BCTMP still has a big part to play in the world's papermaking capabilities, especially now that European influences are spreading

By Eric Cannell and Richard Cockram

The Future of BCTMP

Just over a decade ago, North America witnessed the heyday of market bleached chemi-thermomechanical pulp (BCTMP). Growth in the consumption of this particular grade was strong and Millar Western (Whitecourt), Fibreco, Temcell II, Cascades (Port Cartier), and Consolidated-Bathurst (Bathurst) had all recently started up. On top of that, four more mills were either under construction or in the design phase and BCTMP was selling for $630/air dried metric ton.

Worldwide, BCTMP capacity totaled 1.9 million tons/yr and was forecast to grow to 2.8 million tons/yr by the end of 1991. An additional 3.4 million tons/yr of capacity was also being considered for installation around the globe.

The popularity of BCTMP was to be found in several inherent advantages that it offered over other chemical pulps. For example, BCTMP offered:

• lower capital costs
  
• twice the yield (85% to 90% versus 42% to 52% for other chemical pulp grades)
  
• lower manufacturing costs
  
• higher bulk, opacity and stiffness
  
• reduced environmental impact.

In 1992 though, the growth bubble burst. The selling price of BCTMP plunged to $330-400/ton as part of an overall collapse in the market pulp sector (Figure 1). Many BCTMP projects were cancelled outright, some capacity was integrated and several lines shut down. Contrary to the rosy forecasts, the magical growth of BCTMP came to a grinding halt. Except for a brief recovery in 1995, the selling price of BCTMP remained at these low levels, until recently that is.

Current capacity

It has been nine years since a greenfield BCTMP mill has started up and today the worldwide capacity of market BCTMP stands at 2.4 million tons/yr. Table 1 presents a listing of market BCTMP mills that are currently in operation, not including capacity that is integrated and not available for sale on the open market.

Figure 1: Average Annual Pulp Prices 1983-2000

Some 74% of the world's market BCTMP capacity is located in Canada. Low energy costs and proximity of wood species suitable for BCTMP production are the primary reasons. Another 20% of the BCTMP capacity is located in the Nordic countries, most of which dates from the early days of BCTMP (ie 1978-1985). As might be expected, BCTMP capacity in the Nordic countries has dropped since 1989 due to integration and shutdowns. Nordic capacity stands at 495,000 tons/yr compared with 505,000 tons/yr in 1989 - a 2% drop. On the other hand, Canadian capacity has increased from 1.04 million tons/yr to 1.80 million tons/yr, with most of this 70% increase having occurred in 1990 and 1991.

Contrary to earlier expectations, the production of market BCTMP has not taken hold in Latin America. In fact the 90,000 tons/yr of BCTMP capacity that existed in Mexico and Brazil in 1989 has been removed from the market. Several projects have been proposed, such as the Parana River project in Paraguay, but none have come to fruition. Tremendous potential still exists for the production of radiata pine and eucalytus BCTMP in Latin America.

See Related Table:,

Grades and end-uses

At present, 52% of BCTMP capacity is devoted to softwood grades, while 48% is focused on hardwood grades. This is a major shift from 1989 when softwood grades accounted for more than 90% of the installed capacity. The growth of hardwood BCTMP is primarily due to its successful application in printing and writing grades, especially in Europe and Asia. Hardwood BCTMP grades are currently made from aspen, birch and maple.

In 1989, the so-called 'woodfree rule' limited the amount of mechanical pulp that could be included in a 'free-sheet', or woodfree, grade to a 10% maximum. But between 1996 and 1998 this rule was, for all intents and purposes, eliminated in North America from all of the various paper standards.

The removal of this barrier helped boost the utilization of hardwood BCTMP. With the exception of 'permanent papers', buyers and sellers of free-sheet are at liberty to base paper purchase specifications solely on functionality. Nonetheless, some prejudice against mechanical pulps lingers. For permanent papers, ANSI/NISO and ISO standards limit the lignin content to 1%, thereby excluding the use of BCTMP. Research is ongoing in an attempt to have this limitation lifted.

