Gas cleaning systems, including scrubbers, are essential components in industrial processes for removing particulate matter, gases, and pollutants from gas streams before they are discharged into the environment. Scrubbers are devices that use a liquid (usually water or another solvent) to capture and remove contaminants from the gas stream through absorption or chemical reactions. There are several types of scrubbers used in gas cleaning systems.
2. PRODUCER GAS
• A combustible mixture of nitrogen, carbon monoxide, and hydrogen, generated
by passing air with steam over burning coke or coal in a furnace and used as fuel.
Typical components of producer gas
• Producer gas is usually a mixture of mainly carbon monoxide (CO), hydrogen
(H2), methane (CH4), and the inert species carbon dioxide (CO2), nitrogen (N2),
water steam (H2O), and small amounts of Argon (Ar).
• Apart from the main species, the gas contains various minor species, tars, and
particulates depending on the feedstock and the production method.
3. GASIFICATION CHEMISTRY
H2O + heat
Charcoal + tar (200-300 °C)
heat no air
H2O & CO2(900 °C)+ charcoal & TAR
CO,H2 (1200 °C)
Unburned carbon
Charcoal is also used catalyst like
filtration process
4. MINOR CONTAMINANTS H2S, NH3, COS, AND HCN
• Sulphur content of biomass is generally low, but varies with the type of biomass.
• During ordinary combustion, the sulphur is mainly released as SO2, but in the
reducing environment present during gasification, the sulphur is instead released
as hydrogen sulphide, approximately 30–150 ppm
• Nitrogen content of the biomass is released as ammonia; NH3.
• Hydrogen sulphide reacts with carbon dioxide in the gas phase, forming
carbonyl sulphide
• The formed COS can react further with H2S, forming carbon disulphide CS2.
Similarly, ammonia reacts with carbon monoxide CO
• There might be other reaction pathways by which COS and HCN are formed
from the solid biomass during gasification and from tar decomposition
5. Tars are polyaromatic compounds formed when heating the biomass and during the breakdown of its main constituents, i.e.,
cellulose, hemicelluloses, and lignin.
Primary products: mixed oxygenates (e.g., organic acids, aldehydes, and ketones) formed at low temperature,
approximately 400 °C
Secondary products: phenolic compounds formed at approximately 600 °C
Tertiary products: methyl derivatives of polynucleous aromatics (alkyl-PNA) (e.g., methyl acenaphthylene, methyl
naphthalene, toluene, and indene) formed at approximately 800 °C
Quaternary products: at higher temperatures, the tertiary products (alkyl-PNA) are stripped of their substituents and
benzene, naphthalene, acenaphthylene, and pyrene are formed, at approximately 900 °C
Tar classification
− Class I: Heavy GC undetectable Tars
− Class II: Heterocylic compounds (phenol, cresol)
− Class III: Aromatic compounds (toluene, xylenes, not benzene!)
− Class IV: Light Polyaromatic compounds (2-3 rings, naphthalene, indene)
− Class V: Heavy Polyaromatic compounds (4-7 rings, fluoranthene, pyrene)
TAR CLASSIFICATION
8. Basic type Equipment
Dry Cyclone, rotating particle separators (RPS),
Electrostatic precipitators (ESP),
bag filters,
baffle filters,
ceramic filters,
fabric/tube filters,
sand bed filters, adsorbers, etc.
Wet Spray chambers,
packed column scrubber (wash tower),
impingement scrubbers,
venturi scrubbers,
Orifice scrubbers
OLGA, etc.
CLASSIFICATION OF MECHANICAL/PHYSICAL GAS
CLEANING SYSTEMS
9. GAS CLEANING
Dust and ash removal techniques
Schematic of a candle filter
• Cyclones generally remove particles from 1 mm
down to 5 µm in size and work with dry
particulates.
• Cyclone operate actually (up to 900–1000 °C) to
avoid chilling the gas
• Ceramic filter material that allows gases to pass
but not the particulate matter
• Operated at temperatures up to 500 °C and can
effectively remove particles in the 0.5–100 µm
range
• Removing the filter cake or by back flushing
with steam or nitrogen.
Cyclone separator
10. GAS CLEANING WITH COARSE AND FINE
SAND FILTER
• Before the sand bed filter
the producer gas is
quenched by water
injection
• The clean gas leaves the
filter at about 5 to 25 C
• However cleaning of the
sand filter is the greatest
disadvantage
11. WET SCRUBBERS
Wet scrubbers are PM control devices that rely on direct and irreversible contact
of a liquid (droplets, foam, or bubbles) with the PM.
