Minimization of exergy loe in combution procee with an illutration of a membrane combution Markku J. Lampinen*, Ralf Wikten, Arto Sarvi, Kari Saari and Marjut Penttinen Aalto Univerity, Department of Energy Technology, Applied Thermodynamic, Sähkömiehentie 4J, P.O. Box 14400, FI-00076 Aalto, Finland * Correponding author. Tel: +358-9-47023582, +358-500-448418, E-mail: Markku.Lampinen@tkk.fi Abtract: The efficiency of internal combution engine and ga turbine procee are free from Carnot limitation a they are not performing cycle procee the initial tate of the thermodynamic ytem i the fuel with air, wherea the final tate i the flue ga, whoe chemical compoition i different than fuel and air. Therefore, a we how here, the theoretical thermodynamic efficiencie of ideal combution engine and ga turbine procee can be very high, the ame a it i for fuel cell. The entropy generation analyi, what we have done for the internal combution engine and ga turbine, how that they uffer for relatively low efficiencie becaue of the exergy loe in the combution procee, i.e. for the reaon that the combution reaction take place quite far from the equilibrium tate. We have tudied everal different combution procee in the Exergyproject of MIDE-program to find a method for decreaing the entropy generation in the combution. If the entropy generation can be reduced, by any mean, then a a reward, we get the outlet preure of the flue ga higher than the inlet air preure without uing any compreor which in turn would then increae the efficiency eentially. We preent here a theoretical decription of a membrane combution method which, with the aid of gaification, uit for bioenergy. Shortly, it can be decribed a a molecular cale oxygen ga compreor driven by the combution reaction, where the affecting force i amplified by the electric field acro the membrane. Keyword: Entropy, exergy, combution, membrane, efficiency, combution engine, ga turbine proce 1. Introduction In adiabatic combution proce the outlet temperature of the flue ga depend on the air factor λ and the fuel. It doe not depend on the preure a far a we conider the flue ga a an ideal ga, becaue then the pecific enthalpy of the pecie (i) depend only on i t temperature: hi = hi (T ). In very high preure near to the critical preure, where the ideal ga aumption i no longer valid, the preure affect alo on the pecific enthalpy. Hence, the preure of the outflow ga doe not follow from the energy balance, but it depend on the manner how the combution proce i realized. So we need the econd law of thermodynamic to analyze thi. The pecific entropy of the ga pecie (i) depend on the temperature T, and alo on it partial preure p i, i.e. according to the ideal ga model i ( T, pi ) = i ( T, p0 ) R ln( pi / p0 ), which how that the higher the preure p i the maller i the entropy i. Here the reference preure p0 = 1 bar and the ga contant R = 8.314 J / molk. The entropy generation in the adiabatic combution i = nii n j j 0, (1) out in from which we ee that the higher i the outlet preure, the maller i the entropy generation Intead of the entropy generation we may a well peak of the exergy lo defined a. T ( ) = exergy lo, (2) 133
which decribe the lo of mechanical work in chemical combution reaction, or the lo to increae the preure of the flue ga by the chemical reaction. Temperature T ( ) i defined with the aid of the real final tate (B) and the ideal ientropic final tate B ) a follow [1]: H ( B ) H ( B) T( ) (3) S( B ) S( B) Uually the flue gae from the combution chamber in the ga turbine procee flow out approximately at the ame preure a the inlet flow of the air, and we peak then about combution at contant preure. In our earlier paper [2] we have hown that in the conventional combution with contant preure the entropy generation i very high and the exergy lo ( T( ) ), depending on the air factor and the fuel, i about half of the heat value of the fuel. In the claical form of the Guoy-Stodola, the exergy lo i defined a To, where T o i the lowet temperature of the urrounding with which the ytem i in thermal contact. A we have hown in our earlier paper [2] the choice of the temperature T o for Eq.(2) doe not give the accurate value for the lot of the work and for the efficiency of combution engine (for Eq.