Unprocessed biomass comes in a variety of sizes, shapes and other properties, ranging from a gooey puddle of microscopic algae to the massive trunk of a large oak tree. If any feedstock is to be used effectively for combustion, it usually needs some form of processing to ensure that it is suitable for effective use in combustion equipment. The most common types of processing for combustion include drying, resizing, and densification.
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Dry fuel burns better than wet fuel because the water in fuel must be "boiled off" before the fuel can heat up enough to combust. Dry fuel is also desirable for storage because it is less susceptible to rot and microbial degradation. Some processes (i.e. pelletizing) are also very sensitive to moisture levels and may require careful treatment. The three most common methods for drying biomass (in order of increasing cost and effectiveness) are 1) ambient air drying, 2) forced air drying, and 3) kiln drying. One unusual approach to drying, called Torrefaction, partially burns the fuel, using the heat from the flames to dry the remaining fuel.
The size of a biomass fuel has a big impact on how it performs in a combustion system. Not only are its combustion characteristics affected, but the feed and handling equipment for a combustion system often works only with fuel of a specific size. The main methods for resizing biomass are sawing, splitting, chipping, shredding and grinding. A key consideration in sizing is fuel is consistency in shape and size, particularly in the case of woodchips or pellets, to enable the fuel handling system to operate with greater ease.
While some biomass fuels, most notably wood, are fairly dense on a weight and energy basis, many types of biomass are relatively light. This makes them difficult to transport and handle in an effective way. However, it may be possible to densify the biomass to make it more compact and easy to use. The most common methods used are pelletizing, briquetting, and cubing. Densification also increases the size of the biomass fuel, which may also be beneficial.
It is remarkable that biomass can be transformed into a variety of other types of solid, liquid or gaseous fuel. Sometimes it is worthwhile to transform the fuel before combusting, if the transformed fuel is easier to handle or use. Most farmers will not transform their biomass on-site, but instead sell their biomass to a processing company. However, it is worthwhile to understand the ways their feedstock can be altered for different uses.
Some possible products from biomass include:
Other articles in this Combustion Series:
Woody Biomass Processing: