Ways to Burn Wood For Industrial Use

Direct burning is the most familiar way of producing energy from wood. Forest products industries are the main users of wood as fuel on a large scale, because they have much woody debris left over from milling and harvesting. The basic strategy is to reduce leftover wood to walnut-size or smaller particles. This size reduction is usually done by a "hog." Large pieces enter the top of the hog and are forced down by rotating hammers. The small pieces fall through spaced bars, which control their size. This particulate fuel is then stored in piles at the mill, and from there reclaimed to fire boilers that produce steam for heat and power. The largest wood-fíred boilers commonly used in pulp and paper plants or in small utility plants produce enough steam to generate 35 to 50 megawatts of electrical power. This is enough electricity to drive machines demanding 35,000 to 50,000 horsepower—or to heat, cool, and light 10,000 to 14,000 households in the South. Such a boiler consumes about 500,000 tons of green wood fuel annually, or about three 20-ton truckloads an hour, 24 hours a day, 365 days a year. Wood has a lower heating value than coal or oil per unit of weight. In other words, it takes more pounds of wood than of coal or oil to produce a given amount of energy. Wood has its advantages, however. For example, wood has a much lower ash content than coal and, unlike coal, contains little or no pollution-producing sulfur. Chief drawbacks of wood are high moisture content and bulkiness. Most freshly cut wood is approximately half water by weight. Such green wood ^ves off only about half as much heat as dried wood. If wood is burned when green, furnace efficiency is lowered because heat energy is required to vaporize the moisture in the wood and much of this heat is lost up the stack. If fired with dry fuel instead of