Biofuels from Algae

In the previous post, I described work that I had done on making biofuels from lignin related materials, but I have also looked at algae, and here hydrothermal processing makes a lot of sense, if for no other reason than algae is always gathered wet. There are two distinct classes: microalgae and macroalga. The reason for distinguishing these has nothing to do with size, but rather with composition. Microalgae have the rather unusual property of being comprised of up to 25% nucleic acid, and the bulk of the rest is lipid or protein, and the mix is adjustable. The reason is microalgae are primarily devoted to reproduction, and if the supply of nitrogen and phosphate is surplus to requirements, they absorb what they can and reproduce, mainly making more nucleic acid and protein. Their energy storage medium is the lipid fraction, so given nutrient-rich conditions, they contain very little free lipids. Lipids are glycerol that is esterified by three fatty acids, in microalgae primarily palmitic (C16) and stearic (C18), with some other interesting acids like the omega-three acids. In principle, microalga would be very nutritious, but the high levels of nucleic acid give them some unfortunate side effects. Maybe genetic engineering could reduce this amount. Macroalgae, on the other hand, are largely carbohydrate in composition. Their structural polysaccharides are of industrial interest, although they also contain a lot of cellulose. The lipid nature of microalgae makes them very interesting when thinking of diesel fuel, where straight-chain hydrocarbons are optimal.

Microalgae have been heavily researched, and are usually grown in various tubes by those carrying out research on making biofuels. Occasionally they have been grown in ponds, which in my opinion is much more preferable, if for no other reason than it is cheaper. The ideal way to grow them seems to be to feed them plenty of nutrients, which leads them to reproduce but produce little in the way of hydrocarbons (but see below) then starve them. They cannot shut down their photosystems, so they continue to take on carbon dioxide and reduce the carbon all the way to lipids. The unimaginative thing to do then is to extract the microalgae and make “biodiesel”, a process that involves extracting the lipids, usually with a solvent such as a volatile hydrocarbon, distilling off the solvent, then reacting that with methanolic potassium hydroxide to make the methyl esters plus glycerol, and if you do this right, an aqueous phase separates out and you can recover your esters and blend them with diesel. The reason I say “unimaginative” is that when you get around to doing this, you find there are problems, and you get ferocious emulsions. These can be avoided by drying the algae, but now the eventual fuel is starting to get expensive, especially since the microalgae are very difficult to harvest in the first place. To move around in the water, they have to have a density that is essentially the same as water, so centrifuging is difficult, and since they are by nature somewhat slimy, they clog filters. There are ways of harvesting them, but that starts to get more expensive. The reason why hydrothermal processing makes so much sense is it is not necessary to dry them; the process works well if they are merely concentrated.

The venture I was involved in helping had the excellent idea of using microalgae that grow in sewage treatment plants, where besides producing the products from the algae, the pollution was also cleaned up, at least it is if the microalgae are not simply sent out into the environment. (We also can recover phosphate, which may be important in the future.). There are problems here, in that because it is so nutrient-rich the fraction of extractable lipids is close to zero. However, if hydrothermal liquefaction is used, the yield of hydrocarbons goes up to the vicinity of over 20%, of which about half are aromatic, and thus suitable for high-octane petrol. Presumably, the lipids were in the form of lipoprotein, or maybe only partially substituted glycerol, which would produce emulsifying agents. Also made are some nitrogen-rich chemicals that are about an order of magnitude more valuable than diesel. The hydrocarbons are C15 and C17 alpha unsaturated hydrocarbons, which could be used directly as a high-cetane diesel (if one hydrogenated the one double bond, you would have a linear saturated hydrocarbon with presumably a cetane rating of 100), and some aromatic hydrocarbons that would give an octane rating well over a hundred. The lipid fraction can be increased by growing them under nutrient-deprived conditions. They cannot reproduce, so they make lipids, and swell, until eventually they die. Once swollen, they are easier to handle as well. And if nothing else, there will be no shortage of sewage in the future.

Macroalgae will process a little like land plants. They are a lot easier to handle and harvest, but there is a problem in obtaining them in bulk: by and large, they only grow in a narrow band around the coast, and only on some rocks, and then only under good marine conditions. If the wave action is too strong, often there are few present. However, they can live in the open ocean. An example is the Sargasso Sea, and it appears that there are about twenty million tonne of them in the Atlantic where the Amazonian nutrients get out to sea. However, in the 1970s the US navy showed they could be grown on rafts in the open ocean with a little nutrient support. It may well also be that free-floating macroalgae can be grown, although of course the algae will move with the currents.

The reason for picking on algae is partly that some are the fastest-growing plants on the planet. They will take more carbon dioxide from the atmosphere more quickly than any other plant, the sunlight absorbed by the plant is converted to chemical energy, not heat, and finally, the use of the oceans is not competing with any other use, and in fact may assist fish growth.

