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Novel high rate system for biogas production from energy crops
Background and aims
In recent years the use of energy crops such
as maize or grass silage has become an important factor for the
production of renewable biomethane. However, typical biogas
digesters in use today can only deal with energy crops under certain
conditions and often in an inefficient way. In order to prevent
flotation, fermentation of energy crops in classic digesters needs
continuous and intensive stirring. A 2005 study by the German Agency
for Renewable Energy (FNR) indicates that agitation consumes up to
10 % of the energy produced by the plant.
As an additional effect, excessive mixing
could effect the digestion process negatively by disturbing
interactions of different bacteria groups. When fermentation
residues are discharged, an equal loss of bacteria is inevitable,
leading to further reduction of process performance and stability.
As a consequence, a classic reactor for digesting energy crops
can only handle around 3 to 4 kilograms of volatile solids (VS) per
cubic meter working volume and day. Higher reactor loadings induce
self inhibition, caused by accumulated intermediates like volatile
fatty acids (Linke and Mähnert 2005).
In order to overcome these problems Leibniz
Institute for Agricultural Engineering Potsdam-Bornim developed the
novel up-flow leach-bed (ULB) process. It follows a completely
different strategy by making use of and by stimulating flotation in
stead of preventing it. This system promises both lower energy
demands as well as higher biogas production rates.
State of the art
Regarding high rate systems for biomethane
production, several reactor types are known which use immobilised
bacteria. Corresponding systems are for example the up-flow
anaerobic sludge bed reactor (UASB), the expanded granular sludge
bed (EGSB) reactor and the fixed bed reactor. All these high rate
systems are designed for the treatment of high strength wastewaters
and are therefore not suitable for direct use of solid biomass.
Thus, in order to obtain compatibility, energy crops must be
liquefied at first, resulting in a two stage two phase digestion
system. A corresponding system, consisting of a fixed bed reactor in
combination with four individual batch-operated solid-state reactors
was already developed and brought to practical application (Linke et
al. 2006).
However, in spite of having substantial
advantages a continuous two-stage two-phase system for energy crops
has not been described before. In the continuous process chemical
conditions are more stable, thus stimulating bacterial activity.
Furthermore, the continuous operation allows higher loading rates
and better process control. Regarding the conditioning and usage of
biogas, handling becomes much easier when gas production and gas
quality are both stable.
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