MEASUREMENT OF MERCURY IN FLUEGAS

Abstract

To determine the sum of all mercury species in flue gas by continuous emission monitors (CEM) different methods have been developed. Widely used is the condensation of flue gas and subsequent reduction of Mercury containing compounds into elemental Mercury, followed by measurement of gaseous Mercury at 254 nm. The reduction is performed in a wet chemistry system, by solutions of Stannous chloride or ascorbic acid and waste water is produced. Moreover higher sulphur dioxide concentrations in flue gas may increase the absorption at 254 nm and cause incorrect high Mercury readings. 

To overcome the problems caused by cross sensitivity towards sulphur dioxide and by wet chemistry we have developed the novel Hg-CEM analyser. In this instrument Mercury compounds are decomposed by a catalyst at elevated temperature. The resulting gas containing elemental Mercury flows over an amalgamation trap, from which Mercury is desorbed by nitrogen in regular intervals. The concentration of Mercury in the gas is measured at 254 nm again.

The HgCEM instrument was extensively tested by German authorities and TUV-approval according to German and European regulations has been completed. In this paper we report our test measurements with the HgCEM at a waste incinerator. Long term operation experience with the instrument is reported.

Introduction 

Despite efforts to avoid the use of Mercury in consumer products in Europe, low concentrations of the element and its compounds are emitted in the flue gas of many industrial high temperature processes. The main sources in Europe (1995) are coal combustion in power plants (89.2 t/a), residential heating (89.3 t/a), oil combustion (7 t/a), waste incineration (9.7 t/a), cement production (26.3 t/a), Lead and Zinc production (15.4 t/a), Caustic Soda production (41,3 to/a) and Pig-Iron production (10.21 t/a)1.

Depending on flue gas compositions various distributions of elemental Mercury (Hg0) and Mercuric Chloride (HgCl2) have been reported by Hall and co-workers2. First generation Mercury CEM’s used wet chemical reactions to convert Mercuric Chloride to elemental Mercury and subsequent measurement at 245 nm. Commonly used reagents to accomplish the reduction were Stannous Chloride, Sodium Tetrahydroborate and Ascorbic Acid. Due to
the presence of reactors and tubings associated with wet chemistry the first generation Mercury CEMS suffered problems related to tubing pluggage, corrosion and inefficient conversion of mercuric chloride to mercury.

The continuous monitoring of Mercury emissions at German waste incinerators is required by the 17th BImSchV (Federal Imission Control Ordinance) of 19993. Continuous monitoring is under discussion for combustion processes, which substitute fuel by using mercury containing wastes, such as the co-incineration of sewage sludge in coal-fired power plants or in cement kilns. Here we report on the development and testing of a novel Continuous Emission Monitor for Mercury (Hg-CEM) in flue gas of a waste incinerator.