Currently Prevalent Biomass Combustion Technology
By far the most common means of converting biomass to usable heat energy is through straightforward combustion, and this accounts for around 90% of all energy attained from biomass. There are a number of different technologies available that can be used for biomass combustion and the main ones can be categorised under two headings: Fixed bed combustion systems and fluidised bed combustion systems.
There are two prominent types of fixed bed combustion: underfeed stokers and grate firings. With these methods of combustion air is primarily supplied through the grate from below, and initial combustion of solid fuel takes place on the grate and some gasification occurs. This allows for secondary combustion in another chamber above the first where secondary air is added.
Fluidised bed furnaces operate in quite a different manner from fixed bed furnaces and have a number of advantages associated with them. There are two main types of fluidised bed furnace, Bubbling Fluidised Bed (BFB) and Circulating Fluidised Bed (CFB).
Bubbling Fluidised Bed (BFB) Furnaces
BFB’s have two main advantages in terms of fuel size and type over more traditional fixed bed systems. Firstly they can cope with fuel of varying particle size and moisture content with little problem, and secondly they can burn mixtures of different fuel types such as wood and straw. BFB’s are only a practical option with larger plants with a nominal boiler capacity greater than 10 MWth.
Circulating Fluidised Bed (CFB) Furnaces
CFB’s deliver very stable combustion conditions but it comes at a cost. Due to their larger size compared to other combustion methods the cost is relatively high and there are problems involved with fuel size, which must be very small, and the difficulties involved in running them at partial load. All of this means that they are really only feasible for plants with a boiler capacity of over about 30MWth.
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