Supply Chain - Renewables & Energy Management - Biogas

Deephams Sewage Treatment Works (STW) is located in Edmonton, North London. In November 2018, Thames Water Utilities Limited (TWUL) issued take-over to the AECOM, Murphy, and Kier (AMK) joint venture for the replacement of the entire sewage treatment stream. The project increased the treatment capacity to 989,000 population equivalent and met the new final effluent flow to full treatment capacity of 5,750 l/s and the discharge consent quality concentration of 10/5/1/1mg/l (solids/BOD/NH₃-N/P). As described in the UK Water Projects 2018 article Deephams STW - A630 Major Project, the project utilised integrated fixed film activated sludge (IFAS) media in the temporary and permanent phases of the works to increase the biological mass in the aerobic phase of the activated sludge process.

Biogas yield

Since the commissioning of the new sewage stream, TWUL operational excellence has resulted in the yield of biogas exceeding the best estimates of its generation from the anaerobic digestion plant. The unintended consequence being that both the CHP and boiler demands are met but regular flaring of excess gas occurs. To avoid these environmental emissions, the biomethane project at Deephams STW was born. During 2020, TWUL created a design, build, maintenance and operation framework agreement for the provision of biomethane plants and in March 2021 a contract was awarded to SGN Commercial Services Limited for the project at Deephams STW. The plant is designed for a maximum 1,150Nm³/hr flowrate of biogas, with a maximum biomethane injection rate to the Cadent medium pressure grid of 825Sm³/hr after enrichment with the addition of propane. [caption id="" align="alignnone" width="1200"] Deephams STW Biomethane Plant flow diagram[/caption]

Engineering technical solution development

The biomethane plant gas-to-grid treatment stream comprise the following stages:

• Biogas transferred from the anaerobic digestion plant.
• Pre-treatment and dewatering.
• Compression and methane separation.
• Enrichment, gas quality monitoring, odorisation, and injection into the grid.

The design concept was to allow flexibility for the biogas to be diverted to either the biomethane plant or to the retained CHP capacity. Thereby, permitting biogas to feed the biomethane plant during normal operational periods but to be diverted to the CHP during high electricity demand periods from the STW plus fulfil any obligations to export electricity off-site to the electricity grid during high consumer demand periods. [caption id="" align="alignnone" width="1200"] Membrane unit - Courtesy of SGN Commercial Ltd[/caption] Biogas transferred from the anaerobic digestion plant: Upstream of the gas holder, a connection was provided to supply the biomethane plant. The biogas header operates at between 5–25mbar. Biogas is saturated with water vapour, and its composition is 50-60% methane (CH₄), 40-50% carbon dioxide (CO₂), and small amounts of hydrogen sulphide (H₂S), siloxanes, nitrogen (N₂), Oxygen (O₂), and other trace components such as volatile organic compounds (VOCs). Compression and methane separation: The pre-treated biogas is then analysed and if it is within specification transferred to the compression stage (two 600Nm³/hr compressors capable of 50% turndown ratio) which creates a driving force for membrane separation of the CO₂ and CH₄. The compressed biogas is further cleaned to reduce oil, water and any remaining contaminants to negligible levels to protect the membrane system’s operation and performance. Highly selective Evonik Sepuran membranes supplied by DMT Environmental Technology are used to permeate the CO₂ from CH₄. The polymer of which the membrane is formed allows the larger CH₄ molecule to be retained within the gas stream whilst the membrane polymer which is subjected to heat and pressure allows the smaller CO₂ molecule to permeate through its structure and separate from the CH₄. The CO₂ passes through a vent and is then discharged to atmosphere. [caption id="" align="alignnone" width="1200"] Compressor room - Courtesy of SGN Commercial Ltd[/caption] Enrichment, gas quality monitoring, odorisation, and injection into the grid: The resultant biomethane gas (retained methane) is checked for compliance with the Gas Safety Management Regulations (GSMR); once satisfactory, it is conveyed to the grid-entry-unit for final conditioning. The biomethane has a lower calorific value, approximately 5%, than the indigenous network natural gas so requires enrichment with propane (a heavier hydrocarbon with a higher calorific value). Following the addition of propane, the gas passes through the Honeywell UK limited gas-to-grid entry unit where it is tested. If it is of acceptable quality for onward conveyance into the Cadent gas network, it is metered and undergoes its final conditioning phase with the addition of a sulphur compound or ‘stenching’ agent to introduce the distinctive ‘natural gas’ odour. In the event of there being a problem in the Cadent gas network there is a remote operated valve (owned by Cadent) that can shut off the supply of biomethane gas from Deephams STW. If at any phase of the process stream ‘gas’ is out of standard or specification the biomethane gas is rejected by the grid-entry-unit. These rejected gases are then either diverted to the waste gas burner (WGB) or returned to the biogas storage.

