The optical and electrical measurement of aerosols produced by normalized fires

J. Aerosol S c l . , Vol. 20, No. 8, pp. 1509-1512, 1989.

0021-8502/89 $3.00 + 0.00 Pergamon P r e s s plc

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O P T I C A L AND E L E C T R I C A L

MEASUREMENT

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D e v e l o p m e n t Department, Capscr 38, Rue M a g l c i r e Dcuville, 78270 Cravent, France

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Intrcductlon Tc e n c o u r a g e trade of a u t o m a t i c fire d e t e c t i o n e q u i p m e n t a m o n g s t the E u r o p e a n countries, has led to a h a r m o n i z i n g of the r e q u i r e ments and test methods. Sc if a new fire d e t e c t i o n p r i n c i p l e is proposed, the E u r o p e a n s t a n d a r d s offer a way tc d e t e r m i n e its usefulness. The d e t e c t o r s m a i n l y used in a u t o m a t i c fire d e t e c t i c n e q u i p m e n t are e i t h e r based on the d i f f u s i o n of light when c o n f r o n t e d with the fire aerosol, the o p t i c a l detectors, or on the a t t a c h m e n t of gasecus ions p r o d u c e d by a r a d i o a c t i v e scurce to the aerosol particles, the so called i c n i z a t i c n detector. The s t a n d a r d s d e a l i n g with these d e t e c t o r s d i f f e r f u n d a m e n t a l l y . The EN 54-7 uses an a r t i f i c i a l smcke in the form cf a fine p a r a f f i n e oil mist and was a d a p t e d in such a way that it is only c o n v e n i e n t for these two types of detectors. Two s m c l d e r i n g and four open n o r m a l i z e d fires are d e s c r i b e d in the EN 54-9 s t a n d a r d as well as the test c h a m b e r to be used. In both s t a n d a r d s the m e a s u r i n g e q u i p m e n t used is the same, n a m e l y an opacit o m e t e r and a p u m p e d v e r s i o n of i o n i z a t i o n detector, the m e a s u r i n g i o n i z a t i c n chamber, MIC. B e c a u s e the r e s p o n s e of the MIC is a f f e c t e d by e l e c t r i c a l charge c a r r i e d by the a e r o s o l particles, it was d e c i d e d that c p a c i t c m e t r i cal o b s e r v a t i o n cf real fires of the EN 54-9 s t a n d a r d was the best chcise tc o b t a i n data on which the p e r f o r m a n c e cf the E l e c t r c s t a t i c C h a r g e d A e r o s o l Sensor, ECAS, could o b j e c t i v e l y be Judged.

Combustion aerosols The c o m p l e x i t y to m e a s u r e the electric content carried by c o m b u s tion g e n e r a t e d a e r o s o l s is p r a b a b l y one of the reascns for the conf u s i o n and m y s t i f i c a t i o n that s u r r o u n d s this subject. C o m b u s t i o n a e r o s o l s are in g e n e r a l a s y m e t r i c a l l y c h a r g e d by a c o m p l e x p r o c e s s i n v o l v i n g t h e r m a l and c h e m i - i c n i z a t i o n , Calcote et al. 1988. That is is the case for d i f f e r e n t types cf c o m b u s t i o n is c o n f i r m e d for diesel e x h a u s t smoke by K i t t e l s o n et al. 1986, for cig a r e t t e smoke, Roos et al. 1989 and for open s h i p b o a r d fires by Kinsey et al. 1986, who uses a n e g a t i v e l y c h a r g e d mist to speed up ele c t r o s t a t i c a l l y the c o a g u l a t i o n of water p a r t i c l e s and the smcke, i n c r e a s i n g so g r a v i t a t i o n a l settling. The charge levels i n v o l v e d seems to d e p e n d on the mode of g e n e r a tion. F l a m i n g fires p r o d u c e s charge levels close to the mean Boltzm a n n e q u i l i b r i u m charge. P y r o l y t i c a l l y p r o d u c e d p a r t i c l e s have generally lower charge levels and it can take more than a hour to reach the mean e q u i l i b r i u m charge level, B u r t s c h e r et al. 198@. P a r t i c l e d i s p e r s i o n d e p e n d s also on the type of fire. In a flaming fire p a r t i c l e s are m i x e d and swept away t h e r m a l l y from the prod u c t i o n zone, w h i l s t in case of p y r o l y s i s the low t h e r m a l m o m o n t u m and charge levels makes that very high p a r t i c l e d e n s i t i e s can occur , g i v i n g high c h a r g e - v o l u m e ratios. This means that there is a electrical fire s i g n a t u r e in both fire modes, and that they can be detected electrically.

