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Gas Turbines
Fuels Flexibility
Gas Turbine Engineering
BHEL - Hyderabad
Gas Turbine Fuels ! Gaseous fuels
! Liquid fuels
• Natural gas
• LPG
• NGL
• Coal gas, Blast furnace, Coke oven gas, etc.
• HSD
• LDO
• Distillate#2
• Naphtha
• Kerosene
•Ash bearing Fuels
Gaseous Fuel : Types
Classification by Calorific Value
Calorific
Value ( Kcal /nm3 )
Typical fuels
Primary Gas Composition
Very High 44,500 to 10,700 LPG, NGL Propane and Butane
in liquid formHigh 10,700 to 7,100 Natural gas Methane
Medium 7,100 to 2,700 H2 , CO , CH4
Low 2,700 to 900 Coal gas ( air blown) H2 , CO , N2
Very Low Below 900 Blast furnace gas CO , N2
Refinery gas
Coke oven gas
Coal gas ( O2 blown )
Significance of Fuel Properties - Gaseous fuels
• Heating Value
• Flammability limits
• Fuel temperature
• Fuel Pressure
• Hydrogen
-Effect on emissions, output and combustor stability.
-To maintain stable combustion Ratio of flammability limits to be minimum 2.2
- Avoid Condensation.Min. superheat of 28 deg C above dew point.
-To maintain stable pressure ratios across nozzle, sizing of gas stop and control valves.
-Potentially hazardous. Requires special fuel handling systems including inert gas purging.
Gas Fuel Specifications
Minimum Heating value 100 - 300 Btu / Scf
Wobbe index variation ±5 %
Minimum super heat 28 deg C
Minimum flammability ratio 2.2 / 1
Contaminants ( ppmw )
Sodium + Potassium 1.0
Vanadium 0.5
Magnesium 2.0
Lead 1.0
Calcium 2.0
Sulfur 30.0
Total Particulates 30
Particulates above 10 microns 0.3
Gas Fuel Contaminants
• Trace Metals
• Sand, Clay, Rust
• Oil & Hydrocarbon
liquids
• Water & Salt Water
• Tar, Napthalene
Benzol, H2S, NH3
- Hot Corrosion , Life Issues , Turbine foulingNa, K, Ca, V , Pb , Si , Ni
- Hot corrosion, fouling deposits, erosion and plugging of Gas orifices.
- Load spikes, HGP hardware damage.
- Blow out of flame, Hot corrosion.
- Plugging, Caking in Gas nozzles & HGP components.
Gas firing - Concerns
• Avoid liquids
• Particulate
- Load spikes, Damage to HGP components.
- Plugging of fuel nozzles. High emissions, high exhaust spread.
Liquid Fuel : Types
! Distillates
! Ash Bearing Fuels
• Light Distillates : ( < 1.8 Cst )
- Naphtha , Kerosene
• Heavy Distillates : ( > 1.8 < 25 Cst )
- HSD , LDO , Distillate#2
• Crude Oils , Blends : ( ~ 160 Cst )
- Crude oils , Marine diesel fuel , #4 Burner Fuel
• Residual Oils : ( ~ 900 Cst )
- LSHS , FO , Bunker Oil
Significance of Liquid Fuel Properties
• Aromatics
• Olefins
• Sulphur
• Nitrogen
• Hydrogen
• Carbon Residue
• Water
- Excess smoke , Radiation
- Gum formation
- Sox, Reaction with K, Na for hot corrosion
- Nox due to FBN
- Less tendency to smoke , Operational Hazard
- Higher atomisation pressures, Nozzle clogging
- Fouling of fuel handling components
Significance of Liquid Fuel Properties
• Viscosity
• Pour Point
• Fuel gravity
• Distillation
• Flash Point
• Vapour Pressure
• Compatibility
• CFPP
- Pumping,atomisation,lubrication,treatment
