No abstract

Spent coffee grounds (SCGs) have been extensively investigated as a feedstock to produce fuels, specialty chemicals and materials. Whilst a few reports have used cascade processes to generate several products from SCG, this work takes the novel approach of using integrated subcritical water extraction (SWE) and hydrothermal carbonisation (HTC) to derive three products: a bioactive extract, a protein isolate (SCG PI) and solid fuel. SWE and HTC processes were optimized producing an antioxidant rich extract, with the chlorogenic acid (CGA) content and antioxidant activity determined. The protein content was quantified via total amino acid analysis, giving the first SCG specific elemental nitrogen-to-protein conversion factor of 7.90. HTC was then performed on the residual solids from SWE, the protein extraction and the raw feedstock. This biorefinery approach gave higher quality products than previously reported in single product systems. For example, pretreatment reduced nitrogen in the hydrochar (N = 0.23% wt, HHV = 33.30 MJ/kg) relative to the control (3.03% wt, HHV = 31.31 MJ/kg). Limiting biorefinery processes to the pretreatment and HTC preferentially increased protein content (33.0% vs 16.9% wt) and yield (53.0% vs 23.9%) of the protein isolate, rendering a hydrochar with a higher yield and HHV compared with hydrochar derived following upstream SWE process (33.30 vs 26.92 MJ/kg, 16.3% vs 14.7%, respectively). This work goes towards the complete utilisation of SCGs within a biorefinery, highlighting the potential of subcritical water processing to produce commercially viable products across the value chain.

As an increasingly abundant lignocellulosic biomass with high moisture content, spent coffee grounds (SCG) are an ideal feedstock for hydrothermal carbonization (HTC). The few examples that have converted SCG into a solid fuel hydrochar have yet to address the char's prohibitively high nitrogen content (∼3 wt %)−a barrier to commercialization due to NOx emission regulations. In this work, an alkaline pretreatment is presented that reduces the N content prior to carbonization under regimes optimized for maximum calorific value (HHV), energy yield, and a conflation of both fuel properties. Characterization of hydrochars and secondary chars, the extractable volatile phase, revealed the highest calorific value for the secondary chars (max HHV = 40.69 MJ/kg) and a minimum N content of 0.1 wt % in hydrochars derived from the alkali pretreated feedstock. Char thermal stability and oxidative reactivity were also determined, with pretreated primary chars exhibiting superior combustion reactivities, ignition, and burnout temperatures. Management of HTC process water is both an environmental and operational challenge due to the presence of phytotoxic organic components and the energy expenditure associated with heating large volumes of water. To this end, the process water was recirculated for up to five cycles resulting in an increase in HHV, energy, and solid yield as well as compositional change of the char products.

Purpose Use of spent coffee grounds (SCG) in horticulture has deleterious effects on plant health and yields, suggesting that processing of SCG is necessary prior to field application. To this end, two products of an SCG based biorefinery were investigated: primary chars from the hydrothermal carbonisation (HTC) of SCG and enzymatic protein hydrolysates. Methods Primary chars were produced under various HTC regimes from raw SCG, and alkaline pre-treated SCG. Primary chars were evaluated in germination toxicity tests and under soil stress conditions using Arabidopsis thaliana (Arabidopsis). Proteolytic enzymes were screened in production of SCG protein hydrolysates; biostimulant activity in the growth of Saccharomyces Cerevisiae, germination of Arabidopsis thaliana under normal and cold conditions and auxin-like activity was assessed. Results Toxicity assays of primary chars with land cress (Barbarea verna L.) showed a maximum 35% increase in root length, relative to the control. In Arabidopsis growth trials, outstanding performances were recorded at 100 t/ha for primary chars produced from alkaline pre-treated SCG: rosette diameter and dry weight increased by 531 and 976%, respectively, relative to the control. SCG protein hydrolysate from Bacillus lichenformis proteolysis (glycine present at 47% of total) gave 140% increase in Arabidopsis seeds with expanded cotyledons, relative to the control. Auxin-like activity was also measured in the extracts. Conclusion Cascade process design was used to valorise a major waste stream. Removal of phytotoxic components from SCG hydrochars enhanced plant growth, while biostimulant activity of SCG protein hydrolysates was observed. Graphical Abstract