Spontaneous Landfill Fires: Investigation and Simulation
Shadi Moqbel*1 and Debra Reinhart
2
1 Civil Engineering Department, Al-Isra Private University, Airport Road, Al-Isra Post office P. O. Box 22, 33
Amman 11622, Jordan. Contact: Phone: +962-6-4711710 ext. 2452. Email: [email protected] 2
Civil, Environmental & Construction Engineering Department, University of Central Florida, P.O. Box 162450,
Orlando, Florida 32816-2450, USA
* Corresponding author
ABSTRACT
Spontaneous combustion landfill fire incidents are critical problems for landfill operators and
owners. Their occurrence might be occasional or rare but their consequences can be severe and
catastrophic to the landfill and surrounding communities. This study explored the causes of
spontaneous landfill fires and heat generation through a questionnaire and a numerical model. A
questionnaire was created and distributed to landfill owners and operators. The questionnaire
layout was organized to allow landfill operators/owners to add their observations on
spontaneous fires, detection and extinguishing methods, comments and notes. The numerical
model has been developed to simulate temperature rise in landfill. Four scenarios have been
presented using the model showing the general conditions for heat generation in landfills. These
scenarios include: anaerobic condition, aerobic condition, high temperature condition, and
spontaneous combustion. The model shows significant contribution of chemical oxidation and
moisture content in controlling the fire.
1. INTRODUCTION
Landfill fire incidents are critical problems for landfill operators and owners. Their occurrence
might be occasional but their consequences can be severe and catastrophic to the landfill and
surrounding communities. Fires in landfills are divided into two categories, surface fires and
subsurface fires. Surface fires involve recently buried or un-compacted refuse, situated on or
close to the landfill surface. Although fire is an exothermic reaction, the ignition temperature of
any combustible material, in this case solid waste, must be reached through a spark, pilot flame
or other heating mechanism (Rynk, 2000). Sources of ignition or triggers for surface fire vary
among deliberate action, accidents, and spontaneous combustion. Subsurface fires take place
deep within the landfill or inside large waste piles. Subsurface fires are often caused by
spontaneous combustion resulting from self-heating of solid waste.
Self-heating takes place as a result of the degradation and decomposition of solid waste. Solid
waste is primarily composed of degradable organic material that decomposes exothermically.
Degradation processes can be both chemically and biologically mediated, however biological
reactions predominate at temperatures below 65oC (Storm, 1985). Unless hot loads were
dumped at the landfill, biological reactions are solely responsible for increasing the solid waste
temperature from ambient temperature to 65 oC. Chemical reactions do not have significant
contribution to heat generation in this range because of the low oxidation rate of solid waste. As
the temperature of the solid waste increases chemical reactions become more influential and
become the primary source of heat generation at temperatures higher than 65oC. If this heat is
not dissipated efficiently, temperature will rise until it reaches the ignition temperature of waste
causing fire to initiate.
Spontaneous ignition occurs when material is heated beyond the ignition temperature. The
presence of heat, oxygen and fuel (i.e. solid waste) in the landfill constitute the necessary
elements of a fire. However, the progressive self-heating reaction for a fuel-oxidizer (solid
waste-oxygen) mixture will take a period of time before the ignition temperature is reached
(Glassman, 1996).
Although, surface fires are predominant among landfill fires, damage associated with surface
fires are far less costly than subsurface fires. The effects of subsurface fires can extend beyond
landfill boundaries and damage associated with deep spontaneous fires can be devastating. A
landfill slope failure resulted in the catastrophic deaths of 147 persons in Indonesia in February
2005. This failure was due to a smoldering landfill fire that damaged the landfill reinforcement
(Koelsch et al, 2005). Fires may have long-term negative effects on landfill gas production as a
result of inhibition of the methanogenic organisms by combustion products, high temperature,
and drying of the affected area. Moreover, subsurface fires can affect the integrity of the landfill
cap due to settlement, desiccation and firefighting operations (Lewicki, 1999).
This paper focuses on subsurface fires that result primarily from spontaneous combustion. The
purpose of this paper is to review the causes and major factors influencing heat generation in
landfills and simulate temperature rise in landfill under four scenarios representing general
conditions for heat generation in landfills. These scenarios include: anaerobic condition, aerobic
condition, high temperature condition, and spontaneous combustion.
