Understanding Fuel, Combustion, and Calorific Values

2. Introduction A fuel is defined as any substance used to produce heat or power by combustion. Any chemical process accompanied by the evolution of light and heat is called combustion. It is simply the reaction of substances with oxygen and converts chemical energy into heat and light. Plants via photosynthesis Solar energy Chemical energy (carbohydrate)

4. Characteristics of a Good Fuel It should ignite easily. The temperature of the fuel at which ignition starts and continues to burn without further addition of heat is called ignition temperature. It should be moderate for a good fuel. Very low ignition temperature leads to fire hazard and very high ignition temperature disfavors the starting of fire. It should give out a lot of heat, that is, its specific heat should be high. It should have low smoke and combustible matter such as ash. It should not give out harmful combustion products. This property depends on the nature of elements present in the fuel. It should be inexpensive and readily available. It should be easy to store and transport. It should have low ash content. Ash reduces the calorific value of the fuel, causes hindrance to the flow of air and heat, reduces the specific heat and leads to unwanted disposable problems.

5. Calorific Value Units British Thermal Unit (BTU) It is 1/180 of the amount of the heat necessary to raise 1 lb of water from 32oF to 212oF. 2. Calorie (cal) It is 1/100 of the amount of heat necessary to raise 1 g of water from 0oC to 100oC. 1 BTU = 252 cal and 1 cal/g = 1.8 BTU/lb Gross and Net Calorific Values 1. Higher calorific value (HCV) or gross calorific value 2. Lower calorific value (LCV) or net calorific value

6. Determination of Calorific Value • Theoretically Determination • Dulong’s formula for calculating the calorific value from the chemical composition of the fuel may be written as follows:

7. Experimentally Determination • Bomb calorimeter • For calorific values of solid and liquid fuels • Known amount of fuel is burnt at constant volume • Temperature of surrounding water increases as heat is produced. • Quantity of heat and calorific values are calculated.

8. water equivalent (w) “The water equivalent of a substance is equal to the amount of water needed to absorb the same amount of heat as that substance does for one degree of increase in temperature.” In other words, For any given substance the equivalent mass of water W having the same specific heat as the given substance is called its Water Equivalent. Therefore, water equivalent of a body equals the product of its mass and its specific heat of apparatus. Water equivalent = mass x specific heat of apparatus

9. Boy’s calorimeter • Gas or volatile liquid burns at constant rate. • Water flowing at constant rate absorbs the heat produced. • Calorific value is calculated from volume of water, increase in temperature and volume of gas/liquid burnt.

10. Junker’s calorimeter Control of rate of burning of gaseous/liquid fuel and water circulation is maintained. The combustible products are released at nearly the atmospheric pressure. Calorific value is calculated from amount of water passed, volume of gas burnt, the steady rise in temperature and mass of the condensed water flowing out.

11. Combustion Combustion is a chemical process accompanied by evolution of light and heat.

12. Calculations Calculation of theoretical air for combustion of a fuel requires the following points: Percentage of oxygen in air by volume is 21% and 23.2% by weight. Stoichiometric equations involved in combustion

13. Flue Gas Analysis • It comprises the gaseous products of combustion of fuel. Its analysis helps in finding out the correct quantity of air to be supplied in a furnace. • Orsat’s apparatus

14. Solid Fuels – Coal Formed from dead plants buried for several million years.

15. Uses of Coal As a primary fuel:Coal is used to produce steam through heat and combustion, which is again used for running turbines to generate electricity in power plants. As a secondary fuel:The product of burning coal in the absence of air is of metallurgical importance. The byproducts are useful in making plastics, tar and synthetic fibers and also used making steel in industries. Pulverized Coal Pulverized coal generally refers to coal in powdered form obtained by crushing, grinding or pulverizing coal. As the surface area of pulverized coal is large, the volatile matter present in it comes quickly in contact with air and is released, enabling the combustion of fixed carbon. This increases the calorific value of the coal and enhances its quality.

