CO~ REDUCTION POTENTIAL OF FUTURE COAL GASIFICATON BASED POWER

Transcription

1 ECN-RX CO~ REDUCTION POTENTIAL OF FUTURE COAL GASIFICATON BASED POWER GENERATION TECHNOLOGIES D. JANSEN A.B.J. OUDHUI$ H.M. VAN VEEN FIRST INTERNATIONAL CONFERENCE ON C02-REMOVAL 4, 5, 6 MARCH 1992, AMSTERDAM

2 KEYWORDS CO~ Removal, Power Generation, Coal Gasification, Molten Carbonate Fuel Cell, Ceramic Membranes, System Analysis, IGCC, IGMCFC 2

3 CO~ REDUCTION POTENTIAL OF FUTURE COAL GASIFICATION BASED POWER GENERATION TECHNOLOGIES D. Jansen, A.B.J. Oudhuis and H.M. van Veen Netherlands Energy Research Foundation ECN, Business Unit Fossil Fuels, P.O. Box 1, 1755 ZG Petten, The Netherlands. ABSTRACT Assessment studies are carried out on coal gasification power plants integrated with Gas Turbines or Molten Carbonate Fuel Cells (MCFC) without and with CO2-removal. System elements include coal gasification, high temperature gas-cleaning, molten carbonate fuel cells or gas turbines, CO-shift, membrane separation, CO~ recovery and a bottoming cycle. Various system configurations are evaluated on basis of thermodynamic computations. The energy balances of the various system configurations clearly indicate that Integrated coal Gasification MCFC power plants (IGMCFC) with CO2-removal have high efficiencies (42-47% LHV) compared to IGCC power plants with CO2-removal (33-38% LHV) and that the CO2-removal is simplified due to the specific properties of the molten carbonate fuel cells. ]GMCFC is therefore an option with future prospective in the light of clean coal technologies for power generation with high energy efficiencles and low emissions. INTRODUCTION The growing awareness of the risk of a climate change due to greenhouse gas emissions and the fact that in the foreseeable future coal will continue to play an important role as key source of energy, has triggered all over the world the development of so called clean coal technologies for power generation with high efficiencies and low emissions. ECN responded to this by launching a new energy research and development programme ENGINE (ENergy Generation In the Natural Environment) which has a long-term prospect and a pollution-free character. The ENGINE sub-programme "H~-CO~ Technology" (Boswinkel et al., 1990) addresses the use of fossil fuels, in particular coal, for the (co)-production of electricity and hydrogen in combination with removal and storage of CO2, the development of inorganic membranes for H~-CO~ separation (Veen van et al., 1989) and the development of Solid Polymer Fuel Cells for transportation applications. ECN is also the main contractor in the Dutch Molten Carbonate Fuel Cell development programme (Joon et al., 1992) which include the demonstration of a 250 kw unit fuelled by coal gas. It is obvious that, as part of the two mentioned R&D programmes, assessment studies are carried out on coal gasification Power plants integrated with gas turbines or molten carbonate fuel cells (MCFC) without and with CO,-removal using inorganic membranes. These studies mainly concern the energy efficiencies of these technologies for Power generation, their potential to reduce the CO2-emission and the electricity production costs. To generate overall material and energy balances the processes are modelled in detail on the ASPEN PLUS TM process simulator. In this paper we will discuss the results of these assessment studies and compare these results with data for the CO2-removal in IGCC power generation systems taken from other sources. 3

4 SYSTEM DESCRIPTION Integrated Coal Gasification Combined Cycle with High Temperature C02-Removal First the feasibility of the use of ceramic membranes for high temperature gas separation, to reduce the CO~ emission of an IGCC power plant with a High Temperature Gas-cleaning system (HTG), has been studied (Jansen et al., 1991). An IGCC power plant with HTG, which is currently under development, is chosen as starting point, because: 1) it has an optional higher efficiency than an IGCC power plant with a low temperature gas-cleaning system (46.4% versus 43.6% LHV (Alderliesten et al., 1990)) and 2) an optimized IGMCFC power generation plant without or with CO2oremoval will also include HTG. A typical IGCC power generation plant with HTG and CO~-removal by means of ceramic membranes is shown in fig. 1. It consists of four basic sub-systems: the gasifier, the high temperature gas-cleaning system, the CO,-removal system (including the CO-shift, the membrane separation unit and the CO~ conditioning section) and the Power production system which includes the gas turbine and the Heat Recovery Steam Generator system (HRSG). These sub-systems will be described in more detail. Fig.1. Simplified flowsheet of an IGCC power plant with CO2 removal (Oudhuis ~1., 1991). The Gasifier and Syn_oas Cooler. The entrained bed gasifier developed by Shell is used for the conversion of coal into fuel gas. In this gasifier pulverized coal is gasified at a pressure of 30 bar and a temperature of 1500 C using 95% pure oxygen from the air separation plant and steam, producing a mixture which mainly consists of CO and H2. The minerals in the coal feed melt at the gasification temperature and leave the gasifier mostly as molten slag. At the top of the gasifier the fuel gas is quenched with cooled recycled gas from the syngas cooler. In the syngas cooler the gas is cooled to 350 C after which it is cleaned in the high temperature gas-cleaning system. Heat from the gasifier wall and the syngas cooler is used to generate saturated, high pressure steam in the HRSG system. 4

