Smelting Solar Steel for Mass Production and Recycling

The following is a public-domain invention for the smelting and recycling of steel using concentrated solar power, as well as the manufacture of the coke used in blast furnaces. The production of glass and cement, the kiln drying of wood and the recycling of glass, aluminum, copper and plastic by similar means will be covered in a followup article.

Why is it critical to produce or recycle these materials in a renewable manner? In addition to basic questions of climate change, most people are unaware of how rapidly the supplies of non-renewable, high-energy coal needed to power conventional processes such as steel production are depleting, despite the devastation of mountaintop-removal mining. Hence, alternatives will soon be needed, and now -- thanks to this innovation -- the public has a few more in hand.

Solar Steel

The world’s existing solar furnaces already demonstrate the principle which makes solar-based steel production possible. The largest solar furnace in the world – a paraboloid concentrator mirror 40 meters by 54 meters that is illuminated by a further 63 heliostat mirrors – can reach 3,000 degrees Celsius in the Pyrenees and melt steel placed at the focus of all that combined solar energy. Unfortunately, that focus is apparently about the size of a cookpot.

In order to use solar energy to manufacture high-melting-point materials, say, steel, we need a system that generates a great deal of raw heat, and that distributes it a bit more effectively simply focusing it on a fixed point. Frankly, if our solar furnace’s output, however impressive, is confined to a cookpot, then our steel production may end up confined to one as well.

So visualize instead that your prospective foundry is in an area with strong sunlight, such as a desert. There, on a relatively flat plain, you have either a large, round building or a large, round, partially excavated hill. Certain external features of this site are critical to the solar furnace we will be powering inside of it, out of sight.

Since it would be very difficult to heat iron ore or molten metal directly with sunlight, what we will do instead is use a column of superheated air to do our work for us. The problem with trying such a column using an ordinary solar furnace, even one as monumental as the one in Font Romeu, France, is that the constant airflow could not be easily heated by just one mirror acting on a single point as the air roars through it. Instead, we have to get creative and mobilize more energy to affect the entire airflow along the whole length of its passage, giving us the time and raw heat necessary to achieve the kind of effect we are looking for.

So, visualize this plant as though you are there, facing this hill or round building. To your right you can see, among other things, a long, dark path approaching the foundry, covered in a long, half-cylinder of glass. This glass is actually double-layered to increase its insulative properties, and may even have a few relatively small mirrors focused on it. It is open at its far end, where unheated air flows into the system. What this transparent “tail” does is draw desert air by way of a continuous suction (one easily amplified by an air pump further along the passage). Light shining on the black “path” inside is absorbed by that dark, heat-emissive material and radiated back out as heat. The glass traps that heat inside the ever-hotter tunnel (however imperfectly), and the flow of air keeps it moving towards your foundry. Much like a dark car sealed up on a hot summer day, especially in a particularly warm latitude, this area quickly becomes unbearably hot. As the tunnel gets closer to the plant, larger, more powerful heliostat mirrors concentrate their light on sections of the path. The rays of these curving mirrors are oriented to reach their focus inside the tunnel, sparing its glass housing the full power of their concentrated rays on any one point, even as temperatures rise to ever more formidable levels.

The air then flows up to the outside wall of your foundry itself, and begins circling up the side. The tunnel it follows regularly penetrates the hill/building, but not deeply, insulating the airflow for much of the distance to retain its heat, but exposing the passage occasionally for yet more solar heating. Periodically the wall or rammed earth beside the tunnel will open up. These openings may, in fact, include more large, double-paned, domelike windows, but in any event will be able accept more sunlight that can be focused on a point on the lining of the rising air pipe. In the wall of that tunnel, you may find it wise to set a slab of dark material that is highly heat-emissive and extremely resistant to heat. Obsidian may serve this purpose, or some other tough, black material. The tunnel will constantly spiral upward as it moves, causing the hot air to naturally flow up its open passage.

Finally, as you approach the final bend in your tunnel, the two great paraboloid concentrator mirrors will make their presence felt. Each of these mirrors will have its own set of heliostat mirrors directing sunlight towards them, and each will be focusing all their light continuously on the last and second-to-last exposed points on the tunnel’s route. They will be tipped up slightly (thus trapping the solar-born heat they transmit against heavily insulated ceilings), and each will be hitting an embedded, dark object that will be emitting heat in turn back into the passage. Obviously, we have been using this succession of solar heaters on the airstream to steadily increase its temperature as it rises. At this stage, if you have done everything right, your air should be ready to go into the blast furnace.

Conventional Versus Solar Foundry Design

The “hot blast” of pre-heated, oxygen-enriched air entering a modern blast furnace ranges from 900ºC to 1300ºC and passes over a mixture of iron ore, coke and limestone. Oxygen reacts with the coke to form carbon monoxide (CO) and greater heat, boosting temperatures to almost 2,000 ºC – hot enough to melt the ore. The carbon monoxide then reacts with the molten ore by stripping away its oxygen and forming carbon dioxide and molten iron. The limestone, meanwhile, combines with the impurities to form liquid slag, which is easily removed as it floats on top of the liquefied metal.

