Nitrogen oxides (NOx) are produced during combustion processes and can harm human health and the environment. Selective catalytic reduction (SCR) is a process that uses a catalyst to convert NOx in exhaust gases into less harmful nitrogen and water. SCR systems inject ammonia or urea into exhaust to facilitate the reaction on the catalyst. Proper operation of SCR systems and monitoring of emissions helps control NOx and improve air quality.

1. of NOx from Exhaust Stack

2. Nitrogen Oxide Nitrogen oxide is typically any binary compound of oxygen and nitrogen , or a mixture of such compounds: Nitric oxide (NO), nitrogen(II) oxide - extremely toxic Nitrogen dioxide (NO 2 ), nitrogen(IV) oxide - extremely toxic Nitrous oxide (N 2 O), nitrogen (I) oxide - undesirable Dinitrogen trioxide (N 2 O 3 ), nitrogen(II,IV) oxide Dinitrogen tetroxide (N 2 O 4 ), nitrogen(IV) oxide Dinitrogen pentoxide (N 2 O 5 ), nitrogen(V) oxide Source: Wikipedia

3. NOx NO x is a generic term for mono-nitrogen oxides (NO and NO 2 ). These oxides are produced during combustion , especially at high temperatures. At ambient temperatures, the oxygen and nitrogen gases in air will not react with each other. In atmospheric chemistry the term NO x is used to mean the total concentration of NO plus NO 2 . Source: Wikipedia

4. Industrial Sources The three primary sources of NO x in combustion processes: Thermal No x - produced when nitrogen and oxygen in the combustion air supply combine at high flame temperatures. Thermal NOx is generally produced during the combustion of both gases and fuel oils. Fuel No x - produced when nitrogen in the fuel combines with the excess oxygen in the combustion air and is only a problem with fuel oils containing fuel bound nitrogen. Prompt NO x - Attributed to the reaction of atmospheric nitrogen, N 2 , with radicals such as C, CH, and CH 2 fragments derived from fuel. Formed during the early, low temperature states of combustion and is insignificant. Source: Wikipedia & http://www.bacharach-training.com/combustionzone/nox1.htm

5. Health Effects NO x react with ammonia , moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases such as emphysema , bronchitis it may also aggravate existing heart disease. Source: Wikipedia

6. NO x react with volatile organic compounds in the presence of heat and sunlight to form Ozone . Ozone can cause adverse effects such as damage to lung tissue and reduction in lung function mostly in susceptible populations (children, elderly, asthmatics). Ozone can be transported by wind currents and cause health impacts far from the original sources. Health Effects Source: Wikipedia

7. NO x (especially N 2 O) destroys ozone layer . This layer absorbs ultraviolet light , which is potentially damaging to life on earth. NO x also readily react with common organic chemicals, and even ozone, to form a wide variety of toxic products: nitroarenes , nitrosamines and also the nitrate radical some of which may cause biological mutations . Health Effects Source: Wikipedia

8. 1999 National Emissions by Source: Nitrogen Oxides 1999 National Emissions by Source: Nitrogen Oxides (On-Road Mobile Sources) 1999 National Emissions by Source: Nitrogen Oxides (Nonroad Mobile Sources Source: http://www.epa.gov/oms/invntory/overview/pollutants/nox.htm

9. Selective Catalytic Reduction Means of converting nitrogen oxides , also referred to as NO x with the aid of a catalyst into diatomic nitrogen , N 2 , and water , H2O. known to reduce the NOx emissions by nearly 70-95%. SCR provides emissions after-treatment well into the exhaust stack. Commercial selective catalytic reduction systems are typically found on large utility boilers , industrial boilers , and municipal solid waste boilers and have been shown to reduce NOx by 70-95%. [1] More recent applications include diesel engines , such as those found on large ships, diesel locomotives , gas turbines , and even automobiles . Source: Wikipedia.com About.com & Ezinearticles.com

10. SCR – The Process Engine: The NOx reduction process starts with an efficient CRD engine design that burns clean Ultra Low Sulfur Diesel (ULSD) and produces inherently lower exhaust emissions—exhaust that is already much cleaner due to leaner and more complete combustion. Diesel Exhaust Fluid (DEF) tank and pump: Under the direction of the vehicle’s onboard computer, DEF is delivered in precisely metered spray patterns into the exhaust stream just ahead of the SCR converter. Source: About.com

