Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159

Contents lists available at ScienceDirect

Process Safety and Environmental Protection

jou rn al hom epage: www.elsev ier .com/ locate /psep

Coal dust emissions: From environmental control to riskminimization by underground transport. An applicativecase-study

B. Fabiano ∗, F. Currò, A.P. Reverberi, E. PalazziDICheP – Chemical and Process Engineering Department “G.B. Bonino”, University of Genoa, Via Opera Pia 15, 16145 Genoa, Italy

a b s t r a c t

Coal dust emission from open coal storage parks and handling plants can represent a significant part of the overall

atmospheric emission contribution in different industrial activities, such as power stations or coke production plants.

The emissions are strictly connected to technical characteristics of the handling system, chemico-physical properties

of coal and environmental conditions. In this paper, we present a full-scale operational solution to the problem of

limiting coal dust emissions during transportation from the harbor site to the final utilization plant. To this end, a

simple mathematical framework is developed in order to evaluate explosion risk inside the enclosure and set-up

proper technical and managerial layers of protection suitable to minimize the risk.

© 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Coal accidents; Dust pollution; Explosion; Emissions; Transportation

follows: firstly a statistical study on coal dust explosions is pre-

1. Introduction

In many developed countries, coal is still one of the mainenergy sources, to be used either for heat generator furnaces,or for electrical energy production by power plants. Coal isextracted from mines often far away from the final utilizationplant and, among different transportation means, trains andships are most frequently used. It is well-known that through-out the whole coal handling cycle (unloading and loading ofships, trains and trucks, loading onto, removal from piles andconveyor belt transfer stations) dust emission represents aserious concern. The emission depends on different param-eters related to the specific activity or to the characteristicsof the material, e.g.: particle size distribution, coal type, mois-ture content, pile configuration, dumping height, as well as onweather related parameters, e.g.: wind speed, relative humid-ity etc. Considering open storage, pile morphology influencesthe emission rate: flat-topped stockpile configurations couldlead to a significant decrease of the dust emission rate com-pared to crested ones (Turpin and Harion, 2009). A conveyorbelt, or a small wagon ropeway is often used as a meansof transport from the harbor, or railway temporary storage

site, to the final utilization plant. Even if the coal chemical

∗ Corresponding author. Tel.: +39 010 3532585; fax: +39 010 3532586.E-mail address: brown@unige.it (B. Fabiano).Received 11 July 2012; Received in revised form 9 November 2012; Acc

0957-5820/$ – see front matter © 2013 The Institution of Chemical Engihttp://dx.doi.org/10.1016/j.psep.2013.01.002

composition does not contain any hazardous pollutants, emis-sions from wagons is capable of producing a considerableenvironmental impact particularly when crossing residentialareas, causing nuisance due to dust deposition and contribut-ing to the PM10 atmospheric concentration (see Fig. 1). Theproblem is only partially faced by adopting a fully confinedconfiguration of the transportation wagons, as the loading andunloading phases from them still represent critical issues. Inthis paper, we present a full-scale operational solution to theproblem of limiting coal emissions consisting in the designof an underground mini-tunnel with an open pipe conveyor,properly equipped and instrumented. It stands to reason thatthis technique allows us to face decisively air pollution causedby dusts by applying an inherent safety prevention crite-rion. Clearly, as discussed by Kletz (1993), one must considerso-called unforeseen side-effects of protecting environment.The proposed design option involves a thorough risk anal-ysis, quantitative risk evaluation and planning of suitablecontrol and prevention measures to mitigate explosion/firerisk by both reducing the probability and the magnitude ofthe adverse consequences. The remainder of this paper is as

epted 3 January 2013

sented. Secondly a theoretical approach to the risk evaluation

neers. Published by Elsevier B.V. All rights reserved.

Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159 151

Nomenclature

a empirical coefficient defined in Eq. (1)A area of the tunnel transversal section, m2

b width of sedimentation area, mbc belt width, mdp particle mean diameter, � mg gravitational constant, m s−2

h tunnel height, mL belt length, mmd flux of the i-th granulometric class by sedimen-

tation, g s−1

mi flux of the i-th granulometric class due to con-veyor belt effect, g s−1

ms flux of the i-th granulometric class by suspen-sion mechanism, g s−1

m′′si

specific dust emission, g m−2 s−1

n empirical coefficient, defined in Eq. (1)Rep Reynolds numberv air velocity, m s−1

v0 maximum wind velocity without any observ-able effect on the dust suspension, m s−1

vi∞ sedimentation velocity of the i-th granulomet-ric dust class, m s−1

vn net linear velocity of particles, m s−1

vr relative velocity, m s−1

Va volumetric flux, m3 s−1

Greek letters˛ sloping angle, –�a air density, kg m−3

�i density of the i-th granulometric class, kg m−3

�p density of coal dust, kg m−3

�a kinematic viscosity of air, m2 s−1

iddre

1

DiraoreobDtpioawidM

Fig. 1 – Small wagon ropeway starting from the harbortowards industrial end-users and crossing residential

17.4% of the recorded accidents are connected to design and

Fig. 2 – Immediate cause of accidents connected to coal andcoal dust, elaborated from FACTS database (Unified

nside the transport tunnel is proposed lastly a completeesign of the transport system is presented, with a detailedescription of the plant design and of the safety control andisk prevention systems, together with data on attained dustmission rates.

