|
Closed cycle operations, the EPA’s Cluster Rule, and fiberline yield are all currently issues driving chemical consumption in the pulp mill and bleach plant for North American mills. The efforts throughout the fiberline are focused on making operations as efficient as possible—reusing chemicals where possible and reducing the amount of chemicals necessary to achieve pulp properties.
The following discussion of technological and business drivers for fiberline chemical consumption is based on interviews and data collected from such sources as Neil McCubbin of McCubbin & Assoc. (pulping chemicals), Eka Chemicals Inc. (sodium chlorate), and Air Liquide America (oxygen and ozone).
PULPING CHEMICALS. While many mills are pursuing ways to improve fiber yield in their pulping operations, changes to the cooking process have very little effect on pulping chemical consumption. The primary driver of pulping chemical use currently, according to McCubbin, is closed cycle operations. In effect, mill efforts at minimizing effluent releases through closed loop technologies is reducing the amount of chemicals needed in the process.
“What really makes a big difference is how efficient the recovery cycle is or how closed the loop is,” McCubbin says. “Currently, loop closure can range from 90% to 99+%, and 99% closure uses a tenth as much chemical as 90%.” He gives an example of chemical losses off the washer that 30 years ago were approximately 50 kg/ton and now are on the order of 11 kg/ton.
Odor control efforts made by the industry have had an effect on makeup chemical use in the pulping process, with sulfur losses being controlled to the point where many mills need more sodium makeup than sulfur. “That means instead of using only saltcake as has been done traditionally, more mills are using some saltcake and some caustic,” he says.
McCubbin provides a comparison of one mill in the 1960s that reported emitting 1,700 ppm of hydrogen sulfide (H2S) in the recovery stack, compared with normal discharges today being under 50 ppm and as low as 5 ppm. “If sulfur isn’t going into the atmosphere, then you don’t have to buy as much of it,” he adds.
Another issue that is driving down mill purchases of saltcake is the rapid increase in use of chlorine dioxide in the bleaching process, which produces byproduct saltcake. “In practicality, most kraft mills today have more byproduct saltcake from the bleach plant than the pulp mill needs,” McCubbin says. “And those mills that don’t have excess soon will because of the regulations pushing them to 100% ClO2 substitution or close to it.”
McCubbin again uses the historical perspective to see how saltcake use has shrunk. “When I started in this industry, 100 kg/ton of saltcake makeup was not uncommon for a pulp mill, and a small amount of that came from the mill’s chlorine dioxide plant with the rest being purchased. Today, mills are down to numbers around 10 to 20 kg/ton of saltcake makeup, and all of that, plus some, is produced by the chlorine dioxide generator.”
He adds that the exception is probably less than 10 mills that aren’t currently using chlorine dioxide in their bleaching sequence. The implication is that with the Cluster Rule pushing all mills to chlorine dioxide substitution or totally chlorine free (TCF) operation, any mills that currently purchase saltcake will probably discontinue that practice in the next few years.
As for caustic, McCubbin says, more efficient pulping operations mean that losses are decreasing constantly. “Of course, when you look at mill purchases of caustic, you don’t know how much is going to pulping and how much to bleaching, but for pulping, caustic use will drop because mills are getting tighter.”
The best management practices (BMP) portion of the Cluster Rule is supported by other government regulators and is also advanced by mills seeking to reduce their color reductions, so spill recovery is becoming more important. “Every gallon of liquor a mill recovers is an amount less of caustic it has to buy, because the caustic a mill buys for pulping is all to make up losses.”
SODIUM CHLORATE. According to Eka Chemicals, demand for sodium chlorate was weaker through the third quarter of 1998 due to pulp mills taking summer outages that lasted longer than previously expected. Demand has fallen back a small amount but is anticipated to be up compared with the same period last year. In the near term, demand for sodium chlorate is expected to grow at a modest rate. With the Cluster Rule becoming law, 100% substitution without oxygen will strengthen overall demand.
To date, not all North American pulp mills have converted to ECF (100% ClO2 substitution). Approximately 60% of North American pulp mills have converted or have the ability to produce ECF pulp. About 50% of all bleached chemical pulp produced in the U.S. is exclusively ECF. Somewhere between 80% to 95% of pulp mills in Canada have the capability to produce ECF pulp, and almost 60% of Canadian bleach plants produce ECF pulp exclusively.
Eka Chemicals expects demand to increase to 2.1 million short tons after the three-year compliance period, which should be set in motion near the end of the fourth quarter of 2000. Currently, mills that are direct dischargers (mills discharging into a navigable body of water) are reissued permits by the states. Compliance with best available technology (BAT) limits is mandated as soon as their state permits are reissued. Most of the permits have expired and are waiting to be reissued.
States have the authority to allow a mill up to three years to meet the revised permit limitations. However, EPA urges states not to provide the additional time unless the mill agrees to participate in the Voluntary Advanced Incentive Program. It is stated by EPA in the Federal Register that mills should be required to meet the new requirements by April 15, 1999, or on the date their permit is reissued, whichever is later, unless the mill agrees to participate in the incentive program. Existing indirect dischargers (mills discharging into a publicly-owned treatment facility) must comply with pretreatment standards by April 16, 2001.
The Voluntary Advanced Incentive Program encourages mills to adopt oxygen delignification systems and reduce water consumption. Incentives include longer compliance schedules for adsorbable organic halides (AOX) reduction, public recognition of their efforts, and reduced monitoring requirements. June 15, 1999, is the deadline for mills to enroll in one of the program’s three tiers, with compliance dates set to accompany the three tiers as 2004 for Tier I, 2009 for Tier II, and 2014 for Tier III.
