Making clean, high-quality fuels from low-quality oil

New findings released by MIT researchers could help energy companies implement a long-recognized process for converting heavy, high-sulfur crude oil into high-value, cleaner fuels such as gasoline without using hydrogen — a change that would reduce costs, energy use, and carbon dioxide emissions. The process involves combining oil with water under such high pressures and temperatures that they mix together, molecule by molecule, and chemically react.

More than a third of the world’s energy needs are met using oil, and our reliance on that convenient, high-energy-density resource will likely continue for decades to come, especially in the transportation sector. But converting crude oil into lightweight, clean-burning, high-quality fuels such as gasoline, diesel, and jet fuel is getting harder. Moreover, newly discovered oil contains ever-higher concentrations of sulfur, a contaminant that, when burned, produces gases that are now strictly regulated because they interfere with pollution-control systems in vehicles and contribute to acid rain and smog.

Processes now used to upgrade and desulfurize heavy crude oil are expensive and energy-intensive, and they require hydrogen, which companies typically produce from natural gas — a high-cost process that consumes valuable gas resources and releases high levels of carbon dioxide (CO2). So there’s a lot of interest in finding alternative processes for converting low-quality crude oil into valuable fuels with less residual coke and for removing the sulfur efficiently and economically without using hydrogen

One approach calls for using water rather than natural gas as the source of the hydrogen molecules needed for key chemical reactions in the refining process. Ordinarily, oil and water won’t mix, so the molecules can’t “see” one another and chemically react. But using “supercritical” water solves that problem. Under extreme conditions — specifically, at pressures and temperatures above 220 atm and 375 degrees Celsius — water goes into a supercritical state in which it is as dense as a liquid but spreads out to fill a confined space as a gas does. Add oil to supercritical water (SCW) and stir, and the two will mix together perfectly, setting the stage for the desired chemical reactions — without any added hydrogen from natural gas.

Industrial and academic researchers have demonstrated that mixing heavy oils with SCW produces lighter hydrocarbons (compounds of hydrogen and carbon atoms) containing less sulfur and forming less waste coke. Combining those new insights, the researchers are developing new computational tools to help guide energy companies that want to implement the new process. “