Emissions by combustion of solid fuels in domestic stoves
Citation for published version (APA):Zeedijk, H. (1986). Emissions by combustion of solid fuels in domestic stoves. In H. F. Hartmann (Ed.),
Proceedings of the 7th World Clean Air Congress 1986: held at Sydney, Australia, August 25 - 29, 1986 (pp. 78-85)
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EHISSIONS ВУ COHBUSTION OF SOLID FUELS IN DOHESTIC STOVES Н. Zeedijk
Eindhoven University of Technology, Dept. Chem. Engineering, Lab. of Instrumental Analys1s, РО Вох 513, 5600 МВ Eindhoven, The Netherlands.
SUММARY .
Firewood and 5 di fferent coal types have been burnt in three st.ove types and the air pollut.ing emiss1ons of gases and smoke particulat.e were det.ermined. The smoke part.icles were analy:z.ed for elements and
polycyclic aromat.ic hydrocarbons (РАН). The emission t.ests were
per-formed under full, half and fourth load conditions, and with а smoul-dering fire.
The reproducibilit.y of the measurements appeared t.o Ье poor, caused
Ьу а dominant influence of small differences during t.he tests. Hore-over, the emission could vary broadly within the period between two successive additions of fuel.
The coalstove wi th automatic continuous addi tion of fuel caused the smallest emissions and the woodstove the highest. For t.he fireplace the emissions are hardly de_pendent on the burn rate, but for the other two stoves the high em1ssions at low burn rate can Ье attribu-ted to oxygen defi с iency of the fire, and the same i s the case for the woodst.ove at full load condit.ion as а result of fast fuel pyroly-sis.
The Ьi t.uminous coals were the most polluting fuels and the brown
coals the least polluting ones.
INTRODUCTION
The last 25 years has seen the memory of the notorious 'London smog' trpe fading as а result of an improvement of the air quality in many
b1g cit.ies. This has been caused Ьу replacing the burning of solid
fuels in domestic stoves Ьу other fuels, mainly oil products and
natural gas. Illustrative of this development is, for instance, that the concentration of the cancerogeneous ben:z.o( а) pyrene in the atmo-sphere of the Dutch city of Rotterdam decreased from an average
50 µg/1000 m3 to below 0.5 µg/1000 mJ, in spite of а growing
traffic emission.
The oil crisis of 1973 could have been а turning-point in this
fa-vouraЫe course. Since then the sales of stoves for solid fuels have increased significantly both in the U.S. and Europe, following the increase of fuel prices. In the U.S. the fuel of concern is mainly
firewood [1]. At the moment firewood is the principal fuel for
heating in 10% of the residences and the firewood consurnption rises at а rate of 10% every year. The disadvantages for air quali ty have already been catalogued for some years [2]. The most important air pollutant.s originat.ing from firewood burning are smoke particula- te and hydrocarbons and in many regions in t.he U.S., where the fire-wood consurnption is high, t.here can Ье violat.ion of the PHlO st.an..:.. dard. Action is required and has started already in Oregon with an oЬliged emission certification of woodstoves [3].
The development in Europe is less turbulent t.han in t.he U.S., but. in forest. districts firewood is also а dominant fuel. Even in а densily populated count.ry like t.he Netherlands, that i s poor in fi rewood stocks, 1501000 woodstoves have been sold during the last. 5 years. This is equ1valent. t.o ЗТ.. of the residences. The short.age of firewood can cause the use of other solid fuels like browncoal briquettes and coal.
The results of former researches for estaЫishing t.he emission
fac-tors of the burning of solid fuels in domestic stoves аге often poor-lJ соmрагаЫе as а result of variations in fuels, stoves and
combus-t1on procedures. А wide range of emission factors have been puЫished
combustion procedures have been standardized as we11 as possiЫe
during the tests. FUELS AND STOVES
The composition of the 6 fuels used is given in ТаЫе 1. Besides the composit~on, also the size of the fuel particulates is of importance. The firewood wai а hardwood of the following dimensions: length 30 ст, width 7 cm and height 5 cm. The browncoa1s were in the form of briquettes of 0.5 kg weight а piece. The coals were nuts of 2 to 3 ст.
