General Principles

3.1 Biomass can be converted into energy by simple combustion, by co-firing with other fuels or through some intermediate process such as gasificationii. The energy produced can be electrical power, heat or both (combined heat and power, or CHP). The advantage of utilising heat as well as or instead of electrical power is the marked improvement of conversion efficiency - electrical generation has a typical efficiency of around 30%, but if heat is used efficiencies can rise to more than 85%. This chapter describes these technologies, and considers the amount and types of generation that would be needed to meet the renewable targets discussed in chapter 1 (paragraph 1.2).

3.2 In each type of plant, the overall reaction for a fuel of mean composition Cx Hy Oz is CxHyOz + (x+y/4-z/2)O2 -► x CO2 + (y/2) H2O

The total energy released by this reaction is independent of whether the fuel is burned in a combustion plant, pyrolysed (i.e. heated to decompose the fuel) or gasified (i.e. heated in a flow of a gas, usually air or steam). If the gas and char from pyrolysis or gasification are then burned, the overall reaction is the same as the above; the differences in performances between combustion and pyrolysis or gasification lie in the way in which the heat is released and utilised.

3.3 Biomass differs from other fuels in several respects, ofwhich two are particularly significant for heat, CHP or power plants using biomass. The calorific value - i.e. the heat released by burning a specified mass of fuel- is relatively low. Furthermore, the water content of the combustion gases is relatively high, both because of the hydrogen present in the fuel (see above) and because most biomass fuels contain some degree of moisture which evaporates when the fuel is burned111. To recover the energy retained in the water vapour, it is necessary to use a condensing heat exchanger which converts the water vapour to liquid and recovers the latent heat of evaporation; this is currently considered an undesirable degree of complication for simple heat and simple CHP plants. However, the overall efficiency is generally improved if the biomass is dried before firing, to reduce the water content of the combustion gases.

ii Some types of biomass can be converted to energy through other means, such as anaerobic digestion to produce methane or fermentation to produce ethanol. These methods are not well suited to the lignocellulosic materials being considered here and are therefore not included in this report.

iii Two measures of calorific value are used: the Gross Calorific Value (GCV, or higher heating value), which measures the heat released when the fuel is burnt and the water is condensed out of the combustion gases as a liquid and the Net Calorific Value (NCV, or lower heating value), which measures heat release on the basis that the water remains in the vapour phase. The difference between GCV and NCV is higher for biomass than most other fuels, and is widened by increasing moisture content.

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