University of Strathclyde Biomass Installation Feasibility Tool

Biomass Boilers

The heart of a biomass system is the boiler. Biomass boilers in many ways are very similar to traditional boilers, with efficiency around 90%. They do have some important particularities, though. One important characteristic that must always be taken into consideration is the turn-down ratio of these boilers.

More specifically when using wood as fuel, it is necessary to have it very dry to initiate and maintain the combustion at a high level of efficiency . This is done using the energy generated by the boiler itself (or during start-up period through a secondary source of heat), stored in the thermal lining, to dry the wood chips or pellets entering the combustion chamber. This is the reason why below a given power output, the boiler loses efficiency, the combustion process usually is not complete and the emission levels increase. The relationship between the maximum and the minimum power output for which the best efficiency is maintained is called turn-down ratio. For wood chip and wood pellet boilers this value is around 30%, being able to reach 25% when dealing with very dry fuel.

Because of the limited range of power (defined by turn-down ratio) in which a biomass boiler may operate, more specifically because the need for the pre-drying stage inside the boiler (performed using the thermal lining), biomass systems are not suitable to deal with peak demands. Sizing a boiler based on the peak demand would cause its minimum power output to be too high, maybe even close to the mean demand, forcing the boiler to work most of the time with low efficiency, or not work at all. Is important totake it into consideration when selecting the boiler. In most cases a backup system must be present - usually gas, oil or other fuel to support the biomass system during the peak demand.

The need to pre-heat the fuel in order to make it work efficiently also limits the biomass boilers in terms of the possibility to work in a switch-on, switch-off scheme. Thermal buffers may be sized in a way that (especially during the low demand seasons) the boilers will be turned on just to refill them, a couple of times a day.