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Research Article

Catalytic Flue Gas Treatment from NitridingProcesses

Efficient control and minimization of emissions from technical processes is ofmajor concern in industrial development and process operation. The technicalprocess in the focus of the present contribution is the nitriding process of metallicspecimen. The ammonia content in nitriding process flue gases reaches up to618 g·m–3 (80 vol.-%) and needs to be reduced to less than 30 mg·m–3 (40 ppm)to fulfill present regulations. Exhaust gases from nitriding processes today areburnt in flares without emission control where fuels need to be added that pro-duce additional exhaust gas components. The objective of this investigation is todevelop an alternative gas cleaning route for nitriding processes based on catalyticdissociation of ammonia. The decomposition was studied for different catalystsat varying process conditions. With these results a dissociation pilot plant wassuccessfully tested in a technical-scale nitriding process.

Keywords: Catalytic gas cleaning, Flue gas cleaning, Gas nitriding, Process emissions

Received: June 6, 2009; accepted: October 12, 2009

DOI: 10.1002/ceat.200900291

1 Introduction and Statement of theProblem

In 2005, Germany’s ammonia emissions attained approx.619·103 tons [1] that need to be reduced to 550·103 tons by2010 [2]. The official standards for the German national lim-itation of emissions from industrial processes are based on therequirements of the Federal Immission Control Act (Bundes-Immissionsschutzgesetz) [3] and the Clean Air Guidelines(TA-Luft) [4]. The limit established here for ammonia in wastegases is 30 mg/m3 (40 ppm). In addition, the mass flow rate of0.15 kg/h must not be exceeded within an hour during normaloperation. Waste gases from nitriding processes contain 30–80% ammonia depending on the process conditions and must besubjected to a post process flue gas treatment to reduce theammonia content. This generally occurs in gas burners withaddition of fuels (e.g., propane). As this burning process is notregulated, unburnt ammonia is also released together withCO2 and NOX. Typical ammonia emission levels exceeding theestablished limit often occur, e.g., in the start-up process.

This contribution investigates an alternative system for thecontrolled reduction of the ammonia content in waste gasesfrom nitriding processes with the target of reducing NH3 emis-

sions to comply with the limit established in the TA-Luft. Theaimed waste gas purification process should require smalltechnical and financial expenses, produce no other pollutants,and needs to be tailored to the process and material character-istics of the nitriding process. Possible purification processeshave been evaluated with respect to the practicability of theirapplication in nitriding systems where catalytic ammonia dis-sociation was selected for use in further investigations.

1.1 Process Conditions in Nitriding Systems

The furnace atmosphere in nitriding processes can differ sig-nificantly depending on the composition of the sub-assembliesand the desired nitriding result. Besides high concentrations ofammonia it may also contain nitrogen, hydrogen, and – in therelated nitrocarbonizing process – carbon carriers. It has to beassumed that the vaporizing residues of cleaning agents, anti-corrosion agents, and cooling lubricants used in the pretreat-ment of the sub-assemblies also enrich the atmosphere withfurther unknown carbon compounds. Depending on the ac-tual size of the furnace, the ammonia volumetric flow rates inpure gas nitriding processes may be in the range from1 to 12 m3

N/h. In the nitriding furnace the partial dissociationof ammonia at 820 K obtains hydrogen and nitrogen. The dis-sociation grade in the nitriding furnace is generally between 20and 80 % [5] depending on the process conditions and con-trol. Therefore, the residual NH3 content can be as high as80 vol.-%.

Chem. Eng. Technol. 2010, 33, No. 1, 145–154 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com

Heidi Ludewig1

Brigitte Haase2

Udo Fritsching3

1 Bremer Energie Institut,Research V, Bremen, Germany.

2 Hochschule Bremerhaven,Process Engineering andTechnology, Bremerhaven,Germany.

3 Universität Bremen,Fachbereich 4Produktionstechnik, Bremen,Germany.

–Correspondence: Heidi Ludewig ([email protected]),Bremer Energie Institut, College Ring 2, Research V, D-28759 Bremen,Germany.

