(195d) From Landfill to Cement: Developing a Negative Emission Hydrothermal Pathway to Sequester Waste Carbon

The 2021 greenhouse emissions for the United States were reported to be 6.4 billion tons of CO₂-eq. Nearly 12% of all emissions are related to two industries: 1) Municipal and Industrial Solid Waste Management and 2) Cement Production. Waste management primarily relies on landfilling to sequester waste, however, once capped landfills become a large emitter of methane (CH₄) which has 30 times the global warming potential of CO₂. On the other hand, ordinary Portland cement (OPC) requires calcium oxide (CaO). To produce CaO at the rate and quantities needed for modern cement demands, calcium carbonate (CaCO₃) is calcined to remove CO₂ and produce CaO. This step alone generates 50% of all emissions from cement production. To offset the massive production of greenhouse gases, strategies often focus on direct air capture (DAC) or point source capture (PSC) which look to take dilute or low-pressure CO₂, concentrate it, and then either sequester it or utilize the recovered CO₂ for alternative processes. Focusing these conditions requires significant energy inputs that inherently draw away from the removal capabilities. However, by using a more carbon dense feed such as organic municipal waste and coupling it with a downstream highly CO₂-selective precipitation, the CO₂ removal potential greatly increases.

We took a two-pronged approach to validate this system for commercial deployment. Experimentally, food waste was driven to high purity CO₂ at temperatures and pressures well within the defined supercritical point for water with a stoichiometric excess of hydrogen peroxide to act as an oxidant. Next, a batch pressure reactor was used to demonstrate the enhanced kinetics of mineralization for a high pressure CO­₂ feed. The high pressure CO₂ was then mixed with a buffered aqueous solution with free calcium ion (Ca²⁺). Elevated pressures (>100 psi) of CO₂ were found to achieve 10% conversion to CaCO₃ in 10 minutes. Atmospheric CO₂ (P_CO2 = 0.87 psi) under the same reaction conditions was found to have negligible conversion. Lastly, the resulting mineral has been used as part of the aggregate material to produce alternative cement mortars. Using the experimental results, an extensive lifecycle analysis revealed that when these industries are combined, dry food waste can remove ten times its weight in CO₂ rather than emitting back from a landfill.