Laboratory investigations into a new method of reducing nitrogen oxide in the presence of free oxygen

0360-3199/93$6.00+ o.oo PergamonPress Ltd. © 1993InternationalAssociationfor HydrogenEnergy.

Int. J. Hydrogen Energy, Vol. 18, No. 5, pp. 433~.38, 1993.

Printedin Great Britain.

LABORATORY INVESTIGATIONS INTO A NEW METHOD OF REDUCING NITROGEN OXIDE IN THE PRESENCE OF FREE OXYGEN A. SCHMOLZand W. BOEGNER Kompetenzzentrum Physikalisch-ChemischeGrundlagen, Forschungsinstitut I Mcrcedcs-Bcnz, Daimler-Bcnz AG, Stuttgart, Germany (Received for publication 20 October 1992)

Abstract--This paper reports on investigations into a new method of reducing nitrogen oxide in the exhaust gases of engines which are operated under lean air/fuel conditions, as are, for instance, hydrogen-operated engines. The proposed method is called the "ARD-technique", which means Adsorption, Reduction or Reaction and Desorption. As compared with today's conventional catalytic processes, application of the "ARD-technique" means a completely new procedure in exhaust gas treatment. We present the results of laboratory research work using the "ARD-technique" in combination with commercially available adsorbers, oxygen-containing synthetic exhaust gases and gaseous hydrogen as the reduction medium. Depending on the relevant experimental parameters such as the temperature, the flow conditions and gas compositions, the measurements yield values between 50% and more than 90% for the obtainable NOt conversions.

1. INTRODUCTION AND PROBLEM DESCRIPTION It is well-known that exhaust gases containing oxygen cannot be purified using conventional catalysts developed for applications with k = 1, see for example the paper by Held et al. [ 1 ]. The main problem is the chemical reduction of nitric oxide in the oxidizing environment of free oxygen. While this problem is crucial for the diesel engine [2], it may arise with hydrogen-operated engines, too. With respect to the environment, hydrogen-operated engines have several advantages concerning greenhouse gases, stemming from the lack of carbon in the fuel. NO/, however, is produced in the same way as in engines using conventional fuels. Therefore, the reduction of NOx is an important task and every effort is made to prevent NOx formation from the very beginning using various countermeasures [ 3 ]. If, at the very end, these engines cannot be operated in such a way that present and future legislative limits for NOx emissions are met by properly adjusting the combustion process, additional exhaust gas treatment will be necessary. With this situation in mind, we have recently proposed a new method of reducing NO in gas mixtures containing 02, called the "ARD-technique" [4], which, as compared with today's conventional catalytic processes, means a completely different procedure in exhaust gas treatment. The present paper describes, in detail, the principles of this technique and then reports on laboratory investigations using this new method to reduce NO in synthetic gas mixtures simulating the exhaust gases of engines which are

operated under lean air/fuel conditions, as are, for instance, hydrogen-operated engines.

2. PRINCIPLES OF THE "ARD-TECHNIQUE" "ARD-technique" is a term containing the abbreviations of the names of the major steps of this new concept _for exhaust gas treatment, namely Adsorption, Reduction or Reaction and Desorption. The ARD-technique belongs to the class of adsorptive separation techniques frequently used in the chemical industry and becoming more and more important for the removal of polluting components from air and gas waste streams, as these purified gas streams must fulfil the requirements of emission regulations [ 5, 6]. The main idea of the ARD-technique is a separation in space and time of the relevant chemical steps mentioned above. According to this basic idea, an adsorber will be treated periodically and alternately either with the oxidizing exhaust gas or with a gas stream containing an appropriate reduction medium. This principle is explained in Fig. 1, showing the time dependencies of these two gas flows. One complete ARD cycle has a time length t~ (t c = ARD cycle time) and is composed of two time periods of equal length: the first part of the treatment is called the "adsorption phase", the second part of the ARD cycle is named the "reduction phase" or "reaction phase". During the first phase (the loading phase), adsorption of NO molecules takes place; during the second phase (the regeneration phase), there is reactive desorption, i.e. the

433

434

A. SCHMOLZ and W. BOEGNER

t

AdSorp tio n reaction cycle sorption ~ hase

o

i

~Z

t. .'~

I

I i I i

~

I I I I

R e a c t i o n _ . . ~

0.5 tc

!

phase

I

Three-wa, valves

< :Lol o II e¢ ]Ms No,No211 I

Flow control~

t '---"~

Fig. 1. Principle of the ARD-technique. The figure shows the time dependence of the two gas flows during the two phases of the ARD-technique.

