atti e notizie nota tecnica · 2016. 9. 27. · la metallurgia italiana - n. 10/2012 51 atti e...

La Metallurgia Italiana - n. 10/2012 51

Nota tecnicaAtti e notizie

Recent developments of energy savingand environmental protection

in the steel industry

Carlo RaggioR&D Director - Tenova Spa

Via Monte Rosa, 93 - Milan Italy

Iron & Steel is the second major industryin terms of energy consumption (See figure1). Energy constitutes a significant portionof the cost of steel production, ranging from20% to 40% depending on process, localconditions, etc. Thus, improvements inenergy efficiency result in reduced pro-duction costs and thereby improved com-petitiveness.The Iron & Steel industry has alreadygone a long way in the direction of effi-ciency; sophisticated energy managementsystems ensure efficient use and recoveryof energy throughout the whole steelma-king process.On the environmental side, CO2 emis-sions intensity (t of CO2/t of crude steel)also vary from plant to plant, and countryto country.As a whole, the Iron & Steel industry is animportant source of GHG emissions, ac-counting for 6.8% of world carbon dioxide(CO2) emissions and 3,0% of GHG emis-sions(1). It is the third largest industrial sour-ce of GHG after chemical and cement sec-tor (Baumert, 2005).Out of the sector’s estimated total emissionsof 2.507Mt CO1 Unless specified otherwi-

(1) Unless specified otherwise, all CO2 and GHGemissions figures in this paper include both di-rect (on-site) emissions from fuel combustion andthe use of coal and lime as feedstock and indi-rect emissions from generation of the electrici-ty and heat used in the steelmaking.

se, all CO2 and GHG emissions figures inthis paper include both direct (on-site)emissions from fuel combustion and theuse of coal and lime as feedstock and in-direct emissions from generation of theelectricity and heat used in the steelma-king.-equivalent in 2006, 80% were direct(on-site) emissions and 20% were indirectemissions from the generation of the pur-chased electricity.In 16 years time (1990 – 2006) CO1 Unlessspecified otherwise, all CO2 and GHGemissions figures in this paper include bothdirect (on-site) emissions from fuel com-bustion and the use of coal and lime as fe-edstock and indirect emissions from ge-neration of the electricity and heat used inthe steelmaking. emissions per tonne ofcrude steel decreased by 7%, from 2.2 t ofCO1 Unless specified otherwise, all CO2 andGHG emissions figures in this paper in-clude both direct (on-site) emissions fromfuel combustion and the use of coal andlime as feedstock and indirect emissionsfrom generation of the electricity andheat used in the steelmaking. per t of cru-de steel, to 2.0 (see table 1).Also in this field, the industry is not spa-

ring efforts to limit its footprint and, on ave-rage, today, 1,9 t of CO2 are emitted for eve-ry t of steel produced.CO2 emissions intensities vary from plantto plant due to differences in the techno-logies used, finished products, plant ope-rating efficiencies, plant maintenance,quality or iron ore and coal and the carbonintensity of the electricity. The strongestfactor affecting emissions intensity is theoverall processing method.Today there are two main steel making pro-cesses that account for 98% of the world ste-el production: the integrated blast furna-ce-basic oxygen furnace (BF-BOF) route andthe scrap/DRI-based electric arc furnace(EAF) route (see figure 2).The BF-BOF steelmaking process is moreemission intensive than the scrap/DRI-EAFroute, though estimates vary considerably(see table 2).The international effort to mitigate andadapt to climate change is coordinated andregulated by the Kyoto Protocol (negotiatedin 1997 and entered into force in Februa-ry 2005) and Copenhagen Conference(2009). The efforts established are not pre-scriptive and do not set out binding GHG

Fig. 1 – World industrial sector energyconsumption by major energy-intensiveindustry. Source: IEA Data Services,World Energy Balances (2009), web sitewww.iea.org.

Table 1 - Source: WSA, IEA Energy Stati-stics.

Fig. 2 – Steel Production Route Mix,2011.

Table 2 – kg of CO2

per t of crude steelby process - Note:The high and low-end ranges indica-te CO2-free andcoal-based electri-city, and accountfor country averagedifferences. Sour-ce: IEA, 2007.

