FINAL TECHNICAL REPORT
September 1, 1997, through June 1, 1999
Project Title: EFFECTS OF CHLORINE IN COAL ON FURNACE WALL CORROSION UNDER LOW-NOX CONDITION
ICCI Project Number: 97-1/4.1B-1
Principal Investigator: M.-I.M. Chou, Illinois State Geological Survey (ISGS)
Other Investigators: J. M. Lytle (ISGS); S. C. Kung, McDermott Technology Inc. (MTI)
Project Manager: Ken K. Ho, ICCI
ABSTRACT
British literature published since the late 1960s has shown accelerated furnace-wall corrosions of utility boilers when using coals with Cl content of 0.3% or greater. The US experience, on the other hand, has indicated that Cl in high-Cl coals is not a major cause of corrosion of either the boiler-tubes or furnace-wall. In recent controlled laboratory studies at McDermott Technologies, Inc., tests run under simulated substoichiometric combustion conditions indicated no significant effect on the corrosion rate with variations of hydrogen chloride in the simulated flue gas. Nevertheless, because of the British data, many US boiler manufacturers have recommended a maximum Cl level at 0.3% for burning US coals. This limitation decreases the market potential of high-Cl Illinois coals. Based on the conflicting UK and US experiences in burning high-Cl coal, as well as knowledge in substoichiometric combustion, it appears that chlorine in coal may not be fully responsible, it at all, for the accelerated corrosion observed on the boiler-tubes and furnace-wall. Other factors, such as sulfidation in combination with high heat flux, may play a major role in boiler corrosion.
This is the first year of a two-year project. The overall purpose of this research is to determine the significant effect, if any, of chlorine in coal on corrosion of the furnace-wall under substoichiometric (low-NOx) combustion conditions. The results from pilot scale combustion tests will be used to compare the corrosivity of a high-Cl (0.4 to 0.6%) Illinois coal and a low-Cl (<0.2%) Illinois coal. The comparative results will help clarify the potential effect of chlorine on furnace-wall corrosion when coal is burned under reducing/sulfiding conditions.
In order to conduct the pilot-scale combustion tests under low-NOx conditions, modification of the Stoker boiler system at MTI was completed during the first year. A low-chlorine Illinois coal (<.20% Cl) and a high-chlorine Illinois coal (>.40% Cl) for the combustion tests were also chosen. In addition, the first combustion test on the low-Cl coal was completed. Samples obtained from the test were preserved and are to be analyzed in year-2 for metal corrosion and for the effects of chlorine and/or deposited ash. The second test for the high-Cl Illinois coal will be conducted in year 2. The results of these test from combusting coal under reducing conditions will be integrated with those of our previous results from combusting coal under oxidation conditions into a database. The established database can be used for the boiler manufacturers to consider changing their Cl limits for burning high-Cl Illinois coals in their boilers, or can be used to benefit the utility industries and the boiler manufacturers in selecting boiler or furnace-wall construction materials.
EXECUTIVE SUMMARY
The purpose of this research is to determine the significant effects, if any, of chlorine in coal on corrosion of the furnace-wall under substoichiometric (low-NOx) combustion tests. The corrosivity of a high-Cl (0.4 to 0.6%) Illinois coal and a low-Cl (<0.2%) Illinois coal will be measured and compared. The results obtained will help clarify the potential effect of chlorine on the furnace-wall corrosion when coal is burned under reducing/sulfiding conditions. The results of this investigation, along with those of the previous investigations, will provide data that are beneficial to the utility and coal industries and the boiler manufacturers.
Many British studies have associated accelerated fireside corrosion found on furnace-wall of utility boilers with the high-Cl content in coal (Bettelheim et al. 1980). Their corrosion data suggested that the corrosion rate of boiler tubes increased proportionately with increasing Cl concentration in coal. Based on the results of these studies, US boiler manufacturers and utilities consider coals containing more than 0.3% Cl to be potentially corrosive. This 0.3% limit primarily was based on engineering studies extrapolating the British coal data to the probable corrosion behavior of US coal. The 0.3% limit on Cl level has discouraged the burning of many Illinois BAsin coals in utility boilers.
