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Conditioning of Biogas

The Biogas released from the digester is saturated with water vapors and contains various other gases. In addition to methane (CH4) and carbon dioxide (CO2), various amounts of hydrogen sulphide (H2S) is also present in the biogas.
Hydrogen sulphide is a toxic gas, with an odor, similar to that of rotten eggs. It forms sulphuric acid in combination with the water vapors in biogas. The sulphuric acid is corrosive and can cause damage to the CHP engines and gas pipelines. Hydrogen sulphide must be removed from the biogas and the biogas must be dried to prevent the corrosion.
The manufacturers of CHP units have a minimum requirement for the properties of the gas. The combustion properties must be ensured, to prevent damage to the engines. Similarly further gas up-grading and treatment measures are required for utilizations of biogas as a vehicle fuel or in fuel cells.

Desulphurization

There is an average content of up to 3 000 parts per million (ppm) of H2S in dry biogas produced from AD of animal dung. The biogas produced by co-digestion of animal manure with other substrates can contain various levels of H2S. Conventional engines used for CHP generation need biogas with levels of H2S below 700 ppm, in order to avoid excessive corrosion and rapid and expensive deterioration of lubrication oil.
Desulphurization or Removal of H2S from biogas can be done by various methods, either biologically or chemically, either inside or outside the digester. Desulphurization depends on the content of H2S and the production rate or throughput rate throughout the desulphurization equipment. The throughput rate can vary greatly, depending on the process. Higher biogas production and thus high throughput rates is caused after new feedstock is fed into the digester. Throughput rates up to double of the normal production can occur for short durations. Thus to ensure complete desulphurization, it is necessary to use desulphurization equipment with high capacities which is almost double the average production rate.
Biological oxidation is one of the most popular methods of desulphurization, which is based on introduction of a small amount of air into the raw biogas. This way, the hydrogen sulphide is biologically oxidized either to solid free sulphur or to liquid sulphurous acid (H2SO3)
2H2S + O2 -> 2H2O + 2S
2H2S + 3O2 -> 2H2SO3
Many a time, the sulphur precipitate produced, is added into the storage tanks and mixed with digestate, so as to improve the fertilizer properties of the digestate.

Biological desulphurization done inside the digester

Biological desulphurization can be done inside the digester, as a cost-effective method. This requires oxygen and Sulfobacter oxydans bacteria, to convert hydrogen sulphide into free sulphur. Sulfobacter oxydans is present inside the digester as the AD substrate already contains the necessary nutrients for the metabolism. The oxygen is provided by injection of air in the top of the digester, done by a small compressor pump. This substrate is most often acidic in nature hence there is chance of corrosion. Due to this, the process is often carried out in separate reactors.

Biological desulphurization done outside the digester

Biological desulphurization can be done outside the digester in desulphurization tanks. This method provided control of the desulphurization process and the precise adjustment of oxygen addition. The reactor is similar to a scrubber, consisting of a porous filling which can be randomly packed plastic elements where microorganisms can grow, a sump, a pump and nozzle arrangement, allowing regular showering of the filling. The H2S is oxidized through a biological process to free sulphur, by upstream injection of a small amount of air.
Showering is done for washing out acidic products and supplying nutrients to the micro-organisms.
Hence the sump must contain a high alkalinity liquid, rich in essential nutrients for the microorganisms. A rinsing procedure, where the filling elements are showered through with an air and water mixture, has to be carried out at regular intervals to prevent free sulphur deposits from closing the reactor filling.
Biogas cleaning is, some cases, relies on the formation of a floating layer in the storage tank, on which the microorganisms can grow and perform the oxidation. A floating layer can usually be maintained with a low mixing intensity and operating the tank as buffer storage. This method is cost effective, but quite unreliable as well, as floating layers are rather unstable, i.e. sinking overnight and at times resurfacing some days later. Hence periods with low efficiency of H2S removal may occur.

Chemical desulphurization done inside the digester

Another alternative for Desulphurisation is by adding a chemical substance to the feedstock mixture, inside the digester. This way, the sulphur is chemically bounded during the AD process, preventing the release of hydrogen sulphide into biogas. Thereby, sulphur is not lost, but remains in the digestate.

Chemical desulphurization done outside the digesters

Chemical biogas desulphurisation can take place outside the digester, using a base such as sodium hydroxide. The method needs special equipment. Another chemical method to reduce the content of hydrogen sulphide is to add commercial ferrous solution to the feedstock. Ferrous compounds bind sulphur in an insoluble compound in the liquid phase, preventing the production of gaseous hydrogen sulphide. The method is rather expensive. A cheaper alternative is to supply co-substrate organic waste containing ferrous materials and to use the ferrous addition only if required.

Drying of Biogas

The relative humidity of biogas inside the digester is 100%, so the gas is saturated with water vapors. To protect the CHP equipment from wear and damage, water must be removed from the produced biogas. The quantity of water contained by biogas depends on temperature. A part of the water vapors can be condensed by cooling of the gas. This is frequently done in the gas pipelines transporting biogas from the digester to the CHP unit.
The water condensates on the walls of the sloping pipes and can be collected in a condensation separator, at the lowest point of the pipeline. A sufficient length of the pipe is required for the effective biogas separation of vapors. If the gas pipelines are placed underground, the cooling effect is even higher. For underground pipes, it is very important to be placed on a stable foundation, to guarantee the incline of the pipes, which can be affected by sinking or moving ground. The condensation separator must be kept frost free and easily accessible, so as to be regularly emptied. In addition to the removed water vapors, condensation also removes some of the undesirable substances such as water soluble gases.
Another possibility of biogas drying is by cooling the gas in electrically powered gas coolers, at temperatures below 10°C, which allows a lot of humidity to be removed. In order to minimize the relative humidity, the gas needs to be warmed up after cooling, in order to prevent condensation in the gas pipelines.

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