Carba, an American climate tech company founded in 2021, develops carbon removal technology using low-temperature pyrolysis (torrefaction) to convert biomass waste into biochar, which can then be buried to seal carbon in place for generations. This process is energy neutral, requiring near zero additional power.
(This article contains 5 diagrams and 1478 words.)
Challenges
Carbon emissions
Since the early 1900s, carbon dioxide (CO₂) levels in the atmosphere have increased by 50% due to human activities. When fossil fuels (such as coal, oil, and natural gas) are burned for energy production, transportation, and industrial processes, CO₂ is released into the atmosphere. This excess CO₂ acts as a greenhouse gas, trapping heat and causing the air and ocean temperatures to rise. CO₂ emissions play a crucial role in driving climate change.
This warming effect has caused the global average temperature to rise by about 1.1 ºC since the pre-industrial period. This has led to rising in the frequency and intensity of extreme weather events, melting of polar ice caps and glaciers and rising sea levels, shifts in species ranges and increased risk of species extinction, agriculture and food security, and ocean acidification.
To mitigate these impacts, the Paris Agreement aims to limit global warming to well below 2 ºC above pre-industrial levels. The Intergovernmental Panel on Climate Change (IPCC) estimates that a “carbon budget” of about 500 GtCO₂, which corresponds to about ten years at current emission rates, provides a 66% chance of limiting global warming to 1.5 ºC.
Biochar carbon removal
Unlike Direct Air Capture (DAC), which requires significant energy to concentrate CO₂ from the atmosphere, the method of biochar carbon removal leverages the natural process of photosynthesis to pre-concentrate carbon in biomass. This method begins with plants absorbing atmospheric CO₂ and converting it into biomass during photosynthesis. Then, waste biomass is converted into a stable form of carbon (biochar) via pyrolysis, specifically torrefaction.
The biochar is buried underground, acting as a carbon sink. Proper burial environments avoid aerobic decomposition of biochar back into CO₂, ensuring stability for >1,000 years, akin to natural coal formations. This long-term carbon storage contributes to the reduction of CO₂ in the atmosphere, helping to mitigate climate change. In addition to its role in carbon sequestration, biochar improves soil performance by enhancing the retention and diffusion of water and nutrients.
Carba Technology
Carba has developed a low-temperature pyrolysis (torrefaction) reactor system for converting biomass waste into biochar. This process involves heating the biomass at a low temperature (300–450 ºC) and transforming cellulose and hemicellulose molecules into a stable biochar. The biochar is then buried underground, effectively sequestering the carbon for thousands of years. The company's method is energy neutral, requiring almost no additional energy, and is regarded as the most energy- and material-efficient use of biomass waste.
Process flow of Carba biochar carbon removal
The diagram below depicts the process flow of Carba’s biochar carbon removal.
First, biomass sourcing involves collecting various types of plant and tree waste like agricultural residues, forestry waste, urban yard waste, and food waste from local sources to minimize transport emissions. Collected biomass is prepared before entering the reactor, like drying and/or chipping. Chipping reduces biomass to uniform particle size for efficient reactor feeding and pyrolysis. Biomass is fed batch wise into the torrefaction reactor.
Next is the biomass torrefaction process. The torrefaction reactor is ideally located at or near the burial sites. The reactor uses a twin screw conveyor to mix and heat the biomass. The temperature range is 300–450 ºC to minimize cracking and maximize solid carbon yield. The pyrolysis vapors (syngas, methane) generated during torrefaction are burned to provide heat to the reactor. This makes the torrefaction process self-sustaining (autothermal).
After torrefaction, the biochar product is collected. The reactor yields about 80% of the input biomass as biochar.
Finally, long-term storage involves burying the biochar in anoxic conditions. The burial depths are ideally 0.6–0.8 meters and oxygen levels are <1–2%. Storage methods include burial in abandoned mines, landfills, or purpose-dug pits, and covering with materials like soil or clay.
For example, mines are frequently abandoned without reclamation, which can lead to environmental problems. Landfills produce leachate-containing toxins including perfluoroalkyl “forever” chemicals (PFAS), heavy metals, and hydrocarbons. While torrefied carbon is buried in a mine or landslide, it can absorb environmental contaminants.
Carba torrefaction reactor system
The diagram below depicts Carba’s low-temperature torrefaction reactor system.