BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS FACULTY OF
BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS ENGINEERING PARTICLES Authors: Dr. Bajnóczy Gábor Kiss Bernadett Tonkó Csilla
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Particles Properties: n solid and/or liquid state n small size n varied composition n cause significant environmental damage Aerosol: solid and/or liquid particles, dispersed in gas Smoke: solid particles, dispersed in gas Fog: liquid drops dispersed in gas Classification: n n Alive or lifeless material n alive: bacterium, fungi, spores n lifeless: soot/smut, dust, sea salt, etc. (deal with them) particles origin: n Primary: direct in the course of some process, e. g. condensation, erosion n Secondary: from gaseous material
Harmful effects of particles n n Particles penetrate deeply into the respiratory system: n Bigger than molecule, but not in settling size range n Get deeper than molecule (slow Brownian motion) Increase the effect of toxic gaseous air pollutants: SO 2, NO 2 etc. absorb at the particles surface - get deeper n Increasing turbidity of atmosphere n Visibility decreasing n Warming effect of solar radiation is restricted n Gaseous material → solid particles. n NH 3 + SO 2 → ammonium sulfate
Natural source of particles n n Primary emission (the particles get direct into the atmosphere) Secondary emission (the particles formed from gaseous state in the atmosphere) • (1. ) The maximum quantity is natural origin from seawater • seawater surface → bubble → burst → → wind → salt crystals after evaporation Bubble burst → salt crystals • background aerosol.
Natural source of particles n (2) Volcanic eruption: n variable quantity – sometimes significant amount is emitted into the atmosphere n Volcanic ash: due to small size, long residence time in the atmosphere n (3) Natural source of hydrogen sulphide → sulfuric acid and ammonium → ammonium sulphate particles n (4) soil erosion n (5) forest fire. wind dust
Particles from human activities n Different industrial processes (stone crushers, grinding, metallurgy, cement and lime production, etc. ) and the result of energy generation from coal. n Less, but not negligible: waste and agricultural burning and transportation.
Size of particles n Size range: 0, 0002 – 5000 μm Atmosphere pollution: 0, 001 – 100 μm. n Size depends on source: n n n >10 μm: usually from mechanical processes (grinding, breaking, erosion etc. ) 10 – 0, 1 μm: result of burning process In atmosphere: two main group n 1. 2. 10 μm ≥ particles PM 10 2, 5 μm ≥ particles PM 2, 5 Another classification: 1. Huge particles: diameter > 1 μm 2. Large particles: 0, 1 = < diameter > = 1 μm 3. Aitken particles: diameter < 0, 1 μm
Particle size in micrometer Dangerous range for the lungs Particle size in micrometer
Formation of particles Soot n Carbon-rich (55 -80 %), agglomerate of 10 -80 nm particles n Size: may be higher than 10 μm n Compound: carbon, hydrocarbon, S and N content compounds, microelement n Carbon content compound → soot
Simplified process of soot formation Initial process is similar to PAH formation, thus the soot always contains carcinogenic PAH substances. n Soot in flame Soot-C + + O → CO OH* → CO + H NO → CO + N H 2 O → CO + H 2
Formation of particles Ash n Origin: coal and biomass firing n The fuel does not contain ash. It forms from the ash forming compounds during the combustion. n Coal firing: a. / acidic ash forming: Si. O 2, Al 2 O 3 b. / basic ash forming: Ca. O, Fe 2 O 3, Mg. O n Biomass firing The ash forming materials mainly Si. O 2 and KCl n The ash forming materials may imbedded in the solid fuel or may be individual, separated particles.
