PARTICLES

PARTICLPARTICLESES

Authors: Dr. Bajnóczy GáborKiss BernadettTonkó Csilla

BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS

DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS ENGINEERING

FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING

The pictures and drawings of this presentation can be used only for education !

Any commercial use is prohibited !

ParticlesParticlesProperties: solid and/or liquid state small size varied composition can cause significant environmental damage

Aerosol: solid and/or liquid particles, dispersed in gasSmoke: solid particles, dispersed in gas Fog: liquid drops dispersed in gas

Classification: Alive or lifeless material

alive: bacterium, fungi, spores lifeless: soot/smut, dust, sea salt, etc. (deal with them)

particles origin: Primary: direct in the course of some process, e.g. condensation, erosion Secondary: from gaseous material

Harmful effects of particlesHarmful effects of particles Particles penetrate deeply into the respiratory

system: Bigger than molecule, but not in settling size

range Get deeper than molecule (slow Brownian motion)

Increase the effect of toxic gaseous air pollutants: SO2, NO2 etc. absorb at the particles surface - get deeper

Increasing turbidity of atmosphere Visibility decreasing Warming effect of solar radiation is restricted

Gaseous material → solid particles. NH3 + SO2 → ammonium sulfate

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

• background aerosol.Bubble burst → salt crystals

Natural source of Natural source of particlesparticles

Natural source of particlesNatural source of particles (2) Volcanic eruption:

variable quantity – sometimes significant amount is emitted into the atmosphere

Volcanic ash: due to small size, long residence time in the atmosphere

(3) Natural source of hydrogen sulphide → sulfuric acid and ammonium → ammonium sulphate particles

(4) soil erosion dust (5) forest fire.

wind

Particles from human Particles from human activitiesactivities

Different industrial processes (stone crushers, grinding, metallurgy, cement and lime production, etc.) and the result of energy generation from coal.

Less, but not negligible: waste and agricultural burning and transportation.

Size of particlesSize of particles

Size range: 0,0002 – 5000 μm Atmosphere pollution: 0,001 – 100 μm.

Size depends on source: >10 μm: usually from mechanical processes (grinding, breaking,

erosion etc.) 10 – 0,1 μm: result of burning process

In atmosphere:two main group1. 10 μm ≥ particles PM10

2. 2,5 μm ≥ particles PM2,5

Another classification:1. Huge particles: diameter > 1 μm2. Large particles: 0,1 = < diameter >

= 1 μm3. Aitken particles: diameter < 0,1 μm

Dangerous range for the lungs

Particle size in micrometer

Particle size in micrometer

Formation of particlesFormation of particles

Soot Carbon-rich (55-80 %), agglomerate of 10-80 nm particles Size: may be higher than 10 μm

Compound: carbon, hydrocarbon, S and N content compounds, microelement

Carbon content compound → soot

Simplified process of soot Simplified process of soot formationformation

Initial process is similar to PAH formation, thus the soot always contains carcinogenic PAH substances.

Soot in flameSoot-C + O → COSoot-C + OH* → CO + HSoot-C + NO → CO + NSoot-C + H2O → CO + H2

Formation of particlesFormation of particles

Ash Origin: coal and biomass firing The fuel does not contain ash. It forms from the ash forming

compounds during the combustion. Coal firing: a./ acidic ash forming: SiO2, Al2O3

b./ basic ash forming: CaO, Fe2O3, MgO

Biomass firing The ash forming materials mainly SiO2 and KCl

The ash forming materials may imbedded in the solid fuel or may be individual, separated particles.

Behavior of ash forming particlesBehavior of ash forming particles Individual inorganic particles quickly melt

and evaporate and will condensate in amorf or crystal form on the cooler part of the boiler (heat exchanger)

Similar, but delayed process takes place with the imbedded inorganic particles.

Metal evaporation from the carbon

containing fuel, due to the metal oxide reduction by carbon at high temperature.

