The Advantages of External Digester Heating
Anaerobic digestion (AD) is a well-established technique for the production of renewable energy (biogas) and the management of organic wastes such as sewage sludge, manures and crop wastes. However, many digester designs still incorporate heating elements within the tank itself (or in the floor or the walls), an outdated design which causes a number of operational difficulties and can lead to unnecessary and expensive downtime.
Anaerobic digesters are normally designed to operate in one of two temperature regimes: mesophilic (between 30°C and 38°C) or thermophilic (50°C to 57°C). Irrespective of which temperature regime is utilised, in many situations some form of digester heating is likely to be required to achieve optimal biogas yields.
In the past the heating system (often comprised of a number of hot water pipes) has been located on or around the floor of the main digester. While this results in good initial heat transfer and creates thermal currents in the digestate to assist mixing, the accumulation of sediment quickly diminishes these properties and regular cleaning (which requires the digester to be emptied each time) is necessary to maintain satisfactory performance with such systems. In some cases, heating units have actually been incorporated into the concrete walls or floor of digesters adding to the potential problems and expense should something go wrong.
The solution is to locate the heating unit outside of the digester, an approach which was first used in Illinois in 1946. Since then, external digester heating based on heat exchangers has been used successfully in AD plants around the world. Over the years the design of heat exchangers has improved considerable and is epitomised by the HRS DTI Series of corrugated tube heat exchangers which are ideally suited for the external heating of digester fluids and sludge – and which have been successfully used in a number of such applications.
The biggest advantage of external heating is that it can be checked, cleaned or serviced at any time without the need to empty (or enter) the digester, a process that is both costly and potentially dangerous. However, there are a number of other benefits, including the fact that external systems can be designed so that one heat exchanger array heats more than one digester, and that the improved thermal performance reduces heating requirements and improves the overall energy efficiency of the AD plant. Where units are constructed of materials such as stainless steel, operating life is considerably improved compared to internal heating units, and routine maintenance is straightforward.
The HRS DTI Series is a true counter-current heat exchanger, meaning that the product flows through the inner tube and the service fluid flows through the surrounding shell. HRS’s corrugation technology increases both heat transfer and efficiency over standard smooth tube heat exchanger designs and minimises fouling, and important consideration when dealing with materials such as digester sludge. Multiple units can be interconnected and have the options of frame mounting, insulated and cladded in stainless steel.
To discover why a number of AD plants are replacing their original internal heating systems with external heating using DTI Series heat exchangers, please talk to us today.