Flue gas denitrification project liquid ammonia storage and evaporation system

1. Denitrification technology.

 Comparison of five process technologies

The "Emission Standard of Air Pollutants for Thermal Power Plants" GB13223-2011 was implemented on January 1, 2012, in which the emission limit of nitrogen oxides reached the "world's strict level" - 100mg/Nm3.

Currently, there are five main process technologies for controlling NOx emissions in power plants, namely selective catalytic denitrification (SCR), selective non-catalytic denitrification (SNCR), electron beam, corona discharge and wet method.

SCR flue gas denitrification technology has a high denitrification rate of more than 80%, can complete the catalytic reduction reaction within the temperature range of 300℃ to 400℃, is simple to operate, and currently occupies a dominant position in the world's denitrification process.

The principle of SCR denitrification technology is to react NOX and NH3 in the flue gas to generate N2 and H2O through a catalyst in the presence of oxygen within a suitable temperature range, thereby achieving the purpose of removing NOx from the flue gas. Its basic reaction equation is: 4NO+4NH3+O2→4N2+6H2O6NO2+8NH3→7N2+12H2O. Its basic working process is to spray the reducing agent NH3 at the appropriate position of the boiler flue. The reducing agent NH3 is evenly mixed with the flue gas and then passes through a denitration reactor filled with a catalyst. Under the action of the catalyst, NOX and NH3 undergo a reduction reaction to generate N2 and H2O.

2. Selection of denitration reducing agent.

The reducing agent required for the SCR denitration reaction is ammonia, which can be obtained from three chemical raw materials: liquid ammonia, ammonia water or urea.

(1) Liquid ammonia method. Liquid ammonia is used as a denitration reducing agent. It is mainly to send liquid ammonia to the liquid ammonia evaporator for heating and evaporation to become ammonia and send it to the denitration system. Therefore, the liquid ammonia system is relatively simple and has low energy consumption.

However, liquid ammonia is a flammable, explosive, toxic and corrosive substance. GB12268-2012 "List of Dangerous Goods" classifies liquid ammonia as Class 2.3 toxic gas, with a hazardous material number of 1005. According to GB18218-2009 "Identification of Major Hazard Sources of Hazardous Chemicals", ammonia storage exceeding 10t is a major hazard source. Therefore, in the transportation, storage and use of liquid ammonia, in addition to taking necessary preventive and protective measures, the design, construction and management should be strictly based on the relevant standards and specifications formulated by the government, local governments and industries.

(2) Ammonia method. Ammonia produced from ammonia is not common in SCR flue gas denitrification systems. This is mainly because the purchase concentration of ammonia is only about 25%, the power plant denitrification system uses a large amount of ammonia, and the transportation cost is high. In addition, since flue gas denitrification is gaseous ammonia, the heating and vaporization energy consumption is large, and the operating cost is the highest among the three. And like liquid ammonia, it is a hazardous drug. Therefore, since the 1990s, ammonia has rarely been used as a denitrification reducing agent in practice.

(3) Urea method. Properties of urea: Urea is an artificially synthesized organic substance that is widely present in nature and has relatively stable physical and chemical properties. However, solid urea is easy to absorb moisture. When the relative humidity in the air is greater than the hygroscopic point of urea, it absorbs moisture in the air and deliquesces.

The urea ammonia production method is rarely used due to the high initial investment. Currently, there are two technologies: urea pyrolysis and hydrolysis.

1) Urea pyrolysis technology.

Using auxiliary energy (fuel, electric heating, etc.) at a temperature range of 350-650℃, the atomized urea solution is directly decomposed into ammonia.

CO(NH2)2 → NH3 + HNCOHNCO + H2O → NH3 + CO2 The advantages of urea pyrolysis are: compact equipment and fast load regulation response. The disadvantages are: low urea conversion rate to ammonia, large urea and fuel consumption, and easy deposition of by-products produced by incomplete pyrolysis.

2) Hydrolysis technology. The urea solution of a certain concentration is hydrolyzed under the conditions of pressure of 0.45-0.65MPa and temperature of 140-160℃ to release ammonia.

CO(NH2)2 + H2O←→NH2-COO-NH4 ←→2NH3 ↑+ CO2↑The advantages of urea hydrolysis are: mature technology, wide application in the power industry and domestic and foreign chemical industries, mild reaction conditions, no other auxiliary energy except low-pressure steam from power plants, and low operating costs. The disadvantages are: certain requirements for the corrosion resistance of equipment materials, and the dynamic responsiveness of ammonia production load needs to be optimized through process design.

3. Liquid ammonia storage and evaporation system process.

Liquid ammonia is delivered by a liquid ammonia tank truck. The pressure difference between the liquid ammonia tank truck and the ammonia tank and the ammonia unloading compressor are used to increase the pressure to transfer the liquid ammonia from the tank truck to the ammonia tank for storage. The pressure difference between the liquid ammonia storage tank and the liquid ammonia evaporator or the liquid ammonia delivery pump (used in winter for northern projects) is used to transport the liquid ammonia in the liquid ammonia storage tank to the liquid ammonia evaporator to evaporate into ammonia gas (steam and electricity can be used as heat sources). The pressure is stabilized by the gas ammonia buffer tank and then sent to the denitrification system.

Ammonia gas discharged urgently from the liquid ammonia storage and evaporation system is discharged into the ammonia dilution tank, and after being absorbed by water, it becomes dilute ammonia water and is discharged into the wastewater pool, and then pumped to the industrial wastewater treatment station in the plant area for treatment.

Before the system is put into operation, shut down for maintenance, and before the system is resumed after maintenance, it needs to be purged and replaced with nitrogen to ensure that ammonia air explosion gas will not be formed in the entire system.

