Debunking So-Called Wood Pellet Facts

Two highly inaccurate statements are often made about using wood pellets as a substitute for coal in power generation:
By William Strauss | July 29, 2015

Two highly inaccurate statements are often made about using wood pellets as a substitute for coal in power generation: CO2 released from wood pellet combustion is greater than CO2 released from coal combustion, and using wood pellets for heat or power creates a carbon debt that takes decades to repay. 

The Manomet study, released in June 2010, codified both of those so-called facts about using wood for fuel.  Since then, both the “pellets are worse than coal” and the “carbon debt” arguments have become engrained in anti-biomass literature. 

Below are reasons why those statements, often presented as facts, are inaccurate.

Pellets do not release more CO2 in combustion than coal.  Coal started its life a very long time ago, as biomass.  As it turns out, on a dry basis, coal and wood yield similar results in terms of CO2 produced (kilograms of CO2 per unit of potential energy). But wood and coal do not have zero moisture content (MC). 

It is water in the solid fuel that causes CO2 emissions to increase over the dry weight basis.  It takes energy to evaporate water in wood or coal and convert it to vapor (steam).  All of that energy is sent into the atmosphere in the form of water vapor and is lost. To get a million Btus of useful energy from solid fuel, a larger mass is necessary to compensate for the losses.  More wood or coal per unit of useful energy means more CO2 per unit of useful energy.

The analysis of carbon emissions from wood and coal will vary, depending on the grade and MC of the coal.  At 45 percent MC for wood—the level used by the Manomet study and a common MC for green wood chips—and 15 percent MC for subbitunimous coal, the combustion of wood yields about 34 percent more CO2 per unit of useful energy than power generated from sub-bituminous coal. 

But green wood chips are not suitable for use in most coal power plants.  Most power generated from coal in the U.S.  is at power plants that pulverize and send it to burners on the boiler sidewalls.  Wood pellets pulverize easily, and millions of tons annually are used as coal substitute in pulverized coal power plants around the world.  The correct wood fuel for comparison with coal is wood pellets.

At lower moisture contents, CO2 released by combustion is less, because more energy is available to do useful work. Wood pellets at 6 percent MC result in less CO2 emissions from combustion than all grades of coal under otherwise equal circumstances.  Wood pellets release less CO2 per unit of useful energy than coal.  Furthermore, even green wood chips that release more CO2 in combustion than coal have no carbon debt.

There is no carbon debt.  If wood used for pellets comes from working forests in which the aggregated stock of wood held in the forests is not shrinking, then the carbon stock in those forests is not being depleted.  If that constraint is met, every ton of carbon emitted from chip or pellet combustion is absorbed contemporaneously.

For example, a working forest is harvested annually, and each harvested plot is replanted. The tree farmer harvests one plot per year—the forty-year-old, mature plot. The carbon sequestration rate is 10,000 tons the first year. There are 40 separate plots at 40 stages of growth, from seedling to mature, and each plot sequesters carbon every year at a declining rate, as the trees mature. The entire forest sequesters 152,640 tons per year, every year. The accumulated carbon in the mature, 40-year-old stand exactly equals the carbon accumulated every year by all the younger stands.  Although 152,640 tons of carbon are released from the 40-year-old-plot when used as fuel pellets, 152,640 tons of carbon are sequestered in the same year by each of the other plots, including the replanted plot on the site of the most recent harvest. 

Demand for forest products are continuous. Harvesting, replanting, and regrowth happens daily, not annually. Carbon released by the continuous use of pellets consumed daily for power generation is sequestered immediately by the continuous regrowth that occurs in balance with the harvest.  Working forests can renew forever, if they are managed properly.  Some of each harvest, the larger-diameter, straight logs, will be used to produce lumber. So the amount of carbon released by pellet combustion is less than the amount sequestered. 

The anti-biomass literature is wrong on wood pellets.  Wood pellet combustion releases less CO2 than coal combustion, and as long as there are sustainability criteria that ensure the aggregate stock of carbon in the working forests is never lowered, there is no carbon debt.

Author: William Strauss
President, FutureMetrics Inc.
[email protected]