-

ating plant replaced by the CHP plant,Ėch pp ‒ flux of the chemical energy of fuels consumed in the po-

wer plant replaced by the CHP plant,Ėch cog ‒ flux of the chemical energy of fuels consumed in the

CHP plant.


The consumption of chemical energy in a replaced heating plant and a replaced power plant is calculated from the following relations applying the values of efficiency of reference unit accor-ding to the Directive 2004/8/EC [1,2]:

(2)

(3)

where:Q͘c cog ‒ demand for the flux of cogenerated heat by the consu-

mers,EUFhp G ‒ gross energy utilization factor of the replaced (referen-

ce) heating plant, η'ht ‒ efficiency of transporting the heat concerning the he-

ating plant,εh hp ‒ index of internal consumption of heat concerning the

heating plant,Nel c cog ‒ demand for cogenerated electric power by the consu-

mers, EUFpp N ‒ net energy utilization factor of electricity production of

the replaced (reference) power plant, η'et ‒ efficiency of electricity transmission from the replaced

power plant.

The flux of the chemical energy of fuels consumed in the cogeneration part in a CHP plant results from the equation:

(4)

where:EUFcog G ‒ gross energy utilization factor of a CHP plant,εh cog ‒ index of internal heat consumption in a CHP plant, ηht ‒ efficiency of transporting the heat from a CHP plant,εel cog ‒ index of internal power consumption concerning

a CHP plant, ηet ‒ efficiency of electricity transmission from a cogenera-

tion plant.

Assuming that the efficiencies of transporting the heat and electricity are the same in a CHP plant and separate production of heat and electricity, the flux of the chemical energy of fuels saved thanks to cogeneration results from the equation:

(5)and

(6)

(7)

where:Q͘c ‒ total demand for the flux of heat by the consumers,αcog ‒ coefficient of the share of cogeneration,σ ‒ power to heat ratio,Nel cog G ‒ gross cogenerated electric power,Q͘cog G ‒ gross cogenerated flux of heat.

Formula (5) constitutes the base for a comparison of the following variants of the cogeneration systems:• referencesystem(CHPplantwithanextraction-back-pres-

sure turbine),• CHPplantintegratedwithaCO2 processing unit (CPU) and

an installation of waste heat recovery (back-pressure turbine operating at elevate back-pressure in comparison with the reference system).The Eqs. (1)-(7) constitute the algorithms concerning clas-

sical CHP units. These algorithms have been applied in the case of two variants mentioned above.

Reference CHP plant

A reference system in a comparative analysis is a CHP plant with an extraction-back-pressure turbine (Fig. 3) with the coefficient of the share of cogeneration αcog = 0.665.

The basic part of the demand for heat is covered by the basic heat exchanger (BHE) whose share in covering the ma-ximum demand amounts to 0.5. The subpeak part of the ma-ximum heat demand is covered by the peak heat exchanger (PHE) fed with extracted steam, the share of which in covering this demand amounts to 0.17. The remaining part of the maxi-mum heat demand amounting to 0.33 is covered by the peak water heater.

The gross energy utilization factor [11] of a CHP plant with an extraction-back-pressure turbine is expressed by the equ-ation [17]:

Fig. 2. Classical CHP plant with a back-pressure turbineFig. 1. Separate production of heat and electricity

Ėch hp

Ėch pp

WH

Nel

Q̇

Ėch cog

Nel

Q̇


(8)

where:σref ‒ power to heat ratio in the reference CHP plant,ηE sb ‒ energy efficiency of the steam boiler,ηme ‒ electromechanical efficiency of the turbogenerator.

The results presented in Fig. 4 are obligatory for the following input data: ηht = 0.95; εh hp = 0.015; EUFhp G = 0.88 [1,2]; σref = 0.48 ; ηme = 0.97; ηel cog ref = 0.11; ηh cog ref = 0.025; EUFpp N = 0.454; ηme = 0.97; ηE sb = 0.92. The share of cogeneration (abscissa in Fig. 4) should be calculated basing on the procedure of optimizing or applying adequate empirical equations [19, 20].

