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Journal of Hazardous Materials 175 (2010) 920–927

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

xperimental determination of self-heating and self-ignition risks associatedith the dusts of agricultural materials commonly stored in silos

lvaro Ramíreza,∗, Javier García-Torrentb, Alberto Tascónc

BIPREE Research Group, Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid, SpainLaboratorio Oficial Madariaga, Universidad Politécnica de Madrid, C/Alenza 1-2, 28003 Madrid, SpainDepartamento de Ingeniería y Ciencias Agrarias, ETSI Agraria, Universidad de León, Av. Portugal 41, 24071 León, Spain

r t i c l e i n f o

rticle history:eceived 23 March 2009eceived in revised form 21 October 2009ccepted 23 October 2009vailable online 4 November 2009

a b s t r a c t

Agricultural products stored in silos, and their dusts, can undergo oxidation and self-heating, increasingthe risk of self-ignition and therefore of fires and explosions. The aim of the present work was to deter-mine the thermal susceptibility (as reflected by the Maciejasz index, the temperature of the emissionof flammable volatile substances and the combined information provided by the apparent activation

eywords:elf-heatingelf-ignitionust explosiongricultural materials

energy and the oxidation temperature) of icing sugar, bread-making flour, maize, wheat, barley, alfalfa,and soybean dusts, using experimental methods for the characterisation of different types of coal (nostandardised procedure exists for characterising the thermal susceptibility of either coal or agriculturalproducts). In addition, the thermal stability of wheat, i.e., the risk of self-ignition determined as a functionof sample volume, ignition temperature and storage time, was determined using the methods outlinedin standard EN 15188:2007. The advantages and drawbacks of the different methods used are discussed.

ilos

. Introduction

Respiration processes are presented in all organic materialstored in silos. When these processes are in the presence of oxygent is called aerobic respiration. Carbohydrates, proteins and fats arehen broken down to carbon dioxide, water, and energy. The energyiberated during respiration is used by the cells to fuel metabolicrocesses. Dry mature grains in storage experience a very lowetabolic activity. However, immature grains freshly harvested or

rains with high moisture content have a higher activity. Duringhese processes heat is released and if a sufficient heat exchange isot produced between the reaction system and the external envi-onment, the increase in temperature further increases the ratef these reactions, and the product becomes ever warmer—a phe-omenon known as self-heating [1]. The increase in temperature

nitially occurs as a result of a biological activity which normallyoes up to 55 ◦C and exceptionally up to 75 ◦C; bacteria, insects,ites and moulds contribute to this increase besides the metabolic

rocesses in the grains. Afterward a chemical oxidation processtarts up to at least 150 ◦C. Self-heating not only may lead to theecomposition of the material [2,3], with the release of flammableases but it also may be responsible of major fires and risk of

∗ Corresponding author. Tel.: +34 91 3365625; fax: +34 91 3365625.E-mail addresses: alvaro.ramirez@upm.es (Á. Ramírez), jgtorrent@qyc.upm.es

J. García-Torrent), alberto.tascon@unileon.es (A. Tascón).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.10.096

© 2009 Elsevier B.V. All rights reserved.

explosion if the ignition point is reached—a phenomenon knownas self-ignition [4]. Different physical and chemical factors such asthe oxygen available, the rate of heat loss or the rate at which heatis generated are involved.

Even the dust associated with agricultural materials, which canaccumulate around conveyer belts, around machinery, and in ware-houses etc., can undergo self-heating and self-ignition [5,6].

Therefore, it is important to characterise the risk of self-heatingand self-ignition of these materials before to design any prevent-ing measure in these type of storage facilities (limitation of storagevolumes, recirculation of the material to control the storage tem-perature, design of protection devices, cleaning and maintenanceprocedures. . .) [7,8].

In order to safely be transported and stored, the self-heatingand self-ignition phenomena in coals have been theoretically andexperimentally studied by many researchers since the earliest stud-ies of Semenov [9] and Frank-Kamenetskii [10] until the numerical3D simulations carried out in these days [11–20]. This research hasalso pointed to the study of different types of wood products andbiomass [21–23]. However, not enough research has been carriedout to characterise the risk of self-heating and self-ignition of agri-cultural products. Therefore, there is still a lack of data regarding

these properties for agricultural materials stored in silos as well asthe dusts.

