Environmental Engineering Reference
In-Depth Information
Many biomethane potential (BMP) tests (see Chapter 3) have been carried
out with extruded biomass, and additional methane production was found
for most of the treated substrates. Hjorth et al. (2011) analysed the effect of
the extrusion process on anaerobic degradation of straw, unensiled grass,
the solid fraction of manure from a screw press, the solid fraction of manure
after flocculation and deep litter from cattle. After 28 days of BMP tests, the
extruded straw showed up to 70% more methane production than the
untreated straw and after 90 days of BMP tests, the methane yield of
extruded straw was 11% higher than that of the untreated. This shows that
biogas production from straw was faster when extrusion was used to pre-
treat it, but the data had a very high standard deviation so the additional
benefit may be smaller than presented. For the extrusion process,
approximately 10-15 kW per tonne of substrate is needed; this is a similar
value to the parasitic electrical demand of a CSTR digesting slurry (Murphy
and McCarthy, 2005).
A major problem with extrusion pre-treatment technology is the screws,
which have to be changed after a few months due to abrasion. As with other
mechanical pre-treatment technologies, stones or metallic materials in the
substrates severely reduce the lifetime of the screws. This has a negative
impact on the economics of the extrusion process.
Thermo-chemical pre-treatment
During thermo-chemical pre-treatment, the effects of thermal and chemical
influence are combined. Different kinds of bases and acids can be used, but
ammonia (e.g. the AFEX process) or different kinds of solvents (e.g. the
organosolv process) are also used. Temperatures from 60 to 220
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Chave
been studied. Pre-treatment temperatures of more than 160-200
C showed a
drop in methane production, depending on the input material (DiStefano
and Ambulkar, 2006; Delge´ nes et al., 2000; Penaud et al., 1999).
The thermal influence during alkali pre-treatment of waste activated
sludge leads to a higher chemical oxygen demand (COD) solubilisation
(100%) and a higher gas yield (20%) when compared with alkali pre-
treatment (Kim et al., 2003). Inhibition effects of alkali addition are similar
to alkali pre-treatment without thermal influence.
Zhang et al. (2011) analysed the thermal pre-treatment of cassava with
acid addition. Sulphuric acid was used in concentrations of 1.32-4.68% (w/w)
and the temperature was 150-170
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￿ ￿ ￿ ￿ ￿ ￿
C. The reaction time was 10-36minutes.
A 57% higher gas yield was found for pre-treated cassava compared with
untreated. The pre-treatment parameters that obtained the maximum gas
yield were 160
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C, 3% H 2 SO 4 and 20minutes retention time (Zhang et al.,
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2011).
The influence of thermal, chemical and thermo-chemical pre-treatment on
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