JPS6127402A - Method of washing waste-heat recovery boiler - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention The present invention relates to the cleaning power of an exhaust heat recovery boiler equipped with a denitrification device that uses ammonia as a reducing agent.

<Technical background of the invention and its problems> In recent years, there has been a strong demand for efficiency improvement in power plants from the viewpoint of energy and resource conservation. As a means of improving efficiency, increasing the capacity of power plants, increasing
This includes higher pressure. Furthermore, combined cycles, which combine a gas turbine and a steam turbine, are attracting attention because they are significantly more efficient than conventional steam turbines alone. In the combined cycle, an exhaust heat recovery boiler is used as the steam generator used in the steam turbine. Needless to say, in this exhaust heat recovery boiler, the efficiency becomes higher as the temperature of the exhaust gas discharged from the chimney is lowered as much as possible.

Therefore, it is used until the exhaust gas temperature approaches 100°C.

On the other hand, from the perspective of environmental conservation, SOx and NOx in exhaust gas are
Concentrations are becoming increasingly tightly regulated.

In particular, when heavy oil is used as fuel, the combustion of sulfur compounds and nitrogen compounds contained in the heavy oil generates 130x and NOx.
occurs. Desulfurization equipment and NOxO as a measure to reduce SOx
x Denitrification equipment is often used as a countermeasure against low electricity. The mainstream method of recent denitrification equipment is to add ammonia to the exhaust gas and pass it through a catalyst, thereby converting NOx into harmless gas. The reaction is as follows.

6NO+4NH, -5N, +6H, 06No2-1-
f3 NH,→7N, +12H20 However, in the case of the ammonia reduction method, since residual ammonia exists, ammonium sulfate is produced by the following reaction with SO3 in the exhaust gas.

2NH1+ SO, +H, O-go (NH, ), 80°The generated ammonium sulfate is unstable at high temperatures of about 200°C and becomes acidic ammonium sulfate through the following reaction.

(NH,), 80. -GoNH, H80, +NH.

(NH4), 80. +803+H20 nib 2NII, I
S.O.

This acidic ammonium sulfate has the property of precipitating at around 150°C, and in exhaust heat recovery boilers that are used until the exhaust gas temperature approaches 100°C, the acidic ammonium sulfate adheres to the outer surface of the heat exchanger tube of the economizer. Significantly reduces performance. Conventionally, in order to prevent performance deterioration, a method has been adopted in which acidic ammonium sulfate is dissolved by periodic cleaning. However, when acidic ammonium sulfate is dissolved in water, it releases hydrogen ions, which significantly lowers the pH of the washing water. For this reason, it was experienced that one heat transfer tube was corroded during cleaning.

<Objective of the Invention> An object of the present invention is to provide a method for cleaning an exhaust heat recovery boiler that prevents corrosion of heat transfer tubes due to cleaning water and can be cleaned even during operation and stoppage.

<Summary of the Invention> The method for cleaning an exhaust heat recovery boiler according to the present invention is characterized in that alkaline cleaning water containing caustic soda and ammonia is used as the cleaning water.

<Embodiments of the Invention> Hereinafter, the method for cleaning the exhaust heat recovery boiler 2 according to the present invention 1 will be explained as follows.
The explanation will be given using Fig. 3 and Fig. 2.
This conventional vertical heat recovery boiler will be explained. In Fig. 3, water is supplied to the energy saver 2 by a water supply pump 1.
1 and heated by the exhaust gas 7. heated. The heated feed water enters drum 3. Circulation pump 4 for drum water
The water is then sent to the evaporator 5, where it becomes a two-phase flow of water and steam, and returns to the drum 31 again. The saturated steam separated by the drum 3 enters the superheater 6, becomes superheated steam, and is sent to a turbine (not shown).

The NOx contained in the exhaust gas 7 is converted into harmless N! by the denitration catalyst 8 installed between the evaporator 5 and the superheater 6. becomes. Note that the denitrification catalyst 8 is placed at a position between the evaporator 5 and the superheater 6 because it is most efficient at a temperature of approximately 300 to 400°C. Since the ammonia added as a NOx reducing agent is present in the exhaust gas as residual ammonia, it reacts with SOx in the exhaust gas to become ammonium sulfate.

