【正文】 s/ options available for greenhouse gas reduction [7]. Principally, cofiring operations are not implemented to save energy but to reduce cost, and greenhouse gas emissions (in some cases). In a typical cofiring plant, the boiler energy usage will be the same as it is operated at the same steam load conditions (for heating or power generation), with the same heat input as that in the existing coalfired plant. The primary savings from cofiring result from reduced fuel costs when the cost of biomass fuel is lower than that of fossil fuel, and avoiding landfill tipping fees or other costs that would otherwise be required to dispose of unwanted biomass. Biomass fuel at prices 20% or more below the coal prices would usually provide the cost savings needed [8]. Apart from direct savings in fuel cost, other financial benefits that can be expected from cofiring include the following: M costs due to direct cofiring were estimated at $ 外文文獻(xiàn) 及譯文 10 Analysis of the external cofiring option required a preliminary design of a Circulating Fluidized Bed (CFB) boiler. The required thermal input for the steam generated by the biomass boiler was determined using a turbine efficiency of 88%. A thermal design of the boiler was done using CFBCAD in order to calculate the efficiency of the CFB boiler and used in conjunction with the turbine efficiency to calculate the required biomass fuel flow rate. The capital costs of the external cofiring were determined using a detailed cost assessment. This included the estimated costs of engineering design work, project management, boiler fabrication, civil footing, secondary ponents, controls and instrumentation, and erection and missioning. This cost estimate was based on previous work done by Greenfield Research Inc. on the feasibility of a subpact, biomassfired CFB boiler for placement within an existing PCfired plant. The capital cost of the CFB boiler worked out to $139/kWth. The Oamp。 Fuel flexibility: Biomass as a fuel provides a hedge against price increases and supply shortages of coal ore. In cofiring, biomass can be viewed as an opportunity fuel, used only when the price is favorable. Various pollutionreduction incentives: As cofiring, through synergetic effects, reduces the SOx, NOx and heavy metal emissions, the plant could claim the applicable pollutionreduction incentives offered by government agencies. M costs were conservatively estimated at $5/MWhth. . Gasification cofiring In the analysis of the gasification cofiring option, a generation loss of % was taken, thus reducing the plant capacity factor to %. The product gas produced by the gasifier can cause problems in the backpass of the boiler with increased tube corrosion/fouling. This would lead to a slight increase in time for boiler maintenance and repairs, and hence the lower capacity factor. The capital cost of the gasification cofiring option was calculated based on the analysis of Antares [14]. Antares proposed a capital cost estimate of 382 USD/kWe. The capital cost was then found using the heating rate of the existing coalfired plant. The Oamp。 Earning of renewable energy tax credits: The use of biomass as an energy source to displace fossil fuel can be eligible for special tax credits from many governments. Financial incentives for plant greenhouse gas (GHG) emission reduction: A cofiring plant that uses biomass to replace an amount of coal in an existing boiler will reduce almost an equal amount of CO2 emission from the plant. M costs of the gasifier were estimated at $6/MWhth. 外文文獻(xiàn) 及譯文 11 5. Economic evaluation criteria The economic evaluation of each cofiring option was based on any savings/increase in fuel cost arising from the price difference of coal and biomass, and ine generated through the sale of emissions credits, both carbon and sulphur. As biomass is a carbon neutral fuel, any reduction in coal use can be see as a subsequent reduction in CO2 produced. A further reduction in carbon emissions could be gained if the PC plant uses a sorbent based scrubber. Sorbents such as limestone, used to capture sulphur dioxide produced by coal bustion, release additional carbon dioxide in the capture process adding to the plant’s carbon emissions. As the sulphur content of biomass is nearly zero, sulphur produced from coal bustion is reduced by the corresponding cofiring carbon dioxide and sulphur dioxide produced from the offset coal were calculated using the following equations: where [CO2] is the carbon dioxide offset by cofiring (tons/year), [C] is the carbon fraction in coal, [SO2] is the sulphur dioxide offset by cofiring (tons/year), [S] is the sulphur fraction in coal andmco is the amount of coal displaced by biomass (tons/year). From bustion stoichiometry [15], to capture every mole of sulphur, mol of calcium is needed, and production of 1 mol of calcium is associated with the generation of 1 mol of CO2. The effect of NOx reduction is a little more plex. An increase in the volatile content of a fuel busted in a PC burner could potentially reduce the NOx produced, but it would not reduce the thermal NOx in direct cofiring. The reduction would therefore be small when pared to reductions in CO2 and SO2. In external cofiring using a CFB boiler or gasifier, NOx emissions from the plant would be reduced due to the lower bustion temperatures found in CFB furnaces. Actual NOx reductions through decreased coal firing are dependent on the PC burner design and would be difficult to quantify. Gases produced from the gasification of biomass could be used to reduce the NOx emissions from PC plants if the gas was used in a NOx reburning situation. The analysis 外文文獻(xiàn) 及譯文 12 indicates that the cost of CO2 reduction through direct cofiring works out to 33$/ton of CO2. For t