Dryer Section Energy System Measurement and Energy-saving Potential Analysis for a Paper Machine

Transcription

1 Dryer Section Energy System Measurement and Energy-saving Potential Analysis for a Paper Machine Jigeng Li, Lingbo Kong and Huanbin Liu State Key Laboratory of Pulp and Paper Engineering South China University of Technology, Guangzhou, Guangdong 51064, China Abstract In this paper, a detailed dryer section audit was conducted on a paper machine to evaluate its energy performance. The methodology and instruments used for field measurements are introduced as an important part of dryer audit. The deficiency points which limit drying performance of the paper machine are identified based on the tested data. It was found that significant energy savings can be achieved in the studied paper machine. 1. Introduction Papermaking is an energy and capital intensive process. For a paper mill, raw materials and energy costs are the top two largest manufacturing costs. It is estimated that about 20-30% of total operating costs is consumed by energy in the form of steam and electricity. Thus, many paper mills are pursuing energy efficiency ways to reduce their energy cost. This is especially driven by the rising cost of energy and environmental awareness in recent years. As one of the most important parts in the papermaking process, the dryer section has a great impact on the overall energy performance [1]. Although only approximately 1% of original water is removed in the dryer section, it still remains the most costly and energy intensive unit in the papermaking process [2]. The dryer section costs 40% of total capital investment and consumes more than 60% of total energy used in a paper machine [3]. In addition, the dryer section is also a complex unit in which mass and heat transfer simultaneously [4]. The paper drying is accomplished by steam and condensate system, air system and heat recovery system. Since the changes of one part will affect the other parts of the system, the dryer section may deviate from the optimal level without a thorough understanding of the current drying performance. This is also often ignored by operators due to their lack of energy management experience. Moreover, the operators are interested in the paper quality and production capacity rather than energy cost. Indeed, some key parameters are seldom monitored nor adjusted when the operating conditions change. Therefore, reducing energy consumption by improving dryer performance has been given a high priority when energy efficiency programmes are implemented on paper machines. It has been also the topic of some recent publications to improve the energy performance of the dryer section, especially from the industrial perspective [5-11]. Mattinsson found that a reduction of energy use would be achieved if the fresh air s heating was regulated [5]. Hill presented five essential rules that can help papermakers identify the areas and actions in the dryer section for further measurable opportunities for energy improvement [6]. Some energy saving opportunities in paper drying were studied by Reese, Wedel and Chaloux respectively [7-9]. Since the dryer fabric plays a role in both heat and mass transfer in the paper drying process, Lang studied the effect of dryer fabric on the dryer section energy consumption [10]. Laurijssen et al. conducted some research on how the energy efficiency of multicylinder dryers can be optimised and found that 32% of the dryer section s heat usage could be reduced [11]. In this paper, a systematic audit of a paper machine dryer section is implemented to analyse its current dryer performance. Based on the survey results, significant energy efficiency improvement opportunities are identified. It was also recommended that this method to be used to analyse other similar paper machines for improving their energy efficiency. 2. Methodology To have a better understanding of the dryer section for a paper machine, field testing has to be carried out firstly to evaluate its present drying performance. Process parameters, environmental parameters and physical geometric parameters are to be determined before analysing the dryer section s performance Process parameters The process parameters are the most important variables that affect the energy performance of a paper machine dryer section. For the same dryer section operated under different process parameters, the overall energy consumption may be varied significantly. The mass transfer driving force for paper drying is the difference between partial pressure of water vapour at the paper web surface and in the surrounding air [12]. The vapour pressure at the web surface and in the surroundings is related to the web temperature, moisture content and air humidity. In this context, the heat transfer is a function of the difference between temperature of steam and paper web [1]. Therefore, the process parameters such as the steam pressure, web moisture content, web temperature and air humidity are Measurement + Control Vol 45/8 October

