substitution of primarv fuels bvwaste materials in …

Substitution of Primary Fuets by Waste Materials in High Temperature Processes

27

SUBSTITUTION OF PRIMARV FUELS BV WASTE

MATERIALS IN HIGH TEMPERATURE PROCESSES

Beckmann, M.I) ; Scholz, R. 2);

I) Clau5lhalln51itute of Em'ironmental Engineering Ltd. (CUTEC-ln51itut GmbH)~IInstitule of Encrgetic Process Engineering and Flic! Technology orlhe Technical Uni\'l~'fsi{yC'lausthal

Table of Contents

I Introduction .

2 Balances of Firings and Industrial Furnaces including Heat RecovelY

3 Potential ofFuel Substitution by Wastes .

4 Effects of Slibstinllion on Process Management (Examples)

5 References .

6 Nomenclature ...

7 Figures ...

... 2

.3

5

10

13

.. ... 15

.. ... 16

Proc. wiss. Konferenz "Einsatz von industriellen und kommunalen Abfallen im Zementherstel-lungsprozeß", 15.-16. 10 1998, Oppeln/ Polen, [SSN 0860-9160,pp. 70 - 93.

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Textfeld

Beckmann, M.; Scholz, R.: Substitution of Primary Fuels by Waste Materials in High Temperature Processes. In: Proceedings Konferenz "Einsatz von industriellen und kommunalen Abfällen im Zementherstellungsprozeß", 15.10.-16.10.1998, Opole (PL), S. 70-96. ISSN 0860-9160

Substitution of Primary Fuels by Waste Materials in High Temperature Processes Page 1

SUBSTITUTION OF PRIMARV FUELS BV WASTE

MATERIALS IN HIGH TEMPERATURE PROCESSES

Beckmann, M. I) ; Scholz, R. 2);

I)Clausthallnstitute ofEnvironmental Engineering Ltd. (CUTEC-lnstitut GmbH)2) Institute ofEnergetic Process Engineering and Fuel Technology oflhe Technical University Clausthal

Table of Contents

1 Introduction 2

2 Balances of Firings and Industrial Furnaces including Heat Recovery 3

3 Potential of Fuel Substitution by Wastes 5

4 Effects of Substitution on Process Management (Examples) 10

5 References 13

6 Nomenclature 15

7 Figures 16

SUBSTITUTION OF PRIMARY FUELS BY WASTE MATERIALS IN HIGH TEMPERATURE PROCESSES

AbstractThe energetic and material utilisation of waste has been the focus of much attention for a longtime. A lot of references are found in literature with regard to the specific application e.g. the co-combustion of sewage sludge, the substitution of primary fuels with plastic wastes in theincineration process of the cement industry etc. Furthermore, comparisons of the individualapplications with each other, as weil as comparisons with state-of-the-art thermal wastetreatment methods should be discussed. Naturally, when comparing these processes, the entiretyof required additional material and energy expenditure must be included under equal boundaryconditions. Here, among other things, special attention must be paid 10 the uniqueness of therespective ca se of application that results from the specific properties of the waste materialssupplied. This holds especially tme when examining the effects of replacing primary fuels withsubstitute fuels from waste (secondary fuels) e.g. -in the case of a combustion process- on heattransfer conditions, flow conditions and the related distribution of temperature, material transportand specific energy expenditure. Apart from material aspects, substitute fuels must also satistYrequirements pertaining to fuel- and heat-related aspects.

This article deals especially with demands conceming fuel- and heat-related aspects. For a betterunderstanding, first of all general aspects of heat transfer in firings and industrial furnaces areexplained. The so-ca lied energy exchange ratio is important when evaluating a given case of fuelsubstitution. This ratio expresses the value of a substitute fuel in relation to that of the primaryfuel and must be taken into consideration when the comparison is based on balancing equations.Moreover, by resorting to the example of the clinker process in the cement industry, this articleaddresses various consequences of fuel substitution on e.g. kiln temperatures, flue gas massflows etc. from a process engineer's perspective. Using an example from the steel and ironindustry, fuel substitution is considered for the case of a natural gas-fired, coke-free cupola.

1 IntroductionTechnical development in the field of waste management has lead 10 a situation today, in whichseveral concepts are available for the treatment of waste. Increasingly, these also include thematerial and energetic utilisation of partial fractions in processes of material treatment or energyconversion.

Figure I-I illustrates these possibilities for domestic and industrial wastes, plastic wastes andsewage sludge. ClassitYing the options into the three process steps

• mechanical-biological treatment (pre-treatment),

• thermal waste treatment and

• production processes,

figure I-I confines itself to mentioning the respective basic operations. For each process step,further divisions are possible according to the different processes or, in the case of theproduction processes, according to the branch of industry. On this note, multiple references canbe found in literature, for the mechanical-biological processes [e.g. I to 6), as weil as forprocesses of thermal waste treatment [e.g. 7 10 15) and production processes [e.g. 16 to 23).


In figure 1-1 the arrows exemplifying material and energetic utilisation take into consideration

• not only the classical incineration of untreated waste (combustion-postcombustionprocesses with grate systems [e.g. 14], but also

• the generation of combustible gases by means of gasification, after having pre-treatedthe waste, e.g. for substituting primary fuels in material processing (calcinator stagein the clinker process [18]),

• the use of high calorific fuels (BRAM) and sewage sludge for co-combustion inpower stations [23,24],

• the separation of plastics and metals for material utilisation [25,26]

• etc.Moreover, figure 1-1 shows the need for investigating the entire course of the process frombeginning to end after having dealt with a single process step. These investigations have beencarried out for certain sorts of waste. For example, the energetic utilisation of plastic waste in thecement industry [27] or the treatment of domestic and industrial waste through mechanical-biological methods combined with thermal treatment processes deserve mention. Currentresearch projects serve to diminish the deficits connected with the balancing of mechanical·biological treatment methods compared to thermal treatment methods and production processes.As a rule, the aforementioned comparisons are based on identical boundary conditions.Occasionally, this may require including additional reference processes or substitute processes.Furthermore, attention needs to be paid to process conditions changed by application of wastefractions in processes of material treatment and energy conversion as weil as the thereby ensuingdemand for appropriate optimisation of the process management.

Against this background, the following, first of all, investigates the general case of fuelsubstitution in processes of material treatment carried out in so-called industrial fumaces.Attention shall then be paid to the examples mentioned in the abstract, i.e. from the cementindustry and steel and iron industry. Statements concerning fuel substitution can similarly betransferred to other processes of the raw material industry (lime, glass, ceramics etc.) as weil asto processes of energy conversion in general, i.e. high temperature processes.

2 Balances of Firings and Industrial Furnaces including HeatRecovery

The term "industrial furnace" comprises all fossil fuel-fired or electrically heated apparatuses, inwhich solid, liquid or gaseous materials are subjected to thermal treatment on an industrial scale.In this context, the focus is on fossil fuel-fired industrial furnaces.

