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KIMIA BAHAN BAKAR
Combustion or burning is a complex sequence ofexothermicchemical reactions between a fueland anoxidantaccompanied by the production of heator bothheatand lightin the form of either a glow or flames.
Chemical reaction equation of combustion:
Example: Fuel: Hydrocarbons (C and H)
Oxidant: air (oxygen and nitrogen)
Combustion product gases: CO2or CO and H2O
Fuel Combustion
productgases
HeatOxidant++
Combustion of wood (cellulose) thatproduce flame, heat and ash/carbon
http://en.wikipedia.org/wiki/Exothermichttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Oxidanthttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Flamehttp://en.wikipedia.org/wiki/Image:Et_baal.jpghttp://en.wikipedia.org/wiki/Flamehttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Heathttp://en.wikipedia.org/wiki/Oxidanthttp://en.wikipedia.org/wiki/Fuelhttp://en.wikipedia.org/wiki/Exothermic5/20/2018 Bab 3 Kimia Bahan Bakar
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In a complete combustion reaction, a compound reacts withan oxidizing element, such as oxygenor fluorine, and theproducts are compounds of each element in the fuel with the
oxidizing element. For example:CH4+ 2O2CO2+ 2H2OCH2S + 6F2CF4+ 2HF + SF6
A simpler example can be seen in the combustion ofhydrogenand oxygen, which is a commonly used reactionin rocket engines:
2H2+ O22H2O(g) + heatThe result is water vapor
In the large majority of the real world uses of combustion,
the oxygen (O2) oxidant is obtained from the ambient airand the resultant flue gasfrom the combustion will containnitrogen:
CH4+ 2O2+ 7.52N2CO2+ 2H2O + 7.52N2+ heat
When air is the source of the oxygen, nitrogen is by far the
largestpart of the resultant flue gas.
http://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Fluorinehttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Flue_gashttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Flue_gashttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Fluorinehttp://en.wikipedia.org/wiki/Oxygen5/20/2018 Bab 3 Kimia Bahan Bakar
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In reality, combustion processes are never perfect orcomplete. In flue gases from combustion of carbon(as incoalcombustion) or carbon compounds(as in combustion ofhydrocarbons, woodetc.) both unburned carbon (as soot)and carbon compounds (COand others) will be present.Also, when air is the oxidant, some nitrogen will be oxidizedto various nitrogen oxides(NOx).
In reality, combustion processes are never perfect orcomplete. In flue gases from combustion of carbon(as incoalcombustion) or carbon compounds(as in combustion ofhydrocarbons, woodetc.) both unburned carbon (as soot)
and carbon compounds (COand others) will be present.Also, when air is the oxidant, some nitrogen will be oxidizedto various nitrogen oxides(NOx).
http://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Compoundhttp://en.wikipedia.org/wiki/Hydrocarbonshttp://en.wikipedia.org/wiki/Woodhttp://en.wikipedia.org/wiki/Soothttp://en.wikipedia.org/wiki/Carbon_monoxidehttp://en.wikipedia.org/wiki/Nitrogen_oxideshttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Compoundhttp://en.wikipedia.org/wiki/Hydrocarbonshttp://en.wikipedia.org/wiki/Woodhttp://en.wikipedia.org/wiki/Soothttp://en.wikipedia.org/wiki/Carbon_monoxidehttp://en.wikipedia.org/wiki/Nitrogen_oxideshttp://en.wikipedia.org/wiki/Nitrogen_oxideshttp://en.wikipedia.org/wiki/Carbon_monoxidehttp://en.wikipedia.org/wiki/Soothttp://en.wikipedia.org/wiki/Woodhttp://en.wikipedia.org/wiki/Hydrocarbonshttp://en.wikipedia.org/wiki/Compoundhttp://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Nitrogen_oxideshttp://en.wikipedia.org/wiki/Carbon_monoxidehttp://en.wikipedia.org/wiki/Soothttp://en.wikipedia.org/wiki/Woodhttp://en.wikipedia.org/wiki/Hydrocarbonshttp://en.wikipedia.org/wiki/Compoundhttp://en.wikipedia.org/wiki/Coalhttp://en.wikipedia.org/wiki/Carbon5/20/2018 Bab 3 Kimia Bahan Bakar
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Types ofcombustion
Rapid combustion
Slow combustion
Complete combustion
Incomplete combustion
Turbulent combustion
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Complete combustion:In complete combustion, the reactant will burn in oxygen,
producing a limited number of products. When a hydrocarbonburns in oxygen, the reaction will only yield carbon dioxideand water. When a hydrocarbon or any fuel burns in air, thecombustion products will also include nitrogen. Whenelements such as carbon, nitrogen, sulfur, and iron areburned, they will yield the most common oxides. Carbon will
yield carbon dioxide. Nitrogen will yield nitrogen dioxide.Sulfur will yield sulfur dioxide. Iron will yield iron(III) oxide. Itshould be noted that complete combustionis almostimpossible to achieve. In reality, as actual combustionreactions come to equilibrium, a wide variety of major and
minor species will be present. For example, the combustion ofmethane in air will yield, in addition to the major products ofcarbon dioxide and water, the minor product carbon monoxideand nitrogen oxides, which are products of a side reaction(oxidation of nitrogen)
http://en.wikipedia.org/wiki/Hydrocarbonhttp://en.wikipedia.org/wiki/Nitrogen_dioxidehttp://en.wikipedia.org/wiki/Sulfur_dioxidehttp://en.wikipedia.org/wiki/Iron(III)_oxidehttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Iron(III)_oxidehttp://en.wikipedia.org/wiki/Sulfur_dioxidehttp://en.wikipedia.org/wiki/Nitrogen_dioxidehttp://en.wikipedia.org/wiki/Hydrocarbon5/20/2018 Bab 3 Kimia Bahan Bakar
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Incomplete combustion:Incomplete combustion occurs when there isn't enoughoxygento allow the fuel (usually a hydrocarbon) to reactcompletely with the oxygen to produce carbon dioxide andwater, also when the combustion is quenched by a heat sinksuch as a solid surface or flame trap. When a hydrocarbonburns in air, the reaction will yield carbon dioxide, water,carbon monoxide, pure carbon (soot or ash) and various
other compounds such as nitrogen oxides.
