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: shadi.moqbel@iu.edu.jo 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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