Fighting oil spills with fire
Traditional methods for cleaning up oil spills, like collecting the oil with mechanical skimmers and breaking it up with chemical dispersants, are often ineffective and can harm marine life. Oil spill experts have long believed that the fastest and most effective way to remove oil from water is to burn it. But while decades of research has shown that in-place, or in-situ, burning (ISB) can remove up to 90 percent of spilled oil, the clean-up method has seen only limited use.
The most extensive real-world application took place following the Deepwater Horizon disaster in 2010. About 5 percent of the 4.9 million barrels of crude that spilled into the Gulf of Mexico was burned in a large-scale operation overseen by an interagency group. The campaign was deemed a success, but it also reinforced problems with ISB that the earlier research had identified.
For one, the oil mixes with water to form an emulsion that can be difficult to ignite. Second, cold water acts as a heat sink, making it challenging to sustain a fire. Third, soot formation in the center of the fire limits its temperature and leads to incomplete combustion and thick black smoke laden with particulate matter that can be a hazard for first responders.
In March 2017 a technology that overcomes all of these limitations – and holds the promise of making ISB a truly viable option for cleaning oil spills – underwent its first large-scale, real-world test. Called the Flame Refluxer, the system, developed over the course of more than three years by a research team at Worcester Polytechnic Institute (WPI) in Worcester, Mass., is designed to make ISB easier to conduct, more efficient, and cleaner. Simple, inexpensive, and easy-to use, the Flame Refluxer could revolutionize how spills are mitigated.
The Birth of an Idea
Much of the early interest in ISB arose from worries about how to clean up oil spills in the Arctic. In situ burning seemed the only practical solution in an inaccessible region covered by ice and snow much of the year. But while the approach made sense, there was no guarantee that oil would burn while sitting on top of ice or icy water. In 2014 the U.S. Department of the Interior’s Bureau of Safety and Environmental Enforcement (BSEE) awarded WPI the first of a series of awards that now total more than $1.5 million for research on ISB. The initial grants funded laboratory studies, led by Ali Rangwala, professor of fire protection engineering, aimed at answering fundamental questions about the combustion of oil in Arctic conditions.
Starting with test burns on disks of ice the size of hockey pucks and advancing to burns of one-meter-square pools of crude oil surrounded by a field of ice blocks, the team made a number of groundbreaking discoveries, including the fact that burning oil sinks into ice, creating widening pockets that thin the oil out, making it more difficult to burn. They also found that ice and frigid water create thermal inertia, making the oil harder to ignite and the fires more difficult to sustain.
Rangwala wondered if there might be a way to warm the oil to make it more volatile. “Typically, with in situ burns, about 95 percent of the heat goes up into the atmosphere by buoyancy,” he says. “I knew that if we could transfer some of that heat to the fuel we could create a feedback loop, enhancing the burning rate, creating still more heat, to enhance the burning rate further, and so on.”
He started experimenting with metal rods – which act as collectors, absorbing heat from the flames – and heaters, promoting vaporization of the oil. The next step was to find a way to more effectively spread the refluxed heat through the oil. That led to replacing the heater portion of the rod with a blanket made from a layer of copper wool sandwiched between two sheets of copper mesh. The final innovation was transforming the simple rods into metal coils to increase their surface area and their contact with the flames.
Tests of prototypes of various sizes and with varying numbers and configurations of coils helped refine the design and demonstrated a key benefit of the Refluxer. Heat trapped in the blanket produces nucleate boiling in the oil, which leads to much greater vaporization, hotter flames, and lower soot production. The next step, with a $1 million award from BSEE, was to test the Flame Refluxer outside under real-world conditions.
Trial by Fire
The large-scale tests were conducted at the Joint Maritime Test Facility on Little Sand Island in Mobile, Ala., which is operated by the U.S. Naval Research Laboratory and the Coast Guard Research and Development Center. There, in the 1970s, the Coast Guard built a large steel burn pan (33 by 11.6 meters and 1.5 meters deep). Seriously damaged during Hurricane Katrina, the pan was restored and ready for service again by the fall of 2015. A year later, WPI would become the first university granted use of the reopened facility.
Over the course of a week, the WPI team conducted six test burns – overseen by Rangwala and postdoctoral researcher Kemal Arsava – inside of a test stand consisting of a metal ring 1.4 meters across, resting on four legs just over a meter tall, which was positioned inside the burn pan. A network of pipes delivered crude oil to outlets beneath the stand while a camera below the waterline helped keep the oil layer at about one centimeter. The stand was equipped with dozens of thermocouples to measure the temperature of the flames, oil, and water, as well as instruments to monitor heat flux. An innovative emissions sampling system used a wind sock to funnel smoke from the fires to an analyzer that recorded levels of oxygen, carbon dioxide, and carbon monoxide.
The first burn, a baseline test without the Flame Refluxer, was followed by a test with the blanket alone (no coils) and three tests with various coil configurations: one with 24 20-centimeler-tall coils arranged into a ring at the outer edge; one with 48 20-centimeter-tall coils arranged into two rings; and one with an outer ring of 24 10-centimeter-tall coils and an inner ring of 24 20-centimeter coils.
Even without glancing at the data, it was clear that the Refluxer had made a difference. In contrast to the thick black smoke that billowed from the baseline burn, the smoke from the Refluxer tests was thin and gray, and it dissipated quickly. And when the fuel flow was shut off, the baseline fire burned on for another five minutes, but left behind about 20 kilograms of unburned oil (or about 32 percent of the oil used in the test). Because so much heat was retained in the porous blanket, it took more than an hour for the last flames to die out after the Refluxer burns, and just a kilogram (or less that 2 percent) of the oil used in the test remained unburned.
In their final report to BSEE, the WPI team reported that the Flame Refluxer had performed at least as well as expected. In the best test cases (the tests with the blanked and two rings of coils), the Refluxer increased the burning rate by six times. (Rangwala says he believes that if the Refluxer is scaled up from 1.4 meters in diameter to 18 meters, that a 10-fold increase in burning rate can be achieved). With the faster burning rate, the Refluxer consumed about five times more oil per minute than the baseline burn. And in addition to the 15-fold reduction in unburned residue, the Refluxer cut carbon emissions by 50 percent.
“We have high hopes for this technology, which can make a good technique even better,” Karen Stone, oil spill response engineer at BSEE, said as the week of tests wrapped up. “There is more work to be done, particularly in thinking about how the technology can be deployed in practice, but we are very pleased with what we’ve seen this week.”
As he thinks about where the research may go next, Rangwala says he is interested in finding ways to further refine and optimize the Refluxer. But regardless of what shape the device finally takes, Rangwala says the research will continue to be motivated by the same goals that have driven his team until now. “The longer you let oil linger in the environment, the greater the danger becomes,” he says. “That’s what our work on ISBs is aimed at: reducing the clean-up time to a minimum, and removing the maximum amount of oil from the environment. With each advance, we get closer to those objectives.”
For more information, go to www.wpi.edu/+FPE