Industrial dust control methods and equipment, selection of particulate control device and air quality standards of Nepal

Industrial dust controls methods and equipment

There are the different method available for dust control and various types of equipment used in each method are given below.

Source correction method

• Raw material changes (substitution of raw materials)
• Process changes (Process modification)
• Equipment modification or replacement
• Maintenance of equipment

Cleaning of gaseous effluent

• Particulate control equipment (wet absorption & dry adsorption methods)
• Gases and odor control equipment (chemical alteration of the pollutant through combustion and catalytic incineration)

The equipment used are

GRAVITATIONAL SETTLING CHAMBER

• Used for removing particles exceeding 50 μm in size from the polluted gas stream.
• The particles having higher density obey Stoke’s law and settle at the bottom of the chamber from where they are removed ultimately.
• The several horizontal shelves or trays improve the collection efficiency by shortening the settling path of the particles.

Cyclones

fig: Cyclones

In this method, dust-laden gas is whirled rapidly inside the collector shaped like a cylinder or cyclone. The swirling motion thus creates centrifugal forces that cause the particles are to be thrown against the walls of the cylinder and drop into a hopper below. The gases left in the middle of the cylinder after the dust particles have been removed moves upward and exit from the cylinder. Cyclones are typically used for the removal of particles 50 microns (µm) or larger.

Electrostatic precipitation (ESPs) fig: Electrostatic precipitation

It is used in relatively large, low-velocity dust collection devices that remove particles in much the same way that static electricity in clothing picks up small pieces of lint. Transformers are used to develop extremely high voltage drops between charging electrodes and collecting plates. The electrical field produces in the gas stream as it passes through the high voltage discharge introduces a charge on the particles, which is then attracted to the collecting plates. Periodically the collected dust is removed from the collecting plates by a hammer device striking the top of the plates.

FABRIC FILTERS (BAGHOUSE FILTERS)

• A stream of the polluted gas is made to pass through a fabric that filters out the particulate pollutant and allows the clean gas to pass through.
• The particulate matter is left in the form of a thin dust mat on the insides of the bag. This dust mat acts as a filtering medium for further removal of particulates increasing the efficiency of the filter bag to sieve particles of 0.5 μm.
• For efficient filtration and a longer life, the filter bags must be cleaned occasionally by a mechanical shaker to prevent too many particulate layers from building up on the inside surface of the bag.

WET COLLECTOR – SPRAY TOWER

• Water is introduced into a spray tower by means of a spray nozzle.
• As the polluted gas flows upwards, the particulates (>10 μm) present collide with the water droplets being sprayed downward from the spray nozzles.
• Under the influence of gravitational force, the liquid droplets containing the particulates settle to the bottom of the spray tower.

VENTURI SCRUBBER

• It removes the particulates of size 0.5 to 5 μm associated with smoke and fumes very efficiently.
• Polluted gas speed passing through venturi = 60 – 180 m/s.
• A coarse water stream is injected upwards into the throat where it gets atomized due to the impact of high velocity of the gas.
• The particles collided with liquid droplets get entrained in the droplets and fall down to be removed later on.
• To separate the droplets carrying the particulate matter from the gas stream, this gas-liquid mixture in the venturi scrubber is then directed into a separation device such as a cyclone separator.

CYCLONE SCRUBBER

• High-pressure nozzles generated a fine spray that intercepts the small particles which are in the polluted gas.
• The centrifugal force throws these particles towards the wall from where they are drained downwards to the bottom of the scrubber.

CONTROL OF GAS

• Combustion (Incineration)

– Direct flame incineration

– Thermal incineration

– Catalytic incineration

• Adsorption
• Absorption

INCINERATION

• Incineration is applied to effluent streams containing combustible gases in which the volume flow rate of the gas is large and the concentration of the combustible contaminant is relatively low.
• Incineration is to control,
  • Malodor ants such as mercaptans and H2S.
  • Organic aerosols and visible plumes such as those produced by coffee roaster and enamel bake ovens.
  • Combustible gases produced by refineries.
  • Solvent vapors produced by a variety of industrial processes.

DIRECT FLAME INCINERATION

• This system is employed when effluent gases are between the LEL and UEL.
• Flare incinerator is a direct flame type system used in petrochemical plants and refineries.
• It is used for combustible waste gases that cannot be conveniently treated any other way.

THERMAL INCINERATION

• It is utilized for the treatment of effluent gases which have a combustible concentration below the LEL.
• Efficient thermal incineration requires the provision of supplemental fuel to bring the reaction chamber temperature to 750 to 800°C.
• It is used to treat effluent streams from a number of industrial processes including varnish cooking, paint bake ovens, meat smoke houses, fat rendering plants, paint application and resin manufacturing.

CATALYTIC INCINERATION

• Effluent gases with combustible concentrations below the LEL can also be treated by incineration that employs catalytic substances.
• The preheated gases (300-475°C) are discharged to a catalytic bed, where combustion occurs.
• As effluent gases enter the catalytic bed, they are adsorbed on the surface of the catalyst, increasing the concentration of reacting substances.
• Combustion efficiency: 95-98%.
• It is smaller than thermal incinerator and has less retention time (20-50 times).

