Fuels

Solid fuels

(“Fuels and Fuel Technology”W. Francis and M.C. Peters)

Liquid fuels

Gaseous fuels

(these slides include some notes by Mário Nina)

Diagram of fuels

Solid fuels have the lowest ratio of hydrogen to carbon and may contain an appreciable fraction of oxygen

Despite the wide variety of solid fuels, their composition is located in typical bands

Coal presents the largest reserves of fossil fuels and are more distributed than any other reserves

Classification of solid fuels

Solid fuels can be classified into natural fuels and manufactured(or artificial) fuels

Natural solid fuels

wood

peat

lignitebituminous

(often peat and lignite are considered vegetable coal, while bituminous and

Manufactured solid fuels

hard coalbituminous

anthracites

wood charcoal

peat charcoal

lignite coke

coal briquettes

coke (from coal)

low-temperature

medium-temperature

high-temperature

bituminous and anthracite coal are considered mineral coal)

Analysis of solid fuels

Composition is characterized by two types of analysisUltimate analysis

Proximate analysis

Ultimate analysis - mass fraction of the elements present:

C, H, O, N, S and ashes (nowadays it includes Cl, Ca, ... )

Nitrogen (NP1012), Sulphur (ASTM D3177)

Proximate analysis - mass fractions according to the decomposition:

Nitrogen (NP1012), Sulphur (ASTM D3177)

Moisture (determined at 110ºC)

Volatile matter (gases released by heating to 950ºC) - NP3423

Fixed carbon (computed from the difference to the remaining values)

Ash (residue obtained when fuel is burned in air) - NP1019

Note: NP – Portuguese Standard

Ultimate analysis

The composition of the fuel is characterized by:

Moisture

Volatile matter

Fixed carbon

Ash content

Bases of reporting

The following bases are generally used for reporting analysis of solid fuels:

As received (Ar - including moisture and ash)

Dry basis (Db - dry base by excluding moisture)

Dry ash free (Daf - excludes moisture and ash)

Usually the proximate analysis is Ar and the ultimate analysis is Daf

The conversion between bases is straightforward, e.g.

xCAr = xCDaf * (1 – xAshAr - xHumAr)

Dry ash free (Daf - excludes moisture and ash)

xVolDb = xVolAr/(1 - xHumAr)

(7.1)

(7.2)

Characteristics of wood

Wood is a solid fuel with less maturity and can be considered renewable

Moisture in the trees varies between 25 to 50%,and after drying at open air is 10 to 15%

The ash content is low (0,5%)

Wood consists of cellulose (including hemi-cellulose) (51-60%), lignin (44-32%), resins (<2%), and water soluble (3-6%), apart from moisture

The calorific value ranges from 17 to 19 MJ/kgAr, with cellulose being 16 MJ/kg and resins 32.5 MJ/kg

Wood can be burned directly or converted to gas or charcoal

Combustion of woodThe combustion of wood begins with heating, release of moisture, and pyrolysis (which releases volatile matter)

6 CO + 6 H2 + 6 O2 → 6 CO2 + 6 H2O

The combustion of wood may occur (mainly!) with flame or at the air-solid interface. This depends on the temperature at which pyrolysis occurs

Pyrolysis of cellulose:

C6H12O6 → 6 CO + 6 H2

k1

6 C + 6 H2O + 6 O2 → 6 CO2 + 6 H2OC6H12O6 → 6 C + 6 H2Ok2

predominance of smouldering

predominance of combustion with flame

k1

k2

k

T

Carbonization and gasificationof wood

The combustion of wood can be inefficient and a large amount of volatile matter may not be burned. Moreover the use of gaseous fuel is generally more versatile

The wood heated at low temperature (<350 ° C) under oxygen deficiency gives rise to approximately 40% of charcoal and 12 to 17% volatile matter gives rise to approximately 40% of charcoal and 12 to 17% volatile matter (≈ 30 MJ⋅kg-1)

