Summary: This article studies the soot fouling on SCR reactor in power plant. It reveals mechanism of fouling and its harm to SCR. A comparison of steam soot blower and acoustic soot blower is made in different respects.
SCR (selective catalytic reduction) technology is patented invention by American company Eegelhard. The reaction principle is to spray NH3 at the back-end of flue, under aerobic condition, NOx in flue gas would be turned into nonhazardous N2 and H2O when passing through catalyst by a fast reduction process. Due to the usage of catalyst, reaction temperature is reduced a lot. SCR denitrification technology is featured with no by-product, no secondary pollution, simple device structure, high efficiency (higher than 90%), reliable operation, easy maintenance, etc. Now, SCR is widely used by coal fired power plant to control NOx emission.
SCR reactor is normally installed between economizer and air preheater, where dust content is high. Dust has great influence on the catalyst activity.
2. SCR soot deposit
2.1 Soot deposition
Fly ash deposition on SCR reactor is a complex physical & chemical process, involving processes like heat transfer, mass transfer, particle motion, adhesion of fly ash, etc.
Flue gas temperature in SCR is normally 300-400℃. Soot deposit on SCR reactor is loose. Fly ash generated during combustion would float to upper stream of SCR reactor and concentrate along with flue gas. Finally, they cohere and fall down on the surface of catalyst to form clogging. Loose soot deposit is interfered by gravity force, VDW force, surface tension force. Bonding strength of this type deposit is 50-250Pa, it is easy to remove.
Acid vapor and water vapor in flue gas would condensate and adhere with fly ash to form soot deposit. Matters containing Na, K, Ca, Si in soot would react with acid and become sulfate. This would increase stickiness and thickness of soot. As alkali metal content is high, catalyst poisoning happens at a high chance. Compared to loose soot, this type of soot is interfered by chemical force additionally. This sticky soot deposit is not easy to remove. But if temperature inside SCR reactor is higher than the condensation temperature of acid vapor, this type of soot won’t occur.
CaO would settle down in the micro-channel of catalyst. Due to mass transfer and adhesion, CaO would adhere and react with SO3 to form CaSO4.
2.2 Influence of soot deposit on the catalyst
Influence of soot deposit on the catalyst activity includes chemical poisoning and physical poisoning.
Chemical poisoning means loss of activity due to chemical absorption by active site of catalyst. Alkali metal like Na, K could react directly with active ingredient of catalyst and make them lose activity.
Abrasion of catalyst and clogging & blocking by fly ash is physical poisoning. Fly ash in flue gas would cause abrasion of outer wall or even inner wall. Surface area of catalyst is reduced. Clogging would block the channel to active site. Effective volume of catalyst is reduced.
The key to stable operation of SCR reactor is to clean soot deposit on catalyst and keep its activity. Now equipment widely used for SCR cleaning is steam soot blower and acoustic soot blower.
3. SCR soot cleaning technology
Due to its safety and reliability, steam soot blower is conventional technology. Acoustic soot blower was invented in 1968 in Sweden, and introduced to China in 1980s and 1990s. Now acoustic soot blower is also used for SCR.
3.1 Working principle and cleaning strength
Working principle of steam soot blower is to spray steam of certain pressure and temperature at a high speed onto heating surface to clean soot deposit. The power of sprayed steam decides the effectiveness of soot blowing.
Working principle of acoustic soot blower is to generate sonic wave of certain sound pressure and frequency by metal diaphragm with compressed air. The sonic wave is enlarged by horn. Sound energy is transmitted to soot deposit. Soot would fall down or is taken away by flue gas under the vibration caused by sonic wave.
Steam soot blower uses steam as cleaning medium and blows directly to soot. Its cleaning strength is higher. It is more effective for the low melting point and sticky soot. It is very suitable for furnace where soot is sticky. Acoustic soot blower uses sound energy to clean soot with lower strength. It is not as effective as steam soot blower for slag and sticky soot.
3.2 Cost on operation, maintenance and equipment
Sonic soot blower consumes compressed air, which is available in power plant. The investment in equipment is lower.
Sonic soot blower is featured with simple structure, reliable performance, easy start & stop, safe operation. Cost on compressed air is only 10-20% of steam consumed by steam soot blower. Operation cost is much lower. Once sonic wave is sent into SCR reactor, it could automatically transmit to the soot deposit. No need for big and complex retraction & rotation mechanism. The only wearing part is the metal diaphragm, whose service life is normally longer than 2 years. So the money spent on sonic soot blower is less.
3.3 Safety & Reliability
Filling ratio of two types of soot blower differs quite a lot. Due to channel in catalyst is small, structure is irregular, steam as cleaning medium has lower filling ratio. There is dead corner for steam soot blower.
Due to reflection and refraction, sonic wave could always go inside micro-channel of catalyst and effectively prevent soot deposit, no matter the installation location and direction of sonic soot blower. there is no dead corner.
Steam used as blowing medium would increase humidity and dew point of flue gas. Steam blowing on surface of catalyst would cause fatigue damage. Long time operation under this situation would make catalyst lose its function. Sonic soot blower is non-contact cleaning, it has no side effect like this.
If frequency of sonic wave is close to the inherent frequency of pipes, it would cause resonance of pipes and damage. Within certain frequency range, the noise of sonic soot blower is harmful for human body.
Working radius of two soot blowers is different. Steam soot blower transfers energy by the motion of steam. The motion will face various resistance, dissipation of energy is big. Sonic soot blower transfers energy by the vibration of air molecules. Air molecules has no directional migration except the elastic vibration around the equilibrium position, dissipation of energy is nearly zero. So working radius of sonic soot blower is bigger.
SCR reactor is installed at high dust content area, where soot fouling happens easily. Soot would cause abrasion and clogging of catalyst, and active site of catalyst would absorb chemical composition of soot. Activity of soot is greatly influenced. The key to stable operation of SCR reactor is to clean soot deposit.
Now both steam soot blower and sonic soot blower are used. After the comparison, we can conclude that sonic soot blower could make catalyst clean and keep the activity of catalyst to the maximum. From the view of safety, reliability or economic benefits, sonic soot blower is very suitable for SCR reactor soot cleaning.