Environmental Engineering Reference
In-Depth Information
Table 4.1
Biogas yield of steam-exploded substrates compared with untreated
substrates
Biogas yield (m
3
CH
4
/t fresh matter)
Slaughterhouse residues Maize silage
Straw
Reeds
Before treatment
450
200
280
210
After treatment
500
250
400
350
Source: Coramexport (2011).
formed. These products can inhibit anaerobic digestion. Many studies in the
field of ethanol production have shown high furfural and hydroxymethyl-
furfural (HMF) production as a result of long retention times and high
temperature. Benjamin et al. (1984) and Bochmann et al. (2011) demon-
strated that these products have a negative impact on anaerobic digestion.
Another negative aspect is that the recovery of heat from this pre-treatment
is impossible. However, using steam explosion allows substrates such as hay
or straw to be used for biogas production. Bauer et al. (2009) analysed steam
explosion tests of straw and showed calculations of ethanol and biogas
potentials. Table 4.1 shows gas yields with and without steam explosion.
Extrusion
Extrusion is a process adapted from other industries such as metal and
plastic processing industries. In these industries, material is fed into an
extruder and conveyed by screw along a tube, where it is exposed to high
pressure, temperature and shear forces. The material is subsequently pushed
out of a hole of specific shape to form the final product, which could be a
pipe or a sheet. Biogas substrates in extruders are subjected to the same
forces, causing tough fibres to break and the plant cells to lyse. In addition,
as the substrate leaves the extruder, the sudden drop in pressure causes
evaporation of intracellular water, as in steam explosion .
Extrusion tests for biogas substrates typically use twin-screw extruders
where the screws rotate counter wise. Extruders are available at ratings from
11 kW to a 55 kW; substrate output is in the range 0.9 to 4.0 t/hour
respectively. Depending on the consistency required at the end, the substrate
can be placed under a pressure of up to 300 bar at temperatures from 60 to
300
C. For biomass with a total solid content of 30 to 35%, the temperature
should not exceed 100
8
C due to water evaporation and substrate drying.
Extrusion increases the specific surface area of biomass, which allows
easier access by hydrolytic enzymes to the chemical bonds. The increasing
availability of the biomass results in faster methane production. This
facilitates higher organic loading rates in the reactors.
8
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