New Four-Step Process for the Direct High-Yield and Economic Production of Renewable Jet and Diesel Fuels From Wood Processing Waste
|Integrated conceptual lignocellulosic biorefinery for the production of renewable chemicals and liquid fuels. The four-step process to make straight-chain alkanes is highlighted in the shaded box. Source: Xing et al. Click to enlarge.|
Researchers at the University of Massachusetts, Amherst and the University of Maine have developed a novel four-step process for high-yield, economic production of jet and diesel fuel-range alkanes from hemicellulose hydrolysates derived from northeastern hardwood trees. The hydrolysate is representative of a byproduct that could be produced by wood-processing industries such as biomass boilers or pulp mills in the northeastern US. A paper on this work by George Huber and his colleagues was published online in the RSC journal Green Chemistry.
The process is an extension of early work by Huber and James Dumesic that first presented a catalytic process for the conversion of biomass-derived carbohydrates to liquid alkanes (C7–C15).
The four-steps of the process are:
- Combined acid hydrolysis into xylose and acid-catalyzed biphasic dehydration of xylose into furfural;
- aldol condensation of the furfural extract through which the alkane precursor F-Ac-F is formed through the reaction of furfural with acetone;
- low-temperature hydrogenation of the F-Ac-F dimer to thermal stable hydrogenated dimers (H-FAFs). In this step, three types of double bonds of F-AC-F are saturated and the final hydrogenated dimers contain merely spiro and alcohol forms of dimers; and
- High-temperature hydrodeoxygenation of the hydrogenated dimer solution and H2 to produce jet and diesel fuel range alkanes over a bifunctional catalyst.
The researchers obtained experimental yields of 76% for jet fuel range alkanes, corresponding to a weight-percent yield of 0.46 kg of alkanes per kg of xylose (monomer and oligomers) in the hemicellulose extract. The theoretical yield for this process is 0.61 kg. Currently, the low-yielding steps are dehydration and hydrodeoxygenation. For the dehydration step, they were able to obtain a yield of 90% with model xylose solutions, and the kinetic model indicates that a yield of 95% can be achieved.
Therefore it should be possible to obtain yields close to 95% for this process by further optimizing the reaction system. Yields higher than 95% are very challenging due to the undesired decomposition and polymerization reactions. The yield for the hydrodeoxygenation step could also be improved from 91% to 95% with improvements in the catalysts and reactor design.
We predict that the overall yield for jet and diesel fuel range alkanes could be increased up to 88% with these modest process improvements. The straight alkanes produced in our process can be further upgraded via the hydroisomerization process to form branched alkanes. The straight and branched alkanes together can either be directly sold as chemicals or liquid fuels, or sent to a refinery as additives to make the desired jet and diesel fuels by blending with other hydrocarbons.
Currently the alternative approach to the synthesis of straight and branched alkanes for synthetic jet and diesel fuels is the Fischer–Tropsch process, using synthesis gas derived from natural gas. As such, our process provides another way to make jet and diesel fuels range alkanes from waste hemicellulose-derived solutions.—Xing et al.
The team performed a preliminary economic analysis for this process and concluded that jet and diesel fuel range alkanes can be produced from between $2.06/gal to $4.39/gal depending on the feed xylose concentration in the hemicellulose extract, the size of the plant capacity and the overall yields.
Rong Xing, Ayyagari V. Subrahmanyam, Hakan Olcay, Wei Qi, G. Peter van Walsum, Hemant Pendse and George W. Huber (2010) Production of jet and diesel fuel range alkanes from waste hemicellulose-derived aqueous solutions. Green Chem. doi: 10.1039/c0gc00263a