fuel explosions - wydział...

FUEL EXPLOSIONS

EXPLOSIONS

Explosion or chemical explosion is a violent chemical

reaction with intensive heat evaluation, capable to

selfsupporting propagation in space.

Necessary conditions for explosion occurrence: a) gaseous, b) dust

GASEOUS AND DUST EXPLOSIONS

gas air

ignition

a)

airdust

limited space

mixing

ignition

b)

Initiation of an explosion occurs as a result of ignitionof combustible mixture.

Ignition of a combustible mixture may have a following character:– forced ignition, – self-ignition.

INITIATION OF EXPLOSION

CHARACTER OF INITIATION OF EXPLOSION

Self accelerating character of explosion initiation (ignition) occurs when the temperature of the combustible mixture reaches such value that accelerating development of the chemical reaction appears without any outer stimulation.

This preliminary explosion stage may have thermal or chain character.

Forced (external) ignition can be caused be different sources which produce enough energy to start combustion.

The necessary condition of ignition is to attain the required temperature of a combustible mixture by delivery a sufficient amount of energy.

IGNITION

Minimum of ignition energy (MIE) is one of the majorparameters characterizing the explosion properties of a fuel. It depends on the type of fuel, mixture content and conditions of ignition.

Ignition delay τin is a time interval (delay) after which the pressure rise of explosion in observed as an effect of burning.

For conventional fuels (e.g. gasoline) the ignition delay isin range of20–40 ms.

IGNITION DELAY

THERMAL EXPLOSION

Container with exothermically reacting substance

( )ot2 TTSq −= α

Rate of heat loss

Thermal equilibrium between the container and surroundings occurs when the rate of heat

evaluation q1 is equal to the rate of heat lossq2

q1 = q2

CONDITIONS FOR THERMAL EXPLOSION

a) Critical conditions of thermal explosion: q1 = q2 and dq1/dT = dq2/dTb) Temperature behaviour in the container depending on the walls

temperatureTs

time

CRITICAL TEMPERATURE OF EXPLOSION VS. SIZE OF A CONTAINER

Substance Size of container, mm Tcr, °C

Wool51204610

177125109

Activated coal

51204610

1189060

Tetryl142149

167153136

THERMAL EXPLOSION LIMITS for H2 + O2 REACTION

Thermal explosion limits on the temperature – pressure plane

slowreaction

explosion

CHAIN EXPLOSIONS

Oscillations during ignitions of aldehyde in oxygen

LIMITS OF CHAIN IGNITION

Limits of chain ignition in the mixture of propane and oxygen

slowreaction

explosion

FORCED IGNITION

History of the temperature profiles in combustible material ignited by hot

body (5 – flame)

hot b

ody

combustiblematerial

Diesel oil

gasoline

Influence of flow velocity u on minimum energy of electric spark E j

MINIMUM IGNITION ENERGY OF ELECTRIC SPARK

EXPLOSION LIMITS

Explosion limits mean the same as ignition limits.

Explosion limit is a minimum (maximum) content of fuel in the mixture, in which

explosion is possible.

EXPLOSION LIMITS OF MIXTURES

nnm VPVPVP

V/...//

100

2211 +++=

Explosion limit of the combustible mixture can be calculated from the Le Chatelier formula knowing the explosion limits of the components of the mixture:

where: Vm – lower or higher explosion limit of the mixture, Pi – concentration of a single, combustible component of the mixture (∑ Pi = 100%), Vi – lower or higher explosion limit of a single component.

Explosion limits

hydrogen H2carb. monox. CO

methane CH4ethane C2H6

propane C3H8n-buthane C4H10n-pentane C5H12n-hexane C6H14ethylene C4H10acetylene C2H2benzene C6H6

methanol CH3OHethanol C2H6OH

gaslower upperupperlower

% of gas in mixture with oxygen % of gas in mixture with air

explosion limits in air and in oxygen for chosen gases

GASODYNAMICSOF EXPLOSIONS

Explosion in stoichiometric mixture of air-propane, central ignition: a) flame size, b) pressure vs. time

pres

sure

edges of container

MECHANISM EXPLOSION PROPAGATION IN DUCT WITH OBSTACLES

Flame development after obstacles

FUMES

velo

city

pro

file FRESH

MIXTURE

recirculationzone

fron

tof

flam

e

afterburning in pits

MECHANISM OF TURBULENT FLAME ACCELERATION IN CHANNEL WITH OBSTACLES

ignition COMBUSTION

turbulence

instabilityof flame

dilution of gas–rise of pressure

flow of gas

Detonation is the combustion wave induced by the preceding shock wave.

