Reducing Mercury Emissions from Small-Scale Coal-Fired Boilers Used in Residential Heating

Abstract

Aim: The aim of the research was to demonstrate that the use of low-emission carbon fuels (obtained using the initial thermal conversion of coal feedstock) in residential heating also makes it possible to reduce mercury emissions from small-scale coal-fired boilers. Project and methods: The publication presents the results of mercury emission tests conducted using five different small-scale coal-fired boilers and five different coal fuels. The research was carried out under laboratory conditions, but also using heating devices of residential users. They covered a wide range of operational parameters, both energy and emission. The flux of coal fuels burned ranged from 2 to 12.2 kg/h, with an equally wide range of boiler efficiencies obtained – 67.6–88.5%. Results: The test results presented in the article show that the amount of emissions of pollutants limited by the criteria of the PN-EN 303-5+A1:2023-05 standard and the ecodesign, namely carbon monoxide, nitrogen oxides, dust and organic substances, depends mainly on the design and operating conditions of the device in which the specific solid fuel is burned. There is a group of pollutants emitted into the atmosphere, for which the amount of emissions depends primarily on the quality of the fuel burned. These pollutants include sulphur oxides and mercury, whose emissions depend primarily on the combustible sulphur and mercury content of the fuel being burned. Conclusions: Experimental studies were carried out to verify what portion of Hg contained in coal during its combustion in domestic boilers with manual and automatic fuel feeding remains bound in bottom ash, and what is emitted into the atmosphere. The content of Hg in bottom ash, regardless of the boiler and fuel used, was at a similar low level, reaching a maximum of about 6% of Hg initially contained in the fuel. As studies have shown, more than 94% of mercury contained in coal fuels burned in small-scale coal-fired boilers is emitted into the atmosphere, contributing significantly to the deterioration of the environment. Replacing traditional coal with low-emission carbon fuels (e.g., such as BC fuel) would significantly reduce mercury emissions from small-scale coalfired boilers, by up to 90% compared to current emissions. Setting a legal requirement for the permissible level of mercury content in coal fuels used in domestic boilers, for example, at a maximum value of 0.05 mg/kg, would reduce mercury emissions from these devices by at least half. Keywords: mercury emission, small-scale coal-fired boilers, low-emission carbon fuel