Methane from anaerobic treatment of waste water to produce syngas to act as a fuel

Methane from anaerobic treatment of waste water to produce syngas to act as a fuel

An anaerobic treatment system is a complex three-step process that produces methane gas (in addition to other products) from the biological digestion of sewage waste. The first stage is the hydrolysis of lipids, cellulose, and protein. Extracellular enzymes produced by the inhabiting bacteria breakdown these macromolecules into smaller and more digestible forms. Next, these molecules are decomposed into fatty acids such as propionic, acetic, and butyric acid. This decomposition is performed by several facultative and anaerobic bacteria such as Clostridium, Bifidobacterium, Desulphovibrio, Actinomyces, and Staphylococcus. Finally, methanogenic bacteria such as Methanobacterium, Methanobacillus, Methanococcus, and Methanosarcina digest these fatty acids, resulting in the formation of methane gas.

The production of methane gas is the slowest and most sensitive step of the anaerobic digestion process because it requires specific environmental conditions for the growth of methanogenic bacteria. These bacteria can only digest effectively at a pH of 6.6-7.6, and if the growth of the acid forming bacteria is excessive, there will be an overproduction of acid leading to a decrease in the pH causing many problems. Also, the methanogenic bacteria have a limited temperature range for optimum performance, usually in the mesophilic range (90 - 105 F). Often this requires pre-heating of the waste before entering the digester .Once the methane gas has been collected from the reactor, it must be cleaned and separated from other biogas constituents such as carbon dioxide, hydrogen sulfide, and excess moisture. Hydrogen sulfide is corrosive to metal piping and may damage gas engines and therefore must be removed by scrubbing the gas with an iron oxide sponge or a gas scrubber. Metal ions added to the sludge before anaerobic treatment can also reduce the hydrogen sulfide content, forming insoluble salts which are removed during digestion. This methane can then be transformed into syngas which could easily act as a fuel/energy generator. The process of conversion of methane to syngas is known as Autothermal reforming.

AUTOTHERMAL REFORMING:

Autothermal reforming (ATR) uses oxygen and carbon dioxide or steam in a reaction with methane to form syngas. The reaction takes place in a single chamber where the methane is partially oxidized. The reaction is exothermic due to the oxidation. When the ATR uses carbon dioxide the H2:CO ratio produced is 1:1; when the ATR uses steam the H2:CO ratio produced is 2.5:1

The reactions can be described in the following equations, using CO2:

2CH4 + O2 + CO2 3H2 + 3CO + H2O + Heat

And using steam:

4CH4 + O2 + 2H2O10H2 + 4CO

The outlet temperature of the syngas is between 950-1100 C and outlet pressure can be as high as 100 bar.

The main difference between SMR and ATR is that SMR uses no oxygen. The advantage of ATR is that the H2:CO can be varied, this is particularly useful for producing certain second generation biofuels such as DMEwhich requires a 1:1 H2:CO ratio

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