Skoglund et al.
10.3389/fther.2023.1282028
FIGURE 6 Carbon fl ows from the mill that are affected by the integration of a carbon capture process or lignin extraction process. Results for different mill con fi gurations and two scenarios: (A) minimizing fuel use, and (B) maximizing back-pressure steam turbine power generation. Note that the carbon fl ow associated with the core pulp product (ca 25 ton C/h) is not included since it is the same for all con fi gurations and scenarios.
streams. When designing an actual heat exchanger network, a more careful selection of temperature difference contributions for individual process and utility streams is likely necessary to ensure that the design can come close to the estimated energy targets while ensuring operability and an economically sound distribution of heat transfer area within the heat recovery system. However, for this study, varying the (global) minimum temperature difference for heat exchange had no signi fi cant effect on the overall energy targets for minimum heating and cooling requirements, which are the main results of the pinch analysis. In short, the potential improvements in heat recovery that could be achieved by allowing for a smaller temperature difference would be minor compared to the total energy requirements. The case study mill, as any pulp mill, is subject to varying conditions that affect the operation and heat fl ows in the mill. This includes, for example, seasonal variations in feedstock temperature and defrosting requirements, make-up water temperature, district heating demands, etc., which all increase the heat demand during winter. Since this study evaluates the mill during typical normal operation represented by annual mean operating data, it does not capture seasonal variations and their effect on the required utility boiler capacity. Furthermore, there are also short-term variations in the steam balances of the mill due to, e.g., variations in feedstock properties, various process disturbances, and cleaning and maintenance schedules, which typically are managed by a utility boiler. Altogether, the effect of the different sources of variations in the mill is large enough that the existing power boiler, at times, during winter, is fully utilized, while it can be completely shut down during the summer. Consequently, the existing power boiler has a rather large spare capacity available during summer, which can be used to cover additional steam demands for carbon capture integration, but almost no spare capacity during winter. Limited and varying availability of additional capacity in the existing power boiler can be managed in different ways. One option
without lignin extraction). However, due to the unfavorable energy balances of this mill con fi guration, it would also result in more carbon being emitted. If only considering the carbon ef fi ciency of the process, this mill con fi guration would perform worse than the carbon capture con fi guration without lignin extraction, since the increase in emitted carbon is higher than the increase in recovered carbon in products. A simple carbon ef fi ciency would, however, not capture the usefulness of the recovered carbon and the value of the different product streams. Obviously, the potential for high-value applications is expected to be much higher for lignin than for both CO 2 and the solid biomass fuels used in the utility boiler. The analysis of direct impacts on carbon fl ows in the mill would also not capture the indirect effects on CO 2 emissions resulting from the changes in mill ’ s (net) electricity balance. Comparing the results for scenarios A and B in Figure 6 shows how the maximization of electric power generation clearly increases the amount of emitted (biogenic) carbon (from the mill site) compared to the scenario where fuel use is minimized, due to increased emissions from the utility boiler. However, in a wider systems perspective, the effects on CO 2 emissions would be strongly dependent also on the emissions intensity of grid electricity generation, which would be replaced by the electricity from the mill. Note that the results for carbon fl ows (similarly to the results for energy balances) are affected by the assumption of 90% capture of CO 2 from the fl ue gases. With a lower capture rate, it would be possible to avoid increased wood fuel use and associated emissions from the utility boiler. On the other hand, less carbon would be captured and the emitted carbon from the recovery boiler would be higher.
4 Discussion
For the pinch analysis presented in this paper, a minimum temperature difference for heat exchange of 10 K was assumed for all
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