Flue Gas Treatment 145

2 State of the Art

Catalytic dissociation of ammonia is selected as a suitable pro-cess for the system concept as it produces no air pollutantsand may achieve an NH3 conversion of 99 % [6]. The basic re-action mechanism has been described in the relevant literature[7–12]. Above 750 K ammonia is thermodynamically instableand dissociates on catalytic surfaces according to:

2 NH3 � N2 + 3 H2 (1)

The reaction is associated with doubling the gas volume atconstant pressure. The temperature which is thermodynami-cally required to reach the equilibrium at which materials dis-sociation meets the required limit can be calculated using theequilibrium constant K:

K � xN2� xH2

� �3�p2

�xNH3�2 (2)

lnK � �DRG

RT(3)

where xi = pi/p are the amount fractions of the substances am-monia, hydrogen, and nitrogen in the equilibrium, DRG is thefree reaction enthalpy, R is the general gas constant, and T isthe temperature. Tab. 1 shows the calculated free reaction en-thalpies, the equilibrium constants, and the partial pressures ofammonia reached at equilibrium for different process temper-atures.

The aimed equilibrium partial pressure of 4 Pa (40 ppm) isnot reached below 1200 K. Prerequisite for the scale-up processis knowledge of the dependency of the reaction rate rDiss onthe process parameters of temperature, ammonia concentra-tion, and catalyst material. A first-order reaction rate equationwas determined empirically for the dissociation of ammonia athigh temperatures [11], thus:

rDiss � � 1

V

∂N

∂t� �cNH3

�t� � k (4)

The derivative∂N

∂tdescribes the change in the amount of the

substance in relation to time. The phrase -cNH3(t) in mol·L–1 isthe ammonia concentration decreasing in the direction of flow,k is the rate constant of the reaction in s–1, and V is the reac-tion volume. For a plug-flow reactor the integral for irrevers-

ible reactions of first order with increasing volume can be cal-culated [13] as:

�1 � a� � ln1

1 � X� a � X � k � sM (5)

The rate of the ammonia dissociation is definitively fixed bythe most inhibiting, i.e., the slowest elementary step in the re-action. Ganley et al. [14] and other authors subdivided thegross reaction into six elementary reactions:

NH3,g � NH3,ad (a)

NH3,ad � NH2,ad + Had (b)

NH2,ad � NHad + Had (c)

NHad � Nad + Had (d)

2 Had � H2,g (e)

2 Nad � N2,g (f)

As the recombination of the atomic nitrogen in combinationwith desorption of the N2 molecule (step f) requires the high-est activation energy, this sub-step is regarded as being theslowest [6]. Other authors suspect that the temporally definedsub-step is dependent on the temperature. At temperatures be-low 650 K, the recombination and desorption of nitrogen arelimiting the rate, and beyond 750 K the breaking of the N-Hbonds [10]. McCabe [15] investigated the dissociation of am-monia on nickel wires and determined that above 1000 K thereaction order increased from zero to first order, combinedwith a change of the rate-depending step. He proved that therate is limited by the rate of nitrogen desorption at low tem-peratures and by the ammonia adsorption on the substrate athigh temperatures. The influence of the catalyst material onthe ammonia dissociation process not yet agreed. Amongother factors, the activity of the catalyst is strongly influencedby the size of the surface, integrated promoters, and thegrid structure of the metal [10]. Choi [16] observed an in-crease in activity when the size of the catalyst particles was in-creased and the total surface area decreased, respectively,while Jedynak et al. [17] observed a much higher turnover fre-quency when smaller iron particles were employed. Li et al. [7]determined that beside the catalyst material influencing the

activation energy of the reaction also the manufacturingprocess (e.g., particle size distribution on the support) isrelevant. In a plug-flow reactor, the residence time s ofthe gas in the reaction zone also decides the ammoniaconversion; a short residence time reduces the conversion[12, 19].

2.1 Calculating the Required Conversion in theWaste Gas Purification

The maximum ammonia volume fraction in the nitridingwaste gases is approx. 80 vol.-% and must be reduced to30 mg/m3

N to meet the TA-Luft threshold. The required

www.cet-journal.com © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2010, 33, No. 1, 145–154

Table 1. Equilibrium partial pressure of ammonia at different tempera-tures.

