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Biogas Design
Design of Biogas from definitions to digesters and gas holders
Biogas design (that is the design of biogas production plants/facilities) varies widely, based on the organic
materials available for use as the feedstock, and the scale of production.
Biogas itself is a type of biofuel formed by the anaerobic, or oxygen-free, decomposition of organic materials by
bacteria and it is almost completely methane (chemical composition CH4). Organic materials typically include human,
animal and plant waste, and energy crops
(crops that are grown for the express purpose of fuel production). The gas formed from this process may be used for
heating purposes or, in its concentrated form, for vehicle propulsion. It is essentially the same as the natural
gas used in dometic homes when delivered through city gas mains, although it needs cleaning up before it can be
used for this purpose.
While biogas design differs in the particulars the core of the reactions which take place always require
methanogenic organisms and those organisms must be provided with suitable conditions to grow and thrive in adequate
numbers in order to get good methane production.
At the heart of the biogas design is the bioreactor, which is known as the digester, where fermentation and methane
gas production takes place - it is indeed the same process as brewing, but the organisms differ.
The digester is an airtight container in which the waste is conveyed and decomposed, and the digester given that it
must provide a home to "good" mirco-organims, the ones which will produce the methane biogas with the least odor
and highest output rate. Research and experience has shown that there are two sets of organisms which work well,
and these have been shonw to be classified into two sets by the temperature ranges at which they work best.
Thus in biogas design the process engineer talks of digesters as being either mesophilic or thermophilic. Those are
just fancy words for describing the two types of microrganisms according to the environmental temperature ranges at
which they work. Mesophilic organisms tend to be those which can survive in normal warm to hot temperatures whereas
the thermophlics like it really hot.
Bacteria within the digester tank breaks down the waste and, as it decomposes, gases such as carbon monoxide,
methane, hydrogen, and nitrogen, are released. It is best to keep the slurry or sludge in the tank at a slightly
basic pH to ensure faster decomposition.
A major disadvantage for anaerobic digestion is the sensitivity and some would say instability of he biogas design
due to ammonia toxicity which can inhibit methanogenesis.
Operators and researchers have suggested that biogas design be optimised over a range of C:N and pH to minimise
ammonia and to maximise biogas yield. We recommend that anyone embarking on biogas design should start by running
their on trials with their proposed feedstock materials and vary the green and woody materials balances to find the
best C:N ratio.
For example in one reported case the optimum conditions were found with a blend that provided C:N of 15 and pH of
6.5 and the cumulative biogas yield was highest at this optimum blend. At higher pH of say 8.5, un-ionised ammonia
present would be high and the negative effect of ammonia toxicity would be seen.
If the pH is too low (say at the pH of 4.5) in biogas design, the unionised ammonia would be at a minimum, but high
VFA concentrations (a family of high odor acidic compounds called Volatile Fatty Acids) will inhibit the
methanogens.
Biogas design must also take care to consider the make-up of contaminants in biogas streams which varies widely,
depending on the source of the stream. It includes contaminants such as volatile organic compounds, and often
siloxane. The source of siloxane contamination in landfills and waste water systems has reportedly been traced to
glossy parchment paper, industrial lubricants and personal care products such as hair sprays, shampoo products, and
deodorants.
The presence of siloxane can have very costly and damaging effects on gas combustion biogas design equipment, and
thus great care is needed when considering suitable engines etc for biogas energy production.
by Steve Last - 12 January 2010
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Source: http://www.anaerobic-digestion.com
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