Large-Scale Mercury Control Technology Testing for Lignite-Fired Utilities - Oxidation Systems for Wet FGD

Abstract

Mercury (Hg) control technologies were evaluated at Minnkota Power Cooperative's Milton R. Young (MRY) Station Unit 2, a 450-MW lignite-fired cyclone unit near Center, North Dakota, and TXU Energy's Monticello Steam Electric Station (MoSES) Unit 3, a 793-MW lignite-Powder River Basin (PRB) subbituminous coal-fired unit near Mt. Pleasant, Texas. A cold-side electrostatic precipitator (ESP) and wet flue gas desulfurization (FGD) scrubber are used at MRY and MoSES for controlling particulate and sulfur dioxide (SO 2) emissions, respectively. Several approaches for significantly and cost-effectively oxidizing elemental mercury (Hg 0) in lignite combustion flue gases, followed by capture in an ESP and/or FGD scrubber were evaluated. The project team involved in performing the technical aspects of the project included Babcock & Wilcox, the Energy & Environmental Research Center (EERC), the Electric Power Research Institute, and URS Corporation. Calcium bromide (CaBr 2), calcium chloride (CaCl 2), magnesium chloride (MgCl 2), and a proprietary sorbent enhancement additive (SEA), hereafter referred to as SEA2, were added to the lignite feeds to enhance Hg capture in the ESP and/or wet FGD. In addition, powdered activated carbon (PAC) was injected upstream of the ESP at MRY Unit 2. The work involved establishing Hg concentrations and removal rates across existing ESP and FGD units, determining costs associated with a given Hg removal efficiency, quantifying the balance-of-plant impacts of the control technologies, and facilitating technology commercialization. The primary project goal was to achieve ESP-FGD Hg removal efficiencies of >55% at MRY and MoSES for about a month. Hg in the lignite coal fired in MRY Unit 2 varied from 0.05 to 0.25 ppm and averaged 0.112 ±0.014 ppm (dry coal basis). Most of the Hg was associated with Hg-rich (2.28 ppm) pyrite grains that ranged in concentration from about 1 to 6 wt% (on a dry coal basis). During routine power plant operations, total Hg concentrations at the ESP and FGD inlets varied from about 12 to 16 µg/dNm 3 , whereas at the stack, concentrations were consistently at approximately 13 µg/dNm 3 , indicating that the ESP and FGD were very inefficient at removing Hg primarily because Hg 0 was dominant. MgCl 2 and CaCl 2 injections were relatively ineffective in promoting Hg 0 oxidation and capture in an ESP-FGD. Relatively low additions of SEA2, however, significantly improved the Hg removal efficiency of the ESP-FGD, although the goal of 55% Hg removal was not achieved using as much as 75 ppm SEA2 (dry coal basis). Most of the Hg removal occurred in the ESP, suggesting that SEA2 addition promoted the conversion of Hg 0 to particulate-bound Hg. SEA2 addition (50-100 ppm, dry coal basis) combined with 0.15-lb/Macf PAC injection was performed for a month during which Hg removal efficiencies generally ranged from 50% to 65%. The mobility of Hg in MRY fly ashes sampled before and after PAC and SEA2 injections was evaluated using a synthetic groundwater leaching procedure augmented wit...