Tidal Current Energy

Biomass
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production of methane-rich biogas from landfill sites as a result of organic material
decomposition results in a significant contribution to atmospheric methane.
At many larger landfill sites, the collection of landfill gas and the production of
electricity using various types of gas engines has been shown to be profitable,
and hence the application of such systems has become widespread. The benefits
are significant, as the methane gas produced becomes a useful energy carrier
rather than a gas that would otherwise contribute to a build-up of greenhouse
gas emissions in the atmosphere [14]. New landfill sites are often specifically
developed in a configuration which encourages anaerobic digestion. In these new
sites, a pipe system for gas collection is designed and implemented prior to waste
deposition. This collection system optimizes the gas output, which can be as high
as 1000 m 3 �h �1 [4]. A variant on engineered landfill systems with gas collection
systems is bioreactor cells, where the biological processes of breaking down the
waste and biogas production are enhanced through process optimization [1].
4.1.2. Hydrolysis/fermentation
Fermentation is used commercially on a large scale in various countries to
produce ethanol from sugar and starch crops, as well as ligno-cellulosic biomass
feedstocks. Typically, the biomass is ground and the cellulose and hemicelluloses
are converted by enzymes to sugars; subsequently, the sugars are
converted by yeast to ethanol. The best known biomass feedstock for ethanol
production is sugar cane, but other organic materials can be used, including
wheat and other cereals, sugar beet, Jerusalem artichoke and wood.
The choice of biomass is important as feedstock costs typically make up 55–80%
of the final alcohol selling price. To date, starch-based biomass remains less expensive
than sugar-based biomass, but requires additional processing [1]. Lignocellulosic
materials such as wood and straw are readily available, but are more
complex due to the presence of longer-chain polysaccharide molecules. Hence
these ligno-cellulosic biomass feedstocks require pretreatment (acid, enzymatic
or hydrothermal hydrolysis) to depolymerize the cellulose and hemicellulose to
monomers, which will subsequently be converted by yeast and bacteria employed
in the process [16,26]. Lignin in biomass is refractory to fermentation and, as
a by-product, is typically employed as boiler fuel or as a feedstock for other
thermochemical conversion processes converting the residual biomass to other
fuels and products [31]. The purification of ethanol by distillation is an energyintensive
step. Production of approximately 496 liters of ethanol per metric
tonne of dry corn is feasible. Hydrolysis techniques for alternative feedstocks are
currently at the pre-pilot stage [26].
4.2. Thermochemical conversion processes
4.2.1. Combustion
Combustion is a thermochemical process that is widely used on a variety of
scales. Direct combustion is a mature and well-established technology with
numerous operating plants around the world. In combustion, the biomass fuel