Abstract
In this paper, we propose a realistic model for gas distribution of an advanced municipal wastewater treatment works and through minimisation of the total cost of gas distribution we perform retrospective optimisation (RO) using historical plant data. This site is the first in the UK with a mixed operational strategy for biomethane produced on site: to burn in combined heat and power (CHP) engines to create electricity, burn in steam boilers for onsite steam use or inject the biomethane into the National Grid. In addition, natural gas can be imported to make up shortfalls in biomethane if required. Implemented using a novel mixed integer linear programming (MILP) approach, to ensure a fast and robust solution, our results indicate the plant operated optimally within accepted tolerance 98% of the time. However, improving plant robustness (such as reducing unexpected breakdown incidents) could yield a significant increase in gas revenue of 7.8%.
Highlights
In 2015, Northumbrian Water Limited (NWL) was the only wastewater company in the UK to use all sludge after wastewater treatment to produce renewable electricity (Northumbrian Water )
To perform retrospective optimisation (RO) for this site, we develop a model of the process and use mixed integer linear programming (MILP) to determine optimal historic performance that can be compared to actual historic operations
For a daily biogas production of 40,000 Nm3, the site should be operated according to the strategy in Figure 3 for optimal cost efficiency: to inject all biomethane into the National Grid, use natural gas in the combined heat and power (CHP) engines at 100% load to generate electricity on site and use natural gas in the steam boilers to create steam as required
Summary
In 2015, Northumbrian Water Limited (NWL) was the only wastewater company in the UK to use all sludge after wastewater treatment to produce renewable electricity (Northumbrian Water ). It has added a gas-togrid injection plant at its sewage treatment works (STW) in Howdon, Tyneside, whereby biomethane/biogas produced from anaerobic digesters is purified prior to injection into the National Grid (Martins & Wilson ). The site typically digests up to 40,000 tonnes of sewage sludge (dry solids) annually producing renewable biogas. The STW can be considered to have two sections: effluent water treatment, and sludge treatment. Sludge treatment occurs at the advanced anaerobic digestion (AAD) plant. The relatively new processing techniques (CAMBI thermal hydrolysis) involved in the AAD plant results in significant operational challenges to maximise the economic performance
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