A number of experimental techniques can be used to charac-terise the thermal susceptibility of bulk solids and thermal stability.Some commonly used methods are crossing point measurements,

Á. Ramírez et al. / Journal of Hazardous Materials 175 (2010) 920–927 921

Table 1Identification, description and place of collection of the tested dusts.

Dust material Place of collection

Icing sugar Storage sitesMaize Silo wallsWheat Pneumatic suction conduit coming from a

grain reception siteBarley Silo walls

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2

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TM

Table 3Variables determined to characterise thermal stability.

Variable Initials Test apparatus used

Reaction with H2O2 (Maciejaszindex)

MI Dewar flask

Temperature of emission offlammable volatiles

TEFV Tube oven

ThermogravimetryWeight increase WIBH ThermobalanceTemperature of combustiononset

TIC Thermobalance

Temperature of maximumloss of weight

TMWL Thermobalance

Differential scanning calorimetryOnset temperature ofexothermic reaction

TOE Differential calorimeter

Final temperature ofexothermic reaction

TDE Differential calorimeter

Alfalfa Filter bagBread-making wheat mix Pneumatic conveyor lineSoya Silo walls

xygen adsorption, isothermal and adiabatic calorimetry and tem-erature differential for attempting to predict the spontaneouseating tendencies in laboratory experiments, as reviewed by Car-as and Young for coals. Thermogravimetric (TG) and differentialcanning calorimetric (DSC) analyses are available for estimatinghe risk of self-ignition, as others arebased on isothermal oven tests.hese techniques have been used by the Laboratorio Oficial Joséaría Madariaga (LOM) to assess the risk of self-ignition associatedith different types of coal [24–26]. The aim of the present workas to examine the thermal behaviour of two powdered materials

nd the dusts of five agricultural materials. Since no standardisedrocedures exist for use with such materials, those employed byhe LOM to characterise the thermal susceptibility (the propensityo oxidise [as reflected by the Maciejasz index], the thermal degra-ation [as reflected the temperature of the emission of flammableolatile substances] and self-ignition [as reflected by the apparentctivation energy and the oxidation temperature]) of different coalsere followed.

In addition, the thermal stability of wheat, i.e., the risk ofelf-ignition, determined as a function of sample volume, ignitionemperature and storage time, was determined using the methodsutlined in standard EN 15188:2007 [27]. A simpler method is thene followed by the United Nations (UN) procedure for classifica-ion of dangerous goods for transportation [12], originally based onust one basket size (1 dm3) and one temperature (140 ◦C).

The advantages and drawbacks of the different methods usedn this research are discussed and quantitative data provided thathould help establish safer storage and handling conditions forgricultural products. This work completes the results obtained inprevious study examining the sensitivity of these materials to

gnition and the severity of their explosions [28].

. Methodology

The study materials were the dusts of seven agricultural prod-cts commonly stored in silos: maize, wheat, barley, alfalfa,oybean, icing sugar and bread-making flour. These dusts were col-ected from different places at a number of silo storage installations

Table 1).

The moisture content of each was determined using a halogennalyser; the particle size distribution of each was determined byaser diffraction (see Table 2).

able 2oisture content and mean particle size.

Dust material Moisture content (%) D50 (�m)

Icing sugar 0.2 17.9Maize 13.5 215.1Wheat 7.7 36.4Barley 7.7 33.9Alfalfa 5.6 39.0Bread-making wheat mix 13.4 55.6Soya 10.5 51.7

Temperature of onset rapidreaction

TRE Differential calorimeter

Temperature of self-ignition SIT Isothermal oven

A number of variables (see Table 3) were examined in order todetermine the thermal behaviour of these products.

Since no standardised procedures exist for determining the ther-mal susceptibility of agricultural products, the procedures used bythe LOM for the characterisation of the thermal susceptibility of coalfrom different parts of Europe were followed [29–32]. The LOM hasalso used the same procedures to investigate the thermal suscepti-bility of lycopodium powder, olive residue, dry sludge and animalwaste (LOM, unpublished results).

