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Infrasound soot cleaning VGB PowerTech 8 l 2013

Authors

Kurzfassung

Online Rußblasen mit Infraschall

Das Unternehmen Infrafone wendet seit über 30 Jahren Infraschalltechnik als Rußblastech­nik an. Die von Infrafone eingesetzte Rußblas­technik steigert die Effizienz sowie die Verfüg­barkeit und Reisezeit von großtechnischen und auf Schiffen installierten Dampferzeugern. Im vorliegenden Beitrag wird die von Infra­fone eingesetzte Infraschalltechnik sowie deren Wirkungsweise als Rußbläser dargestellt und beschrieben. Es schließt sich ein Vergleich bzw. eine Abgrenzung zur Hochton­Reinigungs­technik an. Abschließend werden anhand von Fallstudien Ergebnisse und Einsparpotenziale durch Infrafone­Rußblastechnik aufgezeigt. l

Ph.D. Èric Torra i FernàndezDevelopment EngineerM.Sc. Martin EllebroDirector EngineeringInfrafone ABStockholm/Sweden

Introduction

The paper is to provide a deeper insight and better understanding of infrasound cleaning. The authors try to summarise the most important technical aspects of infra-sound cleaning of heat exchange surfaces.Infrafone, with headquarters in Stock-holm, Sweden, is the oldest company in the field of soot cleaning using infrasound, with more than 30 years of experience. The technical development has resulted in a very powerful product and acoustic model-ling software that is unique. Infrafone has installed infrasound cleaning equipment on 65 waste-to-energy and bio-fuel boil-ers (most of them during the last 8 years), 11 coal-fired boilers and 900 ships. In power plants and heating plants, the technology is mainly used for cost-efficient on-line cleaning of soot (fly-ash) off econ-omisers, tubular air preheaters, catalysts, ducts, electrostatic precipitators (ESP), and Ljungstrom air preheaters. Cleaning of superheaters is possible if the soot build up is not sticky. On marine cruise ships and cargo ships the technology is used for on-line cleaning of exhaust gas economisers, ducts, and oil fired boilers.

Infrasound

The frequency of sound is related to the wavelength λ according to the expression λ= c/f. The wavelength is the distance between two points of the same phase in consecutive cycles of a wave. The speed of sound in air is approximately 340 m/s at room temperature. Hence high frequen-cies of sound have short wavelengths and low frequencies of sound have long wave-lengths. The infrasound cleaners are using infra-sound as an acoustic cleaning method. The infrasound is the sound produced at very low frequencies, typically less than 20 Hz. At room temperature a sound wave with 20 Hz will have a wavelength of around 17 m. However, a sound wave of 250 Hz will only have a wavelength of 1.36 m. From a cleaning point of view, low frequen-cy sound has several advantages. One is that infrasound is omnidirectional, which means it spreads in all directions. Further-more, sound absorption in tube bundles is very low at these frequencies. This means that the sound produced by infrasound cleaners can reach all parts of a tube bun-dles in for example an economiser, which in many cases means a lower and more stable differential pressure compared to steam soot blowing.

Online soot cleaning using infrasoundÈric Torra i Fernàndez and Martin Ellebro

A third property of infrasound that is ad-vantageous from a soot cleaning point of view is the high degree of turbulence that infrasound creates in the flue gas stream. The typical flue gas velocities at heat ex-changers in power plants are low and the flue gas flow has a low degree of turbu-lence. This results in areas with a very low flue gas velocity near the surface of the tube bundles. Soot particles accumulate in these areas with low gas velocity and grow to large deposits with time. For efficient re-moval of the soot particles attached to the heat exchangers or other surfaces with in-frasound, the particle displacement of the flue gas has to be maximised. The particle displacement is the magnitude that is relat-ed to the distance of the movement of the particles when excited by a sound wave. The particle displacement is inversely pro-portional to the frequency. Thus, low fre-quency sound as the infrasound results in large particle displacement. The oscillation with large particle displacement creates a high degree of turbulence in the flue gas and this high degree of turbulence around the tubes of a heat exchanger helps to keep the tube surface clean ( F i g u r e 1 and 2 ). Infrafone’s infrasound cleaner is producing sound just for a few seconds every few min-utes. This means that the turbulence pro-duced by the sound wave occurs typically only during 1 to 2 seconds. The system is a continuous and dry on-line cleaning method since it operates every few min-utes. The infrasound cleaners cannot clean surfaces where the soot build up is sticky or hard, i.e. they keep clean surfaces already cleaned. In summary, the main advantages of using infrasound for cleaning are:

– Large wavelengths, – Low sound absorption, – Omni-directionality, – Large particle displacement,

high turbulence, – Dry cleaning method, – Non-abrasive, and – Continuous cleaning method.

