Clean heat a hot topic

Modifying burners to flameless combustion can drastically reduce the emissions of industrial furnaces. Research is ongoing, while industry cautiously innovates.

Lasers scan the base of the flame. (Photo: Sam Rentmeester/FMAX)

“Imagine a steel cube, the size of a housing block, filled up with burners spewing six meter long flames along steel pipes filled with chemicals that need to be heated up. Or think of the steel furnaces where thick slabs of steel roll through a hall, waiting to be reheated by a 100-megawatt fire before moving on to the next processing step.” When prof.dr Dirk Roekaerts (multi-scale physics, Applied Sciences) talks about furnaces, he has huge installations in mind. Perhaps equally large is the challenge that he and his colleagues face: to make these furnaces more environmentally friendly by reducing the emissions (of NOx especially) and to increase their efficiency.

Last week Roekaerts' colleague, associate professor Bassam Dally from the centre for energy technology at the university of Adelaide, Australia, gave a lecture on Mild combustion -  a clever acronym that stands for moderate or intense low oxygen dilution and signifies a special type of burning that is also known as flameless oxidation or Flox. Characteristically, fuel is not – as is normally done – mixed with cold air, but rather with warm exhaust gases containing just a few percent of oxygen. The effect is a less sudden jump in temperature and a delayed ignition, resulting in a more equal spread of the heat, lower peak temperatures and reduced noise levels. “Oftentimes, you only hear the hiss of the gas flowing in,” says Dally. Roaring flames will soon be regarded as an old-fashioned, wasteful practice. In fact, often there is no visible flame at all, but rather only a steady orange glow radiating from the oven. “At lower temperatures the radicals don't emit radiation, and hence there is no light,” Dally explains.

In Delft, two different groups are working on Mild combustion. PhD student Ernst Oldenhof performs measurements on the 'fundamentals of the flame' in prof. Roekaerts' group at the faculty of Applied Sciences. The research at the energy technology group (Mechanical, Maritime and Materials Engineering) on the other hand, is more applied. A big closed steel box containing three burners of 100 kilowatt each has been built here to mimic industrial processes and help develop adequate computer simulations.

Oldenhof's lab is located in the basement of the Applied Sciences building. Once inside, you see a pale orange flame quivering in the stream of hot but low-oxygen air that surrounds the fuel jet. Laser beams cross each other at the base of the flame. “We want to know how a flame is formed and how it stabilises”, Oldenhof explains. “Turbulence and chemical reactions are occurring, but how do they influence each other?” To find out, Oldenhof measures the velocity and temperature of the gas and the presence of radicals (highly reactive half-burnt fuel remnants) by shooting laser beams at various points through the flames at different mixes of fuel and oxygen.
In the furnace at the energy technology group, Mild combustion is achieved by recirculation of the exhaust gases. “This spreads the reaction evenly throughout,” Dr. Wiebren de Jong explains. “It eliminates peak temperatures of 1500 °C and more”.

Lower temperatures are vitally important for NOx-emissions reduction, Roekaerts explains. The emission target for burning methane is set at a concentration of 10 ppm (parts per million).  But high temperature industrial furnaces (for glass melting for example) can reach up to 1000 ppm.
Roekaerts and De Jong both work with Corus and Shell to optimise their furnaces. “We do have suggestions for how to adapt their burners,” says Roekaerts, “but this will change the spread of heat.” This seems less of a problem at Corus, where slabs of metal merely need to be reheated, than it is at Shell's petrochemical plant, where a different heat distribution might influence the chemical processes.

In Australia, industry is even more cautious to adopt the new technology, Dally says. And, he adds, as long as there are no legal limitations on the exhaust gases, there is little incentive for industry to change.