The charcoal (already with very little volatile matter) burns without such losses. The absence of flame can also be an advantage

The wood heated at high temperature (1000 to 1200 ºC) under oxygen deficiency leads to approximately 20% of charcoal and high quantity (≈ 2,75 m3⋅kg-1) of volatile matter (carbon monoxide, carbon dioxide, nitrogen, hydrogen, and methane)

Formation of coal

Peat

Lignite

Sub-bituminousBituminous

Anthracite

Type and composition of coals

Type of coal Carbon

% mass

Hydrogen

% mass

Age (years*106)

Anthracite 93 - 95 3.8 - 2.8 210 - 250

Carbonaceous 91 - 93 4.25 - 3.8 210 - 250

Bituminous 80 - 91 5.6 - 4.35 150 - 180

Sub-bitum. 75 - 80 5.6 - 5.1 60 - 100

Lignitous 60 - 75 5.7 - 5.0 20 - 60

Peat 50 - 60 6.1 - 5.8 1

Wood* 46 - 51 6.2 - 5.9 0

* Wood is considered renewable (non-fossil)

Typical compositions of solid fuelsFuel

Composition in % mass (dry base, ash free)

Carbon Hidrogen Nitrogen Sulfur Oxigen Volatile

matter Ash

Cedar 49.0 6.4 − − 44.6 0.4

Pine 52.6 6.1 − − 41.3 0.1

Red pine 53.6 5.9 0.1 − 40.4 0.2

Birch 49.9 6.5 − − 43.6 0.3

Quercus (Oak) 50.5 6.6 − − 42.9 0.2

Populus (Poplar) 51.9 6.3 − − 41.8 0.7

Wood (typical) 52.2 6.1 0.1 − 41.6 79.3 1.5

Peat 57.5 5.5 1.6 0.4 35.0 68.0 8.0

Lignite 74.8 4.8 1.2 1.2 18.0 46.8 29.0 Lignite 74.8 4.8 1.2 1.2 18.0 46.8 29.0

Semi-bituminous coal 72.5 4.8 1.5 4.0 17.2 47.5 17.5

Bituminous coal (high content of

volatile matter)

85.0 5.6 1.5 1.4 6.5 36.7 7.0

Bituminous coal

(medium content of

volatile matter)

89.0 5.0 1.7 0.6 3.7 22.9 3.0

Bituminous coal

(low content of

volatile matter)

89.8 5.0 1.6 0.8 2.8 16.8 5.2

Anthracite A 89.8 4.0 0.9 2.6 2.7 8.7 19.5

Anthracite B 93.0 3.7 1.3 0.7 1.3 7.5 8.0

Charcoal 93.0 2.5 0.8 0.1 3.6 10.0 1.0

Coke 93.0 3.0 1.0 1.0 2.0 8.0 7.0

Mass balance

The mass balance equations are generally made in mass basis

1kg Caol + s kg Air -> (1+s) kg Products

(xC C + xH H + xN N + xS S + xO O + xAsh Ash + xHum H2O) + λ xOstoic (O2 +

+ 3,31 N2 ) → xC CO2 + (xH + xHum) H2O + xS SO2 + (xN + 3,31 λ xOstoic) N2 + 2 → 2 2 2 stoic 2

+ xAsh Ash + (λ -1) xOstoic O2

Note that 3,31 = 1 – 0,232

0,232

The equation can also be written without ash (which is a solid product), and can also be written on a molar basis for air and gaseous products.

where xOstoic= xC + xH + xS - xO32

32

32

12

16

2

(7.3)

(7.4)

Calorific value

The calorific value can be expressed in different mass bases (Ar, Daf, Db), with regard to the LHV and HHV at constant volume or pressure

Its determinations follows the standard ASTM D2205

The enthalpy of formation of coal is close to zero and the calorific value can be calculated from the enthalpy of formation of combustion productsbe calculated from the enthalpy of formation of combustion products

There are several approximate expressions

HHV ~ 33,8 xC + 144,3 (xH - xO/8) + 9,4 xS (Dulong)

