Biomass Combined Heat and Power Catalog of Technologies

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Biomass Combined Heat and Power Catalog of Technologies ( biomass-combined-heat-and-power-catalog-technologies )

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EPA Combined Heat and Power Partnership Biomass CHP Catalog designed biomass steam system has a reasonable expectation of operating in the 92 to 98 percent availability range.53 Operating Advantages and Disadvantages Stoker and fluidized bed boilers have specific operating advantages and disadvantages with biomass fuels depending on the fuel characteristics and site requirements. Biomass fuels are extremely variable in terms of heating value, moisture content, and other factors that affect combustion. Wood and most other biomass fuels are composed primarily of cellulose and moisture. As discussed previously, the high proportion of moisture is significant because it acts as a heat sink during the combustion process. The latent heat of evaporation depresses flame temperature, taking heat energy away from steam production, and contributing to the difficulty of efficiently burning biomass fuels. Cellulose, in addition to containing the chemical energy released in combustion, contains fuel-bound oxygen. This oxygen decreases the theoretical air requirements for combustion and, accordingly, the amount of nitrogen included in the products of combustion. A few general guidelines for direct firing of wood and biomass in boilers include: • Maintain stable combustion, which can be achieved in most water-cooled boilers with fuel moisture contents as high as 65 percent by weight, as received. • Use of preheated combustion air reduces the time required for fuel drying prior to ignition and is essential to spreader stoker combustion systems. Design air temperatures will vary directly with moisture content. • A high proportion of the combustible content of wood and other biomass fuels burns in the form of volatile compounds. A large proportion of the combustion air requirement, therefore, is added above the fuel in stoker and other conventional combustion boilers as overfire air. • Solid chars produced in the initial stages of combustion of biomass fuels are of very low density. Conservative selection of furnace section size is used to reduce gas velocity and keep char entrainment into the flue gases and possibly out the stack at acceptable levels. To ensure smooth fuel feeding, biomass fuels have to be carefully sized and processed. As discussed above, the moisture content of wood and other biomass waste can vary over a wide range, from 10 percent to more than 60 percent. To ensure steady heat input into the boiler using volumetric feeders, efficient homogenization of fuel with different moisture contents at the fuel yard is a necessity. Biomass-based fuels can increase the risk of slagging and fouling of heat transfer surfaces and, in some cases, the risk of fireside corrosion as well. Potassium ash content is relatively high in fresh wood, green particles, and fast-growing biomass, which causes the ash to melt at low temperatures and leads to a tendency for fouling and slagging. Additionally, biomass fuels can contain chlorine, which, together with alkalis, can induce aggressive corrosion. Table 5-3 provides a comparison of combustion characteristics and fuel issues for stoker and fluidized bed boilers. Stoker boilers have been around for a long time and are a relatively basic technology, whereas fluidized bed technology is newer and more complex, but offers more flexibility and operating control. Fluidized bed systems offer significant operating flexibility because they can operate under a wide range of load conditions. The thermal inertia of the bed material allows it to withstand changes in moisture and heating content of the fuel without negative impacts. Additionally, the low fuel 53 Energy Products of Idaho, a company that provides fluidized bed boilers, has reported operating availabilities of 98 percent for their units, . 5. Biomass Conversion Technologies 37

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