1. Introduction
A conventional Tissue paper plant, featuring a Crescent Former Tissue machine (designed with two virgin fibre stock preparation lines with two refiners, approach flow with two layer headbox, single press solution, duo-system gas hood etc.), the total energy consumption can be broadly divided as follows:
• total electric power consumption:- 900-1000 kWh/ton;
• total gas consumption in the hood:- 1100 kWh/ton;
• total gas consumption for generating steam to Yankee:- 800 kWh/ton.
The “average” consumption of a tissue plant is therefore around 2.8-2.9 MWh/ton. If one considers a plant using recycled paper as raw material, the energy consumption further increases. A great amount (more than 2/3 of the total consumption) of energy is used for drying the tissue. Obviously, distribution of this thermal energy depends on the balance between the hood and the Yankee in the mill. Considering, the last few years, an average increase in the cost of energy in the World has been 25% for electricity and 50% for gas, hence in conventional mills, the target will be focused on the lower values of overall energy consumption and drying process in particular.
2. Thermal energy used in the tissue process
To define the necessary thermal energy required for paper drying, one can refer to Tappi formula TIP 0404-05 which is as follows:
Rw = B x S x W x [ L – 1]
E
Rw = Rwhood +RwYD
• = total evaporation [ Kg /sec ] determined by the drying balance between hood and Yankee;
• B=basis weight at the reel [ g /m2 ];
• S=machine speed [ m/sec ];
• W=paper width [m];
• L=dryness at the hood outlet [%];
• E=dryness at the hood inlet [%].
If we consider the terms B, S,Wand L as constants, linked to technological and production parameters, the total load for evaporation is therefore tied to the value of dryness at the hood inlet ‘E’ determined by the press action. The greater the value, the lesser will be the total evaporation value and therefore the “work” to be done by the hood and the Yankee. In a “conventional” installation, a two press solution, which guarantees dryness at the hood inlet of approximately 42% and is advantageous in terms of thermal energy saving, compared to a single press solution, which guarantee an average value of 38% only. This has an impact on tissue quality and increases electric power consumption.
Furthermore, considering both drying supplies at hood and Yankee, it can be said that the Yankee “system” is, more efficient than the hood in terms of energy. There is an interesting solution which allows considerable energy saving. This technology is the combination of a large diameter press (TT SPR 1430) with steel Yankee Dryer cylinder (TT SYD). The large diameter press, having the prospect of working with an “increased” nip, both in terms of width and linear pressure, is able to reach values of dryness at the hood inlet, comparable to those of a double press. Whereas the steel Yankee cylinder is able to supply a greater evaporation coefficient compared to a conventional cast iron cylinder, thus making the drying of paper more “efficient” and economical.
Opportunity for Yankee head insulation with possible increase in its efficiency, brings further energy saving. In the following paragraphs the two equipments are described as well as results obtained at some important installations by Toscotec Tissue Division are also explained.
2.1 Large diameter suction press roll (TT SPR 1430)
As previously mentioned, the concept of this press is to fit the growing need for tissue products with high thickness and softness, with energy saving demands. This new concept of suction press roll with diameter greater than 1400 mm, has the characteristics of generating a wider nip compared to a conventional press roll. The approximate value of this nip will increase by 30%. Maintaining the advantages of a traditional press roll, such as low maintenance cost, reduced investment cost and quick installation, the suction press roll allows an increase in paper softness and bulk, without having the added constraints involved in managing a shoe press. The large diameter suction press roll is designed to operate with a maximum nip load of 120 kN/m, instead of 90-95 kN/m typical of a conventional press roll, thus it is possible to choose between operating at full power for an “economic” product grade and at reduced pressure to enhance softness and thickness of a higher quality product. The increase in terms of percentage of dryness at press outlet with larger nip is at least 2-3% compared to a conventional press roll with consequent energy saving.
A further advantage is also obtained by application of double blade on the press roll, which limits “rewetting”. This advantage means an improved moisture profile and an increase in degree of dryness of at least 1%. It is important to underline the fact that, in terms of energy, a decrease of 1% of dryness at the press outlet could be translated into approx. 400kg H2O/h more to evaporate in the drying process (~55kg H2O/t of Paper) with consequent increase in specific consumption of thermal energy.
2.2 Steel Yankee Dryer cylinder (TT SYD)
Traditionally, Yankee cylinder has always represented a critical element in tissue machines. The combined action of Yankee and hood is responsible for drying the most part of tissue. Paper drying is an energy intensive process and the Yankee cylinder supplies a relevant contribution, thus any possible energy saving connected to it is of great interest to all Tissue manufacturers. In tissue production with conventional crescent former machines, after the paper is formed and transferred to the cylinder by the presses, the drying process is mainly constituted by a change in the state of water contained in paper. This change is carried out by heat transfer, as a result the steam condensation inside the Yankee and hot air blowing in the hood. Thanks to these two actions, the water trapped inside the fibre matrix passes from liquid to vapour state and is then extracted by the hood exhaust.
In past years some companies started building steel Yankee cylinders, however many limitations in terms of thermal stability, maintenance in working conditions as well as deficiencies in the manufacturing process limited its development and this product wasn’t introduced in the tissue sector.
