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Self Introduction. • Name: Muhammad ShozabMehdi • Qualifications: • BSc. Chemical Engineering (2003), NFC Institute of Engineering and Technology (IET), Multan, Pakistan. • MS leading to PhD in Chemical Engineering (2013), Pakistan Institute of Engineering and Technology, Islamabad, Pakistan. • Research in Chemical Engineering Laboratory (LGC), Toulouse, France. • Research Interest: Hydrodynamics and Mass Transfer in Multiphase flows. • Office: FMSE-xxx, Ext: xxxx, Email: xxxxx@giki.edu.pk • Teaching Assistant: Mr. Jahanzaib Ahmad Ansari, Ext: xxxx, Email: xxxxxxxxxxxx@giki.edu.pk
CH 212: Fuel and Combustion Spring – 2014
Introduction Fuel: Anything which burn to give heat in presence of oxygen. Combustion: The process of burning fuel. Burning is the intense chemical reaction with the emission of heat and other exhaust gases when fuel reacts with oxygen.
Introduction (contd.) • Where does the energy comes from the combustion reactions? • The energy stored in the chemical bonds that hold the carbon and hydrogen atoms together, releases when bonds are broken and atoms are rearranged. • How does this energy stored in fuel? • This energy stores in fuel by the process of photosynthesis which converts sunlight into chemical energy and storing it in the bond of sugar. Plants needs Carbon dioxide, Water and sunlight to make sugar. The overall reaction is: 6CO2 + 6H2O + Sunlight -----> C6H12O6 + 6O2 • These plants when became dead, buried under the soil, as more and more soil deposited over them for thousands of years, they became compressed and then under high temperature and pressure converted into fossil fuel.
Importance • Almost 70% of energy used in the world came from combustion sources.
Importance (contd.) • Heat for homes comes directly from combustion. • Electricity for homes generated by burning fossil fuel. • Our transportation system relies almost entirely on combustion. • Air craft are entirely powered by fuel burning. • Most trains are powered by diesel engine. • Most of the domestic vehicles use gasoline. • Industrial process rely heavily on combustion. • Iron, steel, aluminum and other metal refining industries employ furnaces for producing raw products. • Cement industry is the heavy user of heat energy delivered by combustion. • Other example of industrial combustion devices are boiler, refinery, glass melters and solid dryers etc.
Course Description • Various conventional and non-conventional fuels; their characterization and processing including refining, coal gasification and natural gas treatment. • Various aspect of combustion and related balances. • Fuel economy, burners classification and design. • Flame analysis and its various regimes and temperatures; Interface energy balances and heat distribution; oxygen diffusion and flame front; flammability limits and flame quenching. • Furnaces, boilers and engines (I.C & G.T). • Emission control (SOx & NOx).
Books (Text & Reference) • Flame and Combustion by J. F. Griffiths and J. A. Barnard, 3rd Ed. 1995 • An Introduction to Combustion; Concept and Applications by Stephen R. Turns, 2nd Ed. 2000 • Elements of Fuel, Furnaces and Refractoriesby O. P. Gupta 4th Ed. 1997
Before Mid • Various aspect of combustion and related balances.
After Mid • Various aspect of combustion and related balances. • Fuel economy, burners classification and design. • Flame analysis and its various regimes and temperatures; Interface energy balances and heat distribution; oxygen diffusion and flame front; flammability limits and flame quenching. • Furnaces, boilers and engines (I.C & G.T). • Emission control (SOx & NOx).
Marks Distribution • Assignments (Nos?): ?% • Quizzes (Nos?): ?% • Midterm exam: ?% • Final Exam: ?% • Attendance: ?%
General Classification of Fuels • Fossil fuels:Those which have been derived from fossil remains of plants and animal life and are found in crust of earth. For example coal, petroleum and natural gas etc. • By-product fuels:Those which are co-product of regular manufacturing process and of secondary in nature. For example coke oven gas from manufacturing of coke and blast furnace gas from making of iron. • Chemical fuels:Those which are of an toxic in nature and normally not used in conventional processes. For examples hydrazine, ammonium nitrate etc. • Nuclear fuels:Those which release heat by fission (uranium, plutonium etc.) or by fusion (deuterium, tritium etc.)
Classification based on Occurrence • Solid Fuels:Wood, charcoal, coal, coke etc. • Liquid Fuels:petroleum and its products (gasoline, kerosene, diesel, furnace oil, lubricating oil etc), Liquid fuel from coal liquefaction etc. • Gaseous Fuels:Natural gas, refinery gas, gas from coal gasification, wood gas, bio gas etc.
