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Technology and application of energy saving reformation in SCR flue gas denitrification by urea pyrolysis

2017-10-9 16:23:29

The biggest feature of this technology is energy saving, energy consumption reduction, elimination of the huge losses caused by urea pyrolysis heat source failure, can greatly reduce the operation cost of denitration, and strengthen the enthusiasm of denitration in thermal power enterprises.

This technique has high denitration efficiency, mature technology, stable operating characteristics of various flue gas, good adaptability, and the exhaust emission concentration is lower than 50mg/m3, can meet the needs of the national environmental policy and the increasingly stringent energy-saving emission reduction work, widely used in chemical industry, building materials, electric boiler, papermaking, printing and dyeing, metallurgy industry etc. waste incineration flue gas industry, gas and other gas and industrial denitration facilities, play an important role in the control of atmospheric pollution in NOx.

The main denitration technology of flue gas in coal-fired power plants at home and abroad is urea pyrolysis SCR (selective catalytic reduction) technology. Due to the technical points of the use of hot air as the heat source, and the need for electric heaters and other high-power equipment, this paper focuses on the urea pyrolysis energy saving technology and application.

2015, three ministries issued on the issuance of "full implementation of coal-fired power plant ultra low emissions and energy saving transformation work program" notice. By 2020, all have the conditions for the transformation of coal-fired power plants to achieve ultra low emission (i.e., oxygen content in the benchmark 6%, dust, sulfur dioxide, nitrogen oxide emissions are less than 10, 35, 50 mg / m3).

The new coal-fired power generation project in principle to use and more than 600 thousand kilowatts of ultra supercritical units, the average coal consumption of less than 300 grams of standard coal / kWh (hereinafter referred to as g / kWh), until 2020, after the transformation of existing coal-fired generating units is lower than the average coal consumption of power supply 310 grams / kwh. With the continuous improvement of national environmental laws and regulations, NOX, as one of the main pollutants of coal-fired boilers, has gradually attracted the attention of society and enterprises. Therefore, the analysis of the application of flue gas denitration technology has certain guiding significance for the technical selection of energy saving transformation projects in china.

1. selection of reducing agent for denitrification

At present, the most commonly used reductant preparation methods are generally 3? Species: liquid ammonia method, ammonia water method, urea method. For safety and practicality, urea method is more widely used.

Urea ammonia production is generally divided into hydrolysis method and pyrolysis method. Hydrolysis method is to decompose urea in the form of aqueous solution. The pyrolysis method is a urea solution directly heated and atomized rapidly.

1.1 hydrolysis

Urea is hydrolyzed and hydrolyzed to ammonia and carbon dioxide at a certain temperature. By controlling the rise and fall of the reaction temperature, the quantity of ammonia gas mixture is controlled so as to adapt to the change of boiler load. Urea hydrolysis technology mainly includes AOD, U2A and SafeDeNOx Three methods.

The main performance is: the urea hydrolysis process will generate some acidic intermediates (such as ammonium carbamate, etc.) may cause serious damage to the oxide film on the surface of stainless steel, the corrosion rate of the system is accelerated, more than 190 DEG C, the stainless steel material (such as 304SS) will suffer severe corrosion, when more than 220 degrees, even with titanium such as corrosion resistant materials, the system will suffer corrosion. The blockage of high concentration urea aqueous solution leads to the formation of insoluble two urea and other condensation substances, which is the cause of the urea hydrolysis system being easy to block.

1.2 pyrolysis

The pyrolysis of urea ammonia technology using high temperature air or flue gas as a heat source, the urea water solution is atomized rapidly decomposed into ammonia, ammonia concentration into the SCR reactor as a reducing agent into the mixed flue and the flue gas, in the presence of catalyst the reduction of nitrogen oxides into harmless nitrogen and water.

Its disadvantages and prone to failures are:

1) the electric heater consumes much and runs high. During the operation of urea pyrolysis unit, due to the low dilution temperature, large flow rate and large ammonia demand, the heat absorbed by urea will be absorbed more heat, and the more heat will be provided by the electric heater;

2) the product of pyrolysis furnace tail is faster. The pyrolysis furnace of high working temperature (450-600 DEG C), in use because of the tail pipe at the urea accumulated too much, leading to the outlet flow reduction system for ammonia content is insufficient, direct result of pyrolysis furnace outage cleaning, affects the reliability of denitrification device. If the thermal air flow in the pyrolysis furnace is low or the temperature is low, the urea solution can not be completely pyrolyzed, and the deposition is formed at the tail.

Compared with urea hydrolysis and pyrolysis technology, pyrolysis technology has some advantages over hydrolysis technology, especially in operation response time and corrosion. But there are also such as heat consumption, power consumption, easy to plug the pyrolysis furnace tail phenomenon, therefore, the focus on the furnace urea direct injection technology are introduced. This technology is a good solution to the shortcomings of urea pyrolysis ammonia system.

Introduction of urea direct injection technology in 2. furnace

Ammonia process of direct injection boiler boiler including urea, urea solution preparation system, urea solution supply and control device, and is located in the boiler room to the urea solution injector group, injector group and urea supply and control device for pipeline connection, the injector arranged in the boiler room steering group.

In the urea solution preparation of urea solution system in urea solution through supply and control device through the pipeline will transport the urea solution to set and boiler steering indoor urea solution injector group, urea solution injector group from urea droplet and gas mixing chamber, in turn is heated and decomposed into ammonia. The mixture of ammonia and flue gas passes through boiler Province in turn