Temperature (K) DRG(kJ·mol–1)

Equilibrium constantof ammonia K

Equilibrium partialpressure pNH3 (Pa)

298 33 1.6·10–6 94·103

500 –9.7 1.0·101 8·103

800 –79.9 1.7·105 80

1200 –181.3 7.8·107 4

1300 –207.7 2.2·108 2

146 H. Ludewig et al.

ammonia conversion XNH3 in the waste gas purifi-cation for this can be calculated as [13]:

XNH3�

1 � cNH3

c0�NH3

1 � a � cNH3

c0�NH3

�6�

where c0�NH3(mol·L–1) is the ammonia concentra-

tion before dissociation, cNH3(mol·L–1) is the con-

centration after dissociation, and a is the coeffi-cient of volume expansion. The required ammoniaconversion is XNH3 = 0.9999 (99.99 %).

3 Experimental Section

3.1 Laboratory System

Fig. 1 shows the setup of the laboratory-scale experimentalsetup for ammonia dissociation, comprising a gas mixingstation, the plug-flow reactor, and the gas analysis.

The catalytically active materials and parameters of plugflow reactor can be seen in Tab. 2.

3.2 Measuring Program

3.2.1 Influence of Temperature

The gas mixtures and temperatures used are listed inTab. 3.

3.2.2 Influence of the Catalyst Material

The influence of different catalyst materials on the ammo-nia conversion was compared at the reaction temperatureTR = 1273 K and different ammonia volume fractions atthe reactor entrance, see Tab. 4.

For X4CrNi18-10, the volume flow rate at the entrance was3.6 LN·min–1, for nickel oxide it was 2.6 LN·min–1, and for purenickel it was 3 LN·min–1. It should be noted, however, that withthe different volume flows, the same volume fractions give dif-ferent ammonia substance flows. To ensure the comparabilityof the results, the measured results are represented subject tothe ammonia flow rate at the reactor entrance.

3.2.3 Influence of the Volumetric Flow Rate(Residence Time in Reactor)

The influence of the residence time s on the disso-ciation process was investigated at a reaction tem-perature TR = 1273 K and 80 vol.-% ammonia. Thevolume flow rates employed were 1.0; 1.5; 2.0; 3.0;3.5 LN·min–1.

4 Results and Discussion of the LaboratoryTests

4.1 Influence of NH3 Volume Fraction andTemperature

The influence of the temperature on the dissociation process isshown in Fig. 2 for the pure nickel catalyst as an example.

At reaction temperatures of TR = 973 K, the NH3 volumefractions UNH3 increase much more steeply at the reactor exit


Figure 1. Setup of the laboratory-experimental station.

Table 2. Parameters of catalysts and plug flow reactor.

Catalysts

X4CrNi18-10(chips)

Nickel oxide(balls)

Nickel(wire)

Volume of catalysts VCat (cm3) 7 30 22

Surface of catalysts ACat (cm2) 1061 129 284

Mass of catalysts m (g) 38 41 180

Reactor

Length L (cm) 22

Inside diameter D (cm) 2.2

Total volume Vreactor (cm3) 83

Free volume Vf (cm3) 76 53 61

Table 3. Experimental parameters using nickel wire as catalyst.

Nickel (wire)

Temperature TR (K) 973 1173 1273

Ammonia volumefraction

U0,NH3

(vol.- %)0.1 / 0.5 / 1 / 5 /10 / 20 / 30

0.5 / 1 / 5 / 10 /20 / 30 / 40

10 / 20 / 30 / 40


than at 1173 K and 1273 K. The ammonia volume fraction ofU0,NH3 = 30 vol.-% fed into the reactor is reduced toUNH3 = 89·103 ppm (� 8.9 vol.-%) at TR = 973 K, to 1144 ppm(� 0.11 vol.-%) at TR = 1173 K, and to 158 ppm (� 0.01 vol.-%) at TR = 1273 K. At U0,NH3 = 40 vol.-% NH3 at the reactorentrance and a reaction temperature of 973 K in the reactor,the NH3 volume fraction at the reaction exit exceed the uppermeasuring limit, so there no valid result could be achieved. Ata reaction temperature of 1173 K and a maximum ammoniavolume fraction, the mean value at the reactor exit isUNH3 = 1643 ppm, and UNH3 = 423 ppm at 1273 K. The am-monia conversion subject to the ammonia volume fraction atthe reactor entrance U0,NH3 is shown in Fig. 3.

