Skoglund et al.
10.3389/fther.2023.1282028
would be to use a utility boiler to generate enough additional steam to allow for co-generation of electricity in steam turbines, while providing the necessary amount of steam to the process. This would increase the fuel use at the mill, but the marginal electrical ef fi ciency of additional fuel use would be close to 100%. While the results of the integration assessment are illustrated graphically for the mill con fi guration with carbon capture but no lignin extraction in Figure 4, the minimum heating and cooling requirements were determined numerically for all mill con fi gurations. The results are presented in Table 6, which also shows the heat generation from the recovery boiler in each case. Table 6 shows that if the pulping process is modi fi ed to extract lignin from the black liquor, the recovery boiler would not generate enough heat to meet the minimum heat requirements of a carbon capture process integrated with the mill. For this mill con fi guration, there would be a de fi cit of heat from the recovery boiler, even if no co-generation of electricity is assumed. This is mainly due to the reduced heat production in the recovery boiler when the extracted lignin is no longer being combusted, thus reducing the heating value of the black liquor fuel sent to the recovery boiler. To balance the de fi cit of heat additional hot utility must be provided, e.g., by a power boiler.
FIGURE 5 Illustration of the integration between the processes and the steam turbine cycle, represented as split GCCs. The background process GCC represents ideal heat integration between heat sources and sinks in the mill and in the capture process, and includes unavoidable high-temperature heat from the recovery boiler.
3.2 Effects on fuel demand and power generation potential
foreground curve and the GCC of the pulp process is shown as the background curve. The shifted pinch temperature of the carbon capture and liquefaction process is 122 ° C, and since most of the excess heat from the pulping process is available below 100 ° C, the opportunity for heat integration between the two processes is very limited. Figure 4 shows that the high-temperature heat generated by combustion of black liquor in the recovery boiler of the mill has the potential to cover the combined heat demand of the carbon capture process and the mill. This will, however, require that almost all the high temperature excess heat is used for process heating. Currently this heat is used to generate high-pressure steam, which is run through a back-pressure steam turbine to co-generate electricity while also providing steam for heating the mill ’ s processes. If the heat demand of the carbon capture process should also be covered by the recovery boiler heat, this would require that the steam turbines are by-passed. Consequently, this would come at the cost of greatly reducing the electricity production. An alternative
For further investigation of utility boiler requirements and potential steam turbine power generation for the different mill con fi gurations, the integration of a steam turbine cycle was also estimated using foreground/background analysis (see Section 2.4). Two cases representing different trade-offs between fuel use and electricity generation were investigated for each mill con fi guration scenario. In the fi rst case, minimized fuel use is prioritized, and in the second case, maximized back-pressure steam turbine electricity generation is prioritized. For the mill con fi guration without lignin extraction where carbon capture is integrated, the maximized co- generation scenario is illustrated graphically in Figure 5. For this and the other mill con fi gurations and optimization scenarios, the results were derived numerically, and are presented in Table 7. In Table 7 it is shown that, if minimizing fuel use, it is only necessary to use a utility boiler when both lignin extraction and carbon capture are implemented in the mill, which is in line with the results from Table 6.
TABLE 5 Overview of investigated mill con fi gurations and optimization scenarios for the heat integration analysis.
Mill con fi guration
Description
Base case
Existing mill processes, assuming annual average heat fl ows and temperature data
Carbon capture
Existing mill with a heat integrated carbon capture and liquefaction process
Lignin extraction and carbon capture
Mill with lignin extraction plant and with a heat integrated carbon capture and liquefaction process
Optimization scenario
Description
Minimized fuel use
The use of additional (biomass) fuel in utility boilers is minimized. Co-generation of electricity is considered only if enough heat is generated in the recovery boiler to allow for combined heat and power production
Maximized power generation
Power generation in back-pressure steam turbines is maximized given that back-pressure steam from the turbine should match the minimum process net heating demand. No condensing turbine operation is considered
Frontiers in Thermal Engineering
09
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