Processing Biogas from Anaerobic Digestion into a Commercial Methane Product
The biogas collected from the top of the reactor will require compression for most external sales uses, however, before that can be done the gas has to be purified to remove a number of chemicals which might cause corrosion and sometimes also for odour removal.
The biogas produced by any methane digester is primarily methane and carbon dioxide, with traces of hydrogen sulphide, and other gasses.
Now it is of course possible to use the biogas as it is for heating and for cooking, and for many biogas plants in the developing nations this is all those users seek to use the biogas for.
Nevertheless, in developed countries there will be economic advantage in selling the biogas for general commercial distribution and use. Both in the US and in some Scandinavian countries the owners of AD Plants have sold the gas both into the natural gas distribution system in their locality and as Liquefied Natural Gas (LNG) for automobile fuel use. Some have even gone further to manufacture biodiesel, but this is very rare.
Unfortunately, the use of raw biogas in heating equipment and in internal combustion engines without first cleaning it, would be unacceptable as the raw biogas would cause corrosion due to the presence of small amounts of hydrogen sulphide and water vapour. Costly failures of components would be the result.
In fact biogas produced in an anaerobic digester comprises the following constituents:
- Methane (60 to 70 %),
- Carbon dioxide (30 to 40 %),
- Various toxic gases in trace quantities, including hydrogen sulphide, ammonia, and sulphur-derived mercaptans (which are also highly odorous),
- 1 to 2% water vapour.
The carbon dioxide in the biogas does not combust so it lowers the overall heating (calorific) value of the gas.
In the paragraphs which follow we have set out the common treatment methods for biogas from agricultural AD. It should be noted that the biogas from the digestion of animal wastes does not have some of the contaminants of biogas from some landfills or some municipal waste water (MSW) treatment plants. As a result some MSW biogas may be harder to clean up, than described below.
Hydrogen sulphide is corrosive and the smell it produces is that of "bad eggs", and is uniquely unpleasant. It is also dangerous as it is highly toxic to man. However, it can be removed from the biogas by various methods, the following are some reported possible techniques:-
- the injection no more than 9% volume of air into the biogas in the gas reservoir,
- the addition of iron chloride to the digester feed
- passing the biogas through wood chip impregnated with iron oxide (iron-sponge)
- passing the raw biogas through activated carbon.
Carbon dioxide can be removed by scrubbing in vertical column packed-bed tower, during which the carbon dioxide is dissolved in the water.
The final stage is then the removal of moisture. One popular way that this is done is by condensing the water vapour by passing the biogas through a cooled radiator.
The benefit from carrying this additional cleaning stage is a premium energy product, which as natural gas supplies become less available as supplies dwindle will become much more valuable. Natural gas reserves already are, for example, largely worked out as far as the UK’s North Sea supplies are concerned.
Many products produced from waste process technologies suffer from the problem that the market for those products is uncertain. This is where Anaerobic Digestion has a huge advantage over so many other waste process technologies. It is surely inconceivable that the AD producers commercial quality methane product will ever be unable to find a profitable market.
|