HHV ~ 34,1 xC + 132,3 [xH - (xO + xN)/11] + 6,8 xS – 1,5 xAsh

(Mason and Gandhi, 1983)

(in MJ⋅kg-1 and dry basis. The second includes the effect of ash xAsh)

(7.5)

(7.6)

Classification of coals

In order to characterize the coals and reconciling various nomenclatures there are several classifications

ASTM (USA)

NCB (UK)

EC for bituminous coals

Seyler’s classification

Besides rank (maturity or age) and composition, the rankings include:

Swelling index (variation of the diameter in the pyrolysis)

Agglomerating character (it also indicates plasticity)

Coking and Caking indexes (behaviour of coal according to the rate of

heating – slow and fast, respectively)

ASTM classification

NCB classification

The National Coal Board uses numeric codes associated with volatile content, and coking power of clean materialclean material

This concept is extended in the classification of the European Economic Community

E.C.E. classification

Diagrams with analysis

ProximateUltimate

A – anthracite, B – semi-anthracite. Bituminous (1% to 5% moisture),

C – bituminous agglomerating, D – bituminous partialy agglomerating,

E – bituminous non-agglomerating, F – semi-bituminous (3% to 20%

moisture), G – lignitous (12% to 25% moistur),

H – peat (20% a 35% de humidade).

Diagram of Seyler (adapted)

Seyler’s classificationRank Carbon (%)

Hidrogen

(%)

Volatile

matter (%)

CV

(MJ/kg)

B.S.

Swelling

index

Anthracite > 93.3 3.0-3.8 5-10 32.9 1

Carbonaceous

Semi-anthracite 93.3-91.2

3.8-4.4 10-14 37.4 1

Semi-bituminous 4.4-5.0 14-20 33.7 3.5

Bituminous

Meta- 91.2-89.0 4.4-5.4 20-28 33.5 9

Orto- 89.0-87.0 4.7-5.6 28-31 33.1 9

Para- 87.0-84.0 4.9-5.7 31-36 32.0 6

Lignitous Lignitous

Meta- 84.0-80.0 5.0-5.7 36-42 30.5 2

Orto- 80.0-75.0 5.0-5.7 42-49 28.4 1

Lignite < 75.0 5.0-5.7 49-59 25.0 1

The bituminous coal may distort during the volatilization

The volatile matter content is given by:

VM = 10,61 xH – 1,24 xC + 84,15

ln(VM) = 0,23364 xH – 0,02706 xC + 2,579

except for anthracite:

Other important properties

Temperature of melting of ashes

at different stages: IDT, Hemi, Fluid

Indexes of grading Hardgrove (HGI) and of abrasiveness

Indexes of Fouling and Slagging Indexes of Fouling and Slagging

(deposit formation based on the ashes)

Apparent density (1.2 – 1.8 103 kg⋅m-3)

Porosity (varies during combustion)

Internal area (~ 100 m2⋅g-1)

Other propertiesFuel

Higher

CV

(MJ/kg)

Initial

moisture

(% mass)

Temperature

range for

melting of

ashes (K)

Swelling

index

Hardgrove

index

Apparent

density

(kg/m3) (A/F)stoich, mass

#

Cedar 19.62

Non

applicable

5.92

Pine 20.72 20

1300-1600 Non

applicable

6.37

Red pine 21.07 50 6.46

Birch 20.18 6.10

Quercus (Oak) 20.53 8 6.23

Populus (Poplar) 20.89 6.34

Wood (typical) 20.88 48 6.31

Peat 20.93 93 53-79 700-1100** 7.0 Peat

Lignite 27.80 47 1310-1480 66-82 400 9.48

Semi-bituminous coal 28.97 26 100 640-800 9.25

Bituminous coal

(high content of volatile matter)