TABLE 1 Wood Brown Anthracite Bituminous Bituminous coal coal ло. 1 coal no. 2
C-content То 50 67 92 87 75
H-content То 6 6 3 5
s
Ash-content То 1 8 3
s
7Water-content То 13 10 2 2.5 10
Heating value KJ/kg 18.5 25.1 34.8 35.0 29.3
The following stoves bave been used: а typical woodstove (that can Ье
made suitaЬle for the combustion of coals Ьу inserting а grate). а
fireplace and а coalstove supplied with fuel bunker for automatic cont1nuous addition of fuel nuts. In tbis stove type it was
impossi-Ыe to use logs and briquettes.
There are big differences in combustion in tbe tbree stove types. In the fireplaces there is free access of air oxygen to the f1re. The excess a1r is large and the carbon dioxide content in the flue gases as low as l-2'J.. Ву tbis the heating efficiency is low: 5-15'1о. The only possiЫe control of beat production is to add more or less fuel
into the fire.
In the woodstove tbe airflow is limited and controlled Ьу valves. Additional control is possiЫe Ьу tbe rate of fuel addition. So the same heat production of the stove сал Ье obtained Ьу different valve adjustments resulting in а varying carbon dioxide content of the flue gases.
In contrast to the woodstove, in the coal stove the combustion air is flowing through the fuel bed as а result of а pressure drop over the bed caused Ьу draft. The size of tbe fire is constant and independent of fuel consumption. Its geometry is fixed Ьу the fuel filling system. The supply of combustion air is controlled Ьу а thermostat in the room that must Ье heated.
EXPERIHENTAL
During а combustion experiment the stove and chimney (steel pipe of 15 cm diameter and а length of 6 meters) is placed freely on а Ьig
balance, that registers the total wei~ht. In this manner the fuel burn rate сал Ъе determined Ьу we1ghing with an accuracy of
О. 2 kg/hr. The flue gases leavi ng the chimney top are Пowi ng in а
steel suction funnel. Together with excess of laboratory air at room temperature the flue gas is flowing into а rnixing room in which, Ьу
rneans of baffles, the gas is rnixed up. The temperature of the mix is below 100°С and non-volatile compounds condense • rnainly on the sur-face of the smoke particles that are present. The added laboratory air is dry (R.H. approx. 40~) and this prevents condensation of water vapour.
The samples for gas analys i s are taken frorn the chirnney or from the rnixing room. In both samples the carbon dioxide content is determined and the ratio gives the dilution factor. In the wet gas sample from the chimney only the staЫe pollutants are analyzed: carbon monoxide, h~drogen and the lower hydrocarbons. In the dry gas sample from the rn1xing roorn the instaЬle gases are analyzed: sulphur dioxide, nitro-gen oxides. Also from the mixing roorn, samples of smoke particles are taken Ьу filtrating the gas over glass fiber filters for the analysis
of polycyclic aromatics and over millipore filters (polyvinylacetate) for the analysis of elements. From prior studies it was k.nown that the smoke particles are predominantly smaller than 1 micrometer, so that the non-isokinetic sampling of them will not have caused а
con-sideraЬle error.
Analysis of the polycyclics was performed Ьу HPLC [S,6] and of
elements Ьу proton induced X-ray emission analysis [7 ,8]. The
determination of the concentration of sulphur dioxide was performed
Ьу both а coulometric instrument and а pulsed fluorescence analyzer
(Thermo Electron), of nitrogen oxides Ьу chemiluminescence (Bendix
monitor)1 and of the organic gaseous compounds, carbon monoxide, car-bon diox1de and hydrogen Ьу gas chromatography.
ТНЕ KEASUREМENTS
The combustion of solid fuels is often а batch process.
intervals fresh fuel must Ье added to the fire, which means stove door must Ье opened. VariaЫes in this process are the of an addition and the time interval between two additions, will call 'the burn cycle'.
At time that the quantity which we In this study, as we already mentioned, the aim was to determine the influence of stove and fuel type on emissions, and this is the reason
that the combustion procedure was standardized as much as possiЫe.
Only the burn rate was varied as follows:
- full load = maximum heat production. This means normall_y 2.0 to
3.0 kg/hr fuel added in З portions in burn cycles of 20 m1nutes. half load = half of full load. Performed also in burn cycles of 20 minutes. 10 8 6 4 2
fourth load = one fourth of full load. Only 2 burn cycles of 30 minutes in an hour.
smouldering. This means that in full load conditions, exactly 10
minutes after the last fuel addi tion, the air supply into the stove is shut off Ьу closing the valves. However, 1t 1s
impossi-Ыe to prevent all air entry because of leakages. А sample was
taken during а lapse of 1 minute. Due to this the procedure could not Ье done with the fireplace.
