Bio Dynamic improve efficiency at their Colwick AD site with Microbial Desulphurisation

Bio Dynamic process 50,000 tonnes of food waste each year & produce 2 MW green energy at their Colwick site in Nottingham.

Technical director Gary Burgess was determined to avoid the conventional methods of H2S control such as ferric dosing & activated carbon. Not only are these techniques expensive to install there is a significant annual operating cost associated with them.

The desulphurisation system at the Bio Dynamic site features the Allison Engineering Biogas analyser which is supplied with its own air compressor and uses a combination of PID and Fuzzy Logic control to automate the injection of small quantities of air (0.5 to 1%) into the gas stream within the digester. The oxygen converts the hydrogen sulphide (H2S) to elemental sulphur and water, a process which is completely automatic and does not require any operator input and so eliminates additional operational and maintenance costs.

The H2S concentration in biogas from anaerobic digestion (AD) can be up to 2,000 to 3,000 ppm. When the biogas is burnt in a CHP plant to generate electricity the H2S breaks down, producing a weak sulphuric acid that can cause corrosion within the engine, leading to increased maintenance costs and downtime. For this reason many engine manufacturers stipulate a maximum H2S concentration in their warranties.

One of the most common ways of reducing the H2S concentration is to inject iron chloride into the digester slurry or feed substrate, converting the H2S into iron sulphide particles.

Whilst this is very effective it requires additional process control functions with dedicated pumps and storage vessels and therefore has associated on-going operational costs.

Technical Director Gary Burgess said:

“Protecting the CHP plant operations from excessive H2S levels is crucial to any successful AD site. We wanted a desulphurisation system that was easy to install, easy to maintain yet capable of reducing H2S concentration to less than 300 ppm. The Allison system has enabled us to maintain levels of less than 100 ppm and at minimal operational cost.”

From small agricultural sites to larger industrial AD plants, accurate and dependable online gas composition analysis is an essential part of this process and the Allison range provides a wide choice of options. The range comprises the Awi-Eco, a low cost, compact discontinuous analyser which provides 50 x CH4 and CO2 measurements per day, with 4-20 mA output and USB connection.

For more sophisticated monitoring and analysis requirements, the Awi-Flex provides measurement of CH4, CO2, H2S, O2 and H2. This system provides up to five sample points with Ethernet, Profibus, Modbus and USB connection. Most importantly, it can also be used to take flow, temperature and pressure inputs and undertake automated desulphurisation control.

All Allison analysers use a rugged, touch screen industrial interface which allows easy access to recorded data, sensor parameters and system diagnostics. This ensures that all information is instantly available, remotely.