A survey jointly conducted by EPRI and ICCI (Doane et al. 1994) indicated that some US utilities have had decades of experience burning high-Cl coals in the PC-fired boilers. Although fireside corrosion problems have been reported, most of them could not be directly related to the presence of Cl in coal. Furthermore, the corrosivity of two Illinois coals, one with a high-Cl content of 0.31% Cl and the other with a low-Cl content of 0.16% was determined by Monroe et al. (2994) under pilot-scale combustion tests. The results showed that the corrosion rate of the high-Cl coal was actually slower than the corrosion rate of the low-Cl coal. In that study, resistance corrosion probes instead of conventional probes were used in the tests, and corrosion rates were based on measurements of electrical resistance over the period of the tests. Also, during the tests for the high-Cl coal, equipment malfunctions caused higher temperature excursion which increased the relative corrosion rates. Resistance probes are particularly sensitive to short temperature increases and are also sensitive to the temperature gradients along the probes. These temperature gradients of the probes were unavoidable during the tests in both oxidizing zones and reducing zones. Though the overall results show that the high-Cl Illinois coal may not be corrosive under the test conditions, the results could not be used as a basis for redefining recommended limits of chlorine in coal for boiler combustion. This is because the data were not fully convincing due to the reasons mentioned earlier on the high temperature excursion and the sensitivity of the resistance probes subjected to temperature variations.
Our previous study (Chou et al. 1998) focused on high-temperature superheater/reheater tube wall corrosions which occurred under oxidation conditions. Pilot-scale combustion corrosion tests were conducted on a high-Cl Illinois coal, a high-Cl British coal, and a low-Cl baseline Illinois coal. The corrosion rates were measured using conventional probes under oxidizing conditions as would be experienced by superheater tube wall in a conventional boiler. The results showed no evidence of a correlation between coal Cl content and rate of corrosion, and suggested that high-Cl Illinois coals, like low-Cl coal, could be successfully used in utility boilers if other coal components or boiler properties were understood and controlled.
A recent review published by British authors (James and Pinder 1997) correlated the high-corrosion wastage observed on the furnace-wall of UK boilers with the high-Cl in UK coal. However, they also suggested that factors other than coal chlorine should be considered in the accelerated corrosion mechanism. It was suggested that the accelerated corrosion took place mainly on the furnace-wall where substoichiometric combustion and high heat flux coexisted. The presence of reducing gases implies that insufficient oxygen is supplied to the combustion zone by improper air/coal mixing, and a high heat flux dictates the existence of a high metal temperature on the furnace-wall. Under insufficient oxygen conditions, sulfur in coal is primarily converted to sulfide (H2S) instead of sulfur oxide (SOx). The H2S gas is very corrosive and readily sulfidizes the conventional furnace-wall alloys. The sulfidation of the furnace-wall alloys is further escalated by high metal temperature. Therefore, when a large H2S concentration and a high metal temperature coexist, accelerated corrosion wastage on the furnace-wall is expected. A combination of these conditions could have accounted for the majority of the corrosion wastage experienced on the furnace-wall of UK boilers.
The conflicting UK and US experiences in burning high-Cl coal, as well as knowledge in substoichiometric combustion suggest that the role of Cl in coal on furnace-wall corrosion is not fully understood. It appears that chlorine in coal may not be fully responsible for the accelerated corrosion observed on the furnace-wall. Other factors, such as sulfidation and high metal temperature, would also play an important role in the corrosion mechanism. In general, the smaller UK boilers may favor the formation of substoichiometric combustion conditions and flame impingement in the lower furnace, thus leading to sulfide attack at a high metal temperature.
