“Economic factors have led to new investments in pulp manufacturing, especially in plants producing market pulp, focusing on fiber lines with increasingly larger capacities,” says the sales director. Antti Suoknuuti From Valmet's dryer department.
"Currently, single-line fiber lines have already exceeded the daily production level of 5 tons. This has been achieved with new equipment solutions, higher process consistencies and larger equipment sizes. We cannot forget about new automation solutions that enable even large plants to run as smoothly as possible," Suoknuuti continues.
According to him, the drying line at the end of the fiber line has had to adapt to the corresponding development. After the beginning of a single-line fiber line, a two-line drying solution is not the most attractive option in terms of investment or operating costs.
The cutter after the dryer has also required a lot of experimentation and development work.
"Radically increasing the cutting width has not been a simple matter, but has required progress in small steps. From the typical web speed of around 150 m/min twenty years ago, the latest production lines have already reached web speeds of over 250 m/min, and cutting widths have grown from around eight meters to over ten."
Investment targets include large mass market lines
The latest investments in high-capacity pulp drying lines are aimed specifically at market pulp mills that drive the same quality.
The options for the dryer's wet end solution include a Fourdrinier-type flat wire, where dewatering is enhanced by a so-called Top-Former section on the top, or a solution based on double wire technology developed especially for the mass market.
The Fourdrinier type is also suitable for special stocks due to its flexibility. In the double wire solution, the stock feed from the headbox goes directly and closed between two wedge-shaped narrowing wire spaces, with water being discharged both upwards and downwards. Thanks to the efficient precipitation, the length of the wire section is also shorter than in the case of a flat wire.
Both Andritz and Valmet have developed their own applications for this technology. In Valmet's solution, the double wire section is equipped with advanced automatic dewatering control. To maximize wet end dewatering in new plants, the press section usually has a combi press-shoe press combination, with either one or two shoe presses.
Depending on the type of pulp, an efficient press section can achieve a dry matter content of up to over 55 percent before the web is guided to the actual drying section.
From cylinder drying to air dryers
To achieve a final dryness of around 90 percent, the remaining water in the pulp web must be evaporated using heat. Until the 1980s, traditional cylinder drying technology was mainly used, in which, like paper machines, cylinders heated by steam dried the pulp web.
"We gradually switched to using convection-based air dryers in the drying section, which are more cost-effective and energy-efficient in terms of investment costs. In them, the pulp web moves back and forth in layers in the drying cabinet, supported by heated air, evaporating water," says Suoknuuti.
Record-breaking drying lines under construction
Currently, the largest drying machine projects utilizing new innovations are the largest drying line in Europe, which will be installed at Metsä Group's Äänekoski bioproduct mill, and the two largest lines in the world, which will be installed at Asia Pulp and Paper's OKI project in Indonesia.
The Äänekoski line, which will start up in 2017, has a design production of 2 tonnes of softwood or hardwood pulp per day. The line's web width at the cutter is 700 metres. Two parallel fibre lines will be completed for the OKI pulp mill, which will start up this year, each with a production of 8,25 tonnes of hardwood pulp per day. The machines' cut web width is 4 metres.
"Machine widths are already such that we are unlikely to see any growth in the near future. Capacity increases are possible by further increasing both the web weight and the machine speed. Increasing the weight requires investments in better control of the evenness of web formation, both with equipment technology and automation solutions. However, operational reliability is of paramount importance for the customer. A realistic drying machine speed in the future could be closer to 300 m/min," Suoknuuti believes.
Text: Leif Lindberg
