Effect of SCR operation variables on mercury speciation Manuela Rallo a,⇑ , Barna Heidel b , Kevin Brechtel b , M. Mercedes Maroto-Valer a a Energy and Sustainability Research Division, Faculty of Engineering, University Park, University of Nottingham, Nottingham NG7 2RD, United Kingdom b Universität Stuttgart, Institute of Combustion and Power Plant Technology – IFK, Department Fuels and Flue Gas Cleaning, Pfaffenwaldring 23, 70569 Stuttgart, Germany highlights " We investigated the SCR effects in terms of Hg oxidation. " We investigated the impacts of acid gases on Hg oxidation in a SCR system. " We identified several Hg species in by-products by means of a novel methodology. " Increasing information on Hg retention in by-products was achieved. article info Article history: Received 10 February 2012 Received in revised form 18 May 2012 Accepted 22 May 2012 Available online 29 May 2012 Keywords: SCR Mercury speciation SO 2 oxidation Coal combustion Ash abstract It is important to understand how existing air pollution control devices can remove mercury as co-benefit. This paper presents the results of a test programme at a 20 kW th PF pilot scale unit and a lab scale micro- reactor to investigate the catalyst performance on the concentration and speciation of mercury in coal combustion flue gas. The SCR temperature, the ammonia dosing rate and the SO 2 concentration were varied; increasing the temperature the mercury oxidation rate decreased, being the optimal temperature for a high mercury oxidation rate between 300 and 350 °C. The oxidation of SO 2 , as undesired side reaction, is strongly dependent on temperature. The optimal temperature range for low SO 3 concentrations down- stream of the SCR is below 360 °C, while the reduction of NO x by NH 3 is faster for higher temperatures. A higher ammonia dosing rate leads to a high loading of the catalysts active sites and reduced mercury oxidation rate. Increasing the SO 2 concentrations only slightly enhanced the mercury oxidation rate due to the higher acid loading of the catalyst, which promoted the DeNO x -reaction and made more active sites of the catalyst available for the mercury oxidation. Increasing SO 2 concentration at the inlet of the SCR leads to higher concentrations of SO 3 downstream of the SCR, while the relative oxidation rate of SO 2 is lower at higher SO 2 concentrations. Ashes were characterized by X-ray Fluorescence and Thermal Decomposition– Atomic Fluorescence Spectrometry to investigate mercury content and occurrence, respectively. Measur- able mercury release began at approximately 200 °C. It was also found that the ash samples showed the occurrence of only insoluble/partially soluble mercury. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Coal-fired power plants are one of the largest sources of mer- cury emissions to the environment [1]. Although the concentration of mercury in coal is low (approximately 0.1 ppm on average), when coal is combusted the combination of high combustion tem- peratures and the volatility of mercury causes the presence of ele- mental mercury vapor in the flue gas. This vapor may be oxidized via homogeneous (gas–gas) and heterogeneous (gas–solid) reac- tions. Hence, during coal combustion mercury can be present in different oxidation states, (Hg 0 ) elemental, (Hg 2+ ) oxidized and (Hg p ) particulate-bound mercury. While elemental mercury is volatile, relatively inert and virtually insoluble, oxidized mercury is water-soluble and easily captured onto ash. Although several mercury-specific control technologies are currently under develop- ment, initial mercury emission reductions can come as a co-benefit of exiting controls used to remove PM, SO 2 and NO x [2]. The degree of the mercury removal co-benefit will vary significantly depend- ing on the type of coal burned and the specific control-technology configuration [3]. Many factors affecting the oxidation processes in coal combustion, including coal chlorine content, gas temperature, surface reaction with ash and unburned carbon, and plant operat- ing conditions. Modern power plants are typically equipped with air pollution control devices (APCDs), such as staged combustion burner 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.05.080 ⇑ Corresponding author. Tel.: +44 0115 951 4198; fax: +44 0115 951 4115. E-mail address: Manuela.Rallo@nottingham.ac.uk (M. Rallo). Chemical Engineering Journal 198–199 (2012) 87–94 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej