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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 471h St, New York, N.Y GT-383 The Society shall not be responsible for statements or opinions advanced In papers or rescussico at meetings of the Society or of its Divisicass or Sections, or printed in Its publications. Discussion is printed only if the paper is published in an ASME Journal. Authorization to photocopy for internal or personal use is granted to libraries and other users registered with the Copyright Clearance Center (CCC) provided $3/article or TA/page is paid to CCC, 222 Rosewood Dr., Danvers, MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright C 1998 by ASME All Rights Reserved Printed in U.S.A. ZERO EMISSION MATIANT CYCLE ,a Pk MATHIEU, R IVIHART Universite de Liege, Institut de Mecanique Dpt of Nuclear Engineering and Power Plants 21, Rue Ernest Solvay - B-4000 Liege - Belgium Fax : pmathieu@ulg.ac.be ABSTRACT In this paper, a novel technology based on the zero CO2 emission MATIANT (contraction of the names of the 2 designers : MATHIEU and IANTOVSIC1) cycle is presented. This latter is basically a gas cycle and consists of a supercritical CO2 Rankine-like cycle on top of regenerative CO2 Brayton cycle. CO2 is the working fluid and 02 is the fuel oxidizer in the combustion chambers. The cycle uses the highest temperatures and pressures compatible with the most advanced materials in the steam and gas turbines. In addition, a reheat and a staged compression with intercooling are used. Therefore the optimized cycle efficiency rises up to around 45% when operating on natural gas. A big asset of the system is its ability to remove the CO2 produced in the combustion process in liquid state and at high pressure, making it ready for transportation, for reuse or for final storage. The assets of the cycle are mentioned. The technical issues for the predesign of a prototype plant are reviewed. KEY-WORDS: zero CO2 release, CO 2 cycles, CO2 removal, 02 combustion NOMENCLATURE ASU : air separating unit CC : combined cycle GT : gas turbine HP : high pressure (bar) LHV : low heating value (MI/kg) LP : low pressure (bar) MBA : mono ethanol amine MP : medium pressure (bar) : high pressure cycle P2 : upper intermediate pressure (bar) P3 : lower Mb:Mediate pressure (bar) P4 : low pressure cycle PC : pulverized coal PFBC : pressurized fluidized bed combustor TIT : turbine inlet temperature CC) outlet recuperator temperature at upper intermediate pressure ( C) INTRODUCTION Among many approaches to the CO2 emissions mitigation extensively investigated in the scientific literature [Blok et al., 1992][Riemer, 1993] and [Riemer et al., 1995], we propose here an original concept, of zero-release of pollutants in the atmosphere as a competitor with systems using monoethanolamine scrubbing or membrane techniques and cutting the CO2 releases by 80 to 90%. The removed CO 2 is then disposed of in proper sites, like in aquifers, in deep oceans or in depleted oil and gas fields. Of course, zeroemission is an ideal and desirable target and leakages are unavoidable. This is discussed here. In this paper, 2 concepts are introduced : 1. The stack downwards. The common features to zeroemission cycles are the use of CO2 as the working fluid and 02 as the fuel oxidizer. In order not to release CO2 in the atmosphere, we make a knot in the stack or better, we turn the power plant upside down (see fig. I). So the emissions are avoided and the combustion products are dealt with as effluents, possibly contaminated with NOx, S0x, particles, toxics. The objective is to keep the effluents under total Control. Such zero-emission cycles all require an air separation unit (ASU). (lantovslci et al, 1997, a, b, c). Presented at the International Gas Turbine & Aeroenglne Congress & Exhibition Stockholm, Sweden June 2 June 5, 1998 This paper has been accepted for publication in the Transactions of the ASME Discussion of It will be accepted at ASME Headquarters until September 30, 1998