Wet scrubbers are generally classified by the method that is used to induce contact
between the liquid and the PM, e.g. spray, packed-bed, plate.
Advantages
They can collect flammable and explosive dusts safely, absorb gaseous pollutants,
and collect mists.
Scrubbers can also cool hot gas streams.
Disadvantages
It have the potential for corrosion and freezing.
The use of wet scrubbers can lead to water and solid waste pollution problems.
12. • Spray chambers are very simple, low-energy wet scrubbers.
• In these scrubbers, the particulate-laden gas stream is introduced into a
chamber where it comes into contact with liquid droplets generated by
spray nozzles.
• These scrubbers are also known as pre-formed spray scrubbers, since the
liquid is formed into droplets prior to contact with the gas stream.
• The size of the droplets generated by the spray nozzles is controlled to
maximize liquid-particle contact and, consequently, scrubber collection
efficiency.
• The common types of spray chambers are spray towers and cyclonic
chambers.
SPRAY CHAMBERS
13. • Spray towers are cylindrical or rectangular chambers that
can be installed vertically or horizontally.
• In vertical spray towers, the gas stream flows up through
the chamber and encounters several sets of spray nozzles
producing liquid droplets.
• A de-mister at the top of the spray tower removes liquid
droplets and
• wetted PM from the exiting gas stream. Scrubbing liquid
and wetted PM also drain from the bottom of the tower in
the form of a slurry.
• Horizontal spray chambers operate in the same manner,
except for the fact that the gas flows horizontally through
the device.
SPRAY TOWERS
14. • A cyclonic spray chamber is similar to a
spray tower with one major difference. The
gas
• stream is introduced to produce cyclonic
motion inside the chamber. This motion
contributes to higher
• gas velocities, more effective particle and
droplet separation, and higher collection
efficiency.1
• Tangential inlet or turning vanes are
common means of inducing cyclonic motion.
CYCLONIC SPRAY CHAMBER
15. • It consist of a chamber containing layers of variously-shaped packing material, such as
raschig rings, spiral rings, and berl saddles, that provide a large surface area for liquid
particle contact.
• The packing is held in place by wire mesh retainers and supported by a plate near the
bottom of the scrubber.
• Scrubbing liquid is evenly introduced above the packing and flows down through the
bed. The liquid coats the packing and establishes a thin film.
• In vertical designs, the gas stream flows up the chamber (countercurrent to the liquid).
• Some packed beds are designed horizontally for gas flow across the packing
(crosscurrent).
• In packed-bed scrubbers, the gas stream is forced to follow a circuitous path through the
packing, on which much of the PM impacts. The liquid on the packing collects the PM
and flows down the chamber towards the drain at the bottom of the tower.
PACKED BED SCRUBBERS
16. • A mist eliminator (also called a "de-mister") is
typically positioned above/after the packing and
scrubbing liquid supply.
• high PM concentrations can clog the bed, hence,
the limitation of these devices to streams with
relatively low dust loadings
• packed-bed scrubbers are more suitable for gas
scrubbing than particulate scrubbing because of
the high maintenance requirements for control of
PM.
17. • It accelerates the gas stream to atomize the scrubbing liquid and improve gas-liquid
contact.
• In a venturi scrubber, a "throat" section is built into the duct that forces the gas
stream to accelerate as the duct narrows and then expands.
• As the gas enters the venturi throat, both gas velocity and turbulence increase.
• The scrubbing liquid is sprayed into the gas stream before the gas encounters the
venturi throat.
• The scrubbing liquid is then atomized into small droplets by the turbulence in the
throat and droplet-particle interaction is increased.
• After the throat section in a venturi scrubber, the wetted PM and excess liquid
droplets are separated from the gas stream by cyclonic motion and/or a mist
eliminator.
VENTURI SCRUBBER
18. • The performance of a venturi scrubber
depends on the velocity of the gas
through the throat which can be done by
varying the throat diameter.
Advantages
• simple in design
• easy to install
• low-maintenance requirements
• High collection efficiency for small PM
19. • Orifice scrubbers, also known as entrainment or self-induced spray scrubbers,
force the particle-laden gas stream to pass over the surface of a pool of scrubbing
liquid as it enters an orifice.
• With the high gas velocities typical of this type of scrubber, the liquid from the
pool becomes entrained in the gas stream as droplets.
