New Power Plant Concept for Moist Fuels, IVOSDIG

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1 ES THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 91-GT E. 47 St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections. or printed in its publications. M Discussion is printed only if the paper is published in an ASME Journal. Papers are available ^^L. from ASME for fifteen months after the meeting. Printed in USA. Copyright 1991 by ASME New Power Plant Concept for Moist Fuels, IVOSDIG HULKKONEN, M. RAIKO, M. AIJALA Imatran Voima OY SF Vantaa, Finland ABSTRACT Imatran Voima Oy (IVO) is developing an advanced gas turbine power plant process for wet fuels as peat, biomass, and lignite. The process consists of a steam injected gas turbine, an air blown gasifier and a pressurized fuel dryer. The dryer operates at steam atmosphere and is integrated to the process to produce injection steam for the gas turbine. The heat needed for drying is taken from the exhaust gases of the gas turbine. The process is patented and called IVOSDIG. INTRODUCTION Imatran Voima Oy (IVO) is in the process of developing a new type high-efficiency gas turbine power plant for moist fuels. The plant is based on IVOSDIG (Imatran Voima's Steam Drying, Injection, Gasification) process which consists of pressurized fuel drying and gasification and gas turbine coupling (Raiko, 1989). Through the process it is possible to reach a fairly high efficiency in power generation, and as the power production only takes place in the gas turbine, the plant is also economical in investments. The R&D activities aim at commercialization of this technology in the latter part of the '90s. This paper deals with the IVOSDIG process and its features in general, and introduces the plant concept designed for peat. PROCESS DESCRIPTION The idea behind the IVOSDIG cycle is to dry the moist fuel in a high pressure steam dryer from where the steam being released during drying of the fuel is conducted into the gas turbine where it generates electricity while expanding together with gas. Thus it is a question of a gas turbine power plant provided with steam injection. Here most of the injection steam, contrary to the ordinary, is taken from the released steam during drying of the fuel. In the gas turbine combustion chamber the steam replaces the excess-air whereby the air flow and the compressor power are reduced, consequently the net power from the process is increased. The IVOSDIG process combines the Brayton and Rankine cycles so that the only power machine used is the gas turbine and the steam turbine with condensers is not needed at all. This resembles the already earlier developed Cheng process. The energy needed for the drying is taken from the waste heat boiler located after the gas turbine and no net energy from the process is wasted for the drying of the fuel. In addition, it is possible to develop additional steam for injection into the gas turbine, and, if necessary, also district heat and process steam. In the dryer the fuel is dried from % initial moisture content (wet base) down to % final moisture content after which the dry fuel is fed into the gasifier. In the gasifier, the fuel is gasified, i.e. it reacts with the understoichiometric air quantity forming product gas suitable for fuel in the gas turbine. Prior to gas turbine, the gas is cooled and cleaned from the dust particles. The simplified flow diagram of the IVOSDIG process is given in Figure 1. Presented at the International Gas Turbine and Aeroengine Congress and Exposition Orlando, FL June 3-6, 1991

2 RECIRCULATION BLOWER EXTRACTION STEAM (EVAPORATED FROM FUEL) 'S GAS CLEANING PRESSU- RIZED DRYER 50 EFFICIENC Y (%) WET FUEL PRESSU- DRY FUEL RIZATION FEEDING COMBUSTION BOOSTER CHAMBER 45 :7 AIR WASTE HEAT BOILER INITIAL MOISTURE CONTENT OF THE FUEL (%) DRYING STEAM SUPERHEATER et power output Moisture content after drying: 25 Efficiency = Fuel gas cooling: 900->400 C fuel input (LHV) Air temp.: -5 C ADDITIONAL STEAM Gas turbine inlet temp C GENERATOR Carbon conversion: 98 Aux. power: 3 % of the output Figure 2. Power generating efficiency. Fig. 1. IVOSDIG process flow diagram. The pressurized fuel drying and gas turbine coupling unique in the IVOSDIG process can also be applied to the gas turbine and combined cycle processes based on direct combustion. In this paper, the presentation is restricted to the gas turbine process based on fuel gasification only. PROCESS FEATURES AND APPLICABILITY The IVOSDIG process features include, besides the simple process construction, also a high efficiency in power production, suitability to wet and low calorific value fuels, small size of equipment, and low investment and operating costs which lead into fairly economical price of electricity in small magnitudes already. The power generating efficiency of the process may exceed 50% specified according to the lower heating value (LHV) of the fuel. The efficiency is obtained from the equation where Pgen Paux (1) mf Hu Pgen = Generator power Paux = Auxiliary power mf = Fuel flow (kg/s) Hu = Fuel lower heating value (MJ/kg) For the peat concept to be introduced later, the power production efficiency as function of the initial fuel moisture content is shown in Figure 2. The figure shows that the efficiency grows heavily as the initial fuel moisture content grows. Improvement in the efficiency along with the increase in the moisture content is due to the definition of the fuel input according to the lower heating value of the fuel. With peat the high moisture content facilitates peat production which lowers the fuel production costs. Thus the price of wet fuel per energy unit (FIM/MWh) is more economical. The price of peat in Finland is for the moment about half of the price of natural gas. The process efficiency calculated according to fuel higher heating value (HHV) remains nearly constant regardless the moisture, and is abt. 35 %. As can be deduced from the figure depicting the efficiency IVOSDIG process is suitable for moist and very wet fuels, such as peat, biomass, lignite and waste sludges. The upper moisture limit of fuel is obtained from the energy in the waste heat boiler necessary for drying of the fuel. The maximum moisture content is % whereby, in practice, nearly all of the waste heat boiler output goes for drying of the fuel. With lower moisture content fuels it is possible to produce in the waste heat boiler additional steam either for injection or for use elsewhere. In mere power production all the steam to be generated is conducted to the gas turbine, thus the entire steam flow produces electric power. Because of the waste heat boiler realized on one pressure level the flue gas final temperature remains, however, fairly high (abt. 200 C). 2

