Recent developments in blast furnace operation, characterised by long campaign life combined with ever increasing productivity and high rates of fuel injection, require high efficiency top gascleaning equipment to cope with the changes in top gas composition, dust loading, pressureand temperature fluctuations and which match the campaign life of the furnace. Theadvantages of the annular gap scrubber in comparison with other venturi scrubbers, includesystem design, operation and performance.

Blast furnace gas cleaning systems

RAW MATERIALS AND IRONMAKING

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AUTHOR: Alex LajtonyiDanieli Corus

At Corus IJmuiden (formerly Hoogovens)there are two blast furnaces in

operation. No.6 with an 11m hearth diameterproduces approximately 3Mt/yr of hot metal,while No.7 with a 13.8m hearth diameterproduces approximately 4Mt/yr. Pulverisedcoal injection (PCI) is in excess of 200kg/tand the burden mix is 50% sinter and 50%pellets. Both blast furnaces are equipped witha dust catcher followed by a Bischoff annulargap wet scrubber, and top gas energy recoveryturbine (see Figure 1).

The gas cleaning systems share a commonwater recycling and blow down treatmentsystem (see Figure 2). Prior to clarification thewaste water is aerated by compressed airinjection in an open reactor to remove CO2,increasing the pH from 5–6 to 7. Clarificationis achieved by operating two out of threeinstalled clarifiers. Caustic soda is added priorto the clarifier to raise the pH to 7.8 wherezinc and lead solubility is at a minimum. Theclarifier overflow has a suspended solidsconcentration of a maximum of 100mg/l andis returned to the blast furnaces withoutpassing over a cooling tower at a temperatureof 40ºC.

A blow down stream of 100m3/hr istreated by two ‘Dynasand’ filters in series,reducing the outlet suspended solidsconcentration to less than 50mg/l.Dynacyclones are used to classify thethickener underflow into large and small particles. Thesmall particle fraction, which is high in zinc, is stored ina temporary site for possible future extraction byleaching. The large particle fraction, is lower in zinc andhigher in iron and is recycled to the blast furnace via thesinter plant to the extent permitted by the zinc-inputrestriction.

BLAST FURNACE OPERATIONAn important by-product of the blast furnace process isblast furnace gas, used as fuel for heating blast air inthe hot blast stoves and as supplemental fuel for steamand power generation. The primary function of the blastfurnace gas cleaning system is to remove particulatematter from this gas. In addition, the system also cools

To Sinter Plant

Dustcatcher

ScrubbingWater

DischargeWater

To Waste DumpTo Sinter Plant

Make-upWater

Water Treatment Plant

Sludge

Hydro Cyclone Separation

BischoffScrubber

Demister

Top GasRecoveryTurbine

PowerPlant

GG

Cooling TowerScrubbing

Water

From BF/Scrubber

Clarifier

Buffer Tank

First Stage

Filterpress

Sinter Plant

Sludge

Storage

Second Stage

Filter

r Fig.1 Gas cleaning system

r Fig.2 Recycle water treatment

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the gas to reduce its moisture content, thus increasingits calorific value. The recovered sludge containsrelatively high quantities of iron and carbon and canthus be recycled as, in our case, through the sinter plant.

Knowledge of the blast furnace process is necessaryfor the proper design and operation of the gas cleaningsystem. Burden preparation and type, pellet or sinter,will affect the quantity and particle size of the dust.Also, the choice and preparation of fluxing materialscan affect the water chemistry in the scrubber system.Calcium can be introduced to the water system from limestone used as a fluxing agent. Coke and fuel injectants will contribute sulphur, chlorides,ammonia and nitrogen oxides. Typical gas analysismeasured at two blast furnaces with PCI are given inTable 1.

Level of production and wind rates affect particulateloadings in the gas cleaning system. Top pressure and

scrubber pressure drop will affect dust collectionefficiency, as well as the adsorption of dissolved gases inthe scrubber water.

