Emissions control

Emissions control includes the techniques employed to reduce or eliminate the emission of substances which can harm the environment or human health. The air is considered to be polluted when it contains certain substances in concentrations high enough and for durations long enough to cause harm or undesirable effects. These emissions are therefore controlled through emissions reductions and through continuous monitoring of the emissions to ensure the reduction and elimination methods are working.

Emissions monitoring involves on-going collection and use of measurement data or other information for assessing air purification processes against a standard or status in order to meet emissions or environmental requirements. Air quality regulatory requirements are monitored with two different functions: ambient air quality monitoring and stationary source emissions monitoring. Through emissions monitoring, industrial plants can measure and monitor discharged amounts of pollutants in order to meet the necessary requirements.

In ambient air quality monitoring involves the collection and measurement of samples of ambient air and pollutants to evaluate the status of the atmosphere as compared to clean standards and historical information.

In stationary source emissions monitoring collects and uses measurement data at individual stationary sources of emissions, with these sources including facilities, manufacturing plants, processes, emissions control device performance and work practices.

The main emission sources where emission reduction systems are found include power generation, cement, chemical and petrochemical, pulp and paper, manufacturing, mining and metals. In these settings, pollution control equipment or emissions reduction and control systems are used to regulate and eliminate the emissions of potentially hazardous substances. This is typically done with fans or blowers directing industrial exhaust and emissions into air pollution control equipment and systems. These equipment or systems remove or reduce air pollutants through the use of one or more of the following processes and their related methods:

• Combustion, or destroying the pollutant
• Conversion, or chemically changing the pollutant, usually to a less harmful compound
• Collection, or removing the pollutant from the waste air before it releases into the environment

Electrostatic precipitators, also called electrostatic air cleaners, are devices using an electric charge to remove certain impurities from air or other gases in smokestacks and other flues. These impurities can be either solid particles or liquid droplets in the emissions. Electrostatic precipitators were originally designed for recovery of valuable industrial-process materials, and are now used for air pollution control and removing particles from waste gases.

Example of an electrostatic precipitator and the flow of particulate matter past the negative charge plates to the positive charge plates.

These systems work by passing dirty gas in the smokestack through two electrodes. The shape the electrodes take depend on the type of precipitator used, but can be metal wires, bars, or plates inside a pipe or the smokestack itself. One of the electrodes is charged with a high negative voltage and the plate causes particulates inside the waste gases to obtain a negative charge as they pass. Further along the pipe, a second electrode carries a similarly high positive voltage. The previously charged negative particles are pulled towards the positive electrode and stick to it. These plates require occasional cleaning and the particulate can be collected from these plates.

There are multiple types of fabric filters, including:

• Pulse-Jet cleaned type
• Mechanical shaker cleaned type
• Mechanical shaker cleaned type with sonic horn enhancement

Each type works to capture particulate matter less than or equal to 10 micrometers in aerodynamic diameter, particulate matter less than or equal to 2.5 micrometers in aerodynamic diameter, and hazardous air pollutants in particulate form, such as metals.

Image of a typical fabric filter emissions control system, also known as a bag house.

Typical equipment design efficiencies are between 99 to 99.9 percent, which is especially true for newer machines. Whereas older equipment offers a range of actual operating efficiencies between 95 to 99.9 percent. The factors determining fabric filter collection efficiency include gas filtration velocity, particle characteristics, fabric characteristics, and cleaning mechanism. In general, the efficiency increases with increasing filtration velocity and particle size.

One of the more common emissions control devices in manufacturing and processing facilities, air scrubbers employ a physical process - scrubbing - to remove particulates and gases from industrial emissions. These scrubbers have two main categories: dry scrubbers and wet scrubbers.

Dry scrubbers, or dry adsorption scrubbers, inject dry, neutralizing chemical agents, such as sodium bicarbonate, into the emission stream, and cause gaseous pollutants contained to go through a chemical reaction which either neutralizes or converts the pollutants. Once complete, filters within the scrubber chamber collect and remove the spent agents from the cleaned gas. These collected agents can, in some cases, be washed and reused. Typically, dry scrubbers are used to remove or counteract acid gas within industrial emissions.

Example of a wet scrubber.

Wet scrubbers, or wet adsorption scrubbers or wet collectors, use liquid solutions, typically water, to collect and remove water-soluble gas and particulate pollutants from industrial emissions. The wet scrubbing process either passes a gas stream through a liquid solution or injects a liquid solution into a gas stream. As these streams contact, the liquid solution absorbs the pollutant and removes it from the emissions.

Example of a cyclone system.

A cyclone removes particulates by causing the emissions airstream to flow in a spiral path inside a cylindrical chamber. The dirty air enters the chamber from a tangential direction at the outer wall to form a vortex as it swirls within the chamber. Through the inertia, the larger particulates move outward and are forced against the chamber wall. Slowed by friction, these particulates slide down into a conical dust hopper at the bottom of the cyclone while the cleaned air swirls upward in a narrower spiral through an outlet at the top of the inner cylinder.

Cyclones are best at removing relatively coarse particulates. They can achieve efficiencies of 90 percent for particles larger than 20 micrometers. By themselves, cyclones are not sufficient to meet stringent air quality standards, and are often used as pre-cleaners and followed by a more efficient air-cleaning system.

Thermal oxidizers, or thermal incinerators, are combustion devices that control volatile organic compounds (VOCs), carbon oxide (CO) and hazardous air pollutants (HAPs) emissions by combusting them to carbon dioxide and water. Thermal oxidizers are designed to include high temperature (high enough to ignite the organic constituents in the waste stream), residence time (sufficient for the combustion reaction to occur), and turbulence or mixing of the combustion air with the waste gas.

