What’s up with C5 and C6 sugars? Given that a significant portion of the several billion dollars spent on renewable fuels spending involves these molecules, it is not surprising that progress is fast and furious.
The goal of the biomass fuels and chemicals industry is to produce billions of gallons of fuel while avoiding impacting the world’s food supply. Converting cellulose to sugar for fermentation or conversion to other products will play a critical role in this effort. Fermentation of C6 sugars to produce ethanol is currently the major source of renewable liquid fuels. C5 and C6 sugars can also serve as precursors to platform chemicals that can be converted to a wide variety of products. Several of these sugar to chemical processes are being commercialized. Continued advances in sugar production and conversion are key factors in the future of biomass-based fuels and chemicals.
Evaluating progress requires determining the potential impact of nehe w technology on the volume of sugar produced per ton, the volatility in pricing, and the value of the overall production process. Feedstock price and availability are the major drivers of the cost of production.
Economics
Cellulosic sugars compete against those extracted from corn, sugar beets, and other sources. The cost of the cellulosic sugar produced should be equivalent, or lower than, sugar from traditional sources. As of yet, the cellulosic sugars are not cost-competitive, averaging 30-50% above ethanol produced from extracted sugars. However, these costs are highly variable. The production cost of cellulosic ethanol in the six commercially operating cellulosic ethanol plants varies by 40%. As the technology improves it is widely predicted that the goal of matching extractive sugar costs will be achieved in the near future.
The significant contributors to the cost of cellulosic sugar production are feedstock, pretreatment, and capital costs with each contributing about 20-40% of the costs. Enzyme costs have been decreasing rapidly. For example, the enzyme cost per pound of cellulosic ethanol has come down 72% between 2008 and 2012 and have continued to fall. Improvements in activity and process technology which allow more effective use of the enzymes.
Capital costs are the other major portion of the cost of cellulosic ethanol production. Again these costs are highly variable depending on plant size, location, and technology. So far the volume of cellulosic ethanol on the market has been well below the nameplate capacity of the new commercial plants started in the past several years. While this will likely change in the future, the productivity of the plants needs monitoring to access the real cost of production.
Feedstocks
Feedstocks are estimated to comprise about 45% of the cost of cellulosic ethanol. Cellulose feedstocks fluctuate widely in price depending on the type and location. The data suggests that progress in harvesting practices, smart plant sizing, and cogeneration opportunities can greatly impact the value of cellulosic sugar production. Developing alternative feedstocks with lower costs and establishing an infrastructure and stable markets are still a priority.
The US DOE has set a target feedstock price of $60 per ton which using industry rule of thumb would translate to $0.06 per pound of sugar. This price does not include transportation or pretreatment. The current cost of woodchips, which are the only commercially marketed biomass, is about $80 per ton at harvest and increases to over $120 per ton when processing and delivery costs are included. This would correspond to sugar production cost of $0.12 per ton based on raw materials alone. The market price of industrial sugars has varied from $0.11 to $0.19 per pound in the last year.
In past months there have been several publications featuring methods for producing sugars from seaweed and algae. These are other potential sources of sugars which have not be explored that might lower eventually lower prices.
Role of regulation, subsidies and societal factors
The current low prices of oil and the trend towards lower traditional sugar prices has negatively impacted biofuel development. In addition, there is sufficient elasticity in the supply of traditional sugar to allow new supplies to open up if prices drop. As a result, government subsidies and mandates will continue to play a key role in the future in both traditional and cellulosic ethanol. These subsidies are a reflection of the societal demand for reducing the use of fossil carbon sources rather than economics.
Current subsidies provide up to $2.64 price benefit for cellulosic ethanol. This subsidy more than makes up for increased production costs and is a major driver for short term investment. The initial market for cellulosic ethanol is likely to be in areas with mandated renewable contents like California and the EU. However, the fate of these subsidies and mandates is uncertain, and all technologies have a goal of being price equivalent with traditional routes.
The situation for chemical products derived from sugars in the long run is less clear. A number of countries have renewable material content mandates. However, a large portion of the impetus is a combination of societal demand for the renewables coupled with economic advantages in delinking material costs from fossil fuel prices in the long term.
Beyond Ethanol
The current economics of cellulosic ethanol make it difficult to make a profit without mandates or significant subsidies. However, it is possible to improve on the profitability of C5 and C6 sugars production by using them as a starting material to produce higher value products. The value of the chemicals far exceeds the feedstock costs. One strategy is to use these products to help justify and finance the initial implementation of new cellulosic sugar technology. A limitation is that volumes of these chemicals are often fairly low.
There are numerous potential products that can be derived from C5 and C6 sugars. Many of these products are at various stages of commercial development. Evaluating the the techno economics of each is a continuing process.
Initially, these new processes will use extracted sugars since there is little price advantage and significant risk in using cellulosic sugars. Many of the major petrochemical and specialty chemical manufacturers have very active programs in this area.
One of the materials closest to commercialization is butanol. Butanol can be used as a diesel fuel additive which is of higher value than ethanol as a gasoline additive. Recently, alcohol derived fuels have been certified for use in jet fuels. The price differential is great enough to justify conversion of existing ethanol production to butanol. The butanol fermentation is more sensitive than ethanol production, and cellulosic sugars are not yet widely accepted as feed. However, a number of commercial projects are moving forward as the renewable diesel mandates begin to increase in the next few years.
Using C5 and C6 sugars as the starting material for the production of succinic and adipic acid is also well on the way to wide scale commercial applications. Methods for converting ethanol to olefins and diesel are also being explored. Recently commercial production of a selective catalyst has been reported.
