Skoglund et al.
10.3389/fther.2023.1282028
FIGURE 3 Simpli fi ed fl owsheet of lignin extraction process through precipitation.
2.4 Heat integration assessment
TABLE 4 Assumed effects of lignin extraction in the mill.
Description
Unit
Value
To investigate how the integration of carbon capture would affect the heat balances and power co-generation potential at the pulp mill, an approach based on Pinch Analysis (see e.g., Kleme ลก , 2023) was used. Speci fi cally, pinch-based tools were applied to establish targets for heat recovery through optimized heat cascades as well as corresponding minimum utility requirements for different mill con fi gurations. Next, the theoretical targets for minimum hot and cold utility, as well as targets for minimum fuel use and maximized co-generation were compared between the different mill con fi gurations. The approach closely follows the method proposed by Svensson et al. (2019) for estimation of excess heat and co-generation targets. One of the strengths of applying an energy targeting approach is that the graphical representation of the optimized heat cascade, the GCC, can be conveniently used for visualizing the potential for heat integration between a process (e.g., the mill) and its utility system or another process (e.g., a capture process). This is commonly referred to as splitting the GCC (see e.g., Kemp and Shiun Lim, 2020). Basing the analysis of integration potential on energy targets also avoids the need to collect information regarding existing process heat recovery solutions in the mill, as well as the need to know how the heat recovery system for the new processes will be designed. However, it should be noted that the GCC represents heat fl ows in a process with maximum internal heat recovery and does not consider any constraints of the process layout and design besides the minimum temperature difference for heat exchange. Such ideal heat integration is unlikely to occur in an actual process plant, and especially not in an existing plant that has been revamped regularly over a long period of time. On the other hand, the assumption about ideal heat recovery allows for a consistent and fair comparison between different con fi gurations, avoiding the risk of comparing one optimized con fi guration with a non- optimized one. Furthermore, the heat integrated process may
Increased evaporator steam load
MWh/t lignin 0.028
Decreased recovery boiler steam production
MWh/t lignin 5.42
2 due to avoided combustion a
t CO
2 /t lignin 2.27
Reduced fl ue gas CO
CO 2 added to precipitation vessel
t CO 2 /t lignin 0.28
a Based on an assumed carbon content of lignin of 65.1 wt%.
combusted in the recovery boiler, thereby reducing the steam production in the boiler as well as the amount of CO 2 (available for capture) in the fl ue gases. The lost heat production must either be met by reductions in steam use in the mill processes or steam turbines, or by increased steam production (with corresponding CO 2 emissions) in utility boilers. Moreover, the lignin-lean black liquor and wash water streams returned from the lignin extraction plant means more water must be evaporated in the evaporation section, which consequently requires a higher heat demand. The CO 2 that is added to the precipitation vessel is assumed to react with the cooking chemicals of the liquor and form carbonates. The carbonates will remain with the liquor until the causticization unit, and the carbon fi nally is released as CO 2 withthe fl ue gas of the lime kiln. Since the CO 2 in the lime kiln is captured, this will result in a small recirculation of CO 2 . Overall, since the fl ue gases are assumed to be mixed before being sent to the capture plant, the increased release of CO 2 fromthe lime kilns and the changed reaction conditions in the recovery boiler are assumed to have only a minor effect on the concentration of CO 2 in the mixed fl ue gases. In this work, it is assumed that this effect is small enough to not affect the heating and cooling requirements of the carbon capture and liquefaction process per tonne of captured CO 2 . A summary of the effects of lignin extraction on mill heat and carbon balances is presented in Table 4.
Frontiers in Thermal Engineering
07
frontiersin.org
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