Skoglund et al.
10.3389/fther.2023.1282028
FIGURE 1 Simpli fi ed process fl ow chart of the kraft pulp process.
of the carbon that enters with the black liquor is also converted to carbonates, which leave the recovery boiler together with the other inorganic chemicals as a smelt, which is dissolved in water to form so-called green liquor. In the causticization section, green liquor is mixed with burnt lime (calcium oxide) to form white liquor. This is achieved in a series of reactions, where the burnt lime fi rst reacts with the water of the green liquor, forming calcium hydroxide (slaked lime), which in turn reacts with the sodium carbonate of the green liquor to form sodium hydroxide and calcium carbonate. The calcium carbonate is separated from the liquor and sent to the lime kiln to be regenerated into burnt lime, and the liquor is now regenerated into white liquor, that can be reused in the cooking. The regeneration of burnt lime from calcium carbonate in the lime kilns requires energy at high temperatures. At the studied mill, this energy is supplied using tall oil pitch as fuel. Tall oil pitch is a biofuel, which is a residue from the processing of crude tall oil into tall oil. When the calcium carbonate is oxidized to calcium oxide, carbon dioxide is formed, which will exit the lime kiln with the fl ue gas. Through the series of reactions explained above, this carbon originates from the organic components in the black liquor and is thus considered to be biogenic carbon dioxide. Since the lime kiln fl ue gas contains carbon dioxide both from the oxidization of the lime and from the combustion of (bio-) fuel, the carbon dioxide concentration of the fl ue gas is usually high. Pulp mills are energy-intensive industrial plants that consume large amounts of heat and electricity. However, like most modern,
TABLE 1 Steam header properties for the steam utility system.
Steam utility headers Pressure [bar(g)] Temperature [ ° C]
High-pressure steam
56
440
Medium-pressure steam
11
190
Low-pressure steam
3
145
energy-ef fi cient market kraft pulp mills, the case study mill is essentially self-suf fi cient in heat supply from internally generated fuel by-products. In fact, the mill currently operates completely free from fossil fuels. Most of the steam demand (including steam for turbines, which cover a large share of the mill ’ s electric power demand) is covered by steam generated through black liquor combustion in the mill ’ s recovery boilers. To a smaller extent, the mill ’ s steam demand is currently also supplemented by combustion of bark and other internally generated wood fuels in the power boiler. The steam generated in the boilers is fed to a high-pressure steam header. This steam is expanded through a back-pressure, extraction steam turbine, or, if necessary, through direct expansion valves to medium-pressure and low-pressure steam headers. Pressure and temperature speci fi cations for the steam headers are presented in Table 1. In the mill, medium-pressure and low-pressure steam is used in various parts of the process, mainly to supply heat through heat exchangers but also by mixing the steam with process streams directly.
Frontiers in Thermal Engineering
04
frontiersin.org
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