PROCESS CHANGES AND CORROSION OF MATERIALS IN RECOVERY BOILERS

Transcription

1 PROCESS CHANGES AND CORROSION OF MATERIALS IN RECOVERY BOILERS Black Liquor Combustion Coloquium Park City, Utah May 13, 23 Doug Singbeil and Joey Kish Paprican Jim Keiser ORNL 2 1

2 3 Recovery boiler evolution: leading cause of corrosion? 1934* ~199* * Steam, B&W (1992) 4 2

3 PULP AND PAPER RESEARCH INSTITUTE OF CANADA Adapting materials solutions to process problems began with Windsor # 2 Decade Process change Consequence 193 s Modern recovery boiler Introduction of water-cooled tubes 196 s Higher pressure, energy efficient boilers Corrosion of carbon steel wall tubes Introduction of composite tubes 197 s First closed-cycle mill Increased throughput Superheater corrosion Near-drum thinning 198 s Higher solids firing Composite tube corrosion and cracking 199 s Improved air systems Corrosion in mid-furnace locations 5 Corrosion problems are seldom anticipated solutions are reactive Carbon steel thinning Lower furnace Floor tube cracking Superheater wastage Near drum tube thinning Cracking and corrosion at air port openings Cut line corrosion 6 3

4 How to build a recovery boiler with predictable corrosion problems? Boiler design Process understanding Materials selection Manufacturing practices Fuel variability 7 Case histories Near drum thinning of generating bank tubes Cracking of composite tubes on floors and at primary air port openings Corrosion of lower furnace carbon steel waterwall tubes 8 4

5 Near-drum tube thinning affected many two drum boilers in mid-198 s Moderate rate of corrosion on tubes at interface with mud drum Differences in boiler design reflected in location of corrosion Change in process operation was shown to mitigate the problem 9 1 5

6 The most effective solution to near- drum thinning lay in process change 11 Corrosion of carbon steel wall tubes precipitated a change to higher cost materials Need for increased energy efficiency raised boiler operating pressures Result was the gradual acceptance of composite tubes as new standard Burning liquor on walls found to be detrimental 12 6

7 Corrosion rates increase with higher pressure operation Normalized Corrosion Rate SA-21 34L Temperature o C 13 Pyrolysis gases can be very corrosive to carbon steel, but not stainless steel Normalized Corrosion Rate SA o C 4 o C H 2 S CH 3 SH (CH 3 ) 2 S Sulphur Source 14 7

8 Release of pyrolysis gases against wall probably overshadows most other causes Relative Corrosion Rate SA-21 carbon steel 32C 4C 3-4C 4C Environment 15 Alloys with greater nickel content can corrode more quickly when exposed to H 2 S Normalized Corrosion Rate SS34L SS31 IN825 IN Temperature o C 16 8

9 Very localized corrosion has been observed on alloy 625 weld overlay openings LEFT TUBE RIGHT TUBE Towards Adjacent Towards Adjacent Weld passes Above BMT Overlay Thickness (mils) 17 Laboratory corrosion tests have identified environments corrosive to Deaerated Aerated Corrosion Rate (mpy) L A825 A625 Alloy 18 9

10 Cracks in composite wall and floor tubes pose different risks to boiler operation Several mechanisms are responsible for cracking 34L SS readily cracks inservice, alternative alloys may be more resistant Process changes can reduce risk of cracking 19 Cracks in air port openings are more likely to grow into carbon steel 2 1

11 A 6 C excursion with steam blanketing on the ID can cause tensile stresses in carbon steel stresses in the carbon steel are compressive in normal operation stresses in carbon steel are tensile after the temperature excursion 21 Metallurgical condition of an alloy influences resistance to cracking Maximum Crack Depth (µm) % Cold Worked 5% Cold Worked & Solution Annealed 34L A825 A625 Alloy 22 11

12 Operating practices that direct liquor away from the walls reduce thermal activity on air port surfaces Cracking 5 Non-cracking Temperature ( C) LH2-1 LH2-2 LH2-5 LH2-6 Temperature ( C) RH3-1 RH3-2 RH3-5 RH3-6 3 : 2: 4: 6: 8: 1: 12: 14: 16: 18: 2: 22: : 35 Time (24h) 3 : 2: 4: 6: 8: 1: 12: 14: 16: 18: 2: 22: : Time (24h) 23 Additional materials issues Waterside problems scaling circulation flow induced corrosion stress assisted corrosion Mechanical design and fabrication Fatigue at headers, tube ties Sootblower-related problems Thinning of tubes at intersection with drums and headers 24 12

13 Conclusions Corrosion in recovery boilers occurs in micro-climates Compatibility of composite tube alloys with boiler environment is unresolved Burning and pyrolysis on surfaces should be avoided to minimize corrosion 25 Challenges for the future Process changes have profound effect on corrosion how do we anticipate the consequences in advance? Recovery boilers are amongst the few chemical reactors in the world to be built largely from carbon steel. Can the industry afford an all-alloy boiler? 26 13