In addition to the basic functions of cleaning andcooling of the top gas, the scrubber and/or the top gasenergy recovery turbine also controls the top pressure ofthe blast furnace. For smooth and stable furnaceoperation the top pressure must be controlled and held asconstant as possible. The gas cleaning system designmust, therefore, be fully integrated with the operation ofthe blast furnace.

During operations such as charging, tapping,equalisation and change of stoves, there are inevitablyvariations in gas flow and pressure. The fluctuations areusually of short duration but of considerable amplitudeand are felt at the top of the blast furnace as well asdownstream throughout the gas system. The scrubberand/or the energy recovery turbine must be designed tohandle and control such fluctuations.

GAS CLEANING SYSTEM DESIGNTrouble free scrubber operation, even during roughfurnace driving conditions, is a prerequisite to economiciron production. The design of the Bischoff Annular Gapscrubber has been fully optimised, through experiencegained in over one hundred installations world-wide, toprovide high reliability and superior performance. Thesingle tower construction comprises the pre-scrubber/cooler and the RS, or annular gap scrubberstages, and is followed by a high efficiency, externalmoisture separator.

Pre-scrubber/cooler The pre-scrubber/cooler stageis an open vessel located in the top half of the scrubbertower. The centrally arranged spray nozzles installed onnine levels (see Figure 3) offer virtually no obstruction tothe gas flow and are provided with large internal

Blast furnace A BCO %Vol. 22.9 20–25CO2 %Vol. 17 20–25H2 %Vol. 2.6 1–5N2 %Vol. 57.5 50–58H2S mg/m3 8–23 10–40SO2 mg/m3 23–43 20–50NH3 mg/m3 4–20 5–25Cl- mg/m3 26–200 100–250NO2

- mg/m3 1NO3

- mg/m3 2–7 5–15Br- mg/m3 6–12HCN mg/m3 50–150

r Table 1 Blast furnace gas analysis with PCI

r Fig.3 Pre-scrubber/cooler

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RAW MATERIALS AND IRONMAKING

openings to produce a 90° hollow cone spray pattern.The nozzles are designed to spay in two directions(upwards and downwards) intercepting the adjacentnozzles’ hollow cone spray at about the middle of thevessel cross-section. In this way a fine water dropletspectrum is produced to achieve efficient cooling andparticulate removal.

Almost the entire cooling as well as the removal of thecoarse fractions of the blast furnace dust takes place inthe pre-scrubber/cooler stage. About 80% of the rawgas dust particulate content is captured here at anegligible pressure drop of approximately 100mm (4in)water gauge.

The spray nozzles are fitted with stainless steelreplaceable orifice inserts installed in the cast iron body(see Figure 4). The smallest cross-section in the bodycavity is the tangential inlet of 40mm wide by 150mmhigh. The size of the orifice inserts’ opening variesbetween 40 and 65mm. These large openings ensureclog-free operation and make the nozzles insensitive tothe amount of suspended solids in the recycle scrubbingwater. The connection between the pre-scrubber and theraw gas main is designed to allow wetting of thetransition zone between the dry-wet sections. Thisprevents encrustation often found in installations where

the dry-wet interface constantlyshifts. High lime content in the blastfurnace dust and calcium carbonate-saturated scrubbing water inevitablyleads to the formation of hard crusts,which can grow to considerable size,when the transition zone is notpermanently wetted.

RS-Scrubber The RS, or annular gapscrubber is a special version of theventuri scrubber and is designed toremove the finest dust fractions.Adiabatic expansion of the gas in thisstage provides additional cooling.There are normally three unitsinstalled in the lower part of thescrubber tower. These units consist ofthe circular gas inlet ducts,converging and diverging outer shellsin which the adjustable conical RSelements are installed (see Figure 5).The asymmetric cone angles of thefixed diverging sections and theadjustable scrubber elements providea decreasing opening in the direction

of the flow and produce large changes in flow crosssection with relatively small movements of the scrubberelements.

