Biomass Cofiring and Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 364

(due to its lower density and higher reactivity) but to a comparable size for
transportation through coal pipes and combustion in a PC burner. Raw biomass,
due to its tenacious fibrous nature, is difficult to grind to such fine sizes.
Grinding raw biomass is not only energy intensive but it is also difficult to
obtain finer particle sizes in an adequate amount for a given energy input. Such
high-energy requirement may be a result of the plastic behavior of biomass.
The pulverizing mills are designed for certain energy input. So, the out-
put of such a mill reduces when called upon to grind biomass and that
reduces with increase in several parameters like moisture content, degree of
fineness, as well as tenacious nature of the material. For this reason, when
raw biomass is fed through the coal feeder for pulverization in a mill
designed for coal, the output of the mill reduces accordingly. If the plant has
spare mill capacity, it can maintain the thermal input in the furnace but at
the expense of additional energy consumption by the mills.
10.6.3 Storage
Raw biomass, agriculture waste in particular, tends to have high-moisture
content. This characteristic makes transportation, handling, and storage of
biomass difficult. If the local climate is dry and windy, open-air storage
could help reduce the moisture of the biomass, but that is not the case for
most plants. Some power plants go for expensive covered storage to prevent
wetting of the biomass from rain or snow. Even drying of the biomass on-
site will not prevent hygroscopic fresh biomass from future absorption of
moisture from its surroundings while in storage. Additionally, the moisture
within the biomass attracts fungal attacks and causes rotting while in storage.
Such problems are not present to this extent for coal.
10.6.4 Fouling, Agglomeration, and Corrosion
The ash content of average biomass is lower than that of average coal. The
ash in biomass is fine, while that from coal is in general coarse. For the low-
ash mixed feed, one could expect a lower rate of ash deposition on boiler
surfaces while cofiring biomass, but the reality is different. Many biomasses
(herbaceous and agriculture waste in particular) contain high fraction of
alkali and chlorine in their ash ( Table 10.1 ). As a result, the fouling and cor-
rosion rate of boiler heating surfaces could increase with the amount of bio-
mass cofired. The alkali metals from biomass ash evaporates at combustion
temperature and it subsequently condenses on cooler boiler surfaces down-
stream enriching alkali in the metal to ash interface. Fouling from such
deposits could be hard enough to be beyond the cleaning capacity of stan-
dard soot blowers. The chlorine in biomass leads to hydrochloric acid in the
flue gas that increases chlorine in metal-ash interface increasing the corro-
sion potential.