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Simultaneously treat many mixed-waste streams

By Mary Page Bailey |

A team of researchers at Worcester Polytechnic Institute (WPI; Worcester, Mass.; www.wpi.edu) are investigating hydrothermal liquefaction (HTL) to simultaneously treat a variety of municipal waste streams and create biofuels. “HTL is an emerging technology for the conversion of various types of aqueous streams containing organic components, including biomass, food, sewage sludge and many others,” explains WPI chemical engineering professor Michael Timko. In HTL, the feedstream is heated while system pressure is maintained at a level sufficient to maintain water in its liquid state. At these conditions, depolymerization, dehydration and hydrolysis reactions occur, yielding an energy-dense oil that can be upgraded as diesel blendstock, along with a carbon-rich solid char, a CO2-rich gas and an aqueous phase. A major benefit of HTL is speed — its reactions take just minutes, whereas conversion via anaerobic digestion, the incumbent technology, can require multiple days. Hydrothermal liquefaction also avoids other challenging aspects of anaerobic digestion, including incomplete digestion of lignin and crystalline cellulose, microbial susceptibility to toxins and the production of byproducts that necessitate additional purification. Since the technology handles aqueous streams, no drying is required, which further improves process economics.

mixed-waste streams

Timko’s team is looking at a variety of catalytic platforms for their HTL process, including bauxite tailings, also known as “red mud,” an abundantly available waste product of alumina processing. “Red mud has diverse acid and base sites that are appropriate for promoting depolymerization and recombination reactions that help funnel carbon through desirable oil-producing pathways and away from byproducts,” says Timko. The team has tested the process with different combinations of yard and food waste to produce oils. “In principle, any aqueous stream with greater than about 10 wt. % organic content is worth investigating. Plastics upcycling is an emerging area we are currently exploring,” adds Timko. The project currently uses 100-g reactors, but plans are in place to expand to kilogram-scale and also evaluate continuous processing. Timko notes that an important consideration with HTL scaleup is to balance the economies of scale for larger-scale facilities versus maintaining modular reactors, which benefit from reduced feed-transportation costs.

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