Biogas Energy Production Potential of Grass
under Anaerobic Digestion: Review
Yalemtsehay Debebe1* and Girma Gonfa2
1Jimma agricultural Research center, Ethiopia
2Addis Ababa Science and Technology University, Ethiopia
Submission: March 25, 2019; Published: April 24, 2019
*Corresponding author: Yalemtsehay Debebe, Jimma agricultural Research center P.O. box 192 Jimma, Ethiopia
How to cite this article: Yalemtsehay D, Girma G. Biogas Energy Production Potential of Grass under Anaerobic Digestion: Review. Agri Res& Tech:
Open Access J. 2019; 21(2): 556159. DOI: 10.19080/ARTOAJ.2019.21.556160
Because of the recent increase demand of energy, interest in environmental issues and depletion of fossil fuel, the demand of renewable energy sources as substitute of fossil fuel is rapidly increasing. Production of biogas under anaerobic digestion as methane can be used to replace nonrenewable energy resources in both heat and power generation and as a vehicle. For biogas production various organic feedstocks are used. Most often used are animal manure, food waste, agricultural wastes and crop residues, aquatic waste, forest residues, municipal solid wastes and grass. The review is based on secondary data obtained through various sources such as textbooks, journals, documents and conference papers with the objective of providing an overview of biogas energy production from biomass, thus providing information toward anaerobic digestion contributing to methane rich biogas production. The review shows that biogas production from biomass as a good fuel alternative because the source biomass can be grown renewably and decrease environmental stress.
Energy is an essential factor in development since it stimulates and supports a country’s economic growth and development . The global energy demand is growing rapidly, and about 88 % of this demand is met at current time by fossil fuels. The depletion of fossil fuel and the impact of greenhouse gases on the environment initiated the concern in searching and exploring alternative sources of energy . These sources are described as renewable energies such as solar energy, wind energy, different thermal and hydro sources of energy, and biomass . Biomass residues available from agricultural and forest processing constitute a potential source for biofuel production. Currently there exist five forms of biofuels such as bioethanol, biodiesel, biomethanol, biohydrogen and biogas .
The production of methane rich biogas (biomethane) involves a complex biochemical reaction that takes place under anaerobic conditions in the absence of oxygen and the presence of highly pH sensitive microbiological catalysts that are mainly bacteria . The anaerobic digestion process is a fermentation process where organic raw materials such as manure, food waste, and sewage sludge, organic industrial waste and energy crops used as substrates for biogas production. From energy crops the grass has the greatest potential for biogas production (587.5m3 methane production/tone of dry organic matter) . The production of
biogas through anaerobic digestion offers several environmental advantages compared to fossil fuels, because available and cheap resources to feedstock, relatively easy and cheap technology for production, extra values of digestate as a fertilizer , avoiding health problems, reduce greenhouse gas (GHG) emissions , reduce deforestation , reduce workload from women and children . The objective of this review is to provide an overview of biogas energy production from biomass, thus providing information toward anaerobic digestion contributing to methane rich biogas production.
Anaerobic digestion is a natural decomposition processes in which microorganisms break down biodegradable material in the absence of oxygen . Anaerobic digestion is widely used as a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production, helping to replace fossil fuels. The nutrient-rich digestate which is also produced can be used as fertilizer. Fig .1 shows that the digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbondioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid,
along with additional ammonia, hydrogen, and carbon dioxide.
Finally, methanogens convert these products to methane and
carbon dioxide  (Figure 1).
Biogas can be produced from biodegradable substrates such as
animal manure, food waste, agricultural wastes and crop residues,
aquatic waste, forest residues and municipal solid wastes .
According to  a brief description of the various feedstocks is
provided in this section.
Animal manure is considered a promising way for biogas
production and fertilizers through the anaerobic digestion process
. There are significant populations of livestock in many
countries. Livestock produce large amounts of manure which
are suitable substrates for AD. Manure is a mixture of faeces and
urine, and its chemical composition varies markedly depending on
the chemical characteristics of the feed consumed by the animal.
Animal manure comprises huge amounts of lignocelluloses,
polysaccharides, proteins, and other biomaterials.
A number of crops and activities produce residues that can be
used as a feedstock for biogas production . Many forage and
sugar beets, due to their high organic dry matter yield per hectare
which produce high biogas yield . Biogas also produced from
non- food crops like Justicia schimperiana (Hochst. ex A. Nees) T.
Ander (JS) .
Compared with other feedstocks, grass has suitable and
promising characteristics as energy. Crop for biogas production
due to their fastest growing rates even in infertile land, low
cultivation costs, higher accessibility, and consumption of whole
plants. In addition, the usage of grassland as a renewable source
of energy during biogas production will provide considerable
quantity of environment protection, owing to the capability of
grass to sequester carbon into the soil .
Napier grass is a complex material that is composed of
cellulose, hemicellulose and lignin. Commonly, cellulose and
hemicellulose primarily contain glucose and xylose, respectively.
They can be fermented to produce renewable energy using several
microbial processes . As shown in Table 1, the maximum
methane content and methane yield of 5% and 122.4ml CH4/g-
TVS remove was obtained using napier grass as the substrate .
Vetiver (Vetiveria zizanioides) is a perennial tufted grass
belonging to the Poaceae family and the common domesticated
cultivars used around the world are non-invasive. It has become
known as a miracle grass with diverse environmental applications,
including: a source of scented oil from its roots, fodder for
livestock, soil and water conservation, rehabilitation, remediation,
waste water treatment and a raw material for biogas production
Experiment conducted in China on vetiver grass as raw
material for biogas production and the result point out that the
production potential of biogas were 310mL.Ts and 332Ml/g. vs
of the grinding process group and 322 and 349 of the shearing
process group  (Table 2).
Elephant grass is a species native to African grasslands and is
low in water and nutrient requirements, allowing it to make use of
otherwise uncultivated lands . Elephant grass possesses several
advantages that make it suitable as bioenergy crop; elephant grass
is perennial, it can be vegetative propagated and it can withstand
repeated cutting/harvesting and regenerates  other features
that made elephant grass suitable for bioenergy purposes include
the possibility of multiple harvest per year.
Demand and dependency on fossil fuels have namely been
increasing since the industrial revolution. It has become clear
that fossil energy sources of the earth are finite, moreover, their
use causes more and more damages to the environment (climate
change, air pollution, greenhouse effect, etc.). In this context, biogas
from biomass will play a vital role in future. Biogas is a versatile
renewable energy source, which can be used for replacement of
fossil fuels in power and heat production, and it can be used also
as gaseous vehicle fuel. Grass biomass growing is importance as it
offers extensive environment protection, owing to the capability
of grass to sequester carbon into the soil matrix as a result CO2
emissions reduced. Furthermore, various socioeconomic profits
are possible to achieve without compete with food production. Due
to their fastest growing rates even in infertile land, low cultivation
costs, higher accessibility, consumption of whole plants.
On the other hand, farmers can use grass for biogas
production feedstocks and its digestate as fertilizer because it is
readily available, it doesn’t require any pretreatment, thus helping
farmers to reduce their energy cost. Even environmental concerns
such as deforestation and ecological damage will not be an issue
as the grass which will be cut will grow in a short span of time.