Acoustic modelling

Infrafone has developed an acoustic mod-elling software that is used for simulating the sound propagation in great detail. An acoustic model is designed for each boiler where an infrasound cleaner is to be in-stalled. All the dimensions of the boiler and the heat exchangers are introduced in this model. Flue gas temperature distribu-tion and flue gas velocity are also entered since these are related to wavelength of the

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VGB PowerTech 8 l 2013 Infrasound soot cleaning

sound and attenuation of sound energy in the tube bundles. By using the acoustic model it is possible to choose the optimal installation position, the optimal size, and number of sonic cleaners as well as the fre-quency of sound in order to obtain the re-quired acoustic power in the desired clean-ing area.The model is always confirmed by carrying out sound pressure level measurements in-side the boiler ( F i g u r e 3 ).

Infrafone’s infrasound cleaners

The infrasound cleaner consists of an at-tachment socket, a diffuser, a resonance tube, a resonance chamber and a pulsator. The attachment socket is welded directly to the flue gas duct. The resonance tube can be designed in different shapes in order to fit in the installation position and sur-roundings (F i g u r e 4 and 5 ). The frequency of the infrasound can be fine-tuned by a moveable plate inside the resonance chamber. The total length of the sonic cleaner can be adjusted in order to obtain the desired frequency simply by

moving the resonance plate along the reso-nance chamber.The pulsator is placed on the top of the resonance plate and is connected to a com-pressed air source. The compressed air should have a pressure of 6 to 8 bar(g). The pulsator produces the air pulses that gen-erate infrasound. It consists of a cylinder, a piston, and a titanium spring. The axial mechanical moving system is very simple and can be easily maintained. There are no rotational mechanisms involved and no electric or electronic instrumentation is required.A unique feature of the infrasound cleaner allows the pulsator to auto-regulate itself by the positive feedback produced by the reflected acoustic waves in the resonance tube. In this way, the maximum acous-tic power is always generated by the in-frasound cleaner independently of load changes and changes in flue gas tempera-ture in the boiler system. Without posi-tive feedback, a change of some degrees in temperature inside the boiler means a decrease in acoustic power emitted by the infrasound cleaner.

The acoustic power delivered is propor-tional to the square of the cross section area of the open end of a sonic cleaner. Inf-rafone has several infrasound cleaner sizes. The attachment socket’s diameter of the biggest and most powerful sonic cleaner is 1,500 mm. The infrasound cleaner is very powerful, so to prevent any movement of the sonic cleaner, reinforcements around the at-tachment to the boiler wall is generally recommended. Infrafone has designed a vibration damper that is tuned in for each sonic cleaner, so vibrations transmitted to the boiler are kept low. Infrafone also has an automatic control system that mini-mises the operating time of the infrasound cleaners, which is also crucial for obtain-ing high cleaning effect and low vibration levels.

Comparison with sonic horns

Sound at the audible frequency range is used by sonic horns which typically use sound frequencies of around a few hun-dred Hz, i.e. the area which they can keep

Laminar flow

Fig. 1. Laminar flow.

Turbulent flow

Fig. 2. Turbulent flow.

Cleaning effect

Fig. 3. Acoustic model.

Attachmentsocket

Pulsator

Resonancechamber

Resonance tube

Diffuser

Fig. 4. Infrasonic cleaner.

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Infrasound soot cleaning VGB PowerTech 8 l 2013

clean is small due to the short acoustic wavelengths.Furthermore, the sound frequencies used by sonic horns are not omnidirectional and the sound absorption is much higher for this sound frequency range than for in-frasound. For this reason, sonic horns are producing a local cleaning effect close to the installation position. Due to the limited cleaning range, several sonic horns must be installed. However, just one, or in some cases two, infrasound cleaners are usually installed to cover a large cleaning area.Furthermore there is a limit for how large cross-section at the cleaning area that can be cleaned by sonic horns due to the long distance from the installation locations at the walls to the middle parts of e.g. an economizer.

