Combustion characteristics of commercial burner for ...

Back to Table of Contents THIRD EUROPEAN COMBUSTION MEETING ECM 2007

Combustion characteristics of commercial burner for home appliance G. Toniato1, C. Accordini1, A. D’Anna2∗, M. Commodo2, P. Minutolo3, R. Pagliara2 1

2

Riello S.p.A. – Burner Division – Italy Dipartimento di Ingegneria Chimica Università “Federico II” di Napoli – Italy 3 Istituto Ricerche sulla Combustione – CNR- Italy

Abstract An experimental study of the combustion characteristics of new burners used for home appliances is presented with the aim of evaluating the effect of burner configurations and operating excess air on the emissions of gaseous pollutants and particulate matter. Advanced in-situ optical diagnostics and ex-situ measurements of the particulate concentration and size distribution function are used in three burner configurations: two premixed and one diffusive burning methane as fuel. The premixed burners are differentiated by the combustion head: a Knitted metal and a drilled cylindrical head have been used. For each burner configuration, various combustion condition have been analysed by changing the air/fuel fraction from the stoichiometric value up to 31% excess air. Measurements have shown that particulate matter with size in the 1nm – 10nm size range is formed in all the examined conditions. The emitted mass concentration of these compounds is very low, of the order of 0.01ppm. They are formed in large number concentrations in the flame region but are also strongly oxidized in the post-oxidation region of the flames. Particles in the 10nm – 100nm range are not formed in the examined conditions . Introduction Methane is generally believed to be the cleanest burning hydrocarbon fuel, nevertheless, in practical system, non-optimized combustion conditions could promote undesirable by-products formation and pollutant emission in the atmosphere. Due to the wide application of methane burners, it is of great interest to develop even more sophisticated practical combustors that reduce pollutant emission. In this work we present an experimental study of the combustion characteristics of new burners used for home appliances. The aim is to evaluate the effect of burner configurations and operating excess air on the emissions of gaseous pollutants and particulate matter. Three configurations have been studied. Two premixed and one diffusive burning methane as fuel. The premixed burners are differentiated by the combustion head: a knitted metal fibre manufactured into a unique patented “sock” without any welded seams, and a drilled cylindrical head. The diffusion burner is a five-tube injector of gaseous hydrocarbons [1]. For each burner configuration, various combustion condition have been analysed by changing the air/fuel fraction from the stoichiometric value up to 31% excess air. Experimental The burners have been operated in free atmosphere and in commercial boilers. The flames stabilized in free atmosphere have been characterized by thermocouple measurements (Pt/Pt-Rh thermocouple with a bead diameter of 300μm) and by in-situ optical techniques for the identification of the main flame structure and the formation of particulate matter. ∗

Corresponding author: [email protected] Proceedings of the European Combustion Meeting 2007

Laser induced emission measurements have been performed at different heights above the burner surface to follow the flame evolution. The fifth harmonic (λ0=213nm) of a pulsed Nd-YAG laser has been used as the exciting source. The energy of the laser pulse is kept constant at 1.5mJ and the pulse duration is 8ns. Detection is by a gated ICCD camera and the signal is focused onto a 280 μm entrance slit of a spectrometer. The choice of the fifth harmonics far in the UV allows us to readily excite electronic transitions in a variety of aromatic intermediates resulting in fluorescence (LIF). Also, 213nm light is readily absorbed by soot particles producing laser induced incandescence (LII) if soot is present. The emission signal at 305nm is used to detect OH concentration along the flame axis. A correction is applied for the wavelength-dependent sensitivity of the camera. Calibration of laser light scattering signal at 213nm and of laser induced emission signals is made by measuring the scattering signal for cold ethylene. Both LIF and LII signals have been calibrated on a premixed ethylene/air flame where the concentration of high-molecular-mass precursor particles and soot have been obtained by broadband UV-visible absorption measurements. Scattering measurements have been also performed. Known the volume fraction of particulates in the flame, the mean size of the particles can be retrieved from the scattering measurements. The concentration of particulate matter exhausted from the flame has been measured by collecting the particles by means of a water-based sampling technique and analyzed by absorption and fluorescence measurements in water suspensions. Combustion gases, sampled by a probe in the exhaust pipe, are cooled in order to condense combustion water and drawn through