Industrial furnaces and firings for energy conversion have in common the process of combustion.However, compared to industrial furnaces where the combustion process is controlled inaccordance with the load or input material to be treated, firings are optimised, e.g. in the ca se ofsteam genera ted in a boiler, principally with respect to the exploitation of energy. Figure 2-1schematically illustrates this difference [28]. With regard to heat generation and exchange, thesimilarity between a boiler and an industrial furnace principally allows aspects of balancing, heatrecovery and fuel substitution to be transferred from an example, treated in the following, of amaterial conversion process to energy conversion processes. In the case of directly frred


industrial fumaces, fuel substitution is always linked to the question of product compatibility.Fuel with an ash content may then also require a respective material substitution.

Industrial fumaces are employed in almost all significant branches of the raw material industry.Examples include the buming of clinker in rotary kilns in the cement industry (figure 2-2),buming of lime stone in shaft fumaces, buming of ceramic products for tbe building industry intunnel fumaces, generation of pig iron in blast fumaces, smelting of scrap in a cupola(figure 2-3) etc.

Commonly, energy balances form the basis for evaluating industrial fumaces, both for plantcomponents as weil as for the entire plant (preheating, combustion and cooling processes etc.).For a better understanding, figure 2-4 contains an example of air and fuel preheating and acontinuous fumace process with a chemical or physical conversion process in the load (burningof lime, smelting of metals etc.). For the example in figure 2-4, the total balance reads:

IhF ·h" = rilS . [,MRc +Cs]J .(SSl -SS/)]+liIC 'CCJ, .(SCJ -S,)+ LQ, (2.1).

In the general case depicted here, heat from the hot flue gas is exchanged in a recuperator topreheat the combustion air and, in a second recuperator connected in series, to preheat the fuel.Especially in the context of fuel substitution, the question arises, to what extent air andlor fuelpreheating makes sense. The entering and exiting mass flows ofair, fuel and gas (riIA,lhF,lilc)needed for the balancing equations are connected with each other via the specific stoichiometricair requirement obtained througb the combustion equation and by neglecting the solid inertcomponents in the fuel (ash, eq. (2.3)):

mA = "A. ·'min . InFrilC = (1 + A. .lm,,)·riIF

Thus, the efficiency factor of the air recuperator amounts to:

rilF ·A.·lm;n .cA]J .(SAl -SAI)

11AH= rilF .(1 + A..lm,.)·CCI' .(SCI-S,)

(2.2)

(2.3).

(2.4).

(2.5).

Accordingly, that ofthe fuel recuperator reads:

IhF .cF]J .(Sn -SFI)11FH =. ( ) ( )mF' 1+ A. ·lm,n . cGl,' B Gl - S,

As seen in eqs. (2.4) and (2.5), the heat capacity flow ratios of both recuperators are less thanone. Thus, in a counterflow arrangement, air can be only heated to a maximum temperature ofSA2 = BGI and fuel to a maximum temperature of Sn = SG2. Therefore, the maximum efficiencyfactor of air and fuel preheating results, with tbe simplified assumption that the heat capacities ineqs. (2.4) and (2.5) are, in pairs, approximately equal and for the limiting case of astoichiometric plant operation with A.= 1,0 for SAI = Sn =Sa, in [29]:

Imin

11 AH,ma:c ::;:;:1+ imin

1llFH,ma:c ::;:;:1 + t

min

(2.6)

(2.7).


(2.8).

Tbe greatest possible overall efficiency factor oftbe beat recovery plant amounts, with eqs. (2.6)and (2.7) for tbe special case of adiabatic recuperators, to:

J+lmm +/~inllHR,m4' = ( )1

J + Imin

Figure 2.5 sbows tbe maximum efficiency factors (eqs. (2.6) to (2.8)) over tbe stoicbiometric airrequirement. Improvement of llHR.max by fuel prebeating is only acbieved in tbe case of fuels witba low stoicbiometric air requirement, tbus, as a rule, only in tbe case witb fuels of low calorificvalue.

Peculiarities in tbe context of heat recovery must obviously be taken into consideration wbenreplacing a primary fuel of bigb calorific value witb a substitute fuel of low calorific value. Tbefollowing section takes a closer look at tbis kind of fuel excbange.

3 Potential of Fuel Substitution by Wastes

Usually, replacing fuels witb waste will immediately lead to tbe question of tbe influence of tbewaste materials on tbe conditions of tbe respective process. Especially tbe effects of usingsubstitute fuels will be exarnined on process temperatures, flue gas amounts, noxious substancesor noxious matter loads and specific energy consumption in tbe case of industrial furnaces or,respectively, efficiency factors in tbe ca se of energy conversion plants.

Not until tben can possibilities be discussed of optimising the process witb cbanged boundaryconditions due to use of substitution. Tbus, tbe assessment of a fuel depends not only on tbe kindof fuel itself, but it is also essentially influenced by tbe manner of operation of botb plant andbeat recovery.

Moreover, for tbe individual process, tbe ensuing mass, energy and material balances must beexarnined in tbe context of the entirety of tbe respective overall concept in tbe form of totalbalances [e.g. 15], i.e. among otber tbings, tbe absolute figures of energy saved by means ofsubstitution need to be compared with energy expenditures for required waste pre-treatment etc.

The effects of substitution can essentially be derived from tbe combustion properties such as netcalorific value hn, stoicbiometric or tbeoretical air requirement im;,,, specific flue gas amountVC.m;n, calorific combustion temperature Beal etc.

At frrst sight, the balancing equation (2.1), along witb figure 2-4, sbows that the flue gas loss ofa given fuel is bigher, the higher tbe amount of flue gas mG in relation to tbe net calorific valuehn or tbe bigber the flue gas temperature BCl . Tbe increase of flue gas loss means less fuelenergy can be utilised for the process, the specific energy expenditure rises.

Less apparent is tbe influence of the calorific combustion temperature on process conditionsconnected with tbe aforementioned quantities. For discussing tbis quest ion, in a frrst step weexamine one section of a continuous industrial fumace and - for the time being to explain tbeprinciple - for reasons of simplification liken it to a continuously stirred reactor element (CSR)(figure 3-1 A, Jeft). Tbe supplied energy results from the conversion of fuel with air taking intoaccount air and fuel prebeating. For the time being, tbe basic considerations disregard theinfluence of athermal dissociation balance and tbe theoretical combustion temperature Sth that isdetermined accordingly. Tbus, tbe caloric combustion temperature Seal can be used for obtaining


the gas enthalpy flow. In an ideally mixed kiln chamber (CSR) this enthalpy flow effects a heatflow Qcs. to the load with the constant surface temperature SSI and the temperature of the gas inthe stirred reactor element settles at a constant temperature that equals the exil temperature SCI .

From that, assummg adiabatic conditions, follows generally (SSI '" Ss, SCI '" Sc):

MG = Qcs. (3.1)

with MG = lilG • CC.caIC . (Scal - Sc) (3.2)

and Qcs. = u'" . As .(S C - Ss) (3.3),

where in eq. (3.3) the heat transfer coefficient u~ includes convection and radiation and Asdenotes the surface of the load.

When replacing the primary fuel (PF) with the substitute fuel (SF), the demand exists that theload temperature Ss and the kiln output, i.e. the exchanged heat flow Q, remain unchanged

~=~ 0~immediately yielding:

(3.5).

