Coal Gasification Development for IGFC (EAGLE Project)

Transcription

1 Coal Gasification Development for IGFC (EAGLE Project) Sadao Wasaka Energy and Environment Technology Development Department New Energy And Industrial Technology Development Organization Tokyo, Japan Masao Sotooka Hajime Miki EAGLE Technology Development Group Technology Development Center Wakamatsu Research Institute Electric Power Development Co Ltd Kitakyusyu, Japan 1

2 ABSTRACT It is proved that the IGFC (Integrated coal Gasification Fuel Cell Combined cycle) system which consists of a gasifier, fuel cells, gas turbine and steam turbine etc achieves high thermal efficiency (53% or more, net) according to our feasibility study. The purpose of the EAGLE (coal Energy Application for Gas, Liquid and Electricity) project is to develop the technology to produce coal gas for fuel cells. EAGLE coal gasification pilot plant has been operated to develop a gasifier and establish a gas clean-up system for fuel cells since July This paper describes the result of the feasibility study on an IGFC system with a molten carbonate fuel cell, the outline of our gasification pilot plant EAGLE and the status of its operation experience. Keywords: Coal, Gasification, Fuel Cell, IGFC 1 INTRODUCTION In Japan, it is anticipated that coal will continue to provide a stable energy source into the 21 st century. Meanwhile, along with the heightened interest in environmental issues in recent years, particularly the CO 2 problem, there are increasingly strong calls for efficient usage of coal and reduction in the resulting environmental burden. Therefore it is required to improve coal power generation s efficiency in Japan. The efficiency of conventional coal power generation is approximately forty percent. The efficiency is improved by the IGCC (Integrated coal Gasification Combined Cycle) system which consists of a gasifier, gas turbine and steam turbine etc. On the other hand, high efficiency direct power generation technologies, such as MCFC (Molten Carbonate Fuel Cell) and SOFC (Solid Oxide Fuel Cell), are expected as the next generation power generating technologies. And it is proved that the IGFC (Integrated coal Gasification Fuel Cell Combined cycle) system which consists of a gasifier, fuel cells, gas turbine and steam turbine etc achieves higher efficiency than IGCC according to our feasibility study. When coal is used in fuel cells, the coal must be supplied to fuel cells after converting it into an ash free fuel gas. The objectives of EAGLE coal gasification project are to develop an optimum coal gasifier for fuel cells and to establish a clean up system, which purifies the gas to a level acceptable for fuel cells. Figure 1 shows the development schedule. A feasibility study of the IGFC system was conducted in fiscal Basic and detail designs of the EAGLE pilot plant with a coal feed rate of 150 tons per day were drawn up in fiscal The construction work, including the manufacturing of a gasifier and other main facilities of the EAGLE pilot plant was started at the Wakamatsu Research Institute in The EAGLE gasifier has been intermittently operated since March 2002 after its unit test, and accumulated coal gasification operation time reaches over 1,290 hrs for about 20 months Feasibility study & Preliminary test Design Construction Operation Evaluation Figure 1 Development Schedule 2 IGFC FEASIBILITY STUDY 2.1 Process Flow The IGMCFC (Integrated coal Gasification MCFC combined cycle) system is composed of a coal gasification unit, a gas clean up unit and a power block including the MCFC unit as shown in Figure Coal Gasification Pulverized coal is transported with nitrogen to the gasifier where coal is converted into raw syngas with a gasifying agent (95 percent pure oxygen) at 2.5MPa and 1,200~1,600 C through chemical reactions. Meanwhile, molten ash is discharged from the bottom of the gasifier into a water quench. Raw syngas exits the gasifier unit after its heat is recovered with a syngas cooler (SGC) at 450 C, and is forwarded to a gas clean-up unit. The char in raw syngas is removed with a cyclone and a filter, and then recycled to the gasifier. 2