Tenova:Layout 1 17/10/12 16:30 Pagina 51

52 La Metallurgia Italiana - n. 10/2012

Atti e notizie

emission targets.In parallel to the Kyoto Protocol and Co-penhagen Conference, Governments ofevery country have issued national primarypolicies aiming at mitigating GHG emis-sions.On average, Kyoto Protocol and Copenha-gen Conference’s target, to be achieved ona worldwide basis, is to reduce GHG gasesby 15-20% by 2020 vs. 1990 levels (rangedepending on countries). In 2009, total glo-bal CO2 emissions increased to 31.3bn t, al-most 40% since 1990, the base year of theKyoto Protocol (see figure 3).The assessment excludes CO2 emissionsfrom deforestation and logging, forest andpeat fires, from post-burn decay of remai-ning above-ground biomass, and from de-composition of organic carbon in drainedpeat soils.

Fig. 3 – Global CO2

emissions fromfossil fuel use andcement productionper region, 1990-2009. Source: Oli-vier and Peters,2010 – www.pbl.nl.

Collectively, the countries that signed theKyoto Protocol reduced CO2 and greenhousegas emissions in 2009 by 7% compared to1990, the base year for the Protocol.Most of the decrease, though, has taken pla-ce due to the financial crisis. Greenhousegas emissions could rapidly increase to-ward pre-recession levels as industrializedcountries grow out of recession.In November 2011, the United Nations Cli-mate Change Conference took place in Dur-ban, South Africa. The most importantachievement was the adoption of a mandatefor parties to the UN Frameworks Con-vention on Climate Change to negotiate anew legal agreement by 2015. The newagreement would take effect from 2020.For the first time all of the world’s majorgreenhouse gas emitters, including the Uni-ted States, China, India and Brazil, accep-

Fig. 4 – HYL ZR DRprocess flow she-et.

Fig. 5 – “MinimalCO2 Emission Sche-me” (~80% selecti-ve CO2 removed).

ted to enter into this new international le-gal framework for reducing carbon pollu-tion. In addition, as a bridge to 2020, anumber of developed countries intend tomake new emissions reduction commit-ments under the Kyoto Protocol when itscurrent commitment period ends in 2012.Following the a.m. global strategy Chi-na’s12th Five-Year plan (5YP) emphasizesenvironmental issues and clean technolo-gy posing a challenge to the iron & steel sec-tor regarding energy usage and pollution.In the last January 2012 the State Councilof China published a work plan for emis-sions control of GHG by 2015.Considering all the above:• Climate change is a global environ-

mental challenge which can be addres-sed only with a strong global action. Dur-ban made a significant breakthroughwith the past;

• The iron and steel industry will play animportant role;

• Innovation will give a fundamental con-tribution and Tenova is fully committed,in the value chain steps where active,with R&D activities and efforts in orderto give an important portion.

• Tenova’s new technologies meet themost updated environmental require-ments and at the same time focus on pro-cesses competitiveness and flexibility.

In the next pages, the most important in-novations of our technologies in the fieldof energy efficiency, CO2 and NOx emis-sions abatement are described.

DIRECT REDUCTIONCO2 abatementIn 2006, a strategic alliance was formed byTenova and Danieli & C. for the design andconstruction of gas-based DR plants underthe new ENERGIRON trademark. ENER-GIRON® is the innovative HYL direct re-duction technology jointly developed by Te-nova and Danieli, and whose name derivesfrom the unique DRI product which di-stinguishes this technology from other avai-lable processes. The ENERGIRON® te-chnology is characterized by a signifi-cant environmental contribution in termsof CO2 abatement.ENERGIRON ZR DR process is the most ad-vanced Tenova HYL technology, based onzero reforming (reductants are generatedby “in-situ” reforming inside the shaft fur-nace). Figure 4 represents the process flowsheet.The selective CO2 removal, based on che-mical absorption (amines, hot carbonatessolutions) which is inherent with the ZR DRprocess, becomes a real GHG abatement ifthe removed CO2 is commercialized.A new development of the ZR DR process,recently patented and referred to as the



