Encyclopedia of Renewable Energy - James G. Speight

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considering the electrons and consists of two protons and two neutrons bound together into a particle identical to a helium-4 (4He) nucleus and are typically the product of the alpha decay process. The symbol for the alpha particle is α or α2+, and because alpha particles are identical to helium nuclei, they are also sometimes written as He2+ or 42He2+ which indicates a helium ion with a +2 charge (missing two electrons). Once the ion gains electrons from its environment, the alpha particle becomes a normal (electrically neutral) helium atom 42He.

      Alpha decay typically occurs in the heaviest nuclides, and, theoretically, it can occur only in nuclei somewhat heavier (higher atomic number) than nickel atomic (number = element 28), where the overall binding energy per nucleon is no longer a minimum and the nuclides are therefore unstable toward spontaneous fission-type processes. In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitters being the lightest isotopes (mass numbers 104 to 109) of tellurium (atomic number = element 52).

      When an atom emits an alpha particle in alpha decay, the mass number of the atom decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom decreases by two, as a result of the loss of two protons, and the atom becomes a new element. Also, unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus that can support it. The process may leave the nucleus in an excited state after which the emission of a gamma ray then removes the excess energy.

      See also: Beta Decay, Gamma Decay, Nuclear Energy.

      Alpha Particle

      An alpha particle (also termed alpha radiation or alpha rays) is a helium nucleus stripped of its orbital electrons and which is emitted from a radioactive atom with a velocity of about 1/20 that of the speed of light and with energies ranging from 4 to 9 MeV. The alpha particle was the first nuclear radiation to be discovered; beta particles and gamma rays were identified soon thereafter.

      Alpha particles cause ionizations in matter when they are deflected by the positive charge of a nucleus and pull the orbital electrons (attracted by the alpha’s positive charge) along with them. Alpha particles also cause excitation along their path by pulling inner orbital electrons to outer orbits. Energy is then given off by the atom as fluorescent radiation (low energy x-rays) when the electrons drop back down to the inner orbital vacancies.

      Because of its relatively large mass (2 neutrons and 2 protons), high electrical charge (2+) and low velocity, the specific ionization of an alpha particle is high and, as a result, it creates many ion pairs in a very short path length. Because of this, it loses all of its energy in a short distance. The range in air is only several centimeters even for the most energetic alpha particles.

      See also: Beta Decay, Beta Particle, Gamma Decay, Nuclear Energy.

      Alternate Fuels

      Alternate fuels (renewable fuels, advanced fuels, synthetic fuels which include liquid and gaseous fuels, as well as clean solid fuels) are any materials or substances that can be used as fuels, other than conventional fuels. Alternate fuels are derived from resources such as coal, oil shale, or tar sands, and various forms of biomass – in the last case the erm that is more appropriate is renewable fuels. Renewable fuels are making headway into the fuel balance by reducing dependence on imported oil. Often, renewable fuels produce less pollution than crude oil-derived gasoline or crude oil-derived diesel fuel.

      Examples of renewable fuels and renewable energy sources are: (i) biodiesel, which is diesel fuel derived from vegetable oils and animal fats. It usually produces less air pollutants than crude oil-based diesel, (ii) biogas which is produced from the anaerobic digestion of diverse organic waste sources using various methods, (iii) biomass liquids: produced by biomass-to-gas-to-liquid conversion and by pyrolysis processes which involve the conversion of biomass, such as wood and agricultural residues, (iv) ethanol or other alcohols produced domestically from corn and other crops and produces less greenhouse gas emissions than conventional fuels, and (v) hydrogen which is produced from biomass, (vi) nuclear power, or (vii) other sources such as solar energy, tidal energy, and wind energy and hydroelectric power.

      Biodiesel from plant sources is similar to diesel, but has differences that include higher cetane rating (45 to 60 compared to 45 to 50 for crude oil-derived diesel fuel), and it acts as a cleaning agent to get rid of dirt and deposits. As with alcohols and gasoline engines, taking advantage of the high cetane number of biodiesel potentially overcomes the energy deficit compared to ordinary number 2 diesel.

      See also: Biodiesel, Biogas, Hydrogen, Synthetic Fuel.

      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.

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