Abstract
ABSTRACT Micro-nanobubbles can spontaneously generate hydroxyl free radicals (OH). Urea is a cheap reductant and can react with NOx species, and their products are nontoxic and harmless N2, CO2 and H2O. In this study, a Wet Direct Recycling Micro-nanobubble Flue Gas Multi-pollutants Removal System (WDRMRS) was developed for the simultaneous removal of NO, SO2 and Hg0. In this system, a micro-nanobubble generator (MNBG) was used to produce a micro-nanobubble gas–liquid dispersion system (MNBGLS) through recycling the urea solution from the reactor and the simulated flue gas composed of N2, NO, SO2 and Hg0. The MNBGLS, which has a large gas–liquid dispersion interface, was recycled continuously from the MNBG to the reactor, thus achieving cyclic absorption of various pollutants. All of the investigated parameters, including the initial pH and temperature of the absorbent as well as the concentrations of urea, NO and SO2 had significant effects on the NO removal efficiency but did not significantly affect the SO2 removal efficiency, whereas only the initial solution pH and NO concentration affected the Hg0 removal efficiency. The analysis results of the reaction mechanism showed that ·OH played a critical role in the removal of various pollutants. After the treatment by this system, the main removal products were Hg0 sediment, and NH4+ which could be easily recycled. The use of this system (MNBGLS) for the simultaneous removal of NO, SO2 and Hg0 is a new technology application and research. Recycling process based on MNBGLS succeeded in simultaneously removing NO, SO2 and Hg0. The system (MNBGLS) can provide a reference for commercial applications. The removal products are relatively simple and beneficial to recycling, which can reduce the cost of waste gas treatment.
Highlights
Nitrogen oxides (NOx) and sulfur dioxide (SO2), the major air pollutants, are the main precursors for acid rain and photochemical smog, and mercury (Hg), which is considered to be one of the most toxic heavy metals, has received increasing attention because it has persistence, bio-accumulation and neurological toxicity (Wei et al, 2009; Fang et al, 2011; Zheng et al, 2007; Rallo et al, 2012; Ye et al, 2014; Xie et al, 2013)
Liu et al (2015) conducted the simultaneous removal of nitric oxide (NO), SO2 and Hg0 by using a Fenton-like reagent based on Fe3+ in a spray reactor, and the results showed that · OH
No matter whether urea was added or not, the SO2 removal efficiency was greater than 99.6% and the Hg0 removal efficiency was maintained at approximately 86%
Summary
Nitrogen oxides (NOx) and sulfur dioxide (SO2), the major air pollutants, are the main precursors for acid rain and photochemical smog, and mercury (Hg), which is considered to be one of the most toxic heavy metals, has received increasing attention because it has persistence, bio-accumulation and neurological toxicity (Wei et al, 2009; Fang et al, 2011; Zheng et al, 2007; Rallo et al, 2012; Ye et al, 2014; Xie et al, 2013). As a mature method for NOx, SO2 and Hg removal in flue gas, the combined system of selective catalytic reduction (SCR) (Krishna and Makkee, 2005; Wu et al, 2008), wet flue gas desulfurization (WFGD) (Sharma et al, 2012) and activated carbon injection (ACI) (Rallo et al, 2012; Yang et al, 2007) has been widely applied This system can achieve the deep removal of NOx, SO2 and Hg0, it still has some disadvantages such as the high cost of construction and operation, high occupational area, high complexity, low stability, ammonia leakage, and secondary pollution (Wang et al, 2019; Guo et al, 2012). The wet liquid phase oxidation-absorption process which can oxidize and absorb NOx, SO2 and Hg0 in one-step, is the main direction of the future research on the integrated removal of multi-pollutants from the flue gas (Wang et al, 2019)
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