The recovery boiler is a critical, and certainly the most expensive, component of paper mills that use the Kraft process to produce pulp from wood chips. This most widely used pulping process enables the production of dark paper used for wrappings, envelopes, and grocery bags and, after bleaching, white paper used for printing or personal cleanliness.
Recovery boilers, however, have a problem: they can fail unexpectedly. Over the past 40 years in North America, about two boilers a year have exploded. Sometimes workers are injured or even killed, and the boiler usually is damaged. The downtime resulting from a boiler failure may cost a paper mill as much as $1 million a day. Continuing concern about the problem has led the paper industry to work with ORNL and other research institutions.
In the Kraft pulping process, wood chips are treated with a mixture of caustic soda and sodium sulfide in the digester vessel to separate wood fibers from lignin, the complex polymer that binds the fibers together. The concentrated waste stream from the digester, called black liquor, contains both lignin and spent pulping chemicals. From this black liquor, the recovery boiler separates chemicals for reuse in the pulping process and burns the lignin to produce steam. Steam is used to provide heat to various processing units, such as digesters and paper-making machines, and to produce electricity for use in other parts of the paper mill.
The steam is produced in the array of tubes that form the heat exchangers and walls of a recovery boiler. Because of the boiler's highly corrosive environment and high temperature and pressure, these tubes are subject to catastrophic failure. If a tube ruptures, high-pressure water can be released into the boiler. There on the floor, the water contacts molten chemicals. Because these chemicals are as hot as 800°C, the water instantly vaporizes upon contact, causing an explosion that can severely damage the recovery boiler.
The cause of the failure of bimetallic tubes in the lower portion of most recovery boilers is not known. However, failure mechanisms such as thermal fatigue, stress corrosion cracking, and corrosion fatigue have been proposed. Because of the nature of the stainless steelclad carbon steel tubes (known as composite tubes in the boiler industry), high residual stresses can be generated when the tubes are heated. In all of the failure mechanisms suggested, residual stresses are an essential part of the environment required for each mechanism to work.
To help the paper industry unravel the mystery, a multidimensional project is being conducted that involves ORNL and U.S. and Canadian paper institutes in cooperation with boiler tube suppliers, boiler manufacturers, and most major paper companies. This program involves an industry-wide survey to define all that is known about tube failures, a thorough characterization of the environment to which the tubes are exposed, residual stress measurements to define the stress state of composite tubing, computer modeling to predict the stresses experienced by the tubes during boiler operation, and laboratory fatigue and corrosion tests in environments like those of recovery boilers. It is hoped that this work will lead to alternative materials or operating procedures to prevent tube cracking.
As part of the effort to unravel the mystery, Xun-Li Wang and Camden Hubbard of ORNL's Residual Stress User Center are working to identify stresses in boiler tubes that could lead to a tube failure.
"Cracks have been found in composite steam tubes after service," Hubbard says. "We think some cracks form from stress corrosion cracking brought on by residual stresses introduced during manufacture and made worse by thermal and fatigue effects and mechanical stresses from pipe bends and other constraints. Our early results show significant differences in residual stresses in boiler tubes from different manufacturers and with different clad coatings. We hope to identify the materials that hold up the best. We will provide data for computer models that simulate the elevated temperature operation and use of improved tube materials that may extend the service life of recovery boilers."
"The paper industry," Hubbard says, "hopes the research will lead to improved materials that will have two benefits for recovery boilers. One is enhanced safety by decreasing the chances of tube failure and boiler explosions. The other is increased efficiency by allowing increases in boiler operating temperature and pressure and by reducing downtime."
Neutron diffraction measurements are being made to characterize stresses inside straight tubes and in tube bends and welds. The measurements are being made at room temperature; in the future, they will be made at higher temperatures. Stresses on tube surfaces under fieldlike conditions are being measured using X-ray diffraction.
In addition, ORNL is using advanced computer modeling to simulate the operation of a recovery boiler and calculate internal stresses at high temperatures typical of normal and abnormal operation. The hope is that by modeling the materials, components, and stresses in a recovery boiler system, scientists will be able to better understand causes of failure and to predict time of failure under various conditions. The computer modelers will use data from ORNL's Residual Stress User Center, such as room-temperature residual stress measurements in steam tubes, to help make their model predictions more accurate.
The ORNL researchers are working with a team of researchers from a group of 11 pulp and paper companies (including Weyerhaeuser, Georgia-Pacific, International Paper, and Union Camp) and five major boiler suppliers, including Babcock & Wilcox and ABB Combustion Engineering. Besides ORNL, research sites for the project include the Institute of Paper Science and Technology in Atlanta and the Pulp and Paper Research Institute of Canada in Vancouver, British Columbia.
The work is sponsored by the Advanced Industrial Materials Program, Office of Industrial Technologies, Energy Efficiency and Renewable Energy, U.S. DOE. It is hoped the capabilities brought to bear on these problems will benefit the paper industry.
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