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Net energy yield comparing study for Biomass to methane
and ethanol
Huang Weidong
1
, Xia Weidong
2
1
Environmental Division, Department of earth and space science, University of Science and
Technology of China, Hefei (230026)
2
Department of energy science and engineering, University of Science and Technology of China,
Hefei (230026)
E-mail:huangwd@ustc.edu.cn,xiawd@ustc.edu.cn
Abstract
A detailed comparative analysis of biomass conversion efficiency for ethanol and methane is provided
according to the conversion yield and energy consumption of the two conversion processes. The results
demonstrate that net energy yield for methane is 0.50-0.83MJ/MJ fuel more than ethanol during
biomass conversion, as all small biomolecule including hydrolysate from carbohydrate, protein and fat
can be digested to methane, and only carbohydrate hydrolysate can be fermented to ethanol; the energy
consumption for evaporation of less residue in digestion is less than in fermentation, and the energy
consumption in removing CO
2
from biogas through freezing for methane purification is much less than
that in removing water from bioethanol through distillation for purification of ethanol. It was
demonstrated that the comparative analysis is more credible than Life Cycle Analysis in screening
conversion method for biomass.
Keywords: Digestion, Fermentation, Life Cycle Analysis, Comparative Analysis, Net Energy Yield,
Biomass
1. Introduction
As biomass is the only renewable material and energy source[1], it will become the main source
of fuel and chemicals [2] in the future to cope with global warming and depletion of fossil fuel
resources. One of the key problems is how to converse biomass to biofuel. Although it has been
thought that fermentation to ethanol can contribute to energy and environmental goals[3], the energy
return on investment of ethanol has been hotly debated for more than twenty years[4-7] through Life
Cycle Analysis (LCA). In LCA, all processes in the entire lifecycle of biofuel are evaluated including
the manufacture of inputs (such as fertilizer), biomass production, transportation from farm to
production facilities, and then biofuel production, distribution, and use. These processes are each
complex and may be expected to change in the future, some processes are lack of data, making any
evaluation challenging[3]. Here we show that a comparative method can demonstrate that bio-
conversing biomass to methane consumes less energy and provide fuel with more energy than to
ethanol.
2. Methods
We defined net energy value (NEV) of biofuel from biomass as fuel energy transformed from
biomass minus energy consumption during the biomass conversion process. It is written as:
NEV(MJ/L fuel)=(E
f
-E
c
) (1)
Where E
f
is fuel energy (MJ/kg) conserved from biomass, E
c
is the consumed energy during the
conversion process (MJ/kg).
The Net Energy Yield (NEY) E for biomass conversion process is defined as:
E=(E
f
-E
c
)/E
f
(2)
The energy of fuel or feedstock is not included in the energy consumption for only energy storage
form is changed during the conversion process. NEV and NEY only describe the energy variance of