Ions in combustion exhaust as soot monitor

J, Aerosol Sci., Vol. 21, Suppl. Printed in Great Britain.

i, pp. S579-S582,

1990.

0021-8502/90 $3.00 + 0.00 Pergamon Press plc

IONS IN COMBUSTION EXHAUST AS SOOT MONITOR

H. BURTSCHER,

Laboratory

A. GLINZ and M. OCHS

for Solid State Physics, ETH-ZQrich, Switzerland

CH-8093 ZQrich,

ABSTRACT A simple device to measure the ion concentration in the exhaust pipe of an oil burner is introduced. Measurements show that the ion concentraton is related to the setting of the air/fuel ratio respectivley the soot emission of the burner. However, the ion concentration is not dominated by ion attachment to the soot particles in the stack pipe. Changes occuring in the stack pipe due to changed soot concentration are of minor importance compared to changes in the ion emission from the flame, when the alr/fuel ratio is altered.

KEYWORDS Combustion aerosol;

charged soot particles;

ionlsatlon in combustion

INTRODUCTION In every combustion process high concentrations of ions are produced mainly by chemiionisation (Keil et al., 198A). With increasing distance from the reaction zone of the combustion, the ion concentration rapidly drops by recombination and ion attachment to particles. The charge distribution established thereby on the particles depends on the kind of combustion and may be far from the Boltzmann equilibrium charge at ambient temperature. A measurement of the charge distribution allows for example to disti~guish between smoldering and open fires. This has been shown in experiments by Burtscher et al. (1986) with several test fires as they are used to test and compare smoke detectors . The charge distributions observed there are qualitatively explained by a simple model, assuming that the particles are initially charged to equilibrium at a high temperature. This charge distribution is then altered by coagulation and ion attachment. Similar observations were made by Moon (1984) for Diesel particles. In the case of an oll burner the particle charge was found to depend on the air/fuel ratio respectively the soot production of the burner. These experiments also indicate that the particles are charged to equilibrium in the hot zone. Depending on ion and soot concentration this distribution is frozen at a high level (for sooting conditions) or continuously adapted to the temperature during cooling of the exhaust gas (for nonsooting conditions, where enough ions are available to maintain the equlibrium). Beside the particle charge, the ion concentration in the exhaust gas also depends on the burning conditions and is related to the amount of soot produced by the combustion as will be shown in the following.

EXPERIMENT A very simple device is used to estimate the ion concentation in the exhaust gas. As shown in Fig. i., a metallic rod, at which a voltage U is applied, is placed in the exhaust pipe. Due to the electrostatic field, ions and charged particles of one polarity are precipitated at the rod. The current, produced by these ions is measured by a sensitive current amplifier. As the ion mobility is much higher, they may selectively be precipitated by choosing a small enough voltage. The efficiency ~ of this sensor can be estimated by a configuration with two sensors of the same type at different locations in the stack pipe (at a distance LA) as shown in Fig.2.

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Fig.l.

Sensor to measure the ion concentration

in the exhaust gas

Sensor 2

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I'II E x h a u ~ gas

Fig.2.

Measurement of the precipitation distance L A .

efficiency by two ion sensors at

First 12 is measured with the voltage at sensor i switched off, then the difference ~12, when the voltage is applied to sensor i, is determined. ~ is given by the relative change in 12: - AI2/I 2 The absolute

ion concentration

(i)

n then is: n - I/(eQ~)

(2)

where Q is the flow rate of the exhaust gas. In Fig. 3 ~ is plotted versus the applied voltage U. If the sensor is operated at low enough voltage, mainly ions and only very few charged particles will be precipitated and ~ and U are in a linear relation. In the following measurements 15 V are applied.

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RESULTS The ion concentration has been measured at different locations increasing distance from the burner the concentration decreases meter, as can be seen in Fig. 4.

in the stack pipe. With to about 50% after one

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Ion concentration as function of the distance from the burner for 3 settings of the air/fuel ratio, corresponding to Bacharach soot numbers of 0.5, 2 and 3.

This decrease may be due to ion diffusion to the wall, to ion recombination or attachment of ions to particles. A simple estimation immediatly shows, that diffusion is negligible. Comparing recombination and attachment rates, obtained from measurement of the particle size distribution, shows that attachment to particles is dominant. Only in the case of clean combustion (Bacharach number 0.5) recombination is not completely negligible. This is also corroborated by the shape of the curves in Fig. 4. They fit better to an e-X-functlon, as expected if attachment is dominant, than to i/x typical for recombination. For increasing soot the fit becomes even better. The absolute ion concentration as function of the air/fuel

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ratio A shown in Fig. 5. exhibits a very significant decrease with decreasing A. Compared to this decrease, the losses by recombination and attachment in the stack pipe, discussed above, are negligible. This means that the observed decrease cannot be due to ion losses after the combustion. It has to be caused by a reduced ion emission from the flame itself. Eighter less ions are produced if the flame is sooting or they attach to soot particles already in the flame. As the soot concentration in the flame is very much higher than the concentration actually emitted the second effect can be significant. The ion concentraton in the gas would then be a measure for the soot concentration in the flame.

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lon concentration in the exhaust gas of the oil burner as function of the air fuel ratio A

CONCLUSIONS According to Fig. 5 the extremely simple measurement of the ion concentration yields a quantity which is related to A and may be used as measure for A. The ion sensor can therefore be used to monitor A or control it in a feedback control loop. For this application the fast response of significantly less than a second of such a sensor is of advantage. Comparison of results obtained from different burners shows that the absolute ion concentration varies significantly from burner to burner, the shape of the curve however is always the some. For every individual burner the measurement was reproducible.

REFERENCES Burtscher H., Reis A. and Schmidt-Ott A. (1986) Aerosol Sci., 17, 47-51

Particle Charge in Combustion Aerosols,

Keil D.C., Gill R.J., Olson D.B. and Calcote H.F. (1984) Ion Concentation in Premixed Acethylene-Oxygen Flames near Soot Treshold. In: The Chemistry of Combustion Processes T.M. Sloane, Ed.), Am. Chem. Soc., Washington, pp. 33-43 Moon K.C. (1984) Charging Mechanisms of Minnesota

of Submicron Diesel Particles.

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