The Evaluation of the Oxycombustion for Power Cycles

Transcription

1 5th WSEAS Int. Conf. on ENVIRONMEN, ECOSYSEMS and DEVELOPMEN, enerife, Sain, December 14-16, he Evaluation of the Oxycombustion for Power Cycles GHEORGHE DUMIRASCU Engineering hermodynamics, hermal Machines, and Refrigeration Deartment "Gh. Asachi" echnical University of Iasi, Blvd. D. Mangeron, 59 61, 75-Iasi, ROMANIA Abstract: he sustainable energy systems require zero emissions into environment. he main emission s comonent is CO that contributes mainly to the global warming. he goal of zero emissions follows two ways: reduction of carbon intensity factor, e.g. Clear Sky initiative in USA, and the cature of CO, resectively the Euroean move towards. On a global scale, the growth of oulation will lead to exansion of needs that will cause an increasing imact on environment by exhausting consume of resources, and raw materials, accomanied by a much more use of energy and these will ruin intensively the environment by harmful emissions. he CO cature from fossil fuelled ower lants is technically feasible, but it nevertheless remains to find viable solutions for actual retrofitted or new large-scale ower generation systems, workable economically and environmentally. he aer seeks to evaluate the cycles based on atmosheric/ressurised oxy-combustion in recycled/rich CO medium, that have the benefit to use the high efficient combined cycle technology. he oxy-combustion has the advantage to suly a very rich in CO exhaust flue gases stream, allowing consequently a more effectual CO cature&sequestration. Power lant rocesses based on ressurised oxy-combustion, roducing enriched CO flue gas ready for disosal, are neither demonstrated on a ilot scale nor have the necessary comonents tested out in semi-technical or ilot lants. Investigations by the IEA GHG rogramme have estimated cost, efficiency and emission data for various tyes of ower stations, including both current and future technology roviding reresentative base cases for studies of CO cature and storage. According to this study, ower lant rocess otions CO cature are: -conventional ower lants chemical CO scrubbing of the atmosheric flue gas; -integrated oxygen/steam-blown gasification of fossil fuel and combined cycle (IGCC) lants CO shift and searation of CO and H before combustion; -combustion/gasification of fossil fuel (esecially coal) in an atmoshere of ure oxygen and recycled CO. he robability of further technical research and develoments in each of the main tyes of lant has also been examined. his aer evaluates the oxy-combustion rocesses in recycled/rich CO medium. Keywords: oxy-combustion based ower cycle evaluation, first laws efficiencies 1 Introduction In the field of the design of new co generation energy lants and of otimization of existing ones, there has been a substantial effort dedicated to the reowering techniques, new fuels, renewable and recovered energy, new suitable working fluids, second law otimization, modelling, and simulation, study of technical solutions to diminish the non desired oerating influences. he co generation systems that combine a gas turbine engine to a steam engine are very sensitive to the intake state arameters of the ambient air. he convenient oeration mode must assure a constant ower outut at constant rm. his aer erforms an evaluation of the oxycombustion rocess in recycled/rich CO streams. At resent, it is almost imossible to decide which technologies of CO cature & sequestration are alicable to large ower lants for the future. aking all known facts into account, it aears to be a romising solution to use ure oxygen instead of air, either to roduce H by gasification or for a direct combustion rocess. he arrangement of ower generation systems might be foreseeable for the next future, and it is obvious that there still stay behind a larger contribution of fossil fuels. Noticeably, to meet the demands for a cut