2.1. Determination of thermal susceptibility

2.1.1. Calculation of the Maciejasz index (MI)The MI reflects an organic product’s susceptibility to self-

ignition, and is obtained by reacting the sample with hydrogenperoxide (H2O2) and then recording any change in temperature.If the substance reacts with oxygen an exothermic reaction willoccur and the temperature of the system will rise. The MI is aninverse function of the time (t) (in minutes) necessary for the sam-ple to increase in temperature by 65 K from the initial ambienttemperature when reacts with the oxygen peroxide Eq. (1):

MI = 100t

(1)

When the MI is >10 there is a risk of self-ignition.In the present work, 5 cm3 of distilled water were added to 10 g

samples in a Dewar flask, double-walled with the space betweenthe two walls exhausted to a very high vacuum, stirring until ahomogeneous paste was obtained. Thirty millilitres of H2O2 werethen added from a burette and the temperature change measuredover time. All tests were performed in sextuplet (Fig. 1).

2.1.2. Determination of the temperature of emission of flammablevolatile compounds (TEVF)

This is the temperature at which flammable volatile substancesare released during the thermal degradation of organic matter. TheTEFV is determined by heating a portion of the sample to differ-ent temperatures and then approaching it with a glowing wire (anignition source) (Fig. 2).

In the present work, samples of 2 g were placed in a test tubeinside an aluminium block which was heated to the known Mini-

mum Ignition Temperature with dust deposited on a layer (previouslydetermined [28]). When the sample reached this temperature, theemission of volatile flammable substances was tested by approach-ing the sample with a glowing wire. If no flame appeared, thetemperature of the sample was increased by 10 K and the process

922 Á. Ramírez et al. / Journal of Hazardous Materials 175 (2010) 920–927

Fig. 1. Diagram showing the stages for determining the Maciejasz index.

Table 4Thermogravimetry test variables measured.

Variable Value

◦

rpisw

2

pamp

omta

atWt

tacu

Fig. 3. Determination of the TIC, TMWL, WIBH variables from a thermogravimetricanalysis with air carried out with a sample of wheat dust.

Initial temperature 30 CFinal temperature 800 ◦CHeating rate 5 K/minIsothermal time at 800 ◦C 10 min

epeated. When a flame did appear, the temperature of the sam-le was reduced by 10 K and the process repeated. Each new test

nvolved a fresh sample. The temperature at which flammable sub-tances were emitted was taken as the test temperature 10 K belowhich a flame appeared (as confirmed by three tests).

.1.3. Thermogravimetric analysis (TG)This is used to determine the weight loss experienced by sam-

les when subjected to programmed heating. Thermogravimetricnalyses were performed with a TG-DSC Mettler Toledo TG-50odel. The temperature range was 303–1073 K. Experiments were

erformed under a carrier air flow at a heating rate of 5 K min−1.A 70 �l crucible filled with the sample is weighed, placed in an

ven and heated at a linear heating rate. Several variables can beeasured in this kind of test, including the temperature of combus-

ion onset (TIC), the temperature of maximum weight loss (TMWL),nd the weight increase at the beginning of heating (WIBH) (Fig. 3).

The TMWL is a clear indication of the reactivity of the productnd represents the yield of volatile matter produced by pyrolysis;he higher the temperature, the lower the product’s reactivity. The

IBH is a reflection of oxygen adsorption during initial heating andhe initial oxidation of the sample.

The results obtained can vary substantially according to theype of test used and the test conditions. To make tests repeat-ble, standardised conditions should be used, but unfortunately noonsensus exists regarding what these should be. Table 4 shows thesual conditions used by the LOM for the analysis of coal [24–26];

Fig. 2. Diagram showing the stages for the determination of the t

Fig. 4. Determination of the Tcharac variable in a thermogravimetric analysis carriedout in the presence of an oxygen stream in a sample of wheat dust.

these were followed in the present work. The reproducibility ofthese tests are high because they are not influenced by the personthat carry out the test. The uncertainty of the results obtained is±5 K.

