Bio-Gas Process Control And Measurement

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If you live near a large sewage treatment plant as some of us have done sometime in their lives, you will, no doubt, be aware of the characteristic smell of sewage sludge being processed in big enclosed tanks with a gas holder on top that stand prominently above the rest of the plant.  What you may not have realised is that one of the important by-products of this treatment process is the bio-gas methane.  The resulting sludge is then de-watered and used as soil conditioner that is a it is a bit like peat that doesn’t smell much at all.

For 40 years or so,  anaerobic digestion in large enclosed reactors has been producing the bio-gas methane in sufficient quantities on medium and large sewage treatment plants to both provide the heating source for the anaerobic process and a modest surplus at times to supplement district heating schemes or to run on-site generators.

So what is anaerobic digestion?  Simply put, it is the processes that use micro-organisms in the absence of air to break down putrescible organic matter into bio-gas, carbon dioxide, solid and liquid residues that can be used on the land as a safe soil conditioner/fertiliser.

Producing bio-fuel as a substitute for fossil fuels is not new.  Indeed, it is quite possible, with a few engine modifications, to use vegetable oils as a partial replacement for diesel fuel.  However, this process is not as ecologically friendly as  producing bio-gas in view of diverting large acreages of of arable land for growing fuel instead of food.   Bio-gas production uses waste products like manure, domestic sewage, silage.

There are two general types of anaerobic digestion, mesophilic and thermophilic; the former operates between 25ºC and 40ºC and the latter between 55ºC and 60ºC or even higher.    The microbes that are used differ in each case but the outputs are similar in composition but the quantities are greater with thermophilic digestion.   Earlier sewage treatment plant digestors used the lower temperature mesophilic process only for economic reasons and for the fact that they were designed to render a large quantity of problem material into a small amount of safe and “sweet” smelling one.  The bio-gas was a handy by-product.

With the perceived need to produce bio-gas as the main product, the economic model altered in favour of maximum bio-gas produced for the minimum solids input to the process.  Therefore, some modern plants use both processes in series to ensure that all the cellulose and other hard-to-breakdown organic material such as wheat stalks and woody bits are broken down to produce the optimum useful bio-gas.

This type of processing is usually very expensive to construct and therefore it is very important to insulate the vessels to retain the correct temperature as well as  ensuring the ‘health’ of the microbe populations.   Therefore, controlling the process using Bio-Gas Process Analysers has become very sophisticated.

The gaseous outputs from a biogas plant include Methane (CH4), Carbon Dioxide (CO2), Nitrogen (N2) a small amount of Hydrogen (H2),  and a trace of  Hydrogen Sulphide(H2S).  This latter gas is highly toxic, corrosive and it has a very strong odour in trace quantities.   It is this gas, above all others that gives sewage plant it’s characteristic odour.  Thermophilic digestion is less likely to produce this gas than the mesophilic process.

The bio-gas analyser uses 5 gas sensors to measure each of the above-mentioned gases.  In the above diagram, the gas sample from the digestor comes in from the right hand side.  There is a flame (Deflagration) arrestor followed by a water content sensor.  The five sensors then detect the five gases.  Before these sample gases can be released, they need to be filtered for H2S before exhausting into the atmosphere and to protect the sensitive pressure sensors.  The outputs are digitised and the signals are conditioned and amplified so as to be compatible for integration into fieldbus control systems or for monitoring locally via Ethernet connections on a PC.

To measure these gas flows, a thermal flow switch or  mass flow transmitter is used.  By measuring and monitoring the relative quantities of each gas used, the health of the various process stages can be determined.  These gases being of distinctly differing molecular weights, are drawn off at different points of the digestion process.  In all cases,  H2S needs to be filtered out to avoid contamination of the product gases.

The thermal mass flow transmitter uses the principle of varying flows cause different cooling of a heated probe.  Again, these transmitters are designed to be incorporated into  the common fieldbus process control system.

You can see that anaerobic digestion has come a long way since the rather crude sewage digestors used in the last century.  The economic and ecologic reasons for producing bio-fuel from waste or by-products of agriculture are, however, relatively new.  It will be difficult to see how extensive bio-fuel processes will become in the twenty-first century as burning anything produces the greenhouse gas CO2.  We will watch this debate with interest.

Hasta mañana