5 High Temperature Gas-cleaning System. In the high temperature gas-cleaning system, particles, sulphur components (H2S, COS), chlorides and fluorides are removed from the fuel gas to such an extend that the gas can be handled in the other sub-systems and that the final emissions comply to the environmental requirements. Most of the fly-slag in the gas is removed with cyclones. This fly slag is recycled back to the gasifier. For the removal of halogens nacolite is injected in the gas. Reaction products and un-used nacolite are removed again with cyclones. For the final dust removal ceramic candle filters are used. The desulphurisation process is based on the iron oxide on silica carrier sorbent which has very good cleaning efficiencies. With financial support of the EC (JOULE Programme) ECN, KEMA, VEG, RUU and Foster Wheeler are currently developing this high temperature ( C) desulphurisation process for IGCC. When successful this process can also be used in the IGMCFC power generation system without any additional sulphur polishing step to avoid poisoning of the fuel cell. The regeneration step of the sorbent is aimed at the direct production of elemental sulphur, which is another new aspect of this process. Therefore this process was selected for the high temperature desulphurisation step. The operation conditions of the high temperature gas-cleaning system are 26 bar and 350 C. CO2-Removal System. Before the fuel gas enters the H~ CO2 membrane separation unit the CO in the gas is converted with steam to CO2 and H 2 (CO-shift reaction). In order to get a sufficient CO conversion at high temperature, two high temperature shift reactors are used. In the first step 80% of the CO is converted. In the second step another 10% of the original CO is converted. Therefore the total CO conversion efficiency is more than 90%. Between the two shift reactors the gas is cooled. The heat released is used in the HRSG system. The outlet temperature of the second shift reactor is 370 C. The shifted fuel gas is then entering the ceramic membrane separation unit in which the H 2 and CO 2 are separated. In fact the H~ is being removed at high temperatures with ceramic (molecular sieve like) membranes. These membranes are being developed on lab-scale at ECN 1. CO,-removal with ceramic membranes (molten salt imbedded) is not possible at the moment as indicated in an earlier study (Jansen et al., 1991_). D~.velopment of these membranes will take considerable time. The recovered H~ is burned in the combined cycle. The remaining CO~ rich stream is sent to the compression/drying section where it is compressed to 110 bar for disposal in e.g. depleted gas or oil fields. The CO2-removal system is integrated in the HRSG system Gas Turbine and HRSG System. The recovered H 2 is burned in the gas turbine. For this study gas turbine data taken from literature sources are used. The integration of the gas turbine with the steam bottoming cycle (HRSG) is the same as used in the IGCC power generation plant with HTG operating at 350 C (Alderliesten et al., 1990). The H~ is burned with air and expanded producing electricity. The exhaust gas is used for steam raising in the HRSG system. The steam generated in the gasifier wall and syngas cooler is superheated in this part if the HRSG system. i The membrane system currently developed by ECN consists of a tubular ceramic support which is coated with an intermediat~ layer and a top layer. The latter, which consists of "/-alumina and has a pore size of about 4 ran, is the actual membrane layer. The most promising applications lie in the high temperature separation (> 300 C), where Knudsen diffusion is probably the only segregative transport mechanism. Work is now directed towards the development of inorganic membranes with smaller pore sizes tu increase the selectivity of the membranes. The results of permeability measurements for different gases and of separation experiments at elevated temperatures ( C) indicate that with ongoing R&D a separation factor for H2/CO 2 mixtures of at least 25, necessary for 88% H a recovery, can be achieved. 5