35.82 1.5 1830 + 3-7.5 57-62 11.42

Bituminous coal

(medium content of

volatile matter) 36.37 3.5 1610 + 8.5 44-54 11.80

Bituminous coal

(low content of volatile

matter) 26.68 3.8 8.5-9 68 11.93

Anthracite A 34.66 6.3 1520

Non

applicable

25-35 11.59

Anthracite B 36.28 1.0 11.91

Charcoal 34.75 2.0 300-600 11.40

Coke 34.33 8.0 > 1600 370-510 11.64

Variation of properties

Apparent density Specific internal area

Coal preparation

The coal preparation is intended to obtain a fuel with the appropriate characteristics to the equipment and application concerned

separation of coal types (hardsand brights)

screening of different sizes

hand-picking (D > 100 mm)

crushing of larger sizes (avoiding creating very small particles)

cleaning (to remove inorganic impurities)

drying (when small-sized coal are washed

blending (to modify properties of a coal

Storage of coal

Coals deteriorate during storage by low-temperature oxidation

Factors affecting deterioration by oxidation:

volatile matter content ⇒ oxidation

surface / volume ratio ⇒ oxidation

ventilation of the coal pile ⇒ oxidationventilation of the coal pile ⇒ oxidation

If the temperature does not exceed 50 ºC for lignite and 80 ºC for the bituminous spontaneous ignition does not occur. However, coal deteriorates:

– decrease in calorific value CV

– decrease in C and H

– increase in O

– reduction in size grading

The critical period for the occurrence of spontaneous ignition is 4 to 5 weeks after extraction. Coal stored for more than six months usually will not be subject to spontaneous ignition

Summary (Fuels and Fuel Technology - W Francis & M C Peters - 1980)

Classification of liquid fuels

Liquid fuels can be classified into natural fuels and manufactured(or artificial) fuels

The natural fuels are essentially those derived from petroleum (though they result from human intervention). They are sometimes classified as light(petrol, oil, diesel, ...) and heavy (heavy fuel-oil and bunker). The former are mainly used in engines, and the latter in boilers and large (and slow) engines

Manufactured liquid fuels

alcohols

biofuels

products from the synthesis of hydrocarbons

products from the synthesis of coal

mainly used in engines, and the latter in boilers and large (and slow) engines

...

Hydrocarbons (and alcohols)Almost all of the liquid fuels are formed by hydrocarbons or oxygenated hydrocarbons

Paraffin (alkanes)

n-alkanebranched-chain

Olefin (alkenes) - paraffin with double bondsOlefin (alkenes) - paraffin with double bonds

Alkyne (alkynes) - paraffin with triple bonds

Naphthenic (cyclo-paraffin)

Aromatic single ring

multiple rings

Petroleum oil and crudePetroleum oil is generally considered to be formed from the decomposition of plants (mostly marine - and also animals) in the near absence of oxygen. Once extracted and with no gas nor solids, is usually called by crude

Typical composition

CarbonHydrogenNitrogen

80 a 89 %12 a 14 %0,3 a 1 %Nitrogen

SulphurOxygen

0,3 a 1 %0,3 a 3 %2 a 3 %

Petroleum oil classification

Paraffinic

Naphthenic

Asphaltic(aromatic)

Fractional distillationCrude oil is distilled in distillation towers. The heavier fractions are distilled again at sub-atmospheric pressures

Gasolina Etileno

Gas and lightfractions

LPG

Ethylene

Gasoline

GAS

Óleo base mineral

(source: BP)Distillation tower

Gasoline

Light gas oil

Heavy gas oil

Lubricating oils

Bitumen

LIQUID

Gas

Gasoline

Kerosine

Light Dieselfuel

MediumDiesel fuel

HeavyDiesel fuel

LightDiesel fuel

Watervapour

Naphtha

Watervapour

C1

to C

4

C1 to C3Fractional distillation at atmospheric pressure and at low pressure

Although much depends on its type (paraffinic, naphthenic, asphaltic), an average crude oil could lead to