2.1.4. Thermogravimetric analysis in the presence of an oxygenstream (TG–O2)

This is used to determine the characteristic oxidation temper-ature. In conventional TG, as above described, the loss of weightexperienced by the sample occurs slowly, and it is therefore difficultto assign a single oxidation temperature (Tcharac). The procedurefollowed in the present technique is similar to the above described,however, employs an oxygen stream rather than an air stream, and

the reaction therefore takes place quickly (Fig. 4).

This allows a Tcharac to be assigned. This value allows differentdusts to be classified in terms of their ignition risk.

Table 4 shows the usual conditions used by the LOM for theanalysis of coal; these were followed in the present work.

emperature of emission of flammable volatile compounds.

Á. Ramírez et al. / Journal of Hazardous Materials 175 (2010) 920–927 923

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2

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arorttw

tLroo

2

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l

ig. 5. Determination of TOE, TRE and TDE variables from a differential scanningalorimetry test carried out with a sample of wheat dust.

.1.5. Differential scanning calorimetry (DCS)This is used to determine the heat exchange that occurs in a

ust sample. This heat exchange is measured relative to an inerteference material and in accordance with an underlying temper-ture programme. The sample and inert reference material areeated in a well-insulated furnace and the temperature differenceetween the sample and inert reference material is recorded. Theeat flow is a differential that is proportional to the temperatureifference between the sample and the reference and is calculatedy multiplying the data by a calibration function. Endothermic orxothermic reactions will be produced depending on whether theeat is absorbed or emitted.

Tests were performed with a Mettler Toledo, DSC-25 model withcontrol device. DSC was registered from 303–823 K with a heat-

ng rate of 20 K min−1. The inert reference material was CaCO3,ecause it has thermal properties that remain constant throughouthe temperature range studied in this work.

The test procedure consists of heating the sample in a 50 �lluminium crucible and recording a number of variables; the expe-ience of the LOM is that the minimum temperature required for thenset of an exothermic reaction (TOE), the maximum temperatureeached during that exothermic reaction (TDE), and the tempera-ure required for the onset of a rapid exothermic reaction (TRE) arehe best variables for characterising coals [24–26]. These variablesere therefore recorded in the present work (Fig. 5).

Table 5 shows the initial temperature, the heating rate, the finalemperature and the isothermal time conditions suggested by theOM [25–26]. These conditions were used in the present work. Theeproducibility of these tests is high because they are independentn the person that carries out the test. The uncertainty of the resultsbtained is ±5 K.

.1.6. Determination of the apparent activation energy (Ea)The Ea of a sample is calculated at the point of maximum weight

oss using the data from the conventional thermogravimetric anal-

sis with air stream and the Cumming’s equation [33] Eq. (2):

n(

− 1w

· dw

dt

)= ln A − Ea

RT(2)

Table 5DSC test variables measured.

Variable Value

Initial temperature 30 ◦CFinal temperature 550 ◦CHeating rate 20 K/minIsothermal time at 550 ◦C 10 min

Fig. 6. Diagram showing the installation used to carry out thermal stability analysis.

where w is the weight of the unburnt sample (g), dw/dt is theinstant velocity of loss of weight (g s−1), A the frequency factor,Eathe apparent activation energy in J mol−1, R the universal gasfactor (8.314 J mol−1 K−1), and T the absolute temperature (K). Thisequation relates the activation energy to the rate of weight loss, andprovides an estimate of the Ea from the slope of the least-squaresline fitted to the test data. The Ea, which is based on first orderreaction characteristic equations, provides a means of represent-ing the ease with which self-heating takes place [34] together withthe Tcharac.

In the present work, the speed of the loss of weight of thesamples recorded in the thermogravimetric analysis was deter-mined and plotted against the corresponding temperatures. Thisprovided a cloud of points that obeyed the Cumming’s equation.Least-squares adjustment was then performed to obtain a sloperepresenting the Ea.

2.2. Thermal stability analysis of wheat dust

The thermal stability of a material is based on the determinationof self-ignition temperatures in samples of different volumes inan isothermal oven that reproduces environmental temperatures(Fig. 6).

