“If you can provide inexpensive lipids, we can turn them into aviation fuel, diesel, and gasoline without too much trouble.”
The two primary approaches to using thermochemical process to convert biomass into fuels and chemicals are gasification and pyrolysis. Gasification produces syngas, a mixture of carbon monoxide and hydrogen that is already used in the petrochemical industry. Pyrolysis produces bio-oil, a thick, corrosive mixture that in some respects resembles crude oil, and charcoal, which can be used as either an energy source or a carbon sequestration agent. Both approaches yield intermediates that are then used as feedstocks for further processing. An advantage of using thermochemical processing, as opposed to biological processing, is that it can more readily break down lignocellulosic materials in a controlled manner to produce high concentrations of desired intermediates. An important obstacle to thermochemical processing is the inorganic contaminants in biomass, which can foul the catalysts used to convert syngas or bio-oil into fuels and chemicals, said Robert Brown, founding director of the Bioeconomy Institute at Iowa State University.
The choice of whether to use a biochemical route, which uses enzymes and microorganisms to generate desired products, or a thermochemical route, which uses heat and catalyst to generate product, depends on the type of feedstock being processed, said Brown. The three major classes of biomass, he explained, are lipid-rich biomass, which historically has been soybeans but that would include algae in the future; lignocellulosic biomass; and waste biomass, which is a mixture of all different kinds of feedstocks.
The beauty of lipid feedstocks is that they are nearly hydrocarbons that are not difficult to turn into aviation fuel, diesel, and gasoline, Brown noted. In fact, petroleum companies have developed and proven large-scale processes for making fuels from lipid biomass, but all of these operations have been shut down for one reason, said Brown—the high cost of the feedstock. If ongoing research can successfully develop cheaper feedstocks, such as those that could grow on marginal land, thermochemical conversion of those feedstocks into fuels will be attractive. Algae may prove to be one of those feedstock sources, but algae also produce high levels of protein that will need to be dealt with in an economically viable manner, by turning it into either fuel, which will involve catalytic removal of nitrogen, or food.
Lignocellulose is naturally recalcitrant to degradation, but thermochemical approaches can break down lignocellulose in a controlled manner that produces high concentrations of desirable molecules. See Figure 4-1 for lignocellulose’s structure. The question that needs to be asked, said Brown, and one that he does not have an answer to, is whether efforts should focus on lignocellulosic or lipid feedstocks. In essence, this comes down to a decision as to what kind of plant should be used to deoxygenate carbohydrates—a petrochemical-type plant that produces carbon dioxide and water as the waste stream, or a green leafy plant that also deoxygenates and decarboxylates sugars in situ.
Gasification and Pyrolysis
Turning to the concept of thermochemical processing, Brown explained that the basic idea is that a feedstock is depolymerized into what DOE calls a feedstock intermediate and what Brown calls a thermolytic substrate. The thermolytic substrate then undergoes some type of upgrading, either through biological or chemical processes, to produce a biofuel. The two major types of thermochemical processes are gasification and pyrolysis.
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4 Fuels and Chemicals from Biomass via Thermochemical Routes “If you can provide inexpensive lipids, we can turn them into aviation fuel, diesel, and gasoline without too much trouble.” Robert Brown INTRODUCTION diesel, and gasoline, Brown noted. In fact, petroleum com- panies have developed and proven large-scale processes for The two primary approaches to using thermochemical making fuels from lipid biomass, but all of these operations process to convert biomass into fuels and chemicals are gasifi- have been shut down for one reason, said Brown—the high cation and pyrolysis. Gasification produces syngas, a mixture cost of the feedstock. If ongoing research can successfully of carbon monoxide and hydrogen that is already used in the develop cheaper feedstocks, such as those that could grow petrochemical industry. Pyrolysis produces io-oil, a thick, b on marginal land, thermochemical conversion of those feed- corrosive mixture that in some respects resembles crude oil, stocks into fuels will be attractive. Algae may prove to be and charcoal, which can be used as either an energy source one of those feedstock sources, but algae also produce high or a carbon sequestration agent. Both approaches yield levels of protein that will need to be dealt with in an economi- intermediates that are then used as feedstocks for further cally viable manner, by turning it into either fuel, which will processing. An advantage of using thermochemical process- involve catalytic removal of nitrogen, or food. ing, as opposed to biological processing, is that it can more Lignocellulose is naturally recalcitrant to degradation, but readily break down lignocellulosic materials in a controlled thermochemical approaches can break down lignocellulose manner to produce high concentrations of desired intermedi- in a controlled manner that produces high concentrations ates. An important obstacle to thermochemical processing of desirable molecules. See Figure 4-1 for lignocellulose’s is the norganic contaminants in biomass, which can foul