Deephams Biomethane Project: Supply chain - key participants

• Biomethane plant design/delivery: SGN Commercial Services
• DSEAR EPD: SLR Consulting
• Civil/electrical engineering: Bridges Limited
• Pipelines: South East Eco Utilities Limited
• Biogas upgrading plant: DMT Environmental Technology
• Gas-to-grid entry unit: Honeywell UK Limited
• Siloxane & H₂S removal filters: CPL Activated Carbons
• Propane tanks: Flogas Britain Ltd

[caption id="" align="alignnone" width="1200"] Honeywell UK Limited grid entry unit - courtesy of SGN Commercial Ltd[/caption]

Non-engineering technical challenges

As with most projects, the complexity of its implementation was not solely restricted to the engineering solution. Multiple other challenges were confronted and delivered, including:

• Third party agreements.
• Permitted development rights.
• Waste operation environmental permit.
• Application of Thames Water standards and gas industry standards.
• Renewable energy grants.
• Future opportunity.

Third party agreements: Engagement with Cadent was a key delivery component to ensure timely compliance with their guidelines; a pre-requisite for the biomethane to be injected into the national gas transmission grid. The first key component being to obtain a Grid Connection Agreement which permitted the physical connection to the medium pressure main; the second being granted the Network Entry Agreement which required the installation of Cadent approved plant to demonstrate the provision of ‘in-specification biomethane’ being ready for injection into the grid. This was achieved by utilising the Honeywell UK limited gas-to-grid entry unit; a tried-and-tested solution that Cadent had previously approved. [caption id="" align="alignnone" width="1200"] (left) Cadent-owned remote operated valve and (right) the electrical HV room - Courtesy of SGN Commercial Ltd[/caption] Permitted development rights: A key driver in achieving timely commissioning of the biomethane plant was for the project to be implemented under TWUL’s permitted development rights as a statutory sewerage undertaker. TWUL requested of London Borough of Enfield (LBE) an EIA Screening Opinion under Regulation 6 of the Town and Country Planning (Environmental Impact Assessment) Regulations 2017 for the proposed biomethane plant to agree that an EIA was not required (as if full EIA is required, permitted development rights are removed). LBE agreed and issued an EIA Screening Opinion confirming that full EIA was not required. TWUL hence notified LBE that the scheme would be implemented under their permitted development rights as Statutory Sewerage Undertaker. Subsequently, LBE agreed that the development constituted Permitted Development under Part 13 B (f) of the regulations, as none of the exclusions to development permitted under Part 13, Class B applied. Waste operation environmental permit: The waste operation environmental permit for the CHP plant was varied to include an ‘R3: recycling/reclamation of organic substances (which are not used as solvents)’ activity for upgrading the biogas to biomethane, in a biomethane plant, for injection into the national gas transmission grid. In essence, the change had the effect of extending the CHP’s green permit boundary line. As a result of biogas being converted to biomethane, compliance with the requirements of the end-of-waste quality protocol for biomethane was a prerequisite. It defined the point at which the biogas (which is termed as a waste) ceases to be waste; and the by-product biomethane can be used as a product without the requirement for waste management controls as the point-of-injection into the Cadent gas network. [caption id="" align="alignnone" width="1200"] Propane tanks - Courtesy of SGN Commercial Ltd[/caption] Application of TWUL asset standards and gas industry standards: As the design developed, it was necessary to agree the order of precedence of TWUL corporate asset standards and gas industry standards. It was concluded that the conditioning plant up-front of the biomethane plant is typical of the gas conditioning plant for CHP unit, hence, TWUL adopted corporate asset standards to that area. Once the biogas meets the 15bar high pressure forwarding gas compressors (the ‘compression’ phase described above), then international and gas industry (IGEM) standards were adopted. Renewable energy grants: It was identified that the project qualified for the Renewable Heat Incentive (RHI) accreditation scheme. A 20-year long-term financial support programme for renewable heat that closed on 31 March 2022. Notwithstanding the benefit of the project contributing towards TWUL goal of net zero carbon emissions in 2030, the financial benefits of meeting the RHI timeline were a driver. If the deadline was not met, it was planned on selling the biomethane into the Road Transport Fuel Obligation (RTFO) scheme. A non-guaranteed revenue stream dictated by market forces of supply and demand for the biomethane commodity. Following the closure of the RHI, TWUL is currently seeking an amendment to the Green Gas Support Scheme (GGSS) to allow retro-fits. This would support biomethane gas-to-grid projects like the Deephams STW project being built at other TWUL sites. Future opportunity: To further enhance the benefit of such biomethane plants; investigations are on-going into the practicality of adding a carbon dioxide capture plant into the process. A key driver for its implementation will be whether validation is granted that the carbon dioxide is of food industry quality; plus assessing whether the technology currently available is commercially viable. [caption id="" align="alignnone" width="1200"] Deephams STW Biomethane Plant - Courtesy of SGN Commercial Ltd[/caption]

