J. Aerosol Sci., Vol. 26. Suppl 1, pp. $221~.~222, 1995
Elsevier Science Ltd
Printed in Groat Britain
Peroamon
0021-8502/95 $9.50 + 0.00
Sublimation of Urea Particles at High Temperatures
G.N. SCHADING t, C.C.M. LUIJTEN ~, AND P. ROTH t
t Institut fiir Verbrennung und Gasdynamik
Gerhard-Mercator- Universit6t Duisburg, ~ 70~8 Duisburg, Germany
Faculteit Technische Natuurkunde
Technische Universiteit Eindhoven, 5600 MB Eindhoven, Netherlands
Keywords
Particles Sublimation; Shock Waves; Light Scattering Diagnostic
The sublimation of solid urea particles suspended in a carrier gas is of greatest interest
with respect to the reduction of nitric oxides produced during combustion. Caton and Siebers
[1] investigated the decomposition of dry urea at high temperatures and showed that urea
decomposes into ammonia and isocyanic acid. Both molecules axe able to reduce the NO
content of exhaust gas under certain conditions. In practice, a generated urea aerosol could
be added to the hot exhaust gases of a combustion system in order to reduce the nitric oxide
concentration. The rate of removal depends on temperature due to both the temperature
dependence of the urea particle sublimation and temperature-dependent rate coefficients of
the NO removal reactions.
In the present investigation the sublimation of urea particles suspended in argon was
investigated at high temperatures. The aerosol was generated by heterogeneous condensation
in a specially designed aerosol generator. Before each experiment the particle size distribution
function was determined by an opticle particle counter. The shock tube technique was
applied to study the aerosol sublimation process under well defined temperature and pressure
conditions. A shock wave running through the gas/paxticle mixture causes a gasdynamic
heating of the aerosol and initiates the sublimation of the urea particles. The time-dependent
decrease of the particle size was followed by light scattering measurements. A He-Ne-laser
was directed through the shock tube perpendicular to the tube axis. The scattered light was
detected under three fixed angles (20 ° , 30° , and 45°). An example of measured scattered
light intensities obtained from an individual experiment is illustrated in Fig. 1. After shock
arrival, the intensity signals increase due to aerosol compression followed by a signal decrease
caused by sublimation of the particles. The intensity signals obtained were compared with
calculated Mie intensities resulting in time-dependent values of the particle radius. Results
are shown in Fig. 2 where the square of the particle radius is plotted as a function of time.
The initial decrease of particle size can be approximated by a straight line illustrating the
S221
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G . N . SCHADINGet al.
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1000
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Fig. 1: Example of light scattering intensities
obtained from a shock heated urea particle ensemble.
'
T
600
--
T
800
--
1000
//J,s
Fig. 2: Evaluation of the light scattering intensities in terms of particle size showing the timedependent decrease of the urea particle size.
correctness of the r~-law described by:
dr2p
d-7
-
DV, G *
2Mv
Pv pOpR----~p -
K
For longer reaction times Knudsen effects result in an increasing deviation from the linear
decrease. The observed sublimation behaviour agrees quite well with the evaporation theory
for particles which was described in detail by Timmler and Roth in [2]. For constant temperatures and material properties the sublimation rate dr2p/dt = - K should have a constant
value which was confirmed by our experiments.
The evaluation of all experiments performed under different temperature conditions
showed a strong temperature-dependent sublimation behaviour of the urea particles. Pressure effects were neglected due to the small variations in the experimental pressure values.
A comparison with theoretical considerations are in progress.
Acknowledgement
This work originated in the Sonderforschungsbereich 209 of the Universit~t Duisburg. The
financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The
authors like to thank N. SchlSsser for the help in conducting the experiments.
References
[1] J . A . C a t o n a n d D.L. Siebers: Comparison of Nitric Oxide Removal by Cyanuric Acid
and by Ammonia. Combust. Sci. and Tech. 65, 27%293 (1989).
[2] J. T i m m l e r a n d P. Roth: High-Temperature Evaporation Rates of Solid KBr and
NH4C1 Aerosol Particles. Part. ~ Part. Syst. Charact. 8, 79 (1991).