Percentage in flue gas
•
.
.
ComЬustiЫes smoke (g/hr) 40 oxygen ' time(min) t--4..
2 4 6 8 10 12 14 16 18 20 4 8 12 16 20 24 28 Figure 1.VARIATI0N OF ТНЕ EHISSIONS DURING ТНЕ BURN CYCLE
The ernission of а sol id fuel stove wi thout continuous fuel addi tion
varies with the time during а burn cycle. Directly after addition of
fresh fuel pyrolysis products are forrned as а result of the warrning
up of the fuel. In oxygen shortage or at too low fire ternperatures
'pyrolysis gases сап escape and cause ernission of pollutants.
Particu-larly this is the case in the first part of а burn cycle and the
quantity ·of the ernission is dependent of the arnount of volatiles of
the fuel.
Airtight stoves сап Ье actuated in rnany manners resulting in the same
burn rate depending on the positioning of the air valves. At low in-flow of cornbustion air with the fire mass large or at high inin-flow of
air with а srnall fire, the burn rate is equal. The first case has
advantages for the stove user, because the time interval between two
fuel addi tions сап Ье long. However I frorn the view of clean
cornbus-tion it сап Ье better to create an intensive srnall fire.
Illustrative of the variation in ernissions during the burn c~cle is
the ernission of smoke particulate and combustaЫes in а big f1re and
high burn rate. In that case the ~yrolysis is fast and the inflow of
air moderate. For firewood а defic1ency of cornbustion air is the
ori-gin of an ernission peak (figure 1).
RESULTS
а. Sulphur dioxide. The emission of this cornpound is mainly
determi-ned Ьу the sulphur content of the fuel, but also Ьу the partition
between sulphur dioxide in the gasphase and sulphate in the ash of the combusti on. On average, the erni ss ion factors in g/kg fuel
are found for the different fuels as presented in ТаЫе 2.
Ь. Nitrogen oxides. The cornbustion temperature in small residential
stoves is low and Ьу this the thermal forrnation of nitrogen
oxi-des is low. However, they can Ье formed Ьу the occurrence of
ni-trogen containing cornpounds in the fuels. The measured emission factors are low, they vary slightly depending on the stove type,
but they are independent of the burn rate (ТаЫе 2).
TABLE 2 EHISSI0N FACT0R in g,/kg,
Sulphur dioxide Nitrogen oxide Carbon monoxide
Fuel Hean value, Wood Fire- Coal Иеаn value,
all stoves stove place stove all stoves
Wood о.
s
0.4 0.3 69 Browncoal 1 1.2 1.2 2.0 S4 Browncoal 2 6.9 2.0 1.9 83 Anthracite 12.3 3.4 1.4 4.9 319 Bitwn. coal 1 17.9 2.4 4.9 7.9 163 Bitwn. coal 2 6.0 1.2 2.8 4.0 138с. Hydrogen. Usually the emission of this component can Ье neglected
wi th exception of the full load and smouldering condi tions, when
for diverse fuels, except wood. cons ideraЫe amounts of hydrogen
were found in the flue gases, up to an ernission of 20 g/kg fuel.
d. Carbon monoxide. The emission of this compound is strongly
fuel-related, but hardly stove-related. In the fireplace the emission factor is nearly independent of the burn ratej in the woodstove the ernission was lowest under halfload and 1n the coal stove
under full load. ТаЫе 2 gi ves the average erni ssion factor fог
coal fuel and it is obvious that the coals, especially anthra-cite, have the highest emissions.
е. Paraffinic hydrocarbons. Quantitatively the rnost important
repre-sentative of this ~roup is methane, which justifies the separated
propane, given in emission case for load.
butanes, pentanes and hexanes and the total of them i s the ТаЫе З. Especially under smouldering conditions tbe
of paraffinics can Ье very high, but this is also the
the coal stove at fourth load, and the woodstove at full Surprising is the unexpected high methane emission of anthracite
combustion in the fireplace, but this can Ье explained Ьу the
fact _that in that case the fire was bad and smouldering.
f . Olefinic hydrocarbons (ТаЫе З). The main components were etbene,
propene, butenes and pentenes. Anthrac i te combustion always
pro-duces а low emission. For the other fuels the emission is high in
а smouldering fire, and again for the woodstove at high burn rate
and for the coal stove at the lowest burn rate.
