Fluidized Bed Combustion of Coal, Renewable Fuels, and Waste: Current Status and Developments - Novel Combustion Concepts for Sustainable Energy Development - page 227

direct negative effects to human health. The sulfur in the coal is oxidized to SO 2
which subsequently contributes to acid rain formation and direct adverse effects of
the human health. Beside high-quality coals which are low in ash, also coals with
high amounts of ash and moisture exist. These coals are signi

cantly lower in their
energy contents and usually used locally because of their availability and lower
costs.
Renewable Fuels like biomass, wood, wood waste, and bark are signi cantly

lower in their calori c values (typically about 15 MJ/kg) compared to coals and are

considered CO 2 neutral. They are usually low in ash and sulfur; however, signifi- -

cant nitrogen contents are possible depending on the source of the biofuel. Also
high moisture contents are possible depending on the conditions of harvesting and
storing. Because of their origin, decomposition and changes in their quality with
storage may be of importance.
Waste is a very heterogeneous fuel and may differ in many ways. Some waste
which is high in plastics show very high calori c values, for example, up to about

46 MJ/kg if polyethylene or polypropylene is considered. However on the other
hand, municipal solid waste (MSW) may be in average as low as about 10 MJ/kg.
Also, the calori

c value of sewage sludge is low, depending strongly on the water
content. Waste may be rich in chlorine forming HCl which is acidic and corrosive
and a hazardous pollutant. In addition, heavy metals are usually present in waste in
higher quantities and may be found in the fly ash or bottom ash of the combustion
system.

2 Fluidized Bed Combustion Technology

The fl fluidized bed technology is positioned between a fixed bed like a grate furnace

where the fuel particles are rather ow where the

fuel particles are in a state of pneumatic transport (refer also to Lackner et al. 2010 ).
In

fixed on the grate and an entrained fl

fluidized bed technology, the fuel particles are mixed with bed material usually
made of ash particles and some additional silica sand particles and they are in a state
of

fl

fl fluidization somehow similar to a fl fluid, similar to a liquid. The combustion air

comes from below to fl fluidize the fuel together with the bed material. Because of the

vigorous fluidization, excellent mixing and high heat transfer rates are obtained.

This leads to high combustion rates and high ef

fl

ciencies. The bed material also acts
like a heat storage and this characteristic is ideal to handle a wide range of different
fuels with different moisture contents and qualities. The combustion temperatures
are usually in a lower range about 850 ° C, resulting in practically no thermal NO x

formation. Therefore, fluidized bed combustion is usually low in NO x . The NO x

generated is from the fuel nitrogen as described in the introduction.
If the fuel is rich in sulfur, limestone can be added into the

fl

fluidized bed
combustor. Then the limestone undergoes calcination and reacts with the SO 2
generated during fuel combustion. This is an in situ desulfurization of the com-
bustion process and decreases SO 2 emissions from the combustion chamber

fl