Biomass Cofiring and Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 367

literature on biomass, it is easy to speculate that piled biomass runs the
risk of self-ignition as much as the coal does. Several fires that occurred
in biomass plants highlight this risk.
3. Biomass-coal mixture: Biomass being reactive, its mixing with coal dust
adds another dimension to the explosion issue. Biomass is indeed more
reactive than most coal, petcoke, or anthracite. Thus, explosion hazard for
such mixtures has to be evaluated. Presently, very little data is available
forcing the designer to assume the mixture to be all coal or all biomass in
their assessment. The explosion behavior of the coal

biomass mixture is
at the moment largely unexplored. This aspect is discussed further in
Section 10.7.4 .

10.7 COFIRING WITH TORREFIED WOOD
Torrefaction could offer some relief to the above problems and make bio-
mass cofiring more viable. Such relief is due to the following intrinsic prop-
erties of torrefied biomass.

1. The fuel preparation for cofiring greatly benefits from the torrefaction
of the biomass feed because this process makes biomass more brittle
and less fibrous. The least cost option for cofiring uses the existing
pulverization mills and feeds biomass directly into them along with
coal. Though torrefaction cannot make biomass as grindable as coal, it
makes significant improvement in the grindability of the biomass. As a
result, the existing mills can grind the required amount of biomass
without requiring additional energy. This allows the boiler to feed its
burner with the required amount of coal and biomass to match the fur-
nace heat input of the existing boiler. Furthermore, in cases where
separate mills are used for coal and biomass, the improved grindability
of torrefied wood allows the mills to produce particles of right size and
in right quantity.
2. Capital investment for covered biomass storage could be a major compo-
nent of the total cost of biomass cofiring upgrade of an existing plant.
The carrying charge of that could tip the economic balance against cofir-
ing. Even that may not prevent dried biomass from picking up additional
moisture from the atmosphere and cause a health hazard due to fungal
attack on biomass. Torrefied biomass, being relatively hydrophobic, does
not pick up moisture even when stored outdoors and experience very little
fungal attack. Thus, it obviates the need for expensive covered fuel
storage allowing the plant to use parts of the existing coal yard to store
the biomass.
3. Torrefaction acts like a quality leveller for multiple fuel feed. The dif-
ference between different biomass feedstock is reduced through torre-
faction. Thus, while the quality of the delivered biomass supply is