2 2. Grave-into-the-cradle We take the fossil fuels from the bowels of the earth, we must send there back the products of their combustion. In the MATIAN'T concept, the removed CO2 flow is liquid and at high pressure. We can consequently use it in separated from the extracted mixture CO2/oil mixture, can be reinjected in the depleted well. Hence the waste (CO2) in its final storage is replacing the fuel which produced it in the fuel site. So a closed fuel/co 2 cycle is set up (see fig.!). This applications where pressurized liquid CO2 is required. It is, for example, the case in the oil extraction. CO2 is used to increase the oil recovery efficiency. The higher the CO2 injection pressure, the higher this efficiency. The oil well is then more depleted and the CO2, has to be considered as a concept, the technical feasibility and the associated technical issue are the subject of a separate study. N2 CO2 CLOSED LOOP CLEANED WATER Figure 1: stack downwards and grave-into-the-cradle concepts CYCLE DIAGRAM AND LAYOUT OF THE PLANT The MATIANT gas cycle is shown on the T-s diagram in figure 2 and the layout of the corresponding plant is in fig.3 [Iantovslci and Mathieu, 1996 a) and b)1, [lantovski et al, 1997 a)]. It comprises a supercritical CO 2 Rankine-like cycle ( ) combined with a regenerative CO 2 Brayton cycle with reheat ( ). The combustion takes place along the isobar 7-8. The fuel is injected at the proper pressure P2 in the burners as well the mixture of the working fluid CO2 and the 0 2 at 7. At the exit 8 of the combustion chamber (1300 C), the mixture CO2/H0 2 is expanded along 8-9 and then reheated up to the temperature 10, in a second combustion chamber where other fractions of fuel and 02 are injected at P3. At 10, in stochiometric proportions, the fluid contains the combustion products, namely 8% CO2 and 6%H20 in addition to the 100% CO2 circulating along the total cycle. This fluid is expanded in These two expanders produce electricity. The fluid is then cooled from 11 to 12 in a recuperator where it gives up its heat to 2 CO 2 gas flows : first at the higher cycle pressure P I and after expansion in 5-6 at the pressure P2 of the combustion chamber. CO2 is expanded along 5-6 in a steam-like turbine and generates electricity. As to 02, its total throughput is 10% of the CO2 mass flow rate whose around 7% is injected at 7 and another 3% is injected at 9 for the reheat process. It can be reheated from its temperature at the ASU outlet up to the temperature at the inlet of the burners 7 and 9 of the combustion chambers. The gas generator is split into its 2 components : an expander with its associated turbogenerator and the compressor driven by a motor. At the recuperator outlet, the water is condensed in a cooler and extracted in a CO2/1120 separator. The remaining gas is then CO2 and it is compressed in a 3 staged compressor with intercooling along 1-2 up to the CO 2 saturation line in order to carry out a compression as close as possible to an isothermal process. Then CO 2 is condensed and the liquid CO2 at 3 (70.5 bar, 29 C) is easily compressed to a high pressure through a pinup with a low energy consumption. At 4, the excess CO2 (about 8% of the recirculated CO2 mass 2

3 flow rate) is removed in liquid state through a valve, without any energy consuming and costly system as it is the case in a CO2 scrubber installed in the flue gas. Basically, the effluents, possibly contaminated, remain under control. At this stage of the modelling, the cooling of the hot parts of the cycle is not included yet but in this analysis, a cycle efficiency penalty of 2.5 % pts to take it into account is adopted by similarity to an air GT.[Bolland et al, 1992,1993). Of course, the fluid in the recuperator contains small quantities of Ar and N2 coming from the MU and of 02, not used in the combustion process because an excess of oxygen is necessary in practice to make the combustion complete. They are present in the cycle and have an impact on the efficiency. However, the used MU delivers 0 2 at a purity of 99.5% with a specific electricity consumption of 0.28 kwh/kg 02 at 5 bar. Depending on the natural gas composition, the major part of the non-condensable gases are 142 and argon. As their adiabatic exponent y is much higher than that of CO2, so is it for the resulting mixture. The compression asks then for more electricity than for pure CO 2 but more power is provided in the high temperature expanders 8-9 and These gases are vented from the condenser at the lowest cycle pressure and temperature. Normally, they would take some CO 2 with them in the atmosphere. Also, with an oxygen purity of 99.5%, the amount of non-condensable gases should be very low (but of high commercial value, like argon). On the other hand, the pressure in the condenser could not be significantly affected by their presence. Figures about the effects of the inert gases will be given in a next step of this study. Another option to limit a possible CO 2 leakage is to avoid a condenser in the cycle and extract the non-condensable gases when the CO 2 is supercritical (point 4, fig. 2). Then there is a small accumulation of non-condensable gases in the cycle but for instance, the accumulation of Ar in the cycle remains lower than about 1% of the total mass flow rate. T ( C) S (kj/kg.k) figure 2: Cycle representation in (T-s)

