A couple of years ago there was a flurry of interest in hydrothermal carbonisation, another way to make char from biomass.

Earlier we referred to carbon-negative energy systems that rely on gasification and biochar sequestration: biomass is gasified which results in a carbon monoxide and hydrogen rich gas that can be used for energy or transformed into ultra-clean synthetic biofuels via the Fischer-Tropsch process, whereas a fraction becomes bio-char that can be stored in soils (using a technique known as 'terra preta'). Similar techniques can be build around pyrolysis processes (earlier post). In such systems, soil fertility would be gradually enhanced, 'historic' CO2 would be sequestered and clean biofuels could be used to power our societies.

Only biomass can be used for the creation of such carbon-negative energy systems that clean up our emissions from the past. Other renewables are carbon-neutral at best, meaning they can only reduce future CO2 emissions - something many scientists think is not enough to avert dangerous climate change.

Maria-Magdalena Titirici, Arne Thomas and Markus Antonietti of the Department of Colloid Chemistry at the Max Planck Institute of Colloids and Interfaces, now describe a new, highly efficient though 'low-tech' way to use biomass as a tool to clean up past emissions. Their research appears in an open access article in the New Journal of Chemistry, in which they suggest creating "turbo-rainforests" based on fast-growing energy crops that are grown, turned into bio-coal via a process known as hydrothermal carbonization (HTC), and then stored into 'carbon landfills', while deriving energy from the process. The technique can be practised on an ultra-large scale, and can thus be described as a geo-engineering option - one that is actually technically and economically feasible.

Importantly, in contrast to other biomass carbonisation techniques that require dry biomass, the hydrothermal carbonisation process is a highly efficient 'wet' process that avoids complicated drying schemes and costly isolation procedures. The resulting carbonaceous materials also open a new field of chemistry, full of novel possibilities and challenges that may lead to the development of new (nano)materials: . . .

Finally, to summarize the outcome of the optimization trials, catalyzed HTC required only the heating of a biomass dispersion under weakly acidic conditions in a closed reaction vessel for 4–24 h to temperatures of around 200 °C. This is indeed an extremely simple, cheap and easily scalable process. Besides that, HTC has a number of other practical advantages. HTC inherently requires wet starting products or biomass, as effective dehydration only occurs in the presence of water, plus the final carbon can be easily filtered from the reaction solution. This way, complicated drying schemes and costly isolation procedures can be conceptually avoided. In addition, under acidic conditions and below 200 °C, most of the original carbon stays bound to the final structure. Carbon structures produced by this route—either for deposit or materials use—are therefore the most CO2-efficient.

Once activated, HTC is a spontaneous, exothermic process. It liberates up to a third of the combustion energy stored in the carbohydrate throughout dehydration (due to the high thermodynamic stability of water). . .

Therefore, we strongly believe that the carbonization of fast growing plants is currently the most efficient process for removing atmospheric CO2, binding it into depositable carbon or even as useful solids.

For a negative atmospheric CO2 balance, the generated carbonaceous materials have to be deposited on a large scale, and potential carbon landfills may lay the foundations for chemical starting materials of the next century.

Another quite attractive application with immediate impact is their use as water- and ion-binding components to improve soil quality. This is a chemical process that is also found in nature, and carbonaceous soil is presumably the largest active carbon sink on earth. The proposed terra preta, i.e. artificial coal-enriched soil as a potential carbon sink of global dimensions, has already been mentioned in soil research, improving soil quality and plant growth at the same time. Instead of clearing the rainforest for questionable palm oil production, such a carbon-reinforced "turbo-rainforest" would produce at least 10 times the energy, but stored in carbon, whilst also being CO2-negative for the climate and supporting biodiversity at the same time.

There have been some projects funded and even one that would operate in a deep abandoned mine shaft where atmospheric pressure is higher, saving the need to pressurize the reaction vessel, and the heat generated would be used to generate power.

But some question the whole concept.

The world already has a huge amount of coal in storage, and the burning of that coal will be responsible for a great deal of global warming. So if you want to use plants as a way to do something about climate change, the best thing you can do is not engineer them to make new coal, but simply burn their biomass for energy and leave an equivalent amount of coal safely unperturbed. Burning the carbon stored in plants before microbes can eat it all up and leaving coal in the ground makes a lot more sense than trying to store away the carbon.

And this in turn illustrates a larger lesson that bears strongly on tackling the carbon-climate crisis. The fact that a great deal of energy was stored away in fossil fuels over time has conditioned people to think of energy itself as something embodied in fuels. But energy, which cannot be created or destroyed, is far better seen in terms of flows than of stores.

An extraordinary amount of solar energy flows through the earth-system, coming in as sunlight, leaving as infrared radiation. On its way through the system it runs through many different channels, like the wind and the waves and the carbon cycle. The challenge of the carbon-climate crisis is to put to work these flows and others — the flow of heat stored for billions of years in the interior of the earth, and of energy stored away earlier still in the nuclei of radioactive elements — in ways that make civilization independent of the fossil fuels stored away in the crust.

The question of how to use the biosphere against global warming is thus better seen in terms of harvesting energy from the carbon cycle, rather than storing away carbon. And there is much that can be done here. Biomass already supplies a lot of energy — a large part of the world cooks with it, for example — but the ways in which it is used are terribly inefficient. New agronomy, new crops and new technologies can all add to the flow of energy out of the plant and into the cooker battery, hot water or whatever. In that way, bioenergy can be substituted for fossil fuel.

In itself, expanding the carbon cycle this way cannot be the whole solution. While the world’s supply of solar energy is for all intents and purposes unlimited, its supply of well-watered, arable land is not. And although photosynthesis is so impressive in its workings as to be almost miraculous, it is not particularly efficient. Even in a plantation geared to nothing but the maximal growth of biomass, it is difficult to store away much more than 2 or 3 percent of the solar energy that hits the leaves, and it’s hard to see how that can be improved on a great deal. (There might be better efficiencies with algae and bacteria, but as yet they come with other problems.)

At that sort of efficiency — and bearing in mind that a lot of land is needed to feed people, and that a fair bit of the rest houses ecosystems we value and much more of the rest isn’t really cultivatable — bioenergy can never be more than a small-to-medium part of the energy supply in a fossil-fuel-free world.

But there is no need for a single solution; the more there are on offer, the merrier. And bioenergy, done properly, has a poetry of its own. To know how to use the mechanisms that keep the atmosphere alive in order to prevent any further harm to humanity is good in itself.

I see it a bit differently. CO2 levels have risen and will rise further. Plants like this and would like a great deal more. With correspondingly increased nutrient levels the amount of carbon in plant tissues at any given time will rise. The carbon cycle will be unchanged but the flows will be greater, with more carbon being drawn down, held for a time, and then released again.

At any given moment there will be more carbon tied up in biomass than in the recent past, which does reduce the amount in the air by some amount. And if some of that biomass is used for energy generation of the sort that produces a char residual - such as in some CHP pyrolysis systems - and the char is used as a soil amendment, then even more biomass can be produced. (I won't repeat why this is so here, see earlier posts if necessary, but you ought to know by now.)

This won't be enough to drop CO2 levels back to those of the last century as some would like, but it is a better use of biomass than simply burning it for its energy and so displacing some amount of fossil fuel use. Switching to other energy sources, such as the geothermal and nuclear sources mentioned above, will be required to reduce the use of fossil fuels, though they will always be the smart choice for some purposes.

NB - See a site search on stocks and flows to get some idea of why Oliver's analysis is compelling for me, though I have a somewhat different view. Stocks, flows and accumulation have been a minor theme here.