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1510

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aerosol

ROOS and D. DUTERTRE-LADUREE

sensor

The f a c t t h a t h i g h v o l t a g e discharge g a p s are s h o w i n g s t r a n g e behavicur when ccnfrcnted with pclluants, is k n o w n f o r a v e r y l o n g t i m e . By u s i n g a c e r t a i n e l e c t r o d e size and configuration, a gap has b e e n c c n s t r u c t e d showing a remarkable behaviour when confronted with a combustion aerosol. The p r i n c i p l e is u s e d as b a s e f o r t h e E l e c t r o s t a t i c Charged Aerosol S e n s o r , w h i c h s c h e m a t i c measuring configuration is s h o w n in fig u r e I. In f i g u r e 2 we s h o w t h e m e c h a n i c a l adapticn of t h e p r o t o t y p e Its r e a c t i o n to f o u r o p e n a n d one p y r o l y t i c f i r e is s h o w n in t h e f i g u r e s 3 to 7. In t h e s a m e f i g u r e s we see a l s o the r e a c t i o n cf t h e normalized cpacitcmeter, p l a c e d in t h e t e s t r o o m as d e s c r i b e d in t h e European EN 5 4 - 9 s t a n d a r d . For most fires the quantity of c o m b u s t i ble was h i g h l y r e d u c e d in o r d e r to p r e v e n t saturation of t h e E C A S . The b e h a v i c u r of t h e s e n s o r c a n be e x p l a i n e d as f o l l o w s . The w o r k i n g p o i n t of t h e d i s c h a r g e g a p is n e a r t h e e n d cf t h e n a t u r a l saturation current region, by a p p l y i n g a high voltage between the electrodes. The e l e c t r i c field produced in t h i s w a y w i l l c a u s e c h a r g e d aerosol particles to d r i f t t o w a r d s an e l e c t r o d e carrying an o p p o s i t e sign. When the particles reach the electrode, a n u m b e r of t h e m w i l l a d h i r e to it, f i r s t as r e s u l t of C o u l o m b f o r c e s a n d l a t e r w h e n d i s c h a r g e d by v a n d e r W a a l s or c a p i l l a r i c forces. The current collected in t h i s w a y in in g e n e r a l q u i t e l o w a n d n e a r to t h e i n p u t l e a k a g e curr e n t of e x i s t i n g electrometer amplifiers, making it d i f f i c u l t tc u s e However after sometime a m o r e or l e s s i n s u l a t i n g layer will cover the electrode. T h e t o p of it w i l l b e c o m e c h a r g e d in c a s e t h a t t h e l e a k a g e cf c h a r g e t h r o u g h t h e l a y e r is o v e r c o m p e n s a t e d by the i n c o ming charge from the aerosol particles. T h i s w i l l p r o d u c e an a d d i t i o n a l electric field, that when added tc t h e i n i t i a l e l e c t r i c field will surpass a certain level, capable to c a u s e b r e a k d o w n of t h e p o r o u s l a y e r , p r o d u c i n g an e l e c t r o n avalance. These highly mobile c h a r g e s are s w e p t i n t o t h e g a p , m u l t i p l y i n g themselves by i o n i z i n g collisions with gas molecules, making the gap tc o p e r a t e as if in t h e T o w n s e n d discharge region. The in t h i s w a y amplified collection current is e a s e l y m e a s u r a b l e . The prccess h a s i c t s of s i m i l a r i t i e s with certain problems found in e l e c t r o s t a t i c precipitators. Here the breakdown cf the c o l l e c t e d d u s t l a y e r is c a l l e d b a c k i c n i z a t i c n or b a c k c c r c n a a n d is k n o w n f o r its d e g r a d a t i o n of the o p e r a t i n g voltage a n d t h e l o s s cf c o l l e c t i o n efficiency, M a s u d a et al. 1977. An i m p o r t a n t difference is t h a t in e l e c t r o s t a t i c precipitation the a e r o s o l particles a r e c h a r g e d by a c o r o n a d i s c h a r g e , whilst the Electrostatic Charged Aerosol S e n s o r E C A S m a k e s u s e cf t h e n a t u r a l particle charge and operates in t h e s a t u r a t i o n current level mode. S i m i l a r tc b a c k i c n i z a t i c n the layer breakdown causes minute light impulses at the e l e c t r o d e s . Natural gaseous ions and captured aerosol p a r t i c l e s will break off gradually the p o r o u s l a y e r , p r e v e n t i n g so t h a t t h e e l e c t r o d e s should become clogged, a f o r m of a u t o c l e a n i n g When studying the obtained results, we n o t i c e t h a t t h e r e is in general a remarkable similtude in t h e r e s p o n s e of the E l e c t r o s t a t i c Charged Aerosol Sensor ECAS and the cpacitcmeter. Probably due to the m u c h s m a l l e r v o l u m e m o n i t o r e d by t h e s e n s o r , its r e s p o n s e has a less integrated character t h a n t h a t of t h e c p a c i t c m e t e r . Interesting is the f a c t t h a t an a l c o h o l fire produces a usuable signal level, question t h u s t h e i d e a t h a t s u c h t y p e of f i r e h a s s u c h a complete combustion that only neutral gaseous molecules are r e l e a sed, M e i l i , 1952. The b e h a v i c u r of the s e n s o r tc t h e o n l y p y r o l y t i c f i r e , TF2, h o w ever locks less promising, with a violent reaction of t h e o p a c i t o meter and a small delayed recaction of t h e s e n s o r . It h a s b e c o m e highly probable that this behavicur w a s c a u s e d by an i n s u f f i c i e n t screening cf t h e ac s i g n a l p r o d u c e d by t h e h i g h v o l t a g e oscillator. This produced probably a modulation of t h e e l e c t r i c f i e l d , u s e d to let p a r t i c l e s e n t e r t h e gap, r e d u c i n g so c o l l e c t i o n efficiency. Inc r e a s e in g e n e r a t o r - e l e c t r o d e distance and a more effective shielding makes that the sensor produces a usefull signal when confronted with a pyrolytically generated aerosol.

