Air Pollution Control Devices For Industrial Steam Boilers

Air Pollution is a mixture of solid/liquid particulate matter and gases suspended in the air. Air pollution kills approximately 7 million people around the world. In India, it is estimated that 1.67 million people die from long-term exposure to outdoor or indoor air pollution.

Some of the primary sources of air pollution are vehicular emission, thermal power plants, construction dust, waste burning, and using poor-quality fuels for cooking.

Air pollution control devices are equipments that help to prevent different types of pollutants from entering the atmosphere out of chimneys or stacks.

Types Of Industrial Air Pollutant

We can classify air pollutants into two main groups – particulate and gaseous. Dust, fly ash, smog, fog, soot, and fumes are different types of particulate matter. The examples of gaseous pollutants are carbon monoxide, hydrocarbons, oxides of sulfur, and nitrogen.

These pollutants are primary air pollutants. The primary pollutants interact with one another to form secondary air pollutants such as ozone.

Air Pollution From Industrial Steam Boilers

Power plants are one of the top industrial polluters. Coal is one of the most widely used fuels in industrial steam boilers. The combustion of coal releases a lot of energy. During the chemical reaction, a lot of toxic pollutants and heavy metals are released into the environment. Other fuels such as biomass fuels also emit harmful gases. Some of the air pollutants released by power plants are:-

  • Mercury
  • Sulfur dioxide
  • Nitrogen dioxide
  • Particulate matter

These pollutants can be extremely harmful to humans and can cause asthma, bronchitis, and other lung diseases. Air pollution has been linked to acid rain, crop damage, and climate change. It is imperative to use air pollution control devices to reduce the amount of pollutants released into the atmosphere. In India, air pollution is regulated through the Air Act of 1981 and Environment Act 1986.

With rising pollution in India, the central and state governments have become more stringent. As a result, power plants, cogeneration units, and factories using industrial steam boilers have started investing in air pollution control devices.

Types Of Air Pollution Devices

Electrostatic Precipitators

The combustion of the fuel produces flue gas. This gas contains tiny particles of soot. An electrostatic precipitator (ESP) is a pollution control device that uses static electricity to remove particulate matter such as soot and ash from the flue gas.

The operating principle of an ESP is quite straight forward. The flue gas passes through two electrodes – a positively charged electrode and a negatively charged electrode. The negatively charged electrode causes the small particulate matter to acquire a negative charge. The negatively charged particulate matter gravitates towards the positive charge and sticks to it. The particles lose their negative charge and fall. The soot and ash are collected from the hoppers. The accumulated soot and ash need to be cleaned from the electrodes after some time.

There are two types of ESP – dry ESP and wet ESP. Dry ESP is used to collect dry fly ash and particulate matter from boilers and incinerators. Dry ESP collects and transports in dry conditions. Wet ESP uses water to flush the particles from the collecting electrodes. Wet ESP is commonly used in the textile, paper, and metallurgy industries.

Electrostatic precipitators are high-efficiency devices and can remove 99% of ash and soot. ESP has a high collection efficiency of 99% for particle size ranging from 0.05-5µm. They are easy to operate and reliable. ESP can handle particles of different sizes.

Some of the disadvantages of the ESP include high capital costs, large space requirements, and the inability to control gaseous pollutants.

Multi Cyclone Dust Collector

A multi-cyclone dust collector (MDC) is a separation apparatus used to remove particulate matter. The MDC has several cyclones arranged in parallel with one common inlet and outlet. The multi-cyclone dust collector uses centrifugal force to separate the dust particles from the flue gas. The flue gas with particulate matter enters the MDC in the tangent direction. The particulate matter hits the MDC surface due to centrifugal action and falls through the hopper in a spinning motion. The flue gas leaves the multi-cyclone dust collector because of internal pressure. The efficiency of the MDC depends on the centrifugal force created and the residence time to allow the dust to settle down.

