COMBUSTION ENGINEERING PROBLEMS
1.) The gravimetric analysis of dry exhaust gas from an internal combustion engine is as follows:
CO2 = 20%; O2=5.0%; N2= 75%. The constant pressure specific heats for each composition of gas in kcal/kg-C are:
CO2 = 0.203
O2 = 0.219
N2 = 0.248
Determine:
a.) Molecular weight in kg/kg-mol
(ans.: 30.4)
b.) Specific Gravity, if the molecular weight of air is 28.97 kg/kg-mol.
(ans.: 1.049)
c.) Specific Gas Constant, J/g-K
(ans.: 0.235)
d.) specific heat at constant pressure, in Kcal/kg-C
(ans.: Q =745.18 Kcal/kg of fuel)
2.) In the combustion of fuel oil, C12H26, there are 20kg of flue gas formed per kg of fuel burned. Determine:
a.) the theoretical air-fuel ratio on a mass basis; (b) the excess air per kg of fuel oilburned.
(ans: 15kgair/kgfuel;e=25%)
3.) An 8-cylinder, 4-stroke cycle compression ignition oil engine delivers 1000KW at 500rpm. Other data:
Fuel composition:
C12H26
Orsat analysis of dry flue gas: CO2=7.0%, 02=11% , N2=82%. Flue gas temperature: 250⁰C & Brake Thermal Eff =30%
Take air supplied for combustion as 100Kpa and 20⁰C. Determine:
a.) the volume of air supplied for combustion
(ans.: 1.664m³air/sec)
b.) the thermal energy loss due to dry products of combustion
(ans.: Qdg =451.53 KW).
c.) the heat lost due to water vapor from hydrogen.
(ans.:Q watervapor =225 KW)
4.) In boiler design it is desirable to have the flue gas exit temperature above the dew point temperature of the
products of combustion. The gravimetric analysis of the flue gas formed in the combustion of a certain fuel is:
CO2= 20.35% O2= 3.61% H20 = 4.20% N2 = 71.84%. Assume that the air infiltration and leakage are negligible,
determine: (a.) the molecular weight of the flue gas (b.) the Specific gravity, (c.) specific gas constant in J/Kg-K and the
dew point temperature of the products of combustion. (Ans.: MW=29.635, SG=1.023, R = 280.55, t = 39.37⁰C ).
5.) Methane is burned with dry air. The molar analysis (by volume) of the products on a dry basis is : CO2= 9.7%; 02=
2.95%; CO= 0.5% (implies incomplete combustion) and N2= 86.85%. Determine: (a) the A/F ratio on both a molar and
mass basis (b) the percent theoretical air (c.) the dew point temperature if the pressure is 1 bar
(ans. : 19.52 kg air/kg fuel; excess air (e) = 13.235 %; (c.) t= 56.59⁰C)
6.) A natural gas has the following volumetric analysis, in percent: CO2= 0.4; N2 = 3.4; C2H6 =4.1; CH4 = 92.1. The
natural gas is burned in a steam boiler furnace with 20% excess air. For complete combustion, calculate: (a.) the
theoretical volume of air required; (b.) the actual air-fuel ratio by volume; and (c.) the volume of (flue) gas formed per
kg. of natural gas. (ans.: 1.9855 kmol of oxygen/kmol natural gas; actual 11.3448 m³ air/m³ natural gas; 12.34m³ flue
gas/m³nat gas).
7.) The dry exhaust gas from an oil engine has the following gravimetric analysis: CO2 = 21.6%, O2=4.2%, N2=74.2%
Specific heats at constant pressure for each component of the exhaust gas in Kcal/kg-C are:
CO2 = 0.203
O2 =0.219
N2 = 0.248
`Calculate: (a.) the molecular weight in kg/kg mol; (b )the gas constant in J/kg-K (c.) the specific gravity if the molecular
weight of air is 28.97 kg/kg-mol ; (d) the specific heat at constant pressure in KJ/kg-C ( ans.30.561; 0.272; 1.0549; 0.237)
8.) A steam generator burns fuel oil with 20% excess air. The fuel oil may be represented by C14H30. The flue gases
leave the air preheater at 0.31 MPa. Find the minimum stack temperature to avoid condensation (ans.: t = 73⁰C).
9.) A typical fuel oil, C8 H18, is burned at the rate of 0.065 kg/s with 50% excess air. The fuel and air enter the
combustion chamber at 25⁰C and 101.325Kpa. For complete combustion determine:
(a.) the theoretical a/f ratio by volume. (b.) actual (a/f ) ratio by weight (c.) the volume of air required for combustion
(d.) volumetric analysis of wet flue gas( e.) the gravimetric analysis of dry flue gas( f) the DPT of the flue gas.