Conclusion

The Deephams STW biomethane plant gas-to-grid solution is helping Thames Water Utilities Limited achieve its goal of net zero carbon emissions across all operations by 2030. Within the first year of operation, it has produced an average of 550m³/hr of biomethane being injected into the gas grid with 97% availability. This is enough gas derived from the biomethane plant to heat 4,250 homes in the London Borough of Enfield. The potential for roll out of similar projects across TWUL’s estate is large; estimates indicate that there are twenty-four potential STW sites that could install biomethane plants to generate enough gas to heat over 50,000 homes. The Deephams STW biomethane plant project remains a suitable engineering example where technology provides a viable opportunity to produce renewable energy whilst helping to reduce carbon emissions.

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.

Cardiff and Afan WwTWs are in South Wales and are owned and operated by Dŵr Cymru Welsh Water. The Cardiff WwTW serves a population equivalent of 974,869 with a flow to full treatment of 6,060 l/s, and Afan WwTW located on the TATA Steelworks site at Port Talbot, has a flow to full treatment of 2,423 l/s - a combined flow capable of filling an Olympic size swimming pool in just five minutes. At both sites, surplus activated sludge is pumped to a sludge treatment building where it is mixed with polymer, thickened, and then pumped (along with imported sludge) to a Cambi advanced aerobic digestion (AAD) process for further treatment. Methane produced in the Cambi AAD is used in the CHP engines to produce electricity.

Background

The CHP plants at both Cardiff and Afan treatment works were constructed in 2010, however the CHPs and associated ancillary assets at both sites face continued degradation, exposing Operations to volatile energy prices and increased maintenance costs. These assets are not currently performing to their designed capability, meaning the potential energy recovery savings are not being fully realised.

Samples of the biogas entering the CHPs have highlighted that the siloxane composition is at unacceptable levels, causing a risk to the CHP condition. The scope will include for replacing the existing siloxane plant with a new carbon filter which will increase delivered pressure of biogas to the CHPs and boilers to help increase performance.

[caption id="attachment_13304" align="alignnone" width="1200"] (left) Cardiff WwTW and (right) Afan WwTW - Courtesy of Google Earth[/caption]

Project drivers

Morgan Sindall Infrastructure was appointed as principal contractor and principal designer to successfully deliver a scope of work at both sites to meet the following main drivers for the project:

• Increased CHP plant availability.
• Ability to run all CHP engines and boilers while using the gas clean-up equipment.
• Operational health & safety: the ability to work safely on boilers while the associated CHP boiler is running.
• Reduce energy and maintenance costs.
• Operational health & safety at Afan WwTW: relocate the CHP exhaust bursting discs to outside of the operational building.

Being one of the designers working for Morgan Sindall Infrastructure on Welsh Water’s AMP7 framework, and because of their previous experience on similar projects, Arcadis were selected by Morgan Sindall as the design consultants for the Cardiff and Afan WwTWs CHP Optimisation Project.

Due to the commonality between the two sites, both projects were developed using the same principal contractor, design consultants, sub-contractors, and supply chain. This helped realise significant design and programme savings across the two sites.

CHP process description

For both sites, at each CHP engine the hot exhaust gases are directed to a dedicated composite steam boiler to recover heat. Each CHP package includes a biogas booster pump which is used to increase the pressure of the biogas supplied to the CHP engine.

[caption id="attachment_13326" align="alignnone" width="1200"] ADD process diagram - Courtesy of Dŵr Cymru Welsh Water[/caption] The pipework from the biogas booster pump to the container includes connections for a gas analyser and an in-line gas meter, as well as a safety slam-shut valve. The gas analyser monitors the levels of methane, carbon dioxide, oxygen, and hydrogen sulphide in the biogas. The readings are used by the CHP control system to adjust the engine performance, or to trip on high levels. The in-line gas meter monitors the flow rate of biogas to each CHP engine and the gas slam-shut valve is used to isolate the container in the event of a gas leak or fire alarm.

