7.2 NOx Control in Flue Gas

 

As much as 40 % of the total quantity of NO_x  ejected into the atmosphere from Thermal power plant.

Nitrogen Oxides are produced in two ways during fuel combustion:

1.      1. Chemical reaction between aerial Nitrogen and Oxygen in the high temperature combustion zone (above 1600 °C).

2.      2. Oxidation of Nitrogen content in fuel. This can take place below 1600 °C.

       Some of the techniques for NOx control are as below:

Technique	Description	Advantage	Disadvantage	Impact to consider	Applicability	NOx Reduction
1. Low excess air (LEA)	NOx reduction by reducing oxygen availability	Ease of operation. Amenable to up gradation	DeNOxing potential is low.	High CO emission. Flame stability. Flame length which should not be too high to avert licking of furnace wall.	All fuels	1 - 15 %
2. Low NOx Burners (LNBs)	Staged combustion thru internal staging thus reducing peak flame temp.  an O2 availability	Low operating cost Compatible with FGR as a backup technology to augment deNOxing.	Moderately high capital cost. Efficiency varies with fuel composition & characteristics, burner design, waste stream etc.	Flame length. FD Fan capacity. TD flame stability. Design compatibility.	All fuels.	30 – 50 %
3.Flue Gas Recirculation (FGR)	AS much as 20-30 % of FG is recirculated back into the furnace and thus reducing oxygen concentration in the combustion zone.	High NOx reduction potential for NG and other fuels with lox fuel bond nitrogen.	Moderately high capital cost. Moderately high operating cost. Affects system pressure. Affects heat transfer.	FD fan capacity. Pressure drop in burner nozzle. Furnace pressure. TD flame stability.	Gaseous Fuels All low Nitrogen fuel.	40 – 80 %
Selective catalytic reduction (SCR)	Ammonia/ Urea solution dozed into the flue gas stream upstream of Pt honeycomb catalyst bed (V2O5 based on TiO2)	High deNOxing potential	Very high capital cost. High operating cost. Extensive duct work is required to and from the reactor. Increased pressure drop taxes more ID fan power. Reduced efficiency ammonia sulphate precipitation. Requires water wash of air heater.	Space requirement. Ammonia slippage. Hazardous waste disposal problem.	Gaseous Fuels All low sulphur solid and liquid fuel.	70 – 90 %
Selective Non Catalytic Reduction (SNCR)	Injection of Ammonia/ Urea solution into the furnace	Low capital cost. Simple operation. Nontoxic dosing agent. Moderate NOx removal Simple system that involves no hazard.  Low injection energy	Moderately high capital cost. Essential a thermal deNOxing process. Depends on temp. NOx reduction potential drop at lower load. Design must consider boiler operating condition and design.	Temp profile. Furnace geometry. Residence time.	All fuels	25 – 50 %
Chemical Reaction	NO + NO2 + 2NH3 ------------------> 2N2 + 3H2O

 Reference:

-          Boiler Operation Engineering by P. Chattopadhyay.