INTERIM FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

Project Title: EFFECTS OF CHLORINE IN COAL ON BOILER CORROSION

ICCI Project Number: R95-1/1.2A-1

Principal Investigator: M.-I.M. Chou, Illinois State Geological Survey (ISGS)

Other Investigators: J.M. Lytle, R.R. Ruch, and K.C. Hackley (ISGS); S.C. Kung, Babcock & Wilcox (B&W); L.L. Baxter, Sandia National Labs-Combustion Research Facilities (SNL-CRF).

Project Managers: K.K. Ho, ICCI; P.M. Goldberg, U.S. DOE

ABSTRACT

British literature, correlating superheater/reheater corrosion in PC boilers with the total chlorine (Cl) content in coals, has led many US boiler manufacturers to set their recommended Cl level at 0.25% to 0.3% for burning US coals. However, Cl-related boiler corrosion has not been reported by US utilities burning high-Cl Illinois coals. This means other factors, such as alkali metals, sulfur, or boiler parameters, may be responsible for accelerated corrosion. The goals of this study are to: 1) measure the rate of corrosion caused by two high-Cl coals (UK and IL) under identical boiler conditions for a duration which will give a reliable comparison, 2) obtain data on the concentration and occurrence of chlorine, sodium, sulfur, and potassium compounds in the gas and solid phases produced during combustion, and 3) define the nature of Cl in both Illinois and British coals and the factors, if any, that could affect its behavior or rate of corrosion during combustion.

Two of the three long-term pilot scale combustion tests were completed on Rend Lake Illinois coal and Gascoigne Wood British coal. Analyses of the corrosion samples are in progress. Chemical fractionation data showed that Gascoigne Wood coal contained a considerably greater amount of mobile alkali metals than the Rend Lake coal, and suggested that if these elements caused corrosion, then the Gascoigne Wood coal could exhibit greater corrosion, fouling, and slagging tendencies than the Rend Lake coal. The characterization results indicated that when Gascoigne Wood coal was heated under air at low temperature (<350C), some of the chlorides in coal produced NaCl crystals. When heating temperature was increased, the NaCl crystals were decomposed and the formation of organic chloride species with subsequent release of Cl into gas phase were observed. When Rend Lake coal was heated under air at low temperature, however, no solid NaCl was observed , but upon further heating at high temperature, formation of organic chloride and subsequent release of Cl occurred similar to the Gascoigne Wood coal. The results clearly indicated that the behavior of chlorine in the Illinois coal was somewhat different from that in the British coal.

The work at B&W begun in July1997. A purchase order for the third, low-Cl, coal was just completed, and the pilot-scale combustion test will be started soon after the coal is processed and delivered to the B&W. The project was extended to December 31, 1997 in order to complete the test.

EXECUTIVE SUMMARY

Many British studies have correlated accelerated fireside corrosion of heat exchanger tubes in utility boilers with the high-Cl content in the fuel coal. Their correlations implicated that the corrosion rate of boiler tubes increased proportionally with increasing Cl concentration in the fuel coal. Based on these correlations, U.S. boiler manufacturers and utility operators consider coals containing more than 0.3% Cl to be potentially corrosive and have set limits on the Cl content (<0.3%) of coal to be used in their boilers. These limits were based primarily on engineering studies in which the British coal data were extrapolated to the probable corrosion behavior of U.S. coals. The limits on the Cl level have discouraged the burning of high-chlorine Illinois Basin coals in utility boilers.

A recent survey jointly conducted by EPRI and ICCI indicated that some U.S. utilities have 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. This contradiction in published results suggests that the role of Cl in coal on boiler-tube corrosion is not fully understood. It is possible that the level of Cl in coal is not as harmful as predicted, or the corrosivity of high-Cl Illinois coal is less severe than that of British coal, or other coal properties such as sulfur and potentially volatile alkali metals in coals are the major causes of boiler corrosion. This discrepancy may also be attributed to differences in boiler design and operation between the US and UK utilities, such as boiler superheater and reheater temperatures. Tests are needed to show how the corrosivity of high-Cl Illinois coals actually compares with that of British coals of similar Cl contents.

Many researchers have associated high-temperature superheater/reheater corrosion with an alkali-sulfate-driven mechanism. It is possible that the existence of Cl in coals may have an impact on the liberation of the alkali metals in coals, which in turn may promote the production of complex alkali trisulfates and thus increase the rate of boiler corrosion. This mechanism could be induced by the association of alkali metals with Cl in coals or by the capability of Cl to catalytically react during coal combustion with minerals that contain alkali metals. Tests are required to determine the concentration of alkali metals, their volatility and their ability to form corrosive coatings during combustion. In addition, the chemical or physical properties of the UK and IL coals may be different, and the differences could directly or indirectly contribute to their corrosion abilities. A study is also needed to reveal the modes of occurrence of Cl, its availability (mobility), and its chemical association in the two coals and their behavior during combustion.

