Evaluation of consolidated bioprocessing as a strategy for production of fuels and chemicals from lignocellulose

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There are a number of reasons why it is imperative to look at alternatives to fossil fuels as sources of energy and chemicals. Of immediate concern is the major contribution that the burning of fossil fuels is making to greenhouse gas accumulation. While power stations might address this through carbon capture, this is not feasible for distributed systems such as transport and chemicals. Therefore, there is a strong drive to find renewable sources of liquid fuels and chemicals. Currently, this is being addressed using glucose obtained from maize or wheat starch, which is having a negative impact on the price of starch for food. We urgently need to find ways to use the more abundant carbohydrates found in woody material and grasses (lignocellulose), either purpose grown or as wastes. However, this is currently uneconomic due mainly to the cost of releasing the useful carbohydrate from woods and grasses (pre-treatment). This project will address the economics of pre-treatment by exploring the benefits of an approach called 'Consolidated Bioprocessing' (CBP). Current methods rely on acids or alkalis to break down either the lignin and/or some of the long chains (polymers) of carbohydrates found in woody material, followed by treatment with enzyme cocktails, generally referred to as cellulases, but typically containing a complex mixture of enzyme activities, to produce monomeric carbohydrates. These cellulases are typically bought from one of the two major enzyme producers and add a significant cost and greenhouse gas contribution to the process. The concept of CBP envisages that the micro-organism that produces the desired end-product also produces some/all of the enzymes necessary to break down the carbohydrate polymers, so reducing these additional costs. In order to be able to investigate the benefits and disadvantages of CBP within the timescale of the programme we intend to work with a group of bacteria known as Geobacillus spp which grow best in the 50-70oC temperature range and already have the ability to use some of the intermediates produced from polymer degradation. In this way we should only have to engineer in a limited array of novel activities, and we already have the genetic tools and organisms containing the relevant additional enzymatic activities that we require. The project will focus on using miscanthus, a high-yielding grass under consideration in the USA and Europe as a purpose-grown lignocellulosic feedstock. This will be pre-treated by two established methods that leave the polymeric carbohydrates largely intact but disrupt the lignin. Having constructed suitable bacterial strains for CBP the intention is to generate information that allows us to judge the practically and economics of the process when operated at a large scale. This will be done largely by generating models, based on information gathered from small scale operations, and applying appropriate scaling-up criteria. Factors such as the consequence of including significant amounts of solids in the fermentation process and the 'cost' to the organism of producing extra enzymes will be examined. Because CBP is still largely a concept, there are a number of areas of process development which may impact overall practicality and economics which will need to be addressed during the project. By doing this we intend to produce the first genuine assessment of the practicalities of CBP and guidelines on areas that may need further improvement.