Results and Discussion Temperature, OH emission and Laser Induced Emission measurements have been performed along the axis of the two premixed flame burners at two power conditions, namely 8kW and 16kW, changing the excess air fed to the burner from the stoichiometric conditions (0% excess air) to 31% excess air. Figure 1 reports the OH and particulate concentration profiles measured along the axis of the knitted metal fibre burner fuelled with methane at 16kW and 0% excess air. OH emission (in arbitrary units in the graph) reaches a maximum close to the burner exit and decreases to the equilibrium value in 10mm from the burner surface. Maximum OH concentration is well correlated with the

maximum temperature measured in the flame and is representative of the main oxidation region of the flame. Close to the burner exit, a fluorescence signal has also been measured. It is initially very low and sharply increases reaching a maximum value at 20mm, i.e. just downstream of the flame front. The laser induced incandescence signal in this flame condition is below the detection limit. 0.20

2100

1900 0.15 1700

T, K

volume fraction (ppm)

a reservoir containing deionised water, placed in an ice bath. This type of sampling procedure allows the collection of very small particles which have more affinity with water respect to soot. Water samples, put in a standard 1cm path-length quartz cell, have been analyzed by light absorption and UV-induced fluorescence measurements. The steady state absorption spectra were recorded using a deuterium lamp in the 200-500 nm wavelength region. The steady state fluorescence spectra were preformed using the forth harmonic (266 nm) of a Nd:YAG pulsed laser with a pulse duration of 7ns. The size distribution functions of the particles in the exhaust pipe have been determined by measuring the differential mobility of charged particles with a nano differential mobility analyzer TSI Model 3936 SMPS. This apparatus is specifically designed to measure particles in the 3–50 nm range. The SMPS system consists of a diffusion charger (Kr-85 Bipolar), a nanodifferential mobility analyzer (NDMA, type TSI 3085) and an ultrafine condensation particle counter (UCPC, type TSI 3025A). A suction probe is used to sample particles from the exhaust gases diluted with an air flux. The expected loss of particles from coagulation is very low due to the low number concentration of particles in the analyzed conditions. The loss of particles due to diffusion during the time interval in the probe has been estimated to be below 10% for particles in the 3–10 nm. The SMPS was operated in high-flow mode (aerosol flow set at 1.5 L/min and sheath flow at 15.0 L/min). The total counting time required for each sample was about 3 min. On-line standard measurements have been also performed on the exhaust gases for the determination of the concentrations of CO, unburned hydrocarbons and NOx. CO2 measurements have been used for the determination of exhaust gas dilution in the exhaust pipe. Measurements have been also performed at the exhausts of commercial boilers equipped with the same burner used for the laboratory measurements. Determination of gaseous compounds and of particulate matter in the water condensed from the combustion gases have been performed.

0.10

1500 0.05

exh

1300

OH 0.00 0

50

100

150

1100 200

h, mm

Figure 1: Thermocouple temperature, OH emission and particulate volume fraction measured at different height from the burner surface (metal burner in premixed conditions at 16kW and 31% excess air). Also reported is the concentration of particulate in the exhaust pipe.