Design solution for both sites

• Removal of siloxane plant and installation of carbon gas cleaning.
• Chiller installation to include:
• Shell and tube heat exchanger.
• Recuperator.
• Lagging to exchanger and recuperator.
• Inlet and outlet temperature probes.
• Condensate pot (drained locally).
• Parker WFB2380CSB3180A-316 biogas cooler/separator.
• Parker ICE183B4003501P15FF0LA000 biogas process chiller.
• Parker HDF220BE biogas float drains.
• Gas booster to ensure boiler and combined heat and power operation, to include:
• Control panel booster and chiller with SCADA access and HMI.
• Gas booster motor 1 and 2 starters.
• VSD control.
• Pressure and temperature protection.
• Pipework and manifolds.
• Interconnecting pipework between the booster set and chiller including valves and purge points.
• Safety measures to enable a CHP to be operated and allow boiler safe isolation.
• Gas flow meter installation (on each digester).
• Afan Works only: Relocation of the CHP exhaust bursting discs to outside the existing boiler house.

[caption id="attachment_13308" align="alignnone" width="1200"] New biogas booster set install at Cardiff WwTW – courtesy of Morgan Sindall[/caption]

Design figures

• Maximum biogas throughput:
  • Cardiff WwTW: 3,000 Nm³/hour.
  • Afan WwTW: 2,500 Nm³/hour.
• Assumed biogas composition:
  • CH₄/CO₂/N₂/O₂): 65/35/0/0 %.
• Assumed biogas methane content range: 60-70%.
• Dehumidified gas temperature from chiller: 5⁰C.
• Combined siloxane level required following carbon filter treatment: 10 ppm.

Supplier selection

Early in the project it was recognised that the management of multiple key suppliers and lead-in times of up to six months were two key risks to a successful delivery. Selecting the same suppliers, contractors and designers across both projects, together with a clear collaborative approach from the start helped mitigate these risks.

1. Endress+Hauser was selected as suppliers of the biogas flow meters due to their proven accuracy at a similar project completed at Five Fords WwTW in North Wales.
2. CC Jensen Ltd was selected as a specialist carbon filter supplier they based on their work on previous CHP installations.
3. Dunphy Combustion Ltd was selected based on their technical capabilities and previous performance at Cog Moors WwTW.
4. Uniflare Group was selected based on programme delivery and the previous work undertaken at Cog Moors WwTW.

[caption id="attachment_13305" align="alignnone" width="1200"] Installation of foam probe and suppression nozzles at Cardiff WwTW - Courtesy of Morgan Sindall[/caption]

Cardiff & Afan CHP Optimisation: Supply chain - key participants

• Principal contractor & designer: Morgan Sindall Infrastructure
• Consulting designer: Arcadis
• Mechanical installation & biogas scrubber supplier: CC Jensen Ltd
• Electrical installer: Protocol Control Systems Ltd
• Biogas flow meter installation: Marches Biogas Ltd
• MCC modifications: GPS Group
• Systems integration: GPS Links
• Biogas boosters & chillers supplier: Uniflare Group
• Steam boiler dampers & ducting supplier: Dunphy Combustion Ltd
• Biogas flow meter & control panel: Endress+Hauser

Project benefits

As part of its response to climate change, and to reduce greenhouse gas emissions, Welsh Water have invested in a renewable energy generation and energy efficiency portfolio, split as follows:

• 45% hydro electric
• 45% Combined heat & power
• 5% wind energy
• 5% solar energy

Source: latest figures taken from dwrcymru.com - June 2024 [caption id="attachment_13309" align="alignnone" width="1200"] Installation of new Endress+Hauser prosonic gas flow meters at Cardiff - Courtesy of Morgan Sindall[/caption]

At Cardiff WwTW, the current average power generation is 20.25 GWh, and at Afan WwTW is 10 GWh. The target to achieve a 3 GWh improvement at each site will generate a combined revenue of £930,000 per annum and contribute significantly to Welsh Water’s renewable energy totals while reducing greenhouse gas emissions. The carbon benefit is assessed to be approximately 1,000 tonnes/year for both sites combined.

Project status

At the time of writing (June 2024), both projects are progressing well, and at Cardiff WwTW the civils works are now complete. The mechanical installation is expected to be complete by the end of June, with handover back to Welsh Water targeted for August 2024.

The Afan project trails Cardiff, providing the opportunity to benefit from any lessons learnt during the Cardiff installation. The work at Afan WwTW is scheduled to be commissioned in August and handed back to the Welsh Water to operate in October 2024.

These projects will meet all the client drivers to improve operational health & safety, reduce energy and maintenance costs, and increase CHP availability and output. The carbon benefits will also contribute to Welsh Water’s overall carbon reduction strategy and journey to net zero.