Nota tecnica

“Minimal CO2 Emission Scheme” (see fi-gure 5), assures the possibility to removeup to 80% of total carbon input as selecti-ve CO2. By the incorporation of a PSA-typePhysical Adsorption System, the carbona-ceous compounds are separated from therecycling gas (after CO2 absorption), feedingthem back to the reduction circuit andusing the separated H2 as fuel instead oftail and/or natural gas.For example, using this new scheme for arecently proposed project for a 1.6 Mt/y DRplant in Europe, only 19% of carbon emis-sions would be non-selective CO2 ventedinto the atmosphere instead of the 30% inthe conventional case without PSA.The unique CO2 emission reduction whichis proper to the ZR DR process can be alsokey to reduce emissions in the traditionalBF-BOF route by charging pre-reducedDRI into the BF-BOF. For a 23% - 38% DRIcharge, production increase is about 20%- 28%, respectively and with 23% lower CO2

emissions.

Energy efficiencySpecifically the ZR scheme is the most ener-gy efficient DR technology available; in factin terms of energy savings this technolo-gy has been refined over the years to whatis now the lowest consumption of energy-per-t of DRI of any DR process on the mar-ket.The overall energy efficiency of the processis optimized by:• the higher operating pressure (6-8

barA), which optimizes the power con-sumption;

• the higher reduction temperature (abo-ve 1050°C), which increases the reduc-tion process kinetics;

• “in-situ” reforming inside the shaft fur-nace, which avoids an external energyconsumer (reformer);

• the various energy recovery units in theplant.

Therefore, the DRI product takes most ofthe energy supplied to the process, with mi-nimum energy losses to the environment.The overall energy efficiency of the ZR pro-

cess is around 87%, compared with lessthan 75% for other DRI technologies. For na-tural gas-based plants, the requirements(including selective CO2 removal and pro-duction of high metalized and high-carbonDRI) are now only 9,84GJ and 70 kWh pert of DRI.More importantly, the process provideseven greater energy savings for the steel-making process, thanks to its inherent abi-lity to produce highly metalized DRI in ex-cess of 94% with high carbon content in theform of iron carbide. Furthermore, the pro-duct can be continuously transported fromthe DR plant to the electric arc furnaceusing the reliable Tenova HYL HYTEMP®

System for pneumatic hot transport and fee-ding to the EAF. This retains the inherentheat from the DR process of around 600°C,delivering the product already hot to thefurnace.The combination of hot DRI and its high car-bon content in excess of 3.5% provides che-mical energy to the melting process, thusreducing power-on-time from around 44 mi-nutes for cold DRI to around 31 minutes,and reducing electrical energy require-ments in the EAF from 530 to around 380kWh per ton of liquid steel. The benefitsprovided to steelmakers by hot charging ahigh-carbon DRI product represent millionsof dollars in annual operating cost savingsby reduced energy costs, as well as in-creased productivity in the furnace.

Flexibility in using different energysourceTenova HYL DRI technology also allows theuse of Coke Oven Gas (COG) and Syn-Gas(gas from coal gasification). In both casesthe basic ZR scheme can be used withoutany modification. For example, consideringthat COG has about 25% methane, while na-tural gas has +95% hydrocarbons, thereforethe use of COG makes the process even ea-sier. For coke oven gas-based plants, ZR DRprocess consumption per ton of DRI needsare only 10,05GJ and 90 kWh, includingCOG compression and for Syn-Gas-basedplants 9,42GJ and 70-90 kWh.

Fig. 6 – “Use ofCOG and syn gasas reductant in ZRscheme”.

Fig. 7– DRI plant at Emirates Steel Indu-stry in Abu Dhabi.