.1. Coal accident statistics

ust explosions still account for several fatalities and vastnsurance claims: around 50 explosions are reported each year,anging from small deflagrations to detonations. They are usu-lly associated with the grain and mining industries, but canccur whenever a process uses combustible particulate mate-ials, either as feedstocks, intermediates or products (Shaht al., 2004). In the following, we provide a statistical surveyn accidents occurring in the coal sector utilizing the dataank FACTS managed by the Unified Industrial & Harbor Fireepartment in Rotterdam-Rozenburg (NL). It must be stated

hat, sometimes, for minor accidents, without damage to peo-le and/or without relevant damage to structures, forms are

ncomplete. From the beginning of this century to 2011, a totalf 98 accidents connected to coal were found in the database:mong them, 72 events refer to coal (19 with fatalities and 28ith injuries) and 26 to coal dust (8 with fatalities and 12 with

njuries). On the whole, unwanted events with coal and coal

ust were responsible for 171 and 39 fatalities, respectively.aking reference to the main accident scenario classification,

areas.

the most common scenario corresponds to Explosion (56.0%),followed by Fire (30.8%) and Release (13.2%). The subsequentanalysis of concurring causes according to three macroar-eas, namely plant/process, environment and organization(Fabiano and Currò, 2012), yields that plant/process accountedfor 28.3% of total entries, organization 60.8% of the entries,while the remaining 10.9% are connected to environment. Thedetailed distribution of the immediate cause of accidents isdepicted in Fig. 2. Among complete records, the main directcauses are “Human error” and “Technical failure/malfunctionof component or equipment”, corresponding to 37% and28.3% of the entries respectively. It must be noticed that

Industrial & Harbor Fire Department inRotterdam-Rozenburg (NL)).

152 Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159

Fig. 3 – Accident location for coal and coal dust events,elaborated from FACTS database (Unified Industrial &

Harbor Fire Department in Rotterdam-Rozenburg (NL)).

management errors, i.e. accidents connected to human fail-ure at a different stage. This classification was made in orderto distinguish between accidents identified as the ultimateactions of front line personnel (in the individual context orig-inated by a moment’s aberration, or poor training, or lackof supervision) from those related to historic unsafe acts bydesigners, trainers and/or managers. Accident location forevents connected to coal and coal dust is depicted in Fig. 3,where one can notice that the majority of entries under bothcategories is connected to power and process plants. The dis-tribution of accidents among the different activities, for bothcoal and coal dust is depicted in Fig. 4. These last resultsare in agreement with Abbasi and Abbasi (2007) showing thatthe highest explosion frequencies in the Federal Republic ofGermany in industries processing coal were recorded in con-veying systems and storage units including silos and hoppers,respectively accounting for 26% and 22% of the accidents sur-veyed. Similar results were also given by Nifuku et al. (2000)reporting a survey focused on coal dust explosions in Japanover the period 1952–1995 and evidencing a figure of 13 acci-

dents causing 7 fatalities and 41 injuries. It must be remarkedthat data on dust explosions collected in Germany over a

Fig. 4 – Activity/plant unit involved in coal and coal dustaccidents, elaborated from FACTS database (UnifiedIndustrial & Harbor Fire Department inRotterdam-Rozenburg (NL)).

long period, revealed that electrical equipment is a minorsource of ignition (3.2% of recorded cases), while other sourcessuch as mechanical sparks/friction (32.7%), static electricity(8.5%) and glowing clumps (12.7%) are of much greater impor-tance (Plumb, 2010). In designing the transportation system,consideration was given to the above-mentioned statistics,from one side trying to remove or reduce opportunities forhuman failure, from the other side increasing the reliability ofsafety devices, by redundancy and management systems (e.g.adequate test frequency procedure of safety instrumentedsystems).

1.2. Theoretical approach

The modelling approach here outlined is based on the evalu-ation of the minimum explosion concentration of coal dustdetermined on experimental basis and the calculation bya simplified mathematical model of the maximum attainedconcentration in the tunnel, both during transport and stand-by of the conveyor belt, as a function of the longitudinalairflow.

