On any given day, four small power plants in rural California are busily turning trash into something akin to gold: 120 megawatts of renewable, low impact electricity.

Owned and operated by the privately held Greenleaf Power LLD, biomass plants in the towns of Scotia, Tracy, Mecca and Wendel collect and burn woody and agricultural castoffs that have traditionally ended up in landfills and open burns:  tree trimmings, agricultural waste, and clean construction and demolition debris.

“We’re focused on one segment of waste that nobody wants and we’re recycling it into energy,” says Hugh Smith, president and CEO of the Sacramento-based company.

“It makes all the sense in the world to take materials that have already been used once and turn them into electricity or fuel that can be used twice. And it’s a direction we should be going as a society.”

Greenleaf is an example of the small but burgeoning biomass industry which is turning previously unusable wood and other agricultural residue into energy, either by burning it or by converting it into biofuel -- a technically demanding kind of alchemy that scientists are still trying to perfect.

The biomass power business is also providing jobs in an industry hard hit by a drop in the market for lumber, paper and other wood products. Greenleaf employs 120 people directly, and 460 indirectly to collect and deliver biomass materials.

Some scientists say converting wood—even wood waste-- to energy raises significant questions about environmental impacts, but many scientists contend that removing and recycling wood waste provides a greener source of fuel than coal.

From sawdust to electricity

There is a staggering amount of waste in producing wood products. For all of the wood that ends up in newspapers or two-by-fours, a huge amount gets left on the ground. Using data from the U.S. Forest Service, researchers at the Energy Biosciences Institute recently estimated that each year loggers leave behind about 47 million metric tons of so-called forest residue—stumps, branches, tree tops and other leftovers—in Eastern forests alone. Of all of that material, roughly 20 to 30 million metric tons would be relatively easy to collect without undue expense. The U.S. Department of Energy has also estimated the available forest waste from the timber industry and wood thinning to prevent catastrophic firestorms to be as much as 100-plus tons

About 40 percent of milled logs end up as either sawdust, trimmings and other odds and ends, representing another source of potential energy, says Steve Kelley, Ph.D., head of the department of forest biomaterials at North Carolina State University. “When you cut a cylinder into rectangles, you lose a lot of good wood in that process,” he says.

Some renewable energy plants run on woody waste from many other sources as well including wood, brush and stumps from clearing projects in town, unused pallets, trees that have been damaged by storms or infested with insects, trees too small for lumber, and agricultural leftovers such as rice hulls and even—in California – fruit pits and grape skins. Some, like Greenleaf, don’t use any whole trees at all. And, like most other plants,  Greenleaf accepts no garbage and draws the line at construction waste such as painted wood, plywood, and treated lumber, all of which could release pollutants.

The easiest way to turn sawdust or leftover wood into energy is to burn it – something that lumber, pulp and paper companies have been doing in their mills for a long time. Wood scraps can be chipped up or compressed into pellets that can be shipped to utility companies where it is burned much like coal. In fact, the pellets are often “co-fired” with coal. Pound for pound, wood pellets only pack about 60 to 70 percent as much energy as coal, but that’s still enough to spin a turbine and generate electricity.

Burning wood waste is a simple process, but getting the debris to the power plant can be a logistical nightmare--and an extra cost, economically and environmentally in fuel alone. Smith says Greenleaf tries to avoid that by using products from within 50 or so miles of the plants. Waste from San Francisco, for example, goes to the Tracy plant– 63 miles away.

Forest residue can be another issue. Nobody is going to make an extra trip into the forest for wood residue, says Kelley of NCSU, so forest leftovers generally have to be gathered and chipped at the time of harvest, which takes equipment and planning. As a result, many pellet mills depending on other sources of wood, including whole trees that are too small for paper or lumber. “There are five pellet mills in North Carolina, and they aren’t living on branches and twigs,” he says.

Such challenges pale compared to the monumental task of turning wood waste into ethanol, a process that has been notoriously expensive and inefficient. But there have been some encouraging signs: The energy company INEOS Bio is producing commercial amounts of ethanol from wood waste and other vegetative matter at its Indian River BioEnergy Center in Vero Beach, Fla. The wood waste is heated at low levels of oxygen to produce as much carbon monoxide as possible. The company’s patented bacteria then feed on a mixture of carbon monoxide and hydrogen to produce ethanol. The company hasn’t disclosed how much ethanol it has already generated, but it has a stated goal of producing eight million gallons each year..

