Mineral Elements in Wood

In addition to oxygen, hydrogen, nitrogen, and carbon, trees are composed of many other mineral elements. While these elements do not produce energy during combustion they do affect the energy content of woody biomass. On average, hardwoods have a higher concentration of these mineral elements than softwoods; but the presence of these minerals is more affected by the site where they are grown than their age, species, or size.

Nitrogen is a component of all fuel systems and during the combustion process it is oxidized into NOx. When emitted from combustion facilities at relatively low levels, NOx may have a useful fertilizing effect on forests, however as emission levels increase, NOx produces adverse health effects and increases the acidification of water and soils. Nitrogen concentrations in wood are typically less than that in coal and oil (BROKEN-LINK Alakangas et al. 1987), thus NOx emissions from wood-based fuels are lower and more environmentally friendly than fossil fuels.

Calcium and potassium are the most abundant mineral elements found in woody biomass but other trace elements such as sulfur, chlorine, and heavy metals are also present. Sulfur emissions from combustion of fuels cause extensive damage to ecosystems and buildings, so fossil fuels are often graded by the amount of sulfur present. As occurs for nitrogen, sulfur is oxidized during combustion to form SOx. This compound can have serious environmental effects and causes the acidification of soils and water. The sulfur content in woody biomass resources is much lower than fossil fuels (Richardson et al. 2002). Equipment to remove sulfur during or resulting from the combustion of fossil fuels is expensive. As emission standards become more stringent, woody biomass is considered a viable alternative fuel for reducing SOx emissions associated with fossil fuel sources.

Most chlorine in trees is found in the foilage as an essential component in chlorophyll. Although only present in trace amounts, its ability to form alkali compounds with potassium and sodium can create serious problems for boiler equipment during combustion as a result of oxidation and corrosion (Riedl and Obernberger 1996). Eliminating foilage from woody biomass feedstocks has been shown to reduce corrosion problems, as has co-firing biomass resources with higher sulfur content fuels such as peat or coal (Orjala et al. 2000).

Heavy metals may vaporize during combustion. The remainder contributes to ash formation. Should levels of heavy metals be high, recycling of ash as fertilizer is restricted by environmental legislation, since the metals may leach into ground water or be absorbed by crops. More modern furnaces are often equipped with filters to reduce heavy metal emmisions but it is best that painted or treated wood waste be avoided as a fuel source considering the higher heavy metal concentrations found in these sources.