The Pros and Cons of Burning Wood for Heat and Other Biomass Issues

The Pros and Cons of Burning Wood for Heat and Other Biomass Issues

I must admit it is nice building a fire in the fireplace and warming up around it, smelling the aromatic wood and hearing the crackling fire. Even cutting, splitting, stacking, and hauling wood can be good exercise and a good way to clean up debris and put it to good use at the same time. Potential hazards of dead trees and limbs do need to be cut.

Is Wood Burning Carbon Neutral?

Proponents of wood heat often note that it is carbon neutral. However, on short time scales it is not, since it can take quite a while for a tree to decompose and release its carbon to the atmosphere, typically several decades, compared to the short time it takes to burn it. Burning wood releases other greenhouse gases, notably nitrogen oxide (N2O), methane, carbon monoxide (which has an indirect climate effect), and some types of particulate matter. In one scenario I read, 1kg of burnt wood releases 1900g of CO2, 200g of N2O adjusted to be CO2 equivalent, and 70g of methane adjusted to be CO2 equivalent. With the addition of carbon monoxide and particulate matter, that makes burning wood about 85% carbon neutral (15% more greenhouse gases would be released overall than sequestered), without considering the fossil fuels used in cutting, processing, and delivering the wood. I am not sure how correct that is as some nitrogen and methane might be released in decomposition of the wood anyway. If one adds in processing the carbon neutrality falls even further. Of course, all these variables will be quite variable. The bottom line is the burning wood for heat is not carbon neutral. It is, however, carbon lean. There is an obvious and significant benefit to greenhouse gas emissions. However, as the Wiki entry notes: “Wood burning creates more CO2 than biodegradation of wood in a forest (in a given period of time) because by the time the bark of a dead tree has rotted, the log has already been occupied by other plants and micro-organisms which continue to sequester the CO2 by integrating hydrocarbons of the wood into their own life cycle.” This means that not all of the CO2 and hydrocarbons would be released into the atmosphere. In terms of kgCO2eq per MWh delivered, wood is slightly better than natural gas and significantly lower than oil. This is due to the high energy density of oil. However, natural gas by far has the lowest pollution of the three. 

Limits of Wood-Burning as a Sustainable Practice 

Burning wood is not sustainable on a mass scale for obvious reasons. There do exist wood-burning power plants. Germany has many as do other countries, including the U.S. Most are small scale. Firstly, the amount of wood required to replace 10% of coal burning in the U.S (as of 2009) would have required double the annual consumption of wood. In a mere few years forest depletion would become quite evident, as is currently happening in the southeastern U.S. where forest products, both waste and cut logs, are being processed and shipped to Germany for use in their “renewable” wood-burning power plants. Secondly, an increase in the use of wood among home-owners is only sustainable among widely-spaced rural homes. Otherwise the smoke and particulate matter in towns would become problematic as it has been in places like Australia and New Zealand. There in winter it can make up most of the air pollution. Thirdly, there is the problem of fires. Having had a chimney fire the very first time we made a fire in the fireplace of our home, I can attest that it was not fun. Lucky for us, we shut the flue quickly and the fire went out. Toxic indoor smoke can also be a problem, especially in less efficient open fireplaces. It is horrifically problematic in the developing world where unvented or poorly vented wood and dung fires cause sickness and the premature death of millions of people. This is one reason some people advocate the immediacy of the reduction of energy poverty over the immediacy of reducing greenhouse gas emissions. There are several things that can be done with wood products, woodburners, fireplaces, and plants that can make the burning more efficient, less toxic, and less emitting. These include burning at different temperatures, better venting, more efficient combustion, processing out moisture to make it less toxic and lighter to transport, and the use of catalytic converters (required in some jurisdictions). Pellet stoves boast efficiency and less toxic emissions, but the pellets must go through significant energy-using processing. 