The RS elements are guided by a non-lubricatedbushing and operated by hydraulic cylinders. Theoperating rod penetrating through the scrubber shell issupported by a lubricated bushing and is sealed by astuffing box and grease fill. Central spray nozzles are ofthe same design as installed in the pre-scrubber/coolersupply scrubbing water to the RS scrubbers. Anabrasion- resistant wear collar takes the impact of thewater spray and the water is then guided into thescrubber throat, where it is intensively mixed with thepre-cleaned gas. Due to the long throat, long contactpath and long retention time the final cleaning isachieved with low gas velocities and thus, the energyconsumption of the annular gap scrubber is the lowestamong the competing venturi scrubbers.

Demister The gas cleaning system is complete with anexternal mist eliminator (see Figure 6) arranged in theclean gas main immediately downstream of the scrubbervessel. Internal demisters, common in scrubbers builtprior to 1982, are either not efficient enough or highlysensitive to normal swings in the quality and chemistry a

q Fig.5 RS-Scrubber

r Fig.4 Spray nozzle

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of the cleaned gas and scrubbing water.The external demister features highefficiency and is furnished with non-clogging, maintenance-free internals,designed to match the campaign life ofthe furnace.

CONSTRUCTION MATERIALSRecent developments in blast furnaceoperations such as the introduction ofhigh rates of pulverised coal andnatural gas injection, and in some casesthe injection of granulated plasticwastes in combination with restrictedblow-down from the closed loop recyclewater system, resulted in an increasedconcentration of chlorides andsulphites in the blast furnace gas, aswell as in the recycled scrubbing water.These changes in the gas and waterchemistry must be fully considered inthe selection of materials andprotective coatings of a wet gascleaning system.

For example, the pre-scrubber shell isnormally protected by an epoxy resinpaint system and the scrubber inletmay be constructed from stainless steel,or fitted with acid-resistant tiles. Thespray header pipes internal to the pre-

scrubber are fabricated from stainless steel. In theRS scrubber section the circular gas inlet pipes are

of stainless steel construction, while the converging anddiverging outer shell and the RS element are cast froma corrosion and abrasion-resistant white iron alloy witha hardness of 600 Brinell.

All other components, such as the actuating and guiderods, guide bushing housing and stuffing box assemblyare made from stainless steel.

OPERATING CHARACTERISTICSThe characteristics of the annular gap scrubber are bestdescribed as follows:

` Multiple dust removal mechanisms` Minimum scrubbing water requirements` Superior top pressure control` Proven performance and best efficiency

Because of its unique design, the annular gap scrubberalso offers the least space requirement along with thelowest energy consumption and the lowest noiseemission.

DUST REMOVAL MECHANISMThe separation of dust particles from the blast furnacegas requires the application of a force that producesdifferential motion of the particle relative to the gas andsufficient retention time for the particle to migrate to thecollecting surface. The principal separating mechanismsin an annular gap scrubber are inertial interception,turbulent (Brownian) diffusion and flow line interception(see Figure 7).

Inertial interception is characterised by the differentinertial forces of the different masses. When the dust-laden gas flows around the collecting water droplet, thedust particles of larger mass do not follow the flow linesof the gas stream. These particles, propelled by theinertia force, strike and penetrate the water droplet andthus are removed from the gas stream.

Turbulent diffusion is highly effective in removingsmaller dust particles from the gas stream. Smallparticles, particularly those below about 0.3μm in

Inertial Interception

Turbulent (Brownian) Diffusion

Flow Line Interception

Streamline

Unintercepted Particle

Intercepted Particle

r Fig.6Demister

r Fig.7 Dust removal mechanisms

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diameter (see Figure 8), exhibit considerableBrownian movement and do not moveuniformly along the gas streamline. Theseparticles diffuse from the gas stream to thesurface of the water droplets and arecollected. This collection mechanism can onlyfunction in scrubbers that promote turbulentflow of gas-liquid mixture, operate at lowvelocity and provide sufficient retention time.

Flow line interception only functions if thegas stream line passes within one particleradius of the collecting water droplet. Thedust particle travelling along this stream linewill touch the water droplet and will becollected without the influence of inertia, or

turbulent diffusion.