20 to 30 % of gasoline

30 to 45 % of intermediate fractions

25 to 50 % of residual fuel oil

The vacuum distillation allows more light and intermediate fractions

Crude

BoylerAtmosphericdestillationtower

BoylerLow-pressuredestillationtower

ResidueButhane Propane

Buthane extraction unit

Propane extraction unit

Cracking processes allow to obtain lighter grades

thermal cracking – heating (≈ 500 º C and 25 bar) of heavy oil above its decomposition temperature, yielding 50 to 70% of low-quality gasoline

thermal reforming – similar to thermal cracking but with heating at higher temperatures, yielding 50 to 70% of gasoline of better quality

catalytic cracking – heating in the presence of a catalyst of aluminium and silica, yielding good quality gasoline

hydrocracking – heating in the presence of catalysts and hydrogen, yielding large amounts of kerosine and diesel fuel

Kerosine – mixture of heavier hydrocarbons than those of gasoline. Used in gas turbines, ram-jets, heating and lighting

Gasoline – mixture of light hydrocarbons, mainly used in internal combustion engines. Additives are added to gasoline to suit the required applications

Types of liquid fuels

Light Diesel fuel – mixture of heavier hydrocarbons than those of kerosine. Light Diesel fuel – mixture of heavier hydrocarbons than those of kerosine. Used in high-speed diesel engines

Medium Diesel fuel – the next mixture. Used in medium-speed diesel engines

Residual fuel – distillation residue. Very viscous (needs pre-heating). It has a high amount of sulphur, and some contain metal compounds (which give rise to adverse reactions). It is used in boilers and in very large (and slow) Diesel engines

Heavy Diesel fuel – the next mixture. Used in low-speed diesel engines

Some typical values for liquid hydrocarbons

FuelMean

composition (approx.)

LHV

(MJ⋅kg-1)Density (kg⋅m-3)

Tboiling (ºC)

(at 1.0 bar)

Kinematic viscosity

(cSt @ 50ºC)

Gasoline C8H15 44.0 750 30 to 200 0.8 (@ 20ºC)

Kerosine C10,5H20 44 800 150 to 300 -

Light Diesel fuel C H 43.2 810 210 to 235 2.5 (@ 20ºC)

Properties

Light Diesel fuel C11H19 43.2 810 210 to 235 2.5 (@ 20ºC)

Medium Diesel fuel C12,3H22 43.0 840 185 to 360 2.8

Heavy Diesel fuel C14,5H25 42.8 900 > 200 8

Residual (fuel-oil) > C14,5H25 40 950 - 180 to 380

Notes: – almost all values given show appreciable variations. Only for Tboiling

variations were presented– references used are varied and not always very coherent, which is mainly

due to the wide variety of hydrocarbon mixtures that these fuels may have

Note that some properties vary very little and others vary widely

Calorific values vary very little (43 to 44 MJ⋅kg-1, except for residual fuels) as well as the stoichiometric mass air/fuel ratio (typically 14.4 to 14.6)

The density also varies little, and only in residual fuels is a little higher, approaching the water’s (which raises major problems in the separation of water from the fuel !)

Important differences are found in volatility (illustrated in the table by the boiling temperature), and viscosity. This is very important in the formation of boiling temperature), and viscosity. This is very important in the formation of drops (and droplets), and therefore in the characteristics of sprays. The more viscous fuel must be preheated for pumping and/or injection

Another important difference is the sulphur content of the fuel, substantially increasing from gasoline (≈ 0.07 wt%) to heavy diesel (≈ 1.7%). The fuel-oils have much higher values (2.5 to 4%)

Another important difference in engines is the ease (or difficulty) of spontaneous ignition. This behaviour is characterized by the Cetane and the Octane Numbers

Alcohols are oxygenated hydrocarbons where a hydrogen atom was replaced by an OH radical

Alcohols

There are many alcohols, but the most common are methanol and ethanol

Methanol is produced as a product of the carbonization of wood, or through a synthesis process 2 H2 + CO → CH3OH

The process yields about 75% methanol and 25% heavier alcohols, which are then separated by fractional distillationare then separated by fractional distillation

Ethanol is obtained from fermentation of sugars, followed by fractional distillation

Methanol and ethanol are good fuel for spark ignition engines. But they have a high latent heat of vaporization, which can cause problems when starting from cold. They are good solvents, so their blend with gasoline provides a cleaner operating engine. However, they cause some problems of corrosion and attack the elastomers.

