NOx Control
Typical NO-NO2 emission ratios
from combustion sources
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
NOx FORMATION MECHANISMS
• The most universal of these is thermal NOX formation. In thermal NOX
formation, NOX is formed by the reaction of N2 in the combustion air with
combustion reactants such as O and OH radicals. Thermal NOX is emitted
from virtually all combustion sources.
• Fuel NOX is formed when the nitrogen bound in the fuel is burned.
Obviously, the amount of NOX formed from this mechanism is a function of
the amount of nitrogen in the fuel. Coal and residual oil have significant
amounts of nitrogen that can generate half or three quarters of the total
NOx emissions. Distillate oils such as #2 or diesel oil typically have very
little nitrogen. Natural gas has no fuel nitrogen, and therefore no NOX is
formed by this mechanism.
• Prompt NOX, the third formation mechanism, forms NOX by converting
molecular nitrogen to NO via intermediate products. This reaction occurs
in the early phase in the flame front with hydrocarbons and is observed in
laboratory research studies. While prompt NOx may be responsible for
some of the NOx from practical combustors, it isn’t normally considered
when dealing with NOx emissions control
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Relation of thermal NOX formation and temperature
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
The main factors affecting the quantity of NOX formed by
thermal fixation are:
(1)the flame temperature,
(2) the residence time of the combustion gases in the peak
temperature zone of the flame,
(3) the amount of oxygen present in the peak temperature zone
of the flame.
Thermal NOX reduction is therefore accomplished by various
combustion modification techniques that either reduces the
peak flame temperature or the oxygen in the primary zone, or
both. These methods include:
(1)reducing the local oxygen concentration at the peak flame
temperature,
(2) reducing peak flame temperature,
(3) decreasing the furnace release rate
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Fuel NOx
• NOX generated from organically bound nitrogen
contained in fuel is termed fuel NOX. Organic nitrogen
(not N2) in the fuel burns along with the carbon and
hydrogen. It either forms N2 or it forms NOx. Although
only about 50% or less of the fuel nitrogen converts to
NOx, fuel NOx can constitute most of the total NOx
emissions from coal or any fuel with a high nitrogen
content (excluding N2).
• How much of the fuel nitrogen coverts to NOx depends
on the oxygen levels in the flame. Consequently, the
reduction of flame oxygen level is a key element in
reducing the emissions that are formed by the fuel NOx
mechanism.
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Possible paths of fuel nitrogen contained in coal
particles during combustion
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Staged Combustion
• Staged combustion burners, the most common type of low
NOx burners, achieve lower NOx emissions by staging the
injection of either air or fuel in the near burner region.
• The division of combustion air reduces the oxygen
concentration in the primary burner combustion zone,
lowering the amount of NOx formed and increasing the
amount of NOx reducing agents. Secondary and tertiary air
complete the combustion downstream of the primary zone,
lowering the peak temperature and reducing thermal NOx
formation.
Low NOx Burner
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Source: www.ctre.iastate.edu/educweb/CE524/NOx.ppt
Source: www.ctre.iastate.edu/educweb/CE524/NOx.ppt
Reburning in a Utility Boiler
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Flue Gas Recirculation (FGR)
Flue gas recirculation (FGR), also called exhaust gas
recirculatoin (EGR), works by mixing some flue gas with
the incoming combustion air.
This increases the mass flow through the combustion zone
and decrease the concentration of O2 available for
combustion.
Increasing the gas flow in the combustion zone decreases
the temperature, because the same amount of energy is
distributed to a larger thermal mass.
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Two versions of the concept of
drawing gas from the exhaust
ducts and mixing it with
combustion air.
The upper path, sometimes
called “induced FGR” uses the ID
fan inlet suction to draw gas from
the stack. This increases the flow
through the fan, but otherwise
there is no energy penalty.
The lower path, “active FGR”
requires a dedicated fan and
pumps relatively hot gas across is
significant pressure differential
into preheated combustion air which means that the FGR fan is
fairly large. Active FGR systems
use a significant amount of
power.
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Low Excess Air (LEA) combustion
As shown in the figure, we can minimize
diffusion flame NOx by reducing the excess air
as far as possible without excessive CO or
smoke.
This highlights the need to control air flow
precisely on any combustion system that is
attempting to minimize NOx. Air flow needs to
be maintained near the minimum practical
(not quite smoking) level. The automatic air
flow control system needs to do this
continuously as the boiler load changes.
