Skoglund et al.
10.3389/fther.2023.1282028
FIGURE 2 Flowsheet of the carbon capture process assumed for the integration study. Adapted from Voldsund et al. (2019), licensed CC-BY-4.0.
TABLE 3 Heating and cooling requirements of the carbon capture and liquefaction processes.
T in [ ° C]
T out [ ° C]
Duty [kJ/kg captured CO 2 ]
Carbon capture
Stripper reboiler
116.5
116.6
2940
CO 2 -lean
fl ue gas HX
59.9
70
54
Lean solvent cooler
53
40
513
Stripper condenser
98.9
28
843
DCC cooler
66
28
1934
Wash water cooler
65
28
375
Liquefaction
Compressor train intercooling 2
140.9
38
109
Compressor train intercooling 1
131
38
97
Cooling refrigerant - intercooling stage 2
105.3
38
465
Cooling refrigerant - intercooling stage 1
90.6
38
42
Cooling duty
57.5
28
11
fi lters. The crude lignin from the fi rst press fi lters is then further puri fi ed by further lowering the pH value, washing and dewatering it in a second press fi lter. A major advantage for lignin extraction of having a CO 2 capture unit in the mill is that the need for CO 2 for the lignin precipitation can be ful fi lled by captured CO 2 , thereby avoiding the need to purchase CO 2 . In this study, it was assumed that 0.12 tonnes of lignin is extracted from the black liquor per air-dried tonne of pulp. This is well within the range of feasible extraction rates reported in literature (Välimäki et al., 2010), and similar to plants in commercial operation (Haaker, 2015). The major effect of lignin extraction on the mass and heat fl ows of the mill is connected to the operation of the recovery boilers. When lignin is extracted from the black liquor, it will no longer be
modeling of the liquefaction plant is described in detail in (Deng et al., 2019). The carbon capture and the liquefaction processes require both heating and cooling in different parts of the processes and these thermal energy requirements are summarized in Table 3.
2.3 Lignin extraction
The predominant way of extracting lignin from black liquor is by precipitation (Pola et al., 2022). A simpli fi ed process fl owsheet is shown in Figure 3. The lignin is extracted by removing a part of the black liquor fl ow from the evaporation plant and acidifying it with CO 2 to lower the solubility of lignin in the liquor. The precipitated lignin is then separated from the remaining liquor through press
Frontiers in Thermal Engineering
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