Combustion Engineering (Mid-Term Examination) Mechanical Engineering Department-UEP.

2020

Part I. Multiple Choice (Choose the best answer). Elements of Fuels & Combustion (2 points each, 40%)
1.) Dry air can be approximated as ____% Oxygen and ______% Nitrogen by mol numbers.
a.) 21% &79% b.) 76.8% &23.2% c.) 79& & 21% d.) 23.2% & 76.8% e.) 70% & 30% f.) 30% & 70%.
2.) The product(s) of complete combustion of gaseous hydrocarbon
a.) CO2 & water b.) carbon monoxide c .) carbon dioxide, water and ammonia d.) water, CO, CO2
3.) The unit for expressing API (American Petroleum Institute) gravity is
a.) gm/cc b.) dimensionless no. c.) degree d.) none of the above.
4.) If the fuel composition is known, the ratio between the nitrogen & oxygen that is supplied is
a.) equal b.) variable c. ) fixed d.) increasing e.) decreasing
5.) When water in the products of combustion is in liquid form, the heating value is known as
a.) higher heating value b.) lower heating value c.) low & medium heating value d.) average heating value.
6.) Amount of heat liberated by complete combustion of a unit weight or volume of fuel is:
a.) heating value b.) latent heat c.) sensible heat d.) heat of vaporization.
7.) A gas produced by the combustion of fuel oil and cannot be found in the flue gases is:
a.) CO2 b.) N2 c.) H2 d.) O2
8.) Percent excess air is the difference between the air actually supplied and theoretical air, divided by
a.) sufficient air supplied b.) deficiency air supplied c.) actual air supplied d.) theoretical air supplied.
9.) The exact theoretical amount of air needed to burn a unit amount of fuel, as determined from the combustion reaction
a.) actual air b.) excess air d.) theoretical air e.) supplied air
10.) The amount of air supplied for combustion that is in excess of what is theoretically required
a.) theoretical air b.) excess air c.) actual air d.) supplied air e.) sufficient air
11.) When 1 mol of Carbon combines with 1 mol of Oxygen equals
a.) 2 mols of carbon dioxide b.) 1 mol Carbon and 1 mol CO2 c.) 1 mol of CO d.) 1 mol of CO2.
12.) An Orsat Apparatus is used for
a.) volumetric analysis of the flue gas ; b.) gravimetric analysis of the flue gas
c.) smoke density of the flue gas ; d.) all of the above
13) A chemical reaction in which heat is given off
a.) combustion reaction b.) heat reaction c.) exothermic reaction d.) endothermic reaction
14.) A chemical reaction in which heat is absorbed
a.) combustion reaction b.) heat reaction c.) exothermic reaction d.) endothermic reaction
15.) The gaseous products of combustion of a boiler which contains CO2, CO, O2, N2 and water is called____.
a.) producer gas b.) flue gas c.) product gas d.) universal gas e.) inert gas.
16.) What is the chemical formula of an aromatic type of hydrocarbon fuel?
a.) CnH2n-6 b.) CnH2n-2 c.) CnH2n d.) CnH2n+2
17.) What is the general formula of a paraffin type hydrocarbon fuel?
a.)CnH2n b.) CnH2n-6 c.) CnH2n+2 d.) CnH2n-2
18.) Is a property of fuel in which fuel composition analysis is given on a mass basis: the relative amounts of moisture
content, volatile matter, fixed carbon & ash. a.) ultimate b.) proximate c.) priori d.) posteriori
19.) It is the calorific or heating value obtained when the water in the products of combustion is in liquid state
a.) Higher HV b.) Lower HV c.) Gross Calorific value d.) Net calorific value
20.) It is the calorific or heating value obtained when the water in the products of combustion is in vapor state
a.) Higher HV b.) Lower HV c.) Gross Calorific value d.) Net calorific value

Part II. Problem Solving.Write theCombustion Reactions (Please write your solution on your paper ) 60%
A.) 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
(19.52 kg air/kg fuel; excess air (e) = 13.235 %; (c.) t= 56.59⁰C)
B.) 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 (30.561; 0.272; 1.0549; 0.237)

PreparedBy: David D. Parial,MA,PME,PhD-Mat Units(Instructor) RecommendingApproval:Rem N. Laodeno, DEng’g (Dept.Chair)

Approved: Benjamin D. Varela, PhD (Dean, COEngineering)