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Team:Edinburgh/Biorefinery
From 2011.igem.org
Biorefinery
A feasibility study of the engineering design and economics of a biorefinery plant concerned with the synergistic breakdown of cellulose.
Contents |
What is a biorefinery?
A biorefinery is a processing plant that converts biological raw materials into a variety of products that can be used as industrial intermediates or sold directly to consumers. Biorefineries offer flexibility in both the types of products that can be made and the raw materials that can be used; flexibility that conventional chemical/food plants aren’t able to offer. However, for this technology to be successful it has to be integrated within a bio-based economy where its raw materials can be sourced both sustainably and cheaply.
Using synthetic biology, by creating BioBricks and incorporating them into genetically designed microorganisms, it should be possible to make products from biological feedstocks that range from food additives to chemicals.
This document develops a design for a biorefinery based on our iGEM project of synergistic cellulose breakdown. We examine one engineering design for this biorefinery with a detailed process flow diagram. An analysis of the economics of this plant is also made, investigating its required capital and projected profits. This detailed economic and technical investigation is part of our bigger feasibility study, and we see it as related in important ways to questions about what role synthetic biology might have in the future, and whether biorefineries are a realistic short-term application area for this technology.
Raw Materials
Building a biorefinery, and the investment required to do so, can only be justified if the procurement of its raw materials is done both economically and sustainably. There are very important questions about where these biological feedstocks should come from. Nicholas Peyret, from Scottish Enterprise, a public body which encourages economic development, asks,
“Can we take advantage of Scottish feedstock?” (Interview with Nicholas Peyret, 25/08/2011)
He says that more research is needed to analyse the potential of different Scottish feedstocks. But in principle it makes more sense than shipping biomass across the world for example, growing sugarcane in Brazil to export for industrial use in the United States or Europe. Eric Hoffman, Biotechnology Policy Officer for Friends of Earth asks,
“What happens if we start switching more and more land which is currently used in agriculture production and start shifting that for feedstock for synthetic bugs?” (Interview with Eric Hoffman, 11/08/2011)
His concerns shouldn’t be ignored in a bid to make synthetic biology commercially successful in the short term, but should be addressed with a careful rationale and with humility.
The raw material to be used in our proposed biorefinery is open for debate as the biorefinery is not constrained in the exact material but only in its source. Lignocellulose is the primary structural material in plants and is made up of hemicellulose, cellulose and lignin. The biorefinery will separate lignocellulose into its three constituents, and from cellulose then into glucose and the desired product.
Traditionally those sourcing lignocellulose biomass would look to softwoods, hardwoods and grasses. Biomass such as switchgrass, corn stover and sugar cane are popular options. But what if the biomass sourced is already waste and can be recycled? We suggest that old newspapers, furniture, straw and other lignocellulose- based waste could be possible input feedstocks. Commercially, this method could be significantly cheaper, but a challenge nonetheless to the contractors to find reliable sources at the necessary volumes. The biorefinery proposed by the Edinburgh iGEM team aims to use up to 700 kg/h of biomass, to make it commercially worthwhile. Current biorefineries operate at around 500 kg/h, with company Elsam Essential energy (European commission, 2004) looking do develop pilot lignocellulose plants of up to 1000 kg/h of biomass. 700 kg/h was assessed to be both realistic in the short to medium term and ensure good returns to any potential investor.
Science and Engineering
Pre-treatment
One of the biggest challenges for any biorefinery is cost-effective pre-treatment of raw materials. Vom Stein et al (2011) have proposed a ‘one step fractionation of lignocellulose components by the selective organic acid and catalysed depolymerisation of hemicellulose in a biphasic system’. Our proposed biorefinery process incorporates this method using oxalic acid at 140 degrees Celsius at a pressure of 20 bar, which is moderate compared to other methods. Steam is fed into reactor 1 (R-1) along with oxalic acid, controlled by a series of proportional, integral, derivative (PID) control mechanisms used to regulate flow, level and temperature. Here the depolymerisation of hemicellulose to soluble sugar in aqueous phase occurs, while the cellulose-pulp (which is more crystalline) remains solid and unreachable to the acid catalysis. Crystallisation is used to separate the oxalic acid from the aqueous stream and then after further processing, it is recycled back into reactor 1.
In a second organic phase involving bio-based compound 2-methyltetrahydrofuran (2-MTHF), ‘lignin is directly separated from the pulp and the soluble carbohydrates by in situ extraction’ (Vom Stein et al, 2011). 2-MTHF allows for relatively easy separation and recycling using distillation due to its low boiling point, 80 degrees Celsius.
Synthetic biology is currently not incorporated within the pre-treatment phase of the proposed biorefinery. Assuming the efficiency of the Vom Stein et al (2011) process is high and cost effective, is there any reason for it to be? One of the lessons that can be learnt from previous research and even previous iGEM teams, is synthetic biology always the best option for the problem, and does a problem even exist in the first place? With pre-treatment finding the optimum chemical solution will be far easier than developing one that involves microorganisms, as survival of the microorganisms comes into question with high temperatures and pressures needed to convert biomass into cellulose. Having said that, if synthetic biology can result in a method, possibly with the use of enzymes that can reduce the operating temperature and pressure then it has the potential to significantly change the design of future biorefineries and even future chemical plants. This means biorefineries built today should be flexible enough to incorporate tomorrow’s synthetic biological advances.
Lignin treatment
Lignin makes up approximately 30% of lignocellulose, and the potential to convert it into chemical feedstock is significant. The proposed biorefinery utilises this potential and aims to convert lignin into phenols using hydrous pyrolysis. Hydrous pyrolysis refers to the thermal decomposition of organic compounds when heated in the presence of water. However, pure lignin has a low melting point (ranging from 80-200 degrees Celsius) and the process is very energy-intensive. Furthermore, lignin is a complex bio-polymer, which makes it difficult to convert it into valuable monomeric chemicals. Aston inventions an enterprise subsidiary of Aston University cite the process’s potential if a sufficiently large-scale and efficient method for lignin conversion is developed but warn of lignin’s, ‘resistance to chemical, thermal and biological degradation’ (Aston University, 2010) .
However what if a technology can be developed that operates at ambient temperatures and bypasses the need for energy intensive work? Lead researcher Dr Michael E. Scharf from the University of Florida believes creating synthetic recombitant enzymes produced out of genes from host termite tissues. ‘These enzymes include two cellulases that act on cellulose and hemicellulose, and laccase that acts on lignin’ (Materials world, 2011). He recognises that efficient and inexpensive cellulases are already used in industry, but inexpensive and efficient processes for the treatment of lignin and hemicellulose are yet not available. Therefore, ‘In this respect termite digestomics has already offered significant insights.’ (Scharf and Tartar, 2008). There is potential genetic engineering and synthetic biology can impact every stage of the biorefinery process more than what we propose with synergistic (enzymes working together) cellulose degradation. This should be treated as both exciting for the field in years to come and with great caution. Those designing the biorefineries of the future will have to think harder on how ‘synthetic biological based biorefineries’ will impact society. Biorefineries in that every stage has an element influenced by the field of synthetic biology.
