US pulp and paper production is primarily driven by the kraft pulping process, which accounts for approximately 73% of all papermaking processes. [4]. The kraft pulp and paper industry, more than any other industry, has great potential for achieving carbon neutrality through the capture of its CO₂ emissions [1], [5]. Moreover, the CO₂ captured in the industry has various utilization options internally for tall oil manufacture, lignin separation, production of precipitated calcium carbonate (PCC), brown stock washing, near-neutral bleaching, and pH control for stock preparation [6]. Additionally, the captured CO₂ can be utilized to produce bio-derived plastics, carbon-neutral fuels, and a range of other chemicals that are currently derived from fossil-fuel-based feedstocks. A comprehensive analysis of these applications has been explored by Gulzar et al. [7]. However, the energy-intensive nature of CO₂ capture has limited the adoption of CO₂ capture and utilization in the pulp and paper industry. Furthermore, the biogenic nature of most of its emissions excludes these emissions from CO₂ emission regulations.
The three main CO₂ emission sources from the kraft process, the recovery boiler (RB), the bark boiler (BB), and the lime kiln (LK), have been reported to emit 76%, 14%, and 10% of the total CO₂ emitted, respectively. [8]. However, the partial pressure (or composition) of CO₂ in BB and RB is relatively low (12% and 13% mole fraction, respectively [8]), limiting their potential as a capture source. The high CO₂ content and non-biogenic nature of lime kiln emissions made it an attractive and potentially cost-effective capture source. Consequently, it has been the primary focus of most research works [6, 9, 10]. However, the large volume of CO₂ from other sources, especially the recovery boiler, can be cost-effective when comparing the unit cost per tonne of captured CO₂.. Moreover, the utilization of this purely biogenic CO₂ can significantly reduce the carbon footprint of its derived products.
This study evaluated the cost of CO₂ capture from all major flue gas sources in the kraft pulp as well as four different combinations of these sources (BB+RB, BB+LK, RB+LK, and BB+RB+LK). Two sets of flue gas data were evaluated: one sourced from a hypothetical mill as reported by Onarheim et al. [8] and the other a real data provided by a kraft mill. A chemical absorption method was employed using monoethanolamine (MEA) as the solvent. The use of chemical absorbents for CO2 capture is a mature technology, and these absorbents are better suited to handle low-pressure CO2 in effluent gases [11, 12]. MEA is a suitable solvent compared to other solvents due to its high selectivity, fast kinetics, high absorption capacity, recovery, purity, low cost, and biodegradability [13 - 15]. The CO2 capture process was simulated using Aspen plus V14 and the KEMEA (Kent-Eisenberg) data package. The whole process was divided into three sections: the dehydration section, the capture section, and the compression section. The flue gas was quenched, and 99% of its constituent water was removed in the dehydration section before the capture process. Radfrac columns with structured packings were used for the absorber and stripper, and a rate-based model was used for the capture process. The captured CO₂ with >98% purity (wt.%) was then compressed to 150 bars and thus liquefied for transportation and storage.
To determine the optimal process configuration with the lowest capture cost, a derivative-free optimization (DFO) and cost analysis were carried out. RBFOpt solver [16] was used for the optimization, while the cost models and parameters developed by Turton et al. [17] were used for techno-economic analysis (TEA). The RBFOpt solver was executed in a Python script and integrated with Aspen Plus for optimization. Key optimization variables included the number of absorber, stripper, and direct contact cooler stages, absorber and stripper inlet temperatures, CO₂ loading and MEA concentration, and CO₂ recovery rate. An optimization of these eight variables was done to minimize the capture cost.
The results showed that CO₂ capture from RB ($59.7 per tonne of CO₂ captured) was more cost-effective compared to BB ($71.1), LK ($89.8), and BBLK ($65.5). This cost decreased when RB was combined with other sources. Overall, the capture cost from all sources combined (BB+RB+LK) yielded the lowest cost ($59), outperforming individual sources or any of the other combinations. These findings highlight the significant influence of flue gas volume on capture economics. While the recovery boiler’s low CO₂ concentration might seem disadvantageous, its large flue gas volume compensates for it, reducing the overall cost per tonne of CO₂ captured. Therefore, CO₂ capture efforts should be focused on combined flue gas capture rather than a single source.
Due to the adverse environmental and health effects of MEA, future research will focus on the optimization and techno-economic analysis of alternative solvents, and the results obtained will be compared to this study. This comparative analysis will extend to a comprehensive life cycle assessment aimed at evaluating the overall environmental impact of the chosen solvents. Moreover, considering the significant influence of the stripper's energy requirements on the reboiler heat duty, which subsequently affects the overall capture cost, future investigations will prioritize the reduction of this heat duty through process intensification strategies. A decreased capture cost is expected to enhance the economic viability of utilizing carbon dioxide for value creation while simultaneously mitigating the CO₂ emissions.
References
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