At 973 K and smaller ammonia fractions in thegas mixture the conversion increases to almost90 %, reaches a maximum at 1 vol.-%, and falls to<70 % as the ammonia fraction is increasedfurther. The conversion is thus dependent on theammonia volume fraction in the gas mixture up tothe maximum conversion. Thereafter, the conver-sion becomes limited, particularly because of thelow temperature. The calculated ammonia conver-sion for 900 and 1273 K only decreases slightly inthe regions investigated and is always beyond 99 %.At the maximum ammonia volume fractionU0.NH3 = 40 vol.-% the calculated conversion isXNH3 = 99.4 % at 1173 K and XNH3 = 99.8 % at1273 K. In addition, the constant conversion showsthat the NH3 volume fraction fed in at this temper-ature has a negligible impact on the dissociationprocess. The ammonia conversion increases whenthe reaction temperature is increased. Li et al.showed that the rate of ammonia dissociation withruthenium and nickel catalysts almost tripled when

the reaction temperature was increased by 200 K [7]. More-over, it can be established that the difference between the con-version at 973 K and 1173 K is considerably larger than the dif-ference between those at 1173 K and 1273 K. This means thatthe influence of the temperature reduces, the higher the reac-tion temperature is. Yin et al. [8] calculated an equilibriumconversion for reaction temperatures between 523 and 773 Kand concluded that the influence of the temperature on theconversion becomes less as the temperature increases. If onetakes into account that the required conversion of 99.99 % de-mands almost complete dissociation, the reaction temperatureto form the basis for the scale-up process must be high.

4.2 Influence of the CatalystMaterial

Fig. 4 shows the ammonia volumefractions UNH3 at the reactor exit de-pending on the NH3 flow rate qn0,NH3

at the reactor entrance at a reactiontemperature of 1273 K. The thresholdfor ammonia is also shown.

At a reaction temperature of 1273 Kthe trend lines of the measured NH3

volume fractions UNH3 for the catalystsX4CrNi18-10 and nickel oxide overlap,though the NH3 values measured whenusing X4CrNi18-10 are slightly lowerthan with nickel oxide. The gradient ofthe trend lines is slight across thewhole substance flow region; atqn0,NH3 = 8 mol·h–1 the highest NH3

value measured was 71 ppm. The trendline for pure nickel is, as shown in Fig.4, much steeper than those of the othertwo catalysts and the NH3 value mea-


Table 4. Experimental parameters to investigate the influence of the different cat-alyst materials.

Catalyst Volume flow rate NH3 volume fraction Ammonia flow rate

qV,0 (LN·min–1) U0,NH3 (vol.- %) qn0,NH3(mol·h–1)

Nickel oxide 2.6 10406080

0.712.834.245.65

Nickel wire 3 10203040

0.821.632.453.26

X4CrNi18-10 3.6 1020406080

0.991.993.985.957.95

Figure 2. Ammonia volume fraction in the effluent gas as a result of different reaction tem-peratures.


sured of UNH3 = 400 ppm at qn0,NH3 = 3 mol·h–1 is approxi-mately tenfold those of the other two catalyst materials.

The conversion calculated for the three catalysts versus theammonia flow rate at the reactor entrance at 1273 K is illus-trated in Fig. 5.

All conversions are beyond those at 1173 K (not shownhere), but the difference is small in comparison to the differ-ences between 973 K and 1173 K. The conversions there in-creased at up to 40 % via the temperature increase, while the

temperature increase from 1173 K to1273 K merely resulted in an increasein conversion, e.g., of 0.06 % at themaximum ammonia substance flowfor X4CrNi18-10. In conclusion, thetemperature only has a small influenceon the catalysts’ conversion at high re-action temperatures, and the true lim-iting component is clearly the specificactivity of the catalyst material. Whennickel is used, the conversion is notapproximately constant, but rather de-creases significantly as the ammoniasubstance flow increases. Nickel is thusnot suitable as a catalyst material inthis application. The conversion trendsfor nickel oxide and X4CrNi18-10 arealmost identical. The required conver-sion of 99 % is achieved with both cat-alysts, although for the residence timeselected for the laboratory unit thiswas only sufficient up to an ammoniaflow rate of qn0,NH3 = 4 mol·h–1. To in-crease the conversion at higher ammo-nia flow rates, the reaction temperaturecould be raised, yet the influence re-mains small and this would also re-quire the use of highly thermoresistantmaterials. For the dimensioning of theprototype in the scale-up process, analternative would be to analyze thereactor size extensively and thereby ex-tend the residence time of the gases inthe reactor, which would in turn in-crease the conversion.

