A new generation of renewable fuels is on the horizon
By Wayne Seames
A new generation of technologies to generate renewable fuels is nearing commercialization. Some of these are focused on producing ethanol and other alcohols from cellulosic biomass using fermentation technologies. These alcohol-based fuels can be used as a substitute for gasoline.
Another group of technologies is focused on producing fuels that replace kerosene and diesel fuels. These technologies take advantage of the chemical composition of crop oils, such as camelina, to generate organic chemical mixtures that are more similar to existing kerosene (jet fuel) and diesel products than current biofuels such as biodiesel. Crop oils contain a group of chemicals known as triacylglycerides (TG).
A TG molecule consists of three carbon chains ending in a carboxylic acid group, with each carbon chain (known as a fatty acid) connected to a glycerol backbone.
Plants and animals naturally synthesize TG as a means to store energy, as do some algae and bacteria.
Two process schemes are nearing commercialization for the production of fuels to replace kerosene and diesel: hydrotreating and noncatalytic cracking. Both process schemes manipulate TG oils to generate renewable fuels and by-products.
As the name implies, hydrotreating involves the reaction of TG oils with hydrogen. The TG oil and hydrogen are fed into a reactor where a combination of heat, pressure, and time induce chemical reactions that will (i) remove the fatty acids from the glycerol backbone and (ii) replace the carboxylic acid group on the fatty acids with a hydrogen atom, producing hydrocarbons.
A catalyst is typically used to increase the efficiency of the hydrotreating reactions. Some versions of the hydrotreating process also use a catalyst to induce some of the fatty acids/hydrocarbons to rearrange to introduce side chains onto the base carbon chain, a process known as isomerization. The catalyst formulation is also used to encourage any double bond-connected carbon pairs to transform into single bond-connected carbon pairs. After hydrotreating/isomerization, the reactor outlet mixture is separated into product fractions. In some versions, reactions to cleave some of the carbon bonds are performed during or interspersed with the purification steps to decrease the average carbon chain length of the fuel. This carbon bond cleavage process is known as cracking. Hydrotreating processes typically produce diesel, kerosene, propane, and syngas products.
Recently, a kerosene product known as synthetic paraffinic kerosene (SPK, a renewable kerosene product with limited aromatics content) was produced from camelina oil and used by the US Air Force and Navy in full-scale performance tests. The SPK was mixed 50:50 with petroleum-derived military specification-grade JP-8 jet fuel, then tested in current military aircraft. Based on the success of these tests, a number of commercial airlines have begun testing 50:50 SPK blended fuels in their aircraft (see inform 22:497–499, 2011).
Commercial production facilities based on hydrotreating process technology are likely to be in service within the next couple of years. These fuels will supplement existing petroleum sources without requiring substantial changes in the infrastructure supporting current fuel generation, storage, and supply systems.
The other process that is nearing commercialization is the University of North Dakota’s noncatalytic cracking process (patents pending). In this process (Fig. 1), TG oil is fed into a reactor where heat, pressure, and time are used to induce cracking reactions in the TG molecules.
This generates a complicated mixture—we’ve identified more than 250 separate chemical compounds in the reactor products—that is dominated by short-chain fatty acids, paraffins (hydrocarbons with all single-bond carbon-pair connections), and aromatics (compounds containing a six-carbon ring with three double-bond carbon-pair connections). The reactor product stream is then separated into intermediate product fractions and further processed into a final suite of fuels and by-products. The noncatalytic cracking process typically produces diesel, kerosene, naphtha, light hydrocarbon fuels, and syngas. Another by-product stream is a suite of very heavy, viscous compounds, typically labeled as “tars.” These are long-carbon-chain chemicals that are formed when multiple fatty acid fragments, produced during cracking, combine. These tars can be recovered and converted into purified carbon products such as high-purity granulated carbon for spark plug rods and carbon nanotubes or into a mesophase pitch that can be spun into carbon fibers.
There are some advantages in noncatalytic cracking. First, an external hydrogen source is not required. The cracking process generates hydrogen as it produces aromatic compounds. This hydrogen can be recovered from the syngas and used to convert the carboxylic acid groups in fatty acids into hydrocarbons, where required.
Second, the first step in the process—cracking—does not use a catalyst. Because of this, the process can tolerate more impurities in the feedstock TG oil than processes that use a catalyst, such as hydrotreating processes. This is not a concern for edible crops, such as soybeans or corn, since these sources will likely treat their TG oils for human consumption. But this can substantially reduce the costs for extraction and treatment of nonedible crops such as camelina. This feature also means that noncatalytic cracking facilities will be feedstock flexible, capable of changing or blending TG oil feedstocks in response to market conditions.
Third, the noncatalytic cracking process produces fuel products that contain aromatics in concentrations that are similar to those contained in petroleum jet and diesel fuel products. For example, a complete Jet A commercial jet fuel can be produced solely from this process. There is no requirement to mix this renewable jet fuel with petroleum jet fuel, as with SPK. Thus, these fuels can completely replace or be indiscriminately blended with existing petroleum fuels without impacting the infrastructure supporting the generation, storage, and transportation of existing fuels.
One of the challenges for renewable fuel producers is the low gross profit margins that can be realized in the fuels market. Petroleum refining is extremely efficient, and fuel sales prices can be sustained at levels that are challenging for renewable alternatives. The noncatalytic cracking process provides greater flexibility to generate higher margin by-products than many other processes. For example, instead of converting all of the short-chain fatty acids generated during cracking into hydrocarbons, they can be extracted from the cracking reactor product, purified, and sold as separate chemical products.
As with hydrotreating, commercial facilities are expected within the next few years. Both noncatalytic cracking and hydrotreating processes will initially use crop oils such as camelina as their feedstocks. Technologies to produce TG oils from other sources of TG oils, such as microbe- and algae-derived oils, are expected to evolve to allow high volumes of oil to be cost effectively produced. When this occurs, both noncatalytic cracking and hydrotreating pathways will be able to accommodate these feedstocks as well.
Researchers are pursuing a number of other strategies for the generation of renewable fuels. One pathway is often labeled the “thermochemical pathway.” Biomass material is reacted under specific conditions of heat, pressure, and time to break down the TG oil through pyrolysis or gasification. These reaction processes are similar to cracking. The gasification versions generate a syngas that is then fed to catalytic reaction steps that induce the recombination of the syngas molecules into larger molecules to generate liquid transportation fuels. Another strategy is to use yeasts, microbes, or algae to directly synthesize hydrocarbons that can be purified into fuels.
Wayne Seames is the Chester Fritz Distinguished Professor of Chemical Engineering and Director of the Sustainable Energy and Supporting Education (SUNRISE) program at the University of North Dakota, Grand Forks, USA, where he has served on the faculty since 2000. He is the lead inventor of a suite of technologies used to convert triacylglyceride oils into renewable fuels and chemicals. He can be contacted at firstname.lastname@example.org.