Page 169 - New Trends In Coal Conversion

132 New Trends in Coal Conversion

of <6 mm) than pulverized boilers (Agbor et al., 2014). They can achieve high fuel-to-
steam efficiency, typically over 90%, even with challenging, low-grade fuels. One
problem that is exclusive to this design is the probability of bed agglomeration occur-
ring when biomass with a high alkali/alkaline earth metal content is used.
Cyclone boilers. They are designed with large, water-cooled burners that are placed
in a horizontal position, and its external furnace can reach combustion temperatures in

the range of 1650 and 2000 C. The boiler allows the fuel’s mineral matter to form a
slag capturing the oversized particles and to combust the fine and volatile fuel particles
in suspension. The intense heat that radiates from this design burns up the layer of slag
formed. For optimum performance, the ash content of the biomass fuels must exceed
6%, volatiles should be greater than 15%, and, except in a dried form, the moisture
content of the fuel must not be less than 20% (Agbor et al., 2014). This technology
is suitable for biomass cofiring, although a few modifications may be necessary to
enhance the feeding and mixing of the biomass and the coal.
Gasifiers. The gasification technology is used in indirect cofiring systems. Fixed bed
gasifiers require mechanically stable fuel particles of limited size (10e30 mm) to facil-
itate passage of gas through the bed (Dai et al., 2008), and they are generally used in
small-scale applications (<10e15 MW e power capacity). The fluidized bed gasification
has been identified as the most effective gasification technology for indirect biomass
cofiring, where both bubbling fluidized bed gasification and circulating fluidized bed
gasification can be applied because they permit the use of a wide variety of biomass fuels
and waste-derived fuels. An efficient performance of fluidized bed gasifiers requires
relatively small fuel particles to ensure good contact with bed material. Entrained
flow gasifiers convert mixtures of fuel and oxygen into a syngas at high temperatures

(>1200 C, even as high as 2000 C) in very short periods of time (a few seconds)

and at high pressures (50 bar). To achieve reliable feeding and high conversion of the
feedstock, particles should be smaller than 1 mm (Maciejewska et al., 2006).
Direct cofiring results in slightly higher efficiencies (around 2% points) than indirect
and parallel cofiring due to the conversion losses in the biomass gasifiers and boilers.
The overall efficiency of direct cofiring falls with higher percentages of biomass due
to fouling and slagging, associated corrosion, especially in pulverized coal-fired or
grate-fired boilers. The overall efficiency of direct cofiring in coal-fired power plants
with fluidized bed boilers is less sensitive to higher levels of biomass, although high
levels require more sophisticated boiler and fuel handling control systems. In general,
cofiring in modern, large, and highly efficient coal power plants results in a biomass con-
version efficiency that is significantly higher than what can be achieved in small
(<10 MW) and medium-scale (10e50 MW) dedicated biomass power plants with effi-
ciencies of 14%e18% and 18%e33%, respectively (IEA-ETSAP and IRENA, 2013).

5.6 Environmental issues: flue gas and ash from cofiring

Biomass cofiring with coal is a useful tool for the reduction of emissions in the power
generation industry, which enables plants that generate electricity through coal power