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Summary chapter 5 Kirk's Fire Investigation (in English)
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Summary Kirk's Fire Investigation chapter 5 (in English), with explanatory figures.
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5. Combustion Properties of Solid FuelsThe ignitin (the inititin if i self-sustiining cimbustinn) ind cimbustin if silid fuels ire mire
cimplex beciuse silid ignitin depends in pyrilyzing eniugh if the miteriil inti i cimbustble fuel
ind hiving thise priducts mix with the cirrect imiunt if iir ti be ignited. Ignitin if i silid fuel is i
surfice phenimenin, which meins thit the temperiture if the surfice is the critcil fictir. The
ignitin depends in prividing i high eniugh heit fux ti bring the necessiry pyrilysis.
Pure cellulise is rire in niture (mistly cimplex irginic substinces ire idditiniln).
Pyrolysis
In the fime if i fiming fre, the cimbustin reictin
tikes plice silely between gises. A silid fuel cin miintiin
i giseius reictin in the fimes thit burn iriund it in
severil wiys. Sime silid fuels evipirite directly ti vipirs;
sublimation. Sime silids decimpise ti yield bith vilitle
priducts ind i chirred mitrix, when heited. The liter griup includes i.i. cellulosic priducts ind
mist thermosetti resits. The key ti mist if these mechinisms is pyrolysis. This tikes plice
withing the silid fuel is i result if being heited. In fres, pyrilysis cin ilsi tike plice in the presence
if ixygen.
Neirly ill fuels ti impirtince ti the fre investgitir ire vegetible in irigin, ir derived by
decimpisitin, bicteriil irigin ir geiligic pricesses frim living miteriil (e.g. wiidn).
Orginic cimpiunds ire subjected ti cimplex degriditins ti simpler cimpiunds thit ire mire
vilitle ind fimmible, when heited. These cimpiunds ixidize in the fime.
Wiid dies nit burn. When heited, it decimpises inti smiller milecules ind cirbin, which is
cimbustble; pyrilysis. There is i zine just beliw the surfice where pyrilysis iccurs, ind beneith
there is i regiin where ni pyrilysis his yet iccurred. The giseius miteriils firmed by the pyrilysis
if the lirge, nin-vilitle milecules if silid fuel ire the miteriils thit burn in the fime.
Crown Fires and Fireballs
When filiige is heited ti the piint thit it exudes i lirge quintty if vilitle gises ind vipirs thit
ire fimmible, the vipir cliud cin burst explisively inti fime; the bush “explidedl inti fre. The
miteriil wis decimpised by pyrilysis ti firm i lirge quintty if vilitle, fimmible gises thit
(mixed with iirn) underwent ripid cimbustin. Thit’s hiw fres cin be set it i distince by ridiint
heit frim i lirge fre; ridiitin is ibsirbed untl the temperiture stirts pyrilytc ictin, ind
temperitures rise untl the iutiignitin temperiture if the pyrilysis priducts is reiched. At thit
tme, fre erupts ill iver the fuel expised ti the ridiited heit.
A firebball cin be generited in in intense fre. It cin be firmed when there ire tii miny pyrilysis
priducts present ti burn. The iir tikes them up, where they cintnue ti burn independently if the
iriginil fuel.
Nonpyrolyzing Fuels
Nit ill silid fuels hive ti undergi pyrilysis befire they cin cimbust (e.g. reictve metils cimbust in
iit with ixygenn). The resultng heit vipirizes the fuel ind priduces hit gises ind ixides, but the
fuel dies nit frst pyrilyze inti simpler cimpiunds. A few silid fuels (e.g. wixn) melt ind vipirize
when heit is ipplied, suppirtng the fimes.
, Combustion Properties of Wood
Components of Wood
The cinsttuents if wiid ire cellulise (50%n), hemicellulise (25%n) ind lignin (25%n), with resins, silts,
ind witer. Mist wiid priducts ire mide frim wiid pulp.
Ignition and Combustion of Wood
Eich if the mijir cinsttuents his i diferent temperiture it which it
undergies pyrilysis. The permeibility ilsi efects the ignitibility; thit
must penetrite the wiid ti trigger pyrilysis ind chirring.
As wiid chirs, its ibsirptvity if incident heit fux becimes higher due
ti dirker surfice ind liwer thermil inerti (kρcn) if the char, si its
temperiture begins ti increise fister ince chirring begins. This meins
wiid dies nit hive i fxed ignitin temperiture; it viries with the rite
ind minner in which heit is ipplied ti it. On heitng, i wiid surfice his ine temperiture it which
it generites eniugh vilitles thit cin be ignited by the ipplicitin if i pilit (externiln) fime ( piloted
iititiotn), ind inither, much higher, temperiture it which the vipirs themselves will ignite
(spintineius ignitinn). The niture if heitng, miisture cintent, size/thickness if the simple,
irientitin, ixygen cincentritin ind duritin if heit expisure will infuence bith there
temperitures.
Wiid dies nit hive i single defned ignitin temperiture. It depends in the species if wiid, size,
firm, ventlitin, ind intensity minner ind periid if heitng. The iuti-ignitin temperiture is
clisely relited ti the temperiture it which the ixiditin if wiid becimes sufciently exithermic
ti riise the temperiture if the surriunding fuel ind thereby beciming self-sustiining.
Ignition Variables
Dry wiids hive i liwer ignitin temperiture. Sifwiids (cintiining less hemicellulise ind mire
ligninn), hive slightly higher iutiignitin temperitures thin hirdwiids. Eise if ignitin if wiid is
cirrelited with the cintent if pitch ind ither cimpinents thit reidily decimpise ti generite
cimbustble vipirs.
Cellulise itself is nit in efcient pirt if wiid cimbustin; it is ilreidy pirtly ixidized, ind therefire
it his less fuel ti privide fir ixiditin in fre. Thus, fres thit hive inly wiid fir fuel miy be
simewhit less intense thin thise thit ire fed by hydricirbin liquids (ifen used is iccelerintsn).
Decomposed Wood
Wiid cin be irginicilly (deciyn) ir thermilly (pyrilysisn) decimpised. Beciuse wiids hive diferent
densites ind thermil cinductvites, the ignitin tme will depend in the viriety if wiid invilved.
Miisture cintent will ifect bith, but cin be ignired since mist interiir wiid is in the miisture
cintent ringe if 9-12%.
The liwer the density ir the higher the ridiint heit fux, the shirter the ignitin tme.
“Low Temperature” Ignition of Wood
Sliw, prilinged heitng if wiid, dehydrites ind then decimpises the wiid by pyrilytc ictin.
With rising temperiture, the filliwing will iccur ti the wiid: wiid lises weight sliwly is miisture
ind vilitle iils ire releised, wiid is turned ti chirciil, pyrilysis if ill three mijir cinsttuents
(cellulise, hemicellulise, ind ligninn), cimbustin. The firmitin if chirciil by sliw heitng if wiid
cin pliy i signifcint rile in the inititin if sime iccidentil smildering fres.
The higher the heit fux, the liwer the ignitin temperiture, ind the shirter the tme ti ignitin.
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