Wood fuel has several environmental advantages compared with fossil fuels. Wood can be continually replenished, which leads to a sustainable and dependable supply. However, proper forest management must be practiced to ensure that growing conditions are not degraded during biomass production.There is little net production of carbon dioxide from wood combustion, because the CO2 generated during combustion of wood equals the CO2 consumed during the lifecycle of the tree. Transporting the material using petroleum generates excess CO2.Wood fuel contains minimal heavy metals and extremely low levels of sulfur; wood fuel is no threat to acid rain pollution. Particulate emissions from wood are controllable through standard emission control devices such as bag houses, cyclone separators, and electronic precipitators. Bottom ash is minimal. Usually, wood ash is less than 1% of the weight of the wood, and sometimes ash may be used as a fertilizer.The principal economic advantage of wood-burning systems is that wood fuel is usually less expensive than competing fossil fuels. However, the price of wood for use as fuel can be extremely variable. Sometimes when surplus supplies of wood residues are available at nearby forest product manufacturing plants or municipal solid waste handling facilities, the cost can be very low or even negative. Transportation for delivering from the supply site to the wood combustion or wood processing unit is the primary expense of wood fuel.