i structure. The question that needs to be asked, said Brown, the catalysts used to convert syngas or bio-oil into fuels and and one that he does not have an answer to, is whether chemicals, said Robert Brown, founding director of the efforts should focus on lignocellulosic or lipid feedstocks. Bioeconomy Institute at Iowa State University. In essence, this comes down to a decision as to what kind of plant should be used to deoxygenate carbohydrates—a THERMOCHEMICAL ROUTES TO FUELS AND petrochemical-type plant that produces carbon dioxide and CHEMICALS water as the waste stream, or a green leafy plant that also deoxygenates and decarboxylates sugars in situ. The choice of whether to use a biochemical route, which uses enzymes and microorganisms to generate desired prod- ucts, or a thermochemical route, which uses heat and catalyst Gasification and Pyrolysis to generate product, depends on the type of feedstock being Turning to the concept of thermochemical processing, processed, said Brown. The three major classes of biomass, Brown explained that the basic idea is that a feedstock he explained, are lipid-rich biomass, which historically has is depolymerized into what DOE calls a feedstock inter- been soybeans but that would include algae in the future; mediate and what Brown calls a thermolytic substrate. l ignocellulosic biomass; and waste biomass, which is a The hermolytic substrate then undergoes some type of t mixture of all different kinds of feedstocks. u pgrading, either through biological or chemical processes, The beauty of lipid feedstocks is that they are nearly to produce a biofuel. The two major types of thermochemical hydrocarbons that are not difficult to turn into aviation fuel, processes are gasification and pyrolysis. 21
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22 OPPORTUNITIES AND OBSTACLES IN LARGE-SCALE BIOMASS UTILIZATION FIGURE 4-1 Lignocellulose structure. SOURCE: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. Gasification, Brown explained, is the thermal decomposi- ing of natural gas, which while not a renewable resource is tion of organic matter into flammable gases, using either a a domestic resource that would move the country away from bubbling fluidized bed reactor or an entrained flow gasifier imported petroleum and reduce greenhouse gas emissions to produce syngas. These technologies have been commer- Figure 4-1 to petroleum. compared cialized for coal, but in some respects they work evenbitmapped uneditable better The other major thermochemical technology is fast with biomass. Though gasification is in theory an equilibrium pyrolysis, which rapidly heats biomass in the absence of process at very high temperatures and long residence times, oxygen to produce three products: syngas, charcoal, and a in practice equilibrium is obtained rarely and so the process liquid mixture of organic compounds and water known as generates tar, char, and small amounts of contaminants that bio-oil that is recovered from pyrolysis vapors and aerosols agronomists would like to claim as nutrients. These contami- (see Figure 4-3). Charcoal, also known as biochar, can be nants, which include small amounts of alkali metals, sulfur, used as a carbon sequestration agent. In combination with nitrogen, and chlorine that must be removed before upgrad- the bio-oil, biochar presents an opportunity for producing ing in order to prevent poisoning of catalysts. Removal of carbon-negative fuel, said Brown. each contaminant requires its own catalyst, adding substan- Fast pyrolysis, he explained, is characterized by residence tial costs to any gasification process. times of 0.5 to 2 seconds, a very high rate of heating at A significant advantage of the gasification process is that moderate temperatures of 400–500°C, and the production of there is no question of what to do with lignin, as it is turned a liquid that looks like petroleum but smells like barbecue into syngas, too. Gasification can also handle virtually any sauce. Typically, 60–70 percent of the weight of biomass is feedstock, including waste streams, and produce a uniform converted to bio-oil (see Figure 4-4). The yield of biochar intermediate product for upgrading. It can also be used to ranges from 13 to 15 percent and that of syngas is in the produce heat, power, fuels, or chemicals and allows for 13–25 percent range. The syngas can be used as an energy energy integration into biorefinery operations. source to support this process. Technical challenges are particularly challenging in Pyrolysis chemistry is poorly understood, and Brown terms of developing technologies that can cost-effectively stressed the need for chemists and chemical engineers to remove contaminants from the gas stream (Figure 4-2). study this process. Studies at Iowa State have found that Gasification operations must also be integrated with fuel pyrolysis converts cellulose into products in a number of synthesis operations, which is not a simple matter given competing parallel pathways. This work has also shown that fuel synthesis occurs at high pressures and under that alkali present in biomass acts as a powerful catalyst exacting stoichiometries. From a commercial perspective, that produces undesired light oxygenates, and so research is gasification only works economically at large scale, which needed to understand how to control or suppress this process. translates into high capital costs that could be as high as Chemists can play a critical role in this research and help $10 per gallon of annual plant capacity. Brown added that produce a more valuable product in the end. (To see a figure biomass gasification must also compete with steam reform- of pyrolsis chemistry, please see Figure 4-5.)
