1. Clean Combustion Technologies
Clean Combustion Technologies
Overview
Energy and Furnace Technology
Wlodzimierz Blasiak, Professor Royal Institute of Technology (KTH) School of Industrial Engineering and Management Department of Materials Science and Engineering Division of Energy and Furnace Technology

2. Legislation in Sweden

3. Carbon monoxide It is the product of incomplete combustion and is:
Flammable (from 12,5 % up...)
Colorless,
Odorless gas,
Easy to mix with air,
Extremelly toxic (from 50 ppm can produce symptoms of poisoning),
- ALWAYS BE VERY CAREFUL and do measure it if you want be ...

4. Carbon monoxide – combustion (after-burning)
CO is subsequently slowly oxidised to CO2 by the reactions:
CO + OH = CO2 + H
H + H2O = H2 + OH
CO + H2O = CO2 + H2
Conversion of CO to CO2 in the post-flame zone gases is termed after-burning and depends on process design:
- cooling of flue gases,
- oxygen availability,
- residence time,
- water content.

5. Carbon monoxide – destruction is a must !
Destruction of most hydrocarbons occurs very rapidly at temperatures between 550 C and 650 C. Possible exception is methane which is stable molecule and require higher temperature (750 C) for oxidation in a few tenths of a second. It has been reported that the time required for the oxidation of CO is about 10 times the time needed for oxidation of hydrocarbons to CO. (slow reaction !) In the absence of water CO is extremely difficult to burn. Incinerator experience shows that temperatures of C are required with an actual residence time at this temperature of 0.2 – 0.4 seconds and 4 – 5 % O2 as a minimum to achieve nearly complete oxidation of CO to CO2. Units with poor mixing patterns exhibit outlet CO concentrations higher than 1000 ppm though temperatures are at 750 – 800 C level.

6. Thermal NO (nitric oxide) formation
The formation rate of thermal NO is dependent on;
the reaction temperature,
the local stoichiometry,
the residence time.

7. Summation on NOx formation
The NOx formation is depending on combustion conditions.
As with all chemical processes, the rate of formation of NOx is, among other things, a function of temperature and residence time.
NOx formation is reduced by both lowering the flame temperature and shortening the residence time of the combustion gases,
Lower (uniform !) flame temperature can be obtained by:
mixing the fuel with large excess of combustion air,
Control of mixing (eliminate ”hot spots”)

8. Available Technologies
Removal of the source of pollution (sulphur, nitrogen, ..) from fuel,
Pre-combustion approach removes impurities such as sulphur, from the coal before it is burnt. Among possible methods one may distinguish coal cleaning and upgrading, coal blending, coal switching and bioprocesses.
2. Avoiding the production of the pollutants during combustion (so called primary measures or in-furnace measures),
3. Removing the pollutants from the flue gases by “end of pipe“ technologies prior to emission.

9. Primary measures of NOx reduction – strategy of NOx reduction during formation/combustion
Control of concentration of oxygen contacting with fuel (air excess control) through air staging and mixing of fuel and air.
- Control of oxygen concentration distribution in whole volume of combustion,
- Low but high enough (to complete combustion) oxygen concentration
Control of combustion temperature (flame) through increase of combustion zone as result flue gas recirculation (Dilution).

10. NO species versus stochiometry (pulverised coal combustion)

11. Why control of temperature, oxygen concentration and time is so important ?
Thermal NO - strongly depends on temperature), less dependent on O2.
- reduction at first through limitation of temperature and oxygen avialbaility as well as residence time).
Fuel NO – strongly depends on O2 and much less on temperature.
- reduction through limitation of oxygen during first stage of combustion (during devolatilisation),
- and through monitoring/control of coke residue combustion it means through control of oxygen concentration, temperature and residence time along the coke residue particles way.

12. Methods to limit formation of NO during combustion process (primary methods)
Combustion air staging through:
- Air staging (basic method),
- Fuel staging,
- Flue gas recirculation (internal, external). Does not reduce very much efficiency (change of relation between convection and radiation) but may create operational problems,
- Injection of water/steam … (risk of efficiency drop and corrosion).

