Author's Personal Copy ARTICLE in PRESS

Author's personal copy ARTICLE IN PRESS

Fire Safety Journal 45 (2010) 69–81

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Fire Safety Journal

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Pile burning of cutting debris in stands of hazel (Corylus avellana): An experimental study of smouldering combustion towards the validation of a burning protocol

Elsa Pastor a,n, Yolanda Pe´rez a, Alba Agueda a, Marta Miralles b, Eulalia Planas a a Department of Chemical Engineering, Centre for Technological Risk Studies, Universitat Politecnica de Catalunya, Diagonal 647, 08028 Barcelona, Catalonia, Spain b GRAF, Divisio´ Operativa de la DGPEIS, Generalitat de Catalunya, Ctra. Universitat Autonoma s/n, 08290 Cerdanyola del Valles, Spain article info abstract

Article history: Smouldering combustion in burning piles was experimentally investigated by studying temperature Received 16 March 2009 changes in six piles of 2 m of diameter of cutting debris of hazel (Corylus avellana) for three days after Received in revised form extinction. The piles were monitored using an IR camera and K-type thermocouples. The experiment 9 October 2009 was designed in order to study how the maximum temperature of the charcoal might be influenced by Accepted 15 October 2009 the individual and interaction effects of both the quality of extinction and the elapsed time until the Available online 13 November 2009 start of extinction of the piles. The piles that were properly extinguished (i.e. using a high-pressure, Keywords: homogeneously distributed water flow of 50 l/min for 4 min) had a uniform temperature profile and did Forest fires not have significant hot spots. The temperature reached equilibrium with the environment in less than Safety 10 h after extinction. In contrast, a smouldering front moved throughout the poorly extinguished piles, Smouldering temperatures which had a wide temperature distribution and hot spots of up to 700 1C. A simulation of windy IR monitoring conditions after three days of experiments on a poorly extinguished pile showed that the reactivation of charcoal combustion was possible. It gave a high-risk scenario to cause a wildfire, with hot smouldering embers being transported by wind flow. The results are of interest to improve pile burning protocols so that the number of wildfires caused by such practices may be reduced. & 2009 Elsevier Ltd. All rights reserved.

1. Introduction to the environment increases. As a result, the fuel cools down by radiation and convection until the combustion self-extinguishes. Pile burning of cutting debris is a silviculture technique that is Under favourable conditions, however, smouldering combustion widely used in the entire Mediterranean Basin mainly by can continue for hours or days, consuming charcoal at very low agricultural workers to eliminate slash and crop stubble and to velocities without smoking. The phenomenon is therefore un- push back the forest to make room for agricultural expansion [1]. detectable to the naked eye. In this practice, special care must be taken with residual hot spots Pile burnings are usually linked to wildfire lighting since that keep burning without flame, such as in blacking-out smouldering combustion creates a major hazard of reignition. operations where smouldering combustion might be present. Recent statistics [3] show that in Catalonia (NE Spain) 14.2% of the Smouldering combustion is associated with the surface wildfires declared between 1994 and 2005 were caused by oxidation of char that remains after flaming combustion. There agricultural burnings (debris and slash burnings being the most is no thermal decomposition of the fuel, since all of the volatiles representative activities). This category represented the main have disappeared during flaming combustion. The heat of cause of wildfires out of the fire summer season and the second combustion released in the non-flaming phase can account for a most important cause accounting the entire years. Similar figures significant percentage of the total: about 30% in the case of wood are found in the rest of Spain where 10.4% of the wildfires within [2]. Smouldering combustion can generally continue as long as the period 1994–2003 were due to agricultural burnings as well heat is conserved on the reacting surface, due to radiation [4] or in Portugal where agricultural and forest debris burns exchanges between incandescent surfaces and a contribution of represented the 20% of the total fires between July 2004 and oxygen that is sufficient to keep the reaction active. When a pile of December 2005 [5]. smouldering charcoal is opened out, the exposure of the surfaces Therefore, rigorous, science-based burning guidelines are needed. There are only a few examples of quantitative standards for pile burning—for instance, the one proposed by the NSW Rural

n Corresponding author. Tel.:+34 934011090; fax: +34 934011932. Fire Service [6]. These procedures provide some recommendations E-mail address: [email protected] (E. Pastor). about the dimensions and locations of piles. However, little

70 E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81

Nomenclature T2ext external thermocouple measurement halfway along a radius of the pile (1C)