Aspen is the predominant fiber source for hardwood BCTMP. While aspen BCTMP is typically produced at 200-250 mL Csf and 85% ISO, a range of brightness and freeness is available. Depending on the aspen grade, its tensile strength ranges from 80% to 100% of bleached hardwood kraft (BHK), while its bulk is always higher. This grade can be used by non-integrated mills to replace BHK in commodity coated woodfree grades at an application rate of 5% to 15% of the base sheet. The reported benefits include reduced manufacturing costs, improved formation, better opacity, increased bulk, greater sheet stiffness and lower porosity. Lower porosity improves coating holdout so that a higher filler load can be achieved without loss of bulk or sheet rigidity.

In uncoated free-sheet such as copy, forms and books, aspen BCTMP is typically applied in the 10-30% range, especially in non-integrated mills. Its use reduces manufacturing costs and improves bulk and formation. Aspen BCTMP is also used to replace a percentage of the chemical pulp in the outer plies of multi-ply packaging board.

Currently, Tembec is the only supplier of maple and birch BCTMP. Birch is also used in printing and writing grades, giving similar benefits to aspen. Birch has a higher bulk and a slightly lower tensile strength compared with aspen, making it the premier BCTMP grade for printing/writing grades. It is typically produced at 500 mL Csf and 85% ISO. Unfortunately it has limited availability.

Maple BCTMP is used in the center ply of multi-ply packaging board, replacing softwood chemical pulps. Benefits include reduced manufacturing costs, increased bulk, greater sheet stiffness and improved creasing and folding properties - less 'springback' for example. The higher bulk of maple compared with chemical pulps, translates into lower fiber requirements for the same basis weight. Maple BCTMP is produced at 400 mL Csf and 78% ISO. Part of Temcell's maple BCTMP production capacity is integrated with Temboard and is used in the production of multi-ply packaging board.

Softwood BCTMP is primarily used in tissue, toweling and fluff products, partially to replace bleached chemical pulp or secondary fiber. Birch and maple BCTMP are also used. Benefits include reduced manufacturing costs, greater absorbency and improved bulk. For tissue and toweling, the higher bulk of BCTMP permits a reduction in basis weight at constant caliper for significant reduction in fiber requirements.

The fluff market for BCTMP failed to grow as originally forecast due to the introduction of super absorbent chemicals. The use of such chemicals allows thinner diapers at equivalent absorbency, with a subsequent reduction of the fiber component in the product.

Softwood BCTMP is also used in copy, book, forms and the inner plies of multi-ply board. Softwood/hardwood blends are also available. These grades are used as a groundwood extender or to replace reinforcement pulps in lower brightness sheets to increase opacity or improve sheet formation.

On the market

Statistics for the production and consumption of BCTMP are kept a closely guarded secret by producers. This makes it extremely difficult to get accurate figures, even from trade associations. Based on the latest shipment figures published by the Canadian Pulp & Paper Association (CPPA), we believe current world consumption of market BCTMP is close to 2.2 million tons/yr, of which 52% is softwood and 48% hardwood. Europe remains the largest consuming region (Table 2), but Asia could soon overtake it.

To put this into context, world consumption of all bleached market pulps is around 40 million tons/yr, giving BCTMP a market share of a little over 5%. BCTMP consumption patterns vary considerably in different regions, for both historical and structural reasons.

Taking Europe first, BCTMP is used in a wide range of paper and board grades, but still only accounts for 7% of bleached market pulp consumption. Non-integrated woodfree paper producers have not seized the opportunity presented by aspen BCTMP to produce lower grammage grades with no loss of brightness, bulk, or opacity. They have been deterred by early 20th century definitions of woodfree papers, based on the quality of groundwood pulps at that time, and by exaggerated fears of yellowing. Metsä-Serla did take up the challenge with a new grade based on integrated aspen refiner pulp and aimed directly at traditional woodfree end uses, but its lead has not been followed.