-CHP plant integrated with a CO2 processing unit and installation of waste heat recovery

In the case of an integrated CHP plant with a CO2 proces-sing unit and installation of waste heat recovery the cogenerated part is a back-pressure turbine with parameters of back-pressure steam elevated to a level corresponding to the parameters of extracted steam in the reference system. The basic demand for heat is covered by the recovered waste heat from the humidity separator in the installation of the chemical absorption of CO2 and the interstage cooling system of CO2 compressors. The re-covered heat is charged with the consumption of chemical ener-gy similarly as the basic part of heat produced in the reference system. In this case the principle of replaced process has been applied based on the partial energy efficiency of the basic part of heat production [9]. Fig. 5 presents the calculation diagram of CHP plant integrated with CO2 processing unit and waste heat recovery installation.

Equation (1) in the case of integrated CHP plant with waste heat recovery has the form:

(10)

where the respective items denote, analogically as in Eq. (1), the consumption of chemical energy in separate and cogenerated systems equipped with a CPU unit with waste heat recovery.

In the case of the separate production of heat and electricity we have:

(11)

(12)

Fig. 3. Schematic diagram of reference CHP plant; BHE – basic heat exchanger; PHE – peak heat exchanger; PWH – peak water heater;

Q͘DHS – demand for heat in district heating system

Substituting Eq. (8) in Eq. (5) and assuming that ηet = η'et as well as ηht = η'ht , we get:

(9)

The non-dimensional equation (9) is a convenient mathe-matical form for analyzing the energy effects of heat and elec-tricity cogeneration. The index of energy savings is the most adequate measure of effectiveness of heat and electricity coge-neration. The greater share of cogeneration, the higher are the values of the index of energy savings of chemical energy of fuel. Fig. 4 presents the index of savings of the chemical energy of fuels as a function of the share of cogeneration. In this paper savings of the chemical energy of fuels related to the production of heat have been preferred which is justified in large-scale CHP plants where usually heat is the main product. Then electricity is the by-product charged by the consumption of fuel based on the principle of the replaced process (separate production of electri-city in reference power plant). This approach is called “method of equivalent power plant” [16, 17] and is legitimized in the USA as a lawful act known as PURPA (Public Utility Regulatory Poli-cies Act) [21, 18]. An explication of this approach is the exergy method in the context of evaluating the true value of heat on the different levels of thermal parameters [22, 16]. Such an approach to evaluate the energy effectiveness of heat and electricity co-generation does not exclude the index of PES (Primary Energy Savings) preferred by the Directive on the promotion of cogene-ration [1, 17].

Fig. 4. Index of the savings of the chemical energy of fuel

0.50 0.55 0.60 0.65 0.70 0.75Share of cogeneration αcog

0.40

0.35

0.30

0.25

0.20

( )–ΔĖch

Q̇c


and

(13)

where:EUF int

hp G ‒ gross energy utilization factor of a heating plant inte-grated with a CPU and waste heat recovery installa-tion,

EUF intpp N ‒ net energy utilization factor of a power plant integra-

ted with a CPU and waste heat recovery installation, ͘Qreg cog ‒ flux of heat for regeneration of the solvent in the case

of a CHP plant, ͘Qrec cog ‒ flux of recovered waste heat from a CPU unit in a

CHP plant, σint ‒ power to heat ratio of an integrated CHP plant with

waste heat recovery (σint = σex , because the outlet pa-rameters of back-pressure steam of integrated CHP are equal to extracted parameters in the reference system),

σex ‒ power to heat ratio of extracted steam of the referen-ce CHP plant.

The numerator in Eq. (12) denotes the demand for po-wer by the consumers provided by the cogeneration system integrated with a CO2 processing unit and installation of waste heat recovery. The same power must be delivered from an po-wer plant integrated with CO2 processing unit and installation of waste heat recovery in the case of separate production of electricity.

