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affects emissions. This section produces direct emissions of CO 2 , which are included in the EU ETS scheme. The facility under study produces 55,000 tons per year of uncoated carton board using 100% recycled paper as a raw material. The grammage ranges from 200 to 650 g/m 2 , andthe unit used for grammage is the surface density, according to ISO 536 [52], the international reference in this industrial sector. The machine speed varies between 1.30 m/s for heavy paper and 4.16 m/s for light paper. Figure 1 shows the installation studied. Note that the emission level of the plant under study is close to the corresponding product benchmark. The emissions associated with the factory are in accordance with the benchmark or even lower than the benchmark. Before the study, no special actions were carried out to maintain the isolation facilities of the enclosure hood. Due to maintenance operations, some panels had deteriorated, and the adjustment between them due to disassembly and assembly operations was far from optimal. Regarding the cold water system, the maintenance was outsourced, and control and tracking of the associated KPI or main parameters were carried out only monthly by the external company. The air exchangers of the enclosure hood were checked only once a year without cleaning and maintenance routines. 2.2.1. System of Cold Water Added to the Boiler Circuit The drying section heats the paper sheet to remove the remaining moisture by thermal processes. The energy is supplied from steam generated in the boiler. When the latent heat of the steam is transferred to heat the paper sheet, the steam condenses into water, which is also known as ‘condensate’. The factory has a closed circuit to recover all the condensate generated in the dryer section. Most of this condensate is used to heat the air blown to the dryer hood and is then fed to the boiler feedtank. The feedtank is the major location where cold reserve water and condensate return meet. The feedtank provides a reserve of water to cover any interruption of the cold water supply and should have sufficient capacity above its normal working level to accommodate any surge in the rate of condensate return. In the dryer section, the steam passes through various regulation valves to adjust the dryer temperature and enters the cylinder section through a rotating joint; here, the steam changes state back to condensate, which is high-quality hot water. Condensate is an ideal boiler feedwater. From an economic perspective, reuse is desirable; however, in practice, reuse is not possible due to rotating joint and valve losses, and there will typically be water loss from the boiler via blowdown. A high condensate return rate can occur at start-up when the condensate is lying in the plant and pipework is suddenly returned to the tank, where it may be lost to the drain through the overflow. This subindicator affects the boiler efficiency, hood heat recovery, condensate recovery, and steam transport. The condensate return rate represents the addition of cold water needed by the feedtank to maintain the feedtank level and to supply the steam demanded by the dryer section. The return of condensate represents considerable potential for energy savings in energy and CO 2 emissions. Blowdown is necessary for the correct quality and amount of steam to be generated by the boiler. The boiler generates steam, and any impurities in the boiler feedwater that do not boil off with the steam concentrate in the boiler water. The dissolved solids become increasingly concentrated, and the steam bubbles tend to become more stable, failing to burst as they reach the water surface of the boiler. There comes a point (depending on the boiler pressure, size, and steam load) at which a substantial part of the steam space in the boiler becomes filled with bubbles, and foam is carried over into the main steam. This is undesirable because this steam is excessively wet as it leaves the boiler and contains a high level of dissolved and perhaps suspended solids, which can contaminate and possibly
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