2. METHODOLOGY
2.1 Landfills Survey
A web-based questionnaire was created and distributed to US landfill operators through The
Solid Waste of North America (SWANA) and National Solid Waste Management Association
(NSWMA). The purpose of the survey was to collect unrecorded observations or fire treatment
methods. The survey was divided into two parts; landfill description part that investigated
landfill operation type, solid waste types, and historical records of previous fires. The second
part is subsurface fire incidents description which investigated the events associated with fire
incidents, the detection methods used, and extinguishing methods applied.
2.2 Energy Model
An energy balance model was developed to simulate the heat generation in the landfill. Many
simplifying assumptions were necessary; therefore an exact prediction of environmental
conditions leading to spontaneous combustion is not possible. However, this simulation does
help to explore the conditions of heat generation in a landfill and the major factors influencing
the energy balance.
2.2.1 Model Description
A one-dimensional finite-difference model of the temporal distribution of heat inside the landfill
was developed using Microsoft Excel 2003. The model was applied to a 1
area longitudinal section of a landfill drawn between the landfill side slope and a vertical gas
collection well located 10 m from the side slope. The model simulates pulling air into a landfill
by overdrawing on the well. A diagram of the simulated
presented in Figure 1
The energy balance equation is represented by Equation 1 (El
pC∂
∂ρ
difference model of the temporal distribution of heat inside the landfill
was developed using Microsoft Excel 2003. The model was applied to a 1-m
area longitudinal section of a landfill drawn between the landfill side slope and a vertical gas
collection well located 10 m from the side slope. The model simulates pulling air into a landfill
by overdrawing on the well. A diagram of the simulated section and unit volume cell is
The energy balance equation is represented by Equation 1 (El-Fadel et al, 1999):
Genpgg Hx
TvCTk
t
T∆+
∂
∂+∇=
∂
∂ρ2
difference model of the temporal distribution of heat inside the landfill
m2 cross-sectional
area longitudinal section of a landfill drawn between the landfill side slope and a vertical gas
collection well located 10 m from the side slope. The model simulates pulling air into a landfill
section and unit volume cell is
Fadel et al, 1999):
(1)
Where ρ is specific weight (kg/m3), Cp and Cpg are specific heat capacity for solid and gas
(J/kg.oK), respectively, T is temperature (
oK), t is time (s), k is the thermal conductivity
(W/m.oK), and ∆HGen is the net heat generated (J/m
3.s). The term on the left represents energy
storage. The first term on the right side represents heat conduction, the second term denotes heat
loss due to convection and the third term represents heat generation.
Boundary conditions are provided in Equations 2 and 3:
)( ∞−−=∂
∂TTh
x
Tk t>0, x = xo (2)
0=∂
∂
x
T at x = 0 (3)
Initial conditions are provided below:
At t = 0, T = Ta
Where Ta is ambient temperature (oK).
Heat generation due to anaerobic degradation was related to methane generation potential as
described by Vesillind et al (2003) and EPA (2005). Heat generation equations are provided in
Equations 4 and 5:
DQH anan =∆ (4)
)exp( tAMLAD anoan −= (5)
Where: ∆Han is anaerobic heat generation rate (J/m3.s), Qan is heat of anaerobic degradation
(J/m3CH4), D is methane production rate (m
3/s), Aan is a first order rate constant (s
-1), Lo is
methane generation potential (m3/kg wet waste), M is mass of wet waste (kg, and t is time (s).
In this model heat generation from aerobic reactions is expressed by heat released from
degradation of glucose (C6H12O6) as a surrogate substrate as shown in Equation 6.
dt
dCQH
OHC
aeae
6126=∆ (6)
The biodegradation rate of glucose is expressed by (Wallner et al 2003):
fyC
RT
EA
TR
EA
dt
dCOOHC
s
a
s
a
OHC
26126
6126
22
11
exp1
.exp
−+
−
= (7)
Where: ∆Hae is aerobic heat generation rate (J/m3.s), Qae is heat of aerobic biological
degradation (J/kg C6H12O6), A1and A2 are frequency factors (s-1
, unitless), Ea1 and Ea2 are
activation energy (J/mole), yO2 is oxygen volume fraction, f is oxygen consumption coefficient,
and 6126 OHCC is concentration of glucose (kg/m
3).
Further, heat generation due to chemical oxidation was expressed according to the Arrhenius
relation, Equation 8:
−=∆
RT
EAQH Ch
ChChCh expρ (8)
Where: ∆HCh is chemical heat generation rate (J/m3.s),QCh is heat of chemical degradation (J/kg
C6H12O6), ACh is the frequency factor (s-1
), and ECh is activation energy (J/mole).
