Thermal Gasification of Biomass Introduction

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esidual char from the pyrolysis step. The resulting fuel gas is cooled and cleaned in aventuri scrubber and is fed to a 12 litre Perkins engine rated at 100-120 kW e . Thewaste heat from the engine exhaust is used to heat the first stage pyrolysis reactor. Theefficiency of electricity production is estimated to be 25%. Tests in the 50 kW th DTUPilot Plant showed that if the feed is dried to 30-35% moisture content the processmay consume the entire condensate and there may not be any need to provide a wastewater treatment facility. The pilot plant tests produced a fuel gas with a heating valueof 6 MJ/Nm 3 at a thermal efficiency of about 90%.FinlandLahti Kymijärvi ProjectIn 1997-98, the Lahden LÄMPÖVOIMA Oy, installed a 60 MW th capacity atmosphericpressure Foster Wheeler (formerly Ahlström) CFB biomass gasifier, at a cost ofapproximately US$15 million at its 200 MW e fossil fuel fired power station. Thispower plant was originally built in 1976 to use fuel oil. In 1986 the burners in theboiler were converted to natural gas and a natural gas turbine cycle was added. Thebiomass gasification plant was installed primarily to use locally available fuels andwaste materials including plastics. The gasifier is a single gasifier vessel with a cycloneand an air preheater for heating the gasification air to approximately 400°C. The LCVgas is cooled from approximately 830-850°C to 700°C before it is transported in apipeline to the boiler. The raw gas has no adverse effecton the performance of the boiler. Emissions are reducedand the heating surfaces in the boiler stay relativelyclean. The reported gas composition (in volume %) is:12.9% CO 2 ; 4.6% CO; 5.9% H 2 ; 40.2% N 2 ; 33.0%H 2 O; 3.4% C x H y . The heating value of the LCV gas isapproximately 2.0-2.5 MJ/Nm 3 . The NOx emissionswere reduced by 5% (permitted level is 230 mg/MJ forboth NOx and SO 2 ) and the dust emissions werereduced by half because of increased conductivity ofdust. However HCl emission increased by a smallquantity of 5 mg/Nm 3 . The present breakdown of fuelsin the boiler is approximately: 11% LCV fuel gas fromthe gasifier, 69% coal, 15% natural gas to boiler, and5% natural gas to gas turbine. The annual average totalefficiency is approximately 80%, the fuel to powerefficiency with gas turbine in operation is 35%. The gasturbine has increased the efficiency by 4% points. Theplant supplies 200 MW e power to the national grid(110 kV line round the town) and 250 MW th heat tothe town (100,000 inhabitants) and surrounding houses(main pipe 700 mm). The district heating system wasconstructed in 1958.Courtesy of Lahti Kymijärvi Project, Finland9
BIONEER ProcessCourtesy of BioneerKauhojoki, FinlandThe BIONEER gasifier is an updraft moving bed gasifier, producing tarry LCV fuel gas.The gasifier consists of a refractory lined vessel with a rotating cone-shaped grate.Biomass fuel is fed from the top, from where it flows downwards through drying,pyrolysis, gasification and combustion zones. The residual ash is discharged from thebottom by the rotating grate. The temperature of the combustion zone is regulated byhumidifying gasification air. Air and steam are fed as the gasification media throughthe grate. Since updraft gasification produces a raw gas with significant amounts oftar, the gas cannot be either transported long distances or directly used in IC engines.In the existing BIONEER plants the gas is burnt in a close coupled boiler to generatesteam and hot water for district heating. During the mid 80s, VTT and BIONEERconducted extensive tests with a variety of feed stocks (eg wood chips, forest wastes,peat, straw, RDF pellets, and coal and RDF mixed with wood chips) in a 1.5 MW thpilot plant located at BIONEER’s Hämeenlina works. A typical gas composition with41% moisture content wood chips consists of 30% CO, 11% H 2 , 3% CH 4 , 7% CO 2 ,and 49% N 2 , with a HHV of 6.2 MJ/Nm 3 . The tar content of dry product gas isestimated to be in the range of 50-100g/Nm 3 . Between 1985 and 1986, whenfuel oil prices were high, eightcommercial BIONEER plants, withcapacities ranging from 4-5 MW th , werecommissioned, five in Finland and three inSweden. Four plants are operated withwood or wood and peat mixtures while therest are operated with peat only. Most ofthe gasifiers are in operation at smalldistrict heating plants to providecirculating hot water. The BIONEERplants are completely automated andoperated with minimal personnel costs.Ahlstrom Corporation bought the BIONEER company originally owned by YITCorporation. After Foster Wheeler acquired Ahlstrom, a 6.4 MW th plant was installedat Ilomantsi, in eastern Finland in 1996. The estimated investment cost for districtheating applications is about 350 kECU/MW th and the operating cost is about 17ECU/MWh.Wisa Forest Pyroflow Gasifier10In 1981, Ahlstrom Corporation developed the first 3 MW th capacity pilot CFB gasifierfrom its successful CFB pyroflow combustion technology at the Hans AhlstromLaboratory at Karhula. The first commercial Ahlstrom Pyroflow CFB gasifier wascommissioned in 1983 at the Wisa Forest Pulp and Paper Mill in Pietarsaari, Finland.The fuel for the 35 MW th (about 150 t/day of biomass) gasifier is primarily bark andsawdust, sized up to 5cm, and dried at 150°C to about 15% moisture content. Thebiomass is fed from the side into the circulating sand of an air-blown CFB gasifiermaintained at about 900°C. The hot fuel gas at 700°C, is fed directly to a lime kiln.The objective of replacing 85% of the fuel oil for the lime kiln was achieved within a
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Thermal Gasification of Biomass Introduction

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