"A FLUIDIZED BED AIR BIOMASS GASIFICATION CHP PLANT WITH"
A FLUIDIZED BED AIR BIOMASS GASIFICATION CHP PLANT WITH AN EXTERNALLY FIRED EVAPORATIVE GAS TURBINE CYCLE 1. Aim The overall aim of the project is to demonstrate an innovative plant for the production of heat and power from biomass residues. The project aims in particular at the demonstration of a reliable wood fueled CHP plant using an air blown fluidized bed gasifier and an externally fired gas turbine cycle. External or indirect firing occurs by firing the product gas in the exhaust of the gas turbine, and by routing the heat to the turbine through a metallic high temperature gas/gas heat exchanger or ‘air heater’. The power and the efficiency can be boosted by evaporation of hot water into the gas turbine cycle. The design capacity of the plant is 500 kWe, whereas the commercial target is the 2 to 5 MWe power range. 2. Introduction The indirect gas turbine firing is an intermediate solution between external firing of steam turbines and internal firing in gas turbines or reciprocating engines. The main incentive to proceed in this way is to reduce the gas cleaning problems encountered in internal firing and to take advantage of the potential higher efficiency of a gas turbine cycle. The main problem with the gas cleaning is the efficient and environmentally safe removal of tars. With the indirect cycle, the tars are not condensed but are burned with the hot gas in the combustion chamber of the heat exchanger. This eliminates one of the most critical problems of biomass gasification. The critical component in such a process becomes the air heater which can be made in metal or ceramic materials. The considered heater is metallic both for technical and economical reasons: ceramic pressurized heaters are not available yet and would anyway represent a too expensive component. The VUB campus provided an ideal customer for the installation and testing of this technology. Photo 1 shows the gasifier during start-up when the fuel gas is flared. Photo 1: The gasifier under operation flaring the gas 3. Technical Description Gasifier : The wood is fed from a one day capacity silo to the fluidized bed gasifier at a flow rate of some 400 kg/h. The air factor ranges between 0.25 and 0.3 producing a gas with a calorific value of 3.5-4 MJ/kg (excluding tars). The bed temperature is about 700°C and drops to about 625°C in the freeboard. The freeboard has an expanded volume in relation to the bed in order to increase the residence time of the fly ash and thus minimize carbon loss. The biomass feeding point is located close to the distributor in order to allow sufficient residence time for the particles. The producer gas is dedusted by a single cyclone. A dust content lower than 500 mg/Nm3 has been specified. Alkali and chlorine contents are depending on the nature of the biomass but should be limited. The gas pressure is 400 mm H2O. The expected gas temperature is about 600°C which is high enough to avoid tar condensation. Gas turbine cycle The direct firing of the product gas from biomass in a small gas turbine is still difficult to achieve and external firing is considered as alternative (Figure 1). The external firing reheats the turbine exhaust gas (air) and the energy is recovered in front of the turbine by a high temperature air heater. Although large in size, the cost of the metallic heat exchanger itself does not exceed 10% of the total project cost and it keeps the product gas out of the turbine. air heater combustor 900°C Fuel gas 800°C 350°C compressor gas turbine Figure 1: Indirect firing of a gas turbine using an air heater Both efficiency and power of the gas turbine cycle can be improved by injecting steam or water gas. In the present system, which is too small for combined cycles and where no steam is produced, injection and evaporation of preheated liquid water is considered for boosting power and efficiency. Direct injection of water in the cold side tubes of the air heater is designed, but not realized in the present stage. The water injection arrangement is shown on Figure 2 which shows the complete plant arrangement. In this arrangement, a topping combustor was considered in the original project but has not yet been realized due to excessive cost. On the same figure, the low temperature recuperation for district heating purposes is included. Photo 2 shows the turbine with the arrangements for indirect firing. The front tubes of the air heater are made of Haynes 120 alloy (NI-3Co-33Fe-25Cr-2.5Mo-2.5W) The remaining tubes are made of Hastelloy 800 HT and SS 253MA. The hot collector and hot duct to the turbine are made of two concentric tubes. The inner tube is made of sleeving pieces of 321 Steel. low temperature high temperature stack recuperation air heater main combustor wood pellets air add. firing pressurised feeding atmospheric hot water water fluidised bed gasifier ~ gas turbine air air and vapor Liquid water Figure 2 : Plant layout 4. Performance of the Plant : Since normal operation was not yet achieved, it is not possible to give final data about global performance. The project has however demonstrated some typical problems related to the external firing which affect the performance, and which are under consideration in the continuation of the project. The consumption of the peripherals is also known. Gasifier performance : So far only the composition of the product gas and the ashes have been measured, with exclusion of the tar content in the product gas. Only a few percentage of the carbon is not converted and found in the ash bin, which means that the conversion rate of the gasifier is pretty high. The gas analysis is found to correspond to the typical values in literature. Since the tar content is not yet measured, it is not possible to mention the exact calorific value of the gas. Since the gas is kept hot and tars are burnt in the turbine cycle, most of the energy from the gasifier is recovered and the losses mainly result from the non-converted carbon (a few %), the heat losses and the peripheral consumption. Expected overall performance : The planned performance is summarized in figures 9 and 10, where efficiencies are shown versus amount of water injection in the case of no top firing (Figure 9) and maximum top firing (Figure 10). The open cycle efficiency (24% in this case) can be Photo 2 : Gas turbine with external achieved, either through additional firing firing arrangements of Natural Gas (figure 5), or through injection of water (figure 4). 24 69 70 24,5 80 58 800C TIT 25 50 60 70 25,5 43 total 60 1000C TIT 50 Efficiency 50 total Efficiency 40 40 30 electric 30 20 electric 20 10 10 0 0 0 2 4 6 8 10 0 2 4 6 8 10 12 water in turbine % water in turbine Figure 3: Target performance in Figure 4: Target performance with absence of top firing maximum top firing 5. Economic Performance There is not enough operating experience so far to give a reasonable figure of the economic viability of the externally fired system. A 2.5 MWe plant investment cost of about 5 M€ has been estimated, based on the experience obtained. The total project cost for the present 500 kWe plant were of 2.5 MECU. The substantial specific cost reduction is mainly obtained by scaling effect and by replacing the expensive aero derivative gas turbine by large low cost marine turbochargers. The plant can be considered as consisting of two parts which have only minor interaction with each other: the gasification island and the power cycle. The gasifier requires only minor modifications and is virtually ready for commercial application, provided the product gas can be used in hot conditions. This is certainly the case in different applications, where the product gas can be used directly in boilers for production of steam, in combination with other fuels. The power cycle is not ready for commercial application, but the use of air heating at low power to heat ratio’s in new or existing boilers could be identified. Possible sites for such application of external firing are looked for. 6. Project Identifiers Project: BM/367/92-BE Owner: Vrije Universiteit Brussel, (VUB) Pleinlaan 2, 1050 Russels Belgium Tel: +32-2-6292393 Fax: +32-2-6292865 Contractors: VUB, SEGHERS BETTER TECHNOLOGY, DECOMETA, Technology: SEGHERS BETTER TECHNOLOGY (Gasifier) VOLVO AERO TURBINES (Gas Turbine), DECOMETA (Heat Exchanger) Total Cost: 2,680,124 ECU EC Support: 896,226 ECU