Biomass as Fuel in Small Boilers (PDF 6.4 - APO Asian Productivity ...

5. RETROFITTING OF COAL-/OIL-FIRED BOILER WITH
BIOMASS AND CHECKLIST FOR ENHANCED BOILER
EFFICIENCY
5.1 BOILER RETROFITTING
With the escalating cost of fossil fuel, most enterprises in small and mediumscale
industries, using coal or oil for steam generation, are keen to switch over
to biomass fuel to reduce steam generating cost. Acquiring a new biomass
boiler requires high investment and is difficult for small enterprises. Most
small-scale industries are interested in retrofitting measures, with which they
can adapt an existing boiler for switching over from fossil fuel to biomass. One
of the major drawbacks of fuel substitution in an existing boiler is de-rating, or
reduced steam generation capacity, of a retrofitted boiler. The de-rating factor
depends upon the type of boiler. For example, in an oil-fired package boiler,
the biomass combustion chamber is designed externally, and the de-rating
factor will be larger than for boilers where the furnaces is installed internally,
as in the case of a stationery grate water tube boiler.
The heat transfer rate in a boiler is a combination of radiation and
convection, which constitute 30% and 70% respectively. As the radiation heat
transfer increases, boiler steam generation capacity also improves. This is
one of the major problems in fire tube boilers, where radiation heat transfer is
affected due to the external furnace. Sometimes the de-rating factor is as high
as 25-30%.
The other factor in de-rating is the slow combustion press for small tyoes of
biomass with uneven size and higher moisture content, such as woody mass,
bagasse, etc. Burning this type of fuel in a small boiler requires high excess air
and reduced flame temperature.
The de-rating factors also accelerate due to manual removal of ash in most
biomass burn boilers (particularly in stationary grate furnaces), where the
combustion air supply is on which cools down furnace temperature.
Table 5-1 depicts the selection of types of furnaces for different boilers for
retrofit.
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