Biogas – What it is and How it is Used

Biogas is the gas produced by the anaerobic digestion of organic materials. It takes place in what is known as a biogas digester. These are becoming increasingly common sights in the countryside. They are easily recognized once you have seen one or two. The main fermentation tanks where the anaerobic digestion takes place is usually circular in plan, green in colour (to make them inconspicuous), and at least 5 to 6 metres high around the perimeter. In diameter they may be from 15 metres to 35 metres, and over the top of them is placed a circular plastic membrane in which the biogas collects.

Chemically, biogas comprises a mixture of methane at 55-65%, and carbon dioxide at 35-45%, the remainder being small quantities of other gases including hydrogen sulphide (H2S) and ammonia (NH4). The biogas contains water vapour at saturation point as well. Most biogas plant tanks and pipework systems are made of steel. Steel is easily corroded by H2S in water, as this is a strong acid even a small quantity of H2S can cause bad corrosion. The secret of successful biogas production without costly downtime, is avoiding corrosion. Achieving this is dependent upon the operator keeping the hydrogen sulphide present, as low as possible to prevent corrosion. Many biogas plant design and build contractors have their own methods for minimizing this corrosion, and sponge filters made of iron, and carefully controlled air-injection, are just two of the methods used. One or two biogas plant contractors avoid concerns about corrosion by using non-corrodable stainless steel as the material of construction. This can be a wise move, but does cost more money during construction of the biogas plant.

Biogas – How It is Used

There is a straightforward use for the biogas. It can be burnt. It has a high calorific (heat) value, it burns cleanly, and is just the same as many other gases. At its simplest, all that is needed is a simple stove gas-burning ring. If there is a factory or any other user of heat, they may just burn the gas to heat a workspace. However, it is best when considering the efficient use of the energy at this level, to use it to run a gas engine which will convert the energy into electricity, and also heat. That heat, is nothing other than the heat which is taken out from the cooling jacket of the engine to keep it cool. It is normally hot enough to run through pipework and to supply radiators to heat the space in homes and factories. When this is done it is known as combined heat and power (CHP). As the cost of energy rises and society seeks to reduce the harmful emission of carbon into the earth’s atmosphere, more and more CHP installations at biogas plants are being built.

However, the cost of by piping this heat, with all the insulation needed, is high. To keep construction costs viably low, the users of the heat provided in CHP schemes need to be close to be anaerobic digestion plant. An example of a very good CHP scheme can be when a landfill is constructed in the void left by the extraction of clay for brick production. If brick production continues at the site, the electricity, and the waste heat (CHP) can be used to heat up the bricks and save a lot of fossil fuel consumption, while also reducing the cost of brick production.

Of course, if the biogas is used to generate electricity and there is no user for electircity nearby, it will be necessary to connect it to the local electricity grid. At some locations this connection may not be possible within a reasonable distance, and/ or the costs charged by the electricity company may not be affordable. When that happens the biogas can be used as vehicle fuel.

This can be done at several different levels. At its cheapest, the biogas may just be given a minimum of clean-up and used in cars and plant operated by the biogas plant owner. In such a case it may make economic sense to keep the clean-up to a minimum and accepts that using it will simply be at the cost of additional corrosion maintenance in those engines. After-all, the energy will be very cheap and usually taxed at a very low rate.

At the next level of sophistication, the most sophisticated proprietary biogas upgrading equipment may be installed to produce a refined (upgraded) biogas which is to all intents and purposes, equivalent to compressed natural gas (CNG). However, at this intermediate level of upgrading quality the CNG would not pass the rigorous tests applied for its commercial and retail sale as, CNG quality gas. Nevertheless, the use of this gas may be entirely intended for the biogas plants owner’s sole use, to fuel his local fleet. In such cases, there is no need for formal certification of this gas quality. Such biogas upgrading projects are becoming very popular, and indeed these are the sorts of installations which supply the biogas to many of the much publicized “green” buses and “green” trucks seen on our highways nowadays.

The highest level of biogas upgrading, which requires not only the highest quality upgrading, but also additional monitoring and certifying of the biogas quality, is the production of grid quality compressed natural gas (CNG). This gas is so pure and guaranteed to be consistently pure, that it is to all intents and purposes the same as natural gas when it is taken that out of gas wells.

This gas is very valuable, commands the highest price, and as long as there is a high pressure gas-main nearby, can be injected into the local natural gas supply. The proponents of sustainable living, point out that this use of biogas is the most sustainable and highly efficient way to use renewable biogas. That is because when electricity is generated there is a large power-loss in the cables and overhead lines, amounting to about 30 per cent in many cases before it reaches houses and factories where it is used. This is unavoidable and is inherent in all electricity generation and distribution systems.

The choice of the best biogas use is made for each site by considering first of all the other there is a local user for the energy (for example in a factory), second what it would cost to connect the biogas plant to the electricity network, and thirdly whether there is an opportunity to connect to a pressure gas pipeline. If none of those are suitable, it is always possible to simply use the biogas to power transport vehicles.

Biogas Safety Considerations

Finally, no article about what biogas is, and how it is used, is complete without pointing out that biogas (methane) is an explosive gas and at all times during process design, commissioning, operation and maintenance, great care must be taken to avoid accidents. All biogas plants, must be fitted with appropriate safety equipment. In Europe it is a matter of complying with a number of statutory regulations and guidance documents. The main regulations stem from the ATEX regulations and are based in each EU country upon the EU ATEX directives. Typical equipment includes gas-flares to burn-off gas during emergencies, flame arrestors, automatic emergency cut off valves, carbon monoxide detectors, and special AREX certified electrical and electronic equipment designed so that it does not create any sparks or have any hot components.

In addition to all the necessary equipment needed to provide for safe operation of these plants when explosive gas is present, the operator of every biogas plant will also need to take care of the human factor. During certain maintenance activities, and in certain areas of the biogas plant, staff must always take precautions that their actions do not create the risk of an explosion. Clearly, such actions of smoking are outlawed, but also actions which may cause a spark, such as casual use of a hammer causing a spark, must be guarded against. For this reason there are guidelines and good practise manuals that must be used at all times when present in the vicinity of biogas plant equipment. Explosions are rare, but unfortunately the consequences are very serious when one does occur. That means that all biogas plant operative staff must be trained to work safely, and extreme caution is essential wherever working upon biogas plant maintenance.

Anaerobic Digestion of Wastewater Treatment Sludge
Centralised Anaerobic Digestion Plants

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