Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 47

In the interval of 20-60% moisture content (wet basis) the weight of raw biomass needed, to
produce heat in order to dry 1 kg of upgraded fuel to 10% moisture content, is more than doubled
for each 10% going from the bottom of the interval to the top. The higher moisture content the
lower net gain. At high moisture contents the concept without recovery of condensation heat starts
to break down completely.
On the other hand, Wahlund et al . (2002) reported that biomass drying could be achieved
with low energy input at a bio-based CHP facility. In this case, the biomass was dried using
pressurized steam and a heat exchanger was used to transfer heat from the 'dirty' steam generated
by moisture in the wood to 'clean' steam that could be used to generate power. In addition, the
heat released during the condensation of the 'dirty' steam was recycled and reused in the drying
process. Additional heat exchange below the vapor condensation point could further reduce the
specific energy of drying and increase the net output of bioenergy in this process. Thus, the most
effective drying process accounts only for heat exchange losses in the system.
These examples illustrate that controlling the moisture content of biomass is crucial for its
industrial use and that the heat needed for drying should be reused in as many ways as possible in
order to continuously increase the net output of upgraded fuel. This requires the development of
integrated processes that allow for the efficient use of surplus heat; many current industrial and
energy conversion processes produce a lot of surplus low-value, low-temperature heat that could
be exploited for drying biomass.
There are of course other ways to reduce or overcome problems arising from the moisture
content of biomass. One is delayed harvesting, which has been demonstrated for the rhizome
grass, reed canary grass ( Phalaris arundinacea L.), and for industrial hemp, by cropping in
spring instead of autumn (Xiong et al ., 2009). During late fall and winter, the above-ground
biomass of these grasses dies, and over the course of the early spring, it dries out, at which point it
is harvested. A notable advantage of such delayed harvesting is the low ash content of the biomass
obtained and the favorable composition of the ash that is formed. Other systems used in forestry
involve storing covered piles of logging residues in such a way that they self-dry over time in the
air. A drawback of these outdoor storage systems is that some biomass is lost on standing due to
biological and other degradation. In addition, infestations of mould and other bio-contaminants
are hard to avoid and may be harmful downstream in the value chain.
2.6.2 Wet processing
Novel systems have been developed to avoid having to dry biomass completely. These typically
focus on carrying wet biomass through all the steps in energy generation. Thus, the biomass
is stored after pretreatment at low temperature with bio-preservatives such as polysaccharide-
degrading enzymes (which can be added by treatment with yeasts) that maintain its condition
and facilitate subsequent enzyme treatments (Passoth et al ., 2009). It can then be subjected
to fermentation e.g. ethanol and biogas production in sequence (Dererie et al ., 2011). It is also
possible use hydrothermal upgrading processes e.g. hydrothermal carbonization of lignocellulosic
biomass (Hoekman et al ., 2011). Gasification of wet biomass feedstocks using supercritical
water oxidation has been studied and may become more practical following the discovery of new
catalysts that improve reaction efficiency and product yield (Azadi and Farnood, 2011; Robbins
et al ., 2012).
2.6.3 Health aspects
Biomass in and of itself can be hazardous and cause health problems. Notably, it releases volatile
species such as terpenes, aldehydes, and ketones that are collectively known as volatile organic
compounds (VOCs). Banerjee (2001) has shown that during softwood drying, there are three
mechanisms involved in the release of α -pinene and other terpenes. Moreover, dissolved terpenes
accounted for approximately 0.1% of the total quantity of vapor released during softwood drying.
Wajas et al . (2007) used headspace solid-phase micro extraction to analyze VOC emissions from