Combustion technology

Combustion Technology Overview

  • Combustion Reaction: The process of fossil fuels (coal, natural gas) reacting with oxygen, producing heat, carbon dioxide, and water. It is always an exothermic reaction.

Key Concepts

Heat of Combustion

  • Heat of Combustion: The heat released when a substance undergoes complete combustion with oxygen.

Stoichiometric Air

  • Stoichiometric air: The exact amount of air needed for complete combustion of fuel.

  • Combustion reactions can be represented by chemical equations detailing reactants and products.

Composition of Air

  • Air consists of approximately 79% nitrogen and 21% oxygen, affecting how combustion is calculated.

Calculation of Theoretical Air

Importance of Oxygen in Combustion

  • For perfect combustion of carbon:

    • 1 kg of carbon requires 2.667 kg of oxygen to produce 44 kg of CO2.

  • The oxygen required for burning other components like hydrogen and sulfur varies:

    • 1 kg of hydrogen requires about 0.301 kg of oxygen.

    • 1 kg of sulfur requires around 0.05 kg of oxygen.

Total Oxygen Calculation

  • Example coal sample composition:

    • Carbon: 73%, Hydrogen: 4.5%, Sulfur: 5%.

  • Total required oxygen = 1.9469 + 0.301 + 0.05 = 2.298 kg.

Air Requirement Calculations

  • Theoretical air needed can be derived from:

    • Air required per kg carbon: 4.32 x (C x 2.667 + (H x 8 - O) + S) kg.

Volume Calculations

Volume of Oxygen Required

  • Volume of oxygen: Determined in Nm³ per kg of fuel, with conversions to account for gases.

  • Example: 1 kg carbon requires about 1.886 Nm³ of oxygen.

  • Volume of air required = Volume of oxygen / 0.21.

Excess Air Calculation

  • Excess Air: Often, more air is supplied than theoretically needed to ensure complete combustion, typically around 50% more.

  • Too little air may result in incomplete combustion, producing carbon monoxide. Excess air can dilute flue gases and reduce efficiency.

Gaseous Fuel Combustion Example: Methane

  • Chemical Reaction: CH4 + 2O2 → CO2 + 2H2O.

  • Excess air is crucial for complete burning; if 5% excess air is used, the resulting flue gas composition can be determined from the combustion equation.

Flue Gas Composition and Stoichiometric Ratio

  • Stoichiometric ratio: The ratio of actual air to stoichiometric air used in combustion.

    • Calculated using the required air for burning methane compared to actual supplied air.

Example Calculations in Combustion

  • Calculation of excess air based on actual flow rates versus those needed for stoichiometric combustion. For methane burning at given flow rates, excess air can be calculated as a percentage, leading to optimal efficiency.

References

  1. Applied Combustion, Second Edition, Eugene L. Keating, CRC Press, 2007.

  2. Fuels and Combustion, S. Sarkar, 3rd Edition, University Press, 2009.