WATER REQUIREMENTSAfter primary separation in the dust catcher, or cyclone,the blast furnace top gas is scrubbed with water in theannular gap scrubber to obtain the desired residualclean gas particulate concentration. The quantity ofwater required for scrubbing is relatively low and thusthe gas cooling requirements normally determines thetotal water flow rate. The water circuits are optimisedthrough internal water re-circulation to minimise thecapacity of the recycle system. For example, to cool thetop gas from 150°C to 38°C, 3.0 l/Nm3 water at atemperature of 30°C is required when the furnace isoperated at 2.5 bar top pressure (see Figure 9).

Normally, clean gas temperatures of 35–40°C aredesirable in order to minimise water vapour in the gas. Ifa top gas recovery turbine (TRT) is in operation, it isdesirable to keep the gas temperature high and thusmaximise the enthalpy gradient that can be converted touseful energy. This enthalpy gradient, for a given inlet andoutlet pressure, is directly proportional to the turbine inlettemperature. Therefore, under normal conditions and toachieve a clean gas temperature of 35–40°C downstreamof the turbine, the top gas is cooled to 50–60°C. Theadiabatic expansion of the gas across the turbine providesthe additional cooling.

Cooling of the top gas to 50—60°C is achieved byreduced cooling water flow, or bypassing the coolingtower of the recycle system. As in the previous example,if a TRT inlet gas temperature of 50°C is to bemaintained, the total scrubbing water requirements arereduced to 1.4 l/Nm3.

PERFORMANCEThe performance of a blast furnace gas cleaning systemis generally evaluated on the basis of outlet dust

RAW MATERIALS AND IRONMAKING

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0.1

1

0.0010.010.11

AVERAGE DIMENSION OF THE DUST PARTICLES ( μm)

RELA

TIVE

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RT O

F SE

PARA

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S BY

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By contact and Diffusion

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SPECIFIC WASH WATER FLOW RATE, (l/m³n)

CLE

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S TE

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RATU

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t water = 30°Ctop pressure = 2.5 barDp scrubber = 2300 mm Ws

250 °C

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150 °C

100 °C

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100 125 150 175 200 225 250

Δp [mbar]

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r Fig.8 Dust removal by inertia and diffusion

r Fig.9 Gas temperature vs water flow rate

r Fig.10 Dust concentration vs. pressure drop a

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gas energy recovery turbines. On the other hand, thethree parallel scrubber elements are also suitable forsingle-stage pressure reduction when the turbine is notinstalled, or is out of operation.

The saturated moisture content of the gas istemperature-dependent. In addition to water vapourthe gas also carries entrained, or free water dropletswhich are removed in the demister. High temperaturein combination with high entrained moisture reducesthe calorific value of the gas and, thus higherenrichment gas rates are required for firing the stoves to maintain the desired flame temperature (seeFigure 12).

Gas temperature can be controlled by the quantity ofcooling water used in the cross-flow pre-scrubber/cooler section, while the properly sizeddemister limits the entrained moisture to below5g/Nm3. Excessive amounts of dust and moisture areknown to cause damage to and shorten the campaignlife of the hot blast stoves, boilers and top gas energyrecovery turbines.

Top pressure control The annular gap scrubber is notonly a highly efficient dust collection device, but alsoprovides superior top pressure control. The pressure atthe furnace top is kept constant by the scrubberelements, operated via hydraulic cylinders, or by the topgas energy recovery turbine. When the turbine is inoperation, the scrubber is switched to differentialpressure control mode to maintain the required gascleaning efficiency.

The scrubber and turbine are installed to operate inseries with the blast furnace, but the turbine is arrangedin parallel with the clean gas main (see Figure 13). Forsafety reasons a quick acting valve is located in theturbine supply main to close within one to two secondsof turbine failure.

The main shut-off valve then opens automatically but iscontrolled to ensure that pressure at the furnace topremains as constant as possible. Simultaneously, the toppressure control function carried out by the turbine inletvalve, or by variable stator blades, is switched back to theannular gap scrubber.

During normal start-up and shut-down of the turbinethe changeover is smooth enough to avoid pressurevariations at the furnace top and in the clean gas main.