In order to determine the gross energy utilization factor of the heating plant integrated with CPU and waste heat recovery installation it has been assumed that heat demand for regenera-tion of the solvent is produced in the steam boiler and electrici-ty of driving the CO2 compressors, fans and pumps is provided from the system power plant integrated with CPU and installation of waste heat recovery:

(14)

where:ηE wh ‒ energy efficiency of the water heater,ηE sb ‒ energy efficiency of the steam boiler, Q͘reg hp ‒ flux of heat for regeneration of the solvent in the case

of a heating plant, Q͘rec hp ‒ flux of recovered heat from a CPU unit in the heating

plant, EUF int

el hp ‒ electric power concerning an integrated heating plant (consumption of electricity for compressing of CO2 and driving fans and pumps in a CPU).

In Eqs. (12), (13), (14) only the cogeneration part of heat is taken into account. It was assumed that peak part of heat for di-strict heating system in CHP plant and heating plant is produced in the boilers with the same energy efficiency.

The flux of heat for the regeneration of the solvent results from the equation:

(15)

where:Ṗhp ‒ the flux of coal feeding the replaced heating plant integra-

ted with a CPU unit and waste heat recovery, kg/s,q̅reg ‒ unit consumption of heat for the regeneration of the so-

lvent, MJ/kg of coal.

Electricity power consumed by a CPU unit:

(16)

where:ecom ‒ unit consumption of electricity for CO2 compressors, MJ/kg

of coal,eFP ‒ unit consumption of electricity for driving the fans and

pumps in the CPU unit, MJ/kg of coal.

Introducing (15), (16) in (14) we get:

(17)

Formula (17) is applied in calculations of useful effects re-sulting from the realization of cogeneration.

Fig. 5. Calculation diagram of the back-pressure CHP plant integrated with a CO2 processing unit (CPU); FPC – fans, pumps and compressors; BHE – basic heat exchanger; PHE – peak heat exchanger; PWH – peak water heater; sb – steam boiler; fg1 – flue

gasses from the boiler; fg2 – flue gasses leaving CPU (the so-called vent); Nel FPC – power driving fans, pumps and compressors;

Q͘reg – heat flux for regeneration; ͘Qrec – heat flux from waste heat recovery installation; Ėch – flux of chemical energy of fuel


The flux of the chemical energy in an integrated CHP plant with a CO2 processing unit and waste heat recovery installation is calculated from the equation:

(18)

The gross EUF of an integrated CHP plant with waste heat recovery is described by the equation:

(19)

Applying the similar equation as (15) in the case of inte-grated CHP plant the relative flux of heat for regeneration of the solvent results from the equation:

(20)

Basing on general equations (11), (12), (13) and (18), the ratio of savings of chemical energy of fuels to the demand for heat by the consumers takes the form:

(21)

The some input data were obtained from simulation calcu-lations using professional code Thermoflex [18]. Non-dimensio-nal equation (21) allows convenient multivariant calculations of the energy effects of cogeneration heat and electricity.

Results of the analysis of chemical energy savings in a CHP plant integrated with CPU

and waste heat recovery installation

Basing on algorithms presented in sections 3 and 4 the cal-culations of savings of chemical energy of fuels concerning the analyzed variants of cogeneration units have been realized.

The following sets of input data have been assumed [23]:• referenceCHPplant

αcog = 0.665; ηht = η'ht = 0.95; εh hp = 0.015; EUFhp G = 0.88 [1,2]; σref = 0.48; ηme = 0.97; εh cog ref = 0.025; ηE b = 0.92; EUFpp N = 0.454; εel cog ref = 0.11.

• CHPplantintegratedwithaCPUunitandinstallationofwa-ste heat recoveryηE wh = 0.88; ηE sb = 0.9; LHV = 23 MJ/kg; ηet = η'et = 0.9; εel cog = 0.215 [9]; σex = 0.464 [9]; ecom = 0.668 MJ/kg of coal; eFP = 0.24 MJ/kg of coal.

