Choice of the NOx control technology is dependent on many factors, including regulatory requirements, technical feasibility, cost and public perception. Figure 1, compiled based on publications [1-5], presents a summary of reduction techniques that are capable of high NOx removal efficiencies.
* Glass furnaces -- Replacement of combustion air by cryogenically distilled oxygen was proven to be very efficient for NOx reduction in container and press-blown glass manufacture. This technique allows increases in glass production capacity but requires high capital and operating costs because of firing system modification and oxygen generation. Installation of low-NOx burners gives modest results because of exceptionally high process temperature requirements. The selective catalytic reduction system can be less expensive, especially for large flat or container glass furnaces, and can provide a DeNOx efficiency in excess of 90 percent.
* Cement kilns -- Additions of small amounts of steel slug to the raw kiln feed (CemStar method) leads to lower fuel consumption and NOx emission reduction by approximately 30 percent. Mid-kiln tire firing reduces NOx emissions by 30 to 55 percent but can only be employed by a fraction of the industry that uses older long kilns without preheaters or precalciners. Low NOx burners, which provide for up to 40 percent reduction, can be used in kilns equipped with precalciner. The selective non-catalytic reduction method was applied for precalciner kilns with efficiency of up to 70 percent. The SCR catalytic method is tested in Europe in pilot scale with a potential to increase the efficiency up to 90 percent.
* Gas fired industrial heaters -- Using low and ultra-low NOx burners (ULNB) is a common approach employed on many refinery and chemical industry heaters. The efficiency of modern ultra-low NOx burners compared to typical conventional burners, determined as a reduction of NOx emissions, can be as high as 80 percent. In terms of emission rate per unit of heat duty, the emission can be decreased from 0.1 - 0.15 lb/mmBTU to 0.02 - 0.03 LB/mmBTU. Those high efficiencies seem not to be attainable for hydrogen fired pyrolysis and reforming furnaces using the ULNB only. The most modern systems combine it with flue gas recirculation. The emission below 0.02 - 0.03 LB/mmBTU can be obtained using selective catalytic reduction or a combination of this method with ULNB.
* Stationary engines -- The popular Low Emission Control technology (LEC) modifies lean-burn engines to higher air/fuel ratios. It can achieve very high emission reduction, in excess of 90 percent, compared with current generation of gas fired stationary engines. The average emission level for a LEC system is about one LB/bhp. The LEC systems may not be applicable to all engine models, making SCR a reasonable alternative choice. Some emerging after-treatment approaches such as NOx adsorption trap and high efficiency SNCR2 have also demonstrated very high levels of emission control. The SCR is likely to be a technology of choice for compression ignited (diesel) engines.
In most applications, the lowest NOx emission can be achieved by combining combustion / process modifications with an after-treatment system.
SCR Technology Advancement
Selective catalytic reduction provides for highest NOx removal efficiency for many industrial applications (Figure 1). However, it causes concerns related to secondary emission of ammonia reductant (ammonia slip), handling and storage of ammonia reductant, process control at variable flow rate, NOx concentration and temperature, and high equipment and control system costs. Significant advances have been made during the past ten years to address these concerns. Improvements deal with SCR catalyst, reductant delivery, control system and heat recuperation.
* SCR catalyst -- Catalyst vendors offer highly active and selective NOx reduction catalysts with proven performance and longevity in different industrial applications. The vanadia/titania catalysts have demonstrated the ability to perform satisfactorily under harsh conditions of coal fueled utility boilers, with 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 those catalysts is very infrequent. For relatively small units emitting clean gases (diesel and natural gas engines), catalyst monoliths with small channels have been recently developed. One of the recent developments is a vanadia/titania catalyst made as trilobe shaped extrudates having a diameter of 1 to 2 mm. Such catalysts show high activity at exhaust temperatures as low as 250 degrees Fahrenheit.
* Alternative SCR reductants -- Gaseous ammonia is a dangerous chemical and is difficult to transport and store. Aqueous solution of ammonia is much easier to handle and is now used routinely. Continuing the trend of switching to safer reductants, 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, small, metered amounts of gaseous ammonia are produced to take part in the SCR reaction.
* Ammonia slip reduction -- Uneven distribution of ammonia/urea reductant and mixing it with gas streams creates a possibility for localized pockets of ammonia concentration and excessive ammonia slip. Preparing perfectly mixed gas stream eliminates such a possibility. Latest developments of ammonia / urea injection subsystems include thorough investigation of reductant injection and mixing using Computation Flow Dynamic (CFD) modeling tools and the optimization of such subsystems on the basis of simulation results.
An alternative method to reduce ammonia slip places a bed of oxidation catalyst downstream of the SCR catalyst to destroy essentially all secondary ammonia emission. However, the oxidation must be selective, i.e. it must favor the formation of N2 over the formation of NOx. Base metal oxidation catalysts are better suited for this purpose as they provide for much higher selectivity to N2 over noble metals. Alternatively, vanadia/titania catalysts have an ability to adsorb and store significant amounts of ammonia. To utilize this capability, a process technology has been suggested which employs a catalytic reactor with periodical flow reversal and adding ammonia between two layers of a catalyst bed. Ammonia is used for NOx reduction, but the downstream catalyst stores a part of it. Flow reversal prevents the ammonia from leaving the bed by redirecting the 'wave' of adsorbed NH3 back to the center of the bed. As a result, better ammonia utilization and significantly lower slip can be achieved.
* Pre-oxidation of NO to NO2 -- This technique is a recent development for diesel engine exhaust treatment. It is based on much higher reactivity of NO2 compared to NO. Oxidation of NO to NO2 upstream of the SCR catalyst substantially increases the performance of the vanadia catalyst, thus shifting the lower limit of SCR reaction temperature window from 500 to 400 degrees Fahrenheit. Pre-oxidation of NO to NO2 is achieved over a bed of monolithic noble metal catalysts.
* Temperature control and heat recovery -- Temperature control is essential for treatment of gas streams having low and/or variable temperatures. In small systems, a simple temperature control by firing a burner is often sufficient. The burner provides for quick and accurate temperature adjustment. Thermal efficiency of treatment of cold gases can be improved using a recuperative heat exchanger. Further improvement in thermal efficiency can be achieved by regenerative heat exchange, such as in flow reversal reactors that also offer a lower ammonia slip advantage. The systems with heat recovery are employed in the applications characterized by high dust content in the flue gas that requires wet or dry particulate matter removal, associated with cooling the gas. Examples of such applications include glass manufacturing and cement kilns.
Figure 1. High efficiency NOx reduction method