Abstract
Spent coffee grounds (SCG) are the organic residue obtained from the coffee brewing process, and are produced in industrial, commercial and residential settings. Approximately 9 million tonnes of SCG is now produced globally, yet is still largely disposed of in landfill or sent to low value food waste processing, such as Anaerobic digestion. However, SCG contain a huge abundance of potentially valuable biomolecules, including fatty acids, proteins and polysaccharides that could be further valorised to produce fuels and chemicals.To fully exploit the value from the biomass then an integrated biorefinery can be established. This type of biorefinery includes multiple platforms that produce an array of products, maximising the potential value from a feedstock, while reducing the waste produced. This is normally accomplished by fractionating the biomass or reusing the waste streams in subsequent conversion processes to yield increased high-value production in addition to the main product obtained in the biorefinery. As such, integrated biorefineries can normally handle a large variation in the feedstock entering the system.
In this thesis, SCG were evaluated as a potential biorefinery feedstock towards the production of biofuels and higher-value chemicals. Initially, SCG were considered as the only feedstock in a biorefinery configuration aimed at the production of 5-hydroxymethylfurfural (HMF) as the main product. The feedstock was firstly processed in an organosolv fractionation to separate the biomass into an aqueous fraction, a hydrophobic bio-oil and a solid material. It was found that the organic and aqueous fractions were rich in lipids and depolymerised hemicellulose, respectively. These fractions were characterised and presented the potential to be further used in some biorefinery processes. The solid fraction obtained from the organosolv fractionation was rich in cellulose. This fraction was further processed in a combination of enzymatic hydrolysis and enzymatic isomerisation to yield fructose, which was dehydrated to yield HMF.
Following on the success of demonstrating that SCG can effectively be used as a feedstock in a HMF biorefinery, SCG were tested as a potential blending agent with a microalgae strain of Scenedesmus acutus. The blends of these two biomasses were tested in a biorefinery designed towards the production of bioethanol, lipids, a biocrude oil and biochar. The configuration of this biorefinery was initiated with an acid pretreatment to depolymerise the biomass macromolecules into fermentable sugars. The produced sugars were then fermented into bioethanol using a strain of Saccharomyces cerevisiae. This fermented broth was then submitted to a lipid extraction followed by an extraction of the produced bioethanol, the presence of which aided in the lipid extraction process. The solid extracted stillage obtained was finally submitted to a hydrothermal liquefaction to produce biocrude and biochar, The use of blends of SCG with microalgae demonstrated higher production of carbohydrates and slightly lower lipids extracted than the microalgae when processed alone in the same biorefinery configuration. This study demonstrated that blends of SCG with microalgae can effectively tackle the seasonality problem associated with microalgae-based biorefineries.
Microalgae production has numerous advantages, including the rapid production that can be carried out in non-arable land. However, this requires a relatively large area to install the microalgae production facility, the technology and investment to do this and a higher value portfolio of products to make it economically viable. An alternative, that is gathering a large amount of research interest recently, is the production of fuels from macroalgae (seaweed) that can take place both in freshwater and saltwater ecosystems. This flexibility presents an opportunity towards this biomass type as there are large areas of saltwater that can be explored in the macroalgae production. However, just like in microalgae, macroalgae also present a seasonality problem. To this end, two strains of macroalgae, Ulva lactuca and Chorda filum, were blended with SCG and tested in an integrated biorefinery approach designed for the production of HMF. The feedstock was initially submitted to an acid dehydration yielding HMF, which was subsequently extracted. The HMF-free stream was then submitted to a second acid dehydration to produce more HMF, which was also extracted in a downstream process. The resulting solid stream obtained was rich in lipids, lignin, protein and unreacted carbohydrates. This stream was finally processed in a hydrothermal liquefaction to produce biocrude and biochar. The blends of SCG and macroalgae proved to effectively mitigate the availability of macroalgae during low production seasons. In addition the combination of a lipid rich SCG source, coupled with the high C6 content of macroalgae presented improved conditions towards the production of HMF, with no isomerisation needed to produce the HMF and higher yields of HMF in the combined system for when SCG and microalgae when processed separately.
Ultimately, SCG is a highly suitable feedstock for the production of biofuels and chemicals in an integrated biorefinery concept. The relative lack of supply of SCG, albeit all year-round production, can be mitigated against by blending with alternative feedstocks to produce an array of suitable components including HMF.
| Date of Award | 17 Feb 2021 |
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| Original language | English |
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| Supervisor | Chris Chuck (Supervisor) & Alfred Hill (Supervisor) |