2 Dryer Section Energy System Measurement and Energy-saving Potential Analysis for a Paper Machine very crucial to the heat and mass transfer process in paper drying. In addition, the supply and exhaust air properties have also a direct influence on the dryer energy consumption. Steam pressure, flow and differential pressure can be extracted from the existing distributed control system (DCS). Assuming that the steam in the cylinders is saturated, the steam temperature can be calculated using the Antoine equation. Cylinder surface temperature has a great impact on the dryer curves. It provides an indication of the heat transfer capacity of the dryer section and is one of the measuring process parameters. However, few mills monitored the dryer temperature profile alone in the entire dryer section. In this paper, the contacted thermocouple sensor that is manufactured by Swema are employed to test the cylinder temperatures. Paper moisture content is measured using the NDC 8100-F system with a 104-D sensor. The paper temperature measurement is proposed by Maggard [13]. It should be noted that the instrument we used in the field measurement is thermal infrared imaging viewer. The temperature test points are located on the web immediately out of each cylinder. The air condition in the pocket and hood is one of the most important parameters that affect dryer performance. However, they are seldom monitored and controlled in some mills, especially for those established decades ago. They are tested using the Testo 400 robust humidity probe and vane/temperature probe. The pocket measuring points is located in the area where the web is leaving each cylinder. Dew point is the temperature at which the air becomes saturated and water vapour begins to condense to liquid water. It is a function of air temperature and relative humidity and can be calculated using Equation (2) [14]. The recommended dew point is lower than the air dry bulb temperature within the hood [15]. The air conditions, especially the supply air and exhaust air humidity, influence the air consumption greatly. In general, 8-10kg air is required to remove 1kg water vapor for a closed hood whilst the value for semi-open hood is between 20-30kg air [14]. It is recommended that it be monitored online for these key parameters in order to see their influence on energy efficiency Environmental parameters The environmental parameters have a close relationship with the overall energy performance. Indeed, the hot supply air consumption for removing per unit of water evaporated varies with environmental conditions. Besides, the energy used for drying in the summer is lower than that in the winter conditions. Therefore the measured environmental parameters should include the air temperature and humidity in the hood, indoor and outdoor. Testo 400, a robust humidity probe, is used for measuring these parameters. However, with the small range of temperature involved in paper drying section, the specific heat is considered constantly Physical geometric parameters In this work, the paper drying process is characterised with two distinctive zones: cylinder-paper conductive drying zone and paper-air convective drying zone. Here the effects of dryer fabric on heat and mass transfer are not considered. The overall drying area should be determined before calculating the evaporation. The distance between the adjacent cylinders Figure 1: The structure of double-felted dryers horizontally (L 1 ) and vertically (L 2 ), as well as the cylinder diameter (D) can be measured from the drawings. For example in the double-felted configuration in Figure 1, the contact drying length (L cp ) and free draw length (L pa ) can be readily calculated. 