Continuous measurement of mercury emissions in combustion plants (Overview and new measuring method)

Introduction

Continuous measuring devices have been used for continuous recording of Hg emissions in waste incineration plants since the early Nineties. Operators of combustion plants must maintain a 24-hour measured value of 30 μg/m³ mercury and a half-hourly average value of 50 μg/m³ in accordance with the 13th and 17th BImschV. In order to meet these requirements, measuring devices used for the measuring ranges 0 – 45 μg/m³ and 0 – 75 μg/m³ must be suitability tested and certified. Apart from waste incineration plants, more and more cement plants and power plants that use substitute fuels for example, fall within the scope of this Directive.

Sample gas conditioning, reduction and existing measurement technology

Existing measurement technology is based on extractive gas sampling, conversion, possibly amalgamation and UV measurement (Fig. 1). Sample gas conditioning plays a special role for mercury measurement. The reason for this is that UV analyzers with atomic absorption spectroscopy usually used can only detect metallic mercury. The flue gas does however not only contain metallic mercury but also oxidized mercury compounds, mainly mercury chloride (HgCl, HgCl2). These compounds cannot be detected directly and must be reduced to metallic mercury through appropriate measures in sample gas conditioning. The total mercury in the flue gas can first be recorded after this reduction. 

Various methods are used for mercury chloride reduction (Fig. 2). One method uses the classic wet chemical reduction, for example, with a tin chloride solution, as also used in laboratories for reference measurements. Another method being used more and more over the past years is the so-called dry reduction. Dry reduction uses converters in gas sampling which convert the oxidized mercury compounds to metallic mercury either at low temperatures (approximately 250° C) or high temperatures (approximately 700° C). The methods used each have advantages and disadvantages, and are rated by operators very differently regarding operating, efficiency and reliability.