Some results obtained by simulating calculations applying the Thermoflex code [18] have been gathered in Table 1. They are used to calculate energy savings in CHP plants integrated with a CO2 processing unit and waste heat recovery installations.

Fig. 6. Relative savings of chemical energy of fuels

Fig. 6 illustrates the results of calculations of the savings of chemical energy of fuels concerning the analyzed CHP plants: refe-rence unit and CHP plant integrated with a CO2 processing unit and installation of waste heat recovery. The indices of relative energy savings of the chemical energy of fuels are similar in the case of the reference CHP plant and integrated CHP plant with a CO2 pro-cessing unit and installation of waste heat recovery. The chemical energy consumption in CHP plants, both reference plant and a CHP plant integrated with a CO2 processing unit and installation of waste heat recovery as well as a heating plant and systems power plant are calculated in the same conditions concerning the integration with CO2 capture and compression. The characteristic values of co-generation, viz. coefficient of the share of cogeneration and power to heat ratio are the same or nearly the same (power to heat ratio) in reference and integrated CHP plants. This explains the similar values of energy savings. Slightly lower values of relative savings of chemical energy of fuels going together with the diminishing unit consumption of heat for regenerating the solvent result from the lo-wer cogeneration of heat and electricity.

Systems effects of partial compensation of the increased internal consumption

of heat thanks to cogeneration

This problem can be very simply explained on the example of heat losses from the district heating network [17]. The losses of heat during its transport from the heating plant or CHP unit to consumers leads to an increase of the gross production of heat and in consequence to an increase of fuel consumption. In the case of heating plant this dependence is quite simple:

(22)

where:ΔEch hp ‒ increase of the chemical energy of fuels in the heating

plant due to heat losses in the district heating network,Qloss ‒ heat losses in the district heating network, EUFhp ‒ energy utilization factor of the heating plant supplying

district heating network.

4.0 3.4 3.15Reference CHP plant

0.4

0.3

0.2

0.1

0.0

–ΔĖch

Q̇c

Integrated CHP plant with heat recoveryqreg

MJkg CO2

0.3810.342 0.323 0.308


If, however, the heat is produced in a CHP plant, heat los-ses in the district heating network also lead to an increase of fuel consumption but simultaneously influence the increase of the production of electricity in the cogeneration process. In this way we can partially compensate the increase of fuel consump-tion because in condensing power plant (separate production of electricity) the savings of the chemical energy of fuel has take place. This is a system effect of the cogeneration. Therefore, we can write:

(23)where:ΔEch sys ‒ the systems increase of the chemical energy of fuel

consumption due to the of heat losses in the district heating network thanks to cogeneration,

ΔEch CHP ‒ the increase of the chemical energy of fuel consump-tion in the CHP plant due to heat losses on district heating network,

‒ ΔEch pp ‒ the decrease of the chemical energy of fuel consump-tion in the replaced power plant thanks to additional cogeneration of electricity in the CHP plant.

The increase of the consumption of the chemical energy of fuel in a CHP plant is as follows:

(24)

where:ΔEel cog ‒ additional cogeneration of electricity due to cogenera-

tion of heat covering the heat losses in district heating network,

EUF CHP ‒ energy utilization factor of a CHP plant.

The additional cogeneration of electricity leads to savings of the chemical energy of fuel in the replaced power plant

(25)

where EUFpp denotes gross energy utilization factor of the repla-ced power plant.

In Eq. (25) it is assumed that the efficiency of electricity transmission and relative internal power consumption are the same in the replaced power plant and the CHP unit.