This flood of extremely hot air is the only thing you will be replacing in your blast furnace. Everything else will remain the same, including the inputs of coke, limestone and ore. (But see the technique for recycling scrap steel below, which requires only the air and steel.)

If you are using the “basic oxygen process” to produce bulk steel, then no further modifications are needed in your foundry save for driving the oxygen lance into your metal and providing the mechanical power to pour and move the molten steel and slag. Use steam heated by the cooling metal (by way of pipes of water vaporized by their proximity to the molten mass) to provide the force to turn your steel vessel directly (employ vanes or better still a turbine moved by the vapor) combined with an electrical assist, as necessary. The steam can drive a generator also, of course, and thus provide that additional electricity.

You can control the temperature of the blast furnace’s incoming air by restricting how much of the superheated air flows into it once that air has surpassed the temperature you need, be it 900ºC or 1300ºC. That reduced airstream can be mixed with a flow of cooler air as a simple method of temperature regulation. Meanwhile, the rest of the original, still undiluted air you have been heating can be directed to other purposes requiring higher temperatures, such as the coke ovens mentioned below.

This means of regulating air temperature can also be used to heat steam turbines, or – in the case of particularly large operations – to slowly turn on excess capacity elsewhere in your plant… whether the coke ovens, an additional blast furnace, or something unrelated to your main work, such as glassmaking. Alternatively, if your airstream can not quite reach the temperature levels required (because your environment lacks sufficiently strong sunlight, or you happen to be operating on a particularly overcast day), you can use another, more conventional energy supply to augment your solar-heated airflow, such as natural gas.

Waste heat in conventional plants is usually removed through water cooling. In our case we might use it to power turbines, using water or ethylene-glycol mixtures to carry the energy away, or else we might store the raw heat in molten salt for nighttime heating. The turbines, incidentally, could again provide mechanical power as well as electricity, driving gas compressors and other critical equipment. You could also wrap a coil full of discharged gas from the blast furnace or excess airflow around other pipes to pre-heat fuel and air inputs.

Recycling Steel

Cheaply recycling steel with this solar system is indeed a “blast from the past,” because the simplest recycler design looks like nothing so much as an ordinary blast furnace. The principles are very simple.

Less than 1600ºC will liquefy iron or any normal carbon steel. This detail is important, because if you can reach those temperatures consistently in your solar-heated airstream (say, in a desert location) you can easily melt scrap steel into liquid, making the recycling of steel possible at an incredibly low cost. How do you do this?

Because most steel actually has a lower melting point than raw iron, all you need to do is heat your airstream to roughly that 1600ºC level and then channel it straight into a structure very similar to an iron blast furnace. But in the top of that furnace, instead of dropping in iron ore, coke and limestone, you simply pour in your scrap steel. Meanwhile, your airflow, much like the waste gases of a blast furnace, travels up the entire length of the vessel before leaving, pre-heating all of the incoming metal as it goes. Since none of your steel can resist liquefying at 1600ºC, you just have to reach or exceed those temperatures in your incoming airblast.

Simply alter the bottom of the structure so that the bottom of the furnace slopes down to a narrower, open pipe, which your scrap will only flow down once it has liquefied. If you’re worried about getting stray bits of augered metal down that pipe before they’re properly melted, merely drop some larger pieces in first at the beginning of each day. Once they have melted down they will unblock that discharge pipe, and at that point none of those smaller bits of scrap will slip through a superheated pool of molten metal unaltered.

A Point Worth Considering

Many people make a mistake in considering renewables – the assumption you have to generate electricity with them in order to accomplish any important work. Other applications are often seen as quaint or useful for the technologically backward, but not relevant to advanced societies and especially not to large cities or industrial projects. But using mechanical or thermal energy directly is frequently the most efficient way to use your renewable source of power.

Coke Ovens

How is coke, a key element in iron manufacture, created? To quote the World Bank, “In the coke-making process, bituminous coal is fed (usually after processing operations to control the size and quality of the feed) into a series of ovens, which are sealed and heated at high temperatures in the absence of oxygen, typically in cycles lasting 14 to 36 hours. Volatile compounds that are driven off the coal are collected and processed to recover combustible gases and other by-products. The solid carbon remaining in the oven is coke. It is taken to the quench tower, where it is cooled with a water spray or by circulating an inert gas (nitrogen), a process known as dry quenching. The coke is screened and sent to a blast furnace or to storage.”

One piece of that process could be helped by our solar airflow – heating the ovens themselves. During the “heat of the day” high-yield solar heat sources can use their excess thermal output to heat coke ovens in addition to driving the site’s blast furnaces. Alternatively, coke production can be performed separately, either nearby or at an entirely different location.

The production of coke has many environmental problems besides the fuel normally used in its manufacture, which is one reason I have also included a steel recycling process in this series which naturally requires no coke whatsoever. But for those foundries wedded to the process, a solar variant should at least modestly reduce their pollution output.