.
TMB flame   
BORON: A BETTER ENERGY CARRIER THAN HYDROGEN?
Graham Cowan
...

If far-flung energy users each possess several tens of kilograms of the somewhat costly, somewhat toxic substance diboron trioxide (also known as boria), and occasionally send some to a central power station that can reduce it to its elements, that station can, by returning only the boron, transmit back one continuous kilowatt of power for each 5.1 kilograms per day of boria received.

That rate of active material shipment is much larger than in the analogous hydrogen-economy power coupling, even though a water-splitting power station needs not 5.1 but 6.8 kilograms of water per kilowatt-day transmitted. The Earth's atmosphere is an enormous water vapour bank the station can borrow from and the users repay, and therefore the only shipping task is distributing the hydrogen. Its mass is 0.76 kilograms per kilowatt-day.

Elemental boron is now produced at very high prices, 23 to 400 times more for a pound without oxygen than for a pound with, $360 to $6,400 per contained kilowatt-day.

Its use as a potentially mass-market fuel has not been demonstrated. One promising approach is a gas turbine that would burn it in a large excess of pure high-pressure oxygen. Problems would include fabricating boron -- brittle, refractory, very hard -- in a form amenable to service as fuel, making the combustion chamber and rotor of materials that neither pressurized oxygen nor boria will attack, providing the oxygen, and extracting the boria (for the user to send to the power station again).

Poisoning deaths have occurred with boric acid, which forms spontaneously when boria and water meet at normal environmental temperatures:

(1/2) B2O3 + (3/2) H2O(l)    --->    B(OH)3, DG = -22.03kJ/mol

Its acute oral LD50 in rats is listed as 3 to 4 grams per kilogram of body weight (*1). Taking into account the 0.563 mass fraction of boria in boric acid, and supposing boric acid acts similarly in rats and humans, the extrapolated acute LD50 by ingestion of boria is 1.7 to 2.2 grams per body kilogram, i.e. 75 grams for someone weighing 100 pounds. This is twice the acute toxicity of ingested sodium chloride.


Motivation

To enable noncombustion primary energies -- nuclear, direct solar, hydraulic, wind -- to take over the jobs petroleum and natural gas now do, without adopting their safety standards.

After these hydrocarbons have been taken out of the ground, where they were not in contact with air, it is best to keep them still shut away from it for as long as they are in processing, distribution, or storage. Every week this is attempted on a worldwide scale of roughly 100 million tonnes, seldom without at least one disastrous lapse.

Elemental hydrogen is produced on a scale four orders of magnitude smaller, and also kept apart from air with great but occasionally inadequate care. Lethal failures occur, perhaps even somewhat more often than once in 10,000 weeks. It is therefore just as incongruous to imagine nuclear power stations producing it as to imagine them producing gasoline.

Water's great abundance is sometimes called an advantage of hydrogen power. But there are other very abundant, very strongly bound oxides. Let us pick one whose deoxidation yields a safer fuel. Corundum and quartz fit the bill, and maybe periclase. The oxophilic elements in them are respectively aluminum, silicon, and magnesium. They do not have to be kept out of contact with air, yet when they burn they burn very strongly.

Burning regenerates the oxides as fine particles that would not be as inoffensive to dump as the original minerals. If they were first consolidated into gravel, then they might not be too much of a nuisance. A large urban intersection thronged with cars powered by oxidation of metal or silicon would see the accumulation of approximately one cubic metre per day.

Hot-pressing equipment to make that gravel on board cars would be expensive. Perhaps some kind of glue could be used, or bags. In any case, these energy carriers would either accumulate after use as large heaps of oxide, or go back to the power station to be deoxidized again.

But if they go back then conceivably they can be used many times, and more expensive, lighter varieties of atom can work. Two elements are lighter than magnesium and, when uncombined, share aluminum's and silicon's trait of burning strongly but not readily: beryllium and boron.

The pursuit of safety has thus with apparent perversity led to toxic elements. In boron's case there are reasons to think that appearance is false, and the prospect that its use will give much safer combustion power than hydrogen or hydrocarbons can is real.