DEFLAGRATION AND DETONATION

Mechanism of detonation propagation is based on adiabatic compression of the combustible mixture by a

shock wave, so that after a few microseconds self--ignition occurs and the combustion wave follows the

shock wave.

STRUCTURE OF DETONATIONfront of detonation

front of shock wave end of reaction zone

fumesfreshmixture

flame congestion wave

freshmixture

flamecorrugation turbulization

turbulent flame next congestion waves

flame explosion zone

detonation waveretonation wave

fumes

DEFLAGRATION TO DETONATION TRANSITION

Not in every combustible mixture a detonation can occur.There are detonation limits, which are much narrower then explosion limits

Detonation limits

Combustible gas

in air in oxygen

lower,% vol.

higher,% vol.

lower,% vol.

higher,% vol.

Methane CH4Propane C3H8Isobutane (C4H10)Ethylene C2H4Acetylene C2H2Diethyl ether (C2H5)2OHydrogen H2Hydrogen+carbonmonoxide H2+COCarbon monoxide COAmmonia NH3

4,12,1

5,54,52,815,019,0

12,515,0

15,49,5

11,5100,04,563,559,0

75,028,0

10,03,22,83,53,52,715,017,2

38,025,4

56,037,031,173,0100,040,090,091,0

90,075,4

Detonation limits in air and in oxygen for chosen gases

CHARACTERISTICS OF

EXPLOSION/FIRE PROPERTIES

OF GAS AND DUST

GAS AND DUST EXPLOSIONS

Major explosibility parameters of gases and dusts:

– maximumpressure of explosion pmax,

– maximum rate of explosion pressure rise (dp/dt)max

or K factor:K = (dp/dt)maxV1/3,

– explosion limits,

– detonation limits,

– temperature of self-ignitonTz,

– minimum energy of ignitionEmin .

EXPLOSIBILITY PARAMETERS

Maximum pressure of explosionpmax is the highest pressure recorded during explosion in the closed container.

MAXIMUM EXPLOSION PRESSURE

ignition

Record of the explosion pressure in a closed container –

determination ofpmax and (dp/dt)max

EXPLOSIBILITY RANK OF DUSTS

00–2020–30> 30

KMPa ⋅ m/s

nonexplosive dustsweakly explosive dustsstrongly explosive dusts

very strongly explosive dusts

Dust characteristic

0123

Dust explosibilityrankP

No Parameter Unite Value

1.2.3.4.5.6.7.

Temperature of self-ignition of dust cloud TscTemperature of ignition in dust layer TZOLower explosion limit LELMaximum pressure of explosion pmaxMax. rate of explosion pressure rise (dp/dt)maxReduced max. rate of explosion pressure rise KstRank of dust explosibility P

oCoC

kg/m3

MPaMPa/s

MPa⋅m/s-

5602650.090.829.98.11

FIRE/EXPLOSION PROPERTIES OF BITUMINOUS COAL

Powierzchnia właściwa S, m2/kg

0 200 400 600 800 1000 1200 1400 1600

Mak

sym

aln

e ciś

nie

nie

wyb

uch

u p m

ax,

bar

6

7

8

9

10

PW1, V/A = 3,70PW2, V/A = 3,61PW3, V/A = 1,99PW4, V/A = 3,04PW5, V/A = 2,47

max

imum

pres

sure

of e

xplo

sion

, pm

ax, b

ar

specific surface, s, m2/kg

MAXIMU M PRESSURE OF EXPLOSION VS. SPECIFIC SURFACE OF COAL DUST

pyłów węglowych PW1, PW2, PW3, PW4 i PW5.