PC

PC

Fig. 2. Experimental set-up in the laboratory. The main parts are the gas mixing unit, the reactor with the adsorber and the analytical part. adsorbed molecules react with the reduction agent and the reaction products will desorb. At the end of the regeneration phase, the adsorber will be able to extract NO molecules from the exhaust gas again. For the experiments in the laboratory, the exhaust gas flow and the gas flow containing the reducing agent are both simulated and properly prepared by appropriate mixing of gases from bottles. The effectiveness of the ARDtechnique realized in the laboratory may be deduced from the decrease of the NO concentration relative to the known NO input. 3. EXPERIMENTAL SET-UP IN THE LABORATORY Figure 2 shows the experimental set-up in the laboratory. It essentially consists of three main parts: on the left side is the gas mixing unit, where gases of the two compositions are prepared; at the top right is the electrically heated reactor which contains the adsorber; and on the lower right side is the computer-controlled analytical part. Gases or gas mixtures contained in bottles are used to prepare the different compositions of the gas mixtures necessary for the two phases of the ARD cycle. The proper mixing and the periodical feed of the reactor is achieved by mass-flow control devices and PC-operated three-way valves. The electrically heated reactor containing the adsorber is fed alternately with the appropriate gas mixtures according to the ARD cycle. The composition of the gases leaving the reactor is investigated in the analytical part of the experimental set-up using a mass spectrometer (MS) and a chemical luminescence detector (CLD). The mass spectrometer may be operated either manually or in combination with an external PC. This PC is not only used for the mass spectrometer control, but also for the data acquisition. The chemical luminescence detector allows the continuous and simultaneous measurement of the NO and NO, concentrations, and, by subtracting the first from the second

value, gives the NO2 concentration, too. The data acquisition and control system used is of a modular type and is combined with several appropriate PC software packages. 4. MEASUREMENT METHODS

4.1. Principle of measurement The principle of the ARD-technique and also, therefore, the principle of the measurement, has been shown in Fig. 1. According to the basic idea of the ARD-technique, an adsorber is alternately and periodically fed with gas flows containing NO in the adsorption phase and a reduction medium in the reaction phase, respectively. In the case of hydrogen engines, we can use the engine fuel at the same time as the reduction agent for the exhaust gas treatment. Therefore, for the present experiments in the laboratory, the bottle providing the reduction medium contains a mixture of argon with hydrogen. Later, this reduction medium is easily available onboard the vehicle. The primary data obtained by the analytical devices are the concentrations of the different gas components at the exit of the reactor. The effectiveness of the adsorber may be deduced from a comparison of the gas composition given by the measurement and the known gas composition at the entrance of the reactor. The focus of interest concentrates on nitrogen oxides. The reduction of NOx during an ARD cycle is given by the difference between the amount of NO fed into the reactor at the input, minus the amount of NOx leaving the reactor in the same time. Assuming a constant gas flow 1;', both these quantities may be obtained by integrating the appropriate concentrations within the limits of one ARD cycle. In order to get a characteristic quantity for comparison purposes, the NOx conversion is defined as the reduction of NOx, as given before, divided by the amount of NO at

REDUCING NO~ IN THE PRESENCE OF

435

0 2

In the present series of measurements, the standard value of the exhaust gas composition during the adsorption phase

the reactor entrance:

was:

KNo~ = NOx conversion

Ar + 2~o 02 + 1900 ppm NO for the adsorber "Doduco 200"; and

Reduction of NOx NO at the reactor entrance

Ar + 29/o 0 2 + 1968 ppm NO for the adsorber "Doduco 400".