(4) below), only an upper limit, and therefore, a hown in [2] we ue the correct temperature T ( ) intead of T o. 2. Ientropic combution The ideal adiabatic combution proce i uch that there i no entropy generation, = 0. In the language of thermodynamic it i called an ientropic combution, and it mean a combution proce which proceed via equilibrium tate. In the following we dicu how important for the efficiency it i to keep the entropy generation a mall a poible in order to have a good efficiency. 2.1. Combution engine For the combution engine the following general equation for the efficiency can be derived [1] T( ) η = 1 (4) [ H ( B ) H ( A)] where i the generation of entropy in the whole combution engine proce and [ H ( B ) H ( A)] i the heat value of the ientropic combution proce from A to B, which i defined o that S ( A) = S( B ). In the denominator of Eq.(4) there i the enthalpy difference [ H ( B ) H ( A)] becaue our ytem i a cloed ytem which make tranformation proce in the engine during 720 o degree of rotation of the crank haft. The entropy generation (J/K) mean correpondingly the entropy generation during 720 o degree of rotation. ( 134
We tudied [1] the exergy loe of the dieel engine proce, hown in Fig.1, and we found that 79% of all exergy loe took place between the combution proce tep 4-5-6-7. The whole efficiency of the dieel engine wa 47.5%. Therefore, if we could eliminate the exergy loe of combution, then the exergy loe left were 21% x 52.5% = 11% and the efficiency of the dieel engine would be η = 89%. It can be o high a the theoretical efficiency can be even one a we ee from Eq.(4). The reaon for that i that the efficiency i not limited by Carnot formulae becaue the proce i not a cycle proce. The efficiency i under the ame type of limitation a fuel cell, but for the combution engine the reference proce i an ideal reverible adiabatic proce wherea for the fuel cell it i an ideal reverible iothermal proce. Therefore, the maximum work out here i H ( S = cont), wherea for the fuel cell it i G ( T = cont, p = cont). How would then the ideal proce without any exergy loe in the combution tep look like compared to Figure 1a? Firt of all, the engine would be then a two troke engine, but without having any dead volume. Suppoe firt that the piton i at left (with volume=0) and the hot flue ga tart to fill the cylinder by puhing the piton to the right in Fig.1b by contant preure (ay at 230 bar a in Fig.1a) from the point 4* to the point 5*. At that point the inlet valve i cloed and the ientropic expanion proce tart, during which the preure and the temperature become lower. The length of the piton troke i uch that the point 9* i reached. After that the outlet valve i opened and the flue ga i flowing out to the turbocharger at contant preure (3 bar in Fig.1a and 1b). All the flue ga i puhed off at contant preure to the zero dead volume and then the proce i repeated by filling the cylinder again by hot flue ga with a high preure. The turbine unit of the turbocharger i rotating the haft of the compreor which i feeding air into the pecial external membrane combution chamber. That external membrane combution chamber i aumed to produce the flue ga out at high preure without any remarkable additional work (only the compreion work for fuel feeding) and theoretically with zero entropy generation. The efficiency of thi type of a combution engine hown Fig.1b i much higher than the engine in Fig.1a becaue the compreion work (3-4 in Fig.1a) i not needed and becaue the expanion (point 8) continue down to 3 bar wherea it top now (Fig.1a) at 12 bar, which mean that more expanion work can be taken out to the crank haft by the proce. 135
Fig. 1a. Turbocharged dieel engine proce. Combution take place between 4-5-6-7. 1b. Turbocharged external membrane combution engine. Combution only between 2*-4*. 2.2. Ga turbine proce Alo for the ga turbine procee the exergy loe in the combution chamber are of crucial importance. In a methane ga driven power plant in Finland the ga turbine give out 94 MW of haft power with the flue ga inlet at 1100 o C and preure 11 bar (ab). The compreor driven by the ga turbine take 54MW which mean that the haft power delivered to the generator i 94 MW 54 MW = 40 MW. By reducing ufficiently the entropy generation in the combution unit the outlet preure of 11 ba r could be achieved without uing the compreor at all, and thu the whole turbine power could be tranferred to the generator which of coure, would increae the efficiency eentially. The principal power proce baed on the ue of the theoretical ientropic combution chamber i hown in Fig.2. Fig. 2. Illutration of an ideal theoretical ga turbine proce where the combution take place ientropically [1]. Figure 2 how the theoretical limit for the ga turbine proce if the entropy generation i zero in the combution chamber and alo in the turbine. If = 0 in the combution chamber, then a hown in Fig.2, the outlet preure will be a high a p out = 746 bar. Thi i, of coure jut a theoretical number, but it how that there i a great potential to improve the conventional combution proce. For intance, to achieve preure ratio p / = 11 in the out p in 136