TIR temperature of piles measured by IR monitoring (1C) T3int internal thermocouple measurement at the edge of 1 TIRmax maximum temperature measured by IR monitoring the pile ( C) (1C) T3ext external thermocouple measurement at the edge of 1 Np pile number the pile ( C) t relative time (h:min) N northern quadrant of the pile RH relative humidity (%) S southern quadrant of the pile

Uw wind speed (m/s) W western quadrant of the pile T1int internal thermocouple measurement at the centre of E eastern quadrant of the pile the pile (1C) L Theoretical distance reached by a lofted ember (m)

T1ext external thermocouple measurement at the centre of V0 theoretical initial velocity of the embers (m/s) 2 the pile (1C) g gravity (m/s ) 1 T2int internal thermocouple measurement halfway along a y mean shooting angle of the lofted embers ( ) radius of the pile (1C)

attention is paid to how extinction and surveillance should be 2.2. Experimental design performed. Some European forest or environmental agencies have defined pile burning procedures [7] but there are still some The pile burning protocols currently used in the Mediterranean unanswered questions regarding the efficacy of these protocols. Basin call for roughly the following succession of tasks: fuel The study described in this paper was undertaken to arrangement, pile ignition, fuel supply, fuel consumption phase, investigate smouldering combustion in charcoal from pile burn- opening-out of embers, extinction phase (i.e. mopping up) and the ings of leafy fuels in order to improve burning protocols according start of surveillance [17]. Bearing in mind this sequence and to scientific criteria. Although a lot effort has been done by the taking into account the first specific aim of the study, a 2 2 scientific community to study flaming combustion in wildfires, factorial experimental design was used, the two factors being (a) smouldering combustion has received historically little attention the elapsed time until the start of the extinction and (b) the and its fundamentals remain mostly unknown [8]. Nevertheless, it quality of the extinction. is noteworthy to mention the laboratory studies by Frandsen The first factor, a numerical variable, considers the time in [9,10], Miyanishi and Johnson [11] and Rein et al. [12] and the field minutes elapsed from the end of the opening-out of the embers in approaches by Hille and Stephens [13] and Rabelo et al. [14] all a pile to the start of extinction with water. This variable is mainly developed to better understand the controlling mechanisms of related to the residual diameter of the charcoal and thus to the smouldering of organic soils. Within this framework, only a few fuel available for smouldering. The longer the time elapsed theoretical or laboratory studies [15,16] have particularly ad- between the opening-out of the embers and the beginning of dressed the reignition hazard regarding smouldering combustion extinction, the smaller the residual diameter; hence, less fuel is and to the best of our knowledge, no experimental studies have available for smouldering. The second factor, a qualitative been conducted in real practice scenarios. variable, considers the water flow used and the person’s ability The specific aims of this study were as follows: firstly, to to put out the fire (i.e. the uniformity of the wetting along the experimentally analyse the changes in time and space of the perimeter and over the area of the pile, and the degree to which temperature of the charcoal from piles of tree cutting debris, embers are stirred up in this operation). which are influenced by the individual and interaction effects of These factors were split up into two levels, ‘high’ and ‘low’, as both the quality of extinction and the elapsed time until the start shown in Table 1. A ‘high-quality’ extinction means that this task of extinction; and secondly, to study the possible reactivation of is performed well, i.e. the person wets the pile homogeneously combustion in the charcoal by simulating a windy environment, with a water flow of 50 l/min for 4 mins, using high water three days after the experimental burnings. pressure to generously shake the embers; a ‘low-quality’ extinction means that extinction is done poorly, i.e. 4 min of low water pressure with a flow of 15 l/min, heterogeneously distributed, without surrounding the perimeter of the pile. A ‘high’ level in terms of elapsed time between the opening- 2. Materials and methods out of the pile and the start of extinction was set at around 80 minutes. During this time, it could be reasonably expected that 2.1. Study area some extra fuel would be consumed and some cooling effects would be seen. The ‘low’ level of elapsed time was set at 30 min, The study was carried out on a 6000 m2 terrace (geometric as the abovementioned effects were assumed to be less intense centre: UTM E337859, N4575294; elevation: 642 m) called Lo Pla after this period of time. del Mas d’en Bella, which is approximately 150 m long and 40 m wide. It is located along a narrow hollow oriented east to west in the Brugent valley of the Prades mountains, about 40 km north- west of Tarragona (Catalonia, Spain). The spot had been used for Table 1 hazel harvesting since the first plantation was established 35 Variables and associated levels of the 2 2 experimental factorial design. years ago. There were approximately 100 trees, with an average Variable Level + Level height of 2 m, and with approximately three or four hazel stumps for each live tree. The entire stand was cut during the second Quality of extinction High Low fortnight of November 2006, seven weeks before the experiments. Elapsed time until extinction 80 min 30 min Author's personal copy ARTICLE IN PRESS

E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81 71

With this factorial design, the experimental matrix shown in randomly covered all of the groups envisaged in the factorial Table 2 was developed. It was made up of six piles that were design. Pile 6 served as a ‘control’ pile, without any extinction, for identical in terms of dimensions and fuel load, but with different comparison purposes. Given the desire to conserve real-life pile values for the two factors under consideration. Piles 1 through 4 characteristics in the experimental design, and given also the

Table 2 Experimental matrix.

