07 Gaseous Fuels

8/13/2019 07 Gaseous Fuels

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Gaseous Fuels


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1. Introduction

There are numerous factors which need to be

taken into account when selecting a fuel for

any give application.

Economics is the overriding consideration-thecapital cost of the combustion equipment

together with the running costs, which are

fuel purchasing and maintenance.


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2. Natural Gas

Natural gas is obtained from deposits in

sedimentary rock formations which are also

sources of oil.

It is extracted from production fields andpiped (at approximately 90 bar) to a

processing plant where condensable

hydrocarbons are extracted from the rawproduct.


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It is then distributed in a high-pressure mains

system.

Pressure losses are made up by intermediate

booster stations and the pressure is droppedto around 2500 Pa in governor installations

where gas is taken from the mains and enters

local distribution networks.


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The initial processing, compression and

heating at governor installations uses the gas

as an energy source.

The energy overhead of the winning anddistribution of a natural gas is about 6% of

the extracted calorific value.


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The composition of a natural gas will vary

according to where it was extracted from, but

the principal constituent is always methane.

There are generally small quantities of higherhydrocarbons together with around 1% by

volume of inert gas (mostly nitrogen).


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The characteristics of a typical natural gas are:

Composition (% vol) CH4 92

other HC 5

inert gases 3Density (kg/m3) 0.7

Gross calorific value (MJ/m3) 41


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3. Town gas (Coal Gas)

The original source of the gas which was

distributed to towns and cities by supply

utilities was from the gasification of coal.

The process consisted of burning a suitablegrade of coal in a bed with a carefully

controlled air supply (and steam injection) to

produce gas and also coke.


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This is still the gas supplied by utility

companies in many parts of the world and

there is continuing longer-term development

of coal gasification, since it is one of the mostlikely ways of exploiting the substantial world

reserves of solid fuel.

It was first introduced into the UK and theUSA at the beginning of the 19th century.


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The gas was produced by heating the raw

coal in the absence of air to drive off the

volatile products.

This was essentially a two-stage process,with the carbon in the coal being initially

oxidized to carbon dioxide, followed by a

reduction to carbon monoxide:C + O2CO2

CO2+ C2CO


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The volatile constituents from the coal were

also present, hence the gas contained some

methane and hydrogen from this source.

An improved product was obtained if waterwas admitted to the reacting mixture, the

water being reduced in the so-called water

gas shift reaction:C + H2O CO + H2


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This gas was produced by a cyclic process wherethe reacting bed was alternately blown with air andsteam- the former exhibiting an exothermic, and thelatter an endothermic, reaction.

A typical town gas produced by this process has thefollowing properties:Composition (% vol) H2 48

CO 5

CH4 34CO2 13Density (kg/m3) 0.6Gross calorific value (MJ/m3) 20.2


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A more recent gasification process,

developed since 1936, is the Lurgi gasifier.

In this process the reaction vessel is

pressurized, and oxygen (as opposed to air)as well as steam is injected into the hot bed.

The products of this stage of the reaction are

principally carbon monoxide and hydrogen.


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Further reaction to methane is promoted by a nickel

catalyst at temperatures of about 250-350:

CO + 3H2CH4+ H2O

The sulfur present in the coal can be removed bythe presence of limestone as follows:

H2+ S H2S

H2S + CaCO3CaS +H2O +CO2


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4. Liquefied Petroleum Gas (LPG)

LPG is a petroleum-derived product

distributed and stored as a liquid in

pressurized containers.

LPG fuels have slightly variable properties,but they are generally based on propane

(C3H8) or the less volatile butane (C4H10).


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Compared to the gaseous fuel described

above, commercial propane and butane have

higher calorific values (on a volumetric basis)

and higher densities. Both these fuels are heavier than air, which

can have a bearing on safety precautions in

some circumstances.


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Typical properties of industrial LPG are given below:

Gas Propane Butane

Density (kg/m3) 1.7-1.9 2.3-2.5Gross calorific value (MJ/m3) 96 122

Boiling point (at 1 bar) -45 0


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5. Combustion of Gaseous Fuels

5.1 Flammability Limits

Gaseous fuels are capable of being fully

mixed (i.e. at a molecular level) with the

combustion air.

However, not all mixtures of fuel and air are

capable of supporting, or propagating, a

flame.


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Imagine that a region of space containing a

fuel/air mixture consists of many small

discrete (control) volumes.

If an ignition source is applied to one of thesesmall volumes, then a flame will propagate

throughout the mixture if the energy transfer

out of the control volume is sufficient to causeignition in the adjacent regions.


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Clearly the temperature generated in the

control volume will be greatest if the mixture

is stoichiometric, where as if the mixture goes

progressively either fuel-rich or fuel-lean, thetemperature will decrease.

When the energy transfer from the initial

control volume is insufficient to propagate aflame, the mixture will be nonflammable.


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This simplified picture indicates that there will

be upper and lower flammability limits for any

gaseous fuel, and that they will be

approximately symmetrically distributed aboutthe stoichiometric fuel/air ratio.


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Flammability limits can be experimentally

determined to a high degree of repeatability

in an apparatus developed by the US Bureau

of Mines. The apparatus consists of a flame tube with

ignition electrodes near to its lower end


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Intimate mixing of the gas/air mixture is

obtained by recirculating the mixture with a

pump.

Once this has been achieved, the cover plateis removed and a spark is activated.

The mixture is considered flammable if a

flame propagates upwards a minimumdistance of 750 mm.


