Can we burn metal as fuel?
Updated: Dec 12, 2021
Even if we stick solar panels on every roof in the world, we’d still need a way to store energy to use when it gets dark. And a way to use that energy later to do more than just power a lightbulb—to do things like power construction equipment or move a cargo ship across the Pacific. Metal powder may be a great way to do it.
The steady heartbeat thump of the Stirling engine falls silent and we coast to stop at the foot of the massive pile of trash. The van hisses and pings as its internals cool, its steel pipes relaxing and contracting as it settles to ambient temperature. I kick open the rusty door and hop down onto the spongy soil of the landfill. It stinks, sweet and sour and sick, even through the microfilters of my mask.
“I hope the scanner is right,” My partner’s muffled voice drifts over the garbage-strewn landscape.
“Hope so, looks undisturbed,” I respond. The long-range scanner picked up some ferrous metal in this heap and the chances are good it’s right. We had to cross the river using the folding bridge to get here. Most scrappers don’t have a bridge and nobody’s gonna ferry scrap on that rank river. I can smell its stench from here—turpentine and rotting fish.
I pull the metal detector from the van’s cargo hold and fit it with the spike attachment. If there’s anything in there, it’ll be deep. My partner deploys the watch drone, sends it buzzing overhead to keep an eye out for raiders or other scrappers. Maybe they couldn’t get the steel outta here, but they may be lurking nearby to protect it. He flips a switch on his goggles and peers through the drone’s eyes 100 feet above us. “I don’t see anything. Lidar, infrared is clear. If anyone’s hiding they’re hiding real good.”
I approach the pile and start lancing it with the detector. It buzzes and crackles in my ear, singing its familiar song. I wade into the pile, prodding as I go. About half-way up the pile I hit gold, or iron, rather. The detector squawks and squeals in delight. It’s positively ecstatic—there must be something big in there. I call back down to my partner. “Get the shovels.”
We dig through the soggy trash to our treasure, a nearly perfect bed frame. It’s covered in a thick layer of brown paint and is dripping with garbage, but it should recycle well. We cut it by hand into smaller chunks, feed a few pounds of it into the grinder. Even with the paint mixed itn, it should burn well enough to get us back to the solar mill. Hopefully it won’t foul the combustion chamber too bad. Just had it scrubbed clean a week ago.
We pack up our gear and bid farewell to the mound. Some people scorn our ancestors for being so wasteful. I’m grateful. This life is easier than digging in the mines. And one day all this will return to the earth and plants will grow again. Sure, things didn’t turn out well the first time, but second time’s a charm.
Scavenging for scrap iron among the wasteland ruins of civilization may sound far-fetched, but that’s the future some scientists envision. A researcher at McGill University in Montreal imagines a civilization that runs on sunlight and iron powder. A team at a Eindhoven University in the Netherlands have built a working power plant that burns iron powder. Others think the cars of tomorrow will run on aluminum. Of course none of them predict the scavenging through ruins bit, that’s all me, but they do see metal as a means to move renewable energy around the planet. Wait, what does that even mean?
More sunlight energy hits the surface of the earth in an hour than all of humanity uses in an entire year. And it’s totally free, and without repercussions. Of course collecting and storing all that energy is hard. Really hard.
Even if we stick solar panels on every roof in the world, we’d still need a way to store energy to use when it gets dark. And a way to use that energy later to do more than just power a lightbulb—to do things like power construction equipment or move a cargo ship across the Pacific. Fossil fuels are really good at that kind of stuff because they’re extremely energy dense. A gallon of diesel contains about 139,000 btus, or enough energy to bring 115 gallons of water to a boil in less than an hour. It’s a tremendous amount of energy packed into very little space. It’s portable and relatively easy to store. Finding a carbon-neutral alternative is almost impossible. You can use batteries to store and transport captured solar energy, but they aren’t anywhere near as energy dense as diesel. The batteries in the long-range Tesla Model 3 store 75 KWh of power, which is equivalent to about two gallons of diesel. And they weigh more than 1,000 pounds. Plus batteries are expensive, sensitive to temperature fluctuations, and eventually wear out.
Clearly, batteries can’t compete with fossil fuels—yet—and they may never reach the energy density of diesel. But there are plenty of ways to store the sun’s energy. You could use sunlight energy to split hydrogen out of seawater, then either burn that hydrogen in an engine or use it in a fuel cell to do work. Hydrogen burns super clean, producing nothing but water vapor as a byproduct. It’s also far more energy dense than gasoline or diesel. But it’s way less dense overall.
