Environmental Controls: Understanding and Controlling NOx Emissions
Over the past decade, the U.S. Environmental Protection Agency (EPA) has taken several steps to reduce NOx emissions. In 1990, the Clean Air Act Amendment recognized the problem of interstate transport of ground-level ozone. Following this action, the EPA established regional rules tightening NOx budgets. The recently finalized NOx State Implementation Plan (SIP) Call substantially limits summer season NOx emissions in 22 states east of Mississippi. The California South Cost Air Quality Management District has also reduced allowable NOx emissions, and, more recently, Texas promulgated even more stringent standards. One of the distinctive features of the new rules is that they include small and medium size stationary sources along with the large power generation units that have been the traditional focus of NOx control. Many U.S. industries, including glass and ceramic manufacturing, must begin complying with the new standards starting in 2003, with complete compliance required by 2007.
An emission trading and banking system was adopted in the U.S. that allows each individual plant to implement a flexible strategy. If the plant opts to reduce emissions below the prescribed allowance (tons of NOx emitted per summer season), it can bank or sell the difference (or credit). Therefore, installing an effective NOx reduction technology may become profitable for plants choosing the over-controlled emission option.
Emission Sources OverviewTable 1 lists the most important small/medium stationary emission sources. The NOx release numbers are estimates based on EPA publications. The general process industry sector includes chemical, petroleum, metallurgical, pulp and paper, cement, ceramic and glass manufacturing industries. Stationary internal combustion engines are used mainly for gas pipeline transportation and as back-up power generators. Combined with small and medium-size boilers and turbines, these sources could contribute about 15% of the total nationwide NOx emissions.
The majority of industrial processes generate NOx as a byproduct of the combustion of fossil or synthetic fuel. In combustion systems, the temperature is raised high enough to oxidize atmospheric nitrogen and thus produce NO, which can be oxidized further to NO2. Both compounds are atmospheric pollutants and are collectively referred to as NOx.
Two major mechanisms exist for NOx formation in combustion processes: thermal and fuel. The thermal NOx formation mechanism involves thermal dissociation and subsequent reaction of nitrogen and oxygen molecules in combustion air, while the fuel NOx formation mechanism is based on oxidation of fuel-bound nitrogen contained in certain fuels.
NOx Control TechniquesReducing atmospheric pollution by nitrogen oxides is accomplished by two primary methods: source emission reduction and end-of-pipe exhaust gas treatment. The first method employs various techniques for combustion modification. The second method uses custom process solutions that include industrially accepted technologies, such as selective catalytic or non-catalytic reduction, and emerging technologies, such as adsorption/catalytic systems.
The most widely used NOx control technology, selective catalytic reduction (SCR), involves mixing the exhaust air with gaseous reagent, typically ammonia or urea, and passing the homogenous mixture over a bed of catalyst designed for reaction completion at the airstream temperature. The catalyst promotes a reaction between NH3, NOx and the excess oxygen (O2) in the exhaust stream, forming nitrogen (N2) and water (H2O) (see Figure 1).
Selective non-catalytic reduction (SNCR) uses the same principle of reacting NOx with ammonia in the presence of oxygen, but uses gas-phase free-radical reactions instead of a catalyst to promote reactions. In order to achieve a high NOx reduction, the gas mixture must be kept within a relatively narrow and high temperature range of 1600-2100?F. No solid or liquid wastes are created in the SNCR process. While SNCR performance is specific to each unique application, NOx reduction levels ranging from 30 to 70% have been reported.
A recent progression in system design is the hybrid SNCR/SCR system approach. By combining the two technologies, improvements can be seen in chemical and catalyst use, making the hybrid combination often more flexible and effective than the sum of its parts. A hybrid system uses lower temperature SCNR injection to provide improved NOx reduction while generating higher ammonia slip. The residual ammonia slip is the reagent for a smaller SCR reactor that removes the slip and reduces NOx, while limiting the costs associated with a larger catalyst volume.