Powierzchnia właściwa S, m2 / kg

0 200 400 600 800 1000 1200 1400 1600

Wsk

aźn

ik w

ybu

cho

woś

ci K

st,

max

, m

ba

r / s

0

50

100

150

200

250

300


expl

osib

ility

fact

or, K

st,m

ax, b

ar*m

/s


K FACTOR VS. SPECIFIC SURFACE OF COAL DUST


0 200 400 600 800 1000 1200 1400 1600

Min

imal

na

tem

per

atu

ra z

apło

nu

war

stw

y py

łu

T5m

m, o C

100

150

200

250

300

350

400

Rys. 6 Zależność minimalnej temperatury zapłonu warstwy pyłu od powierzchni



tem

pera

ture

of i

gniti

on in

laye

r, T

5mm

, o C

TEMPERATURE OF IGNITION IN LAYER VS. SPECIFIC SURFACE OF COAL DUST


0 200 400 600 800 1000 1200 1400 1600

Min

imal

na e

ner

gia

zao

łonu

ob

łoku

pył

u Wm

in, m

J

0

1000

2000

3000

4000

5000

6000

7000

PW1, V/A = 3,70PW2, V/A = 3,61PW5, V/A = 2,47


min

imum

ene

rgy

of ig

nitio

n of

clo

ud, W

min, m

J

MINIMUM ENERGY OF IGNITION OF CLOUD VS. SPECIFIC SURFACE OF COAL DUST

PROTECTIVE MEASURES AGAINST the OCCURRENCE and EFFECTS OF EXPLOSION

There are two types of measures reducing explosion hazard:

1. Preventing explosion protection.

2. Explosion protection through design measures.

REDUCTION OF EXPLOSION HAZARD

– prevention of explosive fuel/air mixtures,

– prevention of effective ignition sources,

– prevention of autoignitions (hot surfaces),

– static electricity,

PREVENTIVE EXPLOSION PROTECTION

1. Explosion pressure-resistant design for the

maximum explosion pressure

2. Explosion pressure venting

3. Explosion suppression

4. Diversions/arresting explosions

EXPLOSION PROTECTION THROUGH DESIGN MEASURES


EXPLOSION FLAP


SAFETY MEMBRANE


EXPLOSION FLAP

THORWESTEN VENT


FLAME TRAP


SUPPRESSION EXPLOSION SYSTEM

frontof flame

opticaldetector

pressuredetector

control unit

pyrotechnical valve


EXPLOSION SUPPRESSION OF PIPELINES

front of flame

detectorcontrol unit

pyrotechnicalvalve

extinguishingpowder


EXAMPLES OF PROTECTING MEASURES AGAINST

EXPLOSIONS

EXPLOSION PRESSURE VENTING

Boiler

Coal mill

Coal silo

Membrane of explosion pressure

venting



MEMBRANE OF EXPLOSION PRESSURE VENTING

Roof of coal dust separator


DECOMPRESSION OF DUST SEPARATOR


Roof ofcoal dustseparator

Membranes

SELF-IGNITION OF COAL IN PILES

SELF-IGNITION OF COAL DURING ITS STORAGE

The phenomenon of spontaneous ignition (self-ignition) of coal when it is storage in silos, bunkers, containers and piles, and during its milling in coal mills is not well understood.

Hazard of self-ignition occurs also when organic materials are stored, like: grain, hay and straw, and municipal wastes.

This phenomenon is also important during food processing. Also some explosives undergo self-ignition.

DEVELOPMENT OF SPONTANEOUS IGNITION WHEN SOLD EXOTHERMIC

MATERIAL IS STORED

a) Development of self-heating during storage of solid fuels, b) temperature development during self-ignition in pile, Tz – temperature of self-ignition

tem

pera

tura

udział tlenu

autoutlenianie

tlenie

żarzenie

spalaniepłomieniowe

piroliza

tem

pera

ture

oxygen level

pyrolisis

flamecombustion

glowing

smouldering

self-oxidation

Okres inkubacji

Tz

T

t

Okres samonagrzewu

incubation time self-heatingtime

a) b)

EFFECTS OF OXIDATION OF COAL IN PILES

a) Coal weathering

�loss of reactivity,

� partial loss of volatile matter.

b) Lowtemperature oxidation of coal

�Rate of lignite oxidation in pile Rśr = 0,09 %/24h.

c) Self-heating of coal

�Self-heating of coal (temperature range of 60−80 °C),

�Loss of v.m. and hydrogen in coal

�After crossing of the temperature of self-ignition (>100 °C ) a stage of glowing occurs, which can go into the fire.

