Some time ago I made a number of posts on biofuels, by concentrating on what I saw as the pros and cons of individual technologies, but by themselves, while you may have your own ideas as to their usefulness, one of the more important points is they lacked perspective. Of course it is hard to get perspective of such a wide field in a 600 word post. Another odd thing about those posts was I did not get around to posting about hydrothermal liquefaction, or hydrothermal hydrogenation, which, in my opinion, are likely to be the most useful technologies. Of course I am also biased, because these are the areas in which I have actively worked and published on and off over the last 35 years. The reason I got into those areas was that early in my career, while working for the main New Zealand government chemical research lab, I was given the job, and a useful travel budget, to try to survey what were the possibilities, and to unravel what the more promising (if any) options were. As a consequence of that, I have now repeated the exercise (without the travel budget!) and put my conclusions into another ebook that I am publishing on July 31.
 
The important aspect about such a survey is that it must explain why it is important to develop biofuels and to do that, numbers have to be put on the assertions. I feel that is the biggest problem with current work in this area. It is true, and I conclude this, that there is no single 'magic bullet', and that a very large number of resources will have to be used, and there is no harm in using resources that are available, even if, by doing so, you will be doing something that is not general. But it is also important to end up with a limited range of fuels. There is no point in having 120 different fuels on the market, when a given motor can only reasonably operate on one. Now, if you put numbers on resources, you very quickly find that if you want to eat, and you want to retain something of the natural land-based environment, you cannot replace oil from the land. There is simply insufficient area that is reasonably useful. Accordingly, I conclude that eventually we have to utilize the oceans. Now the problem here is that we have very little truly adequate technology to do this with. On the other hand, we know that in principle we can grow the algae. Problems include getting past "in principle". There have been clear demonstrations of growing macroalgae in deep water, but the experiment by the US navy started in the 1970s got wrecked in a storm, and when, at the time, the price of oil collapsed, the project was stopped. That does not mean it cannot be restarted, but it will require more work to solve the obvious problems.
 
So why do I think hydrothermal liquefaction is such a desirable technology to chase? Largely because it can process any biomass and with some reservations, provided one adjusts the methodology to be suitable for the resource. It then produces either drop-in fuels, or fuels than need a little more processing, however, once one gets to the liquid state, it is much easier to transport the "pre-fuel" to a refinery for upgrading. Can we totally replace oil? Probably not. Probably we shall have to reduce the wasted travel, but in principle we can come reasonably close. And while I most certainly do not claim to have all the answers, I am putting what I have out there.

My theory of planetary formation differs from the standard theory in three ways. The first two are that the standard theory has no mechanism to reach the initial position that it assumes, which is an even distribution of planetesimals (asteroid-sized bodies) with respect to distance from the star. (There is a lowering of concentration due to greater circumference as r increases.) My theory requires accretion to be due to chemical means (which includes physical chemistry), which means the distribution of accreting bodies is highly temperature dependent. The third major difference is that the standard theory has everything accreting through the collision of similar-sized bodies, so it becomes very slow; my theory requires accretion to be continuous and proportional to the gravitational cross-section, hence major bodies grow very quickly or not at all. The maths show that once one body in a region is significantly larger than any of the others, it alone tends to grow, by sweeping up all the smaller objects.
 
In a previous post, I used my theory of planetary formation to predict the properties of the two planets around Kapteyn’s star. Over the last few weeks there have been further papers in accord with my theory, and against the standard theory of planetary formation. In one (Science 344: 1150) the use of the hafnium/tungsten chronometer showed that the iron meteorite parent bodies formed over an interval of 1My, and within 0.1 – 0.3 My of calcium aluminium inclusions. However there is also evidence that the latter formed over about 3 My (Science 338: 651) so the iron meteorite bodies were amongst the earliest bodies in the solar system to form, at least in the zone of the rocky planets. That is required by my theory, because iron meteorites had to form at least within the first My, and probably over an even shorter time.
 
My theory requires rocky planets, with the possible exception of Mercury, to accrete through water acting as an initial setting agent for silicaceous cements, which gives the initial body enough strength prior to gravity becoming strong enough. This requires the water to be here initially, and not through cometary bombardment. There have been some papers recently that argue that seismic evidence suggests a zone between 410 and 660 km deep that contains 1 – 3% water (Science 344: 1265). That cannot get there by cometary impact, and had to have been there initially, which is well in accord with my theory.
 
Growth of moons should follow the same rule in my mechanism, and a recent paper (Icarus 237: 377) gave interesting support, in that the moments of inertia of Callisto and Titan inferred from gravity data suggest incomplete differentiation of their interior, which implies cold accretion. Simulations show accretion rate plays only a minor role, and the fraction brought by large impactors plays a more crucial role. The simulations show that a satellite exceeding 2,000 km in radius may accrete without experiencing significant melting only if its accretion is dominated by small impactors and if more than 10% of satellite mass was brought by satellitesimals larger than 1 km, global melting for large bodies like Titan and Callisto cannot be avoided.
 
On the other hand, there was one paper that suggests an alteration to what I put in the ebook is required. When I wrote the book, all available evidence stated that the isotope ratios of most elements in Moon and Earth samples were the same. This was a problem, because in the giant impactor scenario, since isotope ratios seem to be dependent on radial distance from the star, the impactor should have had different isotope compositions. My suggestion was to support a previous proposition, namely that the impactor formed at the same radial distance, specifically at one (or both) of the Lagrange positions L4 or L5. If so, the Moon would have formed towards the end of Earth’s accretion (because it needs Earth to have a significant gravitational field at these positions) and because the rocky planets were supposed to accrete by adding additional material as it headed starwards, there should have been a slight difference in oxygen isotope ratios. On this proposition, the impactor would not accrete many small iron containing bodies either, because the lunar feed would lie outside the zone where iron melted.
 
So, overall, I remain reasonably happy. The very first forms of this theory were  laid down in the mid 1990s, and of course I started to keep track of evidence for or against. Some of the evidence as interpreted by the authors naturally supported the standard theory, but so far nothing I have seen has contradicted fundamentally what I started with, although of course there were some minor adjustments. There is a slightly weird feeling when your theory contradicts what everyone else thinks, and nothing falsifies it over twenty years, and also some of what has deeply puzzled almost everyone has a natural explanation.