Why monitor biogas? (From Geotech in the UK)

Portable biogas analysers are ideal for manual sampling or interval monitoring for varying requirements for one or more sampling points as regularly as needed. Geotech's BIOGAS 5000 is a portable biogas analyser with a variety of gas range options, including CH4, CO2, O2, H2S, NH3 and H2. Portable biogas monitoring can assist with process control and analysis.
	Fixed biogas analysers can be used for continuous or automated sampling at user-defined monitoring intervals for single or multiple sampling points. The GA3000 PLUS from Geotech can be used for up to 3 sample points for CH4, CO2, O2, CO, H2S and H2. Fixed gas analysis can be crucial for CHP protection and revenue generation

Reasons for monitoring biogas

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

• Changes in the mixture of gases produced during AD, particularly if they are sudden, can damage a CHP engine, whether that is long-term wear and resulting high maintenance costs, or short term damage. The costs of engine maintenance contracts and spare parts / call-out charges can be weighed against the cost of biogas monitoring.
   
• High levels of unmanaged H2S can cause increased engine wear. CHP engine suppliers can supply information about what difference unmanaged H2S can make to engine maintenance costs and life.
   
• Damage caused by sudden fluctuations in methane levels, e.g. a burst cylinder head caused by a sudden change in feedstock, can cost tens of thousands of pounds each time. Equally or perhaps even more importantly, unplanned engine downtime means power is not being generated and income lost. This may vary from a few hundred 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. On the other hand, 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/or toxic, so there are good safety reasons for monitoring them.
  
   

Which gases need to be monitored

The key gas is methane (CH4). As well as providing an indication of AD output, it should also be measured in order to protect a CHP engine. It is obviously highly explosive and engines are sensitive to sudden changes in methane levels, as well as requiring a minimum CH4 level to restart.

Hydrogen sulphide (H2S) is often produced during AD, particularly with high protein feedstocks such as maize. As H2S turns into sulphuric acid in the engine, high levels can cause damage. AD plants with high H2S often use H2S scrubbers to remove the H2S before the biogas reaches the engine. In this case it is a good idea to monitor H2S before and after scrubbing to keep an eye on the effectiveness of the scrubbing process.

Carbon dioxide (CO2) and oxygen (O2) are also useful indicators to monitor. The CH4 / CO2 balance can help confirm effective AD. O2 levels show that the process is anaerobic and there are no inward.