GWE Biogas opens AD plant
31 May 2011
Yorkshire facility designed to handle 50ktpa of food waste
Driffield, UK – 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 had to decide between mesophilic and thermophilic digesters, which operate at different temperatures, according to Mathew Girking, joint managing director of the company.
Mesophilic systems operate in a temperature range between 25-45C°, whilst thermophilic systems operate in a temperature range between 50-70 C°. 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 70C°.
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, said Girking. The wet AD system, he explained, is better suited to feedstocks in slurry form, which can be pumped. In such systems solid materials are effectively 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 in 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on continuous flow processing, in which new feedstock is added to the process at the same time as processed material is withdrawn.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.
The management of batch flow systems is more complicated than continuous flow systems, noted the MD.
Batch flow tends to be associated with dry AD systems, whilst continuous flow is associated more with wet systems. As reliable processing is required, and because a wet system is to be utilised, he said.
The overall AD process occurs in a number of stages and to ensure that each stage is as efficient as possible it is often beneficial to carry out the process in multiple digesters.
As GWE Biogas wanted a very efficient process it opted to employ multiple digesters at the plant rather than the less costly option of using a single digester, said Girking.
The GWE Biogas plant utilises vertical tanks – rather than horizontal tanks – at the Sandhill facility as it is a “wet” AD system.
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.