(3.8).

Eq. (2.3) links the gas mass flows lilC,PF and lilG.5F from the fuel conversion with the respectivefuel mass flows lilpF and mSF while disregarding the solid inert component of the fuel (ash).

In introducing the energy exchange ratio E :lilSF • h, SF

E = . (3.6),lilpF • !I',PF

which, from an energetic point of view, expresses the value of a substitute fuel in relation to theprimary fuel, eq. (3.6) can be re-arranged to yield the ratio of fuel mass flows:

mSF hn,PF-=E·- (3.7)JnpF hn,SF

while eqs. (2.3) and (3.6) yield the ratios of flue gas flows:

rilc.SF = E. h,.PF • (I + A. SF ·lm",SF )

rilC.PF h,.SF . (I + A. PF ·/m".PF )

The ratios of fuel and flue gas mass flows are important for further consideration of suchboundary conditions that are linked to the plant itself such as fuel supply, flow conditions in thekiln chamber, material transport etc. Often these deterrrune the maximum rate of substitution(replacement ofprimary with substitute fuel), which is dealt with at a later point.

Further detailed, with eqs. (2.3), (3.2) and (3.6), the energy exchange ratio for a continuouslystirred reactor reads:

c .(S -S )(I A..I ). G,PF,calGI cal,fF GI.PF

+ PF min,PF hE _ ,.fF

~- (S S)(1 A..I ). CC,SF .ca/GI' cal,SF - GI.SF

+ SF min,SF hn,SF

(3.9).

Substitution 01 Primary Fuels by Waste Materials in High Temperature Processes Page 7

During the following examinations we assume that the conditions of heat exchange (a", . AsJare not influenced by the substitution (static examination). At a later point, the influence ofchanged conditions of heat transfer is separately dealt with in an advanced step. From the staticexamination assumed for the time being folIows:

(3.10).

With the accepted simplifications, the energy exchange ratio ECSR.& can be obtained for a givenreplacement of a primary fuel with a substitute fuel.

As a rule, substitute fuels with lower calorific values than the primary fuels also achieve lowercalorific combustion temperatures. Therefore, when employing substitute fuels, in order tobalance the smaller difference ofpotential between the calorific temperature and the balancing orgas temperature (Seal - Sc), compared to that of the primary fuel, the gas mass flow lile mustincrease accordingly. Thus, compared to the primary fuel, the use of substitute fuel at first entailsa higher total energy expenditure. The energy exchange ratio expresses the ratio between bothtotal energy expenditures. In the context of substitute fuel employment, the aim of optimisationis, by appropriate plant arrangements and heat recovery etc., to attain a smallest possible energyexchange ratio. This is explained in further detail at a later point.

Figure 3-2 shows ECSR.& according to eq. (3.9) over a required gas temperature Sc for the ca se ofreplacing a primary fuel that has a calorific value of hn.PF = 30 MJ/kg with a substitute fuel withhn.sF= 11, 15,35 and 45 MJ/kg. The energy exchange ratio increases, the higher the requiredbalancing temperature and the lower the calorific value of the substitute fuel as compared to theprimary fuel (curves I to 4 in figure 3-2).

Conversely, in the case of the substitute fuel having a higher calorific value than the primaryfuel, the energy exchange ratio assurnes va lues less than one (curves 5 to 8). Thjs, for example,is the case when replacing coal with high calorific plastic wastes in the primary firing of theclinker buming process (cf. figure 2-2). When comparing the energy exchange ratios in figure 2-2 for a given substitute fuel with and without air preheating, the significance of heat recovery(e.g. recovery of heat from the load in the grate cooling-clinker buming process) becomesevident. In the ca se of a substitute fuel with hn.sF = 11 MJ/kg (curves 1 and 2), gas temperaturesSc> 1700 °C can only be achieved with appropriate preheating of air.

Until now, the gas temperature 9G with substitution was assumed constant. However, due tochanged mass flows and compositions of the gas, fuel replacement also leads to changedconditions of heat transfer (convection and radiation), thus, varying gas temperatures(Se,PF oF SC.SF) are attained. The following considerations refer to the case of an exclusivetransfer of heat by means of convection. The general ca se reads:

QcsRa =a.As·(Se-Ss) (3.11).

With Stanton's number:

the transferred heat flow becomes

QCSR.a =me ,ce '1+~/SI·(SCQ1-SS)

(3.12),

(3.13).

(3,15)

(3.14),

(3,16),

PAGE 8 SUBSTITUTION OF PRIMARY FUELS BY WASTE MATERIALS IN HIGH TEMPERATURE PROCESSES

Hence, the energy exchange ratio ECSR,a is expressed by the respective fuel-related quantities h",

I"'b" Seal, the process-related, fixed temperature of the load Ss, and finally by Stanton's number.the latter characterising the heat transfer conditions:

1 (ScaIPF-SS)(1 + APF ,lmm,PF Y. cG PF ( 1) h

1+-- nPFStpF

E~.= ()1 Sca',SF - S5

(1 + ASF ,lmin,SF Y. CG,SF '( 1)' h1+-- n,SF

StSF

In order to evaluate eq, (3,14), StpF and StSF require further examination,

Heat transfer by convection is commonly described by functions of the following kind:

Nu = k ' Re' , prb.( LZ )'With a = 0,8 and assuming constant material-related quantities, the ratio of Stanton numbersreads:

(, )O'} ( I (' )O'}SI PF _ n,IG,SF _ , '"PF' 1+ "-SF ,lmin'sF)

-. - ECSR,Cl ---------St SF nlG,PF h''sF ,(1+ I,PF ,Imin,PF)

As a rule, a vast amount of data concerning operation with primary fuel is available, allowing therelated Stanton numbers StpF to be calculated, Experience shows that, in the case of the hightemperature applications especially dealt with here, the Stanton numbers range from StpF = 2 toStpF = 4, This stipulation enables the calculation of an energy exchange ratio that takes intoaccount the means ofheat exchange-here, convection- EcsR,a ,with eqs, (3,14) and (3,16),

Figure 3-3 presents ECSR,a over the ratio of calorific values h",sFlh",PF for StpF = 2 and a requiredtemperature of the solid of Ss = 1500 oe as weil as with air preheating (SA = 800°C) ascompared to the previously discussed ECSR,3 for the static examination, The boundary conditionsfor this example roughly resemble those of the combustion zone of a clinker burning process(primary firing) or those of a smelting processes in the steel and iron industry, Here, acomparison of the two curves for ECSR,3 and ECSR,a shows that consideration of changed heattransfer conditions by substitution leads to ECSR,a < ECSR,3' However, only heat exchange byconvection was assumed, Similarly, heat exchange by radiation can be included, In this case, theKonakov number takes the place of Stanton's number, stating the ratio between heat capacityflow of the gas and the heat flow transferred by radiation, In the framework given here we willrefrain from going into further detail concerning radiation, however, it should be mentioned thata deterioration of the radiation properties of the gas from substitute fuel combustion is possibletoo, This could lead to less favourable conditions for heat exchange (i,e, ECSR,a > ECSR,3) whencompared to the use of prirnary fuel, thus resulting in possible gas temperatures SG,SF > SG,PF,

In the first step discussed above, only one section of an industrial furnace was examined undersirnplified conditions Uust one continuously stirred reactor element) with regard to the energyexchange ratio of fuel substitution, However, as mentioned above, the evaluation of a fueldepends not alone on the kind of fuel but also on process management and heat recovery,Therefore, the amount of additional energy expenditure in the ca se of fuel substitution, as

Substitution 01Primary Fuels by Waste Materials in High Temperature Processes Page 9opposed to primary fuel use, required primarily for allaining high process temperatures, e.g. in abuming or calcining process, is not necessarily "lost".