Example of a type of thermal oxidizer.

Thermal oxidizers can use a lot of fuel for operation, and in order to reduce fuel usage thermal oxidizers have some form of heat recovery. This heat recovery can either be recuperative or regenerative. In recuperative heat recovery, heat is recovered by passing the hot exhaust gases through a non-contact air-to-air heat exchanger. In regenerative heat recovery, hot exhaust gases and cool inlet gases are passed through a fixed bed, typically employing ceramics.

Catalytic oxidizers, or catalytic incinerators, are oxidation systems that control VOCs and HAP emissions. Catalytic oxidizers use a catalyst to promote the oxidation of VOCs to carbon dioxide and water. The catalyst allows oxidation to occur at lower temperatures than for thermal oxidation, with catalytic oxidizers generally between 650 to 1000 degrees Fahrenheit.

Otherwise, catalytic oxidizers work in the same way as thermal oxidizers, and include forms of heat recovery in order to reduce fuel usage required for the oxidation. These recovery methods are the same for catalytic oxidizers and thermal oxidizers.

Most often used in manufacturing and metalworking spaces, mist collectors work to reduce the aerial matter of dangerous chemicals in a working environment. A mist collectors main function is remove droplets from the filtered airstream. To accomplish this task, a collector must coalesce small drops into larger ones, then drain the collected coolant from the filters. Mist collectors capture droplets in several ways, including electrostatic precipitation, inertial separation, and filter media.

Filter mechanism types.

Once droplets adhere to the fibers in a filter media mist collector type, they coalesce with other droplets on the fibers. When the coalesced droplet is large enough, the force of gravity pulls the droplet down along the fiber where it drains.

Biofilters are commonly used to treat pollutants emitted from industries, including wastewater treatment facilities, printed circuit board manufacturing plants, meat rendering plants, pet food industries, and general chemical and petrochemical industries. The pollutants emitted from industries include VOCs and odorous compounds, such as hydrogen sulfide, ammonia, methyl mercaptan, and dimethyl sulfide. The biofilter designs and configurations vary depending on the airflow volume being treated. But in most cases, biofilter applications need pretreatment steps involving physical, chemical, or other methods prior to the biofiltration steps.

Carbon capture involves capturing carbon dioxide at its emission source. From here, it can be transported to a storage location or can be reused in the production of hydrocarbon based materials. When stored, captured carbon is usually stored deep underground. These technologies are reportedly capable of capturing up to 90 percent of CO2 emissions from power generation and other industrial processing.

Once captured, carbon is often stored, but it can also be converted and used as a raw material. This can be for the production of urea, methanol, polycarbonates, cyclic carbonates, and specialty chemicals. These techniques have been adopted by the chemical industry with the goal of reducing its greenhouse gas intensity based on a ratio of net greenhouse gas emissions to production.

Carbon markets aim to reduce carbon emissions in a cost-effective manner by setting limits on emissions and enabling the trading of emissions units, which are instruments representing emission reductions. Trading enables companies which reduce emissions to be paid to do so by higher-cost emitters, and thus lower the economic cost of reducing emissions. The carbon markets are intended to help incentivize companies and countries to develop technologies and schemes to reduce carbon emissions, and in turn to reward those companies or countries which have successfully reduced their emissions below the cap in place.

Visualization of a carbon market

These systems, in some cases, work as a "cap-and-trade" system used by the European Union and California, in which the government puts a cap on the emissions and businesses can sell any extra allotment of emissions cap to other businesses. The first carbon market scheme was developed under the United Nations 1997 Kyoto protocol. This market collapsed over concerns over environmental efficacy and corruption. The idea of carbon markets have been revived in the 2015 Paris Agreement on climate change, under Article 6, which creates a new scheme to trade credits between countries signed up to the agreement in a way similar to companies can trade credits in different regions.

A carbon credit is a permit that a company holds and which allows it to emit a certain amount of carbon dioxide or other greenhouse gases. One credit permits the emission of a mass equal to one ton of carbon dioxide. Carbon credits work on a cap-and-trade program similar to those used by carbon markets. Except in the case of carbon credits, companies that pollute are allowed to continue to pollute up to a certain limit. The limit to which companies are "allowed" to pollute without penalty under this type of system is reduced periodically. And this slow reduction is intended to work towards incentivizing companies to reduce their emissions and these companies with space under their allowance can in turn resell their emissions allowance. These carbon credits are in turn traded on carbon markets.

Carbon credits are sometimes also called carbon offsets. Both carbon credits and credit offsets work to help a company either artificially increase its carbon emissions limits or else artificially reduce its overall carbon emissions. Carbon credits are tradeable, between companies, and markets, similar to securities markets, have developed to further help companies trade these credits. Whereas carbon offsets are purchased to artificially reduce the emissions of a company.

Visualization of carbon offsets.

Similar to carbon credits, carbon offsets are measured by tons of carbon dioxide equivalent, except than instead of representing when a company emits less carbon than their limit, carbon offsets are created when a company decides to invest in something that reduces greenhouse gas emissions outside of their everyday operations. These investments are sometimes known as carbon projects, and generally involve building wind turbines, supporting solar farms, or investing in forest preservation and reforestation efforts. And these projects are typically located in lower-developed countries. These offsets offer companies another way of offsetting their carbon emissions instead of actively reducing their emissions.