Trees for Carbon Capture, and Subsequent Problems

A little over fifty years ago, a 200 page book called The Limits to Growth was published, and the conclusion was that unless something was done, continued economic and population growth would deplete our resources and lead to global economic collapse around 2070. Around 1990, we predicted that greenhouse gases would turn our planet into something we would not like. So, what have we done? In an organized way, not much. One hazard with problem solving is that focusing on one aspect and fixing that often simply shifts it, and sometimes even makes it worse. Currently, we are obsessed with carbon dioxide, but all we appear to be doing is to complacently pat ourselves on the back because we shall be burning somewhat less Russian gas and oil in the future, oblivious to the fact that the substitute is likely to be coal.

One approach to mitigate global warming involves using biomass for carbon capture and storage (See Nature vol 609, p299 – 305). The authors here note that the adverse effects of climate change on crop yields may reduce the capacity of biomass to do this, as well as threaten food security. There are two approaches to overcoming the potential food shortage: increase agricultural land by using marginal land and cutting down forests, or increase nitrogen fertilizer. Now we see what “shifting the problem” means. If we use marginal land, we still have to increase the use of nitrogen fertilizer. This leads to the production of nitrous oxide gas, and these authors show the production of nitrous oxide would be roughly three times as effective as a greenhouse gas as the saving of carbon dioxide in their model. This is serious. All we have done is to generate a worse problem, to say nothing about the damage done to the environment. We have to leave some land for animals and wild plants.

There is a further issue: nitrogen fertiliser is currently made by reacting natural gas to make hydrogen, so for every tonne of fertilizer we will be making something like a tonne of CO2. Much the same happens if we make hydrogen from coal. Rather interestingly for such a paper, the authors concede they may have over-estimated the problems of food shortages on the grounds that new technology and practices may increase yields.

Suppose we make hydrogen by electrolysing water? Ammonia is currently made by heating nitrogen and hydrogen together at 200 times atmospheric pressure. This is by no means optimal, but higher pressures cost a lot more to construct, and there are increasing problems with corrosion, etc. Hydrogen made by electrolysis is also more expensive, in part because electricity is in demand for other purposes, and worse, electricity is also made at least in part by burning fossil fuels, and only a third of the energy is recovered as electricity. When considering a new use, it is important to not that the most adverse in terms of cost and effectiveness must be considered. Even if there are more friendly ways of getting electricity, you get favourable effects by doing nothing and turning off the adverse supply, so that must be assigned to your new use.

There is, however, an alternative in that electricity can directly reduce nitrogen to nitride in the presence of lithium, and if in the presence of a proton-donating substance (technically an acid, but not as you would probably recognize) you directly make ammonia, with no high pressure. So far, this is basically a laboratory curiosity because the yields and yield rates have been just too small, but there was a recent paper in Nature (vol 609, 722 – 727) which claims good increased efficiency. Since the authors write, “We anticipate that these findings will guide the development of a robust, high-performance process for sustainable ammonia production.” They do not feel they are there yet, but it is encouraging that improvements are being made.

Ammonia would be a useful means of carrying hydrogen for transport uses, but nitrogen fertilizer is important for maintaining food production. So can we reduce the nitrous oxide production? Nitrous oxide is a simple decomposition product of ammonium nitrate, which is the usual fertilizer used, but could we use something else, such as urea? Enzymes do convert urea to ammonium nitrate, but slowly, and maybe more nitrogen would end up in the plants. Would it? We don’t know but we could try finding out. The alternative might be to put lime, or even crushed basalt with the fertilizer. The slightly alkaline nature of these materials would react in part with ammonium nitrate and make metal nitrate salts, which would still be good fertilizer, and ammonia, which hopefully could also be used by plants, but now the degradation to nitrous oxide would stop. Would it? We don’t know for sure, but simple chemistry strongly suggests it would. So does it hurt to do then research and find out? Or do we sit on our backsides and eventually wail when we cannot stop the disaster.

