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7/30/2019 New Tech Gas

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NEW TECHNICAL POSSIBILITIES FOR REDUCTION OF COKE OVEN

EMISSIONS

Michael HEIN, Friedrich HUHN, Frank ROSSA,Deutsche Montan Technologie GmbH, Essen, Germany

Heinz OPDENWINKEL, Joachim STRUNK, Deutsche Steinkohle AG, Herne, Germany

This paper was first presented at the 5th European Iron & Cokemaking ConferenceStockholm 2005

Abstract

A research and development program was initiated to reduce the emissions from coke ovens.Main targets were the improvement of the charging process with particular consideration ofthe levelling procedure and the reduction of door emissions.

For reduction of the charging emissions a new levelling system was developed and is since2000 in operation at the Prosper coke plant. A new kind of door sealing system wasdeveloped to reduce the effective raw gas pressure, and hence the emissions at the door seal.This is achieved by means of a pressure equalisation duct arranged around the door concerned

and connected with the gas collection chamber. Furthermore the flexibility of the oven doormembrane was improved by means of a new multilayer-membrane. A test door was built andapplied for 2 years at the Prosper coke plant. After the successful test operation 20 doorsealing systems of the new design were installed in 2004 at the Prosper coke plant. After adescription of the design principles the paper will mainly describe the practical experienceusing the 2 new emission reduction systems at the Prosper Coking Plant, Bottrop.

Introduction

BaP Emissions in Coke Plant Operation

Fugitive emissions from the operation of a coke battery cannot be avoided completely. Theseemissions contain small concentrations of polycyclic aromatic hydrocarbons (PAH). Because

most of the PAHs are carcinogen, these compounds are in the focus of public discussions; asa guiding reference compound for PAH the Benzo(a)pyrene (BaP) is usually considered [1].Depending on the terms and conditions of coke oven operation, coke oven age, technicalstandard and maintenance status of the overall plant, the product-specific BaP emissions ofthe coke plants currently operated in Europe range between 10 and 100 mg/tcoke [2].Generally, from health protection reasons the demands by the public for a further reduction ofindustrial emissions were markedly strengthened in the past decade; in the EU the fourthdaughter directive [3] was recently fixed in which a target value for the Benzo(a)pyrene (BaP)ambient air concentration of 1 ng/m is set effective from 2013. Member states musttranspose it into national law by 15 February 2007. The European Commission will report onits implementation by 31 December 2010. It is a great challenge for the coke making industryto cope with the BaP ambient air concentration target value, at least concerning theconcentration in the direct vicinity. It is questionable whether state-of-the-art emission

reduction technologies are sufficient to prevent leakages to the necessary extent.

This situation was the starting point for DMT to derive more advanced possibilities foradditional emissions reductions. Special consideration was laid on the realisation of primarysolutions for emission control, to which a higher potential for improvement may be attributedthan to only an abatement of the effects. Primary solutions mean process technical measuresto reduce the relevant pressure gradient that is the driving force for any emission. Meanwhilethe developed solutions have found its way into industrial practice.

Reduction of door emissions


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State of the art

In the 80s at most German coke plants the technological transition from rigid doors tosystems equipped with flexible diaphragms was carried out [4-8]. Several different solutionswere invented:

Efficient insulations reduce the heating-up and deflection of the door body. Gas channels with large cross-sections behind the door body reduce the pressure

gradient between the inside and outside of the sealing edge. Flexible sealing elements allow to a certain extent a movement of jamb and door

throughout the coking cycle.

Despite of all the improvements made leakages can be observed quite frequently in the firsthour after charging. The gaps causing leakages in the beginning of the coking time have to besealed off by the workmen; to this end additional measures e.g. manual sealing with fibrousmaterials or slurries or readjustment of the sealing edges have to be undertaken. To facilitatethis work several modern coke plants have installed so-called service cars allowing an easyaccess to the whole outer door area. In every case all this means that despite of a very highstatus of development of the doors a sufficient tightness can be achieved not without thisadditional regular work.

Further cleaning work is necessary by means of the cleaning machinery to remove thecondensates that are deposited at the seal and the jamb and would give rise also to theformation of gaps. However, every mechanical work at the door seal is a curious combinationof heavy work and precision mechanics with tolerances of tenth of a mm; this combination

bears a high risk of damaging the sealing elements. To improve the situation any need forfurther maintenance handling should be lowered.

These problems that are typical for more or less all oven doors could also be observed at thelight weight doors which are installed at the Prosper coke plant of DSK (Deutsche SteinkohleAG). This door type has been in use on DSK coking facilities for a longer period of time fortesting purposes and has proved its worth (Figure 1). The door body itself comprises of

several segments of steel that can be adjusted to the relevant door jamb contour by means ofeccentrically adjustable wedges. The pre-adjustment of the segmented door body of a 7 mdoor alone allows for a certain compensation of the jamb deformation. The sealing effect is

brought about by a diaphragm. A multitude of elements similar to plate springs aredistributed over the entire frame and act on the diaphragm. The seal itself consists of amaterial that easily withstands the high temperatures and corrosive conditions. The weight ofthe door body is 15 % lower than of a conventional cast iron body.