TABLE За EHISSION FACTOR i n g,/kg,:ii:
Methane Paraffinics Olefinics
~ ~ full smoul ~ ~ full smoul ¼
-~
Woodstove Wood 11.0 1.7 3.7 41 2.2 O.lS 0.1 S.7 3.7 1.0 Browncoal 1 0.1 о.о S.l S2 о.о о.о 0.1 2.S 0.3 о.о Browncoal 2 3.6 2.6 13.0 S7 0.7 0.2 0.9 9.9 1.9 2.2 Anthracite 3.2 S.9 39.0 99
o.os
0.1 о. 7 2.9 0.04 0.6 Bitum.coal 1 4.8 7.3 17.0 37 0.3 0.6 1.2 2.2 2.8 6.2Bitum.coal 2 4.6 8.9 2S.O 134 0.6
o.s
3.9 26.7 о. 7 S.2FireElace Wood 5.1 5.4 3.0
-
0.6 0.8 0.5-
3.4 1.7 Browncoal 1 1.6 3.6 10.4-
0.1 0.4 1.6-
3.4 1.6 Browncoal 2 5.0 3.9 2.3-
0.2 о.з 0.1-
0.9 1.6 Anthracite 30.1 53.6 S8.8-
1.1 2.0 2.5-
о.о о.о Bitum.coal 1 8.9 10.0 5.0-
1.8 2.3 0.1-
2.6 3.2 Bitum.coal 2 6.2 14.7 4.3-
1.3 4.6 0.9-
1.4 4.3 Coalstove Anthrac ite 41.0 27.0 0.3 17.6 0.4 1.0 о.о 0.1 0.4 0.15 Bitum.coal 1 49.О 8.4 2.7 36.0 21.7 6.0 0.5 8.4 16.2 2.9 Bitum.coal 2 42.0 7.5 7.4 26.4 15.7 3.1 2.7 5.5 17.1 2.5 TABLE 3ЬOlefinics Aromatics Smoke ¼articulate
full smoul ~ ~ full smoul ~ _ full smoul
Woodstove Wood 3.6 23.5 3.7 2.1 0.5 3.7 2.9 4.3 1.1 13.О Browncoal 1 3.S 11.4 0.3 0.1 1.2 6.1 0.5 0.4 1.8 о.о Browncoal 2 11.1 22.8 0.7 1.0 5.7 9.3 2.7 0.3 0.7 0.8 Antbracite 2.6 0.3 0.4 0.3 1.8 о. 7 3.4 1.3 0.5 0.1 Bitum.coal 1 12.0 3.8 1.0 2.9 5.1 2.6 10.0 15.0 42.0 0.1 Bitum.coal 2 16.5
ss.o
1.8 4.3 6.3 12.7 11.0 17.5 16.0 о.о FireElace Wood 2.S-
0.9 0.6 0.6-
7.4 12.2 3.9 Browncoal 1 6.3-
0.3 0.6 2.6-
1.7 4.3 14.4 Browncoal 2 0.6-
0.6 1.1 0.4-
6.5 2.8 1.6 Anthracite о.о-
0.8 1.1 1.2-
2.8 5.3 16.5 Bi tum. coal 1 3.7-
1.0 1.0 1.3 - 21.2 25.9 19.3 Bi tum. coal 2 1.3-
0.7 1.5 0.5 - 19.7 27.2 53.1 Coalstove Anthracite о.о 0.3 0.7 0.3 0.05 0.1-
1.2 2.1 15.7 Bi tum. coal 1 1.1 18.7 3.1 0.7 0.3 4.9 53.0 16.2 16.0 4.3 Bitum.coal 2 1.7 16.1 4.3 1.1 0.5 4.6 89.2 12.2 12.4 8.6*-
For"''
full and smoul average valuesfor ½ the mean of 2 aveгage values.
g. Aromatic hydrocarbons. The frincipal compound is benzene. In lower concentrations also to uene, ethylbenzene and the xylenes are found.
h. Smoke particulate. The bituminous coals produce а lot of particu-late in all stoves under all conditions, with exception of the smouldering condition. The fireplace emits more particulate than the woodstove for the same fuels.
