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3 Fuels and Chemicals from Biomass via Biological Routes “We have to drive a substantial amount of cost out of both operating and capital costs in order to make this work.” Chris Somerville “It should be hard to displace a mature trillion dollar industry.” Chris Somerville INTRODUCTION A closer look at these costs reveals some surprises, said Somerville. Lignin drying boilers and wastewater treatment Using biological systems to convert biomass into fuels account for 55 percent of the capital costs. A 50-million- and chemicals is already feasible, but the costs of doing so gallon facility built using the NREL process could produce make the resulting products uncompetitive economically up to one billion gallons a year of wastewater that contains at present with those produced from petroleum. According as much as 2 percent solids, necessitating the construction to Chris Somerville, professor of alternative energy and of a large wastewater treatment facility. Boiler costs are so director of the Energy Biosciences Institute at the University high because it must handle solids, and constructing a olids s of California in Berkeley, currently, biological conversion boiler for a 50-million-gallon facility makes little sense processes are done in batch mode, which has a substan- economically. In fact, it might be more efficient to eliminate tial impact on capital costs and throughput. Successfully the boiler, pelletize the lignin and other solids, and sell them developing continuous flow processes could have a marked to a coal power plant. Figure 3-1 shows the contribution to positive effect on the economic competitiveness of biomass- the overall cost by process area and capital, operations, and derived fuels and chemicals. Creating such processes, fixed costs. however, will require significant advances in pretreatment Turning to process costs, feedstock costs are reason- and separations technologies, and realizing those advances able, about $0.74 per gallon, but pretreatment, hydrolysis, requires recruiting chemists and chemical engineers to attack and ellulase enzyme total about $0.83 per gallon, with c these problems. total operating costs of about $2.15 per gallon of ethanol. S omerville said that given that ethanol has two-thirds the BIOLOGICAL ROUTES TO FUELS AND CHEMICALS energy density of gasoline, this scheme does not work eco- nomically. A substantial amount of cost must be driven out The biological conversion of cellulosic biomass into of both operating and capital in order to make this work, and fuels and chemicals should be straightforward, said the first step that is needed to do that, he said, is to realize Chris Somerville, but as companies have begun building that all of the existing processes are fundamentally batch c ommercial-scale bioconversion facilities it has become processes. Though he admitted that this is a radical view, he clear that putting theory into practice is more challenging believes that ethanol production facilities need to follow in than expected. One challenge is dealing with the complexity the footsteps of petrochemical plants and turn to continuous of the overall process of converting lignocellulosic biomass processes, which the chemical industry has shown is the only into ethanol. Using the National Renewable Energy Labora- way to operate economically at commercial scale. tory (NREL) Aspen model for bioconversion as an example, he noted that there are 21 unit processes that require engi- neering solutions and equipment. As a result, the capital Turning Batch Processes into a Continuous Process recovery costs for a bioconversion plant are substantial, Batch processes, he explained, are inherently inefficient totaling about $1 per gallon of ethanol. because they never operate at an optimal place in terms of sugar concentration, lignin inhibition, or microorganism 17
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18 OPPORTUNITIES AND OBSTACLES IN LARGE-SCALE BIOMASS UTILIZATION DW1102A Capital Recovery Charge Raw Materials & Waste Process Electricity Grid Electricity Total Plant Electricity Fixed Costs Feedstock + Handling: 74.1¢ Pretreatment/CondiƟoning: 28.8¢ EnzymaƟc Hydrolysis and FermentaƟon: 20.2¢ Cellulase Enzyme: 33.8¢ DisƟllaƟon and Solids Recovery: 12.1¢ Wastewater Treatment: 33.5¢ Storage: 2.3¢ Boiler/Turbogenerator: (Net) 4.3¢ UƟliƟes: 6.3¢ -$0.40 -$0.20 $0.00 $0.20 $0.40 $0.60 $0.80 FIGURE 3-1 Contribution to minimum selling price of corn-derived ethanol from each process area. SOURCE: Humbird et al. (2011). NREL/TP-5100-47764 (Somerville presentation). performance. Batch processes produce dilute fuel that needs block today is implementing lignin removal at the beginning to be concentrated, catalysts and microorganisms are basi- of the process in a way that minimizes polysaccharide loss cally burned after each batch is processed, and wastewater and recycles the lignin solved at very high efficiency. generation is substantial. In addition, the entire process is In reviewing some of the approaches being explored today, shut down after each batch is completed, during which time Somerville said that each has limitations. Acid pretreatment capital equipment sits unused. is inexpensive, but it removes valuable hemicelluloses as well In the ideal process scheme, sugars are loaded into the as lignin and it also produces some downstream inhibition. fermentation tanks at an optimal rate and fuel is removed Various ionic liquids will dissolve cellulose and also separate continuously. One scheme for reaching this optimum is to use polysaccharide-lignin mixtures, but ionic solvents are cur- a plugged reactor concept based on the continuous removal rently too expensive and would require recycling efficiencies of fuel (see Figure 3-2). This concept relies on liquid/liquid of 99.999 