31
Gas Combustion
Figure 31-13 (A) Primary air is induced into the air shutter by the velocity of the gas stream from the orifice. (B) Ignition of the gas is on top of the burner. (C) Incomplete combustion yields yellow “lazy” flame. Any orange color indicates dust particles drawn in with the primary air
Standard Air
• 20.9% oxygen • 78% nitrogen • 1% other gasses
Complete Combustion • CH4 + 3O2 = Heat + 2H2O + CO2 + O2
• Where
– CH4 = 1 cubic foot of methane gas (natural gas)
– 3O2 = 3 cubic feet of Oxygen – Heat 1027 BTU’s of energy produced from
the chemical reaction – 2H2O = 2 cubic feet of water Vapor – CO2 = 1 cubic feet of carbon dioxide – O2 = 1 cubic foot of excess oxygen
Incomplete Combustion
• CH4 + 3O2 = Heat + 2H2O + CO (+/- O2) – Where CO = Carbon Monoxide
High-Efficiency Gas Furnaces (cont'd.)
• Annual fuel utilization efficiency rating (AFUE) allows consumer to compare furnace performance before buying – Furnace efficiency ratings are determined by
amount of heat transferred to the heated medium • Conventional furnace: 78-80% AFUE • Mid-efficiency furnace: 78-83% AFUE • High-efficiency furnace: 87-97% AFUE
Gas Combustion
• Requires fuel, oxygen, and heat – Ignition temperature for natural gas is 1100-
1200ºF – Perfect combustion produces carbon dioxide,
water vapor, and heat • Poor combustion produces carbon monoxide, soot, and
other products
– Flame should be blue with orange tips • Yellow tips indicate carbon monoxide
Combustion Diagram The ideal operating range is a setting with excess air.
Incomplete Complete
Excess Fuel Excess Air
Flue
gas
com
pone
nts
Idea
l ope
ratin
g ra
nge
of
burn
ers
CO
CO 2
O 2
Eff .
Stoi
chio
met
ric li
ne
Courtesy of Bill Spohn, Testo
What is flue gas?
carbon hydrogen sulfur oxygen
nitrogen water
Oxygen (20.9%) water vapor Nitrogen (79%)
Carbon dioxide CO2 Carbon monoxide CO Sulfur dioxide SO2 O2 - balance Nitrogen oxide NOx Water vapor H2O Smoke (oil systems)
fuel-residual ashes
fuel air
How is it formed?
Courtesy of Bill Spohn, Testo
What is Carbon Monoxide ? • CO originates from incompletely (oxidized) burnt
carbon (fuel). • It is very dangerous for human and animals, because
it prevents the absorbstion of oxygen in the blood stream.
• Reasons for the formation: - fuel rich mixture - Improper venting
- too early cooling of the flame (1128 F) CO is expressed in parts per million (ppm).
Courtesy of Bill Spohn, Testo
CO Sources • Fuel burning furnaces and
hot water heaters • Fuel burning boilers • Fuel burning space
heaters • Kitchen ranges & ovens • Auto emissions
– Attached garages
• Fireplaces • Tobacco smoke
Courtesy of Bill Spohn, Testo
Characteristics of CO
• Odorless • Colorless • Tasteless • Mixes well in air
– Does not stratify – Follows air flow in a
structure
• Poisonous
Courtesy of Bill Spohn, Testo
CO Health Effects • 35 ppm NIOSH Permissible Exposure Limit – 8 hours • 200 ppm NIOSH Ceiling– 15 minutes • 200 ppm Slight headache with 2-3 hours • 400 ppm Headache within 1-2 hours • 800 ppm Sickness & twitching of limbs within 1-
2 hours; unconsciousness in 2 hours • 1,600 ppm Headache within 20 minutes; death
within 2 hours • 3,200 ppm Death in 30 minutes • 6,400 ppm Death in 10-15 minutes • 12,800 ppm Death in 1-3 minutes
Courtesy of Bill Spohn, Testo
CO Testing Fuel Burning Appliances
• Sample from entry of home to exit of home
• Sample around all un-vented appliances
• Sample before draft diverter of atmospheric devices
• Sample where you may suspect CO
Graphics © COAD 1996
Courtesy of Bill Spohn, Testo
Types of Furnaces • Atmospheric – Natural Draft
– Buoyancy Creates NEGATIVE Pressure in Flue
• Induced Draft – Fan Creates NEGATIVE Pressure in Flue
• Power Draft –Power Vent – Fan Creates POSITIVE Pressure in Flue
• Condensing – Sealed combustion
Calculating Combustion Efficiency
Natural Gas Light Oil Heavy Oil Propane
Wood
Ambient Temp (combustion air)
Oxygen O2
StackTemp
fuel Air Temp
The analyzer uses the oxygen and net temperature measurements in fuel specific equations to give the efficiency, CO2 and excess air readings.
Courtesy of Bill Spohn, Testo
Combustible Gas Leak Detection
Spillage, Flame Roll-Out, Backdrafting
Testing for CO in the Appliance
• Test undiluted CO
Testing Draft
Notice the hole in the boiler flue pipe
Testing Draft • Some combustion analyzers test for pressure as well as CO.
This one records pressure (draft) in Inches of Water Column, IWC
Testing Condensing Furnaces • Test under Steady State Operating
Conditions • Sample around burner (CO) • Sample efficiency in plastic vent
pipe (MFG or authority) • Or sample in stack termination
(CO)
Follow Manufacturer’s Specs
Graphics © COAD 1996
• Only gas pressure and primary air can be adjusted in the field
• Gas/air mixture is important – 0-4% natural gas will not burn; 4-15% natural gas
will burn but can explode; 15-100% natural gas will not burn or explode
– Limits of flammability vary for gases
• Extra primary air supplies better combustion
Gas Combustion (cont’d.)
What Information is Important?
• Combustion Efficiency – 02, Temperature, Fuel
Reduce Fuel Cost - Savings
To determine Operating Condition
Diluting effects of excess air (NO, NO2, SO2, CO) Weight of pollutant (lbs/Mbtu)
• Carbon Dioxide (CO2)
• O2 Reference (3%, air free)
• Emission Conversions
• Excess Air Extra Air for Combustion
Courtesy of Bill Spohn, Testo
Typical Readings
• Gas Fired Power Burners – Oxygen : 3 % - 6 % – Carbon Dioxide: 8.5 % - 11 % – Stack Temp : 275 ºF to 570 ºF – Draft: -.02WC to -.04WC in the
Stack – Carbon Monoxide: <100 ppm (diluted) – Always Follow Mfr‘s Specifications
Courtesy of Bill Spohn, Testo
Gas Fuels
• Natural gas – 90-95% methane and other hydrocarbons – Lighter than air (specific gravity = 0.60) – Colorless, odorless, and not poisonous
• Displaces oxygen and can lead to suffocation • Odorants are added for detection purposes
– Produces about 1050 Btu per cubic foot when burned with air
Gas Fuels
Figure 31-12 A digital manometer being used to measure gas pressure in inches of water column Courtesy Ferris State University. Photo by John Tomczyk