The use of steel plates instead of cast iron fusion allows significant reduction in shell thickness and therefore resistance to heat transfer. The result is an increase in drying capacity achieved through steam condensation inside the Yankee, which is known to be the most efficient paper drying system and often the least expensive. The energy yield of a cylinder is further improved by the possibility of operating at higher steam pressures; the development of an efficient system of condensate extraction allows furthermore reduction in steam pressure differential necessary for proper evacuation of the cylinder. The CD moisture profile on the tissue is also optimized, thanks to a uniform and constant condensate removal and thermal exchange taking place in the cylinder.
On some reference Yankee installations, it was possible to verify that heat transfer coefficient and condensation capacity of the SYD cylinder, on an average, at least 30% higher, compared to a cast iron cylinder with the same dimensions and operating conditions. Considering that drying could be equally shared between hood and cylinder, the total drying capacity of a tissue machine equipped with Steel Yankee Dryer is higher by 10- 12% (see Fig. 3 and 4) compared to traditional design with a cast iron dryer. Alternatively the same productive capacity can be obtained by increasing the contribution of the cylinder with obvious benefits in terms of energy saved on the hood heating.
A further possible significant contribution to reduction of energy consumption is the prospect to insulate the cylinder heads. This enables saving of approximately 5% on steam consumption in the cylinder. In case of a rebuild or modification of a tissue machine, the steel Yankee also has another advantage, which is to have a larger covering on the grooved surface compared to a cast iron cylinder and is therefore able to dry wider paper with same cylinder size. Another advantage is to have a lesser weight of about 30%, compared to a cast iron solution, which reduces civil work in installation and the related costs. Thanks to the metal coating, the surface of the cylinder is homogenous in terms of roughness, it isn’t porous and its hardness is far superior to a cast iron cylinder. These factors contribute in increasing the creping process efficiency with obvious benefits in productivity of the paper machine and with consequent saving on the coating consumption, thanks to absence of porosity on the cylinder and to even distribution of the product on its surface.
At the same thermal conductivity in cast iron and steel, the shell thickness reduction means a drastic reduction in resistance to thermal exchange and therefore an increase of 30% of thermal transmission efficiency of the cylinder. This increases productivity and saves energy.
Plant Case Study
Listed below are a few interesting examples of the significant achievements resulting in huge energy savings, obtained with this technology from application of the steel Yankee cylinder and the large diameter press roll in actual Tissue plant conditions:
Case 1: Installation of a new Yankee cylinder TT SYD 12FT
• Reference data:
– Paper width at the reel: 2,75m
– Test reference pressure: 5 bar
– Conventional single press design
– Test basis weight reference: 16,5 g/m2
• Evaporation rate of a 12ft cast iron cylinder at 5 bar:~60 kgH2O/hr-m2
• Evaporation rate of TT SYD at 5 bar:~ 80 kgH2O/hr-m2
• Comparative increase in Yankee cylinder evaporation rate:~34%
• Comparative reduction in thermal energy consumption:~64kWh/ton
• Comparative reduction in thermal energy consumption at maximum working pressure: ~6,5%
Case 2: Rebuild of a cast iron cylinder with a TT SYD 12FT steel cylinder
• Reference data:
– Paper width at the reel: 3,7m
– Test reference pressure: 5 bar
– Double press design
– Test basis weight reference: 35 g/m2
• Evaporation rate of the former 12ft cast iron cylinder at 5 bar: ~68,5 kgH2O/hr-m2
• Evaporation rate of new TT SYD at 5 bar:~94 kgH2O/hr-m2
• Comparative increase in Yankee cylinder evaporation rate: ~37%
• Comparative reduction in thermal energy consumption: ~71kWh/ton
• Comparative reduction in thermal energy consumption at maximum working pressure: ~7,3%
Case 3: Installation of a Yankee cylinder TT SYD 15FT combined with a TT SPR 1430
The energy consumption of the drying section (hoods+Yankee cylinder) verified on a Toscotec machine installation in Germany and equipped with a TT SYD 15FT and large diameter press roll TT SPR 1430, reported values less than 1700 kWh/ton.
The following were the operative conditions and results:
• Reference data:
– Daily production: 97,3 tpd
– Paper width at reel: 2,68m
– Basis weight at reel: 19,8 g/m2
– Raw material: 100% virgin pulp
– Test reference pressure: 6 bar;
– Single press design: dia. 1430 mm;
– Yankee: steel dia. 15 ft (4572 mm);
– Linear nip load: 120 kN/m;
• Paper dryness at reel: 95%;
• Hood blowing temperatures: 470°C wet end; 470°C dry end;
• Measured steam consumption: 1,24 ton/ton~791 kWh/ton (considering boiler efficiency of 90%);
• Measured gas consumption: 88,3 Nm3 /ton ~ 883 kWh/ton (gas calorific value: 8600 Kcal/Nm3 );
• Total energy consumption gas + steam: 791 + 883 = 1674 kWh/ton
It is interesting to note that in case of using SYD combined with TT SPR 1430, energy consumption values reach closer to those with double press configuration, however maintaining all the quality features like softness of tissue etc.
Conclusions
Conventional technology for the production of tissue paper has shown that it continues to have strong validity and the energy consumption could be significantly reduced by the use of reliable tools. Toscotec’s long time experience in the optimization of the production process for tissue and paper industry, and in particular in the drying area, has highlighted the fact that energy saving is possible through smart and easy to run applications.