Sustainable biodiesel biorefineries for the green succinic acid production

Access to files

•   FULL-TEXT.PDF (pdf)

Abstract

There is a huge global challenge to establish alternative forms of energy in order to cope with the increasing worldwide energy demand, currently based on finite fossil fuel reserves. In the transportation sector, renewable liquid fuels, such as bio-ethanol and biodiesel which are made from biomass and are substitutes for the petroleum-derived gasoline and diesel, have received increasing interest. In spite of their recent development, the biofuel industries cannot compete with conventional liquid fuels because of their higher costs. Decisive changes are required to improve their economic sustainability, such as the establishment of novel processes that utilize their by-products for the production of value-added chemicals. In this study, the bioconversion of glycerol, which is the main by-product of the biodiesel industry, to succinic acid by using the bacterium Actinobacillus succinogenes has been investigated both experimentally and computationally. Initially, the cells were adapted to accept a glycerol rich environment by performing a series of experiments. Cells from the best experiment from each run were used as inocula for the next experiment. Batch fermentations were then performed in small scale anaerobic reactors (SARs) and in lab-scale bench top reactors (B-TRs) by using the new ‘adapted’ strain. The maximum succinic acid yield, productivity and final concentration obtained from this bioprocess were found to be 1.29 g/g, 0.27 g/L/h and 29.3 g/L, respectively. Moreover, cells have also grown successfully in both synthetic and biodiesel-derived crude glycerol, indicating that it is not necessary to remove the impurities that biodiesel-derived glycerol contains. Subsequently, an unstructured model that accounts for substrate and product inhibition was developed in order to predict the behaviour of experiments starting from different initial conditions. Model predictions were found to be in good agreement with experimental data obtained for both systems (SARs and B-TRs). Batch and fed-batch systems were optimized using the developed model to obtain high succinic acid productivity. Optimization results showed that productivity increased by 31% for batch and 79% for fed-batch systems. The corresponding optimal values were computed to be equal to 0.356 g/L/h for batch and 0.488 g/L/h for fed-batch systems. A semi-mechanistic model for the fungal fermentation on solid state rapeseed meal (i.e. the other main by-product of the biodiesel industry) was also constructed for small scale tray bioreactors. This fermentation targets to increase the nutrient factor of the rapeseed meal by decomposing its macromolecules to simple compounds which can then be used as a generic medium. The developed model effectively predicts the fungal growth, the temperature fluctuations and the moisture content inside the bed and the produced extracellular enzymes that break the complex compounds of rapeseed meal (i.e. proteins) to free amino acids. The economic sustainability of biodiesel production was investigated by the construction of a plant model of an integrated biodiesel biorefinery for the production of fuels (biodiesel) and chemicals (succinic acid) in Aspen Plus®. For a biodiesel plant with capacity of 7.8 ktons per year, it was found that the plant’s profitability can be increased by 60% (considering a 20 years plant life and an interest rate of 7%) if a fermentation and recovery process for producing succinic acid is added. The integrated biorefinery scheme demonstrated the highest profits (€ 9.95 M.) when compared with other scenarios which either purified or disposed of the glycerol. These results illustrate the critical role of glycerol when it is utilized as a key renewable building block for the production of commodity chemicals. It is clear, based on this work, that future studies targeting the sustainable development of biodiesel biorefineries should focus their investigation on novel bio-processes, like the succinic acid fermentation, supplementing the production of fuels with the co-production of platform chemicals.

Keyword(s)

Biofuels; Integrated Biorefineries; Succinic acid; Sustainable Industries

Bibliographic metadata

Institutional metadata

Record metadata