Pile number Quality of extinction Elapsed time until extinction Fireﬁgther

1 +A 2 A 3+ + C 4+ B 5+ C 6X X X

Fig. 1. Experimental layout. Author's personal copy ARTICLE IN PRESS

72 E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81 amount of fuel available for burning, one replica was also created. 2.3. Measurement of fuel characteristics and meteorological This was randomly chosen from among the four options conditions and numbered Pile 5. An arbitrary criterion was applied to assign a firefighter (A, B or C) to each pile. Pile 6 did not need a The fuel was set up 15 days before the day of the burning. The firefighter, since extinction was not performed there. Fig. 1 shows dry bulk density of the cut hazel branches (fine fuel measuring a diagram of the entire experimental layout and a view of the less than 6 mm in diameter and coarse fuel measuring less than study area. 15 mm in diameter) was estimated as follows. A certain volume of The six piles were burned in mid-January. All of the piles fuel was arranged in a parallelepiped shape about 1.5 m wide, were ignited simultaneously (15/01/2007 at 12.00 p.m.). The same 1.5 m high and 2.4 m long and measured in the field. Afterwards, amount of fuel was used in the arrangement of each pile and later the fuel moisture content of both the fine and coarse fuels was when feeding the piles during flaming combustion. The fuel determined. A value of 6.73 kg/m3 was obtained. The bulk density supply lasted 40 min, and the embers were opened out 1 h and of piles made up of cut hazel logs with a diameter of less than 40 min after the end of this task. Piles 2, 4 and 5 were 10 cm was determined by following the same procedure. This extinguished 30 min after opening-out, whereas piles 1 and 3 time, a frustum with a volume of 2.56 m3 was arranged, and a were extinguished 80 min after opening-out. Fig. 2 shows piles 4, value of 81.73 kg/m3 was obtained. The fuel available for the 5 and 6 burning after the entire fuel supply had been experiment was then split up into equal parts and distributed used. throughout the experimental area as follows: the hazel branches Once all of the piles had been extinguished, they were were arranged in six parallelepipeds measuring approximately monitored at frequent intervals until midnight. Two observations 45 m3 each (3 m wide, 10 m long and 1.5 m high). These piles had were made the following day, one in the morning and one in the the same height and therefore the same compactness as the piles afternoon. On the second and third day after burning, further made in order to measure the bulk density. As a result, each of the monitoring of the still-smouldering piles was performed. The piles had around 300 kg of dry fine and coarse fuel available for effect of the wind was simulated for one of the piles on the burning. Also, the total supply of logs was divided into six afternoon of the third day. Table 3 summarizes all of the actions frustums of about 200 kg of dry fuel each. Fig. 3 shows the fuel performed over the four days of experiments. available for pile 5.

Fig. 2. Burning piles 4, 5 and 6 after ﬁnishing fuel supply. Fig. 3. Fuel available for pile 5.

Table 3 Time schedule of the experimentation.

Date Absolute time (h) Relative time (h:min) Action

15/01/07 12:00 pm 0 Ignition of the piles and start of fuel supply 15/01/07 12:45 pm 0:45 End of fuel supply 15/01/07 2:25 pm 2:45 Opening-out of embers 15/01/07 2:55 pm 2:55 Extinction of piles 2, 4 and 5 15/01/07 3:00 pm 3:00 Monitoring of piles 2, 4 and 5 15/01/07 3:45 pm 3:45 Extinction of piles 1 and 3 15/01/07 3:50 pm 3:50 Monitoring of piles 1 and 3 15/01/07 4:30 pm 4:30 Monitoring of all the piles 15/01/07 5:30 pm 5:30 Monitoring of all the piles 15/01/07 11:30 pm 11:30 Monitoring of all the piles 16/01/07 10:15 am 22:15 Monitoring of all the piles 16/01/07 1:15 pm 25:15 Monitoring of all the piles 17/01/07 12:45 pm 48:45 Monitoring of piles 1, 2 and 6 18/01/07 1 pm 73 Monitoring of piles 1, 2 and 6 18/01/07 1:40 pm 73:40 Wind simulation in pile 2 Author's personal copy ARTICLE IN PRESS