Now, this is a bit confusing, so hang on to your chair. Hydrogen contains about 61,000 BTUs per pound vs about 21,000 BTUs per pound for gasoline. But its density is so low that you can’t really carry that much of it. Even when it’s cooled and compressed into a liquid, a gallon of hydrogen has only 9.45 percent the mass of gasoline. A gallon of gas has a mass of about six pounds, but a gallon of liquid hydrogen has a mass of just .567 pounds. So you just can’t carry that much hydrogen around with you.
Plus, hydrogen is the lightest and smallest element, which makes it really difficult to store. You can’t just pump it into any old container. Hydrogen tanks are meticulously engineered to withstand high pressures and to fail gracefully—you know, not explode catastrophically. That makes them expensive and difficult to manufacture. The fuel tanks in the Toyota Mirai, one of the world’s only production hydrogen fuel cell vehicles, hold about 5kg of hydrogen at 10,000 psi of pressure. That’s a lot of pressure. For comparison, most car tires hold air at about 35 psi. Hospital oxygen tanks can usually handle around 2,000 psi. To hold hydrogen at 10,000 psi, Toyota made the Mirai’s tanks out of an exotic carbon fiber-reinforced plastic. They say the tanks can withstand just about any kind of traffic accident without bursting, but still. 10,000 psi is a lot of pressure.
Hydrogen may end up being a piece of the zero-carbon energy pie, but we still need other ways to store and transport renewable energy–something that has less of a chance of blowing up. Ironically, the answer may be found in fireworks.
Metal powders have been used in pyrotechnics for ages. They add brilliant colors, sparks, and other razzle-dazzle to fireworks shows. And they can do that because metals are extremely energetic. It’s strange to think about something like aluminum as being energetic, but it really is. Especially when you light it on fire. Aluminum has been used in solid rocket boosters for decades. The solid-rocket boosters on NASA’s Space Launch System (SLS) use a mixture of aluminum powder and ammonium perchlorate to blast stuff into orbit. But the best way to burn aluminum is to douse it in water.
Pure aluminum reacts violently with plain old water. It rapidly oxidizes, producing heat and hydrogen. But wait, why don’t soda cans spontaneously melt down? When aluminum is exposed to air, it instantly oxidizes, forming a layer of aluminum-oxide that doesn’t react with much of anything. It’s stable, and super useful for storing stuff—even water.
But if you can manage to prevent that layer of aluminum-oxide from forming, it’ll react violently with water, producing a ton of heat and hydrogen. You can run that hydrogen through a fuel cell where it’s combined with oxygen to generate electricity. Fuel cells work kind of like batteries: they generate electricity through a chemical reaction, but they need a constant stream of fuel. In this case, aluminum provides a constant stream of hydrogen for the fuel cell. The reaction can be started and stopped quickly and easily, just add or subtract water. The system will generate electricity until it runs out of aluminum to react. Then you can take the aluminum oxide out and recycle it back into pure aluminum to be used over again. Of course you’d use a renewable energy source like solar or wind to do it.
A company called Trolysis has supposedly developed an aluminum fuel cell system that does just that. Trolysis uses a proprietary catalyst to strip away the non-reactive aluminum oxide layer from the aluminum. Once that layer is gone, the pure aluminum underneath is free to react with the water and the system is up and running. Trolysis said in 2018 that they’d release a commercial version of their system sometime in 2019, but the year’s almost up and the company website is currently under construction.
But there’s a much easier way to use aluminum to store renewable energy: You can use it to make batteries. Aluminum-air batteries have a tremendous energy density—potentially up to eight times the energy density of the lithium-ion batteries in your phone and an even higher energy density than gasoline. They react aluminum with the oxygen in regular-old air to generate a current. But there are problems. First, they’re not rechargeable. The reaction uses up the aluminum anode, which has to be replaced if you want to use the battery again. You can think of an aluminum-air battery as a kind of chemical engine that uses aluminum to make electricity. Second, the reaction creates a thick goop around the aluminum that can make the battery die prematurely.
Still, you’d think that researchers around the world would be working overtime to make aluminum-air batteries. Aluminum is one of the most common elements on earth and its energy density is much greater than lithium. Even if you have to rebuild the batteries, they make a lot of sense. But we’ve invested so much time and energy into lithium-ion batteries that it’s tough to change course.
