【正文】 ity 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 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 fouling 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 shows the photograph of one such unit built by Greenfield Research Inc., for a 220MWe Pulverized coalfired boiler in India. In this specific case, the piggyback boiler fired waste fuel from the parent boiler as that was the need of the plant. . Gasification cofiring Cofiring through gasification involves the gasification of solid biomass and bustion of the product fuel gas in the furnace of the coalfired boiler. This approach offers a high degree of fuel flexibility. Since the gas can be injected directly into the furnace for burning, the plant can avoid expensive flue gas cleaning as one would need for syngas or fuel gas for diesel engines. As the enthalpy of the product gas is retained, this results in a very high energy conversion efficiency. If the biomass contains highly corrosive elements like chlorine, alkali etc., a certain amount of gas cleaning may be needed prior to its bustion in the important benefit of injection of gas in the furnace is that it serves as a gasover firing designed to minimize NOx. Although less popular, indirect or external and gasification cofiring options have certain advantages, such as the possibility to use a wide range of fuels and easy removal of ash. Despite the significantly higher capital investment requirement, these advantages make these two options more attractive to utility panies in some cases. 3. Current status of biomass cofiring There are a number of cofiring installations worldwide, with approximately a hundred in Europe, 40 in the US and the remainder in Australia and Asia Fig. 2 [9,13]. Most of these installations employ direct cofiring, mainly because it is the simplest and least cost option. Examples include the 635 MWe EPON Project of Gelderland Power Station in Holland which uses direct cofiring with waste wood and the 150 MWe Studstrup Power Plant, Unit 1, near Aarhus, Denmark cofiring straw. Gasification cofiring is also an attractive option. Three examples of the plants operating on this type of cofiring are: the 137 MWe Zeltweg Power Plant in Styria in Austria, the AMERGAS biomass gasification project at the Amer Power Plant in Geertruidenberg, Holland, and the Kymiarvi power station at Lathi in Finland. The majority of biomass cofiring installations is operated at biomass: coal cofiring ratios of less than 10%, on a heat input basis. The successful operation of these plants shows that cofiring at low ratios does not pose any threat or major problems to the boiler operation. Fig. 2. Worldwide cofiring plant locations For hi