Technology for Air Pollution Control – Part1
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Transcript of Technology for Air Pollution Control – Part1
TECHNOLOGY FOR AIR POLLUTION CONTROL – PART1
TECHNIQUES WITHOUT USING EMISSIONS CONTROL DEVICES Process Change
Wind, Geothermal, Hydroelectric, or Solar Unit instead of Fossil fired Unit.
Change in Fuel e.g. Use of Low Sulfur Fuel, instead of High Sulfur fuel.
Good Operating Practices Good Housekeeping Maintenance
Plant Shutdown
COMMONLY USED METHODS FOR AIR POLLUTION CONTROL
PARTICULATE
Cyclones Electrostatic Precipitators Fabric Filter Wet Scrubbers
GASES Adsorption Towers Thermal Incernation Catalytic Combustion
SOX CONTROL
GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS
Change to Low Sulfur Fuel Natural Gas
Liquefied Natural Gas
Low Sulfur Oil
Low Sulfur Coal
Use Desulfurized Coal and Oil Increase Effective Stack Height Build Tall Stacks
Redistribution of Stack Gas Velocity Profile
Modification of Plume Buoyancy
GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS (CONTD.) Use Flue Gas Desulfurization Systems
Use Alternative Energy Sources, such as Hydro-Power or Nuclear-Power
FLUE GAS DESULFURIZATION SO2 scrubbing, or Flue Gas Desulfurization processes can be
classified as: Throwaway or Regenerative, depending upon whether the recovered sulfur
is discarded or recycled. Wet or Dry, depending upon whether the scrubber is a liquid or a solid.
Flue Gas Desulfurization Processes The major flue gas desulfurization ( FGD ), processes are : Limestone Scrubbing Lime Scrubbing Dual Alkali Processes Lime Spray Drying Wellman-Lord Process
LIMESTONE SCRUBBING
LIMESTONE SCRUBBING Limestone slurry is sprayed on the incoming flue gas.
The sulfur dioxide gets absorbed The limestone and the
sulfur dioxide react as follows :
CaCO3 + H2O + 2SO2 ----> Ca+2 + 2HSO3-+ CO2
CaCO3 + 2HSO3-+ Ca+2 ----> 2CaSO3 + CO2 + H2O
LIME SCRUBBING
LIME SCRUBBING The equipment and the processes are similar to those in
limestone scrubbing Lime Scrubbing offers better utilization of the reagent. The operation is more flexible. The major disadvantage is the high cost of lime compared to limestone.
The reactions occurring during lime scrubbing are :
CaO + H2O -----> Ca(OH)2
SO2 + H2O <----> H2SO3
H2SO3 + Ca(OH)2 -----> CaSO3.2 H2O
CaSO3.2 H2O + (1/2)O2 -----> CaSO4.2 H2O
DUAL ALKALI SYSTEM Lime and Limestone scrubbing lead to deposits inside spray tower. The deposits can lead to plugging of the nozzles through which the
scrubbing slurry is sprayed. The Dual Alkali system uses two regents to remove the sulfur
dioxide. Sodium sulfite / Sodium hydroxide are used for the absorption of
sulfur dioxide inside the spray chamber. The resulting sodium salts are soluble in water,so no deposits are
formed. The spray water is treated with lime or limestone, along with make-
up sodium hydroxide or sodium carbonate. The sulfite / sulfate ions are precipitated, and the sodium hydroxide
is regenerated.
LIME – SPRAY DRYINGLime Slurry is sprayed into the chamber
The sulfur dioxide is absorbed by the slurry
The liquid-to-gas ratio is maintained such that the spray dries before it reaches the bottom of the chamber
The dry solids are carried out with the gas, and are collected in fabric filtration unit
This system needs lower maintenance, lower capital costs, and lower energy usage
WELLMAN – LORD PROCESSSchematic process flow diagram – SO2 scrubbing and recovery system
WELLMAN – LORD PROCESS This process consists of the following subprocesses:
Flue gas pre-treatment. Sulfur dioxide absorption by sodium sulfite Purge treatment Sodium sulfite regeneration. The concentrated sulfur dioxide stream is processed to a
marketable product.
The flue gas is pre - treated to remove the particulate. The sodium
sulfite neutralizes the sulfur dioxide :
Na2SO3 + SO2 + H2O -----> 2NaHSO3
WELLMAN – LORD PROCESS (CONTD.) Some of the Na2SO3 reacts with O2 and the SO3 present in the flue
gas to form Na2SO4 and NaHSO3.
Sodium sulfate does not help in the removal of sulfur dioxide, and is
removed. Part of the bisulfate stream is chilled to precipitate the
remaining bisulfate. The remaining bisulfate stream is evaporated
to release the sulfur dioxide, and regenerate the bisulfite.
