Effect of Flue Gas Recirculation on the Formation of Fine Particulate Matter in a Domestic Pellet-Fired Boiler

Transcription

1 Effect of Flue Gas Recirculation on the Formation of Fine Particulate Matter in a Domestic Pellet-Fired Boiler U. Fernandes, M. Henriques and M. Costa Mechanical Engineering Department, Instituto Superior Técnico Technical University of Lisbon, Lisbon, Portugal Joint Working Group Meeting Soot and PAHs Sorrento, Italy, April

2 Emissions of fine particulate matter (PM) can seriously affect human health. Among the major contributors of fine PM to the atmosphere are the small-scale biomass-fired boilers, which are widely used around the world. In biomass combustion fine PM is produced from incomplete combustion (i.e., unburned matter and/or soot) and from vaporization and condensation of easily volatile ash elements. In principle, the presence of PM in the flue gas can be minimized through combustion modifications. Combustion modifications (e.g., temperature) may, however, enhance the vaporization of the volatile ash elements, which may originate higher ash based PM emissions How about flue gas recirculation? Motivation 2

3 Domestic boiler Pellets Flue gas T1 Termocouple K 12 bit A/D Water network T2 T3 Probe HC analyzer NOx analyzer O2 analyzer CO and CO2 analyzers Air Weighbridge RS 232 BOILER 220 V LPI Condenser Water in Silica gel Water out Cotton filter Diaphragm pump Zero gas and span gas 3

4 Schematic of the domestic boiler with flue gas recirculation 4

5 Pellet characteristics Parameter Value Proximate analysis (% wt, ar) Volatiles 80.5 Fixed Carbon 10.9 Moisture 7.3 Ash 1.3 Ultimate analysis (% wt, daf) Carbon 46.0 Hydrogen 6.2 Nitrogen 0.5 Sulphur < 0.01 Oxygen 47.3 Parameter Value Ash analysis (% wt, db) SiO Al 2 O Fe 2 O CaO 26.2 SO MgO 4.3 P 2 O K 2 O 11.5 Na 2 O 2.5 Cl 0.04 Other oxides 2.3 Low heating value (MJ/kg) 17.1 Average dimensions (mm) Diameter 6 Length 18 5

6 Boiler operating conditions Thermal input (kw) Pellets load (kg/h) Oxidant O 2 (vol %) CO 2 (vol %) Flue-gas O 2 (vol %) EGR (%) Flame stability unchanged with flue gas recirculation 6

7 Emission data (without fgr) PM emissions were dominated by particles with sizes below 2.5 μm and above 10 μm Source: Fernandes, Costa (2012). Particle emissions from a domestic pellets-fired boiler. Fuel Process. Technol., 103,

8 PM composition I (without fgr) Analysis revealed the presence of ultra fine particles composed mainly of O, K, Cl, Na and S that is, particles that resulted from the inorganic material in the fuel ashes... Source: Fernandes, Costa (2012). Particle emissions from a domestic pellets-fired boiler. Fuel Process. Technol., 103,

9 PM composition II (without fgr) and sub-micrometer sized particles/agglomerates contained mainly C, but also O, Ca, Mg, K and P that is, particles that resulted from incomplete combustion (soot) Source: Fernandes, Costa (2012). Particle emissions from a domestic pellets-fired boiler. Fuel Process. Technol., 103,

10 Emission data I Low pressure three-stages cascade impactor 10

11 Emission data II At high flue gas recirculation rates (>30%) PM2.5 decrease, but CO emissions Increase (flame stability and emissions of HC and NO x ) 11

12 PM size distribution (13-stages low-pressure cascade impactor) At high flue gas recirculation rates (>30%) fine PM (< 100 nm) disappear 12

13 Summary and on-going work Our preliminary results indicate that high flue gas recirculation rates may lower fine PM emissions, while not affecting the flame stability and maintaining relatively unchanged the HC and NO x emissions. The CO emissions, however, increase. PM composition analyses are currently being performed and inflame measurements of temperature, gas species concentration and PM are planned. Flue gas recirculation reduces homogeneous condensation? Flue gas recirculation promotes heterogeneous condensation? What is the effect of the flue gas recirculation on soot emissions? Ratio between carbon and in organics in PM? How about air staging combined with flue gas recirculation? 13