In the present work, tests were performed only with wheat dustdue to resource limitations and following the procedures outline instandard EN 15188:2007 [27]. The change in the temperature ofthe sample with respect to the temperature of the oven is recordedover time. Three different curves may be obtained (Fig. 7):

Curve A: subcritical. At an experimental temperature TA thesample becomes hotter, approaching the temperature of the oven.The sample does not produce heat by itself, and no ignition isobserved.

Curve B: critical. The temperature of the sample slightly sur-passes that of the oven (TB) is slightly exceeded for time, but then

tends towards the oven temperature.

Curve C: supercritical. Heat production in the sample surpassesits heat losses. Eventually, non-stationary conditions are reachedand the sample temperature increases rapidly over that of the oven(TC), and ignition occurs.

924 Á. Ramírez et al. / Journal of Hazardous Materials 175 (2010) 920–927

Fi

ttantm

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tii

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3

3

acTs

Table 6Maciejasz index (MI) and temperature of emission of flammable volatile substances(TEFV).

Material MI TEFV (◦C)

Icing sugar No reaction Melts at 190Maize No reaction 270Wheat No reaction 280Barley No reaction 280Alfalfa No reaction 300

ig. 7. Thermal behaviour (sub- and supercritical) of the dust materials in thesothermal oven.

The experiment is repeated for different sample volumes, raisinghe oven temperature until combustion occurs—the self-ignitionemperature (SIT). The SIT is the mean of the lowest temperaturet which self-ignition occurs and the highest at which ignition doesot occur (the latter determined by three confirmatory tests), andhe difference between the temperatures recorded for TB and TC

ust be ≤5 K Eq. (3):

IT = TB + TC

2(3)

The SIT is recorded for different oven temperatures, along withhe time required for the oven temperature to be suprassed (thenduction time [Tind]). As the volume of the sample tested isncreased, the SIT becomes lower and the Tind longer.

In the present work, the SITs of different volumes (50, 150, 350nd 1500 cm3) of the test material were determined. The size ofhe samples (log[V/A] where V is volume and A is surface area witheposited dust) was then plotted against the inverse of the SIT1/SIT) in an Arrhenius diagram, as well as against the Tind. Regres-ion curves for the results were plotted to show the transitionetween stationary and non-stationary behaviour in the differentamples. The uncertainty of the results obtained is ±5 K.

. Results and discussion

.1. Thermal susceptibility

MI and TEFV variables were determined to provide informationbout self-ignition because of they have been extensively used foroals and data is available for numerous types of coals [25–26].able 6 shows the MI and TEFV results for the different materialstudied. No reaction with H2O2 was recorded for any of the materi-

Fig. 8. DSC assay: (a) wheat d

Bread-making wheat mix No reaction 290Soya No reaction 330

als, suggesting they do not easily oxidise and therefore do not easilyself-ignite. This, however, was not confirmed by the results of theTG and DSC analyses (see below). The TEFVs recorded ranged from270 ◦C for maize to 330 ◦C for soybean dust. The icing sugar meltedat 190 ◦C.

Table 7 shows the results for the TG and DSC variables recordedfor the present materials and, for the sake of comparison, thosestudied by the LOM (lycopodium powder, olive residue, dry sludgeand animal waste). The icing sugar dust was apparently the mostsusceptible to self-ignition; its TIC was 212 ◦C and its TMWL 220 ◦C.Since icing sugar dust melts (at 190 ◦C) before its TIC is reached thepores between the particles will disappear, leaving the individualparticle surface reaction zone with no oxygen supply; self-ignitionshould therefore not take place in large stored volumes of thismaterial. The highest TICs and TMWLs were those of the wheatdust and bread-making flour. Intrinsic characteristics of the testedmaterials are emphasized in the thermogravimetric analysis car-ried out in the presence of a current of oxygen and as a result it wasobtained that Tcharacs were higher than TICs practically in all cases.Both types of thermogravimetric analysis carried out are needed toprovide information about the ignitability of the tested materialsand both temperatures should be seen as complementary to eachother.

Table 7 also shows that the onset of an exothermic process(TOE) began in some materials at 80–100 ◦C. These are the low-est temperatures registered at which the first stage slow processof an exothermic process starts for organic products generally. Thehumidity of the material is still decreasing until these temperaturesare reached—endothermic process. Afterward oxygen starts tointeract with the material at its surface producing oxo-compoundsand despite the loss of humidity of the material continues, theprocess becomes exothermic at an initial stage.

