A systems approach to thermochemical conversion and carbon sequestration from microalgae
thesisposted on 29.03.2022, 01:26 by Scott Grierson
supply and demand, coupled with mounting evidence regarding anthropogenic climate change and the need to find sustainable alternatives to consumption of fossil oil, place biomass resources in a position of unique prominence given the fundamental carbon capture mechanism inherent in plant photosynthesis. In navigating a transition to a sustainable energy future, biomass also offers the prospect of leveraging existing infrastructure, knowledge and investments while potentially reducing the greenhouse gas intensity of many of our activities, through refining of transport biofuels and renewable electricity production. However, the availability of terrestrial biomass resources other than agricultural or forestry wastes and weedy species for these applications is highly contentious, especially where human population is booming and food production is likely to assume increasing amounts of productive land. Microalgae are an aquatic biomass alternative that can be cultivated in a range of water sources and climatic conditions, promising high productivity per unit area, without the need to occupy productive land. Furthermore, the pyrolysis processing of microalgae presents an opportunity to combine a highly productive source of biomass with the means to produce renewable bio-oil and biogas, in addition to biochar, that can be used to sequester carbon in soil. Taken together, this offers the potential to deploy a solution that may be able to net reduce atmospheric carbon dioxide levels, while producing considerable economic and societal value and avoiding food commodity conflicts. Six species of microalgae (Tetraselmis chui, Chlorella like, Chlorella vulgaris, Chaetocerous muelleri, Dunaliella tertiolecta and Synechoccocus) were initially selected for study, representing a broad cross-section of physical characteristics and known behaviour under cultivation. The objective of this preliminary investigation was to ascertain differences in thermal conversion behaviour between these microalgae species under slow pyrolysis conditions. The samples were first analysed with a Computer Aided Thermal Analysis (CATA) technique at a standard heating rate of 10°C/min. For all species, the energy required to achieve thermal conversion was found to be approximately 1MJ/kg. Gas chromatography was then applied to measure the evolution of biogas compounds with temperature. The heat of combustion of the biogas compounds was estimated to vary significantly between species, ranging from 1.2 to 4.8 MJ/kg. Pyrolysis oil product yields were also estimated at 500oC. The oils produced at this temperature were collected and their molecular weight distribution assessed by Matrix Assisted Laser Desorption/Ionisation (MALDI). The species were found to produce up to 43% by volume of bio-oils. In all samples the char fraction remained above one third of total sample weight. The oil and char derived from the slow pyrolysis of the unicellular marine green alga Tetraselmis chui were then further analysed in detail, using a variety of techniques. The pyrolytic oil fraction exhibited a wide variety of fatty acids, alkanes, alkenes, amides, aldehydes, terpenes, pyrrolidinines, phytol and phenols, with a high heating value (HHV) of 28 MJ/kg. The biochar produced has a HHV of 14.5MJ/kg and reveals a number of properties that are potentially valuable from an agronomic point of view, including high cation exchange capacity (CEC), large concentration of N, and a low C:N ratio. The quantity of C in T. chui biochar that can be expected to stabilise in soil amounts to approximately 9%/wt of the original feedstock, leading to a potential net reduction in atmospheric CO2. Examining ways to innovate the microalgae cultivation and processing value chain includes a focus on the most efficient and economical means to extract the liquid oil fraction from the microalgae species. Additional work compares the use of organic solvent, supercritical carbon dioxide (SC-CO2) and pyrolysis to assess their relative capacity to derive oil from the marine microalgae, Tetraselmis chui (T. chui). SC-CO2 technique was shown to be least effective in natural oil extraction from T. chui due to the polarity of compounds but nevertheless demonstrates the feasibility of this concept. The results revealed that pure solvent extraction produces the most complete extraction of natural oil at just under 15% by weight. Subsequent pyrolysis of post-solvent extraction residue and examination of by-products suggest that extraction of natural lipids prior to thermal processing increases the total quantity of bio-oil yield production by more than 11%. Life cycle assessment (LCA) of a microalgae biomass cultivation, bio-oil extraction and pyrolysis processing regime is a useful means to gauge the likely environmental impact of this prospective new development on an industrial scale. Coupled to thermal conversion via slow pyrolysis, the prospect of biologically ‘sequestering’ carbon derived from microalgae biomass as biochar, added to soil, is considered. However, an intensive closed culturing photobioreactor system coupled to a pyrolysis process incurs a net increase in global warming impact and life cycle impact, notwithstanding biochar application to soil. Results indicate that up to 50% of environmental impact in certain categories stems from the upstream influence of fertiliser production. Energy used in flue gas delivery and pumping during cultivation is also considerable, suggesting that current practice in closed cultivation systems does not yet adequately trade-off biomass productivity against operating intensity. Drying of the harvested microalgae biomass for pyrolysis processing is potentially a major hurdle in terms of process viability also. Overall, utilisation of nutrients derived from waste streams, integrating renewable energy and capture of process heat for more efficient drying are essential levers for reducing the environmental impact of this proposition before it can be declared of net benefit to society.