【正文】 sting of softwood trees for the pulp and paper industry takes place, making hardwood very cost effective. For coal, a low ash bituminous type coal was considered, typical of the fuel type used in the specific pulverized coal boilers. Table 2 presents the results of the ultimate analysis of coal and biomass. For all three cofiring options, the energy input remains the same, and was determined using the overall plant generation and heat rate: where, Qplant is the plant heat input MJ , Pplant is plant electrical generation MWh and HRplant is the plant heat rate MJ/MWh . The heat input required from the biomass was calculated at 10% of the overall heat required by the plant. The amount of coal that would be offset through the cofiring of biomass, in a year, was found through the following equation: where, mco is the mass of coal offset by cofiring tons/year , fbf is the biomass cofiring fraction, HHVcoal is the higher heating value of coal MJ/ton , and CF is the plant capacity factor. The capacity factor specifies as to what extent the installed capacity of the plant is utilized, either for technical reasons, or for operational reasons. Technical reasons, leading to technical availability of the plant, may be less than 100% due to forced shutdown or routine maintenance. The higher the reliability of the cofiring option, the higher is this factor. Direct firing means, which could interfere with the operation of the existing plant, could result in lower CF. . Direct cofiring Biomass firing in coal plants can result in increased tube corrosion/fouling or problems in the fuel pulverization and feed system, leading to increased maintenance and down time for the plant. This reduces the CF further. In the analysis of the direct cofiring option, a generation loss of 1% was therefore considered, which reduced the plant capacity factor to 79%. The capital cost associated with the implementation of direct cofiring was calculated using a value of 279 USD/kWth, from Cantwell [7]. The increased Oamp。M costs due to direct cofiring were estimated at $ 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。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。M costs of the gasifier were estimated at $6/MWhth. 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