Wood-Chip Heating Systems - Biomass Energy Resource Center

WOOD CHIP HEATING SYSTEMS
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gasifi er is a pre-combustion device that cooks the wood
fuel in an oxygen-starved environment, producing
unburned combustible gases. These gases are then
cooled and cleaned to produce a medium or low Btu
content gas, which can be used much like natural gas or
liquid propane gas. The resulting bio-gas can be stored,
transported, and used in applications remote from the
gasifi er that produced it.
Potential uses for bio-gas include combustion for
heating and steam production, and fueling internal
combustion engines for a variety of applications.
Bio-gas can also be used as a feedstock for chemical
processes, and it may replace petroleum-based
feedstocks in some cases.
More importantly, biomass gasifi ers are expected to
lead to a dramatic increase in the effi ciency of burning
wood to produce electricity. Currently, the only commercially
available way to produce electricity from solid
biomass fuel is to burn the wood fuel to create steam for
a steam-turbine driven generator. The overall effi ciency
of this process for steam-based electric production
is very low and the maintenance cost is high. Wood
gasifi ers promise to produce electricity at a higher
effi ciency, with lower emissions and with less expensive
operating costs, compared to a steam boiler approach.
System effi ciency can be further boosted when the
gasifi er is in a combined-heat-and-power (CHP)
application. With a gasifi er CHP system, it is easier to
capture and use the waste heat than with a steam CHP
system.
Gasifi ers for power and CHP applications are
currently under development in a wide range of sizes.
Smaller gasifi ers will initially be used to fuel internal
combustion engines to drive electric generators.
Heat can be extracted from the engine coolant and
the engine exhaust to provide hot water as a useful
byproduct. Large wood gasifi ers will produce bio-gas
fuel for combined-cycle gas turbine systems for power
and thermal energy production. In this application,
burning bio-gas turns the blades of a combustion
turbine to drive an electricity generator. Hot exhaust
gases are captured to create steam that in turn drives
a steam-turbine power generator. Thermal energy is
captured from the steam turbine outlet to provide either
low-pressure steam or hot water. Both small-scale
and large-scale biomass gasifi cation will signifi cantly
out-perform wood-burning steam systems for power
production.
Further in the future, product gas from biomass
gasifi ers will be used in microturbines and in fuel cells.
These technologies will further increase effi ciency and
reduce emissions in producing power and heat from
biomass.
1 There are two major categories of gasifi ers, and numerous
sub-categories of each. The fi rst category, close-coupled
gasifi ers, refers to combustion appliances that produce
heat by separating the com bus tion process into two
stages in connected, adjacent, combustion chambers.
Among close-coupled gasifi ers there are variations: small
combustors that fi re into residential-sized boilers or
furnaces; a class of small cordwood boilers; and the twochamber,
commercial-sized gasifi ers discussed in Chapter
Six. In the second category are those gasifi ers that produce
bio-gas, which can be used in a variety of ap pli ca tions.
This section of the guide discusses the second type of
gasifi er.