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AUTHOR: Alfred R. Hampel Siemens VAI Metals Technologies GmbH
& Co (Siemens VAI)
BOF steelmaking comprises several operating steps,including
charging, oxygen blowing and tapping; allof which involve emissions
of hot and dust-laden gas.Usually, two different off-gas systems
are installed; forfume evacuation and to keep the converter
environmentcool and clean.
Major quantities of hot gases are emitted during theoxygen
blowing phase, mainly in the form of hot carbonmonoxide. Specially
designed primary off-gas systemscollect these gases and cool the
partial or fully combustedcarbon monoxide. To evacuate the hot
fumes producedduring the other converter operation steps such as
scrapcharging, hot metal charging, steel tapping and slagdumping,
doghouses and secondary off-gas cleaningsystems are normally
installed (see Fig. 1).
For a number of steel shops the primary off-gas
systemaccomplishes the secondary ventilation via a by-passfunction.
However, the evacuation performance is very
poor during converter charging and is not carried out atall
during the blowing phase.
In the majority of steel shops independentsecondary off-gas
cleaning systems are installed. Theyensure the required evacuation
capacity with properlydesigned induced draught (ID) fans and meet
therequired clean gas dust content using fibre bag filters.These
systems are usually designed not only toevacuate the converter
environment during charging,tapping and de-slagging, etc., but also
other dustemission sources on the hot metal side
(reladling,desulphurisation, de-slagging and scull cutting) aswell
on the liquid steel side (ladle treatment stations,ladle furnaces,
etc.).
DESIGN DEVELOPMENTS IN SECONDARY DE-DUSTING SYSTEMSFor many
years the design criteria for secondary off-gascleaning systems
were rather simple, with the evacuationperformance based on
converter tapping size only.Comparing all emission sources, the
gases evacuatedduring hot metal charging via the charging hood
show
the highesttemperature profilesand are characterisedby high gas
flows.Considering thelimitations with respectto the filter
inlettemperature, the hotcharging gases have tobe cooled so that
thefilter bags are notdamaged byoverheating. Themaximum
allowabletemperature for polyesterfilter bags should notexceed
160C. This is
The off-gases released during steelmaking operations,
particularly during the charging of low-quality scrap with its
inherent hydrocarbon impurities, frequently overloads the exhaust
andcooling capability of conventional off-gas treatment systems.
The resulting combustion of the emittedgases occasionally leads to
uncontrolled fume emissions and high gas temperatures which can
causeexcessive damage to the system, especially to the bag filters.
An advanced off-gas treatment systemby Siemens VAI eliminates these
problems thorough optimised plant design and hot metal
pouringspeed, leading to increased operational safety and shorter
tap-to-tap times.
BOF secondary off-gas cleaning systems
ar Fig.1 Schematic view of secondary off-gas cleaning system
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achieved by mixing the hot charginggases with fumes coming from
`cold`evacuation sources and by activating,so-called, emergency
cooling air flapsas a source of cold ambient air formixing. System
designs which achievethe required temperature by mixinghigh and low
temperature evacuationgases (cooling by mixing) can beclassified as
`conventional`. Figure 2shows different conventional
off-gascleaning systems where the ratio of theconverter suction
volume and theconverter tapping capacity arecompared. The
horizontal axis showsthe converter tapping weight of theplant
indicated by name in the field.The vertical axis shows the total
suctionvolume in m3/h. In order to enable aneutral assessment of
the results, bothSiemens VAI -supplied off-gas systemsand off-gas
systems from other suppliers - referenced withthe denomination OX1
to OX9, were investigated.As clearly seen, most of the reference
points of theassessed systems are located within two parameter
lines:
` Upper parameter line, which represents a highevacuation
performance of 5,400m3 off-gas/hr/ttapping weight
` Lower parameter line representing the lowevacuation
performance of 2,100m3 off-gas/hr/ttapping weight
The figures `xx MW` of the different reference pointsdefine the
cooling performance of the off-gas system inMegawatt (MW), and
represent the physical magnitude ofthermal power which is absorbed
from the off-gas suckedthrough the system. It can be explained as
follows:
` The actual volumetric gas flow VA [m3/h] refers viathe gas
temperature to a certain gas mass flow VM[Nm3/h]
` The air and fumes have a certain specific heat Cp[kJ/Nm3.C].
