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Combustion and Fuels

This chapter covers the various types of fuels used in combustion processes, including solid, liquid, and gaseous fuels, along with their characteristics. It also explains the stoichiometry and analysis of combustion reactions, the application of the first law of thermodynamics in combustion, methods for calculating heat using enthalpy tables, and the determination of adiabatic flame temperature. Additionally, it discusses chemical equilibrium and the use of Gibbs free energy in analyzing equilibrium composition.

Sections

Introduction to Solid, Liquid and Gaseous Fuels

This section introduces various types of fuels—solid, liquid, and gaseous—and discusses their characteristics.

1 Section Overview

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1.1 Solid fuels: Coal, lignite, wood, coke

This section provides an overview of solid fuels, highlighting key types such as coal, lignite, wood, and coke, along with their characteristics and applications.

1.2 Liquid fuels: Petrol, diesel, kerosene, fuel oils

This section explores the properties and characteristics of liquid fuels such as petrol, diesel, kerosene, and fuel oils, highlighting their significance in combustion processes.

1.3 Gaseous fuels: Natural gas, LPG, biogas, producer gas

This section covers the various types of gaseous fuels, including natural gas, LPG, biogas, and producer gas, highlighting their composition and applications.

1.4 Fuel characteristics

This section discusses various characteristics of fuels, including calorific value, moisture and ash content, volatility, and ignition temperature.

1.4.1 Calorific Value (Higher and Lower Heating Value)

This section explores the concepts of calorific value, including the distinction between Higher Heating Value (HHV) and Lower Heating Value (HHV), and their importance in fuel evaluation.

1.4.2 Moisture content, ash content

This section delves into the importance of moisture and ash content in fuels, impacting their combustion efficiency and energy output.

1.4.3 Volatility, ignition temperature

This section discusses the important concepts of volatility and ignition temperature, which are crucial in understanding fuel characteristics and combustion processes.

Stoichiometry of Combustion

This section covers the stoichiometric principles involved in the combustion processes of hydrocarbon fuels, including key ratios and concepts like excess air and equivalence ratio.

2 Section Overview

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2.1 Combustion reaction of hydrocarbon fuel

This section discusses the combustion reaction of hydrocarbon fuels, including the stoichiometry involved and key concepts like air-fuel ratio, excess air, and equivalence ratio.

2.2 Stoichiometric air-fuel ratio (AFR)

This section defines the stoichiometric air-fuel ratio (AFR) and discusses its significance in combustion reactions.

2.3 Excess Air

This section discusses excess air in combustion processes, highlighting its calculation and importance in combustion efficiency.

2.4 Equivalence ratio (ϕ)

The equivalence ratio (ϕ) is a critical parameter in combustion analysis that compares the actual air-fuel ratio to the stoichiometric air-fuel ratio.

Exhaust Gas Analysis

Exhaust gas analysis is crucial for assessing combustion efficiency using tools like the Orsat apparatus to measure flue gas constituents.

3 Section Overview

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3.1 Measures volume fractions of CO₂, O₂, CO in flue gases

This section discusses the measurement of gas composition in flue gases, focusing on CO₂, O₂, and CO content to assess combustion efficiency.

3.2 Used to determine combustion completeness and excess air levels

This section details exhaust gas analysis techniques and their role in assessing combustion completeness and excess air levels.

First Law Analysis of Combustion Reactions

This section evaluates the energy changes involved in combustion reactions through the First Law of Thermodynamics.

4 Section Overview

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4.1 For steady-flow combustion at constant pressure

This section explores the principles of steady-flow combustion at constant pressure, highlighting energy changes and the application of enthalpy in combustion reactions.

4.2 For closed systems

This section covers the first law of thermodynamics as applied to combustion, specifically in closed systems.

4.3 Internal energy and enthalpy values

This section discusses the concepts of internal energy, enthalpy, and their relation to combustion reactions, emphasizing the importance of these values in energy calculations.

Heat Calculations Using Enthalpy Tables

This section discusses heat calculations based on enthalpy tables, emphasizing the standard enthalpy of formation and heat of reaction.

5 Section Overview

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5.1 Standard enthalpy of formation (ΔHf0)

The standard enthalpy of formation (ΔHf0) is the heat change when one mole of compound forms from its elements in their standard states.

