Experimental Evaluation and Field Application of a Salt Method for SO₃ Measurement in Flue Gases

Abstract

In this study, an SO 3 measurement technique was evaluated and developed. In the method, a salt is used to capture gaseous SO 3 /H 2 SO 4 . Various salts were tested to evaluate the suitability to measure SO 3 /H 2 SO 4 without interference from SO 2 . Salts tested include NaCl, KCl, K 2 CO 3 , and CaCl 2 . The salts were tightly packed into a Teflon tube, and the gas was fed through the salt tube with subsequent reaction between SO 3 /H 2 SO 4 and the salt with formation of sulfates of the respective salt. After the measurement, the salt was dissolved in water, and the solution was analyzed for sulfate ions. The SO 3 /H 2 SO 4 concentration in the flue gas could then be determined because the gas volume flowing through the salt was measured together with the amount of sulfate bound in the salt. The method was tested in laboratory conditions, in a 100 kW th test unit during airfiring and oxy-fuel combustion, and in an industrial boiler. A first attempt to continuously measure SO 3 /H 2 SO 4 indirectly with an FTIR, by measuring the release of HCl in the sulfation of KCl, was also made. The conversion of SO 3 to H 2 SO 4 in flue gas conditions is discussed. It was found that at the measurement conditions almost all SO 3 is present as H 2 SO 4 . Therefore, the laboratory study was made with gaseous H 2 SO 4 instead of SO 3 . The laboratory tests showed that all salts captured all H 2 SO 4 . The best selectivity toward H 2 SO 4 was shown for NaCl and KCl; no significant amount of SO 2 was captured in these salts. An in situ implementation of the salt method using KCl as salt was used during heavy oil combustion in a Kraft recovery boiler. The salt method showed to be an accurate, inexpensive, and easy way to measure SO 3 /H 2 SO 4 in flue gases.

Sulfur trioxide formation/emissions in coal‐fired air‐ and oxy‐fuel combustion processes: a review

“…Tests with NaCl and KCl showed comparable results to the CCM. Less convincing results were observed with K 2 CO 3 and CaCl 2 …”

“…Flue gas containing SO 2 and SO 3 passes through the dry layer of NaCl and the sulfur trioxide reacts with NaCl . This method was applied to detect SO 3 concentration in a 100 kW rig at Chalmers University . A schematic of the salt method is shown in Fig.…”

“…A schematic of the salt method is shown in Fig. , where salt is fixed in the tube with glass wool . With a decrease in the gas temperature to approximately 500 °C, SO 3 starts to react with water vapour to form gaseous sulfuric acid, which then reacts with NaCl to form Na 2 SO 4 and NaHSO 4 .…”

“…, where salt is fixed in the tube with glass wool . With a decrease in the gas temperature to approximately 500 °C, SO 3 starts to react with water vapour to form gaseous sulfuric acid, which then reacts with NaCl to form Na 2 SO 4 and NaHSO 4 . The reactions occurring during the salt method are given as: …”

“…In this case, the exit gas temperature should be kept between 200 and 400 ºC. However, at 200 ºC, only about one‐half the SO 3 was converted to HCl . Chamberlain et al .…”