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CombustionSynopsisIntroductionTypes of CombustionCombustion StoichiometryThree Ts of CombustionEnergy Efficiency OpportunitiesThe Chemistry of Coal CombustionCoal Combustion SystemsQA2IntroductionCombustion is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species.

The release of heat can produce light in the form of either glow or a flame.QA3

Complete combustion is almost impossible to achieve.

As actual combustion reactions come to equilibrium, a wide variety of major and minor species will be present, such as carbon monoxide, hydrogen and even carbon (soot or ash).

Additionally, any combustion at high temperatures in atmospheric air, which is 78% N2, will also create small amounts of several nitrogen oxides, commonly referred to as NOx.QA4QA5FIRETHE FIRE TRIANGLEOxygenHeatFuelTypes of CombustionComplete CombustionIn complete combustion, the reactant burns in oxygen, producing a limited number of products.

When a hydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, sulfur will yield sulfur dioxide, and iron will yield iron(III) oxide. QA7Incomplete CombustionIncomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water.

QA8Rapid combustionRapid combustion/explosion is a form of combustion, otherwise known as a fire, in which large amounts of heat and light energy are released, which often results in a flame.

This is used in a form of machinery such as internal combustion engines and in weapons. QA9Spontaneous combustionQA10Combustion in Turbulent EnvironmentCombustion resulting in a turbulent flame is mostly used for industrial application (e.g. gas turbines, gasoline engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.QA11Micro-combustionCombustion processes which happen in very small volumes are considered micro-combustion. Combustion in Microgravity EnvironmentCombustion processes behave differently in a microgravity environment than in Earth-gravity conditions due to the lack of buoyancy. QA12

Flame in Earths atmosphereFlame in Micro-GravityCombustion StoichiometryQA13Combustion in OxygenQA14

Combustion in AirQA15

Combustion in Air (Cont.)Nitrogen is considered to be a temperature reducing diluter that must be present to obtain the oxygen required for combustion.

Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the flue gases.

It also increases the volume of combustion by-products, which then have to travel through the heat exchanger and up the stack faster to allow the introduction of additional fuel-air mixture.QA16Air-Fuel RatioQA17Air-Fuel Ratio (Cont.)QA18

Formation of NOx in CombustionThermal NOxOxidation of atmospheric N2 at high temperatures

Formation of thermal NOx is favorable at higher temperature

Fuel NOxOxidation of nitrogen compounds contained in the fuel

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Formation of CO in CombustionIncomplete CombustionDissociation of CO2 at high temperature

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Three Ts of CombustionQA21The objective of good combustion is to release all of the heat in the fuel. This is accomplished by controlling the "three T's" of combustion which are:

Temperature high enough to ignite and maintain ignition of the fuelTurbulence or intimate mixing of the fuel and oxygenTime, sufficient for complete combustionQA22QA23

(a)(b)(c)(a) Perfect, (b) Good & (c) Incomplete combustionEnergy Efficiency OpportunitiesQA24Pre-heating of the Combustion OilThe entire tank may be preheated. In this form of bulk heating, steam coils are placed at the bottom of the tank, which is fully insulated

The oil can be heated as it flows out with an outflow heater. QA25Temperature control of Combustion OilThermostatic temperature control of the oil is necessary to prevent overheating, especially when oil flow is reduced or stopped. The temperature at which oil can readily be pumped depends on the grade of oil being handled.

Oil should never be stored at a temperature above that necessary for pumping as this leads to higher energy consumption.QA26Assignment: (Submission date: 05-03-2015)What are the combustion products if the fuel contains O, S, Cl or other elements?In combustion process, which oxidant is better to use, O2 or air ?What is the Air-Fuel ratio for stoichiometric combustion of methane and benzene, respectively?Consider the combustion of methanol in an engine. If the Air-Fuel ratio of the actual mixture is 20, is the engine operating under rich or lean conditions?QA27The Chemistry of Coal CombustionQA28Coal is burned in three ways: As large pieces in a fixed bed or on a grateAs smaller or crushed pieces in a fluidized bed As very fine particles in suspension

Particle size is found to be the most important parameter with respect to the dominant reaction mechanism and other thermal behavior (i.e., rate of heating)

QA29Combustion ProcessAs the coal particles are heated, moisture is driven off the coal particles.The coal particles undergo devolatilization and release volatile organic constituents. The volatile matter is combusted in the gas phase (homogenous reaction). This can occur prior to and simultaneously with step 4Combustion of the char particlesA surface (heterogeneous) reactionQA30Char CombustionQA31Char combustion is a slow process and therefore determines the time for complete combustion in a furnace

The combustion of the char begins with chemisorption of oxygen at active sites on char surfaces. The decomposition of the resultant surface oxides mainly generates carbon monoxide (CO)

Fresh reaction sites are continuously exposed as the surface oxides are decomposed.QA32The combustion of char involves at least four carbonoxygen reactions

Along with the oxidation of non-carbon atoms

Which may be followed byQA33

Coal Combustion SystemsQA34The manner in which coal is burned and the devices in which it is burned are primarily determined by:

The desired unit size or capacity (i.e., required hourly steam production or electricity generation)&Coal type and qualityQA35Fixed-Bed CombustionOverfeed

Underfeed

Crossfeed

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Fluidized Bed CombustionCirculating Fluidized Bed

Bubbling Fluidized Bed QA37References:Thermal Equipment: Fuels and CombustionGlassman, I. and R. A. Yetter, Combustion, Elsevier Science USA. 2008Miller, B. G., Coal Energy Systems, Elsevier Academic Press. 2005http://en.wikipedia.org/wiki/CombustionQA38