CO2 mineralization in gas scrubbers using basalt slurries

Motivation

• BMBF:
  5-15 % of current greenhouse gas emissions are unavoidable
• IPCC:
  Negative emissions are needed to limit the rise in temperature
• Leopldina:
  To achieve greenhouse gas neutrality, ... emissions ... must be offset by removing CO₂ from the atmosphere
• Global emissions currently just under 40 Gt per year
  Removal of at least 4 Gt per year is required in the long term

Options

Idea

Combination of Direct Air Capture and Accelerated Weathering in a gas scrubber using basalt slurry

Idea/potential

• Basalt is the most common rock in the Earth's mantle
• Basalt typically contains
  10 mass % calcium oxide (CaO) and
  10 mass % magnesium oxide (MgO)
• CaO and MgO react with CO₂ to form calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃)
• 1 ton of basalt absorbs 188 kg of CO₂ as carbonates or
• 376 kg as dissolved hydrogen carbonates Hydrogen carbonates only exist in aqueous solution and are not to be considered for CO₂ storage

Preliminary work

• Qualitative experiments with wash bottle
  - Wash bottle 250 ml
  - Sludge 30 %
  - 2.5 l/min air 747 ppm 548 ppm > 15 min
• Literature research
• Calculations
• Preparation of patent specification
• Development of test apparatus

Idea/execution

• Mining and crushing basalt
• Grind basalt to 50 µm
• Transport basalt to the gas scrubber
• Prepare 30 % basalt powder slurry
  z. B.: 230 kg basalt + 770 kg water (+ 27 kg table salt)
• Batch operation
  Pump the sludge in the scrubber with air supply until the pH stops falling
• Dewater and dispose of sludge

Yield/kinetics

• What determines the speed?
  - Mass transfer of CO₂ from air to water
  - Diffusion in the water to the grain boundary
  - Diffusion in the grain
  - Dissolution of CaO/MgO in water
  - Reaction between CaO/MgO and CO₂
  - Waking up of carbonate layers on the grain
  - Degree of grinding
  - Composition of the basalt
  - Temperature
• In the absence of knowledge:
  Mass transfer of CO₂ from air to water

Measurement of CO₂ capture

• Measure air volume flow and CO₂ concentration at the inlet and outlet (also records hydrogen carbonates)
• Gravimetric
  Raw material
  - Drying up to 200 °C weighing, firing > 850 °C weighing
  - Mass difference equal to contained_CO2
  Reacted sludge
  - Drying up to 200 °C weighing, firing > 850 °C weighing
  - Mass difference equal to contained + absorbed CO₂
• Alternative to firing: Expel the CO₂ with hydrochloric acid
• Wet chemical
  Raw material
  - Dry to 200 °C, dissolve and titrate with hydrochloric acid above pH 8.3 to pH 4.3
  Reacted sludge
  - Dry to 200 °C, dissolve and titrate with hydrochloric acid over pH 8.3 to pH 4.3
  Difference gives the absorbed CO₂
• IR spectroscopy