By PPI Special Report
Clearly there is more to this story than fine-tuning. It speaks of a "root and branch" review of technology, management and design. Of course cogeneration, which has been a feature at LC Paper for many years, is at the heart of this project. But this alone does not explain the radical results which LC and its suppliers achieved at Besalú, Girona, in Spain. Energy efficiency and water saving were fundamental aspects of the project, while at the same time allowing maximum production flexibility to facilitate swift product changeover and new product development.
Hood held the key
"We knew that the tissue machine hood held the key to the most meaningful potential energy savings," says Moreno. "So we sat down with Milltech's Luca Linari with a blank sheet of paper to discuss just how we could operate our tissue machine as efficiently as possible from an energy point of view, and integrate most effectively with our cogeneration facility."
Linari and Moreno started with the basics: What influences the energy efficiency of a tissue machine? This is the fundamental question to address when embarking on any new tissue machine project.
On the stock preparation side, fiber composition, in other words the ratio of long fiber to short fiber, is the place to start, particularly as mills need to review their furnish constantly according to the mood of the pulp market and customer requirements. Then there are water consumption, refining quality and intensity, fiber retention and stock air content to consider, as well as white water temperature. Most of these are of course interdependent to a greater or lesser extent. Fiber choice, refining intensity and quality, for example, are inseparable.
On the machine side, energy efficiency is affected by formation, which itself is largely defined by the diameter of the former, the design of the wire and the type of felt used. Other contributors are the vacuum system - ring or fan pump - machine speed, press section design/diameter, yankee cylinder and steam plant design, and yankee hood and air system specification.
The brief moments the web spends between the yankee cylinder and hood are particularly influential when it comes to energy performance. The choice of yankee coating, the web temperature and dryness ratio all bear great responsibility for the final result.
LC Paper in Besalú, Girona in Spain specializes in machine glazed paper, which can be printed on-line, and tissue. There is one paper machine dedicated to each. Able to trace its history back to the 19th Century, LC Paper 1881 has a history of pioneering new technology. In 1973, for example, it invested in the world’s second dispersion plant for recovered paper.
The key energy strategies
LC Paper and Milltech adopted a series of critical measures which resulted in considerable energy efficiencies. LC Paper is in the vanguard of tissue mills increasing the weight of the tissue it produces and consequently reducing the number of plies. By changing from two 17 g/m2 plies to one of 30 g/m2 , the mill has been able to reduce its raw material consumption by 11.8%, and slash energy use by 30%. As a consequence, carbon emissions have dropped as has the use of chemical additives, which has helped cut NOx levels.
This product innovation has been matched by process initiatives. With low intensity refining, increased stock temperature at 44ºC (in a closed POM water circuit - the first of its type on a tissue machine), improved retention (to reduce water consumption), improved moisture profile using a steam box, and an increased press load, savings in energy consumption of up to 20% have been possible. Increased paper adhesion to the Yankee combined with a higher ratio of drying on the Yankee side, have yielded 18% in energy savings from that part of the line. To put it simply, the yankee has taken on greater responsibility for drying compared with the hood.
Yankee/hood strategies
Individual strategies in the Yankee/hood area include the use of the exhaust gases from the cogeneration system in the dry end hood, which avoids using the burner to dry the sheet. In addition, the air process system has been optimized according with operating parameters by means of the reverse cascade/parallel circuit. It should also be pointed out that there are several heat recovery stages to recover exhaust energy. These include: the steam recovery generator, which generates high pressure motive steam with a booster burner; the economizer on the steam generator; the air to air heat recovery for hood processes; a low pressure steam generator for the steam box; plus the vacuum fan, which is used to recover heat for ventilating the roof. Hood balance has been improved by optimizing the blowing distance and exhaust humidity has been increased. The burner and combustion chamber - designed specially for this project are highly influential, and the hood wrap angle has been optimized, and is now asymmetrical.
Thanks to its vision and Milltech's intimate knowledge of the hood dynamics, LC Paper is in a position to supply tissue with the efficiency and flexibility which many manufacturers can still only imagine. The energy results speak for themselves.
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