NO fed into the r e a c t o r - NOx leaving the reactor

During the reaction phase the gas mixture consisted of:

NO fed into the reactor

Ar + 2% H2. ZNO.in X I/dt -

=

ZnOx.o,tx l/dt

=0

frt~

=0

ZNO.in x I?dt

100%.

x

=0

Integrating the appropriate concentrations within the limits of one ARD cycle for I;" = constant:

tc ;'~

KNox = ;(so.i. x } -x

=o ZNO..o,, ×dt

}

tc /~(NO. in X

2

For practical reasons of measurement with the mass spectrometer, argon has been used as the carrier gas for both periods of the ARD cycle. An experiment was started by simultaneously flushing the adsorber with the pure carrier gas argon and heating it at the same time up to the desired measurement temperature. Then, starting with an adsorption phase, a minimum of at least 10 to at most about 20 ARD cycles were performed at constant temperature. Figure 3 shows a typical example of the time dependences of the NO input and the concentrations of the nitrogen oxides NO and NO2 in the gas flow leaving the reactor.

100%.

Quantitative evaluation is made by using the areas, XNO,.,n/ou, = NO~ concentrations. 4.2. Selection of adsorbers The previous paper [4] describes results for an adsorber in the form of Pd pellets (Degussa catalyst type E 22 P/D). The active substance was 5% Pd on 3,-A1203 spheres with diameters of 4 - 6 mm. In the present work, we have expanded those investigations into a detailed study of NOx conversion using, again, commercially available adsorbers: in this case, two different versions of the reduction catalyst type V1586 from Doduco. The noble metals used for these catalysts are Pt and Rh on a cordierite support coated with stabilized aluminum oxides. These adsorbers were the honeycomb type. They had either 200 or 400 cells per square inch and therefore are named "Doduco 200" and "Doduco 400", respectively. Both adsorbers were of cylindrical shape with the following dimensions: length, 6 4 - 6 5 mm; diameter, 2 4 - 2 5 mm. The volume was about 30 cm 3, the mass 19.6 g for the adsorber "Doduco 200" and 16.5 g for the adsorber "Doduco 400". With this choice we have - - together with the previously published measurements - - investigated both adsorbers in the form of pellets as well as honeycomb type adsorbers. 4.3. Experimental conditions It is obvious that, apart from the properties of the adsorber, the composition of the exhaust gas and the kind and amount of the reducing agent are of great importance to the ARD-technique.

5. N O x C O N V E R S I O N : M E A S U R E M E N T S A N D RESULTS The NOx conversion data depend on the properties of the adsorber, the compositions of the two gas flows, and on external physical parameters. The most important external parameters for the measurements are the temperature T, the time duration of one ARD cycle to, and the gas flow I;' or the space velocity R. Since for the measurements of this investigation the compositions of the two gas flows were Typical courses of concentrations Ar+1953 ppm NO+2% O2 ~

Ar+0.9% H2

2500

2000 E

NO,

e~ co

1500

g

1000

in

NOx, out

tO 500

0

10

20

30

40

50

Time (s) Fig. 3. Time dependencies of the NO input and the contents of the nitrogen oxides NO. NO 2 and NO~ in the gas flow leaving the reactor.

436

A. SCHMOLZ and W. BOEGNER

kept constant, we can directly deduce the influence of the different external parameters. The following three sections show the NO+ conversion data as a function of each of these parameters.

Doduco 400; 30 cm 3 Ar/1968

of 2.0 N1 rain -L combined with an ARD s; and of 3.5 Nl min -~ combined with an ARD s.

