Biodiesel is a renewable transport fuel produced largely from terrestrial oil seed crops which, if used as an alternative to fossil diesel, can reduce our dependency on fossil fuels. Brazil is one of the largest biodiesel producers in the world, yet cultivation of the predominant feedstock, soybean, puts pressure on highly biodiverse ecosystems as well as threatening land ownership and access. In order to improve the environmental, social and economic sustainability of biodiesel production in Brazil, new feedstocks are being investigated.
The successes and weaknesses of the Brazilian “Programme for Biodiesel Production and Use” were analysed and the opportunity to introduce a new, potentially more sustainable feedstock was identified. Heterotrophic microalgae were investigated as an alternative feedstock, due to suggested benefits over other feedstocks such as high growth rates and lipid yields, potentially reducing production costs and energy inputs. To investigate the feasibility of supplying nutrients from different waste streams, the microalga Chlorella vulgaris was cultivated in a synthetic wastewater medium with addition of an organic carbon feedstock, either pure glucose, molasses from the sugar industry or crude glycerol from the biodiesel industry. The harvested biomass was converted to biodiesel by transesterification of hexane extracted lipids or by in situ transesterification to investigate the difference in yields. The properties of the biodiesel were then analysed to assess its quality. The life cycle energy use and greenhouse gas emissions were calculated and compared with autotrophic microalgae, followed by a whole systems analysis to identify risks and challenges to integrating heterotrophic microalgae into the biodiesel industry in Brazil. The analysis found that the biodiesel programme in Brazil has made compromises to allow family farmers to contribute to the feedstock matrix, and the programme would face sustainability challenges if it were scaled up. Therefore a sustainable alternative feedstock would be required to provide for an increase in feedstock demand.
Heterotrophic microalgae were selected as they may be capable of introducing additional social benefits, particularly associated with improving sanitation and waste management. Heterotrophic cultivation growth trials demonstrated that biomass densities of up to 3 g l-1 d-1, with a lipid content of 48% could be achieved where crude glycerol was the organic carbon source. The fatty acid methyl ester composition of the transesterified lipids and other fuel characteristics were determined using correlations based on the FAME composition, including a new technique for predicting cetane number. The results suggest that in situ transesterification can lead to higher biodiesel yields than extraction and transesterification, and that the algal biodiesel quality from either technique was comparable with soybean biodiesel. The rate and quality of the oil produced is significant as there is potential to integrate this oil into the existing blend as an economical product.
The energy ratio calculated for heterotrophic microalgae showed a potentially positive balance could be achieved when waste nutrients were utilised. This was compared to autotrophic microalgae feedstock, and found advantages for the heterotrophic systems due to lower energy and water requirements during cultivation. The opportunities and risks of integrating microalgae into the existing system for biodiesel production in Brazil, identified by the whole system analysis, determined that the existing infrastructure could be utilised, but highlighted the role of policy decisions and investor confidence in stimulating further development and potential deployment of microalgal feedstocks for biodiesel. However, the barriers to future development are significant and the gap between research and commercialisation must be bridged by working at the interface of different disciplines, in order to produce a truly sustainable biodiesel feedstock.
