Bab 3 Kimia Bahan Bakar

5/20/2018 Bab 3 Kimia Bahan Bakar

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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/Exothermic


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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/Oxygen


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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/Carbon


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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/Hydrocarbon


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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_dioxide


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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_equation


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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!

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