Energies 2023 , 16 , 746
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fuels, and products and capturing biogenic carbon. Hence, in addition to targeting the cut of industrial emissions, the PPI’s potential to provide additional CO 2 savings must be exploited. 3.4.1. Additional Renewable Electricity Production The PPI can increase its renewable electricity production. A recovery boiler is most importantly a process component that regenerates used cooking chemicals in a kraft pulp mill, but it is additionally a large steam and electricity producer. The most important criterion for a recovery boiler has been high availability. Later, the mitigation of air emis- sions became an issue. Currently, the increasing production of renewable electricity is a key trend. An increase in steam parameters and black liquor dry solids contents and heat recovery from flue gases as well as improved preheating of both water and air can lead to notable growth in the electricity generation. According to Vakkilainen et al. [56], electricity generation in modern recovery boilers increased by 20% within 10 years, and thus, veritable opportunities for higher electricity production exist. Sweden is an example of a country which has notably increased electricity generation in the PPI [21]. The most important reasons behind the shift are an increase in the electricity price due to an electricity market reformation as well as an introduction of the EU ETS and a green electricity certificate system that supports the production of renewable electricity [57]. Thus, future energy markets as well as incentives for renewable electricity production most probably affect the development of electricity production in the PPI. Table 5 presents recovery boiler parameters for an average European mill and for a modern mill. Based on rough calculations presented by Valmet [58], it can be estimated that if all European recovery boilers operated as modern boilers, renewable electricity production could increase by roughly 7 TWh. If the electricity is used for replacing current European electricity production with an emission intensity of 296 gCO 2 /kWh [59], an annual savings of approximately 2 MtCO 2 could be achieved. It is noteworthy that the additional energy production in the PPI conflicts with the energy efficiency target that aims to limit energy consumption to a certain value; i.e., in spite of the increased production of excess energy, the input energy to the mill might not change.
Table5. Recovery boiler parameters: a European average and a modern mill.
Black Liquor Dry Solid Content
Steam Pressure and Temperature
Steam Generation per Input Dry Solids
Recovery Boiler
European avg 1
86bar/486 ◦ C 110 bar/515 ◦ C
77% 83%
3.64kg 4.35kg
steam /kg ds steam /kg ds
Modern 2
1 Estimated using collected recovery boiler data. 2 Estimated based on [58].
Despite the potential to increase renewable electricity production, alternative routes could lead to reduced generation. Processing of biomass to high-value products decreases available combustible material. For example, lignin separation has become an attractive option as lignin can be used as a versatile raw material or biofuel. Separating 30% of the lignin in black liquor decreases the electricity production in the recovery boiler by approximately 29% [60]. In addition, possible electrification may have an effect on onsite electricity production; i.e., if electricity is used for heating purposes, production of electricity in combined heat and power (CHP) units decreases. 3.4.2. Renewable Fuels and Materials The PPI can play a notable role in creating bioeconomy. Its side streams can be utilized in the production of biofuels (e.g., biogas and biodiesel) and bioproducts (e.g., textile fibers, fertilizers, and lignin derivatives). As an example, Figure 5 presents a simplified balance and opportunities of a northern stand-alone mill producing 600,000 ADt/a of softwood pulp. The mill generates approximately 340 kg/ADt and 760 kg/ADt of wood residues and dissolved lignin, which indicates the potential for enlarging the product portfolio to
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