Microsoft Word master thesis report fina doc

 2.2.5. Selective non catalytic reduction (SNCR) 
SNCR is a postcombustion technique consisting in injecting ammonia or urea into combustion 
flue gases. A reaction with NOx occurs to produce nitrogen and water. There are not many 
experiences to evaluate effectiveness of this technique. 
2.2.6. Selective catalytic reduction (SCR) 
SCR is another postcombustion technique consisting in injecting ammonia into the 
combustion zone in presence of a catalyst to reduce NOx into nitrogen and water. This 
method allows achieving NOx emission reduction by 75 to 90%. This technique is rather 
common. 
Both SNCR and SCR are influenced by sulfur content of the flue gas. 
The table below (table 2-1) summarizes available techniques for NOx control for process 
heaters. It uses information from the “Alternative Control Techniques Document - NOx 
Emissions from Industrial/Commercial/Institutional (ICI) Boilers” published by the US EPA.
This table can help in a first approach for identifying the best technology to use in function of 
the type of boiler and the fuel. However, many other parameters are to be taken into account 
such as the NOx emissions threshold wanted, the budget, etc. 

---

7
Fuel / boiler 
NOx control 
% NOx 
reduction 
Advantages Drawbacks 
Residual oil / 
watertube 
LNB 
20 - 50 
- Relatively inexpensive 
- Minimal furnace modification, retrofit feasible for old units 
- Many designs and vendors available. 
- Specific emissions data from industrial boilers with LNB are lacking. 
- Staged air burners could result in flame impingement on furnace walls of 
smaller units. 
FGR 
4 - 30 
- Available 
- Best suited for new units 
- Requires extensive modifications to the burner and windbox. 
- Possible flame instability at high FGR rates. 
SCA 
5 - 50 
- BOOS applicable for boilers with multiple burners only. 
- Retrofit is not feasible or not available for all design types. 
LNB + FGR 
N.A. 
LNB + SCA 
N.A. 
SNCR 
40 - 70 
- Commercially offered. 
- Not widely demonstrated on large boilers. 
- Elaborate reagent injection, monitoring, and control system required. 
- Must have sufficient residence time at proper temperature. 
SCR 
N.A. 
- Applicable to most boiler designs as a retrofit technology . 
- Available but not widely demonstrated. 
Distillate oil 
/ watertube 
LNB 
20 - 50 
- New burners generally applicable to all boilers. 
- Comercially available. 
- Specific emissions data from industrial boilers equipped with LNB are lacking. 
FGR 
20 - 68 
- Available. 
- Best suited to new units. 
- Requires extensive modifications to the burner and windbox. 
- Possible flame instability at high FGR rates. 
SCA 
17 - 44 
- Limited application except BOOS, Bias and OFA for large watertube. 
LNB + FGR 
N.A. 
- Most common technique. 
LNB + SCA 
N.A. 
- Common technique. 
SNCR 
40 - 70 
- Commercially offered 
- Not widely demonstrated on large boilers. 
- Elaborate reagent injection, monitoring, and control system required. 
- Must have sufficient residence time at proper temperature. 
Natural gas / 
watertube 
WI 
50 - 77 
- Thermal efficiency loss of 0.5 to 2.5% and CO increase is expected. 
SCA 
15 - 50 
- BOOS applicable for boilers with multiple burners only. 
LNB 
40 - 85 
- Popular technique. Many designs and vendors available. 
- LEA LNBs more applicable to single-burner systems. 
- Staged air burners could result in flame impingement on furnace walls. 
FGR 
50 - 75 
- Requires extensive modifications to the burner and windbox. 
LNB + FGR 
55 - 90 
- Most popular technique for clean fuels. 
LNB + SCA 
N.A. 
- Applicable principally to multi-burner boilers. 
SNCR 
10 - 40 
SCR 
80 – 90
- No data available. 
Residual oil / 
firetube 
LNB 
30 – 60 
- Staged air could result in operational problems. 
SCA 
50 
- Technique not practical unless incorporated in new burner design. 
Distillate oil/ 
firetube 
LNB 
20 - 50 
- Several designs are available. 
- Specific emissions data from industrial boilers with LNB are lacking. 
FGR 
55 - 75 
- Effective technique for clean fuels. 
- Requires extensive modifications to the burner and windbox. 
Natural gas / 
firetube 
SCA 
5 
- Technique not practical unless incorporated in new burner design. 
LNB 
30 - 80 
- Several designs are available. 
- Specific emissions data from industrial boilers with LNB are lacking. 
FGR 
55 - 75 
- Effective technique used in many applications. 
LNB + FGR 
N.A. 
- Most popular technique for very low NOx levels. 
Table A-2-1: Comparison of available techniques for NOx control for process heaters

---

8
2.3. Control techniques for SOx emissions reduction 
On the contrary to NOx, SO
x
emissions are directly linked to the initial sulfur content of the 
fuel and the combustion parameters do not influent on the amount of SOx emitted. Two 
strategies can be used to reduce SOx emissions: the formation prevention (low sulfur fuel 
usage, fuel desulfurization) or the flue gas desulfurization (wet or dry scrubbing, dual-alkali, 
spray drying, Wellman-Lord process, etc.).
There are many postcombustion flue gas desulfurization techniques. Almost all techniques are 
based on the acid-alkaline reaction between SO
2
(and SO
3
) and an alkaline agent such as often 
lime or limestone, caustic soda, magnesium hydroxide or ammonia. Other techniques are 
selective adsorption or absorption.
Flue gas desulfurization is mostly used in thermal power plant. Few refineries have a flue gas 
desulfurization, except in Japan where principally dry processes are used. The principles of 
four major techniques are given below.