System Description

Primary Hogger

Primary Hogger

Mechanical Vent Exhauster

System Boundary for Biomass F&adstock Handling System

Valve

Mechanical Vent Exhauster

System Boundary for Biomass F&adstock Handling System

Valve lUUWvmWAWAl

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Figure 1. Biomass co-firing retrofit schematic for a pulverized coal boiler system.

Co-firing is the simultaneous combustion of different fuels in the same boiler. Many coal- and oil-fired boilers at power stations have been retrofitted to permit multi-fuel flexibility. Biomass is a well-suited resource for co-firing with coal as an acid rain and greenhouse gas emission control strategy. Co-fring is a fuel-substitution option for existing capacity, and is not a capacity expansion option. Co-fring utilizing biomass (see Figure 1) has been successfull y demonstrated in the full range of coal boiler types, including pulverized coal boilers, cyclones, stokers, and bubblin g and circulating fuidized beds [1]. The system described here is specifically for pulverized coal-fred boilers whic h represent the majority of the current fleet of utility boilers in the U.S.; however, there are also significant opportunities for co-fring with biomass in cyclones. Co-fring biomass in an existing pulverized coal boiler will generally requir e modifications or additions to fuel handling, storage and feed systems. An automated system capable of processing and storing sufficient biomass fuel in one shift for 24-hour use is needed to allow continuous co-fring while minimizin g equipment operator expenses. Typical biomass fuel receiving equipment will include truck scales and hydrauli c tippers, however tippers are not required if deliveries are made with self-unloading vans. Biomass supplies may b e unloaded and stored in bulk in the coal yard, then reclaimed for processing and combustion. New automate d reclaiming equipment may be added, or existing front-end loaders may be detailed for use to manage and reclai m biomass fuel. Conveyors will be added to transport fuel to the processing facility, with magnetic separators to remove spikes, nails, and tramp metal from the feedstock. Since biomass is the "flexible" fuel at these facilities, a 5-da y stockpile should be sufficient and will allow avoidance of problems with long-term storage of biomass such as mol d development, decomposition, moisture pick-up, freezing, etc. [2].

Fuel processing requirements are dictated by the expected fuel sources, with incoming feedstocks varying from gree n whole chips up to 5 cm (2 inches) in size (or even larger tree trimmings) to fine dry sawdust requiring no additiona l processing. In addition to woody residues and crops, biomass fuel sources could include alfalfa stems, switchgrass , rice hulls, rice straw, stone fruit pits, and other materials [3]. For suspension firing in pulverized coal boilers, biomass fuel feedstocks should be reduced to 6.4 mm (0.25 inches) or smaller particle size, with moisture levels unde r 25% MCW (moisture content, wet basis) when firing in the range of 5% to 15% biomass on a heat input basis [2,4] . Demonstrations have been conducted with feedstock moisture levels as high as 45%. Equipment such as hoggers , hammer mills, spike rolls, and disc screens are required to properly size the feedstock. Other boiler types (cyclones , stokers, and fluidized beds) are better suited to handle larger fuel particle sizes. There must also be a biomass buffe r storage and a fuel feed and metering system. Biomass is pneumatically conveyed from the storage silo and introduced into the boiler through existing injection ports, typically using the lowest level of burners. Introducing the biomass a t the lowest level of burners helps to ensure complete burnout through the scavenging effect of the upper-level burner s and the increased residence time in the boiler. Discussions with boiler manufacturers indicate that generally n o modifications are required to the burners if the biomass fuel is properly sized [1].

The system described here, and shown in Figure 1, is designed for moderate percentage co-firing (greater than 2% o n a heat input basis) and, for that reason, requires a separate feed system for biomass which acts in parallel with the coal feed systems. Existing coal injection ports are modified to allow dedicated biomass injection during the co-firing mode of operation. For low percentage co-firing (less than 2% on a heat input basis), it may be possible to use existing coal pulverizers to process the biomass if spare pulverizer capacity exists. If existing pulverizers are used, the biomass i s processed and conveyed to the boiler with the coal supply and introduced into the boiler through the same injectio n ports as the coal (i.e., the biomass and coal are blended prior to injection into the boiler). Using existing pulverizer s could reduce capital costs by allowing the avoided purchase of dedicated biomass processing and handling equipment, but the level of co-firing on a percentage basis will be limited by pulverizer performance, biomass type, and exces s pulverizer capacity. The suitability of existing pulverizers to process biomass with coal will vary depending o n pulverizer type and biomass type. Atritta mills (pulverizers which operate much like fine hammermills), for example, have more capability to process biomass fuels [3].

Drying equipment has been evaluated by many designers, and recommended by some. Dryers are not included her e for three reasons: (1) the benefit-to-cost ratio is almost always low, (2) the industrial fuel sources that supply most co-firing operations provide a moderately dry fuel (between 28% and 6% MCW), and (3) biomass is only a modes t percentage of the fuel fired. Although drying equipment is not expected to be included initially, future designs ma y incorporate cost effective drying techniques (using boiler waste heat) to maintain plant efficiency while firing a broader range of feedstocks with higher moisture contents.

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