Selective catalytic reduction of nitrogen oxides (SCR-DeNOx
) has been widely used for the treatment of flue gas streams in coal-fired power plants in China, owing to its superior denitrification efficiency and lower ammonia emissions [1
]. However, when the flue gas contacts the catalyst, the adsorption and desorption of the reactants are affected, the specific surface area of the catalyst is reduced, and thus the number of active catalyst sites derease, leading to the deactivation of the catalyst. SO2
, fly ash, and alkali metals contained in the flue gas may decrease the denitrification capacity of the catalyst. SO3
reacts with NH3
to form (NH4
], and CaSO4
]. These small particles, with a particle size <10 μm [5
], with sticky ammonium sulfate particles, especially ammonium bisulfate, may clog the micropores of the catalyst surface [6
] and stain and corrode the downstream devices of the SCR, such as the air preheater [8
]. The combined presence of metal sites and SOx
may also result in irreversible loss of active sites via metal sulfates formation [10
]. Fly ash particles hitting the catalyst surface can degrade the catalyst. The particles can also block the microporous channels on the catalyst surface [13
], and the deposition of fly ash on the surface of the catalyst may contaminate and shield the catalyst [14
], which may prevent the NOx
, and O2
in the flue gas from reaching the active site of the catalyst [5
]. The alkali metal oxide decreases the number of active sites on the catalyst and poisons the catalyst by combining with the active acid sites of V2
, which also reduces the ammonia adsorption on the catalyst surface, decreasing the denitrification activity of the catalyst [15
Two main technical methods are generally accepted for addressing the problem of denitrification capacity decrease of the SCR-DeNOx catalyst caused by the flue gas. The first involves the use of fresh catalyst in the overall replacement of deactivated catalyst when the denitrification catalyst activity decreases to a certain level. The cost of this method is prohibitive for the boiler users. The second method is returning the deactivated catalyst back to the plant and then reinstalling it in the denitrification reactor. However, the deactivated catalyst must be removed from the boiler’s denitrification reactor. This requires long-distance transport and secondary installation, which not only requires time and effort, but also causes secondary mechanical damage to the catalyst.
With the aim of solving these problems by increasing the activity of the deactivated catalysts, a method of in situ activation of the denitrified catalyst based on the activating solution is proposed in this study. The activating solution was used to activate the deactivated catalyst in the SCR denitrification reactor to effectively load the active ingredients. It in the in-situ activation process, it is important that the activating solution be efficient, cost effective, and does not cause secondary pollution. The acidity of an in situ activating solution cannot be too strong as it may damage the denitrification reactor. The solution should not only result in the recovery of the denitrification catalyst activity, but should be able to be recycled. Several studies were carried out that resulted in important achievements. For example, Li et al. [18
] studied the effect of 1-hydroxy ethylidene-1 and 1-diphosphonic acid solution on the removal of CaWO4
, caused by the high content of CaO, from the catalyst surface to activate the denitrified catalyst. However, this activation method was not able to recover the strong acid sites and supplement the active sites. In addition, the effect of flue gas was not considered, and a higher vanadium content must be maintained to ensure the denitrification efficiency. Dong et al. [19
] investigated the effect of the acidity of the precursor solution on the activity of the denitrified catalyst. The results showed that with increasing acidity of the precursor solution, more vanadium species and active sites would form on the surface of the catalyst. However, the optimum acidity of the activating solution and the vanadium content in the catalyst activity recovery have not been reported. Although these investigations provided valuable information about activating the SCR denitrification catalyst, only a few studies have reported in situ activation of the catalyst, and the activating solution suitable for in situ activation has not been investigated. Not only is there no activating solution suitable for in situ activation, but no research on the effect of the flue gas in in situ activation process exists.
This study aimed to find an efficient and inexpensive activation solution suitable for in situ activation. An in-situ activation modeling device was designed to activate the deactivated catalyst. The denitrification activity of the catalyst was tested using an activity measuring device. First, the effects of the active ingredients, including vanadium, tungsten, molybdenum, and oxalic acid, and the flue gas conditions, including space velocity, oxygen concentration, ammonia to nitrogen ratio, and the initial concentration of NO, on the in-situ activation of the catalyst were studied. In addition, the effects of these different reaction conditions on the deactivated catalysts were analyzed by scanning electron microscopy (SEM), specific surface area analysis (BET), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). Finally, an efficient activating solution for in situ activation was obtained. The in situ activating modeling experiment provides a powerful platform for activating the deactivated catalysts and is broadly applicable to a variety of systems and experimental conditions. We expect that these results can pave the way for the application of the above catalysts to in situ denitrification situations.
In this study, the performance of several activating solutions, during the in situ activation of SCR-DeNOx catalysts, was investigated. Remarkable changes were observed in the deactivated catalysts after the activation process. The texture, morphology, and surface chemistry of the activated catalysts were investigated. We drew the following conclusions from the results of this study:
(1) The deactivated catalysts were effectively activated by the in-situ activation method, and the active material was found to be highly loaded. The active ingredients, V2O5, WO3, and MoO3, of the activating solution effectively increased the NO removal rate of the activated catalysts; the activation effect of the L2 solution, which contained 1.0% V, 9% W, and 6% Mo, was best, increasing the NO removal rate by 32%.
(2) The result of the flue gas conditions of the NO removal rate of the activated catalyst exhibited that the effect of space velocity on the NO removal rate in the high temperature zone (>370 °C) was not as obvious as that in the low temperature zone (<370 °C). The NO removal rate effect at a low space velocity was stronger than that at a high space velocity. When the oxygen concentration was <4%, the NO removal rate of the catalyst was effectively promoted. The NO removal rate of the catalyst increased sharply with increasing ammonia concentration at the ammonia to nitrogen ratio of <1. The initial concentration of NO had a slight effect on the NO removal rate.
(3) The fresh, deactivated, and activated catalysts were characterized by SEM, BET, FTIR, XRD, and EDS analyses. The results showed that the gaps between the activated catalyst particles increased owing to the washing and activation by the activating solution, which removed the toxic substances, resulting in a more evident internal structure. The activation process is beneficial for increasing the specific surface area and total pore volume of the catalyst. The active ingredient can be effectively loaded, and the vanadium oxide was disseminated on the catalyst in an amorphous or highly dispersed form. Activating solutions could effectively increase the active ingredients of the deactivated catalyst and enhance the NO removal rate as a result.
Our findings suggest that the in-situ activation method of denitrified catalysts, based on an activation solution, has the advantages of having a rapid activation period and prevents the disassembly and removal of the catalyst, which can solve the deactivation problem of denitrified catalysts as well. We expect that the in-situ activation and activation device have potential applications to SCR-DeNOx. In fact, it should be noted that very little optimization work has been performed on these devices. Further mechanistic studies and development of durability, activating process, and corrosion behavior are ongoing in our laboratory.