This research explores the potential to re-purpose dry flue gas desulfurization (FGD) process technology, designed initially to remove sulfur dioxide (SO2) from coal-fired power plants, for direct air capture of CO2, while generating a solid byproduct that is the major ingredient for urea and nitrogen-based fertilizer. The envisioned patent pending1 process utilizes common dry FGD reactors that employ either the spray dryer absorber (SDA) or circulating fluid bed (CFB) concept. The proposed process uses simple dry, gas phase chemistry first explored at the Oak Ridge National Laboratory (Li, 2003) to convert carbon dioxide (CO2) and ammonia (NH3) to particulate matter, the latter a mixture of ammonium bicarbonate (NH3HCO3) and carbamate (NH4)2CO3. This particulate matter can be removed from a gas or air stream by the existing filtering hardware (possibly with modifications) following an SDA or CFB reactor. Notably, this dry, gas phase chemistry is distinctly different from aqueous-phase chemistry previously explored and commercialized (e.g., the chilled ammonia process). The byproducts of this dry chemistry support reactive capture of CO2 and combined with carbon-free water electrolysis and an electrified version of the Haber-Bosch process to produce ammonia (NH3), generate urea for crop fertilization at lower CO2 compared to conventional means. The strategic importance of reducing imports increases the market for fertilizer produced local to the point-of-use, supplementing the CO2 benefit of using direct air capture paired with green hydrogen and ammonia. This approach leverages the historical investment in coal-fired acid rain control technologies and produces fertilizers to satisfy the local demand all while maintaining employment at retired coal-fired power generation sites and providing revenue to local cities. The technical objective of Phase 1 of this work is to define the process conditions for the dry, gas phase reaction of NH3 and CO2 to remove CO2 from ambient air and maximize ammonium carbamate as the solid byproduct. These conditions are anticipated to be replicated at commercial scale in the highly turbulent mixing environment and extended residence times within SDA or CFB reactor, and fabric filter process equipment. The results will inform the design of an apparatus to thoroughly mix ammonia in air at the SDA or CFB reactor inlet, provide extended residence time for rapid gas phase reaction to form solid ammonium compounds, and subsequently collect the material as a particulate. Detailed tests will be conducted at laboratory bench-scale to define the role of (a) mixing of ammonia in air, (b) residence time, and (c) air temperature to achieve high CO2 removal and maximize carbamate content. Both physical and chemical features of the collected particulate matter that could affect removal in commercial, large-scale filter hardware will be measured. The results from Phase 1 will be the input for a demonstration-scale series of modules that could be tested in a Phase 2 program, which if successful will be further scaled and demonstrated at an actual commercial site in a Phase 3. Members of the proposed project team have collaborated for decades on similar tasks specifically directed to designing and testing bench-scale and pilot-scale facilities and transforming these results into design guidelines. This historical work - although mostly addressing controlling emissions of nitrogen oxides (NOx) also focused on extrapolating results of testing at bench- and pilot-scale to commercial equipment. The project team has extensive experience with monitoring, injection, and mixing of trace quantities of ammonia in gas streams as much work focused on ammonia-based selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) controls for NOx.