The temperature range covered was between about 100 and 400°C. The results of these measurements are shown in Figs 4 and 5 for the two adsorbers "Doduco 200" and "Doduco 4007 respectively. It can be seen that all these curves are very shallow and show a maximum at low temperatures, i.e. near 200°C for the adsorber " D o d u c o 2 0 0 " and at about 160°C for the adsorber "Doduco 4 0 0 " . With respect to the achievable NOr conversion, we have to prefer the adsorber with 400 cells in. -2, with a maximum of about 80% at a temperature as low as 160°C. For 100°C this adsorber still has an NOx conversion value of 50%. Its counterpart with 200 cells in.-2 yields a maximum value for the NO+ conversion of a little more than 70%. The low temperature data are 4 0 - 5 0 % at II0°C. In order to take advantage of the maximum NOx conversion in all cases, the temperature of the respective

Doduco 200; 30 cm 3 Ar/1900ppm NO/2% O2 " . Ar/2% H2 100

80-t.~

60

--

o

40

-

O~

z

20

-

0

[]

R = 4000

l/h;

tc=

×

R = 7000

I/h;

t¢ = 4 0 s

I 50

100

I 150

200

I

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H2

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60

_

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--

z 20

--

0

[]

R = 4000

l/h;

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x

R = 7000

l/h;

t c=

I

I 50

100

150

200

40 s

I

I

I

I

250

300

350

400

Temperature (°C) Fig. 5. NOx conversion as a function of the temperature for the adsorber "Doduco 400" and for two combinations of the ARD cycle time and space velocity. maximum of each curve was chosen for all further investigations, i.e. 200°C for the adsorber "Doduco 2 0 0 " and 160°C for the adsorber "Doduco 4 0 0 " .

5.2. Dependence on the ARD cycle time Since the ARD-technique is a discontinuous method using two periodically alternating gas flows of different compositions, the measure of the discontinuity, i.e. the ARD cycle time, is an important parameter and was investigated next. The NOn conversion was measured as a function of the ARD cycle time between 20 and 90 s for both adsorbers for their optimum temperatures. Figure 6 shows the NOx conversion as a function of the ARD cycle time for the adsorbers " D o d u c o 2 0 0 " and "Doduco 4 0 0 " . The temperature for the first case was 200°C, while that for the second one was 160°C. The parameter in the figure is the gas flow or the corresponding space velocity. In Fig. 6, the gas flows used were 2.0 NI ro_in-1 and 3.5 NI m i n - t , the associated space velocities being 4000 and 7000 h-1. The general trend is that, for a constant gas flow/space velocity, the NOt conversion decreases with increasing ARD cycle time, and, for a constant cycle time, the smaller gas flow yields the higher conversion data. 5.3. Dependence on the gas flow~space velocity

60 s

250

02"

80

As usual, argon was used as the carrier gas for the gas mixtures during both periods of the ARD cycle for all experiments. Using the standard values for the gas compositions as given in Section 4.3, we started with a variation of the temperature for two conditions: a gas flow time of 60 a gas flow time of 40

NO12%

100

5.1. Temperature dependence of NOv conversion

(1) cycle (2) cycle

ppm

I

I

350

400

Temperature (°C) Fig. 4. NOx conversion as a function of the temperature for the adsorber "Doduco 200" and for two combinations of the ARD cycle time and space velocity.

In addition to the conditions shown before, we performed measurements for both adsorbers using the following values for the gas flow: I? = 1.0, 2.0, 3.5 and 4.5 NI min -~. The corresponding values for the space velocity are R = 2000, 4000, 7000 and 9000 h ~. For all four settings of the gas flow, two values for the ARD cycle time were chosen, 40 and 60 s. The temperature was 200°C for the adsorber "Doduco 2 0 0 " and 160°C for the adsorber "Doduco 4 0 0 " . The results are shown in Fig. 7 for both adsorbers.

REDUCING NOx IN THE PRESENCE OF 02 Doduco; 30 cm 3 Arlca 1900 p p m N O / 2 % 0 2 "

6. DISCUSSION

_ Ar/2% H2

6.1. Comparison of adsorbers

100 4 0 0 0 l / h 7O0O l / h ~ 4000

l

/

h

~

~

80

.~

60

o o

40

Z

_

ta : : 0 : + :

_

20

0

Doduco Doduco Doduco Doduco

200 200 400 400

I

I

I

I

I

20

40

60

80

100

Cycle time (s) Fig. 6. NOx conversion as a function of the ARD cycle time for two values of the gas flow. Adsorber: "Doduco 200"; temperature 200"C. Adsorber: "Doduco 400"; temperature 160°C. Doduco; 30 cm 3

Arlca 1900 ppm NO/2% 02 " v Ar/2% H2 100

Tc=40 t c = 40

s

8O

.~.

