lectures - gas emissions - b&w

8/12/2019 Lectures - Gas Emissions - b&w

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ENVIRONMENTAL MANAGEMENTENGINEERING

Gas emissions

Prof. Fulvia CHIAMPO

Academic Year 2013-14


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CHARACTERISTIC PARAMETERSFOR FLUE GASES

Composition (% by volume)

Temperature (C)

Pollutants (mg/Nm3)

Micropollutants (g/Nm3)

State of the pollutants (solid, vapor, gas)

Dust (mg/Nm3)

Particle size distribution

Boiling point of the pollutants (C)


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ELEMENTS OF LEGISLATIONFOR FLUE GAS EMISSIONS

The legislation considers the main pollutants emitted with flue gases.

The limits of pollutant concentration change according to theindustrial activity that produces the pollutants themselves.

Normally, the main pollutants considered by legislations are:

- DUST-SO2

-NOx(NO and NO2)

-CO

-HCl and HF (hydrogen chloride and hydrogen fluoride ! Not ACIDS)

-METALS

..


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.....

-Gas and vapor organic substances, expressed as total organiccarbon (TOC)

-PCDD and PCDF (polychlorodibenzodioxins andpolychlorodibenzofurans).

Taking into account that the emissions are gas, the limits must bedefined at a fixed temperature and at a fixed pressure, very often at a

empera ure o , an a a pressure o , a.The emission limits, also those of gases (for example, SO2) arealways expressed in mass/ volume.

The modern legislations contain also:

-the monitoring plan-the control plan

and of course

- penalties (fine and/or arrest).


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PCDDs and PCDFsDIBENZODIOXIN STRUCTURE

75 congeners ofPOLYCHLORODIBENZODIOXINS

DIBENZOFURAN STRUCTURE

135 congeners of

POLYCHLORODIBENZOFURANS


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REMOVAL/SEPARATION

In general, a force must be applied to separate two systems; it canbe:

-gravitational (it is FREE but its efficiency is satisfactory just for

large and heavy particles)

-mechanical (centrifugation, filtration)

-

-intermolecular attractive (adsorption).

C1 C2 C1= INLET MASS CONCENTRATIONC2= OUTLET MASS CONCENTRATION

SEPARATION EFFICIENCY =


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SOLID PARTICLE (DUST) REMOVAL

SEPARATION EFFICIENCY


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CYCLONES

vIN= 15-40 m/s


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INVOLUTE CYCLONE


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AXIAL CYCLONES

Battery of vane axial cyclones


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CYCLONE

DP(m)

0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-15 15-20 20-30 30-40 40-50 50-75

DP,MEAN

(m) 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 12.5 17.5 25 35 45 62.5

fD 1 3 4 5 4 3 3 5 12 15 18 7 8 7 3 2

0 0 0 0 0.05 0.15 0.2 0.35 0.5 0.8 1 1 1 1 1 1

*D

T= fD

fD* = fD(1 )/(1 T)

For this cyclone, T= 0.66 = 66 %

cIN= 3 g/m3 Q = 1200 m3/h

With T= 0.66, cOUT= 1 g/m3 and

SEP= (3 1) 1200 = 2400 g/h REMOVED PARTICLES


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CYCLONE

Q = vINS = vINDODO/2 = vINDO2/2

With vIN= 25 m/s, DO= 0.16 m

DC= 5 DO= 0.8 m L = 8 DO= 1.28 m

30

f(D)

f(D)*

0

10

20

0 10 20 30 40 50

fD,

fD*

DP,MEAN(m)


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FILTRATION

Woven filters


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FILTRATION

Felted filters


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FABRIC FILTER WITHVENTURI CLEANING SYSTEM


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vF= 0.01 0.06 m/s

DB= 0.15-0.7 m

LB= 1.5-4m


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FABRIC FILTER

DP(m) 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-15 15-20 20-30 30-40 40-50 50-75

DP,MEAN(m)

