Environmental Engineering Reference
In-Depth Information
biomass. They found that vapors from different species had unique chemical compositions and
that 64-98% of the eluted compounds were monoterpenes. Arshadi et al . (2009) have studied
the emission of volatile aldehydes and ketones from wood pellets. Long-term exposure to high
levels ofVOCs may cause health problems. Thus, gaseous substances such asVOCs from biomass
should also be monitored regularly, especially in industries that handle biomass. In addition, good
ventilation should be maintained.
It is possible that stored biomass may undergo self-heating, which presents a risk of explosions
and fires. If the moisture content of the biomass is sufficiently high, this initial self-heating
often has biological causes, as thermophilic microorganisms increase the temperature of the fuel
while decomposing it. At low moisture levels, which tend to suppress microbial growth, biomass
may adsorb vapor from the ambient air, which can trigger self-heating due to differential rates
of adsorption and vapor condensation. These initial heating processes can trigger auto-oxidation
of the biomass, raising its temperature to the point that increasing quantities of VOCs and non-
condensable gases like CO, CO 2 and CH 4 are released, greatly increasing the risk of explosions
and/or fires. The temperature of stored biomass should therefore be monitored continuously and
if it starts to rise, preventative measures should be taken to reduce the risk of fire.
Of the non-condensable gases, CO is the most hazardous because it prevents oxygen uptake by
humans even at low atmospheric concentrations. Unprotected people exposed to an atmosphere
containing around 800 ppm of CO will lose consciousness within 2 hours. At 12,800 ppm, death
occurs within 1-3 minutes. Svedberg et al . (2004, 2008 and 2009) have studied VOCs and
also cases where people were killed by emissions of non-condensable gases from biomass. These
typically occur in environments where biomass is stored in rooms with no or very little ventilation,
i.e. limited exchange of air. Such places are found in ships transporting biomass, in silos and in
storage areas, etc. The solution is simple: increase the amount of ventilation and fresh air. People
working in such environments must ensure that the levels of both CO and O 2 are safe (it is not
enough to measure O 2 alone) before entering any space where biomass is stored in an enclosed
area with little or no ventilation. Because CO can leak out to other rooms, the air in adjacent
spaces should also be checked before entering. The safety measures for fuel pellets discussed in
the Pellet Handbook (Obernberger and Thek, 2010) are valid for most biomass and should be
implemented as a matter of course.
Another problem associated with handling biomass is the presence of dust as well as moulds
and other microorganisms that grow on biomass. These are also hazardous. Explosions can occur
if the concentration of dust in the air becomes too high, and prolonged inhalation of dust is
harmful. Furthermore, microorganisms can be pathogenic and produce toxins and allergenic
spores; exposure to these for even relatively short periods may result in sickness.
2.6.4 Bulk handling
It is well known that industrial processes involving solid biomass are significantly more problem-
atic than those involving only gases or liquids. This point was highlighted three decades ago by
the RAND Corporation (Merrow et al ., 1981) and is still valid. The general conclusion of these
reports was that the start-up costs and processing times for fluid processing plants exhibited little
variation, with most being within 20% of the mean, whereas those for plants based on solids were
200 to 300% greater. After completion, fluid-processing plants typically operated at around 90%
of the designed throughput whereas solid-based plants tended to operate at around 50%. This is
mainly due to the variable flow ability of solids compared to fluids. One should also keep in mind
that viscoelastic solid biomass is harder to handle than hard solids such as gravel.
Feeding problems are a common reason for delayed start-ups and failure to sustain the designed-
for production capacity. In comparison to fluid-based indusial processes, relatively little is known
about biomass flow ability in different industrial processes and during internal transportation. As
such, it will be important to study the tribology and rheology of solid biomass-derived materials
in order to further their widespread adoption in industrial processes.
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