16. The advantages of pulverized coal are as follows: It provides easy transportation and storage. Even high grade coal having high ash content, such as found in India, could be used satisfactorily. It can easily undergo combustion in small percentage of excess air with less wastage of heat, and has high thermal efficiency. It does not form clinkers (incombustible residue) and provides an oxidizing and reducing atmosphere inside the furnace for metallurgical purposes. The disadvantages of pulverized coal are as follows : It involves the extra cost of pulverizing and sieving. Special types of burners are required for good mixing of air and fuel. During the process of grinding, crushing and pulverizing, finely divided ash is generated in form of fly-ash causing air pollution and causing problems associated with its disposal.

17. Proximate and Ultimate Analyses of Coal It includes the determination of moisture content, volatile matter, ash and fixed carbon. In this analysis, the data varies with the procedure adopted for study. The content of moisture, volatile matter and ash are experimentally determined, while that of fixed carbon is calculated. Proximate Analysis: It provides the information about practical utility of the coal Moisture content: Lesser the moisture content, better is the quality of coal.

18. 2. Volatile matter: Lesser the volatile matter, better is the rank of coal. For this heat the dried coal with lid in muffle furnace for 7 min. at around 900 oC. 3. Ash content: Lower the ash content, better is the quality of coal. For this heat the dried coal (free from volatile matter ) without lid in muffle furnace for 0.5 h at around 750 oC. 4. Fixed carbon: Higher fixed carbon content, better is the quality of coal.

19. Ultimate Analysis Analysis of coal for C, H, N, S, O & ash Carbon and hydrogen: Greater the percentage of carbon and hydrogen, better is the quality of coal. Nitrogen:Presence of nitrogen is undesirable. N2 + H2SO4 Ammonium Sulphate  treat with NaOH  NH3 gas  Collect in Known volume of standard acid  Back titrate with NaOH using Methyl orange indicator Note: Volume of acid consumed by NH3 is in liters.

20. Analysis of C & H e.g., CaCl2 e.g., KOH

21. Analysis of N Step 2: Treatment with Base Step 1: Digestion (at around 400 oC) N- Sample + Conc.H2SO4 + K2SO4 + CuSO4 Reactions: Degradation: Sample + H2SO4→ (NH4)2SO4(aq) + CO2(g) + SO2(g) + H2O(g) Liberation of ammonia: (NH4)2SO4(aq) + 2NaOH → Na2SO4(aq) + 2H2O(l) + 2NH3(g) Capture of ammonia: B(OH)3+ H2O + NH3→ NH4++ B(OH)4– Back-titration: B(OH)3+ H2O + Na2CO3→ NaHCO3(aq) + NaB(OH)4(aq) + CO2(g) + H2O

22. Sulphur:Presence of sulphur is highly undesirable. Ash:Determined the same way as in proximate analysis. Oxygen:Lower the percentage of oxygen, better is the coal. S Sulphate  treat with BaCL2  BaSO4

23. Significance of Ultimate Analysis Greater the percentage of carbon and hydrogen, better is the coal in quality and calorific value. However, hydrogen is mostly associated with the volatile matter and hence, it affects the use to which the coal is put to. Nitrogen has no calorific value and hence its presence in coal is undesirable. Thus, a good quality coal has very little nitrogen content. Sulphur is usually present to the extent of 0.5–3.0% and is derived from ores like iron pyrite and gypsum, etc., mines along with the coal. Sulphur, although contributes to the heating value of coal, on combustion it produces acids which have harmful effects of corroding the equipment and also causes atmospheric pollution. Oxygen content decreases the calorific value of coal. High oxygen content coals are characterized by high inherent moisture content, low calorific value and low coking power. Moreover, oxygen is in combined form with hydrogen in coal and thus, hydrogen available for combustion is lesser than actually present.

