Encyclopedia of Renewable Energy. James G. Speight

      Alternate Fuels – Gaseous Fuels

      Gaseous fuels are those fuels that are in the gaseous state under ambient conditions. In some circumstances, the definition of gaseous fuels may also include the low-boiling hydrocarbons such as pentane but such fuels are considered to be liquid fuels.

      Biogas (i.e., gas from biological or alternate sources) is a clean and renewable form of energy, and the most important biogas components are methane (CH₄), carbon dioxide (CO₂), and sulfuric components (H₂S). The gas is generally composed of methane (55 to 65%), carbon dioxide (35 to 45%), nitrogen (0 to 3%), hydrogen (0 to 1%), and hydrogen sulfide (0 to 1%). Biogas could very well substitute for conventional sources of energy (i.e., fossil fuels) which are causing ecological–environmental problems and at the same time depleting at a faster rate. Due to its elevated methane content, resultant of the organic degradation in the absence of molecular oxygen, biogas is an attractive source of energy. The physical, chemical, and biological characteristics of the manure are related to diet composition, which can influence the biogas composition. Raw natural gas is approximately 70 to 95% methane, but biogas is approximately 55 to 65% methane. The biogas composition is an essential parameter because it allows identifying the appropriate purification system, which aims to remove sulfuric gases and decrease the water volume, contributing to improve the combustion fuel conditions.

      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.

A specific source of biogas is landfills. In a typical landfill, the continuous deposition 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 in the atmosphere, which has stronger greenhouse gas impact than the carbon dioxide emitted from the power plant. This makes landfill gas utilization in general an attractive greenhouse gas mitigation option, which is being increasingly deployed in world regions.

      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.