Alternative Fuels Benefits
• Cement plant point of view • Cheaper kiln fuel resource
• Lower NOx emission
• Wastes elimination point of view (comparing to other elimination methods) • Utilize its heat capacity
• Full destruction of the material
• Do not have ashes (incorporated to the clinker)
Alternative Fuels used at Cement Industry
• Animal Meal
• Shreaded Plastics
• Wood chips
• Saw dust
• Biomass – husks
• Tires chips
• Fluff – municipal waste
• Chicken feathers
• Glycerin
• Textiles
• Solvents
• Sewage sludge
• Oil Sludge
• Cellulose, paper, board
Firing Alternatives Fuels
• Modern Kilns • Start using calciner for AF firing
• Bigger particles sizes of AF
• High Substitution – Main Burner
• Old Kilns – No calciners • Main burner
• Mid kiln tires firing
• Small portion in the kiln back
Alternative Fuels Selection
To be considered when firing AF: - AF preparation - Constant feed / conveying - Chemical analysis - C, H, O, N, S, Cl - Moisture, Volatiles - Flame adiabatic temperature - Refractory infiltration - Sulfur balance - Cl balance - Presence of heavy metals P, Hg, Ti ... - Lower clinker production (O2 and H2O) - Maximum and Minimum particle dimension - Shape of the particle
Picture: Courtesy of Aixergee
Videos: Courtesy of LEAT Bochum / Aixergee
3D plastic 2D plastic
Slow ignition Fast ignition Super fast ignition
Low volatile High volatile High volatile
Long time burn out Fast burn out Very fast burn out
Keeps shape Becomes a sphere Forms droplets
High ash content Low ash content Low ash content
sphericityparticlerealofarea
volumesamethewithsphereofarea
Importance of particle shape:
COMBUSTION:
• Particles with lower sphericity have more surface area for the same volume, i.e. they tend to present better heat and mass tranfer
• Particles with lower sphericity have lower terminal velocity, thus they have higher “flying” time
C (mm)
L (mm)
A (mm)
Superficial area (mm2)
Particle volume (mm3)
Equivalent sphere area
(mm2)
I 1.0 5.0 10.0 130.0 50.0 65.6 0.50
II 3.0 3.0 5.5 84.0 49.5 65.2 0.78
Combustion of Solid Wastes Overview
• Unstable flame
• Wrong kiln thermal profile
• Higher kiln inlet temperatures
• Fuel falling onto clinker bed
• Reduction zone near clinker
• Excessive Sulphur recirculation
• Rings formation
• CO emissions
• Clinker quality
• Cement Strength
• Operational problems
• Environmental problems
When higher substitution rates are attempted with inadequate technology:
Firing Alternatives Fuels
Burners development
One or more pipes above the burner
• High oxygen zone
• Low control
• Low mixing
• Low oxygen zone
• High control
• High mixing
One or more pipes inside the burner
Old Concept New Concept
High Momentum Burner
11
Advantages of High flame momentum : • Kiln operations becomes more stable & improved
fuel efficiency for hard to burn fuels • Improved clinker reactivity and shorter sintering
zone • Lowered volatility and recirculation of sulfur in kiln
gases (control of surfur into the clinker) • Tendency for ring formation is lower • Clinker granulometry more uniform Requirement: • Quick mixing of secondary air to the flame, to allow
the complete combustion which is iniciated by primary air.
• Primary air jets need to accellerate the secondary air.
• Internal recirculations are controlling the ignition distance of the fuel
Combustion SECONDARY AIR
• High oxygen availability: 95-100% of required amount
• Low control and mixing • Low velocity
PRIMARY AIR • Low oxygen availability: 8-12% of
required amount • High control and mixing • Ignition fuel zone • Flame stability
Combustion
Secondary Air
Secondary Air
Primary Air
Recirculated
Gases
Lofting
Air
supply
RDF Pipe – easily
removable
Axial Air
Swirl Air
Coal / Petcoke
Igniter
Flame Scanner
Natural Gas
Alternative Fuels
Alternative Fuels
With Lofting Air
Burner Concept
• Primary air pressure – 250 up to 650 mbar(g) • Minimum coal/petcoke transport air - up to 5,0 kg/kg of air • 8 to 12% of stoichiometric combustion air • AF (solids) – with Lofting air • Easy to burn AF – up to 4,0 kg/kg of air • Hard to burn AF – up to 2,5 kg/kg of air
Axial Air
• Injection of solid alternative fuel through a ring channel
• Ring channel wide enough to avoid blockages
• Even fuel and air distribution
• Wear protection
• High momentum
• High flame control
• Up to 100% solid alternative fuel
New concept for solid
alternative fuels injection:
Through the burner:
HIGH CONTROL
Near secondary air:
HIGH OXYGEN
Mixing of the fuel and air occurs primarily as a result of jet entrainment
A free jet can entrain as much of the surrounding air until it’s velocity is the same as its surroundings and thus is able to expand unimpeded
U
Um
Uo
r
do
Nozzle
Potential
Core
Mixing
Region
Transition
Region
Air Assisted Dispersion of RDF and other similar fuels
Lofting
Air
supply
RDF Pipe – easily
removable
Lofting air
solid
dispersion
pattern
Lofting Air
Effect of lofting air on volatiles burnout - RDF
Lofting air OFF
Lofting air ON
RDF volatiles mass fraction (coloured regions show RDF concentration)
Burner tip
Lofting air OFF
Lofting air ON
Flame temperature
The effect of lofting air on flame temperature map
Lofting Air
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