Skoglund et al.
10.3389/fther.2023.1282028
TABLE 6 Key energy fl ows of the kraft pulp mill for the three different process con fi gurations.
Base case
Carbon capture
Lignin extraction and carbon capture
Recovery boiler primary steam production [MW]
286
286
248
Recovery boiler high temperature excess heat [MW]
110
1
0
Minimum hot utility requirements [MW]
0
0
26
Minimum cooling requirements [MW]
107
175
162
TABLE 7 Estimated minimum utility boiler heat production and maximum potential power generation from the back-pressure steam turbine under different assumptions about the trade-off between fuel use and electricity generation.
Optimization scenario
Base case Carbon capture Lignin extraction and carbon capture
Minimized fuel use in utility boiler
Utility boiler steam production [MW]
0
0
25.7
Back-pressure power production [MW]
50.6
1.3
0
Maximized back-pressure electricity production Utility boiler steam production [MW]
0
61.2
85.6
Back-pressure power production [MW]
50.6
62.5
59.9
In the case study mill, existing boiler and turbine capacity is currently fully utilized, at least during certain periods of the year (see further Section 4 for a discussion regarding seasonal variations). Consequently, any increase in targeted fuel use or power generation would require investment in new boiler or turbine capacity, respectively. While the capacity of the existing power boiler (95 MW) does not represent the varying (spare) capacity available to cover new heat demands, the estimated utility boiler steam requirements can be compared to this value to get a rough idea about the need for new boiler investments. Overall, the results show that the integration of a carbon capture plant strongly affects the energy balances of the mill, and if lignin extraction is implemented in the mill where carbon capture is integrated, the energy balances become even more constrained. In the case where lignin extraction is implemented in the mill, the theoretical energy targets show that additional fuel use in a utility boiler would be required independently of the trade-off between fuel use and power co-generation.
However, when back-pressure electricity production is maximized, both the case with only carbon capture and the case with both lignin extraction and carbon capture require signi fi cant use of a utility boiler. The required new steam production can be compared to the current steam production in the recovery boiler (286 MW) which would be reduced to 248 MW if lignin is extracted. Note that for the base case, i.e., for the pulp mill without carbon capture and lignin extraction, additional utility boiler use would not be required (based on the theoretical energy targets) even when back-pressure electricity production is maximized since the heat available from the recovery boiler is suf fi cient to cover the demands of the integrated mill and steam cycle. The potential power generation varies somewhat between the process con fi gurations when the back-pressure electricity production is maximized, depending on how large the heat requirements are for the integrated processes. The two process con fi gurations with carbon capture (with or without lignin extraction) show a very similar increase in electricity production potential when comparing maximized power generation with minimized fuel use. Consequently, the corresponding increase in utility boiler use is also very similar. The results should also be put in relation to existing boiler and turbine capacities in the mill. Because of the signi fi cant effect on utility boiler requirements if aiming for maximized co-generation, the integration of a carbon capture process is likely to require new boiler capacity in these scenarios, and since the power co- generation potential increases compared to the base case, potentially also new turbine capacity. Available capacity in existing boilers and turbines will, therefore, be important input for deciding to what extent minimized fuel use or maximized power generation should be prioritized. The optimal solution is likely to be a trade-off, where, power co-generation is maximized within the capacity constraints of existing boilers and turbines, and additional hot utility requirements are met with minimized fuel use as the main priority.
3.3 Carbon fl ows for the different mill con fi gurations
Figure 6 shows differences in estimated biogenic carbon fl ows for the three different mill con fi gurations, both in the scenario when fuel use is minimized (Figure 6A) and in the scenario when back- pressure steam turbine power generation is maximized (Figure 6B). Note that the fi gure only shows carbon fl ows of product and exhaust streams, which are affected by the integration of the carbon capture and/or the lignin extraction plant. Figure 6 shows that if carbon capture is integrated in a mill with lignin extraction more biogenic carbon would be converted to products (compared to integrating carbon capture in a mill
Frontiers in Thermal Engineering
10
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