The temperature increase t [C] fromambient temperature to the
maximum filter inlettemperature (which has to be controlled to
preventdamage) multiplied by the specific heat Cp results inthe
specific thermal energy content Espec [kJ/Nm3].The specific thermal
energy content [kJ/Nm3]multiplied by the mass flow [Nm3/h] results
in thetotal energy flow [kJ/h] equivalent to thermal powerPth
expressed in kilowatts [kW] or Megawatts [MW].Espec = Cp x t, Pth =
VM x Espec
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As an example, the figures for the reference point ofvoestalpine
Stahl (VAS), Linz, Austria (160 t tappingweight) show a cooling
capacity of 43 MW. This meansthat as long as the total thermal
power emission of theconverter operation (charging, tapping,
de-slagging, etc.)remains below 43 MW, the off-gas system will
notbecome overloaded with respect to the temperature andvolume of
fumes.
CONVERTER HEAT EMISSIONS As indicated above, the bulk of the
thermal power loadto be absorbed by the secondary system comes
fromthe converter during hot metal charging. The scrapusually
contains combustibles, mostly in the form ofhydrocarbons such as
paint, plastics, grease, oil, etc., aswell as galvanised and
tin-coated metals. The specificheating values of these substances
are of the order of80 - 90% of the energy content of the same
quantityof oil by weight. During charging of hot metal onto
thescrap in the converter, the hydrocarbons are heatedand the zinc
or tin is vapourised. However, they cannotcombust due to the lack
of oxygen inside the converterduring the charging period. Pyrolysis
(decomposition)gases (e.g., CO, H2, CH4 compounds) and/or vapoursof
Zn or Sn are produced which carry the energy in achemical phase.
The pyrolysis and vapourisingprocesses are endothermic, with the
required energybrought in by the hot metal during pouring.
Thecomposition of the pyrolysis gases depends on thedecomposition
temperature. At a lower temperaturethe dominating elements are CH4
compounds and at ahigher temperature the equilibrium moves to
higher
r Fig.2 Comparison of converter suction volume and tapping
capacity inconventionally designed steel shop ventilation
systems
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formulae are similar to those already outlined for thecooling
capacity of a plant. The further reactions takeplace in the
sequence of process steps depicted in figure3. The gases and
vapours leaving the converter comeinto contact with the ambient air
supplying thecombustion oxygen and begin to combust. The
chemicalenergy converts to thermal energy and the permanentgas flow
creates the thermal power emission of theconverter (Pconv).
For the design of the off-gas system with respect to therequired
cooling capacity (Poff), the maximum thermalpower emission of the
converter (Pconv, peak,) is the keydesign factor. Only in cases
where the figure Poff is higherthan the figure Pconv, peak, can the
off-gas cleaning systemsafely absorb and remove the thermal power
emission ofthe converter. If this is not the case, the charging
hoodcannot remove all of the fumes, and secondary
emissionsaccumulate in the hood which lead to fugitive
emissions.
The combusted hot gases above the converter rise andalso suck
air into the combustion zone, increasing thetotal gas volume.
Dilution air is required as excesscombustion air in order to avoid
incomplete combustionof the emitted gas concentrations.
INFLUENCE OF SCRAP QUALITYIn the past the major portion of scrap
for BOF steelmakingcame from the steelworks as return scrap; with
externalscrap accounting for only a minor portion of the
charge.This situation has changed during the past 10 to15 yearsas
clean return scrap from the steelworks continuallydecreased as a
result of better process control in theupstream and downstream
facilities. External scrap in the
H2 concentrations with increased explosion risk. Thepresence of
water charged with the scrap (liquid or ice)increases the
generation of hydrogen.
