Biomass Combustion and Co-firing

Small-scale appliances for space heating

Small-scale
appliances for space heating

Wood fires have been used as a local heat source for thousands of years,

A microprocessor controlled woodlog
stove with downdraught combustion and separate chamber where
secundary combustion takes place (Courtesy of Fröling, Austria)

progressing from open pit to semi-open pit (a fireplace) to enclosed pit
(a stove).

The interest in using wood for heating purposes is increasing. Besides
heating, wood-burning appliances are also used for cooking, for producing
a pleasant atmosphere, and for interior decoration. Domestic wood-burning
appliances include fireplaces, fireplace inserts, heat storing stoves,
pellet stoves and burners, central heating furnaces and boilers for wood
logs and wood chips, and different kinds of automatic wood chip and pellet
appliances.

Over-fire boilers are commonly used to burn logs and are relatively
inexpensive. In such a boiler, a fuel batch is placed on a grate and the
whole batch burns at the same time. The stove or boiler is normally
equipped with a primary air inlet under the grate and a secondary air
inlet above the fuel batch, into the gas combustion zone. Wood is fed from
above and ashes are removed from a door below the grate. These boilers
work on the principle of natural draught and, as the fuel bed is cooled by
fresh fuel, the initial CO-emissions can be relatively high.

Very strict emission limits in some countries have made it necessary to
introduce down-draught boilers. Here, unburned wood gases released by wood
placed on a ceramic grate are forced by a fan to flow downward through
holes in the grate. Air is introduced below the grate in the secondary
combustion chamber, where the gases flow along ceramic tunnels, and final
combustion takes place at high temperatures. By using lambda control
probes to measure and control flue gas oxygen concentration, staged air
combustion, and even fuzzy-logic control, very low emissions are achieved.
Naturally, down-draught boilers are much more expensive than conventional
boilers.

Two wood pellet boilers, used to heat a school in
Denmark

A recent innovation in space heating is automatic pellet combustion.
The excellent handling properties of pellets mean that the fuel is gaining
popularity rapidly in Sweden, Denmark, and Austria. In other countries,
the interest in pellet burners is just awakening. Pellet burners are of
special interest since they can replace an oil burner in an existing
oil-fired boiler.

If the burner-boiler combination is well designed, efficiencies over
90% can be achieved at nominal thermal output. At part load, and varying
load, the efficiency decreases but for the best burners efficiencies over
86% have been obtained.