SCR denitration

SCR denitration

The process of removing nitrogen oxides from combustion flue gas, and the importance of preventing environmental pollution, has been pointedly raised as a worldwide problem. The more mainstream processes in the world are divided into: SCR and SNCR. There is not much difference between the two processes except that the reaction temperature of SCR is lower than that of SNCR due to the use of catalysts. However, from the perspective of construction cost and operating cost, the investment of SCR is at least several times that of SNCR, or even More than 10 times. Desulfurization and dust collector manufacturers

concept:

In order to prevent excessive NOx pollution to the environment after coal combustion in the boiler, the coal should be denitrified. It is divided into pre-combustion denitration, combustion process denitration, and post-combustion denitration.

Denitrification technology

According to the formation mechanism of nitrogen oxides in cement kilns, there are two types of technical measures for nitrogen reduction and emission reduction in cement kilns:

One is to manage from the source. Control the formation of NOx during calcination. Its technical measures: ① adopt low-nitrogen burner; ② stage combustion in the decomposition furnace and pipeline to control the combustion temperature; ③ change the batching scheme and use mineralizer to reduce the clinker sintering temperature.

The other type is governance from the end. The technical measures to control NOx emitted from flue gas: ① "Stage combustion + SNCR", which has been piloted in China; ② Selective non-catalytic reduction (SNCR), which has been piloted in China; ③ Selective catalytic reduction (SCR) , Europe has only three lines of experiments; ③ SNCR/SCR combined denitrification technology, there is no successful experience in domestic cement denitrification; ④ biological denitrification technology (in the research and development stage).

In short, the development of cement denitrification in China is still in the stage of exploration and demonstration, and a scientific summary has not yet been carried out. Whether various design process technology routes and equipment facilities are scientific and reasonable, reliable denitrification efficiency, operating costs, cement energy consumption, and secondary pollutant emissions will all be tested by practice.

System engineering of denitrification:

It is not a matter for cement enterprises to adopt the "SNCR" method for denitrification. It is subject to many constraints. It not only involves production, circulation, distribution and consumption, but also involves multiple government departments such as industry, agriculture, commerce, transportation, public security, energy, price, environmental protection, safety supervision and quality inspection.

With the SNCR method for denitration, the reducing agent is a consumable (but for SCR denitration, the consumption of catalyst is more). Cement denitrification generally uses urea or ammonia water (do not choose liquid ammonia—dangerous goods) as the reducing agent, but urea and ammonia water are produced by the conversion of synthetic ammonia, but the comprehensive energy consumption per unit product of synthetic ammonia is quite high (see Table 2 for details) .

Taking the denitrification of 18 2500t/d lines in Chongqing as an example, if the background value of NOx emission is about 1000 mg/Nm³, the NOx emission should be reduced to below 500 mg/Nm³ For SNCR denitration, if ammonia water (concentration 25%) is selected as the reducing agent, the annual consumption of ammonia water will be 62,280 tons. It is equivalent to consuming 15,570 tons of synthetic ammonia → it will inevitably increase the comprehensive energy consumption of the chemical plant by 25,691 tons (standard coal) → the Economic and Information Commission will inevitably increase the natural gas consumption and electricity consumption indicators of the chemical plant → will inevitably increase the synthetic ammonia water pollutant discharge (chemical oxygen demand) of the chemical plant. 23.36 tons of ammonia nitrogen, 9.3 tons of ammonia nitrogen, 0.047 tons of cyanide, 10.90 tons of SS, 1.56 tons of petroleum, 0.031 tons of volatile phenol, 0.156 tons of sulfide, and 155,700 tons of drainage) → It will inevitably involve the environmental protection department’s total control of pollutants in synthetic ammonia production enterprises. Quantity indicators → Involves how the price department determines the price of ammonia water and urea used for denitrification in the cement industry and how to determine the price of additional natural gas used by chemical plants → Involves increasing the social transportation volume of ammonia water and urea to cement plants and the safety and smoothness of public security traffic → Involves agricultural supervisors The department transfers urea and ammonia water out of the agricultural system → involves the municipal government’s energy conservation and emission reduction office to increase the energy consumption per unit of product energy consumption and waste emissions of cement enterprises. Cement product quality standard inspection, etc.

Technology:

Selective non-catalytic reduction technology (SNCR)

The selective non-catalytic reduction method is a method of reducing NOx in the temperature range of 850-1100 °C without using a catalyst. The most commonly used drugs are ammonia and urea.

Generally speaking, the denitration efficiency of SNCR can reach 25% to 40% for large coal-fired units and 80% for small units. Because this method is greatly affected by the boiler structure size, it is mostly used as a supplementary treatment method for low-nitrogen combustion technology. Its engineering cost is low, the layout is simple, and the floor area is small, which is suitable for the renovation of the old plant, and the new plant can be used according to the boiler design.

Selective Catalytic Reduction (SCR)

SCR is the most mature flue gas denitrification technology at present, it is a kind of after-furnace denitrification.

The method, first completed commercial operation in Japan in the late 1960s and 1970s, is to use a reducing agent (NH3, urea) under the action of a metal catalyst to selectively react with NOx to generate N2 and H2O instead of being oxidized by O2, so called "selectivity". The popular SCR processes in the world are mainly divided into two types: ammonia SCR and urea SCR. Both of these two methods utilize the reduction function of ammonia on NOx, and reduce NOx (mainly NO) to N2 and water which have little effect on the atmosphere under the action of catalyst, and the reducing agent is NH3.

Most of the catalysts used in SCR use TiO2 as the carrier and V2O5 or V2O5-WO3 or V2O5-MoO3 as the active component, and are made into three types of honeycomb type, plate type or corrugated type. SCR catalysts used in flue gas denitration can be divided into high temperature catalysts (345℃～590℃), medium temperature catalysts (260℃～380℃) and low temperature catalysts (80℃～300℃). Different catalysts have different reaction temperatures. . If the reaction temperature is too low, the activity of the catalyst will decrease, resulting in a decrease in the denitration efficiency, and if the catalyst continues to operate at low temperature, the catalyst will be damaged; if the reaction temperature is too high, NH3 will be easily oxidized, and the amount of NOx generated will increase, which will also cause The phase transition of the catalyst material degrades the activity of the catalyst. Most SCR systems at home and abroad use high temperature, and the reaction temperature range is 315℃～400℃.

Advantages: This method has high denitrification efficiency and relatively low price. It is widely used in domestic and foreign projects and has become the mainstream technology of power station flue gas denitrification.

Disadvantages: The fuel contains sulfur, which can generate a certain amount of SO3 during the combustion process. After the addition of the catalyst, under aerobic conditions, the generation of SO3 greatly increased, and combined with excess NH3 to generate NH4HSO4. NH4HSO4 is corrosive and viscous and can cause damage to tail flue equipment. Although the amount of SO3 generated is limited, its impact cannot be underestimated. In addition, the catalyst poisoning phenomenon can not be ignored.