Pretreatment
Pretreatment of the biomass is critical to improve the yield and quality of the C5 & C6 sugars. There continue to be numerous reports of new concepts of feed pretreatments via hydrothermal, steam, ammonia, acids and bases. The amount of open and prior art in this area suppresses the patent activity. The key will be to evaluate these methods to find the ones that increase the yield and quality of the sugars with the lowest costs.
One focus of research is contacting the biomass with catalysts and enzymes. New enzymes that degrade cell walls have been shown to greatly enhance the rate of cellulose conversion. A recent patent application describes incorporating the genes for expressing these enzymes into plant DNA.
The use of organic solvents to swell biomass and extract selective components continues to be explored. The solvents can open up the biomass structure allow more accessibility and selectively extract lignin components and forming an enriched sugar stream for fermentation. Ionic liquids are also being employed to separate ligand derived compounds since their charged structures are particularly attractive to phenolic groups. Residual ionic liquids have been found to be toxic to some organism. However, a recent disclosure patent disclosure reported new strains that can tolerate them.
Product Quality and Fermentation Inhibitors
Cellulosic sugars often contain impurities and coproducts that interfere the fermentation process. These impurities prevent cellulosic sugars from being drop-in replacements for current commercial sugars. This is a particular concern when using some of the new high performing microbes for sugar production and the use of the sugars for fermentation to specialty products. One of the major trends in research is confirm that the new supplies of cellulosic sugars can be used in these processes.
There continue to be a number of new methods proposed for removing or limiting these impurities. Purification of the hydrolase broth containing the sugar with membranes and ion exchange resin was recently disclosed. A new class of microorganism that selectively metabolizes the lignin derived aromatics in the presence of sugar was reported. However, these purification steps add to the cost of using cellulosic sugars.
New Technology
Advanced biofuels need to fully utilize biomass resources including cellulose, hemicellulose and lignin. Methods for hydrolyzing and valorizing the hemicellulose and lignin utilization continue to be investigated. These will need to be commercialized to improve the process economics.
A survey of the patent applications published in the last few months found disclosures of over 30 new inventions from 20 different companies or institutions. Keeping track of this fast-moving technology is a daunting process. These applications provide a window on early-stage technologies and which areas are viewed as critical by the research community. What follows is a summary of the areas where advances have been reported. However, there is no guarantee that the patents will issue or that the technologies will be commercialized.
Process Improvement
There continue to be suggestions for new process approaches that have the potential to lower costs and improve throughput. A recent disclosure discussed partial hydrothermal conversion of the cellulosic material to a liquid extract and a fibrous portion. The fiber fraction is separated and is subjected ted to enzymatic liquefaction and saccharification. Improved methods for removing solids and separating alcohol from the hydrolysis broth which lowers the amount of liquid used in the process has also been suggested.
Another disclosure described methods for adapting existing pulp and paper production equipment to allow continuous enzymatic hydrolysis of cellulose. Repurposing pulp processing plants is a particularly attractive route to lower the capital required for producing biofuel.
New Microbes and Enzymes
There is significant progress in identifying more robust enzyme systems capable of continuous operation at higher temperatures, pH, resistance to inhibition. The ability to operate at higher temperatures is of particular interest because it increases enzyme activity and allows better integration with pretreatments that often involve higher temperatures. There are a number of commercial and academic groups applying genetic engineering methods to modify known enzymes and/or to insert nucleotides into plant cells to improve processability. Several new variants of cellobiohydrolases have been reported in the last three months, and it is likely there will be continued improvements in the biological catalysts.
One of the most important trend is discovery of new biological systems for processing the hemicellulose portion of the biomass. Microbes that can that use C-5 xylose sugars as a fermentation feed continue to be found. Processing both the cellulose and hemicellulos in a single reactor can have an impact of up to 40% on the cost of the ethanol. Currently attempts to promote the xylase activity have not proved economical because of interference with the C6 sugars pathways. Several new microbes have been found to be active and these are the basis of continued development work. Recently, specific polypeptides were identified as bacterial xylose isomerases that can complete a xylose utilization pathway so that yeast can use xylose in fermentation.
Chemical Catalysis
A large portion of the commercial and academic groups in the biofuel area have focused on biocatalytic approaches which fit with traditional fermentation to produce ethanol. However, the original methods for industrial sugar productions from ethanol involve acid hydrolysis. This process is more complicated for lignocellulosic materials since the acid also breaks down the lignin making separation more difficult. Also, disposal and/or recycling the waste waters is an issue. However, there are still many proposed catalytic materials including both homogeneous and heterogeneous catalysts.
The C5 and C6 sugars can be converted to fuels by a number of chemical catalyzed reactions like hydrogenation, dehydration and reforming to yield furfurals, furans, levulunic acid and polyols. Ethanol itself can be converted to olefins and hydrocarbons, and a number of recent patent disclosure and commercial announcements have recently appeared proposing these approaches.
Downstream carbon-carbon bond formation reactions are needed to produce fuel products. The most common C–C coupling reactions to be used in a biorefinery are Fischer-Tropsch synthesis, aldol condensation, ketonization and oligomerization. The net results can be drop-in nonoxygenated blending components indistinguishable from current fuel components.
Final Thoughts
Prior estimates that cellulosic ethanol would be cost competitive by this year were clearly optimistic but it is clear that the costs will continue to be lowered over time. Given that renewable carbon sources will definitely be a part of our future it is critical to understand the details of the sugar to fuels and chemicals. Following the worldwide effort to develop processes to produce and use C5 & C6 sugars is a formidable task involving a wide range of expertise. It is often difficult to sort through competing claims and get to the gist of what differentiates the different technologies.
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