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5.2 Burning Velocity

The burning velocity of a gas-air mixture is

the rate at which a flat flame front is

propagated through its static medium, and it

is an important parameter in the design ofpremixed burners.

A simple method of measuring the burning

velocity is to establish a flame on the end of atube similar to that of a laboratory Bunsen

burner.


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When burning is aerated mode, the flame has

a distinctive bright blue cone sitting on the

end of the tube.

The flame front on the gas mixture istravelling inwards normally to the surface of

this cone.


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If U represents the mean velocity of the gas-air

mixture at the end of the tube and is the half-angle

of the cone at the top of the tube, then the burning

velocity S can be obtained simply from:

S = U sin ()

This method underestimates the value of S for a

number of reasons, including the velocity distribution

across the end of the tube and heat losses from theflame to the rim of the tube.


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More accurate measurements are made with a

burner design which produces a flat, laminar flame.

Some typical burning velocities are:

Fuel Burning velocity (m/s)

Methane 0.34

Propane 0.40

Town gas 1.0Hydrogen 2.52

Carbon monoxide 0.43


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Burning velocity should not be confused with

the speed of propagation of the flame front

relative to a fixed point, which is generally

referred to as flame speed. In this case, the speed of the flame front is

accelerated by the expansion of the hot gas

behind the flame.


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5.3 Wobbe Number

This characteristic concerns the

interchangeability of one gaseous fuel with

another in the same equipment.

In very basic terms, a burner can be viewedin terms of the gas being supplied through a

restricted orifice into a zone where ignition

and combustion take place.


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The three important variables affecting the

performance of this system are the size of the

orifice, the pressure across it (or the supply

pressure if the combustion zone is at ambientpressure) and the calorific value of the fuel,

which determines the heat release rate.

If two gaseous fuels are to beinterchangeable, the same supply pressure

should produce the same heat release rate.


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This ratio is known as the Wobbe number of agaseous fuel and is defined as:

Some typical Wobbe numbers are:Fuel Wobbe number (MJ/m3)

Methane 55Propane 78Natural gas 50Town gas 27

3

0.5

Gross calorific value (MJ/m )

Relative density (air=1)


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The significant difference between the values for natural gas and

town gas illustrates why appliance conversions were necessary

when the UK changed its mains-distributed fuel in 1966.

Example 1:

Calculate the Wobbe number for a by-product gas from anindustrial process which has the following composition by volume:

H2 12%

CO 29%

CH4 3%N2 52%

CO2 4%


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Solution:

The gross calorific values are:

CO 11.85 MJ/m3CH4 37.07 MJ/m

3

H2 11.92 MJ/m3

The calorific value of the mixture:CV=(0.1211.92)+(0.2911.85)+(0.0337.07)=5.98 MJ/m3


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The relative density of the mixture is

calculated by dividing the mean molecular

weight of the gas by the corresponding value

for air (28.84). The mean molecular weight of this mixture is:

(0.122)+(0.2928)+(0.0316)+(0.5228)+(0.0444)=25.16


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The relative density is thus

25.1628.84=0.872.

The Wobbe number is then:

5.98/(0.872)0.5=6.36

The Wobbe number of a fuel is not the only

factor in determining the suitability of a fuel

for a particular burner. The burning velocity of a fuel is also

important.


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In general, any device will operate within a

triangular performance map, such as that

sketched in Fig. 7.3 (next slide).

Outside the enclosed region, combustioncharacteristics will be unsatisfactory in the

way indicated on the diagram.


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6. Gas Burners

6.1 Diffusion Burners

The fuel issues from a jet into the

surrounding air and the flame burns by

diffusion of this air into the gas envelope(Fig. 7.4, next slide).


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A diffusion flame from a hydrocarbon fuel has

a yellow color as a result of radiation from the

carbon particles which are formed within the

flame. The flame can have laminar characteristics or

it may be turbulent if the Reynolds number at

the nozzle of the burner is greater than 2,000.


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Pratical burner operate in the turbulent

regime since more efficient combustion is

obtained in this case because the turbulence

improves the mixing of the fuel with air. Industrial diffusion burners will have typical

supply gas pressures of 110 Pa.


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Diffusion burners have the following positive

characteristics:

(a) Quiet operation

(b) High radiation heat transfer (about 20% ofthe total)

(c) Will burn a wide range of gases (they

cannot light back)

(d) Useful for low calorific value fuels


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6.2 Premixed Burners

The vast majority of practical gaseous burners mix

the air and fuel before they pass through a jet into

the combustion zone. In the simplest burners, such as those that are used

in domestic cookers and boilers, the buoyancy force

generated by the hot gases is used to overcome the

resistance of the equipment. However, in larger installations the gas supply

pressure is boosted and the air is supplied by a fan.


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The principle is illustrated by the flame from a

Bunsen burner with the air hole open, and is

shown diagrammatically in Fig. 7.5

(next slide).The gas and air are mixed between the fuel

jet and the burner jet, usually with all the air

required for complete combustion.


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The velocity of the mixture through the burner

jet is important.

If the velocity is too low (below the burning

velocity of the mixture) the flame can lightback into the mixing region.


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If the velocity is too high the flame can lift off

from the burner to the extent where it can be

extinguished by, for instance, entrainment of

additional (secondary) air around the burner. The flame from a premixed burner will emit

very little heat by radiation but, because of its

turbulent nature, forced convection in a heatexchanger is very effective.

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07 Gaseous Fuels

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