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 12.5 17.5 25 35 45 62.5

fD 1 3 4 5 4 3 3 5 12 15 18 7 8 7 3 2

0.85 0.95 1 1 1 1 1 1 1 1 1 1 1 1 1 1

f*D 50 50 0 0 0 0 0 0 0 0 0 0 0 0 0 0

T= fD

fD* = fD(1 )/(1 T)

For this filter, T= 0.997 = 99.7 %

cIN= 3 g/m3 Q = 1200 m3/h

With T= 0.997, cOUT= 0.009 g/m3 and

SEP= (3 0.009) 1200 = 3589.2 g/h REMOVED PARTICLES


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FILTER

Q = vINS AND with vIN= 0.04 m/s, S = 8.3 m2

S = NDBL

With DB= 0.15 m and L = 1.5 m, we obtain N = 11.8 N* = 12 bags

60

f(D)

0

20

40

0 25 50

fD,

f*D

DP,MEAN(m)


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ELECTROSTATIC PRECIPITATOR


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The gas velocity must be in the range of laminar flow, to avoidsolid particle detachment.


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ADSORPTION


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ADSORPTION is an exothermic surface phenomenon that occurs

between a solid (adsorbent) and a compound (adsorbate) distributedin a gas or in a liquid (solution).

It is a MASS TRANSFER OPERATION.

Adsorbate can be bonded on the adsorbent through weak forces,

such as van der Waals and London forces (physical adsorption more common), or through an exchange of electrons(chemiadsorption) with chemical reaction.

.

Physical adsorption is reversible, with exothermicity 4-40 kJ/moladsorbate.

Chemiadsorption is nonreversible, with exothermicity > 200 kJ/moladsorbate.

Very often, the solid is activated carbon.

Other common adsorbents are: silica gel, zeolites, syntheticpolymeric adsorbents.


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CHARACTERISTICS OF ACTIVATED CARBON

Their origin can be vegetal (very common, from weak woods) ormineral (their use is limited).

According to their morphology, they can be:- Powdered activated carbon (PAC): particle size in the range 1-150m

- Granular activated carbon GAC : article size in the ran e 0.5-4 mm

- Extruded activated carbon: particle size in the range 0.8-4 mm

Typically the activation, to say the creation of activated sites, isthermal (to say, using heat), even if chemical one is sometimes

carried out.

Thermal activation is carried out at 850-1000 C, with a gase ousactivating agent (very often steam).


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CHARACTERISTICS OF ACTIVATED CARBON

Activating agent, duration and temperature of operation influence theadsorbent characteristics, namely:

-

the inner surface area; typical values are 400-1500 m2

/gADSORBENT- the pore volume; it can be in the range 0.1-0.8 ml/gADSORBENT

-the porosity (% void); never > 50 % for structural reasons.

,

-micropores: d < 2 nm

-mesopores: d = 2-50 nm

-macropores: d > 50 nm.

For physical adsorption, lower the operation temperature higher thequantity of adsorbed compounds.


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FREUNDLICHS ISOTHERM

x = kAD

c1/n

[massADSORBATE/massADSORBENT]

kAD= adsorption capacity[mg/g ] [l/mg]1/n

1/n = adsorption intensity [-]

The isotherm describes the

ADSORPTION EQUILIBRIUM.


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REGENERATION AND REACTIVATION

OF ACTIVATED CARBON

When the adsorption capacity is exhausted, to say the most

activated sites are bonded to molecules, the molecules must beremoved through desorption.

The operation is called regeneration; it exploits the parameters

Steam or hot air can be used to desorb Volatile Organic Compounds(VOCs) in air treatment applications.

For wastewater treatment carbons, regeneration is seldom practicedsince a low adsorption capacity is obtained due to the large

molecules of adsorbate that are not volatilized.Reactivation is similar to the activated carbon manufacturing, withburnoff of the adsorbate and partially of the carbon surface.


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DESIGN OF ACTIVATED CARBON PLANT

For gas treatment, granular carbon is used (powdered carbon is usedjust for masks), distributed in circular columns.

Freundlichs isotherm gives the maximum quantity of adsorbate thatcan be adsorbed by a unit mass of activated carbon.

The gas velocity is in the order 0.05-0.4 m/s, to give a contact time of3-4 s.