in CO emissions, on one hand it becomes necessary to raise the overall efficiency of ower generation rocesses and on the other hand to settle roer rocedures for CO cature. Besides coal and natural gas, oil lays a minor role for ower generation in the countries of the EU, therefore in diminishing the CO the researches should focus on coal and natural gas. able 1 shows an overview of rocess rinciles for fossil fuelled ower generation low CO emissions. he table includes combustion (1a) as well as gasification rocesses (1b) at atmosheric or higher ressure, subject of CASOR and ENCAP EC-Projects. he targets are: -to identify the "best scheme" for high efficiency oxycombustion-based ower lants for CO cature; -to develo and re-design rototyes of critical comonents and to test them; -to analyse the feasibility

2 5th WSEAS Int. Conf. on ENVIRONMEN, ECOSYSEMS and DEVELOPMEN, enerife, Sain, December 14-16, of the concets at large scale; to develo advanced engineered CO cature otions able 1: Overview of rocess rinciles for CO -reduced ower generation from fossil fuels a: Processes based on Combustion air O and recycled CO atmosheric ressurised atmosheric CO Flue gas scrubbing CO Flue gas scrubbing b: Processes based on Gasification Air searation (O searation by membranes) air ure O O and recycled CO atmosheric ressurised atmosheric ressurised atmosheric ressurised CO-shift CO-shift Air Air searation searation CO flue gas CO flue gas CO /H searation CO /H searation (O searation by (O searation by scrubbing scrubbing membranes) membranes) hermodynamic Processes he studied scheme of the air/oxygen flue gases aths is reresented in Figure 1. he thermodynamic rocesses considered are: 1 oxygen air mixing, 1 flue gases (air oxygen) mixing, 3 adiabatic comression, 3 4 combustion, 4 5 adiabatic exansion, 5 6 exhaust heat use. A.S.U C mechanical efficiency.98 fuel comosition, 15% H and 85%C he numerical code, OXYCOM-UI, was comuted in MAHLAB and consists of modules: MC comute the mixing rocess, COMP comute the adiabatic comression considering variable heat caacities and adiabatic exonents as ratio of enthaly to internal energy variations, COMB comute the combustion rocess in a sequential logical scheme, resectively a combustion out dissociation followed by a dissociation of resulting flue gases, URB comute the adiabatic exansion considering both the variable heat caacities and adiabatic exonents as ratio of enthaly to internal energy variations, and the deendence of the dissociation degree function of the temerature and ressure, BOIL comute the exhaust heat use considering both the variable heat caacities, and the condensation of water vaor below dew oint, and the dissociation degree function of the temerature and ressure PERFOP comute the erformance criteria (e.g.: first and second law efficiencies, entroy generation by irreversibility) and builds the numerical results matrices used in otimization. he dissociation rocess was analyzed by the intermediary of chemical equilibrium constants of selected rimary and secondary dissociation reactions. he combustion mass flow balance scheme is showed in Figure. Mix 1 Air Mix 1 HO Exhaust Heat Use N: (1x) n N O: (1x) n O x n C x n H / CO : (1x) n CO x n C C fuel : n C x recycled N: (1x) n N O: (1x)(n O n C x n H /) CO : (1x)(n CO n C ) CO cature H fuel : n H HO: x n H HO: (1x) n H Fig. 1. he ath flow scheme he emloyed restrictive conditions are: comression ratio 1 to 3; environmental arameters 1 bar, 88 K combustion temerature 1 C isentroic efficiencies,.88/comression and.94/exansion combustion efficiency.97 combustion ressure dro coefficient.98 combustion Fig.. he combustion mass flow balance n kmol s number; x the recycled flue gases fraction.1 Main Basic Equations saturation ressure of the water vaor: 1 3 log VS ( ) A1 A A 3 A 4 A 5 (1)