combution chamber with carbon a fuel and with λ=1, we need to decreae entropy generation from = 366 J / molk only to = 270 J / molk. In the following we will hortly dicu how we could do thi by a membrane combution. 3. Principle of emipermeable membrane combution Ceramic membrane made of yttria-tabilized zirconia (in Fig.3: ZrO 2 /Y 2 O 3 ) ha the property that in ufficient high temperature (800-1000 o C) they tart to conduct oxygen ion (O 2- ). The ionic conductivity depend on the amount of yttria in the tructure. Approximately half of the amount of yttria atom in the crytalline tructure can offer vacancie to be occupied by hopping oxygen ion. Thee material are well known from Solid Oxide Fuel Cell and from lamda-enor ued in car for meauring the oxygen concentration in flue gae. A hown in Fig.3, the oxygen ga i conumed on the urface of the combution ide and therefore, the partial preure of oxygen ga become there lower than on the urface of the airide. Becaue of the ionic form of oxygen, the difference of partial preure generate a potential difference, which can be etimated by Nernt equation (ee Fig.3). The electric field performed by the potential difference i the driving force for the oxygen tranport through the membrane. Depending on the concentration of oxygen ion, the electric volumetric force field (ion charge denity x electric field, N/m 3 ) can be amplified to everal magnitude higher than the partial preure gradient of oxygen ga. Fig. 3. Semipermeable oxygen ga membrane. Electric potential difference acro the membrane i illutrated by auming that the partial preure of oxygen are 0.21 bar (on the air ide) and 0.1 bar (on the flue ga ide). The total preure of the flue ga p(b) i kept higher than the total preure of air p(a) by adjuting the outlet flow of flue ga accordingly. 137
Recently, it ha been tudied the ionic and electronic charge tranport for ingle crytal of yttria-tabilized zirconia with additional nitrogen doping, e.g. [3] and [4]. At temperature above 850 o C, even in the preence of a very mall oxygen concentration in the urrounding ga phae, the nitrogen ion dopant become highly mobile, and thu diffue to the urface where it i oxidized to gaeou N 2 (g). The technical motivation for that tudy [3] ha been to achieve ufficient nitrogen ion conductivity for the development of a nitrogen enor or nitrogen pump. In the membrane combution the driving force for the nitrogen tranportation come alo from the electric field generated by the oxidation of the fuel. The flow of nitrogen ga lower the temperature of the membrane, which merely by oxygen combution would be too high. A contruction uing nitrogen and oxygen ga emipermeable membrane i hown in Fig.4. Fig. 4. Hollow ceramic fiber ued in the membrane combution. Fuel (here the methane ga) i fed in the fiber and air outide of the fiber. 4. Dicuion We have dicued here that the entropy generation in the combution procee i the crucial point that reduce the efficiencie of combution engine and ga turbine procee. We have preented here an illutration of a membrane combution method which, with the aid gaification i quite uitable for bioenergy, and which can reduce the entropy generation by preurizing the combution chamber without uing any external work. Shortly, it can be decribed a a molecular cale ga compreor driven by the combution reaction, where the affecting force i amplified by the electric field acro the membrane. Acknowledgement Thi tudy wa upported and financed by the MIDE-program of Aalto Univerity. We wih to thank prof. Yrjö Neuvo and Dr. Sami Ylönen for the kind upport to thi work. The author wih to acknowledge alo prof. Frank Petteron and prof. Ron Zevenhoven from Åbo Akademi for many ueful dicuion. 138
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