Global demand for long fibers is expected to grow by four million tonnes by 2025 (Farinha, 2014) and several mega softwood pulp mill projects have already been launched in China, Russia and Northern Europe to catch that growth. Those projects are challenging ones since the latest large softwood pulp project was completed in 2004, already 10 years ago, at Stendal in Germany. All greenfield pulp mill projects have been dealing with hardwood pulp production since then.
That 10-year gap has allowed some softwood pulp producers to carefully review state-of-the-art technologies and realize that some new technologies now qualify as mature technologies. As a result, those pulp producers chose ozone bleaching as their base case solution. Indeed, ozone bleaching allows for significant environmental benefits and economic savings in the range of Euro 7-10/air dry tonne (adt).
However, other pulp producers put ozone bleaching to the side from the beginning considering a priori it would not be suitable in the specific case of softwood pulp bleaching. The present article shows such a decision could be based on outdated or mistaken information concerning pulp quality, economics and an environmental viewpoint.
The main argument of some specialists against the use of ozone in the bleaching of softwood pulp is that ozone bleaching would not deliver the same pulp strength properties as conventional ECF bleaching does (i.e. with chlorine dioxide as the main bleaching chemical). This is not true. A first counter-example is SCA Östrand (Sweden) where softwood kraft pulp has been successfully bleached since 1995 with a Z-TCF sequence (ozone and hydrogen peroxide are the only bleaching chemicals). The mill produces premium NBSK pulp characterized by brightness up to 90% ISO, excellent beating properties, purity and high strength. It is used in printing and writing paper, tissue paper and fine paper. The Östrand pulp does not step back against conventional ECF pulp (Germer et al., 2013).
Nevertheless every specialist knows that introducing even a small chlorine dioxide stage in a TCF bleaching sequence eases the bleaching process. And many specialists know that introducing a moderate ozone dose of 5 kg O3/adt in a conventional ECF bleach plant has no significant effect on softwood pulp quality. For high doses such as 6.5 kg O3/adt, Wennerstrom reported in 2008 a tear index decrease of only by 6% (from 16.5 to 15.5 mNm2/g) for a tensile index of 80 Nm/g on Siberian softwood pulp when comparing D-EOP-D-D with Ze-D-D.
Another example is Mondi Ruzomberok in Slovakia where a ZEOP-D-P bleach sequence delivers softwood pulp characterized at 28°SR by a breaking length of 11-12 km, a tear index of 9.5-10.5 Nm2/kg and a stiffness of 125-135 mN (Germer et al., 2011).
Pulp quality is enlightened by its behavior on the paper machine: Mondi Ruzomberok's PM 18 set a world record as fastest paper machine of its kind in 2011 while PM 16 is the Valmet benchmark leader for lowest percentage in broke among uncoated fine paper machines (Germer et al., 2013).
It is also well-known that introducing an ozone dose ahead of the first chlorine dioxide stage in a Z/D combination without intermediate washing is very effective for hardwood pulps. Interestingly the same phenomenon was noticed on softwood pulps as Fuhrmann (2005) and Chirat et al. (2008) reported clear improvements of the tear index of softwood ECF pulps bleached when including a Z/D combination. In fact Z-ECF bleaching with a Z/D combination allows for the highest strength properties.
Today six mills - some belonging to worldwide leaders such as SCA, Mondi, Mercer or UPM - successfully bleach softwood kraft pulp with ozone. It is significant since the development of modern technology has been limited in softwood pulp production by the fact that softwood accounts for less than 25% of the global raw material and that most projects have been conducted on eucalyptus pulps over the past years.
Current softwood pulp mill projects are aimed at joining the million tonners club. The following case study explains why Z-ECF bleaching is more competitive than ECF bleaching for a 1,000,000-tonne/yr fiberline.
Ozone generation is a pure on-site technology requiring only energy and oxygen (usually also produced on site from a VPSA plant). Ozone (O3) is produced from oxygen (O2) in an electrical field at a concentration of 12% by weight 10 kWh/kgO3and 8.3 kgO2/kgO3and then compressed at 10 barg with 1.5 kWh/kgO3. Therefore, delivery at the mixing point of 1 kg pressurized ozone requires 11.5 kWh and 8.3 kg oxygen.
Considering an energy price of Euro 0.04/kWh and oxygen price of Euro 0.08/kgO2, 1 kg of pressurized ozone costs 11.5 x 0.04 + 8.3 x 0.08 = Euro 1.124. Including 5% for operation and maintenance, 1 kg of pressurized ozone costs Euro 1.18. Nevertheless, it should be highlighted that oxygen leaves the ozone bleaching stage unreacted. More than 50% of the mills using ozone bleaching reuse that oxygen in oxygen delignification, EOP-stage or other applications. Therefore, if considering that oxygen is used further, 1 kg of pressurized ozone costs (11.5 x 0.04 + 1 x 0.08) x 1.05 = Euro 0.57.
Ozone cost is in any case cheaper than the average Euro 1.5/kg price of chlorine dioxide.
Moreover, every kilogram of ozone replaces 1.7 kilograms of chlorine dioxide (Grundelius, 1993). Therefore, the use of 5 kgO3per tonne of pulp allows the mill earning 5 x 1.7 x 1.5 - 5 x 1.18 = Euro 6.85/adt if not reusing oxygen or 5 x 1.7 x 1.5 - 5 x 0.57 = Euro 9.9/adt if reusing it. Thus, ozone bleaching can allow a pulp mill to save Euro 7-10 million/yr.
Considering investment costs in ozone generation, ozone compression, ozone mixing as well as automation and civil works, the return on investment (ROI) of ozone bleaching is between 1.5 and 2.5 years.
ECF bleaching effluents are considered to be ecologically friendly because they are compared with the effluent from chlorine bleaching. But looking at specific figures, dioxin production per tonne pulp was divided by only 10 with the implementation of ECF bleaching (Zimmer, 2014) while present pulp mills are much larger than those built 30 years ago. Dioxin rejects are indeed still a concern as ECF bleaching and ECF bleaching effluents are definitely less environmentally-friendly than Z-ECF and TCF ones.
It should also be noted that reduced chlorine dioxide consumption also means a lower OX content in the final product. It is a parameter with growing importance in the developing markets of tissue and food contact papers.
Moreover, the extensive use of chlorine dioxide hampers environmental performance of "mega" pulp mills because it limits the possibility of recycling bleaching effluents to the recovery boiler. It results in difficulties meeting current COD and color regulations so more and more pulp mills are now requested to implement costly tertiary treatment for hard COD or color removal.
Ozone bleaching is a solution that allows mills to meet stricter environmental requirements because when compared with traditional ECF bleaching, it allows a reduction in the AOX load of 2-3 times, the COD load by 20-40%; the BOD load by 25-30%, the color load by 60-70% and the OX content in the pulp by 2-3 times (Germer et al, 2011).
Z-ECF bleaching gives similar softwood pulp quality as conventional ECF bleaching.
A mill with a 1,000,000-adt/y capacity would save a minimum of Euro 7-10 million yearly by investing in ozone bleaching.
ROI is around two years.
OX content in the pulp 2-3 times lower than with conventional ECF so pulp marketability is increased for tissue and food contact applications.
Ozone bleaching allows for reduction in the effluent of AOX load by 50-70%, COD load by 20-40% and color load by 60-70%.
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Emil Germer, SaintPetersburg State Forest Technical University;Alexis Métais, JeanChristophe Hostachy, Xylem Inc.