Introducing in Eq.(23) Eqs. (24), (25) and dividing Eq. (23) by Eq.(22) we get:

(26)

where σ denotes power to heat ratio.Fig. 7 presents the result of the relative reduction of the in-

crease of the chemical energy of fuels due to heat losses thanks to realization of heat and electricity cogeneration in relation to the increase of chemical energy consumption in the heating plant. The following data have been assumed: EUFhp = 0.88; EUFCHP = 0.8; EUFpp = 0.44. The result in Fig. 7 shows that the ra-tio of system increasing the chemical energy of fuel to the incre-ase of the chemical energy of fuel in a heating plant due to heat losses in the district heating network is lower than 1 and drops with the growing power to heat ratio. This is the system effect of partial compensation of an additional consumption of fuel due to heat losses in the transport of heat thanks to cogeneration. Final-ly, we can say that the heat losses in the district heating network are less severe in the case of heat and electricity cogeneration.

A similar approach we can apply in the case of integration of a CHP plant with CO2 processing unit and installation of waste heat recovery. This integration is connected with the demand for an additional amount of heat in order to regenerate the solvent in post-combustion amine installation of CO2 removal. Simulta-neously, however, some part of heat for district heating system is produced in the installation of waste heat recovery. Thus, on the one hand, an additional flux of heat for regeneration of the solvent leads to the additional production of electricity in coge-neration. But, on the other hand, waste heat recovery influences the decrease of cogenerating the electricity. It is also important that the integration of a CHP plant with a CO2 processing unit is

Table 1. Input data obtained from simulations calculus

Parameter Unit Value

Unit consumption of heat for regenerating purposesqr – MJ/kg of removed CO2

4.0 3.4 3.15

q̅r – MJ/kg of coal 7.833 6.658 6.168

Net energy utilization factor of an integrated power plant ‒ EUF intpp N

– 0.416 0.427 0.432

Ratio of the heat of regeneration to the demand for heat by consumers ‒ – 0.485 0.396 0.361

Ratio of recovered waste heat to the demand for heat by consumers ‒ – 0.332 0.268 0.246

– 0.209 0.177 0.163

Ratio of coal consumption in the heating plant to the demand for heat by consumers ‒

kg/MJ 0.0403 0.0393 0.0389


connected with an increase of internal power consumption for driving the fans and pumps and first of all CO2 compressors in CO2 processing unit. So, this problem is more complicated than presented above.

Fig. 8 presents the ratio of the increase of the consumption of chemical energy of fuels concerning an integrated CHP plant in relation to the reference CHP plant. The maximum value of this ratio was obtained in the case when the unit consumption of heat for the regeneration of the solvent amounts to 4 MJ/kg of remo-ved CO2. This ratio decreases and achieves its minimum at the value of unit consumption of heat for regeneration of the solvent amounting to 3.15 MJ/kg of removed CO2 which is the base for calculations of the relative increase of the chemical energy of fuel in the integrated CHP plant. The unfavourable results concerning qr = 4.0 and 3.4 in comparison with qr = 3.15 can be partially com-pensated because simultaneously additional electricity is produ-ced in cogeneration with heat for the regeneration of the solvent. This additional production of electricity is, however, lower than the sum of the decreased electricity production in cogeneration with heat for the district heating system due to waste heat reco-very from the CO2 processing unit and the internal consumption of power for driving the fans and the pumps in CPU unit as well as compressors of CO2. For this reason the net production of electricity is decreased in comparison with the reference power plant, but with the growing demand for heat for the purpose of regeneration of the solvent this decrease is lower (Fig. 9). It is exactly the effect of partial compensation of additional consump-tion of heat in the CO2 processing unit. The greater the demand for heat for regeneration purposes (Fig. 8), the greater is the ad-ditional production of electricity in cogeneration, and therefore the greater is the effect of compensation, viz. reduction of the decrease of the net production of electricity and in consequence the decrease of the consumption of the chemical energy of fuel in the systems power station. This does not mean that the greater the heat for regeneration, the better. We may only speak of a less severe consequence of the great heat for regeneration of solvent in a CO2 processing unit if it is integrated with a CHP plant. This is the system effect of cogeneration.

