Page 285 - Synthetic Fuels Handbook

FUELS FROM CROPS 271

A biomass-fired boiler is a more adaptable direct combustion technology because
a boiler transfers the heat of combustion into steam. Steam can be used for electricity,
mechanical energy or heat. Biomass boilers supply energy at low cost for many industrial
and commercial uses.
Pile burners consist of cells, each having an upper and a lower combustion chamber.
Biomass fuel burns on a grate in the lower chamber, releasing volatile gases. The gases
burn in the upper (secondary) combustion chamber and pile burners must be shut down
periodically to remove ash.
Although capable of handling high-moisture fuels and fuels mixed with dirt, pile burn-
ers have become obsolete with the development of more efficient combustion designs with
automated ash-removal systems.
In a stationary or traveling grate combustor, an automatic feeder distributes the fuel onto
a grate, where the fuel burns. Combustion air enters from below the grate. In the stationary
grate design, ashes fall into a pit for collection. In contrast, a traveling grate system has a
moving grate that drops the ash into a hopper.
Fluidized bed combustors burn biomass fuel in a hot bed of granular material, such
as sand. Injection of air into the bed creates turbulence resembling a boiling liquid.
The turbulence distributes and suspends the fuel. This design increases heat transfer
and allows for operating temperatures below 972°C (1781°F), reducing nitrogen oxide
(NO ) emissions. Fluidized bed combustors can handle high-ash fuels and agricultural
x
biomass residue.
Conventional combustion equipment is not designed for burning agricultural residues.
Straw and grass contain alkali (potassium and sodium) compounds, which are also present
in all annual crops and crop residues and in the annual growth of trees and plants. During
combustion, alkali combines with silica, which is also present in agricultural residues.
This reaction causes slagging and fouling problems in conventional combustion equipment
designed for burning wood at higher temperatures.
Volatile alkali lowers the fusion temperature of ash. In conventional combustion equipment
having furnace gas exit temperatures above 1450°F, combustion of agricultural residue causes
slagging and deposits on heat transfer surfaces. Specially designed boilers with lower furnace
exit temperatures could reduce slagging and fouling from combustion of these fuels. Low-tem-
perature gasification may be another method of using these fuels for efficient energy production
while avoiding the slagging and fouling problems encountered in direct combustion.
Combustion facilities that produce electricity from steam-driven turbine-generators
have a conversion efficiency of 17 to 25 percent. Using a boiler to produce both heat and
electricity (cogeneration) improves overall system efficiency to as much as 85 percent. That
is, cogeneration converts 85 percent of the fuel’s potential energy into useful energy in two
forms: electricity and steam heat.
Two cogeneration arrangements, or cycles, are possible for combining electric power gen-
eration with industrial steam production. Steam can be used in an industrial process first and
then routed through a turbine to generate electricity. This arrangement is called a bottoming
cycle. In the alternate arrangement, steam from the boiler passes first through a turbine to pro-
duce electric power. The steam exhaust from the turbine is then used for industrial processes
or for space and water heating. This arrangement is called a topping cycle.
The direct-fired gas turbine is another combustion technology for converting biomass
to electricity. In this technology, fuel pretreatment reduces biomass to a particle size of less
than 2 mm and a moisture content of less than 25 percent. Then the fuel is burned with
compressed air. Cleanup of the combustion gas reduces particulate matter before the gas
expands through the turbine stage. The turbine drives a generator to produce electricity.
Co-firing biomass as a secondary fuel in a coal-burning power plant using high-
sulfur coal could help reduce sulfur dioxide and nitrogen oxide emissions. Also,
co-firing decreases net carbon dioxide emissions from the power plant (if the biomass