During hot metal pouring the converter emits acontinuous flow of
combustible gases of a certainspecific energy (calorific) content,
depending oncomposition. This energy flow is expressed by means
ofthe magnitude of the thermal power. The respective
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a
r Fig.3 Schematic diagram of thermal powerbalance factors
r Fig.4 Trend of thermal power emission from a 295t
converter
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INTRODUCTORY THEMES
100MW. As a result, their off-gas system was
sufferingpermanently from overheating, from escaped fumeemissions
at the charging hood, and from systemexplosions as a result of
incomplete combustion.
Numerous other steelmakers - mostly in Europe, but alsoin the
U.S.A. - are suffering from the same problemsbecause their systems
are positioned in the`conventional` design group and because they
arecharging cheap, but obviously energy-rich scrap. Figures 5and 6
reflect the situation occasionally faced by steel producers.
ADVANCED DESIGN OF SECONDARY OFF-GAS SYSTEMSStarting in 2003,
Siemens VAI improved the design ofsecondary off-gas cleaning
systems by quantifying and
form of metal cuttings, cans and alsoan increased number of
scrapped carbodies has increased. This type ofscrap is mostly
compressed to bundlesor blocks which typically contain 2 to4kg of
combustibles per tonne ofscrap.
An example highlights the currentsituation in steel shops, which
areincreasingly encountering problems inconnection with thermally
overloadedoff-gas systems, resulting insignificant visible fume
emissionsduring charging and by highequipment temperatures:
` Considering a scrap weight of 50tonnes for charging and
acombustible content of 3.0kg/t,the total weight of combustibles
inthe scrap chute amounts to 150kg
` The energy content of thisquantity of combustibles compared
with oil shows anenergy equivalent of 120kg of oil (150
litres),equivalent to nearly one barrel of oil
` The burning of this energy in less than one minutemeans the
release of 4,800MJ, leading to an averagethermal power emission of
4,800/60 = 80MW
Considering the reactivity of the scrap, the thermal peakpower
development can increase to two or more timesthat of the average,
which means 160MW, and more.Figure 4 illustrates the trend of a
thermal power emissionlife during hot metal charging. The power
peak shows thesame figure as shown in the example, i.e., 160MW.
(Notethat the integration of the thermal power versus time
-equivalent to the shadowed area - expresses the energyreleased
during the observation period. On the basis ofthese figures the
energy content in the scrap can bedetermined.) Even with a lower
hot metal charging rate,an eruptive reaction cannot be avoided.
This data of thermal power emissions by convertersduring hot
metal charging has now to be compared withthe diagram showing the
cooling performance of the off-gas system with a conventional
design (see Fig. 2).The comparison makes it understandable that the
thermalpower development set free by a 50 tonne scrap chargeeasily
exceeds the cooling and evacuation capability of alloff-gas systems
shown in figure 3. The example for 50tscrap charging corresponds
with the reference point ofvoestalpine Stahl (Linz, Austria), which
shows a coolingcapacity of the off-gas system of only 43MW,
comparedto a typical converter emission considerably exceeding
r Fig.5 Uncontrolled combustion and fume emissions during
charging of scrap ontohot metal
r Fig.6 Roof emissions during converter charging
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Each of the two 295t converters wasequipped with the following:
` One evacuation system designed
for both charging hot metalfollowed by scrap charging, andvice
versa. At Sidmar it was decidedto charge scrap first followed by
hotmetal.
` One evacuation system for tapping` One roof extraction system
(RE)` Exhaust system for two ladletreatment stands
At the same time the dust emissionlevel inside the steel
building had to be
kept to below 5 mg/Nm3, the gas-cooling system had to cope with
athermal load of 180 MW and the gas
cleaning system had to ensure a clean gas dust level atthe stack
to less than 10 mg/Nm3. The solution to achieve these targets was
as follows (see Fig. 7):` Installation of an evacuation system with
a capacity
of 2,400,000 Nm3/h for the start of hot metalcharging (at 0C
ambient temperature). During hotmetal pouring, the total gas flow
can reach3,800,000 Nm3/h for all hoods.
