Hydrochar: the spectrum of possibilities
Hydrothermal carbonization offers a wide spectrum of possibilities for carbon-neutral or carbon-negative economies. In this newsletter we offer a framework how to assess and evaluate all the options.
Over the last few weeks in this newsletter, we tried to advocate for hydrothermal carbonization, a technical solution to the carbon emissions problem that is simple to implement, including decentrally in emerging economies, relies on well-established chemical processes, and offers a range of end products that can be returned into a carbon-neutral economy or sequestered to enable a carbon-negative economy.
The array of possibilities can be quite dizzying, so we want to use this week’s newsletter to present a structure that helps us sift through the available options and formulate a strategy of what we should do now, what we can do later, and what we should avoid entirely for lack of a perspective.
A simple framework
Our framework consists of a five-step flowchart: three stages (“input”, “output”, “end product”) connected by two transformations (“process” and “application”). Each step can be mapped out from “low” to “high”, typically translated as low cost to high cost, low quality to high quality, simple to complicated.
1. Inputs
Our primary claim is that we're better off focusing on solid biomass rather than carbon dioxide. Solid biomass includes sludge and other wet biomass, from grape residue to invasive water plants to chicken poop, all rich in carbon.
This is also the first factor to categorize our inputs, from “low” to “high”: biomass comes in different qualities, and roughly speaking, the higher the quality, the higher the cost and the more limited the supply. Sewage sludge and contaminated biowaste would certainly make up the low end of the spectrum, as we might have to consider the contaminants in the process or ultimately in the storage.
Wet biomass is best processed via hydrothermal carbonization, dry biomass such as woodchip is better suited for dry processes such as pyrolysis or torrefaction.
Ultimately we envision a carbon economy where these processes are combined to maximize the range of usable inputs.
2. Processes
The primary focus of this newsletter is hydrothermal carbonization (HTC) of wet biomass into hydrochar. Our more general focus is carbonization of solid biomass, wet or dry, into biochar. This includes dry processes like pyrolysis or torrefaction.
We consider hydrothermal carbonization as the “lower quality” of the processes. This might sound counterintuitive, but it has distinct advantages when it comes to creating a decentralized carbon economy.
We’re dealing with cheaper, more ubiquitous biomass, we offer a process that is simple to implement even in emerging countries, and we produce an output that is more amenable to a range of applications, from sequestration to terraforming to biofuels, than the higher-quality pyrochar.
Scaling dry processes will ultimately run into problems procuring high-quality dry biomass, which has other economic uses. Most of our inputs can be considered waste with little or even negative economic value.
Ultimately we see all processes that turn solid waste biomass as complementary and necessary to create a solid carbon economy.
3. Outputs
The output of our carbonization step, and the intermediate product of the whole process flow, is biochar — charcoal from biomass. We generally call dry biochar “pyrochar” and wet biochar “hydrochar” to differentiate the two main process types.
For most of the applications we consider, pyrochar can be considered the “higher quality” intermediate product compared to hydrochar. This might be a major reason why it attracts more attention, but ultimately this might work to its disadvantage for carbon storage projects.
High quality intermediate products produced from high quality biomass lend themselves to high quality industrial uses, so the economic rationale to sequester them is much smaller, and the overall window of making a carbon-negative economy feasible based on pyrochar alone is much smaller — and this is before even taking to account that the carbon efficiency of HTC is much higher than that of pyrolysis (up to 100% vs 30%).
4. Applications
Applications are all the things we can do with char, from industrial uses to transformation into biofuel and biogas to storage and sequestration. Our main focus here is on storage and sequestration, but as we noted before, the fact that re-used biochar (hydrochar or pyrochar) can contribute to a carbon-neutral economy makes it an intriguing economic proposal even where a carbon-negative storage is not yet feasible.
Often used synonymously, we use storage for mid-term, reversible carbon deposits and sequestration for long-term, hard-to-reverse carbon deposits.
Although our ultimate goal should be to find technologies that rebalance carbon emissions indefinitely, a fast and comprehensive carbon strategy contains a portfolio of storage and sequestration strategies. Reusing char in agriculture might make more sense than overengineered, energy-intensive attempts to push CO2 deep underground. There is no guarantee that highly complex industrial processes get us to a 1.5 degree goal faster than many simple, easy-to-implement, decentralized solutions.
5. End products
The end product or end use of our process is either sequestered, stored, or re-used char. None of these are in a strict sense end states in a circular economy, even long-term sequestered carbons can ultimately be released again, but for our purposes we consider them the end of our process flow.
Our spectrum from “low” to “high” goes both from short-term to long-term storage, mainly driven by the stability of the char in the environment until it gets released into the atmosphere, and from low quality to high quality char, from contaminated char to highly stable char suitable for agricultural use.
Again, hydrochar sits somewhat below pyrochar on this spectrum.
The whole process
A major reason why we mapped out the whole process is that — if we want to design a functioning market around negative carbon credits — we also have to map, record, and audit the whole process in digital form. This includes both the specific technical details and the owners of each step.
Mapping out the process is both a key prerequisite of creating transparency in the market and to start a long-term data collection to find the most efficient processes, applications, and end uses for the available biomass.
This is by far the most promising path to a balanced carbon economy.
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