The Combustion of Hydrocarbon Fuels

1. The Combustion of Hydrocarbon Fuels 朱　信Hsin ChuProfessorDept. of Environmental EngineeringNational Cheng Kung University

2. 1. Introduction • The key to the development of mankind was the discovery of how to manage the combustion process; in other words, the ability to initiate combustion when required and to control and apply the resulting fire. • There are three significant applications- the generation of power, the provision of heat for processes and the provision of heat for control of the built environment.

3. The earliest heating applications were directed towards the improvement of basic creature comforts: keeping habitable space warm and cooking food. • The development of combustion equipment for heating building followed a path of increasing efficiency and improving amenity: primitive “plant” consisted of an open, central solid fuel fire with the flue consisting at best of a central opening in the roof.

4. Combustion plants are fired by a wide variety of fuels, from natural gas through to municipal refuse, but common to all is the combustion of fuels containing the elements carbon and hydrogen-generically known as hydrocarbon fuels.

5. 2. The Combustion Process • Combustion is a chemical reaction between a fuel and oxygen which is accompanied by the production of a considerable amount of heat (it is an exothermic reaction). • The reaction has to be initiated by some source of high-temperature energy (ignition).

6. The most obvious characteristic of the combustion process is the reaction zone, which is usually visible as a flame; the radiation emitted from the flame may be very intense, for example the characteristic yellow color of an oil flame, or it may be quite weak as in the case of the flame from a gas hob on a cooker.

7. We can divide up the combustion of a fuel into several processes: • Bringing together the fuel and air (the reactants) in the correct proportions. • Igniting the reactants. • Ensuring that the flame burns in a stable manner and that combustion is complete. • Extracting useful heat from the process, and • Arranging for the safe disposal of the products of combustion.

8. 3. The Complete Combustion Reaction • Complete combustion has been achieved when no further reaction takes place- all the carbon in the fuel appears in the flue gases as carbon dioxide (CO2) and all the hydrogen in the fuel is burned to water (H2O). • As an illustration of the combustion reaction in the case of a simple but common gaseous hydrocarbon we can look at the combustion of methane (CH4).

9. The bonding rearrangement which takes place when the fuel and air react can be regarded as producing species (carbon dioxide and water vapor) which are thermodynamically at a lower energy level than the reactants. • The transformation to a lower energy level is responsible for the exothermic nature of the reaction. • Next slide (Fig. 1.2)Energy profile of the combustion reaction

11. 4. Quantification of the Combustion Reaction • StoichiometryAt the same temperature and pressure, equal volumes of gases contain equal numbers of molecules. This means that the reaction CH4 + 2O2 → CO2 + 2H2Oalso shows that one volume of methane requires just two volumes of oxygen to produce complete combustion.

12. The equation above can be quantified as:CH4 + 2O2 → CO2 + 2H2O1 vol 2vols 1 vol 2(or 0) vols2: water vapor0: water liquid • Taking as a basis each molecular weight expressed in kg:CH4 + 2O2 → CO2 + 2H2O16 kg 64 kg 44 kg 36 kg

13. A third basis for quantification of the combustion reaction, and which is perhaps the most generally applicable, is the use of molar quantities. CH4 + 2O2 → CO2 + 2H2O1 mole 2 moles 1 mole 2 moles • This last point can be significant as the combustion of carbon:C + O2 → CO2cannot be meaningfully represented in volumetric terms as carbon is a solid fuel.