CHEMICAL RECOVERY

Mills Explore Capacity Options to Extend Recovery Boiler Life

Several years ago, the Black Liquor Recovery Boiler Advisory Committee (BLRBAC) published data showing that the mean age of recovery boilers in the U.S. was more than 20 years. "At the time BLRBAC published its data, at least five new recovery boilers would have had to be purchased and annually installed to keep the mean age of the U.S. 'fleet' constant," according to Olof Strandell, vice president sales and marketing, Kvaerner Pulping in Charlotte, N.C. "That has not happened. Instead, purchases have been closer to two or three per year."

Strandell suggests that more than half of the major recovery boiler improvements or replacement activity in North America is in Canada. "Mills are looking primarily at consolidating their recovery boilers-retiring several older boilers by consolidating the work into a single 6 to 7 million lb/day boiler."

There are new, large, modern recovery boilers being purchased and installed, Strandell reports, but "the major activity, so far as new recovery boilers are concerned, is not in North America but in Indonesia and, to a lesser extent, South America. The Indonesians are buying 6 million and 7 million lb/day boilers, two and three at a time."

Jussi Mantyniemi, director of recovery boiler technology at Kvaerner, adds that "lots of those older unit are relatively small, only about 1 million lb/day. Today the normal capacity of a new boiler is about 7 million lb/day. One boiler built today can replace seven old, smaller units."

What has inhibited recovery boiler replacement in the U.S.? According to Strandell, it has been regulation, capital cost, and successful maintenance. "In North America, when the Cluster Rule was first published several years ago, it seemed that the demise of direct contact evaporators was eminent. The recovery boiler manufacturers expected that the demise would cause a flood of new orders-both replacing old, aged boilers and retro-fitting others with 'low-odor conversions.'"

Companies wisely waited for the winds of regulatory change to settle into a certain direction. As the U.S. Environmental Protection Agency nears promulgation of the rule, it seems that the combustion portion of the rule will not force replacement of direct contact evaporators. The "flood" of replacements and conversions has yet again been postponed.

Strandell offers that "there is a large pent-up demand. The delays in finalizing the Cluster Rule have slowed replacement or improvement of boilers by adding a great deal of uncertainty to long-term planning for industry. The uncertain features of the future rule made for a poor environment for committing capital."

DOING MORE WITH OLDER UNITS. Of the approximately 247 active recovery boilers in the U.S. (in about half as many mills), about 82 have direct contact evaporators (Figure 1). The oldest boilers operating today were commissioned during the mid-1930s. The last boiler with a direct contact evaporator was commissioned in 1974.

"One has to ask 'how is it possible to keep those running and viable?'" says Strandell. "The answer is improved reliability. The answer to 'Why keep them running?' is also simple: capital cost. Modern recovery boilers are hugely expensive. If you look at it, half of the capital cost in a new mill goes into the recovery loop. The other half goes to the fiberline. The cost makes companies hesitant to replace what is working.

"Deciding to replace a recovery boiler is not an easy decision for a mill. If you consider a large mill-one with paper or pulp capacity of several thousand tons per day-the mill is probably operating three or more old recovery boilers. A new recovery boiler-one capable of replacing the old units-will cost nearly $100 million, exclusive of turbogenerators, evaporators, and the other ancillary improvements that may be included in a boiler replacement. It's a substantial investment."

Strandell adds that while a reduction of operating and maintenance costs is certainly rewarding, and old units can reach the reasonable end of their service life in an environment of limited capital and uncertain regulations, "it is quite logical that the decision-makers wait a little while before they make a commitment.

"There is very little that can be done to change the environmental performance or operating efficiencies of old units without a lot of capital investment. In terms of reliability, older boilers cost more to maintain as time goes on. That cost is generally much less expensive than buying a new boiler. But if you can build a capacity increase into the economics of the replacement project-assuming that the recovery plant was the capacity bottleneck-or if you can otherwise increase the mill's capacity with a 'small' amount of capital, the return on the investment may be sufficient."

Through the activities of the American Forest and Paper Association (AF&PA) and BLRBAC, as well as demands by insurance companies, kraft recovery boilers in North America are currently subject to a rigorous inspection and auditing program. According to Strandell, "Boilers undergo extensive mechanical inspection during outages and operational audits during operation to make sure that operators know what they are doing and that the proper emergency shutdown procedures (ESP) are in place.

"The far reaching program that is in place has undoubtedly contributed to the fact that there are substantially fewer critical exposures and smelt-water reactions compared with 10 years ago. This has occurred in light of a boiler fleet that is relatively old and aging."

