熱能與動力工程專業畢業論文外文文獻及翻譯-免費閱讀

　　

【正文】 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 外文文獻 及譯文 4 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 外文文獻 及譯文 5 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 cofiring involves the installation of a pletely separate biomass boiler to produce lowgrade steam for utilization in the coalfired power plant prior to being upgraded, resulting in higher conversion efficiencies. An example of this option is the Avedore Unit 2 project in Copenhagen, Denmark. In Canada, Greenfield Research Inc. has developed a similar CFB boiler design that utilizes a number of units of the existing power plant systems like ID fan etc. to reduce the capital cost. In this system, a subpact circulating fluidized bed boiler is designed specifically to have a piggyback ride on an existing power plant boiler. Since it is not a standalone boiler it does not need many of the equipment or ponent of a separate boiler. This 外文文獻 及譯文 6 unit releases flue gas at relatively high temperature and joins the existing flow stream of the parent coalfired boiler after air heater. Thus, the flue gas from the cofiring unit does not e in contact with any heating elements of the existing boiler, thus avoiding the biomass related fo uling or corrosion problem, which is the largest concern of biomass cofiring. This boiler is totally independent of the parent unit, and as such, any outage in the cofiring unit does not affect the generation of the parent plant. Thus this indirect bustionbased option offers high reliability. The piggyback boiler produces low pressure steam feeding into the process steam header of the power plant. Fig. 1 sho