In the early 2000s British engineer Trevor Jackson built a new kind of aluminum-air battery that nearly eliminated the goop problem. The battery had a high energy density and would be fairly simple to recycle. He presented it to Tony Blair, who turned it down. Blair and his staff told Jackson that the future is lithium ions. Jackson later moved to France, where he received a grant to continue his research. The minute the Euros started rolling in, the UK foreign office asked Jackson to come back. But the UK Technology Strategy Board refused to help Jackson, saying that the Automotive Council Technology Road Map “excluded this type of battery.” So Jackson remained in France. He formed a company called Metalectrique and started looking for investors.
He built a proof-of-concept power pack for the Nissan Leaf, but Nissan was already committed to lithium ion. In tests, Jackson’s aluminium-air power cell gave the Leaf a 1,500-mile range and could be swapped out in 90 seconds. The Leaf’s current range is just 226 miles, if you’re lucky.
Despite this seemingly incredible breakthrough, the UK government still hasn’t welcomed Jackson back into its fold. He’s still looking for investors for Metalectrique and working with auto manufacturer Lotus on some undisclosed projects. There’s an excellent TechCrunch article about Jackson’s plight by Mike Butcher called: “Negative? How a Navy veteran refused to accept a ‘no’ to his battery invention.” It’s truly stunning how difficult it is for someone to introduce new awesome technology that subverts the status quo. People will do almost anything to avoid change. Anyway, go read the article. Link in the show notes.
Aluminum-air batteries sound like the perfect solution to a lot of our energy problems. Aluminum is extremely energy dense, recyclable, and abundant. Lithium, on the other hand, isn’t recyclable. And lithium-ion batteries require some rare-earth metals that are only available on a few places on the planet.
Thankfully, Jackson isn’t the only person researching aluminum-air battery technology. An Isralei company called Phinergy has built a working prototype and researchers in China are also working on aluminum-air tech. If it takes off, it could be a game changer for long-haul transportation. Trucks, trains, and even cargo ships could use aluminum-air batteries instead of diesel fuel. Of course we’d need a standardized and simple way to recycle/recharge the batteries, but that really couldn’t be any more complicated than the process of refining and distributing crude oil, right? I could imagine a car that runs on quick-charge lithium batteries around town, then switches to ultra-long-range, hot-swappable aluminum-air batteries for road trips. That would be pretty cool. Then again, a high-speed train network in the US would also be pretty cool, and I wouldn’t need to take a loan out on a fancy new car. But I digress.
Aluminum is a pretty metal way to store renewable energy, but it’s not the most metal. That honor goes to iron. It has a high energy density, it’s easy to store, and, well, what’s more metal than iron?
So how do you use iron as an energy storage medium? I mean, you can’t just shovel scrap iron into your car’s gas tank, as cool as that may sound. Although that’s not that far off from what’s being proposed. Iron has a super-high energy density, much greater than any battery tech we currently have and even higher than diesel. To release all that energy you need to burn it in a swirling, sparkling tornado of fire.
Jeffrey Bergthorson at McGill University in Montreal thinks that iron powder could be the energy storage medium of the future. Here’s how it works: Pulverize iron into powder, then burn it in a combustion chamber to release its energy. Use the heat to make steam to turn a generator, or to drive a stirling engine. When you burn iron, all you’re doing is converting it to iron oxide, or rust. That rust powder can be collected from the combustion chamber and smelted back into pure iron powder using renewable energy like solar, wind, geothermal, or tidal power. And that’s the iron economy.
Of course there are a ton of challenges to overcome. First, we don’t really know the best way to burn iron. The powder burns well, but it has to be mixed with air just right to burn efficiently. It’s also challenging to collect the iron oxide powder from the combustion chamber. But hey, we figured out how to turn oil into gasoline, then burn it in the mechanical nightmare that is the internal combustion engine, so this shouldn’t be so difficult. I mean, compared to variable valve timing, direct injection, and forced induction, this seems like child’s play.
In fact, a team at Eindhoven University of Technology in the Netherlands built a working iron powder furnace that generates electricity using a Stirling engine. A Stirling engine turns heat into motion by harnessing the compression and expansion of a gas or fluid. They’re closed-cycle engines, which means the working fluid is sealed within the engine. All you need to do is heat one part of the engine to get it moving. The Eindhoven team stuck their Stirling engine in the iron furnace exhaust and used it to spin a generator. It worked, but it’s more likely that they’d use the 1,800-degree Celsius (3,200-degree Fahrenheit) burner to power a steam turbine system, which is more efficient than a Stirling engine at those temperatures. Still, advanced Stirling engines could be used to power freight trains or trucks.
