The energy resources are classified into three categories: fossil fuel, renewable resources, and nuclear resources. It is reported that in 2000, the remaining reserves to annual production ratio for oil and natural gas is only 42 years. The fossil fuel combustion produces harmful air pollutants such as carbon dioxide, nitrogen oxides, sulfur oxides and soot particle, etc. The biomass is a substitute of the fossil fuel and carbon neutral energy. The sulfur oxides emissions are extremely low because of the low sulfur content in biomass. In practice, biomass is usually utilized by direct combustion, gasification, bio-oil production, methanol or ethanol production according to its characteristics and the facility scale.
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