Encyclopedia of Renewable Energy - James G. Speight

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      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. The most important biogas components are methane, carbon dioxide, and sulfuric components. The gas generally composes of methane (55 to 65%), carbon dioxide (35 to 45%), nitrogen (0 to 3%), hydrogen (0 to 1%), and hydrogen sulfide (0 to 1%).

      Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant. Organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so that the process could occur. 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 reactive 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 function 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. 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 in constructing hydrogen collection and transport channels of low cost. A second pathway uses plant material such as agricultural residues in a fermentation process leading to biogas from which the desired fuels can be isolated. This technology is established and in widespread use for waste treatment, but often with the energy produced only for onsite use, which often implies less than maximum energy yields. Finally, high-temperature gasification supplies a crude gas, which may be transformed into hydrogen by a second reaction step. In addition to biogas, there is also the possibility of using the solid by-product as a biofuel.

      The technologies for gas production from biomass include processes such as (i) fermentation, (ii) gasification, and (iii) direct biophotolysis.

      See also: Gaseous Fuels.

      Alternate Fuels – Liquid Fuels

      Liquid fuels are combustible or energy-generating molecules that can be harnessed to create mechanical energy. It is the fumes of liquid fuels that are flammable instead of the fluid. Most liquid fuels in widespread use are derived from fossil fuel sources, but there are several types derived from non-fossil fuel sources – these are hydrogen, methanol ethanol, and biodiesel that are derived from non-fossil fuel sources which are also categorized as a liquid fuel.

      Biofuels are fuels derived from plant materials – are entering the market, driven by factors such as oil price spikes and the need for increased energy security. Examples of solid biofuels include wood, sawdust, grass cuttings, domestic refuse, charcoal, agricultural waste, non-food energy crops, and dried manure. Biofuels are also known as non-conventional fuels or alternative fuels. Alternative fuels can be classified as any fuel that is not derived from conventional sources like natural gas, crude oil, and coal.

      See also: Liquid Fuels.

      Alternate Fuels - Production

      Biorenewable feedstocks can be converted into liquid or gaseous forms for the production of electric power, heat, chemicals, or gaseous and liquid fuels. Main biomass conversion processes are – alphabetically rather than by preference – (i) anaerobic digestion, (ii) direct combustion, (iii) fermentation, (iv) gasification, and (v) pyrolysis. Each process has its own particular aspects, and process application is dependent upon the type of feedstock and the desired product(s).

      The amount of hemicellulose and cellulose in wood and the chemical products desired determine the general type of process that might be used to hydrolyze wood. Hardwoods yield more five-carbon sugars than softwoods. Since, at this time, only the six-carbon sugars from cellulose are readily fermentable, softwoods are desired for ethanol production, but they are not as widely available as hardwoods. Hardwoods are more widely available now, so considerable effort has been expended to develop processes to utilize their unique constituents.

      The main components of wood cells are cellulose (an insoluble substance which is the main constituent of plant cell walls and of vegetable fibers such as cotton. It is a polysaccharide consisting of chains of glucose monomers.), hemicellulose (a class of substances which occur as constituents of the cell walls of plants and are polysaccharides of simpler structure than cellulose.), and lignin (a complex organic polymer deposited in the cell walls of many plants, making them rigid and woody), forming some 99 % w/w of the wood material. Cellulose and hemicellulose are formed by long chains of carbohydrates, whereas lignin is a complicated component of polymeric phenolics. Lignin is rich in carbon and hydrogen, which are the main heat producing elements. Thus, the calorific value of lignin is higher than that of cellulose and hemicellulose (both are carbohydrate derivatives). Wood and bark also contain so-called extractives, such as terpenes, fats, and phenols. The amount of wood extractives is relatively small compared to the amount of extractives from bark and foliage.

      The nitrogen (N) content of wood is approximately 0.75 %, varying somewhat from one tree species to another. For example, nitrogen-fixing alder (Alnus sp.) contains twice as much nitrogen as most coniferous trees. Wood has practically no sulfur (S) and, compared to many other fuels, the wood has a relatively low carbon content (some 50% of the dry weight) and high oxygen content (some 40% w/w), which leads to relatively low heating value per dry weight.

      The collection of solid wastes is usually organized on a communal basis; in developing countries though, it may be organized (to a greater or lesser extent) on an informal basis. The treatment and disposal of solid wastes are definitely connected. Treatment is applied to recover useful substances or energy, to reduce waste volume, or to stabilize waste remains to be dumped or disposed of in landfills. Wastes may be treated before disposal to reduce the volume or to alter the characteristics of the waste which can be achieved by various physical, chemical, and biological processes, while combustion can be used to destroy some toxic organic chemicals. Where a method of waste disposal is not specified, the choice of disposal route will typically depend on (i) the availability of facilities, (ii) volume of waste material, and (iii) hydro-geological characteristics; the influence of industrial and environmental lobby groups must also be taken into account.

      Provided that there is no shortage of land with suitable geological formations, landfill remains the principal final disposal route for the majority of wastes, even in highly industrialized countries. Where there is treatment, it is usually designed to reduce the volume of waste to be landfilled and includes compaction, shredding, baling, and combustion. Most solid wastes will therefore directly be disposed of in sanitary landfills.

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