` Installation of plate-type convection coolers whichcan absorb
the generated heat (see Fig. 8)
` Development and application of a thermal powercontrol system
to control the hot metal pouringspeed in order to avoid any heat
overload of theplant by extremely energy-rich scrap mixtures
The design of the patented coolers requires theapplication of
sophisticated calculation programsbecause the temperature profile
of the gas and the platesvaries considerably. The coolers are made
of differentsized plates which enable the rapid cooling of
exhaustedgases. Even during slopping, the large volumes ofgenerated
gases can pass the cooler without overheatingthe filter bags. As
seen in figure 9, even after 21 2 years ofoperation no major
accumulations of dust on the coolerplates can be observed.
The cooling performance of Sidmar's coolers is depictedin figure
10. The gas temperature trend (red line) shows apeak temperature of
540C at the cooler inlet and atemperature of only 90C at the cooler
outlet. The coolersare designed for a maximum outlet temperature of
250Cat a maximum inlet temperature of 750C whichrepresents a
cooling performance of 95MW per converter.
After the off-gas cooling phase the cooler commenceswith a
regeneration phase, which means the removal of
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qualifying thermal power emissions, by applying highperformance
cooling capabilities, integrating thermalpower emission control
systems and by reducing the flowresistance in the duct systems. The
engineering of thisadvanced design includes the determination of
thecontent of combustibles in the scrap, the maximumthermal power
emission of a converter (based on theamount of scrap charged, the
energy content in the scrap,and the hot metal pouring speed), the
required coolingcapabilities as well as the combustion requirements
witha controlled hot metal pouring speed. The charging hoodsand
duct systems were designed using computationalfluid dynamics (CFD)
techniques.
The first major application was the off-gas systemdesigned and
supplied for the BOF plant of Sidmar inGent, [1] Belgium.
r Fig.8 Sidmar, Gent, Belgium - outside view of coolers
r Fig.7 3D plant layout of advanced secondary off-gas cleaning
system
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a
where double-evacuated charging hoods for twoconverters will be
installed and where one cooler will beprovided for both converters
(red coloured). The filterplant, fans, stack and the major part of
the ductworkremain unchanged. This new system is now underdesign
and will boost the cooling capacity by more than100%.
At voestalpine Stahl the installation of additionalcoolers
within the charging ducts of the off-gas systemboosted the cooling
capacity by approximately 50%and the evacuation capacity by nearly
100% (referred tothe gas mass flow).
The improved heat removal efficiency that results byinstalling
coolers in conventional steel shop ventilationarrangements are
shown in figure 13. Figure 2 has beenenlarged in the ordinate
direction with respect to thesuction volume and a parallel scale
was included toshow the equivalent cooling capacity in MW.
Therespective arrows show the increased cooling capacities
accumulated heat energy by an air stream suckedthrough the
coolers to cool down the plates. As seen infigure 10 the
regeneration phase starts when the outlettemperature (blue line)
shows a higher value than theinlet temperature. Regeneration takes
approximately 25minutes. The air stream sucked through the
coolersduring regeneration additionally evacuates fumesescaping
during blowing and tapping, eliminating theneed for doghouse
doors.
Figure 11 illustrates the thermal power flow, showingthe
distribution of generated power, absorbed powerand passing power as
designed for the Sidmar coolingsystem. Scrap charging releases
180MW of thermalpower, of which, the ducts between the hood and
thecoolers absorb 22%. The gas temperature duringcharging can rise
to 1,000C which results in a largeheat transfer to the cold ducts.
The coolers reduce thegas temperature to 250C by removing a further
50%of the input energy, which is absorbed by the coolerplates. The
tapping exhaust and roofexhaust are included in the
balance(accounting for approximately 25MW).A few further
percentages areabsorbed by the filter, so only 30%(approximately
60MW of the totalinput power passes to the stack. Thecorresponding
exit-gas temperature isbelow 80C.
