Skoglund et al.
10.3389/fther.2023.1282028
hand, the thermal energy systems of these sites may be strongly affected by the integration of new technology, thereby changing the conditions for the ef fi cient use of these renewable resources.
sources (recovery boiler, lime kiln and power boilers) of a chosen, conventional, case study mill, without consideration of simultaneous implementation of other strategic mill development projects. Only a few studies have considered the potential combination of carbon capture and other emerging process developments. For example, Möllersten et al. (2003) included the option of replacing the conventional recovery boiler with a black liquor gasi fi cation plant with pre-combustion CO 2 capture in their study on potential forCO 2 reductions in Swedish pulp and paper industry. Möllersten et al. (2006) also investigated CCS in combination with biomass gasi fi cation as a replacement for the conventional power boiler systems. More recently, Santos et al. (2021) proposed a new process concept based on calcium looping, which would replace a large part of the conventional chemical recovery cycle while also allowing for ef fi cient capture of CO 2 . Solid looping is an emerging concept, which seems to have promising techno-economic potential for carbon capture in several industrial sectors, including pulp and paper (Santos and Hanak, 2022). In a recent paper by Mendoza- Martinez et al. (2022), hydrothermal carbonation (HTC) of bio- sludge generated from wastewater treatment was investigated in a mill that was also assumed to have integrated post-combustion carbon capture on the recovery boiler and lime kiln. The biochar produced from the HTC process offers further potential for carbon removal. Skagestad et al. (2018) investigated the total capture cost for different capture integration cases representing full or partial CO 2 capture in a model mill. For this mill, which represents a modern, energy-ef fi cient mill using best available technologies, a signi fi cant amount of excess steam is available from the recovery boiler. When sizing a partial capture plant to use only this excess energy and no additional fuel, partial capture could be achieved at considerably lower speci fi c capture cost than full capture. In the same paper, Skagestad et al. (2018) also evaluated a CCU case, in which a part fl ow of the captured CO 2 is assumed to be utilized for lignin extraction. For this scenario, they also analyzed the effect of lignin extraction on the capture potential and emissions from the mill. However, for this case, only the partial capture scenario was considered, where the CO 2 capture rate was limited to the amount of CO 2 that could be captured using excess steam from the recovery boiler. This paper aims to investigate the effect of integrating full-scale (90%) carbon capture on the recovery boiler and lime kiln in a pulp mill, which is also assumed to implement a large-scale lignin extraction plant. To our knowledge, this is the fi rst published case study of a real pulp mill for which the combined integration of lignin extraction and carbon capture is analyzed. Compared to the ideal model mills previously studied in literature the mill can be assumed to have a less favorable energy balance, thus being more representative for integration of new technology in mills in the near term. The study illustrates the need to consider the interaction between different technology pathways for the industrial energy and climate transition, where in this case, the pulp mill can contribute to decarbonization by making either biogenic CO 2 or biomass resources available for carbon dioxide removals or for replacing fossil carbon sources in other sites or sectors. The paper offers a different perspective on renewable energy integration focusing on a sector where the majority of the thermal energy supply already comes from renewable energy (in the form of biomass). On the other
2 Materials and methods
Hubbe (2021) reviewed the literature for state of the art approaches to improve the energy ef fi ciency of pulp and paper mills. The extensive review discusses methods based on thermodynamic principles such as exergy analysis and pinch analysis, as well as energy management practices and energy audits. In this study, pinch analysis was used to estimate how the heat balances of a case study mill may be affected by the integration of a carbon capture process, and how this may differ if the mill is also assumed to implement a lignin extraction process. The impact of changes in heat balances on the overall fuel demand and electricity generation potential in the mill was analyzed for two different scenarios: one in which additional (biomass) fuel use is minimized, and one in which the back-pressure steam turbine power generation is maximized. Furthermore, the utilization of the biogenic carbon in the feedstock was analyzed with regards to the amount of carbon that is converted into useful products. The case study mill is described in Section 2.1. Then, in Sections 2.2 and 2.3, the modelling of the carbon capture and liquefaction process and the lignin extraction process are described. In Section 2.4, the pinch-based energy targeting approach is explained, and the different integration cases and optimization scenarios de fi ned.
2.1 Case study mill
For this study, a kraft pulp mill in Sweden producing about 0.5 million tonnes of pulp per year from birch and conifer feedstock was used as a case study. The mill is a stand-alone market pulp mill, which means it does not have integrated paper production. A simpli fi ed fl ow chart of the kraft pulp process is shown in Figure 1. In the cooking plant, lignin is dissolved and separated from cellulose fi bers using white liquor, which is a water-based solution with sodium hydroxide and sodium sul fi de. The separated cellulose fi bers are further processed into the fi nished pulp product, while the liquor with the dissolved lignin and the spent cooking chemicals, referred to as black liquor, is processed in the recovery cycle to regenerate the cooking chemicals and recover the energy content of the organic components (mainly lignin and hemicelluloses) of the liquor. For ef fi cient combustion of the black liquor, the water content must be reduced signi fi cantly. This is done in the evaporation section where the dry solids content is increased from about 20% to 70% – 72%. The evaporation section consists of a series of integrated evaporator effects where the fi rst one uses low pressure steam as heat source, and the following effects use vapor from the previous effect to drive the evaporation. The concentrated black liquor from the evaporation section is sprayed into the recovery boilers, where the organic components are combusted and some of the chemical reactions necessary to recover the cooking chemicals take place. The combustion generates large amounts of biogenic carbon dioxide in the fl ue gas. However, some
Frontiers in Thermal Engineering
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