ρ = Density: S.g. ρ ρ S.g. ρ ρ

INTERNAL COMBUSTION (PART 2)

Fuel is any substance, natural or artificial which upon combustion release heat energy. Combustion is synonymous to oxidation and is the union of oxygen with combustible materials. Classification of Fuels  SOLID FUELS such as coal, coke, wood, charcoal, bagasse, coconut shells and briquetted fuels.  LIQUID FUELS such as crude petroleum and its distillates (gasoline, alcohol, kerosene, diesel, bunker, and other fuel oils)  GASEOUS FUELS such as natural gas, artificial gas, blast furnace gas, liquefied petroleum gas (LPG), methane, acethylene and propane.  ATOMIC FUELS such as natural uranium, thorium and artificially produced fuel like plutonium. PROPERTIES OF FUELS 1. Analysis of Composition  Proximate Analysis is an analysis of the composition of fuel which gives, on mass basis, the relative amounts of moisture content, volatile matter, fixed carbon and ash.  Ultimate Analysis is an analysis of the composition of fuel which gives, mass basis, the relative amounts of carbon, hydrogen, oxygen, nitrogen, sulfur, ash and moisture. 2. Specific Gravity

S .G . fluid = S .G . gas=

ρfluid ρwater

ρ gas ρ air

ρ = density S.G. = Specific Gravity

3. API and Baume gravity units

° API =

141.5 −131.5 S . G .@ 15.6° C

° Baume=

140 −130 S .G . @15.6 ° C

4. Specific Gravity at temperature (t)

S .G .t =( S .G . @15.6 )∗(Correction Factor ) where: Temperature in °C

Temperature in °F

CF = 1- 0.00072 (t – 15.6)

CF = 1 – 0.0004 (t – 60)

5. Higher Value or Calorific Value HIGHER HEATING VALUE OR GROSS CALORIFIC VALUE – is the heating value obtained when the water in the product of combustion is in liquid state. LOWER HEATING VALUE OR NET CALORIFIC VALUE – is the heating value obtained when the water in the product of combustion is in vapor state. Higher Heating Value of Solid Fuels 

ASME Formula for Petroleum Products Qf = 41,130 + 139.6 (°API) kj/kg



Bureau of Standards Formula Qf = 51,716 - 8793.8 (SG)2 kj/kg



Dulong’s Formula Qf = 33,820 C + 144,212 (H – O/8) + 9,304 S kj/kg

Lower Heating Value of Liquid Fuels QL = QH – 9H2 (2442) kj/kg

where: H2 = 26 – 15.6 S.G. (in %)

Combustion of Solid Fuels  

Theoretical Weight of Air – The theoretical weight of air is the exact theoretical amount as determined from the combustion reaction of air needed to burn a unit amount of fuel. If the Ultimate Analysis of coal is available, the theoretical weight of air required to completely oxidize the fuel can be calculated as:

(

W ta =11.5 C +34.5 H −

kgair O + 4.3 S , 8 kg fuel

)

Where: Wta = ideal proportion of air to completely burn the fuel C, H, O, and S are proportions by weight of Carbon, Hydrogen, Oxygen and Sulfur per kg of fuel from the ultimate analysis. 

If the Ultimate Analysis of coal is not available, an approximate formula to obtain the theoretical air – fuel ratio when the heating value of the fuel is known:

/kg ) kg , ( AF ) =W = Q ,(kj 3117 kg /kg) kg , ( AF ) =W = Q ,(kCal 745 kg Q ,(Btu/lb) kg A =W = , (F) 1340 kg h

t

air

ta

fuel

h

t

fuel

h

t



air

ta

fuel

Actual Weight of Air The weight of air supplied for combustion is necessarily in excess of what is theoretically required. The volumetric analysis of the dry flue gas can be used to calculate the actual weight of air.

( AF ) =W ( AF ) =W a

a



air

ta

C x N2 kg air 1 , 32.72 C O 2 +CO kg fuel % Excess air kgair , aa =W ta 1+ 100 kg fuel aa

(

=

)

(

)

Where: Waa = actual weight of air including the excess C = weight of carbon per kg of fuel from the ultimate analysis N2, CO2 and CO are percentages by volume of the component gases in the flue gas as obtained by ORSAT Analysis Analysis of Flue Gas CO2+ CO + O2 +N2 = 100%

The weight of gaseous products liberated during the combustion of fuel with air: Wfg = Waa + Wf kg/hr The weight of dry flue gas formed per kg of fuel burned: Wdg = Waa + 1 – (SO2 + H2O + Ashloss) Where: Waa = actual air-fuel ratio in SO2 =