Other biofuels

The demand for alternative fuels to products derived from petroleum oil has led to development of techniques for production of vegetable oils specifically for combustion (although from ancient times animal and vegetable oils have been used as fuel)

The oils obtained are not compatible with spark ignition engines, but are (up to

These biofuels (particularly biodiesel) are made from oils extracted from plants (sunflower, palm, soybean, rapeseed, ...), using a chemical process called transesterification, which makes the extracted oil to react with an alcohol in the presence of catalysts

The oils obtained are not compatible with spark ignition engines, but are (up to a point) compatible with diesel engines. A great deal of research has been done to allow a mutual adaptation of these oils and diesel engines, mainly for diesel-biodiesel blends

Classification of gaseous fuels

biogas

petroleum

coal

wood gas (distillation or carbonization)

peat gas (distillation or carbonization)

coal gas (carbonization)producer gas (gasification in air)

Natural gas

producer gas (gasification in air)

coal gas (hydrogenation)water gas (gasification in air and steam)Lurgi gas (gasification in O2 and steam)

refinery gas (cracking)oil gas (hydrogenation)oil gas (partial oxidation)oil gas (water gas reaction)

acetylene (reaction of water and carbide)

hydrogen (electrolysis)

Manufactured gas

from coal

frompetroleumand oil shale

Typical composition

CH4 75 to 95 %

Natural gasfrom coal

CH4

C2H6

CO2

93 to 99 %

up to 3 %

up to 4 %

N2 and inerts

up to 4 %

Biogas

CH4

CO2

LHV

≈ 75 %

≈ 25 %

29.6 MJ⋅m-3 (0ºC, 1 bar)

Natural gas from petroleum deposits

4

C2H6

C3H8

3 to 12 %

up to 6 %

BiogasBiogas is obtained from the decomposition of organic matter by bacteriological action in closed digesters, in the absence (or nearly) oxygen. It requires a high moisture content

Organic matter (provided it has high

Pressure can be adjusted with weights

Floating metallic ceiling

above the gasGas output

Pasty manure

Solid waste

Fermentation chamberOrganic matter (provided it has high

moisture content) can be of various origins, either plant or animal, being very common the use of organic manure

In operation the digesters maintain a temperature around 30 to 40 °C

Since the process is not totally anaerobic, the gas has a significant percentage of CO2, but the solid waste is a good fertilizer

Baffle

chamber

Use of Natural Gas

Natural gas has a high calorific value, is uniform in its properties (depending on its source), and contains no harmful impurities (or their content is very low)

Its flame propagation speed is relatively low, requiring special burners

Its flame is not sooty, rendering heat transfer by radiation very difficult.

It has a high octane rating (it is good resisting self-ignition) and it is a good fuel for spark ignition engines (but incompatible with Diesel - except in Dual-Fuel version). It is also a good fuel for gas turbines

This is an advantage when reducing heat loss is a goal, but it is a drawback if heat transfer by radiation is the objective (e.g., boilers); therefore large areas for heat exchange by radiation are required

Natural gas forms explosive mixtures with air (within the Limits of Flammability). Safety devices are required, particularly to allow the starting and stopping the firing systems

However its density is lower than that of the air, so they are easily dispersed in the atmosphere, reducing the risk of formation of explosive mixtures in air