Tenova HYL started the execution of a pro-ject for Jindal Steel & Power Ltd in India fora production of 2.5 Mt/y of hot and coldDRI, using a mixture of syngas and COG inany proportion. This project is a break-through in the DR industry for using a mix-ture of coal based energy sources, whiletransporting hot DRI to adjacent EAF’s withthe HYTEMP system.The plant is designed to produce both hotand cold DRI in any combination; fur-thermore, it will also use syngas from coalgasification and Coke Oven Gas as sourcesof reducing gas and also in any proportion.The energy consumption for the plantwill be on the order of only 2.2 Gcal/ton ofDRI. As a further example of energy effi-ciency, the plant will also use BOF gasesfor fuel to the DR Process Gas Heater andother users.Adding to the excellent flexibility of the ZRprocess is the fact that this very versatileplant being built for JSPL uses the same ba-sic process scheme characteristic of anyENERGIRON ZR plant, whether natural gasbased or other. Thanks to the above bene-fits, ZR direct reduction technology has haddramatic success in the past seven years.Since 2005, nine new plants have been orare currently being built for a total new ca-pacity of 13.65Mt per year. Two additionalprojects for conversion of existing plantsto the ZR process scheme will add another0.80Mt of incremental capacity, for an ove-rall total of 14.5 Mt with this technology insuch a short time span.Among the most recent references EmiratesSteel Industry, located in Abu Dhabi (seefigure 7), is one of the world’s largest pro-jects to date with DRI production of morethan 3.2 Mt/y in two new plants.The technology now exists to be able to buildZR DR modules of 2.5Mt/y in a single unit;



Atti e notizie

as in the case of Nucor Corporation in USand JSPL in India, and work is underway ondevelopment of a 3.0 Mt/y unit.

STEELMAKINGTenova’s approach to steelmaking optimiza-tion and control provides great potential forimproving steel quality, increasing produc-tivity, lowering operational costs, improvedenvironmental performance and safety.Due to the development of novel sensors ,mathematical models and process impro-vements, Tenova makes it possible to ob-tain more efficient operations in the elec-tric arc furnace and in the oxygen con-verter.Tenova is working in two main directionsto achieve these targets:1) Process optimisation to reduce/ im-

prove the energy efficiency of the mel-ting process:− Continuous feeding and preheating

system (Consteel®).

Table 3 – CO2 emis-sion related to a1Mt/y steel plantwith top chargeEAF equipped withTenova technolo-gies.

Fig. 8- EFSOP® ope-rating results (/t ofgood billet).

− Holistic process optimisation basedon real time measurement (EFSOP®)of off-gas composition;

− Dynamic process control based on off-gas analysis and other novel sensor,and process optimization models(iEAF®);

2) Energy recovery from off-gas:− An efficient solution for off-gas heat

recovery to produce steam/electricalenergy (iRECOVERY®); A steel plantof 1 Mt/y production with top char-ge EAF 100% scrap (150 t/h pro-ductivity) equipped with Tenova te-chnologies is capable to cut CO2

emissions by about 65.500 t/y (seetable 3).

The synergic integration of our technologiesin the EAF route is the new Consteel Evo-lution™ which is able to achieve overall CO2

reduction of 80.000 t/y of CO2 (-15.4%).A short overview of all the above Tenova in-novative technologies.

Fig. 9 – iEAF® sche-me.

EFSOP®

In the last decade, real-time off-gas analysiswas the approach taken to optimize theenergy input in the EAF, due in part to itsuse in post-combustion control and in theenhanced understanding of process dy-namics provided by off-gas composition pro-files. The reliability and effectiveness of theEFSOP® system has been demonstrated bythe successes achieved in over 60 instal-lations; world-wide. Typical results arepresented in figure 8.

iEAF®

Tenova’s “Intelligent Electric Arc Furnace”the iEAF®, was the result of a tremendouscollaborative effort between Tenova, TorontoUniversity, CSM Research Centre and Te-naris Dalmine. The iEAF® is an innovati-ve dynamic process control system for thereal time management of arc furnace mel-ting; that is based on EFSOP® off-gas ana-lysis, and other novel sensors, and dyna-mic real-time models of the EAF process.The iEAF® makes it possible to align che-mical and electrical energy usage duringthe melting phase and to pace the processso that both optimal tapping temperatureand carbon content (see figure 11) can beachieved simultaneously.The benefits demonstrated by the 3 in-stallations of the iEAF® that have been ope-rating since the beginning of 2010 showprocess improvements which are roughlydouble those previously achieved by the EF-SOP® system alone. Four new installations,in Italy, Mexico and Canada, are currentlyunderway.

iBOF®Two issues that affect yield and producti-vity in BOF steelmaking are the ability toaccurately predict end-point and the miti-

Fig. 10 - iBOF® scheme.



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Fig. 11 – Energy balance in a traditionaltop charge EAF (kWh/t).

