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CHEC denitration unit

key word:Heat exchange element


Product description

CHEC denitration unit

 

1. SCR denitration reaction principle

In the presence of catalyst, the main reaction equation of SCR denitrification process is as follows when the reaction temperature is 250-450

           4NO+4NH3+O2→4N2 +6H2O            (2-1)

           NO+NO2 +2NH3→2N2 +3H2O           (2-2)

           6NO2 +8NH3 →7N2+ 12H2O              (2-3)

Among them, formula (2-1) and formula (2-2) are the main reaction processes, because 90% - 95% of NOx in flue gas exists in the form of no, and 5% - 10% of NOx exists in the form of NO2. During the reaction process, NH3 selectively reacts with NOx to produce N2 and H2O without secondary pollution, which are emitted with flue gas.

 

2. Layout of SCR denitration device

According to the layout of SCR denitration reaction chamber in different positions of flue gas system at the tail of boiler, the layout of SCR denitration device can be divided into high dust layout and low dust layout. The low dust layout can be divided into high-temperature dust collector low-dust arrangement and desulfurization low-dust arrangement. The layout of SCR denitration device is shown in Fig. 2-1 ~ Fig. 2-3. In comparison, the low dust arrangement has low dust concentration in the flue gas, which can effectively reduce the wear of catalyst and prolong the service life of catalyst. However, due to the need to increase GGH and burner after high-temperature dust collector or desulfurization, the high dust layout is more economical after comprehensive comparison. At present, the existing SCR denitration devices mostly adopt high dust arrangement.

3. SCR denitration process system

In engineering application, the common reducing agents in SCR denitrification system are liquid helium, ammonia water and urea. Most SCR denitrification systems at home and abroad use liquid ammonia as reducing agent. The process system mainly includes flue gas system, liquid ammonia storage, preparation and supply system and waste gas collection system. The main process flow of SCR denitration process with liquid ammonia as reducing agent is shown in Fig. 2-4

3.1. Flue gas system

After the flue gas comes out of the boiler economizer, it enters the SCR reaction chamber through the inlet flue of SCR reaction chamber. A jet grille (AIG) is set on the inlet flue of SCR reaction chamber, which can inject ammonia / air mixture into the inlet flue of SCR reaction chamber to fully mix the ammonia injected and NOx in flue gas. In order to ensure that the flue gas passes through the catalyst bed in SCR reaction room vertically, so as to reduce the abrasion of catalyst caused by flue dust, flue gas guide plates are set at the corners of flue gas. After passing through the guide plate, the flue gas is evenly distributed and flows through the catalyst bed in turn. Under the action of catalyst, NH3 reacts with NOx in flue gas to generate N2 and H2O without secondary pollution, which flows through boiler air preheater, dust collector and desulfurization device with flue gas, and then enters chimney for emission.

The main equipment of the system includes ammonia spraying grid, catalyst, etc.

3.2. Liquid ammonia storage, preparation and supply system

The liquid ammonia is transported by the liquid ammonia tank car, and the gas helium is extracted from the liquid ammonia storage tank by the unloading compressor. There is a pressure difference between the tank car and the liquid ammonia storage tank, so the liquid ammonia in the tank car is squeezed into the liquid ammonia storage tank: the liquid ammonia in the liquid ammonia storage tank is transported to the liquid ammonia evaporator under the action of the tank pressure and self gravity (or under the action of the liquid ammonia supply pump); the gas ammonia is stabilized in the ammonia buffer tank After that, it is piped to the SCR area and mixed with dilution air (usually diluted to ammonia gas (/ air volume ratio of 5%), and then injected into the inlet flue of SCR reaction chamber through ammonia spraying grid.

The main equipment of the system includes ammonia unloading compressor, liquid ammonia storage tank, liquid ammonia evaporator, ammonia buffer tank, dilution fan, ammonia / air mixer, ammonia spray grid, etc.

3.3. Waste gas collection system

Due to overpressure, the safety valve on the outlet pipeline of liquid ammonia storage tank, ammonia buffer tank and ammonia unloading compressor in the liquid ammonia supply system is collected through the collection pipeline and then discharged into the ammonia dilute tank. After being absorbed by water, it is discharged into the waste water pool, and then sent to the wastewater treatment station for treatment through the waste water pump.

The main equipment of the system includes ammonia dilution tank and waste water pump.

 

4. Catalyst of denitration unit

SCR denitration system is relatively simple, and there are few types of equipment. Among them, ammonia unloading compressor, liquid ammonia storage tank, liquid ammonia evaporator, ammonia buffer tank, dilution fan, ammonia / air mixer, ammonia spray grid, ammonia dilution tank, waste water pump, ammonia spray grid, etc. are all conventional equipment in the power industry, which have been widely used in power engineering. The following will focus on the core part of SCR denitration catalyst.

4.1. Type of catalyst

There are four kinds of commercial catalysts used in SCR denitration unit, namely noble metal catalyst, metal oxide catalyst, zeolite catalyst and activated carbon catalyst. Metal oxide catalysts are the most widely used catalysts in SCR denitration of power plants, among which V2O5-WO3 (MoO3) / TiO2 series catalysts based on titanium oxide are the majority.

At present, there are three kinds of catalysts for SCR denitration on the market, which are honeycomb, corrugated plate and flat plate. See Fig. 2-5 for honeycomb type, corrugated plate type and plate type catalyst units. The cross section size of honeycomb catalyst unit is 150 mm x 150 mm, that of corrugated plate catalyst unit is 466 mm x 466 mm, and that of plate type catalyst unit is 464 mm x 464 mm. The cross-section size of honeycomb catalyst module is 1910 mm x 970 mm, that of corrugated plate catalyst is 1880 mm x 946 mm, and that of plate type catalyst is 1882 mm x 954 mm. The module height of each type of catalyst is determined according to the volume required. The plate catalyst module is shown in Fig. 2-6.

4.2. Catalyst composition

The composition of honeycomb, corrugated plate and flat plate catalysts commonly used in SCR denitrification are basically similar, which are composed of TiO2, V2O5, WO3 or MoO3, SiO2, Al2O3, Cao, MgO, Bao, Na2O, K2O, P2O5, etc. among them, TiO2 accounts for 85% ~ 95%, WO3 or MoO3 accounts for 5% ~ 10%, V2O5 accounts for 1% ~ 5%.

4.3. Catalyst characteristics

4.3.1. The honeycomb catalyst is composed of TiO2, V2O5, WO3, etc. after drying, sintering and cutting, the honeycomb catalyst is assembled into a catalyst module which is easy to transport and install. Honeycomb catalyst has the characteristics of high activity, large specific surface area, easy control of length and high recovery rate.

4.3.2. Corrugated plate catalyst is made of glass fiber board or ceramic plate as the base material, impregnated with active material and sintered. Corrugated plate catalyst is characterized by large specific surface area, light weight and low pressure drop.

4.3.3. The plate type catalyst is made of stainless steel wire mesh as the base material, impregnated with active material and sintered. The characteristics of flat catalyst are large unit volume mass, small specific surface area and strong adaptability to high dust environment of flue gas.