As a preliminary remark, it must be observed that theexplosion of an organic dust suspension, such as coal dust,follows three subsequent steps, namely particle heating, itsdevolatilization/pyrolysis and then, predominantly homoge-neous oxidation of the pyrolysis gases. Understanding theimportance of the rate-determining step of the whole com-bustion process is obviously a key to develop adequateprevention and protection measures (Dufaud et al., 2010).The impact of pyrolysis on dust explosions was studied in-depth and Hertzberg et al. (1987) evidenced that, notably forcoal particles, the combustion is controlled by the gas phasecombustion for small particles and low dust concentrations,whereas for high dust concentrations and large particles, thecombustion is controlled by the pyrolysis step. As summa-rized by Amyotte et al. (2010) explosibility data should includethe maximum explosion pressure in a constant-volume explo-sion vessel; size-normalized maximum rate of pressure risein a constant-volume explosion vessel; minimum explosibledust concentration (MEC); minimum ignition energy of thedust cloud and the minimum ignition temperature of the dustcloud.

Dust concentration is connected to different concurrent,or consequential events such as suspension of particulatematters; dumping activities, for instance during unloadingfrom a ship and in temporary storage silos; transportationwith the underground belt; transport of particulate mat-ters by the ventilation system and partial sedimentationof suspended solids. A complete analysis of all differentphenomena connected to the different phases would requirea complex mathematical description (e.g. to be performedby an in-depth and time consuming CFD characterizationof the confined volume). In the following we apply a sim-plified model suitable to attain conservative results of theeffects.

As reported in literature (Smit et al., 1976; De Faveri et al.,1990; Visser, 1991) the emission factors of particulate matter(in terms of specific emissivity (g m−2 s−1)) due to transport ofdusty materials are connected to:

• intrinsic physical dust properties;• operative characteristics;

Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159 153

Fig. 5 – Scheme of the physical model of the slopedm

•

d1f

fptmafotcr1m

bF

m

wmsidie

2

Rsapntctsdc

si i i∞ si i0 i∞

ini-tunnel.

material treatment (no treatment, humidification; humidi-fication with surface agent water solution or coating agents,etc.).

When considering coal dust, the specific emissivity in theumping phase (Smit et al., 1976) ranges from 1 g m−2 s−1 to00 g m−2 s−1, in connection with a dust humidity rangingrom 15% (w/w) to nearly 0.

The emissions are roughly inversely proportional to theourth-fifth power of the moisture content, thus making thisarameter the most important policy instrument in reducinghe environmental impact of dusts (Visser, 1991). When the

oisture content of coal is more than 6.5–7.5% the emissionsre practically negligible for most wind velocities. Emissionsrom coal piles are significantly influenced by wind actionn the pile surface (in addition to material density, mois-ure content, particle distribution and pile geometry): specificoal emissivities, m′′

si, equal to 0.044 and 0.44 (g m−2 s−1) were

ecorded in connection with a wind velocity of 6 m s−1 and0 m s−1 respectively and in absence of any pile surface treat-ent (Smit, 1980).According to different authors, the specific emissivity can

e correlated to the wind velocity by Eq. (1) (Grossel, 1988; Deaveri et al., 1990):

˙ s = a[v − v0(dp)]n (1)

here a and n are empirical coefficients and v0 is the maxi-um wind velocity without any observable effect on the dust

uspension, which increases as the mean particle diameterncreases. A conservative hypothesis on the dust diameteristribution was assumed, also taking into account the exper-

mental results for different coal types, reported by Sapkoat al. (2007).

. Mathematical modelling

eference is made to the physical model of the mini-tunnelchematized in Fig. 5 and to a volume dV characterized by

length dx and a transversal area dA. The following sim-lifying assumptions were made: constant ventilation rateeglecting diffusive effects and wall effects, also in view ofhe small tunnel diameter compared to the overall length;onstant dust concentration in each tunnel section normalo the ventilation flux. These conservative hypotheses (corre-ponding to an ideal case) yield an underestimation of the dust

eposition and, consequently, an overestimation of the dustoncentration in air. In fact, for a given average concentration

in a section, dust particles are more concentrated in the lay-ers near the coal surface, which have a higher sedimentationvelocity.

The net dust flux of the i-th granulometric class leaving thevolume, due to the combined conveyor belt and ventilationvelocities is:

dmi = d(�ivnA) = (v − vi∞ sin ˛)bh · d�i (2)

the net dust flux of the i-th granulometric class leaving thevolume by sedimentation is:

dmd = �ivndAx = �ivi∞ cos · b · dx (3)

the net dust flux of the i-th granulometric class entering thevolume by suspension mechanism is:

dms = m′′sidAx = m′′

si · bc · dx (4)

Under steady-state conditions, under which one can expectmaximum dust concentration, the mass balance can beexpressed as

dmi = dms − dmd (5)

or else

(v − vi∞ sin ˛)bh · d�i = m′′si · bc · dx − �ivi∞ cos · b · dx (6)

from which we can write:

d�i

dx= m′′

sibcb

− �ivi∞ cos ˛

(v − vi∞ sin ˛)h(7)

Making reference to the already mentioned Fig. 5, thesteady-state ventilation velocity corresponds to following sit-uation:

vi∞ <v

sin ˛(8)

so that, from Eq. (7), following conditions may be distin-guished:

d�i

dx

⎧⎪⎪⎪⎪⎪⎨⎪⎪⎪⎪⎪⎩

> 0 if m′′si

bc

b> �ivi∞ cos ˛

= 0 if m′′si

bc

b= �ivi∞ cos ⇒ �i = �∗

i

< 0 if m′′si

bc

b< �ivi∞ cos ˛

(9)

From the different conditions reported in Eq. (9), one caneasily evaluate the average distribution of the coal dust gran-ulometry along the longitudinal tunnel length (X-axis).