3. Basic Situation of the Studied Paper Machine The web dewatering process is completed in a thermal way with groups of steam heated cylinders. The heat is transferred in the form of conduction and convection alternatively. For this case, a paper machine dryer section that consists of 62 iron cylinders was used to illustrate the audit method thoroughly. The basic operating conditions of the studied paper machine are shown in Table 1. It produces paper with the basis weight of 120g/m 2 at an average machine speed of 355m/min. The production capacity is 250 tons per day. Parameters Data Basis weight 120 g/m 2 Machine speed 355 m/min Web width 4000 mm Moisture after press section 55% Moisture after size station 30% Moisture after dryer section 8% Stem flow (mean value) 16.9~19.6 t/h Cylinder diameter 1.5m/1.8m Table 1: Operating conditions The pre-dryer group contained 48 cylinders with the diameter of 1.5m, arranged in four steam groups. The steam pressures in different steam groups are presented in Table 2. The steam and condensate system are operated at different levels to ensure the optimal drying profiles and drainage. The initial web moisture content and temperature is 55% and 40-50%, respectively. The sizing station is used for spreading some non-fibre material to the surface of the web. The final dryer group contained 14 cylinders with a diameter of 1.8m, arranged in three steam groups. The purpose of the final dryer section is to remove the water to the final solids of 92% after sizing. The schematic diagram of the studied dryer section is presented in Figure 2. The condensate from the prior steam group is sent to a condenser, where part of the condensate is 240 Measurement + Control Vol 45/8 October

3 Figure 2: Schematic diagram of the studied dryer section flashed to steam as a lower pressure environment. The flashed steam is then led to the next steam group. The condensate is finally returned to the boiler house which acts as the supply water to the boiler. This kind of steam supply form is the most common multi-phases supply method that has been applied in the majority of the multicylinder paper machines. All of the cylinders are arranged in the traditional double-felted configuration in the studied dryer section. It should be noticed that the steam in the final dryer section was operated at a higher pressure than the pre-dryer section. The operated value in the final dryer section is 2.65Bar as compared to 1.6Bar in the pre-dryer section in Table 2. The dryer section is enclosed by a semi-open hood, where the top half of the cylinders is covered with an insulated hood but not covered at the bottom of the cylinders. The size of the semi-open hood covered in the pre-dryer section is 70m 8m 8m (length width height). There is no hot supply air sent to the pocket for improving mass transfer in the semi-open hood. The air is just suctioned to the hood as a result of negative surrounding pressure in the hood. The air that contained the water evaporated and was exhausted from the hood directly without heat recovery. According to the statistical data of the case paper machine, the specific thermal energy consumption is 10.75GJ/ton paper, which is higher than the best practice value of 9.58GJ/ton paper [17]. In order to improve its energy performance we surveyed the dryer section systematically and then identified the energy saving opportunities for the studied paper machine. 4. Results and Discussion The quantity of dewatering in each part of the paper machine was calculated on the basis of water balance, and the results are shown in Figure 3. It shows that 98.5% of water is dehydrated in the wire and press section and only 1.5% is removed in the dryer section. However, the dryer section is the most energy intensive process in terms of thermal energy consumption, representing more than 80% of thermal energy used in paper making. The steam flow is ranged from 16.9 to 19.6ton/h with an average value of 18.6ton/h. The specific steam consumption could be determined which is tons steam/ton paper. Table 3 presented the geometric parameters of the dryer section from which the contact and convection drying areas can be determined. Then the drying rate Dryer groups Numbers Steam pressure Steam (Bar) temperature ( C) Group 1.1 (#1-6) 6 N/A N/A Group 1.2 (#7-14) Group 1.3 (#15-21) Group 1.4 (#22-48) Group 2.1 (#49-50) Group 2.2 (#51-52) Group 2.3 (#53-62) Total 62 Table 2: Dryer section structure and steam properties Parameter Value Value (Units) (Pre-dryer) (Final-dryer) D (m) L 1 (m) L 2 (m) α 1 ( ) α 2 ( ) β ( ) L cp (m) L pa (m) L cp +L pa (m) Table 3: The geometric parameters can be calculated further. The drying rate of pre-dryer and final-dryer section is 12.25kg/m 2 h and 12.31kg/m 2 h, respectively. It is only half of TAPPI drying rate for corrugating medium. Thus, there are opportunities for improvement in this dryer section, which can be kg H20/h 100% Suction system: kg/h 95.6% Press system: kg/h 2.8% Dryer system: kg/h 1.5% Figure 3: Dewatering process of paper machine Water in paper: 750 kg/h 0.1% Measurement + Control Vol 45/8 October