One of these reasons stems from the fact that unlike beryllia or magnesia or alumina or silica, boria does not emerge from a hot flame as suspended solid particles. It emerges as mist. This promises to let it be gathered more easily and effectively than those other oxides could be. Bags may still have a role to play, but hot pressing and glue don't. Above 500C boria is its own glue.


Boria's Unusual Nature Seems Tailored for Extraction from Combustors

Boria can be liquid above 2,000C and still flow at 400C. There is some discord in published values of its normal boiling point -- 4,600 Rankine (*2), 1,860C (*3), 2,316C (extrapolated) (*4), 2,065C (*5).

However, when boron burns in a 25-fold excess of oxygen, the products cannot, after mixing, be much hotter than 1,200C. At such a temperature virtually all of the boria must be incorporated in droplets. In the middle of the 20th century air-breathing turbines fuelled partly with boron were tested in the apparent hope that these droplets would blow by the turbine airfoils and not stick.

It now seems more reasonable to hope that they will; or at least, it does if turbines can be constructed on a somewhat different plan, with blades poking inward from a hoop rather than hanging out from a hub. If such a turbine airfoil revolves in mist, coats itself with liquid, and doesn't reach all the way in to spin centre, the liquid will feel a centrifugal force field of nonzero strength no matter where it is on the airfoil. It will be conducted radially outward in a flowing film.

Based on assuming laminar flow and constant viscosity, the film depths necessary for as much to flow away as is arriving are expected to be small:

Film depth = (3 * Power * Viscosity /
(Specific energy * Width * Density^2 * Centripetal acceleration)) ^ (1/3)

The 'Width' here is the stream width, about twice the distance from airfoil leading edge to trailing edge (because the stream is on both sides). Suppose the airfoil catches the same fraction of the mist as it converts of the thermal power, and all the catching occurs at the innermost point.. Then the amount of boria flowing past any point farther out is the (Power / Specific energy) part of the above. Specific energy is the ratio of heat produced to boria produced by the boron-oxygen reaction upstream of the turbine: 18.0 megajoules per kilogram.

Suppose an airfoil is 20 millimetres from leading to trailing edge at radius 30 millimetres, i.e. stream width is 40mm, and its share of the thermal power is 1 kilowatt. Let it revolve 250 times per second, so that the film on it at 30 millimetres radius has 74.0 kilometres per second per second of centripetal acceleration. Let the temperature be 600C. This fixes liquid boria's density near 1,600kg/m^3 and its viscosity near 480 pascal-seconds (*6). That is 480,000 times the viscosity of water at 20C. Boria at this temperature is thick, syrupy molten glass.

The film depth works out as 0.22 millimetres, probably small enough compared to the airfoil's 20mm chord that it won't cease to be an airfoil. If the temperature were reduced to 400C the viscosity would rise to 160,000 pascal-seconds. This huge increase may reflect the fact that 400 is below the 450C melting point of (very seldom seen) boria crystals. Because of the one-third power in the film depth formula the predicted increase in depth is only 6.6-fold, to 1.5 millimetres.

Diboron trioxide flowing radially outward in a centrifugal force field is an instance of liquid flowing downhill, a situation sometimes conjecturally linked to the arrival of liquid at the bottom. If that happens here, the bottom will be the turbine airfoil root, where it transmits force and power to the support hoop's inner surface. It seems possible for drain passages to descend further from that surface through the support ring to a ledge where the liquid can, by moving along the axis of rotation, spill over. A stationary scraping blade will take it before it can fly off on a tangent.

Now flowing down the scraper in a much thicker film than on the turbine blade because local gravity is so much weaker, it can make its way to molds. This process will resemble the action of bottle-blowing machinery, except the glass aliquots can be cooler when they arrive -- boria's work point is roughly 560C, while that of soda-lime glass is 1,000 to 1,100C -- and probably won't be blown hollow, just allowed to freeze as massive transparent vitreous lumps.


Virtuous Boria

The toxicological risk of handling boria in this fashion is limited by the low surface-to-volume ratio of the lumps and their hardness (Mohs 5, similar to fluorite or tooth enamel). They are, however, able to dissolve slowly in water, through the boric acid-forming reaction noted previously followed by dissolving of the acid.