For example, with a two-fold staging of the energy input along the treatment path (primary andsecondary firing with the clinker buming process), a balance schema as in figure 3-1 can bederived, again making use of the aforementioned simplified model of continuously stirredreactor elements for the first and second stage and assuming static exarnination (SC,PF = SC.SF).

The single figures 3-1 A present the first and second stage of the original process, i.e. withprimary fuel supply in the respective stages (,iIPFI) and (lilpFJ.PF)' This process is now comparedto a case in which, in the first stage, the primary fuel supply is replaced with substitute fuel lilsFi

according to the exchange ratio ECSR.9 mentioned above (figure 3-1 B). For the second stage thesame requirement applies that does to the first stage, namely, that despite fuel substitution, thetemperature of the load SS2 and kiln output remain the same (eqs. (3.4) and (3.5)). Whenemploying substitute fuel, the higher amount of energy (ECSRI) supplied to the first stirredreactor element, in comparison to the use of primary fuel, now can be utilised with the amountthat lies above the required temperature level of the second stage, and, accordingly, less primaryenergy needs to be supplied to the second stage (second continuously stirred reactor). The energybalance for the second stage yields with SCI.PF = SCI.SF = SCI and SC2.PF = SC2.SF.PF = SC2 (staticexamination):

(1 + A PF '/m;"PF ). lilpFI . CGI.PF,GIGl . (S GI - S Gl) +

(1 + A.PF '/mm,PF )- J11pF1,PF . cG!.PF.caIGl . (S ca/,PF - S Gl) =

(1+ASF ·/ml,.sF)·lilsFI·CGI,SF,GIG1·(SGI-SG1)+

(1 + APF ·1mm.PF ). lilPFl.SF . CG1.PF.caIGl . (S ,.I.PF - S Gl)

(3.17).

( )(3.18).

CGUF,GIG1' SGI - SGl

CG1,PF.caIGl . (S ca/,PF - SOl)

From eqs. (3.7) and (3.9), we obtain the difference of primary fuel mass flows in the secondstage wirh and without substitution in relation to the original primary fuel mass flow in stage I:

rnpF, PF - ,hpF2SF/'>x = -. ,

,hpF1

cGI,PF"aIGI . (S ,aUF - SGI)' CGl,PF,GIGl . (SGI - SGl)

COJ,SF,cafGJ . (S ca/,SF - S GI)' C01.PF,caIGl . (S CU/,PF - [) 01)

With the ratio offuel distribution berween first and second stage in the case ofexclusive input ofprimary fuel

ri-lpF1,PF

Y = t11pFI + nlpF1,PF(3.19)

the energy exchange ratio for the second stage (static exarnination) can be obtained fromeq. (3.18):

lilpFl.SF (1)ECSR1 .• = -.-- = /'>x. 1- - + 1mpFl.PF Y

(3.20).

ECSR,. depends on the respective temperature differences and the original fuel distribution y.Generating the energy exchange ratio for both stirred reactor elements:


leads witb eqs. (3.6), (3.19) and (3.20), as expected, to

ECSRI2,S = (1- y). ECSRI,s + y. ECSR2.S

(3.21),

(3.22).

As in the ca se of the previous example, with a primary fuel that has h",PF = 30 MJ/kg, abalancing temperature SGI = 1900 oe and air preheated to SA = 800 oe, figure 3-4 presents theenergy exchange ratio ECSRI,s over hn.SF•

Furthermore, for a fixed balancing temperature SG2 = 1200 oe, tbe curve for ECSR2,S is included,the course of which, due to the savings achieved in the second continuously stirred reactor,assumes va lues less than I in this presented substitution case. Notwitbstanding the fact thatECSRI2.S is greater tban I, due to tbe possibility of a staged fuel supply, it is still significantlysmaller than ECSRI.S'

In the same way as with a single continuously stirred reactor element, the heat transferconditions can be included for !Wo, namely, by connecting them in series. Moreover, with acascade of stirred reactors, the residence time of the real process can be approximated. However,for a number of industrial furnaces this can also be adequately described by plug flow reactor(PFR) characteristics. In this case, relations can be called on that are known from continuousheat exchange systems. Under adiabatic conditions, the energy exchange ratio of a plug flowelement (single stage) is described by:

SGPF-Sa1- .cG.PF .calG . (S 'al.PF - S G.PF) cG.SF,/wla· (S cal,SF - S a) S ,aUF - S a

E PFR = ( ) . ( ) '" S - SCC,SF,calG' S cal,SF - S G,SF CG,PF,ka/a' S cal.PF - -S a J _ G,SF a

Scal.SF - Sa

(3.23).

The simplification of eq. (3.23), in which the specific heat capacities have been disregarded,shows that, when assuming constant exiting temperatures SG.PF SF for the time being, a decreaseof the substitute fuel's calorific combustion temperature S,al,SF compared to S,al,PF, results in anenergy exchange ratio EpFR with va lues greater than one. When, due to the respective heattransfer conditions, witb employment of substitutes fuels the exiting temperatures settle atSG,SF> SG.PF , EpFR is caused to increase further, while conversely, when SG,SF< SG.PF , theresult is a decrease accordingly.

4 Effects of Substitution on Process Management (Examples)

Witb the above acquired and examined findings, greatly simplified with regard to some generalaspects of fuel substitution, the following examples are discussed conceming the impact of fuelsubstitution on the respective process management:

• clinker burning process in tbe cement industry and

• smelting process in a cupo\a in the stee\ and iron industry.

The underlying presumption in the following considerations is that the substitute fue\s, based ontheir material properties, are principally suitable for substitution both with regard to productquality as weil as to emission of pollutants [e.g. 30].


In Germany, cement clinker is produced mainly according to tbe so-called dry method. As seenin figure 2-2, tbe plant consists of the main components prebeater, calcinator (combustioncbamber), rotary kiln and clinker cooler (grate). Fuel is supplied through three inlets: the primaryfiring (kiln exit, I in figure 2-1), the secondary firing (calcinator, ll) and tbe supplemental firingat the kiln entrance (I1I). In principle, substitute fuel can be supplied at all three inlets, however,the specific boundary conditions need to be adhered to.

