Flammability and explosiveness of fuels prepared by: P. Kobel; last rev.: 2011-03-02

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LABORATORY OF COMBUSTION AND FUELS

Flammability and explosiveness of fuels Introduction Explosion is a violent chemical reaction with intensive heat evaluation, capable to self-supporting propagation in space. Initiation of an explosion occurs as a result of ignition of combustible mixture which occurs when the temperature of the combustible mixture reaches such value, that the chemical reaction develops spontaneously without further external stimulation. Conditions necessary for explosion occurrence: a) for gas b) for dust Main explosiveness parameters of gases and dusts:

- maximum pressure of explosion pmax, - maximum rate of explosion pressure rise (dp/dt)max, - explosiveness factor K = (dp/dt)maxV

1/3, - explosion limits, - detonation limits, - temperature of self-igniton Tz, - minimum energy of ignition Emin.

Lower (upper) explosion limit is a minimum (maximum) content of fuel in the mixture, above (below) which explosion is possible. Explosion limit (lower or upper) of the combustible mixture of n copmponents can be calculated from the Le Chatelier formula knowing the explosion limits of the components of the mixture:

uln

n

uli

i

ulul

ul

L

a

L

a

L

a

L

aL

,,,2

2

,1

1,

......

100

+++++=

where: Ll, u– lower (l) or upper (u) explosion limit of gas mixture, % ai – concentration of a ith combustible component in the mixture, % ,

Li l, u - lower (l) or upper (u) explosion limit of ith component, %

gas air

ignition

air dust

limited space

mixing

ignition

LABORATORY OF COMBUSTION AND FUELS

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Explosion limits of flammable gases

Part 1: Explosion limits (ignition limits) of gaseous fuels 1. Unit for determining the explosion limit of gases

2. Results processing

2.1. Determine the lower (Ll) and upper (Lu) explosion limits of flammable gas using the Le Chatelier formula. The composition of gas will be given.

2.2. Determine the composition of tested gas-air mixture using following formulas

Amount of air in the mixture Vair = 4 * (1 – |pevac| + padd) Amount of gas in the mixture Vgas = 4 * |pevac| Concentration of gas in the mixture Cgas = Vgas/(Vair+Vgas)

1

2 3

4

5

1 –fuel tank 2 – air compressor 3 – vacuum pump 4 – air/fuel mixture tank 5 – explosion chamber (tube) 6 – spark igniter

Solenoid valves

LABORATORY OF COMBUSTION AND FUELS

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2.3. Draw a number line of concentrations and put on it the ignition test result and the explosion limits.

3. Values to be measured

test no pressure drop due to evacuation of air

pressure rise due to adding air

ignition test result

pevac, bar padd,bar true/false 1 2 …

Part 2: Explosiveness of dusts 1. Unit for explosiveness testing

1.1. General view

a. Explosion chamber b. Computer for calculation end control c. Pressure control unit d. Control unit e. Sight

f. Tank for dust sample g. Pressure sensor (not visible) h. Signal amplifier for pressure sensor i. Cover with ignition system j. Air supply system

1

2

3

4

6 7

8

9

10

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1.2. schematic diagram

2. Results processing

2.1. Make charts showing: a) maximum explosion pressure (pmax) vs. concentration of dust (c); b) maximum speed of explosion pressure rising (dp/dtmax) vs. concentration of dust (c);

2.2. Determine lower explosion limit using graphical method (see picture below):

a) on maximum pressure chart make line for background pressure (obtained for concentration c = 0,000 kg/m3);

b) draw an approximation curve through measurements points (it has to begin in origin of coordinate system /point 0,0/);

c) point, where approximation curve crosses background pressure line, projected onto horizontal axis determines lower explosion limit (LEL).

2.3. For highest measured value of (dp/dt)max calculate explosiveness factor K

K = (dp/dt)max ⋅⋅⋅⋅ V1/3

where V = 22,4ℓ = 0,0224m3 is volume of explosion chamber.

2.4. Determine explosiveness rank of tested dust (according to table below). Remember to use

correct units for K .

LABORATORY OF COMBUSTION AND FUELS

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Example of graphical determining of lower explosion limit

Explosiveness ranks of dusts

explosiveness rank Dust characteristic K, MPa⋅m/s

0 1 2 3

non-explosive dusts weakly explosive dusts strongly explosive dusts

very strongly explosive dusts

0 0–20 20–30 > 30

3. Values to be measured

test no concentration of dust

maximum explosion pressure

maximum speed of explosion

pressure rising

c, kg/m3 pmax, bar dp/dtmax , bar/s 0,000

1 ... ... ... ...

background pressure

LEL