CHEMICAL RECOVERY

Northern California bleached kraft market pulp mill minimizes its environmental impact with a unique collection, treatment approach

By KIRK FINCHEM, Technical Editor

L-P Samoa Targets Odor-Free Mill with NCG System Modernization

The result was one of the most advanced and comprehensive—in terms of operability, collection efficiency, and process safety—odor control systems in the pulp and paper industry. Now, the mill is once again at the forefront of emissions control as it further improves the system in an effort to achieve early compliance with new National Emission Standard for Hazardous Air Pollutants (NESHAP) requirements.

Prior to the recent improvements, the mill’s NCG system was fairly conventional—a single collection header that conveyed the gases to either the lime kiln or to a 1991-era thermal oxidizer for combustion. The system upgrades allowed the mill to separately collect three gas streams:

• High-volume, low-concentration (HVLC) gases. These flammable gases—primarily from tank and sewer vents—are collected at concentrations below their lower explosive limits. While they can have an unpleasant odor, they typically pose low process safety risks.

• Low-volume, high-concentration (LVHC) gases. These flammable gases are collected at concentrations above their upper explosion limit (UEL), have an unpleasant odor, are acutely toxic, and can present significant explosion and safety hazards if not properly handled.

• Stripper-off gases (SOG). These gases include methanol and total reduced sulfur (TRS) compounds that have been stripped from foul condensates. Containing about 50% methanol and 50% water vapor, the TRS (and other noxious compounds) contamination gives the stream a foul odor, can be acutely toxic, extremely odorous, and explosive.

This separation scheme maximizes process safety while minimizing the likelihood of exposing personnel, and the surrounding community, to both nuisance and toxic odor emissions. The system is shown schematically in Figure 1, which includes improvements currently under construction that are expected to start up later this year.

COLLECTING THREE STREAMS. Anton Jaegel, L-P Samoa’s environmental manager, says, “The mill is limited to two minutes of cumulative venting per day. It’s a very tight, restrictive limit. More than that constitutes a permit violation. And we comply with the limit.”

The HVLC system, which collects vent gases from the turpentine sewer, brownstock spill tank, and brownstock washer filtrate tanks, is continually swept with air to keep the concentration of explosive gases below 25% of the lower explosive limit. Cooled in a small shell and tube heat exchanger prior to treatment, the HVLC gases are treated in the mill’s thermal oxidizer boiler, lime kiln, or—as an emergency backup system—flare stack.

Typically, the mill uses both of its primary treatment systems simultaneously in a “lead-lag” arrangement to ensure process reliability and flexibility. “We always co-fire NCGs to both systems,” says Jaegel.

“One unit—the lead device—takes about 90% of the gases,” he adds. “The other—the lag unit, usually the lime kiln—takes about 10% of the gases. If, for some reason, the lead unit goes down, the line to the lag unit is already hot and operating. As a result, atmosphere during the transition is not necessary or common.”

Mill operators have the flexibility to adjust the apportionment of gas treatment as required by mill process operations. By co-firing the HVLC gas stream in this manner, operators can adapt to the failure of one treatment system by routing all gases immediately to the other system, already in operation. If both treatment systems fail simultaneously—a fairly unlikely event—the HVLC gases are vented to a high-point vent until a treatment system returns to operation or until the mill is shut down in an orderly fashion.

The LVHC system collects vent gases from the evaporator seal tank, concentrator seal tank, foul condensate tank, turpentine decanter, and the digester area (including the blow tank, steaming vessel, and No. 2 flash tank). Digester area vent gases are condensed and cooled in the primary and secondary condensers prior to combining with the other LVHC gases. Similarly, vents from the foul condensate tank and concentrator seal tank are cooled in small gas coolers before passing into the LVHC collection header.

This main collection header is isolated from the individual source streams with flame or detonation arrestors, automatic isolation valves, safety relief valves, and burst discs. Steam eductors located at each of the treatment systems provide the motive force in the header, “pulling” the LVHC gases with sufficient velocity through the arrestors and into the treatment systems.

