The global market for bioproducts is robust and growing, as noted by several recent studies, including a 2016 report from Zion Research that estimates a 2015 renewable chemicals market of $50 billion, with a five-year compound annual growth rate of 11% , and BP’s annual world energy review that pegs 2015 global biofuels production at 74.2 million metric tons oil equivalent . The Biotechnology Innovation Organization released a 2016 report that summarizes these studies .
However, thermochemical conversion of fossil fuels still dominates. For instance, BP’s review put the total consumption of gas, oil and coal for transportation, power, industrial use, non-combusted (primarily as a feedstock for chemicals) and buildings at 11,306 million mt oil equivalent — or 150 times biofuels’ production. Thermochemistry benefits from about 150 years of technology development for efficiently converting these feedstocks to energy, fuel and chemical products.
Some early forms of industrial bio-based processing are starting to reach a certain level of maturity — particularly in the field of conversion of starches and sugars to ethanol, the production of biodiesel from plant- or animal-based lipids, and the making of organic acids such as citric acid and lactic acid by fermentation. This is leading to the building of larger plants, declining production costs, and manufacture of multiple products at a single facility.
This advancing maturity of bioprocessing is enabling a new paradigm: the integration of a conventional or modified thermochemical conversion step to upgrade a bio-based feedstock or a product from a bio-based process. This effectively leverages the centuries of knowledge in chemistry, engineering and catalysis encapsulated in thermochemical processing to advance the bio economy. The emergence of these integrated schemes has the potential to produce fuels and chemicals that can reduce carbon footprint and increase profitability. In addition, the product from these integrated streams often is more easily accepted by the market and existing infrastructure.
Such integration marries bioprocessing’s advantages with mature thermochemical processes.
Bioprocessing advantages. A bio-based feedstock in many cases is an advantaged carbon source. For instance, the growing concerns over sustainability and carbon footprint have prompted a variety of government incentive and regulatory programs that favor fuels and chemicals from plant-based carbon . This is spilling over to the private sector with consumer and shareholder demands driving an increasing focus on sustainable products. However, faced with the reality that the market won’t pay a premium for “green” products, bio-based routes must offer a cost or performance advantage. In some cases, the carbon in these feedstock routes is cheaper than fossil carbon, although this greatly depends on market forces. Tipping fees or low cost hauling charges associated with municipal solid waste, waste oils and other forms of waste organic matter are spurring interest in technologies that can convert these feedstocks into fuel and chemical products. In a similar manner, waste carbon from industrial sources can be “reused,” playing an important role in growing interest in the circular economy.
Reference 5 explores in some detail the processing conditions of biological routes versus thermochemical ones. For instance, biological materials impose limitations on the operating temperatures and pressures of reactors. This restriction against elevated temperatures and pressures actually provides an advantage: the carbon can be delivered to the reactor with much lower energy costs than a conventional thermochemical route. In addition, biological processes have greater tolerance to many species like hydrogen sulfide and ammonia that poison conventional catalysts, thereby simplifying and decreasing the cost of feedstock decontamination. Finally, biological routes often can cope better with changes to feedstock composition and are more forgiving than conventional thermochemical processes.
Thermochemical processes. The lower temperatures and pressures in biological processes limit the reaction rate. In addition, the catalyst “density” in a biological process is much lower than in a thermochemical route. Therefore, a thermochemical process requires a much smaller reactor volume than that of a similar biological conversion step. For instance, a very good biological process may run with a productivity of 100 g (product)/L (reactor volume)/d. In contrast, a typical refinery hydrocracker provides a “productivity” exceeding 30,000 g/L/d (to use the nomenclature of the bioprocessing industry). In other words, the refining process can produce more than 300 times the amount of product per unit of reactor volume.
