Thermodynamic, economic and environmental assessment of energy systems including the use of gas from manure fermentation in the context of the Spanish potential

One of the prospective technologies that can be used for energy generation in distributed systems is
based on biogas production, usually involving fermentation of various types of biomass and waste. This
article aims to bring novelty on the analysis of this type of systems, joining together thermodynamic,
economic and environmental aspects for a cross-cutting evaluation of the proposed solutions. The
analysis is made for Spain, for which such a solution is very promising due to availability of the feedstock.
A detailed simulation model of the proposed system in two different cases was built in Aspen Plus
software and Visual Basic for Applications. Case 1 involves production of biogas in manure fermentation
process, its upgrading (cleaning and removal of CO₂ from the gas) and injection to the grid. Case 2 assumes
combustion of the biogas in gas engine to produce electricity and heat that can be used locally
and/or sold to the grid. Thermodynamic assessment of these two cases was made to determine the most
important parameters and evaluation indices. The results served as input values for the economic
analysis and environmental evaluation through Life Cycle Assessment of the energy systems. The results
show that the analysed technologies have potential to produce high-value products based on low-quality
biomass. Economic evaluation determined the break-even price of biomethane (Case 1) and electricity
(Case 2), which for the nominal assumptions reach the values of 16.77 V/GJ and 28.92 V/GJ, respectively.
In terms of environmental assessment the system with the use of biogas in gas engine presents around
three times better environmental profile than Case 1 in the two categories evaluated, i.e., carbon and
energy footprint.