4.3 Influence of Gas VolumeFlow and Residence Time

The influence of the residence time onthe ammonia dissociation process wasinvestigated by varying the volumeflow rate at a constant NH3 volumefraction in the gas mixture. The aim ofthis was to determine the minimumrequired residence time for sufficientammonia conversion to satisfy the TA-Luft limit. The mean residence time sM

of the gas in the reaction zone Vreactor is determined by the vol-ume flow rate qV where:

sM � Vreactor

qV(7)

As the dissociation of ammonia is associated with the in-crease in gas volume this must be taken into account when cal-culating the residence time of the gas in the reactor. The in-


Figure 3. Ammonia conversion due to different reaction temperatures.

Figure 4. Ammonia volume fraction at the outlet of the reactor using different catalysts.


crease in the volume flow rate subject to the ammonia conver-sion X results from:

qV�t� � qV�0 � �1 � X� (8)

with the volume flow qV,0 at the reactor entrance. The meanresidence time of the gas in the reactor is thus calculated withthe mean volume flow rate q V,M:

qV�M � 1

t�� t

0

qV�t�dt �9�

At almost complete conversion, themean volume flow is approximately 1.9times that of the volume flow at theentrance qV,0. The following figuresshow the influence of the gas volumeflow rate on the ammonia conversionin the reactor for the catalystsX4CrNi18-10 and nickel oxide. Theachieved conversions with nickel werenot sufficient for being taken into ac-count in the scale-up process.

Fig. 6 shows the NH3 volume frac-tion UNH3 at the reactor exit subject tothe gas volume flow rate qV,0 at a reac-tion temperature of TR = 1273 K.

When X4CrNi18-10 is used as thecatalyst, the NH3 volume fractionsmeasured UNH3 are below the limit es-tablished in the TA-Luft of 40 ppm upto a volume flow rate of approximately2.5 LN·min–1. Increasing the flow rate

further results in values of up to fourtimes higher (approx. 160 ppm). Withthe nickel oxide catalyst the limit ismet up to 2 LN·min–1. Below a flow rateof 1.5 LN·min–1 with nickel oxide theNH3 volume fractions UNH3 with20 ppm are just half as large as forX4CrNi18-10. For volume flow ratesexceeding 1.5 LN·min–1, however, theNH3 volume fractions UNH3 increasemore strongly with nickel oxide thanwith X4CrNi18-10 and at 100 ppm at avolume flow rate of 3 LN·min–1 they arealready double.

The calculated ammonia conversionsat a reaction temperature of 1273 K forX4CrNi18-10 and the nickel oxide cat-alyst are given in Fig. 7. The requiredammonia conversion is represented bythe dashed line in the figure.

The required conversion is achievedat approximately 2.8 LN·min–1 withX4CrNi18-10. With nickel oxide, theNH3 conversion at 1.5 LN·min–1 of99.996 % is higher than with

X4CrNi18-10, but the required conversion of 99.99 % is al-ready met at 2 LN·min–1.

The trends for each conversion are always similar in shape:the conversion remains relatively constant up to a certain flowrate and then falls off steeply. Arabzcyk and Zamlynny [12] in-vestigated the conversion trends at increasing space velocityand also recorded a tendency to decrease. However, this didnot include the time in which the conversion remains constant,which may be down to the much lower reaction temperature


Figure 5. Ammonia conversion using different catalysts.

Figure 6. Ammonia volume fraction at the outlet of the reactor depending on the volumeflow rate.


(500 °C). Liang et al. [18] also recorded a decrease in the con-version at increasing space velocity. The dropping conversionshows that the reaction rate does not increase proportionallyto the substance quantities introduced, but rather is limited bymass transfer limitation. The ratio from empty reactor volumeto total volume is defined as porosity and is significantly lowerwith nickel oxide than with X4CrNi18-10 so that the meanresidence time at the same volume flow is significantly shorter.For this reason, the following shows the NH3 conversion de-pending to the mean residence time of the gases in the reactor.