IMPROVING EFFICIENCY AND CAPACITY. While mills wait for the capital to invest in new recovery units, those mills are working to improve recovery system efficiency, sometimes with mixed results. Some of the more typical projects include economizer replacements and a few conversions to low odor operation. The economizer replacements were driven by reliability-trying to eliminate leaks-and fuel efficiency concerns. The low odor conversions were generally capacity increase projects.

"Some of the older boilers that had two-level air delivery systems have been upgraded to three-level air," says John Rickard, power and recovery staff engineering with Jacobs-Sirrine Engineering in Greenville, S.C. "While people have spent a lot of money doing it, the quantitative improvement has been problematic to achieve."

W. Densmore Hunter, director of technology strategy at Weyerhaeuser, agrees. "During the past several years, we've made a number of improvements to our boiler operations. We've improved our air delivery systems and modified our sootblowing and cleaning systems. But boilers are so individual in their operating characteristics. Improving their performance requires much more than simple changes in air delivery systems, such as going from two- to three-level air. We've also focused a lot of our attention on operating procedures."

Rickard points out, "To increase both efficiency and capacity, older boilers that include direct contact evaporators can-and have been-converted to low odor units, which increases their allowable solids firing range from less than 65% to between 68 and 75%. That change increases both solids throughput and steam output."

"But," he cautions, "as a conversion is made to low-odor operation, the mill must consider carefully the potential changes in precipitator operations to make certain that the unit can remove the new dust. Specifically, this can involve particulate chemistry changes with the increased firing temperature of a higher solids unit.

"The hotter bed drives off more sodium as a fume. At the same time, with the higher solids there is less entrainment of saltcake in the flue gas. The result is both a finer particulate and a shift in particulate chemistry-more sodium and less sodium sulfate."

Rickard also emphasizes the effect a conversion can have on boiler pluggage. "The flue gas leaving the furnace is much hotter because there is less water to evaporate. The ash takes longer to cool down as it passes through the superheater, boiler bank, and economizer. If you've got good spacing and sootblowers, there won't be a problem. The reality is that 'good' conditions are uncommon, thus mill experience with conversions can vary considerably.

Hunter adds, "Our understanding of the recovery boiler process continues to improve. Certainly our ability to deal with fouling, fuming, and carryover is improving as we get new and better insight."

According to Rickard, one option that may become more popular is switching old recovery boilers to power-boiler service. "The strategy is to go ahead and put in a new, large recovery boiler and, at the same time, convert the old boiler to power-boiler service, which means considerably less aggressive service. Some recovery boilers that are reaching the end of their economic life as recovery boilers may have 20 or 30 years of useful life as power boilers."

The biggest advantage to this strategy comes in permitting. Building a new recovery boiler may trigger a Prevention of Significant Deterioration (PSD) review-a process most mills would prefer to avoid. However, as Rickard points out, "if the mill is operating both an old power boiler and an old recovery boiler, the combined permitted emissions of the two units may exceed the emissions of a larger, modern recovery boiler operating in combination with the converted recovery boiler. Therefore the expensive, time-consuming PSD review could be avoided."

THE FUTURE. "Of course we have some concerns over the longer term about rebuilding or replacing our aging recovery boilers," says Hunter at Weyerhaeuser. "That is why we are working with Oakridge National Laboratory and a number of other companies to resolve issues around improved boiler materials. Our intent is to improve the life of existing boilers for as long as we can operate them effectively. At the same time, we are interested in new technology options such as black liquor gasification." As part of its fiberline modernization project at New Bern, N.C., Weyerhaeuser is starting up a Kvaerner CHEMREC gasification system to supplement its existing recovery system.

Hunter continues to be very optimistic about the long-term potential for black liquor gasification. "But it's a major change, and the path toward that is going to take time and considerable effort," he says. "It's not an easy thing to do because so much is at stake.

"The real opportunity for black liquor gasification is when it is used in conjunction with combined cycle-power generation. Until gasification technology is suitably mature and until there is a high level of confidence about the ability to deliver a product gas to a commercial gas-fired turbine, companies are naturally going to be conservative. The investment to do that is very large and it has to work."

"But," adds Hunter, "the payoff will be a significant improvement in the power to steam balance in a pulp mill. Gasification also offers a significant opportunity for enhanced environmental performance. While the technology may help overcome some of the maintenance issues associated with conventional recovery boilers, there are still major technology hurdles to overcome.