Behavior of ash forming particles n Individual inorganic particles quickly melt and evaporate and will condensate in amorf or crystal form on the cooler part of the boiler (heat exchanger) n Similar, but delayed process takes place with the imbedded inorganic particles. n Metal evaporation from the carbon containing fuel, due to the metal oxide reduction by carbon at high temperature. n The metal steam is oxidized by the oxygen content of flue gas forming metal oxide particles mainly in PM 2. 5 range n The non volatile inorganic matters form PM 10 particles. npotassium chloride crust in heat exchanger – frequent by burning of herbaceous plants
FATE OF PARTICLES FROM THE ATMOSPHERE n n All particles deposit on the soil surface. Differences are only in the residence time in the atmosphere n n The residence time depends on the particle size Two possibilities to deposit on the soil surface n Dry deposition n Wet precipitation
FATE OF PARTICLES FROM THE ATMOSPHERE DRY DEPOSITION upward air currents counteract the deposition larger particles will have the opportunity to reach the soil by dry sedimentation Dry sedimentation particle size μm Sedimentation velocity cm/sec < 0. 1 negligible 0, 1 8 x 10 -5 1, 0 8 x 10 -3 10 0, 3 100 25 20 % of the atmospheric particles leave the atmosphere by dry deposition
FATE OF PARTICLES FROM THE ATMOSPHERE WET PRECIPITATION WASH OUT RAIN DROP RAIN OUT Particle size > 0. 1 μm Particle size < 0. 1 μm Falling rain or snow collects particles from the atmosphere and carries them to the earth‘s surface Particles serve as site (nuclei) on which water condenses or ice form These small particles, Aitken particles are so small that due to the impact pressure in front of the falling raindrop, the Aitken particles bypass the drop.
EFFECT OF PARTICLES ON PLANTS n Reduction of photosynthesis: particles on the leaf surface reduce the irradiation. n n This effect was significant in the vicinity of former cement factories. The dust settled on the surface of leaves or fruits may contain toxic matters. The consumption of these plants by humans or animals might be unhealthy
HEALTH EFFECT OF PARTICLES particle size > 7 -10 μm n Particles, which penetrate the human body: n themselves toxic n content adsorbed toxic material n Harmful effect – size-dependent Most of them fix in nose or deposit on upper trachea Cilia movement → deposited particles travels up → pharynx
HEALTH EFFECT OF PARTICLES particle size in range 0, 1 - (7 -10) μm Most dangerous particle size range: Gets down into the alveoli Settles on the gas exchange surface decreasing the oxygen – carbon dioxide exchange There is no cleaning mechanism in the alveoli mining disease: silicosis particle size < 0. 1 μm There is no sedimentation during inhalation and exhalation.
Particle elimination techniques n Taking into account 1. 2. Gas flow parameters: limited opportunities at high temperature and pressure than for atmospheric pressure and below 200 °C Particle parameters: size and material quality BASED ON EXTERNAL FORCE filter-bed with granulates gravitational settling chamber cyclon electrostatic precipitator BASED ON BARRIERS wet dust separator decreasing particle size bag filter
Efficiency of particle separators Efficiency of elimination < 1μm 1 -3 μm 3 -10 μm > 10 μm Electrostatic precipitator 96 98 99 99, 5 Bag dust filter ~100 Venturi-washer >70 99 >99 Multi cyclone 11 54 85 95
Gravitational settling chamber n The simplest, but least effective removal: particle size >50 μm used in water purification
CYCLONE n ~15 -30 m/sec gas velocity n centrifugal force is applied n several g/m 3 particle content can be decreased under 0, 1 g/m 3 n Advantage: n Wide temperature range, even T >1000 °C. n High efficiency (η=95%) and cheap n Disadvantage: particle size < 10 μm, efficiency decreases drastically n Better efficiency: more cyclone serial or parallel connection (multi cyclone) purified gas dusty gas particles
ELECTROSTATIC PRECIPITATOR n Flue gas cleaning method of plants using solid fossil fuels. Advantage: n simple structure without moving parts n good efficiency n low electric energy demand n efficiency: 0, 2 -0, 5 μm particles : 97 -98% particles size >2 μm : >98% n removal efficiency is influenced by temperature (preferred interval: 120 – 200 °C) and the specific resistance of particle
Theory of electrostatic precipitator Ionization voltage < voltage gradient < breakdown voltage
105 - no charge up excess of Ca. O, Mg. O, Si. O 2, Al 2 O 3 1010 - BEST RANGE 105 quick charge loss excess of Fe 2 O 3, Na 2 O, H 2 O specific electric resistance [ohm*cm] ELECTROSTATIC PRECIPITATOR
Bag dust filter n n n The best efficiency dust separator (> 1μm particles: ~99%) Application: 120°C – 200°C Filter cake thickening results in better efficiency Advantage: n n Efficiency of elimination doesn’t depend on the electrical properties of particles. Due to the adsorption on filter cake elimination of gaseous pollutants: e. g. dioxins. Disadvantage: n n Filter cake thickening → filter resistance increases → batch filtration technology elimination of filter cake with shake or back pressure
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