Classification based on Nature of Fuel • Primary Fuels:Those which occurred in nature i.e. wood, coal, natural gas and petroleum. • Secondary Fuels:Those which are derived from primary fuels e.g. fuel oil and kerosene derived from petroleum, coke oven gas derived from coal etc. Secondary fuels are further classified into: • Manufactured:Those which are manufactured for some specific purpose e.g. coke made for iron making, gasoline made for internal combustion engine, producer gas made for industrial heating etc. • By-Product:Those which are co-product of regular manufacturing process e.g. tar, refinery gas etc.
Rank of Coal: It denote the maturity of coal. So peat the most immature coal has a lowest rank while the anthracite the most mature coal has highest rank. • Metamorphism of coal: The process of conversion of lignite to anthracite is called metamorphism of coal or coalification. • Carbonization of coal: Heating of coal in absence of air at high temperature to produce coke, tar and gases is called carbonization of coal. • Gasification of coal: Heating of coal in insufficiently less amount of air plus steam to produce a gas rich in CO and H2 is called gasification of coal. • Proximate analysis of coal: Finding out the weight percent of moisture, volatile matter, fixed carbon and ash content in coal. The analysis is useful in deciding the utilization for a particular purpose. • Ultimate analysis: Finding out the weight percent of carbon, hydrogen, nitrogen, oxygen and sulphur of pure coal free from moisture and inorganic constituents. The analysis is useful in designing of coal burning equipments.
Calorific value: The quantity of heat liberated by combustion of unit quantity of fuel is called its calorific value. • Gross calorific value: Where the heat obtained from condensation of water vapours in the flue gases is also include. • Net calorific value: Where the heat obtained from condensation of water vapours in the flue gases is not include. • Flue gas: The gaseous product of combustion of a fuel. • Heat capacity: Amount of heat required to raise the unit weight of substance by one degree. • Specific heat: It is the ratio of heat capacity of a substance to the heat capacity of water (Cp > Cv). • Ignition temperature: It is the minimum temperature at which the fuel ignites. • Flash point: It is the minimum temperature at which the fuel give enough vapours which produces a momentary flash when exposes to flame. • Pour point: It is the minimum temperature at which fuel keeps its flowing nature when cooled under specific conditions.
Wood Domestic fuel used in tropical countries where forest are abundant and other fuels are not easily and cheaply available.
Main combustible components: cellulose and lignin (compounds of carbon, hydrogen and oxygen) • Minor combustible components: resin and waxes. • Non combustible components: Water (25-50% in freshly cut and 10-15% in air dried) • Ash content: very low (<1%) but because oxygen content is very high (upto 45%) therefore its calorific value is very low. • Calorific value: 4000-5000 kcal/kg. • Density: 650 kg/m3.
Composition of Air-dried Wood Proximate Analysis Ultimate Analysis Carbon: 50% Hydrogen: 6% Oxygen: 44% • Cellulose: 50% • Lignin: 30% • Moisture: 15% • Water soluble: 2.5% • Resin & Wax: 2% • Ash: 0.5%
Properties of wood • It is clean, readily ignites, burn with long clean flame in excess air and leaves only small amount of ash. • Wood is a solid fuel as dense as coal but to obtain same amount of heat as any given weight of coal can produce, you have to burnt wood, four times the weight of coal. • It is used to produce charcoal.
Charcoal Carbonization of wood at 600oC
Stages Involved in Carbonization of Wood Stage 1: At 100-120oC ---> moisture expelled results in moisture free wood. Stage 2: At 275oC ---> initial decomposition takes place results in formation of little distillate gas containing acetic acid and water. Stage 3: At 350oC ---> active distillation of wood takes place results in emission of liquid (acetic acid, methyl alcohol, tar etc) and gases (CO, CO2, N2, H2, CnHn). Stage 4: At 350-600oC ---> slow evolution of residual volatile matters from the wood charcoal left in 3rd stage.
Products of Carbonization of Wood • Charcoal: Solid product left after the carbonization of coal. • Hot gases: Cooled to separate • Wood gas • Liquid in two layer • Upper layer is Pyrolignious acid (mixture of acetic acid, acetone and water). • Lower layer is wood tar (fractionated to separate many chemicals).
Uses and Composition of Charcoal • Used for removal of obnoxious and coloring material from solution, gases, vapors and petroleum products by adsorption on its surface. • Used as a feed stock for gasification to make producer gas which is used as a fuel for domestic and industrial use. • Used as raw material for production of carbon sulfide. • Charcoal contains 80% carbon, 15% oxygen and nitrogen, 2% hydrogen and 3% ash.
Merits and Demerits • Because of porous in nature it has high specific surface area as compared to coal. • Ash content is very low. • Calorific value is high as compared to wood. • Mechanical strength is very poor.