NOX CONTROL
BACKGROUND ON NITROGEN OXIDES There are seven known oxides of nitrogen :
NO
NO2
NO3
N2O
N2O3
N2O4
N2O5
NO and NO2 are the most common of the seven oxides listed
above. NOx released from stationary sources is of two types
GENERAL METHODS FOR CONTROL OF NOX EMISSIONS NOx control can be achieved by:
Fuel Denitrogenation
Combustion Modification
Modification of operating conditions
Tail-end control equipment
Selective Catalytic Reduction
Selective Non - Catalytic Reduction
Electron Beam Radiation
Staged Combustion
FUEL DENITROGENATION
o One approach of fuel denitrogenation is to remove a large part of the nitrogen
contained in the fuels. Nitrogen is removed from liquid fuels by mixing the fuels
with hydrogen gas, heating the mixture and using a catalyst to cause nitrogen in
the fuel and gaseous hydrogen to unite. This produces ammonia and cleaner
fuel.
This technology can reduce the nitrogen contained in both naturally occurring
and synthetic fuels.
COMBUSTION MODIFICATION Combustion control uses one of the following strategies:
Reduce peak temperatures of the flame zone. The methods are : increase the rate of flame cooling decrease the adiabatic flame temperature by dilution
Reduce residence time in the flame zone. For this we change the shape of the flame zone
Reduce Oxygen concentration in the flame one. This can be accomplished by:
decreasing the excess air controlled mixing of fuel and air using a fuel rich primary flame zone
CATALYTIC COMBUSTION
CATALYTIC EMISSION CONTROL
MODIFICATION OF OPERATING CONDITIONS The operating conditions can be modified to achieve
significant reductions in the rate of thermal NOx
production. the various methods are: Low-excess firing
Off-stoichiometric combustion ( staged combustion )
Flue gas recirculation
Reduced air preheat
Reduced firing rates
Water Injection
TAIL-END CONTROL PROCESSES o Combustion modification and modification of operating
conditions provide significant reductions in NOx, but not enough to meet regulations.
For further reduction in emissions, tail-end control equipment is required.
Some of the control processes are:
Selective Catalytic Reduction
Selective Non-catalytic Reduction
Electron Beam Radiation
Staged Combustion
SELECTIVE CATALYTIC REDUCTION (SCR)
Schematic process flow diagram – NOX control
SELECTIVE CATALYTIC REDUCTION (SCR) In this process, the nitrogen oxides in the flue gases are reduced to
nitrogen
During this process, only the NOx species are reduced
NH3 is used as a reducing gas
The catalyst is a combination of titanium and vanadium oxides. The
reactions are given below :
4 NO + 4 NH3 + O2 -----> 4N2 + 6H2O
2NO2 + 4 NH3+ O2 -----> 3N2 + 6H2O
Selective catalytic reduction catalyst is best at around 300 too 400 oC.
Typical efficiencies are around 80 %
SELECTIVE NON-CATALYTIC REDUCTION (SNR) At higher temperatures (900-1000oC), NH3 will reduce NOX
to nitrogen without a catalyst.
At NH3 : NOX molar ratios 1:1 to 2:1, about 40-60%reduction is obtained.
SNR is cheaper than SCR in terms of operation cost and capital cost.
Tight temperature controls are needed. At lower temperatures, un-reacted ammonia is emitted. At higher temperatures ammonia is oxidized to NO.
ELECTRON BEAM RADIATION This treatment process is under development, and is not
widely used. Work is underway to determine the
feasibility of electron beam radiation for neutralizing
hazardous wastes and air toxics. Irradiation of flue gases containing NOx or SOx produce
nitrate and sulfate ions. The addition of water and ammonia produces NH4NO3, and
(NH4)2SO4 The solids are removed from the gas, and are sold as
fertilizers.
STAGED COMBUSTION
STAGED COMBUSTION PRINCIPLE
Initially, less air is supplied to bring about incomplete
combustion
Nitrogen is not oxidized. Carbon particles and CO are released.
In the second stage, more air is supplied to complete the
combustion of carbon and carbon monoxide.
30% to 50% reductions in NOx emissions are achieved.
CARBON MONOXIDE CONTROL
FORMATION OF CARBON MONOXIDE
Due to insufficient oxygen
Factors affecting Carbon monoxide formation:
Fuel-air ratio
Degree of mixing
Temperature
GENERAL METHODS FOR CONTROL OF CO EMISSIONS Control carbon monoxide formation.
Note : CO & NOx control strategies are in conflict.
Stationary Sources Proper Design Installation Operation Maintenance
Process Industries Burn in furnaces or waste heat boilers.