Heat loss due to water evaporation is expressed in Equation 9:
−=∆
RT
EAQH Eva
EvaEva expρ (9)
Where: ∆HEva is heat loss rate (J/s),QEva is latent heat of evaporation (J/kg H2O), AEva is the
frequency factor (s-1
), and EEva is activation energy (J/mole)
A number of assumptions were applied to the model including: (1) the landfill section was
modeled as five reactors in series, (2) the solid waste was assumed to have a homogeneous
composition across the simulated section, (3) the thermal conductivity was constant, (4) the
biodegradable material and organic materials were assumed to be 30 % and 80% by weight of
the MSW in the landfill, respectively (Tchobanoglous et al, 1993), (5) air flow through the
landfill was assumed to be uniform; however, in reality preferential pathways develop, allowing
gas to flow in some sections of the landfill faster than others creating local differences in oxygen
concentration, (6) air flow was assumed to be driven by the pressure difference between the gas
collection well and the landfill side slope; the pressure gradient was assumed to be 25 cm-H2O =
2490 N/m2 over the entire modeled section (Tchobanoglous, 1993), (7) solid waste air
permeability was assumed to be within the range of 1.6x10-13
to 3.2x10-11
m2 (Jain et al, 2005),
(8) because oxygen is consumed during aerobic degradation of solid waste, oxygen
concentration was assumed to decline across the modeled section starting with 21% by volume
at the landfill side slope and reaching 10% by volume at the collection well, (9) aerobic
degradation was assumed to cease at moisture content of 15% by weight, (Fleming, 1991), while
anaerobic degradation was assumed to cease at moisture content of 25% by weight (Reinhart and
Townsend, 1998) (10) water evaporation was limited by gas-water equilibrium, and (11) aerobic
degradation rates increased up to the lethal temperature limit for microbes (lethal limit for
aerobic microbes was assumed to be 65oC (Storm, 1985), while the anaerobic microbe lethal
limit was assumed to be 55oC, (Reinhart and Townsend, 1998)). Typical values for model input
parameters for MSW landfills were obtained from literature; parameters values and their sources
are described in Table 1.
2.2.2 Chemical Oxidation Kinetic Parameters
Literature includes numerous work on heat generation kinetics during biological degradation
(Yoshida eta al, 1999; Lefebvre et al, 2000; Lanini et al 2001; Wallner et al 2003; Yesiller et al,
2005; Gholamifard et al 2008). On the other hand, oxidation kinetic data for solid waste in
landfills are rarely found in literature. To estimate the chemical oxidation kinetic parameters,
experimental data from Moqbel (2010) were used. Moqbel (2010) subjected samples of solid
waste samples in a cylindrical mesh basket inside a furnace to gradual heating at a rate of
3oC/min. Temperatures were recorded for the furnace, sample surface, and sample center.
Temperature records from this experiment were analyzed to provide kinetic data for the energy
balance model. The energy balance equation represented by Equation 1 was applied to the solid
waste samples inside the furnace, simulating temperature rise inside the sample. Due to the
shape of the mesh basket holding the waste sample inside the furnace, the energy balance
equation was converted to cylindrical coordinates, as shown in Equation 10.
Genpgg Hr
TvCTk
t
TC ∆+
∂
∂+∇=
∂
∂ρρ 2 (10)
Boundary conditions assumed are provided in Equations 11 and 12:
)( ∞−−=∂
∂TTh
r
Tk t>0, r = ro (11)
0=∂
∂
r
T at r = 0 (12)
Initial conditions assumed are provided below:
t = 0, T = Ta
Where: r is the radial distance, ro is the sample radius, T∞ is the furnace temperature, and Ta is
the ambient room temperature.
To estimate the heat generated from the solid waste during gradual heating, additional
assumptions were made: (1) thermal conductivity was constant and the temperature profile
symmetrical around the center, (2) the air inside the furnace was completely mixed with
temperature increase at rate of 3oC/min, and, since the air flows around the sample more than
through it, the furnace convective heat loss inside the sample is negligible. Accordingly, the
second term from the right side of Equation 10 was neglected.
