Burners
Source: http://www.jacksonvillelime.com/
Burners
Source: The Cement Plant Operations Handbook-page 71
Burners
• Burning system
• Provide flame (heat) at the right location for clinker formation
• Initiate and maintain ignition source (without pilots)
• Mix fuel and air
• Volatilize solid and liquid fuels
• Proportion fuel and air
• Supply fuel and air at the proper rates and pressures to facilitate all functions with safety at the rate required for the
process
• Burner=Burning System
• Proper flame position and shape (High flame temperature)
• Effective combustion
• Proper air/fuel ratio
• Good mixing (“If it’s mixed, it’s burnt”)
• Maintain ignition source
• Maintain acceptable clinker quality
• No reduced burning conditions
• Correct clinker crystal structure
Swirl Air Effects
• Swirl shortens flame “stand-off”
• Longer reducing zone close to burner tip creates
HC radicals that can lower NOx
• Swirl shortens flame length and increases flame
diameter
• Heat releases in a shorter distance may increase
flame temperature and make coating thinner
• Larger diameter flame may impinge on coating
• The effect of swirl air is to open the primary jet
and increase the rate of entrainment
Burner Position
• Burner should be aligned along the kiln axis or few inches away from the load to
prevent impingement
3%
Position Comment
2A or 2B Best heat exchange
between the flame and
the feed
3A Too close to feed. Fuel
may enter feed bed
1C, 2C, Flame impingement
1B upon the coating
(shorten refractory life)
Burner Insertion
• Deeper insertion may lower NOx and flame impingement
Physical model of secondary air pushing the flame to top of the kiln
Source: the Cement Plant Operations Handbook page 42
Burner Insertion
Burner Metrics
• Burners are rated in units of power based on the total input heat capacity of
the fuel.
• Typical units are GJ/h, BTU/h, kcal/h, KW
• Burner specific momentum is the power residing in a moving object.
• Momentum = mass x velocity
• Measure all air going through burner (Kg/s)
• The conveying velocity of pulverized coal should be in excess of 30 m/s (6000 fpm).
Minimum velocity is 25 m/s if horizontal duct is not too long
• Calculate tip velocity (m/s)
• To avoid flame flashback, the burner pipe velocity should be a minimum of 35 m/s
(6800 fpm)
• Calculate MMBTU/h and convert it to MW
• Momentum = (Mass of air + Mass of Coal) * tip velocity / MW
• Units is N/MW
• Typical values for cement kiln burners range from 3.5 to 12.0 N/MW
Burner Aerodynamics
• Primary Air has three major roles in Primary Air = Axial/Jet Air + Swirl air +
burners: Conveying Air
• Transport and control the rate of
fuel/air mixing
• Flame stability
• Cool the burner wall to help prolong its
life
• RCRIT is the minimum ratio of
momentum required for a
recirculatory jet
• Rcrit ~ 1.5
• Also known as Craya-Curtet Parameter
• The rate of entrainment into the jet and thus the fuel/air mixing is
controlled by
Momentum of Burner Jet
RCRIT
Momentum of Secondary Air
Burner Aerodynamics
• For a confined jet (rotary kiln flame), the jet is now constrained in two ways:
• The amount of surrounding fluid fed to the jet is limited to the secondary air flow
• The expansion of the jet is physically constrained by the kiln walls
• If the jet has momentum in excess of that required for complete entrainment of the secondary air stream,
then jet recirculation will occur.
• This occurs when after all available air has been entrained into the jet, the jet will pull back exhaust gases from
further up the kiln.
Recirculating Flame
Burner Aerodynamics
Optimized Burner
Recirculation due to increased
burner momentum (this protects the
kiln refractory from the flame)
Burner Aerodynamics
Optimized Burner
Recirculation due to increased
burner momentum (this protects the
kiln refractory from the flame)
Types of Flame
• Long and lazy R<Rcrit
• Evident by high CO values
• Long burning zone
• Highest SHC
• Impinging R=Rcrit
• Stabilized condition can cause
premature brick failure
• Difficult to form and keep coating
• Ring formation
• Recirculating R>Rcrit
• Improved clinker quality, coating
formation and brick life.
