1. ATMOSPHERE PROTECTION TECHNOLOGY April 22nd, 2013

2. CONTROL of NITROGEN OXIDES

4. What are NOx?.... What are NOy ? There are three primary sources of NO x in combustion processes: thermal NO x fuel NO x prompt NO x Thermal NO x formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. O + N 2  NO + N.... limiting is the concentration of thermally arising atomic oxygen N + O 2  NO + O N + OH·  NO + H·

5. Fuel NO x formation takes place predominantly during the combustion of fuels, such as coal, which have a significant nitrogen content, particularly when burned in combustors designed to minimise thermal NO x. Nitrogen in fuel is oxidized, but NH· radicals can be formed which reduce back the NO x to N 2.... NO x are then under stoichiometry Only around 20% of the char nitrogen is ultimately emitted as NO x, since much of the NO x that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.

6. Prompt NO x are normally considered negligible. This third source is attributed to the reaction of atmospheric nitrogen, N 2, with radicals such as C, CH, and CH 2 fragments derived from fuel. Occurring in the earliest stage of combustion and on the borderline between the flame and flue gases, this results in the formation of fixed species of nitrogen such as NH (nitrogen monohydride), HCN (hydrogen cyanide), H 2 CN (dihydrogen cyanide) and CN · (cyano radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt NO x is especially minimal and it is generally only of interest for the most exacting emission targets. There is good evidence that prompt NOx can be formed in a significant quantity in some combustion environments, such as in low-temperature, fuel-rich conditions and where residence times are short. Surface burners, staged combustion systems, and gas turbines can create such conditions Prompt NOx are most prevalent in rich flames. The actual formation involves a complex series of reactions and many possible intermediate species. The route now accepted is as follows: N 2 + CH·  HCN + N· N· + O 2  NO + O HCN + OH·  CN· + H 2 O CN· + O 2  NO + CO A fourth source, called feed NO x is associated with the combustion of nitrogen present in the feed material of cement rotary kilns, at between 300° and 800 °C, where it is also a minor contributor.

9. Control techniques There are two measures of control techniques for NOx emissions: A.Primary measures - Combustion controls.... 3 generations B.Secondary measurements - Flue gas treatment Usually combination of both measures are used. It can be, however, quite difficult to evaluate the overall process, as there are many possibilities within both of these measures.

10. A.Primary measures First generation 1.Low-excess air firing

12. 2. Off-stoichiometric combustion (OSC) = staged combustion = overfire air

13. 3. Burners out of service (BOOS)

14. 4. Burners in different modes 5. Flue gas recirculation Recirculation of the flue gases has two effects: a) the oxygen concentration on the primary flame zone is decreased b) additional nitrogen absorbs heat and reduces the peak flame temperature

16. 6. Reduce air preheat Combustion air is often preheated in a recuperator with the heat from the flue gases. This saves energy, however, it also raises the peak flame temperature because the combustion air absorbs less heat from the combustor prior to reacting with the fuel.Reducing air preheat lowers the flame temperature.

17. Second generation.... 30 – 50%, mainly new kettles 1.Low-NOx burners

19. 2. Flue gas recirculation 3. Overfire air

22. Third generation.... Upto 80%, totally new kettles Ultra Low-NOx burners

23. B.Secondary measures - Denitrification of flue gases Used, if primary measures insufficient

24. Dry – Wet denitrification Wet denitrification unimportant from the technological point of view, NO is rather non-reactive, washout by Fe 2 O 3 or Chelaton and other media – very low efficiency. Better: First oxidation to NO 2, then washout. Selected catalytic reduction (SCR) – Selective Noncatalytic Reduction (SNR)

25. Selective catalytic reduction (SCR)

27. Catalyst’s lifetime... Dependent on the fuel used.... 3 factors: poisoning by trace elements (As)... At 1400°C oxidation to volatile As 2 O 5, then sedimentation to fly ash and the catalyst dust and other particles sedimentation erosion After 1,25 year abou 1/3 of the catalyst is replaced.

28. Selective noncatalytic reduction (SNCR) Ammonium water or Urea Main reactions: 4NH 3 + 4NO + O 2  6H 2 O + 4N 2 NH 2 – CO - NH 2 + 2NO + 0,5O 2  CO 2 + 2N 2 + 2H 2 O Temperature window 800 – 1100°C, if exceeded NH 3 gives NO, at lower t reaction slows down and the concentration of NO in flue gases increases. Important: the size of ammonia droplets

29. Minority reaction: 4NH 3 + 4NO + O 2  6H 2 O + 4N 2 Also occuring: reactions without oxygen access: 4NH 3 + 6NO  6H 2 O + 5N 2 8NH 3 + 6NO 2  12H 2 O + 7N 2 Unwanted side-reactions NH 3 + O 2  NO, NO 2, N 2 O... mainly at t ≥ 1200°C, no catalyst SO 2 + O 2  SO 3.... SO 3 + NH 3 + H 2 O  NH 4 HSO 4 NH 3 as a reductant..... Exxon-NH 3 (USA) Thermal Denox (Europe)

30. Important: Transport of ammonia or urea.... pipes inserted to the walls of the kettle have to be cooled down, at 1000°C they would burn down

31. Low-temperature oxidation with absorption

32. Low-temperature oxidation process.

34. Have A Nice Day!