Heat From Biomass

Biomass which has been grown and harvested must be delivered within defined quality standards if it is to be efficiently utilized by biomass boilers to deliver heat. Important quality characteristics are:-

Each boiler works within the quality constraints that the manufacturer sets, although even when operating within these limits the performance of a boiler may vary. Studies have shown, for example, that as the moisture content of the fuel increases, net heat output declines, boiler efficiency declines, gaseous outputs increase and the boiler needs more frequent servicing.

For most boilers the biomass fuel is delivered in a chipped or pelleted form. Generally larger boilers, with greater diameter delivery augers, will be able to handle larger chip sizes. The uniformity of chipped material can be improved by grading to remove both oversized and undersized material. A high proportion of dust can cause particular issues due to the explosion risk from dust laden air.

The most important quality characteristic is the moisture content. Fresh timber or willow at the time of harvest will generally have a moisture content of about 55 percent. Timber or willow rods left to air dry naturally will reach an equilibrium with ambient conditions within several months resulting in moisture contents below 30 percent. Artificial drying with warm air ventilation can bring the moisture content to below 15 percent while wood pellets will generally have a moisture content well below 10 percent. The lower the moisture content the greater will be the output of heat per kilogram of dry matter. Only relatively large boiler systems (>500kW) are specified to work satisfactorily at moisture contents of above 50 percent, and even then ignition of the material maybe more difficult. Most medium and small scale boilers are specified to perform satisfactorily with biomass fuels below 30 percent moisture, and prefer material of about 20 percent moisture.

Achieving uniformly dry woodchip using an on-floor drier can be difficult as the lower layers dry quickly while the top layer initially becomes wetter. If any moulding and compaction occurs in the upper layers the airflow can be restricted reducing the rate of drying. In some cases it may be necessary to remix the material after achieving the desired average moisture content and then allow the whole batch to equilibrate. Due regard must be given to running the drying system only when efficient drying can be achieved, and during very cold spells of weather this may not be possible.

A number of biomass fuels such as miscanthus and grain contain a relatively high amount of chlorine which can give rise to a range of problems including rapid corrosion of the boiler and flue linings, the formation of excessive amounts

of clinker in the grate, and the emission of unacceptable dioxins in the flue gases. These issues can be avoided if properly specified boilers are used which have been set up correctly to ensure that sufficiently high combustion temperatures are achieved. In some cases it may also be necessary to add lime to the fuel to counteract the acidity.

In trials being carried out at AFBI Hillsborough using a range of biomass fuels available locally including SRC willow chips, wood pellets, Miscanthus, forest brash and chipped spruce, all the fuels have performed well with heat outputs of between 3.5 and 4 kWh per KG fuel dry matter. The ash content has varied from as low as 0.22 percent from wood pellets and 0.4 percent from chipped spruce to 1.2 percent from SRC willows and 1.9 percent from Miscanthus.

Correctly set-up biomass boilers can achieve measured heat output efficiencies of close to 90 percent, and can run in automatic mode with relatively little attention provided the fuel is of consistently high quality. Nevertheless it must be understood that biomass boiler systems do require significantly more management input than would be the case for oil or natural gas fired systems.

For further information contact Lindsay Easson on (028) 9268 2484 or lindsay.easson@afbini.gov.uk

CAFRE have a range of Biomass Boilers. Follow the links below for more details