Opportunities for reducing mercury emissions in the cement industry

Abstract

The cement industry is a major source of mercury emissions to the atmosphere. In order to reduce mercury emissions and mercury content in cement industry products, it is necessary to achieve a thorough understanding of its distribution in all stages of the process. Cement production can be divided into two main stages: clinker burning and grinding. These underwent a two-year balance investigation. The main raw material used in the clinker burning process is limestone, in which mercury concentration varied from 16.2 to 33.4 μg/kg. Heat for the clinker burning process is obtained from solid fuels: fine coal, containing from 30.3 to 44.0 μg Hg/kg, alternative fuels – from 170.6 to 408.4 μg Hg/kg and rubber waste, containing from 60.2 to 68.9 μg Hg/kg. The major additive used in both clinker and cement production is gypsum. Concentration of mercury varied widely depending on the supplier, ranging from 199.6 to 467.1 μg/kg. Ferrous additives were the raw material with highest mercury content – 3695.4 μg Hg/kg on average. Average mercury concentration in products was as follows: clinker – 4.0 μg/kg, furnace cement – 12.0 μg/kg, portland cement – 10.1 μg/kg. Performed mass balances enabled determination of mercury flows in production processes: limestone with ferrous additive – 119.4 mg Hg/h, rotary kiln-produced clinker produced – 0.21 mg Hg/h, furnace cement – 22.2 mg Hg/h, portland cement – 12.5 mg Hg/h. Indexes of mercury emissions to air were also determined for production processes: clinker burning – 8.952 mg Hg/Mg of clinker, furnace cement production – 4.085 mg Hg/Mg of cement and portland cement – 8.561 mg Hg/Mg of cement. Average mercury content in flue gases was 5.82 μg/m3, with a variability range of 0.4–81.5 μg/m3, including approximately 20% of oxidized mercury Hg2+. Applying additional mercury emission reduction methods, i.e. adsorption on activated charcoal and SCR or SNCR will enable a reduction of emissions to 4.924, 2.247 and 4.709 mg Hg/Mg respectively and average mercury concentration in flue gas to approximately 2.5 μg/m3.