Combustion
DEFINITIONS
Flame
The Process of Combustion of Inflammable materials producing Heat and Light
and (often) smoke
Combustion
The act of burning something
A Process in which a substance reacts with Oxygen to give Heat and Light
A state of violent disturbance and excitement
Joule Cycle
Compressed greater mass flow rate
inject more fuel
Cycle
Compressed air to the combustor
inject fuel
Typical
Gas Turbine Performance as a
Function of P and T
Hot gas to inlet of turbine
Pressure ratio determined by the
Pressure Ratio 30+ final stage
compressor
compressors
Temperature from the turbine
Compressor shock problem due to
transonic fuel
Increasing
Temperature and Pressure
Performance increases
Turbine needs high cooling
Combustor
pressure
drop increase
Torque or thrust
generated
from losses
Efficiency
turbine reduced
Considerations
Combustor Boundary Conditions
Inlet - fuel, air and second air
1. Fuel
a. Compositions
b. Available Pressure
c. Temperature
d. Mass Flow
2. Air ( from compressor )
a. Pressure
b. Temperature
c. Mass Flow
d. Flow Pattern laminar transition turbulent
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e. Cooling Requirements combustor itself needs cooling, some decay
through walls, therefore need to inject secinair into combustor cooling
and dilute and make lean combustion better. Air left is lean
3. Stability/Transient Performance injection introduce unsteady
phenomena
Outlet
1. Exit Temperature - assess efficiency of the combustion high T to first
turbine efficiency better
2. Exit Temperature Profiles: six around ring hotspots impinging on
turbine uniform is better
a. Radial Temperature Distribution ( RTDF )
b. Orbital Temperature Distribution ( OTDF )
3. Exit Pressure
4. Exit Flow Profiles velocity profile links to flow distortion
5. Emissions
a. NOx
b. CO2
c. CO
d. Unburned Hydrocrabons ( UHCs )
Combustion Technology Drivers
[1]Increased Component Life turbine life as hot gas impinges on turbine
blade
[2]Reduced Maitenance and Inspection
[3]Cost
[4]Low Emissions NOx
[5]Fuel Flexibility
[6]Higher Cycle Efficiency
Some Chemistry
The simplest hydrocarbon fuel is pure methane, with a basic representation of the
combustion reaction being:
Stoichiometric
Equivalence
ratio ( )
Rich
Lean
Correct balance of Fuel and Oxygen so that theoretically all the
Fuel may be consumed leaving no Oxygen
Ratio of actual mass of Fuel to Stoichiometric Fuel Mass
Requirement
More Fuel than Stoichiometric ( > 1)
More Air than Stoichiometric ( < 1)
Conventional Gas Turbine Combustion
Diffusion Flame Combustor:
Turbulent non-premixed flame
-
Ease to control
Good turn-down
Few problems with combustion dynamics
Maximum Temperature in
Flame - approximately
Stoichiometric Flame
Temperature
More surface area vapour
better
Swirl increase the air
fuel mixing
Fuel Flexibility
Fuels used in Gas Turbines:
Natural Gas - Diesel - Kerosene - Synthetic Gases from Biomass, Coal and Wastes
(Wood Gas) - Landfill and Sewage Gas - Mines Gas (Coal Bed Methane) - High
Hydrogen Gases - Wellhead Gases - Coke oven Gas - LPG (Liquid and Gaseous) Naphtha - Crude Oil
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Carbon Dioxyde
NOx
Oxides of Nitrogen (NOx) - Nitric
Oxide (NO) - Nitrogen Dioxide
(NO2) - Nitrous Oxide (N2O)
3 quoted NOx formation
Mechanisms
1.
