Optimisation and characterisation of hydrochar production from spent coffee grounds by hydrothermal carbonisation

Current ways of managing spent coffee grounds (SCGs) are uneconomical and have significant environmental impacts. Alternative approaches, such as hydrothermal carbonisation (HTC), which can utilise their rich organic matter for energy recovery purposes are essential. Here we present, a response surface methodology (RSM) for understanding the combined interactive effects due to the main HTC operating conditions, i.e. reaction temperature and residence time, as well as optimising them to produce hydrochar from SCGs of maximal yield and calorific value. The fuel properties and combustion behaviours of hydrochar were further evaluated to assess its suitability to replace coals for energy applications. Depending on the operating conditions, the atomic carbon content and calorific values of the hydrochar were significantly improved: by 11.2–30.7% and 15.8–44.7% respectively. The highest hydrochar calorific value recorded, 33.5 MJ kg−1, resembled that of anthracite and dry steam coal, generally used in the UK. At optimal conditions of ∼216 °C and 1hr - guided by the RSM - a maximal hydrochar yield of ∼64% and a calorific value of 31.6 MJ kg−1 are feasible. Using this as a benchmark, the 500,000 tonnes of SCGs generated annually in the UK has the potential of replacing 4.4% of the coal used for electricity generation in the country.