Stirling engines are really slow to respond to changes in power output, so they wouldn’t be great for directly powering cars. But it’s easy to imagine a stirling engine coupled to a generator that powers an electric motor or charges a set of batteries. A sort of Stirling-hybrid vehicle that could run on a combination of battery power and iron powder. It seems bizarre and far-fetched, but let me remind you that we currently dig up black goo from the depths of the earth, cook it in a nearly unfathomable alchemical process to make a clear liquid that burns at high temperatures—then we burn that magical liquid in a devilishly complex machine just to get to the grocery store and back. Our current modes of transportation are far from simple and it’s not a stretch to imagine alternatives to gasoline and diesel. We just need to decide to do it.
More than 80 percent of the greenhouse gas emissions in the US are related to transportation—moving people and stuff around the country. If we want to rapidly reduce greenhouse gas emissions, we’ll need to change how we get around town. Electric cars are definitely a great solution, but they have a limited range and they’re pretty expensive. Unless we develop better and cheaper battery technology, they probably won’t replace all the gasoline and diesel cars on the road. Iron powder-burning hybrids may be able to compete with gasoline and diesel cars on price and range, but they’d require an extensive infrastructure of iron powder recycling stations. Again, easily doable with current technology, even if it’s a daunting task.
All of these alternative energy technologies work better when deployed on a big scale. A giant iron-burning power plant would be more efficient than a million little iron-burning hybrid cars. And a giant pyramid of aluminum power cells would be easier to manage than a huge network of aluminum power cell stations. Likewise, a bus full of people is more efficient (per person) than a dozen cars full of people. And a train full of passengers is even more efficient than that.
Hold onto your hats listeners, because I’m gonna get a little eco preachy. Fast forward a few minutes if you don’t want to be lectured at. I totally get it, I don’t like being lectured at either. And actually I don’t like lecturing. Quite frankly lecturing people or even ordering them around makes me woozy. But some things need to be said.
We’ve gotten used to fast, easy, and wasteful forms of personal transportation. We use 3500-pound magical iron chariots just to pick up a few few bags of groceries. The amount of energy we use just to get around is insane. Do cars give people a tremendous amount of freedom to move about as they wish? Are they fun? Do they represent American individual empowerment, creativity, and ingenuity? Yes to all those things. But when you do the math, they’re just silly. A single gallon of gas contains 120 million joules of energy. Most modern cars get around 22 miles per gallon around town. That means they burn about .045 gallons of gas to go a mile. That’s about 5 million joules of energy just to drive a mile. Now a joule is a meaningless unit of measurement to anyone who’s not a scientist, so we can translate that to kilocalories (or calories), roughly. Driving a mile in a car burns about 1,200 calories. Walking that mile would burn just about 100 calories.
Of course that’s an unfair comparison because you can’t walk at 60 miles per hour while carrying four bags of groceries. But it does illustrate that going anywhere in a car burns up a TON of energy. And I didn’t even count all the energy that goes into making cars, tires, roads, traffic lights, parking lots, all that stuff. It’s tremendous.
Now before you delete my podcast forever, hang on. I’m not pointing fingers. I drive a car—I have since I was 17. It’s an indispensable part of my life, and it’s probably an indispensable part of yours. We didn’t have a choice. We were born into cities built around the automobile. We weren’t born into well-designed, walkable cities with lots of mass transit options. Or ones where workplaces are anywhere near our houses. We have a truly inefficient and insane system, but it’s not our fault. But we need to recognize it as inefficient and insane if we’re ever going to change it. There are more efficient, safer, and less stressful ways to get around. And there are better ways to organize cities—and life in general—so we don’t have to move around as much. Replacing our current internal-combustion cars with electric cars, or iron-burning cars, probably isn’t sustainable. My guess is that if we want this planet to be a nice place to live, cars will have to go away. Bikes, on the other hand, will still probably be a thing. And maybe race cars and go karts if people are still into them. We love going fast and having fun, and I really don’t want to take that away from people. I also want to have a nice place to live.
Okay, rant over.
Universities and research labs across the world are working on various metal-burning systems, so I wouldn’t be surprised to see them in commercial use in the coming years. Will they save us from the climate apocalypse, ushering in a new carbon-neutral era of prosperity and wealth? Probably not, but they could be a piece of the puzzle.
That’s it for this episode. There’s a ton more to read about metal powder out on the interwebs. Check out the giant list of links in the show notes. I threw a lot of math and numbers at you in this one. The sources for all that stuff is there.
Next time I want to do a deeper dive into the hydrogen economy, and my experience with hydrogen fuel cell vehicles. I actually got to drive a few early General Motors prototypes and they were super awesome. Can’t wait to tell you all about it.
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