A similar effect occurs in the case of trigeneration techno-logy with an absorption cooler [24]. The factor of increasing the cogeneration effect is the greater, the lower COP of the absorp-tion cooler. This does not also mean that the worse the COP, the better, but only that the integration of the absorption cooler with a CHP plant causes a partial compensation of greater driving heat of the absorption cooler (lower COP).

Conclusions

The integration of a CHP plant with a CO2 processing unit requires some supplementing of the algorithms concerning the thermodynamic indices. The saving of chemical energy of fuels thanks to cogeneration is a very important index. The modified algorithms was elaborated for integrated CHP plants with CO2 processing units and waste heat recovery installations. The re-sults of evaluation were compared with the reference CHP plant without a CO2 processing unit.

Integration of a classical CHP plant with a CO2 processing unit and installation of waste heat recovery does not worsen the energy effectiveness of cogeneration. These energy effective-ness may be considered to be similar.

Fig. 7. Relative reduction of increase the chemical energy of fuels due to heat losses thanks to realization of heat

and electricity cogeneration

Fig. 8. Relative increase of the chemical energy consumption in relation to the reference CHP plant; ΔEch min concerns unit

consumption of heat for regeneration amounts to 3.15 MJ/ kg of removed CO2; index “i” concerns 4.0, 3.4 and 3.15, respectively

Fig. 9. Relative decrease of power in relation to the reference power plant; | ‒ΔNel | max concerns unit consumption of heat for regeneration amounts to 3.15 MJ/ kg of removed CO2;

index “i” concerns 4.0, 3.4 and 3.15, respectively

4.0 3.4 3.15

6.0

5.0

4.0

3.0

2.0

1.0

0.0

ΔEch i

ΔEch min

Integrated CHP plant with heat recoveryqreg

MJkg CO2

5.36

2.14

1

qregMJ

kg CO2

4.0 3.4 3.15Integrated CHP plant with heat recovery

|–ΔNel |i

|–ΔNel |max

1.0

0.8

0.6

0.4

0.2

0.0

1

0.79

0.19

0.90

0.80

0.70

0.60

ΔEch sys

ΔEch hp

0.30 0.40 0.50Power to heat ratio σ


In the cogeneration system a partial compensation of the increased consumption of fuels takes place due to integra-tion with CO2 processing unit. It is the system effect resulting from the production of electricity in cogeneration with heat for regenerating purposes concerning the solvent. This elec-tricity replaces electricity production in system power plant. The results concerning partial compensation of increasing the internal consumption of heat confirm the opinion deal with possibility of effective integration of CHP plants with CO2 processing units in comparison with separate production of electricity and heat.

A similar way of this analysis can be applied in the case of power stations adapted for heat production and integrated with CO2 processing units. This analysis may be based on some results obtained in [10] concerning the energy indices of power plant adapted for heat production and integrated with the CO2 processing unit.

Acknowledgements

The results presented in this paper were obtained from re-search work financed by the National Centre of Research and Development in the framework of Contract SP/E/1/67484/10 – Strategic Research Program – Advanced technologies for energy generation – Development of technology for highly effi-cient zero-emission coal-fires power units integrated with CO2 capture.

REFERENCES

[1] Directive 2004/8/EC of the European Parliament and of the

Council of 11 February 2004 on the promotion of cogeneration

based on a useful heat demand in the internal market and amen-

ding Directive 92/42/EC.

[2] Guidelines for Implementation of the CHP Directive 2004/8/EC.

Guidelines for implementation of Annex II and Annex III. Europe-

an Commission GD TREN. Draft Final November 2006.

[3] Ogawa T., Ohashi Y., Yamanaka S., Miyaike K.: Development

of carbon dioxide removal system from the flue gas of coal

fired power plant, Energy Procedia, Vol. 1, Issue 1, 2009,

721-724.

[4] Mangalapally H. P., Hasse H.: Pilot plant experiments for post

combustion carbon dioxide capture by reactive absorption with

novel solvents, Energy Procedia, Vol. 4, 2011, 1-8.