Table 1: Values for solid waste parameters
Parameter Value Reference A1 3800 s
-1 Wallner et al 2003
A2 1E45 Wallner et al 2003
ACh 0.15 s-1
Simulation
Aan 0.05 yr-1
Vesiland et al, 2002
AEva 2.8x106 s
-1 Rostami et al, 2003
Cp 1939 J/kg.K Yoshida eta al, 1999
Cpg 1010 J/kg.K Incropera and Dewitt, 2002
E1 38260 J/mol Wallner et al 2003
E2 285204.5 J/mol Wallner et al 2003
ECh 40000 J/mol Simulation
EEva 19.5 Kcal/gmol Rostami et al, 2003
f 0.00141 Wallner et al 2003
h 10 W/m2.K Incropera and Dewitt, 2002
k 0.1 W/m.K Yesiller et al ,2005
Lo 0.17 m3/Kg wet waste Vesiland et al, 2002
Qae 14944375 J/kg glucose Wallner et al 2003
Qan 15696 J/ m3-CH4 Tchobanoglous et al, 1993
QCh 11600 kJ/kg Tchobanoglous et al, 1993
Qev 2257000 J/kg Incropera and Dewitt, 2002
ρ 750 kg/m3 Vesiland et al, 2002
ρg 1 kg/m3 Incropera and Dewitt, 2002
The solution for the model was obtained iteratively to determine the values of A and E.
therefore, the solution obtained is not unique; different combinations of A and E values can
produce similar results. Results from model verification show a good agreement between data
set and simulated data as shown in Figure 2. Values for A and E for the chemical heat generation
for a synthetic MSW sample were found to be 65000 kJ/mole and 2200 s
Figure 2: Temperature profiles for the MSW samples and
3. RESULTS AND DISCUSSION
3.1 Survey responses
Thirty seven responses were received regarding landfill fire. Out of these 37 responders, only 22
reported incidents of subsurface fires. The remaining responses described surface fires.
results indicate spontaneous fires are not l
operation. Subsurface fire responses came from landfills including both cells construction
aboveground and underground. Landfill fires occurred in lined and unlined cells. Also,
responses came from conventional landfills as well as landfill practicing leachate recirculation.
Responders reported experiencing spontaneous subsurface fires in all types of waste;
commercial, municipal, industrial, and construction and demolition. Considering the different
properties of the different solid waste types, variation in the solid waste did not change the
possibility of fires occurrence. Age of waste was also reported in the survey, landfill cell’s ages
at fire varied between 11 months and 19 years.
3.1.1 Pre-fire events
In this part of the survey, events that lead to the spontaneous combustion fire (i.e. high
temperature, intrusion of air, and changing moisture content) were described. Events prior to fire
included rain, strong winds, dumping of hot loads, or aggre
Approximately 33% of the fires were noted after dumping of hot loads, 27% after strong winds
and 7% after aggressive gas extraction, 13% after rain and 13% reported fires after hot weather
conditions, and 7% reported other ev
fouling in the sparks arrestor.
for a synthetic MSW sample were found to be 65000 kJ/mole and 2200 s-1
, respectively.
Figure 2: Temperature profiles for the MSW samples and center temperature simulation
RESULTS AND DISCUSSION
Thirty seven responses were received regarding landfill fire. Out of these 37 responders, only 22
reported incidents of subsurface fires. The remaining responses described surface fires.
results indicate spontaneous fires are not limited to any waste type or landfill structure or type of
Subsurface fire responses came from landfills including both cells construction
aboveground and underground. Landfill fires occurred in lined and unlined cells. Also,
conventional landfills as well as landfill practicing leachate recirculation.
Responders reported experiencing spontaneous subsurface fires in all types of waste;
commercial, municipal, industrial, and construction and demolition. Considering the different
properties of the different solid waste types, variation in the solid waste did not change the
possibility of fires occurrence. Age of waste was also reported in the survey, landfill cell’s ages
at fire varied between 11 months and 19 years.
In this part of the survey, events that lead to the spontaneous combustion fire (i.e. high
temperature, intrusion of air, and changing moisture content) were described. Events prior to fire
included rain, strong winds, dumping of hot loads, or aggressive landfill gas extraction.
Approximately 33% of the fires were noted after dumping of hot loads, 27% after strong winds
and 7% after aggressive gas extraction, 13% after rain and 13% reported fires after hot weather
conditions, and 7% reported other events. Other events included dumping industrial waste and
, respectively.
center temperature simulation
Thirty seven responses were received regarding landfill fire. Out of these 37 responders, only 22
reported incidents of subsurface fires. The remaining responses described surface fires. Survey
imited to any waste type or landfill structure or type of
Subsurface fire responses came from landfills including both cells construction
aboveground and underground. Landfill fires occurred in lined and unlined cells. Also,
conventional landfills as well as landfill practicing leachate recirculation.
