Biogas Scrubbers – A Technical Review
Biogas Desulphurisation Techniques
Biogas is produced by the anaerobic digestion or fermentation of biodegradable materials such as manure, sewage, municipal waste, green waste, plant material, and crops. Biogas comprises primarily methane (CH4), carbon dioxide (CO2) and small amounts of hydrogen sulfide (H2S), moisture and siloxane.
Hydrogen sulfide (H2S) is a colourless, very poisonous, flammable gas with the characteristic foul odour of rotten eggs. Hydrogen sulphide is formed in the biogas plant by the transformation of sulphur-containing protein, which can be from plants and fodder residues. Inorganic sulphur, particularly sulphates, can also be biochemically converted to H2S in the fermentation chamber. While plant material introduces little H2S into biogas, poultry droppings introduce, on average, up to 0.5 volume percent of H2S, cattle and pig manure about 0.3 volume/percent. Protein-rich waste (e.g. molasses, etc.) can produce large amounts of hydrogen sulphide (up to 3 vol. %).
If untreated biogas is burnt as fuel in a gas engine, the H2S will form a weak sulphuric acid. This weak acid will quickly contaminate the engine lubrication oil and lead to corrosion of the combustion chamber, exhaust system and in various bearings.
This is enhanced by frequent starts, short running times and the relatively low temperatures when starting up and after cutting off the engine. Running engines with gas containing H2S can reduce the service time to the first general overhaul by about 10 – 15%. SO2 from combustion and water vapour both dissolve in the lubricating oil. Under continuous operating conditions, the interval between oil changes is reduced to 200 – 250 hours.
Gas-engine manufacturers normally request 250 ppm in the clean biogas for offering full warranty. Since the engine is one of the most expensive & components of any biogas plant, controlling the H2S can be critically important.
There are several techniques for biogas desulphurisation, the three most common being ferric chloride dosing, the installation of activated carbon filters & biological treatment. Each type of scrubber has its own advantages & disadvantages.
Iron chloride can be fed directly to the digester slurry or to the feed substrate in a pre-storage tank. Iron chloride then reacts with hydrogen sulphide to form iron sulphide salt (particles). This biogas scrubbing method is extremely effective in reducing high hydrogen sulphide levels & the initial capital costs are not prohibitive, since the only investment needed is a storage tank for iron chloride solution and a dosing pump.
However the chemical is a “consumable” & so the operational costs can be high due to the constant demand for new material. Even small AD plants can use £20,000 to £30,000 per year.
Activated Carbon filters
Activated carbon filters work by passing biogas through a moist carbon substrate, during this process there is an adsorption effect whereby the H2S molecules are trapped inside the pore structure of the carbon substrate. The H2S breaks down into elemental sulphur, CO2, H2O & K2SO4.
Because of the very high operational costs this type of biogas scrubber is usually only used when the H2S needs to be reduced to 0ppm. A typical example would be biogas upgrading.
Biological H2S reduction,
Biological desulphurisation uses naturally occurring aerobic bacteria present in air, to breakdown the H2S. The air is injected in small quantities into the biogas in the head space of the digester. Because there are no chemicals involved, there are no operational costs & it is environmentally friendly. If the retention time of the biogas in the digester is greater than 1 – 1.5 hrs, we can expect a reduction of H2S up to 95%.
The use of chemotropic bacterial species (Thiobacillus genus) to condition biogas is well established & most thiobacteria are autotrophic, consuming CO2 and generating chemical energy from the oxidation of reduced inorganic H2S. The result is elemental sulphur & water.
2 H2S + O2 -> S2 + 2 H2O
Ultimately the aim is to keep the air input to a minimum whilst maintaining control of the H2S. However, most biological treatment consists of a blower with a fixed speed fan & a small but constant air flow into the gas space. Alternatively, plant technicians can manually operate the blower whenever they decide it’s necessary.
Neither option is ideal because they do not allow “control” of the H2S concentration in “real time”. Manual operation by the technician relies on someone noticing the H2S increase in the first place & if they are not experienced, too little or too much air may be used. Too little air & the H2S will be unaffected, too much air & it will not only stop the anaerobic process but could have explosive consequences.
The AwiFlex analyser from Awite Gmbh measures O2 & H2S (as well as CH4/CO2/H2) & has its own air blower. It uses a combination of PI & Fuzzy logic control to automatically adjust the air flow from the blower based on the rise & fall of O2 & H2S. It will typically control the H2S with between 0.4 & 1% O2 depending on the H2S concentration, with an upper limit of 2.4% O2.
Using this system, reductions in H2S from 1500ppm to less than 100ppm are achievable. Whilst this is not as effective as the H2S scrubbers mentioned previously, it is enough to maintain the engine warranty.
The obvious concern & most common objection to biological desulphurisation is the fact that oxygen is being introduced to an anaerobic process. In addition, that, under fault conditions, an explosive atmosphere may result. The AwiFlex has a number of safety features that eliminate this possibility & a more detailed explanation is available on request.