There are several ways to increase the reaction rate and achieve effective combustion. One way is to increase the surface area, which can dramatically increase the reaction rate. For example, powdered metals can burn rapidly, but a bar of iron will only rust slowly. Liquid spray combustors (such as those used in boilers, diesel engines, and gas turbines) use small droplets for quick combustion. Small sticks of wood burn much more quickly than large logs. Another common method of increasing the reaction rate is to increase the temperature. As the temperature is increased, the rate of a chemical exothermic reaction increases exponentially, such that heating the reactants to a sufficiently high temperature causes combustion. Because fuel reactions are exothermic, heating can cause a runaway condition under certain conditions. As the reactants are heated, thermal energy is released, and if this energy is released faster than it can be transported away by heat transfer, the temperature of the system will rise, causing the reaction to speed up. The accelerating reaction can cause an even greater rate of energy release that can cause an explosion.

If the temperature of a combustible mixture is raised uniformly, for example, by adiabatic compression, the reaction may take place homogeneously throughout the volume. However, this is not typical. The most commonly observed combustion process involves a flame, which is a thin region of rapid exothermic chemical reaction. For example, as shown in Figure 1.1, a Bunsen burner and a candle each exhibits a thin region in which the fuel and oxygen react, giving off heat and light.

Combustion can be classified by whether the mixture is homogeneous or heterogeneous, depending on if the oxidizer (typically air) and the fuel are premixed or meet only at the point of reaction; whether the fluid flow conditions of the reaction are laminar or turbulent; and whether the fuel is gaseous, liquid, or solid.

In the case of the Bunsen burner (Figure 1.1), the reactants, consisting of a gaseous fuel such as methane and air, are premixed in a tube before ignition. When ignited, the boundary between the fuel–air mixture and the products of combustion forms a cone-shaped reaction zone that is anchored at the lip of the burner. Most of the heat is released at the leading edge of the reaction zone, which is referred to as the flame front. The luminous zone behind the flame front appears blue due to the slower conversion of CO to CO₂. The Bunsen burner is an example of a premixed flame. A candle flame is different from a Bunsen burner flame in that the fuel is not premixed with the oxidizer (i.e., air). The solid candle wax is heated and melted by the flame, whereupon the melted wax is drawn into the wick and vaporized. The vaporized candle wax then mixes with air, which is drawn into the flame by the buoyant motion of the upward flowing products. This type of flame is called a diffusion flame. The relatively slow diffusion of air into the reaction zone produces a yellow flame due to particulate carbon soot that is formed near the flame front due to insufficient oxygen and then mostly burns out in the flame zone.

Premixed flames, as well as diffusion flames, occur in laminar and turbulent flows. Turbulence speeds up the rate at which reactants are mixed and increases the surface area of the flame zone. As a result, turbulence greatly increases the flame speed. Furthermore, the flame can be stationary, as in a gas turbine burner, or propagating, as in a spark ignition engine combustion chamber.

The nature of the combustion also depends on whether the fuel is gaseous, liquid, or solid. Gaseous fuels such as natural gas are easy to feed and mix and are relatively clean burning. Liquid fuels are typically broken into small droplets by being sprayed through a nozzle at high pressures. When heated, liquid fuels vaporize and then burn as a gaseous diffusion flame. Many solid fuels are pulverized or ground before being fed into a burner or combustion chamber. Larger-sized solid fuels are combusted in a fuel bed with air flowing through the bed. When heated, solid fuels (such as wood, switchgrass, or coal) release gaseous volatiles with the remainder being solid char. The char is mostly porous carbon and ash, and burns out as a surface reaction, while the volatiles burn as a diffusion flame. Vaporization of liquid fuel sprays and devolatilization of solid fuels occur much more slowly than gas phase chemical reactions and hence become important aspects of the combustion process. Char burnout, in turn, occurs more slowly than devolatilization.

Understanding each of the factors noted earlier is critical in the engineering and design of practical combustion systems, and each of these factors presents challenges and opportunities. Combustion systems must limit harmful pollutant emissions to the atmosphere. In addition, combustion systems need to take into account carbon dioxide and other emissions responsible for global climate change as well as to address the need for a future based on sustainable energy and carbon-free fuels.