- Pumpability , Heating
- Water washing of fuel
- Volatility
- Explosion proofing , Statutory requirements
- Mixed phase , Pumping
- Precipitation , Separation , Decomposition
- Filtration problems
Liquid Fuel Contaminants
• Trace Metals
• Sediments
• Dirt
• Water
• Non-Fuel Contaminants
- Hot Corrosion , Life Issues , Turbine fouling
- Na, K, Ca, V , Pb , Si , Ni
- Filter maintenance , Fuel component fouling
- Filter Maintenance
- Fuel Component corrosion
Xe = (A/F) Xa + (S/F) Xs + Xf
Xe - Eq. Contaminants in fuel Xa - Concentration in air (ppm weight)
A/F - Air to Fuel ratio Xs - Concentration in steam/Water ( ppm weight)
S/F - Steam( Water) to fuel ratio Xf - Concentration in fuel
Liquid Fuel Specifications
Distillates Ash Bearing
Light Heavy Crudes Residuals
Min.Kin. Viscosity ( 100F) D445 0.5 1.8 1.8 1.8
Max.Kin. Viscosity ( 100F) D445 5.8 30 160 900
Max.Kin. Viscosity ( 200F) D445 - 4 13 30
Flash Point D1298 --------------- Report -------------------
Distillation Temp D93 --------------- Report -------------------
Pour Point D86 --------------- Report -------------------
Sodium + Potassium ( ppm) 1 1 1 1
Vanadium ( ppm Treated ) 0.5 0.5 0.5 0.5
Vanadium ( ppm untreated ) - - 100 500
Lead 1 1 1 1
Calcium ( ppm) 2 2 10 10
Filterable dirt mg/100ml D2276 4 10 Report Report
Water & Sediment( % vol.) D1796 0.1 0.1 1.0 1.0
Water content ( % vol. ) 0.1 0.1 Report 1.0
Effect On Hardware
Distillates Heavy Oils
Light Heavy Crudes Residuals
Explosion Proofing * * * *
Start UP Fuel Very light fuels Some fuels Nearly always Always
Fuel Treatment Usually none Usually none Nearly always Always
Fuel preheating Some in cold Some in cold Nearly always Alwaysconditions conditions
Fuel filtration Always Always Always Always
Atomisation Low Pressure Low pressure Low or High pr. High Pressure
Combustor Standard Standard Heavy fuel Heavy fuel
Turbine cleaning None None Required Required
* : Refer to local applicable codes
Fuel Systems
! Fuel Storage
! Off-base Fuel Forwarding
! On-base fuel System
! Combustion
Fuel Storage
• Number of tanks to be sufficient to provide fuel without interruption ( Filling , Settling and Pumping )
• Initial coarse filtration recommended
• Inlet to tank from bottom with baffling and velocity diffusion
• Use of floating suction
• Re-circulation ,if required, should not agitate the fuel
• Draining the fuel from tank , site specific experience
• Sloping bottoms for proper collection of water
• Floating roof for volatile fuels
• Provision for heating for heavy oils
• Prohibit use of cadmium,zinc and copper
Gauge Hatch
Limit Lift to 75 Deg18” minimum
Fuel InFuel Out
Inlet Diffuser
Floating Suction
Drain Water
Cable for checking Operation of floating Suction
18”
Cone Bottom
Sloping Bottom
Sloping Bottom
5cm in 300cm2.5 cm from bottomStorage Tanks
Typical details
Off-Base Fuel Forwarding
• Generally centrifugal pumps used
• 100% redundancy with auto change-over
• Each pump to have isolation valving
• Filtration in multi Duplex stages.