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FUELS AND CHEMICALS FROM BIOMASS VIA THERMOCHEMICAL ROUTES 23 Oxygen Biomass Syngas Biomass Ash Fluidized Bed Water cooled radiation screen Steam/ Raw Syngas and Oxygen Molten Slag FIGURE 4-2 Gasification can be done in either a low-temperature fluidized bed system (left) or a high-temperature entrained-flow gasifier (right). SOURCE: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. Particulate Figure 4-2 Removal bitmapped uneditable FIGURE 4-3 Removing impurities from biomass-generated syngas is a major challenge. SOURCE: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. From a technical standpoint, the advantages of fast already mentioned, the fundamentals of pyrolysis are poorly pyrolysis are that it occurs rapidly and at atmospheric pres- understood. Commercially, there have been no demonstra- sure. It is a pathway to drop-in fuels or hydrocarbons, and it tions of bio-oil production and upgrading. Also, the pathway can produce multiple products. Commercially, fast pyrolysis to finished fuels is still uncertain, though he remarked that the offers the lowest-cost option for drop-in biofuels today, andFigurefact that there are many possibilities that have not yet been 4-3 bio-oil can be economically produced on a scale as small as explored is what excites him as a researcher. bitmapped uneditable 200 tons per day, which offers opportunities for distributed processing. Small facilities, located near the source of bio- Other Pyrolysis Routes mass, could produce bio-oil that would then be transported to a centralized facility just as is done with petroleum today. Brown then briefly discussed two other types of The primary technical challenges facing fast pyrolysis, p yrolysis—catalytic pyrolysis and solvolysis. Catalytic according to Brown, are that bio-oil is unstable, corrosive, pyrolysis employs catalysts in the pyrolysis reactor or and contains high levels of oxygen and water. Also, as he immediately downstream before bio-oil recovery to produce
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24 OPPORTUNITIES AND OBSTACLES IN LARGE-SCALE BIOMASS UTILIZATION FIGURE 4-4 Bio-oil produced by fast pyrolysis of biomass. SOURCE: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. Figure 4-4 bitmapped uneditable FIGURE 4-5 Pyrolysis chemistry. SOURCE: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. highly reduced molecules that are more stable and more prototypes using solvolysis, one using direct liquefaction, easily turned into fuels. Some researchers, he noted, have the other hydrothermal processing. claimed that they have produced diesel fuel directly via Brown concluded his talk by briefly discussing vari- catalytic pyrolysis, and while this represents a big advance, ous approaches for upgrading thermolytic substrates, and the product still needs some upgrading before it can be used focused his remarks on the solubilized carbohydrates that as a fuel. The main challenge here is that yields are rela- can be produced. These are essentially highly concentrated tively low because of coking. Nonetheless, a large number Figure solutions of sugars, typically greater than 20 weight-percent, 4-5 bitmapped uneditable have to be diluted to use for fermentation. of companies are exploring this approach even though the which would fundamental chemistry is not well understood. Solvolysis is pyrolysis in a solvent, and it has two major Discussion manifestations. One involves direct liquefaction to produce a bio-crude that is similar to bio-oil but more deoxygenated. Paul Bryan asked about the possibility of producing The other approach uses hydrothermal processing to produce methanol as a pyrolysis product and using it as a fermentable sugar and a lignin-like product. The advantages and chal- intermediate. Brown replied that this is a good possibility, lenges with solvolysis are similar to fast pyrolysis with the among probably a dozen others, that the market will need to added challenge of operating at high pressures. Brown noted assess. A participant asked if the high aromatic content of fast that two start-up companies are working on commercial pyrolysis is a problem given that the fuel industry is moving
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FUELS AND CHEMICALS FROM BIOMASS VIA THERMOCHEMICAL ROUTES 25 away from aromatics. Brown responded that aromatics are Turning to the subject of technological and commercial expected to be an important component of fuels for some barriers, the breakout group noted that economic analyses time to come. They are particularly produced by catalytic and life cycle analyses are missing from most thermochemi- pyrolysis using zeolites, but through further research alkanes cal conversion studies, particularly for combined systems. might instead be produced. Some participants suggested that such studies be funded Mark Barteau, from the University of Delaware, asked because there are many interesting concepts being developed if co-processing of biomass with natural gas might be