Page 257 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 243

Biogas production has usually been applied for waste treatment, mainly sewage
sludge, agricultural waste (manure), and industrial organic waste streams (Hartmann and
Ahring, 2005). The primary source, which delivers the necessary microorganisms for
biomass biodegradation and as well, one of the largest single source of biomass from
food/feed industry, is manure from animal production, mainly from cow and pig farms
(Nielsen et al., 2007).
Anaerobic digestion of organic fraction municipal solid waste has been studied in recent
decades, trying to develop a technology that offers waste stabilization with resources recov-
ery (Nguyen et al., 2007). The anaerobic digestion of municipal solid waste is a process
that has become a major focus of interest in waste management throughout the world. In
India, the amounts of municipal solid waste generated in urban areas ranges from 350 to
600 gm/capita/day (Elango et al., 2006). The municipal solid waste stream in Asian cities is
composed of high fraction of organic material of more than 50 percent with high moisture
content (Juanga et al., 2005).
Currently, biogas production is mainly based on the anaerobic digestion of single energy
crops. Maize, sunflower, grass, and sudan grass are the most commonly used energy crops.
In the future, biogas production from energy crops will increase and requires to be based on
a wide range of energy crops that are grown in versatile, sustainable crop rotations (Bauer
et al., 2007).
A specific source of biogas is landfills. In a typical landfill, the continuous deposi-
tion of solid waste results in high densities and the organic content of the solid waste
undergoes microbial decomposition. The production of methane-rich landfill gas from
landfill sites makes a significant contribution to atmospheric methane emissions. In
many situations the collection of landfill gas and production of electricity by converting
this gas in gas engines is profitable and the application of such systems has become
widespread. The benefits are obvious: useful energy carriers are produced from gas
that would otherwise contribute to a buildup of methane greenhouse gas (GHG) in the
atmosphere, which has stronger greenhouse gas impact than the carbon dioxide emit-
ted from the power plant. This makes landfill gas utilization in general a very attrac-
tive greenhouse gas mitigation option, which is being increasingly deployed in world
regions (Faaij, 2006).
In summary, biogas is most commonly produced by using animal manure mixed with
water which is stirred and warned inside an airtight container, known as a digester.
Anaerobic processes could either occur naturally or in a controlled environment such as
a biogas plant. In the complex process of anaerobic digestion, hydrolysis/acidification and
methanogenesis are considered as rate-limiting steps.
Most biomass materials are easier to gasify than coal because they are more reac-
tive with higher ignition stability. This characteristic also makes them easier to process
thermochemically into higher-value fuels such as methanol or hydrogen. Ash content is
typically lower than for most coals, and sulfur content is much lower than for many fossil
fuels. Unlike coal ash, which may contain toxic metals and other trace contaminants,
biomass ash may be used as a soil amendment to help replenish nutrients removed by
harvest. A few biomass feedstocks stand out for their peculiar properties, such as high
silicon or alkali metal contents—these may require special precautions for harvesting,
processing, and combustion equipment. Note also that mineral content can vary as a func-
tion of soil type and the timing of feedstock harvest. In contrast to their fairly uniform
physical properties, biomass fuels are rather heterogeneous with respect to their chemical
elemental composition.
A number of processes allow biomass to be transformed into gaseous fuels such as
methane or hydrogen (Sørensen et al., 2006). One pathway uses algae and bacteria that
have been genetically modified to produce hydrogen directly instead of the conventional
biological energy carriers. Problems are intermittent production, low efficiency, and difficulty