5.2 Heat of Reaction

This section discusses the heat calculations involved in chemical reactions, particularly focusing on the heat of reaction and enthalpy of formation.

5.3 Use of enthalpy tables for

This section discusses the application of enthalpy tables in calculating heat reactions and energy balances in combustion processes.

5.3.1 Sensible enthalpy at temperatures other than standard

This section discusses the concept of sensible enthalpy and its significance in combustion calculations, particularly at temperatures other than standard.

5.3.2 Accurate energy balance in combustion calculations

This section focuses on how to perform accurate energy balance calculations in combustion, emphasizing the significance of enthalpy values and heat of reaction.

Adiabatic Flame Temperature

The adiabatic flame temperature is the final temperature of combustion products when heat loss is zero.

6 Section Overview

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6.1 Defined as the final temperature of products

This section discusses the adiabatic flame temperature, which is the final temperature of combustion products under adiabatic conditions.

6.2 Depends on
6.2.1 Initial Temperature

This section discusses the importance of initial temperature in combustion processes, explaining its impact on adiabatic flame temperature and overall combustion efficiency.

6.2.2 Pressure

This section introduces the concept of pressure in relation to combustion processes and its significance in energy production.

6.2.3 Air-fuel ratio

The air-fuel ratio (AFR) is a critical parameter in combustion processes that directly affects efficiency and emissions.

6.2.4 Degree of Dissociation

The degree of dissociation refers to the extent to which reactants break into products in chemical equilibrium, especially in combustion processes.

Chemical Equilibrium

Chemical equilibrium occurs when the forward and reverse reactions in a chemical process happen at the same rate, leading to constant concentrations of reactants and products.

7 Section Overview

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7.1 Real combustion at high temperatures

This section discusses the behavior of real combustion processes at high temperatures, focusing on the limitations imposed by thermodynamic equilibrium and the impact of dissociation on combustion products.

7.2 At equilibrium, Gibbs free energy is minimized

This section explains how Gibbs free energy is minimized at chemical equilibrium, which is crucial for understanding combustion processes.

Equilibrium Composition Using Free Energy

This section explores how the equilibrium composition of a chemical reaction can be determined using Gibbs free energy and the equilibrium constant.

8 Section Overview

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8.1 Gibbs free energy

This section addresses the concept of Gibbs free energy and its role in determining chemical equilibrium.

8.2 Equilibrium constant

The equilibrium constant is a measure of the ratio of product concentrations to reactant concentrations at equilibrium in a reaction.

8.3 Relation to standard free energy change

This section covers the relationship between Gibbs free energy and standard free energy change in combustion processes.

8.4 Equilibrium compositions found using

This section discusses how to determine equilibrium compositions in combustion processes using Gibbs free energy and equilibrium constants.

8.4.1 Mass balance

Mass balance is a fundamental principle in combustion that involves accounting for the mass of different components before and after a reaction.

8.4.2 Kp expressions

This section discusses Kp expressions in combustion chemistry, focusing on the equilibrium constant related to the partial pressures of reactants and products.

8.4.3 Iterative solution for mole fractions

This section emphasizes the importance of using an iterative approach for the calculation of mole fractions in chemical equilibrium.

Learning Objectives

  • Different types of fuels include solids like coal and liquids like petrol, each having specific characteristics such as calorific value and moisture content.

  • Understanding stoichiometry is essential in calculating the air-fuel ratio, excess air, and the equivalence ratio in combustion processes.

  • The analysis of exhaust gases helps determine combustion efficiency and completeness.

Key Concepts

Calorific Value

The amount of energy produced by burning a specific quantity of fuel, measured as Higher Heating Value or Lower Heating Value.

Stoichiometric AirFuel Ratio (AFR)

The ideal ratio of air to fuel that allows for complete combustion without excess air or fuel.

Gibbs Free Energy

A thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure, relevant for equilibrium conditions.

Adiabatic Flame Temperature

The theoretical temperature of the products of combustion when no heat is lost to the surroundings, dependent on initial conditions and compositions.

Practice Exercises

Total Questions

5

Estimated Time

10 min

Passing Score

70%

Instructions

  • Read each question carefully
  • You can use hints if you need help
  • Complete all questions before submitting