Assessment of the sustainability of bioenergy production from algal feedstock
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Growing concerns regarding the impact of fossil fuel use upon the environment and the cost of production have led to a growth in the interest of obtaining energy from biomass. 1st and 2nd generation biomass types, however, are often criticised for their high energy requirements and environmental impacts. Algal biomass is considered a 3rd generation biomass which does not require arable land for cultivation, typically has a high productivity and can be converted to a wide variety of energy carriers. Despite research on the concept of producing energy from algal biomass dating back to the 1960s there has been limited commercial development and the environmental advantages are still in doubt. This thesis investigated the potential of algal biomass as a source of bioenergy feedstock by considering the cultivation and processing of localised species of algae and applying life cycle assessment (LCA) methodology to algal biofuel production systems. Experiments were conducted to examine the productivity of a wild algal species in wastewater and the potential recoverable bioenergy yields. The LCA studies drew together data from external studies, commercial databases, industrial reports and experimental work to assess the environmental impacts and the energy balance for each system considered. The thesis investigated the generation of biofuel from both freshwater algal biomass and marine algal biomass. For both cases, the current state of research was examined and the gaps determined. Existing studies suggest the high intensity of microalgal biomass production (fertiliser requirements, high energy harvesting) greatly reduces the overall sustainability. Part of this thesis therefore investigated the possibility of a low input system of microalgal cultivation. A recommended approach was suggested using local species cultivated in wastewater as the nutrient source and a conversion strategy based on the characteristics of the dominant species. The practicality and effectiveness of cultivating and processing locally grown algal biomass under low input conditions was determined by experiments that were conducted in the laboratory. Algal biomass was collected locally and cultivated in the laboratory using agricultural effluent as the nutrient source. The productivity of the algae was monitored alongside the uptake of nutrients. The effluent provided a good media for the cultivation of the wild algae and the nitrogen and phosphorous loading of the effluent was reduced by as much as 98% for NH4+ and 90% for PO4³-. The algal biomass was also tested for its potential as a feedstock for bioethanol production as well as biochar alongside pyrolysis oils and gases. Compared to alternative biomass types tested, the algal biomass appeared to be a good candidate for bioethanol production providing a 38% recovery of bioethanol. The biomass appeared a less favourable substrate for energy recovery from pyrolysis but this process could be considered for carbon biofixation. The sustainability of incorporating microalgal cultivation in wastewater treatment was tested by conducting a life cycle assessment of a large scale system. The life cycle assessment used Haifa wastewater treatment plant in Israel as a case study. The study compared algal cultivation with energy recovery to conventional nutrient removal (A2O process) for enhanced nutrient removal within the wastewater treatment plant. It was found that the use of algal ponds for nutrient removal compared favourably to conventional treatment under specific conditions. These conditions were: the algal biomass is converted to both biodiesel and biogas and the algal biomass is converted to biodiesel, bioethanol and biogas. In these cases the energy balance was greater and the global warming potential and eutrophication potential were less. The conventional nutrient removal was, however, found to be the better method in terms of the acidification potential. Despite being the favourable method of nutrient removal the cultivation and processing of algae relies upon several key assumptions: high year round growth of algae, no contamination and access to a high land area for the cultivation ponds. The sustainability of recovering bioenergy from the cultivation of macroalgae was also tested. A life cycle assessment was conducted investigating the energy return on investment and six environmental impacts for three cultivation methods and three process streams to convert the biomass to bioenergy. Cultivation and processing in Chile was used as a case study due to the depth of knowledge and availability of data. The cultivation scenarios were: bottom cultivation of Gracilaria chilensis, the long line cultivation of Gracilaria chilensis and the long line cultivation of Macrocystis pyrifera. The processing streams were: bioethanol, biogas and both bioethanol and biogas. Most of the data used in the life cycle assessment was obtained from studies conducted in Chile and from communication with local fisherman. It was found that the bottom cultivation of Gracilaria chilensis and conversion to bioethanol and biogas produced the best energy return on investment (2.95) and was most beneficial in terms of the environmental impacts considered. Alternative circumstances were also considered which included new research (untested on a large scale) related to the value used for productivity and conversion of the biomass. This analysis indicated that an EROI of 10.3 could be achieved for the long-line cultivation of Macrocystis pyrifera and conversion to bioethanol and biogas alongside very limited environmental impacts. This result relies, however, upon favourable assumptions that have not yet been proven on a large scale. The work conducted in this thesis highlights the potential of recovering energy from algal biomass. The experimental work and life cycle analysis of freshwater algal cultivation demonstrates the importance of using wastewater treatment as added value to the system. Maximising energy recovery by using a combination of conversion techniques was also shown to be key in providing the most sustainable solution. The sustainability of energy produced from macroalgae was established as being preferable to several conventional energy sources. Innovative methods to improve the system were also shown to greatly enhance the concept.