The problem of denitrification transformation of boiler air preheater

Aug 28,2020

Whether it is SCR or SNCR denitrification method, ammonia water, liquid ammonia, urea and other ammonia-producing substances are injected into the flue gas, and they are reduced to nitrogen by the reaction of NH3 and NOx in the flue gas.

However, it is difficult to completely react out NH3. The residual ammonia component reacts with SO3 and water vapor in the flue gas to produce ammonium sulfate and ammonium bisulfate.

Ammonium sulfide [ (NH4) 2SO4] is a solid powder in the flue gas temperature range of the preheater section (below 450 ° C), and it is only produced in large quantities when the residual composition of ammonia is very high (generally when the volume concentration of NH3 reaches dozens of ppm). The amount generated is very small, contained in ash, and has little effect on the preheater.

Ammonium bisulfate (NH4HSO4) exists in all ammonia-containing coal-fired boiler flue gas, and the temperature zone where ammonium bisulfate transitions from gaseous to liquid is exactly in the smoke temperature zone flowing through the preheater part.

Therefore, after the denitrification equipment is added to the unit, the preheater must be modified synchronously, the enamel-plated heat transfer element with high cold end is adopted, and the dual-medium soot blowing system is added.

After the air preheater is modified, it may have the following effects on the performance of the boiler:

1. The effect of flue gas temperature

After the transformation of the out-of-stock system of the unit, the SCR catalyst improved the conversion rate of SO2 to SO3, so the corrosion of the cold end of the preheater was intensified. In order to protect the equipment behind the preheater (such as electrostatic precipitator, flue, etc.), appropriately increasing the exhaust gas temperature of the boiler is conducive to protecting these equipment. In the flue gas, due to the low ammonia content, the flue gas composition does not change much. When the flue gas temperature at the outlet of the economizer does not change much, the preheater passes through the heat exchange surface of the heating end, and the exhaust gas temperature is generally not affected. However, if the blockage of the cold section is not cleaned in time, the exhaust temperature will rise, but it is not enough to endanger the safe operation of the boiler.

When the boiler is under low load conditions, the flue gas temperature decreases and the ammonia escape rate increases, resulting in the drift of the ammonium bisulfate deposition belt to the hot end of the preheater, which may cause the hot end of the preheater to be blocked. Therefore, we generally only calculate the metal temperature field under low load and worst working conditions, so that the other working conditions must be satisfied.

2. The effect of differential pressure resistance

Due to the increase in the total height of the heat transfer elements, the smoke air resistance of the preheater usually increases by 150-200Pa, but if the cold section is blocked, the resistance increases significantly. Usually when the ammonia concentration is below 1ppm, the amount of ammonium bisulfate generated is very small, so the blockage of the preheater is not obvious. For example, the escape of NH3 increases to 2 ppm. The test of Japan AKK shows that the resistance of the preheater increases by about 30% after 6 months of operation. If the escape of NH3 increases to 3 ppm, the resistance of the preheater increases by about 50% after 6 months of operation. This has a greater impact on the fan.

3. Influence on air leakage of preheater

The use of SCR usually increases the negative pressure on the flue gas side of the preheater by about 1Kpa. If a heat transfer element with a low heat transfer coefficient is used as a cold junction element, in order to achieve a smoke exhaust temperature similar to that of a conventional preheater, it is necessary to increase the total height of the heat exchange element of the preheater, which generally increases the smoke-air resistance of the preheater slightly. The increase in the smoke air pressure difference of the preheater inevitably causes the air leakage rate of the preheater to rise. Usually for the 300,000-grade boiler preheater, the calculation shows that the increase in the air leakage rate is 0.5-0.8%; for the 60-700,000-grade boiler preheater, The increase in the air leakage rate is 0.4-0.6%. Since the preheaters currently use a perfect double-channel sealing system, the influence of the smoke air pressure difference is smaller than that of the earlier single-channel sealed preheater, and the air leakage rate of the preheater generally rises slightly.