Fig. 12 – Heat recovery system at Geor-gsmarienhütte plant in Germany.

gation of slopping.Tenova employs an off-gas analysis system(EFSOP®), along with other measured pro-cess variables, as inputs to a non-linear dy-namic model that is used to predict mass,temperature & compositions of the metal,slag and gas phases. This is a real time dy-namic indicator of the bath carbon and tem-perature concentrations alerting the operatorof his progress towards the end of the heat.The Tenova’s patented slop-detection te-chnology gives operators advanced warningof the onset and severity of potential slop-ping. in the converter.High rate sampling and analysis of the vi-bration of the oxygen lance in a converteris a good predictor of the onset of slopping.

iRECOVERY®

Even in an extremely high efficient EAFprovided with all the above technologies alot of energy (about 30%) is lost in the offgas. Figure 13 shows the energy balance ina traditional top charge EAF (kWh/t).An amount ranging up to 30% of it can berecovered with this technology. The Teno-va iRecovery technology, a development ofthe former Evaporative Cooling System

(ECS) well known for reheating furnaces,has proven the massive potential of heat re-covery at the 140t/h EAF of Georgsma-rienhütte, Germany (see figure 14).Since 2009 in this plant the off gas ener-gy is turned into steam; this steam is usedfor vacuum degassing, oxygen produc-tion and for heating purposes in winter. Theelimination of the gas consumed by the for-merly used boiler house, grants savings hi-gher than 1.000.000 €/y (in the new Ge-orgsmarienhütte installation the total ener-gy recovery achieved is in the high side ofthe range reaching the 30%). During thefirst quarter of 2012, the interest for thistechnology is widely spread in the word and3 important contracts have been awardedin Germany and Korea.Tenova R&D activities are in an advancedstage in order to increase the percentageof energy recovered from the off gas anddust and we consider achievable a recoveryup to 70%.An iRecovery waste gas duct works withpressurized water at the boiling pointwhich is led through the piping.The nearly boiling water absorbs the ener-gy from the waste gas by evaporation. In

Fig. 13 - Consteel® installed capacity.

Fig. 14 – Consteel Evolution™ scheme.

a steam drum steam and remaining waterget separated.One of the enhancements of iRecovery isthe continuous availability of steam also forbatch mode furnaces like the EAF.Consteel Evolution™ Process technologyConsteel® is a well known long term pro-ved technology: 32 references already inoperation since 1989 and 8 under way (seefigure 13).Starting from the impressive results ob-tained, Tenova is continuously working inimproving its innovative sustainable te-chnologies, last development being Con-steel Evolution™ (see figure 14).The new system subdivides the pre-heatingtunnel in two sections: the first containshigh-efficiency burners (developed by Te-nova LOI Italimpianti) while the secondcompletes the combustion of the off-gas lea-ving the furnace. The two gas flows mer-ge in an intermediate section, where theyare extracted at high temperature.With this innovative solution, CO2 emis-sions related to the use of very high effi-ciency combustion systems, are more thanbalanced by the reduction in CO2 associa-ted to the less electric energy needed in



Atti e notizie

ConsteelEvolution™ vs. in the traditional Consteel®.CO2 savings depend on countries and inparticular on how the electric energy is pro-duced (nuclear, hydro, coal or gas based).Table 4 reports an expected performancelevel achievable with a Consteel Evolutionfurnace.

REHEATING FURNACEIn the last years Tenova LOI ItalimpiantiR&D efforts were oriented to a drastic re-duction of NOX emissions and the signifi-cant result of less than 60 ppm @ 3% O2 hasbeen achieved. One of the most recent re-ferences are the three 420 t/h walking be-ams Reheating Furnaces (RHF) for TKSnew mill in Calvert Alabama (see figure 15).NOX emissions measured during perfor-

Table 4 - Reports an expected performance level achievable with a Consteel Evolu-tion furnace.

Fig. 15 – Walking Beam furnace at TKSplant in Calvert Alabama.

