Scalable, economical, and stable sequestration of agricultural fixed carbon

Abstract

We describe a scalable, economical solution to the carbon dioxide problem. CO₂ is captured from the atmosphere by plants, and the harvested vegetation is then buried in an engineered dry biolandfill. Plant biomass can be preserved for hundreds to thousands of years by burial in a dry environment with sufficiently low thermodynamic "Water Activity," which is the relative humidity in equilibrium with the biomass. Maintaining a dry environment within the engineered dry biolandfill is assisted by salt that preserves biomass, which has been known since Biblical times. A "Water Activity" <60%, assisted by salt, will not support life, suppressing anaerobic organisms, thus preserving the biomass for thousands of years. Current agricultural costs, and biolandfill costs, indicate US$60/tonne of sequestered CO₂ which corresponds to ~US$0.53 per gallon of gasoline. The technology is scalable owing to the large area of land available for nonfood biomass sources. If biomass production is scaled to the level of a major crop, existing CO₂ can be extracted from the atmosphere, and will simultaneously sequester a significant fraction of world CO₂ emissions.

Keywords: agriculture; anaerobic; biolandfill; carbon capture; sequestration.

Fig. 1.

A simplified version of the bio-landfill technology. It is essential to keep the biomass dry. A key role is played by dual layers of high density polyethylene adding up to 4 mm thickness, as a water diffusion barrier. In 1 y, <1.75 μm equivalent water thickness diffuses through. This rate of water diffusion can be accommodated for thousands of years by the dry salt-biomass mixture which can absorb the water without increasing its own relative humidity (Water Activity) above 60%. Water Activity remaining below 60% suppresses all life, and all bio-degradation. A more complete version of this biolandfill is in SI Appendix, Fig. S2A–C.

Fig. 2.

A graph showing the CaCl₂ salt fraction needed to compensate the presence of excess water in Miscanthus biomass. This is derived from the water sorption isotherms shown in SI Appendix, Fig. S1.2 A and S1.3 B. The goal is to maintain a Water Activity <0.6, equivalent to 60% relative humidity, the blue line. If there is a 17% ratio by mass, total water in very wet miscanthus, (represented by the big blue dot in Fig. 2), the required CaCl₂ salt/miscanthus ratio required is 2%. CaCl₂ is an inexpensive road-de-icing salt, and would contribute <$3 cost per tonne of biomass. To obtain a Water Activity of <0.6, drying miscanthus to a ~0.12 water/miscanthus mass ratio, eliminates the need for salt addition.

Fig. 3.

We generally estimate costs through a bottom up analysis of all inputs. But there is a 2nd method shown here, the revenue received by farmers at the Chicago Board of Trade. We reason that the median prices received by farmers is an upper limit to the cost of production. Food attracts hungry parasites, requiring intensive farming, with costs of US$850-US$1450/hectare shown in the figure. Non-food biomass crops tend toward the lower end of that range.

Fig. 4.

A breakout of bottom-up farming cost and landfill cost for three different biomass crops, per tonne of CO₂ sequestered. The detailed analysis was done in SI Appendix, sections 4 and 5. A tonne of biomass sequesters ~1.83 t of CO₂.

Fig. 5.

The Judean Date-Palm tree called Methuselah, of historical and cultural significance, germinated in 2005 from a 2,000-y-old seed, which had been stored in a dry location adjacent to the Dead Sea. (Photo acknowledgement, Guy Eisner).