Biofuels to Power Transport

No sooner do I post something than someone says something to contradict the post. In this case, immediately after the last post, an airline came out and said it would be zero carbon by some time in the not-too-distant future. They talked about, amongst other things, hydrogen. There is no doubt hydrogen could power an aircraft, as it also powers rockets that go into space. That is liquid hydrogen, and once the craft takes off, it burns for a matter of minutes. I still think it would be very risky for aircraft to try to hold the pressures that could be generated for hours. If you do contain it, the extra weight and volume occupied would make such travel extremely expensive, while sitting above a tank of hydrogen is risky.

Hydrocarbons make by far the best aircraft fuel, and one alternative source of them is from biomass. I should caution that I have been working in this area of scientific research on and off for decades (more off than on because of the need to earn money.) With that caveat, I ask you to consider the following:

C6H12O6  ->  2 CO2 +2H2O + “C4H8”

That is idealized, but the message is a molecule of glucose (from water plus cellulose) can give two molecules each of CO2 and water, plus two thirds of the starting carbon as a hydrocarbon, which would be useful as a fuel. If you were to add enough hydrogen to convert the CO2 to a fuel you get more fuel. Actually, you do not need much hydrogen because we usually get quite a few aromatics, thus if we took two “C4H8” and make xylene or ethyl benzene (both products that are made in simple liquefactions) these total C8H10, which gives us a surplus of three H2 molecules. The point here is that in each of these cases we could imagine the energy coming from solar, but if you use biomass, much of the energy is collected for you by nature. Of course, if you take the oxygen out as water you are left with carbon. In practice there are a lot of options, and what you get tends to depend on how you do it. Biomass also contains lignin, which is a phenolic material. This is much richer in hydrocarbon material, but also it is much harder to remove the oxygen.

In my opinion, there are four basic approaches to making hydrocarbon fuels from biomass. The first, which everyone refers to, is pyrolysis. You heat the biomass, you get a lot of charcoal, but you also get liquids. These still tend to have a lot of oxygen in them, and I do not approve of this because the yields of anything useful are too low unless you want to make charcoal, or carbon, say for metal refining, steel making, electrodes for batteries, etc. There is an exception to that statement, but that needs a further post.

The second is to gasify the biomass, preferably by forcing oxygen into it and partially burning it. This gives you what chemists call synthesis gas, and you can make fuels through a further process called the Fischer-Tropsch process. Germany used that during the war, and Sasol in South Africa Sasol, but in both cases coal was the source of carbon. Biomass would work, and in the 1970s Union Carbide built such a gasifier, but that came to nothing when the oil price collapsed.

The third is high-pressure hydrogenation. The biomass is slurried in oil and heated to something over 400 degrees Centigrade in then presence of a nickel catalyst and hydrogen. A good quality oil is obtained, and in the 1980s there was a proposal to use the refuse of the town of Worcester, Mass. to operate a 50 t/d plant. Again, this came to nothing when the price of oil slumped.

The fourth is hydrothermal liquefaction. Again, what you get depends on what you put in but basically there are two main fractions from woody biomass: hydrocarbons and phenolics. The phenolics (which includes aromatic ethers) need to be hydrogenated, but the hydrocarbons are directly usable, with distillation. The petrol fraction is a high octane, and the heavier hydrocarbons qualify as very high-quality jet fuel. If you use microalgae or animal residues, you also end up with a high cetane diesel cut, and nitrogenous chemicals. Of particular interest from the point of view of jet fuel, in New Zealand they once planted Pinus Radiata which grew very quickly, and had up to 15% terpene content, most of which would make excellent jet fuel, but to improve the quality of the wood, they bred the terpenes more or less out of the trees.

The point of this is that growing biomass could help remove carbon dioxide from the atmosphere and make the fuels needed to keep a realistic number of heritage cars on the road and power long-distance air transport, while being carbon neutral. This needs plenty of engineering development, but in the long run it may be a lot cheaper than just throwing everything we have away and then finding we can’t replace it because there are shortages of elements.

Non-Battery Powered Electric Vehicles

If vehicles always drive on a given route, power can be provided externally. Trams and trains have done this for a long time, and it is also possible to embed an electric power source into roads and power vehicles by induction. My personal view is commercial interests will make this latter option rather untenable. So while external power canreplace quite a bit of fossil fuel consumption, self-contained portable sources are required.

