Designing an AD plant

GWE Biogas has recently opened a £10-million, anaerobic digestion (AD) power plant at its Sandhill site in Driffield, Yorkshire. The facility is designed to convert 50ktpa of food waste from supermarkets, food manufacturers and other sources into biogas.

In designing the plant, GWE Biogas first had to decide between mesophilic and thermophilic digesters, according to Mathew Girking, joint managing director of the company.

Mesophilic systems operate in a temperature range between 25-45ºC, while thermophilic systems operate in a temperature range between 50-70ºC. Thermophilic systems have faster throughputs, produce gas at faster rates and have greater pathogen kill than mesophilic systems.

As reliability is very important to GWE Biogas, a mesophilic system has been chosen for the plant. However, to achieve high pathogen kill, a pasteurisation step has also been included before the digesters, where the incoming material is heated to 70°C.

However, these advantages had to be weighed against the need for more energy, to heat and maintain the temperature of the digester, and the need for greater management to gain reliability, noted Girking.

The GWE Biogas plant is a wet system because, after front-end processing, the food waste is in the form of a slurry, he added. The wet AD system, Girking said, is better suited to feedstocks in slurry form, which can be pumped. In such systems, solid materials are entrained in a water matrix.

By comparison, dry AD systems are more suited to reasonably solid feedstocks, such as straw, that are transported by conveyors and can be stacked in tanks. In dry systems, water is effectively entrained in a solid matrix.

The Yorkshire AD plant relies on continuous flow processing, in which new feedstock is added to the process at the same time as processed material is withdrawn. In batch flow systems the digester is filled up with feedstock, allowed to process the feedstock for a few weeks and is then emptied.

Management of batch flow systems is more complicated than continuous flow systems. Batch flow tends to be associated with dry AD systems, while continuous flow is associated more with wet systems. As reliable processing is required, and because a wet system is to be utilised, GWE Biogas opted for continuous flow processing.

The overall AD process occurs in a number of stages and to ensure that each stage is as efficient as possible the process is often carried out in several digesters. GWE Biogas, therefore, decided to employ multiple digesters at the plant rather than the less costly option of using a single digester.

The GWE Biogas plant utilises vertical tanks rather than horizontal tanks at the Sandhill facility as it is a ’wet’ AD system, the MD further explained.

In horizontal tank systems, a ’plug’ of dry feedstock material is fed into the digester and slowly flows through the tank at a rate equal to the rate new material is fed in. Vertical systems, by contrast, take in feedstock through a pipe on one side of a tank and allow digestate to overflow through a pipe on the other side of a tank, Girking pointed out.