The doors were equipped with a ceramic heat shield which enabled the raw gas to passbetween door body and coke cake. In this way, the gas pressures prevailing in the entire ovenquoin area could be distinctly reduced. Furthermore the door weight is lowered considerably

because of the missing door plug.


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Figure 1 Coke oven door installed at the Prosper coke plant

Improved door sealing

To lower the leakage risk causally it was the target to further decrease the pressure gradient

behind the sealing in combination with the avoidance of any gap from thermal distortion. Toreduce simultaneously the deposition of tarry condensates the direct contact of the seal withraw gas should be reduced. To this end DMT developed a new sealing type and implementedit into the existing door body at the Prosper coke plant. One door was installed in May 2002and tested over a time of 2 years in continuous operation [9-11]. Reduction of the drivingforce for door emissions. To reduce the driving force for emissions, i.e. the pressure gradient,a double sealing was developed that is arranged around the door and forms a duct (Figure 2);

by an opening in the upper part this duct is connected with the free space of the coke oven[12]. This provides the possibility of a pressure equalization over the height withoutoriginating an undesirable pressure increase:

At the beginning of the coking time the pressure behind the inner sealing (in theoven) is quite high because of the strong gas formation. If there would be anyleakage at the inner seal the gas would penetrate through the gap and expand into thegas channel. Here the driving force for emissions, the pressure behind the outersealing is very low such that the gas could not penetrate through the outer seal.

At the end of the coking time the pressure in the oven behind the inner sealing may belower than the ambient air pressure. However, because of the possibility of a gasexchange the pressure behind the outer door seal is increased, such that no air can besucked into the oven. The lower temperature in the gas duct (approx. 160C) is ofadditional advantage. If there is a gap at the inner seal only a small amount of gas inthe gas duct can flow into the oven; no detrimental effect by air flowing into the ovencan take place. The gas duct works like a gas lock. Because of the connection to thefree gas space only the pressure level present there is effective.

Improvement of flexibility of the sealing system

The amount and the transmission of the forces onto the sealing is an important precondition toget a good seat of the sealing. The effect of the forces onto the sealing must not bedeteriorated by a too high stiffness of the diaphragm and the springs; this would lead to theformation of gaps because of thermal distortion. To this end the flexibility in combinationwith the distribution of the bearing pressure was improved using a multi-layer diaphragm [12]as shown in Figure 3.


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Compared to a single diaphragm of similar total thickness this design has the advantage thatthe flexibility is enhanced by more than 300% because the individual layers can slide overeach other - and this without any irreversible deformation. The multi-layer concept also hasthe advantage that any corrosion of one of the sheets will not lead to any leakage, because theother sheets in combination with the deposition of condensate assure the tightness. The forcetransmission is applied by leaf springs to realize a contact pressure uniform over the length.

Figure 2 Oven door area - principle of the new sealing system


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Figure 3 Design of the sealing systemFigure 4 shows the conventional and the practical realization of the improved door. Thesprings are also executed in a multi-layer manner; the force is transmitted near the outersealing. This allows for an ability of tilting of the gas channel and thus for an additionaladaptability.

Corner area of the sealing system

A particular problem of most door constructions is the corner area. These areas usually havea much higher inherent stiffness and often give rise to leakage. Furthermore because of themissing flexibility thermal stresses occur resulting in the creation of fissures. In this area aconstruction based on the application of cup springs provides for a very high flexibility and asufficient bearing pressure (Figure 4, center).

The leveller door at many plants frequently is a source of leakages. This results in particularon the higher temperature level in this area leading to thermal distortions and thus to the

formation of gaps. The flexibility of the existing door was only a tenth of a millimetre at thegiven contact force. For improvement the double sealing and multi-layer diaphragm conceptswere also applied to the leveller door construction (Figure 4, right). Thereby its flexibilitywas enhanced drastically to a level of some millimetres. To prevent any stiffening by thecorner construction and to maintain the flexibility also a special cup spring construction wasused. The 4 side pieces of the sealing are not directly connected or welded and can be pressedwith the cup spring construction each individually onto the jamb.