13. Methods to reduce NO already formed during first stages of combustion
B. Reduction inside combustion chamber
- SNCR (Selective Non Catalytic Reduction) – introduction of ammonia chemicals (ammonia, trona) into combustion chamber,
- Reburning – introduction of secondary fuel (gas, coal, …) which creates CHi or/and NH3 reducing NO.

14. Methods to reduce already formed NOx at the boiler outlet (outside combustion chamber and process)
C. Reduction performed at the outlet of flue gases:
SCR (Selective Catalytic Reduction) – introduction of ammonia chemicals into low temperature flue gases between economiser and air heater.
SCR disadvantages:
- high cost of investment dependent on NOx reduction level,
- high operational cost ,
- risk of ammonia slip,
- catalyst life time,
- storage of used catalysts.

15. Selective Catalytic Reduction

16. Selective Catalytic Reduction - SCR

17. Selective Catalytic Reduction

18. Air Staging, Over Fire Air (OFA)
Mixing
Primary combustion zone
Secondary combustion/mixing zone
Fuel
Secondary air
Primary air
Flue gases

19. New look at Air Staging process (air staging with extensive internal recirculation-mixing)
Intermediate zone
korozja
Mixing
Primary combustion
(l<1)
Secondary combustion
(l > 1)
fuel
Secondary air (OFA, ...)
Primary
air
Flue gases

20. Air Staging with external flue gas recirculation
mixing
Primary combustion zone
Secondary combustion/mixing zone
Flue gases
Secondary air
fuel
Primary air

21. Air staging – secondary air injection methods
Direct injection of secondary air through air nozzles placed on walls:
1. Conventional OFA (Over-Fire-Air) – system of many low pressure nozzles,
Allows primary air reduction down to % of theoretical air required with high risk of corrosion, CO emission and LOI increase
2. Advanced Rotating OFA system – system of high pressure air nozzles asymetricaly placed on walls.
Allows reduction of primary air down to % of theoretical air without creating corrosion or CO and LOI.

22. Air staging - burners

23. Air staging - burners

24. Air staging – boilers, furnaces

25. NOx versus type of combustion chamber

26. System of low pressure nozzles – 1 (conventional OFA)
Main disadvanatge: week control of flow and oxygen concentration by OFA

27. System of many low pressure air nozzles, OFA
Problem seen – low oxygen content, high temperature corrosion of walls

28. Rotating OFA Widok z góry Widok z boku duża prędkość powietrza
Paliwo/powietrze
Paliwo/powietrze

29. Homogenous temperature profile in furnace
From CFD

30. Baseline/ROFA comparison – NOx
From CFD

31. Increased particle residence time and reduced LOI

32. Conventional combustion
Gas reburning in PC boiler
Complete combustion zone
OFA
(overfire air)
Reburning zone
Gas, biomass
20%
coal 100%
coal 80%
Primary combustion zone
Conventional combustion
Gas REBURNING

33. Reburning - theoretical concept

34. Retrofiting to reburning

35. Retrofiting to reburning

36. Reburning and Reb+SNCR

37. NO + CHi  HCN  NCO NH  N N2
NOx reduction via co-firing (reburning)
Biomass combustion is considered CO2 neutral when grown and converted in a closed-loop production scheme
NOx may be reduced by extended fuel staging or reburning (high volatile and low N content in biomass)
NO + CHi  HCN  NCO NH  N N2
SOx reduced by decreased sulphur content in the biofuel
(often proportionally to the biofuel thermal load)
Sulphur content in coal: mg S/MJ, average 217 mg S/MJ
Sulphur content in peat: mg S/MJ, average 127 mg S/MJ
Sulphur content in oil (average): 72 mg S/MJ
SOx reduced by sulphur retention in alkali biofuel compounds

38. NOx reduction by the in-furnace measures

39. Selective Non-Catalytic Reduction - SNCR
SNCR technique employs direct injection of a nitrogenous reagent (normally ammonia – NH3) into the flue gas stream. NOx is reduced by gas-phase, free radical reactions. Process is however effective over a realtively narrow temperature range.
- Ammonia - (NH3) (temperature 900 – 1000 C)
- Urea - (NH2)2CO (temperature up to 1100 C)
4NO + 4 NH3 + O2  4N2 + 6 H2O
At low temperature reaction is very slow and NH3 passes unreacted into the back end of the plant, where it forms corrosive ammonium salts which can also cause fouling.
At high temperature, the injected NH3 is oxidised to form NOx, so that NOx emission can actually increase.

