Processes 2021 , 9 , 1707
10of 20
The study begins by analyzing the historical available data of the indicators related to ‘tCO 2 /t Paper’, the patterns for maintenance and minor reforms that have been proposed and implemented. The introduced indicators have been detailed in terms of their relation with energy-efficiency saving opportunities in the subsections identified by Kong et al. [57] on paper machines (boiler efficiency, the implementation of heat recovery, and exhaust humidity control). Energy-saving opportunities are directly related to the CO 2 emissions. In addition, other energy-saving opportunities such as steam transport and condensate recovery are included in these subsections. The enclosure hood of the drying section is an important part of the paper drying process in terms of energy efficiency but is beyond the scope of this article. For the analysis of CO 2 emissions, the indicators are defined in terms of the main physical variables involved in the process of drying paper identified by Karlsson [31] and related to the consumption of thermal energy, the energy aspects from the theoretical perspective of paper drying [35,37], and the maximization of energy recovery [33,36]. The reforms and maintenance guidelines proposed, based on data provided by the defined indicators, are those that reduce the indicator ‘t CO 2 /t Paper’. The drying process significantly affects the characteristics of the paper produced and the efficiency of the process; thus, it is necessary to analyze whether the improvements and maintenance guidelines are favorable for maintaining product quality and reducing internal rejections. Data are collected from the machine’s control instrumentation and are supplemented by daily collection data from the environmental management system. The factory has only one energy supplier, and all of its energy is used by the paper dryer. In this case, it is possible to assume that the difference between the energy used and theoretical needs are energy losses that can be saved. 2.4. Determination of ‘t CO 2 /t Paper’ This study is focused on a paper manufacturing plant. The data are collected mainly from the paper drying section. The facility has only one natural gas supply source, which is used only in one boiler to generate the demanded steam for thermal paper drying. The direct emissions of CO 2 produced by the plant come only from the combustion of the natural gas mentioned above. This natural gas combustion is included in the EU ETS scheme and, in this case, generates all the plant’s direct CO 2 emissions. Figure 1 shows a schematic process flow in which the boiler (steam generator) can be identified as the only point of natural gas consumption, and the dryer section is the only point that consumes the steam produced by the boiler. We consider the direct emissions of CO 2 produced in the installation from the combustion of natural gas, which is under the EU ETS. The analysis of data availability, paper-produced air dry tons (ADTs) and emissions, are collected with regular frequency and measured in accordance with Spanish law [58–60]. Other variables associated with the processes and the drying section were collected. The variables regarding the temperatures of the paper, steam, and condensate system and balance of air in the enclosure hood (identifying each flow, temperature, moisture, and other associated enthalpy characteristics) are analyzed to find the relations among them and theCO 2 emissions through the considered indicator, ‘t CO 2 /t Paper’. The conditions of the dryer hood, mainly steam pressure in dryer cylinders and inside hood air conditions, affect the evaporation capacity of the drying system and the speed of the process. This issue determines the difference between the theoretical capacity and actual production yielded. This also affects the indicator ‘t CO 2 /t Paper’ and thus the CO 2 emissions. The indicator ‘t CO 2 /t Paper’ is obtained as a direct ratio between CO 2 emissions, determined according to Spanish law, and the tons of paper produced at the plant in the same period. A month is taken as a period to determine the effect of each change in the operating parameters of papermaking. The period of one month provides sufficient data to study and compare to other periods.
Made with FlippingBook Online document maker