Developing a biorefinery from spent coffee grounds using subcritical water and hydrothermal carbonisation

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.

Determination of Relationship between Higher Heating Value and Atomic Ratio of Hydrogen to Carbon in Spent Coffee Grounds by Hydrothermal Carbonization

“…The biochar of HHV of different biomasses is shown in Table 5. Previous HHV studies were compared with the HHV of this study to assess the potential for biochar through HTC, and SCGs were found to have a higher HHV than the other biomass samples, indicating their potential as a valuable fuel source through HTC conversion [32][33][34]. The producing biochar using SCGs has been carried out in a variety of ways, and as a result, the possibility of biochar is fully demonstrated not only in this research's results but also in previous research.…”

Soil Amendments and Biostimulants from the Hydrothermal Processing of Spent Coffee Grounds

“…where: ω(AA) i is the amino acid individual mass fraction (mg/ g); M r (AA) i is the amino acid molecular mass (g/mol); N AAi is the mass of N in the amino acid (mg/g); f n is the nitrogen-to-protein conversion factor, which for SCG is 7.79 [19].…”

“…with 1260 autosampler and variable wavelength detector (VWD). Run methods were as previously published [19].…”

“…This is presumably a result of the disruptive action of NaOH on the morphology of SCG via degradation of polymeric sugars or removal of N containing components (including proteins, peptides and free amino acids) and fatty acids via saponification. A more porous, open structure is observed for PE SCG, aiding the proliferation of biomass fragmentation into comparatively more reactive char products [19,20].…”

“…In this investigation, a previously reported SCG-based biorefinery was modified and two major products were utilised to yield SCG-based substrates for horticultural amendments (Fig. 1) [19]. Specifically, SCG hydrochar products derived from untreated and alkaline pre-treated SCG feedstocks were assessed for their influence on the germination of Barbarea verna L. (land cress) and rosette growth of Arabidopsis thaliana.…”