2-fuels-and-combustion.docx

FUELS AND COMBUSTION

Fuel – any substance, natural or artificial that will burn CLASSIFICATION OF FUELS 1. Solid Fuels (Principal component: Carbon) a. Coal Classification of coal by rank: i. Anthracite – is the oldest coal from a geological perspective. It is a hard coal composed mainly of carbon with little volatile content and practically no moisture. - 86 to 98% carbon - Less than 2 to 14% volatiles - Shine, black, dense, hard and brittle ii. Bituminous - 46 to 86% fixed carbon - 20 to 40% volatiles - Heating value, 25,600 to 32,000 KJ/kg. iii. Sub-bituminous - Brownish- black and homogenous - Heating value 19,300 to 26,750 KJ/kg - 15 to 30% moisture - Low sulfur iv. Lignite – it is the youngest coal from a geological perspective. It is a soft coal composed mainly of volatile matter and moisture content with low fixed carbon. - Like wood-brown and laminar - High moisture (30%) - High volatiles - Heating value 14,650 to 19,300 KJ/kg v. Peat – a heterogeneous material - Up to 90% moisture - Widely found - Expensive to transport b. Wood - Very variable in density - Variable moisture content - High volatile content c. Charcoal - Low density - Low volatiles - Biologically stable d. Bagasse – waste left when sugar is extracted from cane. - 84% volatile matter, 12.5% Fixed carbon (F.C.), 3.5% ash on dry basis 2. Liquid fuels (Principal Component: Hydrocarbon, CnHm) Some common liquid fuels: a. Paraffinic Hydrocarbons i. Petroleum ii. Kerosene iii. Diesel iv. Residual fuel oil b. Aromatic hydrocarbons i. Benzenes - is a clear, colorless, highly flammable and volatile, liquid aromatic hydrocarbon with a gasoline-like odor. ii. Xylenes - is an aromatic hydrocarbon widely used in industry and medical technology as a solvent. It is a colorless, sweet-smelling liquid or gas occurring naturally in petroleum, coal and wood tar, and is so named because it is found in crude wood spirit

iii. Toluenes - It is a colorless, water-insoluble liquid with the smell associated with paint thinners. c. Biological materials i. Vegetable oil ii. Animal oils

iii. Alcohols 3. Gaseous fuels – commonly used in industry, whether distributed by public utilities or produced in isolated plants, are composed of one or more simple gases in varying proportions Commercial gases: a. Hydrogen – refinery “off gases”. b. Methane – natural gas, high BTU gas, biogas c. Propane/butane – cooking gas, rock gas d. Producer gas – low BTU gas, medium BTU gas, high BTU gas 4. Atomic fuels a. Natural uranium b. Thorium and artificially produced fuel like plotunium Two broad types of oils are in use: 1. Straight oils – are produced entirely from the crudes chosen through elimination of undesired constituents by suitable refining processes. 2. Additive oils – are produced by adding to straight mineral oils certain oil-soluble compounds that enhance the lubricating oil properties for use in a diesel engine. SAE three types of lubricating oils: 1. Regular type – suitable for moderate operating conditions. 2. Premium type – having oxidation stability and bearing corrosive- preventive properties making it generally suitable for more severe service than regular-duty type. 3. Heavy duty type – has oxidation stability, bearing corrosion-preventive properties, and detergent-dispersant characteristics for use under heavy-duty service conditions. PROPERTIES OF FUELS AND LUBRICANTS: 1. Analysis of composition A. Ultimate Analysis - Gives the mass percentage of: CARBON, HYDROGEN, OXYGEN, NITROGEN, SULFUR - Ash given as mass of oxides. - Moisture content as a percentage of dry mass. B. Proximate Analysis of Solid Fuels i. Fixed carbon – carbon available for combustion - It is the solid fuel left in the furnace after volatile matter is distilled off. It gives rough estimate of the heating value of coal ii. Volatile matter- gases (apart from water) that can be driven off by heat. - Are the methane, hydrocarbons, hydrogen and carbon monoxide and incombustible gases like carbon dioxide and nitrogen found in coal. iii.Moisture – water that can be driven off up to 110°C. iv. Ash - a non-combustible material left after carbon has burned. - It is an impurity that will not burn. v.Sulfur – determined by sulfur oxides it produces. - SO2 and SO3 are serious air-pollutant. 2. Specific Gravity, Density Specific Gravity – the ratio of the weight of a given volume of oil to the weight of the same volume of water at a given temperature. The density of fuel, relative to water is called SPECIFIC GRAVITY. Specific Gravity, SG of water = 1 Density – the ratio of the mass of the fuel to the volume of the fuel at a reference temperature of 15°C. It is measured by an instrument called a HYDROMETER. Specific Gravity, S.G.liquid = Specific Gravity, S.G.gas =