4 10 G H20 Fut:1_4. _ co2 a: CO2 compressor b : condenser C: CO2 pump d : splitter e : mixer f recuperator g : uncooled HP expander h: combustion chamber 1 i MP expander with internal cooling j : mixer k: combustion chamber 2 1 : 12 expander with internal cooling m: cooler n : water separator unit o and p : fuel compressors Figure 3 : Layout of the plant DESIGN POINT CALCULATION Natural gas being not pure methane, its Low Healing Value, in Europe anyway, is around 42 Mag. A mixture of about 84% of CH4 and 16% of CO2 (mass fractions) is well representing the LHV of this natural gas and is used in the modelling In order to comply with the admissible menbanical stresses criteria for the advanced materials used here, the high pressure level is limited to 300 bar and the temperature of this high pressure gas flow at the recuperator outlet is limited to 600 C (point 5 fig. 2). The higher the pressure the lower this temperature limit Similarly, as the pressure P2 is varied between 40 and 100 bar, the temperature Trnp at the recuperator outlet (point 7 in fig. 2) has to be limited to 700 C but should the marerialc withstand it, this latter could be thither increased. Taking into account the thermal resistance of the materials and the limitation of the cooling techniques the temperature inlet of the MP and 12 expanders is limited to 1300 C as well. Technical issues raised by the pressure and temperature levels are taken into account, limiting among others the pressme P2 to 40 bar. The cycle efficiency is optimized with respect to the pressures P3 and P4. The calculations are carried out using the commercial ASPEN PLUS code. Taking into account the previous constraints, the high pressure PI is equal to 300 bar, P2 to 40 bar and P4 to 1 bar and the recuperator outlet and the LP and MP expanders inlet temperatures are respectively 700 and 1300 C. The optimization process introduces a dependence of the maximization of the heat recovery in the system and of the temperature limitation at the recuperator outlet The resulting P 3 is 9.3 bar, taking an isentropic effectiveness of 0.75 for the fuel compressors, the 02 compressors and the CO2 pump, 0.87 for the expanders and 0.85 for the isentropic effectiveness of the CO2 compressor (centrifugal compressor). Typical values of the pressure losses in the heat exchangers are taken as 5% of the inlet pressure and 3 % in the combustion chambers. Pressure losses in mixers, splitters, etc. are also taken into account. It is assumed that the cycle efficiency penalty due to the cooling of the hot parts of the system is equal to 2.5 % pts in every case. From the calculations, it appears that the cycle efficiency can be increased by increasing P 2 but in the same time by adapting P 3 to comply with a limit of 700 C for T rnp. This is illustrated in fig. 4. For example, for P2=70 bar, the efficiency is 43.3 % then P3=13.5 bar, namely the value calculated to maximize the efficiency complying with the Tn, limit of 700 C. 4

5 The results show that the cycle efficiency has a flat maximum with respect to P3. Indeed, due to the ESCI that P3 is limited by the Top constraint, the optimal efficiency is not very sensitive to 50 T 48 46,81 48, ,5, 48,93 49 ' 24 Cycle efficiency (%) 46 le- CO2 compressor isentropic effectiveness=0,75.0o2 compressor isentropie effcetiveness=0, ,1 44,4 43,3 43, , , P2 (bar) Figure 4: cycle efficiency versus upper intermediate pressure P2 (P3 is optimized at each point) Here, we have to worry about the technical feasibility. When P2 moves upwards, this means that the combustion takes place at a high pressure and boilers operating at too high pressures could be unrealistic. On the other hand, due to the cooling requirements, P2 is limited at the compressor outlet pressure (70 bar), if the cooling is made through CO 2 bleedings from the compressor. In this modeling, the fuel is considered to feed the burners at ambient temperature (17 C) and at 3bar. In reality, the delivery pressure of the natural gas depends on the network and in Europe in particular this pressure can be as high as 40 or 60 bar. Consequently, depending on P2, the fuel compression can be low or even zero. If the combustion chamber is at a pressure lower than that of the network, a recovery in an expander can be considered. In this case however, a preheat of the fuel is required before its injection in the burners. On the cold side, our calculations are made with a CO2 condenser pressure of 70.5 bar, that is a saturation temperature of 29 C. In very cold countries like Desunarlc, Norway, Sweden and Finland, the heat sink (the ocean) is still more favourable. For 1 kws of carbon