As with the HVLC system, the LVHC system’s control valves are aligned in a lead-lag arrangement. And again, as with the HVLC system, when one treatment system fails, all LVHC gases are automatically routed to the other device with no venting of any gases to the atmosphere. If both treatment systems fail, odor and HAP emissions are minimized by directing the LVHC gases to an emergency flare burner until a primary treatment system is returned to service (or until the mill can be shut down in an orderly fashion).

The SOG system, which collects the vent gases from the steam stripping system, is pressurized to about 30 psig by means of a steam generator coupled to the stripper. The pressurization provides the motive force to drive the SOG gases to the treatment systems.

The SOG collection header is both heat-traced and insulated to prevent condensate formation. Further more, a steam-jacketed “knock-out pot” and reboilers prior to the treatment systems ensure that condensate is fired into the treatment device. Since the SOG stream contains a high fraction of methanol, double-block and bleed valves are used to isolate this fuel stream from the treatment systems in the event of failure.

PROXIMITY BREEDS CAUTION. Owing to the mill’s close proximity to the city of Eureka—and the likely immediate impact of odorous emission on nearby residents (i.e., complaints)—the SOG system was designed to avoid venting the highly odorous stream, which might otherwise occur during stripper startups, shutdowns, or the simultaneous failure of both treatment systems. In fact, the SOG stream collection header and stripper can be “bottled up” briefly—until a treatment system returns to service—by cooling the gases and/or by enriching the methanol concentration in the stripping column.

According to Jaegel, the overall performance of the NCG collection system has been excellent. At the 1998 TAPPI Environmental Conference in Vancouver, B.C., Jaegel presented a paper that reported, “The NCG system is comprehensive in terms of the number of pressure flow and temperature sensors, rupture disks, flame arrestors, and relief valves that are required.”¹

The system’s advanced control strategies and interlocks ensure that the NCG streams are handled safely, preventing both personnel hazards and equipment damage. Logic “permissives” and system checkouts—required before any NCG stream is introduced into either the thermal oxidizer or lime kiln—are controlled principally by the burner management system. The programmable logic control-based burner management system interfaces with the mill’s distributed control system (DCS), facilitating both efficient monitoring and operational control.

According to Jaegel’s presentation, operators noted that attempts to “bumplessly” re-route NCGs from one treatment system to the other—particularly when the transition was precipitated by the failure of one treatment system—were occasionally difficult and sometimes “tripped” the other combustion device. This seemed especially true for the SOG system, owing to the formation of a tar-like buildup between the collection header and the lag treatment system. This was remedied by occasionally switching the lead-lag valves to purge the lines. A more permanent solution slowed the ramp time allotted for opening the lag valve. This gave enough time for the contaminants in the collection header to clear and avoid upsetting the other treatment system.¹

Operators also noted that, when the double-block and bleed valves between the SOG header and the treatment devices de-energize to their “failed” default positions, a small puff of SOG and steam was released to the atmosphere through the bleed line.

According to Jaegel, “It turned out that the sequencing of those valves was very important.” Investigation revealed that the smaller bleed-line valve opened more quickly than the larger (upstream) block valve could close, exposing the bleed line to the charged SOG header. This problem was eliminated by sequentially de-energizing and verifying the position switches for each double-block and bleed valve before opening the bleed-line valve.

Complex systems such as the one at Samoa pose several design challenges that should be considered in the context of upgrading an NCG system. For example, the Samoa system is designed to concentrate TRS in the LVHC and SOG gas streams. The elevated concentrations mean that even “small” gas releases can result in severe odor problems for the surrounding community.

Jaegel explains, “A community accustomed to persistent low-level pulp mill odors will welcome the permanent reduction in odorous emissions gained by a modern NCG system and foul condensate stripper. The community goodwill can evaporate quickly, however, if a system upset results in a short-term but intense odorous release. In response, local air quality districts may impose strict permit conditions and penalties.”

MACT. To meet the requirements of the Clean Air Act, the Environmental Protection Agency (EPA) promulgated the National Emission Standards for Hazardous Air Pollutants (NESHAP) for selected pulp and paper subcategories on Nov. 14, 1997. According to Jaegel’s presentation, “Under section 112(d) of the Act, the goal of the NESHAP is to require the implementation of maximum achievable control technology (MACT) to reduce emissions and therefore reduce public health risks associated with pollutants emitted from stationary sources.” Existing kraft mills must comply with the rules within three years of their publication in the Federal Register. HVLC sources must comply within eight years.

