FINAL TECHNICAL REPORT

September 1, 1995, through August 31, 1996

Project Title: EFFECTS OF CHLORINE IN COAL ON BOILER CORROSION

ICCI Project Number: 95-1/1.2A-1

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

Other Investigators: J.M. Lytle, R.R. Ruch, K.C. Hackley, and J.A. Bruinius (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 the 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 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.

This is the first year of a two-year project. Chemical analyses were performed on three British coals and two Illinois coals. A high-Cl Illinois coal, Rend Lake, and a high-Cl British coal, Gascoigne Wood, with similar Cl (0.4%) and sulfur (1-1.3%) contents were chosen for this investigation. A twenty-ton lot of Rend Lake coal chosen for this study was ordered, processed, and distributed. The combustion tests are now in progress. The Gascoigne Wood coal was also ordered and received in the U.S. Combustion tests for both Gascoigne Wood and a low-Cl baseline coal will be initiated in the next project year starting September 1, 1996.

Characterization results indicated some differences and some similarities between the two high-Cl coals. When Gascoigne Wood coal was heated under air at low temperature, some of the Cl in the coal combined with Na to form solid NaCl. With further heating at higher temperatures, the NaCl was decomposed and the formation of an organic chloride was observed. When Rend Lake coal was heated under air at low temperature, however, showed no formation of solid NaCl, but further heating at high temperature showed the formation of an organic chloride similar to the Gascoigne Wood. These tests will be continued to include the analysis of all three coals chosen. Conclusive results from short-term combustion and characterization may be helpful in the interpretation of the pilot scale combustion results.

EXECUTIVE SUMMARY

Many British studies have associated accelerated fireside corrosion of heat exchanger tubes in utility boilers with the high-Cl content in the fuel coal. Their corrosion data suggested that the corrosion rate of boiler tubes increased proportionately with increasing Cl concentration in the fuel coal. Based on the results of these studies, U.S. boiler manufacturers and utility operators consider coals containing more than 0.3% Cl to be potentially corrosive. This 0.3% limit primarily was based on engineering studies in which the British coal data were extrapolated to the probable corrosion behavior of U.S. coals. The 0.3% limit on Cl level has discouraged the burning of many Illinois Basin coals in utility boilers.

A recent survey jointly conducted by the 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 the 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 cause of boiler corrosion. This discrepancy may also be attributed to the difference 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 compare 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 measure the concentration of alkali metals, their volatility and ability to form corrosive coatings during combustion. In addition, the chemical or physical properties of the two coals may be different, and the differences could directly or indirectly contribute to their differences in corrosion abilities. A study is also needed to reveal the modes of occurrence of Cl, its availability (mobility) and chemical association in the two (UK and IL) 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 obtain concentration and occurrence data on Cl, sodium, sulfur, and potassium compounds in the gas and solid phases produced during each stage of 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, analyze the combustion gases using an on-line tunable diode laser technique, and collect/analyze particulate samples during each stage of 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.

Chemical analyses were performed on three British coals (Gascoigne Wood, Thoresby, and Lea Hall) and two Illinois coals (Rend Lake and Jader) obtained from our previous investigations. They were analyzed for the total chlorine, total sulfur, and total ash contents by ASTM methods. A high-Cl Illinois coal, Rend Lake, and a high-Cl British coal, Gascoigne Wood, with similar total chlorine (0.4%) and total sulfur (1-1.3%) contents were chosen for this investigation. To form a basis for assessing the corrosion effect, an Illinois Basin coal, Jader, which contains about 0.16% chlorine will be included in the combustion tests to provide "baseline" data.

This is the first year of a two-year project. In the first year, 20 tons of coal from Rend Lake mine was selected, purchased, processed, and distributed. The Rend Lake coal sample lot contained 0.42% Cl, 1.22% S, and 7.9% ash. The Cl and S data were consistent with data obtained from a previous analysis on a smaller size sample of the same coal. The ash content from the large sample collection (7.9%) was much lower than the content of the sample (15.3%) previously obtained for the preliminary analysis, but it is comparable with the as-shipped coal sample. The combustion tests on the Rend Lake coal are now in progress. The Gascoigne Wood, a high-Cl British coal, was also ordered and received in the U.S. Combustion tests for both Gascoigne Wood and a low-Cl baseline coal will be initiated in the next project year starting September 1, 1996.

The three pilot-scale combustion tests of the three coals chosen will focus on measuring long-term corrosion rates on the superheater stainless-steel alloys most common used in the U.S. and U.K. under identical operating conditions. All three coals will be tested for a duration of 1000 hours, so that a reliable comparison can be made between them. Also, two specific boiler temperatures, which are commonly adopted in the U.S. and the U.K. (1100F and 1200F, respectively) will be employed.

The Multifuel Combustor tests of the three coals will focus on short-term corrosion behavior including the concentration and occurrence in the gas phase of Cl-, Na-, and K- containing compounds produced during each stage of combustion. The concentration and occurrence data of these compounds in both the gas phase and the combustion residues may be helpful in the interpretation of the long-term combustion results.

The nature of Cl in coal was determined by heating the coal, then analyzing the char using chlorine x-ray adsorption near edge spectroscopy (Cl-XANES). The analysis was also followed by low- temperature ashing of the coal, then analyzing the gas with quadrupole gas analyzer (LTA-QGA). The results showed that when Gascoigne Wood coal was heated under air at low temperature around 200C - 350C, some of the Cl in the coal combined with Na to form solid NaCl. With further heating at higher temperatures, the NaCl was decomposed and the formation of an organic chloride compound was observed. When Rend Lake coal was heated under air at low temperature, however, there was not observed formation of solid NaCl, but with further heating at high temperature, an organic chloride was formed similar to the Gascoigne Wood.

On the other hand, when the Gascoigne Wood coal was heated under N₂ at temperatures around 200C - 350C, the formation of solid NaCl was observed, in a manner similar to when the coal was heated under air. However, after further heating at high temperatures, instead of forming an organic chloride, as observed when the coal was heated under air, the generation of solid NaCl continued until most, if not all of, the remaining chlorine was converted. These results indicated that the formation of the organic chloride was associated only with oxidation condition. This study will be continued and extended to include the three coals and both oxidation and pyrolysis conditions. Conclusive results from both MFC combustion and the follow-up characterization may be helpful in the interpretation of the pilot-scale combustion results.