Optical and electrical measurement of aerosols

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Conclusion From the o b t a i n e d results it can be c o n c l u d e d that the electric charge carried by a c o m b u s t i o n aercscl is another fire signature. The e l e c t r o s t a t i c charged aerosol senscr is capable to cbserve this effect m a k i n g that its use as autcmatic fire detector is wcrth tc be studied. References Burtscher, H., Schmidt-0tt, A . ; " P a r t i c l e charge in c o m b u s t i o n aercscls",Aercscls, ed. Liu et al. E l s e v i e r , N e w York,1984. Calccte, H.F, 01son, D.B., Keil, D.G.;"Are icns important scot f c r m a t i c n " , E n e r g y & Fuels, ~, pp 494-504, 1988.

in

Kinsey, J.S., Pendletcn, F . J . ; " E v a l u a t i c n cf charged fog for smoke c l e a r i n g in s h i p b o a r d f i r e s " , R e p c r t N 0 0 0 1 4 - 8 4 - C - 2 1 8 9 , Midwest R e s e a r c h Institute, Kansas City, M0 64110, USA, 1986. Kittelscn, D.B., Mccn, K.C., Ceilings, N . ; " E l e c t r i c a l charge on diesel particles", 2nd Int. Aerosol Ccnf. Berlin, 1986. Masuda, S., Mizunc, A . ; " I n l t i a t i c n ccnditicn and mode discharge",Jcurn, cf Electrcst. ~, pp 35-52, 1977. Meili, E . ; " D @ t e c t e u r Suisse des E l e c t r i c i e n s

d ' i n c e n d i e A chambre ASE, no 23, 1952.

cf back-

d'icnisaticn",Ass.

Rots, R.A., Gcldman, M.;"0n the b e h a v i c u r cf a c t i v a t e d particles in c i g a r e t t e smoke", Eurcp. Aerosol Conf. Wien, 1989.

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