Precise control of excess air is the first and
most important step in controlling emissions
from boilers and furnaces. This can only be
achieved by using an O2 monitor tied into
an intelligent (microprocessor based) control
system.
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
The NOx creation rate typically peaks at excess oxygen levels of 5 – 7% where
the combination of high combustion temperatures and the higher oxygen
concentrations act together. At both lower and higher air/fuel ratios, NOx
production falls off – due to lower flame temperature at high excess air levels
and lower oxygen at low excess air levels.
Source: http://www.cleanboiler.org/Images/RCT45xso.jpg
Reducing NOx with Water Injection
This figure is an example of the effect of water or steam injection
on turbine emissions. Water injection is not commonly used in
other types of combustion sources.
Water injection in a turbine
requires very clean water, but
there is only a small penalty to
engine efficiency. Water
provides cooling that would
otherwise have to be provided
with air - because the
maximum allowable turbine
inlet temperature is far below
the peak flame temperature in
the primary zone.
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
For NO removal:
4 NO + 4 NH3 + O2  4 N2 + 6 H2O
For NO2 removal:
4 NO2 + 8 NH3 + 2 O2  6 N2 + 12 H2O
This reaction is called “selective reduction” because it uses a
reagent to react with the NOx.
Selective Non-catalytic Reduction (SNCR)
• The first of these technologies was Thermal DeNOx. In this
technology, ammonia is sprayed into the post-combustion
area of a boiler or furnace when the temperature is 1650 °F.
Under these condition, a 50% reduction of NOx can be
achieved.
• The reaction is temperature sensitive, and the window is
small. If the temperature is to high, ammonia will convert
to NOx faster than the reduction process. If the
temperature is to low, the reaction with the NOx will not
occur.
• The NOxOUT process uses urea (CO(NH2)2) as a reactant
instead of ammonia. The urea gives the technology a
broader and slightly lower temperature window. Also, urea
can be transported and stored as a solid.
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Selective Catalytic Reduction (SCR)
• The chemical reactions are the same as SNCR, but the
reactions are catalytically driven. The temperature range
depends on the catalyst used:
• Conventional SCR Systems are called base metal systems.
They usually use a Vanadium Pentoxide catalyst. The best
performance will be in the range of 650° to 750°F.
• For low temperature applications, Precious Metal is used,
usually platinum. These systems can be used at
temperatures as low as 350°F, and as high as 550°F.
• High Temperature applications usually use Zeolite catalysts
and have been used successfully at temperatures as high as
1050°F.
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
SCR --Process Design Issues
• NOx Removal Efficiency (impacts costs and
ammonia slip)
• Ammonia Slip (Consideration of catalyst life vs.
ammonia slip)
• SO2 Oxidation to SO3
• System Draft Loss (imposing backpressure on the
combustion device)
• Catalyst Life Expectancy
• Ammonium Bisulfate Formation
• Application Specific Issues
Source: USEPA, APTI 418, NOx Emissions Control from Stationary Sources
Source:
www.ctre.iastate.
edu/educweb/CE5
24/NOx.ppt
Source: www.ctre.iastate.edu/educweb/CE524/NOx.ppt
NSCR - Non-selective Catalytic Reduction
• The catalytic reaction is based on the reaction
between Nitrogen Oxides and a fuel to give
nitrogen and water. The reducing fuel is
generally chosen by site availability & price
(typically Natural Gas, Light Naphtha or
Hydrogen).
• The fuel gas is introduced into the NOx process
upstream where it homogeneously mixes
before entering the catalytic bed. The following
reactions take place:
CH4 + 4NO2 ---> CO2 + 2H2O + 4NO
CH4 + 4NO ---> CO2 + 2H2O + 2N2
• NSCR or three-way catalyst is used in rich-burn
or stoichiometric engines for simultaneous
conversion of NOx, CO, HC, formaldehyde and
HAPs. NSCR catalysts are effective in a wide
variety of engine applications and fuels,
including natural gas, propane and gasoline. A
closed loop air-fuel ratio controller is required
for the three-way catalyst to work effectively.
Source: http://www.netl.doe.gov/technologies/coalpower/cctc/cctdp/project_briefs/limb/limbdemo.html