By integration of Eq. (7) under the boundary condition �i

(0) = �i0, one can write

1vi∞ cos ˛

lnm′′

sibcb

− �i0vi∞ cos ˛

m′′si

bcb

− �ivi∞ cos ˛= x

h(v − vi∞ sin ˛)(10)

or else

bcm′′ − � v cos =

(bc

m′′ − � v cos ˛

)

b b

× (1 − e−[vi∞ cos x/h(v−vi∞ sin ˛)]) (11)

154 Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159

C

E

A

B

D

F

GH

End

User

yout

response actions including fire-fighting. A picture of the

Fig. 6 – Transportation plant la

from which the final expression can be obtained:

�i = �i0 +(

bc

b

m′′si

vi∞ cos ˛− �i0

)(1 − e−[vi∞ cos x/h(v−vi∞ sin ˛)])

(12)

As shown in Appendix 1, the limiting values of the designparameters are fully consistent with the obtained final expres-sion.

In order to evaluate the coal dust deposition rate, weassume that deposition and suspension phenomena occursimultaneously so that the deposition velocity can be calcu-lated from Bird et al. (2007):

v∞=

⎡⎢⎢⎢⎢⎢⎣

118

d2p�−1

a g�p − �a

�aRep=v∞ı

�a≤ 2

0.15dp1.14�−0.43

a

(g

�s − �a

�a

)0.712 < Rep < 500

1.7dp0.5

(g

�s − �a

�a

)500 ≤ Rep ≤ 2 · 105

⎤⎥⎥⎥⎥⎥⎦

(13)

where

�p = 1200 kg m−3

�a = 1.2 kg m−3

� = 1.5 × 10−5 m2 s−1

g = 9.8 m s−2

The evaluation requires the knowledge of the particlesize distribution: as a conservative operative hypothesis weassume a bimodal distribution of the coal dust and obtain thecorresponding deposition velocities summarized in Table 1.

The deposition rate under the same simplifying hypothesespreviously adopted can be calculated as follows:

b − bc ˙ ′′

md,tot =b

Va(�0 − �L) + msbcL (14)

Table 1 – Bi-modal size distribution of coal particles,assumed as calculation hypothesis.

Granulometric class Frequency (%) dpi (�m) v∞(m s−1)

1 50 5 9.1 × 10−4

2 50 20 1.45 × 10−2

from harbor area to end-user.

2.1. Case study description

The design of the transportation system can be seen as aprimary containment system, being designed to keep haz-ardous materials contained within the process. In this respect,the concrete made mini-tunnel given in Fig. 6 is divided intothree stretches, each provided with segregation and venting toatmosphere by proper ventilation systems, suitable to act asmitigative safeguards. On the basis of the geometric and oro-graphic design constraints, the average tunnel cross sectioncorresponds to Ar = 7 m2 and the overall volume of the tunnel,including temporary storage, corresponds to Vt = 12,500 m3.The first tunnel section D, is about 530 m long starting fromship terminal A and the unloading sections B and C. Thesecond stretch, F, of the mini-tunnel starts from temporarystorage silos E. It is 645 m long, located at a depth of nearly15 m below the sea floor, up to the intermediate station (G).The third section of mini-tunnel H is nearly 450 m long andends in close proximity to the end-user.

As previously commented, the first layer of protectionagainst dust explosions is to identify and prevent factorswhich can trigger a dust explosion, namely the ignitionsources which can occur inside the system and include incan-descent material, hot surfaces, sparks, self-heating and staticelectricity. In this respect as preventive systems (Pekalskiet al., 2005) are foreseen ignition avoidance, hot spot detec-tion and extinguishing systems. Further receptor mitigativefactors include occupied building blast resistance, special-ized personal protective equipment, and detailed emergency

Fig. 7 – Picture of the concrete-made mini-tunnel.

Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159 155

Fig. 8 – Picture of the open pipe conveyor belt localizedinside the mini-tunnel.

ct

•

•••

ihmt(taeaoTtptwtfdpwwcasdtdattits0t

Fig. 9 – Scheme of the mini-tunnel section and thetransportation plant equipped with protective preventionsystems (A: video recording system; B: foam wettingsystem; C: transportation belt; D: water curtain/sprinkler

′′ ′′ vr

oncrete mini-tunnel is reproduced in Fig. 7. The main fea-ures are summarized as follows:

total length of the system, including the stations of depar-ture, arrival and intermediate silos: ca. 1700 m;

half-pipe belt speed: 2.5 m s−1; linear belt capacity: 0.06 m3 m−1 of bulk product; total capacity: ca. 500 t h−1.

