Biomass Combustion and Co-firing

Co-combustion

Co-combustion

Co-firing biomass with coal in traditional coal-fired boilers is
becoming increasingly popular, as it capitalizes on the large investment
and infrastructure associated with the existing fossil-fuel-based power
systems while traditional pollutants (SOx, NOx, etc.) and net greenhouse
gas (CO2, CH4, etc.) emissions are decreased.

The least expensive way of cofiring
is by directly adding biomass to the coal belt

The R&D demands arising from co-firing cover the proper selection
and further development of appropriate co-combustion technologies for
different fuels, possibilities of NOx reduction by fuel staging, problems
concerning the de-activation of catalysts, characterisation and possible
utilisation of ashes from co-combustion plants, as well as corrosion and
ash deposition problems.

Fuel Characteristics

The biomass fuels usually considered range from woody to grassy and
straw-derived materials and include both residues and energy crops. The
fuel properties differ significantly from those of coal and also show
significantly greater variation as a class. For example, ash contents vary
from less than 1% to over 20% and fuel nitrogen varies from around 0.1% to
over 1%. Other properties of biomass which differ from those of coal are a
generally high moisture content, potentially high chlorine content,
relatively low heating value, and low bulk density. These properties
affect design, operation, and performance of co-firing systems.

Fuel Preparation and Handling

A biomass fuel handling facility,
which directly meters biomass onto the coal conveyor belts at the
Wallerawang Power Station, Australia (Courtesy of Delta
Electricity, Australia)

Because biomass fuels are hygroscopic, have low densities, and have
irregular shapes, they should generally be prepared and transported using
equipment designed specifically for that purpose. In some cases, however,
they can be directly metered on the coal belt conveyor. Care must be taken
to prevent skidding, bridging, and plugging in pulverizers, hoppers, and
pipe bends.

Effect on NOx emissions when
cofiring wood (top) and switchgrass (bottom) with coal. NOx
emissions can both increase and decrease when cofiring biomass. Fuel
nitrogen content: wood = 0.18, switchgrass = 0.77; coal = 1..1.2 lb
N/MMBtu. (Courtesy Larry Baxter, USA)

Pollutant Emissions

Co-firing biomass with coal can have a substantial impact on emissions
of sulphur and nitrous oxides. SOx emissions almost uniformly decrease
when biomass is fired with coal, often in proportion to the biomass
thermal load, because most biomass fuels contain far less sulphur than
coal.

An additional incremental reduction is sometimes observed due to
sulphur retention by alkali and alkaline earth compounds in the biomass
fuels. The effects of co-firing biomass with coal on NOx emissions are
more difficult to anticipate (see figure).

Ash deposition rate for various
fuels in g deposit per kg fuel. (Courtesy Larry Baxter, USA)

Ash Deposition

Rates of ash deposition from biomass fuels can greatly exceed or be
considerably less than those from firing coal alone. This is attributable
only partially to the total ash content of the fuels. Deposition rates
from blends of coal and biomass are generally lower than indicated by a
direct interpolation between the two rates. Experimental evidence supports
the hypothesis that this reduction occurs primarily because of
interactions between alkali (mainly potassium) from the biomass and
sulphur from the coal.

Carbon Conversion

Experiments on carbon burnout of biomass fuels in coal power plants
show that large, wet or high-density biomass particles may undergo
incomplete combustion. However, this biomass-derived carbon does not
always figure prominently in fly ash analyses because of the relatively
low amount of carbon in biomass, the limited share of biomass usually
co-fired, and the fact that large biomass particles are more likely to
collect in the bottom ash than in the fly ash.

The molar ratio of sulphur to
available alkali and chlorine is an indicator of the chlorine
corrosion potential (Courtesy Larry Baxter, USA)

Chlorine-based Corrosion

High-temperature corrosion of superheaters is of great concern when
burning high-chlorine or high-alkali fuels, such as herbaceous or
intensely cultivated fuels, since species containing chlorine (generally
alkali chlorides) may deposit it on heat transfer surfaces and greatly
increase surface chlorine concentration. However, research has indicated
that the corrosion potential can be reduced if alkali chlorides (primarily
from the biomass) can interact with sulphur (primarily from the coal) to
form alkali sulphates. As a result, highly corrosive alkali chlorides on
superheater tubes are converted to HCl and other gas-phase products that
are less corrosive and that leave the surface relatively easily. The HCl
may condense on lower-temperature surfaces such as air heaters. However,
this problem is generally less serious and more manageable than
superheater corrosion.

Fly Ash Utilization

The majority of the fly ash generated from coal combustion world-wide,
is used as a concrete additive or for other purposes. However, current
standards preclude the use of fly ash as a concrete additive from any
source other than coal.

The technical case for precluding the use of fly ash from co-firing
wood with coal appears to be unjustified. However, the less comprehensive
data available for herbaceous biomass fuels suggest that alkali, chorine,
and other properties may compromise several important concrete properties.

Strict interpretation of many standards that are the basis for
regulations and policy for many institutions would preclude all fly ash
from use in concrete if it contains any amount of non-coal-derived
material, including co-fired fly ash. Though these standards are under
active revision, this may take many years to complete.