Tungsten Trioxide Help China Defend Blue Sky

According to the research institute, the PM2.5 level in the surrounding area of Beijing is decreasing year by year, while the level of New Delhi's PM2.5 in the same period is increasing, and in the winter of this year, New Delhi even appeared the most serious haze in several years. The same thing happened on sulfur dioxide, and the latest issue of the Journal Science report found that since 2007, China's sulfur dioxide emissions have fallen by 75%, while India is up 50%, which makes New Delhi feel frustrated and Indians have accusations of the Modi administration.

According to media reports, 8 provinces and municipalities directly under the central government of Beijing, Tianjin and Hebei provinces and municipalities directly under the central government require shutdown and limited production for most of the high pollution enterprises in Beijing, Tianjin, Hebei and the surrounding areas. The coverage of "blue sky defense war" in autumn and winter has reached 8 provinces and 34 cities. In the long run, stopping and limiting production can only be a means of expediency. After all, this will affect our industrial production and the employment of the masses, and the strengthening of environmental protection measures is the way of sustainable development. It is not difficult to predict that more and more polluting gas enterprises will be strictly required to install flue gas purification equipment in the future, which will be a necessary measure for environmental protection and emission reduction.

How can tungsten help China defend the blue sky? Here we will introduce a widely used flue gas denitrification technology -- SCR technology. SCR, also called selective catalytic reduction technology, is the selective reduction of NOx to harmless N₂ and H₂O by ammonia or other reducing agents under the action of a specific catalyst. SCR has hundreds of thousands of successful applications and ten years of operation experience in the world, and 95% of the denitration devices in Japan and Germany adopt SCR technology.

According to statistics, nearly 99% of China's thermal power generation comes from coal and 1% comes from fuel and gas. The main pollutants are nitrogen oxides NOX and sulfur dioxide SO₂. Most of the thermal power plants in our country begin to adopt SCR system to control gas pollution, and the environmental protection effect is obvious.

In the SCR selective catalytic reduction system, the ammonia storage system, the mixing system of ammonia and air, the ammonia injection system, the reactor system and the monitoring control system are made up. Taking the thermal power plant as an example, the SCR reactor is usually installed between the boiler economizer and the air preheater. The smoke temperature in this interval is beneficial to the SCR denitrification reduction reaction, and the ammonia is injected into the appropriate position of the flue between the economizer and the SCR reactor, and the catalyst is mixed with the flue gas to react with the NOx in the reactor. The catalyst is placed in a box like a solid reactor. The catalyst unit is usually arranged vertically, and the flue gas flows upwards and downwards.

The catalytic system is the most important component in the SCR system, and the SCR catalyst generally uses TiO₂ as the carrier, and the active components are adsorbed and purified with the rare metal oxides such as tungsten trioxide WO₃ and two oxidation two vanadium V₂O₅. The carrier TiO₂, which is usually used in the SCR reaction, does not have strong activity. The main function of the carrier is to increase the specific surface area of the catalyst and the number of microporous structure. The reducing agent is usually ammonia or urea. The most important catalytic work is to be solved by tungsten and vanadium.

Catalyst is the core component of SCR technology. Its cost accounts for about 20%~40% of the total cost of SCR system, and the performance of catalyst directly affects the removal effect of NOx. For the SCR system, the three main commercial catalysts are precious metal type catalysts (Platinum), metal oxide catalysts (tungsten, vanadium), and molecular sieve catalysts.

Titanium tungsten vanadium catalyst is more active, antioxidant and antitoxic. WO₃ and V₂O₅ are the key factors of the catalyst. (1) Tungsten trioxide has a suitable surface area and volume ratio. (2) Tungsten trioxide has a relatively low valence band gap energy, and a suitable band gap structure is beneficial to oxidation and reduction. The reaction is carried out. (3) The stability of tungsten trioxide is high and can endure bad operating environment. (4) The reactive activity of pure V₂O₅ in catalytic reduction is not high, but after it is loaded on TiO2, it will have a good ability to resist SO₂ poisoning.

SCR technology has the advantages of low reaction temperature, high purification rate, reliable operation and so on. The application market of SCR technology will be more and more widely, and the titanium tungsten vanadium catalyst has been commercialized all over the world. Tungsten trioxide has a wide range of sources. Looking at the whole tungsten industry chain, it is not difficult to find that tungsten can be used not only to treat waste water, but also to purify air. Not only is China market demand, but many other new industrial countries are also facing the severe environment brought by industrial development, so the future of the tungsten market is bright. 

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