TABLE 4а
ST0VE/EНISSION Woodstove
EHISSION FACTOR inmg/kg
Wood Browncoal 1 Browncoal 2 Anthracite Bitum. coal 1 Bitum.coal 2 Fireplace Wood Browncoal 1 Browncoal 2 Anthracite Bitum.coal 1 Bitum.coal 2 Woodstove Antrac1te Bitum. coal 1 Bitum.coal 2 TABLE 4Ь STOVE/EHISSION Woodstove Wood Browncoal 1 Browncoal 2 Anthracite Bi tum. coal 1 Bitum.coal 2 Fireplace Wood Browncoal 1 Browncoal 2 Anthracite Bitum.coal 1 Bitum.coal 2 Woodstove Antracite Bitum. coal 1 Bi tum. coal 2 12.S 8.5 14.2 1.8 41.8 36.0 2.3 1.0 1.0 6.0 13.4 16.9 0.4 1.7 1.3 2 15.8 8.2 17.8 1.3 42.2 45.7 2.9 0.4 1.0 2.9 9.2 11.9 0.5 7.3 5.5 11* 12 о о 0.1 О о. 7 О 0.1 О 3.3 о 1.2 О 0.1 О 0.2 0.5 0.9 О 3.8 38 2.8 12.3 3.9 0.9 0.6 9.4 1.4 6.8 1.5 2.5 3 3.7 0.8 5.1 0.5 16.8 17.8 2.0 0.8 0.8 4. 8 7.7 9.7 16.3 1.5 3.8 1.1 33.7 40.5 2.2 ND ND 6.7
s.o
5.2 1.4 2.8 3.5 6.7 2.1 13.1 13 14 24 10.2 о о 8.2 2.1 о о 138 О 44 О 18 5.8 5.2 2.3 13.5 S 340 4.3 о о 142 О 41 3.1 125 0.9 57 3* -
Explanation of the numbers: 1. Fluoranthene 2. Pyrenes
11.2 1.2 2.8 0.4 17.8 11.3 0.3 0.4 0.2 5.5 З.4 2.3 6 4.5 0.2 1.6 0.5 5.3 4.5 0.2 0.2 0.1 0.5 1.9 1.2 7 5.8 0.4 3.1 ND 10.2 8.0o.s
0.2 0.3 2.2 4.3 4.9 2.2 0.4 0.6 2.0 0.1 1.6 1.1 ND 0.3 15 0.3 0.2 0.1 0.2 1.1 0.4 0.1 0.1 0.2 1.5 0.3 о. 5 0.6 6,1 0.4 16 о. 5 0.1 0.4 1.1 2.2 0.8 о 0.1 о 1.8 2.1 0.3 3.1 5.2 1.8 17 о о о о о о 0.4 0.1 о о 00 о о о о 8 1.7 ND 2.0 ND 2.4 3.0 0.1 0.1 ND 0.4 З.8 4.4 9 13.О 1.8 2.0 ND 4.3 2.0 0.3 ND 0.1 4.7 ND ND 0.4 0.5 0.3 0.9 2.0 1.9 18 о о о о 0.1 о о о о 0.1 о о о о о 19 о. 5 0.6 0.3 7.3 1.7 1.8 0.2 0.3 0.3 58 1.7 0.9 0.6 8.3 1 10 4.7 ND 7.0 о. 7 5.S 3.8 0.4 ND ND 3.0 1.5 0.3 о. 5 6.8 0.9 20 21 1.1 О 0.2 9.8 0.1 12.7 0.9 14.5 1.6 81.0 0.4 О 1.2 О 0.1 22 0.1 36 1.1 78 0.7 120 ND 79 27 2.1 4.3 205 1.6 834. Chrysene 5. Benzo(b)fluoranthene 7. Benzo(a)pyrene 8. Dibenzo(a,h)anthracene 10.Indeno(l,2,3,c,d)pyrene
3. Benzo(a)anthracene 6. Benzo(k)fluoranthene 9. Benzo(g,h,i)perylene 11. Bromine
12. Calcium 13. Chlorine 14. Potassium 15. Copper 16. Lead
18. Nickel 19. Iron 17. Hanganese 20. Zinc 21. Sulphur
i. Polycyclic aromatic hydrocarbons (РАН). In contrast to the other compounds the analysis of РАН has not been performed for all the different burning conditions. The results given in tаЫе 4 are mean values of three measurements. Each measurement represents the average emission for one full burn cycle. The analysis is restricted to the compounds with structures built up of 5 and 6 rings. These are the most toxic ones, but also the least volatile
ones. .
