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NFCRC Tutorial: Combustion

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altThe combustion process provides tremendous amounts of energy from a fuel and this energy is converted or transformed producing heat for cooking, making hot water, or to generate steam for manufacturing or turning a turbine to produce shaft power and electricity, to produce mechanical motion as in an auto engine, or thrust as in an aircraft or shaft power as in a land based engine.

altThe chemical energy contained in fossil and biomass fuels which is released by the combustion process consists of reactions with oxygen contained in the air. The fuel bound energy is first converted to thermal energy which in turn may be converted to shaft power and motive power in a reciprocating engine (as in automobiles, trucks, busses, locomotives and ships) or in a rotating engine such as a gas turbine or steam turbine or propulsion energy (as in jet and prop aircraft). The shaft power may also be used to generate electric power which involves turning a generator connected to a reciprocating engine or a gas turbine or a steam turbine. In institutional energy and manufacturing applications where heat energy is required, the combustion process is utilized in boilers and furnaces. Internal combustion engines (diesel engines or gas turbines) in addition to generating electricity may also be used to provide heat energy by recovery of the energy contained in the exhaust gases. Such a dual purpose application is called cogeneration and provides for very efficient utilization of the fuel bound energy. Air conditioning may also be provided from the heat utilizing an adsorption refrigeration cycle such as the lithium bromide cycle for moderate temperature refrigeration or an ammonia absorption cycle for deep refrigeration temperatures. A fuel cell may also be utilized in a cogeneration mode.

Finally, the familiar barbecue is another example of the application of the combustion process.

The combustion of fuels requires the consumption of large quantities of air. For example, 150 Lb of a fuel (oil) requires about 2000 Lb of air and the resulting CO2 introduced into the atmosphere is about 250 Lb. Small quantities of pollutants such as NO, CO and hydrocarbons are also formed, these quantities being negligible from engineering calculations standpoint but very significant from the environmental standpoint.

Combustion Process. The combustion process involves some 1000 reactions to complete the oxidation process forming CO2 and H2O, the ultimate products of combustion. However, pollutants such as CO, HCs, soot, NOx, SO2 are also formed during the combustion process as a result of the various reactions.

Carbon Monoxide (from Natural Gas Combustion). The CH4 molecule is very stable and requires high energy atoms to break loose an H atom forming the CH3 radical which plays a key role in propagating the combustion process. This process includes the partial oxidation of CH4, oxidation of CO, OH reactions and NO formation reactions. CO formation involves a number of steps but is a fast overall reaction while the oxidation of CO to CO2 is very slow and as a result, the auto engine produces significant amounts of CO due to the short residence time. In a gas turbine, however, more residence time is available within the combustor and the CO emissions are much lower. High temperatures and O2 concentrations and large residence times are required for the CO oxidation and involves the reaction with the OH radical formed during the combustion process. High CO emission not only means more pollution but also lower thermal efficiency.

Nitrogen Oxides. The NO is produced from (1) high temperature oxidation of molecular N2 (thermal NO), (2) hydrocarbon radical attack on the molecular N2 (prompt NO), and (3) oxidation of chemically bound nitrogen in the fuel (fuel NO). The thermal NO, i.e., the formation of NO from the N2 present in the combustion air requires the breaking of the covalent triple bond in the N2 and requires very high temperatures and forms by the action of the O radical produced during the combustion process. Increased temperature, residence time, and O2 concentration increase NO emissions which is in direct contrast to the conditions required for CO formation. As much as 60 to 80% of the fuel bound nitrogen present in fuels such as oil and coal forms NO.

The NO formation reaction suddenly takes off around 2800 deg F or 1540 deg C and thus a window of opportunity exists to control NO by staying just below this temperature. Thermal NO formation shows an inverse relationship with respect to HC and CO emissions when the air to fuel ratio is varied. Control strategies include (1) burning under lean conditions, (2) staged combustion with rapid quenching of the flame by the secondary air, (3) pre-mixed burners (ideally, with variable geometry for varying load of the boiler or engine, and (4) flue gas recycle. Post combustion processes are also sometimes applied but have certain disadvantages such as transforming NO into other undesirable species. To meet the ultra low NOx emissions being mandated, the internal structure of the combustion process which is complex and combines fluid dynamics, turbulent mixing, high temperature chemistry and heat transfer need to be understood to develop new solutions without compromising efficiency at full or partial load.

The formation of NO2 is not significant during the combustion process, however the NO oxidizes to NO2 in the atmosphere and thus all NO is potential NO2. NOx refers to NO plus NO2. Another oxide of nitrogen, N2O which is also formed during the combustion has become important in recent years due to its role in the stratosphere as a greenhouse gas. It is formed in significant concentrations (from an environmental impact standpoint) in fluidized bed combustion.

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