The open pipe belt is designed not to disperse materialnto the gallery and not to cause electrostatic charges. Thealf pipe section, depicted in Fig. 8, was designed to mini-ize particle uptake, by minimizing the coal surface exposed

o ventilation, in analogy with results by Turpin and Harion2009). Considering fire hazards, the construction material ofhe belt is self-extinguishing. The system is comprised of anctive system for the minimization of coal dust dispersion,nabled by an aqueous solution containing a wetting agentnd a polymer suitable to form a foam reducing the chancesf the coal particles becoming airborne for at least 20 min.he foam suppression solution was preferred so as to limit

he amount of water applied to the coal. An automatic systemrovides the dosage of the foaming water solution by injec-ion at seven points. Each dosing station must be equippedith suitable control systems and signalling of water level in

he tank. Three dosage points are positioned on the belt andour points are located along the whole height of the interme-iate silos. These last four points act as follows: the injectorsositioned at the upper and lower ends will act continuouslyhile the sprayers located at 1/3 and 2/3 of the total fall heightill be controlled by opacity meters and will intervene only in

ase of an increased dust concentration, so as to avoid haz-rdous dust conditions. The whole control and emergencyystem is fully automatic, requiring human intervention onlyuring maintenance operations. A schematic picture of theunnel section and of the equipped transportation plant isepicted in Fig. 9. The ventilation system is designed to ensure

necessary and adequate ventilation rate is maintained inhe mini-tunnel at a constant underpressure with respect tohe outside environment. The underground transport facilitys equipped with suitable aeration towers, installed close tohe ends of the tunnel and at the intermediate points. Underteady-state conditions a transversal ventilation velocity of

−1

.5 m s is ensured in the whole mini-tunnel by the ventila-ion fans; under emergency conditions auxiliary fans, located

equipment; E: grid pavement and under-floor drain.

at the intermediate station G, ensure an average transversalventilation velocity of 2.0 m s−1.

We hypothesize a constant air flow rate in each of the fourair ventilation points and an average of four air changes perhour under steady-state operations, thus obtaining an averagetransversal air velocity va,tunnel = 0.5 m s−1.

Design results, for all system sections can be summarizedas follows:

v = 0.5 m s−1

bc = 0.7 mb = 2.8 mh = 2.8 m

= 25◦

From the geometric characteristics of the site (see Fig. 6)the length of the different system sections are:

LFH = 1096 mLFG = 645 mLHG = 451 m

Assuming dp,1 = 5 �m and dp,2 = 20 �m, as reported inTable 1, the dust concentration at the beginning of the tunnelis easily calculated on the basis of a conservative assumptionof the specific emissivity during the dumping phase (includingthe wetting effect) m′′

s,base = 0.1 g m−2 s−1:

�10 = �20 = �p

2= 0.285 g m−3

Under the design conditions, the specific emissivity wasevaluated starting from experimental values previously men-tioned, according to Eq. (15), in order to consider the combinedeffect of ventilation velocity and belt speed:

( )3

ms = ms,base vr,base(15)

156 Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159

Fig. 10 – Calculated average particle concentration profiles along the axis of the tunnel stretch FH, for the two granulometricclasses (�1 and �2) and total dust concentration (�T), under steady-state transportation conditions, in connection with an

average air velocity of 0.5 m s−1.

3. Results and discussion

In the following, the results obtained by the application of thedeveloped approach are presented in graphical form.

Fig. 10 shows the dust concentration profile along the sec-tion FH of the transportation system for the two granulometricclasses and the total dust concentration. Reference is made tosteady-state conditions: transport belt speed of 2.5 m s−1 andaverage transversal air velocity of 0.5 m s−1. Under these con-ditions it is possible to calculate a relative velocity vr = 2 m s−1

in connection with a cautious hypothesis of the specific emis-sivity (m′′

s,base = 0.01 g m−2 s−1) based on the results by variousauthors (De Faveri et al., 1990; Fabiano et al., 1994; Calvin et al.,1996; Chang et al., 2003; Ghose, 2004).

In case of no ventilation, the dust concentration isdepicted in Fig. 11, considering an average relative velocity ofvr = 2.5 m s−1 (i.e., the conveyor belt speed).

Under emergency conditions, the activation of emergency

fans located at the intermediate station G, results in an

Fig. 11 – Calculated average dust concentration profiles along theclasses (�1 and �2) and total dust concentration (�T), in connectioventilation.

average relative velocity of respectively vr = 0.5 m s−1, in sec-tion FG, and vr = 4.5 m s−1 in section GH, from which itis possible to derive the concentration trends shown inFigs. 12 and 13 respectively.