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Finally, on the day of the burning, six circular piles about 2 m meter. The temperature was 8 1C and the relative humidity was in diameter and 1 m tall were set up with a bed of hazel branches 83%. There was no wind. at the bottom and some logs on top. A few minutes before Finally, data from the Catalan Weather Service’s automatic ignition, samples were randomly taken from various spots in all of weather station in Prades, located 6 km from the experimental the piles of the fuel arrangements in order to evaluate the fuel site, was also available throughout the experimental period. moisture content (for both fine and coarse fuels). The mean oven- dry weight values were 27.5% and 38% for the moisture content of the fine and coarse fuel, respectively. No significant differences 2.4. Measurement of pile temperatures were found among the piles. The ambient temperature, relative humidity and wind speed After the extinction operations, the temperatures of the and direction were recorded throughout the experimentation days charcoal piles were monitored at frequent intervals using two by a station located at a height of 2 m on an adjacent terrace about different techniques. The first technique was to use a 1 mm K-type 300 m west of the centre of the experimental area. The mean thermocouple to take punctual temperature measurements. The temperature and relative humidity from 12:00 to 5:30 p.m. on the location of the hot zones within the piles, if any, were estimated day of the burning were 8.8 1C and 85%, respectively, with visually—they were usually made up of coarse pieces of charcoal maximum detected variations throughout this period of 70.3 1C and very often contained smoking and smouldering spots. The and 73%. The mean wind was very light during the study period: piles were then monitored by a pair of readings at each of several 1 m/s north-westerly, with a few gentle gusts of up to 4 m/s. spots, one internal reading at a depth of around 10 cm, and an Meteorological conditions at the experimental site were also external reading at the surface of the pile. recorded at the night monitoring time (around 11.30 p.m. on the The second technique was to use an IR camera, whose same day) using a portable anemometer and a thermo-hygro- technical characteristics are summarized in Table 4. The camera was equipped with a frame grabber to control and store sequences of IR images from a laptop computer. The temperature was monitored for 10 s each time – 5 s at the low temperature range Table 4 (20–120 1C) and 5 more seconds to set the medium range Technical characteristics of the IR camera. (80–500 1C) – in order to precisely obtain both the profile and hot spots of each pile. Fig. 4 shows some images from the Commercial name of the equipment AGEMA Thermovison 570-Pro (FSI- monitoring of pile 2. Pile emissivity was assumed to be close to FLIR Systems) Brightness temperature measurement Low:20 to 120 that of a black body and was fixed at 0.95. It is to be said that the ranges (1C) uncertainty related to the emissivity value might generate errors Medium: 80–500 when reading IR temperatures. However, this problem is more High: 350–1500 critical when dealing with flames rather than smouldering fuel. Thermal sensitivity (1C) o0.15 Considering the order of magnitude of temperature and emissivity Field of view (deg) 24 18 reported in this study, note that uncertainties in emissivity of Detector type Focal Plane Array (FPA) around 10% would imply temperature errors lower than 5% for the Spectral range (mm) 7.5–13 Frames per second 4 whole temperature range monitored during the experiments. Operational temperature (1C) 15 to 50 Measurements were taken at eye level 5 m north of the centre Storage temperature (1C) 40 to 70 of each pile. The IR sequences were later analysed using ThermaCAM ResearcherTM software, provided by FLIR Systems.

° 834 °C 115 C

100

AR01 40

<215 °C 15°C

Fig. 4. Monitoring of pile 2: (a) visible image a few instants before opening-out, (c) corresponding IR image, (b) visible image 25:15 h after burning and (d) corresponding IR image. Author's personal copy ARTICLE IN PRESS

74 E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81

Fig. 5. (a) Fan used for wind simulation. (b) Position of the fan in the centre of pile 2. It rested directly on top of the remains of the burnt material.

