BACKGROUND: Mercury emission from coal-fired power plants has caused widespread concern. Halogen is considered to be a good mercury oxidant and many researchers have added it into the coal for mercury removal. However, the mechanism of mercury transformation is still not clear. Effects of NH4Br additive on mercury transformation and removal were studied on a 6 kW(th) circulating fluidized bed (CFB) coal combustor. The main goal of this work is to explore in detail the mercury transformation mechanism in the presence of bromine. RESULTS: Results showed that the concentration of Hg-0 and Hg2+ in the flue gas was 2.003 and 2.434 mu g m(-3), respectively, while Hg-p was 14.866 mu g m(-3) during the raw coal combustion. By adding more NH4Br, the content of Hg-0 and Hg2+ decreased while the content of Hg-p increased. At an NH4Br addition amount of 0.3%, Hg-0 transformation rate and Hg-g removal rate reached maximum values of 78.85% and 76.83%, respectively. Some mercury compounds (such as HgS(red), HgO, HgSO3) were further oxidized by active bromine with NH4Br addition. CONCLUSION: The added NH4Br generates active bromine in the furnace, which promotes both homogeneous and heterogeneous oxidation of mercury. Adding NH4Br into coal benefits mercury removal during coal combustion. (C) 2016 Society of Chemical Industry