In the event of an emergency stop of the turbine, toppressure fluctuations cannot be fully avoided but are keptat low amplitudes and of short duration.

Should the main shut-off valve not open in case ofemergency, the by-pass control valve, which normallybecomes operative when the capacity of the turbine isexceeded, provides additional safety.

0

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Typical Operating Range

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300,000 m³/h

200,000 m³/h

100,000 m³/h

50,000 m³/h

1125

1150

1175

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Clean Gas Temperature (°C)

Flam

e Te

mpe

ratu

re (

°C)

4.5 g/Nm³

16 g/Nm³

37 g/Nm³

r Fig.11 Scrubber characteristics

r Fig.12 Moisture vs. flame temperature

concentration and moisture content. The dust collectionefficiency of the annular gap scrubber depends on thethroat velocity, or pressure drop, and the liquid-to-gasratio. Since collecting particles of given size and massdepends on the level of energy expanded, higherpressure drop across the scrubber will result in increasedefficiency and lower dust outlet concentration (seeFigure 10). The best efficiency at any pressure drop isachieved with a constant liquid-to-gas ratio ofapproximately 0.75–1.0 l/Nm3. As indicated on theperformance curve, a residual dust concentration of5mg/Nm3 (d) is achieved with relatively low pressuredrop of about 200mbar.

The flexibility to handle a wide range of conditions isinherent in the design of the annular gap scrubber(Figure 11). High efficiency dust collection at lowpressure drop makes the annular gap scrubberparticularly suitable for installations on low top pressureblast furnaces, or high top pressure operation with top

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COMPARISON OF SCRUBBERSIn comparison with the short rectangular (or square)throat, variable flap venturi scrubbers the annular gapdesign exhibits major advantages and superiorperformance as outlined below.

Construction Flap venturi scrubbers, whether utilisingthe flooded wall approach or the gap/coverage theory,are designed with relatively short throat and waterinjection nozzles that are often equipped with specialcleaning devices. The water jets in these high-energyscrubbers are deflected by the high velocity gas flowbefore reaching the centre of the throat. On the otherhand, the water injected by the single non-cloggingspray is guided into the throat of the annular gapscrubber where it is intensively mixed with the gas. Thelong and narrowing annular gap (see Figure 14)provides long retention time for efficient cleaning. Thereis no possibility of unwetted gas escaping cleaning inthe annular gap.

Dust removal mechanism The short throat, highenergy, flap venturi scrubbers rely primarily on theinertia force as the only mechanism of dust removal. Thewater droplets injected into the throat, using either thinjets under high pressure or thick jets under low pressure,are deflected and quickly accelerated by the highvelocity gas flow, thereby limiting the path available forinertial interception. High gas velocities in the throatretard homogeneous liquid distribution and, thusreduce dust removal efficiency.

In the annular gap scrubber the atomised water is

guided into the throat where it is intensively mixed withthe gas. The long contact path and long retention timein the annular gap allows the scrubber to operate atrelatively low gas velocities since the coarse dustparticles are removed in the throat by inertialinterception and the finer fractions are captured byturbulent diffusion.

Water-to-gas ratio and efficiency The short throat,high energy, flap venturi scrubbers normally require awater-to-gas ratio of 2–5 l/Nm3 for maximum dustremoval efficiency. The specific water flow rate increaseswith the increase of differential pressure across thescrubber (see Figure 15). High differential pressuremeans high velocity and, hence the increase in specificwater flow rate is to optimise the distribution of waterdroplets in the throat. The water-to-gas ratio in theannular gap scrubber is 0.75–1 l/Nm3 and does notchange with increasing differential pressures (see Figure16). The long contact path promotes complete mixing ofgas and water, even at increased gas velocities.