2.5. Wind effect simulation Table 5 Temperature distribution (% area) at the various relative monitoring times for each pile. The effect of induced wind blowing at ground level was simulated 72 h after the opening-out of pile 2, which was one of Np(t) TIR the piles where hot charcoal could still be found at that time. A 4800 W fan with a diameter of 1 m (Fanenergy V24 Rosenbauer) o25 25–115 115–195 195–315 315–435 435–515 4515 was set and calibrated to provide a 30 km/h wind 5 cm above the 1(3:50 h) 18.7 81.3 (n)(n)– – – ground at a distance of 0.8 m from the source, in the direction of 1(4:30 h) 70.9 29.1 (n)– – – – the induced flow. This velocity would tally with a 10 m wind 1(5:30 h) 98.2 1.8 – – – – – speed of about 50 km/h, considering a wind profile power law 1(11:30 h) 99.3 0.7 – – – – – with a stability class C [18]. This stability applies the simulated 1(22:15 h) 98.0 1.5 0.3 0.2 (n)– – 1(25:15 h) 96.7 2.0 0.7 0.6 (n)(n)– scenario, according to meteorological data recorded by the 1(48:45 h) 43.9 54.5 0.9 0.4 0.2 0.1 (n) Catalan Weather Service. The fan was placed in the centre of pile 1(73 h) 75.3 22.7 0.9 0.8 0.3 (n)(n) 2 with the flow following a north-westerly orientation, as the 2(3 h) 38.2 52.9 2.2 1.7 1.5 1.4 2.1 northern and the western quadrants were the ones where the 2(4:30 h) 67.4 13.4 4.0 11.4 3.4 0.3 0.1 largest amount of smouldering charcoal was found (Fig. 5 and see 2(5:30 h) 68.8 12.2 8.1 6.4 2.0 1.0 1.5 2(11:30 h) 81.5 12.5 4.8 1.0 0.2 (n)– Fig. 1). The fan was turned on for 6 min. Yellow polyester fabric 2(22:15 h) 96.0 3.1 0.4 0.3 0.2 (n)(n) (3.4 m wide, 4.4 m long) was placed on the ground next to the pile 2(25:15 h) 91.5 5.9 1.4 1.1 0.1 – – in the direction of the wind so that any smouldering embers that 2(48:45 h) 65.2 20.7 5.4 5.0 2.6 0.9 0.2 fell on it could be detected easily. The IR camera was placed 2(73 h) 84.4 11.2 2.9 1.1 0.2 0.1 0.1 3(3:50 h) 61.2 38.6 0.2 (n)– – – perpendicular to the wind flow about 10 m away in order to 3(4:30 h) 77.5 22.4 0.1 (n)– – – record the charcoal pile and the path of the embers in the same 3(5:30 h) 99.4 0.6 – – – – – shot. 3(11:30 h) 100 – – – – – – 4(3 h) 86.4 13.6 – – – – – 4(4:30 h) 97.9 2.1 (n)– – – – 3. Results and discussion 4(5:30 h) 99.8 0.2 – – – – – 4(11.30 h) 100 – – – – – – 5(3 h) 72.7 27.3 – – – – – 3.1. Temperature distribution according to IR images 5(4:30 h) 94.4 5.6 – – – – – 5(5:30 h) 99.7 0.3 – – – – – IR monitoring data was analysed to observe the surface 5(11:30 h) 100 – – – – – – 6(3 h) 3.9 39.8 33.3 20.1 2.2 0.4 0.3 temperature distribution and maximum temperature values of 6(4:30 h) 55.1 33.8 9.1 2.0 (n)– – the piles over time. A set of histograms was calculated for each 6(5:30 h) 61.7 27.3 8.7 2.1 0.2 – – pile at each monitoring time (i.e. relative time). Table 5 shows 6(11:30 h) 95.0 4.8 0.2 – – – – what percentage of the area of each pile was at each temperature 6(22:15 h) 99.5 0.5 (n)– – – – 6(25:15 h) 99.7 0.3 (n)– – – – level at the various monitoring times. Based on this data, three 6(48:45 h) 99.8 0.2 – – – – – types of behaviour can be recognized. The first type of behaviour 6(73 h) 96.3 3.7 – – – – – concerns piles 1 and 2. At the first measurement, each of them n presented large hot fractions—81.3% of the area of pile 1 and 61.8% Np means pile number. ( ) % area is less than 0.01%. of the area of pile 2 was above 25 1C. Pile 2 even had hot spots (2.1%) at temperatures 4515 1C. After 73 h, a significant portion of the area of the piles remained hot (4251C). An analysis of the 11:30 h in pile 1 and starting in the period from 11:30 h to various intervening monitoring periods shows that there was a 22:15 h, in pile 2. The percentage of the area above 25 1C was 0.7% reversal of the cooling process starting in the period from 5:30 to in pile 1 and 4% in pile 2 at 11:30 and 22:15 h, respectively. After Author's personal copy ARTICLE IN PRESS