In lbe combustion zone, tbe primary [!ring for the burning of the clinker needs to acbieveapproximate maximum load temperatures around Ss'" 1500 °C or gas temperaturesSG'" 1900 °C (cf. figure 2-1, temperature curve). For example, a question could be, to whatextent coal supplied as primary fuel with hn,PF = 30 MJ/kg can be replaced with a substitute fuelgained from adequately pre-treated waste with hn.sF = 15 MJ/kg. In lbis case, the energyexchange ratio, for the time being, only needs to be examined for tbe first stage. With thesimplified relations derived in chapter 3, figure 3-2 states ECSR.S '" 1,3 for this case. This entailsa 2.6-fold amount of fue! and about 1.5-fold amount of flue gas when using the substitute fuel asopposed to the primary fuel (eqs.(3.7) and (3.8». These circumstances prohibit a completesubstitution, amongst olber things with regard to aspects conceming the plant such as supplyunits, gas velocities in the rotary kiln, sweeping away of particles of lbe load by lbe gas flow etc.Therefore, with regard to justifiable changes of the boundary conditions, only amounts of 30 to50 mass-% could be replaced in this case. With a primary fuel of hn = 30 MJ/kg, a 50 %exchange with substitute fuel would result in a new mean calorific value of izn,SF = 22,5 MJ/kg.

Thus, tbe energy exchange ratio amounts to a mere EI'" 1,1.

In tbe second step, the effects need to be investigated of the substitution in tbe primary firing onthe secondary firing that is still being operated on coal. As previously mentioned, this stagedmanner of supplying fuel enables apart of the additional energy expenditure, due to substitutionand required for the incineration process, to be utilised in the succeeding sections. With cementrotary kiln plants, the share of fuel in the second firing commonly ranges at about y = 0.6. Due tothe limitation of the substitution rate in the first stage, the consequences for the overall processare low. For the example observed here, a total enegy exchange ratio near one is obtained.Therefore, depending on the process management, a smaller overall energy excbange ratio canbe obtained. Beyond lbe scope ofthe simplified relations presented here the determination ofthisratio, however, requires suitable process models and practical research. As a rule, withoutadditional measures for heat recovery, replacement with a substitute fuel of lower calorific va!ueentails an increase of the specific energy expenditure. For example, [31] reports of practicalexperience with a change-over of an incineration process from fuel oil (hn '" 40 MJ/kg) to lignitedust (hn '" 20 MJ/kg). Amongst other things, this modification lead to a greater flame length, ashift of the sintering zone toward the kiln entrance and to a temperature rise at the kiln entranceas well as behind the preheater. Naturally, here, the differing burnout behaviour of lignite dustand fuel oil plays apart. Altogether, tbe outcome was an increase of specific energy expenditureby about 170 kJ/kg.,]. The additional expenditure is atrributed in equal parts to the increase of theflue gas mass flow on the one band and to the fluctuation of grain sizes - and its respective effecton the course of temperature - on the olber hand.

Concerning the secondary firing, due to lower temperatures compared to the primary firing, theenergy exchange ratio in relation to the temperature is of less significance. According to


figure 3-2, required gas temperatures of SG '" 1200 °C entail energy exchange ratios only slightlygreater than one, even when the calorific value is lowered from 30 MJ/kg to 15 MJ/kg. Ofsignificance is, when applying substitution to the secondary f!fing, the quality of the fuel witbregard to the specific flue gas volume and the burnout. Larger-size particles such as tyre cuttingsfall into the rotary kiln's entrance area where they cause a rise in temperature. Throughincomplete combustion, the process of incineration could extend into the preheating zone.Conceming this, figure 4-1 displays results from practical research [32]. Amongst other things,tbe figure shows that, when applying tyre cuttings, the increase of secondary fuel energy inrelation to the clinker mass causes tbe gas temperatures to rise at the kiln entrance. Thistemperature rise in turn effects an increased alkali ne salt vaporisation in the rotary kiln.Conversely, when using a fuel with relatively good burnout properties, the conversion takesplace mainly in the caIcinator's gas rising duct and a reduction of alkaline salt recirculation canbe expected. In the present example, an increase of the secondary fuel share through input ofcoal dust causes, due to a reduction in the primary fuel share, a fall in gas temperature at thekiln's entrance, thereby reducing the release of K20. A reduced amount of alkalines in the cyclefinally leads to a smaller specific energy expenditure. Here, however, a complete burnout of thesecondary fuel in the caIcinator is presumed.

A further example for illustrating energy excbange ratios, the following examines thereplacement of natural gas with gaseous substitute fuels oflower calorific value in the process ofsmelting cast iron, cast iron scrap etc. in a coke-free, natural gas-fired cupola [33,34]. As seen infigure 2-3, the object of interest consists of a counter-current-operated shaft furnace wbere theload proceeds in descending motion. In batches, i.e. discontinually, the load (pig iron, cast ironscrap, steel scrap) is supplied to the kiln through the kiln's feeding opening and is then preheatedto melting point in the first zone. Adjacent to the preheating zone is the melting zone and -inorder to prevent the melt solidif)'ing around the water cooled grate bars- the superheating zone,the latter consisting of ceramic balls. In the combustion chamber natural gas is burnt with air anda supplemental supply of oxygen, altogether in excess of oxygen (A> 1), thereby obtaining gastemperatures of 2100 °C to 2200 °C. Moreover, a burnout of the load takes place in thecombustion chamber as weil as in the shaft, in the magnitude of about 15 % of the overall heatinput. Superheated to a temperature of about 1400°C, the molten iron continually flows via asiphon with slag extraction througb a channel for further processing [33]. Known since the latesixties, the coke-free, natural gas-fired cupola presents, from an economical and ecological pointof view, an attractive alternative to the coke cupola, bowever, amongst other things due to lackof knowledge from a process engineering point of view, it lacked tbe resources to assert itself onan industrial scale [34, 25]. For describing the smelting behaviour of a coke-free, natural gas-fired cupola, an energetic process model has been developed that by now dependably reproducesthe complex, inter-related pro ces ses of the smelting operation, as a comparison withexperimental results conftrrns [33]. Figure 4-2 contains the caIculated course of temperature forboth material and flue gas. Tbis process model also allows an examination of the subject of fuelsubstitution. The principle explanation of the energy exchange ratio in section 3 is based ongreatly simplified relations. Tbe aforementioned process model, however, contains theapproaches regarding heat transfer in a mucb more complex form. When using this processmodel for examining the effects of fuel substitution, the generally explained finding isconfirmed, stating that a decreasing calorific value of the applied substitute fuels entails an

Substitution oi Primary Fuels by Waste Materials in High Temperature Processes Page 13

increase in the energy exchange ratio (figure 4-3). In contrast to the previous example of aclinker burning process, a cupola does not pennit a second stage compensation of the heatcapacity flow, increased due to fuel substitution. The temperatures and mass flows of gas leavingthe preheating zone, designed as a counter-current heat exchanger, increase in the ca se ofsubstitution which, accordingly, means a rise in losses and energy exchange ratio. Of course thethought occurs of, in the case of fuel substitution, utilising the flue gas flow behind thepreheating zone for further heat recovery (e.g. for air preheating). However, apart !Tom theaspects described in this context in section 2 (eq. (2.6)), attention needs to paid to the fact thatthe flue gases leaving the preheating zone contain a relatively high proportion of dust. Similar tothe management of the cement process - where, in the case of substitution, the ratio of heatcapacity flows in the counter-current preheating zone is compensated by a respective smallerinput of secondary fuel - in the present example of the cupola it seems worth consideringextracting a partial flow from the flue gas flow behind the smelting zone with ensuing heatrecovery in order to improve the energy exchange ratio.

