Energies 2021 , 14 , 1161
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The two major products resulting from the raw material preparation process are chips and bark. The chips are the main product. Their quality is of high importance, as the consumption of raw material increases if the chips produced are not homogeneous. Furthermore, a homogenized chip distribution improves the energy efficiency of the system. Another main product is the bark, which is the by-product and is typically used as fuel in the burner for energy production, or can be sold for other purposes off-site [15]. Raw material processing consumes roughly 10% of electricity use [15]. The main energy consumers are conveyor motors, debarking operational motors, and wood chipping motors (see Table 1). Thermal energy is only needed in some mills in cold climates, where the logs have to be de-iced with hot water before debarking [16].
Table1. Specific electricity and heat consumption of the wood processes in pulp and paper production.
Specific Heat Consumption (GJ/t wood )
Specific Electricity Consumption (kWh/t wood )
Reference
Process
8.5 (7–10 KWh/m 3 wood)
Debarking
0.0 0.0
[15,17,18]
Chipping and conveying
30.3
[15,18]
2.1.2. Pulp Production The second step in paper processing is the production of pulp from raw materials (virgin wood or recycled fibers). Wood is usually composed of about 60–65% cellulose and hemicelluloses, the paper’s main fibrous components [15,19]. The remaining material mass consists mostly of lignin, with small amounts of extractives and ash. The fundamental aim of the pulping process is to release the fibers from the lignin that holds the fibers together in the wood. Typically, wood is composed of about 50% fiber, 20 to 30% non-fibrous sugars, and 20 to 30% lignin [15,17]. In the pulping process, wood chips are separated into individual cellulose fibers by removing the wood lignin [15,17]. The pulp is produced by three main methods: mechanical, chemical, and recycled fiber (RCF) pulping (There are another types of pulp production, semi-chemical, dissolving, and non-wood pulp, which are not considered in this paper). • Mechanical pulp : the primary form of pulping is mechanical pulping, which has been extensively replaced by chemical pulping. Mechanical pulping processes separate the fibers from each other by mechanical means, retaining part of the lignin in the paper and increasing the yield (mass of pulp/mass of original fiber source) to around 85–90%. The main kinds of mechanical pulping are the production of groundwood pulping (GW), refiner mechanical pulping (RMP), thermomechanical pulping (TMP), and chemither- momechanical pulping (CTMP). Two main techniques are used to produce mechanical pulping: pressing logs against a rotating grindstone for GW pulping, and feeding the wood chips between metal finishing discs and defibrating them for RMP, TMP, and CTMP. Typically, mechanical pulp is used for low-grade paper such as newsprint and magazines, the fibers with this pulp type are short and weak, and sometimes need to be combined with strong chemical fibers (especially for GW). The main energy source for mechanical pulp is electricity, which is usually converted into heat that can be partially recovered and used in other processes. However, some types of pulp (TMP, CTMP) use steam on the wood chips before they are used in the equipment (see Table 2) [15,17]. • Chemical pulp : The most common pulping process is chemical pulping, divided into two types: kraft (sulphate) pulp and sulphite pulp. In both types, the digester processes the raw material with watery chemical solutions at high temperatures and high pressure to extract pulp fibers. Kraft pulp is the most common kind of chemical pulp, accounting for about 80% of the world’s pulp production. As illustrated in Figure 1, for kraft pulp, the wood chips are mixed with a strongly alkaline solution known as white liquor, which includes sodium
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