As a concluding remark on the concentration profiles,we can observe that the maximum concentration attain-able under all considered conditions is at least an orderof magnitude lower than the minimum explosive dust con-centration (MEC = 30 g m−3), notwithstanding all conservativehypotheses made (Chawla et al., 1996; Dastidar et al., 1997).Clearly, the simplifying assumptions made and the sim-ple analytical approach do not allow for evaluating localdust concentration fluctuations, while providing only aver-age evaluations, suitable to the purpose of the presentpaper.

The evaluation of dust deposition by applying Eq. (15) underthe different operational situations, allows for the quantifica-tion of the frequency of proper housekeeping activities for the

avoidance of hazardous dust level accumulation. Following

axis of the tunnel stretch FH, for the two granulometricn with steady-state transportation conditions without

Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159 157

Fig. 12 – Calculated average dust concentration profilesalong the axis of the tunnel stretch FG, for the twogranulometric classes (�1 and �2) and total dustconcentration (�T), under emergency conditions,corresponding to an average air velocity 2.0 m s−1 providedb

tt

amtficgsmedrm

3

Wtmcapdlicdti

Fig. 13 – Calculated average dust concentration profilesalong the axis of the tunnel stretch GH, for the twogranulometric classes (�1 and �2) and total dustconcentration (�T), under emergency conditionscorresponding to an average air velocity of 2.0 m s−1

provided by emergency fans located at the intermediate

y emergency fans located at the intermediate station G.

his approach and making reference to the possible condi-ions, the following results are obtained.

steady−state conditions and designed ventilation rate md=4.8 g · s−1;

no ventilation md = 11.0 g · s−1;

emergency conditions md = 8.2 g · s−1;

The simulation results determine which design and man-gement criteria must be fulfilled in order to keep theaximum dust concentration inside the mini-tunnel below

he minimum explosible concentration (MEC) under the con-guration of full operability of the transportation belt. Asommented by Amyotte et al., 2009, consideration must beiven to the formation of both dust clouds and dust layers,o as to avoid accidental dislodgement of a layer of dust byan intervention. The potential dust re-suspension from lay-

rs represents a critical factor in the considered undergroundesign: the prevention of this hazard is connected to the keyole of good housekeeping (that can be seen as hazardous

aterial minimization).

.1. Preventive and protective measures

e can observe that under normal or near normal opera-ion, dust concentrations can be kept well below the level that

ight sustain combustion, which is clearly the primary con-ern in design. It must be mentioned that a key feature of dustccidents is often the staged nature of the explosion: a smallrimary deviation in a plant section produces a flow/structuralisturbance that in turns lofts more dust, eventually finally

eading to widespread secondary explosions. In addition, thegnition characteristics for coal with high volatile compoundsontent are reported by Mc Kinnon (1981) as follows for aust cloud: minimum ignition energy 30 mJ; minimum igni-

ion temperature 610 ◦C. Also considering this tendency tognition, the classification of the mini-tunnel was defined in

station G.

a conservative approach according to the requirements ofDirective 94/9/EC as zone 21, as suggested by Fritze (2010). Onthis basis, IEC 60079 series of standards (IEC, 2009) is to bestrictly followed in the design, selection and testing of elec-trical equipment, and all non-electrical equipment must becertified as well according to the ATEX directive. The ATEXdesign option involves additional investment costs that canbe estimated to be in the order of MEur and that could beprohibitive in other industrial applications. Mitigative safe-guards include fire/overheating detection by linear sensorsalong the conveyor belt and warning systems. In case offire detection, under steady-state transportation conditions,an automatic fire extinguisher system is activated, includ-ing sprinkler systems and fire water monitors. In addition,properly designed water curtains (Palazzi et al., 2009) can beactivated so as to limit and segregate the fire zone, allowinga safe way for possible human intervention. A detail of thesystem showing the open-pipe conveyor and designed mit-igative safeguards is given in Fig. 14. Many of the adoptedsafety measures, including condition monitoring and ade-quately planned maintenance, are addressed at improvingequipment reliability and reduced breakdowns.

The systems for ventilation and coal humidification con-trol should be of high integrity, so that they have a highpredefined probability of carrying out their intended func-tion within a predefined time. The control sensing systemmust ensure proper spraying of the solution when the con-veyor is running loaded with coal, while not spraying thesolution if the conveyor is running empty or is stopped. In

detail, the humidification system as a safety instrument canbe subdivided into three functional units, as suggested by

158 Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159

Fig. 14 – Picture of the transportation plant showing details

of the conveyor belt and designed layers of protection.

