Chapter-2

Principal Fuels

Origin

2.3 Fossile Fuels: - Coal Plant Life

- Biomass Plant Life

- Oil Marine Life

- Oil shale Marine Life

- Tar sand Marine Life

- Natural gas Plant and Marine life

Carbohydrates Hydrocarbons Pressure + Heat

Cx (H2O)y ----------------------------------------→ CxHy

Hydrocarbons

Aliphatic

(Chain Compounds)

Alkenes Alkynes Alkanes

Alicyclic

(Ring Compounds)

Aromatic

(Ring Compounds)

Saturated Unsaturated

Alkanes, or

Paraffin Series: CnH2n+2

Methane CH4 Usually, a liqified mixture of those gases is called:

Ethane C2H6 Liquified Natural Gas(LNG)

Propane C3H8 Usually, a mixture of those gases is called:

Liquified Petroleum Gas (LPG) Butane C4H8

. and so on until n = 8 then we reach liquids

.

Heptane C7H16

Octane C8H18 Most of the gasoline is comparable to this.

All are saturated compounds

Appendix I shows reactant properties

Alkenes, or

Olifenes Cn H2n

Ethylene C2 H4

Propylene C3 H6

Butene C4 H8

All are un-saturated compounds

Alkynes, or acetylene series, which are un-saturated

compounds

Alicyclic, which are saturated compounds

Aromatic

Standard Fuels:

Octane Number

For a fuel which exhibits knocking at a certain

compression conditions, it is the ratio of iso-octane in a

mixture of octane and natural heptane such that knocking

properties of the mixture are the same as that given by

that fuel.

Cetane Number

The same, but for Diesel fuel.

Cooperative Fuel Research Engine (CFR)

A variable compression engine that is used to measure

Octane and cetane Number.

Compression ratio may be varied from 4:1 to 14:1.

Reports mass fractions of Reports mass fractions of First 2 columns

Carbon, C Moisture, M of App. E

Hydrogen, H2 Ash, A

Oxygen, O2

Nitrogen, N2

Sulfur, S

HHV, kJ/kg

First 3-8 columns of App. E

Coal Analysis

Ultimate

Moisture and ash-free, or

Dry, ash-free

As-received, or

As-burned, or

As- mined

Proximate

Fixed Carbon, FC

Volatile Matter, VM

Higher Heating Value (HHV)

It is the energy obtained from burning a fuel if the combustion products

are cooled down to the temperature at which water vapor coming out of

the combustion condenses.

For example, if 1 kg of coal is burned on a dry, ash-free basis, its

components ( C, H2, O2 and N2) have higher percentages because they

are free of moisture and ash.

If it is burned on as- burned (or as- received or as- mined) basis, those

percentages are less. Therefore, in order to convert from dry, ash-free to

the other basis multiply by a factor of (1-M-A), called the MULTIPLIER.

Hence, As-burned mass fraction = Dry, ash-free mass fraction × (1-M-A)

Similarily, As burned HHV = Dry, ash-free HHV × (1-M-A)

HHV may be approximately obtained by Dulong's Formula:

HHV = 33,950 C + 144,200 ( H2 - O2/ 8 ) + 9400 S (kJ/kg) (2.4)

If the combustion products are not cooled down to the temperature at

which water vapor coming out of the combustion condenses, the lower

heating value (LHV) is obtained.

HHV – LHV = 2,400( M + 9 H2) (kJ/kg) (2.3)

Example problem 2.1.a

Estimate the as-burned ultimate analysis, evaluate the HHV from the

analysis and from Dulong's formula and find the error% incurred and find

the LHV for Lackawanna, Luzerne County, Pennsylvania coal.

Solution

From App. E, p. 513, find

Dry, Ash-free ultimate analysis As burned ultimate analysis

M = 2.0, A = 6.0

C = 93.5 C = 86.0

H2 = 2.6 H2 = 2.4

O2 = 2.3 O2 = 2.1

N2 = 0.9 N2 = 0.8

S = 0.7 S = 0.6

Multiplier = 1 – M – A = 0.92

HHV = 35,120 kJ/kg HHV = 32, 310 kJ/kg

Which is considered as an experimental value.

Example problem 2.1.a Solution cont'd.

Using Dulong's formula:

HHV = 33,950 (0.86) + 144,200 ( 0.024 - 0.021/8 ) + 9400 (0.006)

= 32,336 kJ/kg, which is considered a theoretical value.

Percent Error in HHV = (Experimental – Theoretical) / Experimental

= (32,310 – 32,336)(100) / (32,310) = - 0.08 %

Note that the minus sign is neglected and the absolute value is taken.

The LHV is obtained from the Eqn.2.3:

HHV – LHV = 2,400( M + 9 H2)

LHV = HHV - 2,400( M + 9 H2)

= 32,310 – 2400( 0.02 + 9 0.024)

= 31,744 kJ/ kg.

