1
Introduction to Combustion

1.1 Introduction

The goal of this book is to provide insight into the modeling and analysis of combustion processes. The term combustion encompasses many high-temperature chemical reactions that involve oxygen. Combustion is a chemical reaction between a fuel and an oxidizer producing heat and light. It is a rapid energy conversion process that converts molecular energy stored in the fuel to thermal energy. The book covers the basic principles of combustion with a focus on the application of the chemical and thermal sciences to better understand combustion processes.

Our society relies heavily on the thermal energy released by combustion to meet a wide variety of needs, as most (85%) of the world's energy use comes from combustion processes. This energy from combustion is primarily used for three purposes: to supply heat, to provide power, and to produce manufactured materials. We use petroleum in the form of gasoline, diesel, or kerosene for transportation, natural gas for heating our buildings and industrial processes, and coal and natural gas for electricity generation.

The mix of U.S. energy production and consumption has changed over time. Fossil fuels have dominated the U.S. energy mix for more than 100 years, but the mix has changed. Currently, in 2023, about a third of the U.S. energy production is petroleum based and another third is natural gas, with the remaining third a combination of renewables, coal, and nuclear energy (Smil 2017).

There are a number of private and governmental initiatives underway to decarbonize, i.e., reduce the amount of greenhouse gas emissions, primarily , from combustion devices. About three-quarters of the worldwide greenhouse gas emissions are due to the combustion of fossil fuels. The goal of decarbonization is to limit global warming to no more than 2.0 °C by 2050. The initiatives include increased use of biofuels and carbon-free fuels such as hydrogen and ammonia and improved combustion and process efficiency. Two technologies have dominated policy discussions about mitigating climate change: renewable energy generation and carbon capture and storage. Renewable technologies for electricity generation now being deployed widely are wind turbines and solar cells. Carbon capture and storage technologies include a process of converting carbon dioxide into fuel. In a two-stage process, the gas is chemically captured and turned into a solid form as calcium carbonate, then heated to drive off the and convert it to carbon monoxide.

1.2 Combustion Applications

• Electrical power generation: Natural gas and coal are burned in the furnaces of electrical power stations to produce steam in order to generate electricity. For many years, most power plants used coal as a fuel, but due to health, economic, and climate change considerations, coal is being replaced by natural gas and also renewable energy sources.
• Transportation: Liquid and gaseous fuels are used as an energy source for transportation by automobiles, trucks, aircraft, and ships due to their high energy density. Gasoline and diesel fuels are the major fuels used in internal combustion engines, and kerosene and natural gas are the major fuels used in gas turbines. There have been significant developments since the late 1960s in internal combustion engine and gas turbine technology in order to reduce their emissions and increase thermal efficiency.
• Process industry for materials manufacturing: Manufacturers of primary metals (iron and steel), nonferrous materials (glass, ceramics, cement, and nanoparticles), chemicals, petroleum products, food, and paper produce these materials through heating and combustion processes. The combustion devices used in materials manufacturing include boilers, furnaces, ovens, and blast furnaces. For example, cement is produced by sintering ground limestone to 1450 °C in large kilns. In the Haber process, hydrogen and nitrogen are reacted together at high temperatures and pressures to produce ammonia, .
• Domestic and industrial heating: Heating of residential homes, commercial buildings, industrial factories, offices, and various types of buildings such as schools and hospitals is accomplished through combustion of natural gas. There is an extensive natural gas network in the United States and many countries which delivers natural gas directly to a domestic and industrial end user.
• Cooking More than 2.5 billion people, about 1/3 of the world's population, rely on solid biomass, kerosene, or coal as their primary cooking fuel. The burning of solid fuels in households for cooking and heating can lead to very low indoor air quality and health issues.

1.3 Organization of Text

In Chapter 1, we introduce various applications of combustion processes, fuels, and emissions from various combustion applications. As discussed below, these applications include electrical power generation, transportation, industrial processes, and heating of domestic and commercial buildings. The major fuels for these combustion processes include natural gas, liquid hydrocarbons (gasoline and diesel fuels), and solids (wood and coal).

Chapter 2 covers chemical thermodynamics, including thermodynamic properties of fuel-air mixtures and equilibrium thermodynamics of multicomponent reacting systems. Chapter 3 reviews chemical kinetics, reaction rates, and the rate of production of combustion products. Chapter 4 introduces detailed reaction mechanisms of various fuels. In Chapter 5, the general conservation equations of mass, momentum, energy, and species for reacting systems are presented. Chapter 5 also introduces turbulence models and turbulent combustion.

In Chapter 6, premixed laminar flames are modeled to determine burning velocities and flammability limits. Chapter 7 examines subsonic deflagration waves and supersonic detonation waves of premixed gases. Chapter 8 covers non-premixed or diffusion flames, including jet mixing and flame length. In Chapter 9, the evaporation and burning of fuel droplets are presented. Chapter 10 discusses the emissions, including nitrogen oxides, carbon monoxide, carbon dioxide, and particulates from combustion processes. The final chapter, Chapter 11, introduces the processes of solid fuel (biomass, i.e., wood and coal) combustion. A Homework Problem section is included at the end of each chapter.

1.4 Solution Methods for Combustion Problems

A combustion problem can be solved using various combinations of analytical, computational, and experimental approaches. The text introduces a number of solution techniques and gives background references for more information. Although the design of many modern combustion systems leans toward calculation and simulation rather than extensive hardware fabrication and testing, diagnostic techniques are still very important.

Indeed, the rise in computational fluid dynamics (CFD) is beginning to allow the evaluation of complex flow and combustion systems over a wide range of design parameters more quickly, inexpensively, and thoroughly than can be accomplished with actual hardware tests. Reactive flows including multistep kinetic considerations have become computationally tractable. For example, there are three major categories of numerical analysis, Reynolds average Navier-Stokes (RANS), large-eddy simulation (LES), and direct numerical simulation (DNS). The predictions of these numerical solution techniques for a diffusion flame are shown in Figure 1.1. A discussion of these methods is provided in Chapter 5.

However, these computational models often require adequate input of kinetic information in the form of sub-models, which are based on specific measurements or basic understanding of the chemical and physical mechanisms derived from experimental observations. In particular, experimental validation of computed results will be necessary to prove the predictive capability of theoretical models and computer codes before they are used for guiding actual designs.

There are a number of software packages available for numerical solution of combustion problems, from commercial, national laboratory, and academic sources. A popular open-source program is Cantera, which is a suite of software tools for problems involving chemical kinetics, thermodynamics, and transport processes. Since Cantera is open source, users can incorporate detailed chemical kinetics and transport models into their calculations. Cantera can be used from Python and Matlab, or in applications written in C/C++ and Fortran 90.

A popular commercial combustion software tool is Chemkin-Pro. Many researchers work with combustion mechanisms in Chemkin format, which has become a de facto standard in the combustion community. The Chemkin software was originally developed in the U.S. Sandia National Laboratories and has since become a standard computational program for combustion engineering as well as in other fields such as fluid dynamics and chemistry. The Chemkin-Pro package is a set of programs which incorporate complex chemical kinetics into simulations of gas-phase and gas-surface reacting flow. The software includes application programs for stirred reactors, plug-flow reactors, premixed and diffusion flames, and opposed-flow diffusion flames. For specific...