This accelerated corrosion wastage on the furnace-wall has been experienced by some US utilities in the pas few years after the implementation of low-NOx burners (Jones 1997). By design, the low-NOx burners create a substoichiometric combustion zone to reduce NOx formation. While NOx reduction is achieved, the substoichiometric combustion mechanism also generates a significant amount of H2S in the flue gases, which can be very corrosive, and through sulfidation, it affects boiler performance. This is clearly indicated in units where medium and high-S coals are burned. These low-NOx combustion environments would be similar to those from insufficient oxidation experienced by the UK boilers. British researchers generally believe that the presence of chlorine in the substoichiometric combustion condition would further elevate the flue gas corrosivity.
However, contrary to the general belief of British researchers, the laboratory studies by MTI (Kung et al. 1994; Kung et al. 1996) indicated that the addition of HCl to the combustion gases at a level equivalent to burning high-Cl coal under substoichiometric combustion conditions would not impose additional corrosion on the furnace-wall. In fact, the presence of HCl in the low NOx combustion gas may retard the sulfide attack. These findings are of potential importance to the Illinois coal industry, especially the State of Illinois which has a large reserve of high-Cl coal. However, the present laboratory corrosion data available at MTI are insufficient to totally quantify the potential beneficial effect from the chlorine in coal on the low-NOx burner applications.
The purpose of this study is to conduct pilot-scale tests using conventional probes to measure corrosion rates under reducing/sulfiding conditions as would be experienced by a water-wall in a low-NOx boiler. The results obtained will help clarify the significance of chlorine on furnace-wall corrosion when coal is burned substoichiometrically.
The specific objectives of this study are:
A. Acquire two Illinois coals, one containing high-Cl, 0.4 to 0.6%, and one containing low-Cl, <0.2%, and process and distribute the coals for characterization and combustion tests.
B. Examine the nature of chlorine, sulfur, and alkali metals in coals and their roles, if any, that could affect the chemistry and mechanism of furnace-wall corrosion during combustion under substoichiometric condition.
C. Conduct two burner-rig corrosion tests in the McDermott Technologies, Inc. (MTI) stoker boiler and collect samples for metallurgical composition and the rate of corrosion examinations.
D. Perform metallographic examination of boiler scale and/or deposit, and measure rates of corrosion from specimen cross sections.
E. Interpret the sampling and analysis results, and compare the rates of corrosion of the high-Cl coal with respect to the low-Cl coal.
This if the first year of a two-year project. This project is a combined effort by research teams from ISGS and MTI. ISGS has acquired coal samples, has coordinated research efforts including sample processing, distribution, and characterization, and has prepared reports for the ICCI. MTI is responsible for conducting Stoker boiler tests, analyzing the corrosion samples obtained, and interpreting the results with input from the ISGS.
During the first year, a Stoker Boiler system at MTI has been specifically designed and modified for conducting corrosion-rate studies that can be related to substoichiometric combustion. Crown II low-Cl Illinois coal (<0.2% Cl) and Rend Lake high-Cl Illinois coal (>0.4% Cl) were chosen for the combustion tests. The combustion test on the low-Cl coal was completed during this project year. The second test on the high-Cl Illinois will be conducted in the second project year. The corrosion samples obtained from the first test were preserved and are to be analyzed after the second combustion test on a high-Cl coal is completed.
Samples obtained from both tests will be analyzed for metal corrosion and for the effects of chlorine and/or deposited ash on the rate of corrosion. The long-term corrosion rates are being measured in the boiler under identical operating conditions on the most common water-wall stainless-steel alloys for a duration (800 hours) that would give a reliable comparison. The results of this study from combusting coal under reducing conditions will be integrated with those of our previous study from combusting coal under oxidation conditions into a database. If the data indicate that Cl in coal is not detrimental for boiler corrosion, the established database can be used by the boiler manufacturers to consider changing their Cl limits for burning high-Cl Illinois coals in their boilers. The data obtained from these studies will also be beneficial to utility industries and boiler manufacturers in selecting boiler or furnace-wall construction materials.