Fig. 8 shows that the required con-version of 99.99 % at NH3 volumefraction U0,NH3 = 80 vol.-% can beachieved by correctly rating the resi-dence time.

When X4CrNi18-10 is used as thecatalyst, the required conversion isachieved at a mean residence time ofapprox. 0.2 s. Extension of the resi-dence time can only result in a slightincrease in conversion up to approxi-mately 99.992 %. Shortening of theresidence time to less than 0.17 s re-sults in a considerable decrease of con-version. The nickel oxide catalystachieves the required conversion at ashorter residence time of 0.18 s. Theconversion increases as the residencetime is extended up to as high as99.996 %. Decreasing the residencetime results in a steep conversion de-crease just as for X4CrNi18-10.

5 Scale-up Process

A prototype for waste gas purificationin nitriding processes has been devel-oped based on the kinetic data fromthe laboratory investigations and withrespect to the following boundary con-ditions:● The pressure loss caused by the

waste gas purification must be smallenough that it does not exceed thedifference between the triggeringpressure of the safety valves in thenitriding furnace (usually 15 mbar)and the furnace’s operating pressure(3–5 mbar).

● The catalyst material should be cost-effective and easily procurable, i.e.,expensive precious metals should beavoided wherever possible.The optimal process conditions weredetermined in the laboratory tests.The following applies:

● The reaction temperature is 1273 Kat the highest ammonia volume fraction of 80 % and themaximum volume flow rate.

● The minimum residence time of the gas in the reactor issM = 0.2 s for X4CrNi18-10 and sM = 0.18 s for nickel oxide.The design of the prototype is based in combination with an

industrial-scale nitriding furnace. The maximum ammoniavolume flow rate which can be fed into the nitriding furnaceis:

q V,nit = 1.5 m3N·h–1.


Figure 7. Ammonia conversion depending on the volume flow rate.

Figure 8. Ammonia conversion depending on the mean residence time.


The ammonia dissociation grade in the nitriding furnaceand the related volume increase must be taken into accountwhen calculating the volume flow q V,pilot which leaves the ni-triding furnace and is fed into the prototype. With a maximumresidual NH3 fraction of 80 vol.-% in waste gases from nitrid-ing processes (in general) the NH3 conversion in the nitridingfurnace can be calculated as follows [19]:

XNH3� 100 � NH3i�A��

�100 � NH3i�A��= 0.11 (10)

Assuming that the gas is already at the reaction temperaturewhen it enters the prototype, the volume flow at the entranceis:

q V,pilot = 7.8 m3·h–1

As in the test unit, a mean volume flow q V,M is calculatedtaking into account the increase in gas volume at a conversionof XNH3 = 99.99 % in the waste gas purification. The mean vol-ume flow q V,M is:

qV�M = 15 m3·h–1.

The following applies to the prototype’s required reactorvolume according to [20]:

Vreactor �qV�M � sM

e(11)

where sM is the experimentally determined minimally requiredresidence time and e is the porosity of the catalyst bed.X4CrNi18-10 chips have a higher porosity than the nickel ox-ide catalyst (also see Tab. 5) and cause less flow resistance.(also see Tab. 6). The pressure loss caused by the catalyst fillingthus can be kept low.

In addition, the higher porosity of the X4CrNi18-10 at anapproximately identical residence time sM of the gas in the re-actor can also indicate smaller dimensions for the system. Thecalculated reactor volumes are 1.5 dm3 for nickel oxide as thefilling and 1 dm3 for X4CrNi18-10. The 50 % larger reactorvolume when nickel oxide is used invokes considerably higherinvestment costs, with the result that, in line with the projectspecification, X4CrNi18-10 is chosen as the catalyst filling forthe reactor. The diameter of the flow reactor is selected as lowas possible to minimize radial concentration gradients andavoid dead spaces in the reactor. The maximum possible

length of the reactor is given by the tube furnace intended forthe heating and stands at 48 cm. Thus, a total volume of 1 dm3

results in an internal diameter of the flow tube of 6 cm. To pro-tect against high-temperature corrosion the reactor is madefrom the nickel-based alloy NiCr23Fe (alloy 601, materialno. 2.4851). The alloy is suitable for continuous operation upto 1420 K.

5.1 Test Program

The varied process parameters for the prototype testing aregiven in Tab. 6.

