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Techno-economic analysis of integrating hydrothermal carbonisation (HTC) in the downstream of the anaerobic digestion process- A Stormossen biogas plant case study
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Researchers conducted a techno-economic analysis of integrating hydrothermal carbonisation (HTC) downstream of anaerobic digestion at the Stormossen biogas plant as a strategy for managing sewage sludge — which contains microplastics, heavy metals, pharmaceuticals, and pathogens — more sustainably. The analysis evaluated the economic feasibility and process integration of HTC alongside existing anaerobic digestion infrastructure as European sludge volumes continue to grow.
Sewage sludge is a byproduct of biological wastewater treatment and volumes produced keep increasing due to the global population growth, industrialization and urbanization. It is estimated that the European Union countries produce about 10 million tonnes of sewage sludge per year on average, and over 1.3 billion tonnes is produced annually globally and is expected to grow to 2.2 billion tonnes by 2025 globally. The sewage sludge produced has a high moisture content of between 97 and 98% and contains nutrients and impurities such as, nitrogen, phosphorus and potassium, organic matter, heavy metals, microplastics, pharmaceuticals, organic contaminants and pathogens. Whilst biogas production using anaerobic digestion (AD) is the best available technology (BAT) for sewage sludge handling, the disposal of the byproduct sewage sludge digestate still poses a challenge. The objective of this study is to assess whether it is technically and economically motivating to integrate the hydrothermal carbonisation (HTC) process in the biogas production plant by looking at the downstream of the biogas production process. The HTC process is an imitation of the natural formation of coal, by reacting high moisture biomass in subcritical water. This phenomenon happens at temperature ranges of 180-250 ˚C for 0.5-72 hours to produce a carbon-rich hydrochar and process water as a byproduct. The hydrochar produced has many applications such as use as solid fuel, catalyst material, fetiliser and process water as a liquid fertiliser or used in the anaerobic digestion (AD) process. Mathematical modelling in Microsoft Excel is used to create a model for the HTC process flows using data from an existing biogas plant in Vaasa, Finland to determine the mass and energy balances. The cost analysis is based on this model, with literature values, pilot plants and assumptions are used as supportive data. Two locations in the downstream of the AD process are selected, giving two types of sewage sludge digestate as feedstock, 5% ds and 10% ds respectively and a total of six scenarios, three for each feedstock type. Both cases analysed showed potential for reduced operational costs compared to the current digestate handling costs depending on scenario. However the turnover from the sale of hydrochar is much lower compared to the sale of the compost as currently done. The price for the process water as a liquid fertiliser is inconclusive as there was no clear data for such. The choice of process flow would heavily depend on what is aimed to be achieved with the HTC plant. Is it to produce hydrochar and its byproducts for other purposes or a cost-effective measure for sewage sludge digestate handling and biogas production improvement? One challenge faced in this study was determing the energy balance as literature values found were given as specific energy demand for the whole HTC system and made it challenging to analyse the system in parts for the six scenarios. The focus in the HTC studies should shift more towards application-specific process parameters and feedstocks for optimal benefits, as these factors easily impact the characteristics of the HTC products and process flow designs.
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