Zero ammonia technology (ZAT), or “NOx trap,” as it is often referred to, is the newest technology that does not require the injection of ammonia or urea. ZAT is a catalytic-based system that converts all of the NOx into NO2 (i.e., it oxidizes the NO) and adsorbs the NO2 onto the catalyst. Portions of the catalyst are isolated from the exhaust stream, and the adsorbed NO2 is reduced to N2 using diluted hydrogen, or some sort of hydrogen reagent gas. The N2 is then desorbed from the catalyst.
SCR Technology AdvancesSelective catalytic reduction provides for the highest NOx removal efficiency for many industrial applications, including ceramic and glass manufacturing. However, concerns have been raised related to secondary emissions of ammonia reagent (ammonia slip), the handling and storage of reagents, process control at variable flow rates, NOx concentration and temperature, and high equipment and control system costs. Significant advances have been made during the past 10 years to address these concerns and improve the catalyst, the reagent delivery and control system, and the operational costs associated with the SCR technology.
SCR Catalyst. Catalyst vendors offer highly active and selective NOx reduction catalysts with proven performance and longevity in different industrial applications. Some catalysts have demonstrated the ability to perform satisfactorily under harsh conditions of flue gas streams containing high sulfur oxide and particulate matter levels. For high dust applications, catalysts have been developed that are supported on stacked ceramic plates, providing large channel size and low dust retention. The cleaning service for these catalysts is very infrequent. For relatively small units emitting clean gases, catalyst monoliths with small channels have also been developed. One of the recent developments is a catalyst made as tri-lobe shaped extrudiate with a diameter of 1 to 2 mm.
Alternative SCR Reagents. Gaseous ammonia is a dangerous chemical and is difficult to transport and store. An aqueous solution of ammonia is much easier to handle and is now routinely used as an alternative. Continuing the trend of switching to safer reagents, many modern SCR systems use aqueous solutions of urea. This reduces handling hazards and costs and removes the psychological barrier associated with ammonia. Urea solutions can be accurately metered and injected into the process ducts using low-pressure pneumatic or pressurized air-free injection systems. As a result of urea hydrolysis in the SCR reactor or within the supply conduit, metered amounts of gaseous ammonia are produced to take part in the SCR reaction.
Ammonia Slip Reduction. An uneven distribution of reagent within the air streams creates a possibility for localized pockets of ammonia concentration and excessive ammonia slip. The latest developments of injection systems have been made by using computational flow dynamic (CFD) modeling tools and by optimizing the systems on the basis of simulation results.
Temperature Control and Heat Recovery. Temperature control is essential for treating gas streams that have low and/or variable temperatures. In some systems, a simple temperature control by firing a burner is often sufficient. The burner provides for quick and accurate temperature adjustment. The thermal efficiency of treating cold gases can be improved by using recuperative heat exchangers for secondary heat recovery. Systems with heat recovery are often used in applications characterized by high dust content in the flue gas, which requires wet or dry particulate matter removal to be associated with cooling the gas. Examples of such applications include ceramic or glass manufacturing and cement kilns.
Reducing NOx EmissionsThe EPA will undoubtedly continue to put pressure on industrial facilities to further reduce NOx emissions. A plant’s ability to comply depends on the volume, depth, density, operating temperature and type of SCR catalyst. The major factors influencing catalyst selection include process gas temperature and a thorough examination of potential contaminants, such as particulate and catalyst poisons. The reagent injection systems need to be designed to ensure complete mixing of the reagent with the flue gas as an aid to the conversion process. Excess residual ammonia (ammonia slip) is environmentally harmful when discharged to the atmosphere through the stack and its level should be minimized.
Finally, for many sources, compliance must be proven at very low NOx and ammonia emission rates. This means that the methods selected for demonstrating compliance will be just as important as the selection of the abatement technology.