E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81 75

100

Area covered (%) 20

<15 15−25 25−35 35−45 45−55 55−65 65−75 75−85 85−95 Temperature95−105 ranges ( 105−115 115−155 155−195 195−235 °C) 235−275 275−315 315−355 355−395 395−435 73:00 435−475 48:45 25:15 475−515 22:15 11:30 >515 05:30 04:30 03:00 Monitoring period (hh:mm)

Fig. 6. Temperature distribution across the surface of pile 6 throughout the experimental period.

that, the size of the hot area started to increase: at 73 h, piles 1 progressive cooling throughout the experiment. In contrast, pile 2 and 2 had values of 24.7% and 15.6%, respectively. Furthermore, decreased in temperature for the first 11:30 h. This is shown in the within this period, both piles reached their maximum among all plot by the translation, within this period, of the relative the monitored values at 48:45 h, when the percentage of hot area percentage area peak from hot to medium temperatures. was 56.1% and 34.8% and hot spots within the temperature After this period, the pile underwent a relative warming until intervals were up to 435–515 1C and 4515 1C for piles 1 and 2, the end of the monitoring period. At 48:45 h, a percentage area respectively. peak (2.8%) can clearly be seen in the 115–155 1C temperature The second type of behaviour was illustrated by piles 3, 4 and range. 5. In this case, at the first monitoring period, the hot area was The progression of the piles’ temperature distribution over smaller than in the first group (38.8% for pile 3, 13.6% for pile 4 time may be related to the meteorological conditions during the and 27.3% for pile 5). They only underwent one cooling process, experiments. Fig. 8 shows the evolution of the hot areas of the which at 11:30 h was completely finished, when the entire area of piles (considered as the fraction of the area above 25 1C), together the piles was below 25 1C. Pile 3 was the hottest of this group, with the mean wind velocity (Uw) and mean relative humidity with 22.5% of its area still above 25 1C at 4:30 h. None of the piles (RH). It is to be said that weather samples were collected every showed significant temperature values greater than 115 1C (only second and then averaged over a time period of a minute. pile 3 reached the temperature level of 115–195 1C at one point, Afterwards, per minute data were averaged over a time period of but with no noteworthy values). No remarkable differences were 1 h. This process was automatically calculated by the weather found between piles 4 and 5, which were extinguished in the station hardware, giving hourly averaged data as final outputs. same way, but by different firefighters. Data were always vector averaged. RH and Uw had inverse trends Finally a third type of behaviour was observed in pile 6. As delayed by around 1:30 h, showing peaks of maximum Uw and expected, since this was the pile left without any sort of extinction minimum RH in the early afternoon (1.7 m/s at 1 p.m. and 74.9% at at all, it was the hottest pile of all at 3 h, with 23% of its area at 3 p.m. for the second day, i.e. at 25 and 27 h relative time; and temperatures greater than 195 1C. This pile underwent a cooling 2.3 m/s at 3 p.m. and 68.1% at 4 p.m. for the third day, i.e. at 51 and process that left just 5% of its area in the 25–315 1C temperature 52 h relative time). At the end of the experimental period, a range by 11:30 h. At 73 h, the increase of the percentage area relative peak can also be observed: 75.4% RH and 3.1 m/s at 1 p.m. within 25–115 1C was not relevant because the maximum on the fourth day, i.e. at 73 h relative time. temperature reached at that stage was 26.8 1C. Some relation can be seen between the meteorological Figs. 6 and 7 show the histograms of piles 6 and 2, respectively. conditions and the behaviour of piles 1 and 2. The first peak Pile 6 was hotter than pile 2 for the first 5:30 h. Pile 6 also showed mentioned above corresponds to the time when the warming Author's personal copy ARTICLE IN PRESS

76 E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81

100

40 Pile 2

Area covered (%) 20

<15 15−25 25−35 35−45 45−55 55−65 65−75

Temperature75−85 r 85−95 95−105 105−115 anges (°C) 115−155 155−195 195−235 235−275 275−315 315−355 355−395 395−435 73:00 48:45 435−475 25:15 475−515 22:15 11:30 h:mm) >515 05:30 iod (h 04:30 03:00 Monitoring per

Fig. 7. Temperature distribution across the surface of pile 2 throughout the experimental period.

