Fuels and Combustion PDF

Faculty of Engineering and Applied Science MECE3260U-Introduction to Energy Systems Fuels and Combustion Dr. Ibrahim Dincer Professor of Mechanical Engineering  OUTLINE Historical Perspectives Impacts of Fossil Fuels Fos...

Faculty of Engineering and Applied Science MECE3260U-Introduction to Energy Systems Fuels and Combustion Dr. Ibrahim Dincer Professor of Mechanical Engineering  OUTLINE Historical Perspectives Impacts of Fossil Fuels Fossil Fuels  Coal  Crude oil  Natural Gas Fuels Combustion Thermodynamic Analysis Closing Points 2  Fossil Fuel Era https://collapseofindustrialcivilization.com/tag/the-fossil-fuel-age/ 3 Historical Perspective on Fossil Fuels Fossil fuels were formed many hundreds of millions of years ago before the time of the dinosaurs. The age they were formed is called the Carboniferous Period (from about 360 to 286 million years ago). Its use began with the Industrial Revolution (1760-1840) 4 Challenges with Fossil Fuels 5  Heating Values Higher heating value (or gross calorific value or gross energy) is determined by bringing all the products of combustion back to the original pre-combustion temperature. Lower heating value (or net calorific value) is determined by subtracting the heat of vaporization of the water in the by-product from the higher heating value results. The lower heating value is typically used in analysis studies. Fuel Phase at room T & P HHV (MJ/kg) LHV (MJ/kg) Hydrogen Gas 141.9 119.9 Methane Gas 55.6 49.9 Gasoline Liquid 47.6 44.7 Diesel Liquid 44.9 42.4 Methanol Liquid 21.2 18.1 6  Coal Source: www.eia.doe.gov Coal exists in a large variety of geologic forms and levels of quality and, more important, specific heating value. Coal can be characterized in a continuous range of ranks, beginning with peat, then lignites (brown coal), then bituminous (soft coal), and finally anthracite (hard coal) and graphite. Coal is used primarily for steam-turbine electricity generation and for coking in the iron and steel industry. 7  Crude Oil Crude oil (petroleum) is a mixture of aliphatic hydrocarbons, mainly alkanes of formula CnH2n+2, and several impurities, with sulphur being the most important (sweet crude: oil with a low sulphur content and sour crude: oil with high sulphur content). The hydrocarbons are classified by their chain lengths. Those with longer chain lengths generally have lower boiling points, which allows for their separation by distillation in refineries. 8  Natural Gas Natural gas (CH4) is found in sedimentary rocks, generally in the presence of crude oil as ‘‘wet’’ gas (associated gas) or separately as ‘‘dry’’ gas (nonassociated gas). Wet gas generally contains many other hydrocarbon gases and several impurity gases that are removed before delivery. Dry gas generally does not contain many other gases besides methane. Natural gas is classified as ‘‘sour gas’’ with high H2S. The composition varies by type and deposit; typical constituents. The heating value of natural gas also depends on the chemical composition. 9  FUELS AND COMBUSTION The total number of moles is not conserved during a combustion reaction! Any material that can be burned to release thermal energy is called a fuel. Most fuels consist primarily of hydrogen and carbon, so-called: hydrocarbon fuels. Gasoline is known as octane, C8H18. Most liquid hydrocarbon fuels are a mixture of numerous hydrocarbons and are obtained from crude oil by distillation. The air–fuel ratio, AF: the ratio of the mass of air to the mass of fuel for a combustion process: AF or AFR = mair/mfuel Here, m = NM, where M is the molar mass. N is the number of moles. 10 11 12  THEORETICAL AND ACTUAL COMBUSTION PROCESSES All the combustible components of a fuel are burned to completion during a complete combustion process. The combustion process is incomplete if the combustion products contain any unburned fuel or components such as C, H2, CO, or OH. The minimum amount of air needed for the complete combustion of a fuel is called stoichiometric or theoretical air. The ideal combustion process during which a fuel is burned completely with theoretical air is called the stoichiometric or theoretical combustion of that fuel. For example: the theoretical combustion of methane is 13 The amount of air in excess of the stoichiometric amount is called excess air. The amount of excess air is usually expressed in terms of the stoichiometric air as percent excess air or percent theoretical air. For example, 60% excess air is equivalent to 160% theoretical air, and 120% excess air is equivalent to 220% theoretical air. Amounts of air less than the stoichiometric amount are called deficiency of air and are often expressed as percent deficiency of air. The ratio of the reduction in volume to the original volume is the volume fraction of the CO2, which is equivalent to the mole fraction if ideal-gas behavior is assumed. 14  Higher heating value (or gross calorific value or gross energy) and Lower heating value (or net calorific value): 15 16 17 18 19 20  Adiabatic Flame Temperature In the limiting case of no heat loss to the surroundings (Q = 0), the temperature of the products reaches a maximum, which is called adiabatic flame or adiabatic combustion temperature of the reaction: ΣHreactants = ΣHproducts The maximum temperatures that occur in these devices are considerably lower than the adiabatic flame temperature, however, since the combustion is usually incomplete, some heat loss takes place, and some combustion gases dissociate at high temperatures. 21  Summary of Fuel Combustion Analysis Reminders: 22 Closing Points:  Fossil Fuels  Fuels combustion  Reactants vs Products  Air–fuel ratio  Stoichiometric or theoretical combustion  Enthalpy of reaction and enthalpy of combustion  Heating values  Thermodynamic analysis  Adiabatic flame temperature 23