Issue 23/2012 - Research on Safety and Economic Performance Assurance Method of Rotary Air Preheater after Denitration Transformation of Thermal Power Plant

China's NOX emissions are second only to the United States, nearly 70% of which comes from the direct combustion of coal, and the environmental pollution caused by China's coal-based power production is a constraint on
the development of the power industry.
In view of the fact that China's energy consumption will continue to grow with the development of the economy in the future, and NOX emissions will continue to increase, the control of NOX will become the focus
of environmental protection management of coal-fired power stations after dust and SOX.
However, with the increase of denitration system, the impact
on other systems and equipment of the unit should also be fully considered.

As far as the currently widely used SCR (selective catalytic reduction) denitration process is concerned, the ammonium bisulfate produced by the system is easy to adhere to the surface of the heat exchange element of the air preheater, which aggravates the corrosion and ash plugging of the heat exchange element, which will affect the resistance of the air preheater and put forward new requirements
for the corrosion resistance and cleaning ability of the air preheater.

First, the background of denitrification transformation of thermal power plants

There are three main methods for NOX emission control measures for coal-fired power plants: (1) fuel denitrification; (2) Improve the combustion method and production process, and denitrate during combustion; (3) Flue gas denitration, that is, NOX control technology
after combustion.
As far as the current situation of China's resource structure and energy policy is concerned, the use of low-nitrogen fuel is difficult to achieve, and there are no reports or explanations
of implementation performance.
Low NOX combustion technology is mature, investment and operating costs are low, has been more used in NOX emission control of thermal power plants, but with the further improvement of environmental protection requirements, it is difficult to meet emission standards by low NOX combustion technology alone, and it is also necessary to consider the installation of NOX removal device technology after combustion to meet the requirements
of environmental protection emissions.

The control technology of NOX emissions after combustion mainly refers to the flue gas denitrification and purification technology, that is, the NOX that has been generated is reduced to N2 to remove NOX in the flue gas, which can be divided into wet denitration and dry denitration
according to the process.
The denitration technologies with industrial achievements mainly include: acid absorption method, alkali absorption method, selective catalytic reduction method, non-selective catalytic reduction method, adsorption method, plasma activation method, etc
.

Among the many denitration process methods, the typical SCR process method using liquid ammonia as the absorbent is a flue gas denitration technology with the most application and the most mature technology
in the world.
The advantages of this method are: due to the use of catalysts, the reaction temperature is low; High purification rate, up to more than 85%; Compact process equipment and reliable operation; After reduction, N2 is vented without secondary pollution
.
However, there are also some obvious disadvantages, such as the presence of some unreacted NH3 in the system and SO2 and SO3 in the flue gas, which generates ammonium
bisulfate that is easy to corrode and clog the equipment.

Second, the influence of SCR denitration system on the air preheater

In order to better control nitrogen oxide emissions, the addition of SCR denitrification devices has become the first choice for
many power plants.
The SCR denitration process uses ammonia as a reducing agent
.
Ammonia is injected into the flue gas at a certain temperature, and under the action of a specific catalyst, ammonia reacts with nitrogen oxides in the flue gas to form nitrogen and water
.

The synchronous construction or reservation of denitration device puts forward new requirements for the design of boilers and auxiliary engines, and the following is a brief description of the impact
of SCR denitration device on the air preheater.

(1) The conversion rate from SO2 to SO3 in the flue gas increases, that is, the amount of SO3 in the flue gas increases, and the acid dew point temperature of the flue gas increases, thereby aggravating the acid corrosion and ash
plugging of the air preheater.
When selecting a catalyst, attention should be paid to the conversion rate of SO2 to SO3, which should be controlled below
1% in general.
If the conversion rate is too high, the corrosion of the air preheater will increase rapidly
.
If the reduction efficiency of the catalyst is very high, although a small amount of catalyst can be used, the conversion of SO2 to SO3 will be higher
.

(2) The evolved ammonia (NH3) in the SCR denitration or SNCR denitration system and SO3 and water vapor in the flue gas form ammonium bisulphate (Ammonium Bisulphate - ABS) condensate, which is moderately acidic, and has great viscosity in the middle and low temperature sections of the air preheater, and is easy to adhere to the surface of the heat exchange element at the cold end of the air preheater, which aggravates the corrosion and ash
plugging of the heat exchange element.
This affects the resistance of the air preheater and places new demands
on the corrosion resistance and cleaning ability of the air preheater heat exchanger elements.

(3) After the SCR denitration system is added to the boiler, the resistance of the flue gas side increases correspondingly by about 1000Pa, which increases the negative pressure of the flue gas flowing through the air preheater, resulting in an increase in the air leakage rate of the air preheater, so the sealing system of the air preheater should also be modified accordingly to reduce the air leakage rate
.
The sealing system of SCR air preheater should be reliable and stable, and there is no need for complex control devices, so as to avoid the control device getting out of control after long-term operation, resulting in an increase in the air leakage rate of the air preheater and affecting the normal operation
of the boiler.

