Wood Pellets Application
Biomass energy refers to energy stored in organic material such as forest and agricultural products. Biomass is a key energy source in the developing world in the form of firewood used for heating and cooking. In the developed world, biomass energy is gaining popularity as a fuel source for electricity generation. One advantage that biomass has over wind and solar power is that it can generate electricity continuously or on a schedule, rather than relying on conditions of the wind or sun. The primary feedstock of biomass for heat and electricity generation is woody biomass. Other sources include corn, soy, crop residues, and grasses. Global production of biomass, waste, and ocean-based sources of energy is predicted to increase more than three times the 2007 level by 2035 (fig. 1).
Wood pellets are one type of biomass energy. Pellets are primarily manufactured from wood waste, including sawdust, shavings, and wood chips, which are byproducts of the manufacture of lumber, furniture, and other forest products. The pellet manufacturing process consists of the following steps: receipt of raw materials, screening, grinding, drying, pelletizing, cooling, screening, and packaging. The raw materials should be dried to the desired moisture content (Brackley and Parrent 2011, Hansen et al. 2009).Once the material is dried, it is ground so that each piece has a diameter that meets proper specifications. In some cases, the raw material is then conditioned in a steam conditioner to soften the wood before it is compacted. This makes the raw material less abrasive to the equipment. The next step is the pelletizing itself. Pellet machines apply pressure to force the raw materials through holes that shape the pellets. Pressure and friction in the pellet machines are used to increase the temperature of the wood, which in turn allows the lignum to soften and the wood fibers to be reshaped into pellets. After the pellets leave the pellet machines, they are air-cooled, the lignums solidify, and the pellet is formed. The dimensions, density, and other factors are specified by standards designed by industry groups.
Wood pellets application
The European wood pellet market can be divided into two segments: electric power generation and heating. Whereas most of the wood pellets shipped to Asia are directed to two markets. The first is the industrial energy sector, where pellets are used for co-firing with coal at power plants and in large boilers. The second is the home heating market, where pellets are used in pellet stoves. Anyway, there are various methods to generate heat and electricity using biomass feedstocks (Pew Center for Global Climate Change 2009):
• Direct firing—Biomass is the only fuel used at a given power plant. The feedstock is fed into a boiler, which in turn powers a steam turbine to generate electricity. The direct firing method attains efficiencies of up to 40 percent.
• Co-firing—Biomass is substituted for a portion of the coal burned in a coal-fired power plant. A coal-fired power plant can be modified to accommodate biomass and use it to supply up to 20 percent of its fuel requirements. This method reaches efficiencies between 33 and 37 percent.
• Cogeneration—Fuel is burned to produce both electricity and heat. As with direct firing, the biomass fuel powers a steam turbine generator. However, unlike direct firing, cogeneration uses the resulting exhaust flow for further electricity generation or heat generation. The advantage of cogeneration is improved efficiencies between 75 and 90 percent.
• Gasification—Feedstock is processed in a hot oxygen-starved area to produce a gas, composed mostly of carbon monoxide and hydrogen. This gas fuels a turbine to produce electricity. This method reaches efficiencies of up to 60 percent.
The article is digested fromThe Asian Wood Pellet Markets written by Joseph A. Roos and Allen M. Brackley