Particulate matter (PM) is the general term used for solid particles and liquid droplets suspended in the ambient air or process gas streams. PM can also be referred to as PM10 and PM2.5 which refer to the aerodynamic equivalent diameter of 10 microns or less and 2.5 microns or less, respectively.
The origin of particulate matter in the ambient air can be natural such as wind-blown dust and volcanoes or human activity such as fossil fuel combustion and automobiles. Particulate matter in the air may also originate from atmospheric reactions between airborne gases such as sulfur oxides, organic compounds and nitrogen oxides.
Some of particulates are visible with the naked eye but most can only be detected with a microscope or a scanning electron microscope. Examples of the typical size ranges for various types of particulates in this diagram.
Particulate matter in the ambient air is harmful for several reasons. Most important is the effect on human health. As a general rule, particulates are the most harmful form of air pollution due to their ability to penetrate deep into the lungs and blood streams unfiltered, causing heart attacks, respiratory disease, and premature death. Particulate matter in the size range of less than 2.5 microns is particularly dangerous in this regard because particles of this size are not well filtered by the upper respiratory system so they can pass into the alveolar membranes causing significant damage.
Particulate matter is also the leading cause of visibility degradation of the ambient air as well as a nuisance that can soil any surface exposed.
Because of its harmful effects the United States Environmental Protection Agency (US EPA) has made the control of PM a priority. With the passage of the Clean Air Act in 1970 the EPA designated PM as a primary pollutant and has focused much of its enforcement activities on ambient air PM reduction.
LDX Solutions has provided technologies to control the emission of PM since 1947.
Brief descriptions of these technologies include:
These devices utilize centrifugal forces to separate particulate matter from a gas stream. They may come as single units or as multiple units otherwise known as multi-clones. Cyclones are very effective in capturing particulates greater than 10 microns in diameter but lose effectiveness on particulate matter smaller than 10 microns. Typically, cyclone collectors are utilized ahead of other particulate collection devices to capture the heavy, coarse particulate matter prior to the secondary device that follows.
Fabric filters, commonly known as baghouses, are very effective in separating even very fine particulates from the carrier gas. Collection efficiencies exceeding 99% are common. However, fabric filters are limited to applications where the gas stream and the particulate matter are dry. Applications with moisture laden gas streams or liquid particulate matter are not normally treated with fabric filters.
Wet scrubbers are sometimes employed for particulate matter control where 1) the efficiency requirements are not stringent, 2) the gas stream is wet and/or 3) simultaneous gaseous absorption are/is required.
Wet ESPs are a proven technology for high efficiency control of fine particulate matter in applications where a wet system is required. This includes applications in which the particulate matter is liquid such as sulfuric acid mist or sticky such as condensable organics.