(4) Due to the increase of the pressure difference at the hot end of the SCR air preheater, the strength and stiffness of the main structural parts of the air preheater must be calculated in the design and strengthened
when necessary.

(5) To control the evolution control of NH3, it should be controlled below
3ppmV under normal circumstances.
If the evolution of NH3 increases, the foreseeable ammonium bisulfate ash plugging will also be very serious
.

(6) After setting the SCR denitration system, the conversion rate from SO2 to SO3 in the flue gas increases, and the escaped NH3 in the system reacts with SO3 to form ammonium bisulfate, and ammonium bisulfate will form condensation in a certain temperature section, which is moderately acidic and has great viscosity, easy to adhere to the surface of the heat exchange element of the air preheater, aggravating the corrosion and ash plugging of the heat exchange element, and this temperature section is just in the middle and low temperature section
of the air preheater.

Third, the cold end heat exchange element of the air preheater is transformed

By analyzing the serious impact of SCR denitration system on the cold end heat exchange element of the air preheater, the cold end heat exchange element of the air preheater should be modified at the same time as the SCR denitration system is installed in the unit, and the selection of the cold end heat exchange element of the air preheater should comprehensively consider the sulfur content of coal, ash-sulfur ratio, and the conversion rate of SO2 to SO3 in SCR to prevent corrosion and ash
plugging of the heat exchange element of the SCR air preheater 。 The cold end heat exchange element should choose a large channel plated enamel element to prevent corrosion, dust plugging and easy washing, and requires uniform coating thickness, good surface finish, good edge wrapping, high enamel adhesion, good component flexibility, low area porosity, strong corrosion resistance, and long
life.

(1) Selection of
enamel plating process method The selection of enamel plating method and process is very important
to obtain enamel heat exchange elements with excellent pressing toughness and high corrosion resistance.
At present, the enamel plating process mainly includes three types
: "dry electrostatic spraying", "wet electrostatic spraying" and "wet dipping".
According to the characteristics of the enamel-plated heat exchanger element and the application occasions, high-quality enamel plating should have excellent corrosion resistance, excellent adhesion to metal substrates, high mechanical and thermal shock resistance, air erosion and wear resistance, and smooth and homogeneous enamel surface
.

The base steel plate should be made of well-made imported decarburized steel to reduce the porosity of the area and improve the service life of
the enamel.
Before the steel plate is plated, it must also undergo alkali washing, pickling, rinsing and other processes to ensure the quality of the substrate, and in order to strengthen the bonding force between the enamel and the steel plate, the surface of the substrate should also be nickel-plated
.
"Wet dipping" process, due to the absence of electrostatic adsorption process, and due to gravity (glaze will have hanging phenomenon), the adhesion, thickness uniformity, surface finish, porosity, edge wrapping rate, etc.
of the product are far from reaching the level of
"electrostatic spraying" process production.
Therefore, it is recommended to use "dry electrostatic spraying" or "wet electrostatic spraying" on the enamel plating
process.

(2) Selection of the waveform of the heat exchange element In the past, the power plant did not distinguish from the high and high temperature section in the waveform selection of the cold end heat exchange element of the air preheater, usually in order to improve the heat exchange efficiency is to use the heating surface of
the compact corrugated plate, but the cold end heat exchange element due to the decrease in wind pressure, the cold end is easy to produce condensation blockage, etc.
, if the use of compact corrugated plate is easy to adhere, the second is once attached, it is difficult to remove (there are purging blind spots), Even if it is repeatedly cleaned with high-pressure water, it is difficult to clean it, which reduces the heat exchange area of the component and reduces the heat exchange efficiency
of the air preheater.
Serious will also repeatedly appear blockage, pressure drop and other operating obstacles, power plant to increase the output of the fan, increase the number of high-pressure water cleaning, etc.
, increase the operating cost of the unit, and even more serious will lead to the unit can not achieve normal output
.

After the SCR denitrification transformation of the unit, ammonium bisulfate that is easy to adhere to the surface of the heat exchanger element of the air preheater will be generated at the cold end of the air preheater, thereby aggravating the ash
plugging of the heat exchanger element.
Therefore, it is recommended to consider the waveform of the large channel with less oblique direction and small resistance in the corrugation selection of the cold end heat exchange element, so that it is straight-through in the direction of flue gas flow, and there is no small ripple, eliminating the oblique air flow to make the channel uniform and regular, and the air flow distribution is uniform
。 It is characterized by large flue gas circulation section, smooth wave type, ammonium bisulfate, dust, cinder, etc.
in the operation of the heat exchanger is not easy to adhere, soot blowing steam and high-pressure water penetration ability is strong, easier to clean, there is no "shrinkage point" or local small gap that is easy to cause dirt accumulation, so the heat exchange surface is not easy to block, and the pressure loss will not rise after long-term operation, effectively reducing the power consumption
of the three major fans.