The calorific value per unit volume of natural gas is very low. Thus, in its The calorific value per unit volume of natural gas is very low. Thus, in its storage or transport compressed or liquefied gas is used. In large pipelines it is typically compressed to 60 to 70 bar (at room temperature). The pressures of storage are around 200 to 250 bar. Its density is much higher when liquefied, so that the transport and storage in the liquid phase is very attractive. However the temperature required is very low (≈ -160 °C). Hence, the vessels must be cryogenic. The temperature is kept very low by letting some of the liquid to vaporize. This is indeed a loss of fuel, which should be burned afterwards

Liquefied Petroleum Gases (LPG)The liquefied petroleum gas are hydrocarbons (or mixtures of hydrocarbons) that are gaseous at normal atmospheric pressure and temperature, but that can be stored in the liquid phase at atmospheric temperature at relatively low pressures

They end up having the usual advantages associated with gas together with the high energy density per unit volume associated with liquids

Main components of LPG

The main sources of LPG are natural gas, crude oil, thermal and catalytic cracking, as well as thermal reforming

Propane Propylene N-butane Iso-butane 1-butene

=

Bottled gases

Commercial liquid butane consists mainly of saturated and unsaturated C4

hydrocarbons, with less than 20% of C3 and less than 2% of C5 hydrocarbons

Commercial liquid propane consists mainly of C3 hydrocarbons (propane and propylene if obtained from refinery gases), with less than 5% of C2

hydrocarbons and less than 10% of C4 hydrocarbons

. Evil-smelling organic sulphides may be added to give warning of leaksThey contain low percentages (<0.02%) of sulphur compounds

Propane is often used in pressurized burners

When the rate of evaporation is intense freezing can occur in the depressurization valve (L = 0.22 MJ⋅dm-3 at atmospheric pressure). In this case it is necessary to heat the valves

Propane is an excellent fuel for spark ignition engines (terrible for Diesel) because of its high octane number and low ratio C/H, resulting in low formation of carbon deposits

It is used in oxy-propane torches. The flame temperature is lower than that of oxy-acetylene torches, but is sufficient for many applications. The flame propagation speed is lower than the one for oxygen-acetylene mixtures, hence decreasing the problems of flashback,, and the flammability limits are much narrower (2 to 10% in air compared with 2.5 to 80% for acetylene),

Butane and propane can be added to other gases to increase the calorific value of the mixture

much narrower (2 to 10% in air compared with 2.5 to 80% for acetylene), highly reducing the risk of explosion

Butane is best suited for domestic applications. Propane is more suitable for industrial applications due to its higher storage pressure

Boiling point (atmospheric pressure)

Specific gravity of the liquid (at 15 ºC)

Density of the gas (relative to the air)

HCV

Latent heat of vaporization (atmospheric pressure)

(ºC)

(MJ⋅kg-1)

- 42

0.51

1.5

50.0

0.43

- 6

0.59

2,0

49.3

0.45

Propane Butane

Latent heat of vaporization (atmospheric pressure)

Latent heat of vaporization (atmospheric pressure)

Flammability limits (% of gas in the mixture)

Stoichiometric air

Ignition temperature

Adiabatic flame temperature

Storage pressure (at 20 ºC)

(MJ⋅kg )

(MJ⋅dm-3)

(m3air / m3

gas)

(ºC)

(ºC)

(bar)

0.43

0.22

2.0 to 9.5

23

500

1950

8.6

0.45

0.22

2.0 to 8.5

30

480

1880

2.0

Wobbe index

CV – calorific value

d

CVW =

d – density relative to the air (section 1.4.11 of NP-927)

Gaseous fuels are grouped into families based on the Wobbe index in

accordance with similar combustion characteristics

(7.7)

This classification seeks to group gaseous fuels with similar characteristics of burning. These characteristics are so distinct from family to family that two different families of gases are not interchangeable, and changing from one family to another requires the conversion of the burning equipment

Therefore, equipment, burners, and gas appliances are manufactured specifically for each indicated family of gases

1st Family (town gas) – Wobbe index from 21.5 to 28.7 MJ⋅m-3.Mainly these are gases obtained from coal and, more recently, from gases of similar characteristics produced from gaseous and liquid hydrocarbons. Their main components are hydrogen, methane, carbon monoxide and inert gases C02 and N2.