The melted icing sugar dust, however, only started to undergo anexothermic reaction at 243 ◦C (results obtained for liquid [melted]samples). Fig. 8 shows the unique behaviour of the icing sugar dustin comparison with the more typical behaviour shown by the wheat

dust. The change in the icing sugar dust’s physical state is clearlyreflected by an endothermic peak.

Table 8 shows the Ea and Tcharac results for the studied materialsand, by way of comparison, for the materials studied by the LOM.

ust, (b) icing sugar dust.

Á. Ramírez et al. / Journal of Hazardous Materials 175 (2010) 920–927 925

Table 7Values of TG and DSC variables recorded for the experimental and LOM-tested products.

Material TG DSC

WIBH (mg) TMWL (◦C) TIC (◦C) TOE (◦C) TDE (◦C) TRE (◦C)

Icing sugar 0 220 212 243 480 410Maize 0 279 268 100 386 242Wheat 0 283 252 95 283 252Barley 0 271 242 80 311 257Alfalfa 0 276 231 92 288 240Bread-making flour 0 282 277 103 341 271Soya 0 265 225 97 306 245Lycopodium 0 282 235 83 314 213Olive residue 0 270 245 100 310 240Dry sludge 0 290 230 95 300 220Animal waste 0 300 180 80 370 -Subbituminous coal 0 365 318 102 291 220Bituminous coal 3 407 392 71 323 240Semi-anthracite coal 2.3 501 303 73 406 265

Table 8Apparent activation energy (Ea) and oxidation (characteristic) temperature (Tcharac)of the LOM-tested materials.

Material Ea (kJ/mol) Tcharac (◦C)

Icing sugar 61.7 239Maize 64.0 289Wheat 65.5 279Barley 65.6 277Alfalfa 67.6 294Bread-making flour 64.0 300Soya 65.2 281Lycopodium 67.7 332Olive residue 68.5 340Dry sludge 72.0 280Animal waste 73.0 280

6

ambwiisb

3

tta

oo

TR

S

Subbituminous coal 74.0 241Bituminous coal 82.0 261Semi-anthracite coal 89.0 375

The Ea of the studied materials ranged from around 61 to around8 kJ/mol.

Fig. 9 shows the Ea and Tcharac values for all these materi-ls graphically, with the proposed risk classification zones clearlyarked. Clear differences in the self-ignition risk can be seen

etween the coals, a consequence of their consolidation times. Theheat, barley, maize, alfalfa and soybean dusts, however, all group

n the high risk zone. The olive residue and icing sugar dust fallnto the middle and high risk categories respectively (although ithould be remembered that icing sugar dust melts at 190 ◦C). Theread-making flour dust was associated with a medium risk.

.2. Thermal stability analysis

This test was only performed with the wheat dust. Table 9 showshe SITs and Tinds for the different volumes tested. The tempera-

ure required to cause ignition fell with the volume of the sample,lthough the Tind increased.

Fig. 10a shows a plot of sample size (log [V/A]) against the inversef the SIT (1/SIT) in an Arrhenius diagram. Fig. 10b shows the sizef the sample plotted against the Tind. The linear regression model

able 9esults of thermal stability test involving wheat dust.

Cell size (cm3) Lowest temperature of ignition (◦C) Highest temper

50 190 185150 180 175350 170 165

1500 155 150

IT = self-ignition temperature

Fig. 9. Risk of self-ignition according to the apparent activation energy and oxida-tion (characteristic) temperature.

with a 90% confidence interval traces the boundary between self-ignition and no ignition.

The results show that a volume of 100 m3 of wheat dust willself-ignite at 65 ◦C after about 14 years have elapsed. A volumeof 0.650 m3 will self-ignite in only 3.3 months, although an envi-ronmental temperature of about 100 ◦C would be needed. By wayof comparison, a 0.650 m3 volume of bituminous coal would self-ignite at 40 ◦C after just 1.9 months [26]. However, external heatsources—such as the sun—could change the Tinds. The temperaturereached in a silo due to the sun will depend on the time of theyear, whether the silo is made of steel or concrete, and whetherit stands alone or in a group of silos, etc. In addition, the specific

weight of the bulk solid or its compaction (which affects the size ofthe solid–air interface), also affect the heating process. Silo shapemay also influence this.