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑞𝑢𝑖𝑑 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟

𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑔𝑎𝑠 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟

𝜌

= 𝜌𝑙𝑖𝑞𝑢𝑖𝑑 𝑤𝑎𝑡𝑒𝑟

=

where: = 1000kg/m3 = 1 kg/li = 62.4 lb/ft3

𝜌𝑔𝑎𝑠 𝜌𝑎𝑖𝑟

Instruments used for measuring specific gravity: Hydrometer,Pycnometer, westphal balance

°API and Baume gravity Units:

141.5

°API = 𝑆𝐺

@15.6℃

− 131.5

140.5

°Baume = 𝑆𝐺

@15.6℃

= 𝑆𝐺

[email protected]̊C = °𝐴𝑃𝐼+131.5 140.5

− 130.5

Other reference: °Baume

141.5

OR:

[email protected] = °𝐵𝑎𝑢𝑚𝑒+130.5

140

@15.6℃

− 130

Specific Gravity with correction factor due to temperature (t) effect: S.G.t = (S.G.@ 15.6) (correction factor, CF) Where: CF = 1-0.00072(t – 15.6)

t = temp in °C

CF = 1-0.0004(t – 60) Therefore:

t = temp in °F

S.G.t = (S.G.@ 15.6) (1-0.00072)(t – 15.6)

3. Heating Value or Calorific value, KJ/kg - Fuel is burned in a bomb calorimeter. - Heat generated is measured. - Higher heating value (HHV) includes latent heat of condensation from water vapour. Higher Heating Value or Gross Calorific Value – is the heating value obtained when the water in the products of combustion is in liquid state. Lower Heating Value or Net Calorific value – is the heating value obtained when the water in the products of combustion is in vapour state. Instruments used in measuring the heating value of fuels: a. Oxygen Bomb calorimeter: for solid and liquid fuels b. Gas calorimeter: for gaseous fuels Calculating higher heating value of fuels by formula: c. Dulong’s formula, used for solid fuels of known ultimate analysis: Qh = 33,820C +144,212 (H - ) + 9,304 S ; KJ/kg d. ASME formula, for petroleum products: Qh = 41,130 + 139.6(°API) ; KJ/kg e. Bureau of Standards formula: Qh = 51,176 – 8,793.8(SG)2 ; KJ/kg Difference between higher and lower heating value: Ql = Qh - 9H2(2442) where: H2 = 26-15(SG) 1 BTU/lb = 2.236 KJ/kg

OR

; KJ/kg ;%

1 BTU = 0.5556 kcal/kg

4. Viscosity of Lubricants - Resistance to flow or the property which resists shearing of the lubricant. Absolute viscosity – viscosity which is determined by direct measurement of shear resistance. Kinematic viscosity – absolute viscosity divided by the density Viscosity index - the rate at which viscosity changes with temperature. Units of Viscosity: For absolute viscosity: 1 reyn =

𝑙𝑏−𝑠𝑒𝑐 𝑖𝑛2

where: 1 poise =

For Kinematic viscosity: 1 stroke = 1

𝑑𝑦𝑛𝑒−𝑠𝑒𝑐 𝑐𝑚2

𝑐𝑚2 𝑠𝑒𝑐

= 0.1 Pa-sec

Viscosimeter – an instrument, consisting of standard orifice, used for measuring viscosity (in SSU and SSF).

