Single-species pulping: simply superior? Several clear advantages show why more mills are adopting this 'singular sensation.'.

In the global pulp market, traditional mixed hardwood and softwood pulps are now competing with single-species pulps such as eucalyptus and acacia, magazine asked several pulping experts for their thoughts on the advantages and disadvantages of single-species pulps compared with traditional mixed pulps.

Some wood species have fiber properties that provide particular values in finished products, and contamination with other species reduces their value. "Producing pulps from these wood species without mixing with other fibers makes sense," says Steven W. McDonald, senior process manager for Metso Paper USA, Atlanta, Georgia. "A mill that has these species in its wood supply with the capability to keep the fibers separated from other species all the way to the pulp dryer or paper machine has an advantage over other mills."

BATCH COOKING

McDonald noted that few mills can produce more than one single-species pulp without also producing large amounts of transition pulp. This process requires separate chip storage and pulp storage. Batch cooking is particularly suitable for single-species operations.

"Some integrated mills cook hardwood and softwood together to supply the desired mixture to the paper machine," said McDonald. "This strategy also avoids the production of low value transition pulp. It requires good control of the chip supply system to deliver the desired mixture to the digester, and optimized process cooking and bleaching conditions for each species mixture."

McDonald added that some wood species do not provide any particular advantage if cooked separately, but they are less expensive and therefore useful as fillers to lower total wood cost. "A disadvantage of single-species pulp production may sometimes be the limitation on using available fiber sources to lower the total raw material cost."

According to McDonald, it is easier to optimize single-species pulping process operations mixed-species pulping. This is an important advantage. "A frequent problem with mixed pulp production is mixture variation," he said. "The pulping rate and bleaching response have very clear differences, especially among hardwoods. Mills that buy mixed-species chips have specifications for chip size distribution and limits on bark and decay, but they have less control over the species mixture and the associated properties of cellulose content, lignin structure, and specific gravity."

DELIGNIFICATION

When mills change the chip furnish mixture, delignification conditions are typically adjusted after kappa control has deteriorated-rather than before the furnish change--because the mixture is difficult to measure or not closely monitored. "This produces more variation throughout the process, less capability to optimize chemical application rates, and variation in final pulp properties," said McDonald. "In this respect, a single-species operation has an advantage over mixed-species pulp production."

Martin MacLeod, senior scientist for Paprican, Pointe Claire, Quebec, Canada, agrees that single-species pulping offers several advantages for most mills. "Single-species pulp will always be the way to go, at least until we can control tree genetics to achieve the exact chemical content and fiber morphology we want," he said. "Many single-species examples exist around the world: eucalyptus species in South America and in Japan with imported wood, various pines (Pinus taeda in the United States, P. sylvestris in Scandinavia, P. radiata in New Zealand and Chile), and aspen in Canada. Let the papermakers set the fiber recipes but give them very predictable, consistent pulps.

"Making mixed-species pulps is inevitably a compromise, and softwood/hardwood pulps are the biggest compromises," MacLeod continued. "Why make wildly 'bimodal' pulps for bad reasons in a digester instead of making uniform ones for good reasons to deliver to a stock preparation department?"

Peter W Hart, senior research engineer for MeadWestvaco Corp., Chillicothe, Ohio, USA, noted that a mill can gain several advantages by pulping single-species trees instead of mixed species. In a single-species cook, the average density variation within the wood chips is substantially lower. Cooking liquor penetrates more uniformly, producing lower kappa number variability within the cook.

"A more uniform cook translates to reduced bleaching costs and better strength properties in the final bleached pulp," said Hart. "Improved uniformity also means better process control capabilities for reduced cooking and bleaching chemicals. This reduces total costs. Pulp yields improve as portions of the resulting pulp are not overcooked. Single-species cooking and bleaching provide mills with many advantages--reduced chemical costs, improved pulp yield, and improved pulp strength."

Panu Tikka, professor at the Helsinki University of Technology, Laboratory of Pulping Technology, Helsinki, Finland, agreed that single-species hardwood pulps are superior to mixed-species hardwood pulps due to the constant and suitable fiber morphology and fiber physics. These characteristics enable fast, mass production of high quality papers