Tiêu chuẩn quốc tế

• Số hiệu / Standard Number: ASTM RR:D05-1028
• Năm ban hành / Publication date: 2002
• Tình trạng / Status: A - Còn hiệu lực
• Tên tiếng Anh / Title in English: Interlaboratory Study to Establish Precision Statements for ASTM D5865, Standard Test Method for Gross Calorific Value of Coal and Coke
• Số trang / Page: 33

Phạm vi áp dụng / Scope of standard

Introduction

Background: The titer correction is needed because when coal is burned in the boiler (constant pressure) the nitrogen is released as NOx, but when coal is burned in the calorimeter, the nitrogen in the air in the calorimeter vessel is converted to HN03. This reaction is exothermic and consequently needs to be subtracted from the BTU content of the coal as determined in the calorimeter.

Recognizing that the nitrogen content in coal is small, the N from coal can contribute approximately 0.3 BTUs and that only approximately 10% of this amount of nitrogen is converted to HN03. Most is released as N2, see NBS report 1119, tech file C-4, August 6 1951. Then the only source of nitrogen that needs to be considered is the nitrogen in the bomb atmosphere. The amount of nitrogen in the bomb, when benzoic acid is combusted is essentially the same as when coal samples are combusted. Hence, in order to convert calorimetry results to boiler combustion conditions (NOx), essentially all of the nitrogen converted to HN03 can be measured by the titration of the bomb washings from the combustion ofbenzoic acid.

In 1994, the task group considered using an average correction for the nitric acid. This did not work because the titer correction varies with the temperature rise or the energy released in the combustion process. The amount of nitrogen in the calibration process and in the test process, are the same at the same energy release.

Then the development of a regression line based on the titer results on varying amounts ofbenzoic acid was considered. It was decided to compare the data calculated for BTUs using the traditional approach and titer calculated for a regression line. This was a turning point because the only variable in the results would be the titer correction. This appeared to work well and quickly showed that the calculated titer based on the regression line seemed to work but there was a difference (sometimes positive and sometimes negative) between the traditional approach and the new calculated titer results.

It was hypothesized that the pH of the combustion ash may be affecting the as measured titer results used in the traditional approach. The calculated titer approach is based on benzoic acid, which has no combustion ash. Forrest Walker investigated this and his experiments showed that different coals produced ashes of different pHs and the variations affected the methyl orange endpoint. The differences could be positive or negative depending upon the ash.

Also Bruce MacDonald showed that the amount of the difference between (whether positive or negative) between the traditional and calculated approach is correlated to the amount of titer used to reach an endpoint when just the coal's ash is titrated or in other words the greater the "ash effect" the greater the difference between the traditional and the calculated titer method. Consequently the traditional method would be susceptible to the effects of the coal ash on the pH of the titer after combustion, where as the calculated titer, which is based on the pH of the combusted benzoic acid (no ash) would solely reflect the formation of nitric acid.

The acidity measured in the bomb washings from the coal and coke test samples (traditional method) includes the acidity of sulfuric acid formed from the sulfur in the material, whereas the acidity of the bomb washings of benzoic acid (calculated method) is solely from nitric acid formed from the nitrogen in the air when the vessel is assembled, and the nitrogen contained in the oxygen used to charge the bomb to the appropriate atmospheres. Consequently the sulfur correction for the calculated method is correspondingly different than the sulfur correction used in the traditional method.