Biochar, hydrochar, or both?
Comparing and contrasting the solid carbonization technologies.
In last week's newsletter we discussed the key reasons to use solid carbon-based processes rather than CO2-based processes for carbon capture and sequestration. Today we're having a closer look at the various processes that can transform solid biomass into carbon end products – mostly char – and its differences and commonalities.
Biochar or hydrochar?
The processes we're interested in all produce char from waste biomass. Two of the processes, pyrolysis and torrefaction, are dry processes ideal for using dry biomass. The third process, hydrothermal carbonization (HTC) is ideal for wet biomass and typically produces wetter end products similar to lignite (brown coal) called hydrochar.
As such, with different source materials and somewhat different end products, the processes differ in scope and are mostly complementary rather than competitive. Depending on the environment, we expect all three to catch on and find widespread adoption in the canon of carbon capture technologies.
There are a few reasons why we put our emphasis on hydrochar and HTC rather than dry biochar processes. For one, wet biomass is ubiquitous and has few other current uses: grass cuttings, thinning and fermentation residues, sewage sludge, pomace and other wet plant residues tend to be low-value or even environmentally problematic byproducts of agricultural production. Compared to this, the original ingredients for torrefaction and pyrolysis: woodchips, dry wood or straw, have competing uses, so they might be expensive or hard to come by.
In addition, hydrochar production via HTC has a much higher carbon efficiency: close to 100% of the carbon contained in the original biomass is converted into hydrochar. For pyrolysis, the typical rate for industrial plants is around 30%.
Biochar and hydrochar?
But beyond finding different uses for the solid carbon processes, there is also the option to use them both, as the example of magnetized bamboo demonstrates. In this recent study bamboo powder was turned into magnetic hydrochar via a succession of hydrothermal and pyrolytic carbonization. So it’s not exactly and either-or question, we expect more interesting and useful combinations to emerge.
Some more readings
Deirdre Lockwood “Researchers turn fish waste into a coffee-scented biofuel” Chemical and Engineering News (2015).
Jianhua Qua et al. “One-pot synthesis of Ca-based magnetic hydrochar derived from consecutive hydrothermal and pyrolysis processing of bamboo for high-performance scavenging of Pb(Ⅱ) and tetracycline from water” Bioresource Technology (2021).
Natalie Wolchover “Work on Earth’s climate and other complex systems earns Nobel Prize in Physics” Quanta Magazine (2021).
Follow us on twitter: @hydrochar.
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