4. Liquid ammonia storage and evaporation system.

Main equipment includes land-based fluid loading and unloading arms, unloading compressors, liquid ammonia storage tanks, liquid ammonia delivery pumps, liquid ammonia evaporators, gas ammonia buffer tanks, ammonia dilution tanks, wastewater delivery pumps, nitrogen purge equipment, cooling spray equipment and eyewash, etc.

5. Safety measures.

(1) According to the properties of liquid ammonia and gas ammonia, it is strictly forbidden to use copper components. Ammonia valves should be used for valves, and the pressure of equipment and pipelines should be determined according to the operating pressure.

(2) The ammonia shed adopts a shed-type building with four open sides and a light roof, which can prevent safety accidents caused by ammonia leakage and avoid the pressure in the tank from increasing due to sunlight.

(3) Each ammonia tank is equipped with two safety valves, and the starting pressure of the two safety valves is different. When one safety valve is taken off and readjusted after starting, there is another safety valve to protect the ammonia tank.

(4) Safety valves are installed at the outlet of the ammonia unloading compressor, the outlet of the liquid ammonia delivery pump, the liquid ammonia evaporator, and the gas ammonia buffer tank. When the safety valve is tripped due to overpressure, the released ammonia gas is not directly vented, but collected in the ammonia dilution tank and absorbed by water to become ammonia water, which is then discharged into the wastewater pool and then sent to the wastewater treatment workshop in the plant area for treatment through the wastewater delivery pump.

(5) A 0.6m high fire embankment is set up in the ammonia tank area. The net distance between the fire embankment and the ammonia tank is 3.0m. The volume inside the fire embankment is larger than the volume of a single tank, meeting the requirements of GB50160-2008 "Design Fire Protection Code for Petrochemical Enterprises".

(6) The overall layout is strictly carried out in accordance with the "Design Fire Protection Code for Buildings" GB50016 and the "Design Fire Protection Code for Petrochemical Enterprises" GB50160.

(7) MCC cabinets are installed in the ammonia area. Two power supplies need to be drawn from the power plant to the ammonia area, one for working and the other for standby. They are interlocked and switched automatically.

(8) According to the "Design Specifications for Electrical Installations in Explosive and Fire Hazardous Environments" GB 50058, the ammonia area is divided into explosion-proof areas. Lighting and electrical facilities in the explosion-proof area are all explosion-proof equipment, and the explosion-proof level is ⅡB. All instruments with remote transmission in the explosion-proof area are explosion-proof instruments, and the explosion-proof level is ⅡB.

(9) According to the provisions of GB50057-2010 "Design Specifications for Lightning Protection of Buildings", the lightning protection level of buildings in the ammonia area is considered as II. Ammonia sheds and electric control rooms are equipped with exposed lightning strips on the four sides of the roof edge and protruding parts. The lightning strip lead-down line is reliably grounded to the grounding grid at no less than 4 points. Ammonia sheds are equipped with a lightning strip grid no larger than 10×10 or 12×8 meters. The lightning strip is reliably grounded to the grounding grid at no less than 6 points, and the grounding resistance is no more than 1.0Ω. The equipment and pipelines in the ammonia area are also reliably grounded.

(10) The ammonia pipeline is cross-connected to prevent static electricity and the pipeline is reliably grounded.

(11) A human body static discharger is installed at the entrance of the ammonia area, and personnel must discharge static electricity before entering the ammonia area.

(12) The liquid ammonia storage and evaporation system is controlled by a DCS or PLC control system. Important monitoring signals or signals related to the unit system interlocking are connected to the unit unit DCS through hard wiring.

(13) The control system power supply requires two power supplies, one of which is provided by the electrical MCC power supply, and the other is provided by the UPS power supply device with the DCS or PLC.

(14) The compressed air for the ammonia area instruments is provided by the compressed air system of the power plant.

(15) The loss of air and power of each pneumatic valve is to ensure the safety of the system.

(16) According to the layout of the ammonia area, an ammonia leak detector is arranged in accordance with the provisions of GB50493-2009 "Petrochemical Combustible Gas and Toxic Gas Detection Alarm Design Specifications", and two signals are sent, one analog signal to the DCS or PLC system, and the other switch signal to the fire alarm system, and interlocked with the sprinkler valve group.

(17) A sprinkler fire sprinkler system is set up in the liquid ammonia storage and preparation area, which has the functions of absorbing the leaked ammonia gas and cooling and extinguishing fire in case of fire.

(18) The unloading area is an open area, which is not convenient for setting up a fixed sprinkler system. Fire water cannons are set up on both sides to protect the liquid ammonia tank truck. (19) A certain number of fire hydrants are set up on the roads around the ammonia area.

(20) A certain number of portable pressure-storage ammonium phosphate dry powder fire extinguishers and cart-type ammonium phosphate dry powder fire extinguishers are set up around the ammonia area.

(21) Ammonia leak detectors are installed in the unloading, storage and preparation areas, and the signals are sent to the DCS control system and fire alarm system. The sprinkler valve group corresponding to the fire alarm command starts the spraying.

(22) A wind vane is installed in the ammonia area so that when a large amount of ammonia leaks, the on-site personnel can escape from the correct direction according to the wind direction.

Hongchang Liquid Ammonia Manufacturer supplies liquid ammonia to major power plants. Choose Hongchang for liquid ammonia for desulfurization and denitrification. We have built a liquid ammonia storage and filling station in western Henan with a storage scale of more than 1,000 tons. We also produce about 50,000 tons of ammonia water in the Luoyang area. We have several dangerous transportation vehicles and can provide better services for different users.