The metal steam is oxidized by the oxygen

content of flue gas forming metal oxide particles mainly in PM2.5 range

The non volatile inorganic matters form

PM10 particles.

potassium chloride crust in heat exchanger – frequent by burning of herbaceous plants

FATE OF PARTICLESFATE OF PARTICLES FROM THE FROM THE ATMOSATMOSPHERPHEREE

All particles deposit on the soil surface. Differences are only in the residence time

in the atmosphere The residence time depends on the particle

size Two possibilities to deposit on the soil

surface Dry deposition Wet precipitation

Dry sedimentation

particle sizeμm

Sedimentation velocitycm/sec

< 0.1 negligible

0,1 8x10-5

1,0 8x10-3

10 0,3

100 25

FATE OF PARTICLES FATE OF PARTICLES FROM THE FROM THE ATMOSATMOSPHERPHEREE

DRY DEPOSITIONDRY DEPOSITION

larger particles will have the opportunity to reach the soil by dry sedimentation

upward air currents counteract the deposition

20 % of the atmospheric particles leave the atmosphere by dry deposition

FATE OF PARTICLES FATE OF PARTICLES FROM THE FROM THE ATMOSATMOSPHERPHEREE

WET PRECIPITATIONWET PRECIPITATION

RAIN OUTWASH OUT

Falling rain or snow collects particles from the atmosphere and carries them to the earth‘s surface

Particles serve as site(nuclei) on which watercondenses or ice form

Particle size > 0.1 μm Particle size < 0.1 μm

These small particles,Aitken particles are sosmall that due to the impact pressure in frontof the falling raindrop,the Aitken particles bypass the drop.

RAIN DROP

EFFECT OF PARTICLES EFFECT OF PARTICLES ON ON PLANTSPLANTS

Reduction of photosynthesis: particles on the leaf surface reduce the irradiation. 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 PARTICLESHEALTH EFFECT OF PARTICLES

Particles, which penetrate the human body: themselves toxic content adsorbed toxic material

Harmful effect – size-dependent

Most of them fix in nose

or deposit on upper trachea Cilia movement→ deposited particles travels up →

pharynx

particle size > 7-10 μm

HEALTH EFFECT OF PARTICLESHEALTH EFFECT OF PARTICLESparticle size in range 0,1- (7-10) μm

Most dangerous particle size range:

Gets down into the alveoli

Settles on the gas exchange surfacedecreasing the oxygen – carbon dioxideexchange

There is no cleaning mechanismin the alveoli

mining disease: silicosisparticle size < 0.1 μm

There is no sedimentation during inhalation and exhalation.

Particle elimination techniquesParticle elimination techniques Taking into account

1. Gas flow parameters: limited opportunities at high temperature and pressure than for atmospheric pressure and below 200 °C

2. Particle parameters: size and material quality

BASED ON EXTERNAL FORCE

BASED ON BARRIERS

cyclon wet dust separator

electrostatic precipitator

decreasing particle size bag filter

gravitational settling chamber

filter-bed with granulates

Efficiency of particle Efficiency of particle separatorsseparatorsEfficiency of elimination

< 1μm 1-3 μm 3-10 μm > 10 μm

Electrostatic precipitator 96 98 99 99,5

Bag dust filter ~100 ~100 ~100 ~100

Venturi-washer >70 99 >99 >99

Multi cyclone 11 54 85 95

Gravitational settling chamberGravitational settling chamber The simplest, but least effective

effective removal: particle size >50 μm used in water purification

CYCLONECYCLONE ~15-30 m/sec gas velocity

centrifugal force is applied

several g/m3 particle content can be decreased under 0,1 g/m3

Advantage: Wide temperature range, even T >1000 °C. High efficiency (η=95%) and cheap

Disadvantage: particle size < 10 μm, efficiency decreases drastically

Better efficiency: more cyclone serial or parallel connection (multi cyclone)

particles

purified gas

dusty gas

ELECTROSTATIC PRECIPITATORELECTROSTATIC PRECIPITATOR Flue gas cleaning method of plants using solid fossil fuels.

Advantage: simple structure without moving parts good efficiency low electric energy demand efficiency: 0,2-0,5 μm particles : 97-98% particles size >2 μm : >98% removal efficiency is influenced by temperature (preferred

interval: 120 – 200 °C) and the specific resistance of particle

Theory of electrostatic Theory of electrostatic precipitatorprecipitator

Ionization voltage < voltage gradient < breakdown voltage

ELECTROSTATIC PRECIPITATORELECTROSTATIC PRECIPITATOR

spec

ific

elec

tric

resi

stan

ce [o

hm*c

m]

105 -105 -

105 -

1010 -

no charge upexcess of

CaO, MgO, SiO2, Al2O3

quick charge lossexcess of Fe2O3, Na2O, H2O

BEST RANGE

Bag dust filterBag dust filter The best efficiency dust separator

(> 1μm particles: ~99%) Application: 120°C – 200°C Filter cake thickening results in better efficiency

Advantage: 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: Filter cake thickening → filter

resistance increases → batch filtration technology

elimination of filter cake with shake or back pressure