The use of gas analysers in anaerobic digestion (AD) plants has sometimes been overlooked as a ‘nice to have’ – but in reality, they are essential for the safety of a plant and help improve and optimise its biological performance. Here, QED Environmental Systems’ customer Avril Banks, Director of AB-EN Environment & Energy, an independent AD expert, explains why she believes plants shouldn’t be functioning without them:
AD plants have a significant role to play in helping the country meet the Government’s target of generating 15 per cent of the UK’s energy from renewable sources by 2020. In order to achieve this, the biogas sector needs to focus on optimising its operation and being more efficient at producing biogas whether it be for conversion to electricity or for injection into the gas grid.
Currently there is quite a disparity in quality and efficiency from one AD plant to the next. Whilst some plants operate very efficiently, there are a large number of plants across the sector that have poor operational performance for many reasons. These issues range from poor feedstock management and quality, to ill thought out design issues and poorly managed maintenance regimes as well as operator competence and engagement throughout the sector.
The overall performance of an AD plant is based on the millions of biological reactions within the digestate and just like in our own bodies, the gas produced varies depending on the particular microbial populations within the tanks. These microbial families are constantly evolving to breakdown the feedstock, so understanding the AD plants reaction to differing feedstocks and feed levels and the quality of the gas being produced from these is vital to optimised performance.
Without an ongoing view of the gas quality it is difficult to operate a plant to its best potential. In-situ monitoring provides a regular and updated record of the plant’s performance. Good systems will even be linked to other control systems within the plant, like aeration. Consistent monitoring also allows the operator to understand the early hydrolysis gas delivery which dilutes the more mature methane production phase and therefore an operator can set the appropriate feed rate dependent on the feedstock in use. This operation will change over time and even with a consistently fed crop, the level of hydrolysis gas can vary from season to season/ clamp to clamp.
A gas analyser also allows the operator to understand the propensity of the feedstock to generate hydrogen sulphide, as some feedstock can generate over 1400ppm of H2S within the first few days.
A system that shows gas quality levels both before and after any filter system is ideal and allows the operator to gauge the effectiveness of the oxygen delivery, ferric additive and filter medium performance. This is imperative as it helps to optimise filter replacement frequency and ensure that the medium being used is fit for purpose. Some activated carbon compositions are more suited to biogas clean up than others and understanding this means that the operator can make operational and financial decisions about their activated carbon purchasing. Understanding the feedstock H2S loading can also be a feature in price negotiations for high H2S producing feedstock.
As well as improving gas quality, there is an important safety aspect to using gas analysers. Hydrogen sulphide released in the AD process can damage equipment and can be harmful to human health. Some materials can give off over 2000ppm of hydrogen sulphide, which not only creates a health hazard and odour nuisance but can also affect the microbial performance upon which the biogas generation is dependent.
In addition, plants need to be aware of the oxygen concentrations within the system. A small amount of oxygen can aid the reduction of hydrogen sulphide, however too much can have catastrophic consequences with the potential for explosion if it reaches the explosive limit.
Also important is the ability of the operators to understand the significance of the data they are being provided.
During the purchase phase and price negotiations, it is often an unintended consequence that some of this monitoring equipment is removed from the scope or is under-specced and this frequently includes gas monitoring systems. Some plants are left with no monitoring at all whereas others are provided with handheld devices which are almost never used at the regularity needed to provide data that will help the smooth operation of an AD plant.
We find that the gas analyser systems may be removed as some of the first cost savings under price discussions as most gas analysers are provided as a third-party delivery like those provided by QED. Unfortunately, the technology provider makes less profit on these items and these are seen as an easy gain for price reduction in final negotiations.
As with all equipment installation, fitting it in during construction is better than in retrofit, however, modern systems such as those offered by QED can be easily fitted into existing systems and, with the help of the technology provider, can also be added into the systems IT interface if required.
To get accurate readings, gas analysers must be properly calibrated, and this is where some users fall down or get frustrated with using them. Gas analysers from some providers don’t have regular service agreements in place and will be sent off site or abroad in some cases for a couple of months before they return from calibration. This leaves the plant to function without an analyser and means that plant operators are then less inclined to calibrate their machinery regularly because of this disruption.
I have been using QED’s gas analysers from its Geotech product range in my work since 1994 and they are calibrated on a fixed system, using the ISO17025 audited automated calibration rigs. Biogas plants can also sign up to an agreement with QED where on request for service the plant receives a “donor” analyser unit to swap in whilst their own analyser is sent for calibration and results in a downtime of just a few minutes.
From my own first-hand experience, it is apparent that gas analysers are an essential tool which should be compulsory in every AD plant across the country. With better monitoring in place, we as a sector, can become more efficient, more productive and improve our safety performance with our aim to establish biogas as a reliable and clean alternative to natural gas.
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