2nd Family (natural gas) – Wobbe index from 37.1 to 52.4 MJ⋅m-3.

3rd Family (LPG) – Wobbe index from 72.0 e 85.1 MJ⋅m-3.

It includes all gaseous hydrocarbons usually supplied as liquids in vessels or bottles, in which the main constituents are butane and propane

2 Family (natural gas) – Wobbe index from 37.1 to 52.4 MJ⋅m .

It includes all natural gases in which the main component is methane, as well as the manufactured gases for replacement of natural gas

Comparison of fuelsSource: Ferreira dos Santos (1986)

From the standpoint of the consumer, readiness and certainty of supply are factors of great importance. Thus, to make a choice of fuel, one must consider the place of production, transport and mode of delivery, and consumption points, in addition to the actual use

Solid fuelsSolid fuels

These, including coal and coke, have the following advantages over other energy sources:

a) their prices are lower;

b) they can be stored in large quantities in hard, flat surfaces;

c) having a low hydrogen content, a larger proportion of their HCV can be used;

d) the average value of sulphur in coal is lower (less than half the average level found for the residual fuel oil), hence creating less pollution and corrosion problems;

e) coal ash has hardly any vanadium, which exists in the oil;

f) coal can be used in its powdered form, whose advantages are similar to those of the oil;

g) they have a greater diversification of suppliers.

However, there are disadvantages related to their handling and use:However, there are disadvantages related to their handling and use:

h) their operation in terms of handling is dirty due to dust. Transportation systems are often subject to halts;

i) they suffer deterioration during storage with reduced particle size and calorific value. In extreme cases, spontaneous combustion can occur;

j) sometimes they have high proportions of mineral matter, which present in the ash can cause serious problems;

k) the presence of sulphur leads to the formation of sulphur oxides during combustion, which accounts for problems of corrosion and pollution;

l) controlling the combustion of solids is more difficult to perform than that of liquid and gaseous fuels, enhanced by a wide range of quality and sizes.

Liquid fuels

With regard to solid fuels, liquid fuels have the following advantages:With regard to solid fuels, liquid fuels have the following advantages:

a) they can be stored more compactly. On the basis of calorific value / weight the oils contain about 50% more energy than coal. Based on area occupied, the oil storage is almost 5 times more compact;

b) handling and transport takes place more easily than solids. Thus, the pumping equipment, transport and control becomes cheaper;

c) the higher calorific value implies a higher power for a given mass of fuel;

d) regulation and control of combustion processes are easier;

e) the burners are more easily adaptable to different calorific powers;

f) start-up is also simpler than in coal plants;

g) minimum excess air is lower, so temperatures are higher. Furthermore, the use of aromatic oils leads to brighter flames and therefore more heat transfer by radiation;transfer by radiation;

h) the supply of fuel is more uniform in composition than that of coal. There are no problems of dust when loading and unloading;

i) oils have very low levels of ashes. There are no problems with pitch, nor is there deposits ("bird nesting") in the boilers;

j) coal has problems of spontaneous ignition during storage. Oil does not;

k) the light distillates (diesel and kerosine) have less sulphur than coal. They are suitable for central heating and domestic applications.

Gaseous fuels

The use and handling of gaseous fuels have great advantages over other solid and liquid fuels:

a) they are clean, and their combustion does not produce ash, dust nor particles that can pollute the atmosphere;

b) any compound of sulphur or sulphur itself that may exist in the gas is easily removed by simple and economic processes before the gas being burned ;burned ;

c) can be burned efficiently with minimal amounts of excess air (50-10%) and produce no fumes if there is good control on combustion;

d) can be burned uniformly without requiring any special process for control and handling;

e) can be easily distributed to various locations using underground piping, as was the case of town gas and is currently the case with domestic natural gas;

f) its purity and calorific value are always well defined;

g) only with LPG special care concerning weather conditions must be taken for storage.