[5] Global CCS Institute, CO2 capture technologies, Post combu-

stion capture - January 2012, last accessed 21.11.2013,

cdn.globalccsinstitute.com/sites/default/files/publica-

tions/29721/co2-capture-technologies-pcc.pdf

[6] MHI’s energy efficiency flue gas CO2 capture technology and

large scale CCS demonstration test at coal-fired power plants

in USA, Mitsubishi Heavy Industries Technical Review Vol. 48

No. 1, March 2011.

[7] Final Report - CASTOR (CO2, from Capture to Storage), Project

report, April 2011, last accessed 21.11.2013, www.cordis.euro-

pa.eu/documents/documentlibrary/124772011EN6.pdf

[8] Lund H., Vad Mathiesen B.: The role of Carbon Capture and

Storage in a future sustainable energy system, Energy 44 (2012)

469-476.

[9] Ziębik A., Budnik M., Liszka M.: Thermodynamic indices asses-

sing the integration of coal-fired CHP plants with post-combu-

stion CO2 processing unit (CPU). Energy Conversion and Ma-

nagement 73 (2013), pp. 389-397

[10] Ziębik A., Budnik M., Liszka M.: Analysis of energy indices of

a power plant adapted for the production of heat integrated with

the amine CO2 processing unit. Journal of Cleaner Production

(in press) 2014

[11] Horlock J.H.: Cogeneration- Combined Heat and Power (CHP).

Thermodynamics and Economics, Krieger Publishing Company,

Malabar Florida, 1997.

[12] Kuramochi T., Ramírez A., Turkenburg W., Faaij A.: Techno-

-economic prospects for CO2 capture from distributed energy

systems, Renewable and Sustainable Energy Reviews, Vol. 19,

2013, 328-347.

[13] Kuramochi T., Faaij A., Ramírez A., Turkenburg W.: Prospects

for cost-effective post-combustion CO2 capture from industrial

CHPs, International Journal of Greenhouse Gas Control, Vol. 4,

Issue 3, 2010, 511-524.

[14] Mago P. J., Smith A. D.: Evaluation of the potential emissions

reductions from the use of CHP systems in different com-

mercial buildings, Building and Environment, Vol. 53, 2012,

74-82.

[15] Uddin S.N., Barreto L.: Biomass-fired cogeneration systems

with CO2 capture and storage, Renewable Energy, Vol. 32, Is-

sue 6, May 2007, 1006-1019.

[16] Szargut J., Ziębik A.: Fundamentals of thermal engineering

(in Polish), PWN, Warsaw, 2000

[17] Szargut J., Ziębik A.: Cogeneration of heat and electricity –

CHP (in Polish). Polish Academy of Sciences, Katowice-Gliwice

2007.

[18] Thermoflex version 22, Thermoflow Inc. www.thermoflow.com

[19] Ziębik A., Gładysz P.: Optimal coefficient of the share of co-

generation in district heating systems. Energy Vol. 45, Issue 1,

September 2012, pp. 220-227.

[20] Gładysz P., Ziębik A.: Complex analysis of the optimal coefficient

of the share of cogeneration in district heating systems. Energy

62 (2013), pp. 12-22.

[21] Danziger R.: Cogeneration technology. The 1991 National Ener-

gy Management Forum – June 24 and 25, 1991, Melbourne,

Australia.

[22] Sama D.A.: Looking at the True Value of Steam. Oil and Gas

Journal, April 14, 1980.

[23] Ziębik A., Budnik M., Liszka M.: Savings of the chemical ener-

gy of fuel in CHP plant integrated with CO2 processing unit

(in Polish). Section 26 in the monograph “Analysis of energy

systems” (Eds. Weglowski B., Duda P.) Technical University of

Cracow, 2013.

[24] Ziębik A., Hoinka K.: Energy systems of complex buildings.

Springer-Verlag London, 2013.

energy effectiveness of heat and electricity cogeneration ... · may be used for preheating the...

Documents