Potential savings from using infrasound cleaning

– Reduced operational and maintenance costs

– Savings on operation of steam soot blow-ing and shot cleaning

– Saved steam can be used for increased production or reduced fuel consumption

– Savings on replacement of tubes in heat exchangers through extended tube life-time

Stable Δp, which means: – Less outages, increased availability,

higher energy production

– Less outages, more burned waste – Less outages, improved working environ-

ment – Reduced fan power consumption – Lower risk for high flue gas velocity in lo-

cal areas – Savings on replacement of tubes in heat

exchangers through extended tube life-time

– Cleaner electrostatic precipitator, cata-lyst

– Compliance with environmental regula-tions

– Savings on replacement of catalyst ele-ments through extended element life-time

Case studies

E.ON Norrköping P14, Sweden, 75 MWth, boiler equipped with circulating fluidised bed combustion (CFB)Commissioning: 2006Cleaning area: Vertical economiser ( F i g -u r e 6 )Fuel: Industrial/household wastePerformance:

– Steam soot blowing reduced from 3 to 4 times/day to once/week

– Increased production of electricity – Less wear on economiser tube bundles – Lower and more stable differential pres-

sure, Δp than with steam soot blowing operating 3 times/day.

SYSAV boiler 1&2 Malmö, Sweden, 2 x 29 MWth, grate firingCommissioning: 2008 and 2009Cleaning area: vertical evaporator and economiserFuel: Industrial/household wastePerformance:

– Shot cleaning of the economiser (flue gas temp 420 to 170 °C) has been turned off

– Lower and more stable Δp over the econ-omiser than with shot cleaning

– The exhaust gas outlet temperature from the evaporator is lower and stable

– The soot deposits in the high-tempera-ture area (flue gas temperature 680 to 600 °C) of the evaporator are now softer and it takes longer time to build up

VAFAB Köping, Sweden, 12 MWth, grate firingCommissioning: 2009Cleaning area: Vertical evaporator and economiserFuel: Industrial/household wastePerformance evaporator:

– The steam soot blowers at flue gas tem-perature range 470 down to 200 °C have been turned off. Previous operation was 3 times/day

– Δp is lower than with steam soot blowing 3 times/day

Fig. 5. Two infrasonic cleaners on top of economisers.

ECO 6

ECO 5

ECO 4

ECO 3

ECO 2

ECO 1

CONV

CONV

SH1

SH1

SH2

SH2

850 °C 350 °C

710 °C 175 °C

INFRAFONESONIC CLEANER

Fig. 6. Norrköping power plant.

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VGB PowerTech 8 l 2013 Infrasound soot cleaning

Performance economiser: – With the previously installed air cleaning

lances flue gas temp increased rapidly – With infrasound technology installed,

air cleaning lances were turned off and the Δp and outlet temperature are now on a stable low level the full season

E.ON Mora, Sweden, 8 MWth, grate firingCommissioning: 2010Cleaning area: Vertical economiserFuel: Industrial/household wastePerformance:

– The boiler has been in operation for two years (since early 2011) without any need for manual cleaning of the econo-miser

– The Δp is still low and stable – The differential temperature is still high

and stable – No other soot cleaning equipment in-

stalled

Mississippi Power Plant Daniel, USA, 2 x 535 MWel

Commissioning: 1999, 2000, 2001, 2005Cleaning area: 4 x Ljungstrom air preheat-ers size 24 ½ and 2 x economiserFuel: Powder river basin coal (with low heat value (8800 BTU/lb.) and high ash content)Performance:

– Since installing the infrasound clean-ers, the use of steam soot blowers for the Ljungstrom air preheaters and econo-mizers for both coal-fired boilers has been eliminated.

EVO Offenbach, Germany, 2 x 110 MWel

Commissioning: 1993, 1994, 1995, 1996Cleaning area: 2 tubular air preheaters, 2 economisers, 2 superheatersFuel: Coal Performance:

– No soot-cleaning device was installed from the beginning. Deposits reduced the heat transfer considerably. Since installing infrasound cleaners the clean-ing results in both boilers are very good. Load changes caused by clogging are no

longer a problem and the thermal effi-ciency has increased by 1.5 %.

Vattenfall Hamburg Tiefstack, Germany, 2 x 150 MWel Commissioning: 1997, 1999Cleaning area: 2 x SCR honeycomb catalyst on two identical boilersFuel: Hard coalPerformance:

– Since installation in the end of the 1990s, the pressure drop over both cata-lysts is being kept stable the whole op-erating season. Infrasound cleaning is the only soot cleaning equipment for the catalysts.

Summary

Infrasound cleaning is a cost-efficient way of keeping low-temperature areas clean, such as economisers, air preheaters, cata-lysts etc. The technique drastically elimi-nates or reduces the need for other clean-ing methods such as steam soot blowing, shot cleaning, air soot blowing, detonation cleaning, and manual cleaning. The return of investment is typically short thanks to e.g. higher availability, increased produc-tion or reduced fuel consumption. l

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