Smells like green spirit: anaerobic digestion plant condition surveys

The boundaries of the Heat Team’s work are often blurred, with our projects sometimes crossing over into PV, EV charging, biomass fuel switching or anaerobic digestion in our search for holistic low-carbon solutions for our clients. This all draws on the wider skills of our team members and the broader Natural Power company.?

To this end, we found ourselves on a warm August morning assembling at a Scottish-based anaerobic digestion (AD) facility with a member of Natural Power’s inspections team. AD plants have become widespread in the UK in recent years due to support through the RHI and FiT schemes and latterly the Green Gas Support Scheme, using a variety of wastes and energy crops to produce combustible methane. This renewable gas allows us to reduce the amount of fossil gas that we burn to generate heat and power.?

Whilst heat is a definitive part of an AD plant, our remit here was to undertake a whole site condition survey with a view to assessing the life expectancy of the site. A collaborative effort between our heat and mechanical inspection specialists enabled us to thoroughly investigate all aspects of the plant’s condition.?

This site was using a combination of municipal food waste and fish processing waste as its feed stock - fortunately for us, no fish waste was on site at the time of our visit! With PPE donned and site induction completed, it was time for a tour through the plant. Starting at feed stock reception and ending up at the grid connection point (taking in de-pack plant, maceration, feed control station, primary and secondary digesters, pasteurisers, final storage, gas clean up and pressure boost, CHP units, pipework, pumps and valves), the team examined the appearance, functionality and state of maintenance of the components of the plant. Once we had followed this up with a detailed examination of the plant records and documented on-site procedures, and interviews with the site operational team , the afternoon was waning, and it was time to decamp back to the office.?

Waiting back at our desks is a raft of digitised documentation from the client on original designs, schematics, maintenance logs and contracts, plus operational data for several years. The job now – to comb through this data looking for trends in performance, repairs and maintenance. What activities had been undertaken, and when? Had repairs and maintenance been undertaken at a higher or lower cadence than expected? Had there been any unanticipated outages or breakdowns? Combining this with the site inspection details we were able to assess whether the individual items of plant were lasting as per the design, and flag where (and at what cost) interventions might be needed.??

Our analysis determined that the plant items at the site were generally performing and being maintained as expected, and we established that some remedial works had already been undertaken to improve safety and pre-emptively extend the life of the digestion tanks. Some areas were in need of additional attention, but these were generally little used items such as back up boilers and supplementary pumps for heating. With some investment in specific plant items, we concluded that the plant would be easily able to reach its design life - and operating for a significantly extended period beyond this would also be within reach.?

If you are interested in hearing more about the work of the Renewable Heat team please get in touch by emailing [email protected].