In recent years studies of aqueous B(III) toxicity to animals have broadened to include consideration of its possible animal essentiality, without very definite results yet, but it is known to have a biological half-life of a day or two. If it isn't a micronutrient, at least it has little potential for cumulative toxicity. So it seemed not unduly risky to measure how quickly boria glass dissolves in a continually refreshed 37C aqueous medium by sucking on a 6mm by 8mm by 1.4mm piece, estimated mass near one-eighth of a gram. It lost all its thickness in 31 minutes, 0.4 microns per second per side. This suggests a lump too big to swallow would have to be sucked on non-stop for about a day to deliver the LD50.

Water vapour contacting a boria glass surface forms boric acid. In the absence of liquid water the resulting tarnish builds up to much less than a millimetre thick, makes the glass look milky, and then prevents further reaction. It is a solid lubricant, so pieces of boria aged in humid air are slippery.


Boria Is Easier to Transport than Hydrogen

For many practical energy-transmission purposes this is true, even though, as previously mentioned, the same energy can be taken up in dissociating the boron and oxygen in 5.1 kilograms of boria as is already in a 6.7 times smaller mass of elemental hydrogen: one kilowatt-day.

The boria could be considered a carrier of energy demand. It is relevant to compare its haulage with that of fuel hydrogen because each is the heaviest cargo, as such, in its system. Hydrogen ash rides the wind and finds its own way to the power station. Elemental boron can't get quite as free a ride as that, but as 31 mass percent of boria, it does have the capability of getting to its destination as a 31 percent load in a vessel that must go there anyway to pick up a full load of boria for the next run.

Boria is easier because it won't burn, won't evaporate, and despite being much heavier than the hydrogen, it takes up much less space. It is 25.6 times denser at room temperature than liquid hydrogen is at its normal boiling point. Loosely packed spheres would be only 0.65*25.6, i.e. 16.6 times denser. A dewar flask that could keep 0.76 kilograms of liquid hydrogen cool would be more than big enough to accommodate 5.1kg of boria ingots, not necessarily spherical, in its bottom half.

Moreover, those same ingots could also go in a sturdy plastic bag that would mass less than a tenth of a kilogram. It is not yet certain what shape the 1.6 kilograms of boron in them will be given when the power station extracts it, but if it is any sort of pellet, the pellets could certainly ride back in the bottom of same bag. Boron is a little denser than boria, insoluble, involatile, infusible, apparently physiologically inert, and unable to burn in air.

Only very large consignments of low-pressure liquid hydrogen, many terajoules' worth, could be accommodated in a container less massive than the contents in the way that plastic bag is. For gigajoules or less, the container is typically ten or more times more massive. Boria plus a bag is lighter.

It has been proposed to use stable hydrides such as that of magnesium as involatile solid forms of hydrogen. As an atomic-scale room temperature hydrogen container, a magnesium atom does nicely shrink hydrogen, down to only 64 percent of the specific volume of liquid hydrogen at -252.87C, but it follows the ten-times-mass-of-contents rule fairly closely -- actually 12 times. As a source of fuel hydrogen, magnesium hydride masses 11.85 kilograms per kilowatt-day.

Magnesium hydride isn't fireproof and neither is the highly expanded, high surface area magnesium metal left behind when the hydrogen is thermally driven off. Interestingly, the fire hazards in this scheme can be much reduced, and the haulage reduced by more than half, if the magnesium is burned along with the hydrogen.

The heaviest load then is the incombustible magnesium oxide travelling to the power station, 4.6 kilograms per kilowatt-day, a little lighter than boria. The hydride leaving the power station is still readily ignitable, but its quantity is reduced to only 3.0 kilograms per kilowatt-day, less than a third as much as in the hydrogen-only case. Magnesia won't ooze out of a combustor and then solidify in void-free masses the way boria can, though.


Boron Pipelines?