Fig. 16 – Energyconsumption com-parison betweenthe new A380 air-craft and a highcapacity traditio-nal walking beamfurnace.

mance tests, conducted under the EPA me-thodology, were 56 ppm @ 3% O2 far belowthe legal limit of 71 ppm @ 3% O2 establi-shed by ADEM (Alabama Department of En-vironmental Management). In parallel,recent R&D efforts have been focalized onthe reduction of energy consumption withthe design of a new type of regenerativeand flameless burners whose performan-ces allow two fundamental advantages:energy saving due to a better thermal ef-ficiency and even lower NOx emissions.Even modern reheating furnaces are cha-racterized by huge energy consumption,comparable to those of apparently far anddistant machines such as big aircrafts. Inthe figure 16 energy consumption of thenew A380 aircraft and a high capacity tra-ditional walking beam furnace (>400 t/h)

Fig. 17 – Annualtotal cost for a fullregenerative fla-meless reheatingfurnace.

Fig. 18– Medium-pipe rotary hearth fur-nace at Tenaris Dalmine plant in Italy.

are compared showing similar rates and hi-ghlighting how energy intensive is slab/blo-om reheating.The most modern high capacity slab re-heating furnaces equipped with Full Re-generative Flameless Burners will have, ona Europe cost basis, an overall annual costof 47.6M€ (see figure17).In this case, the fuel consumption reduc-tion can be in the range of 10%-20% com-pared with a traditional furnace in the sameoperating conditions. Range depends on thefurnace configuration and conditions. Anexample of successful result of this type ofburner is the revamping of Tenaris Dalminemedium-pipe rotary hearth furnace (see fi-gure 18).Tenaris Dalmine rotary hearth furnace re-vamping had the main goal of charging lar-ger blooms (up to 5.300 mm) and to im-prove productivity by about 35%. At thesame time a significant energy saving infuel consumption has been achieved cor-responding to about 15%.In this case the chosen combustion systemincluded the installation of 55 TRGX LOIItalimpianti Full Regenerative FlamelessBurners with very low NOx emissionswell below the legal limits.Furthermore nowadays the energy savingconcept is strongly related to the utiliza-tion/recovery of low calorific value fuelssuch as Blast Furnace Gas (BFG) that is wi-dely available in the integral cycle steelplants often with coke oven gas (COG). Whi-le COG, due to its relatively high heatingvalue (18 MJ/Nm3), is commonly used inreheating furnaces as substitute of natural



Nota tecnica

gas (NG) with heating value 36 MJ/Nm3,BFG with heating value 3.1-3.3 MJ/Nm3 isnormally:• burnt in the flares basically wasting an

energy source;• burnt in boiler or gas turbine for power

generation with an efficiency of 35-40%, while reheating furnaces efficien-cy is 55-60%;

• mixed with other more energetic fuelssuch as COG and/or NG for feeding re-heating furnaces.

The trend to reduce coke plants in Europeforces to maximize the use of BFG as sub-stitute of COG. Since BFG combustion sy-stems based on European technology thatovercome the underline limits are notcommercially available and in any case wor-ldwide the BFG technology have not yet be-come state of the art in reheating furnaceengineering, a specific development isnecessary with the goal to:• maintain furnace throughputs;• obtain better flexibility in use of availa-

ble gaseous fuels;• achieve higher efficiency (>60%) and sub-

sequently reduce CO2 emissions re-spect to burn the BFG in flares or for po-wer generation where the efficiency isonly 35-40%.

The way to use in a more efficient way theBFG is the fuel preheating with regenera-tive systems. This target can be reachedonly solving the safety issues related in ac-cordance to the European standardsTable 5 presents CO2 emission reductioncomparing a traditional 420t/h RHF witha regenerative RHF fed with natural gas

Fig. 19 – CFD si-mulation of a pro-totype burner.

Table 5 – Compa-rison between atraditional 420t/hRHF with a rege-nerative RHF fedwith natural gasand BFG.

and BFG.Research and development is aimed in Te-nova LOI Italimpianti to consider the pre-vious recent results as a starting point fora further reduction of NOx emission levelsfrom its burners in order to comply withthe future standards. A further reductionof NOx emissions requires a big effort interms of massive CFD computing andtrials.

