Sensitive determination of ammonia in exhaust gas of thermal power plant using gas permeation/flow injection system

A flow injection analysis coupled with a gas permeation unit for the sensitive determination of ammonia was investigated to improve the sensitivity and the rapidity of the determination of trace amounts of ammonia in exhaust gas. As gas sampling times are shortened, ammonium concentrations in absorption liquid (boric acid solution) will be lowered; the determination sensitivity of ammonia in the absorption liquid needs to be improved. Various experimental conditions, such as reagent solutions, sample sizes, reaction temperatures, flow rates of the reagent solutions and the length of gas permeation tube, were investigated and optimized. Of these, the reaction temperature and the length of gas permeation tube were found to greatly effect the sensitivity improvement for the determination of ammonia. The effect of the length of the microporous PTFE tube used for the gas permeation system was investigated by varying the length from 5 cm to 50 cm. The results showed that is the 50 cm tube, the peak height was about 6 times higher than in the 10 cm tube, and the percentage of the permeation of ammonia was about 55% and 9% in the 50 cm and 10 cm tube, respectively. Under the optimized conditions, the limit of detection of ammonia was lowered down to on fifth of the previous system. The limit of detection corresponding to the standard deviation of the reagent blank of 3 was 4 μg l-1 of NH4+. A calibration graph was linear from 10 μg l-1 to 500 μg l-1 of NH4+. The relative standard deviations for 50 μg l-1 and 100 μg l-1 of NH4+ were 4.5% and 2.3%, respectively. The proposed method was suitably applied to the determination of ammonia in the exhaust gas of the thermal power plant.