Energies 2023 , 16 , 746
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means that annual fossil CO 2 emissions are around 3–4 MtCO 2 or roughly 10% of total industrial fossil CO 2 emissions of the sector. At the moment, Finland and Sweden have been leading the decarbonization of their lime kilns (Figure 4) [21]. Thus, switching to biofuels is feasible, but it comes with some economical and technical challenges, especially in cases of retrofits. Biofuels have different properties, for example, lower heating value than conventionally used oil and natural gas, which make it difficult to operate existing lime kilns without supporting fossil fuels. It can be expected that new-build lime kilns use biofuels or other renewable alternatives. Moreover, the availability of biomass varies within the mills, which affects the feasibility to use biofuel for decarbonizing the lime kilns.
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Figure4. Estimated current fuel mix in lime kilns of the Finnish pulp and paper industry.
The estimate for the CO 2 emission reduction potential of energy efficiency improve- ment was at 7 MtCO 2 /a. In addition, changes in production mix may lead to lower energy consumptions and therefore lower emissions, but the total fossil CO 2 emission reduction due to energy savings is estimated to remain below 10 MtCO 2 /a. It was esti- mated that the emission reduction potential of dryers and lime kilns are 1–2 MtCO 2 /a and 3–4 MtCO 2 /a, and thus, the decarbonization of processes can lead to an emission reduction of 4–6 MtCO 2 /a. As the European PPI currently emits 34 MtCO 2 /a, the decar- bonization of energy production plays the largest role in the reaching carbon-neutral PPI. As the PPI has access to wood residues and most of the energy supply is already covered using renewables, in the near future, electrification will not play as significant a role in the PPI as in other industrial sectors such as the steel industry. Even though part of the energy demand can be covered by the intensified use of current biofuels, biomass can be considered as a limited resource, and therefore, electricity-based solutions are also needed. High-temperature heat pumps have been seen as a significant part of the future energy supply in the PPI [55]. Increasing competition for renewable electricity and rising prices may be a matter of concern. Obrist et al. [55] claim that the Swiss paper industry that does not produce any chemical pulp can reach net-zero emissions by 2050, but this would lead to an 8–15% increase in paper production costs. Moreover, the re-structuring of energy supply and processes requires significant modifications in the mills. In the long-term future, the role of electricity may increase in the PPI as the increasing share of bio-based residues may be utilized as products. Even though the direct use of electricity is the best solution in terms of efficiency, it is possible that hydrogen will play a role in industrial energy systems as a storage for renewable energy. 3.4. Opportunities to Provide Additional CO 2 Savings The PPI has access to sustainably harvested biomass. The raw materials cannot be fully converted to pulp and paper, and therefore, large bio-based side streams enable the sector to contribute to the decarbonization of other sectors by producing renewable energy,
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