The Application of LCA in Renewable Energy Systems: A Case Study on
Anaerobic Digestion Gas (ADG)
TANTIWIWAT, Jeerayoos
201125026
Currently, the environmental problems associated with human activities are
gradually causing severe impact to mankind. Especially important is the huge dependence
on fossil fuel as energy source. To address this challenge many renewable energy sources
have been developed. However, while these alternatives have the image of being
environmental friendly, if we measure their footprint from a life cycle perspective they
might be still causing great impact on the environment. In this study we analyze a new type
of renewable energy technology namely Anaerobic Digestion Gas (ADG) by using Life
Cycle Assessment in order to determine the feasibility of this technology. The study
proposes different scenarios and compares their performance in two stages: biomass
utilizing methods and power generation methods. The study compares the performance
between the utilization of ADG or other type of biomass source and between the utilization
of fuel via traditional thermal power generation method and via the fuel cell.
ADG is the product of the anaerobic digestion process. Anaerobic digestion is a
series of processes in which microorganisms break down biodegradable material in the
absence of oxygen. This method has been used for industrial or domestic purposes to
manage organic waste and/or to release energy and also been widely used as part of the
wastewater treatment process. The ADG is considered as the renewable fuel because the
process produces a methane and carbon dioxide rich biogas suitable for energy production.
The feedstock sources are typically biodegradable waste such as human bodily waste,
agricultural waste or food processing waste, which can be found in our daily life and cause
environmental problems due to their difficulty of treatment and disposal. This is also
beneficial in terms of CH4 emission reduction, Moreover, ADG can replace fossil fuels in
energy conversion devices and the solid residue, the nutrient-rich digestate, can be used as
fertilizer. For the utilization of ADG, this research proposes the fuel cell, especially Molten
Carbonate Fuel Cell, which directly converts the hydrogen source through chemical
reaction to generate electricity.
In the case study of the application of the ADG in fuel cell, the study uses data from
the biomass utilization, especially in biomass waste such as livestock manure and
wastewater sludge, in Hita city, Oita prefecture and also the fuel cell utilization data is
based on Fuel Cell Utilization Strategy Contributing to the Effective Utilization of Biomass
Resources in Japan. The evaluation is conducted in terms of climate change potential,
landfill use, water pollution and also material and energy consumption along with
production from a life cycle perspective, then the feasibility to apply these types of energy
systems.
In phase 1, we compare the performance between baseline scenario, incinerating the
biomass wastes, with composting and utilizing biomass wastes to produce ADG and utilize
them in gas engine generator to produce electricity. The functional unit is 1 ton of biomass
waste. As the result, in terms of climate change potential, the composting and gas engine
system can reduce the GHG emission for 79.3% and 88.5% in terms of CO2eq respectively.
In terms of landfill use, the composting uses most of landfill space for 0.64 m3/t (we
assume the compost is put into landfill) while the baseline scenario uses 0.11 m3/t and gas
engine system uses 0.06m3/t. In terms of water pollution, the BOD and COD of baseline
scenario is zero due to it is assumed that no combusted waste discharged to water source,
while the BOD of composting system and gas engine system is 0.03 kg/t and 0.02kg/t
respectively. For COD, the gas engine system emits most of COD in 1.25kg/t, while the
composting emits 0.14 kg/t. In terms of energy resources, while the baseline scenario and
composting consume electricity and heat for 70 and 30 kWh/t in terms of electricity and
0.0016 MCal/t of heat for composting, the gas engine system produces electricity and heat
to provide the facilities and has the surplus energy produced for 213.44 kWh/t in terms of
electricity and 313.4 Mcal/t in terms of heat. In phase 2, we compare the performance of
energy systems utilizing ADG between gas engine and fuel cell (in terms of MCFC). As the
result, compared to the gas engine system, the fuel cell system can reduce the GHG
emission for 48.4% in terms of kgCO2 and is able to recover more energy for 18.39 kWh/t
in terms of electricity and 145.96 Mcal/t in terms of heat. For material consumption of fuel
cell, we found that during the raw material and fabrication process of fuel cell, there is still
the emission for 392 kgCO2 (for one 2.5 kW prototype unit).
As the conclusion, although the incineration technology (without energy recovery)
has very poor performance in terms of environment, if including energy recovery, the
energy recovered can compensate with the CO2 emission. For the composting, this type of
technology can reduce pollutant by utilizing the biomass waste to produce fertilizer, which
can be utilized or commercially sold. Gas engine utilizing ADG, is highly evaluated in
terms of producing energy while reducing GHG emission and environmental pollution. By
the way, in terms of environmental factors, the new proposed energy system, fuel cell
utilizing ADG, is expected to have remarkably low emissions, compared to gas engine, due
to methane utilized as hydrogen source with zero emission, carbon dioxide circulated and
utilized in MCFC system and other pollutants removed before being fed into the reforming
system. Thus, this system is expected to greatly contribute to emission reduction. Since the
fuel cell system can utilize ADG to generate electricity power and waste heat from the
system also can be efficiently utilized via co-generation system such as combining with gas
turbine or steam turbine, this system is highly evaluated in an energy aspect. On the other
hand, the fuel cell’s material consumption during the raw material extraction and
fabrication process, especially in heavy metals and rare metals, is in high amount, which
means that there are still high amount of pollutants emitted during these processes. Finally
this new system shows high feasibility to be used in many regions which have enough
biomass resources to be converted into ADG and can be the targets of the introduction of
the fuel cell system in order to replace traditional power generation system.
Keywords: Renewable Energy, Life Cycle Assessment, Anaerobic Digestion Gas, Fuel Cell,
Energy Technology