Trees are an important part of any farm. They reduce erosion and flooding, absorb air pollutants, provide habitat for insect-eating birds, and cast shade to help livestock stay cool. They produce crops, too, whether it's fruit and nuts at an orchard or wood at a tree farm.
But trees also face their own challenges. They can be struck by lightning or stricken by disease, leaving them undesirable for timber or unable to produce marketable fruit. They are subject to storms and weather events that can uproot or otherwise damage them. Even if they're healthy, they may drop too many limbs, generate subpar materials or leave a farmer with a seemingly useless supply of twigs, stumps and other debris after they're harvested.
In some cases, however, this kind of "waste wood" can hold hidden value as a source of biomass energy. It may only be a small splinter of the overall biomass market, and it won't save the world from climate change, but it's still a potentially useful resource for farmers, rural economies and the ecosystems that support all of us.
"On any timber harvest 60% to 65% of all wood harvested is low-grade wood, which ends up as pulp or wood chips for the biomass plants," New Hampshire tree farmer Tom Thomson wrote in a 2019 column for the Concord Monitor newspaper. "Our forest is no different than the garden in your backyard: We both have to weed and thin it if we want a productive garden or, in my case, a sustainable forest for all to enjoy."
What is biomass?
Biomass is a hodgepodge category of fuels, all produced by renewable sources that are alive or were alive recently (at least compared with fossil fuels). It consists mainly of plant matter, and along with other renewable energy sources like wind and solar power, certain kinds of biomass are seen as a more sustainable alternative to fossil fuels like coal. According to The United Nations' Intergovernmental Panel on Climate Change (IPCC), burning biomass to make electricity instead of using fossil fuels is one possible way to reduce CO2 in the atmosphere.
Biomass isn't a silver bullet for the climate crisis, and, like any energy source, it can pose its own problems if used carelessly. Still, as we struggle to diversify our power sources, its potential is worth considering.
Different biomass sources have different strengths and weaknesses, and different end uses. Perennial grasses like switchgrass or giant miscanthus are among the best, for example, thanks to their versatility, high yield and ability to store lots of carbon underground. Trees are some of humanity's oldest fuel sources, yet their role as biomass energy is more complicated. Wood smoke can carry harmful pollutants, and wood is only a carbon-neutral energy source if the right trees are grown the right way in the right places — i.e., with enough living trees to cancel out the carbon emissions from those being burned, and without robbing natural forests of trees that provide food and habitat for wildlife.
Outside of wild ecosystems, tree farmers have the option to raise trees not just for timber products, but also as an energy source. "Some fast-growing trees make excellent energy crops," according to a fact sheet from the Union of Concerned Scientists (UCS), "since they grow back repeatedly after being cut off close to the ground." Examples of trees that can perform well as biomass crops include cottonwood, pine, poplar, sweetgum, sycamore and willow.
Yet these energy plantations aren't the only places where trees can become sources of biomass power. On a smaller scale, we can also squeeze a little extra energy out of waste wood that accumulates on farms of all kinds, capitalizing on a resource that might otherwise be wasted.
What is waste wood?
Like biomass in general, "waste wood" is a broad term that can mean a variety of things. A vegetable farmer might have a small surplus of woody debris after clearing a patch of land for crops, for example, or after a storm damages a forest buffer at the edge of a field. An orchard owner might have a little more lying around on a regular basis, while tree farmers often end up with larger amounts due to low-value timber and wood waste that's collected along with more valuable logs to be sold for lumber.
"Timber harvest residuals typically include tops and limbs too small for lumber production or containing too much bark for pulp use," explains Fastmarkets RISI, a price-reporting and analytics firm for global commodities markets, including the forest products sector. "These are ground or chipped onsite and diverted to energy production. Trees of low value may also be chipped whole for use in energy production, or debarked and chipped for pellet manufacture."
There are many other sources of waste wood, too, such as trimmings and storm-damaged trees from urban areas or prunings and other debris from orchards. It can also come from manufacturing residues (things like wood chips, shavings and sawdust from production of lumber or wood panels) as well as post-consumer wood waste, although these sources may be less valuable depending where they came from.
In the U.K., for instance, Grade A waste wood is visibly "clean," non-hazardous byproducts from the tree farms, packaging waste and off-cuts from untreated wood products, according to the Renewable Energy Hub. Grade B is non-hazardous waste from the production of wood-based panels, while Grade C is non-hazardous waste from construction and demolition sites or recycling centers. Grade A waste wood can be burned in a standard industrial biomass burner, but grades B and C face more restrictions and may require a specialized boiler. Grade D waste wood — any materials that have been treated or otherwise contaminated with any hazardous substance — is likely destined for a hazardous-waste incinerator.