• Shortest burning zone
• Best SHC
Thermal Efficiency
1100
1000
Fuel Consumption (kcal/kg)
900
800
700
600
500
0 1 2 3 4 5 6 7
Oxygen in Kiln Exhaust gases (%)
• Data shows increase in oxygen level from 1 to 5% causes an increase in heat
consumption of more than 10%
• Rule of thumb:
• Every 1% increase in excess oxygen costs 2% in capacity
Thermal Efficiency
5
Flue Gas Heat Loss
2
OPTIMUM
INCOMPLETE OPERATING RANGE
COMBUSTION
1
0
-1 -0.5 0 0.5 1 1.5 2 2.5 3
Oxygen in Kiln Exhaust (%)
Burner Design Recommendations
• Include a port for the insertion of an electrical igniter in the burner
• One highly recommended feature is a backup blower
• To protect the burner in case the main blower fails
Firing Systems
Direct Semi-Direct Indirect
•Simple operation •Moderate primary air (18-25%) •Low primary air (7-10%)
•Relatively clean air passes through the
•Low power consumption fans, less pressure drops and higher fan •High thermal efficiency
Advantages efficiencies
•Higher primary air pressures possible, •Precise control of fuel and
•Low initial investment
hence burner flexibility primary air
•Relatively safe
•Safety concerns over coal dust build-up
•Safety concerns over coal
•High primary air (20-40%) in cyclone and overflow recirculated to
dust storage in bin
hot inlet gases
•Water vapor introduced •If coal is wet primary air requirement
•Complex design
into kiln increases
•Higher power
•High inleakage across mill
consumption
Disadvantages •Kiln susceptible to
•Capital intensive
increased downtime
•Higher fuel rate variability
(time response)
•Coal mill/fan control if
motor coupled
•Alternative fuels use
limitations
Fires and Explosions in Firing Systems
• Fires and/or explosions can occur in firing systems:
• Two components of the combustion triangle are always present:
• Fuel and Oxygen
• The third component, Ignition, must always be eliminated:
• Excessively high operating temperatures (hot surfaces, rotating equipment)
• Spontaneous ignition (condensation of water vapor on coal)
• Sparks (welding, friction, electrical)
• Electrostatic discharge (conveyors, belts, bins)
• An explosion can be defined as the very rapid increase in the system pressure due to
the energy release of a chemical reaction (combustion)
Source: Recommended Guidelines for Solid Fuel Use in Cement Plants, PCA, 2007, page 15
Explosions Factors
• Lower (LEL) And Upper (UEL) Explosion Limits: • Confined Environment:
• Coal mill
• Cyclone
• Storage bin
• Baghouse
• Ducting
Fuel LEL UEL
Coal 60 g/m3 -
Petcoke 125 g/m3 -
Natural Gas 4.4%vol. 17%vol.
Carbon
10.9%vol. 76%vol.
Monoxide
Hydrogen 4.0%vol. 77%vol.
Explosion Limit: Concentration (percentage) of a gas or a vapor (or dust) in air
capable of producing a flash of fire in presence of an ignition source (arc,
flame, heat).
Source: Recommended Guidelines for Solid Fuel Use in Cement Plants, PCA, 2007, pp. 18-21
Safe Mill Outlet Temperature
Note: Table from Standard Handbook of power plant engineering
Safety Measures
• Combustible gas monitoring around system
• CO2 (Inert) suppression system
• Good ventilation to reduce dust concentration below LEL
• Good housekeeping to avoid dust accumulation
Source: http://www.loesche.com/en/products/dry-grinding-plants/coal/flowsheet/
The New Trend: From Coal to Natural Gas
• Traditionally, natural gas firing has been limited to kiln start-up and mill heating
purposes
• With the evolution of natural gas prices compared to coal and/or coke, kiln
operators are pursuing 100% natural gas firing in kilns
• Important process safety aspects to be considered:
• Assessment of kiln burner design and natural gas pipe capability to handle higher natural
gas volume/pressures
• Gas leak detectors to inspect gas trains
• Installation of the following equipment:
• Methane analyzers at the kiln inlet and cooler take off
• Flame detectors (in case of flame off during kiln upset conditions and preheating)
• Interlocks to stop the gas flow when the flame is off (during both start-up and
operation)
• Gas shut-off valves (if absent)
The New Trend: From Coal to Natural Gas
• Traditionally, natural gas firing has been limited to kiln start-up and mill heating
purposes
• With the evolution of natural gas prices compared to coal and/or coke, kiln
operators are pursuing 100% natural gas firing in kilns
• Important process safety aspects to be considered:
• Assessment of kiln burner design and natural gas pipe capability to handle higher natural
gas volume/pressures
• Gas leak detectors to inspect gas trains
• Installation of the following equipment:
• Methane analyzers at the kiln inlet and cooler take off
• Flame detectors (in case of flame off during kiln upset conditions and preheating)
• Interlocks to stop the gas flow when the flame is off (during both start-up and
operation)
• Gas shut-off valves (if absent)