There are still some mills producing mechanical papers with market BCTMP, but the larger mills have invested in integrated BCTMP facilities or deinked pulp (DIP) plants. Tissue producers were keen to take advantage of the bulk and absorbency of BCTMP in the 1980s, but DIP is now the material of choice for most mills. BCTMP is highly effective in the middle layers of multi-ply cartonboards, but most of these products come from integrated machines in the Nordic countries with their own BCTMP lines.

In North America, most of the market BCTMP used in the USA and Canada is for tissue and towel grades. Both Scott Paper and Procter & Gamble were influential in developing grades of softwood BCTMP for absorbent products.

Small quantities of aspen BCTMP are used by non-integrated free-sheet producers, which historically have been constrained by a 10% limit on high yield pulps in free-sheet grades. The recent removal of this barrier will cause a steady increase in BCTMP utilization as the pulp supply tightens and free-sheet producers experience the benefits of BCTMP. Furthermore, when one considers that the production of white paper grades is forecast to grow significantly during the next few years, the demand for aspen BCTMP will become strong.

North American BCTMP producers will have difficulty meeting this increased domestic demand in the short term due to their current supply contracts to European and Asian customers. Assuming no new capacity is built, these producers are likely to shift more volume to domestic customers as their existing contracts expire to take advantage of the better mill net.

The North American paper industry is much more integrated with respect to pulp supply than the European industry, limiting the scope for market BCTMP suppliers. Competition with market DIP has also been a significant influence in the USA.

Looking to Asia, Japanese paper mills import substantial quantities of thermomechanical pulp (TMP) from Canada and New Zealand, but they are relatively modest users of BCTMP. The developing countries, however, increasingly need imported high yield pulps, due to their lack of softwood forests suitable for mechanical pulping.

The rapid expansion of the Indonesian pulp sector has limited the need for BCTMP in woodfree papers in Asia, but the new multi-ply cartonboard machines in China and elsewhere benefit from an inner layer of high yield pulp, which gives both bulk and stiffness to the product.

Economic factors

Average mill manufacturing costs were calculated using NLK's proprietary software program - 'Compete in Pulp and Paper' - and are shown in Table 8. The costs at individual mills vary of course. On average, BCTMP manufacturing costs are $40/ton higher in the Nordic countries than they are in North America for aspen and $22/ton higher for softwood. This disadvantage is largely offset by lower average shipping costs for the Nordic producers.

North American producers have lower fiber and energy costs, but higher chemical and material costs than the Nordic producers. Higher shipping costs for North American mills reflect the fact that the majority of their production is shipped to Europe and Asia.

The selling price for aspen BCTMP has averaged $505/ton over the past 17 years, ranging from a low of $349/ton to a high of $781/ton. During this period, aspen BCTMP sold for an average of $67/ton less than BHK, or $128/ton less than BSK (the dollar values mentioned here have all been corrected to today's dollars based on the Producer Price Index). The capital cost for a 200,000 ton/yr BCTMP plant complete with zero liquid effluent is estimated at $200 million (± 30%).

Technology update

Two of the eight Canadian BCTMP mills built between 1988 and 1991 were based on the APP process. The others were conventional BCTMP. That raises the question of whether one process is better than the other.

Initially, it was expected that the APP process could produce fully bleached market pulp without any bleaching stages after the refiners. As a result, it was claimed that the lack of post-bleach towers gave APP a capital cost advantage over BCTMP. In practice though, the final pulp brightness with this approach was lower than expected and the chemical costs were higher. APP mills consequently added post-bleaching stages.