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Figure 4 Prosper coke oven door; left: conventional; center: equipped with the DMT-sealingsystem and the advanced corner construction; right: new leveller door

Leveller door sealing system

Practical experience

Because of the positive results using the new sealing system at the test door over more than 2years DSK decided to apply the new system at 20 coke side doors. After engineering of thechanges necessary to adapt the sealing system to the coke side conditions the sealings were

pre-manufactured; then the system was installed at existing door bodies in the door shop ofthe Prosper coke plant. Simultaneously the regular maintenance was applied to the door

bodies. Week by week one or two doors were modified. Each modified door was insertedafter a heating up in the preheat box. By means of the existing bolt mechanism the door was

pre-adapted to the door jamb contour. After one coking time the door has accommodated tooperating temperature and a second adjustment was made, in most cases the final one. InDecember 2004 the modification of the doors 51-70 (battery no. 2) was completed (Figure 5);

since then the doors are in regular operation.


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Figure 5 Coke side oven doors of the Prosper coke oven battery no. 2equipped with the new sealing


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Emission reduction

The pressure development behind the door seals reflects a considerable lowering of thedriving force for emissions [9-11]: Behind the sealing of the conventional Prosper door thegas pressure reaches max. values of approximately 1.8 mbar in the first hour of the cokingtime. In contrast to this the corresponding pressure at the outer sealing of the DMT dooramounts only to 1.2 mbar. At the end of the coking time the pressure at the conventionalsealing is near suction at 0 mbar whereas the pressure level at the outer DMT sealing remainsat a save level of 0.2 mbar. Thus the new system lowers the driving force for emissionsdrastically and simultaneously prevents any risk of wall damage by air penetration into theoven at the end of the coking time. The positive pressure at the end of the coking time givesthe operator the opportunity to lower the collecting main pressure with the effect of anadditional emission reduction.

Corresponding to the pressure development the emission reduction is considerable. Really noleakages are visible right from the beginning of the coking time. To assess the emissionreduction using the new sealing type more detailed measurements were performed. Toquantify the emissions the door area was completely enclosed by a polyester foil (Figure 6);subsequently the concentration and the mass flow of Benzo(a)pyrene and organic carboncompounds was determined. To exclude any fortuity the measurements were performed 2 or3 times at each door.


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Figure 6 Measurement of door emission by enclosing the door areaThe results are shown in Figure 7. Whereas the specific BaP-emission of the conventionaldoor attains values up to approx. 5 mg/tcoke, the corresponding value for the DMT-typesealing amounts only to 0.3 mg/tcoke. This considerable reduction was achieved without anymanual sealing off. In contrast to this the conventional doors were sealed off by the workmenmanually using fibrous materials or slurries to seal the remaining leakages after charging. Ithas also to be noted that there is a considerable spread of the values concerning theconventional door, but quite consistent values for the new sealing system.

Figure 7 Specific BaP-emission from CS-doors with the DMT sealing system compared withthe emissions from the conventional door.

Door cleaning

The existing mechanical door cleaner of the Prosper coke plant cannot be applied for themodified doors. However, as it was established already during the operation of the test door,the doors needed only a reduced cleaning work. At time only an occasional manual cleaningmainly at the bottom gas channel is performed. During the remaining time the door isoperated without cleaning. In combination with the lack of the necessity to perform manualsealing it can be stated that cleaning and maintenance can be reduced considerably.

However, to ensure a safe operation even in the long-term an appropriate cleaning device willbe indispensable. At time DSK and DMT are jointly developing an equipment suitable forcleaning of the gas channel sealing.

Low Emission Levelling System

To achieve an even and flat coke oven charge in top-charging processes, wet coking coal ischarged through 4 or 5 coal charging holes into the coke ovens. The charging conesdeveloping under the individual charging holes are levelled-off by means of the leveller bar.During the charging special care is to be taken because substantial charging gas volumesevolve already at the start of coal charging, the exhausting of which into the collecting main,is further prevented through the coal still to be charged. However, to allow charging gases toescape from the coke oven, a sufficient suction is necessary which is generated typically bymeans of high-pressure liquor in the ascension pipe. The coke oven batteries equipped withthe PROven process [13,14] have the system inherent advantage of the collecting main being


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operated under negative pressure. This suction is used very effectively for charge gas transferwithout a need for a high pressure liquor system.

However, as long as the leveller flap is still closed, the charging gases are discharged due tothe adequately effective suction. Usually it is sufficient as long as the gas discharge routesare not blocked by too high a formation of charging cones in the oven. However, to enablelevelling, the leveller door must be opened, causing the negative pressure to collapse. Onaccount of the growing gas pressure in the oven, charging gases or even flames might escapefrom the open leveller flap, the charging holes and the coal charging telescopes. Anadditional problem is the fact that during the levelling period the cross section of the gasspace is more or less blocked by the leveller bar construction itself. Thus a free gas flow isobstructed.

To prevent the occurrence of charging emissions the gas exhausting system the suction lossesin the levelling system have to be minimized. This was achieved by means of a combinationof improvements at the leveller sleeve and the leveller bar [15,16].

