Conventional, first generation biofuels (ethanol and biodiesel) are produced from renewable sources but these biofuels show limitations relating to sustainability, high production costs, performance properties and incompatibility with existing infrastructures. Thus, currently some advanced biofuels (for example biobutanol) based on more sustainable feedstocks and more efficient technologies are being developed in order to overcome these limitations and meet sustainability and fuel quality standards.
The objective of ButaNexT project was to validate an integrated process to achieve a more efficient production of biobutanol using three representative non-food/feed and low cost lignocellulosic feedstocks (wheat straw, miscanthus and treated organic fraction of municipal solid waste -MSW-) and taking into account technical, economic, social and environmental issues.
The idea underpinning the project is based on a holistic approach and comprises the study, validation and optimisation of each independent stage in the conversion process (i.e: pretreatment, enzymatic hydrolysis, fermentation and product recovery) at lab and bench scale using the three selected feedstocks, where, once the individual stages have been optimised and the conversion processes assessed as a whole, the most interesting feedstock in terms of conversion rates has been selected and used to carry out the integration of the individual process stages at pilot scale. Moreover, the development and optimisation of biocatalysts (enzymes and microorganisms) required for the biochemical conversion steps is included as well.
The main lessons learnt during the ButaNexT’s pilot plant integration and scale-up assays have been the following ones:
• Wheat straw and miscanthus have been milled up to 0.5 mm using the micronising prototype designed and constructed by Técnicas Reunidas. In fact, improved productivities were obtained during the commissioning and production step increasing the production flow up to 7 kg/h for wheat straw.
• More than 500 kg of wheat straw slurry have been produced by CENER using an optimized thermal acid pretreatment that consists in subjecting the micronized material to 175ºC with 4% w/w H2SO4 for just 5 minutes generating a pretreteated material more prone to enzymatic hydrolysis of the cellulose content
• More than 500 kg of sugar rich hydrolysate (110 g/l sugars) have been produced by CENER through an enzymatic hydrolysis process using enzymes supplied by MetGen. The Enzyme cocktail used is an improved version that has increased overall enzymatic yields over 80-85%.
• Fermentation process using GBL’s microorganism has been up scaled from 10 litres to 100 litres by CENER with the continuous support of Green Biologics and VITO. Fermentations batches have run successfully even more when the fermentation process is coupled with the pervaporation unit leading to a proper separation of the solvents while being produced.
• According to the results obtained in the fermentation, during the fed-batch mode, glucose and xylose conversion were near completion. To this end, the fermentor was integrated with a POMS pervaporation membrane module for this first time on pilot scale.
• The mass balances of the integrated pilot plant have been updated according to the real data obtained during the assays.
However, it has not been possible to complete a pilot run using wheat straw hydrolysate in the time frames of the project due to some technical problems. Nevertheless, and although the final results have not been as complete as expected, we think the encountered hurdles are critical points that need to be overcome, but are not of a fundamental nature related to the developed technology. Furthermore, the gathered data can be used to properly reevaluate and optimise the pilot-scale design of fermentation integrated with pervaporation.
Finally, it should be noted that while these results represent a useful pilot plant level demonstration, the mass balance analysis shows that significant levels of improvement would be needed to match conversion yields seen in work packages focussed on sub processes or seen elsewhere.