Laurent (2011). Following the classification, sensors (e.g. opti-cal transmitters), logic solver, final elements (e.g. filming liquidvalves), sub-systems, should be assessed for Safety IntegrityLevel (SIL) in conformity with IEC 61511 (2003) requirements.With specific reference to safety instrumented systems (SIS),the safety integrity is defined as the average probability ofa SIS to satisfactorily perform the required safety instru-mented functions (SIF) under all the stated conditions withina stated period of time. A LOPA (layers of protection anal-ysis) method was adopted being a suitable methodology todetermine the target SIL of the safety instrumented functionsimplemented on the BPCS (Basic Process Control System).The liquid storage tank normal fill level, Low alarm leveland Low Low level trip/alarm should be set in compliancewith the coal tank volume and operating levels of the coaltransportation system. Intermediate coal storage level instru-mentation and information display systems should be ofsufficient accuracy and clarity to ensure safe planning andcontrol of coal transfer in safe conditions from harbor to thefinal utilization site. The whole open pipe conveyor insidethe mini-tunnel is to be controlled with temperature probesand infrared camera. The safety function (monitoring sys-tem and related actors) should fulfil the requirement of SIL2.

Lastly, the human error likelihood for coal transfer shouldbe minimized by implementing:

• identification on video-graphics of coal route and receivingintermediate storage;

• monitoring of coal transfer into the conveyor;• automatic shut-off of coal transport in case of temperature

hot spot (by BPCS);• operating procedures for Low level alarm and Low Low level

trip/alarm in the water solution tanks;• warning for the avoidance of dust accumulation should be

given in the context of the need for proper housekeeping.

4. Conclusions

From an environmental point of view, coal dust emissionsduring open transportation raises environmental and healthconcerns especially in residential areas, due to dust depo-sition and the contribution to the PM10 concentration. Thedesign of an underground transportation system with an open

pipe conveyor is presented as a full scale solution for environ-mental control during the handling phase. From the safety

side, statistics here presented and previous studies evidencethat, in spite of technological improvement and developmentof safety management, coal dust explosions in confined con-ditions are still an up-to-date issue, mainly connected toplant/process causes, or organization deficiencies. A simpleanalytical model was developed to verify the safety condi-tions inside the tunnel and address proper preventive andprotective measures against possible dust explosions. Theevaluation of dust deposition rates allows for the quantifica-tion of the frequency of proper housekeeping activities for theavoidance of hazardous dust accumulation. As a clear limi-tation, the model, based on the ideal case assumption, doesnot allow for obtaining local fluctuations of the dust concen-tration, but only average values. The plant design includes thecombination of passive protection type measures, i.e., ignitionavoidance by proper classification and design, as well as activeprotection methods, i.e., dust concentration control maintain-ing it outside the explosive region and safety instrumentedsystems properly designed according to integrity targeting. Alltechnical safety measures must be combined with organiza-tional measures, such as staff training, use of process safetyperformance indicators, both lagging and leading, as well asadequate maintenance plan. The findings of this paper couldapply to other confined solutions, hereby controlling the envi-ronmental impact from emissions.

Appendix A. Appendix 1

The analytical cases corresponding to the limiting values ofthe geometric/operative parameters of the designed mini-tunnel and satisfying boundary conditions can be calculatedstarting from Eq. (12) as follows.

limx→∞

�i = bc

b

m′′si

vi∞ cos ˛�i,eq

limh→0+

�i = �i,eq

lim˛→�/2−

�i = �i0 + bc

b

m′′si

v − vi∞x

h

lim˛→0+

�i = �i0 +(

bc

b

m′′si

vi∞− �i0

)(1 − e− vi∞

vxh )

limvi∞→0+

�i = �i0 + bc

b

m′′si

v

x

h

limvi∞→0+

�i = �i0e−

x

hcot an ˛

⎡⎢⎢⎢⎢⎣

→ �

2

−

→ �i = �i0

→ 0+

→ �i = 0

⎤⎥⎥⎥⎥⎦

References

Abbasi, T., Abbasi, S.A., 2007. Dust explosions-cases, causes,consequences, and control. J. Hazard. Mater. 140 (1/2), 7–44.

Amyotte, P., Domaratzki, R., Lindsay, M., Khan, F., 2010. The roleof material safety data sheets in dust explosion preventionand mitigation. In: Suter, G., De Rademaeker, E. (Eds.), Loss

Prevention and Safety Promotion in the Process IndustriesXIII. Technologisch Instituut vzw, Bruge, pp. 427–430.

Process Safety and Environmental Protection 9 2 ( 2 0 1 4 ) 150–159 159

A

B

C

C

C

D

D

D

F

F

F

G

G

H

myotte, P.R., Pegg, M.J., Khan, F.I., 2009. Application of inherentsafety principles to dust explosion prevention and mitigation.Process Saf. Environ. 87, 35–39.

ird, R.B., Stewart, W.E., Lightfoot, E.N., 2007. TransportPhenomena, 2nd ed. John Wiley & Sons, New York, USA.

alvin, E.M., Emmitt, G.D., Williams, J.E., 1996. A rail emissionstudy: fugitive coal dust assessment and mitigation. In: Proc7th Environmental Virgin Symposium, Virginia Dept. ofEnvironmental Quality and VM Laboratories Inc., Lexington,Virginia, pp. 44–53.

hang, Y.-M., Hwang, J.-S., Chou, C.-M., 2003. Environ. Eng. Sci. 20(4), 265–280.