2 x %S 100

Percent Excess Air

% Excess=

W aa−W ta W ta

Hydrocarbon Fuels Fuel + Air = Products of Combustion

H2O =

9 x %H 100

Ash loss =

%Ash 100

CnHm+ aO2 + 3.76aN2 = bCO2 + cH2O + 3.76aN2 Composition of Air By Weight:

By Volume

O2 = 23%

O2 = 21%

N2 = 21%

N2 = 79%

SAMPLE PROBLEMS 1. A bituminous coal has the following compositions: C = 71.5% H = 5.0%

O = 7.0% N = 1.3%

S = 3.6% Ash = 8.2%

M = 3.4%

Calculate the higher heating value of coal and for complete combustion find the theoretical weight of air required in kgair/kgcoal. 2. A 650 BHP diesel engine uses fuel oil of 28°API gravity, fuel consumption is 0.65 lb/BHP-hr. Cost of fuel is PHP 7.95 per liter. For continuous operation, determine the minimum volume of cubical day tank in cm³, ambient temperature is 45°C. 3. It is required to find the theoretical volume of air at 20 oC and 100 kpa absolute pressure to burn one kilogram of coal. The ultimate analysis of coal-as-fired is as follows C = 65.65%

H = 5.87%

O = 18.61%

S = 1.51%

Mois = 3%

Ash = 5.36%

4. Calculate the higher heating value of liquid Dodecane fuel. The chemical formula of Dodecane is C 12H26 5. Liquid Octane (C8H18) fuel is burned with ideal proportion of air. Calculate the ideal air-fuel ratio by weight. 6. Fuel oil in a day tank for use of an industrial boiler is tested with hydrometer. The hydrometer reading indicates a S.G. = 0.924 when the temperature of the oil in the tank is 35oC. Calculate the higher heating value of the fuel 7. A diesel electric plant supplies energy for Meralco. During a 24 hour period, the plant consumed 200 gallons of fuel at 28oC and produced 3930 kw-hr. Industrial fuel used is 28oAPI and was purchased at P5.50 per liter at 15.6oC. What should the cost of fuel be produce one kw-hr? 8. Methane is burned completely with 25% excess air. Compute the number of moles of nitrogen in the products of combustion per mole of the fuel. 9. A typical industrial fuel oil, C6H32 with 20% excess air by weight, Assuming complete oxidation of the fuel, calculate the actual air-fuel ratio by weight. 10. Ethane, C2H6, is burnt with just twice the stoichiometric amount of air. How much kilogram air is used to oxidize 6 kg of fuel? 11. A furnace burns natural gas with a volumetric analysis as follows: Methane = 85%, Ethane = 12%, Propane = 3%. The gas flow rate is 0.5 m3/s and 25% excess air is required for complete combustion. Combustion air is supplied to the furnace at 25oC and 1 atm pressure. Find the molal air-fuel ratio and the volume flow rate of the flue gas is m 3/s. 12. A gaseous mixture of methane (CH4), ethane (C2H6) and propane (C3H8) has partial pressures respectively of 95 kPa, 105 kPa and 50 kPa. What is the mass percentage of methane in the mixture?

SUPPLEMENTARY PROBLEMS 1. A bituminous coal has the following compositions: C =71.5 % O = 7.0% S = 3.6% W = 3.4% H = 5.0% N = 1.3% Ash = 8.2% Calculate for complete combustion the theoretical weight of air required in kgair/kgcoal. A. 9.80 B. 8.80 C. 7.80 D. 6.80

7.

A circular tank 45 ft long and 5.5 ft diameter is used for oil storage. Calculate the number of days the supply tank can hold for continuous operation at the following conditions. Steam flow = 200lb/hr Steam dry and saturated at 200 psia Feedwater temperature = 230 °F Boiler efficiency = 75% Fuel oil = 34 °API A. 15.21 days B. 16.23 days C. 17.57 days D. 19.26 days

2.

Calculate the higher heating value of liquid dodecane fuel. The chemical formula of Dodecane is C12H26. A. 48,255 kJ/kg B. 55,641 kJ/kg C. 60,221 kJ/kg D. 35,245 kJ/kg

8.

Liquid Octane (C8H18) fuel is burned with ideal proportion of air. Calculate the ideal air-fuel ratio by weight. A. 13.72 kgair/kgfuel B. 14.56 kgair/kgfuel C. 15.05 kgair/kgfuel D. 10.69 kgair/kgfuel

3.

Calculate the minimum volume of day tank of 28 °API fuel having a fuel consumption of 1 kg/s. A. 94.71 m3 B. 97.41 m3 C. 91.74 m3 D. 79.41 m3

9.

