Overview of Wood Energy

William Cash
October 24, 2010

Submitted as coursework for Physics 240, Stanford University, Fall 2010


The advancement of the human race is in large part a result of ancient man's discovery of how to harness fire. The fuel that powered most ancient societies was wood and, despite of all mankind's inventions since then, it still remains the dominant source of energy for over two billion people today. [12] Traditionally, the largest consumers of wood energy have been developing nations, but the global push for less reliance on fossil fuels and fears of global warming is causing developed nations to reconsider the power of the tree. This report presents a brief overview of this renewable resource and its viability as a long-term energy solution.

Current and Potential Application

The most common form of wood energy is still the burning of logs on an open fire used for heating and cooking. The constant maintenance, inefficiency, and availability of gas and electrical heating lead to its decline in developed nations. However, efficient wood pellet furnaces are quickly being adopted in Europe. These stoves burn pellets of wood that are created by pulverizing wood in a hammermill and then drying and compressing it. This process greatly decreases the undesired impurities and moisture in the wood, creating a much cleaner burning product. The pellets are also easier to store than logs and modern pellet furnaces are fully automated. Despite currently being competitive in price with natural gas heating, several tons of pellets must be stored for each heating season and the furnace must be cleaned and loaded with pellets on a regular basis. An impetus for the adoption of these furnaces comes from government incentives to switch to renewable resources. [2]

Government incentives and regulations have also led many industries and utility companies to use wood as a means to generate electricity. The European Union has issued a mandate that 20% of all energy produced must come from renewable sources by 2020 and are requiring greenhouse gas emitters to buy permits for their pollution. [14] Coal-fired plants have begun mixing wood fuel with coal to meet their renewable energy quotas without investing significantly in additional infrastructure. In the Ontario Province of Canada, coal must be completely phased out by 2014; the existing coal plants are currently being considered for conversion to wood pellets. [9] Over the past two decades the use of wood mass to produce power and heat grew 25%, and the production of wood-pellets is predicted to double over the next few years. [9,13]

Although wood is showing potential growth in the aforementioned sectors, wood energy will unlikely have any impact in the field of transportation. Wood simply lacks the energy density and convenience of traditional transportation fuels. Furthermore, solid wood fuels would require an external-combustion engine and lack the instantaneous availability of power supplied by internal combustion and electrical storage media. To be viable, wood would have to be used to create synthetic gasoline and other fuels through a process such as Fischer-Tropsch. This is unlikely to be economically feasible in the near future.

Limits of Supply

The previous section illustrated that wood could theoretically meet all of the common energy applications, but the question still remains as to whether the earth could produce enough wood to sustain today's society. In addition to being an energy source, it is the most commonly used building material and source of nearly all paper products. Forests must also compete for space with cropland and civilization. Thus, it is unlikely that significantly more land for forests will be available in the future. Given these constraints we will attempt to construct a rough estimate of the amount of wood needed to supply the roughly 1020 Joules required every year. [1]

Wood is composed of a variety of hydrocarbon chains with the most prominent being cellulose. For simplicity it will be assumed that pure wood is entirely cellulose. The heat of combustion of cellulose is 1.32 × 107 J/kg. [4] If it is assumed that dried wood has average density of approximately 645 kg/m3, the volumetric energy density is on the order of 1010 J/m3. [3] If it is assumed that all of this energy could be harnessed, the world would require a minimum of 1010 m3 of wood per year to supply the 1020 Joules.

The earth's total growing wood stock was estimated to be 4.34 × 1011 m3 in 2005. [5] Even ignoring tree growth, there is theoretically enough wood to power the world for several decades if it is only used for energy. However, the current demand for wood products is only 2 7times; 109 m3 and the world's forest area is already diminishing at a rate of 200 km2 per day. [5] If inefficiencies are taken into account, a well over ten-fold increase in harvests will be needed. This is likely unfeasible without significantly harming ecosystems, but significant, sustainable increases are likely possible with proper regulation.

Emissions Impact and Sustainability

A large impetus for the adoption of wood energy in recent years is that it has been classified as carbon neutral by the European Union and United Kingdom. [6] The reasoning is that the trees captured the carbon from the atmosphere and that released by burning will in turn be captured by future trees. This however ignores the processes to take a tree and turn it into fuel. All of the equipment required to clear forests and transport wood will likely be powered by fossil fuels for the foreseeable future. A recent study in Massachusetts found that switching from coal to wood biomass would actually increase the total amount of greenhouses gases emitted by 3%. [8] Also, a large portion of the atmospheric emissions are absorbed by the oceans, leading to fears of ocean acidification among other things. [11] However, continually clearing forests could reduce the 100 million tons of methane, a more powerful greenhouse gas, emitted by decaying matter in forests. [10] But, the decay of plant matter plays an important role in fertilizing the soil, which may affect the long-term sustainability of wood plantations. Concerns such as these have lead researchers and the EPA to reevaluate the carbon-neutrality of wood and biomass. [6,7] The EPA's decision is expected in the near future and could have a huge impact on the future of wood energy.

For wood energy to remain sustainable, the rate and method of harvesting must be chosen to minimize the impact on the local ecosystem. If regulations fail to promote the use of wood that is truly renewable, then significant deforestation is possible. This will have long-lasting effects on the environment and biodiversity; the deforestation caused by past civilizations can still be observed throughout the globe.


Wood, like all other energy sources, has many benefits and drawbacks. It is likely to play an important role in the years to come because of its low cost and similarity to coal. Wood may never be able to supply all of the world's energy, but it may see a significant increase as governments try to phase out fossil fuels. The sustainability of wood depends on how we treat it; if it's exploited, it may take hundreds of years to recover.

© William Cash. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] International Energy Annual, U.S. Energy Information Administration.

[2] " Take Advantage of Tax Credits and Rebates," Consumer Reports, October 2010.

[3] J. Chave et al., "Regional and Phylogenetic Variation of Wood Density Across 2456 Neotropical Tree Species," Ecological Applications 16, 2356 (2006).

[4] M. Dietenberger, "Update for Combustion Properties of Wood Components," Fire and Materials 26, 255 (2002).

[5] "Global Forest Resources Assessment 2005," Food and Agriculture Organization of the United Nations.

[6] E. Johnson, "Goodbye to Carbon Neutral: Getting Biomass Footprints Right," Environmental Impact Assessment Review 29, 165 (2009).

[7] R. Johnson, "Biomass Poised to Leap Economic, Political, Regulatory Hurdle," Biomass Power and Thermal, 23 Nov 10.

[8] S. LeBlanc, Mass. Study: Wood Power Worse Polluter Than Coal," U.S. News and World Report, 11 Jun 10.

[9] J. Lorinc, Of Congress, Coal Plants and Biomass," New York Times, 22 Jul 09.

[10} V. A. Mukhin and P. Y. Voronin, "A New Source of Methane in Boreal Forests," Applied Biochemistry and Microbiology 44, 297 (2008).

[11] U. Siegenthaler and J.L. Sarmiento, "Atmospheric Carbon Dioxide and the Ocean," Nature, 365, 119 (1993).

[12] World Energy Assessment: Energy and the Challenge of Systainability (United Nations, 2001).

[13] J. van Loon, Old Wood Is New Coal as Polluters Embrace Carbon-Eating Trees," Bloomberg, June 2009.

[14] M. L. Wald and T. Zeller Jr., Cost of Green Power Makes Projects Tougher Sell," New York Times, 7 Nov 10.