On a final note, in the context of the integrated economy of a steel plant, the energy exchangeratio is presented in figure 4-4 (as a factor A = !fE), for the ca se of substituting natural gas withblast furnace gas, summarising a variety of processes [36]. When using blast fumace gas in thehot blast stoves (cowpers), boilers and fumaces heated from below of coke ovens, an exchangefactor or value factor in relation to natural gas of 85 % to 90 % can be obtained. Applying blastfumace gas in reheating furnaces, i.e. processes with comparatively higher temperatures, thevalue factor attains a mere 30 % to 50 %. From this value factor certain strategies for plantconception and the use of so-called couple energy carriers can be derived. Therefore, inindividual cases of fuel substitution, the need exists for examining which overall concept offersadvantages with regard to energy savings.

5 References[I] Mechanisch-biologische Behandlung von Abfallen- Erfahrungen, Erfolge, Perspektiven.

Zentrum tur Abfallwinschaft, Heft 10, Braunschweig, 1995. ISSN 0934-9243.[2] Turk, M.; Collins, H.-J.: Stoffspezifische Abfallbehandlung. Der Städletag 10(1996), S. 710-714.

[3] Doedens, H.; Cuhls, e.: MBA vor Deponie- neue Erkenntnisse aus laufenden Forschungsvorhaben. Ln:"Planung von mechanisch-biologischen Behandlungsanlagen". VDI- Verlag, Düsseldorf, 1997.

[4] Wiemer, K.; Kern, M. (Hrsg.): Biologische Abfallbehandlung III -Kompostierung-Anaerobtechnik-Mechanisch- Biologische Abfallbehand Iung-Klärschlammverwenung.M.l.e. Baeza-Verlag, Witzenhausen, 1996. ISBN 3-928673-18-1.

[5] Lahl, U.: Status quo und Mindeststandards tur Verfahren zur mechanisch-biologischen Restabfallbehandlung.Fachtagung Umweltinstitut Offenbach GmbH:Biologische Restabfallbehandlung, 19.120. Juni 1997,Offenbach.

[6] Spillmann, P.: Stoffgerechte Behandlung undefiniener Restabfalle durch Kombination biochemischer undlhennischer Behandlungsverfahren ("Bio-Select- Verfahren"). Müll und Abfall 7(1995), S. 416 bis 431.

[7] Christmann, A.; Quitteck, G.: Die DBA-Gleichstromfeuerung mit Walzenrosl.VDI-Bericbte 1192, VDI- Verlag GmbH, Düsseldorf, 1995.

[8] Horn, J.; Manin, J.; Busch, M.; Schaffer, R.: Umweltentlastung durch synthetische Verbrennung inHausmüllverbrennungsanlagen mit dem Rückschubrosl. In: Thome-Kozmiensky, KJ. (Hrsg.): Reaktoren zurthermischen Abfallbehandlung, EF- Verlag tur Energie- und Umwelttechnik GmbH, Berlin, 1993.

[9] LautenscWager, G.: Modeme Rostfeuerung tur die thermische Abfallbehandlung.GVC-Symposium Abfallwirtschaft- Herausforderung und Chance, Würzburg, 17.-19. Oktober 1994.


[10] Lorson, H.; Schingnitz, M.: Konversionsverfahren zur thermischen Verwertung von Rest- und Abfallstoffen.Brennstoff-Wärme-Kraft (BWK) 46 (1994) Nr. 5.

[11] Berwein, H.-J.: Siemens Schwel-Brenn-Verfahren - Thermische Reaktionsabläufe;In: Abfallwirtschaft Stoffkreisläufe, Terra Tee '95, B.G. Teubner Verlagsgesellschaft, Leipzig, 1995.

[12] Redepenning, K.-H.: Thermische Abfallbehandlung mit Kombinationsverfahren- Aufbereitung, Verbrennung,Entgasung, Vergasung. GVC-Symposium Abfallwirtschaft- Herausforderung und Chance, 17.-19. Oktober1994, Würzburg.

[13] Stahlberg, R.; Feuerrigel, U.: Thermoselect - Energie und Rohstoffgewinnung.In: Abfallwirtschaft Stoffkreisläufe, Terra Tee '95, B.G. Teubner Verlagsgesellschaft, Leipzig, 1995.

[14] Scholz, R.; Beckmann, M.; Schulenburg, F.: Waste incineration systems; current technology and futuredevelopments in Germany. 3rd European Conference on Industrial Fumaces and Boilers (INFUB),Lisbon, Portugal, 18.-21. 04. 1995.

[15] Schulenburg, F.; Scholz, R.: Bilanzierung und Bewertung thermischer Abfallbehandlungsverfahren; Einflußunterschiedlicher Abfallvorbehandlungsverfahren. VDI-Berichte 1387, VOl Verlag GmbH, Düsseldorf 1998,S. 17-47. ISB 3-18-091387-8.

[16] Weiss, W.; Janz, 1.: Kunststoffverwertung im Hochofen-Ein Beitag zum ökologischen und ökonomischenRecycling von Altkunststoffen. VOI-Berichte 1288, VDI Verlag GmbH, Düsseldorf, 1996, S. 123-138.ISBN 3-18-091288-X.

[17] Lütge, c.; Radtke, K.; Schneider, A.; Wischnewski, R.; SchifTer, H.-P.; Mark, P.:eue Vergasungsverfahren zur stofflichen und energetischen Verwertung von Abfallen.

In: Born, M.; Berghoff, R. (Hrsg.): Vergasungsverfahren für die Entsorgung von Abf,iIlen.Springer-VDl-Verlag, Düsseldorf, 1998, S. 191-212. ISBN3-18-990035-3.

[18] Albrecht, J.; Gafron, 8.; Scur, P.; Wirthwein, R.: Vergasung von Sekundärbrennsloffen in der zirkulierendenWirbelschicht zur energetischen Nutzung für die Zementherstellung.DGMK Tagungsbericht 9802, Deutsche Wissenschaftliche Gesellschaft für Erdöl, Erdgas und Kohle e.V..Hamburg, 1998, S.115-130. ISBN 3-931850-40-4.

[19] Jennes, R.; Jeschar, R.: Einsatz von brennbaren ReststofTen in industriellen Prozessen. VDI-Berichte 1313,VDI Verlag GmbH, Düsseldorf, 1997, S. 257-268. ISBN 3-18-091313-4.

[20] Sprung, S.: Umweltentlastung durch Verwertung von Sekundärrohstoffen. Zement-Kalk-Gips ZKGInternational 45(1992)5, S. 213-221.

