Furnace Construction & Operations

Furnace Construction & Operations Prepared by Osama Hasan GTE Amm 3 Engro Fertilizers 1

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What is Furnace? Converts Chemical/Electrical Energy to Thermal Energy Examples  Household Furnace  Metallurgical Furnace  Industrial Process Furnace  Non Reactive e.g. start-up furnace  Reactive e.g. primary reformer in ammonia plant 

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Types Draft

Construction

Fuel

Process Type

Burner Arrangement

Heating Method

Tubes Arrangement

Natural Draft

Vertical

Coal Fired

Continuous

Top Fired

Direct

Horizontal Inline

Forced Draft

Horizontal

Gas Fired

Batch

Bottom Fired

Indirect

Horizontal Staggered

Induced Draft

Oil Fired

Side-wall Fired

Vertical Inline

Balanced Draft

Electric Current

Terrace-wall Fired

Vertical Staggered

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Types: Construction

Vertical Furnace

Horizontal Furnace

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Types: Draft The slight pressure difference between firebox and the atmospheric pressure that produces the flow of gases due to temperature difference Forced Mechanical

Induced

Natural

Balanced

Draft

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Types: Burners

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Natural Draft Furnace Air Registers 

Controls the air flow partially

Damper   

Controls the furnace pressure by manipulating the resistance in flow of the flue gas Excessive opening leads to a significant decrease in furnace pressure Excessive closing leads to pressure building in furnace

Reduced furnace pressure:    

 

Increased furnace pressure:

Burner Lifting  Burner backfire Increase in draft  Flue gases leak from peep door Flame-out  Explosion Refractory Damage  Rise in radiation zone temp. Incomplete combustion of fuel  Drop of heat duty in convection coil Radiation zone temperature decreases

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Zones Zones

Tube Banks Radiant

Radiation Shock

Furnace

Convection

Convection

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Zones: Radiation Fire Box 

Area around the burners where gas combustion occurs to produce heat and radiation

Burners 

Devices used to inject, mix and burn the fuel

Refractory 

Wall lining / insulation that accumulates, insulates and radiates heat back to the tubes and increases the heat transfer

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Zones: Radiation Radiant Tubes 

High Pressure and high flow rate process pipelines loaded with catalyst receiving heat via radiation from burners and refractory

Shock Tubes 

Tubes which receive heat via radiation and convection both

Types of Tubes:

Bare

Finned

Studded

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Zones: Convection Also called Heat Recovery Section Arch section 

Space between the convection section and radiant section, where flue gases are mixed to offer uniform heat transfer in convection zone

Convection Coils 

Coils carrying some fluid which recovers heat from the flue gases

Breeching 

Area between the convection section and stack

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Zones: Convection

Arch Section Convection Coil

Breeching Section

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Zones: Convection Vent Stack 

The chimney that exhausts the flue gases from breeching to atmosphere

Exhaust Damper 

Draft regulator which works similar to a butter fly valve to control the air and heat flow

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Burners Raw Gas Burners 





Air and gas is mixed and burnt at spider or burner ring Air inlet is controlled via back and forth movement of air doors Stable operating range is determined by the fuel gas pressure at the burner inlet

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Burners Pre Mix Burners  

Uses a jet of gas to draw the air into aspirator Turbulence is maintained throughout the length of the burner tube due to shape of the aspirator and the velocity difference

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Burners Combination Burner 



Primary Air: Drawn intro aspirator by force of the gas jet. Mixed with gas before it reaches burner spider Secondary Air: Drawn into thimble and meets the mixture at spider.

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Burners Oil Burner / Gun  

Uses steam to atomize oil into a fine mist Stable operating range is determined by fuel oil pressure at the burner inlet and the atomizing steam pressure at the burner inlet

Combination Burner  

Operated on oil or gas or both Startup is done via gas supply

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Refractories 



Heat resistant materials which insulate and protect equipment structure due to their excellent resistance to heat, chemical attack and mechanical damage in high temperatures and/or corrosive environment Low iron-low silica content refractories:    

Insulating castable Insulating brick Ceramic fiber Ceramic module

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Metallurgy In-service degradation mechanisms: Thermal degradation (creep)  The primary damage mechanism for hightemperature service Metal dusting  Metal dusting is a high-temperature corrosion mechanism that occurs in gaseous environments where the carbon activity exceeds one and the metal temperatures are in the range of 450 to 700 °C Stress relaxation cracking  Cracking of a metal because of stress relaxation that occurs during post weld heat treatment (PWHT) or during service at elevated temperature

Furnace Operations

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Impingement Burner Flame touching a tube in the firebox Causes:  

 

Increased gas header pressure Low furnace pressure High draft across furnace Mechanical defect / damage of tubes

Problems: 

 

Hotspot on tube Uneven expansion of the tube Tube rupture

Remedy: 



Installation Peep doors for monitoring Regular inspection

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Flame Color Blue Flame:  

Complete combustion Desired heating value

Yellow Flame:    

Oxygen deficient / starving Carbon monoxide rich Less thermal energy Increased fuel consumption

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Post Burning Causes    

Excess / false air Tube leakage Fuel gas preheat coil leakage Un-burnt fuel carry over

Reaction  

Combustion of un-burnt fuel Oxidation of carbon monoxide

Consequences  

Loss of energy Explosion

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Heat Recovery 

May be used in forced draft furnaces

Recuperative 

Limited heat recovery

Regenerative 

Blowing + heating / cooling of air supply / flue gases

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Instrumentation Skin Couples/TMT 

Check tube wall metal temperatures

Draft Gauges 

Measures draft inside the furnace

Flue Gas Analyzers 

Analyzes for excess oxygen supply

Damper 

Regulates air flow outward

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Startup: Checkup Remove

•Flammable materials •Tools and other sources of danger

Check

•Burner •Pilot Burner Valves •Stack Damper

Open

•Stack Damper •Secondary air registers on furnace

Close

•Peep Holes •Fuel Valves

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Startup: Purging   



Creates a draft and remove all flammable vapors or gas from furnace Purging time should be long enough to ensure all flammable vapors have been removed Purging ensures elements of combustion (fuel, air and heat) are under controlled before we light the burners Natural draft furnace uses steam; 



Steam not only purges the furnace but also preheats it and creates an initial draft

Mechanical draft furnace uses air

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Startup: Lighting the Burners Purging is immediately followed by lighting the burners via pilot / torch  To-Do 

  



Introduce torch via secondary air inlet Crack the burner gas valve Steady the burner flame pattern by air registers adjustment If fuel doesn’t ignite, purge again.

Ensure  

Satisfactory gas header pressure Safety precautions

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Shutdown Reduce fuel / heat

Reduce charge flow

Shutdown all burners

Close Gas header valve

Shut off Gas pilot

Reduce steam

Open Bleeder valves

Open air doors and stack damper

Install blinds in fuel lines

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Thank you Prepared by Osama Hasan Graduate Trainee Engineer Ammonia III – Plant II

[email protected] [email protected]