Plastics and Rubbish

In the current “atmosphere” of climate change, politicians are taking more notice of the environment, to which as a sceptic I notice they are not prepared to do a lot about it. Part of the problem is following the “swing to the right” in the 1980s, politicians have taken notice of Reagan’s assertion that the government is the problem, so they have all settled down to not doing very much, and they have shown some skill at doing very little. “Leave it to the market” has a complication: the market is there to facilitate trade in which all the participants wish to offer something that customers want and they make a profit while doing it. The environment is not a customer in the usual sense and it does not pay, so “the market” has no direct interest in it.

There is no one answer to any of these problems. There is no silver bullet. What we have to do is chip away at these problems, and one that indicates the nature of the problem is plastics. In New Zealand the government has decided that plastic bags are bad for the environment, so the single use bags are no longer used in supermarkets. One can argue whether that is good for the environment, but it is clear that the wilful throwing away of plastics and their subsequent degradation is bad for it. And while the disposable bag has been banned here, rubbish still has a lot of plastics in it, and that will continue to degrade. If it were buried deep in some mine it probably would not matter, but it is not. So why don’t we recycle them?

Then first reason is there are so many variations of them and they do not dissolve in each other. You can emulsify a mix, but the material has poor strength because there is very little binding at the interface of the tiny droplets. That is because they have smooth surfaces, like the interface between oil and water. If the object is big enough this does not matter so much, thus you can make reasonable fence posts out of recycled plastics, but there really is a limit to the market for fence posts.

The reason they do not dissolve in each other comes from thermodynamics. For something to happen, such as polymer A dissolving in polymer B, the change (indicated by the symbol Δ) in what is called the free energy ΔG has to be negative. (The reason it is negative is convention; the reason it is called “free” has nothing to do with price – it is not free in that sense.) To account for the process, we use an equation

            ΔG = ΔH -T ΔS

ΔH reflects the change of energy between each molecule in its own material and in solution of the other material. As a general rule, molecules favour having their own kind nearby, especially if they are longer because the longer they are the interactions per atom are constant for other molecules of the same material, but other molecules do not pack as well. Thinking of oil and water, the big problem for solution is that water, the solvent, has hydrogen bonds that make water molecules stick together. The longer the polymer, per molecule that enhances the effect. Think of one polymer molecule has to dislodge a very large number of solvent molecules. ΔS is the entropy and it increases as the degree of randomness increases. Solution is more random per molecule, so whether something dissolves is a battle between whether the randomness per molecule can overcome the attractions between the same kind. The longer the polymer, the less randomness is introduced and the greater any difference in energy between same and dissolved. So the longer the polymers, the less likely they are to dissolve in each other which, as an aside, is why you get so much variety in minerals. Long chain silicates that can alter their associate ions like to phase separate.

So we cannot recycle, and they are useless? Well, no. At the very least we can use them for energy. My preference is to turn them, and all the organic material in municipal refuse, into hydrocarbons. During the 1970s oil crises the engineering was completed to build a demonstration plant for the city of Worcester in Massachusetts. It never went ahead because as the cartel broke ranks and oil prices dropped, converting wastes to hydrocarbon fuels made no economic sense. However, if we want to reduce the use of fossil fuels, it makes a lot of sense to the environment, IF we are prepared to pay the extra price. Every litre of fuel from waste we make is a litre of refined crude we do not have to use, and we will have to keep our vehicle fleet going for quite some time. The basic problem is we have to develop the technology because the engineering data for that previous attempt is presumably lost, and in any case, that was for a demonstration plant, which is always built on the basis that more engineering questions remain. As an aside, water at about 360 degrees Centigrade has lost its hydrogen bonding preference and the temperature increase means oil dissolves in water.

The alternative is to burn it and make electricity. I am less keen on this, even though we can purchase plants to do that right now. The reason is simple. The combustion will release more gases into the atmosphere. The CO2 is irrelevant as both do that, but the liquefaction approach sends nitrogen containing material out as water soluble material which could, if the liquids were treated appropriately, be used as a fertilizer, whereas in combustion they go out the chimney as nitric oxide or even worse, as cyanides. But it is still better to do something with it than simply fill up local valleys.

One final point. I saw an item where some environmentalist was condemning a UK thermal plant that used biomass arguing it put out MORE CO2 per MW of power than coal. That may be the case because you can make coal burn hotter and the second law of thermodynamics means you can extract more energy in the form of work. (Mind you, I have my doubts since the electricity is generated from steam.) However, the criticism shows the inability to understand calculus. What is important is not the emissions right now, but those integrated over time. The biomass got its carbon from the atmosphere say forty years ago, and if you wish to sustain this exercise you plant trees that recover that CO2 over the next forty years. Burn coal and you are burning carbon that has been locked away from the last few million years.

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”.