11. SCR – The Process SCR Catalytic Converter: This is where the conversion happens. Exhaust gases and an atomized mist of DEF enter the converter simultaneously. Together with the catalyst inside the converter, the mixture undergoes a chemical reaction that produces nitrogen gas and water vapor. Control device: Exhaust gases are monitored via a sensor as they leave the SCR catalyst. Feedback is supplied to the main computer to alter the DEF flow if NOx levels fluctuate beyond acceptable parameters. Source: About.com

12. SCR - Chemistry The chemical equation for a stoichiometric reaction using either anhydrous or aqueous ammonia for a selective catalytic reduction process is: 4NO + 4NH 3 + O 2 -> 4N 2 + 6H 2 O 2NO 2 + 4NH 3 + O 2 -> 3N 2 + 6H 2 O NO + NO 2 + 2NH 3 -> 2N 2 + 3H 2 O Source: Wikipedia

13. SCR - Chemistry With several secondary reactions: 2SO 2 + O 2 -> 2SO 3 2NH 3 + SO 3 + H 2 O -> (NH 4 ) 2 SO 4 NH 3 + SO 3 + H 2 O -> NH 4 HSO 4 Source: Wikipedia

14. The reaction for urea instead of either anhydrous or aqueous ammonia is: 4NO + 2(NH 2 ) 2 CO + O 2 -> 4N 2 + 4H 2 O + 2CO 2 SCR - Chemistry Source: Wikipedia

15. SCR - Catalyst Manufactured from various ceramic materials used as a carrier, such as titanium oxide , and active catalytic components are usually either oxides of base metals (such as vanadium and tungsten ), zeolites , and various precious metals . Source: Wikipedia

16. SCR – Catalyst : Base Metal Catalyst Examples: vanadium and tungsten Lack high thermal durability Less expensive and operate very well at the temperature ranges most commonly seen in industrial and utility boiler applications high catalyzing potential to oxidize SO 2 into SO 3 , which can be extremely damaging due to its acidic properties Source: Wikipedia

17. SCR – Catalyst - Zeolite have the potential to operate at substantially higher temperature than base metal catalysts can withstand prolonged operation at temperatures of 900 K and transient conditions of up to 1120 K have a lower potential for potentially damaging SO 2 oxidation. Source: Wikipedia

18. SCR – Catalyst : Fe- & Cu-exchanged zeolite urea Approximately equal performance to that of vanadium-urea SCRs if the fraction of the NO 2 is 20% to 50% of the total NO x Two most common designs of SCR catalyst geometry used today are honeycomb and plate Plate-type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, but plate configurations are much larger and more expensive. Honeycomb configurations are smaller than plate types, but have higher pressure drops and plug much more easily. Source: Wikipedia

19. SCR - Reductants Remember: A reducing agent (also called a reductant or reducer) is the element or compound in a redox (reduction-oxidation) reaction (see electrochemistry ) that reduces another species . In doing so, it becomes oxidized, and is therefore the electron donor in the redox. Several reductants are currently used in SCR applications including: Anhydrous ammonia Extremely toxic and difficult to safely store, but needs no further conversion to operate within an SCR. Typically favored by large industrial SCR operators Aqueous ammonia must be hydrolyzed in order to be used, but it is substantially safer to store and transport than anhydrous ammonia. Urea the safest to store, but requires conversion to ammonia through thermal decomposition in order to be used as an effective reductant. Source: Wikipedia

20. SCR – The Process

21. SCR - Limitation If unsuitable materials are used in the manufacture of the units, ions can pass from the dispensing materials into the porous head on the SCR unit that spoils the SCR's efficacy and can reduce its lifespan by more than 60%. Ammonia slip or release of unreacted ammonia Slip can occur when catalyst temperatures are not in the optimal range for the reaction or when too much ammonia is injected into the process. Source: Wikipedia

22. NOx Monitoring The purpose of the monitoring is to verify the company's compliance with the air quality standards of the DENR Administrative Order DAO No. 2000-81 (Implementing Rules and Regulations of the Philippine Clean Air Act of 1999) - for Stack Method of Sampling & Analysis : Grab Sampling – Phenolsulfonic Method (US EPA Method 7) DENR ADMINISTRATIVE ORDER No. 46 Series of 1998 – for motor vehicles Method of Sampling & Analysis : Direct Reading

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