6 Integrated Coal Gasification Molten Carbonate Fuel Cell with C02-Removal In figure 2 the system configuration of an IGMCFC power generation plant with CO2-removal is shown. Again four basic sub-systems can be distinguished: gasifier, HTG, power production system (the MCFC stacks and the HRSG system) and H~/CO2 recovery system including the membrane separation unit and the CO2 conditioning section. S ~,,~un tu~blne C02 Fig. 2. Simplified flowsheet of an IGMCFC power plant with CO2 removal (Oudhuis d al, 1991). The gasifier, the syngas cooler and the HTG system are the same as used in the IQCC power plant described above. The only difference is that in the syngas cooler the fuel gas is now cooled down to 600 C after which its is cleaned in the HTG system which is consequently operating at a temperature of 600 C. An expander is used to lower the pressure of the fuel gas to the operating pressure of the MCF C stacks. The MCFC is chosen because it converts H~ and CO and it operates at an average temperature of 650 C, discharging its excess heat to generate high-quality steam. The Molter~ Carbonate Fuel Cell. The MCFC is the primary power generating element in the IGMCFC. Operation conditions were set at 4 bar, an average temperature of 650 C and an average current density of 150 ma/cm 2. The fuel utilisation is 85%. Air and CO~ are used as oxidant in the fuel cell. Cooling is achieved by a cathode recycle loop incorporating a steam boiler to generate saturated high pressure steam. The anode exhaust is fed to the Hz/CO2 recovery system. The recovered H2 is recycled to the anode inlet. Part of the recovered CO2 is fed to the cathode recycle loop in order to raise CO~ partial pressure to improve the fuel cell performance. The MCFC performance was predicted with a in-house developed model (Machielse, 1991). 6

7 includes the air compressor, as well as the recycle blowers. For both systems the COa is pressurized to 110 bar. From the results it is clear that there is a substantial difference in energy efficiency between both systems, caused by the differences in system elements and connectivitles (see fig. 1 and 2). The most remarkable difference is the place of the membrane separation unit. In both systems the gas separation takes place in the stream with the highest CO 2 concentration. In the IGCC system the shifted fuel gas is separated before the gas turbine (after the gas turbine the CO a concentration is decreased, because the fuel gas is mixed with excess air). As a consequence the primary power production will decrease (lower capacity of the expander). In the tgmcfc system however the gas is separated after the fuel cell. This is possible because the fuel gas is not mixed with the oxidation medium. In the anode both the H 2 as well as the CO are oxidized with CO32" resulting in 1 and 2 moles of CO a respectively. In fact an increase of the COa concentration takes place. The COa concentration is further raised in a LT-shift step. The remaining Ha is recovered for 88% with a ceramic membrane and is recycled to the anode inlet. Therefore in the IGMCFC system the primary power production is higher, because of the high fuel utilisation and also because of the higher efficiency of the fuel cell compared with the gas turbine. Another important difference is the required steam amount for the shift reaction In the IGCC system steam is required to achieve the necessary shift conversion, whereas in the IGMCFC system no steam is required for the shift reaction, because of a high H2OiCO ratio in the anode outlet. As a consequence of the difference in size and place of the shift reactor, more steam is available for Power production in the IGMCFC system. Of course not all the CO~ in the IGMCFC system is compressed, part of the CO a has to be mixed with the cathode input for the required cathode reaction. Integration of the shift into the membrane separtion-unit (catalytic membranes) could be a promising alternative to lower the steam demand for the required CO conversion. Both systems can also be compared with their base case, i.e. the same system without CO a- removal. An IGCC Power plant with a HTG system and an IGMCFC power plant has a calculated energy efficiency of 46% and 53% (LHV) respectively (Alderliesten et al., 1990 and Tanaka et al., 1987). There is a substantial difference in efficiency drop between both systems when CO~-removal becomes necessary. The explanation for this is also the difference in the system modifications. In the IGMCFC system, the separation after the anode has a minor influence on the power production. Separation and compression will of course increase the plant consumption and therefore decrease the energy efficiency of the system. In the IGCC system, the Power production is decreased because of the gas separation before the gas turbine. Also the plant consumption will increase because of the CO~ separation and compression and the shift reaction. Therefore the decrease in energy production will be smaller for an IGMCFC system with COa-removal. In table 3 different coal gasification based power generation technologies without and with CO~-removal are compared on basis of efficiency, overall CO a recovery, i. e. percentage of carbon input removed as COa. Also a comparison of the CO 2 emissions per kwh and the electricity production costs is given. For this comparison, an IGCC Power generation plant with the same degree of integration like the 250 MWe IGCC demonstration unit currently under construction in Buggenum (The Netherlands), is used as reference. The starting points for the calculations of the electricity production costs are summarised in table 2. 8