Biofuels as Alternative Fuel Sources

In the previous post I suggested that municipal refuse could be converted to liquid fuels of similar nature to modern hydrocarbon fuels from oil. There are some who think transport can be readily electrified, but three other considerations suggest not. The first is that, as noted by Physics World, a publication by the Institute of Physics, there is a very good chance that a car sold today will still be being used twenty years out. The second is that electric vehicles also have considerable greenhouse emissions. The actual running of the car is free of emissions, but you still have to make the car, which has significant emissions and may exceed that of the standard car; you have to generate the electricity, and the majority of the world’s electricity is generated from fossil fuels, and much from coal; and finally you have to make the batteries, and this is also a serious emitter. There are also parts of the vehicle fleet that will not easily electrify, such as the big road trailers that go into remote Australia, heavy construction equipment, simply because the extra mass to store the batteries would be horrendous, and recharge is not simple in remote places. Shipping will continue to use oil, as will aircraft, and as noted in a previous post, so will much of the vehicle fleet because it is not possible to make satisfactory batteries for replacement vehicles because with current technology there is insufficient cobalt. So what else can replace fossil fuel?

One such possibility is biofuels. The case for biofuels is that in principle their combustion is carbon neutral: their carbon may return to the air, but it came from the air. The energy source is essentially solar, and that is not going to run out soon. The problem then is that biomass is a strange mix of different chemicals. Worse, we eat biomass, and we must not remove our food supply.

The first problem that gives biofuels a bad name is the urge to take the low hanging fruit, especially for tax benefits. Palm oil for biodiesel has made a terrible mess of the Indonesian rain forests, and for what benefit (apart from clipping the tax ticket?) Corn for ethanol similarly makes little sense exceptfor the case where the corn would otherwise go to waste. The problem in part is that corn also utilises so little of the sunlight; most of the plant is simply wasted, and often burned. Seed oils do make sense for specialist uses, such as drying oils in paint, or in cosmetics, or as a feedstock to make other specialist chemicals, but something like “biodiesel” from palm oil makes the overall situation worse, not better. It will never replace the carbon fixed in the rain forest.

Forestry is another interesting case. We are much better off to use the logs for timber in construction: it is worth more, and at the same time it fixes carbon in the buildings. However, if you have ever seen forestry, you will know there is an awful lot of biomass just left to rot: the branches, twigs, stumps, roots, leaves/needles, etc. That is essentially free to use. There are big problems in that it packs extremely badly, so transport costs for any distance are too great.

From a technical point of view, the processes to use woody biomass would come down to the same ones for municipal wastes as noted in the previous post, except that gasification is unlikely to be suitable. A significant plant was put up in the US to gasify biomass and use the gas for a modified Fischer-Tropsch process, but it failed. There were probably several reasons for this, but one is immediately obvious: if we rely on market forces we cannot compete with oil. There are two reasons for that. The first is that the oil is “free” originally, and since liquids can be pumped, it is easily handled, while the second is there is a huge infrastructure to process oil. The cost per unit mass of product becomes lower, usually the ratio of the throughput rates to the power of 0.6. The reason for this is simple. Costs of processing plant are proportional (all other things being equal) to the area of the container, which is proportional to r squared, while the production rate is proportional to volume, which is proportional to rcubed. The huge oil processing plants are extremely efficient, and no much smaller biomass processing unit can have any hope of matching it. The reason for the 0.6 as opposed to 0.66666 is that there are also some extra savings. Control equipment, gauges, etc tend to cost much the same because they are the same, and pumps, etc, tend to be excessively costly when small. The six-tenths rule is, of course, only a rough approximation, but it is a guide.

My approach to this, when I started, was to consider biomass hydrothermal liquefaction. The concept here was that if we merely heated the biomass up with water and simple catalysts under pressure to approaching the critical point, we could end up with liquids. These would have to be refined, but that could be done in a large central plant, while the initial processing could be done in smaller units that could be, with effort, portable. One of the surprises from this was that a certain fraction was already effectively a very high-grade fuel, at least for petrol craft or jet engines without further refining. Exactly what you got depended on catalysts, and a case could also be made to add certain other chemicals to enhance certain products. A lot more work would be needed to get such technology operational, but needing more work is not a reason to discard the concept if saving the world is at stake.

So why did I stop doing this work? Basically, because I felt the desire to change my working environment, and because the funding for this work was drying up. Regarding my working environment, there is a funny story there – well, sort of funny, but it did not feel that way then. The journal Naturedid a quick survey of science in New Zealand. I worked in a Government laboratory, and I had the rather dubious honour of having Head Office describe me in Natureas “an eccentric”. Why? Because I was trying to promote an industry based on a by-product of a synthetic fuels plant constructed at Motunui that would make a key starting material for high-temperature plastics. I suppose some of my antics were unusual, for example there was a program on national television of the dangers of flammable foam plastics, so I went there and pointed out they did not haveto be flammable. I had a piece of foam I had made in the lab that afternoon in the palm of my hand and I fired a gas torch at it so the foam was in my hand, yet yellow-hot on the outside. I held it there for quite some time, until everyone got bored. Anyway, when your employer decides you are eccentric, it felt like time to quit and start up by myself. There were two consequences of this. Before I left, the technical staff made me a brass “eggcup” with a large glass egg in it. One of my prized possessions. The second was I got a letter of apology from Head Office, in which it was explained they did not mean I was eccentric, but rather my ideas were. Not a great improvement, as seen by me. I suppose there was a further example of eccentric behaviour. The laboratory was set up with the purpose of promoting the New Zealand economy by finding new industrial opportunities. I suppose it was somewhat eccentric to actually be following “the Prime Directive”.