40. SNCR – Temperature window for NO reduction (input about 500 ppm NOx, NH3 molar ratio to NO 1.6) ref.

41. SNCR - Selective Non-Catalytic Reduction
Practical problems with SNCR are results of:
Non-uniform temperature distribution at the injection level of NH3,
2. Too short residence time. Optimum about 1 sek but not shorter then 0.3 sek
Not good mixing because of:
NOx concentration is not unform and not stable at the injection level
mixing system does not follow the changes of flow with changes of load.

42. Ammonia slip because of too short residence time and low quality mixing

43. Reburning combined with SNCR (for deep NOx reduction)

44. Reburning and SNCR

45. Reburning combined with SNCR

46. Location of various sorbent inputs in a typical power station

47. De-SOx methods
Wet scrubber systems capable of achieving reduction efficiencies up to 99 percent
Spray dry scrubbers, also known as semi dry, which can achieve reduction efficiencies of over 90 percent
Dry sorbent injection, the lowest cost SOx removal technology and the most appropriate technology if large reduction efficiencies are not required

48. SOx reduction – dry sorbent injection
When limestone, hydrated lime or dolomite is introduced into the upper part of the furnace chamber, the sorbent is decomposed, i.e. decarbonised or dehydrated in accordance with the following reactions:
CaCO3 + heat (825–900oC)  CaO + CO2
Ca(OH)2 + heat  CaO + H2O
and then, lime reacts with SO2 in accordance with the below-described reactions :
CaO + SO2  CaSO3 + heat
CaO + SO2 + ½ O2  CaSO4 + heat
Furnace sorbent injection provides the additional benefit of removing SO3, chlorides, and fluoride from the flue gas as follow:
CaO + SO3  CaSO4 + heat
CaO + 2 HCl  CaCl2 + H2O + heat
CaO + 2 HF  CaF2 + H2O + heat

49. SO2 removal reactions in furnace sorbent injection

50. SOx reduction – dry sorbent injection

51. SO2 removal at different temperature windows for sorbent injection

52. SOx reduction – dry sorbent injection

53. Wet de-SOx methods
Fresh slurry is continuously charged into the absorber. Reduction of sulphur dioxide creates calcium sulphite according to the reaction:
SO2 + H2O  H2SO3
CaCO3 + H2SO3  CaSO3 + CO2 + H2O
An oxidation step, either as an integrated part of the scrubbing process (in situ oxidation) or in separate vessel, can convert the sulphite residue to calcium sulphate:
CaSO3 + ½ O2 + 2 H2O  CaSO4  2 H2O
Overall reaction can be written as follows:
CaCO3 + SO2 + ½ O2 + 2 H2O  CaSO4  2 H2O + CO2
After precipitation from the solution calcium sulphate, is a subject to further treatment (washing and dehydration) and eventually produces a usable gypsum rest product.

54. Wet de-SOx methods

55. CO2 reduction

56. Cofiring strategies and their requirements
Wood firing percentage
(heat input)
Material preparation
Strategy required
Firing strategy required
Boiler investment required
2-5
- co-pulverize with coal
- separate receiving and handling (cyclone)
- fire with coal
- fire in secondary air system (cyclone)
- use existing boiler
10-15
- separate receiving and handling (PC)
- separate receiving, common storage (cyclone)
- separate burners (PC)
- fire with coal (cyclone)
use existing boiler
15-35
- reburning strategy: separate receiving and preparation
- fire above coal burners or cyclone barrels
- use existing boiler heavily modified, overfire air
25-50
- separate receiving and handling of alternative fuels
- fire separately in common, multifuel boiler
- new boiler designed with biomass parameters (e.g. fluidized bed)

57. Co-firing with gasified biomass (reburning)
Introduction of chlorine and alkali compounds into furnace is avoided

58. Thank you