0 Z

60 -

-

[] x 0 +

40

20 m

0

Up to now, we have presented the results of the measurements of the NO, conversion as a function of the temperature, the ARD cycle time and the space velocity for the two Doduco adsorbers investigated. In Ref. [4], similar results for an adsorber in the form of Pd pellets have been published. In this section we will compare directly the efficiencies of the different adsorbers by using the new data of the present paper, together with the corresponding results obtained before. Figure 8 shows the NO~ conversions for the adsorbers "Doduco 200", "Doduco 400" and for the Pd pellets from Ref. [4] for a constant space velocity of 4000 h-1 and for four values of the ARD cycle time. For the smallest ARD cycle time of 20 s, the adsorber "Doduco 400" yields the best result. With increasing values of the ARD cycle time the values for the NOx conversion decrease. The adsorber "Doduco 200" shows a similar behavior, but on a level which is a little lower. The Pd pellets show a decrease, too, but more moderate. Therefore, the result for the longest ARD cycle time is better for this adsorber than the data for the other two adsorbers. In Fig. 9, the ARD cycle time is kept constant (40 s) and the comparison shows the influence of the space velocity on the NO, conversion obtained with the different adsorbers. With only one exception (adsorber "Doduco 200" with space velocity R = 9000 h - l ) , all adsorbers yield a decrease in the NOx conversion with an increase in space velocity. From these data we may conclude:

~

~

Firstly, in nearly all cases, the best results for the NOx conversion are given by the adsorbers from Doduco, especially by the one with 400 cells in.-2.

.

o 0

437

: : : :

Doduco Doduco Doduco Doduco

200 200 400 400

Secondly, it can be seen that the potential of the ARDtechnique is independent of the form of the adsorber. Honeycomb type adsorbers as well as adsorbers in the form of pellets yield - - under comparable conditions - comparable results for the NO/conversion data.

I

I

I

I

I

2000

4000

6000

8000

10000

Space velocity (I/h) Fig. 7. NO, conversion as a function of the space velocity for two values of the ARD cycle time. Adsorber: "Doduco 200"; temperature 200~C. Adsorber: "Doduco 400": temperature 160~C.

In both cases, the NOx conversion decreases roughly linearly with increasing gas flow. The upper curves correspond with the shorter ARD cycle time, the lower ones with the longer tc value. It is worth mentioning in this context that the space velocities realized so far in the laboratory are too small as compared with the real conditions in exhaust gas treatment.

6.2. Approach to technical realization The purpose of the research work performed so far was to prove the feasibility of the ARD-teehnique, in principle at least. The results show that the ARD-technique may yield good NO~ conversion data under the idealized steady-state conditions in the laboratory. For the ultimate application in a vehicle, however, several problems have to be solved. First, we have to match the ARD-technique, which is discontinuous in nature, to the continuous output of exhaust gas of the engine. The technical realization of this problem is subject to discussion. One possible idea is to use two adsorbers alternately: whereas the first adsorber is in the "adsorption phase" and is fed with the exhaust gas from the engine, the second one is in the "reaction phase" and is fed with the reduction medium, and vice versa during the second half of the ARD cycle. This solution involves a great deal of mechanical devices and engineering which

438

A. SCHMOLZ and W. BOEGNER Comparison of adsorbers for 4000 1/h

Ar+ca 1930 ppm NO+2% 02 ~ 100

[ ] Doduco 200 [ ] Doduco 400 [ ] Pd pellets

\

\

0~

s017 i/ I/ t/ v 60 v i,,, t/

g

40

v O

\

\ \ \ ", \

-i

\ x \ \ \ \

;