HEAT TREATMENT FURNACESTenova LOI Italimpianti is applying in itsheat treatment furnaces the most updatedtechnologies and thanks to its R&D effortshas reached very impressive results interms of CO2 savings. In the modernHPH® bell-type annealing furnaces (HPH= High Performance Hydrogen) hydrogenis used as protective atmosphere in com-bination with high convection for betterenergy transfer purposes and in order tomeet the Customers quality requirements.Tenova LOI Italimpianti, thanks to itsR&D efforts, has obtained in this plants im-pressive results for the improvement ofenergy saving.The most important adopted measures tosave energy and increase efficiency are:- Exhaust heat utilization through higher

air preheating- Stack heat recovery during cooling- Exhaust heat usage by preheating of coils- Stack heat usage for hot water/electricity

generation.One of the last innovations introduced inour HPH® bell-type furnaces is a newheating hood (see Fig.20).

Fig. 20 - Energy saving Ultra-Low-NOxHeating Hood for BAF– Higher Air preheating temperature

(≈550 °C) due to extremely enlarged re-cuperators

– 12 % energy saving– New combustion technology (Flame-

less Oxidation, patent pending)– NOx < 100 mg/m³– No hot spots on inner cover– Successful in operation more than 10

months.

It combines a new combustion technologywith flameless oxidation with an increasedair preheating at the same time.The results are NOx emissions in flamelessmode < 50 mg/m3 @ 5% O2 and 12% fuel gasreduction at approx. 550°C preheatedcombustion air with extended inner coverdurability by decreased temperature loadon the burner affected areas of the innercover. Over the whole annealing cycleincl. flame & flameless mode an averageNOx-emission of less than 100 mg/m3 @ 5% O2 can be reached compared to 250-350mg/m3 with conventional combustion te-chnology.

Stack heat recovery during coolingTo date, BYPASS cooling systems of thistype have been installed separately for eachbase. As the charge is cooled by heat ex-changers, the heat transferred to the char-ge during heating is then transferred to thecooling water. However, it is also possibleto couple two neighbouring annealing ba-ses and to transfer waste heat from the hotstack of base (A) to the cold stack of base(B) (Fig.21). This approach is only possibleif the two bases are no longer operated in-dividually but as a pair, which reduces theflexibility of the overall plant. Although theBYPASS cooling systems of the two basescould be coupled directly, an indirect linkvia an additional heat exchanger is prefe-rable because this prevents the controlledatmospheres of the two neighbouring an-



Atti e notizie

nealing bases from becoming mixed. In eve-ryday operation, the coupling of two nei-ghbouring bases reduces the plant throu-ghput by about 15 %, resulting in a slightincrease in specific power requirements (byabout 1.6 kWh/t). At the same time, a fuelsaving of about 20 % is achieved.A further energy saving can be obtained insome cases with a BYPASS cooling as hotwater generator (Fig.22). This is an an-nealing cycle with heating + soaking,slow cooling and fast cooling. The slow coo-ling could be already used for charge pre-heating at the neighbour base. In fast coo-ling there is only a potential to gain lowtemperature heat.

Fig. 21

Fig. 22 - Bypass-Cooling as hot wa-ter generator.

Fig. 23 - Energy consumption trend overthe years per tonne of steel produced.

During the first 4 hours of fast (BYPASS)cooling, it is possible to heat the cooling wa-ter up to 80 °C without any drop in coolingperformance.This represents an average heat flow of 500kW, resulting in 2000 kWh recuperatedenergy per cycle. This energy can be usedfor heating or for electricity generation.This heat recovery system is only availa-ble with BYPASS cooling technology. Theheat recovery amounts to approx. 22.5kWh/t in case of a 88.8 t stack. Using thisnew technology in a HPH® bell-type an-nealing plant with 12 bases the energy sa-ving amounts of nearly 3.8 GWh peryear.

CONCLUSIONIn the last 30 years the steel industry hasreduced its energy consumption per ton-ne of steel produced by 50% (see figure 23).A similar reduction has also been achievedin the impact that the industry has on theenvironment. However, due to this dramaticimprovement in energy efficiency, there isnow only room for marginal further im-provement on the basis of the existing te-chnology.This fits with R&D approach of Tenova thatis oriented to a continuous improvementof its technologies for achieving the bestperformances in terms of energy effi-ciency and environmental sustainability.

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atti e notizie nota tecnica · 2016. 9. 27. · la metallurgia italiana - n. 10/2012 51 atti e...

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