3 5th WSEAS Int. Conf. on ENVIRONMEN, ECOSYSEMS and DEVELOPMEN, enerife, Sain, December 14-16, he coefficients A i, for the fixed range of temeratures, were found by Gauss technique. comosition of humid gases: ( ) rv ( ) r H O kmoles φ VS xv () dry gases kmole φ h VS enthaly of humid gases: for rocesses out dissociation n ri c,id xv L c i dry gases,v 4 th order olynomials were used for c for rocesses dissociation d h d n n j h f, j c, jd j gas V d he N mole fraction made the equivalence of units. entroy of humid gases at equilibrium: s i r i c L xv,i d c dry gases,v 1 R log d ( rv ) ( ) rv R log VS ( ) he state (s ) is defined by: 1bar, 73.15K for dry gases, VS ( ) 61.8 Pa and 73.15K for H O vaor. density of humid air or gases at equilibrium (3) (4) (5) ρ M (6) R.. Calculation of rocesses out dissociation Mixing of humid gases and oxygen, (suosed to be isobaric and isothermal): where inj n O inj (.1 n ) r dry gases O O M M O O.79M dry gases ( 1 ro ) M N N (7) is the number of kmoles of oxygen injected into 1 kmole of dry air. Equation (7) reflects the mass inj conservation law and allows the calculation of n. Remember that φ and x remain constant since the absolute ressure and temerature of the gaseous mixture remain unchanged. By knowing the comosition, the ressure, and the temerature in state 1, we use Eqs. () (6). Adiabatic rocesses O reversible adiabatic rocess: [ s n (, r i, xv)] in [ s n (, r i, xv) ] out (8) he unknown temerature out is calculated by using the Newton-Rahson technique. Constant humidity and dry gas comosition are assumed. he theoretical outut arameters are obtained on the basis of the final comosition, ressure and temerature. irreversible adiabatic rocess: he final state of the irreversible adiabatic comression results as: out in ( h n h n ) rev ηs out in ( h n h n ) irrev (9) out in out in ( ) ( ) ( h ) n h n rev h n irrev h n irrev η he temerature of the final state is calculated using the Newton-Rahson technique. Assuming once again the constant humidity and constant comosition of the dry gases and knowing the enthaly of the final state, the other arameters can be calculated. In addition, the mean adiabatic exonent is calculated on the basis of the reversible adiabatic equation of the ideal gas (knowing the initial and final ressures and temeratures) as the ratio of enthaly to internal energy variations. Heating / Cooling he calculation imlies firstly the determination of final enthaly and subsequently, by a trial and error method the finding of final dew oint and the final humidity, knowing that (r i ) of dry gases remain constant. In this calculus, we get final temerature and comosition. 3 Combustion Calculus Characteristics of the liquid fuel: n C.85 kg C/kg fuel, n H.15 kg H /kg fuel, higher heating value H s 46 kj/kg 3.1 Classical Combustion Calculus he classical combustion equations are emloyed, yielding the roducts as fg combustion CO n CO n [kmole CO /kg fuel] (1) fg combustion recycled H O n H O n HO n [kmole H O/kg fuel] s (11) 3. he Adiabatic Flame emerature In order to calculate the adiabatic flame temerature, a heat balance equation that describes the energy transfer in the ideal combustion chamber. he comressed

4 5th WSEAS Int. Conf. on ENVIRONMEN, ECOSYSEMS and DEVELOPMEN, enerife, Sain, December 14-16, combustion humid gaseous mixture, and the liquid fuel, enter the fictitious combustor, where takes lace the combustion out dissociation. After this combustion, the flue gases dissociate in a fictitious dissociation cell and leave it the adiabatic temerature. he heat balance is: H s where fg ni c,id n j c,jd xl (1) i j dry fg fg i c,id and n j c,jd n i j dry fg are the sensible enthalies of the dry mixture entering and of the flue gases that exit the ideal combustor resectively. hus, since Eq. (1) is function of the unknown temerature fg, the solutions of this equation has been determined by the Newton-Rahson technique, the imosed error for the difference between two successive values of the iteration being.1 K Combustion