Mogden STW: Biomethane Gas to Grid P...
Mogden STW is one of Thames Water’s Tideway treatment works and in AMP6, a combined heat and power (CHP) installation was constructed at the site. Once the ongoing (AMP7) sludge and AD facility upgrade is completed, biogas yield is set to increase again and, in line with its net zero strategy, at the start of 2022, Thames Water opted to add a biomethane gas-to-grid plant at Mogden during AMP7.
Project brief and site-specific challenges
The gas-to-grid plant at Mogden was conceived to complement the AMP6 CHP facility. As such, the plant was sized to process a third of the site’s anaerobic digestion (AD) biogas output. In order to successfully complement the CHPs, the control of the gas-to-grid plant would need to be appropriately integrated into the existing process control system.
With neighbours on all boundaries and an ever-increasing local population, the Mogden STW site is especially constrained for space. The design development and delivery of the gas-to-grid project was further complicated by the aforementioned AMP7 sludge and AD facility upgrade. The layout, plant configuration and plant control would all need to consider construction, interim and legacy phase site arrangements.
Innovative deployment of a fully proven solution
On placement of the design and build order, SGN Green Gas Solutions set about tailoring a well proven membrane upgrader-based solution for the specific requirements of the Mogden project. Fundamentally, the solution is fairly typical of a current generation biomethane gas-to-grid plant.
The biogas supply for the gas-to-grid plant was taken off the existing supply to the CHP engines. To help overcome the challenges routing this supply to the main gas-to-grid plant compound, SGN Green Gas Solutions located the biogas dehydration equipment close to the point of connection. A combination of above-ground stainless steel and buried PE pipework could then be used to convey the dry biogas.
SGN Green Gas Solutions developed an elongated design for the main plant to work given space constraints. The plant layout had to accommodate HV/LV transformers to supply the membrane upgrader and associated ancillaries and an appropriate route was found for the biomethane product pipework.
Construction
A remotely operated valve delineates the biomethane plant from the gas network and due to its location, the SGN team had to design and install a 315mm diameter, PE export pipeline over a complex 1.2km route crossing a road with high traffic flows into a medium pressure (MP) system. Although convoluted, this connection option guaranteed year-round export capacity.
With established supply chain partnerships and relationships, SGN Green Gas Solutions was able to sequence design, procurement, site works and plant installation to optimise the programme and limit the amount of work required on site. The grid connection pipeline commenced in the summer months of 2022.
Mobilisation on the Mogden site was in January 2023 and civils work peaked in June. Package plant items arrived in quick succession throughout August, and by the end of November 2023, site installation and site commissioning were substantially complete.
[caption id="attachment_13204" align="alignnone" width="1200"] Activated carbon pre-treatment units - Courtesy of SGN Green Gas Solutions[/caption]
Operational flexibility and integrated control
Given the aspiration to operate the gas-to-grid plant in conjunction with the existing CHPs, a significant amount of thought went into the control philosophy for the plant and the control interface between the plant and the wider site. Being a large sewage treatment works, Mogden has a sophisticated process control system, power management system and SCADA.
The existing CHPs normall operation includes automatic start, ramp and stop based on the common level of the site’s floating-roof AD tanks. It was agreed that the biomethane plant start, ramp and stop commands would also need to be driven from the existing process control system based on configurable set-points. The values used for these set-points would then allow the site’s process control engineers to optimise the conversion of biogas in real time based on prevailing conditions.
When there is surplus in electricity generation capacity, more of the site’s biogas can be directed to the gas-to-grid plant to be used (and to an extent ‘stored’) in the gas network.
Renewable generation capacity with intelligent features like this help the UK better match energy supply and demand as the country transitions away from fossil fuels.
Commissioning & commercial operation
Commissioning of the gas-to-grid plant commenced in November 2023, with the first green gas being injected in December. From January the plant effectively commenced commercial operation, with optimisation work, performance tests and reliability trials continuing through the spring.
The output from the plant will provide sufficient green gas to supply over 4,000 local homes, offsetting 8,000 tonnes of fossil carbon emissions each year. The project has further shown how gas-to-grid technology can be rapidly deployed and fully integrated in a high-profile municipal wastewater treatment context.