4. Effects of flue gas ash

When there is very little ash in the flue gas, ammonium bisulfate exists in the form of droplets in the liquid phase region. When the ash/sulfur ratio of the fuel is less than 7, the ash can only absorb part of the ammonium bisulfate droplets, but the viscosity of the ash particles is very large., and some pure ammonium bisulfate droplets are adsorbed on the surface of the heat exchange element together; when the fuel ash/sulfur ratio is greater than 7, almost all ammonium bisulfate droplets can be adsorbed when the dust in the flue gas is evenly dispersed and distributed. At this time, the viscosity of the ash is also much larger than that without ammonium bisulfate droplets.

Generally, when the fuel composition satisfies the ash/sulfur ratio greater than 7, the inlet fortification temperature of the cold end heat transfer element of the preheater can be appropriately reduced, and the amplitude is usually 22 ° C (40F).

When the denitrification device (SCR or SNCR) is arranged in the flue at the front of the preheater, such as burning ash fuel (such as coal), it is called a high dust arrangement. The denitrification device is arranged in the flue behind the dust collector or in the flue that does not meet the ash/sulfur ratio greater than 7 (even in the front of the preheater) is called a low dust arrangement.

High ash content does not always mean that the operating conditions of the preheater have become safe. It is also very important to ensure that the residual NH3 is evenly distributed in the flue gas. The distribution uniformity of ammonia injection, reaction and departure from the denitrification unit should be well controlled to avoid localized high concentration areas. In order to ensure uniform flue gas composition, it is necessary to use diversion equipment in the flue.

5. Effect on preheater corrosion

At present, the most common catalytic media used in SCR systems are titanium oxide and vanadium oxide, which can greatly improve the denitrification efficiency. However, some SO2 is also catalyzed to SO3 at the same time. The average data recorded abroad during the service life of SCR catalysts is about 2-3% increased conversion rate. For the original conventional preheater design, for some low-sulfur coals (converted sulfur content below 1.5%), the cold end heat transfer element design only considers the use of ordinary corrosion-resistant materials (usually Corten steel). After the conversion rate is increased, the service life of the cold section heat exchange element of the preheater will be shortened. For the design of exhaust smoke temperature around 130 ° C, the corrosion zone of the cold end of the conventional preheater is only in the range of 100-200mm at the cold end. After increasing the SO3 conversion rate, the sulfuric acid dew point usually rises by 5-10 ° C. The sulfuric acid corrosion zone will rise to 250-450mm, and the 300mm height of the cold section set by the ordinary preheater is not enough. Therefore, some cold end components and sealing components of the preheater rotor (working in the sulfuric acid corrosion zone) must use materials such as Corden steel and NS1, and the heat transfer element itself should use enamel surface as much as possible.

The corrosion of ammonium bisulfate itself is weaker than that of sulfuric acid. From the perspective of foreign use, corrosion is also manifested as electrochemical reaction. Due to its firm adhesion to the surface of the component, it is manifested as pitting corrosion, which is distributed in the range of 600mm-900mm from the cold end of the preheater (varies with the working temperature of the preheater).

6. Analysis of the influence of different operation stages of SCR catalyst on preheater

At present, the guaranteed life of each supplier of SCR catalyst is generally about 3 years. However, at the beginning of SCR operation, due to the good activity of the catalytic medium, the ammonia escape rate can be controlled to a low level (< 2ppm). While the denitrification task is well completed, the negative impact on improving the SO2 conversion rate is also large (the highest record value is 7.7%). At this time, the main task facing the operation of the preheater is to control the cold end sulfuric acid corrosion.

After the catalyst is used for 15,000-20,000 hours, the activity is usually reduced by about one-third. At this time, if a high NOx control level is pursued, only the ammonia injection amount can be increased, resulting in a higher ammonia escape level, which usually reaches more than 5ppm., thereby generating a large amount of ammonium bisulfate.

