Abstract
The use of microalgae biomass for the production of biodiesel has been increasingly investigated mainly because arable land is not required for cultivating this biofuel feedstock. However the large scale production of microalgae biodiesel under present day scenarios is currently considered to be infeasible. This is mainly due to the higher energy and economic costs for microalgae biodiesel production compared with the use of common terrestrial biomass oils (i.e. palm, rapeseeds and soybean oil).
This thesis investigates the production of biodiesel from microalgae biomass using the in-situ transesterification process and the recovery of additional energy from the post transesterified microalgae residues, as a tool for improving the energetics of biodiesel production from this feedstock. The thesis aims to provide preliminary experimental information on biodiesel and subsequently methane production from microalgae which was not previously available in the literature. In addition, a simple renewability assessment of the various in-situ and conventional transesterification methods examined in this study was carried out.
The laboratory experiments examined the influence of important reaction parameters (reaction time, process temperature, reactant concentrations, agitation and the biomass moisture content) on biodiesel production from microalgae using the in-situ transesterification method. The results showed a 5% increase in the biodiesel yields with the use of the in-situ method compared with the conventional transesterification process under similar reaction conditions. Furthermore, results obtained from the in-situ transesterification experiments indicated that continuous reactor stirring, increasing the reacting alcohol to oil molar ratios (≥ 315:1) and increasing the process temperature (≥60°C) led to improved fatty acid methyl ester (FAME) conversions. Total inhibition of in-situ microalgae biodiesel conversion was observed with biomass water content greater than 115% w/w (on the basis of oil weight). However, the optimal excess reacting alcohol requirement of the in-situ process was identified as a factor which could potentially limit any energy gains obtainable with this method, due to the energy demands to facilitate the recovery of the excess alcohol quantities i.e. via distillation methods.
To reduce the process alcohol requirement, modifications to the original in-situ transesterification scheme (via the use of low frequency ultrasound and the integration of co-solvents) were applied. Using similar reaction conditions, the use of ultrasound agitation for the in-situ process was shown to improve the biodiesel yields with reduced molar alcohol to oil ratios (105:1) and reaction times.
Regarding energy recovery from the microalgae residues, the use of the anaerobic digestion process for methane production from the residues post transesterification was examined. The results obtained from the batch anaerobic digestion tests demonstrate that the type of lipid extraction solvent utilised in the conventional transesterification process could inhibit subsequent methane production. A recoverable energy of 8.7-10.5 MJ/kg of dry microalgae biomass residue was obtained after the application of both the conventional and in-situ transesterification processes. Co-digesting the microalgae residues with glycerol led to a 4-7% increase in methane production.
With the use of semi-continuously fed anaerobic reactors, the influence of reaction conditions on the specific methane yield of the microalgae residues was also investigated. These included varying the substrate loading concentrations, co-digestion of the microalgae residues with glycerol, hydraulic retention times and temperatures. It was found that the hydraulic retention period was the most important variable affecting methane production from the residues, with longer periods (> 5 days) corresponding to higher energy recovery. The methane yield was also improved by a reduction in the substrate loading rates and an increase in the proportion of the glycerol fraction co-digested with the microalgae residues.
The raw material and process energy requirements of the up-scaled process were obtained for the different transesterification processes using a commercial chemical engineering software (ASPEN plus®), and a renewability assessment of the various schemes was carried out. The biomass cultivation and biodiesel production process renewability was assessed by comparing the minimum work required to restore the non-renewable resources degraded in the considered process with the useful work available from the main process products. The maximum work obtained from the process products is larger than the restoration work the process is considered renewable. In a present day scenario (with the use of fossil fuel sources for the production of the process raw materials, such as for methanol and sulphuric acid production, and electricity), all the transesterification processes were shown to be non-renewable. The influence of the choice of the electricity generation scheme, raw material source and the type of heating fuels (including heating and drying technology) on the process renewability was also examined The process renewability of the in-situ transesterification of microalgae lipids to biodiesel was found to improve with the use of renewable electricity, reacting alcohols from biomass fermentation and heat pump technology to facilitate the biomass drying and process heating.
Modifying the in-situ transesterification process i.e. with the use of ultrasound agitation and co-solvents was seen to improve the renewability of biodiesel production compared to the mechanical stirred process with the reacting alcohol alone. The use of the ultrasound agitated in-situ process using diethyl ether was seen to be the most renewable process of all the considered transesterification processes. The positive renewability indicators obtained in this study show promise since further process modifications could be used to optimise biodiesel production using microalgae.
The biomass moisture content was shown to negatively influence the alkyl ester production process with the in-situ method. The use of more effective drying methods i.e. heat pump technologies, was shown to have the potential of helping to achieve better process renewabilities. With the in-situ transesterification reacting alcohol requirement being a major process parameter, research into reactor designs which could facilitate the use of lesser alcohol quantities could potentially increase the renewability of biodiesel production via this method. Investigations into optimising the lipid content and productivities of the microalgae biomass could also improve the use of the in-situ method for microalgae biodiesel production. The improved process renewability would be obtained due to the higher biodiesel outputs (and product exergies) achievable with similar work inputs into the process compared with using microalgae with lesser lipid contents.
This work presented in this thesis could provide a basis for further research in the areas of biodiesel production from microalgae using the in-situ process and methane production from the microalgae residues, post transesterification.