Fig. 8. Changes in hot areas over time for all piles. Author's personal copy ARTICLE IN PRESS

E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81 77

Fig. 9. Changes in maximum temperatures over time for all piles.

process is detected in pile 2 and is clearly evident in pile 1. The hot smouldering combustion inside the piles. In contrast, elapsed area in pile 1 begins to grow at 11:30 h with a gradient of 0.12%/h time until extinction did not seem to be relevant. until 22:15 h. However, the gradient from 22:15 to 25:15 h In order to determine whether this assertion was statistically increases by a factor of about 3.3 to reach 0.4%/h. The gradient representative, we used Lenth’s method for analysing factorial for pile 2 for these 3 h is around 1.5%/h, with an increase in Uw of designs [19]. The aim was to study the importance of the effects about 0.5 m/s every hour and a decrease in RH of 4.5%/h. The (i.e. extinction quality as effect A, elapsed time until extinction as second peak is located immediately after the measurement of the effect B and the interaction of both as effect AB) on the behaviour of maximums of the hot areas in piles 1 and 2. Thus, the hot areas in TIRmax over time. We were able to use this technique because the piles 1 and 2 grew as wind speed accelerated. This shows the experimental design was basically unreplicated; only piles 4 and 5 wind’s effect on the spread of the smouldering combustion front. had the same levels of effects. However, in order to use the In contrast, no wind effect was detected in pile 6 because, at that information provided by all of the piles, the analysis was performed stage, very little charcoal was available to glow because it was twice, considering the following groups of experiments: piles 1, 2, 3 being progressively consumed. and4wereusedforthefirsttestandpiles1,2,3and5wereusedfor the second test. Pile 6 was not included in this analysis because it was not extinguished. The results obtained in the two tests were analogous. 3.2. Maximum temperature according to IR images Fig. 10 shows the normal probability plots of the effects for the first test. The analysis confirms that the effect of extinction quality

The TIRmax values of all the piles were extracted from the IR becomes progressively relevant over time. Thus, extinction quality monitoring and its evolution over time, as shown in Fig. 9. The became statistically significant in the period from 11:30 to 25:15 h maximum temperatures of piles 3, 4 and 5 had dropped rapidly at and remained so until the end of the experiment. Note that effect 11:30 h, from 19 to 16 1C. The following morning, they were at 13– A became greater and further from the fitted line over time, 14 1C and stayed at this temperature for the rest of the monitoring whereas effects B and AB tended to be smaller and centred on period. Pile 6, which was not extinguished, showed high TIRmax zero. Therefore, the time factor was found to be not significant, values at the first stage of the experiments (643 1C was the first and no interaction effect was found between the factors. maximum value recorded). It took longer than piles 3, 4 and 5 to reach temperatures close to ambient (at the end of the experiment, its maximum temperature was 30 1C), but its curve 3.3. Front evolution according to thermocouple measurements was always decreasing. The TIRmax for piles 1 and 2 showed different behaviour. Although the first maximum temperature Thermocouples recorded the maximum temperature values in values recorded for pile 1 were lower than the values for pile 6, its the hot zones of the piles over the three days of experiments.

TIRmax was increasing at 11:30 h (relative time), as did its Fig. 11 shows the temperature measurement pairs (internal and percentage of hot area. The TIRmax for pile 2 showed an average external readings from the hottest spot found) of piles 1, 2, 6 and decrease for the ﬁrst 22:15 h, but started to go up again after that 5, plotted over time. T1 readings were taken at the centre of the point. Based on the results analysed so far, we can say that piles, T2 readings were taken approximately halfway along the extinction quality had a major effect on the preservation of radius and T3 readings were taken close to the perimeter. For Author's personal copy ARTICLE IN PRESS

78 E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81

Response is 5:30 h, Alpha = 0.05 Response is 11:30 h, Alpha = 0.05 99 99 Not Significant Not Significant Significant 95 Significant 95 90 90

80 AB 80 AB 70 70 60 60 50 B 50 B

Percent 40 40 30 Percent 30 20 A 20 A 10 10 5 5

1 1 -1000 -500 0 500 1000 -1000 -500 0 500 1000 Effect Effect

Response is 25:15 h, Alpha = 0.05 Response is 48:45 h, Alpha = 0.05 99 99 Not Significant Not Significant Significant 95 95 Significant 90 90

80 B 80 AB 70 70 60 60 50 AB 50 B

Percent 40 40 Percent 30 30 20 A 20 A 10 10 5 5

1 1 -400 -300 -200 -100 0 -600 -500 -400 -300 -200 -100 0 100 Effect Effect

Fig. 10. Normal plot of the effects A (quality of extinction), B (elapsed time until extinction) and AB (interaction effect of A and B) on maximum temperatures at: (a) 5:30 h, (b) 11:30 h, (c) 25:15 h and (d) 48:45 h. First test: piles 1, 2, 3 and 4.