Usually, the ammonia escape rate is controlled by reserving the catalyst for future layers. In the initial commissioning stage of SCR, 2 layers of catalysts (3 layers are also useful) are used. After two years of use, a new layer of catalyst is added (a total of 3 or 4 layers work at the same time). After 3 years of use, replace the catalyst that has reached its working life. This always keeps the ammonia escape rate level below 3ppm.

For SNCR, due to the low reaction efficiency (less than 50%), the ammonia escape rate is very high (above 50-200ppm) in the entire denitrification process, which has a great impact on the ash plugging of the preheater, so the boiler system should try to avoid using SNCR denitrification method.

7. The influence of SCR on the operation and maintenance of the preheater after being put into use

From the analysis of sequence 6, it can be concluded that as the out-of-stock equipment runs for a longer and longer time, the escape rate of ammonia will become larger and larger, which will inevitably cause the condensation of ammonia hydrogen sulfate to increase. In addition to using high-end enamel heat transfer elements at the cold end, strengthening soot blowing is also a necessary means to keep the preheater running normally. A wrong idea is to solve the problem of soot accumulation by increasing the soot blowing pressure and soot blowing frequency. Because the high soot blowing vapor pressure (above 2Mpa) may crack the element, the torn element is bent and deformed, and the debris blocks the channel, so that the subsequent soot blowing effect is completely lost. This method is completely undesirable.

At present, the commonly used cleaning method is to use a dual-medium (steam and high-pressure water) soot blower (semi-retractable or fully retractable), usually one at the cold end and one at the hot end. In normal use, use steam to blow soot to remove the dust accumulated on the upper and lower end faces of the heat transfer element. When the resistance of the preheater rises by 50-60%, rinse with high-pressure water.

High-pressure water flushing can be used in the isolated state of a single preheater, but only at the cold end. The high-pressure water flushing at the hot end is only used when cement samples are blocked in the hot section layer. High-pressure or low-pressure water will produce great temperature stress on the rotor, and even cause serious irreversible deformation of the rotor, which must be carried out carefully.

The nozzles for high-pressure water flushing are carefully selected. Generally, a small diameter (about 1.5mm, water pressure 10-20Mpa) is used, and several nozzles are arranged in a centralized manner to improve the cleaning effect. However, one flushing takes a long time, and it takes about 20 hours for a fully telescopic type (600,000 unit), and the semi-telescopic type time can be halved (the water volume per unit time is doubled).

When cold end in-line water flushing is necessary, it is necessary to ensure that the preheater is completely isolated. Do it when the rotor metal temperature is cooled below 120 ° C. Because even with cold end water flushing, high pressure water can generally penetrate the entire rotor and reach the top of the preheater. Due to the slow cooling of the preheater in the isolation stage, it is difficult for the flue gas side to be completely separated (the baffle cannot achieve 100% isolation). An effective method is to set up a flue gas outlet air bypass to connect the cold secondary air duct and the preheater outlet flue, and run the blower at low load to ensure the rapid cooling of the preheater rotor (usually about 2-3 hours). A simpler approach is to open the preheater flue gas side access door so that the pressure on the flue gas side of the preheater is greater than that of the front flue of the isolation baffle, thereby preventing the flue gas from passing through the preheater rotor during the cleaning stage. When cleaning, the blower of the isolated preheater should be turned on to ensure that the rotor is dried and the flue gas side pressure of the preheater is maintained higher than that on the other side of the baffle. After cleaning, the rotor should continue to be dried with the supply air.

The usual preheater flushing interval is shown in Figure 4. In Figure 4, since Japanese boiler units usually use fuels with lower ash content, ammonium bisulfate is not sufficiently adsorbed by ash, which is manifested in a short cleaning interval; German units use higher coal ash content and a relatively long cleaning interval. However, the ammonia escape rate rises from 2ppm to 3ppm, which will significantly shorten the cleaning interval regardless of the ash content. In recent years, both Japan and Europe have proposed to control the ammonia escape rate level below 2ppm, which will undoubtedly greatly reduce the cleaning requirements for preheaters. If the cleaning interval can be controlled for more than 10 months, preheater cleaning can be included in the normal annual shutdown maintenance.