4 Dryer Section Energy System Measurement and Energy-saving Potential Analysis for a Paper Machine Figure 4: The steam, cylinder and web temperatures Figure 5: Pocket air temperatures Figure 6: Absolute humidity of pocket air Temperature Humidity Relative ( C) (g/kg) Humidity (%) Indoor air Outdoor air Pre-dryer exhaust Post-dryer exhaust Table 4: Measurement results of air conditions achieved by either enhancing the machine speed or turning off some cylinders reasonably at the same production grade. Figure 4 shows the temperature of steam, cylinder and web. The web and cylinder temperatures are increased gradually either in the pre-dryer or in the final dryer section, which is in line with industrial experience. However, there are some questions regarding the cylinders numbered 28, 42, 47 and 54. Since the cylinder temperatures at these points are very low, even close to the web temperature and the cylinders consumed steam, they did not play any role in the paper drying process. Besides, the temperature difference between the steam and the cylinder surface (T s -T c ) was higher than 30 C for cylinders numbered 53, 54 and 55. It also indicates the poor heat transfer with these cylinders. It was recommended to adjust the steam pressures and pressure difference according to the drying curve in order to exhaust the condensate effectively. Figure 5 shows the air temperature of the pocket across the dryer section. It can be seen that the difference of air temperature and dew point is very small at the beginning of the dryer section. This is not ideal for paper drying. In fact, it also means that the condition is poor for paper drying. These questions can also be identified further from Figure 6 and Figure 7. Figure 6 and 7 present the air humidity distribution alone in the dryers. Theoretically, it will be better with a uniform humidity distribution. However, it is impossible for the machine equipped with a semi-open hood. It can be seen that the humidity (either absolute humidity or relative humidity) is higher at the front part of the dryer section. However, it begins to reduce from pocket #25, even decreased to 30 g/kg dry air for pockets # This phenomenon is obvious for the studied dryer section, indicating that there are little water vapour evaporated from the end of the pre-dryer section. Figure 8 shows the absolute humidity of air in the hood at different vertical heights alone in the dryer section. It can be seen that the air humidity is greater at the higher points. Besides, for the space neighbouring cylinders numbered 32-48, the humidity is almost the same as the indoor air. This corresponds well with the information presented in Figure 6 and 7. This can be avoided by either reducing the numbers of cylinders in the pre-dryer section or installing a ventilate system for increasing production later. The relative humidity is also presented with this trend. From the above analysis, it can be concluded that the air system of the studied paper machine was unfavourable for paper drying. The air conditions were investigated further to determine its saving potentials and the measured data is shown in Table 4. In this case, the calculated exhaust air flow is 334,700m 3 /h for pre-dryers and 108,600m 3 /h for finaldryers. The air consumption for removing per unit of water from the hood is 34kg dry airs. The great amount of exhaust air has to be exhausted with the help of enough air fans that would correspondingly use more power energy. If the machine is updated to a closed hood, the above questions would be avoided as only 10kg dry air is required for removing 1kg water. A closed hood requires only one third of air to remove the same amount of moisture compared with a semi-open hood. The power consumption of the fans can also be decreased by one third accordingly. Besides, the higher temperature of exhaust air can also increase the amount of recovered heat. Compared with the semi-open hood, the refurbished programme shows that 24.5% of the energy 242 Measurement + Control Vol 45/8 October