dioxide recirculated the fuel consumption is equal to kg/s, the 02 consumption is equal to kg/s and the mass flow rate of CO 2 removed from the cycle is kg/s. The net cycle efficiency is obtained by : W production W consumption % fuel LHV fuel with Pi = 300 bar, P2 40 bar, P3 = 9.3 bar, P4 =-- 1 bar, and when the cooling demand (An cooling = -2.5% Pis) and the ASU consumption are accounted for. This latter is taken from ASU manufacturers' data as 0.28 kwh/kg 0 2 at a purity of 99.5% at 5 bar. The compression of the gaseous 0 2 at 5 bar up to the pressure of the second combustion chamber (P 3=9.5 bar) adds an electricity consumption of 0.03 kwh/kg 0 2. So taking into account the respective mass flows of oxygen at P2 and P3, the total consumption due to the production and the compression of the 02 injected in the combustion chamber equals to 0.37kWh/kg 02. The good value of the efficiency is not only due to high pressures and temperatures but also to the non-idealty of CO 2. This latter in the low temperature range (0-300 C) and at pressures around 60 bar near the CO 2 saturation line reduces the compressor consumption with respect to that of an ideal gas and nearly offsets the ASU consumption at the design Point 5

6 TECHNICAL ADVANTAGES We have modelled CC plants (reference case) using CO 2 scrubbers for CO2 removal from the flue gas without and with flue gas recirculation. CO2 removal from the flue gas by an absorber/stripper system using MBA is considered currently as the state-of-the-art technology. In addition to be very costly, this chemical plant installed at the exhaust of the plant leads to an efficiency penalty typically of the order of 6 to 8% points for a natural gas fuelled plant. As far as zero-emission is concerned, we have studied CO2 semi-closed CC cycles (Mathieu and Deruyck,1993). We have shown that a CC with recirculation has better performance and lower cost for CO 2 scrubbing than a comparable semi-closed CO2 cycle of that type (Holland and Mathieu,1997). Here we go a step further. The assets of the MATIANT cycle are notably the following ones : 1. good net cycle efficiency (between 40 and 45% when burning natural gas). 2. negligible release of pollutants in the atmosphere. 3. removal of a highly pressurized liquid CO2 flow and possible reuse or final storage without further compression (it is the biggest asset). 4. no use of an industrial gas turbine as such; the compressor and expanders are separated from each others, so there is no danger of surge of the compressor, especially when the LHV of the fuel is lower than that of natural gas, like that of a 5. no need of expensive and energy consuming CO2 scrubber. TECHNICAL ISSUES AND PREDESIGN OF A PROTOTYPE PLANT The ASU has to be integrated in the cycle and in particular the flow of N2 or maybe the rejected heat. A new design or a redesign of known industrial components is required when they operate on CO2 instead of air or water. cycle efficiency (%) 47 _ ,9 44, , , ,6 35 0,65 0,7 0,75 isentropic compressor effectiveness 0,85 0,9 Fig. 5 : cycle efficiency according to 3 stages compressor effectiveness Components already available or needing adaptations 1) the compressor whose design is very important. Indeed, as shown on fig. 4, the cycle efficiency is very sensitive to the compressor effectiveness. As an illustration, a decrease of this latter from 0.9 to 0.75 provokes an efficiency drop of around 5% pts. 2) a CO2 expander without cooling at 600 C, 300 bar, similar to the high pressure section of an UltraSuperCritical Rankine cycle. 3) the gas/gas recuperator at high temperatures and pressures. 4) the supercritical pump. 5) a CO2 condenser and intercoolers with water cooling circuits 6

7 6) the water/co 2 cooler and separator. Components requiring a new design 7) the combustion chamber using 0 2/CO2 instead of air. 8) the temperature expanders (1300 C) with internal cooling of the blades and vanes. The design of gas turbines has previously been investigated in [Mathieu et al, 1994 a and b)] (Mathieu 1995). Corrosion of the materials in presence of CO 2 at high pressures and temperatures must be addressed, as well as possible parasitic chemical reactions of CO 2 with other substances. We are currently integrating the cooling of the hot parts of the cycle in the model. When natural gas will be replaced by a liquid or a solid fuel, an integration of a gasification unit and a clean-up system of the syngas in the MATIANT plant is required. This is the following development of the present study. Other applications of the MATIANT cycle : - electricity and heat cogeneration, for instance district heating (Rasmussen 1997) - use in a Pressurized