The LIF signal can be due to fluorescing gas-phase species or to high-molecular-mass precursor particles as detected in diffusion flames [2]. Excess scattering with respect to gas-phase compounds, not reported in the figure, is measured in the flame region where fluorescence is detected indicating that the fluorescing species are not gas-phase aromatics but are probably high molecular mass compounds. Indeed, scattering is proportional to the number concentration of these compounds and to the sixth power of their size. Fluorescence measurements have been therefore used to estimate the volume fraction of aromatic compounds in the flame. A calibration of the LIF signal has been attempted in premixed flames of ethylene where the concentrations of high-molecular mass compounds and soot were known from broadband extinction measurements. By dividing the excess scattering and the LIF signal an indication of the size can be obtained. A mean size of the order of 2-3nm is estimated indicating that fluorescing species are high molecular mass compounds which contains highly absorbing chromophoric groups. The size values are in close agreement with the size of Nanoparticles of Organic Carbon (NOC) measured in laminar premixed and diffusion flames and at the exhausts of practical combustion systems [3]. The concentration of aromatic particles in this flame reported in Fig.1 has been obtained by calibrating the LIF signal in a laboratory premixed flame [4]. After the maximum value of the order of 0.1ppm reached downstream of the flame front, the volume fraction of the nanosized particles decreases to very low values of the order of 0.07ppm.


10

metal premix

mg/kWh

8

6

4

2

0

0%

6%

19%

31%

10

drilled premix

mg/kWh

8

10% [5]) and the dilution of the combustion gases in the exhaust pipe. The measured value is about one order of magnitude lower than that measured at 80mm above the burner in flame by optical technique indicating a continuous oxidation of these precursor particles in the post-oxidation region of the flame. Figure 2 reports the concentrations of particulate estimated with the water-based sampling technique at the exhausts of the three flame configurations: the two premixed combustion head characterized by the knitted metal fibre and the drilled cylindrical head and the diffusive one. In the following we will name metal premix, drilled premix and diffusive burning to distinguish the three flames. The measurements have been performed changing the excess air in the flame from 0% to 31% at 8kW and 16kW.

6

1.E+08 4

2

0

0%

6%

19%

31%

10

diffusive

1.E+06

1.E+05

1.E+04

8

mg/kWh

dN/dlogDp, cm-3

1.E+07

1.E+03

6

1

10

100

DP, nm 4

Figure 3: Size distribution function of the particles in the exhaust pipe as measured by SMPS analysis (16kW and 19% excess air ∆ metal premix; drilled premix; o diffusive, continuous line ambient air)

2

0

0%

6%

19%

31%

% excess air

Figure 2: Concentrations of particulate (mg/kWh) estimated with the water-based sampling technique at the exhausts of the three flame configurations (light grey 8kWh, heavy grey 16kWh).

Measurements of particulate have been performed in the exhaust pipe by collecting the particles either in the condensed combustion water and in the water trap. The light absorption spectrum of the sample collected from the exhaust of methane combustion presents two strong absorption bands centred at about 210 and 260nm superimposed to a continuous background. The first absorption band is typical of nitrogen-containing compounds, such as nitric acid, deriving from NOx interaction with water in the exhaust system. The second band is typical of aromatic molecules containing 1-, 2-aromatic rings. After the partial evaporation of the sample the second absorption band is almost completely lost, on the contrary, the continuous background of absorption is still present and the absorption spectrum becomes very similar to the light absorption spectrum of NOC particles measured in rich flames [3]. A concentration of particulate in water sample can be estimated; values of the order of 0.03ppm have been estimated for the flame conditions reported in Fig.1 taking into account the collection efficiency of the water-based sampling techniques (estimated to be about

The three burner configurations form very low concentrations of particulate (below 10mg/kWh); the lowest concentration values are obtained with the two premixed burners. The increase of the burner power from 8kW to 16kW slightly increases the amount of particulate particularly in the diffusion burner. The increase of the excess air doesn’t modify significantly the emission of particulate in the analyzed conditions. The size distribution function of the particles have been measured in the exhaust by SMPS analysis. Figure 3 reports the size distribution functions measured at 16kW and 19% excess air with the three burners. Particle diameters range from 3nm, the limit of the detection system, up to 100nm. In the same figure the size distribution function of the particles collected in the ambient air is also reported. In the 10nm – 100nm range the number concentration of the particles measured at the exhaust of the combustion system is of the same order of magnitude of the number concentration of the particles present in the ambient air. We can, therefore, conclude that the oxidation of methane in the conditions of the three burners doesn’t produce particles larger than 10nm. This is in perfect agreement with the optical measurements which show that in all the examined conditions laser induced incandescence signals due to particles larger than 10nm are not measured. However, a large number concentration of very small particles in the


nanometric range are formed. Due to their very low sizes they do not contribute to the particulate emission (mass based concentration). The premixed burners form less particle number concentrations than the diffusive one and even the metal fibre forms less nanoparticles than the drilled cylindrical head. It is worth noting than the mass concentration of these compounds as determined by water-based sampling measurements and SMPS determination are very low, of the order of 0.01ppm. 70