Generally, the equilibrium is not reached, so the activated carbonmass obtained with Freundlichs isotherm represents the minimumquantity to achieve a given result.

The quantity really used in the plant is10-15% more than the calculated oneon the basis of experimental-derivedisotherm relationship


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COMBUSTION

If pollutants are organics, they canbecome fuel and be destroyedthrough combustion.

Combustion is an oxidation process,

at high temperature, with a ratherhigh energy generation.

Process is highly EXOTHERMIC:

q ~ 13-15 kJ/g O2.


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Pollutant combustion is equal to fuel combustion, but the aim is

destroying the pollutant since it is harmful (in ordinary combustionthe aim is energy recovery).

So, the controlling parameters are the same (3 Ts rule):

-temperature: it must be at least 850 C, to have good kinetics

-time: for homogeneous systems (gas systems) it must be over 1 s

-turbulence: fluid velocity inside the combustor influencesturbulence level, with a minimum value of 6 m/s.

Combustion can be carried out in thermal or in catalytic combustors.

The aim and the result are the same, but the operative conditions aredifferent.

Generally, auxiliary fuel is used during operation to maintain theoperative temperature (the quantity of pollutant does not develop asufficient quantity of heat).


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THERMAL COMBUSTORS

They have not limits for inner temperature, and they can be used totreat gas flows containing pollutants in high concentration.

Pollutant concentration influences the combustion temperature;when it is rather high, less auxiliary fuel is used.

In these combustors, auxiliary fuel can be fed directly to thecombustor where it mixes with the contaminated flow.

Thermal combustors are robust equipment, to say they can work in awide variability range for operative conditions.

Flue gases have always high temperatures, so energy recovery ismandatory as much as possible, for example to heat up the inlet flow

through heat exchangers.This common thermal combustors are named recuperative.


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Thermal combustors are named regenerative when energy is

recovered inside the combustor itself, through packing of ceramicmaterial having the function of a heat wheel able to receive heateasily from such gas and store the heat, then give it back readily tothe gas itself.

In this way, auxiliary fuel consumption is very low.The combustor is constituted with 2 beds packed with inert ceramicmaterial, joined through an horizontal combustor chambers.

During operation, the contaminatedgas flows through the first bed ofceramic material and is heated untilreaching a temperature close toself-combustion of the VOC

molecules contained in the stream.VOC thermal oxidation generatesheat in the top part of the bed and inthe combustion chamber.


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After leaving the combustion chamber, the carrier fluid mixed with

the combustion products, meets and flows through the second bed,cooler than the first, thus giving up most of its enthalpy to theceramic packing. Then, the flue gas is discharged through the stack.

After reaching the set-point temperature in the top part of the second

bed, the system is switched to the cyclic operating conditions:-the gas flow is inverted so that it enters the already hot second bed,and after combustion, before discharging, it gives its enthalpy to thefirst bed

-when again the set-point temperature is reached, the gas input is

again changed and sent to the first bed.

Cyclically, each bed works as preheater and heat recovery device.


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CATALYTIC COMBUSTORS

They make use of a catalyst to reduce the activation energy of theoxidation reaction and, consequently, the process temperature.

The maximum temperature range is 150-650 C:-if lower than 150 C, the kinetics is too slow

-if higher than 650 C, the catalyst can sinter.

,

window.

The auxiliary fuel is burnt in a separate heater, then the deriving fluegases are mixed with the polluted gas and sent to the combustor.

In these combustors, process is flameless, differently from thermal

ones.


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CATALYTIC COMBUSTORS

To avoid catalyst clogging due to solid particles, filtration of pollutedgas is mandatory.

In addition, some compounds can be poisons for the catalysts, tosay they bond irreversibly to the catalyst activated sites. Whencatalyst poisoning occurs, catalyst must be replaced and reactivated.

Ver often catal sts are metals or metal oxides the can be:

-supported: they are deposited on support (very often alumina,

Al2O3), in form of pellets (cylinders or spheres, with D = 2-4 mm) ormonolithes (honeycomb)

-pure: in form of powder (250-750m).


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