3 Gasifier Syngas Cooler Cold Gas Cleanup ASU Coal Nitrogen Oxygen Filter Expansion Turbine Anode MCFC Cathode Heat Recovery Steam Generator G Steam Turbine Heat Recovery Boiler Gas Turbine G Catalytic Burner Figure 2 Process Flow Diagram of IGFC 2.3 Gas Clean Up Cold gas clean up must be applied in order to meet the strict tolerance limits of fuel cells. Impurities such as halogens, sulfur compounds and so forth in raw syngas are removed with a water scrubber and an absorber, and then the syngas is finely desulfurized with an adsorbent. Acid gas removed with the absorber is burned in air in a furnace and the sulfur content is recovered as gypsum by the use of limestone. Since the operating pressure of fuel cells is set at approximately 1.5 MPa, to match that of the gas turbine, the pressure energy of the syngas is recovered as power with an expansion turbine. 2.4 Fuel Cell (MCFC) In syngas, the CO concentration is high while the H 2 and H 2 O concentration are low. There is a possibility that carbon will precipitate at the electrode through the following reactions and that the cell performance will drop. CO C + CO 2 H 2 + CO C + H 2 O To prevent the precipitation of carbon, steam is added and anode exhaust gas is recycled to the anode inlet in order to increase the H 2 O and CO 2 concentration at the anode inlet. Anode exhaust gas is burned completely with a catalytic burner. The CO 2 produced in the catalytic burner is supplied to the cathode. Meanwhile, the fuel cell is cooled down with the recycling part of the cathode exhaust gas through a heat recovery boiler (HRB) to the cathode inlet. 2.5 Power Island Cathode exhaust gas, which is a non-calorific gas at 700 C, is sent to a gas turbine combustor. Clean syngas is also sent to the combustor in order to raise the gas turbine inlet temperature to 1,300 C. Waste heat is recovered from gas turbine exhaust gas with an HRSG. And then steam generated with the HRSG along with the gasifier, the syngas cooler and the HRB of the fuel cell is sent to the steam turbine (15 MPa, 538/538 C). 2.6 System Performance The IGMCFC system performance was calculated based on design conditions, as shown in Table 1, obtained from the parameter study. The IGMCFC system performance indicated that the gross and net thermal efficiency would be 59.6 and 53.3 percent (HHV basis) respectively as shown in Table 2. It was clear that considerably higher efficiency could be obtained in comparison with a conventional coal-fired power plant. Table 2 System Performance Table 1 Design Conditions Gross power output MW Gas turbine inlet temperature 1,300 C MCFC MW Expansion turbine 3.1 MW O 2 conc. in gasifying agent 95 vol% Gas turbine MW Steam turbine MW Fuel utilization (one pass) 80% Auxiliary power 65.3 MW Oxidant utilization (one pass) 25% Net power output MW Gross efficiency 59.6% Anode recycling ratio 6.6 Auxiliary power ratio 10.6% Anode recycling ratio 53.3% 3

4 3 PILOT PLANT DESCRIPTION 3.1 General Description Table 3 shows the specifications of the EAGLE pilot plant s equipment and Figure 3 shows the system flow of the EAGLE pilot plant. Table 3 EAGLE Pilot Plant Specifications Coal gasifier Oxygen-blown entrained-flow gasifier (two-stage spiral flow type) Coal feed rate 150 tons per day Gasification pressure 2.5MPa Gas clean-up Cold gas clean-up using MDEA Syngas volume 14,800 m 3 N/h (MDEA absorber outlet) Sulfur recovery Limestone-gypsum wet scrubbing separation Pressurized cryogenic separation Amount of Oxygen 4,600 m 3 N/h Oxygen concentration 95 vol% feed 27,500 m 3 N/h GT power 8,000 kw Pulverized coal is transported by nitrogen to the gasifier, where it reacts with a gasifying agent (95 percent oxygen) at 2.5 MPa and is converted into a fuel gas. Oxygen is produced by cryogenic air separation in the ASU ( Separation Unit). Meanwhile, molten ash is discharged from the bottom of the gasifier into a water quench. The high-temperature syngas exits the gasifier and is forwarded to a gas clean-up unit after heat is recovered by passing it through a syngas cooler, lowering it to 400 C. Char in the syngas is removed by a cyclone and a filter, and transported by nitrogen to the gasifier. Syngas must be cleaned in order to meet the fuel cells strict tolerance. Impurities such as halogens, sulfur and so on in the syngas are removed by water scrubbers and an MDEA (Methyl Die Ethanol Amine) absorber, and the syngas is finely desulfurized through the use of iron oxide. Acid gas removed by the MDEA absorber burns to sulfur oxide in a furnace and the sulfur is recovered as gypsum through the use of a limestone absorber. Cleaned syngas goes to the gas turbine where it combusts to generate electricity. Generated electricity is consumed as auxiliary power for the pilot plant. The plant is also designed so that compressed air from the gas turbine can be supplied to the air separation unit. Cryogenic air separation is introduced to the ASU ( Separation Unit) in consideration of oxygen purity and yielding capacity. Surplus nitrogen produced in the ASU is supplied to the gas turbine to reduce NOx. Coal Gasification Unit Gas Clean-up Unit Pulverized Coal Gasifier Syngas Cooler Precise Desulfurizer COS Converter MDEA Regenerator Acid Gas Furnace Filter Slag Nitrogen Char Gas/Gas Heater Water Scrubber MDEA Absorber Limestone Absorber Oxygen Incinerator Heat Recovery Steam Generator COMP GT G Compressor Rectifier Separation Unit Gas Turbine Unit Stack Figure 3 Flow Diagram of the EAGLE Pilot Plant 4

