Combustion Processes and Combustion Efficiency

Combustion is an exothermic reaction between a fuel and an oxidant.

Combustion of carbon and oxygen to carbon dioxide can be expressed as:

C + O₂ > CO₂

Combustion of methane and oxygen to carbon dioxide and water can be expressed as:

CH₄ + 2 O₂ > CO₂ + 2 H₂O

Combustion of hydrogen and oxygen to water can be expressed as:

H₂ + 1/2 O₂ > H₂O

Typical combustion process efficiency can be summarized to

• home fireplace: 10 - 40 %
• space heater: 50 - 80 %
• residential gas furnace with low efficiency atmospheric burner: 70 - 80 %
• oil burner heating system: 70 - 85 %
• gas powered boiler: 75 - 85 %
• high efficiency gas or oil condensing furnace: 85 - 95 % 

Furnace Performance Indices

The performance of a furnace is usually assessed by the following indices

Thermal Efficiency

μ_t = 100 E_o / E_i                (1)

where

μ_t = thermal efficiency

E_o = energy output (W, Btu/h)

E_i = fuel energy input (W, Btu/h)

Annual Fuel Utilization Efficiency - AFUE

AFUE measures the annual amount of heat actually delivered compared to the amount of fuel supplied to the furnace.

AFUE = 100 E_ao / E_ai                (2)

where

AFUE = annual fuel utilization efficiency

E_ao = annual energy output (W, Btu/h)

E_ai = annual fuel energy input (W, Btu/h)

Steady-state Efficiency - SSE

Steady State Efficiency measures how efficiently a furnace converts fuel to heat once the furnace has warmed up and is running steadily and can be expressed as

SSE = 100 (E_i - E_l) / E_i               (3)

where

SSE = steady-state efficiency

E_i = fuel energy input (W, Btu/h)

E_l = energy loss (W, Btu/h)