Page 53 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

32 Biomass Gasification, Pyrolysis and Torrefaction

2.2.1.1 Energy Crop
Unlike fossil fuels, biomass is a renewable fuel. So, besides its present avail-
ability, which is equivalent to the energy reserve for fossil fuels, one must
consider the annual yield, which is the amount of biomass grown in a year.
For an annual species that grows every year and dies, it is the total biomass
yield of that plant. For perennial species like trees that do not die every year,
it is the annual growth in the plant. Table 2.1 gives the annual growth rates
of some biomass species.
The other important parameter that influences the cost is the heating value
of the fuel and the fraction of the total biomass available for energy production.
Energy crops can provide a biomass plant with an assured supply of feed-
stock over its lifetime and as such it is getting much attention as a commer-
cial source of energy. Its use is similar to a dedicated coalmine feeding coal
to a specific pithead power plant. Instead of using naturally grown trees or
plants in the forest, chosen fast-growing plants are cultivated exclusively for
the supply of energy. Such plants have good energy density, grow fast, and
have a low maintenance cost for cultivation. It takes little water or fertilizer
to grow energy crop. In many cases, they can be grown in abandoned fields.
Growing these plants does not affect the cultivation of food grain. Table 2.2
presents a partial list of such energy crops.

2.2.1.2 Biomass Cost
The delivered cost of biomass is an important parameter that must be
known before the start of the project. It is expressed in terms of energy

TABLE 2.2 Some Examples of Energy Crops
Annual Growth Rate
Energy Crops (dry ton/ha year) HHV (MJ/kg dry)
Miscanthus 13 30 b 18.5 b
SRC willow 10 15 b 18.7 b
Sorghum 0.2 19 c
Switchgrass 2.9 14 c 17.4 b
Alfalfa 1.6 17.4 c
Canary grass 2.7 10.8 c
Kenaf Hybrid poplar 10 a
a
Drapco et al. (2008) p. 252.
b
McKendry (2002), p. 45.
c
Klass (1998), p. 113.