Jaegel further reported in the presentation that the EPA has specifically defined equipment systems within the kraft pulping process (and associated wastewater streams), subject to the MACT standard. These systems are:

• The LVHC vent system

• The knotter and screening system (depending on applicability conditions)

• The brownstock washing system (and associated process units)

• The decker system (depending on applicability conditions)

• The oxygen delignification system.

In some cases—“new affected sources or those pulping systems that have commenced construction or reconstruction after Dec. 17, 1993”—the weak black liquor storage tank vents are also subject to the rules. Since chlorine compounds are not used in the Samoa mill’s bleaching process, it is exempt from the MACT requirements for kraft mill bleaching systems.¹

EARLY COMPLIANCE. While the Samoa NCG system does not currently collect all sources requiring treatment under the new MACT standards, the mill has focused its effort on complying with the rules well in advance of their deadline. This effort is expected to further reduce the facility’s already low TRS emissions by more than 50%.

Working with NLK Consultants in Vancouver, B.C., plans to upgrade the NCG system were developed by L-P that allow for the collection and treatment of additional HVLC sources (Table 1). The large increase in the volume of HVLC gases that the mill plans to collect and treat exceed the capacity of the existing lime kiln and thermal oxidizer systems. The team considered several incremental capacity treatment options, including:

• Recovery boiler (added at the tertiary air ports): The recovery boiler had the advantage of being a reliable combustion source and would capture much of the sulfur, returning it to the liquor cycle, thus improving the mill’s Na:S balance. Further, both the capital and operating costs were relatively low compared with other treatment options. Finally, the option required no new SO₂ scrubber.

On the other hand, recovery boiler treatment of the HVLC gases was expected to complicate boiler operations, and the practice is currently not recommended by the Black Liquor Recovery Boiler Advisory Committee (BLRBAC). This option also required that the existing thermal oxidizer continue as the primary treatment device for other NCG streams.

• Regenerative thermal oxidizer (Ceramic Beds): These units typically enjoy the advantages of high thermal efficiency and low operating and maintenance costs. An added advantage was that a unit could be sized to treat both the HVLC and LVHC streams.

Typical disadvantages include the fact that the units are alkaline-particulate sensitive. And, in the case of the Samoa mill, the need would exist for a new SO₂ scrubber. A new unit was expected to have capital costs greater than those expected for the recovery boiler option, but less than the package boiler option (discussed below). Finally, the mill expected to have an ongoing need to treat the SOG stream in the existing thermal oxidizer.

• Gas-fired power boiler: The option of commissioning a new gas-fired boiler was attractive for several reasons. Such a unit could be designed to treat all of the mill’s NCG gas streams. At present, the mill’s recovery boiler is the only source of steam for the mill. A new gas-fired boiler could provide startup steam—improving operating flexibility—and remove load swings from the recovery boiler. Gas-fired power boilers also typically have a low operating cost for incremental steam production. The principal disadvantage of this option was, of course, high capital cost. It also would have required a new SO₂ scrubber, further adding to the capital cost.

L-P has chosen to treat the additional HVLC gas streams in the mill’s existing recovery boiler. The boiler, an Ahlstrom low-odor unit designed for 3 million lb of black liquor solids/day, has a design pressure of 1,100 psig. To handle the HVLC gases, the existing tertiary air ports will be modified. According to Jaegel, the total HVLC gas flow is less than 20% of the boiler’s total combustion air flow and is about 70% of the tertiary air flow.

Jaegel, in his TAPPI presentation, noted that there are approximately six such Ahlstrom recovery boilers in Scandinavia treating NCGs. The Samoa mill will be the first to adopt the practice in the in the U.S.

Jaegel notes that L-P “expects to have the new portion of the system complete by November 1998.” *

REFERENCES

1. Anton F. Jaegel, Rod C. Ledbetter, Don Manolescu, and Craig Lockhart. “Louisiana-Pacific: Approaching The Odor-Free Kraft Mill,” Presentation at the 1998 TAPPI Environmental Conference, Vancouver, B.C., April 5-8, 1998.