a way today, but there is little thought being given to cost analysis. of shifting the carbon-hydrogen-oxygen ratio without using In the same vein, the breakout group described the need for expensive hydrogen. Brown replied that with natural gas at better coupling of basic, translational, and applied research the $2 level, this idea makes perfect sense and could provide studies and noted that DOE could help fill that gap. a bridge to advanced drop-in biofuels. Helena Chum, from Another area of research that needs to be bolstered, NREL, asked about the optimal scale for biomass process- some group participants noted, concerned the develop- ing, and Brown said that the issue of scale has to balance ment of technologies to efficiently handle biomass solids the fact that larger plants are probably offering economies of different characteristics and to determine how to best of scale as far as process and capital costs are concerned, mix different biomass sources to produce bio-oil with but that larger plants also have increased costs to transport more consistent properties. Some members of the breakout biomass. He guessed that the optimal size for a plant that group also identified the need to develop methods for feed- converted thermolytic substrates into transportation fuels ing biomass feedstocks into reactors at pressure, to design would be somewhere in the 2000–3000 tons per day range catalysts that are more tolerant of the poisons in biomass although distributed processing facilities might be substan- and of water and steam, and to perform separations at lower tially smaller. energy intensity. A basic research question that still needs answering, according to many breakout group members, was whether it might be best to process biomass as fractionated BREAKOUT DISCUSSION components rather than as the natural biocomposite that is This breakout session was led by Douglass Elliott, of the lignocellulosic biomass. Pacific Northwest National Laboratory, and the discussions The breakout group briefly discussed the belief that most began with the participants noting that there are a number of efforts today focus too much on market “push” and not large-scale (greater than 500 tons per day of biomass) and enough on market “pull.” In other words, the many members pilot-scale (under 50 tons per day of biomass) demonstration of the group noted, research was needed to identify products projects, for both gasification and pyrolysis, either under that could be made in an economically competitive manner construction or in the planning stage. The group acknowl- from bio-oil or syngas. One approach to developing market edged that gasification technology, while well proven with pull that is being followed to some extent, the group noted, other feedstocks such as coal, may to struggle to compete is to demonstrate how bio-oil or syngas would integrate with cheap, abundant, long-term supplies of natural gas. seamlessly into existing petrochemical streams. Such efforts Having said that, many breakout group participants con- to develop market pull could help address the problem of curred that gasification is likely to be “omnivorous”; that attracting capital to this area. is, it is likely to be adaptable for use with a wide variety of In terms of skills needed, the breakout group said the main feedstocks. It was suggested, in fact, that while natural gas deficit in training in its opinion was the lack of interdisci- is the feedstock of choice today, it could serve as a bridge plinary coursework and collaboration among engineering, technology to biomass feedstocks in the future. Another chemistry, biology, and plant science investigators. Some suggestion was to integrate a corn stover gasifier into corn breakout group members also noted they would like to see a ethanol plants. Lignin and unconverted sugars could also be greater emphasis in chemical engineering education on using mixed into the corn stover for gasification. It was noted that carbon from biomass as opposed to just from petroleum. such a plant is on the drawing board. Addressing the transportation infrastructure issue, the The breakout group discussed the use of gasification or breakout group noted that there are far more questions natural gas to upgrade the bio-oil produced by pyrolysis and than answers concerning how to best move biomass into thought this was worth exploring. It was noted that Finland a processing system and then move the direct products of has been adding small pyrolysis chambers to existing fluid- biomass conversion into the secondary processing stream. ized bed combustion boilers. The pyrolysis unit uses some Many members of the breakout group concurred that they of the heat from the boiler system and in return feeds char would like to see the emphasis on using existing infrastruc- and other byproducts of pyrolysis back into the boiler’s ture and trying to make products that can be fit into today’s combustion chamber. infrastructure at as early a point as possible.
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