【正文】 gher cofiring ratios, however, it might be necessary to use an indirect cofiring method. 4. Case study methodology The present analysis of cofiring options considers only the economic and emissive effects of cofiring biomass within the plant facility and does not include changes in fuel transportation requirements. In North America, many local sources of biomass are available, and the use of a locally available source of biomass could have benefits beyond those discussed in this paper, in terms of reduced costs and emission generated from transportation of fuel. In areas where the supply of high quality biomass is limited transportation of biomass to the plant would likely be an important part of the economic and environmental costs. The amount of fuel replacement with biomass is generally very low in cofiring because especially in direct firing, the boiler furnace designed for a specific fossil fuel may not respond favorably as there is a major departure in bustion and flame radiation characteristics when some other fuels in used. If cofiring is applied to a fluidized bed boiler, this limit may not be that stringent. The present economic analysis is based on a 150 MW pulverized coal plant located in Eastern Canada. As such, only 10% biomass cofiring rate is considered in all the three different cofiring options examined here. Engineering design of the indirect cofiring system, its capital cost estimation, including fuel requirements for all three options, was carried out through a puterbased analysis. Table 1 lists the inputs of the thermodynamic design. The properties of the biomass fuel used in the analysis were taken as that of the hardwood maple. Hardwood species are widely available in Eastern Canada and are often discarded when harvesting 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 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