    

Characterization of Biomass as Renewable Energy

Biomass is a general category of fuel that includes wood, agricultural waste, biogenic gas from landfills, biogas from homemade and commercial anaerobic digesters, direct incineration of waste, and other forms. It is considered to be renewable energy. However, in the case of wood, the renewability is questionable. As stated earlier, it takes decades for a decomposing tree to release its carbon into the atmosphere and some is re-sequestered, taken up by other life forms. In the case of cutting live trees for fuel, it takes much longer, sometimes up to a hundred years or more to replenish the carbon benefits of a large tree. By one estimate about 50% of designated renewable energy is biomass and about 90% of biomass burned in power plants is wood or wood waste. Since biomass is considered to be renewable, it is eligible for incentives, tax credits, and other subsidies. While this may help companies cash in on their agricultural waste and wood waste it also takes that which goes back into the soil and increases particulate and other pollution. Clearing live forests for wood burning obviously increases atmospheric CO2, yet such projects are also considered renewable and can be subsidized as such. As of 2011 the tax credits and incentives were spurring the building of new wood-fired power plants in the U.S. that would use massive amounts of wood, both waste and live, including clear-cutting. Although some environmentalists praise wood-burning, especially the home variety, others such as the Natural Resources Defense Council have spoken out against the woody biomass industry and favor new regulations to develop best management practices. The EPA has been studying the issue for several years now. Characterization of the different sources of biomass in terms of carbon emissions and pollutants needs further analysis. This is important in carbon pricing as well - how biomass and wood should be evaluated, especially in light of the fact that wood and other biomass have very heavy pollution footprints compared to other forms of "renewable" energy.

Toxicity of By-products of Wood Incineration

Wood-burning pollutants include carbon monoxide, nitrogen oxides, sulfur dioxides, black carbon (soot), fine particulate matter, lead, mercury, and more. Post-burn residues of creosote and ash can also be toxic, ash if concentrated. Ash can also be used modestly on plants as it contains potassium. Wood and dung fires, especially unvented ones, cause health problems and shorten lives among poor people all over the world. The toxic by-products of waste incinerators have been debated among many communities where they have been proposed and/or built.

Pros and Cons of Trash Incineration

Proponents argue that these waste-to-energy projects reduce the amount of land used for landfills, which are a known public health risk. The practice is widespread in Europe, particularly in Scandinavian countries. In some of these countries 50% or more of the waste is burned. In the U.S. about 12% of municipal solid waste is burned in incinerators. Opponents point out that the CO2 produced per MWh is about a third higher than coal, the dirtiest of the fossil fuels. They also say it tends to reduce recycling and composting and point to European statistics for proof. Others disagree. Recycling is part of the “circular economy” which cuts down on the need for raw materials. In some places waste-to-energy projects are eligible for renewable energy credits and subsidies and this designation is being debated in several places. The need and requirement for air pollution control devices cuts down on the profitability of waste-to-energy and incinerators have never been popular in the U.S. due to the greater amount of air pollution. There is much debate about whether waste-to-energy or landfills are better for the environment and climate. Certainly, newer landfills that capture the biogenic methane generated with efficient modern systems are better than the old ones. However, landfill fires can be devastating. Both landfill fires and incinerators release dioxin into the air. The Wall Street Journal article referenced below goes into some detail about the debate.   

References:

Wood Fuel: Environmental Impacts and Potential Use in Renewable Energy Technologies, Wikipedia Entry

Dead Forests Release Less Carbon into the Atmosphere than Expected, news story from University of Arizona, March 22, 2013

Surely Burning Wood Releases CO2?: An Extended Discussion Paper from the Biomass Energy Centre, by Biomass Energy Centre, U.K.

Is Biomass Really Renewable? – by Renee Cho, in State of the Planet, Earth Institute, Columbia University, Aug. 18, 2011

Burning Wood: Issues for the Future – by Martin Crawford, Agroforestry Research Trust, 

Power Hungry: The Myths of “Green” Energy and the Real Fuels of the Future – by Robert Bryce, (Public Affairs), 2010

Biomass Emission and Counterfactual Model (spreadsheet) – from www.gov.uk (last updated 1/2015)

Incinerators: Myths vs. Facts about “Waste to Energy” – factsheet put out by GAIA – Global Alliance for Incinerator Alternatives, Feb. 2012

The Pros and Cons of Burning Waste to Generate Energy – by Arlene Kiridis, Utility Dive news brief, Nov. 18, 2015

Does Burning Garbage for Electricity Make Sense?, in Wall Street Journal, Nov. 15, 2015