Speciation Characteristics and Mobility of Trace Elements Across Ultralow Emission Air Pollution Control Devices

Abstract

In this study, to study the speciation characteristics and mobility of trace elements (As, Se, Cd, Cr, and Pb) across ultralow emission air pollution control devices, bottom ash, fly ash, gypsum, waste sludge, and wet electrostatic precipitator (WESP) slag (as combustion byproducts) as well as coal were sampled from five ultralow emission coal-fired power plants. Results indicated that most of trace elements partitioned to ash and the mobility of trace elements decreased during this process. 37.1%−99.9% of trace elements were bonded in Si−Al−Fe lattice as residual fraction in ash. In wet flue gas desulfurization (WFGD) system, the trace elements in gypsum and waste sludge showed a distinctively higher mobility than in ash. The finer particles captured by WESP had the highest concentrations of trace elements with increased mobility of Se, Cd and Pb. The acid soluble fraction of Cd was very high in gypsum and waste sludge, but the leaching ability was below average level because of the very low absolute concentrations. The potential leaching amount per ton of combustion byproducts considering acid soluble fraction was in the range of 0−7.56g/t (As), 0−5.46g/t (Se), 0−1.77g/t (Cd), 0.028−82.7 g/t (Cr), and 0.012−13.6 g/t (Pb), respectively.

Migration Behaviors of As, Se and Pb in Ultra-Low-Emission Coal-Fired Units and Effect of Co-Firing Sewage Sludge in CFB Boilers

“…ESP and FF are the two main types of particulate control devices installed in coalfired power units to capture particulates from flue gas, which can influence the migration behaviors of the gaseous trace elements and remove particulate trace elements in the flue gas. Previous studies [26,29] confirmed that the synergistic removal efficiency of As, Se and Pb in flue gas by particulate control devices of ultra-low-emission units is over 90%.…”

“…As, Se and Pb compounds connected by weak bonds in coal are oxidized and decomposed in a CFB at a combustion temperature between 800 • C and 950 • C, while As, Se and Pb compounds with high thermal stability can be decomposed and released in the PC boiler with a combustion temperature above 1300 • C. Liu et al [44] found that the volatilization ratio of As in coal was between 17% and 60% when the combustion temperature was 900 • C, and rose to 90% when the combustion temperature was up to 1500 • C. The calculation result of thermodynamic equilibrium by Contreras et al [45] indicated that Se in coal is mainly volatilized to the gas phase SeO 2 at 850 • C. Zhang et al [46] found that Pb in coal usually exists in galena and is mainly released in the form of PbCl 2 or PbS. As a result, the release of Pb can be enhanced with the decomposition of galena within the temperature range between 800 • C and 1200 • C. In addition, fly ash contains a large percentage of metal oxides, such as Fe 2 O 3 and CaO [20,21,26], providing enough absorbability to react with the As and Se existing in the flue gas to form arsenate and selenate compounds (Equations ( 1)-( 4)) [47].…”

“…Although the As, Se and Pb concentrations were extremely low in the liquid fraction, as shown in Figure 4, the mass proportion of the liquid fraction accounted for 86.16-90.98% of the total slurry, so there was still a considerable proportion of As, Se and Pb that migrated to the liquid fraction. Accompanied by a drop in flue gas temperature, Pb in the flue gas entering the WFGD unit is mainly in granular form, which is enriched in fine fly ash particles [26,32]. The fine particles are mainly generated by the fine fly ash that escapes from the particulate control devices.…”

“…Despite the fact that different methods, such as the float-sink test, statistical analysis and direct test analysis, have been suggested to determine the speciation of trace elements, they are hardly in good agreement, mainly due to the complicated chemical structure of coal together with extremely low concentrations of As, Se and Pb. Compared with other methods, the sequential chemical extraction (SCE) procedure, originally developed by Tessier et al [23], was improved and is regarded as a reliable method to determine different kinds of trace elements in coal and its combustion by-products [24][25][26]. This method is dependent on a specific extractant to extract the fraction of elements in samples sequentially according to their differences in solubility.…”

Study on collection of the particles in waste gasification gas by wet electrostatic precipitator

“…The basic working principle of wet ESP includes particle charged, charged particle migration and trapping, removing the dust on the surface of collecting plate [8][9][10]. The details are as follows: (1) The high voltage electric field area in the wet ESP are full of small droplets generating from the centrifugal atomizing nozzle; (2) Then the atomizing droplets will be charged on the condition that corona discharge is produced when the electric field is added an DC high voltage.…”