Most major industrialized urban areas from the U. S. are unable to meet the National Ambient Air Quality Specifications (NAAQS) for ozone. Atmospheric studies have shown that ozone formation is the result of a complex set of chemical words involving volatile organic compounds (VOCs) and nitrogen oxides (NOx). Those studies indicate that many urban areas with VOC/NOx ratios greater tan 15: 1 can reduce ambient ozone levels only by reducing NOx emissions. A large number of states, therefore , are implementing NOx control regulations pertaining to combustion devices in order to achieve compliance with the NAAQS ozone common.
This article discusses the characterization of NOx emissions by industrial combustion devices. It then provides guidance on how to measure the applicable NOx control tech info and select an appropriate control technique.
Most industrial combustion devices have not long been tested to establish their baseline NOx emission levels. Somewhat, the NOx emissions from these units have been only estimated using various factors. In light of up to date regulations, however , it is mandatory that the NOx emissions as a result of affected units now be known with certainty. That should establish each unit’s present compliance status and allow description of fee applicable control technologies for those units intended to require modification to achieve compliance.
It is, therefore , important to examine each combustion device to verify its NOx emissions characteristics. The testing process should be streamlined to provide on time and necessary information for making decisions regarding the applicability in NOx control technologies.
The basic approach is to select one particular device from a class of units (that is, regarding same design and size) for characterization testing (NOx, CO2, and 02). Testing is conducted at three or more load points that represent the normal operating range of the system, with excess oxygen variation testing conducted at each one load point. Figure 1 illustrates the typical characterization test out results. The remaining units in the class are tested of them costing only one load point, at or near full place.
The operational data obtained during testing, in conjunction with the NOx and CO data, are used to define the submission status of each unit, as well as the applicable NOx control technological innovation for those devices that must be modified. In most instances, this approach will allow a number of units to be tested in one day and provide the necessary detailed data the engineer needs to properly evaluate the potential NOx control technologies.
Reasonably available control technologies (RACT) standards for NOx emissions are defined with regards to an emission limit, such as 0. 2 lb NOx/MMBtu, rather than mandating Specific NOx control technologies. Depending on the energy resource fired and the design of the combustion device, a myriad of manage technologies may be viable options. Before selecting RACT for your particular combustion device, it is necessary to understand how NOx emissions are formed so that the appropriate control strategy may be formulated.
NOx emissions formed during the combustion process are a feature of the fuel composition, the operating mode, and the primary design of the boiler and combustion equipment. Each of these parameters can play a significant role in the final level of NOx emissions.
NOx formation is attributed to three distinct elements:
1 . Thermal NOx Formation;
2 . Prompt (i. elizabeth.. rapidly forming) NO formation; and
3. Fuel NOx formation.
Each of these mechanisms is driven by three common parameters – temperature of combustion, time above patience temperatures in an oxidizing or reducing atmosphere, and disturbance during initial combustion.
Thermal NOx formation in gas-, oil-. and coal-fired devices results from thermal fixation of atmospheric nitrogen in the combustion air. Early inspections of NOx formation were based upon kinetic analyses just for gaseous fuel combustion. These analyses by Zeldovich produced an Arrhenius-type equation showing the relative importance of instance, temperature, and oxygen and nitrogen concentrations on NOx formation in a pre-mixed flame (that is, the reactants are thoroughly mixed before combustion).
While thermal NOx formation in combustion devices cannot actually be driven using the Zeldovich relationship, it does illustrate the importance of the big factors that Influence thermal NOx formation, and that NOx formation increases exponentially with combustion temperatures above second . 800°F.
Experimentally measured NOx formation rates near the flare zone are higher than those predicted by the Zeldovich union. This rapidly forming NO is referred to as prompt NO . The actual discrepancy between the predicted and measured thermal NOx character is attributed to the simplifying assumptions used in the derivation of the Zeldovich equation, such as the equilibrium assumption that To = ½ 02. Near the hydrocarbon-air flame zone, the particular concentration of the formed radicals, such as O and OH YEAH, can exceed the equilibrium values, which enhances typically the rate of NOx formation. However , the importance of force NO in NOx emissions is negligible in comparison to winter and fuel NOx.
