7.2.4 Systems for biomass combustion

7.2.4.1 Direct combustion

The direct burning of biomass is the oldest and most traditional use of biomass energy. But recently in many parts of the "civilized" world, under pressure from various energy lobbies on consumption patterns, the use of firewood is perceived as a symbol of poverty! These consumption patterns are really obstacles, which endanger the spread of new, environmentally friendly, sustainable consumption patterns in general, and of renewable energy sources in particular. Today the success of firewood for domestic heating (just as an example of a renewable energy source) depends on the comfort which a fire can give and on the price and costs (including control of emissions) which its use implies.

The direct combustion of biomass means essentially the burning of agricultural, forestry and wood-industrial residues. The nature and appearance of each one of them is very different, requiring a whole range techniques for collection and preparation.

Modern furnaces for biomass need similar technology to those used for other solid fuels, whether in small sizes for domestic heating or for large power stations. However other technologies such as gasification and pyrolysis may be desirable at the larger scale where it is essential to maximise efficiency while minimising gaseous emissions.

7.2.4.2 Domestic scale boilers

Source: ref 7.5 Figure 7.11 Simple boiler with pellet silo

Direct wood burning stoves are widely used in houses where manual charging and ash removal are acceptable. Modern designs give good performance with low emissions.
Where unattended operation for longer periods is required, some form of automatic fuel feed is required. Biomass pellets are one suitable type of fuel in this case. Figure 7.11 shows a typical boiler (B) with screw feed (C) from a pellet silo (A), and delivery (D). Operation of the feed system uses a controller (E).