Elemental boron itself does not seem very well suited to being piped. But much hydrogen now travels short distances through pipelines, as pressurized gas, and it's interesting to consider whether the analogy between it and boria might extend to transportability in pipes. If it does then pipelines can do 76 percent -- one over 1.31 -- of the boron system's tonne-kilometres of active material transfer, and the freighters that do the rest can load all the way up on boron, not just 31 percent.

Boria pipelines would have to be tributary ones bringing the substance from many sources to a few power stations, the opposite to the job hydrogen pipelines would do. The threat to their steel's integrity would be boric acid corrosion, not hydrogen embrittlement. Leaks would be less hazardous, and the pressure behind them might be lower, since boria doesn't need pressure to be dense.

The observed 400 nanometre per second speed at which 37C water corrodes vitreous boria, and the rule of thumb that 10 kelvins less means reaction twice slower, suggest golf ball-sized boria ingots in a cold-water slurry would lose 4mm of radius per day of travel. But this incorrectly treats the water travelling with them as staying fresh. Actually the sparing solubility of boric acid in cold water means a slurry with equal masses of water and boria would reach saturation with most of the boria still solid, even if the boria pieces were smaller than golf balls.

The lubricity of the lumps' boric acid coats might protect the pipe walls from abrasion. One might think this could work for boron too, but at normal environmental temperatures boron is so inert that it is not subject to superficial oxidation. It would scratch like crazy.

If the nearest place to put boria into a pipeline were a few miles away, this would be less inconvenient than the corresponding case with hydrogen, for as noted, it's easier to carry a bag containing 5.1 kilograms of boria than the containment system for 0.76kg of hydrogen.

Large pipelines that now carry natural gas at high pressure might someday have a less demanding task: carrying boria to the boron-deoxidizing nuclear power stations built on the sites where the gas wells once ran.


Fuel Boron Should Hold Still While Flowing Oxygen Devours It

Probably the simplest way one can envision feeding boron to a combustor at a controllable rate is as filament, through a small hole in the combustion chamber wall. Inside the chamber the filament tip will encounter a continuously burning fire that will consume it as fast as it comes.

Very lean combustion has been mentioned, with much of the oxygen present not to react with boron but to take up the heat from the small fraction that does, and be heat engine working fluid. Even at pressure on the order of 10 megapascals this means a very much larger volume of oxygen must flow through or past the fire than of boron.

25 O2 + (4/3) B    --->    (2/3) B2O3 + 24 O2

Supposing the oxygen behaves as ideal gas and starts at 10MPa and 500K, the left side includes about 10 litres per mole of it, and 6 millilitres per mole of boron. So the volume ratio is near 1,700. This means a thin filament will be at the middle of a roughly 40 times thicker cylinder of oxygen flow if they both come to the fire at the same speed.

But it makes better sense for the boron to come in slowly and the oxygen around it to come in faster, in a stream narrower than 40 filament diameters. Then drag from oxygen helps pull the filament in, and the fire consuming its tip is an instance of lancing combustion: turbulently flowing oxygen runs over the burning boron surface, quickly removing boron oxide vapour -- for the flame temperature can be over 4,000C, far above even the highest reported boria boiling point -- and scouring boron atoms and droplets off the surface.

Lancing combustion has recently been recognized as a harmful phenomenon that can happen when high-pressure pure oxygen passes through aluminum regulator valves (*7). But when overcoming the ignition resistance of an ignition-resistant substance is what is desired, lancing combustion is good. It allows a lot of chemical power to be converted to thermal power in a small space.

It is not necessary for all the oxygen to flow past the flame in a single stream. A small excess might do that, and then the rest could be blown in in converging jets downstream of the flame, quickly diluting it. Before dilution the boron-oxygen flame can broadcast a considerable fraction of its power as visible and near infrared light. Putting the mixing jets' impingement point not far downstream of the flame would reduce this fraction.

Continuous filament supports lancing combustion because the part in the chamber is connected to, and held back against the wind by, all the rest still coiled outside, in an environment where it cannot burn. However, today's commercial boron filament is impossibly expensive, and has a tungsten core.

