Since there were limited reports concerned with the synergic effects of H2O, SO2, and HCl on mercury and arsenic speciation under oxy-fuel combustion, this paper utilized the results of the thermodynamic equilibrium calculation with FactSage 5.2 to predict the speciation of mercury and arsenic under oxy-coal combustion. Results showed that the percentages of HgCl2 and HgS were higher under oxy-coal combustion atmosphere than those under air-coal combustion atmosphere within the entire range of temperature. It also indicated that H2O(g) inhibited the generation of HgCl2 and HgS and that the mole percentage of HgCl2 was increased by 1 or 2 orders of magnitude, with the concentration of HCl increased by 5 times or 10 times under oxy-coal atmosphere. Arsenic, As2, and AsN are three dominant arsenic species from 900 to 1400 °C under both air- and oxy-coal combustion atmosphere. Besides, the effects of H2O(g) on arsenic distribution was related to the H2O(g) concentration in the flue gas. These results are important for mercury and arsenic control during the oxy-fuel combustion process.
The in-flight mercury removal performance of ammonium bromide impregnated activated carbon (NH4Br-AC) was evaluated in an entrained flow reactor (EFR) under simulated flue gas. The factors that affect in-flight mercury removal efficiency were explored. The optimum operating parameters were selected to be verified in the EFR under real flue gas, which was derived from the anthracite combustion in a 6 kW circulating fluidized bed (CFB) combustor. The coeffect of NH4Br-AC injection on SO2 and NO emission was also investigated. The results show that the in-flight mercury removal rate of raw activated carbon (R-AC) is significantly improved by the NH4Br modification. Greater sorbent feed rate, longer sorbent residence time, and smaller sorbent particle size are beneficial for improving the in-flight mercury removal rate. In the anthracite combustion flue gas, with the increase of sorbent residence time from 0.59 to 1.79 s, the in-flight mercury removal rate of NH4Br-AC increases from 70.7% to 90.5%. Although the physisorption strengths of SO2 and NO are greater than that of gas-phase mercury, the increase of the Br group on the NH4Br-AC surface improves the mercury adsorption affinity. The reduction rates of SO2 and NO reach 30.6% and 38%, respectively, but the SO3 concentration in the flue gas increases 116% compared to the original emission concentration. The reduction of SO2 and NO in the flue gas is attributed to the chemisorption on the NH4Br-AC surface and the oxidation by the injected O2 existing in the sorbent carrier gas, which promotes more SO3 and NO2 generation in flue gas.
Mn/TiO2 (MT), Ce/TiO2 (CT), and Ce–Mn/TiO2 (CMT) for mercury removal were prepared by an impregnation method, and mercury adsorption tests were conducted in a fixed-bed reactor. Regeneration experiments were carried out in a thermal regeneration reactor, and the effects of temperature and atmosphere on the regenerability were investigated. Surface physicochemical characteristics of fresh, spent, and regenerated CMT were analyzed by means of N2 adsorption–desorption methods, scanning electron microscopy, and X-ray photoelectron spectroscopy. The results showed that CMT had a higher resistance to SO2 poisoning than MT and CT and maintained a high mercury removal capability within a wide range of SO2 concentrations. Optimal thermal regenerability of spent CMT was obtained after thermal desorption at 400 °C followed by N2 + 50% O2 for 2 h. Ten cycles of mercury adsorption–regeneration demonstrated that there was no significant change in mercury removal capacity relative to fresh catalytic sorbent after multiple regeneration cycles. The regeneration of CMT was mainly attributed to the decomposition of mercury compounds and the restoration of Mn4+, Ce4+, and the chemisorbed oxygen on the catalytic sorbent surface. The procedure for the centralized control of mercury emissions from the flue gas by CMT was also analyzed for industrial application.
Particle matter (PM) emitted from coal combustion can cause great harm to human health and the environment. The existence of hazardous trace element arsenic (As) in PM intensifies its toxicity. In this work, four particle sizes of fly ash PM (PM<1, PM1–2.5, PM2.5–10, PM>10) before the electrostatic precipitator (ESP) in a 600 MW coal-fired power plant were sampled by a Dekati low pressure impactor (DLPI). The coal sample, bottom ash, and ash from ESP (ESP ash) were collected simultaneously. Concentrations of total and valent As (As3+ and As5+) in the sample were determined by the inductively coupled plasma-mass spectrometry (ICP-MS) and high performance liquid chromatography (HPLC) coupled with ICP-MS, respectively. The morphological structure and chemical components of the PM surface were characterized by scanning electron microscopy (SEM) and X-ray fluorescence spectrometry (XRF). Results show that the coal used for the power plant is low-sulfur and low-chlorine bituminous coal with 3.851 mg/kg of As. Total As content in the ESP ash is 11.44 times of that in the bottom ash (only 1.637 mg/kg). Arsenic is prone to enrich in ESP ash while dissipates in bottom ash. With the fly ash particle diameter decreasing, the concentration of total As increases from 9.599 mg/kg to 20.088 mg/kg, and the corresponding relative enrichment index increases from 0.68 to 1.42. The main As form in coal-fired fly ash is As5+, which occupies 90.98–98.63% of the total As amount. Toxic As3+ has a little higher ratio in bottom ash with a value of 21.01%. More As physical adsorption and chemisorption active sites on the smaller fly ash PM with a higher specific area contribute to an increase in the concentration of total As and As5+ as the particle sizes decrease. Moreover, the transformation mechanism of As to PM during coal combustion is discussed.
In this article, four kinetic models including intraparticle diffusion, pseudo-first-order, pseudo-second-order, and Elovich kinetic models were applied to explore the internal mechanism of mercury adsorption by activated carbon in wet oxy-fuel conditions. Results indicated that the pseudo-second-order model and Elovich kinetic model could accurately describe the adsorption process, which meant that chemical adsorption played an important role in the adsorption of mercury by activated carbon. The intraparticle diffusion model indicated that internal diffusion was not the only step to control the entire adsorption process and did not have an inhibition effect on mercury removal. Meanwhile, the external mass transfer process is more effective in controlling the mercury adsorption process of activated carbon according to the fitting result of the pseudo-first-order model. According to the obtained kinetic parameters, the intraparticle diffusion rate was improved with the increasing bed height. In addition, the higher temperature inhibited the external mass transfer, which was not conducive to the adsorption of mercury by activated carbon in wet oxy-fuel conditions.
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