Application of flue gas denitrification technology in the iron and steel metallurgy industry: comparison of SCR and SNCR technologies

The iron and steel metallurgy industry is an important part of the national economy, providing an indispensable material foundation for the life of the country and its people. However, with the continuous growth of iron and steel production, the problem of waste gas emission has become increasingly serious. Waste gases generated during the steel production process cause a certain degree of pollution to the atmospheric environment. Among them, nitrogen oxides (NOx) is one of the main pollutants, which is harmful to human health and ecological environment. Therefore, the treatment of waste gas in the iron and steel metallurgy industry is particularly important. Flue gas denitrification technology is one of the effective means to deal with NOx in iron and steel metallurgy exhaust gas, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) technology is currently the mainstream of the two denitrification technologies, which play a pivotal role in the iron and steel metallurgy industry in the treatment of exhaust gas.

Overview of flue gas denitrification technology

Flue gas denitrification technology refers to the process of converting nitrogen oxides in waste gas into harmless substances through chemical reaction or physical methods. According to the different processing principles, flue gas denitrification technology can be divided into three categories: dry, semi-dry and wet. Among them, dry denitrification mainly uses catalysts to promote the reaction between NOx and reductants (e.g. ammonia, urea, etc.); semi-dry denitrification adds an appropriate amount of water on the basis of the dry method to improve the efficiency of the reaction; and wet denitrification is to convert NOx into harmless substances through a liquid-phase reaction.

Selective Catalytic Reduction (SCR) Technology

l Principle

SCR technology refers to the use of reductants such as ammonia (NH3) to react with NOx in a selective chemical reaction under the action of a catalyst to produce harmless nitrogen (N2) and water (H2O). The presence of a catalyst reduces the reaction temperature and increases the reaction rate, resulting in a significant increase in denitrification efficiency.

lApplications

In the iron and steel metallurgy industry, SCR technology is usually applied to the treatment of high-temperature, high-dust, high-sulfur exhaust gas. Since the catalyst has certain requirements on temperature and dust content, it is necessary to pre-treat the exhaust gas, such as cooling and dust removal. In addition, SCR technology can be combined with desulphurisation technology to achieve integrated desulphurisation and denitrification treatment.

l Features

(1) High denitrification efficiency: under suitable reaction conditions, the denitrification efficiency of SCR technology can reach more than 90%.

(2) Higher requirements for exhaust gas conditions: the activity and life of the catalyst are affected by temperature, dust content and other factors, and strict pretreatment of the exhaust gas is required.

(3) Higher investment and operating costs: the need to use catalysts and reducing agents, and the complexity of the equipment structure, resulting in relatively high investment and operating costs.

Selective non-catalytic reduction (SNCR) technology

Principle

SNCR technology refers to the use of reductants such as urea or ammonia to react with NOx in a selective chemical reaction to produce harmless nitrogen and water without a catalyst. The reaction is usually carried out under high temperature conditions, so it is also called high temperature denitrification technology.

lApplications

In the iron and steel metallurgy industry, SNCR technology is suitable for the treatment of low and medium temperature exhaust gases. Since no catalyst is required, the temperature and dust content of the exhaust gas are relatively low.SNCR technology is usually combined with the exhaust system of boilers, heaters and other equipment to realise the denitrification treatment of exhaust gas.

lFeatures

(1) Moderate denitrification efficiency: under suitable reaction conditions, the denitrification efficiency of SNCR technology can reach 60%-80%.

(2) Lower requirements for exhaust gas conditions: due to the absence of catalyst, the temperature and dust content of the exhaust gas is relatively low.

(3) Lower investment and operating costs: As there is no need to use catalyst and complex equipment structure, resulting in relatively low investment and operating costs.

Comparison of SCR and SNCR technology

Denitrification efficiency: the denitrification efficiency of SCR technology is higher than that of SNCR technology, the former can reach more than 90%, and the latter is 60%-80%.

Requirements for the exhaust gas conditions: SCR technology requires high temperature and dust content of the exhaust gas, and requires strict pre-treatment; while SNCR technology requires lower conditions for the exhaust gas.

l Investment and operating costs: SCR technology investment and operating costs are higher than SNCR technology, mainly due to the use of catalysts and reductants and complex equipment structure.

Scope of application: SCR technology is suitable for the treatment of high temperature, high dust and high sulphur exhaust gas; while SNCR technology is suitable for the treatment of low and medium temperature exhaust gas.

SCR technology has the characteristics of mature technology, high denitrification efficiency, stable operation, no secondary pollution, etc. SCR technology is usually chosen when the denitrification efficiency is required to be more than 70%.The denitrification efficiency of SCR technology can be more than 90%.

SNCR denitrification efficiency in pulverised coal furnace is usually 40~50%, denitrification efficiency in circulating fluidised bed is usually 50~70%, mainly used in small boilers and circulating fluidised bed boilers.SNCR technology, no catalyst is needed, but it is required to complete the denitrification reaction within the temperature range of 800~1100 ℃.

Characteristics of Yizhi's denitrification technology

Chengdu Yizhi Technology Co., Ltd. can independently undertake the study, design, modification, commissioning or general contracting of denitrification devices. The technologies mastered mainly include liquid ammonia gasification (urea or ammonia gasification) technology, gas-ammonia distribution and mixing technology, flue gas flow field thermal field and chemical reaction dynamics analysis technology, ammonia fugitive control technology, catalyst selection technology for special operating conditions of the flue gas, personalised transformation of the old unit technology, spray gun positioning and target injection technology. Yizhi Technology's denitrification technology is widely used in the field of boiler flue gas and catalytic regeneration flue gas.

Flue gas denitrification technology plays an important role in the treatment of waste gas in the iron and steel metallurgy industry, and SCR and SNCR, as two mainstream denitrification technologies, have their own characteristics and scope of application. Iron and steel enterprises in the choice of denitrification technology, should consider the nature of the exhaust gas, treatment requirements and investment and operating costs and other factors, choose the most suitable technology programme. Meanwhile, with the increasingly strict environmental protection policy and technological progress, flue gas denitrification technology will continue to develop and improve, contributing to the green and sustainable development of the iron and steel metallurgical industry.