Peat First stage in formation of coal from wood (cellulose)The most immatured coal
Formation of Peat • Peat is the first stage in the formation of coal from wood (cellulose). • It is not strictly a coal or else can be termed as the most immatured coal. • It is formed by gradual decaying (action of bacteria under high pressure and temperature) of remains of plants in moist places.
Properties of Peat • Peat is light brown in color. • Highly fibrous in nature. • With increase in depth the color becomes darker and fibrous structure disappeared. • Composition of peat varies from nature of plants, depth in deposit and age. • Freshly mined peat contains about 90% water and 10% solid. Therefore cannot be used unless air dried. • Calorific value is 650 kcal/kg for freshly mined peat and 5000 kcal/kg for air dried peat.
Composition of Peat Proximate Analysis Ultimate Analysis Carbon: 55% Hydrogen: 6% Oxygen: 33% Nitrogen: 3% Sulfur: 1% • Moisture: 20% • Volatile matter: 50% • Fixed carbon: 25% • Ash: 5%
Gasification of Peat • Peat is gasified in presence of steam and air to produce producer gas. • Steam requirement is very low because peat itself contain sufficient moisture. • Typical composition of gas is: • CO2 – 13% • CO – 18% • H2 – 11% • CH4 – 2.5% • N2 – 55.5% • Caloric value is 1000 kcal/kg • Gas yield is 2500 Nm3/ton of peat
Origin of coal • Coal is a complex mixture of plant substances altered in varying degree by physical and chemical processes. • These processes which changed plant substances into coal has taken million of years and has been accomplished by bacteria, heat and pressure inside the earth’s crust. • Two theories namely “in-situ” theory and “drift theory” have been suggested by geologists regarding mechanism of formation of coal from plant substances. • In situ theory: According to this theory, coal seam occupies the same site where the original plants grew and where their remains accumulated several million years ago to produce coal under the action of heat, pressure and bacteria. • Drift theory: According to this theory plants, tree etc were uprooted and drifted by rivers to lakes and deposited there to form coal during the course of time after they got buried underground. • Stages information of coal: plants/trees peat lignite sub-bituminous coal bituminous coal anthracite coal graphite.
Points in favor of “in-situ” theory: • In the existing peat deposits; the decayed plant substances had accumulated at the place of origin. • Large quantity of fossil fuel have been found under the coal seam. • Composition of coal seam is generally constant over a wide area. If the original decaying plants and tree had been drifted from their original place then they would have a great variation in composition of coal. • Points in favor of “drift” theory: • In coal seam, the percentage of inclined trunks of fossil is much more than the vertical position. It the coal had been formed at the same place where the plants substances decayed then fossil trunks would have been vertical. • To form one meter thick coal seam, ten meter thick seam of peat is required, So for the formation of ten meter thick coal seam should have resulted from hundred meter thick seam of peat, which occur no where. • Seams of coal are made up of different layers separated by layers of clay or sandstone which vary in thickness from merely a film to several meters. As the plants accumulated under water some slits also settled along with them. Thus at later stage some mineral mater got mixed with coal as it formed.
Stages for transformation of wood to coal: • The initial transformation of wood to coal is due to the action of bacteria (aerobic or anaerobic) causing degradation of organic matters (e.g. cellulose, lignin etc. present in wood) and removal of oxygen. • The bacterial action produced acidity which if accumulated prevented further action and coal formation could not proceed. • In some cases the soil was alkaline which neutralized the acidity and bacterial action continued to take place further degradation of organic matter. This explain why coal is found along with certain types of rocks. • Bacterial action takes place in initial stage breaks up organic matters into simpler molecules. • The degraded organic matters, as time passed, gave rise to peat. • Beyond this stage heat, pressure and time became the chief factors for further conversion of peat into different rank of coal.
Lignite • It is the second stage product in the formation of coal from wood. • It is friable and occurs in thick seams (up to 30 meter thick) near the surface of earth. • It moisture content is up to 60 % and calorific value is around 5000 kcal/kg. • On exposes to air the brown color darkens and moisture content reduces to equilibrium value of 10 – 20 %. • On drying lignite shrinks and break up in irregular manner. • It is likely to ignite spontaneously as it adsorbs oxygen readily and must not be stored in open without care. • Composition and property of lignite varies widely. The carbon content is 70 – 75 % and oxygen content is 20 – 25 %. • In large number of cases the ratio of volatile matter to fixed carbon is 1:1. • Raw Lignite is inferior fuel due to high moisture content, low calorific value, small size and bad weathering properties. • Lignite is of economic importance where it is readily available and other fuels do not occur in abundance.