3 In the combined power and heat production the thermal power can be produced in the IVOSDIG process by cooling the flue gases down to a lower temperature or by replacing additional injection steam. The ratio of power and heat production is fairly freely selectable. Limitations to the control of steam flow are mainly created by the flexibility of the gas turbine. The theoretical power and heat output ratio for the peat concept to be introduced later is given in Figure 3. IVOSDIG PROCESS FOR PEAT Peat is in addition to biomass the only indigenous fuel in Finland, and it is rather widely used for power and heat production. Originally the moisture content of peat is very high, being of the order of 90 %, but after solar drying and pretreatment the moisture content is usually reduced to abt. 50% before use in a power plant. However, if it is possible to use the peat with higher initial moisture content, as is the case with IVOSDIG process, the production costs of peat are decreased. This leads to a lower fuel price which with high efficiency makes the plant economical even in small scale. POWER OUTPUT 70 W=Initial moisture content 60^-- = n 1n 9n -in 4n Sn Fn DISTRICT HEATING OUTPUT The peat concept of the IVOSDIG process has been calculated based on KWU's V64.3 gas turbine (Becker and Ziegner, 1988). The turbine has been thought, in this example, to be modified for steam injection by reducing the compressor air flow by abt. 30%. The turbine pressure ratio is 17 and the inlet temperature of the gas is 1120 C (ISO). The fuel is dried in a pressurized flash steam dryer, and the dried fuel is gasified in the fluidized bed gasifier. The simplified process diagram of the peat consept is shown in Figure 1. The fuel used in the process is milled peat with an initial moisture content of 70% (wet basis) and lower heating value of 4.5 MJ/kg (LHV). The fuel composition is shown in Table 1. Fuel moisture content after drying: 25 % Carbon conversion: 98 % Fuel gas cooling: 900->400'C Figure 3. The power and heat production possibilities. The figure shows the connection between the power and heat production the fuel moisture content being the parameter. When needed, the highest district heating output is obtained when the initial fuel moisture content is low. On the other hand, the process efficiency is thus also lower. In the figure, the horizontal section of the curve depicts the output obtained from flue gas cooling which does not directly affect the power output gained. The realization of the IVOSDIG process by using full steam injection requires modification of the gas turbine. The compressor has to be reduced to correspond to a lower than ordinary air flow. Alternatively, the excess air produced by a normal-sized compressor can be conducted to a separate expander which gives back part of the power gone into the air compression. The gas turbine has also to be suitable for low heating value gas firing. Table 1. The properties of peat. PROXIMATE ANALYSIS - Volatile matter 69.4% - Ash 3.9% - Fixed Carbon 26.7% ULTIMATE ANALYSIS (Dry base) - Carbon 55.1% - Hydrogen 5.6% - Oxygen 33.1% - Nitrogen 2.1% - Sulphur 0.2% Calorific heating value 22 MJ/kg The wet fuel is fed into a pressurized dryer where the fuel is dried down to 25 % final moisture content. The dryer is a direct steam dryer where the heat necessary for water evaporation is obtained from the cooling of the superheated steam recirculating in the dryer circuit. The steam is recirculated by means of a high pressure blower and a small amount of the flow is extracted from the circuit as injection steam. After the dryer tube the fuel is separated from the steam flow and fed into the gasifier. The gasifier is of the fluidized bed type which is well-suited for gasifying of very reactive fuels, such as peat (Mojtahedi et al., 1990). The gasification takes place in a temperature of 900 C and 3

