【正文】 savings in fuel cost, other financial benefits that can be expected from cofiring include the following: ??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. ?? 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. ?? Ondemand power production: Unlike other renewable energy technologies .: solar, wind , biomassbased power generation can be made available whenever it is needed. This helps to accelerate the capital investment payoff rate by utilizing a higher capacity factor. ?? An option towards meeting a renewable energy portfolio: Cofiring offers a fast track, lowcost opportunity to add renewable energy capacity economically as it can be added to any coalfired plant immediately, with minimum investment. ?? 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. ?? 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. ??biomass fuels are generally sourced from the areas in the immediate vicinity of the plant to save on transportation costs , the local munities benefit economically from the production of biomass fuels. All these potential benefits are, however, plex functions of local factors such as the price of coal and biomass, government policies, capital investment, and the carbon market in the evaluation of the cost effectiveness of electricity production using biomass cofiring. The present paper discusses the effect of these factors on the viability of different technical cofiring options in coalfired power plants. To illustrate these effects, an analysis of the economic aspects of different cofiring options is performed by considering the case of a 150 MW pulverized coal PC fired power plant in Canada. 2. Cofiring options Biomass cofiring has been successfully demonstrated in over 150 installations worldwide for a bination of fuels and boiler types [9]. The cofiring technologies employed in these units may be broadly classified under three types: i. Direct cofiring, ii. indirect cofiring, and iii. gasification cofiring. In all three options, the use of biomass displaces an equivalent amount of coal on an energy basis , and hence results in the direct reduction of CO2 and NOx emissions to the atmosphere. The selection of the appropriate cofiring option depends on a number of fuel and site specific factors. The objective of this analysis is to determine and pare the economics of the different cofiring options. Brief descriptions of the three cofiring options are presented here. . Direct cofiring Direct cofiring involves feeding biomass into coal going into the mills, that pulverize the biomass along with coal in the same mill. Sometime separate mills may be used or biomass is injected directly into the boiler furnace through the coal burners, or in a separate system. The level of integration into the existing plant depends principally on the biomass fuel characteristics. Four different options are available to incorporate biomass cofiring in pulverized coal power plants [10]. In the first option, the preprocessed biomass is mixed with coal upstream of the existing coal feeders. The fuel mixture is fed into the existing coal mills that pulverize coal and biomass together, and distribute it across the existing coal burners, based on the required cofiring rate. This is the simplest option, involving the lowest least capital costs, but has a highest risk of interference with the coal firing capability of the boiler unit. Alkali or other agglomeration/corrosioncausing agents in the biomass can buildup on heating surfaces of the boiler reducing output and operational time [11]. Furthermore, different bustion characteristics of coal and biomass may affect the stability and heat transfer characteristics of the flame [12]. Thus, this direct cofiring option is applicable to a limited range of biomass types and at very low biomasstocoal cofiring ratios. The second option involves separate handling, metering, and pulverization of the biomass, but injection of the pulverized biomass into the existing pulverized fuel pipework upstream of the burners or at the burners. This option requires only modifications external to the boiler. One disadvantage would be the requirement of additional equipment around the boiler, which may already be congested. It may also be difficult to control and to maintain the burner operating characteristics over the normal boiler load curve. The third option involves the separate handling and pulverization of the biomass fuel with bustion through a number of burners located in the lower furnace, dedicated to the burning of the biomass alone. This demands a highest capital cost, but involves the least risk to normal boiler operation as the burners are specifically designed for biomass burning and would not interfere with the coal burners. The final option involves the use of biomass as a reburn fuel for NOx emission control. This option involves separate biomass handling and pulverization, with installation of separate biomass fired burners at the exit of the furnace. As with the previous option, the capital cost is high, but risk to boiler operation is minimal. . Indirect or external cofiring Indirect cofiri