5.2 Results and Discussion

During test runs 1–4 the nitriding furnace was initially fedwith a small NH3 volume flow rate of qV,nit = 1 m3

N·h–1 and theNH3 volume fraction at the entrance to the furnace graduallyincreased up to 80 vol.-%. The reaction temperature was1173 K. The ammonia measured values recorded at the exit ofthe prototype are shown in Fig. 9.

In test 5, the nitriding furnace was fed with the maximumammonia volume flow rate. The residual NH3 content afterthe nitriding furnace was 80 vol.-% and the reaction tempera-ture in the waste gas purification was TR = 1273 K. The ammo-nia measured value at the exit of the waste gas purificationwas 38 ppm, which means that the objective of meetingthe threshold established in the TA-Luft of 40 ppm hasbeen achieved. The NH3 conversions in the prototype werealso calculated and can be found in Fig. 10 versus the ammo-nia volume fraction at the entrance to the waste gas purifica-tion.

All conversions are above the required value of 99.99 %. Theconversion achieved at maximum volume flow rate with anNH3 fraction of 80 vol.-% totals 99.991 % and thus correlatesthe conversion determined in the experiments on the laborato-ry system (see also Fig. 8). Tests 1-4 showed a higher conver-sion than the respective laboratory tests. The conversionachieved in the laboratory tests at a reaction temperature of1173 K and 40 vol.-% ammonia at the reactor entrance was99.91 % and was thus 0.085 % under the conversion achieved


Table 5. Design data of the prototype.

X4CrNi18-10 Nickel oxide

Mean volume flow rate qV,0(m3·h–1) 15 15

Minimal residence time sM (s) 0.2 0.18

Porosity e 0.9 0.5

Volume of reactor Vreactor (L) 1 1.5

Internal diameter D (cm) 6 7

Length L (cm) 48 48

Table 6. Process variables of the nitriding furnace and pilotplant.

Nr. Volume flow rateat the inlet ofnitriding furnace

Ammonia volume fractionin the waste gas ofnitriding furnace

Reactiontemperature

�V (m3N·h–1) U0,NH3 (vol.- %) TR (K)

1 1 30 1173

2 1 40 1173

3 1 60 1173

4 1 80 1173

5 1.5 80 1273


on the prototype under the same process parameters. In con-trast to the laboratory reactor, which was made from X6CrNi-MoTi17-12-2 (1.4571), the prototype reactor is made of thenickel-based alloy NiCr23Fe (2.4851). There is considerabledifference between the nickel fractions of the two alloys. While1.4571 has approximately 12 % nickel by weight, the nickel-based alloy has up to 63 % by weight. It can be assumed thatthe high nickel content of the internal walls of the test reactorcaused the higher conversion. This conclusion is confirmed by

the results of Choudhary et al. [6] withtwo different nickel contents. They de-termined a conversion of 70 % withNi-SiO2 containing 10 % nickel and97 % with Ni-SiO2-Al2O3 containing65 % nickel. As the tests were con-ducted under otherwise identical pro-cess conditions, it can be assumed thatthe higher nickel content of the catalystcaused the higher conversion.

6 Conclusions

The reduction of the ammonia contentin waste gases from nitriding processesby means of catalytic dissociation re-quires a conversion of 99.99 % to meetthe threshold set out in the Germanregulations. A laboratory unit was usedto determine the design data and pro-cess conditions for a prototype. Theprototype attained the required con-version of 99.99 % and thus the objec-tive of falling below the Clean AirGuideline limit of 40 ppm wasachieved. It was shown that the deter-mination of a minimum residence timein heterogeneous catalyzed gas reac-tions with an increase in volume is asuitable means of achieving a requiredconversion. The transferability to in-dustrial systems was proven by the ef-fective application on the prototype.

Acknowledgements

This study was done within the ProjectFH3 No. 1723X06 sponsored by theGerman Federal Ministry of Educationand Research. H. Ludewig acknowl-edges support from HWP II by theSenate of Bremen, Germany, and theindustrial partners.

References

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[2] Umweltbundesamt, Nationales Programm zur Verminderungder Ozonkonzentration und zur Einhaltung der Emissions-höchstmengen, Programm gemäß § 8 der 33. BImSchV und


Figure 9. Ammonia volume fraction at the outlet of the prototype.

Figure 10. Ammonia conversion of the prototype.


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