FIRE HAZARD OF COAL IN PILES

�Coal belongs to the materials very reactive with oxygen.

�Oxidation reactions in coal is self-accelerating at normal conditions.

�The most capable for self-ignition in piles are: lignite and and young bituminous coal.

�There are no simple method for determination of self-ignition conditions of coal in piles.

MAJOR FACTORS INFLUENCING SELFHEATINGOF COAL IN PILES

Properties of coal Properties of pile

Volatile matter contentMoisture contentPyrite contentEgxynite contentCoal particle mean size

Mass of stored coalContent of small fractions of coal particles Type of backgroundVentilation in the pileShape of the pile (inclination of the slope)

FACTORS WHICH INCREASE HAZARD OF SELFIGNITION OF COAL IN PILES

�High content of small fractions of coal particles

�Moisture

�Pyrite and egyxinite

�Large size of a pile

�Access of oxygen inside the pile

DEVELOPMENT OF THE SPONTANEOUS IGNITION PROCESS IN THE COAL PILE

Stages:

�Incubation stage (preliminary): takes from few to twenty days and undergoes at only slightly increased temperature.

�Selfheating stage: causes increase the temperature inside the pile to 60−80 °C. Its further development is manifested by the appearance of steam above the pile, and by the presence of CO and aromatic hydrocarbons at the approximate temperature100 °C.

�Selfignition: is revealed by the temperature rise in the coal pile and appearance of milky smoke above the pile. Access of air to the coal pile causes its fire then.

TEMPERATURE OF SELFIGNITIONOF COAL IN A PILE

Temperature of self-ignition depends of the type of coal:

– lignite: 150 °C,

– bituminous coal: 200 °C,

– coke: 250 °C,

– anthracite: 300 °C.

MECHANISMS OF SELFIGNITIONOF COAL IN A PILE

I. Incubation stage

� Low temperature oxidation of coal

� Micro-biological metabolism

� Adsorption-desorption of water

� Oxidation of pyrite

II. Selfheating stage

� thermal

LOW TEMPERATURE OXIDATION OF COAL

Oxidation of coal at the low temperature range (approximately up to150° C) can be described by the following physical/chemical mechanism:

Ccoal + O2, gas→ O2 (physical absorption)

O2 (absorbed)→ O2 (chemical absorption), Q I

Ooxycoal→ CO2 + H2O (main reaction), QII

Ooxycoal→ CO, CnHm (aside reaction)

where Ooxycoal is oxygen absorbed at the surface of coal.

CONTROL OF COAL OXIDATION IN THE PILE

The factor controlling rate of coal oxidation in the pile is:

Oxygen content inside the pile

The entrance of oxygen into the pile is governed by two mechanisms:

�The major mechanism of oxygen, products of oxidation and heat transport inside the pile is natural convection.

�Also an important part is played by the wind – in most casesspontaneous ignition took place from the lee side of the pile.

AIR FLOWS IN THE PILE OF COAL

Processes of oxygen transportation in the pile of coal

konwekcjawymuszona

wiatr

powietrze powietrze

Q

O2

CO2

CO wilgoć

konwekcjaswobodna

T>Tot

węgiel

powietrze

forced convection

air

wind

humidity

airair

naturalconvection

coal

HOW TO PROTECT THE COAL PILE AGAINST SELFIGNITION

I. The problems of spontaneous ignition of coal in a pile and its oxidation losses haven’t found satisfactory solution yet.

II. Practically the only factor which allow control of of oxidation processes in a pile is access of air into the pile.

III. The effective technique of preventing spontaneous ignition is using of inert matter in the pile, which means the replacement of air by some neutral gas, like nitrogen. Because of economic reasons this method have found application only to silos, e.g. to a silos with activated coal.

ACTION UNDERTAKEN TO PREVENT SPONTANEOUS IGNITION IN A COAL PILE

I. – periodic empty of a pile (silo, bunker),

II. – decline of a slope of pile side,

III. – the use of curtains,

IV. – cover of the pile by a layer of material impeding access of air,

V. – condensing of a pile,

VI. – monitoringof the process of self-heating in a pile.

PILE OF LIGNITE

COURTAIN IN THE PILE OF LIGNITE

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