Recently, Andritz has proposed using an inter-stage high consistency bleach tower instead. Pulp from the primary refiner would be sent directly to a tower where the residual peroxide could continue the bleaching reactions. Published laboratory results suggest that achievable brightness on hardwood grades would be just 80% ISO - well below the current grade specifications of 85% ISO. Furthermore, the latency has not yet been removed. Latent pulp stored at high consistency can develop 'permanent latency'.

For the moment, a greenfield APP market pulp mill would have to be built with a post-bleach stage (or stages), thereby eliminating its capital cost advantage. APP may have a specific energy consumption (SEC) advantage over BCTMP depending on the alkali/peroxide split between the impregnation stages and the post-bleach towers. As the split is shifted toward the front end, however, savings in SEC may be offset by increased chemical costs. Furthermore, a comparison of the APP and BCTMP commercial aspen pulp quality shows very little difference. We conclude that the differences between conventional BCTMP and APP becomes so small that the selection criteria becomes site specific.

There have been many significant technological advances made in the field of BCTMP production since 1989, many of which are summarized below. One trend here is in TMP refiner lines, which has seen a move to larger refiners and fewer refiner lines. For example, Papier Masson is in the process of installing a single line of refiners (CD-82) with a capacity of 740 tons/day to supply its newsprint machine. For a BCTMP mill that produces only a few grades, such an approach has merit.

However, many BCTMP mills today produce a myriad of grades. Wood species, freeness, brightness and bulk are all changed to generate different grades tailored to customer requirements. In some mills, grade changes take place every few days. A large, single line mill would result in very short run times and excessive amounts of transition pulp. With smaller independent lines, different grades can be produced on each line.

During the past five years, development in refining technology has focused on the reduction of SEC in the production of low freeness TMP for groundwood communication papers. With an SEC of 2,200-3,000 kWh/ton, the potential savings are large. Since the potential energy savings for BCTMP are generally lower, these technologies must be appraised on a case by case basis. In some cases, existing BCTMP mills can be retrofitted to these new technologies.

Closed loop control of the primary refiner consistency facilitates increasing the refining intensity by controlling the consistency to a lower set-point. The blow line consistency can be estimated by the process computer using mass and energy balances, or measured directly by a sensor installed in the blow line.

Refiner plates can be designed to give lower SEC values. The RT-RTS process designed by Andritz reduces SEC by virtue of the increased refining intensity afforded by operating the primary refiner at higher speeds. Pulp strength properties are maintained by pre-heating the chips to elevated temperatures for 10-18 seconds immediately prior to refining and again in a similar manner immediately prior to impregnation. The RT-RTS process has been installed at two mills and an additional four mills are equipped with its predecessor, the RTS process, which is still under evaluation. The RT-RTS process is not applicable to mills using APP.

The Thermopulp process pioneered by Valmet (formerly Sunds Defibrator) reduces energy consumption at the secondary refiner by heating the pulp at 160-165°C for 5-10 seconds between the first two refining stages. This process has been installed in seven newsprint mills, but it is still under development.

Low consistency refining (LCR) immediately after the high consistency refiner(s) can also be utilized. When retrofitted to an existing mill, increased production and energy savings are possible. At least one existing BCTMP mill has implemented this technology.

Increasing refiner throughput by debottlenecking upstream and downstream equipment is also useful.

In detail

Research has shown that high compression (ie 4:1) plug screw feeders (PSF) delaminate the chips, making them more amenable to impregnation. Although the energy requirements of these PSFs are high, the energy input to the refiners is somewhat lower.

The importance of a stable mass flow of chips to the refiners cannot be over-emphasized. We have observed MW load with a coefficient of variance of 10% over 2-5 second intervals. The disc gap swings in unison with the refiner load so that the pulp is over-refined one moment and under-refined the next. Average freeness targets are met, but the average pulp quality (shives, tear, tensile, burst, etc) is adversely affected. Fluctuations in chip feed rate can be due to erratic discharging of chip vessel (silos, bins, pre-heaters), poor level control, defective or worn metering screws, high backflow steam velocity, etc. Chip quality variations aggravate the situation, but are often not the prime cause.