Pressure drop The total pressure drop in any scrubber isdescribed by the empirical formula developed by Hesketh:Δ p = (v2 x σ x A0.133 x L0.78)/C

Where:

Δ p = pressure dropv = gas velocity in the throat (m/s)σ = gas density (kg/m3)A = throat area (m2)

r Fig.13 Control system r Fig.14 Scrubber construction

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L = liquid-to-gas ratio (l/m3)C = empirical constant

This constant greatly simplifies the equation by replacingother factors associated with the pressure losses due toinertia contact, liquid acceleration, the effect of thediffuser and so on. The pressure drop, or energy used forcleaning the gas is due to the inertial contact betweenwater droplets and gas dust particles, and is the same forany scrubber system. The total energy consumption,however, accrues from the sum of pressure losses, whichdepend largely on the scrubber type and construction. Ata given inlet gas condition, the scrubber that operateswith lower throat velocities and lower liquid-to-gas ratiowill require less energy to clean the gas (see Figure 17).

Performance The performance of a scrubber isdescribed as the relationship between the energyexpended and the achieved residual dust concentration.Cleaned gas residual dust concentration of 5mg/Nm3

(dry) is attainable with the annular gap scrubber at apressure drop of approximately 200mbar. In comparison,the achievable gas cleanliness with a flap venturi scrubberoperated at the same pressure drop ranges between 15and 22mg/Nm3 (see Figure 18).

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Specific Water Consumption (l/m³)

Pres

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p (m

bar)

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Inlet Conditionstemperature 60 °C pressure 2 bar adensity 1.9 kg/m³

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annular gap scrubber

r Fig.17 Pressure drop vs water-to-gas ratio

r Fig.18 Dust concentration vs pressure drop

r Fig.15 Water-to-gas ratio vs. efficiency r Fig.16 Water-to-gas ratio vs. efficiency

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CONCLUSIONSIn comparison with other designs, the annular gapscrubber, proven in over 100 installations worldwide,offers distinct advantages as follows:

High dust removal efficiency Clean gas dustconcentration of 5mg/Nm3 is guaranteed. In fact, manyinstallations consistently achieve a gas cleanliness ofbetween 1 and 3mg/Nm3 measured on a dry gas basis.

Low pressure drop The above efficiency is related toa minimum pressure drop of 200mbar measured acrossthe annular gap. Pre-cleaning requires a pressure drop of10mbar and the demister operates at a pressure drop of 20mbar. The total energy expended for cleaning,thus, equates to 230mbar. This low pressure droprequirement makes the annular gap scrubber especiallysuited for installations at low top pressure furnaces aswell as for installations with top gas energy recoveryturbines. On the other hand, the scrubber can alsohandle single stage pressure reduction from high topgas pressure to the required pressure in the work’s cleangas distribution system.

Efficient demisting The proprietary in-line demisterlimits the residual free water content in the clean gas tobelow 5g/Nm3. Low moisture content maximisescalorific value of the clean gas; an important factorwhen used as fuel to fire the hot blast stoves and steamgenerating equipment.

Minimum water requirement The annular gapscrubber operates with a low and constant water-to-gasratio of 0.75 l/m3 of gas. When cooling of the gas is notrequired, as is the case in installations with top gas

RAW MATERIALS AND IRONMAKING

energy recovery turbine, the recycle water systemcapacity can be greatly reduced.

Low energy consumption Low pressure drop gascleaning in combination with the minimum water-to-gasratio reduces the size of the water recycle system andthus minimises energy consumption.

Low operating and maintenance cost The use ofhigh grade materials, the large bore, non-clogging spraynozzles and the simple mechanical design of thescrubber result in extended campaign life withminimum maintenance. Minimum maintenancerequirements and low energy consumption equate toreduced operating cost.

Minimum installation cost The compact, singletower design conserves space, foundation size, thenumber and size of access platforms and stairs, thelength of gas mains, etc, and therefore the cost ofinstallation. Typically, the weight of vessels, gas mains,structural steel and pipework is 30–35% lower thanthat of the competing systems.

Low noise emission The annular gap scrubberoperates with low gas velocities and thus with a low noise level of below 80dB(A) without any special acoustic treatment. This compares with morethan 90dB(A) commonly encountered with otherdesigns. MS

Alex Lajtonyi is Senior Technologist, BF Gas Cleaning atDanieli Corus, IJmuiden, The Netherlands

CONTACT: Alex.Lajtonyi@danieli-corus.com

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