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

有许多原因表明，必须将化石燃料的替代品视为能源和化学品的来源。当前令人关切的是，燃烧化石燃料对温室气体积累起了重大作用。虽然发电站可以通过碳捕获来解决这个问题，但这对于运输和化学品等分布式系统来说是不可行的。因此，存在寻找液体燃料和化学品的可再生来源的强烈驱动力。目前，正在使用从玉米或小麦淀粉中获得的葡萄糖来解决这个问题，这对食品淀粉的价格产生了负面影响。我们迫切需要找到方法来利用木质材料和草（木质纤维素）中更丰富的碳水化合物，无论是专门种植还是作为废物。然而，这目前是不经济的，主要是由于从树木和草中释放有用的碳水化合物（预处理）的成本。该项目将通过探索一种名为“综合生物处理”（CBP）的方法的好处来解决预处理的经济学问题。目前的方法依赖于酸或碱来分解木质材料中发现的木质素和/或碳水化合物的一些长链（聚合物），然后用酶混合物（通常称为纤维素酶，但通常含有酶活性的复杂混合物）处理以产生单体碳水化合物。这些纤维素酶通常从两个主要的酶生产商之一购买，并为该过程增加了显著的成本和温室气体贡献。CBP的概念设想产生所需最终产品的微生物也产生分解碳水化合物聚合物所需的一些/所有酶，从而降低这些额外成本。为了能够在该计划的时间范围内研究CBP的优点和缺点，我们打算与一组称为Geobacillus spp的细菌合作，这些细菌在50- 70 ℃的温度范围内生长最好，并且已经有能力使用聚合物降解产生的一些中间体。通过这种方式，我们应该只需要在有限的一系列新活动中进行工程设计，我们已经拥有了遗传工具和含有我们所需的相关额外酶活性的生物体。该项目将侧重于使用芒草，这是一种高产草，在美国和欧洲正在考虑作为一种专用的木质纤维素原料。这将通过两种已建立的方法进行预处理，使聚合碳水化合物基本上保持完整，但破坏木质素。在构建了用于CBP的合适的细菌菌株之后，目的是产生使我们能够判断大规模操作时该过程的实际和经济性的信息。这将主要通过根据从小规模行动中收集的信息生成模型并适用适当的扩大标准来实现。诸如在发酵过程中包括大量固体的后果和产生额外酶的生物体的“成本”等因素将被检查。由于CBP在很大程度上仍然是一个概念，因此有许多工艺开发领域可能会影响项目期间需要解决的整体实用性和经济性。通过这样做，我们打算对CBP的实用性和可能需要进一步改进的领域的指导方针进行第一次真正的评估。

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.