Matt Hale, international sales manager, HRS Heat Exchangers.
Optimal anaerobic digestion (AD) plant performance can be achieved by fine-tuning the feedstock mixture and retention times to maximize biogas production, as well as through regular servicing of the combined heat and power (CHP) engine to maintain electricity generation.
But aspects can often be overlooked, such as the waste heat from the engine. Some plants use it for digester heating or pasteurization, while others don’t use it at all. This is potentially wasting a resource worth thousands of pounds a year. If you are generating heat separately for a different part of the process, such as drying or concentrating digestate (the organic biofertiliser output from the AD process), then you are also paying for heat which you may be able to get for free.
With biogas combustion heat tariffs under the Non-Domestic RHI between 2.2 and 7.5 p/kWth and heat from an LPG-fuelled boiler currently costing somewhere around 6.6 p/kWth, the price of heat is self-evident. With a typical gas engine producing half its output as usable heat, the heat produced by a 300 kWe engine could be worth as much as £124,000 a year under the RHI before tariff degression is considered.
AD plant uses HRS exchanger technology (centre) for heat efficiency.
Heat exchangers
One solution is to capture and transfer this wasted heat with heat exchangers. These are an established way of gathering the heat generated in one part of a process and transferring it for use in another. But despite their widespread use in industries such as food manufacturing and the chemical sector, they are under-utilized in the AD industry.
There are four main areas of the AD process where heat, which is available and may otherwise be wasted, can be utilized. These are feedstock pre-heating, digester heating, pasteurization and evaporation. Which ones are most suitable or useful will depend on the nature of the production but even if it is not feasible to use the heat as part of the AD process, surplus heat can still be used for other purposes such as drying processes, heating nearby offices or workshops, or hot water for washing down equipment.
Using surplus heat in this way is also free, without the need to buy additional fuel and all of these applications can be fulfilled using a suitable heat exchanger, often with benefits over other technologies such as the tank heating systems often used for pasteurization. A well designed system could recover and use 40% of the heat produced by the plant.
Using heat exchangers in the pasteurization system is more efficient than using tanks with heating jackets as they have a much lower heat requirement — up to half that of some systems.
This is because tank systems have lower heat transfer efficiency and usually dump the hot water after use, rather than reclaiming it.
Dry solid content improvement makes digestate easier and more cost effective to use.
Pasteurisation
Effective pasteurisation of digestate, for example to comply with PAS 110, is possible using surplus heat through heat exchangers rather than the installation of an additional heat source such as biomass boiler which could add hundreds of thousands of pounds to a project.
A well-designed heat exchanger system can provide a continuous pasteurization process which uses less energy than alternative systems, while allowing additional thermal regeneration, or recovery, levels of up to 60%. This saved heat can then be used elsewhere in the process, such as an evaporation plant, helping you to maximize RHI eligibility.
Heat can also be used to separate water from digestate by evaporation. This technique can reduce the overall quantity of digestate leaving the plant by as much as 80%, greatly reducing transport costs associated with the removal of digestate. A well designed system will include measures to retain the valuable nutrients in the digestate while the evaporated water can be condensed and reused.
For example, the captured water can be added back to the feedstock as it enters the digester, making the entire process almost self-sufficient in terms of water use and eliminating liquid discharges from the plant. After evaporation, the treated digestate dry solid content can be as much as 20% (often a fourfold improvement), making it much easier to transport and handle.
Tariff digression, increasing competition and environmental concerns mean that efficiency is directly linked to the operational sustainability of a plant. Incorporating cost-effective heat exchanger systems into the process from the beginning, or retrofitting them to an existing plant, is one of the easiest ways to ensure future financial feasibility and save hundreds of thousands of pounds each year.