Today, nearly all combustion systems, including wood-fired household heating stoves, must satisfy governmentally imposed emission standards for combustion products, such as carbon monoxide, hydrocarbons, nitrogen oxides, sulfur dioxide, and particulate emissions. The emissions standards are set at sufficiently low levels to keep the ambient air clean enough to protect human health and the natural environment. Low emissions are achieved by a combination of fuel selection and preparation, combustion system design, and treatment of the products of combustion. There are challenging engineering tradeoffs between low emissions, high efficiency, and low cost.

Until recently, carbon dioxide emissions were not considered harmful to human health or the environment, but this has changed. In December 2009, the United States Environmental Protection Agency ruled that current and projected concentrations of carbon dioxide in the atmosphere threaten the public health and welfare of current and future generations. Emissions of carbon dioxide are a major cause of global climate change. Regulation of carbon dioxide emissions has broad repercussions for combustion engineers and the energy industry.

Sustainability is likely to be one of the most significant issues of our time. To pursue sustainability means that our lifestyles will change and our society will change. The extent of this impact will be determined in large part by how our energy resources are handled. The United States National Academy of Engineering has written in the Grand Challenges for Engineering:

And while non-combustion-based energy sources (e.g., solar, wind, geothermal, tidal, and nuclear) and conservation are critical to reaching a sustainable energy portfolio, combustion fuels (e.g., oil, natural gas, hydrogen, ammonia, and biomass) will be important energy sources for the foreseeable future.

Today, addressing global climate change is central to our need for a sustainable future. Carbon dioxide levels in the global atmosphere are increasing, and carbon dioxide emissions from the combustion of fossil fuels are the primary contributor to global climate change. Carbon dioxide traps long-wave radiation reflected from the surface of the earth, decreasing global heat loss. Prior to the industrial revolution, the carbon dioxide content of the atmosphere was relatively stable at 280 parts per million (ppm). By 1900, the carbon dioxide level had reached 300 ppm. Beginning in 1958, direct measurements of the atmospheric carbon dioxide concentration have been made at the Mauna Loa Observatory in Hawaii. In 1958, the atmospheric carbon dioxide concentration was 315 ppm. Today, the carbon dioxide concentration is above 400 ppm. If current trends continue, the carbon dioxide concentration in the atmosphere could reach 500–550 ppm by 2050. In addition, the current world reserves of fossil fuels that can be extracted are more than sufficient to increase the carbon dioxide concentration in the atmosphere beyond 750 ppm (Kirby 2009). To address the concerns of global climate change, the Intergovernmental Panel on Climate Change (IPCC) was established in 1988 to evaluate the risk of global climate change. To date, the IPCC has issued five assessment reports. In the Fifth Assessment Report completed in early 2014, the IPCC concluded that the evidence for “warming of the climate system is unequivocal” and that “most of the increase in global temperatures since the mid-twentieth century is most likely due to the observed increase in anthropogenic (human) greenhouse gas concentrations.”

While the critical need to achieve a carbon-neutral future is clear, the worldwide pressure for growth in energy production is overwhelming. The world has a growing population, and much of the world’s population still lacks access to sufficient clean and convenient energy to meet their basic needs. Because of this, energy production and use will increase for the foreseeable future. The energy needs for cooking, heating, transportation, and electric power generation are global, and today in many parts of the world these needs are not being satisfied. For example, 13% of the world’s people do not have access to electricity (Ritchie and Roser 2021). Approximately, 3 billion of the world’s people (40%) use an open fire for cooking (Ritchie and Roser 2021). Replacing one open wood cooking fire with an improved wood cookstove can reduce carbon dioxide emissions by approximately 2–3 metric tons per year. At today’s carbon market prices, this would result in an annual payment of $8–$25 per metric ton of carbon dioxide. And this is for a stove that costs less than $50. While changes such as replacing open fires with improved cooking stoves are important in the short term, no one in the twenty-first century wants to be solely dependent on an open fire and gathering wood to meet their primary energy needs. The challenge before us, as a people, is to meet the energy ne...