Part 3 of Abating Hazardous Air Pollutants: NOx
What is NOx? The term NOx refers to the family of air polluting chemical compounds, Nitrogen Oxides. NOx represents seven different compounds although only one, Nitrogen Dioxide, is regulated by the EPA. Nitrogen Dioxide (NO2) is carefully monitored by the EPA as not only an air pollutant but also for its reaction with the ozone and the creation of acid rain. NO2 is reddish-brown gas with a bitter odor similar to that of Chlorine. NO2 is also commonly referred to as Nitrogen Oxide and Deutoxide of Nitrogen and is highly reactive.
NO2 is an intermediate in the manufacturing nitric acid. Nitric acid is a nitrating agent in the manufacturing of chemical explosives, as a flour bleaching agent, as an inhibitor for acrylates and an oxidizer in rocket fuel. It was used in the Titan rockets which launched Project Gemini, in numerous unmanned space probes and remains in use for space travel in the maneuvering thrusters of the Space Shuttle.
Where is NOx found? NO2 can be found in the environment in natural causes including bacterial respiration, volcanos, lightning and entry from the stratosphere. Scientists estimate that each average flash of lightning turns about 15 lbs of nitrogen into NOx. Each year around the globe there are on average 1.4 billion lightning flashes. That’s a lot of naturally occurring NOx, yet, from these sources NO2 is considered a trace gas in the Earth’s atmosphere where it serves a role absorbing sunlight and regulating the troposphere’s chemistry, specifically in determining ozone concentrations. In comparison to NO2 emissions from naturally caused emissions those from fossil fuel combustions result in over three times as many emission occurrences.
Human causes of NO2 are predominantly from internal combustion engines. Internal combustion engines are used to power automobiles, boats, ships, motorcycles, locomotives, helicopters, airplanes and more. Additional causes are from stoves, butane and kerosene heaters and cigarette smoke. Perhaps the most notable historic causes of nitrogen dioxide produced were by nuclear tests and was the cause of the infamous reddish mushroom clouds. These causes are all related to high temperatures as oxygen and nitrogen do not react at ambient temperatures. At these high temperatures they undergo an endothermic reaction producing oxides of nitrogen aka NO2.
Why do we abate NOx? Individuals regularly exposed to NO2 are at increased risk for occupation lung diseases. Those that are most often affected are those working in agriculture who are exposed to NO2 through decomposing grain in silos. In fact, this is so common that the lung condition caused by this is called “silo-filler’s disease”. Additional lung complications caused by NO2 can cause premature death in extreme cases and in less extreme cases emphysema or bronchitis. NO2 is also known to be a cause of aggregation to those with existing heart disease.
When VOCs and NOx react as a result of heat and sunlight, smog is created. Smog (ground level ozone) causes damage to vegetation and reduces crop yields. When NOx and sulfur dioxide react with other air particles acid rain is formed. Acid rain causes damage to automobiles, buildings and historical monuments. To the environment, acid rain effects are most visible in aquatic environments including streams, rivers, lakes and marshes where exposure to fish and wildlife can be extremely harmful. As acid rain flows through soil it can pull aluminum from soil clay particles and deposit them into aquatic environments. The animals that are most sensitive to acid rain and aluminum are generally the youngest in a given species. At certain pH levels caused by acid rain, fish eggs are not able to hatch and even older fish are not able to survive. Animals with higher sensitivity to pH levels will be effected the most. It is important to keep in mind that ecosystems are holistic and the effects on one species will not be limited to that species. Acid rain not only deposits aluminum into soil but it also strips it of minerals and nutrients that support the healthy growth of plants and trees which can cause their demise. In cases where the plants are exposed to acid rain but do not directly perish, other effects can be that a trees’ foliage is often stripped leaving them with brown and dead leaves and needles. This causes the trees and plants to be more susceptible to the effects of freezing temperatures and less able to absorb sunlight which again can lead to their demise.
While most acid effects are in a wet form, acid can also be found in dust particles. This is referred to as dry deposition. These acids are particularly corrosive to metals, paint and stone causing rapid deterioration. Often the emotional damage cause by acid dust is as great as the financial damage as statues, monuments and tombstones are extremely susceptible to damage.
With all of the effects described in this article it is clear that the abatement of NOx, NO2 and acid rain is essential to the long term health of both the environment and the earths human population. That being said it is also important the costs of environmental protection are prohibitive to profitable business operation. GCES has worked tirelessly for nearly two decades developing pollution control solutions that have reasonable capital costs, low operating costs, little to no maintenance costs and small physical footprints to ensure that the needs of businesses are considered while protecting the environment.