24. Coke Coke is obtained when coal is heated strongly out of contact with the air, the process is called carbonization or coking. Low-temperature carbonization: 500 °C-700 °C; low temperature coke. High-temperature carbonization: 900 °C-1100 °C; metallurgical coke. Comparison of high and low temperature carbonization processes

25. Percentage analysis of a coke and the coal Coke as Metallurgical Fuel Metallurgical coke is preferred over coal due to the following reasons: It is hard, porous and of good mechanical strength. It does not contain much sulphur content. It burns with a short flame.

26. Caking and Coking Coals Coals that produce soft and plastic mass at around 400°C that re-solidifies to form a porous solid are called caking coals. Caking coals that produce solid product (coke) of useful grade are called coking coals. A good coking coal will produce a bright gray, strongly coherent, porous coke. All coking coals are caking coals but not vice versa. High volatile coals are mixed with poor or non-coking low-volatile coals, to yield a denser, stronger coke. Where mixing is practiced, the mixture used depends on the coal used. Usually high- and low volatile coals are mixed to give a mixture having about 30% of volatile matter.

27. Coking Processes Bee-Hive Oven

28. Otto-Hoffmann or Byproduct Oven

29. There are two methods of cooling the coke: dry and wet quenching: Dry quenching: The insulated wall of the oven is removed, so that the inert gases like nitrogen present in the atmosphere are able to come in contact with the coke and cool it naturally over a period of time. Wet quenching: Water is sprayed over the red hot coke for cooling it down. As a result of this, some gases escape from the hot surface of the coke in form of steam and cause pollution. This method also generates a large amount of coke dust. Dry quenching is preferred over wet quenching due to the following reasons: The method is dust free and dry without causing pollution to the environment. The heated gases could be used to recover heat by circulating them through the boilers for steam generation. The coke obtained is strong, dense and free from moisture.

31. Advantages of Byproduct oven. The byproduct coke oven, the valuable byproducts are saved; whereas in the beehive oven, they are allowed to escape into the atmosphere. The byproducts are well worth saving and have many important uses. Their recovery helps to make the process economical. In the purification of the byproduct coke-oven gas by means of a liquid-contact process, pure sulphur can be obtained in a form much finer than the commercial precipitated sulphur. This finely divided sulphur has been found to process superior qualities as a fungicide. Among the other interesting and valuable byproducts recovered by the liquid-purification processes are the thiocyanates of ammonium, sodium, and calcium.

32. Biofuels Biofuels are solid, liquid or gaseous fuels that are derived from living organisms and their waste matter.

33. Biomass Waste material produced by living things There are methods of utilization of biomass in the form of energy. Burn the biomass directly and obtain energy. Convert the matter into ethanol and methanol or it is fermented anaerobically to obtain gaseous fuel, biogas. Biodiesel Produced from transesterification of vegetable oils that contain triglycerides. Biogas The advantages of using gobar gas over heating cattle dung directly in dried state are as follows: The gobar gas produces more useful energy. It provides high device efficiency. It is free from dust and smoke, and environment-friendly. It is used as a domestic fuel as well as an illuminant.

34. Anaerobic digestion tank

35. Semi-Solid Fuels – Some Recent Advances • e.g., Paraffin wax (in candles) • The term semi-solid fuel in modern context refers to non-volatile substances that are environmentally safe and produce no hazardous waste on burning. • These have long shelf-life and their ignition can be easily started and stopped. • The conventional solid and liquid fuels are now being used as formulations in semi-solid form to circumvent the problems associated with their use in the regular form. • The use of coal is associated with many environmental problems, starting from its excavation from mines to gaseous and fly ash emissions. • To overcome these problems, developing countries are transforming coal into gaseous or liquid fuel formulations or converting it into low ash and low sulphur varieties. For example, a solvent-refined, semi-solid form of coal has been prepared by suspending pulverized coal in a solvent and treating it with hydrogen gas at high temperature and pressure. The product compares well with high grade anthracite in combustion properties, is free from ash and has high calorific value of 16000 BTU per pound.