Fluidized Bed Combustor (Jung, 1997). - use in combination with a solar pond [Iantovski et al 1997, - introduction of a MHD unit in the cycle in replacement of the low pressure expander [lantovsld et al, 1997, b)]. - use in a combination with a heat pump or with a refrigeration system - coproduction of dry CO2 ice (1 bar, -80 C; recommended for some storage options) (Mathieu et al, 1998). - use of the high pressure liquid CO 2 extracted flow for oil recovery enhancement - use of chemical looping combustion using metal oxides cycles instead of the ASU. - use of chemical recuperation for efficiency increase. - hydrogen production. CONCLUSIONS The MATIANT concept leads to a power plant design with high performance and no releases of pollutants in the atmosphere. Several applications are obtained by combination with other systems like a gasification unit, a MHD unit, heat pumps, chemical recuperators, oil recovery installations. The concept is also used with technologies like cogeneration, district heating, a PFBC as well as with solar energy (solar pond). This wide field of applications will however only be feasible when conventional components like compressors, expanders, pumps, heat exchanger, recuperator, condenser, will be adapted or redesigned for the operation on CO2 as the working fluid and in more severe thermal conditions. As the power generation techniques using coal as the fuel are larger contributors to the greenhouse effect than those based on natural gas, the MATIANT cycle can be a possible alternative option when the time of an economic change of technology has come. The question is : is it technically feasible and what is its attractiveness in terms of performance (in various regimes), of cost, of environmental impact? Therefore, it is time to design a pilot plant now and this is an appeal towards the potential financial supporters. REFERENCES Blok K., W.C. Tudcenburg, CA. Hendrilcs and Ni Steinberg (1992). Proc. of the First Int. Cont on CO 2 Removal and VoL33, No 5-8 of Energy Conv.& Management, Amsterdam Selland 0. and Mathieu Ph., (1997), "Comparison of two CO2 removal options in Combined Cycle Power Plants", Flowers '97, Florence (Italy), July 1997 Holland 0. and Soother S., "Option Evaluation Study. Gas Fired CC Modified for CO2 Recover, TEA Greenhouse Gas R&D Programme 1993 lantovski E., Ph. Mathieu, a)) "Highly Efficient Zero Emission CO2-Based Power Plant", Proc. Of the ICCDR, Cambridge (Massachusetts, US). lantovski E., Ph. Mathieu, D. Golomb [1996 b)], Int Conf on analysis and utilization of oily wastes, AU20196, Gdansk (Poland) lantovski E., Mathieu Ph., and Nihart 12_, [1997 a)i, Incineration in High Efficiency Zero Emission COrBased Power Plants, Porto (Portugal) lantovsld E., Lhonune R_, Mathieu Ph, (1997b], "Coal Fired MagnetoHydroDynamic Power Plant with Zero-0O2 Atmospheric Emission", Singapore, Sept 1997 lamovski E., Greday Y., Mathieu Ph., [1997 c], "Biomass Fueled CO2 with Zero Emission", Power-Gen '97, Madrid, June 1997 & 'Technologies for Activities Implemented Jointly", Vancouver (Canada), May 97 Jung P., Thesis, (1997), Design of a zero emission Pressurized Fluidized Bed Combustor plant (in French), Liege Mathieu Ph., [1994 a)], "The Use of CO2 Gas Turbine?, Power Gen Europe' 94 Conf., Cologne (Germany), 1994 Mathieu Ph., P.J. Dechamps, N. Pirard [1994 b)], "The use of CO2 in an existing industrial gas turbine". 49 th All (Associazione Tennotecnica Italiana) National Congress, Perugia (Italy) Mathieu Pk (1995). Combined Cycle Project "CO2 Mitigation through CO/steam/non gas turbine cycles and CO/steam/argon gasification". Final Report of the Combined Cycle Project, European Joule II Programme Mathieu Ph., lantovski E., Seifritz W., (1998), "Zero Emission Power Plant with Terrestrial Storage of CO2 -Thy Ice", 1998, Interlaken (Switzerland) submitted for publication at TEA Greenhouse Gas R&D Programme Mathieu Ph. and Demyck J., (1993), "CO2 in Combined Cycle and IGCC Power Plants Using a CO2 Gas Turbine", ASME Cogen-Thrbo Con!, Bournemouth (U.K.), 1993 Mathieu Ph., [1994 a)), 'The Use of CO 2 Gas Turbine?, Power Gen Europe' 94 Con!, Cologne (Germany), 1994 Rasnmssen N. (D GC), (1997), Second International Workshop on zero emission power plants - Ulg 7

8 Riemer P.W.F. (1993), Proc. of the bit. Energy Agency CO 2 Disposal Symposium and vol 34, No 9-11 of Energy Conversion and Management, Oxford Riemer P., 1-L Audus, A. Smith (1993). Carbon dioxide capture from power stations, IE.A Greenhouse Gas Programme, Report Riemer P.W.F. and AY. Smith (1995). Proc. of the Int Energy Agency Greenhouse Gases: Mitigation Options and Vol. 37, No 6-8 of Energy conversion and Management, London 8