4.

It is required to find the theoretical volume of air at 20 °C and 100 kPa absolute pressure to burn one kilogram of Franklin Country coal. The ultimate analysis of coal-as-fired is as follows: C = 65.65% O = 18.61% Mois. = 3% H = 5.87% S = 1.51% Ash = 5.36%

A typical industrial fuel oil, C16H32 with 20% excess air, by weight. Assuming complete oxidation of the fuel, calculate the actual air-fuel ratio by weight. A. 17.56 kgair/kgfuel B. 15.76 kgair/kgfuel C. 16.75 kgair/kgfuel D. 17.65 kgair/kgfuel

A. B. C. D.

9.43 m3air/kgcoal 7.43 m3air/kgcoal 8.43 m3air/kgcoal 6.43 m3air/kgcoal

5.

A 650 BHP diesel engine uses fuel oil of 28 °API gravity, fuel consumption is0.65 lb/BHP-hr. cost of fuel is P 7.95 per liter. For continuous operation, determine the minimum volume of cubical day tank in cm3, ambient temperature is 45 °C. A. 4,372,890 cm3 B. 5,987,909 cm3 C. 5,291,880 cm3 D. 7,352,789 cm3

6.

A logging firm in Isabela operates a Diesel Electric plant to supply its electric energy requirements. During a 24 hr period, the plant consumed 250 gallons of fuel at 80 °F and produced 2700 kw-hrs. Industrial fuel used is 30 °API and was purchased at P 3.00 per liter at 60 °F. Determine the over-all efficiency of the plant. A. 26.08% B. 25.06% C. 29.07% D. 30.01%

10. The ultimate analysis of coal is given below: C = 68.5% S = 1.5% Ash = 12.0% H = 2.5% 3.5% Calculate the higher heating value of coal. A. 25,300 kJ/kg B. 28,300 kJ.kg C. 27,300 kJ/kg D. 26,300 kJ/kg 11. Fuel oil in a day tank for use of an industrial boiler is tested with hydrometer. The hydrometer reading indicates a S.G = 0.924 when the temperature of the oil in the tank is 35 °C. Calculate the higher heating value of the fuel. A. 43,852.13 kJ/kg B. 53,852.13 kJ/kg C. 58,352.13 kJ/kg D. 48,352.13 kJ/kg

12. A steam generators burns fuel oil with 20% excess air. The fuel oil may be represented by C 14H30. The flue gas leaves the preheater at 0.31 Mpa. Find the minimum stack temperature to avoid condensation. A. 73 °C B. 63 °C C. 83 °C D. 53 °C 13. Gaseous fuel mixture has a molal analysis:

H2 = 14% CH4 = 3% CO = 27% O2 = 0.6% CO2 = 4.35% N2 = 50.9% Determine the air-fuel ratio for complete combustion on molal basis. A. 2.130 B. 3.230 C. 1.233 D. 1.130 14. In a boiler design, it is desirable to have the flue gas exit temperature above the dew point. Estimate the dew point temperature of the flue gas produced by combustion having the gravimetric analysis of: N2 = 71.84% O2 = 3.61% CO 2 = 20.35% H 2O = 4.20% Assume that air infiltration and leakage are negligible. A. 39 ºC B. 41 ºC C. 42 ºC D. 43 ºC 15. If the theoretical air-fuel ratio is 15, what is the approximate higher heating value in kJ/kgfuel? A. 46,755 kJ/kg B. 56,755 kJ/kg C. 45,765 kJ/kg D. 55,765 kJ/kg 16. There are 20 kg of flue gases formed per kg of fuel oil burned in the combustion of a fuel oil C12H26. What is the excess air in percent? A. 20.17 B. 16.56 C. 26.67 D. 8.21 17. A diesel electric plant supplies energy for Meralco. During a 24 hr period, the plant consumed 200 gallons of fuel at 28 ºC and produced 3930 kW-hr. Industrial fuel used is 28 ºAPI and was purchased at P 5.50 per liter at 15.6 ºC. What should the cost of fuel be produce one kW-hr? A. P 1.05 B. P 1.10 C. P 1.069 D. P 1.00 18. A fuel oil is burned with 50% excess air. What is the volume rate of flow in m3/min of the wet products at a pressure of 102 kPa and a temperature of 350 ºC when the fuel is burned at the rate of 45 kg/min? Assume that the combustion requirements of the fuel oil are similar to those of C12H26. A. 1865 B. 3526 C. 2462 D. 4563 19. A flue gas has the following volumetric analysis: CH4 = 68% C2H6 = 32%

