The time period between 2003 and 2007 marks a transition line for many U.S. industries to comply with recent NOx emission standards. In 1990 the Clean Air Act Amendment recognized the problem of interstate transport of ground-level ozone. Following this action, the Environmental Protection Agency (EPA) established regional rules tightening NOx budgets. The recently finalized NOx State Implementation Plan (SIP) Call substantially limits summer season NOx emission in 22 states east of Mississippi. The minimum emission factor for fuel combustion units is 0.15 lb of NOx per one mmBTU/hr. In 1988 Californian South Coast Air Management District reduced allowable NOx emission from 0.14 lb to 0.03 lb/(millimeter per British Thermal units per hour (mmBTU/hr)). Texas promulgated even more stringent standards recently.
Adopting the U.S. emission trading and banking system 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. The annual allowance prices have been generally stable when averaged during three years of active trade, although substantially fluctuating during a year. The average price index was around $1,000 per ton of NOx emissions. Some forecasts predict the price could reach tens or even hundreds of thousand dollars per ton at the end of the new rules dateline. Therefore, installing an effective NOx reduction technology may become profitable for plants choosing over-controlled emission option.
Using information on typical NOx emission factors, the minimum plant size affected by the regulations can be estimated at 1,000 to100,000 standard cubic feet per minute (scfm) in terms of gas flow rate. Medium size units have exhaust flow rate of no more than 200,000 - 300,000 scfm.
Emission Sources Overview
The most important small/medium stationary emission sources were estimated based on EPA publications. The general process industry sector includes chemical, petroleum, metallurgical, pulp and paper, cement 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 percent of total nationwide NOx emissions.
Among the sources listed, nitric acid production represents a rare example where nitric oxides are directly involved in process technology. The majority of industrial processes generate NOx as a by product of combustion of fossil or synthetic fuel. In combustion systems, the temperature is raised high enough to oxidize atmospheric nitrogen and thus produce nitric oxide (NO), which can be further oxidized to nitrogen dioxide NO2. Both compounds are atmospheric pollutants and are collectively referred to as NOx.
There are three major routes, or mechanisms for NOx formation in combustion processes. The thermal NOx formation mechanism involves thermal dissociation and subsequent reaction of nitrogen and oxygen molecules in combustion air. The fuel NOx formation mechanism is based on oxidation of fuel-bound nitrogen contained in certain fuels. Of less significance is prompt NOx formation mechanism of reacting molecular nitrogen with hydrocarbon radicals.
The extent of thermal NOx formation depends on combustion characteristics such as oxygen and nitrogen concentrations in flame zone, temperature and residence time. The peak, or maximum flame temperature is the most important parameter that determines the potential for NOx formation.
For industrial emission sources, the flame temperature routinely depends on process temperature requirements. Processes with direct firing treatment of solid materials, such as cement kilns, glass melting and ferrous metallurgy annealing and galvanizing furnaces require minimum combustion temperatures within the range of 2600 degrees to 2800 degrees Fahrenheit. To achieve such temperatures, combustion air is preheated up to 1000 to 1200 degrees Fahrenheit through direct heat exchange with checker brick beds (glass making) or solid product (cement kilns). The severe temperatures result in increased NOx emissions and make it difficult to use low-NOx advanced combustion techniques.
In the chemical industry, pyrolysis furnaces are designed to provide an operating temperature inside the reaction tubes that is necessary for ethane/propane cracking reactions (1900 to 2300 degrees Fahrenheit). The actual temperature outside the tubes, in the furnace firebox, is higher because of finite rate of heat transfer through the tube walls. Tubular furnaces used for catalytic steam reforming of hydrocarbons are also high temperature operations. Higher temperatures result in increased NOx emissions compared with the typical process heaters seen in the petroleum industry. Additionally, pyrolysis furnaces and reformers use hydrogen gas as a fuel source that has a higher flame temperature compared to that seen with the use of natural gas burners, resulting in increased NOx production.
Typical industrial boilers operate at combustion temperature ranges of 2400 to 2800 degrees Fahrenheit producing the steam with the temperature of about 1000 degrees Fahrenheit. NOx emissions from the boilers are greatly fuel dependent. Combustion of natural gas produces NOx essentially via thermal mechanism. Both thermal and fuel NOx are formed during combustion of coal or residual oil that is often rich in nitrogen. In these cases, thermal NOx can contribute to about 50 percent of total NOx emission.
NOx Control Techniques
Reduction of atmospheric pollution by nitrogen oxides is achieved via two major pathways: source emission reduction and tail gas after treatment. The first pathway mainly includes various techniques for combustion modification and NOx absorption improvement in nitric acid production.
Flue or exhaust gas recirculation and inlet water/steam injection reduce flame temperature and NOx formation through the addition of inert nitrogen (or water) in the flame zone. Low-NOx burners (LNB) modify the combustion process through staging of air or fuel, or providing premixed fuel and flue gas into the burner throat. These modifications largely aim to reduce peak flame temperature through the distribution of the flame over a larger space than traditional burners. Ultra-low NOx burners recently developed by some companies combine fuel and flue gas premixing with staging to achieve single digit parts per million (ppm) NOx emissions.
In the end-of-pipe treatment area, NOx control solutions include industrially accepted techniques for selective catalytic or non-catalytic reduction and emerging technologies, such as adsorption/catalytic systems, high-efficiency non-catalytic reduction and ozone liquid phase treatment.
The most widely used technology, selective catalytic reduction (SCR), involves mixing the gas flow with gaseous reductant, typically ammonia, and passing the mixture over a bed of SCR catalyst preheated to the reaction temperature. Nitrogen oxides and ammonia are adsorbed on the catalyst surface and react producing water and molecular nitrogen. The reaction products are desorbed to the gas phase leaving the catalyst surface free for the next reaction cycle.
Traditionally, catalysts made of vanadium pentoxide, V2O5, supported on titania, TiO2, are used, often with additives such as WO3. This composition was chosen for its ability to promote the reaction of ammonia with NOx while suppressing oxidation of ammonia by oxygen. In most cases, the catalyst is shaped as a monolithic honeycomb structure with cell density varying from 30 to 200 cells per square inch. The monoliths provide for low pressure drop at linear velocity in excess of 10 feet per second, which translates into compact catalyst bed design.
Historically, the typical operating temperatures of vanadia/titania catalysts are between 500 and 800 degrees Fahrenheit, and that has satisfied many industrial applications. Low temperature gases can be treated using heat recovery systems, but advances in catalyst technology have created catalysts that can directly treat operating temperatures as low as 250 degrees Fahrenheit. For high temperature applications up to 1100 degrees Fahrenheit, zeolite catalyst provides for high performance and stability. In these times of ever increasing utility costs, properly matching the catalyst to the process temperature will optimize performance and result in lower costs of operation.
Selective non-catalytic reduction (SNCR) uses the same principle of reacting NOx with ammonia reductant in the presence of oxygen, but employing gas-phase free-radical reactions instead of interactions on catalyst surface. In order to achieve high DeNOx efficiency and selectivity, the gas mixture must be kept within a relatively narrow temperature range of 1600 to 2100 degrees Fahrenheit.
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 reductant gas, and desorbed from the catalyst.