issue under investigation is the risk of chloride and alkali ne causing harmful layers on heating surfaces and increased risk of corrosion i n !he boiler. The power plant's fuel handl ing system is designed to accept multiform fuel and the target is to balance !he different fuel characteristics to ensure as stable a combustion process as possible to control emissions and also generate electricity and steam at requi red levels. The plan! ai ms to use about 150 000-200 000 m 3 of logging residue annually. The logging residue is hauled to the plan! as loose material or as bundles (also known as bales). Thi s wood fuel procurement system, based on bundl ing (baling) of forest residues, is i nnovative. Alholmens Kraft expects to use about 300 000 bales of forest residue per year. The plan! is partici pating i n a projeci which ai ms to develop a new logging residue harvesting method, where !he loggi ng residues are bundled (baled) in the field. The mai n idea is to use traditional timber harvesting equipment. Four baling (bundli ng) machi nes are already in use, each producing some 20 bundles per hour. The volume of one bundle is about 0.5 m-3 (450-500 kg, 3.3 m long). An ordinary timber truck is used and it can lake 60 to 70 bundles in one-load. Eventually !here wi ll be 5-6 bal i ng (bundling) chai ns i n action. Overall, !he new plan! can be seen as i ntroducing a "best practice" biomass/fossi l fuel co-fired power plan! concept with extremely diverse fuel range, and is bel ieved to be suitable for repl i cation almost anywhere i n Europe and North America. Alholmens Kraft is competitive as a CHP-plant because coal and peat are taxed when used for heat production. But !he plant has also additional capacity to produce electri city where it cannot use the heat. in that sense the plant is competing with coal power. The plant's electricity is likely to become more competitive i n !he future. Coal capacity is decreasing steadily because no new i nvestments are being MAKALE / ARTICLE made. Coal's future price and taxation level is highly uncertain. At !he same ti me electricity consumption and the demand for green energy are increasing. Plant outline The key process issues were !hat steam and heat had to be suppl ied as economically as possi ble, and with high reliabi l ity i n a wide range of operational situations. Electricity production also had to be able to respond economically to !he base and vari able load demands. A great deal of process analysis was carried out with the turbine suppl ier, in particular duri ng the projeci planning phase, to opti mize turbi ne extractions and process reducti on valves. To achieve the best overall efficiency, high steam parameters were studied along with supercritical steam values. The following conclusions were arrived at: A re-heater was feasi ble because the plan! unit size is sufficiently big !hat i ncreased efficiency compensates tor !he increased i nvestment. The re-heater i ncreases the availabi lity of !he steam supply to the paper mil i i n the case ofa turbine shut down with the boiler continui ng in operation. A maxi mum steam temperature of 545°C tor the HP and RH side was desi rable to mi ni mize !he risk of high temperature corrosion and to enable the use of conventional materials on the heati ng surfaces. A sub-critical pressure, 165 bar, was best. Moving to a supercritical process was estimated to be too expensive i n relation to the benefits. At the heart of !he plan! i s a CFB (ci rculating fluidized bed) boiler with 550 MWth capacity suppl ied by Kvaerner Pulping. CFB technology can cope with the wide range of fuels envisaged, bio-fuels, coal and peat, whi le at the same time achieving low emissions. The i ndividual design features of!he CFB are proven and conventional, but the way they have been put together at Alholmens is i nnovative and usi ng biofuels as !he main fuel for !his size of boiler is bel ieved to be unique. The operational envi ronment at Alholmens means !hat the boiler is effectively half a utility boiler and half an i ndustri al boiler. The utility side requi res higher steam dala and !he i ndustrial side requi res better ECOGENERATION WORLD 57
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