Calculation Dissociation he roducts of the ideal combustion reactions can dissociate due to the high combustion temeratures. he roducts of the dissociation reactions dissociate themselves, leading to a chain of dissociation reactions. Only the most imortant reactions were considered, divided into rimary reactions and secondary reactions (in which some roducts of the rimary reactions articiate among the reactants). Dissociation of Primary Dissociation Reactions Dissociation of CO CO CO O (13) Dissociation of H O Dissociation in hydrogen and oxygen: H O H O (14) Dissociation in hydroxyl and hydrogen: H O OH H (15) N N N (16) Secondary Dissociation Reactions Dissociation of O O O (17) Combination of N and O O N NO (18) Dissociation of H H H (19) he fraction of the reactants converted into dissociated roducts can be calculated from the relations of the kinetic constants of the reactions, which are determined from tables as a function of the temerature. Calculation of the Dissociated Fractions he general exression of a chemical reaction is: ν A A ν B B ν C C ν D D () he reaction take lace in both directions and the control of its direction and intensity is exerted by the temerature-deendent equilibrium constant K, whose exression is: νc νd νa νb νc C νa A νd D νb B νc νd νa νb z z K K z z (1) he rocedure used in the calculation of the dissociation fractions for the considered reactions consists of the derivation of the general exressions of the equilibrium constants for each equation in simlified exressions. hese simlified exressions are necessary since the dissociation reactions take lace simultaneously, and the solution of the resulting non-linear system of equations is very difficult to obtain. Consequently a trial-and-error iterative method is emloyed. he first ste uses the simlified exressions to calculate the initial dissociation fractions, which are utilized as start values for the iterative technique that rovides the final values by means of the general exressions. he calculation rocess imlies information over the temeraturedeendence of the equilibrium constants. he tabulated values of K were interolated by olynomials Calculation of the Flue Gases emerature after Dissociation he heat caacities in the heat balance equation used to determine the flue gases temerature after dissociation are temerature-deendent, thus leading to a comosite equation in terms of temerature (for the heat caacities of O, H, and N, the constant value.7845 [kj/kmole K] is assumed). In such cases, one has to follow one of the two aths: a trial-and-error technique is used, a numerical method for the transcendental equations roots has to be alied. he second way is emloyed here, by using the Newton- Rahson technique. he outcome of the rocedure described above is the flue gases temerature of the flue gases after dissociation and the flue gases comosition (n k ) fg, which deends on final temerature.

5 5th WSEAS Int. Conf. on ENVIRONMEN, ECOSYSEMS and DEVELOPMEN, enerife, Sain, December 14-16, Numerical Results Figures 3 and 4 show the first law efficiencies for the ath flows out recycled flue gases.,45,4,35,3,5 1 3 Air 45% O 7% O 95% O Fig. 3. he first law efficiency of the gas turbine engine function of the comression ratio,45,4,35,3,5 1 3 Air 45% O 7% O 95% O Fig. 4. he first law efficiency of the combined cycle, function of the comression ratio, suosing that the exhaust heat is used in a Rankine cycle having a second law efficiency of about 5% 5 Conclusions he use of the oxy-combustion in a contemorary gas turbine cycle does rovide a first law efficiency higher than those obtained currently for air-based combustion. he aer concisely resents a numerical code useful in evaluation of simle or combined cycle based on oxycombustion. he numerical results are arguable because this work is a first ste in analyzing and otimizing such cycles. his ste took into account only the comlete develoment of the