INTERIM FINAL TECHNICAL REPORT

September 1, 1997, through August 31, 1998

Project Title: EFFECTS OF CHLORINE IN COAL ON FURNACE WALL CORROSION UNDER LOW-NO_X 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 and J. Bruinius (ISGS); S.C. Kung, McDermott Technology Inc. (MTI)

Project Manager: Ken K. Ho, ICCI

ABSTRACT

The purpose of this research is to determine the significant effect, if any, of chlorine in coal on the corrosion of a furnace wall under substoichiometric (low-NO_x) combustion conditions.

British literature published since the late 1960's correlated boiler-tube and furnace wall corrosion of utility boilers with the total Cl content in coals, and showed accelerated corrosion when using coals with Cl levels 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 boiler-tube or furnace wall components. Based on the conflicting UK and US experiences in burning high-Cl coal, as well as knowledge in substoichiometric combustion chemistry, it appears that chlorine in coal may not be fully responsible for the accelerated corrosion observed on the boiler-tubes and furnace walls. Other factors, such as sulfidation in combination with high heat flux can play a major role in furnace wall corrosion.

This study will conduct pilot scale combustion tests to determine the effect, if any, of chlorine in furnace wall corrosion under substoichiometric (reducing) conditions. The corrosion rate for 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 furnace wall corrosion when coal is burned under reducing/sulfiding conditions. The results of this investigation, along with those of the previous investigations, will be beneficial to utility, coal, and boiler manufacturer industries. Also, the produced data base may be useful in modifying the recommended Cl limits for coal to be burned in the boilers and thus increase the marketability of the high-Cl Illinois coal.

Stoker boiler facilities specifically designed by MTI for the combustion corrosion tests required a modification before they could be used in this study for combusting coal under low-NOx condition. The modification process was completed. The low-Cl Illinois coal sample, Crown II, was ordered and shipped to the MTI, and the first combustion test is in progress. The Crown II coal was provided by the Freeman Energy Coal Mining Co. A high chlorine Illinois coal, Rend Lake, was chosen for the second combustion test, and the Rend Lake coal will be provided by CONSOL. This is the first year (ended 6/1/99) of a two-year project. The combustion test for the low-Cl Illinois coal will be completed in the first project year; the test will be repeated for the high-Cl Illinois coal in the second project year.

EXECUTIVE SUMMARY

The purpose of this research is to determine any significant effect of chlorine in coal on corrosion of the furnace wall under substoichiometric (low-NOx) combustion conditions. Pilot scale coal combustion tests are being conducted. The corrosivity of a high-Cl (0.4%) 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, coal industries, and the boiler manufacturers.

Many British studies have associated accelerated fireside corrosion found on a furnace wall of utility boilers with the high-Cl content in coal (Bettelheim 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 manufactures 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 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 highCl content of 0.31 % Cl and the other with a low-Cl content of 0.16% Cl was determined by Monroe et al. (I 994) under pilot scale combustion tests. The results showed that the corrosion rate of the high-Cl coal was actually lower 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 even small temperature increases and are also sensitive to the temperature gradients along the probes. These temperature gradients 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. In this study, conventional corrosion probes are being used to measure corrosion rates over a long period of time (1000 hours) which will provide a reliable comparison on the effect of chlorine content of coal on the rate of corrosion.

During our previous study (Chou 1997), corrosion rates were measured using conventional probes under oxidizing conditions as those experienced by superheater heat exchange tubes. The rates of corrosion for a high-Cl Illinois coal and a high-Cl British coal were compared.

The results showed no correlation between the rate of corrosion and the chlorine content of a coal, and suggested that high-Cl Illinois coals, like low-Cl Illinois coal, could be successfully used in utility boilers if other coal components or boiler properties were first understood and well controlled.

In conjunction to superheater/reheater corrosion with the use of high-Cl coal, 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. 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 1994; Kung 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 potentially beneficial effect by the appearance of chlorine under low-NOx bumer conditions.

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 a high metal temperature mentioned earlier, would also play an important role in the corrosion mechanism.

The purpose of this research is to extend the MTI test results to include real combustion conditions. Pilot-scale combustion tests are being conducted to define the effect of chlorine in coal on furnace wall corrosion under substoichiometric conditions. The results obtained will help clarify the significance of chlorine on furnace wall corrosion when coal is burned substoichiometrically. The results of this investigation on furnace wall corrosion under reducing conditions, along with those of the previous investigations on superheater/reheater corrosion under oxidation conditions, will provide data that are beneficial to the utility, coal industries, and the boiler manufacturers.

The specific objectives of this study are:

A. Acquire two Illinois coals, one containing high-Cl (0.4%) and one containing low-Cl ( 0.2 %). Process and distribute the coals for characterization and combustion tests.

B. Determine the nature of chlorine, sulfur, and alkali metals in coals and their roles, if any, that could affect the chemistry and furnace wall corrosion mechanism during combustion under substoichiometric condition.

C. Conduct two bumer-rig corrosion tests in the McDermott Technology Inc. (MTI) Stoker boiler and collect samples for metallurgical composition and the rate of corrosion examinations.

D. Perform a 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 highCl coal with respect to the low-Cl coal.

The establishment of the subcontract between MTI and ISGS was completed in June 1998. This delay was mainly due to the main contract agreement between ISGS and ICCI that was not finalized until mid February 1998. However, a no-cost extension through June 1, 1999 was approved by the ICCI and DCCA to provide additional time to complete the project.

The stoker boiler which was designed specifically by MTI to conduct corrosion-rate studies was recently modified for substoichiometric combustion tests in this study. One high-Cl (0.4 %) Illinois coal and one low-Cl (0.2%) Illinois coal will be tested. In consultation with other members of the research team and in order to keep continuity with previous research, Rend Lake coal and Crown II coal were chosen respectively as the high and low chlorine coals. Crown II, a low chlorine Illinois coal chosen for the second combustion test was provided by the Freeman Energy Coal Mining Co. The coal was ordered and shipped to the MTI, and the first combustion test is in progress. The Rend Lake, high-Cl Illinois coal sample will be provided by CONSOL free of charge.This is the first year of a two-year project. The combustion test for the low-Cl Illinois coal will be completed in the first year. The test will be repeated for the high-Cl Illinois coal in the second project year. The corrosivity of these two Illinois coals will be measured using conventional probes under identical operating conditions as on the most common water-wall stainless-steel alloys for a duration (1000 hours) that will give a reliable comparison.