Moreover, as already mentioned, thermochemical processes have the advantage of maturity. The chemical engineering profession first was established at the University of Manchester in the U.K. in 1887, followed shortly thereafter by a program set up by Massachusetts Institute of Technology, building upon centuries of operational knowledge about chemicals’ production. The growth of chemical engineering, the concept of unit operations, and best practices for process design and operation largely grew in parallel with the petroleum-based economy. This scale-up experience has been encapsulated in a subset of chemical engineering related to process design, with robust approaches ranging from empirical models based on operating data, rigorous algorithms built up from first principles, and shortcut approaches derived from both empirical and first-principle design methodologies. As a result, while scale-up of new thermochemical technology innovations isn’t trivial, using these tools often enables successful scale-up by two to three orders of magnitude.
Bio-based processes that include a thermochemical conversion step already are emerging. Let’s look at several examples that demonstrate the potential to couple the advantages of both the growing bio-based economy and conventional thermochemical processing to create products that can add value and reduce carbon footprint.
Thermochemical Feedstock Conversion
A growing class of technologies relies on a thermochemical step to convert a bio-based feedstock to a useful fuel or chemical product.
Renewable diesel and jet fuel. Conventional biodiesel is made through a process called transesterification. Triglycerides from an oil or fat are reacted with an alcohol, commonly methanol (resulting in fatty acid methyl ester or FAME) or, in some cases, ethanol (giving fatty acid ethyl ester). Glycerol is produced as a byproduct.
In recent years, an alternative route has developed, adapting hydroprocessing technology common in the refining industry to produce hydrotreated vegetable oils (HVO) and hydroprocessed esters and fatty acids (HEFA). These products are commonly known as “green” or “renewable” diesel. The HVO and HEFA products are more stable than FAME, have better cold flow properties, and can be blended in greater ratios in conventional diesel engines.
Hydroprocessing-based routes also are the driver behind most of the recent activity surrounding alternative low-carbon jet fuels , with hydroprocessed renewable jet routes dominating the test flights to date.
Conversion of sugars to fuel and chemicals. Advances in this area include:
• A two-step process developed by Virent. The company uses an aqueous phase reforming step to transform sugars and other molecules common in bio-based feedstocks into a mixture of products from reactions including reforming, dehydrogenation, deoxygenation, hydrogenolysis and cyclization . The mixed stream from this step then is converted using a modified form of ZSM-5, a catalyst widely used in petrochemical applications. The product from this common thermochemical conversion step resembles petroleum-based reformate, an intermediate fraction that is used for a high-octane gasoline blend stock and as the primary feed to make aromatic bulk chemicals.
• Pyrolysis is a form of thermal conversion in which a carbon-based feedstock such as biomass is heated under controlled conditions, without oxygen, to form an oil along with some gas and char byproducts. Use of this oil directly as a fuel is challenging because it’s acidic, corrosive and oxygenated. One approach under development is coprocessing the oil directly in an existing fluid catalytic cracking (FCC) unit common in many refineries. Honeywell UOP, a leading supplier of technology for FCC units, has explored this route through a joint venture with Ensysn . In this manner, the pyrolysis oil can be converted into gasoline and diesel fractions with partially renewable content.
• Catalytic pyrolysis is an emerging technology being developed to produce a pyrolysis oil that doesn’t pose the same challenges as the oil from conventional pyrolysis. The approach adapts the fluid bed catalyst and process technology common in the refining and petrochemical industry to convert bio-based feedstock directly to fuel and chemical products. Inaeris (formally Kior) is targeting drop-in fuel blend stocks while companies such as Anellotech are focusing on aromatic petrochemicals.
• Thermochemical processing using oxidation chemistry produces a range of oxygenated petrochemicals. Bio-based feedstocks inherently contain oxygen; so, companies are applying a thermochemical step to make the same oxygenated chemicals with either a simplified oxygenation step or elimination of the oxidation chemistry altogether. Rennovia has developed a technology to selectively oxidize glucose, followed by hydrogenation of glucaric acid to produce adipic acid. Bioamber couples a fermentation step to make succinic acid with a thermocatalytic step to produce 1,4-butanediol, eliminating the oxidation step completely. Archer Daniels Midland has a commercial technology for thermocatalytic conversion of glycerin, a biodiesel byproduct, into propylene glycol by hydrogenolysis.