Responders reported experiencing spontaneous subsurface fires in all types of waste;
commercial, municipal, industrial, and construction and demolition. Considering the different
properties of the different solid waste types, variation in the solid waste did not change the
possibility of fires occurrence. Age of waste was also reported in the survey, landfill cell’s ages
In this part of the survey, events that lead to the spontaneous combustion fire (i.e. high
temperature, intrusion of air, and changing moisture content) were described. Events prior to fire
ssive landfill gas extraction.
Approximately 33% of the fires were noted after dumping of hot loads, 27% after strong winds
and 7% after aggressive gas extraction, 13% after rain and 13% reported fires after hot weather
ents. Other events included dumping industrial waste and
3.1.2 Detection methods
Survey results showed that fire was detected primarily by observing smoke or steam emitted
from the surface. Nearly 59% of the responses reported visual observation of smoke (38%) and
steam (21%). About 23% of the responses indicated detection of fire by changes in the landfill
surface, i.e. sudden depression (13%) and cap cracks (10%). Also, 5% of the responses reported
detection by high concentration of carbon monoxide and 3% for interruption in LFG flow. The
remaining 10% reported other methods including high temperature in LFG, and flames and
smoke from leachate collection system at the time of shutdown for maintenance. None of the
responders reported using thermal imaging to detect subsurface fire.
3.1.3 Extinguishing methods
Generally, extinguishing methods were based either on preventing air from accessing the fire
area or cooling the burning material. The study showed that the primary extinguishing methods
are excavation the burning waste (40%) and covering it with soil (29%). Extinguishing by water
has been used regularly (17%), but not as the sole method; it is always combined with soil cover,
excavation, or both. Reason for not being able to extinguish the fire by water only might be
explained by the dual effects of water by cooling the solid waste in one part and changing the
moisture content in another part encouraging continuity of the subsurface fire. Also, channeling
might change the water route away from the fire location inside the landfill. Inert gas injection
was not broadly used, only 3% of landfills reported using inert gas injection. Almost 11%
reported using other methods including covering with foam or geomembrane cover and shutting
down the LFG extraction system.
3.2 Simulation Scenarios
Four scenarios have been selected as the most probable for heat generation in landfills:
anaerobic biodegradation, aerobic biodegradation, aerobic and chemical oxidation, and chemical
reaction. Temperature increases for the scenarios occur at different time scales, therefore, MSW
consumption was used as the basis for comparison of the scenarios.
Gas flow inside the solid waste has an important role in controlling the landfill temperature. Gas
flow rate can control the rate of oxygen supply to promote microbial and chemically-mediated
oxidation. Higher gas flow rate results in more oxygen available for the reactions. In this model,
an oxygen concentration gradient is assumed across the simulated section; therefore, the effect
of gas velocity on oxygen concentration is not seen. Gas flow rate also controls the rate of
moisture evaporation. Also, increasing gas flow rate increases the amount of water lost to
evaporation, resulting in a decrease in landfill temperature. However, increasing gas flow rate
might mean an increase in heat loss by convection. Therefore, a comparison between the heat
loss of water due to evaporation and heat loss by convection and conduction was made as a
function of the extent of waste degradation (Figure 3). The comparison shows the significant
role of the water evaporation in the heat loss.
Figure 3: Profile of Heat Loss Components in the Aerobic Degradation
3.2.1 Scenario 1: Anaerobic Degradation
Operating a landfill under anaerobic conditions has historically been the most widespread
practice. Heat generation under anaerobic conditions occurs at relatively low temperature (25-
55oC). Thus, heat generation is assumed to be caused by anaerobic biological degradation of
organics only with little, if any, contribution from chemical reactions. Results from the model
simulation are presented in Figure 6. The model predicts an increase in temperature from
ambient up to 42oC. Then, temperatures decline as MSW is completely consumed. For this
simulation, initial moisture content is assumed to be 30% by weight dry basis and is sufficient to
support degradation.
3.2.2 Scenario 2: Aerobic Degradation
Scenarios 2 and 3 correspond to the condition of either operating an aerobic landfill or
introducing air unintentionally through an overdraft on the gas collection system. Scenario 2
represents the condition where heat generation is mainly due to microorganisms activities.
Chemical oxidation occurs in this scenario, but it is negligible compared to the aerobic
biodegradation.