percent to work economically. A more promising extraction or an ethanol-selective membrane to achieve approach, one that needs the attention of the chemical sci- fuel separation under continuous operating conditions, and ences, is to develop catalysts that will partially depolymerize depending on the concentration of the sugar input, there will lignin. He noted that there have been some successes reported be little or no waste water to handle. Developing liquid/liquid with model systems, but that this is still an area of research extraction processes and new ethanol-selective membranes that is underexplored. Studies using supercritical water also are areas in which the chemical sciences can make substan- look promising, though the engineering challenges of work- tial contributions. ing at the required high pressures are substantial. In a hypothetical continuous process that Somerville Studies on continuous pretreatment technologies have discussed, biomass would be ground and fed into a lignin examined weak acid/high temperature and strong acid/low removal process that feeds polysaccharides into a poly temperature combinations, as well as the use of aqueous saccharide depolymerization process using enzymes or bases, with or without ammonia and with or without added chemical catalysts that can be recycled. He made note of oxidants. So far, the strong acid/low temperature approach ongoing research that has produced heterogeneous platinum appears to be the most promising, and it produces a very on carbon catalysts with extremely high activities. A con- pure sugar solution after a solid/liquid separation step. Both centrated sugar solution would then be fed in a fermentation hydrochloric acid and the extraction solvent are recycled effi- reactor, with fuel separation and volume adjustment being ciently. Somerville said a major issue is building durable and performed continuously. And while there are challenges safe process equipment that works with strong, concentrated remaining to optimize this process, the only real stumbling acids such as 40 percent hydrochloric acid.
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FUELS AND CHEMICALS FROM BIOMASS VIA BIOLOGICAL ROUTES 19 membrane FIGURE 3-2 Plugged reactor concept for continuous fuel production. SOURCE: Image taken from Somerville presentation. In concluding his talk, Somerville noted two other chal- chemists to change their way of thinking to appreciate reac- lenges that require the attention of chemists and chemical tions that produce mixtures was a real challenge because that engineers. The first is producing optimized depolymerization Figure 3-2 against the paradigm of modern chemical science. idea goes catalysts, whether they are enzymes or chemical catalysts, bitmapped uneditable in very large quantities at an affordable price. The second BREAKOUT DISCUSSION is to develop large-scale anaerobic processes that produce gasoline and diesel fuel from sugars. This breakout session was led by Leonard Katz, asso- ciate professor at the University of California, Berkeley, and a member of the scientific advisory board of Lygos. Discussion The discussion about biological technologies for biomass In response to a question about how much progress conversion focused on whether the biological conversion of was being made on developing a continuous fermentation biomass into chemical and fuel should be conducted in an process, Somerville said that Yong-Su Jin at the University integrated biorefinery or whether it should be broken into of Illinois would be publishing the results of a study dem- components. The argument was made that preparing biomass onstrating efficient continuous fermentation of C12 sugars for processing should be one component, that deconstruction rather than C6 sugars. Tom Richard asked Somerville to to produce sugars would be a second component, and that discuss some of his work on developing new enzymes of conversion of sugar into fuel or chemicals would be a third pretreatment, and Somerville noted that his group has identi- process. Developing these three processes together into an fied enzymes from cow rumen that operate efficiently at high integrated biorefinery was considered by many breakout temperature, but that the major limitation in developing those group members to be too risky at this stage of technology enzymes for industrial use today is the inability to engineer and capacity development. However, the discussion also their favorite production hosts to produce them in the neces- raised the possibility that it might be economically and sary quantities. He remarked that this is a fundamental piece technologically attractive to combine biomass preprocessing of science that needs the attention of the research community. and deconstruction into an integrated process or to combine Somerville then expanded on the need to develop ways of deconstruction and fermentation in an integrated process. converting sugars into fuels. In his opinion, the most promis- Many breakout group members said that determining the ing long-term strategy is to make short-chain alcohols and optimal configuration is an area that needs further study and then use long-established chemistry to produce mixtures analysis. of the longer-chain isoalkanes that are used in jet fuel and One idea that was raised during the discussion was that diesel. He noted that chemists at his institute are exploring there may be a market for lignin as a carbon-neutral source of old chemistries that had largely been forgotten because they energy and that there may be advantages to removing lignin produce mixtures, but since fuels are mixtures of products, from biomass at local facilities close to the biomass source. those chemistries may actually be useful today. Getting Such an approach might improve the economics of biomass