Chemical equation of Combustion
The complete chemical equationfor stoichiometricburningof hydrocarbonin oxygenis as follows:
(g)O2H2y
(g)2COx
(g)2O
4
y4x
(g)yHxC
http://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Carbon_monoxidehttp://en.wikipedia.org/wiki/Nitrogen_oxideshttp://en.wikipedia.org/wiki/Chemical_equationhttp://en.wikipedia.org/wiki/Stoichiometryhttp://en.wikipedia.org/wiki/Hydrocarbonhttp://en.wikipedia.org/wiki/Hydrocarbonhttp://en.wikipedia.org/wiki/Stoichiometryhttp://en.wikipedia.org/wiki/Chemical_equationhttp://en.wikipedia.org/wiki/Nitrogen_oxideshttp://en.wikipedia.org/wiki/Carbon_monoxidehttp://en.wikipedia.org/wiki/Carbon_dioxide5/20/2018 Bab 3 Kimia Bahan Bakar
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Example for propane combustion:Propane: C3H8 (x=3; y=8)
; or(g)O2H28
(g)2CO3
(g)2O
412
(g)8H3C 8
(g)O2H4
(g)2CO3
(g)2O
(g)8H3C 5
The complete chemical equationfor stoichiometricburning
of hydrocarbonin airas oxidant agent (oxidizer) bysimplification is as follows:
where: a = x + y/4
In that combustion chemical equation, air assumed consistof 21%v of oxigen (O2) and 79%v of nitrogen (N2), i.e.,that for each mole of O2in air, there are 3.76 mole of N2.
2N3.76aO
2H2y
2COx
23.76N
2OayHxC
http://en.wikipedia.org/wiki/Chemical_equationhttp://en.wikipedia.org/wiki/Stoichiometryhttp://en.wikipedia.org/wiki/Hydrocarbonhttp://en.wikipedia.org/wiki/Hydrocarbonhttp://en.wikipedia.org/wiki/Stoichiometryhttp://en.wikipedia.org/wiki/Chemical_equation5/20/2018 Bab 3 Kimia Bahan Bakar
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Example for propane combustion:Propane: C3H8 (x=3; y=8)
2N18.8O2H42CO323.76N2O58H3C
Term and its formula in complete combustion byoxidizer of air:Stoichiometric air-fuel ratio, (A/F)stoic
fuelMW
airMW1
4.76a
stoicfuelmairm
stoicFA
Equivalence ratio,
stoicAFAF
FA
stoicFA
Equivalence ratio, , is commonly used to to indicatequantitively wheter a fuel-oxidizer mixture is rich, lean, orstoichiometric. > 1 forfuel-richmixtures, < 1 forfuel-leanmixtures, and = 1 forstoicmixtures
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Percent stoichiometric air
%1001
airexcess%
Percent excess air
%100
airtricstoichiome%
Example:A small, low-emission, stationary gas-turbine engineoperates at full load (3950 kW) at an equivalence ratio of0.286 with an air flowrate of 15.9 kg s-1. The equivalentcomposition of the fuel (natural gas) is C1.16H4.32.
Determine thefuel mass flowrateand the operating air-fuel ratiofor engine!Data:Ar C=12.011; Ar H=1.008; Ar O=15.999; Ar N=14.007
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Thermochemistry of Combustion
Panas
Panas sebagai bentuk energi yang mengiringi reaksi pembakaran
merupakan bentuk energi yang bersifat tidak permanen eksistensinyadalam suatu benda. Oleh karena itu, panas dikatakan sebagai energi
dalam transit dari satu benda ke benda yang lain, atau antara sistem
dengan lingkungannya. Dalam pengertian termodinamika , panas
tidak pernah dianggap tersimpan di dalam suatu benda.