60

NOx, mg/kWh

50

40

30

20

10

0

33 kW

16 kW

9 kW

33 kW

16 kW

9 kW

33 kW

16 kW

9 kW

250

CO mg/kWh

200

150

100

50

0

5

Particulate, mg/kWh

4

3

2

1

0

Figure 4: Emission indexes (mg/kWh) for NOx, CO and nanosized particulate 9kW, 16kW and 33kW at the exhausts of commercial boilers: light grey (measurements performed with a fixed excess air of 31%)

Measurements have been performed also at the exhausts of commercial boilers equipped with the two premixed combustion burners which have been tested on the laboratory rig. Figure 4 shows the emission indexes (mg/kWh) for NOx, CO and nanosized particulate 9kW, 16kW and 33kW. The measurements have been performed with a fixed excess air of 31%. The emission of NOx and CO from the two burners are quite similar. NOx emission remains also practically

unchanged for the three loads. As expected, CO emission increases for increasing load. The emission of particulate is also quite constant for the three loads and burner types. The comparison of commercial boilers exhausts with the measurements performed at the exhaust of the burners in the laboratory rig shows that particulate is continuously oxidized in the post-oxidation region of the boiler reaching values at the exhaust of commercial systems as low as 1 mg/kWh. Conclusions An experimental study of the combustion characteristics of new burners used for home appliances has been presented with the aim of evaluating the effect of burner configurations and operating excess air on the emissions of gaseous pollutants and particulate matter. Advanced in-situ optical diagnostics and ex-situ measurements of the particulate concentration and size distribution function have been used. Three configurations have been studied. Two premixed and one diffusive burning methane as fuel. The premixed burners are differentiated by the combustion head: a metal and a drilled cylindrical head have been used. The diffusion burner is a five-tube injector of gaseous hydrocarbons. For each burner configuration, various combustion condition have been analysed by changing the air/fuel from the stoichiometric value up to 31% excess air. Measurements have shown that particulate matter with size in the 1nm – 10nm size range is formed in all the examined conditions. The emitted mass concentration of these compounds is very low, of the order of 0.01ppm. They are formed in large number concentrations in the flame region but are also strongly oxidized in the post-oxidation region of the flame of commercial boilers. Particles in the 10nm – 100nm range are not formed in the conditions examined. Even the number concentration of the particle measured at the exhaust of the combustion system is of the same order of magnitude of the number concentration of the particles present in the ambient air. The complete absence of particles larger than 10nm is confirmed by the optical measurements which show the complete absence of incandescence signals due to soot particles. References 1. Toniato, G., Zambon, A., Lovato, A., Tomasetto, M., Mazzacavallo, G., 29 Meeting of the Italian Section of the Combustion Institute, paper I-3, Pisa, June 14-17, 2006. 2. D’Anna, A., Commodo, M., Violi, S., Allouis, C., Kent, J., Proc. Combust. Inst. 31:621-629 (2007) 3. Sgro. L.A., Basile, G., Barone, A.C., D’Anna, A., Minutolo, P., Borghese, A., D’Alessio, A., Chemosphere 51(10):1079-1090 (2003). 4. Commodo, M., Violi, A., D'Anna, A., D'Alessio, A., Allouis, C., Beretta, F., Minutolo, P., Combust. Sci. Technol. 179:387-400 (2007) 5. Minutolo, P., Bruno, A., Galla, D., D’Anna, A., D’Alessio, A., Advanced Atmospheric Aerosol Symposium, Milano 12-15 November 2006.