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HEAT TRANSFER
IMPROVING THE QUALITY OF BIOGAS SUPPLY
A Biogas Dehumidification System from HRS Heat Exchangers is boosting energy efficiency and biogas quality at Melbourne Water
ewage and wastewater treatment is an energy-intensive process, but a recent upgrade project at Melbourne Water’s Eastern Treatment Plant is helping to reduce emissions in accordance with company and state objectives. A key part of the upgrade is a Biogas Dehumidification System (BDS) from HRS Heat Exchangers. As well as prolonging the working life of the site’s electricity generation equipment, the technology is also improving energy efficiency and reducing operating costs. Crucially, the upgrade ensures the site can generate around 30% of its own energy needs from biogas. Located in the Melbourne suburb of Bangholme, some 19 miles south-east of the city centre, Melbourne Water’s Eastern Treatment Plant (ETP) treats almost half of all Melbourne’s sewage, an average of 330 million litres a day. The 1,000 ha site is the second-largest wastewater treatment plant in Australia and the largest activated sludge plant in the Southern Hemisphere and produces Class A recycled water from an advanced tertiary treatment system. The ETP has aways generated energy from sewage gas, and the site houses seven power generators which are capable of running solely on biogas – produced through the anaerobic digestion of sludge during the first and second phases of water treatment. In 2020, Melbourne Water began a project with John Holland-KBR Joint Venture to upgrade the biogas handling system at the Eastern Treatment Plant to make it more efficient, resilient and future proof. Expanding capacity, preventing corrosion and sedimentation, and extending the working life of equipment were key aims, with specific requirements to remove liquid droplets and moisture and to minimise the size and cost of any necessary cooling system.
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An energy-intensive process, sewage transfer and treatment is responsible for
around 85% of Melbourne Water’s total greenhouse gas (GHG) emissions, so contributing to the company’s Net Zero commitment was a main target for the project. “However, with existing assets nearing the end of their service life, ensuring that the new asset can service future production growth and plant upgrades was also essential,” said Nick Fung, Senior Project Manager at Melbourne Water. Increasing reliability for the operations team on site and improving safety across both operation and maintenance were further aims. Biogas contains hydrogen sulphide (H2S) gas, which condensates out to form a highly corrosive liquid. Including a system to remove most of the moisture from the gas was therefore essential to minimise the amount of corrosion and degradation of the power station generators, while also limiting operational downtime and reducing the need to import electricity from the grid. HRS Heat Exchangers were one of five companies invited to tender for the moisture removal equipment in the project. The HRS BDS removes water from biogas, protecting combined heat and power (CHP) engines and generator sets from corrosion and cavitation. It condenses more than 90% of the water present in biogas by reducing the temperature to leave a clean, green gas. The addition of heat regeneration technology means the cold biogas produced can be used to pre-cool the incoming warmer biogas. This reduces the load on the final cooling heat exchanger and saves valuable energy. The BDS supplied as part of the project has a maximum capacity of 4,161m3
/hr (4,710kg/hr),
while the inclusion of an energy recovery section subsequently reduces the eventual chiller load by 30%, improving the overall energy footprint of the whole project. This BDS consists of two heat exchangers, a regeneration unit, and a final cooler, supplied
12 JULY/AUGUST 2024 | PROCESS & CONTROL
on two skids. One skid – which is located in a hazardous area zone – contains the heat exchangers, a condensate knockout pot, a condensate drain line, and all the relevant IECEx-rated instrumentation. The second skid contains a buffer tank, standby and duty glycol pumps, and all the relevant glycol line process control valves and instruments. HRS also supplied an additional chiller and detailed process control descriptions, enabling the joint venture engineers to provide overall control and automation systems for the client. Added design complexity came from the fact that the BDS needed to meet the requirements of two different operational stages, each of which contains four duty requirements. This meant a complex thermal design process was necessary to make sure that the supplied unit could meet all eight of the potential design scenarios.
Ella Taghavi, Project Manager and Technical Lead at HRS Heat Exchangers, explained: “The current first phase uses biogas supplied by the compressors and aftercoolers at an average pressure of 66 kPa. In the second (future) scenario, the gas is supplied following a number of additional treatments at an average pressure of 5 kPa and then supplied to the compressors. We therefore had to deal with two very different inlet conditions. “The addition of heat regeneration added further complexity in terms of design. However, thanks to our cutting-edge technology, not only were capital costs reduced as a smaller chiller could be specified, but regeneration is also helping to lower ongoing operational costs.”
Taghavi continued: “Our experience in project management, construction and logistics enabled us to overcome early design challenges and respond to necessary changes during the design and construction process. As a result, we were able to deliver a high quality product that has performed to specification since its installation.” Since it was commissioned in June 2022, the HRS BDS has been performing well and meeting its performance targets, with no breakdowns or interventions.
The upgrade project has improved the reliability and quality of the biogas supply to the power station at ETP, providing more than 36,000 MWh (approximately 130TJ) of thermal energy for process heating. This means a reduced reliance on natural gas, making the operation more self-sufficient and reducing overall emissions.
HRS Heat Exchangers
www.hrs-heatexchangers.com
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