Kapasitas panas
Kapasitas panas dari suatu bahan didefinisikan sebagai jumlah panas
yang diperlukan untuk menaikkan temperatur bahan tersebut sebesar
satu derajat. Tiap-tiap bahan karena memiliki sifat-sifat yang spesifik,
maka kapasitas panas setiap bahan juga akan berbeda-beda
Rumusan dasar dari kapasitas panas adalah sebagai berikut:
dT
QC
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Karena pemanasan dapat dilakukan dengan dua cara
kemungkinan, maka dikenal dua jenis kapasitas panas, yaitu:
1. Kapasitas panas pada volum konstan, atauV
VdT
QC
2. Kapasitas panas pada tekanan konstan, atau
dT
dUCV
P
PdT
QC
dT
dHCP
Kapasitas panas bahan dalam bentuk lain dapat dinyatakan
untuk tiap massaatau tiap molebahan, dikenal: kapasitas
panas jenis(specific heat capacity), c, dan kapasitas panas
jenis molar(molarspecific heat capacity),c
Data-data tentang kapasitas panas dari gas, cair dan padatan
dapat ditemui di banyak literatur, umumnya dinyatakan sebagai
fungsi temperatur. Untuk gas, kapasitas panas dirujukkan untuk
gas ideal.
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2
2
1
CC gz
g
gUWQ
2
2
1
CC gzg
g
UdWQ
Hukum I Termodinamika untuk sistem tertutup:
Hukum I Termodinamika untuk sistem terbuka:
2
2
1
CC gz
g
gHWQ
2
2
1
CC gz
g
gHdWQ
Untuk kasus reaksi pembakaran, Ep dan Ek dapat diabaikan
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Termokimia Panas Reaksi Kimia (contoh: reaksi pembakaran)
Panas reaksi secara umum didefinisikan sebagai panas yang
menyertai keberlangsungan suatu sistem reaksi kimia.
Berdasarkan Hukum I Termodinamika untuk proses alirdenganmengabaikan perubahan energi kinetik dan energi potensial dan
tidak ada kerja mekanik eksternal, dan untuk proses non-aliryang
dioperasikan pada tekanan konstan, diperoleh bahwa Q = H.
Analisa ini yang menjadi dasar umum bahwa kuantitas panas yang
meyertai reaksi kimia (panas reaksi) sama dengan perubahanentalpidari reaksi tersebut.
Panas reaksi standar(HRo) adalah panas reaksi kimia yang
nilainya merupakan selisih antara total panas pembentukan dari
komponen produk reaksi terhadap total panas pembentukan dari
komponen reaktan pada tekanan 1 atm dan temperatur 25o
C.Oleh karena itu panas reaksi standar dirumuskan sebagai berikut:
tan,, reak
o
ifiproduk
o
jfj
o
R HnHnH
Hfoadalah panas pembentukan standar dan ni,jadalah jumlah mol
stokiometri dari tiap komponen yang terlibat dalam reaksi
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Pada reaksi-reaksi aktual/nyata, sering reaksi kimia tidak hanya
dilangsungkan pada temperatur 25 oC. Untuk menghitung panas
reaksi (HR) pada kasus reaksi seperti ini, dikembalikan pada konsep
bahwa perubahan entalpi merupakan fungsi keadaan, sehingga
antara temperatur aktual reaktan dan temperatur aktual produk dapat
dihubungkan dengan temperatur standar.
Reaktan, 25 oC Produk, 25 oC
Produk, T2Reaktan, T1
(1)
(2) (3)
(4)
H1, 2
HRo
H3, 4
HR
Rumusan perhitungan: HR= H1, 2+ HRo+ H3, 4
dTreaktanCHT
P298
2,1
1
dTprodukCHT
P2
298
4,3
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Contoh soal:1. How much heat is required when 10,000 kg of CaCO3is
heated at atmospheric pressure from 50oC to 880oC?C
Pof CaCO
3:
with R = 8.314 kJ kmol-1K-1.(Ar Ca=40.08; Ar C=12.01; Ar O=16)
2. What is the standard heat combustion of n-pentane gas at
25oC if the combustion product are H2O(l)and CO2(g)?Hf
o n-pentane(g) : -146,760 J mol-1
Hfo H2O(l) : -285,830 J mol
-1Hf
o CO2O(g) : -393,509 J mol-1
3. What is the maximum temperature that can be reached bythe adiabatic complete combustion of methane with20%mole exess air? Both the methane and the air enterthe burner at 25oC.
2T5103.12T32.637x1015.572RPC
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Soal Kuis
Sejumlah gas C3H8telah dibakar habisdengan udara (pada STP). Jumlah gasCO2dan H2O yang terbentuk 7,5 liter.
Bila kadar O2dalam udara 18,5%(%volum), hitunglah volum gas C3H8yang telah dibakar dan volum udara
yang diperlukan!