Energies 2021 , 14 , 1161
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2. The Pulp and Paper Industry 2.1. Structure of Pulp and Paper Sector
The paper industry is one of the most expressive industries globally, producing over 400 million tons of paper per year [14]. Pulp and paper mills are highly complex, and involve various process steps, including wood preparation, pulp production, chemical recovery, bleaching, and papermaking, to transform the wood into the final product [15]. Paper mills can be fully integrated or non-integrated mills. Integrated mills include a pulp mill and a paper mill on the same site. These types of mills receive logs or chips and produce paper. In contrast, non-integrated mills produce pulp and paper in separate mills. The non-integrated paper mill uses dried pulp as the feedstock for paper production. The dried pulp is produced in integrated mills or non-integrated pulp mills and sold on the open market. Integrated mills are usually larger, more cost and energy effective, and have cheaper production than separate mills. Nevertheless, smaller non-integrated mills can strategically benefit from being closer to the consumer. Figure 1 shows the main process steps in the integrated paper mill for the two most common steps of pulping (thermomechanical pulping (TMP) and sulphate (kraft) pulping and paper making [16,17].
Figure 1. Overview of wood preparation, mechanical/chemical pulping, and papermaking processes design based on Joint Research Centre (JRC) [17] and ICF [16] reports.
2.1.1. Raw Material Preparation Wood is the primary raw material for the production of pulp. Wood comes to the mill in the form of long raw logs (with bark) in the wood yard. The main processes in the raw material preparation are debarking the wood logs to remove the bark, wood chipping (which reduces the wood logs to small chips), and screening to ensure that the wood chips are the right size for further processing. In general, the wood yard operations are independent of the type of pulping process [15].
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