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1.1. Sustainability and Efficiency in Papermaking Sustainability and efficiency are difficult to implement simultaneously in manufac- turing plants [26]. This difficulty arises from identifying tools needed and quantifying the improvements achieved. This issue was addressed by the indicator identified by Calvo and Domingo [21], ‘t CO 2 /t Paper’. The identified indicator was considered during the drying stage of the papermaking process and indicates the relationships among energy consumption, process efficiency, CO 2 emissions, and machine availability variables. Calvo and Domingo [27] studied the influence of several other factors, including web paper input to the thermal drying section, how it affects the indicator ‘t CO 2 /t Paper’, and whether it affects the measures of efficiency and sustainability in papermaking. Moya and Pardo [19] investigated how adopting the BATs can be a cost-effective contribution to reducing CO 2 emissions and achieving European Union targets through energy-efficient policies. However, Del R í o Gonz á lez [28] noted how difficult it was for the papermaking industry to introduce the BATs or cleaner technologies due to the large investments required and the technical complexity associated with the papermaking process. The intention of the European Authorities with Decision 2010/2/UE [29] is to create a costly EU ETS to force industries to significantly reduce their emissions levels. Ghose and Chinga-Carrasco [30] reviewed CO 2 emissions through a life cycle assess- ment and found that up to 85% of the total energy in papermaking, depending on the paper product, is used for paper drying. The drying process in papermaking affects paper characteristics and cannot be designed by considering only the energy efficiency. Karls- son [31] reviewed the main parameters that affect the evaporation process in the drying phase, and Hostetler et al. [32] studied web temperature throughout the drying process to determine the drying conditions that ensure optimal paper quality at minimal cost. There is considerable information regarding the drying phase. Laurijssen et al. [33] studied the influence of dryer elements on the drying process and proposed actions to decrease heat use in conventional multicylinder drying sections and calculated their effect on energy use. The main optimization measures to be implemented in the drying phase include decreasing the heat used to evaporate water by increasing the air dew point temperature of the dryer section, as noted by Laurijssen et al. [33]. This measure is difficult to implement due to the poor insulation condition of the drying hood. Other measures increase the amount of heat recovered by using the exhaust air to preheat the blown air and water. Ruohonen et al. [34] described the energy required to heat air in the papermaking process and the steam needed to heat the inlet air provided to the drying hood after heat recovery, highlighting the importance of heat recovery systems. The influence in this case is clear; Calvo and Domingo [21] identified the relationship between the external air conditions and energy utilization in the drying stage and found that recovering the energy to preheat blown air requires less steam energy to heat the air hood. By focusing on the conditions in the drying stage, Sivill et al. [35] found that the humidity of the hood exhaust air affects the efficiency of the heat recovery rate and inlet blown air temperature. Heat recovery is used in the drying stage to reduce the energy used to dry the paper. Sivill and Ahtila [36] studied the recovery of energy from the exhaust air, which directly results in reduced CO 2 emissions. Zvolinschi et al. [37] found that regulating the temperature of steam in each dryer section can reduce the energy demand by up to 3%, and they also discussed the effect of humidity in the exhaust air, which can achieve energy savings of up to 35%. Ruohonen et al. [34] indicated that all previously considered options can be used to reduce the CO 2 emissions of a mill. Kong et al. [38] compiled the available information on energy savings, environmental costs, and commercialization status for 25 emerging technologies to reduce the energy use and CO 2 emissions in the paper production process. Including four drying sections (gas-fired dryers, boost drying, and microwaves) may be an alternative in the future for improving the multicylinder drying efficiency; however, this measure is not widely used. In line with the previous study, Kong et al. [39] used the conservation supply curve to measure the potential savings, from both the engineering and economic perspectives of energy, to show potential opportunities
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