Fuel
NOx fuel
not very pure, regenerate through combustion process
2. Thermal NOx
3. Prompt NOx
An additional mechanism, the Nitrous Oxide Route can also occur
Thermal NOx (Zeldovich Mechanism)
Thermal rate of formation strongly dependent on temperature
This Reaction requires the bond between the atoms in the Nitrogen molecule to be
broken
Strong Bond
Slow Reaction except at high temperature
- Maximum Temperature is the system critical in
determining the amount of NOx produced
Time Combustion products spend at High Temperature has a significant effect on the
final level of NOx production
Prompt NOx (Fenimore Mechanism)
Prompt NOx quickly produced within the flame
Complicated method
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Governed by the presence of broken down fragments of Fuel. The CH radical is
produced as a Hydrocarbon Fuel is broken down in the early stages of Combustion. This
reacts with Molecular Nitrogen to form Hydrogen Cyanide (HCN) in the following
reaction
For Lean and near Stoichiometric Combustion ( < 1.2), the following chain
sequence results in the formation of NO
For a Rich Combustion
( > 1.2), the Mechanism becomes more complicated and no longer rapid
Fuel NOx
Minimal for Clean Gaseous and Distillate Fuels normally used in Gas Turbine
Combustion Systems
Some Distillate Fuels can have significant Fuel Bound Nitrogen
Fuel NOx is however significant for Gaseous Fuels derived from sources such as Coal,
Refinery waste and Biomass
Nitrogen in the original Fuel is rapidly converted to Hydrogen Cyanide (HCN) or
Ammonia (NH3)
HCN is converted to NO in the same way as the prompt mechanism
NH3 undergoes a similar Oxidation Process
Note: Under Rich Conditions, the Fuel Bound Nitrogen can be reduced to Molecular
Nitrogen
Nitrous Oxide ( N2O )
NO generation via N 2O as an intermediate
Only Significant under Lean, Low Temperature Conditions
Under very Lean Conditions, the Formation of the CH radical in Prompt
Mechanism may be suppressed - Reduction of NOx Production
Low Temperatures - Low Thermal NOx Production
Nitrogen atom combines with an Oxygen molecule in the presence of a Third
Molecule (M) producing Nitrous Oxide
N2O may
then react with Oxygen atoms to form NO
This Mechanism can occur at relatively
Low
Temperatures and increases at High Pressure
Flame Temperature
Diffusion Flames have maximum Temperatures close to Stoichiometric
Thermal Nox Dominates giving NOx levels typically in excess of 100 ppm (parts-permillion) and often in excess of 200 ppm (for Natural Gas)
Peak ideal scenario as maximum
temperature achieved Lean design less
emissions and used as that temperature too
high causing cooling problems more air can
dilute and lower temp
Other Fuels have different NO x potential
due to different Flame Temperatures
Distillate Fuel produces ~80% more NO x
than Natural Gas
NOx Control
NOx reduction by Clean-up ( Picture of selective catalytic
reduction)
-
Fuel clean-up
Selective Catalytic Reduction (SCR) with the use of Ammonia
SCONOX - Post-combustion Catalytic system removing NOx
and CO without Ammonia
Diluent Injection
Rich Quench Lean (QLR) Combustion
Lean premixed Combustion - Staged Premixed Combustion
Sequential Combustion
Catalytic Combustion
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Diluent Injection
A Reduction in Flame Temperature - NOx Reductions - Water, steam or Nitrogen
Injection into the Combustion Chamber (dilutes) significantly reduces the Peak Flame
Temperature
Rich Quench Lean ( RQL ) reduce the NOx
Cool
Rich
Zone
Rapid Quench to prevent significant NOx
formation
Cool Lean Zone
Lean Premix work out where the lean / rich from fl ow conidtions
To avoid the Local High Temperatures that occur in Diffusion Flames, 2 elements are
required:
1. Operation away from Stoichiometric Conditions (Rich or Lean)
2. Good Mixing of Air and Fuel before Combustion
Rich operation leaves unburnt Fuel and High levels of CO More Air is required to
complete the Combustion Process
The use of well mixed, lean combustion systems is therefore conceptually very
attractive and used in the majority Dry Low NOx (DLN) combustion systems
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To achieve the required performance, a complete redesign of the Diffusion Combustor is
required
All Lean Premix system have the following characteristics in common:
1. Fuel/Air Injection System
Most Systems use multiple Fuel Injection Points - Widest possible fuel
distribution
2. Premixing Zone
Sufficient Time for a Uniform Mixing
Aerodynamics of the System to prevent the Flame propagating into
Mixing zone
3. Flame Stabilisation zone
Lowest levels of NOx achieved at the lowest temperatures - Lowest Equivalence Ratios
Operation close to the Lean Limit for the System
Potential for Flame Failure during Transient event such as Load shedding
substantially
To avoid this problem and to allow stable operation on turndown most systems include
a pilot system that usually introduces a diffusion element to the Combustor for stability
at
part
load
and
during
transients
EV Burner Industrial Gas Turbine
NOx CO Trade-off
Staged Premix
Sequential Combustion - Industrial Gas
Turbine
Extension of staged Lean Premix Principal
-
2 Combustor Turbine pairs in series
First Combustor: Air and Fuel are mixed in a Lean Premix Combustor
Fed to the first (High Pressure) Turbine:
o Energy removed and temperature of combustion products drops
Gases exiting turbine still contain a significant proportion of oxygen
o fed to a second combustor with additional fuel
o spontaneous ignition occurs because of the High Temperature of the gases
from the first stage
Produces lower NO x emissions than would be obtained if all of the Fuel and Air were
added to
the
First
Combustor
Second Combustor is at lower
pressure
Maximum Combustion Temperature is reduced due to the energy removal in the
first stage turbine
Less Oxygen available for NO x formation in the second Combustor
Appropriate choice of Fuel distribution to the 2 Combustors leads to good part load
emissions and load following capability
Challenges with Lean Premix Systems - Industrial Gas Turbine
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