NOx in Non-Utility Industries -- Part II

This the second in a two-part series. Part I, which appeared in our June 2001 issue (see Archives at www.eponline.com), focues on air emission orgins of nitrogen oxides (NOx) in non-utility industries.

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 ratios2. 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 utilizat ion 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 methods.

Process

Emission Reduction Percentages

Glass Furnace

Oxy-firing

80-90

NSCR

40-70

SCR

80-95

Cement Kilns

ChemStar process modification

30-40

Low-NOx burners

45-50

Mid-kiln firing

30-55

NSCR

40-60

Pyrolysis Furnaces and Reformers

Low NOx burners

30-75

Ultra low NOx burners combined

with external FGR

70-80

NSCR

70-80

SCR

70-95

Gas Fired Feed Heaters in Petroleum and Petrochemical Industries (temperature below 1800 ° F)

Flue gas recirculation

50-80

Low NOx burners

30-50

Ultra-low NOx burners

70-80

NSCR

30-80

SCR

70-95

Stationary Engines

Low emission combustion

80-90

Selective catalytic reduction (SCR)

90-95

NOx and Ammonia Monitoring

The methods for sampling and monitoring NOx emissions have taken on significant importance with the development of abatement systems capable of controlling NOx and ammonia to single digit levels. With the requirement for accurate measurement falling below 10 parts per million, the emphasis on proper sampling techniques is critical. Chemiluminescent NOx analyzers have been the method of choice, as required for demonstration of compliance with EPA Method 7E for NOx emissions. Whether used in a Continuous Emissions Monitoring System (CEMS) or for a compliance test, the method is well proven for NOx measurement. The method measures the reaction of NO with ozone that emits chemiluminescence The end result is the total NOx concentration (NO and NO2). The method is extractive and requires sample lines for gas transportation. Since the NO2 component is water soluble, sample conditioning is very important for accurate measuremen t of NOx emissions.

Within the past decade there have been significant improvements in portable analyzers for NOx measurement. There are specific source testing methods in place for portable analyzers, such as CTM-022 and CTM-030 measurement. While chemiluminescent analysis is not available in a true portable system, there are reliable electrochemical based systems with good sample conditioning which run up to six hours on internal batteries. These portable systems are particularly useful for periodic monitoring on stationary engines and turbines. There are a number of high-end portable systems that will monitor single digit NOx emissions for long testing intervals with minimal drift.

The presence of ammonia (NH3) in abatement systems presents a new challenge for emission monitoring. In many cases, the required ammonia slip concentration from the abatement system must not exceed 10 parts per million. Accurate monitoring of low ammonia concentrations is difficult at best. Just as stated above for NOx emissions, the accuracy of ammonia measurement begins with proper transport and conditioning of the gas sample.

Once the conditioned sample is transported to the analyzer, there are options to be considered for measurement of the gas. The traditional non-dispersive infrared (NDIR) analyzer commonly used for CO, CO2 and SO2 measurement lacks the sensitivity required for low level ammonia monitoring. Chemiluminescent analyzers have been developed for an indirect method of ammonia monitoring where both NO2 and ammonia must be converted to NO, then passed through a reaction chamber. The result yields a calculated value from which the NO2 and NOx must be subtracted to derive an ammonia concentration. A direct measurement of ammonia is accomplished with a photoacoustic infrared multi-gas analyzer. Using this technique requires subtracting the concentration of interfering compounds such as CO2, SO2 and H2O from the same analyzer.

In conclusion, the demonstration of compliance for combustion and other stationary sources must be proven at very low NOx and ammonia emission rates. The methods selected for demonstration of compliance are just as important as the selection of the abatement technology.

References:

  1. EPA reports 453-R-94-004, 453-R-93-04, 453-R-94-022, 453-R-94-057, 453-R-94-065
  2. EPA report. Stationary reciprocating internal combustion engines updated information on NOx emission and control techniques. September 2000.
  3. R. K. Agrawal, C. S. Wood. Chemical Engineering, February 2001, pp. 78-82.
  4. Petro-Chem Development Co., Inc. Technical papers. NOx reduction Symposium, Houston Texas, February 7-8, 2001.
  5. Reduction and Control of NOx Emission from High Temperature Industrial Processes, Edited by D. Keith Patrick, Industrial Heating, March 1998, pp. 77-85.



This article originally appeared in the July 2001 issue of Environmental Protection, Vol. 12, No. 7, p. 38.

This article originally appeared in the 07/01/2001 issue of Environmental Protection.

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