Biomass Fuels

Defined as: matter which has recently been produced directly or

indirectly from the photosynthesis process.

They include: Wood and plants

Animals that consume plants

Organic wastes such as garbage, sewage sludge …etc.

Heating values are shown in Appendix C

Classification of biomass fuels are shown at the end of Appendix C.

A diagram of small starved air refuse combustion system is shown in Fig.

2.2.

2.3.5 Petroleum

These are fuels that remain in the distillation column in petroleum

refineries after the removal of LPG and gasoline. They are classified in 6

classes according to the following properties:-

1. Specific gravity

Defined as: Density of oil at 60 oF / Density of water at 60

oF

Expressed as Sp.gr.(60/60oF)

2. API gravity

Related to the Sp. Gr. by the expression:

oAPI = [141.5 / Sp.Gr.(60/60

oF)] – 131.5

Which ranges from 5 to 50, (the higher the lighter.)

The 6 grades of fuel are:

Table of fuel oil grades and specifications

Oil Grade

No. 1

No. 2

No. 3

No. 4

No. 5

No. 6

API gravity

> 35

> 26

Commercial

name

Kerosene Diesel Fuel

Oil

Fuel

Oil

Bunker C

Sulfur

content %

S˂ 0.5

S˂ 1.0

Local

Sulfur

Content, %

S = 1.5

S = 4

3. Heating value

Usually, the HHV is reported because oil has a much less moisture

content than coal.

Heating value is related to API gravity through the following empirical

Equation:

HHV = 17,645 + 54 × oAPI, Btu/lb

HHV – LHV = 1032 (M + 9H2O ), Btu/lb

Fig. 2.6 gives HHV, Sp. Gr., density and total heat of combustion in

terms of API gravity for petroleum derivatives.

4. Flash point

Minimum temperature at which the fuel vapor ignites. If the

ignition flame is taken away, the flame goes off. (See App. G)

For example, For kerosene it equals 130 oF (54.4

o C)

For Diesel: 78- 80 o C

5. Fire point

Minimum temperature at which the fuel vapor burns. The flame

is sustained even if the ignition flame is removed.

For example, For kerosene it equals 78 - 80 o C

For Diesel: 80 - 90 o C

6. Pour point

Minimum temperature at which the fuel flows

7. Viscosity

8. Sulfur content

9. Vanadium content

10. Specific heat

This is important to calculate energy required to heat-up the fuel.

It ranges from about 1.67 to 2 kJ/kg oC.

11. Thermal expansion coefficient

This is important to calculate the actual volume of fuel when

purchased.

For example, if one buys a fuel at a temperature t that is different

from the base temperature to and volume Vo at which the fuel was

made, the volume becomes:

Vt = Vo [ 1 + 0.0004 ( t – to) ]

And if the base temperature is 20 oC, for example, then the density

of the fuel ρo at that temperature becomes

ρt = ρ20 [ 1 - 0.00073 ( t – 20) ]

Solve Problem 2.5 in the book, with the correction that the

gravity given fuel is oAPI and not specific gravity.

Gaseous Fuels

Most common is Natural Gas,

A mixture of Methane, CH4, HHV = 36,730 kJ/m3 = 986 Btu/ft

3

and Ethane, C2H6, HHV = 64,910 kJ/m

3 = 1742 Btu/ft

3

LPG:

A mixture of 25% propane, C3H6 HHV = 106,867 kJ/m3 = 3029 Btu/ft

3

and 75% butane, C4H10

Note that the HHV on volumetric basis increases with the increase of

carbon ratio in the compound, and visa-versa on mass basis.

Manufactured Gas: Town Gas

Coal + heat = Coke + VM ; HHV = 550 – 600 Btu/ft3

Water Gas

Coal + H2O+heat = H2 + CO; HHV = 250 – 325 Btu/ft3

Producer Gas

Coal + H2O + O2 = H2 + CO + N2 + CH4 + C2H6; HHV = 100 – 180 Btu/ft3

Mixtures of Gases

The HHV for a mixture of gases may be calculated if the HHV for each

component is known using the relationship:

(HHVv) Tr, Pr = Σ (HHVv)i, Pr, Tr × Vi

Where:

HHVv is the higher heating value of the mixture on volumetric basis,

(HHVv)i, Pr, Tr is the higher heating value of each component of the mixture

at a reference pressure and temperature pr and Tr

Vi is the mole (volumetric ) fraction of the component.

If the (HHVv) Tr, Pr is known and it is required to calculate it at any other

pressure and temperature, the following relation may be used:

(HHVv) T, P = (HHVv) Pr, Tr × p Tr / pr T

To convert (HHVv) to (HHVm), divide by ρ, where ρ = p × MW / Ru T,

Where MW is the Molecular Weight (or Molar Mass) of gas mixture,

MW = Σ MWi × Vi

Example Problem 2.10