5 balance out the fresh steam consumption. Among the energy conservation opportunities, the most effective way is to update the semi-open hood which can save 24.5% of total energy consumption for the case machine. The first step of energy conservation is to monitor the operating parameters in a timely and accurate manner. There are some parameters that should be measured with sensors embedded in the DCS, such as steam flow to each group, condensate flow out of each group and air conditions within the hood. The audit method proposed in this paper can also be applied to other paper machine dryer sections. It presents a clear picture about the current drying conditions of paper machines and also provides some assistance on how energy efficiency can be improved for the paper drying process. Figure 7: Relative humidity of pocket air Figure 8: Absolute humidity of hood air conservation potential can be achieved, with the specific thermal energy consumption at only 8.12GJ/t paper. There are other energy efficiency improvement opportunities such as preventing steam and condensate leaking, enhancing the cylinder end cap, adjusting the steam pressure difference according to the drying curve and recovering flash steam, etc. Accurate steam flow metering is also very important. Verifying steam use by measuring condensate flow to separators and return to the boiler house is important. By taking web moisture measurements regularly to confirm the amount of water evaporated the adjustment of the operating parameters can be determined. 5. Conclusions A systematic measurement of paper machine dryer section aimed at analysing the energy efficiency for paper drying process was performed in this work. The analysis methodology and related instruments were also illustrated. For the steam and condensate system, it has been observed that too many cylinders were installed at the beginning or the adjustment of steam pressure was not reasonable. This was the main cause for the over drying phenomenon in the end of pre-dryer section. There are three solutions for these problems: reduce the cylinder quantities, reduce the steam pressure of the last steam groups or install a ventilation system. However, the first two solutions are unavailable when considering the possibility of increasing production in the future. For the air system, condensate can occur in the hood, especially at the beginning of the dryer section. The recommendation is to recover the waste heat to 6. Acknowledgements This work was carried out with the financial support from Guangdong Provincial Department of Science and Technology (Project No.2010A ). Lin Zhizuo, Yongjun Yin and Hezi Jiang are also acknowledged for their assistance in the experiment. 7. References [1] K.C. Hill, Paper drying, in Pulp and paper manufacture, 3rd ed., Volume 7, Paper machine operations, Benjamin A, T.;Kocurek, M.J., Editors; TAPPI: Atlanta, [2] A.S. Mujumdar, Handbook of industrial drying, 3rd ed.; CRC Press: New York, [3] C. Yaroslav, K. Aleksander and S. Lester, Laboratory Development of a High Capacity Gas-Fired Paper Dryer, Gas Technology Institute Energy Utilization Center, Des Plaines, IL. Available at, [4] A.B. Etemoglu, M. Can, A. Avcı and E. Pulat, Theoretical study of combined heat and mass transfer process during paper drying. Heat and Mass Transfer 2005, 41(5), [5] J. Martinsson, Energy optimization of paper drying at Munksjö Paper. Department of Chemical Engineering, Lund Institute of Technology, Lund, Sweden, 2005, Master Thesis Thesis. [6] K.C. Hill, Five rules for energy efficiency to improve dryer operations. Pulp and Paper 2006, 80(9), [7] D. Reese, Low-cost dryer update opportunities offer efficiency gains, energy savings. Pulp and Paper 2005, 79(3), [8] G.L. Wedel, Opportunities for Energy Conservation in Paper Drying. In 2006 TAPPI Papermaker s Conference, Atlanta, Georgia: Kadant Johnson Inc.April 24-28, [9] Chaloux, J. Save energy in the dryer section. Paper360 o April/May 2009, [10] I. Lang, Effect of the dryer fabric on energy consumption in the drying section. Pulp and Paper Canada 2009, 110(5/6), [11] J. Laurijssen, F.J. De Gram, E. Worrell and A.Faaij, Optimizing the energy efficiency of conventional multi-cylinder dryers in the paper industry. Energy 2010, 35(9), [12] J.T Paltakari, M.A. Karlsson and O.N. Timofeev, Effect of blow boxes on evaporation during cylinder contact in a paper machine dryer section. Drying Technology 1997, 15(6), [13] J. Maggard, On-line measurement of dryer performance. TAPPI 1995, 78(3), [14] H. Sundqvist, Dryer section ventilation and heat recovery, in Papermaking Part 2, Drying, M. Karlsson, Ed.; Fapet Oy: Helsinki, [15] TAPPI, Dryer section performance monitoring, TIP , [16] K. Asano, Mass transfer-from fundamentals to modern industrial applications; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, [17] BREF-Paper. European Commission, JRC-IPTS IPPC Draft Reference Documents on Best Available Techniques in the Pulp and Paper Industry, Brussels, Measurement + Control Vol 45/8 October