In relation to the volatility we were not sure that оuг sarnpling procedure did not give losses of lower molecular РАН and so we decided to omit the analysis results of them. The emission of РАН
appears to Ье dependent on the fuel as well as the stove type. The highest values are found for bituminous coal fuels combusted in the woodstove and the lowest values fог anthrac ite combusted in the coal stove. Note, that again the bad fire of anthгacite in
the fiгeplace caused an exceptional emission.
j. Elements. The element analyses wi th the aid of proton induced
Х-га~ emission has the lowest detection limit for elements with
atom1c numbers ranging from 40 to 60 (7 ,8]. As expected, the emissions were highest for the different coal types. Only from the fireplace we observed а consideraЫe emission of silicium and from the coal stove of molybdenum. These elements could Ье ducts of the stoves and not of the fuels. Bituminous coal 1 pro-duced an emission of selenium of about 0.1 mg/kg fuel.
It can not Ье explained why the emission factors for the elements can vary in such а way from stove to stove.
DISCUSSION AND CONCLUSIONS
The results show that consideraЫe differences in emissions are occurring depending on the type of fuel and stove, and depending on the burn rate. Prior to making definite conclusions it is necessary to notice the accuracy of the presented emission factors. For doing this а number of measurements were carried out in duplicate and frorn the values found the standard devi ations were calculated (ТаЫе 5). It seems that only the inorganic compounds sulphur dioxide and nitro-gen oxides can Ье measured with reasonaЫe reproduciЬility, but that for the organics the results are far more widespread as сап Ье explained Ьу the accuracy of sampling and analysis.
TABLE 5 Cornponent Sulphur dioxide Nitrogen oxides Carbon monoxide Kethane Paraffinics Olefinics Aromatics Polycyclics Standard deviation (~) 12.4 10.8 45 55 73 68 78 67
In spite of the strict standardization of the combustion test proce~ dure the reproduciЬility of the measurements is poor. It seems that the combust1on of solid fuels is а process which is hardly repro-duciЫe, and since there are nearly infinite realisations of the com-bustion procedure, i t can Ье expected that, in real i ty, the emi ss ion factors will vary more than has been found in this study.
As а consequence of the bad reproducibility of the measurements con-clusions can only Ье made carefully and based on clear differences in emission factors.
Concerning the stoves it is observed, that the coal stove gives less emission than the fireplace and woodstove. The fireplace produces1 as is known from literature f2], less pollutants than tne airt1ght woodstove. It сап Ье noticed, that the emission of the woodstove is maximal at full load burn rate, and is optimal at half load. In
pooгest гesults at quaгteг load. The emission of the fiгeplace is
haгdly dependent of the buгn rate and that is undeгstandaЬle, because
only the siz.e of the fiгe vaгies and not the heat pгoduction рег
square centimeteг .
. The high emi ssion of pollutants of both the woodstove and coalstove
сап Ье attгibuted to incomplete combustion as а гesult of oxygen
de-ficiency of the fiгe when lhe аiг inflow is used to contгol the size
of the fire. In the woodstove, this is also occurгing during а рагt
of the burn cycle caused Ьу а fast pyrolysis of the fuel at higheг
tempeгatuгes (figure 1). This is not happening in the coal stove, because of the automatic continuous addition of the fuel.
The Ьi tuminous coals are found the least clean fuels and the brown
coals the best ones. Fiгewood has а medium posi tion wi th favouraЫe
emission factors for sulphur dioxide, nitrogen oxides and elements.
Compared to emission factors found in the literature, there exists а
remarkaЫe difference between our values for the emi ss ion of
polycy-cli c aromatic hydrocarbons and two Euгopean studies [9,10] and
reasonaЫe accordance wi th American values (11, 12]. The low
emis-sion factors given Ьу the Euгopean studies сап possiЫy Ье attributed
to the fact, that in those expeгiments sampling took only place in
the second рагt of the burn cycle.
ACKNOWLEDGEMENT
А study of this size requires contributions of many people. I thank
them all for their cooperation wi thout mentioning their names, wi th
the exception of Н. Claessens, who was responsiЫe for the analysis
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