Some basic economics of char-based carbon capture

Ultimately, every proposed carbon capture technology must pass the economic feasibility test: Can it compensate for enough carbon emissions at the lowest economic footprint? We offer a first estimate.

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Jan 3

When we started out looking at candidate technologies for a carbon capture ecosystem, our first criterion was that it has to be economically feasible and scalable globally, including in emerging countries.

In other words, it has to be a “Model T” technology rather than a “Humvee” technology. It has to be simple, functional and easily fixable even in rough environments, and it has to simplify the logistics of harvesting, processing and sequestering biomass as much as possible.

In one word, we were looking for a technology that can be deployed decentrally.

Far too many of the currently proposed technologies suffer from being overengineered, with little hope that they can overcome their early-stage limitations and reach a stage where they can be decoupled from an expensive subsidy regime — something only the richest industrial countries can afford.

Turning Christmas trees into carbon

In last week’s newsletter, we teased a few numbers about a potential carbon-negative Christmas tree industry. Let’s elaborate a bit on our numbers we offered to illustrate the economic feasibility of char-based processes (pyrochar or hydrochar).

Taking the European Christmas tree market, we started with 50 million trees at an average weight of 15 kg, for a total available biomass of 750000 metric tons. Assuming a conversion rate of 85% hydrothermal carbonization (HTC) and a carbon content of 50% for the resulting hydrochar, we’ll create some 638 kilotons of hydrochar, containing roughly 319 kilotons of carbon.

At a stoichiometric conversion factor of 3.67, we can compensate the equivalent of 1.17 million tons of CO2. Adding some 75 million Christmas trees in the United States, we’ll manage to compensate somewhat less than 3 million tons of CO2.

Comparing this to carbonization via pyrolysis, we assume a weight reduction from drying to 42.5% of the original wet wood biomass, a conversion rate of 35% from dry biomass to pyrochar, and a carbon content of 85% in the resulting pyrochar, we can compensate for roughly 470 kilotons in Europe, and 686 kilotons in the US.

These are of course only rough estimates, and we foresee that both processes will play a significant role in a future carbon negative ecosystem. The more dry biomass we have available, and the more we consider agricultural reuse, the more we should consider pyrochar as it exhibits a stronger biological stability.

On the other hand, conversion rates and carbon efficiency clearly favor hydrothermal carbonization over pyrolysis. This is particularly relevant if the available biomass is wet and we have no economic reason to remove the water after the process.

In most cases, wet biomass is available at a much lower price than the dry biomass. This is because the latter is very often already used thermally in combustion plants and is unfortunately only carbon neutral and not carbon negative.

Ultimately, as we always like to point out, hydrochar and pyrochar aren’t competitors, but complements in a comprehensive carbon capture strategy.