With oxygen delignification as part of the incentive package, Eka Chemicals expects demand for chlorate to peak at 2.1 million short tons and level off at 1.8 million short tons approximately 7 to 10 years later.
OXYGEN AND OZONE. As mills have shifted away from elemental chlorine bleaching and increased their use of chlorine dioxide, they have also increased their use of oxygen and ozone for both pre-bleach plant delignification and as part of the bleaching sequence. While much of the emphasis has focused on eliminating chlorine-containing bleaching compounds, suppliers are also touting the improvements in yield that come with using oxygen and ozone.
Advances in the conventional bleach plant EO stages allow for application of as much as 20 lb. of O2/ton of pulp in an upflow retention tube at high pressure, high temperature, and long retention time. Benefits include reduced use of other, more expensive bleach chemicals, improved flexibility of the bleach plant with respect to bleached pulp characteristics, and the ability to reduce intensity of the pulping operation.
Advances in the conventional oxygen delignification process have also helped reduce the intensity of the pulping process. This “back up” of the pulping Kappa versus yield curve results in significantly improved fiber utilization.
Changes or improvements in oxygen delignification technology have proceeded in two directions. The first, an established technology aimed at allowing a mill to realize the benefits of oxygen delignification at minimal capital cost, is the “mini O2” stage. This technology uses conditions similar to a conventional bleach plant EO stage allowing for 20% to 30% Kappa reduction. Mills are now using this technology as a way to increase the digester Kappa , allowing a reduction in pulping operation intensity. Total installed cost for a “mini O2” stage is typically $2 to $4 million as compared to $15 to $25 million for a conventional system.
The second path of improvement in oxygen delignification technology is the two-stage oxygen delignification system. This system provides up to 65% reduction in Kappa depending on wood species and allows for a significant reduction in intensity of the digester pulping process.
The application of ozone in the bleach plant enhances the traditional benefits associated with oxygen delignification, such as allowing for lower AOX in mill effluent and providing for improved fiber utilization through a less intense bleaching sequence.
Improvements in ozone implementation in the bleaching process have led to ZD or DZ stages in which ozone and chlorine dioxide are combined in the first bleaching stage. This technology has been proven to be effective both with and without oxygen delignification preceding the bleach plant. ZD or DZ bleaching allows for chemical flexibility in the bleaching process similar to that available when a DC stage was common practice. The ZD or DZ technology employs much of the existing bleach plant equipment, minimizing capital costs to install and maintaining the overall simplicity of the bleaching process.
Previous development of ozone as a component of totally chlorine-free bleaching resulted in the general belief that ozone reduces bleached pulp strength characteristics. However, Air Liquide has studied the effects of the ZD and DZ technology at its Chicago laboratory facilities, as well as at Econotech and Quantum Labs. These reports show that ozone applied at reasonable levels in combination with chlorine dioxide does not impair the mechanical strength of fully bleached chemical pulps. Keys to strength preservation include reasonable ozone application rates, understanding total bleach chemistry, use of bleach plant filtrates within the process, and thorough knowledge of bleach plant carryover limits.
Further optimization focuses on the low pulp consistency (3-4%) ozone application systems instead of the typical medium (10-13%) or high (32+%) pulp consistency systems. Air Liquide states that it has completed very successful pilot plant work on the low-consistency application systems. Results indicate an ozone consumption rate greater than 95% and Kappa drop per unit ozone applied rates similar or better to those achieved with high or medium consistency systems. Further development of low consistency ZD or DZ bleaching systems should greatly reduce the investment costs associated with ozone use for mills operating at low consistency in the first chlorination stage.
Today there are 19 mill installations of ozone worldwide. Three of those mill installations are DZ or ZD technology at medium or high pulp consistency. A fourth ZD installation at medium pulp consistency for E.B. Eddy Forest Products in Espanola, Ont., will start up in October 1998.
The oxygen quantities required for bleach plant EO and oxygen delignification stages typically exceed the lower limit at which onsite generation of oxygen becomes economically and logistically desirable. Ownership, operation, and maintenance of an onsite oxygen generation system by the gas supplier, typically referred to as “over the fence” supply, optimizes the total cost to the mill allowing for greater focus on activities related to pulp and paper manufacture.
Two types of systems are currently available for onsite oxygen generation. The conventional system distills air at cryogenic temperatures (-175ºC to -196ºC) to produce oxygen at 96+% purity. Non-cryogenic systems use adsorption—typically referred to as Vacuum Swing Adsorption or VSA processes—to produce oxygen at 90% to 93% purity. Choosing the best system will depend on the amount of oxygen required, cost of power at the mill site, and other industrial gas needs. Oxygen purity of 90% to 93% as produced by non-cryogenic systems is acceptable for oxygen delignification and bleach plant EO applications.
Ozone “over the fence” provides ozone generation as well as oxygen supply for ozone production and other bleaching applications. In this system, the ability to capture and recycle off gases from the ozone application stages can significantly impact the overall process. Commonly, the oxygen rich vent gas from the ozone stage is recycled back to ozone generators after it is cleaned up due to ozone generator sensitivity to moisture and carbon compounds.
Again this technology has moved forward, as now, with lower oxygen production costs, it is more economical to send the recovered oxygen to other mill processes. Recycling the vent gas back to mill oxygen applications such as oxygen delignification or bleach plant EO stage provides greater value of the oxygen molecules. The recycled oxygen stream can also be applied in wastewater treatment, lime kiln enrichment, white liquor oxidation, and polysulfide pulping liquor production. With polysulfide production, the mill continues to reduce the overall raw material wood entering the process. Significant work on polysulfide pulping liquor production from ozone off gases is currently underway in a joint effort of Paprican and Air Liquide.
|
