These Metal-Based Catalysts Can Boost Biofuel Yields

December 23, 2020 |

By Mike Timko, associate professor of chemical engineering at Worcester Polytechnic Institute

Special to The Digest

A recent study led by a team from Worcester Polytechnic Institute shows that metal-oxide-based catalysts reduce char build up during biofuel conversion processes, increasing bio-crude oil yield and decreasing yields of low-value byproducts.

Have you ever wondered what happens to the banana peel that you threw away after your morning snack? For most of us, it ultimately breaks downs into the greenhouse gases CO2 and methane in another filthy landfill. Other products from the banana’s human-directed biological degradation may even pollute water and soil.  The problem is, this scenario is more than a banana, and more than just you trashing food waste.

Recent estimates by the U.S. Environmental Protection Agency reveal that approximately 120 million tons of energy-rich, food, sewage sludge, and green biomass are wasted each year. This is an otherwise missed opportunity to convert the energy encased in these sources into fuels and nitrogen, which can then be recycled to reduce agricultural reliance on fertilizers.

And, not only are these sources abundant, but they are typically available at much lower costs than biomass, a key economic advantage for biofuel production. Beyond economics, they would allow communities to reduce their usage of fossil fuels, cut the amount of municipal waste going into landfills, provide nutrient sources to supplement fertilizers, reduce water pollution, and more. These are key focuses of the waste-to-energy team at Worcester Polytechnic Institute, along with collaborators from the Woods Hole Oceanographic Institution, Clark University, the National High Magnetic Field Laboratory at Florida State University, Florida-based Mainstream Engineering Corp. and others, which is developing ways to convert wet wastes into environmentally-friendly biofuels.

Utilizing wet food waste requires a fast and efficient conversion technology. Hydrothermal liquefaction (HTL), which is being studied by the WPI team, is a thermochemical process that rapidly decomposes organic wastes with the aid of high-temperature pressurized water (~374 °C and ~22 MPa) into energy-rich, crude-oil-like product, or bio-crude oil. Unlike many other technologies, HTL does not require a dry feed, making it especially suitable for wet feeds, including food waste, sewage sludge, and other wet wastes. The major technical challenge facing HTL is directing carbon to the bio-crude oil, rather than the less valuable byproducts that accumulate in solid char, aqueous, and gas phases. Catalysts offer an opportunity to direct carbon toward the bio-crude oil; however, char accumulation on solid surfaces can rapidly deactivate otherwise promising catalysts.

The WPI team recently reported in a paper, which is to be published by Sustainable Energy & Fuels, that the use of metal oxides impregnated with nickel is effective in directing carbon flux away from the char product.

The base oxides were ceria, zirconia, and their mixtures, selected for their presumptive water stability and promising acid/base properties, an insight provided by previous studies of mixed metal oxides in a report published in 2020 in ACS Sustainable Chemistry & Engineering. Remarkably, the use of these nickel-impregnated oxides under reducing conditions reduced char from 40% to less than 2%, while increasing bio-crude oil yield from 20% to 40%. The higher heating value of these bio-crude oils approached 40 MJ/kg, similar to a petroleum crude.

Through these tests, we found that ceria appears to promote conversion of carbon deposited on catalyst surfaces into carbon dioxide, simultaneously generating gaseous hydrogen by a reaction resembling the water-gas-shift. Zirconia appears to promote a reaction of small organic molecules to form bio-crude via transesterification reactions. Finally, nickel nanoparticles promote gasification reactions that prevent carbon accumulation on the catalyst surface. Interestingly, while hydrogen was co-fed to the reactor to maintain reductive conditions, the reaction is nearly hydrogen neutral. The catalyst structures are stable after 16-hour hydrothermal stress tests and the most effective, nickel on zirconia, was re-used without loss of activity. The WPI team worked with partners at Woods Hole Oceanographic Institution, Clark University, and the National High Magnetic Field Laboratory at Florida State University to study the molecular basis underlying these observations.

Our other team partner, Mainstream Engineering, is now evaluating technology scale-up, distribution, and economic feasibility. One key question we hope to answer through this exploration is: “when does the improved performance of the catalyst justify the added expense?” A secondary objective of economic analysis is to identify the appropriate scale for this technology, given that waste streams are inherently distributed rather than centralized. Combining multiple streams – i.e., food waste and sewage sludge – may be beneficial for achieving optimal scale in some locations, and will lead to new studies on co-processing. Scale-up tests will further help identify possible engineering challenges regarding reactor feed, product recovery, and catalyst stability.

Future work in the area of reductive HTL can establish long-term catalyst stability and evaluate bio-crude oil upgrading. Success in these areas can unlock the potential of food waste and other wet waste streams for the emerging bioeconomy. So, the next time you toss out your banana peel, maybe it will end up in the WPI process where it can be converted into a biofuel, rather than decaying in a landfill to produce greenhouse gases and soil and water pollution.

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