Assume complete combustion with 15% excess air at 101.325 kPa, 21 ºC wet bulb and 27 ºC dry bulb. What is the partial pressure of the water vapor in kPa? A. 9.62 B. 12.81 C. 17.28 D. 15.94 20. The following is an analysis of coal in percent: C = 74 O2 = 8 S=1 H2 = 6 N 2 = 1.6 Ash = 9.4 If burned in a boiler, the coal produces the following Orsat analysis in percent: CO2 = 12 O2 = 6.5 CO = 0.1 The refuse contains 0.008 kg of carbon per kg of coal burned. Determine the percentage of excess air used. A. 45.6% B. 46.5% C. 54.6% D. 56.4% 21. A gaseous mixture has a dew point temperature of 15 ºC. the total pressure is 143.27 kPa. Determine the amount of water vapor present in 100 moles of the mixture. Note: saturation pressure at 15 ºC is 1.7051 kPa. A. 1.10 B. 2.19 C. 1.19 D. 2.0 22. A coal fired utility boiler has the following characteristics: Coal Freed Rate _ _ _ _ _ _ _ _ _ _ _ 6,940 kg/hr Electric Power Rating _ _ _ _ _ _ _ _ 50 MW Refuse Removal Rate _ _ _ _ _ _ _ _ 410 kg/hr Carbon in Refuse_ _ _ _ _ _ _ _ _ _ 30% Ultimate Analysis in percent: C = 76.56 O 2 = 7.70 S = 2.44 H2 = 5.50 N 2 = 1.70 Si = 6.10 Heating value of coal is 32,773 kJ/kg If stack gas particle collectors are required to meet the limit of 0.043 kg particulates per million kg of fuel feed set by Environmental management Bureau under the jurisdiction of the DENR, what efficiency must the collection system have? Molecular weight of Si = 28.09. A. 99.999953% B. 89.999953% C. 79.999953% D. 69.999953% 23. The dry exhaust gas from oil engine has the following gravimetric analysis: CO2 = 21.6% N2 = 74.2% O2 = 4.2% Specific heats at constant pressure for each component of the exhaust gas in kCal/kgºC are: CO2 = 0.203 N2 = 0.248 O2 = 0.219

Calculate the specific gravity if the molecular weight of air is 28.97 kg/kgmol. A. 1.055 B. 1.155 C. 1.255 D. 0.958

O2; 83.1% N2. Calculate the number of atoms of carbon and hydrogen of the hydrocarbon fuel. A. n = 7.26, m = 12.90 B. n = 8.26, m = 7.26 C. n = 8.26, m = 13.9 D. n = 13.90, m = 8.26

24. A coal fired steam boiler uses 3000 kg of coal per hour. Air required for combustion is 15.5 kg per coal at a barometric pressure of 98.2 kPaa. The flue gas has a temperature of 285 °C and an average molecular weight of 30. Assuming an ash loss of 11% and an allowable gas velocity of 7.5 m/s, find the diameter of the chimney. A. 1.88 m B. 1.64 m C. 2.88 m D. 2.64 m

30. The analysis of natural gas fuel used in a gas turbine power plant has the following percentages by volume: CH4 = 80% C2H4 = 5% H2 = 10% CO = 2% and the remaining non-combustible gases. Find the volume of air supplied per hour if the gas consumption is 20,000 m3/hr assuming 40% excess air. A. 248,500 m3/hr B. 348,500 m3/hr C. 260,000 m3/hr D. 360,000 m3/hr

25. A certain coal has the following ultimate analysis: C = 70.5% H = 4.5% O2 = 6% N2 = 1.0% S = 3.0% Ash = 11% Moisture = 4% A stoker fired boiler of 175,000 kg/hr steaming capacity uses this coal as fuel. Calculate the volume of air in m3/hr with air at 60 °F (15.6 °C) and 14.7 psia (101.325 kPa) the coal is burned with 30% excess air. Boiler efficiency of 70% and factor of evaporation of 1.10. A. 212,861.04 m3/hr B. 221,861.04 m3/hr C. 218,261.04 m3/hr D. 281,261.04 m3/hr 26. Methane gas burn completely when supplied with 30 percent excess dry air. The gaseous products are cooled at 32 °C and 1 atm pressure. Determine the mass of moisture condensed per kg fuel. A. 1.302 B. 1.924 C. 1.816 D. 1.614 27. A diesel power plant consumed 1 m 3 of fuel with 30 °API at 27 °C in 24 hrs. Calculate the fuel rate in kg/hr. A. 36.21 B. 26.25 C. 29.34 D. 39.42 28. A coal has the following ultimate analysis: C = 60% H2 = 6% Ash = 5% S2 = 7% N2 = 5% O2 = 9% Moisture = 8% Determine the actual air-fuel ratio using 20% excess air. A. 10.66 kgair/kgcoal B. 11.66 kgair/kgcoal C. 12.66 kgair/kgcoal D. 9.66 kgair/kgcoal 29. The ultimate analysis of a petroleum fuel is 87.1% carbon and 12.9% hydrogen. A certain test of dry products of combustion gives the following analysis by volume: 12% CO2; 0.30% CO; 4.6%