Similar to anaerobic conditions, temperature increases rapidly at the beginning then stabilizes as
waste consumption is completed. In this simulation, temperature reaches 63oC which is higher
than the anaerobic scenario. Results from model simulation are also presented in Figure 4.
Moisture evaporation plays a significant role in the heat generation of the landfill. In this
scenario, gas flow rate was assumed to be 3.5x10-5
m/s. The gas flow rate results in sufficiently
high evaporation rates to produce heat loss necessary to maintain landfill temperatures within a
range suitable for microbes. A higher flow rate would result in greater evaporation rates
producing more heat loss and a decrease in the landfill temperature.
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 2 4 6 8 10 12 14
Chan
ge
in t
emper
ature
∆T
(C
)
Mass Consumption (% of total MSW)Cond. & Conv
3.2.3 Scenario 3: High Temperature conditions (Hot Spots)
This scenario represents the heat generation from biological activities and chemical oxidation
reactions leading to elevated temperatures in the landfill. However, each of these reactions
dominates over a different temperature range. The biological heat generation prevails from
ambient room temperature to 65oC, while chemical oxidation dominates at temperature higher
than 65oC. Simulations have been conducted for two scenarios both of them operating under
aerobic conditions, hot-spot creation and spontaneous combustion. Results from model
simulations are presented in Figure 4.
Initially temperature increases significantly due to the aerobic degradation of the solid waste. As
temperature increases, chemical oxidation becomes more effective and further heat generation
occurs. As in Scenario 2, water evaporation serves as a heat sink; however in Scenario 3 a lower
gas flow rate (7x10-6
m3/s) and, consequently less water evaporation, results in an increase in the
landfill temperature well above lethal temperatures. The maximum temperature exceeded 100oC.
3.2.4 Scenario 4: Spontaneous combustion
In this scenario, further reduction in gas flow rate leads to spontaneous combustion where the
water evaporation rate is not adequate to balance heat generation (See Figure 4). The simulation
was stopped when temperature reached 200oC as further increase in temperature caused the
model to diverge and no solution could be attained. It is assumed that beyond this point
combustion will continue, consuming all combustible material.
Considering simulation results, gas flow rate is a major factor in heat dissipation because higher
gas flow rates carry away more water vapor. Therefore, higher gas flow rate can result in higher
heat loss and as a result lower temperatures in the landfill. Water evaporation appears to be the
controlling factor in spontaneous combustion fires in landfills. However, as previously stated,
this simulation does not consider the effect of gas flow rate on oxygen supply. Lower gas flow
rate will also decrease the amount of oxygen which may limit heat generation.
Figure 4: Temperature Profiles of Modeled Landfill Scenarios at LFG Well
0
20
40
60
80
100
120
140
160
180
200
0 10 20 30 40 50 60
Tem
per
ature
(C
)
Mass Consumption (% of total MSW)
0
20
40
60
80
100
120
140
160
180
200
0 10 20 30 40 50 60
Mass Consumption (% of Total MSW)
Tem
pera
ture
(C
)
Aerobic
Hot Spot
Spontaneous Fire
Anaerobic
4. CONCLUSIONS
Subsurface fires are occasional incidents that can have severe consequences for landfills and
surrounding communities. Spontaneous combustion occurs when materials are heated beyond
their ignition temperature. The survey showed that subsurface spontaneous fires are not
restricted to any landfill geometry, landfill type of operation, or type of waste. Survey responses
suggested intrusion of air, change of moisture content, and increase in temperature due to hot
loads as possible factors encouraging subsurface fires. Oxygen introduction resulting in
biological waste degradation and chemical oxidation is believed to be the main cause of rising
solid waste temperatures and ignition. Moisture content and gas flow rates play an important
role in controlling the temperature and subsequent spontaneous combustion.
A model was created to simulate temperature rise in landfill. Results indicate that moisture
evaporation is the major heat sink in the landfill. The model showed that gas flow has a cooling
effect due to evaporation of water. However, decreasing air flow may limit the amount of
oxygen necessary to promote chemical and biological oxidation of the solid waste and,
concomitantly heat generation would decline. The addition of oxygen-free gas can reduce the
landfill temperature through water evaporation without promoting oxidation. The model showed
that appropriate gas flow rates can control the temperature rise inside the landfill and that
temperatures higher than the biological limit can be maintained in the landfill without initiating
spontaneous fire. The model demonstrated that insufficient heat sinks lead to dramatic increases
in temperature, followed by ignition and combustion.
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