8 Table 2: Starting point for the calculation of the electricity production costs. Plant book life (except membranes and fuel cells) Book life membranes and fuel cells Method of depreciation Interest on debt Fuel price Load demand scenario 25 year 5-10 year annuity 5% (0% inflation) 5 Dfl/GJ 7000 h/year in base load charge; average: 91% load of total load charge Plant investment costs were taken from literature (Alderliesten et al., 1990, Hendriks et al., 1989 and Jansen et al., 1991). The span in the electricity production cost is caused by uncertainties in the investment costs and the life time of the ceramic membranes and MCFC stacks. Table 3: Comparison of different coal gasification based power generation technologies. Type Efficiency CO s recovery Relative CO 2 Relative power plant (%) (%) emission per kwh kwh costs IGCC (Alderliesten et al ) IGCC HTG (Alderliesten et al IGCC with CO2-removal 38.1 (Hendfiks et a1., i989) IGCC with CO2-removal 33.2 (Burcht van der.e3~al.,1991) IGCC with ceramic 34.5 membranes (88% H 2 rec.) IGCC with ceramic 37.5 membranes (95% H2 rec.) IGMCFC 53.1 (Tanaka ~t al ) IGMCFC with ceramic 42.1 membranes (88% H 2 rec.) IGMCFC with ceramic 45.4 membranes (95% H2 rec.) IGMCFC with "optimized" CO,-removal Table 3 clearly indicates that energy efficiency improvement of coal gasification based power generation technologies will result in CO~ emission reduction with only a modest increase of the electricity production costs. However when large reductions of CO= emissions becomes 9

9 necessary IGMCFC power generation systems could be very attractive in terms of CO2-removal cost effectiveness. CONCLUSIONS The energy balances clearly indicate that coal gasification MCFC power plants with CO2- removal have high efficiencies compared with IGCC in combination with CO2-removal. In the case of IGMCFC systems the CO~-removal can be simplified because molten carbonate fuel cells are able to convert CO indirectly to CO 2 and the oxidation medium is not mixed with the fuel or its oxidation products. IGMCFC is therefore an option with a future perspective in the light of clean coal technologies with high energy efficiency and low emissions. Although the development of such power plant has still a long way to go, it shows the direction to more efficient and cleaner power generation from coal with low CO2-emissions. ACKNOWLEDGEMENT Part of the work described in this paper was carried out in the framework of the Dutch National Greenhouse gas R&D programme "SOP-CO2" (Samenhangend OnderzoekPakket CO2-verwijdering en -opslag) on behalf of The Dutch Ministry of Housingi Physical Planning and Environment (VROM). The authors gratefully acknowledge their financial support. REFERENCES Aldediesten P.T. et al. "Systeemstudie hoge temperatuur gasreiniging", Joint system study performed by ECN, KEMA, Stork Boilers and TNO, november (Executive Summary in English is available at Novem, Netherlands agency for energy and the environment) Boswinkel H.H. and van der Laken, R. A.! Outline of a R&D programme H2-CO ~ Technology", Netherlands Energy Research foundation, Petten, The Netherlands, ECN-I July Burcht M.J. van der, Cantle J. and Boutkan V.K. "Carbon Dioxide Disposal from Coal based IGCC S in Depleted Gas Fields" Paper presented at the 10th Annual EPRI Conference on Gasification. San Francisco October Hendriks C.A., Blok K. and Turkenburg W.C. "The recovery of carbon dioxide from power plants",s.vmposium on Climate and Ener_ay. Utrecht. 27th september Jansen D., Oudhuis A.B.J., and van Veen H.M. "CO~-verwijdering bij KV-STEG installaties uitgerust met een heet-gasreinigingssysteem", Netherlands Energy Research foundation, Petten, The Netherlands, ECN-C-gl-021. March Joon K., van der Laag P.C., Jansen D. "The Status of the Dutch MCFC Activities and the Route to Commercialisetion". Proceedin_as of the International Fuel Cell Conference. Tokyo ~!992. Machielse L.A.H. "Simple Model for the Estimation of Isothermal Fuel Cell Performance", Proc. of 180th Meetin_o of the Electro Chemical Society. Modellin_~ of Batteries and Fuel Cells. Phoenix. 13 October Oudhuis A.B.J., Jansen D. and van der Laag P.C. "Concept for coal fuel cell plant with CO2- removal"modem Power Systems. November p

10 Tanaka T., Horiuchi hl., Kaminosono H. "Develoment of Molten Carbonate Fuel Cells at CRIEPI, Technologies and Applications". International Fuel Cell Seminar. The Netherlands. Oct Extended Abstracts p Veen H.M. van, Terpstra R.A., Tol J.P.B.M. and Veringa H.J. "Preparation, characterization and performance of three-layer ceramic alumina membranes for gas separation". Proceedinos of the First International Conference on Inorganic Membranes. ICIM-89. Montpelier. France, p. ~329-~335.!1