CARBON DIOXIDE CONTROL
SOURCES OF CARBON DIOXIDEHuman-Related Sources Combustion of fossil fuels: Coal, Oil, and Natural Gas in
power plants, automobiles, and industrial facilities Use of petroleum-based products Industrial processes: Iron and steel production, cement,
lime, and aluminum manufactures
Natural Sources Volcanic eruptions Ocean-atmosphere exchange Plant photosynthesis
SOURCES OF CO2 EMISSIONS IN THE U.S.
Source: USEPA(x-axis units are teragrams of CO2 equivalent)
CO2 EMISSIONS FROM FOSSIL FUEL COMBUSTION BY SECTOR AND FUEL TYPE
Source: USEPA(y-axis units are teragrams of CO2 equivalent)
GENERAL METHODS FOR CONTROL OF CO2 EMISSIONS Reducing energy consumption, increasing the efficiency
of energy conversion
Switching to less carbon intensive fuels
Increasing the use of renewable sources
Sequestering CO2 through biological, chemical, or physical processes
CONTROL OF MERCURY EMISSIONS
MERCURY EMISSIONS Mercury exists in trace amounts in
Fossil fuels such as Coal, Oil, and Natural Gas Vegetation Waste products
Mercury is released to the atmosphere through combustion or natural processes
It creates both human and environmental risks Fish consumption is the primary pathway for human and
wildlife exposure United states is the first country in the world to regulate
mercury emissions from coal-fired power plants (March 15, 2005).
Source: Seingeur, 2004 and Mason and Sheu, 2002.
Source: Presentation by J. Pacyna and J. Munthe at mercury workshop in Brussels, March 29-30, 2004
Types of Sources
Worldwide Distribution of Emissions
CONTROL TECHNOLOGIES FOR MERCURY EMISSIONS Currently installed control devices for SO2, NOX, and particulates, in a
power plant, remove some of the mercury before releasing from the stack
Activated Carbon Injection: Particles of activated carbon are injected into the exit gas flow, downstream
of the boiler. The mercury attaches to the carbon particles and is removed in a particle control device
Thief process for the removal of mercury from flue gas:It is a process which extracts partially burned coal from a pulverized coal-
fired combustor using a suction pipe, or "thief," and injects the resulting sorbent into the flue gas to capture the mercury.
PARTICULATE MATTER CONTROL
Range: 20 to 40000 mg/m**3First step: Process controlSecond step: Use of collection device
INDUSTRIAL SOURCES OF PARTICULATE EMISSIONS Iron & Steel Mills, the blast furnaces, steel making furnaces.
Petroleum Refineries, the catalyst regenerators, air-blown asphalt stills, and sludge burners.
Portland cement industry
Asphalt batching plants
Production of sulfuric acid
Production of phosphoric acid
Soap and Synthetic detergent manufacturing
Glass & glass fiber industry
Instant coffee plants
EFFECTS OF PARTICULATE EMISSIONS Primary Effects • Reduction of visibility
• size distribution and refractive index of the particles • direct absorption of light by particles • direct light scattering by particles • 150 micro g / m3 concentration ~ average visibility of 5 miles
( satisfactory for air and ground transportation )
• Soiling of nuisance• increase cost of building maintenance, cleaning of furnishings,
and households • threshold limit is 200 - 250 micro g / m3 ( dust ) • levels of 400 - 500 micro g / m3 considered as nuisance
GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS Five Basic Types of Dust Collectors :
Gravity and Momentum collectors Settling chambers, louvers, baffle chambers
Centrifugal Collectors Cyclones
Mechanical centrifugal collectors
Fabric Filters Baghouses
Fabric collectors
GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS (CONTD.)
Electrostatic Precipitators Tubular Plate Wet Dry
Wet Collectors Spray towers Impingement scrubbers Wet cyclones Peaked towers Mobile bed scrubbers
PARTICULATE COLLECTION MECHANISM Gravity Settling
Centrifugal Impaction
Inertial Impaction
Direct Interception
Diffusion
Electrostatic Effects
Tubular Dust Collector Arrangement for an ESP
Overall Collection η
Ci inlet concentrationCo outlet concentration
Note: The smaller the particle, the lower is efficiency of removal.
PROBLEM A cyclone operates removes 75% of particulate matter
fed to it. The filter is then fed to an ESP which operates with 90% efficiency. What is the overall efficiency of this particulate system?
Step 1: Assuming an initial feed of 100%, determine the percentage of
the stream fed to the ESP. 100% - (100% x 0.75) = 25%
Step 2: Determine final composition after ESP. 25% - (25% x 0.9) = 2.5%
Step 3: Determine overall efficiency hoverall = (initial – final)/ initial
= 100% - 2.5%100% = 97.5%