Composition of Lignite Proximate Analysis Ultimate Analysis Carbon: 70 – 73 % Hydrogen: 4.6 – 5.5 % Oxygen: 22 – 26 % Nitrogen: 0.6 – 1.0 % Sulfur: 0.6 – 1.5 % • Moisture: 10 – 30 % • Volatile matter: 40 – 45 % • Fixed carbon: 30 – 35 % • Ash: 3.5 – 7.5 %
Bituminous Coal • It is the most common variety of coal. • It is black and brittle which burns and ignites readily with yellow smoky flame. • It has low moisture content i.e. less than 10 % and carbon content varies from 75 – 90 % where as volatile matter content is 20 – 45 %. • Depending upon the volatile matter content, it is termed as low volatile, medium volatile and high volatile coal. • Its calorific value based on dry mineral free basis goes up to 9000 kcal/kg. • It is used for power generation, coke making, gasification and domestic use.
Composition of Bituminous coal Proximate Analysis Ultimate Analysis Carbon: 68.5 – 79.5 % Hydrogen: 4.5 – 5.5 % Oxygen: 4.5 – 16.5 % Nitrogen: 1 – 1.4 % Sulfur: 0.5 – 1 % • Moisture: 3.5 – 8 % • Volatile matter: 16 – 36 % • Fixed carbon: 49 – 72 % • Ash: 7 – 8.5 %
Anthracite coal • It is most matured coal hence of highest rank. • It is hard and burns without smoke with a short non-luminous flame. • It has high carbon content i.e. 85 – 95 % and low volatile matter content i.e. less than 10 %. • It ignite with difficulty due to low volatile matter content. • Its calorific value may be up to 8000 to 8500 kcal/kg which is less than bituminous coal de to its lower hydrogen and volatile matter content. • It has sub-metallic lustre and sometime even a graphitic appearance. • The chief use of anthracites are in boilers and metallurgical furnaces. • On calcining it gives thermo-anthracite which is a raw material for the production of carbon electrode.
Composition of Anthracite coal Proximate Analysis Ultimate Analysis Carbon: 80 – 86.5 % Hydrogen: 2.5 – 3.5 % Oxygen: 3 – 4.5 % Nitrogen: 0.5 – 1.5 % Sulfur: 0.5 % • Moisture: 2.5 – 3 % • Volatile matter: 3 – 8.5 % • Fixed carbon: 79 – 87.5 % • Ash: 7 – 9.5 %
Significance of constituents of coal Proximate analysis Ultimate analysis Carbon Hydrogen Nitrogen Sulphur Oxygen • Moisture • Volatile matter • Fixed carbon • Ash
Moisture • In general high moisture content in coal is undesirable because: • it reduces the caloric value of fuel • it increases the consumption of coal for heating purpose • it lengthens the time of heating • it increases the cost for purchase and transportation. • Owing to its nature, origin and occurrence, coal is always associated with moisture. • When coal is exposed to atmosphere its external moisture (free moisture) evaporates but apparently dry coal still contain some moisture (inherent moisture). • Air-dried moisture content of coal is determined by observing loss in weight of coal sample on heating at 105 degree centigrade. • Air-dried moisture of coal decreases with increasing rank from a value of 25% for lignite to a minimum value of 0.5% for low volatile bituminous coal.
Volatile matter • Certain gases like CO, CO2, CH4, H2, N2, O2, CXHY etc, are present in coal which comes out during its heating in absence of air. • These gases are called volatile matter of coal. • Coal with high volatile matter content: • Ignites easily i.e. it has low ignition temperature • Burns with long smoky yellow flame • Give more quantity of coke oven gas when it is heated in absence of air. • Volatile matter does not include moisture of coal but it contain moisture that is formed from oxygen and hydrogen of coal during the decomposition. • Volatile matter expressed as per cent on dry mineral matter free basis. • Higher the volatile matter, lower the fixed carbon.
Ash content • Ash is the combustion product of mineral matters presents in the coal. • It is comprises mainly of silica, alumina and ferric oxide with varying amount of other oxide such as calcium and magnesium. • High ash content in coal is undesirable in general. • A coal with high ash content is harder and stronger and has lower calorific value. • Ash content of coal is reduced by its washing. • Coal contains inorganic mineral substance which are converted into ash by chemical reaction during combustion. Ash and coal are therefore not the same. • The bulk of mineral matter of coal is due to clay or shale consisting of alumino-silicates of different composition. Other major constituents may be calcite and pyrites. • When coal burns these mineral matter decomposed resulting loss in weight hence the ash of coal is always less than the mineral matter content.
Fixed carbon • It is the pure carbon present in coal. • Higher the fixed carbon, higher will be the calorific value. • In anthracite where the value of volatile matter is very small, fixed carbon and total carbon are almost same. • In other coals, fixed carbon is always less then total carbon.
