Combustion is a chemical process in which a substance (usually a fuel) reacts with an oxidizing agent, releasing energy in the form of heat and light. It is a key phenomenon in energy production, industrial processes, and daily life activities. Let’s explore the science, types, and applications of combustion in detail.
What is Combustion?
Combustion is an exothermic reaction (a reaction that releases energy) between a fuel and oxygen. The result is the release of heat, light, and combustion products like carbon dioxide (CO₂) and water (H₂O). This process plays a vital role in power generation, heating, and transportation.
The general combustion reaction can be expressed as:Fuel+O2→Products+Energy\text{Fuel} + O_2 \rightarrow \text{Products} + \text{Energy}Fuel+O2→Products+Energy
For example, the combustion of methane gas:CH4+2O2→CO2+2H2O+HeatCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Heat}CH4+2O2→CO2+2H2O+Heat
Types of Combustion:
- Complete CombustionOccurs when there is sufficient oxygen supply.Produces carbon dioxide (CO₂), water, and large amounts of energy.
Example: Burning of natural gas.CH4+2O2→CO2+2H2O+EnergyCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}CH4+2O2→CO2+2H2O+Energy - Incomplete Combustion
- Happens when the oxygen supply is limited.
- Produces carbon monoxide (CO), soot, or other hydrocarbons along with heat. Example: Burning of wood in low oxygen environments.
2CH4+3O2→2CO+4H2O+Heat2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O + \text{Heat}2CH4+3O2→2CO+4H2O+Heat
- Spontaneous Combustion
- Occurs without an external ignition source.
- Results from heat generated within the material due to oxidation or chemical reactions.
Example: Haystacks catching fire due to microbial activity.
- Explosive Combustion
- A rapid form of combustion where energy is released almost instantly, causing an explosion. Example: Explosion of fireworks or gasoline vapors.
Combustion Conditions:
- Combustion Conditions
- Fuel: Combustible material (solid, liquid, or gas).
- Oxidizer: Usually oxygen from the air.
- Heat: A source of ignition to start the process (like a spark or flame).
- Combustion Conditions
- Fuel: Combustible material (solid, liquid, or gas).
- Oxidizer: Usually oxygen from the air.
- Heat: A source of ignition to start the process (like a spark or flame)
Environmental Impact of Combustion:
While combustion is essential for many activities, it has some negative environmental impacts:
- Air Pollution: Emission of carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen oxides (NOₓ), and particulate matter.
- Global Warming: Burning fossil fuels releases large amounts of CO₂, a greenhouse gas that contributes to climate change.
- Acid Rain: Sulfur and nitrogen compounds released during combustion can lead to acid rain, damaging ecosystems.
Combustion: Key Subtopics
1. Stoichiometric Equations for Combustion:
A stoichiometric equation defines the ideal amount of oxygen required to completely burn a given amount of fuel without leaving excess oxygen or unburned fuel. This balanced reaction ensures maximum energy output with minimal pollutants.
Stoichiometric Air-Fuel Ratio (AFR):
AFR=Mass of FuelMass of Air/Mass of Fuel
The stoichiometric AFR for some common fuels:
- Methane (CH₄): 17.2:1
- Gasoline: ~14.7:1
- Propane (C₃H₈): 15.6:1
Example: Combustion of methane
CH4+2O2→CO2+2H2O+Energy
In reality, excess air is often provided to ensure complete combustion. However, too much excess air reduces flame temperature and efficiency.
2.Adiabatic Flame Temperature:
The adiabatic flame temperature is the maximum temperature achieved by the combustion products assuming no heat is lost to the surroundings. It represents an idealized case where all the energy released by the reaction is retained in the products.
Calculation of Adiabatic Flame Temperature
The equation involves energy balance:
∑Energy of Reactants=∑Energy of Products
n practice, tables of specific enthalpies for different gases are used to solve this equation for complex fuels.
- Factors Affecting Adiabatic Flame Temperature:
- Fuel-Air Ratio: A stoichiometric mixture gives the highest flame temperature.
- Type of Fuel: Fuels with higher heating values produce higher flame temperatures.
- Excess Air: Lowers the flame temperature due to dilution by nitrogen.
3.Higher and Lower Heating Values (HHV and LHV)
The heating value of a fuel refers to the energy released during its combustion.
- Higher Heating Value (HHV):
This is the total heat released when the water in the combustion products condenses (i.e., latent heat is included).
\text{HHV} > \text{LHV} ]
- Lower Heating Value (LHV):
It excludes the heat recovered from condensation, assuming water remains in vapor form.
Application: HHV is used when heat recovery systems (like boilers) are in place, while LHV is relevant for internal combustion engines where exhaust gases leave as vapor.
4.Mixture Strength (Air-Fuel Ratio: Lean and Rich Mixtures)
The mixture strength refers to the ratio of air and fuel in the combustion process, which affects efficiency and emissions.
- Stoichiometric Mixture:
Ideal air-fuel ratio with no excess air or unburned fuel. Example: 14.7:1 for gasoline. - Lean Mixture:
Excess air is supplied, making the ratio higher than stoichiometric (e.g., 17:1). It improves fuel economy but may cause misfiring. - Rich Mixture:
Contains more fuel than the stoichiometric amount (e.g., 12:1). It enhances power output but produces carbon monoxide and soot.
Impact of Mixture Strength:
- Lean mixtures: Better fuel economy, lower emissions.
- Rich mixtures: More power, higher emissions.
Practical Problem Example: Combustion of Propane:
Problem:
Q: Calculate the amount of oxygen required to completely combust 2 kg of propane (C₃H₈). The molecular weight of C₃H₈ = 44 g/mol and O₂ = 32 g/mol.
Balanced Equation
C3H8+5O2→3CO2+4H2O
1 mole of propane reacts with 5 moles of oxygen.
Step 1: Calculate the moles of propane:
Moles of C3H8= 2000/44gm/mol=45.45mol
Step 2: Find the moles of oxygen required:
45.45×5=227.25molO2
Step 3: Calculate the mass of oxygen:
Mass of O2=227.25mol×32g/mol=7272g=7.272kg
So, 7.272 kg of oxygen is required to completely burn 2 kg of propane.
