AbstractSpent coffee grounds (SCG) are a lignocellulosic waste product that are amenable for valorisation into a portfolio of products. As a wet biomass, subcritical water is a suitable, energy efficient, medium for extraction of high value bioactives from SCG. The subcritical conditions of hydrothermal carbonisation (HTC) can also transform SCG into porous, carbonaceous solids for energy, catalysis, remediation and sequestration purposes. Though the potential of SCG within the circular economy is well established, limitations to the techno-commercial feasibility of products derived from SCG remain, such as the presence of allelochemicals and high nitrogen content in fuel products. This work sought to address these barriers: first within the biorefinery concept and through subsequent development of the hydrothermal carbonisation platform.
An initial exploration of the literature highlighted only a few examples of SCG based biorefineries where three product classes dominated: high value aqueous extracts, biofuels and biochars. Economic and environmental implications of the types of processes and/or their output, which invariably influence the commercial viability of operations, was largely neglected. Therefore, in this thesis, promising ‘green’ technologies to generate value from SCG were used, resulting in the novel integration of subcritical water extraction (SWE) and HTC platforms for express isolation of bioactives and production of hydrochars suitable for combustion as solid fuels.
However, critical evaluation of the hydrothermal biorefinery highlighted that the nitrogen content of hydrochar was prohibitively high for compliance with fuel regulations, whereas the inclusion of SWE generated low yields of bioactives and hydrochars, as well as reducing fuel quality. To address this issue, the SWE stage was replaced with an alkaline pre-treatment, whereby hydrolysis simultaneously conferred lower nitrogen and superior fuel performance in the hydrochars, whilst the protein isolate product, retained in higher quantities than the bioactive extract, gave a potentially higher value product stream.
Influence of process parameter, pre- and post- treatments on solid fuel composition and performance was explored in a subsequent investigation of the HTC platform- whose biorefinery settings were previously optimised for production of a hydrochar with both maximal calorific value (HHV) and yield (so-called ‘moderate’ regime). Mild (T = 217 °C, t = 1 h), moderate (T= 223 °C, t = 2.75 h) and severe (T = 260 °C, t = 4.5 h) regimes were used to generate hydrochars from alkaline pre-treated SCG and raw SCG in maximum yield (mild) or calorific value (severe) or their composite (moderate regime). Unextracted hydrochars from the severe regime displayed the highest degree of carbonisation, fuel ratio and HHV for both feedstock types. A downstream polar extraction of the hydrochars, to remove the volatile secondary char phase, gave primary chars with upgraded thermal stability as indicated by higher ignition temperatures. For primary chars derived from the alkaline pre-treated SCG feedstock, greater combustion efficiencies were obtained through lower burnout temperatures and higher combustion reactivities. Thus, thermal behaviour and composition are tuneable aspects of hydrochars derived from SCG, with alkaline pre-treatment improving the commercial viability through near removal of nitrogen and upgrading fuel combustion performance.
The liquid phase of the HTC platform was addressed in process water recirculation experiments, with the aim to reduce process effluence whilst maintaining or even improving fuel quality. HHV, energy and hydrochar yield were slightly increased for both feedstock types, with the superior primary char fuel product maximally enriched after 2 cycles for the raw feedstock and 1 cycle for the alkaline pre-treated SCG. High total organic content of the process water, which increased with cycles, necessitates treatment prior to eventual environmental discharge, or valorisation into fertilizer products for example. Techno-economic analysis of the energy and time expended to heat the water vs costs associated with water supply and environmental release is needed to further inform the value of process water recirculation on an industrial scale.
Finally, an alternative application of SCG hydrochars as soil amendments was investigated in Arabidopsis Thaliana (Arabidopsis) growth trials. As observed for the fuels, removal of the volatile secondary char phase enhanced performance, here due to a reduction in phytotoxicity. Highly carbonised primary chars from the severe regime exceptionally supported plant growth under soil stress conditions at higher loadings (100 t ha-1), promoting a rarely reported increase in biomass yield. Inclusion of a bioprocessing platform in the SCG based biorefinery to produce enzymatic protein hydrolysates as potential biostimulants, demonstrated their efficacy in yeast growth, Arabidopsis germination and auxin-activity trials. Further process development and advanced growth trials are expected to establish the merit of SCG-based horticultural amendments.
SCG’s composite physicochemical and chemical properties requires a cascade approach to fully utilise the feedstock. This work has exemplified the amenability of SCG towards a versatile array of products within the circular economy.
|Date of Award
|23 Mar 2022
|Chris Chuck (Supervisor) & Ana Lanham (Supervisor)
- spent coffee grounds
- biomass processing
- hydrothermal carbonisation