Conversion of lignin into a hydrocarbon product for blending with gasoline (2002)

Introduction Lignin is present (15-30 wt%) in all lignocellulosic biomass. Ethanol production from biomass feedstocks will always generate lignin containing residues. Thus, lignin supply will progressively increase as ethanol plants using straw, forest residues, etc., are implemented in the future. Adding value to this residue will significantly enhance the competitiveness of biomass-to-ethanol conversion. Lignin, a cross-linked amorphous phenolic polymer, is the biomass component with highest energy content (9000 11000 Btu/lb compared to 7300 7500 for cellulose). Lignin can be combusted to provide heat and/or power for the ethanol process, and could provide some excess power to generate additional revenue. Given the molecular nature of lignin it is also conceivable to convert lignin into added value end products, especially fuel components. Previous process development studies in our project have focused on converting lignin to reformulated gasolines and gasoline blending components. Lignin-derived products must be compatible with the evolving regulations for transportation fuels. There are specific compositional requirements for gasoline limiting the level of aromatic compounds in gasoline. For Federal Phase II Reformulated Gasoline (RFG), the limits are 1 percent benzene and 25 percent by volume total aromatics. In California, the Phase III RFG specification is an average of 22 percent aromatics or less. For non-RFG gasoline, the aromatic content is limited to the refinery’s 1990 baseline level. Gasoline volatility (90% should distill below 167 C) and Reid Vapor Pressure (RVP, should be 48-56 kPa) are also important specifications. Octane number ((R+M)/2) determines gasoline fuel grade and ranges from 85 to 93. It is the current goal of this project to convert lignin into a hydrocarbon product compatible with blending in gasoline that has an octane number greater than 110. The sulfur-in-gasoline rule, issued in December 1999, greatly reduced the amount of sulfur to be allowed in gasoline by 2005. The new regulations for light-duty vehicles greatly reduced the allowable emissions of NOx, NMOG and particulates. Automakers say these standards can only be achieved if virtually sulfur-free gasoline is available. It is estimated that the required desulfurization of gasoline could reduce aromatics levels in gasoline and create a loss of 0.5 octane numbers. The predicted phase-out of MTBE, an oxygenate and octane enhancer, will exacerbate the nationwide shortage of octane. This may permit introduction of an aromatic octane-blending agent without an increase in permitted aromatics levels. If the ligninderived octane enhancer has the same octane number as xylene (i.e., 106), then 1.7 percent could be blended to remedy the octane loss without exceeding the original aromatics content of gasoline. On this basis, the potential US market for a lignin-derived octane enhancer could be 142,800 barrels per day or 2.2 billion gallons per year. This project is thus targeted to obtain the necessary scientific and technological data with which to evaluate the economics and markets for converting lignin into valuable fuel components. The main source of lignin from Biomass-to-Ethanol processes is the high molecular weight lignin remaining in the solid residue after saccharification and fermentation (SSF) of the dilute acid pretreated biomass. The lignin is soluble in aqueous basic solutions, and can be readily separated by solubilization from the protein, unconverted carbohydrate, and other non-lignin materials present in the residue. The process strategy is to depolymerize the lignin by base catalysis, using moderately high temperatures and short reaction times. To minimize the cost of base recovery only low levels of soluble base are used, just sufficient to dissolve the lignin. The resulting intermediate is a base catalyzed depolymerized (BCD) product that consists of monomeric and oligomeric phenolics. The BCD product is isolated by acidification and then solvent extraction of the raw BCD product. Catalytic hydroprocessing (HPR) is then used to convert the extracted BCD product into a mixture of monomeric aromatic hydrocarbons very similar to components already found in gasoline. HPR utilizes catalysts with hydrodeoxygenation (HDO) and hydrocracking (HCR) activity to remove all remaining oxygen and break the interaromatic C-C linkages. The target characteristics for the hydrocarbon product are: Distillation characteristics –95% should distill 99.5% –S content 65% –No detectable phenols Octane value –Blending octane number ((R+M)/2) > 110 Reed Vapor Pressure -Mandated limits vary -Maximum levels range from 7 10 psi A process flow sheet has been constructed for the process of converting lignin into a high-octane hydrocarbon fuel blending component. From this the production cost for the BCD-HPR product has been estimated at $0.60 – 0.75/gallon. This estimate assumed the plant received a fraction of the lignin from a 600 tpd ethanol plant. Performance targets used in the calculations included that 100% of the lignin in the SSF residue could be solubilized to give a feed with an 8% lignin concentration. The cost of base recovery was minimized by limiting the soluble base concentration to 1% or less in the lignin depolymerization step. The overall yield of hydrocarbon product in the gasoline boiling range was assumed to be 70% of the theoretical yield, equivalent to about 50% on a mass basis. Extraction of the depolymerized lignin intermediate assumed that it could be performed with a relatively low cost solvent such as the BCD-HPR product or a process stream found in a petroleum refinery. The value of high-octane ((R+M)/2 = 110) hydrocarbon fuel blending components has been estimated based on the Annual Energy Outlook projection of $20 – 25 / bbl for crude oil in 2010, and that the value of an octane boosting product is in the range of 0.7 – 1.4 cents per octane gallon. The value of the lignin-derived gasoline blending component should be in the range of $0.97 – 1.14 per gallon based on these assumptions. Thus if performance targets can be met it should be possible to utilize the lignin from a bioethanol plant to enhance the competitiveness of biomassto-ethanol conversion.