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Fuel for Legacy Vehicles in a “Carbon-free” Environment

Electric vehicles will not solve our emissions problem: there are over a billion petroleum driven vehicles, and they will not go away any time soon. Additionally, people have a current investment, and while billionaires might throw away their vehicles, most ordinary people will not change unless they can sell what they have, which in turn means someone else is using it. This suggests the combustion motor is not yet finished, and the CO2emissions will continue for a long time yet. That gives us a rather awkward problem, and as noted in the previous posts on global warming, there is no quick fix. One of the more obvious contributions could be biofuels. Yes, you still burn carbon, but the carbon came from the atmosphere. There will also be processing energy, but often that can come from the byproducts of the process. At this point I should add a caveat: I have spent quite a bit of my professional life researching this route so perhaps I have a degree of bias.

The first point is that it will be wrong to take grain and make alcohol for fuel, other than as a way of getting rid of spare or spoiled grain. The world will also have a food shortage, especially if the sea levels start rising, because much of the most productive land is low-lying. If we want to grow biomass, we need an area of land roughly equivalent to the area used for food production, and that land is not there. There are wastelands, but they tend to be non-productive. However, that does not mean we cannot grow biomass for fuel; it merely states there is nowhere nearly enough. Again, there is no single fix.

What you get depends critically on how you do it, and what your biomass is. Of the various processes, I prefer hydrothermal processing, which involves heating the biomass in water up to supercritical temperatures with some additional conditions. In effect, this greatly accelerates the processes that formed oil naturally. Corresponding pyrolysis will break down plastics, and in general high quality fuel is obtainable. The organic fraction of municipal refuse could also be used to make fuel, and in my ebook “Biofuel” I calculated that refuse could produce roughly seven litres per week per person. Not huge, but still a contribution, and it helps solve the landfill problem. However, the best options that I can think of include macroalgae and microalgae. Macroalgae would have to be cultivated, but in the 1970s the US navy carried out an exercise that grew macroalgae on “submerged rafts” in the open Pacific, with nutrients from the sea floor brought up from wind and wave action. Currently there is work being carried out growing microalgae in tanks, etc, in various parts of the world. In principle, microalgae could be grown in the open ocean, if we knew how to harvest it.

I was involved in one project that used microalgae grown in sewage treatment plants. Here there should have been a double benefit – sewage has to be treated so the ponds are already there, and the process cleans up the nitrogen and phosphate that would otherwise be dumped into the sea, thus polluting it. The process could also use sewage sludge, and the phosphate, in principle, was recoverable. A downside was that the system would need more area than the average treatment plant because the residence time is somewhat longer than the current time, which seems designed to remove the worst of the oxygen demand then chuck everything out to sea, or wherever. This process went nowhere; the venture needed to refinance and unfortunately they left it too late, namely shortly after the Lehman collapse.

From the technical point of view, this hydrothermal technology is rather immature. What you get can critically depend on exactly how you do it. You end up with a thick brown fluid, from which you can obtain a number of products. Your petrol fraction is generally light aromatics, with a research octane number (RON) of about 140, and the diesel fraction can have a cetane number approaching 100 (because the main components are straight chain C15 or C17 saturated hydrocarbons. Cetane is the C16 equivalent.) These are superb fuels, however while current motors would run very well on them, they are not optimal.

We can consider ethanol as an example. It has an RON somewhere in the vicinity of 120 – 130. People say ethanol is not much of a fuel because its energy content is significantly lower than hydrocarbons, and that is correct, but energy is not the whole story because efficiency also counts. The average petrol motor is rather inefficient and most of the energy comes out as heat. The work you can get out depends on the change of pressure times volume, so the efficiency can be significantly improved by increasing the compression ratio. However, if the compression is too great, you get pre-ignition. The modern motor is designed to run well with an octane number of about 91, with some a bit higher. That is because they are designed to use the most of the distillate from crude oil. Another advantage of ethanol is you can blend in some water with it, which absorbs heat and dramatically increases the pressure. So ethanol and oxygenates can be used.

So the story with biofuels is very similar to the problems with electric vehicles; the best options badly need more research and development. At present, it looks as if they will not get it in time. Once you have your process, it usually takes at least ten years to get a demonstration plant operating. Not a good thought, is it?