Skoglund et al.
10.3389/fther.2023.1282028
by Kuparinen et al. (2021) and Kuparinen et al. (2023), the value of the captured biogenic CO 2 depends on future markets for green products that can be synthesized from the CO 2 (e.g., methanol), or the value of carbon removal credits. In the case of using CO 2 for carbon removals (negative emissions), it has also been shown that even though costs for capture and liquefaction dominate the costs for CCS, transport logistics and infrastructure are also deciding factors in the techno-economic evaluation of CCS (Karlsson et al., 2023). Other strategic considerations are also expected to in fl uence the decision, for example, related to sustainability criteria for biomass, development of standards and certi fi cations for negative emissions accounting, as well as the value of fl exibility towards increasingly volatile electricity markets.
would be to invest in additional utility boiler capacity, which would allow to run the carbon capture processes at full capacity all the time (also when lignin is extracted) but would require investments and additional fuel use. This can also be combined with investments in additional back-pressure turbine capacity to enable maximized co- generation of power. Other options would be to reduce electricity production (if not already at zero) by by-passing the turbines, or to run the capture plant at part load, thus reducing the steam demand for solvent regeneration, during periods when utility boiler capacity is limiting. The mill with both carbon capture and lignin extraction would have yet another fl exibility option, through the opportunity to vary the amount of lignin extracted from the black liquor. By running the lignin extraction plant at part load, the heat produced by the recovery boilers can be increased during periods when there is a de fi cit of steam from the power boiler. Temporarily reducing the carbon capture or lignin extraction rate could also be interesting options to make more steam available for the steam turbines when electricity prices are suf fi ciently high to make the value of electricity production higher than the value of CO 2 capture or lignin extraction, respectively. While any fl exibility option may be promising in a context of varying demands or prices, it is important to consider that varying operation would always involve part-load operation during certain periods. Whether this is handled by steam boiler and turbine system, the lignin extraction plant, or the capture plant, this would imply that any capacity invested in will not be fully utilized. With regards to varying operation of the capture plant, its consequences for utilization of the invested capacity in downstream liquefaction, intermediate storage, and transport and logistics chain should also be considered. In this study, CO 2 capture was considered only for the fl ue gases from the recovery boiler and lime kiln, and the capture rate was assumed to be 90%. However, the capture plant could be sized for other combinations of fl ue gases sources and/or other capture rates, which would be associated with different bene fi ts and drawbacks. One potentially interesting option is to capture CO 2 also from the utility boiler to maximize the recovery of biogenic carbon in products instead of releasing it to the atmosphere. This can make a signi fi cant difference in carbon recovery ef fi ciency, especially in the cases with lignin extraction, where the load of the utility boiler is the highest. However, as discussed in Section 2.2, it would be more dif fi cult to establish the optimal design and operation of a carbon capture process applied to the fl ue gases from the utility boiler due to the variations in the utility boiler load. Carbon capture applied on the utility boiler fl ue gases would also further increase the demand for utility boiler capacity and fuel use, due to the increased heat demand. Another interesting option for the mill with lignin extraction could be to design the capture plant for partial capture, by sizing the plant to capture only the amount of CO 2 that can be captured using steam from the recovery boiler. This would result in a steam balance similar to that of a plant with carbon capture but without lignin extraction, but with an important difference in product distribution, due to the extracted lignin. In the end, the deciding factor between these different options forcon fi guration and design of the capture plant will be the expected technoeconomic performance, which in turn depends on the future value of the captured biogenic CO 2 , the lignin product, as well as electricity and fuel prices. As thoroughly discussed and investigated
5 Conclusion
In this study, pinch analysis was used to determine the theoretical minimum heat requirements of a kraft pulp mill with or without lignin extraction, in which a carbon capture process is integrated. The carbon capture process is assumed to capture 90% of the carbon dioxide from the recovery boilers and lime kilns in the mill. The results show that when carbon capture technology is implemented and fuel use is minimized at the case study mill, the heat available from the recovery boilers (286 MW) is suf fi cient to cover the heat demands of the integrated mill and capture processes. However, the amount of high-temperature excess heat available for back-pressure electricity production is greatly reduced, from around 50 MW to almost zero. When carbon capture instead is integrated to a mill where lignin is extracted from the black liquor, the heat produced by the recovery boilers is not suf fi cient to cover the process demands, and approximately 26 MW of additional heat from the utility boiler is required. The increased utility boiler load results in more carbon dioxide being released from the mill since the carbon dioxide emissions from the utility boiler are assumed to not be captured. On the other hand, when extracting lignin, more of the carbon from the biomass feedstock is recovered in useful products, since some of this biogenic carbon ends up in the extracted lignin, and this should have a higher value than captured carbon dioxide. When production of back-pressure electricity is prioritized instead of minimizing the fuel use, additional heat must be supplied by the utility boiler, but an opportunity for generating back-pressure electricity with very high ef fi ciency can be realized. The resulting difference between the electricity-maximization and the fuel-minimization scenario is very similar for a mill with and without lignin extraction. For both mill con fi gurations, prioritization of back-pressure electricity generation leads to a similar increase in electricity production (ca 60 MW), at the cost of a corresponding increase in fuel use and carbon dioxide emissions from the utility boiler. However, in the mill with lignin extraction the boiler capacity is more likely to become a limiting factor, possibly requiring new investments. Overall, the combined integration of lignin extraction and carbon capture has a strong negative effect on the steam balances of a kraft pulp mill, with the capture process signi fi cantly increasing the minimum steam requirement at the same time as extraction of lignin reduces the steam production in the recovery boiler. However,
Frontiers in Thermal Engineering
12
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