numerical code in order to aly it to advanced energy systems using oxy-combustion. Similar calculations made for the so called humid gas turbine engines (using the cooling by adiabatic humidification between comression stages and the injection of water during or after the combustion rocess) can achieve larger first law efficiencies. References: [1] A.F. Massardo, M. Scialò,, hermoeconomic Analysis of Gas urbine Based Cycle, Journal of Engineering for Gas urbine and Power, 1, [] Agazzani, A. F. Massardo, 1997, A ool for hermoeconomic Analysis and Otimization of Gas, Steam, and Combined Plants, Journal of Engineering for Gas urbine and Power, 119, [3] ASHRAE, 199, Systems and Equiment Handbook, Chater 7, Cogeneration Systems [3] B. Horbaniuc, O. Marin, G. Dumitrascu, Oxygen- Enriched Combustion in Suercritical Boilers, Energy, Elsevier, 9, 4, [4] Bammert, K., 1975, A General Review Of Closed Cycle Gas urbines Using Fossil, Nuclear And Solar Energy, Verlag Karl hiemig, Munich, Germany [5] Bathie, W.W., 1984, Fundamentals of Gas urbines, John Wiley & Sons Inc. [6] Consonni, S., Larson, E.D., 1996, Biomass Gasifier/Aeroderivative Gas urbine Cycles, Part AB, echnologies And Performance Modeling, and Performance Calculation And Economic Assessment, J. of Engineering for Gas urbines and Power, Volume 118, , and [7] E. Sciubba, Exergo-economics: hermodynamic foundation for a more rational Resource Use, Int. J. En. Res., v.x, 5 [8] G. Dumitrascu, O. Marin, O. Charon, B. Horbaniuc, he Influence of the Comression Interstage Cooling by Adiabatic Humidification, of the Steam Injection, and of the Oxygen-Enriched Combustion uon the Gas urbine Co-Generation Systems, Cooling, Heating and Power Generation Systems Conference, Paris, 1, vol.ii, 3-9. [9] Gas urbine World, 1996, Handbook, Pequot Publishing Inc. [1] Ishida, M., Jin, H., 1994, A New Advanced Power Generation System Using Chemical Looing Combustion, Energy, 19, [11] M. Bozzolo, M. Brandani, A. F. Massardo, A. raverso, 3, hermoeconomic Analysis of a Gas urbine Plant Fuel Decarbonisation and CO Sequestration, Journal of Engineering for Gas urbines and Power, Vol. 15, , BES PAPER AWARD IGI-ASME. [1] Paisley, M.A., Litt, R.D., Overend, R.P., Bain, R.L., 1994, Gas urbine Power Generation From Biomass Gasification, ASME COGEN URBO, volume 9, [13] R. Anderson, H. Brandt, S. Doyle, K. Pronske F. Viteri, Power Generation 1% Carbon Cature and Sequestration, nd Annual Conference on Carbon Sequestration, Alexandria, VA, May 5-8, 3 [14] R. Knight, M. Obana, C. von Wowern, A. Mitakakis, E. Perz, M. Assadi, B. F. Möller, P. Sen, I. Potts, A. raverso, L. orbidoni, 4, GPOM: hermo-economic Otimisation Of Whole Gas urbine Plant, ASME Paer G4-54. [15]bR.E. Anderson, S.E. Doyle, K.L. Pronske,

6 5th WSEAS Int. Conf. on ENVIRONMEN, ECOSYSEMS and DEVELOPMEN, enerife, Sain, December 14-16, 7 14 Demonstration and Commercialization of Zero-Emission Power Plants, 9th International echnical Conference on Coal Utilization & Fuel Systems, Aril 18-, 4, Clearwater, FL, USA [16] raverso, A. F. Massardo, W. Cazzola, G. Lagorio, 4, WIDGE-EMP: A Novel Web-Based Aroach for hermoeconomic Analysis and Otimization of Conventional and Innovative Cycles, ASME Paer G Nomenclature c mole heat caacity (kj/kmole K) Greek symbols h f enthaly of formation (kj/kmole) η isentroic efficiency h mole enthaly (kj/kmole) ν stoichiometric coefficient H s higher heating value (kj/kg fuel) ρ density (kg/m 3 ) K equilibrium constant of chemical reaction φ relative humidity L water mole latent heat at o C (kj/kmole) M Mass er mole (kg/kmole) Subscrits n number of kmoles fg flue gases ressure V water vaor in air standard state ressure (N/m ) r mole fraction Suerscrits R universal constant of ideal gases d dissociated s n mole entroy (kj/kmole K) fg flue gases temerature (K) standard state temerature (K) Abbreviations xv mole humidity (kmole/kmole dry gases) C comression stage turbine