According to a report presented at an American Council for an Energy-Efficient Economy event, pulp, paper, and paperboard mills account for about 12% of total manufacturing energy use in the U.S. Energy is widely used for processes such as running motors and fans, providing compressed air, processing pulp, and drying paper. Steam is a significant utility used in pulp and paper facilities and we estimate that it accounts for approximately 40% of the total utility demand. Therefore, steam systems provide significant opportunities for energy efficiency.
Steam is a significant utility used in pulp and paper facilities, nationally estimated to be approximately 43% of the total energy demand. Recovery of pretreatment steam during pulping and steam for drying is paramount for reusing energy. While improvements to and maintenance of boilers, insulation of steam and condensate lines, and exhaust heat recovery efforts can lead to approximately 14% energy savings in steam generating systems, a pinch analysis evaluation project can net additional thermal savings throughout a mill’s infrastructure. This type of analysis identifies heat flows between cool streams that are being heated and hot streams that are distributing heat throughout the mill.
Several technologies requiring infrastructure and process changes can yield greater, yet radically variable results, in facilities. For example, low-pressure steam is made during mechanical pulping, which can be recovered in an evaporator/boiler system. A list of eleven energy efficiency improvements for steam systems relative to the amount of pulp manufactured include:
- Recycle low pressure steam. Steam from mechanical pulping can be used in the paper machine dryer section or chip injection by utilizing a heat pump and can save between 33-58% of total natural gas use.
- Retrofit the drying section with infrared (IR) dryers. IR dryers direct heat onto the paper web and can save about 35% of natural gas use; however, the electrical heaters require additional electrical energy.
- Retrofit the press section with hot cylinders. You can use hot cylinders to impulse heat the web before it enters the typical paper machine (PM) drying section and save 5-30% of your natural gas use.
- Retrofit the drying section with a hot-rotating impulse dryer. The roll and a concave nip increases the drying rate and allows the PM to run faster while conserving 21% of total natural gas use.
- Retrofit the drying section with a microwave drying system. Cross or machine directional microwaves can direct heat onto the paper web and reduce natural gas use by 20%; however, the microwaves will require additional electric energy.
- Retrofit the dryer section with a Condebelt dryer. This continuous moving hot steel band is heated by steam or directly heated and could replace typical drying cylinders for paper weights less than 150 g/m2. Savings for this system are estimated to be 16% of total natural gas use.
- Retrofit the drying section with air imp ingement drying. Using either gas burners or steam and amplified compressed air, you can blow hot air across the web to save 10-15% of steam requirements; however this method increases facility electrical demand up to 5%.
- Install drying cylinders with a direct natural gas source. Do this rather than using steam to save up to 11% of total natural gas use.
- Fabricate a closed hood surrounding the PM. Also, optimize ventilation inside the unit reduce thermal energy demands for drying and reduce the volume of air heated. Investment costs are moderate and can yield savings of nearly 8% of natural gas use. Often this hood configuration is coupled with airless drying.
- Pre-heat pulp with a steam shower. Pre-heating steam can reduce the load on the dryer, reduce residence time in the nip, and thus allow increases the machine speed while savings 6% of natural gas use.
- Improve chip screening to remove ‘thick’ wood chips. Doing so can save up to 4% of natural gas use by lowering steam residence times.
Cross-functional technologies can also be employed to reduce energy demands in a pulp and paper mill. Profiling the sheet with IR sensors that feed back to the headbox can help your facility control sheet moisture content precisely and reduce drying inconsistencies. This can potentially allow you to operate the PM faster and have less drying load.
The Lawrence Berkeley National Laboratory estimates that the detection mechanism saves 6.6 therms/tonne of pulp or offsets 7.2% of natural gas use. Another energy-saving initiative introduces a sodium carbonate salt solution to the pulp that displaces water in the pulp, but still allows it to be sprayed effectively out of the headbox. The solution reduces the drying requirement by about 16 therms/tonne, which equates to a 17.7% natural gas savings.
Several technologies requiring infrastructure and process changes can yield greater, yet radically variable results, across the industry. For instance, low-pressure steam is made during mechanical pulping, which can be recovered in an evaporator/boiler system.
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