FINAL TECHNICAL REPORT

September 1, 1995, through March 31, 1998

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 (ISGS); S.C. Kung, McDermott Technology, Inc. (MTI); L.L. Baxter, Sandia National Labs-Combustion Research Facilities (SNL-CRF).

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

ABSTRACT

Many British studies have correlated superheater/reheater corrosion in pulverized coal (PC) boilers with the total chlorine (Cl) content in coals, which has led many US boiler manufacturers to set their maximum recommended Cl level at 0.25% to 0.3%. However, Cl-related boiler corrosion has not been reported by US utilities burning high-Cl Illinois coals. Other factors, such as alkali metals, sulfur, or boiler parameters, have been studied for their effect on corrosion rate. The goals of this study were to: 1) measure the rate of corrosion caused by two high-Cl coals (British and Illinois) and one low-Cl Illinois coal under identical combustion conditions for a duration which will give reliable comparisons, 2) determine 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 coals and its behavior during combustion.

Two specific metal temperatures, which are commonly adopted in the superheater of US (1100F) and the UK (1200F) boilers, and two metal alloys, 304SS and 310SS, were examined during long-term 1000-hour combustion corrosion tests. For the most commonly used 304SS, the rate of corrosion increased significantly with an increase in the tube wall temperature from 1100F to 1200F for all three coals. The rate of corrosion for 310SS, which had a greater chromium and nickel content among the two alloys, and is of interest to boiler manufacturers as a test material for boiler construction, did not follow the trend observed with 304SS for both Illinois coals. The results showed no evidence of a correlation between coal Cl content and rate of corrosion, but showed a correlation between the rate of corrosion and the type of metal and the metal temperature used. The results suggested that different history in corrosivity experienced by the superheater/reheater of UK and US utility boiler burning high-Cl coal could be a result of the difference in the boiler temperatures used in the UK and US boilers. Also, other factors, such as potentially volatile alkali metals, or different alloy compositions of the utility boiler could change the corrosion behavior of a coal. The results also suggested that high-Cl Illinois coals, like low-Cl coal, could be successfully used if other coal properties or combustion conditions could be controlled.

EXECUTIVE SUMMARY

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

A survey conducted jointly by EPRI and ICCI during 1992 (Wright et al., 1994; Doane et al., 1994) indicated that some US utilities have decades of experience burning high-Cl coals in pulverized coal (PC) fired boilers. Although fireside corrosion problems have been reported, none of them could be directly related to the presence of Cl in coal. This contradiction in published data suggested that the role of Cl in coal on boiler-tube corrosion was not fully understood. It was possible that the level of Cl in coal was not as harmful as predicted, or the corrosivity of high-Cl Illinois coal was less severe than that of British coal, or other coal properties such as sulfur and potentially volatile alkali metals in coals were possibly associated with boiler corrosion. The differences in boiler design and operation between US and UK utilities, such as boiler superheater/reheater temperatures, might also have attributed to this discrepancy.

Many researchers have associated boiler tube corrosion with an alkali-sulfate-driven mechanism. It was speculated that the existence of Cl in coals increases the liberation of the alkali metals in coals, which in turn promotes 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 react during coal combustion with minerals that contain alkali metals. Studies are required to determine the concentration of alkali metals, their volatility and their ability to form corrosive substances during combustion and deposition. In addition, the chemical or physical properties of UK and Illinois coals may be different, and the differences could directly or indirectly contribute to their corrosion abilities. It was also necessary to determine the modes of occurrence of Cl, its availability (mobility), its chemical association in the two coals, and its behavior during combustion.

This study focused on how the corrosivity of a high-Cl Illinois coal compares with that of a British coal with similar Cl content at superheater/reheater temperatures. It also focused on revealing the mechanism for corrosive behaviors of these high-Cl coals. The major goals of this study were to: 1) measure the rate of corrosion caused by two high-Cl coals (British and Illinois) and one low-Cl Illinois coal under identical combustion conditions for a duration which will give reliable comparisons, 2) determine 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 coals and its behavior during combustion. The specific objectives of this study were to:

A. 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. Estimate the content of potentially volatile alkali metals in the selected Illinois and British coals using a chemical fractionation (serial-dissolution ash analysis) method.

B. Conduct advanced characterization techniques to define the nature of the chlorine in the British and Illinois coals and the behavior of Cl in coal under combustion condition.