• As the gas velocity and turbulence increases with the passing of the gas through
the narrow orifice, the interaction between the PM and liquid droplets also
increases.
• Particulate matter and droplets are then removed from the gas stream by
impingement on a series of baffles that the gas encounters after the orifice.
• The collected liquid and PM drain from the baffles back into the liquid pool
below the orifice.
ORIFICE SCRUBBER
20. • Some orifice scrubbers are designed with adjustable orifices
to control the velocity of the gas stream.
• the major maintenance concern is the removal of the sludge
which collects at the bottom of the scrubber.
• The sludge is usually removed with a sludge ejector that
operates like a conveyor belt.
• As the sludge settles to the bottom of the scrubber, it lands on
the ejector and is conveyed up and out of the scrubber.
Advantages
• effectively collect particles larger than 2 m in diameter.
• usually have low liquid demands, since they use the same
scrubbing liquid for extended periods of time.
• relatively simple in design and usually have few moving
parts,
21. • An impingement plate scrubber is a vertical chamber with plates mounted
horizontally inside a hollow shell.
• Impingement plate scrubbers operate as countercurrent PM collection
devices. The scrubbing liquid flows down the tower while the gas stream
flows upward.
• Contact between the liquid and the particle-laden gas occurs on the plates.
• The plates are equipped with openings that allow the gas to pass through.
• Some plates are perforated or slotted, while more complex plates have
valve-like openings.
IMPINGEMENT PLATE SCRUBBER
22. CATALYTIC CANDLE FILTER
• Filtration for particles removal and catalytic
cracking of tar from producer gas in one step.
• The ceramic candle filter contains a nickel-
based tar cracking catalyst in the support body.
• That tar removal efficiency between 96% and
98% % for naphthalene and 41% and 79% for
benzene can be achieved with a co-precipitated
catalytic filter disc at a filtration gas velocity of
2.5 cm/s, with 100ppm of H2S at a temperature
of 900 ◦C.
23. ADVANCE CLEANNING SYSTEM
OLGA: Oil Gas Scrubber
• washing and absorption media are
usual referred to as “oil”
350 °C
• Gas is cooled down by the
recirculating oil, which washes out
the condensed heavy tars
• Remove droplets of tars and
oil from the gas, and returns
the drainage to the condenser
• The absorber, the
temperature is just above
the water dew point.
• Absorbed tars and
absorbent oil
• The light tars are stripped off by hot
air
• Regenerating the oil that is fed back
to the absorber
24. ELECTROSTATIC PRECIPITATOR
An electrostatic precipitator (ESP)
is an device that removes dust particles
from a flowing gas (such as air) using
the force of an induced electrostatic
attraction (i.e, like charges repel;
unlike charges attract)
Electrostatic precipitators are highly
efficient filtration devices that allow the
flow of gases through the device, and
can easily remove fine particulate matter
such as dust and smoke from the air
stream.
25. Six major components
oA source of high voltage
oDischarge electrodes and collecting electrodes
oInlet and outlet for gas
oA hopper for disposal of collected material
oAn electronic cleaning system
oAn outer casing to form an enclosure around electrodes
26. WORKING OF ELECTROSTATIC PRECIPITATOR
Several things happen very rapidly (in a matter of a millisecond) in the small
area around the discharge electrode. Electric field is emerged due to dc
terminal arrangement. The applied (-) voltage in discharge electrode is
increased until it produces a corona discharge, which can be seen as a
luminous blue glow around the discharge Electrode.
Due to the formation of corona discharge, free electrons are emitted with high
velocity from discharge electrode.
This fast moving free electrons strikes the gas molecule thus emission of free
electron from gas molecules takes place. The positive ion molecule move
towards discharge electrode by electrostatic attraction
As a result using gas molecule more free electrons are emitted near the
discharge electrode.
Stage - 1
27. Stage - 2
As the electrons leave the strong electrical field area
around the discharge electrode, they start slowing
down. This free electron again strikes the gas molecule
but this time they are captured by gas molecule and
became negatively charged ion.
As the gas molecule are negatively ionized they move
towards the (+) electrode (i.e., collector electrode).
This negative gas ion fills the space of Dust particle
and becoming negatively charged particle.
This particle are captured by collector electrode using
electrostatic attraction.
28. Efficiency
oGeneral collection efficiency is high, nearly 100%
oInstallations operate 98 and 99% efficiency.
oAcid mist and catalyst recovery efficiencies in excess of 99%.
oCarbon black, because of agglomeration tendency collection efficiency less
than 35%.