~r,

V

\

I/ V

\ \

v

0 '~

Z

ix

\

v V

~'

I/ v

\ \

0

>

\

>

\

A N N

•

e I IN.I IN

60s

90s

\

V

20

K"

\

\ \

20s

'1 \

N 40s

Cycle time Fig 8. Comparison of the NO., conversion for the three different adsorbers and four ARD cycle limes. Space velocily 4000 h ~. Comparison of adsorbers for t c = 40 s Ar+ca 1930 ppm NO+2% 02 ~ A r + 2 % H2 100 --

[] []

N~ 80 .~

60

tJ

40

z

0

Doduco 200 Dodueo 400

[] Pd pellets

\

20

2000

i 11111 '

4000

7000

onboard the vehicle, may be used as the reduction medium, too. 6.3. Concluding remarks

I)*

I/

o

Ar+2% H2

9000

Space velocity (l/h)

Fig. 9. Comparison of the NO\ conversion for the three different adsorbers and four space velocities. ARD cycle time 40 s. must operate completely reliably in the environment of the exhaust gas pipe. Another important question is the appropriate supply of the reduction medium. Generally, the reduction medium either has to be stored onboard the vehicle or must be produced in a separate device on the car. With respect to a convenient supply of the reducing agent, the field of hydrogen-operated engines appears to have a very interesting potential for the ARD-technique, since hydrogen, the fuel for the engine, which is available

The removal of NO\ from exhaust gases containing 02 is an urgent problem, with research activities all around the world focusing on it. In this paper, we have presented the so-called ARD-technique, a new and non-conventional method for this task. It is especially suited to hydrogenoperated engines, because in these applications, hydrogen is both the fuel and the reduction medium. If these engines cannot be operated in such a way that future legislative limits concerning their NO\ emission are met by properly adjusting the combustion process, the ARD-technique could provide an additional reduction of nitric oxide emissions. Keeping in mind its several disadvantages and drawbacks, it is, nevertheless, an interesting alternative for NO reduction in H2 engines. For an application within this framework, the adsorber must be optimally matched to the combination of hydrogen engine and hydrogen as the reduction medium.

Acknowledgements--The present work is part of the Euro-Quebec Hydro-Hydrogen Pilot Project (EQHHPP)--Supplementary Task Program/Europe -- and was supported by the Commission of European Communities (CEC). The authors gratefully acknowledge this financial support. Further, we would like to thank Dr K. Lindenmaier, Mercedes-Benz AG and Dr E. Schmidt-Ihn, Daimler-Benz AG, for many stimulating and informative discussions during the initial phase of this project.

REFERENCES 1. W. Held, A. Kfnig, T. Richter and L. Puppe, Catalytic NO\ reduction in net oxidizing exhaust gas. SAE Paper 900496 (1990). 2. G. Lepperhoffand J. Schommers, Behavior of SCR catalysts in the exhaust gases of diesel engines. MTZ 49, 17-21 (1988). 3. Y. Ninomiya, Y. Hosono, H. Hashimoto, M. Hiruma and S. Furuhama, NOx control in LH2-pump high pressure hydrogen injection engines, in T. N. Veziro~lu, C. Derive and J. Pottier (Eds) Hydrogen Energy Progress IX, Proc. 9th World Hydrogen Energy Conf., Paris, Vol. 2, pp. 1295-1304 (1992). 4. A. Schmolz and W. Boegner, NO-reduction in O2-containing exhaust gases, in T. N. Veziro~lu, C. Derive and J. Pottier (Eds) Hydrogen Energy Progress IX, Proc. 9th World Hydrogen Energy Conf., Paris, Vol. 2, pp. 1261-1270 (1992). 5. G. Miiller and M. Ulrich, Abtrennung und Riickgewinnung yon Stoffen aus Abluft- und Abgasstr6men. Chem. lng. Tech. 63, 819-830 (1991). 6. H. Krill, Adsorptive Abgasreinigung -- eine Bestandsaufnahme. VDI-BerichteNr. 730, pp. 417-429 (1989).