The design of pulp screens has improved dramatically during the past five years. Shives in the finished pulp are virtually eliminated by using screens equipped with fine slotted (0.13-0.15 mm), wedge-wire baskets and sized to give low slot velocities. Low pulse rotors reduce energy requirements and contribute to the shive removal efficiency.

These screens also remove most of the dirt, forwarding it to the reject treatment system where the reject cleaners remove it from the process. In fact, the latest design for dirt removal prior to the pulp machine in market kraft pulp mills is based on fine slotted screens. With these points considered, are main line cleaners needed any more? This question must be evaluated on a case by case basis. Candidate mills should meet the following criteria:

• enforced bark content limits in the chip supply
  
• paved chip storage areas
  
• efficient sawdust removal at the chip screens
  
• effective chip wash with captured solids
  purged from the mill
  
• modern screen room
  
• cleaners in the reject treatment system (preferably prior to the reject refiner).

The combined effect of eliminating main line cleaners and conversion to direct coupled screens with low energy rotors results in significant energy savings. Further energy savings are possible by installing a multi-stage screen such as that offered by Valmet. A single screen can replace three stages of screening. At the UPM-Kymmene mill in Rauma, Finland, eight pressure screens were replaced with a single multi-stage screen.

Several twin-line BCTMP mills were built between 1987 and 1992 that used screens to fractionate each line into two streams - one enriched with long fiber and one enriched with short fiber. Similar streams were recombined to produce a low fines pulp and a high fines pulp. It proved to be extremely difficult to balance the production rates on the two lines and still achieve the freeness targets of both products simultaneously. In addition, the two products were dependent on each other - one could not be produced without the other. If sales for product A were strong and sales for product B were weak, it presented an impossible situation and the concept was consequently abandoned.

Technology advances in thickening and mixing equipment allow bleaching at higher consistencies than ever before. In a modern bleach plant the consistency in the second bleach tower has increased to 35-40%. The consistency in the first bleach tower is 12% due to the recirculation of peroxide filtrate from the dewatering presses of the second bleach stage. Bleaching costs are lower due to more efficient bleaching at higher consistency.

Most BCTMP mills have added dedicated pulp washing presses following the final bleach plant dewatering press. This was required to satisfy customer complaints of high dissolved solids in the market pulp. This problem manifested itself in several different ways at the customer's paper machine through high COD losses, product odor and high costs for wet end chemicals. Initially, BCTMP mills were built with one dewatering press after the final bleach stage. This proved insufficient to remove residual chemicals and anionic trash left in the pulp from the bleaching process.

The ideal wash stage consists of a 30-60 minute hot water soak followed by thickening to a high consistency (40-50%). We know of one BCTMP that had such as system and it produced one of the cleanest pulps on the market. This design should be incorporated in all future BCTMP mills.

With an efficient wash stage following the bleach plant, the need for interstage washing is greatly diminished. In fact, the interstage wash stage has been removed in some BCTMP mills. In a typical installation, pulp is diluted after the primary refiner and pumped to screw presses. A portion of the filtrate is sent to effluent treatment, thereby purging resins from the system. Hardwood BCTMP mills experienced high suspended solids losses from this purge. Moreover the screw press baskets tended to plug progressively over time, causing reductions in production rate and eventually forcing the mill down to unplug the baskets.

In a new BCTMP mill, the capital used for an interstage wash would be better spent upgrading the dewatering stage prior to the unbleached high density storage to create a more effective hydraulic seal. Wood species that contain high levels of resins, such as southern pine, will still require an interstage wash to deal effectively with pitch deposits.

All but two of the BCTMP mills use flash dryers. One Nordic mill uses an airborne dryer and one mill in eastern Canada 'can dries' its product on a converted sulfite pulp cylinder dryer. Can dryers have higher operating costs and generally are used when there is a suitable idle paper machine available for conversion to a pulp dryer. This can reduce capital costs.