APPLICATIONS IN EXISTINGPLANTSThe described cooler design is not
onlyapplicable for new steel plants, but isalso highly suitable for
retrofitting toexisting plants. Figure 12 shows thebasic layout of
a steel shop in Germany,
r Fig.10 Inlet and outlet gas-temperature trendswith advanced
cooler designr Fig.9 Cooler plates after 2.5 years of operation
r Fig.11 Thermal power balance of advanced cooler system
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with the cooler installations for the following plants:` Sidmar
(Belgium): +90MW originally installed ` Alchewsk, Ukraine: +75 MW
originally installed` Thyssen-Krupp CSA, Brazil: +65MW originally
installed` voestalpine Stahl, Linz (Austria): +28MW post
installation` Steel shop in Germany: +80MW post installation
The comparison of thermal power versus flow explainsthe amount
of air required as a substitute for cooling airif no coolers are
applied: 1MW cooling capability ofcoolers substitutes 20,000 m3/h
of cooling air for`cooling by mixing` purposes.
SAFETY ASPECTSWith the described advanced design, enough
coolingcapacity is provided so that the plant can operatewithout
the need for emergency cooling air flaps whileassuring a high
enough air flow through the system toensure safe combustion
conditions. With increasingthermal power development the
temperature in thesystem rises and the increasing volumes of gas
create ahigher pressure drop which results in a reduction of theair
mass flow passing through the system. This means areduced oxygen
mass flow despite an increasingdemand for oxygen. Due to this fact
under-ventilatedsystems are quickly overloaded, not only with
respect tothe temperature profile, but also in connection with
theconsumption of available oxygen. The gases emitted bythe
converter are thus incompletely combusted and theremaining
non-combusted gases can lead to potentiallydangerous
situations.
The trend of oxygen consumption can be seen infigure 14 during
two weeks of recordings. The red line`stalactites` show the
remaining oxygen in the gasdownstream of the charging hood. The
content ofoxygen in the air is 21% before charging which drops
during hot metal chargingaccording to the quantity of
oxygenconsumed during combustion. Ifthe remaining oxygen
contentdrops to below 10%, the content ofnon-combusted gases
(visible onthe `stalactites` line in blue) cango higher than 3%,
which is thelowest explosion risk limit. Thecomputer can calculate
the actualpower developmentinstantaneously as well as theactual
oxygen consumption beforereceiving the O2 analysis results(usually
15 - 20 seconds delaytime). The designed thermal power
r Fig.12 Basic layout of off-gas cleaning system fora steel
plant in Germany
r Fig.14 Content of remaining oxygen and non-combusted gases in
the charging duct
r Fig.13 Improved thermal cooling capabilities withinstallation
of coolers
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INTRODUCTORY THEMES
removal by the off-gas system is the set point forcontrolling
the hot metal pouring speed (reduced orinterrupted). This means
that a total consumption ofthe available combustion oxygen can be
avoided withthis type of control.
SUMMARYWith the application of absorption coolers in
secondaryoff-gas cleaning systems the following advantages canbe
achieved:Operational and safety benefits` Higher gas throughput due
to a low gas temperature
profile downstream of the plate-type cooler` Emergency cooling
air is not required, meaning that
the high gas flow will not be diminished by theopening of a
cooling air flap
` Reduced fume emissions at the charging hood` No spark
arrestors are required (hot particles are
cooled to harmless temperatures by the coolers)` In combination
with the charging power control
system there is no risk to operate at under-stoichiometric
combustion conditions and no risk ofexplosion
Commercial benefitsShorter pouring times result, on average,
because a highpouring speed can be applied which is controlled by
thecontrol system. With less contaminated scrap fasterpouring
speeds can be used, resulting in shorter pouringtimes of 40 seconds
or less. This allows more heats to beproduced on a daily basis.
Cheaper scrap, usually with ahigher content of impurities, can be
charged to theconverter without risk of overloading the system.
Theevacuation capacity and cooling performance ofexisting systems
can be increased at relatively lowinvestment costs.
REFERENCES[1] L. Pieters, Secondary Dedusting of the LD
(BOF)Converters at Arcelor Gent, IS 06 Iron and steelConference,
Linz, Austria, October 2006.
Alfred R. Hampel is Senior Expert, EnvironmentalEngineering with
Siemens VAI Metals TechnologiesGmbH & Co (Siemens VAI), Linz,
Austria
CONTACT: [email protected]
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