(ans.: 59.524 mol air/ mol of fuel; 22.59 kg air/kg fuel; 0.83 m³air/s; volumetric wet fluegasCO2=8.53%; 02=6.664%;
N2=75.21%; H2O=9.6%;
gravimetric dry fluegas : CO2=13.93%; N2=78.16%; O2=7.914%; DPT=45.26⁰C.)
10.) A bituminous coal has the following composition:
C = 71.5%;
S=
3.6%
H = 5.0%;
Ash = 8.2%
O = 7.0%;
W=
3.4%
N = 1.3%;
Determine:
a.) the theoretical weight of oxygen in lb/lb of coal
(ans. : (O/F)theo = 2.27267 kg O2/kg of fuel)
b.) the theoretical air-fuel (A/F) ratio
(ans. : (A/F)theo = 10.8222kg air/kg fuel )
c.) the actual air-fuel ratio with 30% excess air.
(ans. : (A/F)a = 14.06889 ).
11.) A certain coal has the following ultimate analysis by weight
C = 70.5%
H2 = 4.5%
O2= 6.0% N2= 1.0%
S= 3.0%
Ash= 11.0%
Moisture=4.0%
A stoker fired boiler of 175,000kg/hr steaming capacity uses this coal as a fuel, calculate:
a.) Weight of air Kg. needed per kg. of coal when this coal is capacity is burned with 30% excess air.
b.) Volume of air in cubic meters per hour with air at 60⁰F and 14.7psia pressure.
[ans.: (A/F)th = 9.52586kg air/kg fuel; (A/F)act = 12.38362kg air/kg fuel) ; V = 212,110.75 m³/hr;
W = 502.7052MT/24hr.]
12.) An investigation on the natural gas coming from one of the abandoned wells, the oil drillers in Bago, Cebu was
made by the Manila Gas Corporation. Sample was taken and the following are the result of the laboratory analysis:
Nitrogen: 2.0% ; CO2= 2.0%; CH4 = 64.2%(methane) ; C2H6 = 31.8%(ethane) . This volumetric analysis was made
with 12% excess air and which was supplied at 90⁰F and relative humidity of 80%. Determine the actual wet flue gas
composition:
a.) Percentage by weight (or gravimetric analysis)
b.) Percentage by volume.(volumetric analysis)
[ans.: MWgas= 21.252kg/kg-mol:
a.) N2= 71.14%; CO2=14.257; H2O= 12.3012; O2= 2.2977%;
b.) ans.: MWgas= 27.6235:
b.) N2= 70.18%; CO2= 8.951; H2O= 18.8779; O2= 1.9835%]
13.) A furnace burns natural gas with a volumetric analysis as follows:
Methane: 85%; Ethane = 12%; Propane = 3%
The gas flow rate is 0.37m³/sec and 25% excess air (e) is required for complete combustion. Combustion air at 25⁰C
and 1 atmosphere pressure is supplied to the furnace by a forced draft fan through a short duct of 0.25m² in area
against a static pressure of 50mm of water. The forced-draft fan has an efficiency of 65%. FIND:
a.) Molal air-fuel ratio
[ans.: ( A/F)theo = 10.0852 ]
b.) the volume flow rate of flue gas in m³/sec.
[ans.: Vfg act = 5.40 m³/s]
c.) the Dew Point of the products of combustion.
[ans.: DPT = 54⁰C]
d.) the power output of the fan motor in KW.
[ans.: Fan BP = 5.5935 KW]
14.) A typical fuel Oil, C8 H18, is burned at the rate of 0.065 kg/s with 50% excess air. The fuel and the air enter the
combustion chamber at 25⁰C and 101.325 kpa. For complete combustion, determine the
a.) Theoretical air-fuel ratio by volume
(ans. A/F)theo = 59.524 mol air/molC8H18)
b.) Actual air-fuel ratio by weight.
(ans.:A/F)a = 22.68 kg air/kg fuel)
c.) Volume of air required for combustion
(ans.= 0.83 m³/s)
d.) The volumetric analysis of wet flue gas
[ ans.: CO2=8.53%; H2O= 9.6%; N2=75.21%; O2= 6.664%]
e.) Gravimetric analysis of wet flue gas.
[ans. : CO2= 13.93%; O2= 7.914%; N2= 78.16%]
f.) The (DPT) the dew point temperature of wet flue gas. (stack gas temperature) (ans.:45.26⁰C, @ P=9.7272Kpa) 10.)
A bituminous coal has the following composition:
C = 71.5%;
S=
3.6%
H = 5.0%;
Ash = 8.2%
O = 7.0%;
W=
3.4%
N = 1.3%;
Determine:
a.) the theoretical weight of oxygen in lb/lb of coal
(ans. : (O/F)theo = 2.27267 kg O2/kg of fuel)
b.) the theoretical air-fuel (A/F) ratio
(ans. : (A/F)theo = 10.8222kg air/kg fuel)
c.) the actual air-fuel ratio with 30% excess air.
(ans. : (A/F)a = 14.06889 ).