• Entire pipelines preferably of SS
• Pressure control with re-circulation to ensure proper flow under all operating conditions
• Relief valves to release locked up pressure
• Adequate provisions for regular sampling
• Sunshades for low lubricity fuels
Water coalescent filters , Magnetic particle removal , 20 Micron filters , 5 micron fine filtration
On-Base Fuel Handling
• Coarse strainer at inlet
• Emergency stop valve
• High pressure fuel pump
• Last chance HP filter
• Flow divider
• Fuel Check valves
• Naphtha purge drain system ( Only for naphtha fuel firing )
• Redundant off-base systems for improved availability
Combustors
• Cannular multiple combustion chambers
• Liquid fuel nozzles with atomisation and purge ports
• Combustion liners suitable for distillates / heavy oils
• Mixed firing of distillates and gases possible
• Flame detection and exhaust spread for control
• ISO ratings slightly lower on liquid fuels
• DLN systems suitable for liquid firing
• Water or Steam injection for Nox abatement
Heavy Fuels : On-site Treatment / Firing
• Trace Metal Removal- Sodium, Potassium, Calcium by water washing / Electrostatic precipitators
- Vanadium by vanadium inhibitors ( Magnesium )
- Lead not removable ( source control )
- Heat tracing for viscosity control
• Peak firing not allowed
• Vanadium inhibition causes HGP deposits
• Turbine water washing essential
• High pressure Atomisation
• Start-up fuel requirement
• Availability lower
Gas Turbine ratings lower on heavy oil firing
NAPHTHA : Key Properties & Issues
• Flash point Deg C : < 25 Fire Hazard
• LEL/UEL % : 0.9 - 6.0 Fire Hazard
• Autoigniton Deg C : 290 Fire Hazard
• IBP Deg C : 30 Volatile
• Viscosity Cst : < 0.7 Low Lubricity• Miscibility with : Hygroscopic Corrosive
Water
BHEL GTs : Special Design Considerations
• Alternate start / shutdown fuel• Naphtha drain and purge• Suitable ventilation system• Leak detection• Lubricity enhancing additive• Suitable fuel storage / delivery system• Operational control
Design Considerations : Start / shutdown fuel
• Machine not to start or shutdown on naphtha
• Use of distillate # 2 , HSD or Natural gas
• Consequences of false start avoided
Design Considerations : Drain / Purge
• Modified design to facilitate drain / purge on GT trip
Dirty Oil / Water drain
Pressurised Naptha drain
Atmospheric fuel drain
Gas TurbineW a t e r Dirty
Oil H.P
Filter
Naphtha
Purge
Drain
To Naphtha Drain Tank
Fuel Pump
Fuel Filter
Stop Valve
Flow Divider
Naphtha Purge And Drain System ( typical)
Design Considerations : Ventilation
• Turbine cooling and reduced risk of accumulated vapours
• Ventilation preferred means of protection
• Hot and cold areas sealed
• Scavenging of heavy vapours
Design Considerations : Ventilation
Design Considerations : Leak Detection
• Detection probes at critical locations
• 20 % LEL / 60% UEL philosophy
• Operator alarm or trip recommended
Design Considerations : Lubricity Additive
• Use of PFD without additive for smaller frames
• Use of mechanical flow dividers with additive
• On-line control of additive dosage
• Fuel pumps with dedicated lubrication
Design Considerations : Operational Control
• Permissive for startup and shutdown sequence with alternative fuels
• Initiation of purge after turbine trip
• Auto-changeover to naphtha after FSNL
• Permissive to start after establishing ventilation
• Mixed firing as per predetermined schedule
• Alarm / trip on detection of naphtha
Naphtha Firing
Naphtha firing experience is predominantly available from Indian installations, most of such gas turbines having been supplied by BHEL. Naphtha when handled as recommended can be a very good fuel for gas turbines. This is ably supported by large number of operating hours of BHEL gas turbines
Mixed Fuel Firing
• Mixed firing of gas and distillate possible
• Minimum limits of gas and distillate as 30 and 10 % of base load
• Mixed firing above minimum load
• Auto change-over to either fuel on trouble
Load
% fuel
Gas 30% 90%
70% 10%
0% 100%
0%100%Liquid
Permissible zone
Summary
• BHEL Gas Turbines are designed to handle variety of gaseous and liquid fuels
• Suitable on-site treatment is provided for handling difficult heavy oils
• Emission control is affected by inbuilt combustion system control mechanism and Nox abatement systems
• Fuel flexibility is further enhanced by having mixed firing of gas and liquid fuels
• Special design and operational features have been added for handling Naphtha firing in India
Thank You