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20 OPPORTUNITIES AND OBSTACLES IN LARGE-SCALE BIOMASS UTILIZATION conversion by reducing the amount of material that would “a dead field” that attracts little interest among researchers need to be transported to larger processing facilities for con- and few funding opportunities, but that this field should be version to sugars while also providing an energy source to reinvigorated if technological barriers are to be addressed. support local industry. What little process engineering research does occur is largely The major technological and commercial barriers to scal- conducted overseas. The same appears to be true for separa- ing up sustainable technologies involve moving from batch tions technology and surface chemistry, and the breakout processing to continuous processing, at least up to the stage group agreed that chemists need to receive better training in of sugar production. It was noted by several breakout group these key technological fields. Some members of the group members that batch processing of biomass to produce sugar highlighted the need for universities to establish biofuels on a scale needed to produce 21 billion gallons of ethanol courses which has already been done at the University of a year by 2022 was untenable economically. Producing that California, Berkeley. Some group members noted the need much ethanol in batch fermentation plants would require 525 for biochemists to receive more training in enzymology, a 40-million-gallon plants at $40 million apiece. field that once flourished. Most breakout group members believe that fuel produc- According to Katz, members of the group said chemi- tion and chemical production should be considered separate cal engineers also need a new set of skills to contribute to pathways, much like they are in the petrochemical industry. the development of a biomass-based industry. Chemical While many breakout group members agreed that fuel pro- engineers today receive very little training in batch process- duction from sugars via fermentation should be done through ing or in the design and operation of continuous enzymatic a continuous process to be truly economical and scalable, processes. Both of these deficits need to be addressed imme- production of most chemicals is best done in batch mode, diately, according to the breakout group members, Katz said. at least based on the extensive experience of the chemical Turning to issues of transportation infrastructure, the indi- industry. It was noted during the discussion from a member vidual breakout group members concurred that the costs of of the group that 90 percent of organic chemicals used today biomass collection and transportation needed to be addressed are made in batch operations. This percentage reflects the if biomass is to make a significant contribution to the produc- production of the many hundreds of specialty chemicals tion of fuel and chemicals. The members of the group said which are done in relatively small batches, whereas the that the biomass industry will have to depend on the exist- top 100 commodity chemicals, which represent by mass ing transportation infrastructure given the huge expense of the bulk of synthesized organic chemicals, are produced in creating a new one. It was noted during the breakout group continuous-flow batches. The chemical industry is already discussion that $4 billion was invested in an ethanol pipeline exploring the production of chemicals from biomass inde- system that is only at 25 percent of capacity now. pendent of fuel production. One idea from the group was that it may be necessary to The breakout group discussed the need to solve techno- subsidize stover collection by secondary harvesting services logical issues involving economical production of enzymes to meet supply considerations if the market develops for to meet a variety of demands. Many members of the group the products of biomass conversion given that there is little concurred that technology development was needed to design economic incentive today for farmers to collect stover. The enzymes with higher activities, that could pretreat biomass group also noted that storage of corn stover or corn cobs, prior to sugar production, that would resist inhibition by lignin which could also be a good source of biomass, is expensive or organic acids, and that will function in alternative environ- and is actually a significant economic barrier that needs to be ments, such as in ionic solvents or under pressure. There was addressed. The breakout group raised the idea of developing substantial discussion, with no consensus, about whether it a slurry-based pipeline system for biomass or a system for was better to build better and less expensive enzymes or to converting biomass into pellets for easier transport. engineer microorganisms to that can perform multiple steps The breakout group concluded its discussions with a in the conversion process. The suggestion was made that the comment about the idea of converting biomass to so-called biomass field could learn from the pharmaceutical industry, drop-in fuels versus expanding the amount of ethanol pro- which makes extensive use of secondary metabolism by duced. The group said that it is hard to compete with ethanol engineered microorganisms to produce high-value products. in terms of net energy return from sugar because ethanol’s The breakout group also noted the need to develop methods high oxygen, low carbon content closely matches that of for conducting large-scale anaerobic fermentation to achieve sugars. For advanced biofuels based on hydrocarbons, fatty more efficient conversion of sugars to product. acids produced by algae or oil crops are likely to be the Addressing the issue of needed skills, the breakout better feedstock. group concurred that fundamental process engineering is