hawla, N., Amyotte, P.R., Pegg, M.J., 1996. A comparison ofexperimental methods to determine the minimum explosibleconcentrations of dusts. Fuel 75 (6), 654–658.

astidar, A.G., Amyotte, P.R., Pegg, M.J., 1997. Factors influencingthe suppression of coal dust explosions. Fuel 76 (7), 663–670.

e Faveri, D.M., Converti, A., Vidili, A., Campidonico, A., Ferraiolo,G., 1990. Reduction of the environmental impact of coalstorage piles: a wind tunnel study. Atmos. Environ. 24A (11),2787–2793.

ufaud, O., Perrin, L., Laurent, A., 2010. State of the art:promotion of early inherently safer design against dustexplosions. Chem. Eng. Trans. 19, 365–370.

abiano, B., Solisio, C., Ferraiolo, A., Bertocchi, D., Pastorino, R.,1994. Environmental and accident risk related to the differentcoal storage techniques. In: Proc 7th IUAPPPA Conference forPacific RIM on Air Pollution and Waste Issue Taipei, vol. V, pp.141–150.

abiano, B., Currò, F., 2012. From a survey on accidents in thedownstream oil industry to the development of a detailednear-miss reporting system. Process Saf. Environ. 90, 357–367.

ritze, J.P., 2010. Explosion protection concepts for storage,transport and grinding of coal. In: Suter, G., De Radaemaker, E.(Eds.), Loss Prevention and Safety Promotion in the ProcessIndustries XIII. Technologisch Instituut vzw, Brugge, pp.455–458.

hose, M.K., 2004. Emission factors for the quantification of dustin Indian coal mines. J. Sci. Ind. Res. 63, 763–768.

rossel, S.S., 1988. Safety considerations in conveying of bulksolids and powders. J. Loss Prevent. Proc. 1 (2), 62–74.

ertzberg, M., Zlochower, I.A., Conti, R.S., Cashdollar, K.L., 1987.Thermokinetic transport control and structural microscopicrealities in coal and polymer pyrolysis and devolatilization.

Their dominant role in dust explosions. Prepr. Pap. Am. Chem.Soc. Div. Fuel Chem. 32 (3), 24–41.

IEC 61511, 2003. “Functional Safety: Safety Instrumented Systemsfor the Process Industry”. International ElectrotechnicalCommission, Geneva, CH.

IEC 60079-10-2, 2009. Explosive Atmospheres – Part 10-2:Classification of Areas – Combustible Dust Atmospheres.International Electrotechnical Commission, Geneva, CH.

Kletz, T.A., 1993. The unforeseen side-effects of improving theenvironment. Proc. Safety Prog. 12,147–150.

Laurent, A., 2011. Sécurité des Procédés Chimiques, 2nd ed.Lavoisier, Paris, pp. 524–534.

Mc Kinnon, G.P., 1981. Fire Protection Handbook, 15th ed.National Fire Protection Association, Quicy MA, 02269.

Nifuku, M., Matsuda, T., Enomoto, H., 2000. Recent developmentsof standardization of testing methods for dust explosion inJapan. J. Loss Prevent. Proc. 13, 243–251.

Palazzi, E., Currò, F., Fabiano, B., 2009. From laboratory simulationto scale-up and design of spray barriers mitigating toxicgaseous releases. Process Saf. Environ. Prot. 87,26–34.

Pekalski, A.A., Zevenbergen, J.F., Lemkowitz, S.M., Pasman, H.J.,2005. A review of explosion prevention and protectionsystems suitable as ultimate layer of protection in chemicalprocess installations. Process Saf. Environ. Prot. 83 (B 1),1–17.

Plumb, K., 2010. Dust: hidden dangers. TCE: The ChemicalEngineer, 833, 43.

Sapkoa, M.J., Cashdollar, K.L., Greena, G.M., 2007. Coal dustparticle size survey of US mines. J. Loss Prevent. Proc. 20 (4/6),616–620.

Shah, A., Brindley, J., Griffiths, J., Mcintosh, A., Pourkashanian, M.,2004. The ignition of low-exothermicity solids by localheating. Process Saf. Environ. Prot. 82 (B2),156–169.

Smit, P.P.A., 1980. Dust concentrations around coal stockpiles. Sci.Total Environ. 15 (3), 207–216.

Smit, P.P.A., Riele, P.H.M, Richards, P.C., 1976. Air pollution by coaldust. In: Benarie, M. (Ed.), Proc. 12th International Symposiumon Atmospheric Pollution. Elsevier Science, Amsterdam, NL.

Turpin, C., Harion, J.-L., 2009. Numerical modelling of flowstructures over various flat-topped stockpiles height:implications on dust emissions. Atmos. Environ. 43,5579–5597.

Visser, G.Th., 1991. A wind tunnel study of the dust emissions

from the continuous dumping of coal. Atmos. Environ. 264,1453–1460.