ADSORPTION

• A variety of contaminant gases can be removed from waste gas streams by being passed through beds or media containing solid sorbents.
• Gases coming into contact with solid sorbents may bond to sorbent surfaces (physical adsorption) or may react with sorbent surface (chemisorption). Both processes are exothermic.
• Sorbents include activated carbons, simple and complex oxides (silica gel), and impregnates (bromine).
• Adsorption is a useful control method where low emissions of very toxic substances and carcinogens are required.

ABSORPTION

• Gaseous pollutants are absorbed when they come into contact with a medium in which they are soluble or with which they can chemically react (scrubbers).
• Collection efficiency is also affected by the following factors: gas solubility, gas and liquid flow rates, contact time, mechanism of contact, and the type of collector.
• Scrubbers experience a number of operating and maintenance problems.

These include corrosion of the scrubber shell and the plugging of packed beds.

• After the medium is removed from the scrubbing system, absorbed materials must be treated and disposed of. For acidic gases waste scrubbing liquids may require neutralization,

Solids removal may require the use of settling ponds. Solids generally must be landfilled.

Selection of particulate controls device

The particulate devices are chosen according to their function and whether to solid or gaseous particulates which are discussed in the earlier topic.

1.8. Air quality standards of Nepal

Air quality is the state of air around us. Air quality Standard refers to the levels of air pollutants prescribed by certain regulations that may not be exceeded during a specified time in a defined area. In other words, it is a general term for the concentration of pollutants for an average length of time set by the Environment Protection Agency. The concerned agency in case of Nepal is MoEST (Ministry of Environment, Science, and Technology).

1. National Indoor Air Quality Standards, 2009

2. National Ambient Air Quality Standards, 2012

Other Laws Regarding Air Quality

• Interim constitution 2007 has guaranteed every person the right to live in a clean environment as a fundamental right and mandate state to make necessary arrangements to maintain the clean environment.
• Environmental Protection Act introduced in 1997 make legal provision to maintain clean and healthy environment by minimizing adverse impacts as far as possible.
• Climate Change Policy 2011 has aimed at moving in low carbon development path, which provides co-benefits for reducing the air pollution.

Status of Air Quality in Nepal

PM10 concentration in the Valley’s ambient air 3-7 times higher than a safer limit. Nepal ranks second last after Bangladesh in terms of air quality and its effect on human health. The graph aside shows the status of air pollution in different parts of Kathmandu City in different parts of a year from November 2002 to December 2006.

Initiatives for Maintaining Air Quality Standards

1) Ambient Air Quality Monitoring System:

MoSTE started monitoring ambient air quality through six monitoring stations in 2002. The equipment, however, stopped functioning in 2007 and in 2013, the Ministry has resumed air quality monitoring in three of the stations: in Bhaktapur, Machhegaun, and Putalisadak by measuring PM10 levels on a daily basis.

2) Vehicle Inspection and Emission Testing:

Nepal initiated activities for monitoring of exhaust emissions in the valley in 1993 and issuance of Green Sticker system was enforced since December 1999. The vehicle emission testing is only limited within Kathmandu valley and is applicable only to three and four wheelers. However, the inspection and emission testing system have not been effectively implemented.

3) Ban on polluting vehicles:

The government banned the operation of heavily polluting diesel three wheelers and import of new two-stroke and second-hand vehicles in 1999, in an effort to combat air pollution in Kathmandu Valley. This initiative prompted the scaling up of electric vehicles (Saja tempo) for public transportation. Ban on the heavily polluting Movable Bull Trench Kiln: In 2003, old polluting brick kilns in Kathmandu Valley were banned by the government following significant protests from the local people. This was followed by the introduction of less polluting technologies, mainly Fixed Chimney & Vertical Shaft Brick Kilns (VSBK).

4) Pollution tax on fuel:

The government started to collect pollution tax of NRs. 0.5 from each liter of petrol and diesel sold in the Kathmandu Valley in 2007. However the provision to deposit the pollution tax to Environment Protection Fund as decided in Financial Act 2002/03 has not been implemented yet and the amount collected has remained unspent.

5) Public transport

The public transport service in Kathmandu Valley is exclusively provided by the private sector. Although it is not efficient it still provides its services to thousands of people, as 28 % of the travel in Kathmandu is on public transport.

6) Operation of electric vehicle:

Around 600 electric three wheelers (SAFA Tempos) are currently operating in Kathmandu Valley in 17 routes providing service to over 10,000 people daily.

References:
1. Mackenzie L. Davis & David A. Cornwell, “Introduction to Environmental Engineering”, McGraw Hill.
2. Gilbert M. Masters, Standford University, “Introduction to Environmental Engineering and Science”, Printice Hall.
3. Stephan Konz, Kansas State University, “Work design”, Grid Publishing Inc., Colombus, Ohio
4. C. S. Rao, “Environmental Pollution Control Engineering”, New age International (P) Limited, Publishers, India.