Evaluation of sulfur trioxide detection with online isopropanol absorption method

Measurement of the SO₃ concentration in flue gas is important to estimate the acid dew point and to control corrosion of downstream equipment. SO₃ measurement is a difficult question since SO₃ is a highly reactive gas, and its concentration is generally two orders of magnitude lower than the SO₂ concentration. The SO₃ concentration can be measured online by the isopropanol absorption method; however, the reliability of the test results is relatively low. This work aims to find the error sources and to evaluate the extent of influence of each factor on the measurement results. The test results from a SO₃ analyzer showed that the measuring errors are mainly caused by the gas-liquid flow ratio, SO₂ oxidation, and the side reactions of SO₃. The error in the gas sampling rate is generally less than 13%. The isopropanol solution flow rate decreases 3% to 30% due to the volatilization of isopropanol, and accordingly, this will increase the apparent SO₃ concentration. The amount of SO₂ oxidation is linearly related to the SO₂ concentration. The side reactions of SO₃ reduce the selectivity of SO₄^2- to nearly 73%. As sampling temperature increases from 180 to 300°C, the selectivity of SO₄^2- decreases from 73% to 50%. The presence of H₂O in the sample gas helps to reduce the measurement error by inhibiting the volatilization of the isopropanol and weakening side reactions. A formula was established to modify the displayed value, and the measurement error was reduced from 25%-54% to less than 15%.