5 3.2 Coal Gasification High concentrations of H 2 and CO and a high calorific value for the syngas are suitable in the application of a coal-gasifier for use with fuel cells. This project has thus employed a dry feed oxygen-blown entrained-flow gasifier. Figure 4 shows the characteristics of the EAGLE project gasifier. The gasifier has the following features: (1) Upper and lower burners are installed tangentially to the cylindrical gasifier sidewall. A spiral flow can occur from the upper stage down to the lower stage, thus making particle residence times much longer than that of a gas stream. This allows high efficiency gasification. (2) Changing the feed rate of oxygen to each stage can control the gasifier s temperature profile. At the upper stage, activated char is formed in lean oxygen conditions. At the lower stage, ash without carbon is fused in rich oxygen conditions. This system makes it possible to obtain both high gasification efficiency and stable operation. Oxygen Coal Upper burner Lower burner CO 2 H 2 O Slag H 2 CO CO 2 Figure 4 Characteristics of the gasifier Temperature ( C) 3.3 Gas Clean-Up This project employs cold gas clean-up in order to satisfy the tolerance limits of fuel cells. Syngas at a temperature of approximately 400 C exits from the char-filter and is heat-exchanged at the Gas/Gas Heater (GGH). Impurities such as halogens and ammonium are removed in a water scrubber, and the gas is then desulfurized in an MDEA absorber. Since MDEA has low absorptivity for carbonyl sulfide (COS), COS must be converted into H 2 S in a COS converter in advance. The clean syngas, which exits the MDEA absorber at approximately 40 C, is heated to approximately 200 C by a steam heater and the GGH and supplied to the gas turbine. Part of the clean syngas is sent to the precise desulfurizer, where it is further desulfurized down to the tolerance limit of the fuel cells or less. 4 DEVELOPMENT TARGET There are two main points in the development target. First one is to develop an optimum coal - gasifier for Fuel Cells. That means higher carbon conversion, and higher heating value of gas is expected. The figure set as a target is shown in next table. The other development target is to establish a clean-up system that purifies the gas to an acceptable level for Fuel Cells. The figure set as a target is shown in next table. Table 4 Target about Gasification Performance Carbon conversion rate > 98 % Cold gas efficiency > 78 % Higher heating value of gas > 10,000 kj / m 3 N Table 5 Target about Purification of Syngas Sulfur Compounds < 1 ppm Halogen Compounds < 1 ppm Ammonium < 1 ppm Particle Matters < 1 mg / m 3 N 5 OPERATIONS EXPERIENCE The first operation of the gasifier was in mid March 2002 and the first raw syngas was produced in that month. The operation characteristic of various equipment of the plant has been solved, and grasped steadily.but many tests were interrupted on account of minor setbacks and plugging at the slag tap hole of the gasifier. We readjusted equipment and made a change of an operation procedure etc. in order to conquer foregoing operation troubles. Table 6 shows operation progress from March 2002 to October 2003.The longest continuous operation hours are 291 hours and the 100% coal throughput rate has been already achieved in 11th operation. After we make sure pilot plant s reliability, we will obtain engineering data for commercial plants and operate using varied coals. We will continue operation runs until June

6 Table 6 Run Progress in October 2003 The number of operations 21 Total Operation hours on Coal 1,295 (hour) Longest Gasifier Continuous Operation hours 291 (hour) Coal Processed 5,463.5 (t) Table 7 shows operation achievement. Through the operations, EAGLE gasifier met some targets such as carbon conversion rate, sulfur compounds and particle matter. Target of heating value of syngas and Cold gas efficiency could be met by optimizing Coal Oxygen ratio. Ammonium and Halogen compounds could be reduced by optimizing water scrubber. Table 7 Achievement Target Achievement HHV > 10,000kJ/m 3 N 9,540kJ/m 3 N Gasifier Carbon Conversion Rate > 98% > 99% Cold gas Efficiency > 78% > 76% Sulfur Compounds < 1ppm N.D(< 0.1ppm) Gas Clean up Ammonium < 1ppm 2ppm Halogen Compounds < 1ppm 4ppm Particle Matters < 1mg/m 3 N < 1mg/m 3 N Others Continuous Operating Hours 1,000hours 291hours Varieties of Coal SUMMARY EAGLE pilot plant test that started March 2002 has proceeded favorably. Continuous operation over 290 hours last October proved that the superiority of the gasifier configuration. There were some troubles, but the cause of major trouble has clarified by present, and it has solved now. Prolonged continuous operation is expected in the future near test operation. As a result of the feasibility study of the IGFC systems, it became clear that the net thermal efficiency of IGMCFC would be 53 percent or more. A success of EAGLE pilot test approaches realization of ultimate high efficiency power generation 1 step. 7 ACKNOWLEDGEMENT EAGLE Project is being conducted as a national Project by our company subsidized by the Ministry of Economy, Trade and Industry (METI) and the New Energy Industrial Technology Development Organization (NEDO). We would like to express our deep appreciation for the support and guidance we have received from all concerned parties, including the Agency of Natural Resources and Energy of METI. Figure 5 view of pilot plant 6