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chapter 5 turbulent diffusion flames - FedOA

chapter 5 turbulent diffusion flames - FedOA

After the maximum value

After the maximum value of the order of 0.1 ppm reached downstream of the flame front, the volume fraction of the OC decreases to lower values of the order of 0.07 ppm. The laser induced incandescence (LII) signal in this flame condition is below the detection limit denoting the absence of soot particles. volume fraction (ppm) 0.20 0.15 0.10 0.05 exh 1300 OH 0.00 1100 0 50 100 h, mm 150 200 104 2100 1900 1700 T, K 1500 Fig. 6.2 Temperature, OH emission (□) and NOC volume fraction (▲) measured at different height from the burner surface (metal burner in premixed conditions at 16kW and 0% excess air). Also reported is the concentration of particulate in the exhaust pipe (ext). 6.3.2 EX-SITU MEASUREMENTS: WATER TRAP AND SMPS Measurements of NOC have been performed in the exhaust pipe, point B in the fig. 6.1, by collecting the NOC in the condensed combustion water. The light absorption spectrum of the sample in water suspensions presents a strong absorption bands centred at about 210 nm 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. The continuous background of absorption is very similar to the light absorption spectrum of NOC measured in rich flames (Sgro et al. [30]). A concentration of these species in water sample has been estimated; values of the order of 0.03 ppm have been found for the flame conditions reported in Fig. 6.2 taking into account the collection efficiency of the water-based sampling

techniques (estimated to be about 10%), (Sgro et al. [82]), and the dilution of the combustion gases in the exhaust pipe. The measured value is about one order of magnitude lower than that measured in situ at 80 mm above the burner by optical technique indicating a continuous oxidation of these species in the post flame region. 6.3.3 EMISSION INDEX FOR CONFIGURATION IN COMMERCIAL BOILER Measurements have been performed also at the exhausts of commercial condensing boilers equipped with the two premixed combustion burners which have been tested on the laboratory rig. In the condensing applications, characterized from a high thermal efficiency, the combustion water is directly condensed into the boiler. Measurements have been performed, in this case, sampling and analysing the water condensed into the boiler, point C1 in the Fig. 6.3, and bubbling in a reservoir containing water the exhaust of the commercial condensing boiler, point C2 in the Fig. 6.3. Hot combustion products Cold exhaust Condensing Boiler Combustion water Point C1 Fig. 6.3 Condensing boiler measurements lay out. 105 Point C2 Figure 6.4 shows the emission indexes (mg/Nm3) for NOx, CO at 9 kW, 16 kW and 33 kW. The measurements have been performed with a fixed excess air of 31%. The emission data 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.

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