Energy recovery from biomass: process simulation and Second Law analysis of an anaerobic digester coupled with an internal combustion engine

Abstract

The paper presents a simulation and an exergy analysis of a power generation system fuelled by the organic fraction of solid urban refuse and food farming waste: biogas is generated in an anaerobic digester (AD) and then burnt in an internal combustion engine (ICE).

Proper thermodynamic models of both components have been developed and implemented into the library of a modular object-oriented Process Simulator, CAMEL-Pro^®.

Mass-, energy- and exergy balances are performed not only for the whole plant but also at a more disaggregated level, to properly allocate the thermodynamic inefficiencies to each component; for the AD an additional distinction is made as to the allocation of the outputs, because the digested substrate may in fact be accounted for either as a plant waste flow or as a plant product.

The results show a good agreement with the available experimental data, so that the model presented here may be considered as having being validated in terms of  mass of biogas per year and net electrical and thermal power output.

Quite surprisingly, a second law analysis reveals a very high exergy efficiency of the anaerobic digester, in the range of 91%. Some discussion of this point is also presented.

Keywords

• digester, biomass energy recovery, biogasification

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