Methods to control particulate Emission

Wet scrubbers for particulate control

Wet scrubbers for particulate control at coal-fired power plants are used in a few coal-fired plants with most of these installations located in the USA to capture fly ash in addition to sulphur dioxide (SO2). In the most widely used venturi scrubber, water is injected into the flue gas stream at the venturi throat to form droplets. Fly ash particles impact with the droplets forming a wet by-product which then generally requires disposal. Wet scrubbing for particulate removal depends on particle size distribution. The system efficiency is reduced as the particle size decreases. The process can also have a high energy consumption due to the use of sorbent slurry pumps and fans. The forceful contact resulting from the droplet dispersal (spray tower), contraction of the gas stream (venturi) or counter current flow (collision), removes some of the particles as well as SO2 from the flue gas. Many of the wet particulate scrubbers are designed to control both SO2 and particulates by utilising the alkaline fly ash as sorbent. Lime is frequently used to boost SO2 removal efficiencies.

Removal efficiency 90-99.9%
Particle size range 0.5 - >100 µm

Electrostatic precipitators (ESP)

Cold side (dry) ESP is located after the air preheater and operates in a temperature range of 130-180°C. The cold side ESP, with fixed/rigid electrodes, makes up a large portion of the current market although ESP with moving electrodes are becoming more widely used. Hot side (dry) ESP, used mainly in the USA and Japan, is located before the air preheater where the operating temperature range is 300-450°C. A 1990 study showed 150 hot side ESP were built in the USA between 1935 and 1990. In wet ESP, a liquid film is maintained on the collection plates using spray nozzles. The process eliminates the need for rapping as the liquid film removes any deposited fly ash particles. Thus, problems with re-entrainment, fly ash resistivity and capture of fine particles become obsolete. However, wet ESP require saturation of the flue gas stream with water, generate waste water and sludge and operate at low temperatures.

Both ESPs and fabric filters are highly efficient particulate removal devices with design efficiencies in excess of 99.5%. Particulate removal efficiencies in ESP and fabric filters can be further improved by flue gas conditioning.

ESPs are the particulate emissions control technology which is most widely used on coal-fired power generating facilities. The trend is expected to continue at least for the next couple of decades. The choice between ESP and fabric filtration generally depends on coal type, plant size and boiler type and configuration. Both technologies are highly efficient particulate removal devices with design efficiencies in excess of 99.5%.

Conditioning the fly ash in the flue gas is an established technique used to restore the performance of an ESP in coal-fired power plants with high-resistivity fly ash resulting from burning low sulphur coals. Elemental sulphur, ammonia (NH3), and sulphur trioxide (SO3) are the main conditioning agents currently used.

Removal efficiency >99->99.99%

Particle size range 0.01- >100 µm

Fabric filters (baghouses)

Fabric filters, which generally operate in the temperature range 120-180°C, have been more widely used since the 1970s, especially at industrial scale. The choice between ESP and fabric filtration generally depends on coal type, plant size and boiler type and configuration. There are three types of fabric filters based on the cleaning mechanisms of each. The two fundamental parameters in sizing and operating baghouses are the air to cloth (A/C) ratio (m/s) and the pressure drop (mm water gauge, Pascals or in.H2O). Other important factors which affect the performance of the fabric filter include the flue gas temperature, dew point and moisture content; particle size distribution and chemical composition of the fly ash.

Both ESPs and fabric filters are highly efficient particulate removal devices with design efficiencies in excess of 99.5%. Particulate removal efficiencies in ESP and fabric filters can be further improved by flue gas conditioning.

Fabric filters are increasing their market share year by year but mainly in industry. The choice between ESP and fabric filtration generally depends on coal type, plant size and boiler type and configuration.

Conditioning the fly ash in the flue gas is an established technique used to restore the performance of an ESP in coal-fired power plants with high-resistivity fly ash resulting from burning low sulphur coals. The benefits of flue gas conditioning in fabric filters include achieving lower emissions at higher bag air to cloth ratio, reducing pressure drop and improving fly ash cake cohesivity thus leading to better dislodgement in larger agglomerates and less re-entrainment. Elemental sulphur, ammonia (NH3), and sulphur trioxide (SO3) are the main conditioning agents currently used.

Removal efficiency >99- >99.9999%
Particle size range 0.01- >100 µm

Mechanical/inertial collectors (cyclones/multicyclones)

In the past, industrial plant operators tended to fit mainly cyclones. More recently, fabric filters have increased their market share in industry in the various processing fields. Cyclones are robust technologies that can deal with the cyclic operation and load changes, which is quite common in these types of plants. However, their efficiency is moderate when compared with ESP or fabric filtration. A cyclone is a cylindrical vessel, usually with a conical bottom. The flue gas enters the vessel tangentially and sets up a rotary motion whirling in a circular or conical path. The particles are 'thrown' against the walls by the centrifugal force of the flue gas motion where they impinge and eventually settle into hoppers.

Removal efficiency 75-99%

Particle size range 1.0-100 µm

High temperature, high pressure (HTHP) particulate control

During the last decade, there have been significant advances towards the commercialisation of combined cycle systems, such as the integrated gasification combined cycle (IGCC) and pressurised fluidized bed combined cycle (PFBCC). Commercial- and demonstration-scale designs are currently being used for power generation in the United States, Europe, and Japan. An important component in combined cycle power systems is a high temperature, high pressure (HTHP) particulate control device. Efficient hot gas particulate filtration is necessary to protect the downstream heat exchanger and gas turbine components from fouling and erosion to meet emission requirements. A range of technologies has been proposed for hot gas particulate filtration but few have been developed sufficiently to enable commercial exploitation in combined cycle power systems. Mitchell (1997) discusses in detail the developments of these technologies.