Unit 2 - Matter and Energy PDF

UNIT 2 - MATTER AND ENERGY FUELS 1.0​ Introduction to Fuels and Combustion Since the beginning of recorded history, fire has been central to our society as a source of heat, light, and security. Our modern fuels, the substances we burn or combust, are available in many diff...

UNIT 2 - MATTER AND ENERGY FUELS 1.0​ Introduction to Fuels and Combustion Since the beginning of recorded history, fire has been central to our society as a source of heat, light, and security. Our modern fuels, the substances we burn or combust, are available in many different forms. We use coal in power plants to generate electricity. We use gasoline to run our cars. We use natural gas or heating oil to warm our homes. We use propane, charcoal, or wood to cook our food at a summer barbecue. We might even use wax to provide light for a romantic candlelit dinner. In each of these cases, using fuels means burning them. This process of combustion causes a difference in energy between the reactants and products of combustion, and that energy is released as light and heat—a flame! A fuel is any solid, liquid, or gaseous substance that may be combusted (burned) to produce heat or work. Sources of fuel date back to prehistoric times, where solids such as grass and straw were burned for heat. The use of coal as a fuel actually dates back to ancient civilizations, where it was used to isolate copper from ore in northeastern China as early as 1000 BC. However, the Industrial Revolution in the late 18th century sparked the large-scale use of coal for steam engines and steelmaking. The development of drilling technology for oil wells in the mid-19th century in the U.S. gave rise to the petroleum industry and mass consumption of petroleum products for transportation, electricity, heating, and even plastics fabrication. Currently, the world’s energy needs are provided by burning fossil fuels, coal, and/or oil. A fuel is considered valuable if it ignites easily at a low temperature and produces a large quantity of heat during its combustion. In addition, fuels should be inexpensively isolated and have properties that allow for their safe and efficient storage or transport. Lastly, a desirable fuel should leave little residue behind after being burned, and produce by- products that are not harmful to human health or the environment. Unfortunately, no fuel satisfies all of these conditions. Humans currently use coal, petroleum products (e.g., gasoline, diesel, propane, etc.), and natural gas as our primary sources of fuel. Contrary to popular belief, these so-called fossil fuels are not the prehistoric remains of dinosaurs. In fact, most of the fossil fuels we use today were formed from decaying plant life that flourished millions of years before the first dinosaurs appeared. Considering how long it takes to convert plant life to fossil fuels, the rate at which we are burning coal, petroleum, and natural gas is not sustainable, at least in terms of having enough of it available to meet current and future energy needs. Burning fossil fuels for energy fails to meet the criteria of sustainability in two ways. First, the fuels themselves are nonrenewable. Once gone, they cannot be replaced—at least within a useful timescale. Second, the waste products of combustion have adverse effects on our environment, both today and in the future. There are three necessary requirements to generate a fire—a source of heat, a fuel, and an oxidizer. When these components are combined, a chemical reaction takes place that releases a variety of by-products and a significant amount of heat. Once a fire is generated, the heat or ignition source is no longer needed. The fire will continue to burn until either the oxygen or fuel source is removed. For instance, fire blankets are used to extinguish a fire by preventing available oxygen from reacting with the fuel source. Regardless of the specific source of fuel or oxidizing agent, the general chemical reaction is the same. ∆ Fuel + Oxidizer → Products The identity of the products will differ, depending on the fuel and oxidizer used for combustion. However, the chemical makeup of these products is rarely straightforward. The great majority of fuels are hydrocarbons, compounds made up only of the elements hydrogen and carbon. 2.0​ Characteristics and Classifications of a Good Fuel A good fuel should satisfy the following requirements: ​ It should have a high calorific value i.e., it should evolve a large amount of heat when it is burnt. ​ Its moisture content should be low so that its heating value should be high. ​ An ideal fuel should have moderate ignition temperature. ​ It should not produce harmful products like CO2, SO2, H2S and other poisonous gases on burning since they pollute the atmosphere. ​ A fuel should have low content of non-combustible matter in the form of ash or clinker. Since the presence of on-combustible matter will enhance the cost of storage, handling and disposal of waste. ​ The combustion of fuel should be controllable so that it can be started or stopped. ​ It should not give any offensive odor ​ It should have moderate velocity of combustion. Fuels may broadly be classified in two ways, i.e ​ according to the physical state in which they exist in nature – solid, liquid and gaseous, and ​ according to the mode of their procurement – natural and manufactured. None of these classifications, however, gives an idea of the qualitative or intensive value of the fuels, i.e. their power of developing the thermal intensity or calorimetric temperature under the normal condition of use, i.e. combustion of fuels in mixture with atmospheric air in stoichiometric proportion. We shall now proceed with the further description of the fuels. A brief description of natural and manufactured fuels is given below: Natural and Manufactured Fuels Natural Fuel Manufactured Fuel Solid Fuels Wood Tanbark, Bagasse, Straw Coal Charcoal Oil Shale Coke Briquettes Liquid Fuels Petroleum Oils from distillation of petroleum Coal tar Shale-oil Alcohols, etc.  