[21] Maury, H.-D.; Pavenstedt, R.G.: Chlor-Bypass zur Erhöhung des Brennstoffeinsatzes aus Müll beimKlinkerbrennen. Zement-Kalk-Gips ZKG International 41(1988)1 I, S.540-543.

[22] Junge, K.: Möglichkeiten und Grenzen der Verwernmg in der Ziegel industrie- Produkt verbesserung-Anforderungen an Ersatzbrennstoffe. 2. Seminar UTECH Berlin '98, Umwelttechnologieforum, Berlin, 1998,S.149-158.

[23] Thielen, W.; Dürrfeld, H.; Kinni, J.; Die Allein- und Mitverbrennung von schwierigen Brennstoffen in derWirbelschicht. VDI-Berichte 1081, VDI Verlag GmbH, Düsseldorf, 1993, S. 285-297.

[24] Schmidt, R.: Stand der Mitverbrennung von Abfallstoffen in Feuerungsanlagen. VOl-Berichte 1387, VOIVerlag GmbH, Düsseldorf, 1998, S. 249-260. ISBN 3-18-091387-8.

[25] Hecka, Chr.; Niemann, K.: Die hydrierende Aufarbeitung von Kunststoffen. VDl-Berichte 1288, VDI VerlagGmbH,Düsseldorf, 1996, S. 123-138.ISB 3-18-091288-X.

[26] Paul, E.: Verschiedene Verfahren des metallurgischen Recyclings. VDI-Berichte 967, VOI Verlag GmbH,Düsseldorf, 1992, S. 39-72. ISB 3-18-090967-6.

[27] Heyde, M.; Kremer, M.: Verwertung von KunststofTabfallen aus Verkaufsverpackungen in derZementindustrie. Fraunhofer-Institut Lebensmitteltechnologie und Verpackung, Freising, 1997.

[28] Jeschar, R.: Überblick über Industrieöfen. Seminar zu Methoden der Energieeinsparung bei Industrieöfen, TUClausthal, Institut für Energieverfahrenstechnik, 1990.

[29] Specht, E.; Jeschar, R.: Beurteilung von Industrieöfen bei Wärmerückgewinnung. Seminar zu Methoden derEnergieeinsparung bei Industrieöfen, TU Clausthal, Institut für Energieverfahrenstechnik, 1990.

Substitution 01Primary Fuels by Waste Materials in High Temperature Processes Page 15

[30] UmweJtverträglichkeit von Zement und Beton. Herstellung, Anwendung und Sekundärstoffeinsatz. Diedeutsche Zementindustrie informiert. Verein Deutscher Zementwerke e.V., Forschungsinstitut derZementindustrie, Düsseldorf, 1996.

[31] Huckauf, H.: Stand und Möglichkeiten der rationellen Energeianwendung beim Zementklinkerbrand.Zement-Kalk-Gips ZKG International 41(1988)4.

[32] Rosemann, H.; Locher, F.W.; Jeschar, R.: BrennslOffenergieverbrauch und Betriebsverhalten von Zement-drehofenanlagen mit Vorcalcinierung. Zement-Kalk-Gips ZKG International 40(1987)10, S. 489-498.

[33] Davis, M.; Weichert, c.; Scholz, R.: Entwicklung eines energetischen Prozeßmodells zur Beschreibung desSchmelzverhaJtens eines kokslosen, erdgasbefeuerten Kupolofens. Veröffentlichung demnäcbst.

[34] Schürmann, E.: Beitrag zum Scbmelzverhalten des kokslosen, erdgasbefeuerten Kupolofens. Gießerei-Rundschau 41 (1994) 9/1o.

[35] Schürmann, E.; ickel, A.: Auswertung von Betriebsdaten zum Schmelzverhalten des kokslos betriebenenmit Erdgas beheizten Kupolofens, Gießerei (1993)12, S. 390-397.

[36] Hoffmann, G.: Energiewirtschaft in der Stahlindustrie. Mitteilungsblan der TU Clausthal, Heft 80, 1995.

6 NomenclatureSymbols Indices (in subscript)A area; exchange factar a ambiem

specific surface area A airspecific heat capacity AH air preheating(in connection with temperature differences

ca' calorificalways mean specific heat capacity)

CF continuous furnaceE energy exchange ratio

" specific enthalpy cl clinker

H enthalpy CSR continuously stirred reaClOr

Ls.s. in standard state F ruel

k constant FH ruel preheating, specific air requirement G flue gas

L length HR heat recovery, loss11I maS5

Q heat max maximum

specific vo)umemin minimum

V valurne 11 net (calorific value)

stage 2 ruel mass in relation to stage I NG natural gasxy stage 2 ruel mass in relation 10 total O} oxygen

PF primary rueI0. heat transfer coefficient PFR plug flow reaClOrD. difference RC reaction/material conversion'1 efficiency factor 5 solidI.. stoichiometric ratio; air ratio SF substitute ruelft temperature [0C]

111 theoretical'I' concentration (volume related)

1,2 stages 1,2Nu usselt numberPI" Prandtl number 0. convection

Re Reynolds numher D. difference

SI Stanton numher radiationft constant gas temperature

Indices (in superscript) (static examination)

a,b,c exponentsflow


7 Figures

productionprocesses')

thermaltreatment

mechanical-biologicaltreatmentwaste

not treated waste fuel gasburning

steam (e.g. fIXing agent)pyrolysis

reduetionhousehold separating equal lono dist. heat(e.g.FElNE-metal)

waste calorific valueerushing gasification electr. energy

synthesis

grading svnthetic nas(e.g.ehern.industy)

industrialwaste ~ combustion combustion

sift lowcalorific process steam (e.g.powerplant)(raction

briquetting FElNE-metal dryingplastics (e.g.paper)

rotting (aerobic) in different ash/slagcombinations heatingsewage

fermentation (e.g. 10ng dist. heat)sludge hi h calorific fradion RDF(anaerobic) general power!FElNE-metal eleetr. energy

=~l in different synthetic granule in different

.' combinations combinaüons

gwaste !1 wastemateriaUr waste material waste maleria: Jl~~n~~~~~~j~ (inert) (inert) (inert)

I:

-+~ITi~~lr~n'.---.!.' wastedump

1} production integrated cycle economic is not regarded (top gas, dross, scrape trom production, PMMA, coke)

Figure 1-1. Basic operations of waste treatment (Examples).

devices ofenergy conversion

devices oflow temperature

processing

devices ofhigh temperature

processing(industrial furnaces)

heat transfer medium

reverse passforeward pass

air

fuelflue gas

airfuel

flue gas

Figure 2-1. Difference between industrial furnaces, devices of energy conversion and devicesof low temperature processing.

Substitution of Primary Fuefs by Waste Materials in High Temperature Processes Page 17

11 rotarykiln inlet

fuel

--rotary kiln furnace

I rotay kilnl

tertiaryair duc!

circulating f1uidized bed furnace

fluidized bedfurnace

product gas

primary air

ash

'Preheaterreverse f10w

Calcinerparalell f10w

rotary kilnreverse f10w

coolerreverse f10w

gas temperature SG

solid temperature Ss

1 2 3' 3 4 5 6

Figure 2-2. Schematic presentation 01the cement process in cobination with a Iluidized bedfurnace inciuding the corresponding temperature path.

