Biomass Cofiring and Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 369

boiler is a little easier than that in a PC-fired boiler because of the fuel flexi-
ble feature of fluidized-bed firing (Basu, 2006).
Raw biomass is highly fibrous in nature. Surface fibers of neighboring
particles lock into each other making it difficult to flow smoothly. These
along with the plastic behavior of biomass causes several problems such as:

1. Handling difficulties
2. Problem with pneumatic transportation in pipes
3. Difficult grinding and pulverization of fine sizes

Co-combustion of biomass would therefore require biomass to be ground
to comparable sizes ( m) and pneumatically conveyed through pipes.

Because of its soft, nonbrittle characteristics, considerably more energy is
required to grind untreated biomass to the above fineness. For example, to
grind a ton of coal to a d 50 around 500

75 μ

B

36 kWh of grinding
energy is required, while that for raw poplar and pine, the energy require-
ment would be 130 and 170 kWh, respectively (Esteban and Carrasco,
2006). There is thus nearly an order of magnitude increase in energy con-
sumption for biomass grinding.
Additionally, the output (tons/h) of a given pulverizer is greatly reduced
while grinding biomass along with coal for cofiring. Reduced mill output
directly reduces the power production of the plant.
Torrefaction addresses the above problems to a great extent by making
biomass particles more brittle, smoother, and less fibrous. An optical photo-
micrograph taken after torrefaction shows (Arias et al., 2008) an absence of
fibrous exterior, sharp ends in the biomass. Thus, the friction created by the
interlocking of the fibers during the handling of a pneumatic transportation
is greatly reduced after torrefaction.

μ m, about 7

10.7.3 Effect of Torrefaction Parameters on Grinding
We note from above that torrefaction reduces the energy consumption for
grinding biomass. This section discusses the grinding issue further.
Torrefaction temperature is the most influential parameter affecting grind-
ing. The higher the temperature at which torrefaction is performed the
lower is the energy requirement for grinding, or for a given energy input,
more amount of fine particles are obtained after grinding. Grinding of tor-
refied biomass gives smaller and uniform size distribution of the product
(Phanphanich and Mani, 2011). The grinding energy requirement for a
specified level of grinding decreases with torrefaction temperature. Here,
one notes that the specific energy consumption reduced from about
250 kWh/ton for raw biomass to about 50 kWh/ton for that

torrefied at

280 C.