Waste harvesting: How much is too much?

Any advancement in wood-to-energy technology is bound to increase the demand for wood chips and scraps, a trend that would undoubtedly lead to increased worries about  environmental damage. The Sierra Club, among other environmental groups, opposes forestry biomass projects that contribute to the destruction of wild or natural forests or in which “excessive amounts of biomass are removed from the land” through in-wood chipping.

Recent studies have lent credence to the fear that removing too much wood waste can damage ecosystems. Researchers in Scandinavia have found that certain species of fungus disappear when dead twigs and branches are removed from the forest.

“Fungi aren’t especially charismatic, but they do have an important function in decomposition,” says Tony D’Amato, Ph.D., associate professor of forest resources at the University of Minnesota.

In a biodiversity experiment, researchers removed woody debris and standing dead trees (snags) from plots in a loblolly pine forest in South Carolina. Compared to plots that weren’t disturbed, the plots that were cleaned of fallen branches and twigs turned out to be much less hospitable to birds, including Eastern towhees, Carolina wrens, and red-headed woodpeckers. “Many birds feed on insects that depend on the wood,” says John Kilgo, Ph.D., a co-author of the study and a research wildlife biologist with the Forest Service’s Southern Research Station. He notes that the research plots were relatively small, especially compared with large-scale logging projects. “The impacts could be even bigger with bigger operations,” he says.

In 2011, the journal Forest and Ecology Management published an analysis of 26 separate studies that examined the impact of removing coarse woody debris from an experimental site after a logging operation. Taken together, the studies suggested that removing branches, stumps, and treetops reduced the number of resident bird species in later years. However, the researchers also noted that experimental plots tended to exaggerate the difference between a typical harvest and a harvest that included removal of woody debris. In actual biomass plantations, the differences tend to be more subtle, suggesting that real-world collection of woody debris might have a smaller impact on local birds and other wildlife.

Deahn Donner Wright, Ph.D., a wildlife biologist with the U.S. Forest Service’s Northern Research Station, has tried to replicate the potential impacts of removing logging residue for bioenergy in the hardwood forests of Northern Wisconsin. In one plot, loggers left behind the tops of all trees after the harvest. In another, one out of five tops were left behind. And in a third plot, all of the tops were removed. Two years later, she has yet to notice any differences in the plants and insects that have moved into the sites.

Still, Wright expects to see more effects in the years ahead. She notes that treetops that are left on the ground provide structure and shelter for insects and amphibians. And, she says, those bits of wood will eventually decompose and add nutrients back to the soil. In areas where nutrients are at a premium, removing treetops could unravel the ecosystem, she says. “You have to be aware of the particular system that you’re in before you start pulling out wood,” she says.

Ensuring that something is left behind

Underscoring the importance of wood waste to a forest, about a dozen states have already established rules that require loggers to leave behind a certain amount of material after logging natural forests. In Minnesota, the first state to enact such a requirement, 33 percent of the woody material has to stay behind, which adds up to five tons of waste per acre. “The science behind that number is inexact, but it should be enough to sustain nutrients in most places,” D’Amato  says. He notes that Minnesota prohibits loggers from removing any waste material in areas with especially nutrient-poor soil.

In practical terms, visions of the bioenergy industry suddenly scouring forests clean were never especially realistic, Kelley says. Although the market for biofuels could theoretically encourage loggers to collect limbs and small trees that they would have otherwise left behind, most modern harvests are clear cuts anyway, he says. And by the nature of the business, some wood will always remain. “Nobody is talking about the drastic scenario of denuding forests for bioenergy,” he says.

Perhaps the bigger concern is that there isn’t enough wood waste to keep utility companies humming. In the words of Forest Futures Project, a 2012 report by the Southern Research Station, “it is unlikely that biomass requirements for energy would be met through harvest residuals and urban wood waste alone. As consumption increases, harvested timber (especially pine pulpwood) would become the preferred (wood) for feedstock.”

But the mountains of other agricultural biomass, mixed with woody waste, could fill the gap and has to go somewhere, says Greenleaf’s Smith.

“On a global scale, we look at it this way,” he says. “With 10 billion people producing (agricultural) waste every day, we can’t continue to pile it up in landfills. It’s crucial to the globe to develop the technology to serve needs in cost-effective and smart ways and turn our waste into reasonable energy more effectively and cleanly that we do today.”

Judith Horstman contributed additional reporting.

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