Recent trends in vanadium-based SCR catalysts for NOx reduction in industrial applications: stationary sources

Abstract

Vanadium-based catalysts have been used for several decades in ammonia-based selective catalytic reduction (NH₃-SCR) processes for reducing NO_x emissions from various stationary sources (power plants, chemical plants, incinerators, steel mills, etc.) and mobile sources (large ships, automobiles, etc.). Vanadium-based catalysts containing various vanadium species have a high NO_x reduction efficiency at temperatures of 350-400 °C, even if the vanadium species are added in small amounts. However, the strengthening of NO_x emission regulations has necessitated the development of catalysts with higher NO_x reduction efficiencies. Furthermore, there are several different requirements for the catalysts depending on the target industry and application. In general, the composition of SCR catalyst is determined by the components of the fuel and flue gas for a particular application. It is necessary to optimize the catalyst with regard to the reaction temperature, thermal and chemical durability, shape, and other relevant factors. This review comprehensively analyzes the properties that are required for SCR catalysts in different industries and the development strategies of high-performance and low-temperature vanadium-based catalysts. To analyze the recent research trends, the catalysts employed in power plants, incinerators, as well as cement and steel industries, that emit the highest amount of nitrogen oxides, are presented in detail along with their limitations. The recent developments in catalyst composition, structure, dispersion, and side reaction suppression technology to develop a high-efficiency catalyst are also summarized. As the composition of the vanadium-based catalyst depends mostly on the usage in stationary sources, various promoters and supports that improve the catalyst activity and suppress side reactions, along with the studies on the oxidation state of vanadium, are presented. Furthermore, the research trends related to the nano-dispersion of catalytically active materials using various supports, and controlling the side reactions using the structure of shaped catalysts are summarized. The review concludes with a discussion of the development direction and future prospects for high-efficiency SCR catalysts in different industrial fields.

Keywords: Catalyst poisoning; NOx removal efficiency; Selective catalytic reduction; Stationary sources; Vanadium-based catalysts.

Fig. 1

Three major families of SCR catalyst [2]

Fig. 2

SCR configurations with typical system temperatures: a high-dust system, b low-dust system, and c tail-end system [11]

Fig. 3

Potential mercury transformations during coal combustion and in the resulting flue gas [16]

Fig. 4

Oxidation of SO₂ to SO₃ in a boiler and SCR (Electric Power Research Institute) [18]. ESP electrostatic precipitator, FGD flue gas desulfurization system, I.D induced-draft, SCR  selective catalytic reduction system.

Fig. 5

NOx removal efficiencies of the synthesized SCR catalysts according to vanadium content measured in the temperature range 150–450 °C a NOx conversion and b N₂ selectivity and N₂O concentration. Reaction conditions: [NOx] = 300 ppm, [NH₃] = 300 ppm, [SO₂] = 0 or 300 ppm, [O₂] = 5%, balance N₂, total flow 500 sccm, and the gas hourly space velocity (GHSV) = 60,000 h⁻¹

Fig. 6

Proposed mechanism of NO reduction and N₂O formation, as well as H₂O/SO₂ suppression effects, with the participation of a Lewis acid sites and b Brønsted acid sites over the Mn/Ti–Si catalyst [43]

Fig. 7

NOx removal efficiencies of the synthesized SCR catalysts according to tungsten content measured in the temperature range 150–450 °C a NOx conversion and b N₂ selectivity and N₂O concentration. Reaction conditions: [NOx] = 300 ppm, [NH₃] = 300 ppm, [SO₂] = 0 or 300 ppm, [O₂] = 5%, balance N₂, total flow 500 sccm, and the gas hourly space velocity (GHSV) = 60,000 h⁻¹

Fig. 8

Structures of the dehydrated surface vanadate phases on TiO₂:a isolated mono-oxo VO₄, b oligomeric mono-oxo VO₄, and c crystalline V₂O₅ nanoparticles on top of the surface vanadate monolayer [78]

Fig. 9

Structures of supported a monomeric and b oligomeric VOx units. c Crystal structure of orthorhombic phase V₂O₅ (Pmnm, No.59). d Raman spectra of bulk V₂O₅ (V₂O₅-ox) and TiO₂-supported V₂O₅ catalysts (1, 3, 5 and 9 wt % V₂O₅/TiO₂) [76]

Fig. 10

Type of monolith SCR catalysts: a honeycomb monolith, b plate, and c corrugated type catalyst

Fig. 11

Catalytic oxidation process of SO₂-SO₃ under the action of V₂O₅

Fig. 12

Correlations between catalyst wall thickness and channel weight and SO₂-SO₃ conversion [124]

Fig. 13

Effect of NH₃/NOx ratio and AV to SO₂-SO₃ conversion [134]

Fig. 14

Influence of NH₃ concentration on SO₃ formation

Fig. 15

N₂ selectivity to a N₂ and b N₂O during NH₃ oxidation as a function of reaction temperature [153]

Fig. 16

Reaction temperature effect of NH3 conversion, operating condition: space velocity at normal conditions (GHSV), 15,000 h⁻¹; linear velocity, 0.1 ms⁻¹; pressure, 0.1 MPa. Feed composition: [NH3] = 500 ppm; [O2] = 3 vol%; [Ar] = balance [153]

Fig. 17

NH₃ oxidation on vanadium catalyst [154]