36. Liquid fuels – Petroleum (Petra= rock; oleum= oil) Petroleum is made (under high pressure & temp.) from the remains of plants and animals buried millions of years ago. Crude oil is naturally found floating upon a layer of brine. It is a non-renewable resource. It contains straight or cycloparaffins. Olefins Aromatics Other organic compounds containing N, O, S etc.

37. Composition of petroleum Petroleum is a dark, greenish brown, viscous liquid that is found underground. It comprises hydrocarbons such as: Straight paraffins or cycloparaffinssuch as methane, ethane, propane, butane, isobutane, pentane, hexane. Olefins such as ethylene, butene, isobutene and acetylene, butadienes. Aromatics such as benzene, naphthalene, cyclohexane, methyl cyclopentane. Some organic compounds containing nitrogen, oxygen and sulphur.

38. Production from Refining of Crude Oil Origin: petroleum is formed when large quantities of dead organisms, usually zooplankton, algae, plants and animals are buried underneath sedimentary rock and subjected to intense heat and pressure. The petroleum obtained by mining (oil drilling) is viscous and dark colored liquid. Due to the presence of sulphur, it has an unpleasant smell. It also contains impurities of sand, brine or sea water. Hence it is called crude oil. The important steps involved in refining of petroleum are: Fractional distillation to give various fractions. Cracking: Conversion of less desirable fractions to valuable products. Purification: Treatment of fractions to remove undesirable substances like S.

39. Fractional Distillation

40. Fractions of petroleum

41. Cracking Cracking is decomposition of high molecular weight compounds (having high boiling points) to low molecular weigh compounds (having low boiling points). OR Conversion of less desirable fractions to valuable products

42. Cracking by application of heat and pressure Cracking by application of catalyst

43. Fixed bed catalytic cracking Temp: around 500 oC

44. Fluidized-bed catalytic cracking Temp: 450-550 oC

45. The fluidized bed cracking has the following advantages over fixed-bed cracking: Better contact with the feed and the catalyst, enabling uniform temperature and efficient heat transfer. The catalyst can be regenerated and used again for the cracking process.

46. Catalytic Reforming Reforming is a process of converting low octane naphthas into high octane gasoline.

48. Knocking (i) Knocking in Spark Ignition Engines (uses petrol) & Octane Number Petrol is used in spark ignition engines. The rapid compression (1/6th or 1/10th ) of the fuel-air mixture heats the engine, spark is produced and it detonates the petrol. This causes a violent jerk to the piston giving a metallic sound called knocking. Order of tendency of fuel to knock: The antiknock value of fuel is expressed quantitatively in terms of octane number. Octane number is the percentage volume of isooctane in the isooctane (2,2,4-Trimethylpentane) & n-heptane mixture that matches the knocking characteristics of the fuel being tested is called the octane number. Initially (Tetra ethyl lead) TEL is added to petrol. Than mixture of TEL (60%), ethylene bromide (26%), ethylene chloride (9%), and a red dye (2%). Molecular structure affects the octane number.

49. (ii) Knocking in Compression Ignition Engines & Cetane Number • Diesel is used in compression ignition engines. • In diesel engines, air is compressed to 1/12th or 1/20th of its volume. This rise in temp. to approx. 300 o C, and diesel is atomized and due to high compression it ignites itself without the supply of spark. • Cetane number represents the spontaneous ignition temperature of a particular diesel fuel. • “It is the percentage of cetane present in a mixture of cetane and alpha-methylnaphthalene which matches the fuel under test in ignition property.” • Cetane no. can be increased by the addition of small amount (1-5%) of additives • like ethyl nitrate, isoamyl nitrate nitronaphthyalene, acetone.

50. Power Alcohol and Synthetic Petrol Power Alcohol Ethyl alcohol is used as additive to motor fuels. When blended with petrol at concentrations of 5–10%, it is called power alcohol. The addition of alcohol to petrol increases its octane number. Manufacture of Ethanol By fermentation Using Molasses as raw material