In the previous posts, I have argued that transport cannot be totally filled by battery powered electric vehicles because there is insufficient material available to make the batteries, and it will not be very economically viable to own a recharging site for long distance driving. The obvious alternative is the fuel cell. The battery works by supplying electricity that separates ions and converts them to a form that can recombine the ions later, and hence supply electricity. The alternative is to simply provide the materials that will generate the ions and make the electricity. This is the fuel cell, and effectively you burn something, but instead of making heat, you generate electric current. The simplest such fuel cells include the conversion of hydrogen with air to water. To run this sort of vehicle, you would refill your hydrogen tank in much the same way you refill a CNG powered car with methane. There are various arguments about how safe that is. If you have ever worked with hydrogen, you will know it leaks faster than any other gas, and it explodes with a wide range of air mixtures, but on the other hand it also diffuses away faster. Since the product is water (also a greenhouse gas, but one that is quickly cycled away, thanks to rain, etc) this seems to solve everything. Once again, the range would not be very large because cylinders can only hold so much gas. On the other hand, work has been going on to lock the hydrogen into another form. One such form is ammonia. You could actually run a spark ignition motor on ammonia (but not what you buy at a store, which is 2 – 5% ammonia in water), but it also has considerable potential for a fuel cell. However, someone would still have to develop the fuel cell. The problem here is that fuel cells need a lot more work before they are satisfactory, and while the fuel refilling could be like the current service station, there may be serious compatibility problems and big changes would be required to suppliers’ stations.

Another problem is the fuel still has to be made. Hydrogen can be made by electrolysing water, but you are back to the electricity requirements noted for batteries. The other way we get hydrogen is to steam reform oil (or natural gas) and we are back to the same problem of making CO2. There is, of course, no problem if we have nuclear energy, but otherwise the energy issues of the previous post apply, and we may need even more electricity because with an additional intermediate, we have to allow for inefficiencies.

As it happens, hydrogen will also run spark ignition engines. As a fuel, it has problems, including a rather high air to fuel ratio (a minimum of 34/1, although because it runs well lean, it can be as high as 180/1) and because hydrogen is a gas, it occupies more volume prior to ignition. High-pressure fuel injection can overcome this. However there is also the danger of pre-ignition or backfires if there are hot spots. Another problem might include hydrogen getting by the rings into the crankcase, where ignition, if it were to occur, could be a real problem. My personal view is, if you are going to use hydrogen you are better off using it for a fuel cell, mainly because it is over three times more efficient, and in theory could approach five times more efficient. You should aim to get the most work out of your hydrogen.

A range of other fuel cells are potentially available, most of them “burning” metal in air to make the electricity. This has a big advantage because air is available everywhere so you do not need to compress it. In my novel Red Gold, set on Mars, I suggested an aluminium chlorine fuel cell. The reason for this was: there is no significant free oxygen in the thin Martian atmosphere; the method I suggested for refining metals, etc. would make a lot of aluminium and chlorine anyway; chlorine happens to be a liquid at Martian temperatures so no pressure vessels would be required; aluminium/air would not work because aluminium forms an oxide surface that stops it from oxidising, but no such protection is present with chlorine; aluminium gives up three electrons (lithium only 1) so it is theoretically more energy dense; finally, aluminium ions move very sluggishly in oxygenated solutions, but not so if chlorine is the underpinning negative ion. That, of course, would not be appropriate for Earth as the last thing you want would be chlorine escaping.

This leaves us with a problem. In principle, fuel cells can ease the battery problem, especially for heavy equipment, but a lot of work has to be done to ensure it is potentially a solution. Then you have to decide on what sort of fuel cells, which in turn depends on how you are going to make the fuel. We have to balance convenience for the user with convenience for the supplier. We would like to make the fewest changes possible, but that may not be possible. One advantage of the fuel cell is that the materials limitations noted for batteries probably do not apply to fuel cells, but that may be simply because we have not developed the cells properly yet, so we have yet to find the limitations. The simplest approach is to embark on research and development programs to solve this problem. It was quite remarkable how quickly nuclear bombs were developed once we got started. We could solve the technical problems, given urgency to be accepted by the politicians. But they do not seem to want to solve this now. There is no easy answer here.