SSU ( Saybolt Second Universal) – number of seconds required for 60ml of oil (@ 37.5 °C) to pass through a standard orifice. SSF (Saybolt Second Furol) – unit used for very viscous liquids using a relatively large orifice). 1 centistoke = 0.308 (SSU-26) 62 SF = 660SSU 5. Other properties of fuels and lubricants: Flash point – the temperature at which oil gives off vapor that burns temporarily when ignited. Fire point – the temperature at which oil gives off vapor that burns continuously when ignited. Pour point – the temperature at which oil will no longer pour freely or the temperature at which oil will solidify. Dropping point – the temperature at which grease melts Cloud point – temperature at which the paraffin elements separate from oil Conradson number (carbon residue) – the percentage by weight of the carbonaceous residue remaining after destructive distillation. Octane number – the ignition quality rating of gasoline, which is the percentage by volume of iso-octane in a mixture of iso-octane and heptane that matches the gasoline in anti-knock quality. Cetane number – the ignition quality rating of diesel, which is the percent of cetane in the standard fuel. COMBUSTION – is the process of chemical reaction between fuel and oxygen (reactants). The process releases heat and produces products of combustion. - The main elements which burn are: carbon, hydrogen and sulfur. The heat released by 1kg or m3 of fuel is called CALORIFIC VALUE. “The oxygen used in combustion processes normally comes from the atmosphere and this brings nitrogen in with it which normally does nothing in the process but makes up the bulk of the gases remaining after combustion”. - It is a special exothermic chemical reaction in which oxygen is a major reactant. Exothermic – means it gives out heat therefore Δh is negative. Endothermic – means reaction absorbs heat and Δh is positive. THREE (3) T’s of Combustion: 1. Temperature – high enough to ignite and maintain ignition of fuel 2. Turbulence or intimate mixing of the fuel and oxygen 3. Time – sufficient for complete combustion. ** the objective of good combustion is to release all of the heat in the fuel”. Air-Fuel Ratio of Solid Fuels: 1. Theoretical air-fuel ratio - the exact theoretical amount, as determined from the combustion reaction, or air needed to burn a unit amount of fuel, kg air per kg of fuel. 𝐴

𝑚

(𝐹)t = air-fuel ratio = 𝑚𝑎 𝑓

Combustion of solid fuel with known ultimate analysis: 𝑂 Theoretical A/F = 11.5C+34.5(H - 8 ) +4.3S

𝑘𝑔

;𝑘𝑔 𝑎𝑖𝑟

𝑓𝑢𝑒𝑙

Where: C,H,O and S are proportions by weight of Carbon, Hydrogen, Oxygen and Sulfur per kg of fuel from the ultimate analysis. 𝐴

2. Actual air-fuel ratio ;(𝐹)a = theoretical air-fuel ratio plus excess air. A/F = (Theoretical A/F)(1+e)

;

𝑘𝑔𝑎𝑖𝑟

𝑘𝑔𝑓𝑢𝑒𝑙

where: e = excess air

Typical combustion reaction of liquid fuels: 1. Molecular weights: C = 12 H=1 O = 16 N =14 Molecular weight of air = 28.92 kg/kg-mol 2. Composition of Air Air by volume consists of: 21% of oxygen 79% nitrogen thus there are 3.76 mols of N2 per mol of O2.

S = 32

Air by weight: Oxygen = 23.20% Nitrogen = 76.80%

𝐴

3. Theoretical air-fuel ratio:(𝐹)t Fuel + air = Products of combustion CnHm + xO2 + x (3.76)N2 = yCO2 + zH2O + x(3.76)N2 Where: x,y, and z represents the number of moles For perfect combustion:

x=

2𝑛+

𝑚 2

2

=𝑛+

𝑚

y=n

4

z=

𝑚 2

𝑚

Theoretical air-fuel ratio = 𝑚𝑎 𝑓

Molal basis: Theoretical air-fuel ratio = x + 3.76x

𝑚𝑜𝑙

; 𝑚𝑜𝑙 𝑎𝑖𝑟

𝑓𝑢𝑒𝑙

Mass basis: Theoretical air - fuel ratio =

32𝑥+28(3.76)𝑥 12𝑛+𝑚

𝑘𝑔

; 𝑘𝑔 𝑎𝑖𝑟

𝑓𝑢𝑒𝑙

𝐴

1. Actual air-fuel ratio: :(𝐹)a Considering the excess air, e Fuel + air = Products of combustion CnHm + (1+e) x O2 +(1+e) x (3.76)N2 = yCO2 + zH2O + (1+e) x (3.76)N2 + e (x)O2 For perfect combustion:

x=

2𝑛+

𝑚 2

2

=𝑛+

𝑚 4

y=n

Where: x,y, and z represents the number of moles Mt = total mols of product = y + z + (1+e) x (3.76) + e (x) 2. Partial Pressure of H2O 𝑧 Partial pressure = 𝑚 𝑃 𝑡

P = pressure Use P = 101.325KPa if not given in the problem 3. Actual Air-fuel Ratio 𝐴 Actual Air-fuel Ratio = Theoretical 𝐹 (1+e) 4. By mass balance in the engine: ma + mf = mg Where: ma = mass of air entering the engine mf = mass of fuel entering the engine mg = mass of gas leaving the engine Gas Constant, R: 8.314 R= 𝑀 R=

1545 𝑀

𝐾𝐽

; 𝑘𝑔−°𝐾 ;

𝑓𝑡−𝑙𝑏 𝑙𝑏−𝑅

where: M= molecular weight of gas

z=

𝑚 2