So, how do we abate NOx and reduce acid rain? There are many options for treating NOx depending on the combination of VOCs in your output. Gulf Coast Environmental Systems will design a system that includes one or more of the following pieces of equipment. These options include a Thermal Oxidizer, Carbon Adsorber, Selective Catalytic Reduction System or a Scrubber.
A Thermal Oxidizer (also known as thermal oxidiser, or thermal incinerator and often referred to as a Direct Fired oxidizer or afterburner) is a process unit for air pollution control that decomposes hazardous gases at a high temperatures and releases heat (which can be recovered), water vapor, and carbon dioxide into the atmosphere. There are many different types of Thermal Oxidizer technologies available to meet specific destruction and business goals including RTOs, TOs, TROs, CATOX, Catalytic Recuperative Oxidizers, DFTOs and the breakthrough Aqueous RTO. At GCES we know Thermal Oxidizers and have over 200 cumulative years in designing, manufacturing, servicing and supporting air pollution control systems and Thermal Oxidizers.
A Carbon Adsorber is one of the most widely accepted air pollution control technologies. Carbon Adsorbers are necessary when you are trying to accomplish VOC abatement, low NOx admissions and solvent recovery simultaneously. In a Carbon Adsorber, a contaminated process stream flows over an activated carbon bed. The carbon acts as an Adsorber removing the VOCs from the process stream, holding then on the carbon surface and in its pores. The VOC free air is then exhausted into the atmosphere. Carbon is most effective when used to adsorb organic materials such as low molecular weight VOCs. Over time a carbon bed will reach its capacity, the concentrated VOCs are then desorbed into a low pressure steam flow and either recycled or sent to a Thermal Oxidizer for destruction. Some important considerations for the selection of a Carbon Adsorber begins with an understanding of your process stream. For a Carbon Adsorber to be effective the process stream must be low in moisture and particulate as carbon is a porous material it will easily clog. If the process stream has either too much moisture or too many particulates GCES may recommend a pre-filtering system and/or a dryer to ensure long term efficiency and effectiveness in VOC treatment.
A Selective Catalytic Reduction System (SCR) is configured with a NOx reducing catalyst and an ammonia injection system. The catalytic reaction of the ammonia across the Selective Catalytic Reducer (SCR) reduces the emission of NOx by converting it into basic atmosphere elements (nitrogen, oxygen, and water). The catalyst comes in varying arrangements depending on the application. The ammonia injection system allows ammonia to be introduced to the process to combine with the process NOx and reduce the amount of NOx emitted from the system. An Ammonia Flow Control Unit (AFCU) and Ammonia Injection Grid (AIG) provide a controlled supply of anhydrous ammonia as dictated by a NOx and Ammonia (NH3) analyzer system. As with Carbon Adsorbers, SCRs are prone to clogging and considerations must be made to ensure effective long term performance. The key in that optimal and lengthy performance is in the system design and a proper understanding of all airflow matter to ensure the correct mix of ammonia during the processing of the VOCs and NOx.
The term Scrubbing is often used improperly to mean the removal of VOCs through a process. It is important to know there are many types of scrubbing, but the most effective for NOx removal is Wet Scrubbing. Wet Scrubbing specifically describes the process of using a liquid (in most cases water) to remove pollutants from an air stream. The liquid of a wet scrubber will collect the particulate matter and gas pollutants from the air. Wet scrubbers are an extremely versatile form of pollution control allowing GCES to custom design each scrubber system to achieve optimal performance in the removal of NOx and VOCs from air released into the atmosphere. The liquid containing the VOCs is collected in the form of droplets and then treated prior to discharge or reused elsewhere in a facilities processing system. Wet scrubbers are extremely effective in their design in that they are often the only system that can be used to treat both particulate matter and gases in a single pollution control devise.
Gulf Coast Environmental Systems offers multiple design solutions to best fit the operational parameters of its customers. In some cases, the desire is for a lower capital investment for a higher operating cost for those systems that do not need to run continuously or the need may be for a slightly higher capital investment with a lower operating cost for those systems that do need to run continuously. Our designs include single stage solutions as well as multistage solution options. GCES offers complete integrated solutions with other air purification systems such as thermal oxidizers, carbon adsorption systems, or filtration packages as needed making us a single source solution for your pollution control needs.
Additional articles in the GCES series ‘Abating Hazardous Air Pollutants’ include:
Part 2: Chlorine Abatement
Part 6: SOx, the compounds of sulfur and oxygen molecules including Sulfur Monoxide, Sulfur Dioxide and Sulfur Trioxide
Part 11: Sulfuric Acid – H2SO4
Part 12: Ethylene Oxide – EtO