Encyclopedia of Renewable Energy - James G. Speight

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eruptions and the rotting of vegetation. They become a problem when humans produce the gases in large amounts and at high concentrations by the burning of fossil fuels. It is arguable which gives off the most acid gases – fossil fuels or natural events such as eruption of volcanoes. The distances that pollutant gases travel means that acid deposition is an international or trans-boundary problem. This means that acid pollutants are not necessarily deposited in the same country where they were produced.

      Acid rain falling over regions with alkaline soils or rocks is quickly neutralized but in areas with little acid-neutralizing capacity is the biosphere sensitive to acid rain. Over North America these areas include New England, eastern Canada, and mountainous regions, which have granitic bedrock and thin soils.

      In areas where the biosphere is sensitive to acid rain, there has been ample evidence of the negative effects of acid rain on freshwater ecosystems. Elevated acidity in a lake or river is directly harmful to fish because it corrodes the organic gill material and attacks the calcium carbonate skeleton. In addition, the acidity dissolves toxic metals such as aluminum from the sediments. There is also ample evidence that acid rain is harmful to terrestrial vegetation, mostly because it leaches nutrients such as potassium and allows them to exit the ecosystem by runoff.

      See also: Acid Rain.

      Acid Gas

      Many gas streams, while ostensibly being hydrocarbon in nature, contain large amounts of acid gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2) and acid gas is natural as or even process gas that contains significant amounts of hydrogen sulfide, carbon dioxide, or similar contaminants. The terms acid gas and sour gas are often (incorrectly) treated as synonyms.

      A gas stream containing hydrogen sulfide or carbon dioxide is referred to as sour and a gas stream that is free from hydrogen sulfide is referred to as sweet. The corrosive nature of hydrogen sulfide and carbon dioxide in the presence of water (giving rise to an acidic aqueous solution) and because of the toxicity of hydrogen sulfide and the lack of heating value of carbon dioxide. However, because gas streams from a variety of renewable sources have a wide range of composition, including the concentration of the two acid gases, processes for the removal of acid gases vary and are subject to choice based upon the desired end-product.

      In addition to hydrogen sulfide and carbon dioxide, a gas stream may contain other contaminants, such as mercaptan derivatives (such as methyl mercaptan, CH3SH, and carbonyl sulfide, COS). The presence of these impurities may eliminate some of the sweetening processes since some processes remove large amounts of acid gas but not to a sufficiently low concentration. On the other hand, there are those processes that are not designed to remove (or are incapable of removing) large amounts of acid gases. However, these processes are also capable of removing the acid gas impurities to low levels when the acid gases are there in low to medium concentrations in the gas.

      See also: Gas Cleaning, Gas Processing, Gas Treating.

      Acid Gas Removal

      Acid gas removal (acid gas treating) is the removal of acidic gases such as hydrogen sulfide and carbon dioxide from gas streams. In addition to removal water and gas stream liquids removal, one of the most important parts of gas processing involves the removal of hydrogen sulfide and carbon dioxide. Gas from some sources contains significant amounts of hydrogen sulfide and carbon dioxide and is usually referred to as sour gas. Sour gas is undesirable because the sulfur compounds it contains can be extremely harmful, even lethal, to breathe and the gas can also be extremely corrosive. The process for removing hydrogen sulfide from sour gas is commonly referred to as sweetening the gas.

      Acid gas removal (i.e., removal of carbon dioxide and hydrogen sulfide from gas streams) is achieved by application of one or both of the following process types: (i) absorption and (ii) adsorption.

      Acid gas removal processes involve the chemical reaction of the acid gases with a solid oxide (such as iron oxide) or selective absorption of the contaminants into a liquid (such as ethanolamine) that is passed countercurrent to the gas. Then the absorbent is stripped of the gas components (regeneration) and recycled to the absorber. The process design will vary and, in practice, may employ multiple absorption columns and multiple regeneration columns.

      Liquid absorption processes (which usually employ temperatures below 50°C (120°F) are classified either as physical solvent processes or chemical solvent processes. The former processes employ an organic solvent, and absorption is enhanced by low temperatures, or high pressure, or both. Regeneration of the solvent is often accomplished readily. In chemical solvent processes, absorption of the acid gases is achieved mainly by use of alkaline solutions such as amines or carbonates. Regeneration (desorption) can be achieved by the use of reduced pressure and/or high temperature, whereby the acid gases are stripped from the solvent.

      Adsorbers are widely used to increase a low gas concentration prior to incineration unless the gas concentration is high in the inlet air stream. Adsorption also is employed to reduce problem odors from gases. There are several limitations to the use of adsorption systems, but it is generally felt that the major one is the requirement for minimization of particulate matter and/or condensation of liquids (e.g., water vapor) that could mask the adsorption surface and drastically reduce its efficiency.

      The precise area of application of a given process is difficult to define, and several factors must be considered: (i) the types and concentrations of contaminants in the gas, (ii) the degree of contaminant removal desired, (iii) the selectivity of acid gas removal required, (iv) the temperature, pressure, volume, and composition of the gas to be processed, (v) the carbon dioxide-hydrogen sulfide ratio in the gas, and (vi) the desirability of sulfur recovery due to process economics or environmental issues.

      A number of processes are available for the removal of hydrogen sulfide from gas streams. These processes can be categorized as those based on physical absorption, adsorption by a solid, or chemical reaction. Physical absorption processes suffer from the fact that they frequently encounter difficulty in reaching the low concentrations of hydrogen sulfide required in the sweetened gas stream.

      Solid bed adsorption processes suffer from the fact that they are generally restricted to low concentrations of hydrogen sulfide in the entering sour gas stream. The development of a short-cycle adsorption unit for hydrogen sulfide removal might help remove part of this low-concentration restriction for the solid bed absorption processes. In general, chemical processes are able to meet the regulated hydrogen sulfide levels.

      The most well-known hydrogen sulfide removal process is based on the reaction of hydrogen sulfide with iron oxide (iron sponge process or dry box method) in which the gas is passed through a bed of wood chips impregnated with iron oxide:

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      The bed is maintained in a moist state by circulation of water or a solution of soda ash.

      The method is suitable only for small-to-moderate quantities of hydrogen sulfide. Approximately 90% of the hydrogen sulfide can be removed per bed but bed clogging by elemental

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