31. A diesel power plant uses fuel with heating value of 43,000 kJ/kg. What is the density of the fuel at 25 °C? A. 840 kg/m3 B. 873 kg/m3 C. 970 kg/m3 D. 940 kg/m3 32. Calculate the calorific value of coal which has the following analysis: Carbon 81%, Hydrogen 5%, Oxygen 5.6%, Sulfur 1% and the remainder is ash content. A. 33,688.35 kJ/kg B. 32,135.45 kJ/kg C. 12,456.34 kJ/kg D. 32,457.45 kJ/kg 33. What is the calorific value and the minimum mass of air required per kilogram of fuel oil which is composed of 85.2% carbon, 12% hydrogen, 1.6% oxygen and 1.2% impurities. A. 13.87 kgair/kgcoal B. 14.55 kgair/kgcoal C. 12.57 kgair/kgcoal D. 15.75 kgair/kgcoal 34. A petrol has the following analysis 85.5% carbon, 14.4% hydrogen, and 0.10% sulfur. Calculate the volume of air at 1.0 bar and 15 °C required for perfect combustion of 1 kg of the fuel. Note: R air = 0.287 kJ/kg-K. A. 12.24 m3/ kgfuel B. 13.45 m3/ kgfuel C. 15.54 m3/ kgfuel D. 14.57 m3/ kgfuel 35. A fuel used in a boiler contains 86.1% carbon, 12.5% hydrogen, and 0.4% oxygen and 1% sulfur. There are 40% excess air supplied to the furnace and the fuel rate is 400 kg/hr. Calculate the mass of air and the heat energy transferred to the air per hour if it enters the air heater at 18 °C and leaves at 130 °C. Note: Cp = 1.00kJ/kg-K. A. 7,974.26 kJ/kg, 890,093.12 kJ/kg B. 8,678.45 kJ/kg, 980,045.12 kJ/kg

C. 7,345.35 kJ/kg D. 8,987.76 kJ/kg

kJ/kg,

670,035.45

kJ/kg,

567,098.57

36. A fuel has the following constituents: Carbon 85%, Hydrogen 13%,Oxygen 2%. When burning this fuel in a boiler furnace the air supply is 50% in excess of the theoretical minimum required for complete combustion, the inlet temperature of the air being 31 °C and funnel temperature is 280 °C. Compute its calorific value, air-fuel ratio and percentage of heat energy supplied. A. 47,133.03 kJ/kg, 21.26 kgair/kgfuel B. 35,897.54 kJ/kg, 12.21 kgair/kgfuel C. 14,576.24 kJ/kg, 17.27 kgair/kgfuel D. 45,457.13 kJ/kg, 12.54 kgair/kgfuel 37. Experiment revealed that the calorific value of an oil fuel by means of a bomb calorimeter, the mass of a sample fuel was 0.75 gram, the mass of water surrounding the bomb was 1.8 kg with some water in the fitting 0.47 kg, and the temperature rises by 3.3 °C. Considering that the specific heat of water is 4.2 kJ/kg-K, compute the calorific value of this oil in Mj/kg.

A. B. C. D.

41,949.6 kJ/kg 35,345.2 kJ/kg 47,457.23 kJ/kg 31,687.45 kJ/kg

38. A fuel consists of 84% carbon, 13% hydrogen, and the remainder incombustible solid matter. Calculate the calorific value of the theoretical mass air required per kg of fuel and the mass analysis of the flue gas if 22 kg of air are supplied per kg of fuel burned. A. 46,795.83 kJ/kg, 14.06 kgair/kgfuel B. 45,787.87 kJ/kg, 15.06 kgair/kgfuel C. 34,576.78 kJ/kg, 14.75 kgair/kgfuel D. 45,567.78 kJ/kg, 15.67 kgair/kgfuel 39. A fuel oil has the following contents: 85.35% carbon, 11.9% hydrogen, 1.6% oxygen, and 1% impurities. Calculate the percentage CO2 in the flue gas for complete combustion. A. 20.94% B. 30.94% C. 35.5% D. 17.56%