When nitrogen is introduced using the fuel, completely different characteristics are observed. The NOx developed from the reaction of the fuel nitrogen with oxygen is certainly termed fuel NOx. The most common form of fuel nitrogen is without a doubt organically bound nitrogen present in liquid or solid powers where individual nitrogen atoms are bonded to carbon dioxide or other atoms. These bonds break more easily in comparison to the diatomic N2 bonds so that fuel NOx formation quotes can be much higher than those of thermal NOx. Additionally , any nitrogen compounds (e. g., ammonia) introduced towards the furnace react in much the same way.
Fuel NOx is substantially more sensitive to stoichiometry than to thermal problems. For this reason, traditional thermal treatments, such as flue gas recirculation and water injection, do not effectively reduce NOx emissions from liquid and solid fuel combustion.
NOx emissions can be controlled either during the combustion process or just after combustion is complete. Combustion control technologies rely on atmosphere or fuel staging techniques to take advantage of the kinetics of NOx formation or introducing inerts that inhibit the structure of NOx during combustion, or both. Post-combustion deal with technologies rely on introducing reactants in specified temperature routines that destroy NOx either with or without the using of catalyst to promote the destruction.
The simplest from the combustion control technologies is low-excess-air operation–that is, lowering the excess air level to the point of some constraint, which includes carbon monoxide formation, flame length, flame stability, and similar matters. Unfortunately, low-excess-air operation has proven to yield only small NOx reductions, if any.
Three technologies that have showcased their effectiveness in controlling NOx emissions are off-stoichiometric combustion. low-NOx burners, and combustion temperature reduction. The earliest two are applicable to all fuels, while the third is applicable to natural gas and low-nitro-gen-content fuel oils.
Off-stoichiometric, or taking place, combustion is achieved by modifying the primary combustion area stoichiometry – that is, the air/fuel ratio. This may be completed operationally or by equipment modifications.
An operational practice known us burners-out-of-service (BOOS) involves terminating the supply flow to selected burners while leaving the air registers open. The remaining burners operate fuel-rich, thereby limiting breathable oxygen availability, lowering peak flame temperatures, and reducing NOx formation. The unreacted products combine with the air from the terminated-fuel burners to complete burnout before exiting the furnace. Determine 2 illustrates the effectiveness of this technique applied to electric utility boilers. Staged combustion can also be achieved by installing air-only kindoms, referred to as overfire air (OFA) ports, above the burner zone. redirecting a portion of the air from the burners towards the OFA ports. A variation of this concept, lance ticket, consists of installing air tubes around the periphery of each burner to supply staged air.
BOOS, overfire air, and puncture air achieve similar results. These techniques are generally related only to larger, multiple-burner, combustion devices.
Low-NOx burners are designed to achieve the staging effect internally. The air and energy flow fields are partitioned and controlled to achieve the wanted air/fuel ratio, which reduces NOx formation and makes for complete burnout within the furnace. Low-NOx burners are applicable lo practically all combustion devices with circular burner creations.
Combustion temperature reduction is effective at reducing thermal N0x but not fuel NOx. One way to reduce the combustion temperature will be to introduce a diluent. Flue gas recirculation (FGR) will be one such technique.
FGR recirculates a portion of the flue gasoline leaving the combustion process back into the windbox. Any recirculated flue gas, usually on the order of 10-20% of the combustion air provides sufficient dilution to decrease NOx emission. Figure 3 correlates the degree of emission damage with the amount of flue gas recirculated.
On gas-fired equipment, emissions arc reduced well beyond the levels ordinarily achievable with staged combustion control. In fact , FGR is just about the most effective and least troublesome system for NOx burning for gas-fired combustors.
An advantage of FGR is so it can be used with most other combustion control methods. Many warehousing low-NOx burner systems on the market today incorporate induced FGR. On these designs, a duct is installed between the add and forced-draft inlet (suction). Flue gas products will be recirculated through the forced-draft fan, thus eliminating the need for just a separate fan.