This ammonium sulfate is unstable at high temperatures, and becomes acidic ammonium sulfate and adheres to the outer surface of the economizer heat exchanger tube. If the performance deteriorates due to this +j observation, the plant is stopped, the denitrification catalyst is removed, and the cleaning pump 9 sends cleaning water to the sprinkler pipe 10 to clean the economizer 2. The cleaning water I used to clean the economizer 2 contains acidic ammonium sulfate, so it becomes low pH water and corrodes the heat exchanger tubes. For this reason, various methods have been studied to reduce the cleaning time as short as possible.

Therefore, in the method of cleaning an exhaust heat recovery boiler according to the present invention, first, the exhaust heat recovery boiler to which the new cleaning method is applied is a horizontal exhaust heat recovery boiler 20 that can be cleaned without removing the denitrification catalyst, as shown in FIG. It is composed. 1 is different from FIG. 3 in that it has a horizontal type and a vertical type, and since the component configurations are the same, the same parts are designated by the same reference numerals.

In the present invention, the performance of the economizer 2 is detected by the temperature sensors 11 and 12 provided at the inlet and outlet of the economizer 2, and when the performance decrease reaches a reference value, the solenoid valve 14 is activated by the controller 13. Cleaning can be performed by opening the water supply and directing a portion of the water supply to the water spray pipe 10. Alkaline volatile substances such as ammonia are added to the boiler feed water to suppress corrosion of the system, and this has the effect of neutralizing hydrogen ions released from acidic ammonium sulfate during cleaning, protecting the heat exchanger tubes.

Furthermore, performance monitoring can be performed without any major performance deterioration.

FIG. 2 shows a horizontal waste heat recovery boiler 20 that is easy to clean, and shows a cleaning method when the boiler is stopped. In FIG. 2, the same reference numerals as in FIGS. 1 and 3 indicate the same parts. In the present invention, a water quality monitoring sensor 15 such as sulfate ions and ammonium ions is provided in the water used to wash the economizer 2, and when the concentration drops below a standard level, the solenoid valve 1 is sent to the controller 131.
In this method, cleaning is completed by opening 4.

<Effects of the Invention> As described above, the cleaning method of the present invention has the advantage that by using alkaline cleaning water as the cleaning water, the released hydrogen ions are neutralized and the heat exchanger tubes can be preserved, and the boiler can be cleaned. The cleaning time can be determined by performance monitoring, and the end of cleaning can be determined by monitoring the quality of the cleaning water.

[Brief explanation of drawings]

Figures 1 and M2 are system diagrams showing a horizontal exhaust heat recovery boiler to which the cleaning method for an exhaust heat recovery boiler according to the present invention is applied, and Figure 3 is a system diagram showing a conventional vertical exhaust heat recovery boiler. . 1...Water pump 9...Washing pump 2...
Energy saver 10...Water pipe 3...Drum
11...Temperature sensor 4...Circulation pump
12... Temperature sensor 5... Evaporator 13.
... Controller 6 ... Superheater 14 ... Pump valve 7 ... Exhaust gas 15 ... Water quality sensor 8 ... Denitration catalyst 20 ... Exhaust heat recovery boiler agent Patent attorney Noriyuki Chika (and others) 1 person) C:J
-4

Claims (3)

[Claims] (1) A method for cleaning an exhaust heat recovery boiler equipped with an ammonia reduction and denitrification device, which comprises cleaning scale adhering to the outer surface of heat exchanger tubes in a low-temperature section of the boiler with an alkaline aqueous solution. (2) The performance of the boiler low-temperature section is monitored, and when the performance deterioration reaches a standard value, cleaning is performed using supplied water or a part of the boiler water and external cleaning water. The cleaning method for the exhaust heat recovery boiler described. (3) The method for cleaning an exhaust heat recovery boiler according to claim 1, wherein completion of cleaning is determined by monitoring water quality after cleaning.