Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Developing steady and dynamic ORP models for mercury emissions control in power plants using WFGD operating data You Lv a, ⁎ , Carlos E. Romero b , Joshua Charles b , Kayla A. Pauvlinch c , Robert W. Watkins c a State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China b Energy Research Center, Lehigh University, Bethlehem, PA 18015, USA c W. H. Sammis Power Plant, Stratton, OH 43961, USA ARTICLE INFO Keywords: Mercury re-emission Oxidation-reduction potential Wet flue gas desulfurization Coal-fired power plants Least squares support vector machine ABSTRACT The oxidation-reduction potential (ORP) is highly related to Hg re-emission in wet flue gas desulfurization (WFGD) systems in coal-fired power plants. Developing an accurate model that describes the ORP characteristics is beneficial to achieve the operation optimization of the WFGD system and Hg re-emission reduction. In this paper, steady and dynamic ORP models were reported based on the least squares support vector machine (LSSVM) technique and using operating data acquired from a coal-fired power plant. For the steady model, input parameters consisted of flue gas flow, flue gas temperature, inlet SO 2 concentration, oxidation air flow, recycle pump currents, and slurry feed flow. For the dynamic model, in addition to the parameters listed above, time delays of these parameters were also considered in the modeling scheme. Both models achieved high accurate predictions. Model sensitivity analysis was conducted to investigate the influence of the input parameters and time delays on ORP. Results show that the parameters are adequate to describe the characteristics of the ORP. In addition, parametric time delays play an important role to represent the dynamic characteristics of the ORP. These models can be extended to design process control strategy and conduct the ORP operation optimization of the WFGD system in power plants to mitigate Hg re-emission from the scrubber. 1. Introduction Mercury (Hg) emissions are an important environmental concern due to its toxicity and accumulation in the environment. Hg, even in low concentrations, can affect human health [1]. Coal-fired power generation is a major Hg pollution source among the utility industry in the US. As a result, the Environmental Protection Agency has promul- gated rules that require coal-fired power plants to reduce Hg emissions [2]. Hg emissions control strategies involve a combination of air pol- lution control device (APCD) co-benefit with process adjustments, dry solid reagent injection, and desulfurization scrubber additives [3–5]. The strategy based on APCD co-benefit with process adjustments for Hg emissions reduction has been proven cost-effective. In this strategy, Hg is processed in the downstream APCDs, such as selective catalytic reduction (SCR) system and the wet flue gas desulfurization (WFGD) system, which have been already installed for nitrogen oxide (NO x ) and sulfur dioxide (SO 2 ) emissions control in coal-fired power plants [6]. Hg in the coal is volatilized after the combustion process, and it pre- dominantly exits the boiler as elemental form (Hg 0 ). Hg 0 is highly vo- latile and insoluble in water, which makes it very difficult to remove from the flue gas [7]. Hg 0 can be converted to oxidized Hg form (Hg 2+ ) in the SCR system and the air preheater (APH) due to the temperature change of the flue gas. Gaseous Hg 2+ that passes to the WFGD system is highly soluble in water and can be effectively removed by the scrubbers [8]. Thus, the WFGD system plays an important role in achieving re- quired Hg emissions reductions in a cost-effective manner. Wet scrub- bing technologies used for Hg emissions reduction have been reported in previous research [9–12]. The WFGD system chemistry is very sensitive to the removal of Hg. The removal process is affected by parameters that include coal prop- erties, boiler load, and WFGD process parameters [13,14]. A portion of the oxidized Hg absorbed into the scrubber liquor can be converted back to Hg 0 , in what is called mercury re-emission. Results obtained from field studies indicate that Hg 2+ captured by the WFGD process can be chemically reduced within the scrubber and re-emitted back to the stack gases as Hg 0 [15]. For this reason, it is important for coal-fired plants to optimize the operation of their WFGD systems to achieve optimal Hg processing while minimizing Hg re-emission. A number of operating parameters have been reported to affect the fate of mercury in WFGD scrubbers. These parameters include pH, https://doi.org/10.1016/j.fuel.2018.07.058 Received 1 May 2018; Received in revised form 27 June 2018; Accepted 14 July 2018 ⁎ Corresponding author. E-mail addresses: you.lv@hotmail.com, you.lv@ncepu.edu.cn (Y. Lv). Fuel 235 (2019) 54–62 0016-2361/ © 2018 Elsevier Ltd. All rights reserved. T