Fuels_Final.pdf

Fuels Fuels come in different types and each type possesses various characteristics. These characteristics avail advantages and disadvantages to be discovered upon completion of discussion. Applications of fuels in industries will be covered to a certain extent. Problems are provided to readers for...

Fuels Fuels come in different types and each type possesses various characteristics. These characteristics avail advantages and disadvantages to be discovered upon completion of discussion. Applications of fuels in industries will be covered to a certain extent. Problems are provided to readers for quantitative appreciation of the topic. Outcome After completion of the topic, the reader should be able to: Understand fuel and combustion Classify fuels Analyze coal Calculate NCV and GCV Introduction Fuel is a substance which, when burnt, i.e. on coming in contact and reacting with oxygen or air, produces heat. Hence, the substances classified as fuel must necessarily contain one or several of the combustible elements: carbon, hydrogen, sulfur, etc. In the process of combustion, the chemical energy of fuel is converted into heat energy. Fuels are broadly classified in two ways: According to the physical state (solid, liquid, and gas) and According to the mode of procurement (natural and manufactured). 1 Solid Fuels Wood Wood is derived from the trunks and branches of trees. It contains about 15% moisture after air-drying. The average composition of wood is largely used as domestic fuel, its application as engineering fuel is restricted. Calorific value varies from 4000 to 6400 Btu/lb. Coal Coal is the most important solid fuel and derived from prehistoric plants. Coal is highly carbonaceous and composed of C, H, and O and non-combustible inorganic matter. Origin: Vegetable matter is first transformed into peat, then to lignite then into bituminous coal and finally to anthracite. The transformation period extends over millions of years through a continuous process. With the progressive transformation of wood, content composition decreases except for carbon content, calorific value, and hardness which increases. Figure 1. Coal Metamorphism 2  Figure 2. Coal Formation Figure 2. Coal-Fired Power Plant Schematic Diagram 3 Analysis of Coals Ranking of coal is done on: a) Proximate analysis: It consists of determination of percentages of volatile matter, fixed carbon, ash and moisture of air-dried coal. b) Ultimate analysis: Consisting of determination of percentages of C, H, O, N, and S. Coke Carbonization or coking bituminous coal leads to the formation of coke. Calorific Value There are different expressions for calorific values: the quantity of heat evolved by the combustion of unit quantity of fuel is its gross calorific value (GCV). Gross or higher calorific value is the quantity of heat liberated by combustion unit mass of fuel in oxygen, the original material and the final product of combustion being at a reference temperature of 25°C and the water obtained in the liquid state, represented by GCV or HCV. Net calorific value (NCV) on the other hand is the quantity of heat evolved when a unit quantity of fuel is burnt in oxygen, the original material and the final product of combustion being at a reference temperature of 25°C and the water obtained from the fuel being at the vapor state. Hence, NCV or LCV (lower heating value) is always less than the gross calorific value by the amount corresponding to the heat of condensation of water vapors i.e., 587.0 kcal/kg. We use Dulong’s formula for calorific value (HCV or GCV) from the chemical composition of fuel. 1 𝐻 𝐻𝐻𝐻 = [8080𝐻 + 34500 (𝐻 − ) + 2240𝐻] 𝐻𝐻𝐻𝐻/𝐻𝐻 100 8 where C, H, O, S are the percentages of carbon, hydrogen, oxygen and sulfur in the fuel. Oxygen is assumed to be present in combination with hydrogen as water. For LCV or NCV, 𝐻𝐻𝐻 = 𝐻𝐻𝐻 − 𝐻𝐻𝐻𝐻𝐻𝐻 ℎ𝐻𝐻𝐻 𝐻𝐻𝐻𝐻𝐻 𝐻𝐻𝐻. 𝐻𝐻𝐻𝐻𝐻𝐻 𝐻𝐻𝐻 = [𝐻𝐻𝐻 − 9(𝐻) × 587 ] 𝐻𝐻𝐻𝐻/𝐻𝐻 (1 part of H gives 9 parts of water and the latent heat of steam is 587 kcal/kg or 2458 kJ/kg) Problem 1 A coal has the following composition by weight C=90%, O=3%, S=0.5%, N=0.5% and ash=2.5%. Net calorific value of the fuel was found to be 8490.5 kcal/kg. Calculate the percentage of H and GCV. 4 Liquid Fuels Petroleum or crude oil is deep brown oil consisting mainly of hydrocarbons, paraffins, naphthenes and aromatics in varying proportions. Practically all metals are found in petroleum, the most common are Si, Fe, Al, Ca, Mg, and Ni. Occurrence: The crude oil has been derived from the organic matter originally present in marine sediments. The dead organic matter settles down to the bottom of shallow seas and lagoons. The settled debris is attacked by anaerobic bacteria, whereby most of the organic compounds are destroyed and the remaining unsaturated fatty oils and fatty acids undergo polymerization. Figure 3. Petroleum and Natural Gas Formation Classification of Petroleum a) Paraffin-base crude composed of saturated hydrocarbons up to C35H72 which are semi-solids, called waxes. b) Asphalt-based crude contains mainly naphthenes and cycloparaohenes with smaller amounts of paraffins and aromatics. c) Mixed base crude contains both the above types of compounds but is rich in waxes. About 90% crude produced at present falls in the last category. 5 Petroleum Drilling: Oil is brought to the surface by drilling holes up to the oil bearing surface. By the Hydrostatic pressure of natural gas the oils are brought up by means of pump. Two coaxial pipes are lowered to the oil reservoir, through the outer pipe compressed air is forced, whereby the oil is forced out through the inner pipe. This crude oil is sent to the refineries for further processing and refining of the crude oils. 6 Figure 4. Mining of Crude oil and use of air-lift pump 7 8 9 10 11 12 13 14 15 16 References Brown L, Holme T. (2011). Chemistry for Engineering Students. Mary Finch. https://ionlights.keybase.pub/books/Chemistry%20for%20Engineering%20Students%2C%202e.pdf Jain J. (2015). Engineering Chemistry. Dhanpat Rai. https://pdfcoffee.com/engineering-chemistry-by- jain-amp-jain-pdf-free.html Images: http://oilandgasprocessing.blogspot.com/2014/03/oil-and-gas-formation.html https://socratic.org/questions/how-does-the-formation-of-coal-differ-from-that-of-natural-gas-and-oil https://jpt.spe.org/legends-artificial-lift-part-3-history 17