Conversion Of Bioprocess Products
Downstream thermochemical steps can transform alcohols produced by fermentation into fuel and chemical products. In most cases, the alcohol is dehydrated to form a corresponding olefin, which then can be further processed conventionally.
• Dehydrating ethanol to form ethylene opens the door to a wide range of downstream conversion products. The ethanol dehydration step provides a bridge between the bio-based route and more traditional chemical processing. Companies such as Petron offer ethanol dehydration technology along with subsequent processing to polyethylene, ethylene oxide, ethylene glycol and other common ethylene derivatives. India Glycols, in collaboration with Scientific Design, produces a range of bio-based glycols using ethanol as a starting material.
• Companies like Gevo and LanzaTech are successfully converting fermentation metabolites to fuel blend stock. Similar to the example above, the starting point is dehydration of the alcohol product to an olefin, followed by thermochemical steps to produce synthetic paraffinic kerosene (SPK). These so-called “alcohol to jet” processing routes are gaining acceptance in the biojet community, providing an alternative means for producing jet fuel with a lower carbon footprint.
• Firms such as Byogy use a similar front end. In this case, the ethanol again goes through a dehydration step but with alternative thermocatalytic steps to make products that are direct replacements for conventional gasoline and jet fuel.
• Other companies also are looking to exploit the growing ethanol infrastructure to manufacture different chemicals directly from ethanol. For example, Greenyug announced in 2016 a project for producing ethyl acetate using ethanol from a co-located wet mill corn processing plant of Archer Daniels Midland .
• Versalis and Genomatica have partnered to produce bio-based polybutadiene. First fermentation gives 1,3-butanediol. This metabolite is dehydrated thermochemically to butadiene, which then goes to a polymerization step.
While these examples show the potential for bringing thermochemical processing into the bio economy, some caveats deserve noting.
Scale. Current bio-based processes simply can’t match the scale of today’s refining and petrochemical technologies. For instance, Jilin Fuel Ethanol has one of the largest ethanol plants in the world, with an annual capacity of approximately 600,000 mt/d (2.3 million L/d or 14,000 bbl/d). In contrast, the Reliance Refinery at Jamnagar boasts a capacity of approximately 1.2 million bbl/d, making it larger by a factor of 85. However, with more time and experience, bioprocessing routes clearly will continue to grow in scale.
Value destruction/yield loss. As already noted, several integrated approaches being developed convert a biological metabolite, typically ethanol, to ethylene, SPK or gasoline-range hydrocarbons. Ethanol is a fungible product, typically with a value tied to the value of gasoline in many cases. Because the oxygen is removed from the final product, there’s a natural loss of yield, making the economics challenging in certain circumstances.
Water consumption. Biological processes necessarily require a significant amount of water. For instance, a typical ethanol titer is 10–15% with the balance water. Industry recognizes the need to optimize water consumption and has made progress in this area. A 2009 benchmarking study estimated that a new 50-million-gal/y ethanol plant consumes 3 gallons of water per gallon of ethanol . Advanced technology can reduce water consumption further, although likely with a tradeoff in capital cost. For instance, Praj touts the ability to maximize water efficiency to a consumption level of 1 gal/gal of ethanol. A more recent concept is exploring the nexus between water, energy and, sometimes, food or land, in an attempt to better understand the constraints and opportunities.
Despite the challenges associated with bioprocessing, the marketplace recognizes the potential to produce more-sustainable products that can reduce the carbon footprint and increase value. The centuries of experience embedded in thermochemical processing gradually is becoming a part of this growing bio economy. The opportunity to use the “best tools in the toolbox” will enhance development of additional innovative process technologies that help lead us to a more-sustainable and profitable future.
MICHAEL SCHULTZ is managing director of PTI Global Solutions, Glen Ellyn, Ill. Email him at firstname.lastname@example.org.
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