The chemistry can be represented by simple equations for low partial pressures of the acid gases:
At high acid gas partial pressure, the reactions will lead to the formation of other products:
The reaction is extremely fast, the absorption of hydrogen sulfide being limited only by mass transfer; this is not so for carbon dioxide.
See also: Gas Cleaning, Gas Processing, Gas Treating.
Ammonia
Ammonia is a valuable chemical that is often produced from renewable sources (biomass) that contain nitrogen. It is recoverable from both the liquid and gas streams and can be readily separated from the liquid, by an ammonia still. The fixed salts must be treated with lime or caustic in a lime leg. The gaseous ammonia may be absorbed with sulfuric acid to produce ammonium sulfate, processed to anhydrous ammonia, or destroyed by combustion.
The standard procedure for the manufacture of ammonium sulfate from a gas stream involves several steps. The gas is first cooled to approximately 32°C (90°F) in an appropriate condensation system. Any tar, which is a very troublesome contaminant of ammonium sulfate (and vice versa), condenses and, in addition, much of the water containing approximately 25% of the ammonia (NH3), primarily as ammonium (NH4+) salts, also condenses. This water is rendered basic (lime treatment), thereby converting the ammonium ion to ammonia, which is recovered by being stripped off in a lime still and placed back in the coal gas stream. The coal gas stream is heated to above its dew point (approximately 65°C; 150°F), and the ammonia is adsorbed in 5 to 10% sulfuric acid solution contained in a lead-lined saturator at a temperature of 50 to 60°C (120 to 140°F); ammonium sulfate crystals precipitate from the sulfuric acid solution.
See also: Petrochemicals.
Anaerobic Digestion
Anaerobic digestion is the decomposition of biological wastes by microorganisms, usually under wet conditions, in the absence of air (oxygen), to produce a gas comprising mostly methane and carbon dioxide.
Anaerobic digestion of animal waste is the primary cause of odors, solids buildup and many diseases in swine, dairy, and poultry facilities, processing plants, municipal waste systems, and septic systems. Animal waste concentrated in pits under slatted floors or collected in holding tanks or lagoons has the natural tendency to involve an anaerobic process. Anaerobic digestion occurs when the anaerobic microbes are dominant over the aerobic microbes. Anaerobic microbes will naturally become dominant in pits or lagoons because of the lack of oxygen in solutions containing heavy concentrations of animal waste, which results in a high biological oxygen demand (BOD). These microbes feed on the animal waste at the bottom of the pits and lagoons. As they digest waste, large amounts of toxic gases are released due to the digestion processes common to the anaerobic microbes.
The anaerobic digestion o f animal waste can be changed to aerobic digestion by proper applications of beneficial aerobic microbes in a highly concentrated form. If aerobic microbes are introduced into an environment that is lacking oxygen, they will begin to build oxygen into this environment as long as they survive and reproduce. Aerobes have the ability to do this in a liquid media or in the soil as long as there is an adequate moisture and food source for them to feed on and reproduce. The process is a multi-stage biological treatment process whereby bacteria, in the absence of oxygen, decompose organic matter to carbon dioxide, methane, and water (Table A-20).
Table A-20 Schematic of the anaerobic digestion process.
| Feedstock | Products | Products | Products |
|---|---|---|---|
| Carbohydrates | Sugars | Carbon acids | Methane |
| Fats | Fatty acids | Alcohols | Carbon dioxide |
| Proteins | Amino acids | Hydrogen | |
| Carbon dioxide | |||
| Ammonia |
In this way, the waste sludge is stabilized and the obnoxious odor is removed. The process can, however, be described adequately and simply as occurring in two stages, involving two different types of bacteria. The process occurs in the absence of air, the decomposition in this case is caused not by heat but by bacterial action. In the first stage, the organic material present in the feed sludge is converted into organic acids (also called volatile fatty acids) by acid forming bacteria. In the second stage, these organic acids serve as the substrate (food) for the strictly anaerobic methane-producing bacteria, which converts the acids into methane and carbon dioxide.
Any organic substance can become subject to anaerobic digestion so long as there are warm, wet, and airless conditions. For example, marsh gas is a product of the anaerobic digestion of vegetation at the bottom of ponds; this gas rises to the surface and bubbles, and the gas is also combustible. With the aid of human intervention, there are two products of this process, biogas and landfill gas. The chemical processes behind the production of these gases are complex. The end result of the process is a well-established sludge in which 40 to 60% of the volatile solids are destroyed. Finally, a combustible gas is produced consisting of 60 to 75% v/v methane and the remainder largely being carbon dioxide. (Table A-21).
Table A-21 Summary of most common chemical reactions in the anaerobic digestion process.
Thus, simplified examples of the chemistry of anaerobic digestion process are as follows:
Organic acids formed in the first stage of the waste treatment process are converted to methane at the same rate at which they are formed. If not, they accumulate and ultimately lower the pH, leading to inhibition of the second stage of the digestion process and digester failure. Temperature must be maintained within certain ranges – heating increases the activity of the anaerobic bacteria, reducing the required digestion time. A pH of 7.0 to 7.5