The most valuable waste wood tends to come directly from trees, as opposed to wood products, and while that can include debris from urban trees, farmers are often in the best situation to take advantage of this market. And along with other forms of biomass, waste wood could provide a much-needed boost for farmers and rural economies overall, according to the UCS.
"Farmers would gain a valuable new outlet for their products," the group explains. "Rural communities could become entirely self-sufficient when it comes to energy, using locally grown crops and residues to fuel cars and tractors and to heat and power homes and buildings."
Uses for waste wood
Much like trees grown on energy plantations, higher-quality waste wood is used for energy in a few ways. It can serve as a feedstock for advanced biofuels, for example, or it can be processed into wood pellets that are burned for energy, either in small boilers and furnaces or at large-scale industrial facilities.
That could help the power sector reduce its coal use, although burning wood for electricity is not free from controversy. There are the aforementioned concerns about wood pellets coming from whole trees in vulnerable ecosystems, for example, especially places like wooded wetlands or old-growth forests. Even if they come from plantations or waste wood, their emissions can still threaten human health, and they're only climate-friendlier than coal if they're part of a system in which live trees are continually offsetting their carbon dioxide output.
To many opponents of this practice, the issue is more about supply: It's impractical to collect enough waste wood to provide a reliable large-scale power source, they argue, so companies may resort to less sustainable sources for wood pellets, including whole trees taken from natural forests. And if those trees are harvested while still green, the mass burning of green wood — which has a high water content — "can end up being as dirty as coal," Mary Booth, industry critic and director of the Partnership for Policy Integrity, told Energy News Network in 2017.
Yet the question of supply depends on the scale at which waste wood is burned for energy. There are large amounts of low-value wood in some U.S. forests, and some experts say it could be responsibly used as an energy source. "In most of the northeastern United States there is quite a lot of low-use wood not utilized in any markets," according to a fact sheet from Penn State Extension. "In Pennsylvania, some studies estimate that six to eight million dry tons per year of sustainably harvestable low-use wood is not being used."
Some critics point out waste wood isn't always "waste" in an ecological sense, because it provides vital habitat for cavity-nesting birds and other wildlife. At the same time, advocates cite the importance of thinning out debris in managed forests, which can reduce fire risk and give trees additional space to grow larger. The biomass industry, they say, offers more economic incentive for that by giving landowners a market for lower-value wood.
"As long as we have forests that grow, we're going to need to take care of them," California tree farmer Gary Hendrix told the American Forest Foundation in 2017, explaining how a new biomass mill and co-generation plant created a market for him to thin out low-grade wood on his property. "The right markets can really help."
If a landowner is located close enough to the right kinds of biomass infrastructure, selling waste wood as a partial power source could be mutually beneficial. The waste could help supplement other sources of wood pellets when it's available, and the landowner could have another source of revenue while also getting rid of unwanted debris in the process. In hopes of making this easier for farmers and forest owners, a research project funded by the European Union aims to decentralize production of biofuels from waste wood by developing mobile production units that can be transported to wherever they're needed.
Because biomass is locally produced, harvested and processed, it often provides an economic boost in rural areas, according to the nonprofit Biomass Energy Resource Center (BERC). Biomass energy tends to keep money spent on fuel within the local economy, the group adds, unlike fossil-fuel systems that generally export fuel dollars.
Aside from selling it, landowners might also be able to capitalize on waste wood as an energy source for their own purposes. Some farmers keep at least some of their biomass to burn in furnaces or boilers for on-farm heating, which can offer higher efficiency, lower energy bills and fewer carbon emissions. In a University of Vermont study that ran from 2008 to 2015, growers who installed biomass greenhouse-heating systems saved an average of nearly $2,700 on net fuel costs per year, with the systems paying for themselves at full cost within 4.8 years.
"The boiler makes hot water which we can use in multiple greenhouses by plumbing it to them in insulated PEX piping. Once in the greenhouse, we convert to hot air with a hot water fan coil, put it in the ground for root-zone heating or on the benches in our mat-heating system for starts," said David Marchant, co-owner of River Berry Farm in Fairfax, Vermont, in an interview about the study with Vermont Food Atlas. "I like it. I keep trying to find something wrong with it, but I can't."
On top of the financial payback, the reduction of CO2 emissions is also a perk for many farmers, adds Chris Callahan, an agricultural engineer with University of Vermont Extension. River Berry's biomass boiler helped the family-owned organic farm avoid 5,910 pounds of net CO2 emissions per year — roughly equivalent to 5,000 miles of car travel, he writes, or the CO2 sequestered by half an acre of pine forest.
In many cases, waste wood that is collected is burned in the open to dispose of it. Using it as part of a responsible biomass plan can play a useful role in helping us phase out fossil fuels. In the meantime, it could also provide valuable benefits for farmers. And while waste wood may only be a small part of that, at a time when climates and economies are undergoing dramatic change around the world, many farmers need all the help they can get.