In his work, Hensel [35] extrapolated correlations between the‘dust volume: surface area with deposited dust’ ratio and the Tind

ature with no ignition (◦C) SIT (◦C) Induction time (min)

187.5 44177.5 92167.5 196152.5 462

926 Á. Ramírez et al. / Journal of Hazardous Materials 175 (2010) 920–927

eat du

forahtaaaidcth

3s

1rDElnahspmitfl

4

iiatsfrIoi

Fig. 10. Extrapolation of wh

or self-ignition, thus providing an explanation for the self-ignitionf coal piles that occurred in the Berlin area during the 1980s. Theseesults were useful to predict induction times for large coal pilesnd explain their self-ignition. In this study, induction times forigher amounts of wheat dust have been predicted by extrapolatinghe results according to the Semenov method. This procedure is alsopplicable to the rest of the agricultural materials of this study. Theuthors have decided to adopt the behaviour model of Semenovmong those outlined in standard EN 15188:2007 [27] because ofts simplicity. On the other hand, it is also assumed the level ofispersion of the results (the regression model used considers aonfidence level of 90%). In this way, considering these assumptionswo different zones, where self-ignition may be produced or not,ave been identified.

.3. Advantages and drawbacks of thermal susceptibility andtability analyses

Thermal stability tests (such as that outlined by standard EN5188:2007) take a long time (several weeks) to complete andequire the use of large samples (sometimes >20 kg). The TG andSC tests (thermal susceptibility tests) for the determination of thea and Tcharac proposed in the present work overcome these prob-ems, however for most organic materials, since TG and DSC areot able to detect slow exothermic reactions and since they usu-lly overestimate the start of exothermic phenomena, they do notave the same reliability than thermal stability tests to assess theelf-heating of samples. TG and DSC tests can be used to select thoseroducts that are more prone to self-ignition when a large numberust be characterised and then study in more detail this property

n those materials by using thermal stability tests. Thermal stabilityests will also allow studying not only finely ground materials suchour or dust as it is in TG and DSC tests but also coarse materials.

. Conclusions

The present results provide information important for prevent-ng the onset of exothermic oxidation processes and self-heatingn the dusts of agricultural materials. None of the tested materi-ls reacted with H2O2 suggesting they do not easily oxidise, andemperatures of over 270 ◦C were required for flammable volatileubstances to be emitted. However, the MI index may not offer use-

ul information in these materials since all those examined wereevealed prone to self-ignition according to TG and DSC analysis.cing sugar dust in particular appeared to present a very high riskf self-ignition, although the low melting of the product wouldmpede its occurring. Maize, wheat, barley, alfalfa and soybean

[

[

[

st thermal stability results.

dusts pose high risks of self-ignition, while the bread-making flourwas associated with a medium risk.

Thermal susceptibility analysis (TG and DSC tests) offers advan-tages over thermal stability analysis (as described by standard EN15188:2007) in terms of the very much smaller sample sizes andshorter test times required. However, they can only be used withfine materials such as dusts. TG and DSC tests could be used toselect the products that are more prone to self-ignition and thencharacterise them by using thermal stability tests.

The data presented in this work could be useful in silo design.However, the thermal stability results should be interpreted withcare since they involve extrapolation. Factors, such as silo sizeand shape, the material from which silos are made, the level ofcompaction of the stored product etc., and external heat sources,should also be taken into account when determining the risk ofself-ignition.

Acknowledgement

The authors thank the CICYT (Interministerial Commission ofScience and Technology) for funding this work (project code:AGL2005-07430-C02-01/AGR) and the Asociación de Fabricantesde Harinas y Sémolas de Espana (AFHSE) and the Harinera EmilioEsteban S.A. for providing test materials.

References

[1] J. García Torrent (Ed.), Seguridad industrial en Atmósferas Explosivas, Labora-torio Oficial J.M. Madariaga, Madrid, Spain, 2003.

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