THINK of a flickering will-o鈥�-the-wisp, or a weak blue flame dancing on your brandy-soaked plum pudding. Now imagine an even fainter flame, one so insubstantial that you can barely see or feel it 鈥� less a flame, more an unusually lively chemical reaction. This is a cool flame, a remarkable phenomenon created by gentle oxidation rather than fully fledged combustion. First identified nearly two centuries ago, cool flames were long regarded as a mere curiosity. But in the last few years they have become one of the hottest things in combustion research.
Engineers are using cool flames to revolutionise heating systems and boilers 鈥� improving fuel efficiency, allowing them to run on a variety of fuels and helping clean up their emissions. Cool flames can also be used as chemical processors to produce hydrogen for use in fuel cells. They are even coming to the aid of vehicle engines, potentially transforming them into cleaner, greener machines.
Humphry Davy was the first to notice cool flames in 1805 when he discovered he could oxidise diethyl ether using hot platinum as a catalyst. Some seventy years later, in his lab in Leeds in the UK, William Perkin achieved the same effect by dropping ether onto a bed of hot sand. After he blocked out all other light sources, he was just able to make out a pale blue glow. Showing that the flame would not scorch, first using his fingers and then with paper, he settled on the description 鈥渃ool flame鈥�.
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A cool flame is distinguished from a conventional flame by its chemistry. Combustion produces heat by unlocking the energy trapped in the chemical bonds of a substance. Once initiated, it is self-sustaining, as some of the energy released heats the material around the combustion zone, initiating further reactions. Reactive chemicals created in the flame also promote combustion by a process called chain branching. For example, one energetic hydrogen atom can react with an oxygen molecule to form two reactive species 鈥� a hydroxyl radical (OH) and an oxygen atom (O). These can then react with other molecules, quickly creating an avalanche of radicals.
But a cool flame is different. It can develop in most vapours of organic chemicals, including aldehydes, ethers and alcohols, and forms spontaneously when the temperature of a mix of air and fuel vapour reaches around 250 掳C. Like conventional combustion, the chemical processes in a cool flame release some heat, and involve a branching process. The reaction can even accelerate to create a normal flame in a process called spontaneous ignition. More usually though, as the temperature rises, branching reactions gradually switch off. They are replaced by slower processes such as the one called chain propagation in which a single reactive species is generated. The reaction rate stops increasing, the temperature stops rising and the cool flame stabilises at around 500 掳C.
So why didn鈥檛 Perkin burn himself? Normal flames reach temperatures of at least 1000 掳C and much of the heat they release is radiated by minute but extremely hot soot particles. However, the chemistry of cool flames means they do not form soot, so heat transfer is very inefficient. To a passing hand, a cool flame would feel no different from momentary exposure to the air in a very hot oven.
A cool flame鈥檚 ability to stabilise at relatively low temperatures is prompting engineers to employ them in a new generation of oil-fired boilers. In conventional systems, fuel is injected into the combustion chamber as atomised drops of oil and the flow is adjusted for optimum burning. This makes it difficult to adjust the boiler鈥檚 power output because combustion becomes unstable, and therefore less efficient when the oil flow rate is altered. You can regulate a boiler鈥檚 output just by switching it off and on, but this too is inefficient.
So a research project called Bioflam, funded by the European Union, has come up with a system in which heavy fuel oil is vaporised, rather than atomised, before burning. The oil is sprayed into an insulated chamber preheated to 250 掳C, and mixed with air. A cool flame develops and, stabilised by the cool flame鈥檚 chemistry and by a small portion of air and fuel-vapour mixture which is recirculated, the temperature in the chamber settles at around 500掳C 鈥� high enough to vaporise the incoming oil drops before they pass into the burner.
The burner is a new design made from a highly porous ceramic structure. Developed by engineers at the University of Erlangen-Nuremberg in Germany, it is riddled with tiny pores 鈥� the perfect environment in which to burn fuel cleanly because it reduces the temperature variations that generate pollutants. And with a stable flow of vaporised fuel, the burner can function like a gas-fired, rather than oil-burning, unit 鈥� that is, it will operate at optimum efficiency over a wide range of power outputs. In tests this was proved true with outputs from 3 to 30 kilowatts.
Thanks to the cool flame and the new burner, the unit produces half the nitrogen oxide emissions of a conventional oil-fired boiler, but is 10 per cent more efficient. Vaporisation also means the design can run on a wide range of fuels from recycled cooking oil to biofuels made from soya and rapeseed. To prove the concept, Bioflam units will be tested in European households this year.
The researchers have also combined a cool-flame vaporiser with a heated catalyst to create a fuel 鈥渞eformer鈥�. This unit converts fuels such as diesel oil into a source of hydrogen that is pure enough to be fed directly into a fuel cell to power electric vehicles. Although the device is still under development, the idea is that it could eventually act as a stopgap until hydrogen becomes widely available at filling stations.
Engineers already know that cool flames are a feature in both diesel and petrol engines. But understanding how cool flames ignite and create full combustion is changing the way they think about internal combustion engines.
In the cylinder of a diesel engine, for example, fuel is mixed with air that is heated by compression. This generates a cool flame, which converts spontaneously to full combustion. Petrol engines, on the other hand, use a spark to produce full combustion, but under certain conditions a cool flame can form. This ignites the mixture prematurely 鈥� a problem called knock 鈥� and is usually prevented by including knock-resistant chemicals such as toluene in the fuel.
However, researchers are designing dual-mode engines that use aspects of both systems. The controlled auto-ignition (CAI) engine works like a diesel engine when idling but when the driver steps on the gas, a spark plug in each cylinder begins operating, allowing the engine to generate more power.
CAI engines have a number of advantages. Firstly, they work with very low fuel-to-air ratios, which means remarkable fuel economy 鈥� as good or better than the best diesel engines. This also creates less soot. In addition, the use of a cool flame to initiate smooth combustion contributes to creating lower temperatures in the cylinder, generating less nitrogen oxides. A CAI engine could also run using a wide variety of fuels.
There are major technical problems to overcome. The CAI engine must be able to operate under a variety of conditions, and the temperature and pressure in the cylinder needs to be controlled to create a cool flame at precisely the right moment. One method under development is to recycle warm exhaust gases, but the overall solution is a mix of sophisticated timing and valve control systems that can measure the ambient temperature and quickly adjust the fuel, air and exhaust gas mix.
Another problem is the fuels used in today鈥檚 vehicles are complex mixtures of hydrocarbons whose cool flame chemistry is not fully understood. Chemists are evaluating different mixtures to see which work best in CAI engines.
And researchers still do not know exactly how cool flames develop into full-blown combustion. An important way to study them is in microgravity because this eliminates convection currents and simplifies the processes of heat and mass transfer, making the physics and chemistry easier to unravel. 快猫短视频s at NASA are already performing these experiments, and they hope to carry out definitive research aboard the International Space Station within the next six years.
Despite the hurdles, Japanese car maker Nissan is already marketing an engine that uses elements of the CAI design. And with fuel cell-powered vehicles still at least a decade away from widespread use, there is lots of time for Perkin鈥檚 chemical 鈥減arty trick鈥� to make a vital contribution to green technology.
