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