Water injection is another method the fact that works on the principle of combustion dilution, very similar to FGR. In addition to dilution, it reduces the combustion air high temperature by absorbing the latent heat of vaporization of your water before the combustion air reaches the primary combustion region.
Few full-scale retrofit or test trials of standard water injection have been performed. Until recently, water injection has not been used as a primary NOx control method on any specific combustion devices other than gas turbines because of the efficiency consequence resulting from the absorption of usable energy to escape the water. In some cases, water injection represents a viable option to consider when moderate NOx reductions are required to achieve complying.
Reduction of the air preheat temperature is another plausible technique for culling NOx emissions. This lowers peak fire temperatures, thereby reducing NOx formation. The efficiency fees, however , may be substantial. A rule of thumb is a 1% proficiency loss for each 40º F reduction in preheat. In some cases this can be offset by adding or enlarging the existing economizer.
Post-Combustion Deal with
There are two technologies for controlling NOx emissions once formation in the combustion process – selective catalytic decline (SCR) and selective noncatalytic reduction (SNCR). Both of these tasks have seen very limited application in the U. S. for exterior combustion devices. In selective catalytic reduction, a fuel mixture of ammonia with a carrier gas (typically compressed air) is injected upstream of a catalytic reactor operating within temperatures between 450º F and 750º F. NOx control efficiencies are typically in the 70-90% percent range, dependant upon the type of catalyst, the amount of ammonia injected, the initial NOx quality, and the age of the catalyst.
The retrofit of SCR on existing combustion devices can be complex and expensive. Apart from the ammonia storage, preparation, and control monitoring specifications, significant modifications to the convective pass ducts may be important.
In selective noncatalytic reduction, ammonia- or urea-based reagents are injected into the furnace exit region, where the flue gas is in the range of 1, 700-2, 000º F. The particular efficiency of this process depends on the temperature of the natural gas, the reagent mixing with the gas, the residence occasion within the temperature window, and the amount of reagent injected in accordance with the concentration of NOx present. The optimum propane temperature for die reaction is about 1, 750°F; deviations from this temperature result in a lower NOx reduction efficiency. Application form of SNCR, therefore , must be carefully assessed, as the effectiveness is very dependent on combustion device design and operations.
As noted previously, selection of applicable NOx control technologies depends on a number of fuel, design, and in business factors. After identifying the applicable control technologies, the economic evaluation must be conducted to rank the technology according to their cost effectiveness. Management can then select the ideal NOx control technology for the specific unit.
It should be documented that the efficiencies of NOx control technologies are not item, but rather multiplicative. Efficiencies for existing combustion devices have already been demonstrated in terms of percent reduction from baseline emissions place. This must be taken into account when considering combinations of technology.
Take into account, for example , the following hypothetical case. Assume a baseline NOx emissions level of 100 ppmv and control technology efficiencies the following: low-excess-air operation (LEA), 10%; low-NOx burners (LNB), 40%; and flue gas recirculation (FGR). 60%. The two to three controls are installed in the progressive order of LEA-LNB-FGR.
It should also he noted that combining same-principle technological innovations (for example, two types of staged combustion) would not make a further significant NOx reduction than either of the mixture, since they operate on the same principle.
It must be emphasized who virtually all of the available control technologies have the potential for badly affecting the performance and/or operation of the unit. Typically the operation data obtained during the NOx characterization testing, therefore , must be carefully evaluated in light of such future impacts before selecting applicable control technologies. Operational restriction such as flame envelope, furnace pressure, forced-draft fan capability, and the like must he identified for each potential technology as well as their corresponding impacts quantified. (Reference (4), for example , analyzes these items, in detail. )
As anyone familiar with combustion steps knows, one technology does not fit all. Careful consideration have got to he used to select the appropriate, compatible control technology or perhaps technologies to ensure compliance at least cost with minimal influence on performance, operation, and capacity.