🪫Chemical Process Balances Unit 9 – Combustion Reactions in Chemical Processes

Key Concepts and Definitions

• Combustion reactions involve the rapid oxidation of a fuel source in the presence of an oxidizer (typically oxygen) to produce heat and light
• Fuels can be solid, liquid, or gaseous and include hydrocarbons (coal, natural gas, gasoline), biomass, and hydrogen
• Oxidizers provide the oxygen necessary for combustion and include air, pure oxygen, and other oxygen-containing compounds (nitrous oxide, hydrogen peroxide)
• Complete combustion occurs when a fuel is burned in the presence of sufficient oxygen, resulting in the formation of carbon dioxide and water as the main products
• Incomplete combustion happens when there is insufficient oxygen, leading to the formation of carbon monoxide and other byproducts (soot, unburned hydrocarbons)
• Stoichiometry in combustion reactions involves balancing chemical equations and determining the quantities of reactants and products based on molar ratios
• Adiabatic flame temperature represents the maximum temperature achieved during combustion, assuming no heat loss to the surroundings
• Flue gas refers to the gaseous products of combustion, which typically include carbon dioxide, water vapor, nitrogen, and other trace components (sulfur oxides, nitrogen oxides)

Fundamentals of Combustion Reactions

• Combustion reactions are exothermic, releasing heat energy as the fuel is oxidized
• The general equation for a combustion reaction is: Fuel + Oxidizer → Products + Heat
• The three essential components of combustion are fuel, oxidizer, and an ignition source (heat, spark, flame)
• The combustion process involves the breaking of chemical bonds in the fuel and the formation of new bonds in the products
• The rate of combustion depends on factors such as fuel-oxidizer mixing, temperature, pressure, and the presence of catalysts or inhibitors
• The air-fuel ratio (AFR) represents the mass ratio of air to fuel in a combustion reaction
  • Stoichiometric AFR corresponds to the exact amount of air needed for complete combustion
  • Rich mixtures have excess fuel (AFR < stoichiometric), while lean mixtures have excess air (AFR > stoichiometric)
• Flame propagation occurs as the heat from the combustion reaction ignites adjacent fuel-oxidizer mixtures, sustaining the reaction

Types of Combustion Processes

• Premixed combustion involves the mixing of fuel and oxidizer before ignition, as in a Bunsen burner or a gas stove
• Diffusion combustion occurs when the fuel and oxidizer are initially separate and mix during the combustion process (candle flame, diesel engine)
• Turbulent combustion is characterized by the presence of turbulent flow, which enhances fuel-oxidizer mixing and increases the rate of combustion (gas turbines, industrial furnaces)
• Catalytic combustion uses a catalyst to lower the activation energy of the reaction, allowing for combustion at lower temperatures (catalytic converters in automobiles)
• Fluidized bed combustion involves the burning of solid fuels in a bed of inert particles (sand, limestone) that is fluidized by the flow of air, providing efficient mixing and heat transfer
• Staged combustion separates the combustion process into multiple stages to reduce the formation of pollutants (low NOx burners)
• Oxy-fuel combustion uses pure oxygen instead of air as the oxidizer, resulting in higher flame temperatures and reduced flue gas volume (used in glass and metal production)

Stoichiometry in Combustion Reactions

• Stoichiometry is used to determine the quantities of reactants and products in a combustion reaction based on the balanced chemical equation
• The general steps for solving combustion stoichiometry problems are:
  1. Write the balanced chemical equation for the combustion reaction
  2. Convert given quantities to moles using molar mass
  3. Use molar ratios from the balanced equation to calculate the desired quantity
  4. Convert the calculated moles back to the desired unit (mass, volume)
• Excess air is often used in combustion to ensure complete burning of the fuel, and the percent excess air can be calculated from the actual and stoichiometric air-fuel ratios
• The equivalence ratio (φ) is the ratio of the actual fuel-oxidizer ratio to the stoichiometric fuel-oxidizer ratio, with φ < 1 indicating a lean mixture and φ > 1 indicating a rich mixture
• Combustion efficiency is the ratio of the actual heat released to the theoretical heat released based on the fuel's heating value, and it can be affected by incomplete combustion or heat losses