Boron developed for fuel use might be small unconnected chain links sintered from pure boron powder. These would behave simply as pellets during distribution and storage. They would resist breakage better than filament and could be easily apportioned, and transferred with a scoop or by hand. But when the fuel feed mechanism linked them together, they would have the same ability as filament to pull back while inside the chamber oxygen pulled them forward. It would be important for the terminal link not to drop pieces of itself, but rather remain linked and in one piece at all stages of consumption.


Providing Pure Oxygen and Handling It at High Pressure and Temperature

These are major difficulties. Combustion chambers and turbine rotors can't be made of anything but noble metals, fluorides, and oxides. Moreover, the oxides must not dissolve excessively in hot liquid boria. Air oxygen purifiers with sufficiently low energy consumption per kilogram of oxygen have been demonstrated, but they aren't small or light.

The use of oxygen in large excess might seem to exacerbate this difficulty, however, the high extractability of liquid boria and the absence of any gaseous combustion products means it will be possible to recycle leftover oxygen. (Note that this was never a possibility with boron-containing fuels that also contained hydrogen or carbon.)

This recycling saves not just the oxygen but also any boria molecules or droplets that fail to fall out immediately after their birth in the flame. They can come around and take their next chance, or the one after.

Boron combustors won't be entirely free of gaseous emissions because their oxygen supply won't be entirely pure. Air-derived oxygen's principal impurity will be nitrogen, which, diluted in high-pressure oxygen and occasionally passing through a very hot flame, will tend to be in the form of nitrogen dioxide.

If a small fraction of the gas flow passes through a polishing circuit the nitrogen dioxide can be captured by a small amount of alkali there, as can carbon dioxide and oxides of sulfur. Oxygen, unreacted nitrogen, and truly inert gases emerging from this polisher can be dumped.

But these are all air gases. Therefore, boron combustors, if they work at all, will be truly zero-emission devices.


Summary

Although they have a much taller first step, boron power systems promise to provide emission-free energy from smaller reservoirs than hydrogen can, using fireproof substances. Boria's high specific binding energy and the undemanding nature as cargoes of both it and elemental boron means they will convey energy lightly and compactly, even in small shipments.

If successfully demonstrated, boron-powered vehicles would show the ability to run on public roads without depending on special fuelling stations, since their ash could be sent away and boron could return by any ordinary freight carrier, even by mail. Operators could blast boron pellets with propane torch flames and show that they don't burn, demonstrating that fuel-fed fires during accidents were not possible. The demonstration vehicles could have fuel/ash reservoirs two or more times larger than would be safe on a hydrocarbon-burning vehicle, and since equal energy would require no more than 1.7 times the size, greater speed and range should be possible.

Despite the initially high cost of fuel boron, there seems to be no reason why a small demonstration fleet wouldn't form the nucleus of a quickly growing group of voluntary early adopters, on whose behalf plans for cheaper, larger-scale boron production would quickly be drawn up.






References

1. Robert B. McBroom, "Boron Oxides, Boric Acid, and Borates", Kirk-Othmer Encyclopedia of Chemistry and Chemical Technology, 3rd Edition.

2. Leonard K. Tower, "Thermal Relations for Two-Phase Expansion with Phase Equilibrium and Example for Combustion Products of Boron-Containing Fuel", Lewis Flight Propulsion Laboratory 1957, p. 35, available at http://naca.larc.nasa.gov/reports/1957/naca-rm-e57c11/naca-rm-e57c11.pdf on 14 June 2001.

3. Editor Robert C. Weast, "Physical Constants of Inorganic Compounds", p. B-62, CRC Handbook of Chemistry and Physics, 60th edition, CRC Press 1980.

4. McBroom, table 4: "Physical Properties of Vitreous Boric Oxide".

5. Mark Winter, "The WebElements Periodic Table of the Elements", available 14 June 2001 at http://www.webelements.com/webelements/index.html.

6. McBroom.

7. NIOSH, "Fire Fighter Fatality Investigation Report 98F-24", available 14 June 2001 at http://www.cdc.gov/niosh/face9824.html.

A different version of "Boron: A Better Energy Carrier than Hydrogen?", with boron and boria photos, is currently available at http://www.eagle.ca/~gcowan/boron_blast.html.