This study focuses on how the corrosivity of a high-Cl Illinois coal compares with that of a British coal of similar Cl content. It also focuses on revealing the mechanism for differences in corrosive behaviors, if any, of these high-Cl coals. The goals of this study are 1) to measure the rate of corrosion for high-Cl coals from the UK and IL under identical boiler conditions for a duration which will give a reliable comparison, 2) to determine concentration and occurrence on chlorine, sodium, sulfur, and potassium compounds in the gas and solid phases during combustion, and 3) to define the nature of Cl in both Illinois and British coals and the factors, if any, that could affect its behavior or rate of corrosion during combustion.

The specific objectives of this study are to:

A. Estimate the content of potentially volatile alkali metals in a set of Illinois and British coals using a serial-dissolution ash analysis method. Choose and acquire twenty tons of one high-Cl Illinois coal, one high-Cl British coal, and one low-Cl Illinois coal for the combustion tests.

B. Conduct advanced characterization techniques to define the nature of the chlorine in the British and Illinois coals and the factors, if any, that could affect its behavior or rate of corrosion during combustion.

C. Conduct three burner-rig corrosion tests in the Babcock & Wilcox (B&W) stoker boiler.

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 two high-Cl coals (UK and IL) with respect to the low-Cl baseline Illinois coal.

F. Conduct Multifuel Combustor (MFC) tests at Sandia National Labs - Combustion Research Facilities (SNL-CRF) on three coal samples, determine volatile components, and collect/analyze particulate samples during the MFC tests.

G. Interpret the MFC sampling and analysis results, and propose mechanisms of corrosive species formation.

H. Consolidate and interpret the results of the MFC tests, the burner-rig tests, and the characterization tests.

Pilot scale combustion tests (1000 hours) for the two high-Cl coals, British Gascoigne Wood and Illinois Rend Lake, were completed at B&W. An analysis of the corrosion coupons and interpretation of the corrosion data are in progress. A purchase order for the third, low-Cl, coal was completed. The pilot-scale combustion test will begin soon after the coal is processed and delivered to B&W. The project was extended to December 31, 1997 to complete this test.

The Multifuel Combustor tests on Rend Lake coal for the analysis of ash depositions was conducted at Sandia National Lab. No evidence was observed by SEM analysis for chloride films on the surfaces of probes that were placed in the Multifuel Combustor. If the chloride films were present, it was possible that they were too thin to be detected by the SEM, or that chlorides completely reacted to form sulfates, which are the thermodynamically favored alkali compounds at the temperature of the probes.

Many researchers have associated high-temperature corrosion with an alkali sulfate driven mechanism and have linked corrosion with the alkali metal content and sulfur content of the coal. However, it is believed that only the potentially volatile portion, not the total, of the alkali metals in a coal is harmful. Alkali metals occurring in mobile forms are considered more likely to vaporize during combustion than those combined in clays or other stable compounds. The former may ultimately lead to deposition of chlorides and sulfates on heat transfer surfaces and lead to corrosion by mechanisms reviewed in many reports (Raask 1985a; Ried et al. 1971) whereas the latter typically remain in the refractory minerals and have no specific role on corrosion. A serial-dissolution chemical analysis method, which combines a sequential wet-chemical leaching procedure and analysis of residuals and solutions, was adopted in this investigation to estimate the potentially volatile alkali metals in both Rend Lake coal and Gascoigne Wood coal. In the method, isolates of water-soluble salts and loosely bound species, ion-exchangeable materials, and acid-soluble materials were obtained sequentially. The water-soluble and ion-exchangeable fractions were used to estimate the potentially volatile portion of alkali metals in the coal. On the basis that the Gascoigne Wood coal has nearly twice as much ash as Rend Lake coal, the results indicated that the Gascoigne Wood coal sample contained considerably more total mobile alkali metals than does the Rend Lake coal sample. If the alkali metals were precursors to corrosion, the Gascoigne Wood coal sample would exhibit greater corrosion, fouling and slagging tendencies than the Rend Lake coal sample.

The staged-heating of a coal sample in a fludized bed combustor was conducted on the two high-Cl coals under two different (oxidation and pyrolysis) conditions. The purpose of these tests was to obtain chars for Cl-XANES analysis to determine the behavior of Cl in coal under the test conditions.The results indicated that when Gascoigne Wood coal was heated under air at low temperature (200C - 350C), some of the ionic chlorides in coal produced NaCl crystals. When heating temperature was increased, the NaCl crystals were decomposed and the formation of organic chloride species coupled with subsequent release of Cl into the gas phase were observed. When Rend Lake coal was heated under air at low temperature, however, no formation of solid NaCl was observed, but upon further heating at high temperature, formation of organic chloride and subsequent release of Cl occurred similar to the Gascoigne Wood coal. The overall results, which clearly indicated that the behavior of chlorine in the Illinois coal is somewhat different from that in the British coal, may help in the interpretation of the results from the pilot scale study.