Thermodynamics of Combustion

• The first law of thermodynamics states that energy is conserved in a combustion reaction, with the heat released equal to the change in enthalpy (ΔH) between the reactants and products
• The enthalpy of combustion (ΔHc) is the heat released when one mole of fuel is completely burned, and it is a measure of the fuel's energy content
• The adiabatic flame temperature (AFT) is the maximum temperature reached during combustion, assuming no heat loss to the surroundings
  • AFT is calculated by setting the enthalpy of the reactants equal to the enthalpy of the products at the adiabatic flame temperature
  • Higher AFTs are associated with more efficient combustion and increased formation of thermal NOx
• The second law of thermodynamics dictates that combustion processes always result in an increase in entropy (ΔS > 0) due to the irreversible nature of the reaction
• The Gibbs free energy change (ΔG) for a combustion reaction determines the spontaneity of the process, with negative values indicating a spontaneous reaction at constant temperature and pressure
• The heat of formation (ΔHf) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states, and it is used to calculate the enthalpy of combustion using Hess's law

Material and Energy Balances in Combustion

• Material balances in combustion processes ensure the conservation of mass, with the mass of the reactants equal to the mass of the products
  • Elemental balances (C, H, O, N, S) are used to determine the composition of the flue gas and the air-fuel ratio
  • The nitrogen balance is often used to calculate the amount of excess air used in the combustion process
• Energy balances in combustion processes account for the conservation of energy, considering the enthalpy of the reactants, the enthalpy of the products, and any heat losses
  • The energy balance equation is: $Q + \sum_{reactants} n_i \cdot h_i = \sum_{products} n_j \cdot h_j + Q_{loss}$
  • $Q$ represents the net heat input or output, $n_i$ and $n_j$ are the molar flow rates of the reactants and products, $h_i$ and $h_j$ are the molar enthalpies of the reactants and products, and $Q_{loss}$ accounts for any heat losses to the surroundings
• The efficiency of a combustion process can be determined by comparing the actual energy output to the theoretical energy input based on the fuel's heating value
  • Thermal efficiency = (Useful heat output) / (Heat input from fuel)
  • Factors that reduce efficiency include incomplete combustion, heat losses, and the presence of moisture in the fuel or combustion air

Industrial Applications and Examples

• Power generation: Combustion of fossil fuels (coal, natural gas, oil) in boilers to produce steam for turbines and generate electricity
• Transportation: Internal combustion engines in vehicles, where the combustion of gasoline or diesel fuel drives the motion of pistons
• Heating and cooking: Natural gas or propane combustion for home heating, water heating, and cooking appliances
• Metal production: Combustion of coke (high-carbon fuel) in blast furnaces to produce pig iron, which is then converted to steel
• Cement production: Combustion of coal or alternative fuels in rotary kilns to heat raw materials (limestone, clay) and produce clinker, which is ground to make cement
• Waste incineration: Combustion of municipal solid waste to reduce volume and generate heat or electricity, while controlling emissions
• Glass manufacturing: Combustion of natural gas in furnaces to melt raw materials (silica sand, soda ash, limestone) and produce glass
• Petrochemical industry: Combustion of hydrocarbon fuels to provide heat for various processes, such as cracking, reforming, and distillation

Safety and Environmental Considerations

• Incomplete combustion can lead to the formation of carbon monoxide (CO), a toxic gas that can cause health issues and even death at high concentrations
• Nitrogen oxides (NOx) are formed during high-temperature combustion through the reaction of nitrogen and oxygen in the air, contributing to air pollution and acid rain
  • Thermal NOx formation increases with higher flame temperatures and can be mitigated through the use of low NOx burners or staged combustion
  • Fuel NOx arises from the oxidation of nitrogen-containing compounds in the fuel and can be reduced by using low-nitrogen fuels or selective catalytic reduction (SCR)
• Sulfur oxides (SOx) are produced during the combustion of sulfur-containing fuels (coal, heavy oils) and can cause respiratory issues and contribute to acid rain
  • SOx emissions can be controlled through the use of low-sulfur fuels, flue gas desulfurization (FGD), or by using alternative fuels (natural gas, biomass)
• Particulate matter (PM) emissions from combustion processes can have negative health impacts and are regulated by air quality standards
  • PM can be reduced through the use of filters, electrostatic precipitators, or baghouses in flue gas cleaning systems
• Greenhouse gas emissions, primarily carbon dioxide (CO2), from combustion processes contribute to climate change and are subject to regulations and carbon reduction strategies
  • Carbon capture and storage (CCS) technologies aim to capture CO2 from flue gases and store it underground to mitigate its environmental impact
• Proper safety precautions, such as leak detection, ventilation, and personal protective equipment (PPE), must be in place to prevent fires, explosions, and exposure to harmful combustion products
• Regular maintenance and monitoring of combustion equipment are essential to ensure efficient operation, reduce emissions, and prevent safety hazards