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Posted: January 22, 2007

Metal fuel

(Nanowerk News) Cheap and constant availability of fuel is a major concern of present day civilization. Different efforts are being directed in this direction with a rising concern about environmental degradation also. Rocketing fuel prices, pollution-choked streets and global warming could change the relationship between transportation and mankind in coming times. People have already experienced importance of concentrates in items like soap, paste, washing powder etc. and it will be of great importance if such concentrates become available in terms of fuels that fullfill qualities like, production of more energy with small amount, cost-effective, easy availability and environmental friendly and will prove to be the fuel of the future. Metal nano-particles are the promising candidates in this direction.
Researcher at Oak Ridge National Laboratory in Tennessee (USA), have come up with a plan to transform the way we fuel engines. Chunks of metal such as iron, aluminium or boron are the thing, they believe can be used as fuels by turnning them into powder with grains just nanometres across and the stuff becomes highly reactive. On ignitition it will release copious quantities of energy. With a modified engine and a tankful of metal, they predict that an average saloon car could travel three times as far as the equivalent petrol-powered vehicle. Better still, because of the way that this metal nano-fuel burns, it is almost completely non-polluting. That means no carbon dioxide, no dust, no soot and no nitrogen oxides. What's more, this fuel is fully rechargeable as spent nanoparticles can be treated with a little hydrogen or something else and the stuff can be burnt again and again. It could spell the start of a new fuel age. All kinds of engines, from domestic heating units to the turbines in power stations, could be adapted to burn metal.
Burning a heap of powdered metal (e.g. iron) releases almost twice as much energy as the same volume of petrol, and replacing iron with boron gives five times as much. When granules of metals such as iron and aluminium come into contact with air, they become coated with a layer of oxide that must be removed before the metal can ignite. To kick off combustion in most metals, we need a heat source with a temperature of at least 2000°C, which is high enough to vaporise the oxide layer and expose the bare, reactive metal beneath. That might be fine for a rocket but it's not so simple for an automobile engine. Another problem is that once the vapourised metal oxide starts to cool, it solidifies and forms ash. High temperatures and clouds of ash present no problems in a oneshot rocket but they create a serious mess for anyone trying to burn metal powder in an internal combustion engine.
Initially when scientists tried burning micrometre-sized iron particles in an internal combustion engine, they modified the engine to work at high temperatures but found that the ash deposited on the pistons, cylinder walls and valves, clogged the engine. Scientists took a fresh look at the problem, using nanoscale particles not much larger than single atoms. In experiments, they found that iron nanoparticles measuring about 50 nanometres across ignited far more easily than the larger granules of iron and heating them to around 250°C or even just a spark, could do the job. The more they looked, the more they realised that the nanoparticles behaved in a very different way compared to the micrometre-sized particles. Nanoparticles burn much more easily because their surface area to volume ratio is huge. Iron reacts very readily with oxygen, so if a lot of it is exposed to air at the same time, oxidation can generate enough heat to spontaneously ignite the metal. To prevent this, nanoparticles are usually given a protective oxide coating during manufacturing. But even with an oxide layer, the huge surface area of these nanoparticles means that with just a little heat, it is easy for oxygen molecules to mix with the metal and trigger combustion. One consequence of this is that once the nanoparticles are ignited, they burn rapidly and the temperature peaks at about 800°C - hot enough to do useful work but not so high as to melt an alloy engine. And crucially, unlike the micrometre-sized particles, nanoparticles don't burn hot enough to vapourise or even melt. They just oxidise, leaving a heap of oxidised nanoparticles. And that means no sticking to the walls of the cylinder, and no clogged engine. The tidy heap of iron oxide left over from the combustion process can be easyly converted back into usable fuel.
Individually, nanoparticles burn in a flash, releasing all their heat in a millisecond or so. But to make the metal fuel useful in engines, the rate of heat production should not be so fast that an engine cannot deal efficiently with the heat produced. So the team attempted to limit how quickly their fuel burnt by pressing the nanoparticles into larger clusters. By creating nanoparticle clusters weighing anything from one to 200 milligrams each, and by adjusting their size, shape and density scientists could control the burn rate. While single particles would burn in just milliseconds, the largest clusters could take from half a second to two seconds. With the first stage of the research complete, the team now plans to design an engine that can run on the fuel. It would be relatively easy to convert engines such as the gas turbines that power jet aircraft and vehicles such as tanks, or even those used to generate electricity in power stations only with a need to find a way to collect the spent fuel. Another option is to use the fuel to power a Stirling engine. A modified diesel engine might be able to burn nanoparticle powder as a fuel, just as a conventional diesel engine uses a mist of diesel fuel. The burnt fuel might be collected using a filter or using an electromagnet, since iron oxide powder is magnetic. The result would be an engine similar to a conventional one, but which emits no carbon dioxide, harmful particulates or even nitrogen oxides. These compounds usually form in combustion at high temperatures, but maintaining combustion temperatures to about 525°C by varying the size of the clusters, formation of harmful compunds can be controlled. However, work is still needed to strike the right balance between temperature, speed of combustion and engine efficiency. A vehicle running on metal fuel should please both drivers and environmental campaigners.
Metal fuel is a more convenient, safer, and more practical energy carrier even than hydrogen. Metal fuel is stable at room temperature, so it is easy to store and transport. Scientists are satisfied that the technology itself is sound, but believes there are fundamental difficulties with metal as a fuel. Although iron is a compact fuel but it is also extremely heavy, and even though its high energy content allows to almost halve the size of a typical 50- litre fuel tank to extract the same energy, a tank of fuel would weigh about 100 kilograms - more than twice as heavy as the petrol it replaces. And because the spent fuel is kept on board, unlike the polluting by-products of conventional fuel, this weight won't decrease. The weight of fuel will also add to the cost of transporting it to and from recycling facilities. Use of aluminium nanoparticles rather than iron will solve the weight problem and we will get about four times as much energy per kilogram. With boron we will get almost six times as much. Of course, since these metals cost more than iron, the fuel would be more expensive in the first place. Clearly it is very early days for metal power and problems like: to build a prototype engine, cost-effectiveness of metal fuel and availability have to be investigated. Experiments to optimise the size of nanoparticles, as well as to investigate the best way to package, inject and collect the burnt stuff in a real engine are being conducted. If becomes practical in the future, scrap yards full of yesterday's automobiles may be transformed into fuel for the vehicles of tomorrow.
Source: Central Chronicle, Dr SS Verma, Dept of Physics, SLIET, Longowal
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