Flash dryers are designed to operate at much lower temperatures than those built in the early 1980s. This results in lower pulp brightness loss in the flash dryers and bales that are easier to repulp. Fuels that burn 'clean' must be used to fire flash dryers to avoid brightness loss due to soot contamination.

In contrast to BCTMP mills, airborne dryers dominate market kraft pulp. We expect to see a higher percentage of airborne dryers in future BCTMP installations as new mills seek to carve out a niche by offering an alternative to flash dried bales.

Yellowing of papers containing mechanical pulp has been a concern since the advent of market BCTMP. An extensive research effort spearheaded by the Mechanical Pulps Network has yielded impressive results. The mechanism by which BCTMP yellows has been determined and additives that inhibit this mechanism have been developed.

Paprican and Ciba Specialty Chemicals have developed a composite inhibitor that reduces the yellowing of BCTMP to near that of kraft pulp. Unfortunately the product must be applied on the surface of the paper sheet. This means that the total paper production must be treated and the BCTMP customer must apply the treatment rather than the pulp mill.

Obviously, this is not a concern for integrated mills. The inhibitor is available from Ciba but has yet to be used commercially. Ciba is searching for a suitable test site. The cost of the treatment may restrict its use to value-added papers. Notwithstanding these disadvantages, this is a significant breakthrough for BCTMP.

Commercially produced 'free-sheet' containing up to 20% hardwood BCTMP does not exhibit meaningful levels of brightness reversion even though laboratory results had suggested otherwise. The addition of 1-2% calcium carbonate to the furnish further stabilizes paper brightness.

The Outlook

Barring major economic disturbances triggered by events such as increasing oil prices and/or a stock exchange meltdown, the pulp sector faces a period of supply limitations and high prices. This is already triggering capacity investments by kraft pulp producers. But so far, nothing significant by BCTMP mills.

As pulp prices increase, the price differential between chemical and BCTMP grades tends to narrow (Figure 1) due to high demand for market pulp. But when prices cycle back down, as they inevitably will, some buyers will revert back to chemical pulps to avoid the inconvenience of using BCTMP. In an attempt to maintain market share, the selling price of BCTMP is lowered further, causing the price differential with chemical pulp to widen. Eventually, the selling price of BCTMP approaches its manufacturing cost and the profits of BCTMP mills evaporate.

For market BCTMP to increase its share of the market pulp sector in the medium to long term, it has to be more cost-effective in all periods of the price cycle. Customers must be given a compelling reason not to revert back to conventional furnish mixes each time the market cycles down. Investors must be convinced that profits can be maintained in all cycles before they will fund the installation of new BCTMP capacity. Below are a few concrete steps that existing BCTMP mills can take toward this objective:

• reduce SEC by one or more of the following - increase production rate, utilize low energy refiner plates, implement closed loop control of primary refiner consistency, install low consistency third stage refining
  
• improve impregnation with high compression plug screw feeders
  
• improve refiner stability
  
• upgrade the screen room to fine slotted wedge wire baskets (0.13-0.15 mm) for shive and mini-shive reduction
  
• in conjunction with the screen room upgrade, consider shutting down the main line cleaners (for energy savings)
  
• add a high consistency bleach tower
  
• add final pulp washing stage(s).

The next major step in grade development will be the introduction of eucalyptus BCTMP. In 1989, we forecast that this development was imminent. Unfortunately, the difficult financial times that engulfed market pulp producers in subsequent years put this development on pause. Now that profitability is returning though, we expect that the "pause' button will be released. We eagerly await the arrival of eucalyptus BCTMP because we forecast that the pulp quality will be greeted enthusiastically in the market place.

Eric Cannell is process specialist, NLK Consultants Inc, Montreal, Quebec, Canada. Richard Cockram is director of the London office of NLK Consultants Inc, and specializes in market research in pulp and paper

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