Mercury capture by boiler modifications with sub-bituminous coals

Abstract

The extent of mercury (Hg) emissions at a particular coal-fired boiler is unit-specific and determined by factors that include the type of coal fired, boiler operating conditions, and the amount of Hg removed at the boiler back-end by the pollution control devices. Mercury speciation, adsorption and removal in the boiler are affected by parameters such as the flue gas and fly ash characteristics, and back-end boiler configuration/operation. Various studies have reported that boiler operating conditions can be optimized to enhance the “naturally” occurring Hg capture in coal-fired boilers.A study was carried out to investigate the impact of boiler operating conditions on Hg emissions at a full-scale coal-fired power plant. Testing was performed at a 810MW coal-fired power plant firing a Northern Powder River Basin (PRB) coal, and equipped with a low-NOx system and a wet flue gas desulfurization (FGD) system for particulate and sulfur dioxide (SO2) control. Speciated (total and elemental, Hg0) Hg measurements and analytical procedures were performed according to the Ontario Hydro Method (OHM)/ASTM D6784-02 Method. OHM measurements were conducted at the air preheater (APH) inlet and the stack. Mercury emissions were also continuously monitored using single-point, non-isokinetic sampling with semi-continuous emissions monitors (SCEM). Readings from these analyzers were used for guidance during boiler control setting manipulations. Individual semi-continuous sampling modules were installed at four single-point locations along the convective pass: APH inlet, FGD inlet, FGD outlet and the stack.The test results obtained in this study showed the benefit of boiler modifications for Hg control and the extent to which the “natural” Hg control could be influenced. Parametric testing indicated a modest, but inconsistent improvement in the stack Hg (HgT) emissions at reduced excess oxygen. This was probably due to the fact that the fly ash unburned carbon levels were very low (less than 1.0%). Modifications to the low-NOx system over-fire registers, burner tilt and FGD liquid-to-gas (L/G) ratio resulted in stack HgT reductions in excess of 25%. The FGD L/G ratio manipulations also resulted in an improvement in oxidized mercury removal across the FGD. However, none of the test data suggested any reduction in Hg0 across the FGD. The combination of optimal boiler control settings for reduced Hg emission operation resulted in a 34.5% reduction in HgT at the stack.