SAN FRANCISCO — Woody biomass provides just 0.94 percent of all U.S. energy  now, supplying the equivalent of 3.5 million American homes. But Bob Cleaves,  president of the Biomass Power Association, a group in Portland, Maine, that  represents about 80 plant-burning incinerators in 16 states, says available raw  material would allow the industry to double its output. New incinerators are  already being planned in many states.