C. Conduct three long-term pilot scale corrosion tests using the stoker boiler facilities at McDermott Technology, Inc. (MTI) .

D. Perform metallographic examination of boiler scale and/or deposit, and measure the 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 Illinois) with respect to the low-Cl baseline Illinois coal.

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

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

H. Consolidate and interpret the results of the MFC tests, the pilot-scale tests, and the characterization tests.

Three 1000-hour pilot scale combustion tests for two high-Cl coals, British Gascoigne Wood and Illinois Rend Lake, and a low-Cl baseline Illinois coal, Crown II, were conducted at MTI. Two alloy materials, 304SS and 310SS, and two specific metal temperatures which are commonly used in US (1100F) and UK (1200F) utility boilers were used. 304SS is the material most frequently used at the hottest superheater section of both US and UK utility boilers, and 310SS is a material of interest to boiler manufacturers for testing purposes.

The results of pilot scale combustion tests showed no evidence of a correlation between coal Cl content and rate of corrosion, but showed a correlation of the rate of corrosion on the type of metal and the metal temperature used. The corrosion rate for 304SS was increased with an increase in the metal temperature used during the combustion for all three coals. Compared with 304SS, the corrosion rate for 310SS showed a similar trend for the high-Cl British coal, but showed a different trend, a bell-shape relationship, for the two Illinois coals. Analysis of the scale and ash layer closest to the probe showed no chloride deposition but indicated the deposition of sulfur. These data suggested that the different field history of the corrosivity caused by burning high-Cl Illinois coal and high-Cl British coal might have been the result of different superheater/reheater metal temperatures used in the US and UK utility boilers. The results also indicated that changing alloy compositions for the hottest superheated section of utility boilers might result in a change of the corrosion behavior of a coal.

The short-term Multifuel Combustor tests were conducted at SNL-CRF on Rend Lake coal for the analysis of ash deposition. No evidence of chloride films was observed by SEM analysis on the surfaces of probes that were placed in the Multifuel Combustor. As mentioned earlier, the results of long-term pilot scale combustion tests at MTI also showed no evidence of chloride deposition on the metal surfaces. These results strongly suggested that the sulfate layers observed probably were not produced by replacement of chlorides; thus, no chloride-laden gas (such as HCl) should be released at the probe surface to enhance corrosion.

Many researchers have associated high-temperature corrosion with an alkali sulfate driven mechanism and have linked the corrosion with the alkali metal and sulfur content of the coal.

The evidence of occurrence of alkali sulfate near the metal surface and the possible effect it could have on the corrosion rate were not apparent from the analysis of ash deposition data.

However, chemical fractionation data which estimated the potentially volatile alkali metal content in coal indicated that Gascoigne Wood coal contained a greater amount of mobile alkali metals than the Rend Lake coal. The data suggested that if these alkali metals were precursors to corrosion (a conventional mechanism), the Gascoigne Wood coal would exhibit greater corrosion, fouling, and slagging tendencies than the Rend Lake coal.

A staged-heating using a fluidized bed reactor was performed on the two high-Cl coals to obtain chars for Cl X-ray absorption Near Edge Structures (XANES) analysis. These tests compared the occurrence of Cl in coals under both oxidation and pyrolysis conditions. The XANES analysis indicated that the Cl in all coals occurred as ions in solution. During heating under nitrogen, some ionic chloride in the British coal formed NaCl crystals while some volatilized over the temperature range. The ionic chloride in the Rend Lake coal, on the other hand, formed NaCl crystals, but as the temperature increased, the occurrence of NaCl was replaced progressively by KCl, which dominated at the hottest temperature examined (600C). The formation of NaCl crystals was also observed during Gascoigne Wood coal heated under air at low temperature, but this was not observed when Rend Lake coal was heated under air at either low or high temperatures.

Overall, the results showed no correlation between the rate of corrosion and the chlorine content of a coal. Other factors, such as potentially volatile alkali metals, or boiler operation conditions, such as boiler tube wall temperatures used in the US and UK, might explain the different corrosion history experienced by the superheaters of US and UK boilers burning high-Cl coals. The results also suggested that high-Cl Illinois coals, like low-Cl Illinois coal, could be successfully used in utility boilers if other coal conponents or boiler properties were understood and controlled.