15.1 Fuels and combustion

Fuel Classification and Properties

Types of Fuels and Their Composition

Fuels are substances that release heat energy through combustion reactions with an oxidizer, typically atmospheric oxygen. A fuel's chemical composition determines its energy content, physical state, and how it behaves during combustion.

• Fossil fuels (coal, petroleum, natural gas) are non-renewable and contain primarily hydrocarbons. They have high energy densities, which is why they dominate large-scale power generation and transportation.
• Biofuels (ethanol, biodiesel, biogas) are renewable, derived from biological sources like plants and organic waste. Their energy densities are lower than fossil fuels, but they can reduce net carbon emissions because the carbon they release was recently absorbed from the atmosphere during plant growth.
• Synthetic fuels (hydrogen, ammonia) are produced through chemical processes rather than extracted from the earth. They can serve as clean-burning alternatives, though producing them often requires significant energy input.

Physical State and Energy Content of Fuels

A fuel's physical state (solid, liquid, or gas) directly affects how it's stored, transported, and burned. Solid fuels like coal require more complex handling and combustion systems compared to liquid and gaseous fuels, which can be pumped, metered, and mixed with air more easily.

Energy content is quantified by heating value, the amount of heat released per unit mass (or volume) during complete combustion. You'll encounter two versions of this:

• Higher heating value (HHV): assumes the water in the combustion products condenses back to liquid, so you recover the latent heat of vaporization
• Lower heating value (LHV): assumes the water remains as vapor, so that latent heat is not recovered

In most engineering applications, LHV is used because exhaust gases typically leave the system hot enough that water stays in the vapor phase. A fuel with a higher heating value per unit mass has a greater energy density, meaning you need less of it to produce the same amount of energy.

Combustion Process and Efficiency

Combustion Fundamentals

Combustion is an exothermic chemical reaction between a fuel and an oxidizer that releases heat, light, and combustion products (primarily $CO_2$ and $H_2O$ for hydrocarbon fuels). Three components must be present simultaneously for combustion to occur, often called the fire triangle: fuel, oxidizer (usually $O_2$), and an ignition source.

Stoichiometric combustion occurs when fuel and oxidizer react in their chemically ideal proportions, with no excess of either reactant left over. For example, the stoichiometric combustion of methane is:

$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

This ideal ratio yields the highest theoretical combustion efficiency and minimal pollutant formation. In practice, real systems almost always operate with some excess air to ensure complete fuel oxidation.

Factors Influencing Combustion Efficiency

Four main factors control how efficiently a fuel burns:

1. Fuel-air ratio: The ratio of fuel mass to air mass entering the combustion chamber. Operating at or slightly above the stoichiometric air requirement ensures complete combustion and maximizes heat release. Too little air causes incomplete combustion (producing $CO$ instead of $CO_2$). Too much excess air dilutes the combustion gases and lowers flame temperature, which reduces thermal efficiency.

2. Combustion temperature: Higher temperatures speed up reaction kinetics and promote more complete combustion. However, very high temperatures cause nitrogen from the air to react with oxygen, forming nitrogen oxides ($NO_x$). This creates a design trade-off between efficiency and $NO_x$ emissions.

3. Fuel-oxidizer mixing: Proper mixing produces a homogeneous fuel-air mixture and promotes complete combustion throughout the chamber. Poor mixing creates local fuel-rich pockets (which produce $CO$ and soot) and fuel-lean pockets (which waste air and reduce temperature).

4. Residence time: This is how long the fuel-air mixture stays at a high enough temperature for the combustion reactions to go to completion. Longer residence times allow more complete combustion, but they also mean larger combustion chambers, which increases equipment cost and size.

Environmental Impact of Fuel Combustion

Atmospheric Pollutants and Greenhouse Gases

Fuel combustion produces several categories of pollutants, each with distinct formation mechanisms and environmental effects:

• Carbon dioxide ($CO_2$) is the primary product of complete hydrocarbon combustion and the most significant greenhouse gas by volume from combustion sources. It's an unavoidable product whenever carbon-containing fuels are burned.
• Carbon monoxide ($CO$) results from incomplete combustion, where there isn't enough oxygen or time to fully oxidize carbon to $CO_2$. It's toxic because it binds to hemoglobin far more readily than oxygen, reducing the blood's oxygen-carrying capacity.
• Nitrogen oxides ($NO_x$) form during high-temperature combustion when $N_2$ and $O_2$ from the air react. They contribute to photochemical smog and acid rain. The dominant formation pathway is thermal $NO_x$, which increases exponentially with flame temperature.
• Sulfur oxides ($SO_x$) are produced when sulfur-containing fuels (coal, heavy fuel oils) are burned. They cause acid rain and respiratory health problems.
• Particulate matter (PM) consists of small solid or liquid particles released during combustion, especially from solid fuels and diesel engines. PM inhalation is linked to respiratory and cardiovascular disease.

Strategies for Reducing Emissions

Emission reduction strategies fall into three broad categories:

Fuel-side approaches:

• Switching to fuels with lower carbon content (natural gas produces roughly 40% less $CO_2$ per unit energy than coal)
• Using biofuels or synthetic fuels to reduce net lifecycle emissions

Combustion-side approaches:

• Staged combustion, where fuel is burned in multiple zones at controlled temperatures to limit $NO_x$ formation
• Flue gas recirculation (FGR), which dilutes the flame with cooled exhaust gas to lower peak temperatures and reduce thermal $NO_x$
• Optimizing mixing and residence time to minimize $CO$ and PM from incomplete combustion

Post-combustion approaches:

• Selective catalytic reduction (SCR) for $NO_x$ removal
• Wet or dry scrubbers for $SO_x$ removal
• Carbon capture and storage (CCS), which captures $CO_2$ from flue gases and sequesters it underground or diverts it for industrial use

Beyond these, improving overall system efficiency through better insulation, waste heat recovery, and process optimization reduces total fuel consumption and, by extension, all associated emissions.