Biogenic carbon capture at pulp mills via sodium spiking and oxy-fuel calcination

Over 13 million metric tons of biogenic carbon dioxide (CO₂) are mineralized yearly in United States (US) pulp mill recovery boilers as molten sodium carbonate. The mineralized CO₂ is released downstream in the rotary lime kiln where it can be captured at a relatively low cost for permanent sequestration. The use of biocarbon and bioenergy at pulp mills enables the possibility for atmospheric carbon removal when biogenic CO₂ is captured and sequestered. We demonstrate the feasibility of capturing biogenic CO₂ via sodium spiking coupled with oxy-fuel calcination in the rotary lime kiln at existing kraft pulp mills. Sodium (Na) spiking elevates the Na ion loading in the kraft chemical looping process by replacing chlorine-based bleaching with a highly alkaline bleaching sequence. Chemical pulping processes are modeled and simulated to understand the technical limits of implementing sodium spiking in existing pulp mills. Each 1 % of sodium added to the bleaching operations increases the rate of CO₂ mineralization by 4 %, and the maximum increase in sodium content in the kraft process is 9 %. Without sodium spiking, estimated CO₂ capture costs are $131 and $107/mt CO₂ for air and oxy-fuel combustion, respectively. With the implementation of sodium spiking, the cost of CO₂ captured decreases by 27 % and 31 %, showing costs of $96 and $74/mt CO₂ for air and oxy-fuel combustion, respectively. Oxy-fuel combustion reduces the costs of CO₂ capture compared to air combustion, but merged deployment with sodium spiking seems to be the most cost-effective pathway to integrate carbon capture.