ABSTRACT
After 39 years of operation, the orifice plate in a black liquor incinerator reactor has seen over 250,000 hours of exposure to operating temperatures in the range of 480-595°C (900-1100°F), and has cracked in numerous locations. This article describes the results of a metallurgical evaluation of plate samples removed from the unit. Although a 0.5Cr-0.5Mo high-temperature alloy was used for the orifice plate, this evaluation showed that the orifice plate had undergone irreversible elevated temperature degradation of its mechanical properties, in addition to the cracking observed. High temperature degradation mechanisms found in the plate include: creep, thermal fatigue, surface cracking, grain growth, temper embrittlement, and low toughness at temperatures below 85°C (185°F). A fracture mechanics evaluation of the plate subjected to the operating stresses and the effects of the existing cracks showed that static loads alone would not induce brittle crack growth and the plate would not collapse. However, below 85°C (185°F) impact loads could be high enough to trigger brittle fracture and a collapse of the orifice plate. Impact loads could originate from process material build-up falling from walls of the reactor chamber or by a power loss where the fluidized bed suddenly rests on top of the plate.
INTRODUCTION
This article describes high temperature degradation experienced in a black liquor incineration unit that has been in service for 39 years. The unit is part of a chemical recovery process in pulping operations and consists of a two-chamber vessel separated by a 7.3 meter (24 feet) diametric membrane (orifice) plate or head with 1889 tubes. Air flows from the lower chamber, through the tubes, to the combustion chamber where black liquor (Na2CO3 and organics) sprayed from the top is combusted at 705°C (1300°F) to remove the organics and recover the sodium carbonate for reuse (Figure 1). Complete combustion is achieved by fluidization and the formation of small pellets of product. The vessel is constructed of carbon steel plate and it is completely insulated from the inside with refractory (cold wall design). The tube containing membrane that separates the windbox from the combustion chamber is covered in the combustion chamber side with a layered refractory of different densities that lies flush with the tubes on top of the plate. The window side of the tube membrane is exposed to preheated air at 480-595°C (900-1100°F). During the last annual inspection of the incinerator unit, the tube membrane was found to have a number of cracks that originated at the tube to plate welds on the windbox side. A change from the original tube membrane design shape of a dished down orifice plate to a reverse curvature upward dome shape was noted where most of the cracking was observed. Some of the cracks extended up to 15 cm (6 in) in length, which is the center tube-to-tube distance (Figure 2). The primary objective of this study was to determine the cause of the cracks and the remaining service life of the membrane plate.
