Some time ago I had posts on biofuels, and I covered a number of processes, but for certain reasons (I had been leading a research program for a company on this topic, and I thought I should lay off until I saw where that was going) I omitted what I believe is more optimal. The process I had eventually landed on is hydrothermal liquefaction, for reasons as follows.
The first problem with biomass is that it is dispersed, and it does not travel easily. How would you process forestry wastes? The shapes are ugly, and if you chip onsite, you are shipping a lot of air. If you are processing algae, either you waste a lot of energy drying it, or you ship a lot of water. There is no way around this problem initially, so you must try to make the initial travel distance as short as possible. Now, if you use a process such as Fischer Tropsch, you need such a large amount of biomass that you must harvest over a huge area, and now your transport costs rise very fast, as does the amount of fuel you burn shipping it. Accordingly, there are significant diseconomies of scale. The problem is, as you decrease the throughput, you lose processing economies of scale. What liquefaction does is reduce the volume considerably, and in turn, liquids are very much easier to transport. But to get that advantage, you have to process relatively smaller volumes. Transportation costs are always less for transport by barge, so that gives marine algae an increased desirability factor.
A second advantage of liquefaction is that you can introduce just about any feedstock, in any mix, although there are disadvantages in having too much variation. Liquefaction produces a number of useful chemicals, but they vary depending on the feedstock, and to be useful they have to be isolated and purified, and accordingly, the more different feedstocks included, the harder this problem. Ultimately, there will be the issue of “how to sell such chemicals” because the fuels market is enormously larger than that for chemicals, but initially the objective is to find ways to maximize income while the technology is made more efficient. No technology is introduced in its final form.
Processing frequently requires something else. Liquefaction has an advantage here too. If you were to hydrogenate, you have to make hydrogen, and that in turn is an unnecessary expense
unless location gives you an advantage,
e.g. hydrogen is being made somewhere nearby for some other purpose. In principle, liquefaction only requires water, although some catalysts are often helpful. Such catalysts can be surprisingly cheap, nevertheless they still need to be recovered, and this raises the more questionable issue relating to liquefaction: the workup. If carried out properly, the water waste volumes can be reasonably small, at least in theory, but that theory has yet to be properly tested. One advantage is that water can be recycled through the process, in which case a range of chemical impurities get recycled, where they condense further. There will be a stream of unusable phenolics, and these will have to be hydrotreated somewhere else.
The advantages are reasonably clear. There are some hydrocarbons produced that can be used as drop-in fuels following distillation. The petrol range is usually almost entirely aromatic, with high octane numbers. The diesel range from lipids has a very high cetane number. There are a number of useful chemicals made, and the technology should operate tolerably cheaply on a moderate scale, whereupon it makes liquids that can be cheaply transported elsewhere. In principle, the technology is probably the most cost-effective.
The disadvantages are also reasonably clear. The biggest is that the technology has not been demonstrated at a reasonable scale, so the advantages are somewhat theoretical. The costs may escalate with the workup, and the chemicals obtained, while potentially very useful, e.g. for polymers, are often somewhat different from the main ones currently used now, so their large-scale use requires market acceptance of materials with different properties.
Given the above, what should be done? As with some of the other options, in my opinion there is insufficient information to decide, so someone needs to build a bigger plant to see whether it lives up to expectations. Another point is that unlike oil processing, it is unlikely that any given technology will be the best in all circumstances. We may have to face a future in which there are many different options in play.