SUBSTITUTION OF PRIMARY FUELS BY WASTE MATERIALS IN HIGH TEMPERA TURE PROCESSES

cooling water

refractory

material

ceramic spheres

grate (water cooled)

natural gas burner

outlet ofiron and slag

preheating zone

melting zone

* superheating zone

~ combustion chamber

Figure 2-3. Sehematie presentation 01a natural gas lired, eokeless eupola lurnaee.

mF hO,F S"continuous furnaceflue gas

mo S01

solid

combustionair mA SA'

mF hOF S"fuel

combustion airmA SAl

flue gasmo SG' fuel preheater

fuel

solid

flue gas

Figure 2-4. Sehematie presentation 01a eontinuous lurnaee with preheating 01air and lue!.


~ 0.8~~~ 0.6

m~ö>- 0.4uI:III'u;:CIl 0.2

\ 1I - -1 --

/ ---- --::=::----- -\'-L-- -- ----\ //~\ / 2 I\ /

/

\ /

1: ~HR,max air and tuel prehealing I' // ---

'< ------ 2: ~AH,max air preheating only I/\I , ------ 3: ~FH,max tuel prehealing onlyI "-

I •....I "- 3I

I "",./III " ,

I -'- ----I ---------------II 1-----------II

246 8stoichiometric air requirement Imin

Figure 2-5. Maximum efficiencies lor the heat recovery using preheating 01air and/or luelcorresponding to the stoichoimetric air requirement 01the luel [29].

10

substitution fuel

A

mG1,PF

Scal,PF

B

primary fuel

______ ~l'L _

n SG1.SF______ :'_51 _

primary fuelmS G2,PF

cal,PF ---------------Sm.PF mG1,PF +mG2,PF

mG1,PF _______ ~2'______SG2,PF

SG1,PF

primary fuelmS G2,PF,SF

cal,PF --------------- . .SG2,PF,SF

mG1 ,SF +mG2,PF,SF

m_______ ~2"- _____S G1,SF

SG2,PF,SFG1,SF

Figure 3-1. Heat transfer in industrial lurnaces; model idea continuously stirred reactorelements.


1000 1200 1400 1600 1800 2000 2200 2400 2600gas temperature SG [0C]

3

~Q.-,

~ 2.5Vl

~~ 2

J}o:; 1.5•..Q)Clc:ca.r:.u~>-~ 0.5Q)c:Q)

o

curve hn•SF 9A

11 MJ/kgI- 1:

-_. 2: 15 MJ/kg--·3: 11 MJ/kg--·4: 15 MJ/kg-5: 40 MJ/kg--- 6: 35 MJ/kg

7: 40 MJ/kg._- 8: 35 MJ/kg

5

primary tuel: hn•PF ~ 30 MJ/kg\',\\

Figure 3-2. Energy exchange ratio by static observation in dependence of the gas temperaturefor different substitution cases.

calorilic value 01substitution luel hn,sF[MJ/kg]10 15 20 25 30

o.~ 2Q)Clc:ca.r:.~Q) 1.5>-~Q)c:Q)

primary tuel: hn•PF ~ 30 MJ/kggas temperature: 9G ~ 1900 oesolid temperature: 9s ~ 1500 oe

air preheating: 9A ~ 800 oeStanton number: St ~ 2,0

EeSRa

0.90.310.2 0.4 0.5 0.6 0.7 0.8

ratio of calorific value hn,SF / hn,PF

Figure 3-3. Energy exchange ratio by static observation in dependence of the ratio of calorificvalues for different heat transfer setups.


30

0.9

primary tuel: hn•PF = 30 MJ/kggas temperature: 3G = 1900oesolid temperature: 3s = 1200oe

air preheating: 3A = 800 oetuel distribution: y = 0,6

calorilic value 01substitution luel hn•SF (MJ/kg]15 20 2510

0.3o0.2

3~

l1..,\ 2.5

~w 2o~Ql 1.5Clc:Cll.r::()

S>-~:!! 0.5Ql

0.4 0.5 0.6 0.7 0.8ratio of calorific values hn,SF / hn,PF

Figure 3-4. Energy exchange ratio by static observation in dependence 01the ratio 01calorilicvalues lor two stage luel leed.

- e -IDulverizJ hard coal /...

---+- tyre cuttings/

/

/

e/ /

- -----II

..------- .-------I

~:----

.... --- --- ~r ..

---- ~--~ ....

-- --- -e_ ---- -,- ....

1?5' 850-l!...c:", 840

.~~830"§~

"'- 8200.'"E'"~~ 810

800

~~.~ nic:::i:0-

'~i 2.5

",'"gEo ~u'"6'::NO><.c:

1.5

c:O 1250.-2.....

~~ 1200(O.S

~~ 1150

E c:1100'" ~-::l",-"'''' 1050Ol~

1000o 0.2 0.4 0.6 0.8 1 1.2

energy of 5econdary tuel [MJlI<g]1.4 1.6

Figure 4-1. Inlluence 01secondary luel leeding on lhe process behaviour.


2400VNG = 320 m3th (i.s.s.)

IjI02,A = 25 vol.-%). = 1,1

rT1s= 7 t1hCl = 1,09

llS = 80 W/(m2K)as = 40 m2/m3

2000

E 1600cf)

l.!!2 1200~~E.Sl 800

400

combustionchamber

meltingzone

superr"atingzone

0.5

preheating zone

f1ue gas

1.5 2 2.5 3furnace length L [m]

3.5 4 4.5

Figure 4-2. Calculated temperature curve 01material and Ilue gas in a natural gas lirded,cokeless cupola lurnace.

51.5

10calorilic value 01the substitution luel hn,sF[MJ/kg]

15 20 25 30 35 40 45

0.9

VNG = 320 m3th (i.s.s.)IjI02,A = 25 vol.-%

). = 1,1rT1s= 7 t1hCl = 1,09

llS = 80 W/(m2K)as = 40 m2/m3

0.310.2

~Q...,

~Li:.~ 1.4=..'"Jo 1.3

~CI>g> 1.2Cl!.s::.u~~1.1GicCI>

0.4 0.5 0.6 0.7 0.8ratio of calorific values hn,SF / hn,PF

Figure 4-3. Energy exchange ratio by static observation in dependence 01the luel substitutionlor the cupola lurnace.


exchange faktor A

[kJ natural gas ]

kJ blast furnace gas

W1,0 1,0 steam boiler

+ blast heating apparatus

1,25 0,8(recuperation operation)

pusher type heating furnace(load per surface unit500 kg/(m'.h))

1,67 0,6soaking pit furnace

pusher type heating furnace

2,5 0,4(load per surface unit650 kg/(m'.h))

pusher type heating furnace(load per surface unit

5,0 0,2 800 kg/(m'.h))

t00 «0

2200 2200 2600 2800 3000 3200

calorific value of blast furnace gas [kJ/m'] A = 11EFigure 4-4. Substitution of natural gas by blast furnace gas by different consurners.

substitution of primarv fuels bvwaste materials in …

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