Gaseous Fuels Natural Gas Coal gas Producer gas Water gas Hydrogen Acetylene Blast furnace gas Oil gas Solid fuels are mainly classified into two categories, i.e. natural fuels, such as wood, coal, etc. and manufactured fuels, such as charcoal, coke, briquettes. The various advantages and disadvantages of solid fuels are given below: Advantages Disadvantages They are easy to transport. Their ash content is high. They are convenient to store without any risk Their large proportion of heat is wasted. of spontaneous explosion. Their cost of production is low. They burn with clinker formation They​ possess​ moderate​ ignition Their combustion operation cannot be temperature. controlled easily. Their cost of handling is high. The liquid fuels can be classified as Natural or crude oil, and Artificial or manufactured oils. Advantages Disadvantages They possess higher calorific value per unit The cost of liquid fuel is relatively much mass than solid fuels. higher as compared to solid fuel. They burn without dust, ash, clinkers, Costly special storage tanks are required etc. for storing liquid fuels. Their firing is easier and also fire can be There is a greater risk of five hazards, extinguished easily by stopping liquid fuel particularly, in case of highly inflammable supply. and volatile liquid fuels. They are easy to transport through pipes. They give bad odor. They can be stored indefinitely without any For efficient burning of liquid fuels, loss. specially constructed burners and spraying apparatus are required. They are clean in use and economic to handle. Loss of heat in chimney is very low due to greater cleanliness. They require less excess air for complete combustion.  They require less furnace space for combustion. Petroleum is a basic natural fuel. It is a dark greenish brown, viscous mineral oil, found deep in earth’s crust. It is mainly composed of various hydrocarbons (like straight chain paraffins, cycloparaffins or napthenes, olefins, and aromatics) together with small amount of organic compounds containing oxygen nitrogen and sulfur. The average composition of crude petroleum is : C = 79.5 to 87.1%; H = 11.5 to 14.8%; S = 0.1 to 3.5%, N and O = 0.1 to 0.5%. Petroleums are graded according to the following physicochemical properties : a)​ Specific gravity b)​ Calorific value c)​ Fish point or ignition point d)​ Viscosity e)​ Sulphur contents f)​ Moisture and sediment content, and g)​ Specific heat and coefficient of expansion Gaseous fuels occur in nature, besides being manufactured from solid and liquid fuels. The advantages and disadvantages of gaseous fuels are given below : Advantages Disadvantages They can be conveyed easily through The cost of liquid fuel is relatively much pipelines to the actual place of need, thereby higher as compared to solid fuel. eliminating manual labor in transportation. They can be lighted at ease. Costly special storage tanks are required for storing liquid fuels. They have high heat contents and hence help There is a greater risk of five hazards, us in having higher temperatures. particularly,​ in​ case​ of​ highly inflammable and volatile liquid fuels. They can be pre-heated by the heat of hot They give bad odor. waste gases, thereby affecting economy in heat. Their​ combustion​ can​ readily​ by For efficient burning of liquid fuels, controlled for change in demand like specially constructed burners and spraying oxidizing or reducing atmosphere, length apparatus are required. flame, temperature, etc. They are clean in use. Very large storage tanks are needed. They do not require any special burner. They are highly inflammable, so chances of fire hazards in their use is high. They burn without any shoot, or smoke and ashes.  They are free from impurities found in solid and liquid fuels. Natural gas is generally associated with petroleum deposits and is obtained from wells dug in the oil-bearing regions. The approximate composition of natural gas is : CH4 = 70.9%, C2H6 = 5.10%, H2 = 3%, CO + CO= = 22% The calorific value varies from 12,000 to 14,000 kcal/m3. It is an excellent domestic fuel and is conveyed in pipelines over very large distances. In America, it is available to a great extent, and so, is quite popular as a domestic fuel. It is now used in manufacture of chemicals by synthetic process. It is a colorless gas and is non-poisonous. Its specific gravity is usually between 0.57 to 0.7. 3.0​ Calorific Values Energy content or calorific value is the same as the heat of combustion, and can be calculated from thermodynamical values, or measured in a suitable apparatus. Calorific value refers to the amount of heat produced by unit volume of a substance by complete combustion. The efficiency of fuel mainly depends on the calorific value. If the value is high, its efficiency will also be high. If the value is low, its efficiency would also decrease. Calorific value is directly proportional to its efficiency. It is very important to have a knowledge of the calorific value of fuel to carry out our day-to-day activities. This knowledge helps us to determine the amount of energy we transport. The gas shippers and suppliers require dis information to bill gas consumers. It also helps to determine transportation charges of gas shippers and suppliers. The calorific value is measured either as gross calorific value or net calorific value. The difference being the latent heat of condensation of the water vapor produced during the combustion process. Gross calorific value (GCV) assumes all vapor produced during the combustion process is fully condensed. Net calorific value (NCV) assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value. The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal while calorific value of fuel oils are much more consistent.