Features of the DMT levelling system

Leveller bar

To avoid the blocking of the gas space during levelling a completely new construction of theleveller bar was designed: As a main feature, the height of the cross-bars was reduced and thetip of the bar was constructed as an open structure, too [17]. From this measure the formationof an integrated gas duct results (Figure 8), which is located above the cross-bars and whichextends from the tip over the entire length of the leveller bar. The statics layout was

calculated in such a way that despite of the open structure of the bar and the lower weight thestability could be increased and the bending be reduced. Detailed temperature measurementsat leveller bars in operation serve as a basis for realistic assumptions.

The coal in front of the open leveller tip is transported and distributed by means of the crossbars instead of being compressed in front of the tip. By way of a tighter arrangement of thecross-bars the distribution capacity of the leveller bar was improved. In addition framework-construction openings were implemented into the side walls of the leveller bar; thus, thecharging gases are afforded access to the integrated gas duct also from the sides. At the sametime, the coal instead of building-up between chamber walls and leveller bar, as mentioned

before, can fall laterally between the cross-bars and is also distributed.


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Figure 8 Integrated gas duct framework construction ofthe new leveller bar (left)

compared to a conventional bar (right)

Leveller sleeve

The second main feature is a special sealing and a counter-suction at the leveller sleeve [17]to avoid a breakdown of the suction in the oven during levelling as occurring usually.


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Figure 9 Scheme of the sealing and counter suction at the leveller sleevesleeve

The sealing encloses the bar in such a manner that a box type profile closed all around, also inthe inner free gas duct results (Figure 9). The inner sealing consists of several loop-shapedstripes of stainless steel sheets. In combination with the cross bars of the leveller, anundesired intake of air into the oven during coal levelling is reduced. The practical realisationat the Prosper plant can be taken from Figure 10.

Figure 10 Leveller bar with integrated gas channel, side openings and the inner sealing


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system at the sleeve

The counter-suction is realized by means of an exhauster at the leveller sleeve. Thisexhauster sucks air from the front opening of the sleeve in; hereby a dead space in terms offlow is created in front of the opened leveller flap. Thus the same effect as with a closedleveller flap is achieved. By means of the sealing and the additional counter suction systemthe suction in the coke oven during charging is made more effective. The system, that wasfirst tested at the Hassel coke plant of DSK, is in operation at one pushing machine of theProsper coke plant since January 2000 and since 2003 also at the second machine.

Results

Levelling uniformity

Because of the improved mechanical construction and the higher stability the bending of thenew bar during operation is considerably lower than that of the conventional bar. This isvalid in cold as well as in hot condition of the bar. In combination with the higher distributioncapacity of the bar the charging heights are on a high level leading to higher charge weightsthan with the conventional bar (Fig.11).


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Figure 11 Charging heights using the conventional (left) and the DMT-type (right)levelling system

Figure 11 Charging heights using the conventional (left) and the DMT - type (right) levellingsystemEmission reduction

The improved pressure conditions caused by the new levelling system leads to a considerableemission reduction compared to the conventional system. To assess the emission reduction

by the new system the dust load and B(a)P (Benzo(a)pyrene)-concentration induring a series of charging procedures using air samplers. The samplers were placed in adistance of 3 m from each charging telescope in the wind direction. Sampling was performedfrom the beginning to the end of each charging procedure. The improvement concerningB(a)P and dust is considerable as shown in Figure 12.


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Figure 12 Dust and BaP concentrations near the charging car during charging procedure

Conclusions

With view to the new EU regulations concerning BaP in ambient air primary measures for acausal reduction of the driving force for emissions at oven doors and during levelling weredeveloped. A new door sealing concept has shown its potential for an important emissionreduction even compared to state-of-the-art doors. Meanwhile, for a customer the sealingsystem was also adapted to a hammer-type door. Two correspondingly modified doors will

be tested in industrial operation shortly. The modification of a leveller door with the DMTsealing system is recently under development for another customer. Also, the new levellingsystem is an effective solution for emission reduction during charging and simultaneously toimprove the throughput of the battery, i.e. the economic situation. In next future the systemwill be implemented at 2 other coke plants.

References

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3. Directive 2004/107/EC of the European Parliament and of the Council of 15

December 2004relating to arsenic, cadmium, mercury, nickel and polycyclic aromatic hydrocarbonsin ambient air

4. W. Eisenhut, Coke oven door-design and efficiency of sealing, The yearbook of thecokeoven managers association, 1985, 124-133

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9. H. J. Giertz, F. Cyris, J. George, F. Rossa, F. Huhn; New coke oven door withpressureequalization duct, ISS Conf. Proc., Indianapolis 2003

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13. H. J. Giertz, F. Huhn, K. Hofherr; New process to avoid emissions: constant pressurein cokeovens, Ironmaking Conference Proceedings 54, 1995, 439-445

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