An energy-focussed approach is just what Asia needs right now.
Marco Dalle Piagge, General Manager of Toscotec in China
Asia is a vibrant place for anyone involved with tissue. Our experience bears this out: we have sold 25 tissue machines to China in the past three years, and in the period October 2012 to February 2103 alone, started up 10 projects. This high level of activity in Asia has taught us many things, but the most fundamental insight is the significance of energy efficiency for the region’s tissue makers. The Steel Yankee Dryer (in our case TT SYD) is the technology of choice in Asia, and is still evolving. We have delivered 40 to this market in the past three years which alone suggests that energy is top of the agenda. But while the benefits of steel as opposed to cast iron have been widely discussed, there are many other aspects of tissue making which offer opportunities to increase energy efficiency to Asia’s tissue makers. In this article, we focus on the interaction between yankee and hood.
Between hood and Yankee. Hood drying requires up to 30 percent more energy per kg of water evaporated than Yankee drying, so as a starting point, it is beneficial to the process to maximise the drying potential of the Yankee, possibly by considering a larger diameter unit, and then specifying the hood according to production priorities and local conditions, especially in terms of available energy sources. C&S Tissue, to which we supplied a total of six new lines last year in Jiangmen, Tanghsan in the northern province of Hebei, and Chengdu in Sichuan, south west China, has been a case in point. Local conditions have dictated that none of the new tissue machines has a gas-heated hood. The hood merely sucks out the mist generated by the Steel Yankee Dryer (SYD), so the dryer is effectively doing all the work. Our answer has been to specify our largest ever SYD at 16ft (4.88m) in diameter. In our experience, Asian tissue manufacturers are becoming more interested in energy and quality rather than just speed and capacity. It is net tonnes or actual output which matters rather than theoretical capacity. In this regard, speed itself can be a distraction, when it is overall efficiency that is the most important figure, as a measure of net energy consumed per tonne of product produced. This is particularly relevant in China, where the lack of gas which C&S has to deal with is a common problem. Guangdong for example, has virtually no gas at all. Most 30 to 50 tpd tissue machines in China rely on coal to generate steam. If mills on sites without access to gas have plans to step up to higher production levels, they will have to consider that coal is simply not able to produce steam at as high a rate as gas. We believe, however, that speeds close to 1,700 mpm are realistic, and with world-class runnability, this can still put a mill in a strong position.
Time to focus on the hood. Given the optimum Steel Yankee dryer, it is the hood which completes the picture in the drying section, and the addition of Milltech to our group of companies has provided a new perspective on this. Whatever the fuel source, the benchmark for all tissue making operations should be that it makes no sense to let the hood’s exhaust simply heat the atmosphere. Heat recovery should be the starting point in planning any new tissue machine or rebuild. This can be broken down into various categories, but the most relevant for Asia is what we call R1, which represents air-to-air heat exchange within the hood system, by which heated exhaust air is utilized to pre-heat the fresh air before the exhaust to the atmosphere occurs. The difference between good and excellent performance is all about stability and balance. Even using conventional equipment, it sometimes may be possible to increase overall energy efficiency by some percentage points. Strategies to achieve this include optimising the balance between yankee and hood drying, and between the contribution of the dry and wet ends of the hood. Stability of the process is another essential optimisation factor: achieving uniformity of dryness, with reduced peak-to-peak dryness variation in the sheet, allows the system to be set for higher overall dryness which requires less energy to maintain and will also result in less rejected paper.
The hybrid solution. One solution, which is about to make its debut in China in an as yet undisclosed location, is the hybrid hood, in which the degree of heating contributed by the hood, and in different sections of the hood, can be easily varied, from zero (suction only) upwards. Typically a hood will blow hot air in the wet end, while in the dry end, the hood’s role is limited to suction. This is a good solution where energy reduction is a priority, with reasonable drying capacity, and in combination with proper heat recovery systems. The best solution for pure energy efficiency, as stated, is to dry with the Yankee alone with the hood just providing suction, but to respect a mill’s needs both for energy efficiency and output, a hybrid solution can be ideal. The optimum balance between Yankee and hood will depend on the grade of tissue produced in terms of drying capacity. For low basis-weight products such as toilet and facial tissue, the most popular grades in most of Asia, the wet-end-only heating concept is generally fine, where the dry end of the hood merely acts as an exhaust system. Other factors may affect efficiency in drying, such as hood cross nozzle box design, hood operating distance to the Yankee surface and hood frame stability at temperature (up to 650°C). Continuous R&D developments are being evaluated in this regard to achieve better machine performance.
Balancing output and input. Depending on priorities most tissue producers in Asia have the option either to increase output for the same or less specific energy input, or to maintain the same level of output but sensibly cut energy consumption. And yankee/hood drying is not the only area to investigate. The favoured raw materials in tissue production have become short and recycled fibres, and Asia is no exception. There are distinct variations within the region, because some parts of Asia are more advanced in gathering recycled paper than others. But throughout, stock preparation has taken on more significance than ever before to deal with new fibres and ensure high quality standards and boost tissue machine runnability. There are significant new opportunities for stock preparation to contribute to overall production efficiency. Bear in mind also that a 1 percent increase in post-press dryness can lead to a reduction of overall drying energy consumption of up to 4 percent, so the press section is an important part of the drying picture, too. While our recent opening of a new manufacturing and service facility in Shanghai for Toscotec is an acknowledgement of the importance of the Chinese market, there are other Asian markets where reduction in energy consumption is increasing in importance, and it is to such markets that we are naturally inclined. South Korean tissue makers are looking closely at their energy profiles, and Japanese producers are starting to rebuild tissue machines with energy consumption in mind. We believe South East Asia will follow shortly, as the benefits of an energy-focussed approach become clear. *