The impacts of a low temperature economizer (LTE) on mercury removal across an electrostatic precipitator and influence of load variation on mercury conversion over selective catalytic reduction (SCR) catalysts were determined at two coal-fired boilers. When the LTE was on, the total and elemental mercury removal efficiency increased by 42.87% and 18.85%, respectively, due to the improvement of adsorption and oxidation capacity of the fly ash at lower temperature. Mercury speciation at the inlet and outlet of the SCR system were analyzed, and the impacts of load variation and catalyst aging on Hg-0 conversion were discussed. The variable loads resulted in simultaneous changes of the gas hourly space velocity, the ambient temperature, and the oxygen content. The results showed the load ratio was significant for Hg-0 conversion by the SCR catalysts and load reduction benefitted Hg-0 conversion. When the load ratios were 100%, 75% and 60%, the Hg-0 conversion were 61.78%, 65.71% and 72.12%, respectively. Moreover, Hg conversion was more significantly affected by the catalyst aging than NOx reduction. Among the three factors, the most important one is the flue gas temperature based on the grey relational analysis. (C) 2014 Elsevier B.V. All rights reserved.