Multi-cyclone dust collectors are ideal if size of suspended particulates ranges from 1 to 100 µm. The multi-cyclone dust collector has an efficiency of 95% for 5-micron particles. Cyclone dust collectors are effective in reducing dust quantities. The equipment is low maintenance, the hoppers need to be emptied periodically. However, they are unable to tackle sticky material. The operating costs involved in getting the required pressure drop may be high.

Bag Filter

Bag filter is a pollution control device that is popularly used with boilers.

It helps to control sulfur dioxide levels and particulate matter in the flue gas. The different types of bag filters are shaker, reverse air, and pulse jet type bag filter. A bag filter contains filtering sleeves. The flue gas enters the bag filter through a suction force provided by the ID fan. The fly ash and particulate matter are unable to pass through the filter and the clean flue gas leaves the bag filter.

The dust-laden bags are cleaned and deposited in the hopper. The dusty bags are cleaned depending on the type of bag filter. The bags can be cleaned through pressurized air at the top of the bag filter, reverse air is blown using the blower or a motor-driven shaft creates waves.

Bag filters have high collection efficiency for both coarse and fine particulates. They are insensitive to fluctuations in the gas stream condition. The maintenance costs are low. They are ideally suited for particulate with very high or low collection efficiency. The flue gas temperature needs to be low as high flue gas temperature can damage the bags. The bags need to be periodically replaced.

Flue Gas Desulfusiation

Flue gas desulfurization (FGD) is a device used to remove sulfur from the flue gas emissions from coal-fired boilers. Absorbents are used to remove sulfur from the flue gas. It can successfully remove 95% of the flue gas. Common absorbents used are sodium sulfate, ammonia, or lime. FGD generally has a two-phase process- remove particulate matter followed by removing SO2. The flue gas first passes through an air pollution device such as a bag filter or ESP and then passes through the absorbers. After several chemical reactions, gypsum is produced in suspension. Pumps are used in every stage of the process.

Wet Scrubber

A wet scrubber is a pollution control device used to remove particulate matter from the exhaust gas. It can effectively tackle exhaust gases with high sulfur content. The wet scrubber introduces a scrubbing liquid (usually water) on the flue gas. The fly ash or gases are absorbed by the scrubbing liquid and clean flue gas leaves the system. The wet scrubber is ideal to remove fly ash and gases (SOX and NOx) from the exhaust gas. The composition of the scrubbing liquid can be altered. Caustic solution (NaOH) for acid-gas control and calcium hydroxide (Ca(OH)2) are common scrubbing liquids.

The different types of wet scrubbers available are chemical scrubber, particulate scrubber, ammonia scrubber, chlorine scrubber, particulate scrubber, and sulfuric acid scrubber. Wet scrubbers can handle high moisture and high-temperature exhaust gas. They can tackle both gases and particulate matter. The scrubber can effectively neutralize corrosive gases. The wet scrubber has a high collection efficiency of over 95%. The scrubbers are easy to operate and can effectively clean exhaust gases with different flow rates.

The scrubbing liquid containing fly ash and corrosive liquid may be difficult to dispose of. The liquid, if dumped into lakes, can cause water pollution. The scrubber has high operating costs with high power costs and the cost of scrubbing liquid.

Selective Non-Catalytic Reduction

Selective non-catalytic reduction aims to reduce nitrogen oxide from the flue gas. Ammonia or urea is added to the flue gas in the boiler when flue gas temperature ranges from 760 to 1090o C.

Nitrogen, carbon dioxide, and water are formed after the reaction.

SCNR has a low cost of operation and can effectively remove the NOx in the flue gas. It can be accommodated in a small area and can be retrofitted easily.

Mago Thermal offers air pollution control devices for industrial steam boilers. The devices help industries in meeting air pollution parameters set by the government. To know more about air pollution control devices for industrial steam boilers in India, contact Mago Thermal at +91-9910490700.