After some 20 years in service, the generating bank and drums of a conventional bi-drum type recovery boiler have a tendency to develop major maintenance requirements and are subjected to frequent unplanned outages for repairs.
Leaks in the boiler bank are often a major reason for low boiler availability and disturbances in the recovery of chemicals and energy from spent liquors in the chemical pulp mill. In order to overcome these problems and also to gain other benefits, the singe-drum concept was introduced in a recovery boiler at the Leaf River Mill (Mississippi, USA) in 1984. As a consequence of this successful project, all major suppliers of recovery boilers adopted the single-drum design as the standard for larger new recovery boilers.
However, as the operating life of recovery boilers is at least 20 years, most of those in operation today are still of the old bi-drum design. This design has natural limitations regarding circulation at increased capacities, as the downcomer tubes of the boiler banks are exposed to more heat and tend to act as risers. Many bi-drum type generating banks also suffer from availability problems due to plugging of the heating surfaces. In recent years, severe corrosion near the mud drum has occurred in several installations, resulting in the need to replace the whole boiler bank section.
For the instances mentioned above and especially when capacity increases are required, a conversion from bi-drum to single drum is a very cost-effective solution.
Case 1 -- Northern Sweden: During its annual shutdown in 1995, the boiler of a pulp mill located in northern Sweden (production about 280 000 t/yr of pulp) was subjected to a thorough inspection. Tube thicknesses were measured using ultrasonic equipment inside the tubes. Visual inspection of the outside of the tubes was also carried out. The boiler bank showed clear signs of what is known as "near mud drum corrosion". Some tubes had lost as much as 4-5 mm of their normal wall thickness and about 100 tubes had lost approx. 3 mm. The corrosion was concentrated to the area just above the drum in a position close to the sootblowers.
As a result of the inspection, mill management decided to replace the old boiler bank. Two alternatives for repair were suggested -- retubing of the existing boiler bank or conversion to single-drum design with a new type of cross-flow generating bank of panel design. The mill decided to opt for the single-drum alternative which, compared to replacement, was both technically superior and required the shortest downtime.
Two similar replacements were studied as references and the technical risk was judged to be low.
The time from order to start-up was approx. eight months. In order to reduce the outage time, a 250-ton crane was used for both dismantling the old boiler bank and installation of the new equipment. The old boiler bank was cut in 50-ton pieces before being lifted out through the boiler house roof. The new generating bank was pre-assembled and erected in large modules that were lifted into place through the roof. The new drum was then installed above the new generating bank, Fig.1. As a spin-off effect of the retrofit (that was mainly carried out for maintenance reasons), the boiler bank's tube panel spacing was almost doubled, considerably lowering the risk of plugging at high loads.
The boiler has operated according to plan ever since start-up.
Case 2 -- Norway: A pulp mill in Norway frequently had unacceptable unavailability due to plugging of the bi-drum generating bank at high capacities. As other time-consuming maintenance work was already planned for the mill, it was decided to convert the boiler to single-drum design as in Case 1 and to take advantage of the fact that the free space between the tubes available for gas passage would be almost doubled after the modification, Fig.2. After the retrofit, unit unavailability was eliminated.
Case 3 -- Southern Sweden: In 1996, a pulp mill in southern Sweden was subjected to over three weeks of unplanned outages due to boiler bank leaks. The boiler had been in operation for 22 years and a decision was made to perform the following upgrade as soon as possible:
Replace the generating bank with a panel type bank and use a single drum
Replace the secondary superheater
Replace the carbon-steel lower furnace and the furnace floor
Install a 4-level air system
Modernize other parts of the boiler
The modification was carried out during a 4-week outage in late 1997, in parallel with other major modernization of the pulp mill. After start-up, the boiler demonstrated the operating characteristics of a modern single-drum boiler equipped with the latest 4-level air system, with potential for future capacity increase.
Case 4 -- Germany: The fastest conversion from bi-drum to single-drum type was carried out in a pulp mill in Germany in 1994 (MgO-Recovery Boiler). In this case, the new drum was installed during the operation of the boiler. The old generating bank, including the two drums, were lifted out and replaced with a panel-type generating bank during a two-week shutdown.
The boiler has been in operation without any problems since 1994, and at higher capacities than those prior to the modification, Fig.3.
Benefits of single Drum conversions: The results gained from recovery boiler conversions from bi-drum to single-drum designs combined with cross-flow panel-type generating banks are the following:
Unplanned stops due to tube leaks are eliminated
The boiler bank inlet is the most critical location in a recovery boiler when it comes to plugging because of the sticky ash, high gas velocity and tight spacing between the tubes. With a new generating bank, the choice of tube spacing and flue gas velocity can be made to obtain a substantial capacity increase for the boiler.
Reduced maintenance costs:
When compared to the old bi-drum boiler concept, the panel-type generating bank requires less maintenance.
The single-drum concept allows for unheated downcomers and an increased number of cyclones for steam/water separation. This, together with a higher drum position, will increase the overall circulation in the boiler. This means better cooling of the furnace water tubes and less risk of tube leaks in the furnace. With a new drum, more steam scrubbers can be installed, reducing the risk of carry-over of salts to the superheater. These salts can cause build-ups on the inside of the superheater tubes with tube leaks as a result.
Conversion of old recovery boilers from bi-drum type to single-drum design has proven to be a cost effective solution for mills requiring increased availability and higher capacity from their existing recovery boilers. Conversion can be carried out during a 3-4 week outage, and there are several years of positive operating result form converted units.
Abstract: The generating bank of an old recovery boiler often limits the capacity and availability of the unit. Many boilers develop leaks in the generating bank after over 20 years in service. The boiler bank is also a major capacity restricting part of the boiler. In Sweden, Norway and Germany, recovery boilers have been converted from bi-drum type to singe-drum type to solve the problem of "near mud drum corrosion" and other maintenance problems and to increase the capacity of the units. The retrofits have been carried out during 3-4 week outages and have provided the results anticipated -- increased availability and higher capacity.
Reference: LJUNGKVIST, K., BERGSTROM, U., LECLAIR, M., HANSSON, B. Converting old recovery boilers from bi-drum to single-drum type. Pulp & Paper Canada. 102(10):T286-288 (October 2001). Paper presented at the 2000 Control Systems Conference in Victoria, BC, May 1-4, 2000. Not to be reproduced without permission of PAPTAC. Manuscript received February 3, 1998. Revised manuscript approved for publication by the Review Panel March 1, 2001.
Keywords: RECOVERY FURNACES, MODERNIZATION, SWEDEN, NORWAY, GERMANY, MACHINE DESIGN, MAINTENANCE.
Résumé: La banque génératrice d'une vieille chaudière de récupération limite souvent la capacité et la disponibilité de l'appareil. Bon nombre de ces banques présentent des fuites après 20 ans. La banque est aussi une partie réductrice majeure de la capacité de la chaudière. En Suède, en Norvège et en Allemagne, les chaudières de récupération ont été converties chaudières à deux corps à chaudières à simple corps afin de résoudre le problème de corrosion du récipient à boues et les autres problèmes d'entretien et d'accroître la capacité des appareils. On a procédé aux mises à niveau pendant des arrêts temporaires de 3 à 4 semaines, ce qui a permis d'obtenir les résultats anticipés -- une disponibilité accrue et une plus grande capacité.