(344c) Decarbonizing Lime Kilns at Pulp Mills Via Oxy-Fuel Combustion/Electrification.

The pulp and paper industry represents 2% of global industrial greenhouse gas emissions, of which the vast majority are biogenically derived. Thus, these biogenic carbon emissions represent an opportunity for carbon-negative pulp and paper processing if captured and sequestered. Despite this potential, the integration of these technologies has yet to be deeply explored in the pulp and paper industry. Recovery boilers, biomass and multi-fuel boilers, and lime kilns are the primary sources of carbon emissions in an integrated pulp mill. The most significant contributor is the recovery boiler, followed by multi-fuel boilers, with lime kilns contributing the least. However, lime kilns have an advantage from a carbon capture standpoint because they have the highest concentration of carbon dioxide (CO₂) emissions and are capable of handling even higher concentrations. Therefore, research on the feasibility of retrofitting or re-designing lime kilns to oxy-fuel combustion or electrification has gained significant attention over the past decade due to its generation of high-purity CO₂ (> 90%) and potential for low-cost CO₂ capture. In addition, multiple Direct Air Capture companies are developing electrified kilns for calcium carbonate (CaCO₃) calcination, and thus this research is directly applicable to such endeavors. The project involves assessing the feasibility of operating pulp mill lime kilns under high CO₂ conditions to elucidate potential barriers to using oxy-fuel or electrified lime kiln technologies. First, a kinetic assessment for the calcination of lime mud (mainly CaCO₃) from a commercial chemical pulp mill under high CO₂ concentrations was carried out. Both constant heating rate (FWO) and isothermal kinetic assessments were performed using TGA under idealistic conditions. Second, insight from the kinetic assessments was used to design experiments with a three-zone splittable, high-temperature, electrified rotary tube furnace under more realistic conditions. The large, rotary tube furnace was confined to operate within conditions acceptable to the pulping industry. A 1D numerical model was then developed in KilnSimu to simulate the calcination baseline (15% CO₂ concentration) and the high CO₂ scenario (90% CO₂ concentration) using a commercial lime kiln as a reference. To demonstrate the technology, three kilograms of lime mud were processed in the rotary kiln under a high CO₂ scenario (90% CO₂), reaching an extent of calcination greater than 95% with 70 minutes of retention time. In a lime kiln operation, producing good quality burned lime is essential. Burned lime (CaO) quality is assessed by its residual CaCO₃ content, availability, and reactivity. The amount of CaO in the burned lime available for slaking represents the lime availability. Moreover, lime reactivity refers to the speed at which the CaO can be hydrated in the slaker, forming calcium hydroxide (Ca(OH)₂), which reacts with the sodium carbonate present in the green liquor to regenerate the sodium hydroxide (NaOH) needed in the pulping process. The obtained burn lime under high CO₂ calcination was causticized with a 12.5% sodium carbonate solution for 2.5 hours at 90 ℃, confirming the NaOH production. Then, the precipitated CaCO₃ was processed again in the rotary kiln under high CO₂ conditions to prove the reproducibility of the calcium cycle. The extent of calcination for the second round was similar to the first result, with a CaO content above 95%. This first-of-a-kind study demonstrates the technical feasibility of repetitively calcining pulp mill-derived lime mud effectively into CaO under conditions expected in commercial-scale, electrified kiln technologies.