4 a pressure of 21 bar. The reactants used are both air and steam. The air is pressurized with the gas turbine compressor to a pressure of 17 bar after which it is compressed to the gasifier pressure by means of a separate booster. The approximated composition of the low-btu product gas formed in the gasifier is given in Table 2. The carbon conversion of the gasifier is 98%. Table 2. Fuel gas composition in peat air gasification at gasifier outlet. GAS COMPOSITION AT GASIFIER OUTLET (Dry gas) - CO 18.9% - CO2 12.3% - H2 16.7% - CH4 2.8% - N2 49.1% - Moisture content 20.1% (Wet gas) - Heating value 4.1 MJ/kg (LHV) After the gasifier, the product gases are cooled down to the temperature of 400 C. Gas cooling is carried out both because of the limitations set by the gas turbine and to condense the vapour phase alkali metals on the surface of the dust particles. The cooling of the gas is realized mainly by mixing the steam released in the drying (and extracted from the drying circuit) with the product gas. A small amount of additional steam and saturated water raised in the waste heat boiler are also injected in the gas stream. The latter is not shown in Figure 1. After the cooling the gas flow is conducted to the ceramic filter unit where the solid particles are separated from the gas flow. Pressurized fluidized bed gasification and hot gas clean-up using peat as fuel has been studied in experimental research projects in Finland (Hulkkonen et al., 1989, Kurkela, Stahlberg, 1991). Finally the cooled and with steam diluted fuel gas is burned in the combustion chamber of the gas turbine. The total heating value of the fuel gas is abt. 3 MJ/kg (LHV). After the gas turbine the flue gases are conducted into the waste heat boiler where the drying circuit steam superheater and additional injection steam generator are located. The power output obtained from the process is abt. 92 MW the fuel input being 183 MW. The power generating efficiency calculated according to the fuel lower heating value is thus 50.7 %. The process parameters of the plant are listed in Table 3. Table 3. Technical data of IVOSDIG peat concept. Dryer (Superheated steam flash dryer) - Pressure 23 bar - Temperature C - Drying capacity 21 kgh 2O/s - Circulating steam flow 223 kg/s Gasifier (Fluidized bed gasifier) - Gasifying agent Air/steam - Pressure 21 bar - Temperature 900 C - Gas heating value 4.1 MJ/kg (LHV) - Carbon conversion 98% Gas Turbine (Hypothetical, modified from KW U V64.3) - Pressure ratio GT inlet temperature 1120 C (ISO) - Gas flow 179 kg/s Compressor - Air flow 125 kg/s - Air temperature -5 C - Thermal input 183 MW (LHV) - Net power output 92 MW - Auxiliary power 4 MW - Thermal efficiency 50% The sulphur dioxide (SO2) emissions of a peat-fired power plant are relatively low owing to the low sulphur content of peat, typically below 140 mg/mj. If a lower level is required for environmental reasons, the sulphur dioxide emissions can be reduced by feeding limestone or dolomite into the fluidized bed in the gasifier. More difficult than the sulphur oxides to retain could be the nitrogen oxides (NOx ). Because of the relatively high nitrogen content of peat and the properties of fluidized bed gasification the ammonia content formed in the gasifier is fairly high (Kurkela, Stahlberg, 1991). In the gas turbine furnace the ammonia further forms nitrogen oxides. If the ammonia level of the product gas cannot be sufficiently reduced, it is necessary to place in the flue gas line e.g. a catalytic cleaning system. The share of so-called thermal NO x formed of the nitrogen in the air is negligent owing to the low combustion temperature. DEVELOPMENT SITUATION AND FUTURE PROSPECTS The IVOSDIG process is patented in several countries, and the development of the peat process aims at commercial level towards the end of the 1990s. Components undergoing development in Imatran Voima are the fuel supply and drying systems which have to be designed specifically for the needs of the IVOSDIG process. As for the gasifier and the gas turbine, it is possible to apply for these the technology developed for the advanced coal gasification combined-cycle plants (air gasification, dry gas cleaning) which is becoming commercial in the near future. 4

5 REFERENCES Becker B., Ziegner M., Die neue Siemens /KWU-Gasturbine V64.3. Motortechnische Zeitschrift 49(1988). Hulkkonen S., Jahkola A., Kurkela E., The Otaniemi Fluidized Bed Test Facility and Research Project. Proc. of the 1989 Int. Conf. on Fluidized Bed Combustion. ASME book No A-1989, Vol I, pp Kurkela E., Stahlberg P., Air Gasification of Peat, Wood and Brown Coal in a Pressurized Fluidized Bed Reactor. Technical Research Centre of Finland. Draft. To be published in Mojtahedi W., Kurkela E., Nieminen M., Release of Sodium and Potassium in the PFB gasification of peat. Journal of the Institute of Energy. September pp Raiko M., Gas Turbine Power Plant Fired by a Water-Bearing Fuel and Method for Utilizing the Heat Value of Said Fuel. US- Patent. Nr