Energies 2021 , 14 , 1161
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hydroxide and sodium sulfide, in the batch or continuous digester for several hours under high pressure and high-temperature (155–175 ◦ C) conditions [17]. After the cooking step in the digester, the hot pulp is transferred under pressure to the blow tank, where softened wood chips decompose into fibers or pulp. Then, in the next stage, in the filtering and washing step, the pulp and the cooking liquor, which is called black liquor, are separated. During the cooking process in the digester, almost half of the wood is not solved, and as a result, black liquor includes organic and inorganic substances. In the next step, black liquor is sent into the chemical recovery system to recover the cooking chemicals and energy. Chemical recovery is an essential part of the chemical pulping process. The chemical recovery system by evaporating the water content of the black liquor in the recovery boiler, on the one hand, recovers the white liquor (by causticizing the green liquor with lime (Figure 1)) and, on the other hand, generates high-pressure steam. The white liquor is returned to the digester, and the steam is used in the heat and power plant (CHP) to generate the process steam (medium-low pressure steam) and electricity. In most cases, the recoverable black liquor’s fuel value is sufficient to make the kraft pulp mills largely self- sufficient in heat and electrical energy. Chemical recovery also offers the mill regeneration of chemical digestion at a rate up to 98%, which leads to a significant reduction in the costs of purchasing process chemicals and energy [15,17]. The production of sulphite pulp is not so common compared to the production of kraft pulp. Pulp produced by sulphite pulp usually has weaker strength properties compared to kraft pulp, although sulphite pulp offers other advantageous properties for some specialty pulp applications (such as textile production). In the sulphite pulping process, the wood chips are digested with the cooking liquor, an acidic mixture of sulphurous acid and bisulphite ion. The acidic solution in the cooking liquor leads to a decomposition of the lignin bonds between the wood fibers. Like the kraft pulp, energy can be recovered in the sulphite process, and the solvent can be regenerate. Due to sulfite pulp’s lower color content, it can be bleached more easily than kraft pulp, but it is not as strong [15,17]. In total, chemical pulping has a low fiber yield, in the range of 40–55%, compared to mechanical pulping, but the pulp produced is of very high quality and is mainly used to produce higher-grade paper, such as office paper. Nowadays, chemical pulp consumption has grown because the demand for printing and writing paper is increasing. In contrast, due to the rising use of social media, newsprint consumption has decreased, causing a reduction of mechanical pulp demand [15]. • Recycled fibers pulp (RCF) : this type of pulp production requires a more straightfor- ward process than the pulping of virgin wood, and consumes less energy than the other types. The main step in RCF pulping is to dissolve the shredded paper in hot water by mechanical means to separate the fibers without damaging and destroying them. For paper grades requiring white pulp, an additional step of de-inking takes place. Recycled fiber pulp is also subjected to mechanical removal of impurities. In some pulp mills, recycled fiber and virgin fibers are used together, and recycle fiber is added to the virgin (wood) pulp [15]. After the pulp has been produced by mechanical, chemical, or RCF processes, the pulp is washed and screened to remove impurities such as remaining pieces of wood and uncooked chips that have not been properly pulped. In the final step, depending on the product’s end-use, bleaching is applied to give the pulp a white color by adding chemical components. This is not an essential process and is only applied to paper types that require good optical properties: graphic paper, whiteboard, etc. An additional drying step is required when the pulp and paper mills are not in the same location (non-integrated mills). The pulp drying process is energy-intensive and not significant for the paper manufacturing process. Therefore, cost and energy savings can be achieved by integrating the pulp and paper mills.
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