Why monitor biogas?

Geotech takes a look at the many reasons to monitor biogas and the benefits for AD operators…

A key reason for investing in an AD plant is due to the biogas that can be produced and its value to the plant as a fuel. The case for monitoring biogas can be made commercially, with some reasons applying to all plants with the strongest case applying to a plant with a CHP engine, particularly where feedstock is varied:

Monitoring gas to protect your plant’s engine:

• CHP engines can cost up to £750,000. Gas monitoring protects your CHP engine from expensive, unplanned downtime due to sudden gas level changes, as well as engine repairs (a sudden change in methane % can cause a valve failure). On a small 0.5MW plant, at around 4% of the CHP cost, an analyser may take five days to pay for itself in avoiding lost downtime. On a 1MW site it could pay for itself in half-a-day's downtime.

• High levels of unmanaged H₂S can cause increased engine wear. CHP engine suppliers have told us the difference unmanaged H₂S can make to engine maintenance costs and life expectancy.

• Damage caused by sudden fluctuations in methane levels, for example, a burst cylinder head caused for example by a sudden change in feedstock, can cost thousands of pounds.  Unplanned engine downtime means power is not being generated and consequential income is lost. Depending on the engine size, this could vary from hundreds of pounds a day for a small engine up to thousands for a larger engine.

• In the absence of other process monitoring, biogas monitoring may be the only way of checking and fine-tuning the AD process. Increased electricity yields can be achieved, potentially worth thousands of pounds over a year. Monitoring also protects against the worst case scenario of a complete digester load failing and needing to be discarded, in which case the costs can be many hundreds of thousands of pounds.

• The gases produced during AD are explosive and potentially toxic, good enough safety reasons to carefully monitor them.

Which gases need to be monitored?

The key gas for AD sites throughout the world is methane. As well as providing an indication of AD output, it is measured in order to protect a CHP engine. As it is a highly explosive gas, engines are sensitive to sudden changes in methane levels, as well as requiring a minimum CH4 level to restart.

Hydrogen sulphide is often produced during anaerobic digestion, particularly with high protein feedstocks such as maize. As H₂S can lead to sulphuric acid in the engine, high levels can cause damage. AD plants with high H₂S often use scrubbers to remove the H₂S before the biogas reaches the engine. In this case it is beneficial to measure H₂S before and after scrubbing to monitor the effectiveness of the scrubbing process.

Carbon dioxide and Oxygen are also useful indicators to monitor. The CH₄ / CO₂ balance can help confirm effective anaerobic digestion. O₂ levels show that the process is anaerobic and there are no inward gas leaks.

Geotech supports AD sites of all sizes by providing portable and fixed gas monitors, used throughout the gas monitoring process. To find out more email, visit the website or call +44(0)1926 338111.