measurements T2 and T3, the cardinal point of the quadrant of the The initial spread of the smouldering front without a pile where the hot spot was located is shown in the ﬁgure as N–S– predominant direction can be explained by the presence of a E–W (see also Fig. 1). Each pair of measurements is compared with charcoal layer distributed heterogeneously within piles 1 and 2,

TIRmax values that correspond to the same monitoring period. and also by an erratic wind flow bearing. However, the The smouldering front moved over time within piles 1 and 2. smouldering fronts of piles 1 and 2 turned west on the last day For pile 1, the hottest spot was first located halfway along the of the experiments, as wind speed was also increasing and locally radius of the southern quadrant inside the pile and moved north heading in this direction. for the first two days. It finally ended upon the surface of the Most of the monitored spots had higher temperatures at a western part of the pile. In contrast, the hottest spot of pile 2 certain depth of the pile, since smouldering combustion can more spread south—it began at the edge of the northern quadrant and easily be preserved inside the piles than outside, where the reached the centre of the pile after 48 h. During this period, the surface is cooled. Nevertheless, at certain instants, wind gusts maximum temperature values measured with the thermocouples could activate combustion on the surface as well if pieces of hot were found either on the surface or inside the pile. For the last charcoal were exposed. The values for TIRmax were generally higher day, the smouldering front also moved west. The hot spot of pile 6 than the values obtained by the thermocouples. This is because IR was always located roughly in the centre. The similar temperature monitoring can scan the entire surface of the piles, even to a values shown before and after 12:00 p.m. on 16 January prove that certain depth, and therefore record the radiation coming from the the pile was cooling homogeneously and that there was no very hottest spot. However, there is no guarantee that IR smouldering front. The hottest spot of pile 5 was initially located monitoring can provide the real value of the maximum tempera- in the northern quadrant but quickly cooled down. By 6 h after ture if the hot spot is located at a certain depth inside the pile. ignition time, the values for pile 5 had already reached This is the case where Tint values are higher than TIRmax values. Nor equilibrium with the environment. The same behaviour was can IR monitoring capture the activation of external hot points by shown by piles 3 and 4 (not plotted). wind gusts (e.g. around 1 p.m. on the second day for pile 2 and Author's personal copy ARTICLE IN PRESS

E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81 79

600 750 700 550 650 W 500 600 N 450 550 500 400 N N 450 350 400 300 350 N N W 250 S 300 200 250 150 200 N

Pile 1 Maximum temperture ( ° C) 150 100 Pile 2 Maximum temperture ( ° C) 100 50 50 0 0

070115 07011512:00 07011620:00 07011604:00 07011612:00 07011720:00 07011704:00 07011712:00 07011820:00 07011804:00 12:00 070115 07011512:00 07011620:00 07011604:00 07011612:00 07011720:00 07011704:00 07011712:00 07011820:00 07011804:00 12:00 Time (day, hour) Time (day, hour) 100 550 T1int 90 500 T1ext 80 450 T2int T2 400 70 ext N T3int 350 60 T3 300 ext 50 TIRmax 250 40 200 150 30 100 20 Pile 6 Maximum temperture ( ° C) S 50 Pile 5 Maximum temperture (ºC) 10 0 0

15 20:00 15 24:00 070115 12:00070115 20:00070116 04:00070116 12:00070116 20:00070117 04:00070117 12:00 070115 12:00070115 16:000701 0701 070116 04:00070116 08:00070116 12:00 Time (day, hour) Time (day, hour)

Fig. 11. Maximum temperatures of: (a) pile 1, (b) pile 2, (c) pile 6 and (d) pile 5. Monitored by thermocouples and compared to IR data. Location of the measurements in the piles: N, northern quadrant; S, southern quadrant; E, eastern quadrant and W, western quadrant.

around 6 p.m. on the ﬁrst day for pile 6) unless it is done at exactly following expression: the same time as the thermocouple measurements. V2 sinð2yÞ L ¼ 0 ð1Þ g

3.4. Wind simulation analysis where V0, the initial velocity of the embers, is assumed to be equal to the local induced wind velocity (8.3 m/s). During the ﬁrst The effect of wind on the reactivation of pile 2 was studied by period of the test (i.e. 100 s) the number of lofted embers detected analysing the information provided by the IR sequence on the in each image was considerably high (a mean of around 30 lofting of embers and the temperatures reached after the test. embers, with a maximum of 80 at 10 s) and the shooting angle MATLAB algorithms were used to extract, for each IR frame, the remained roughly constant at around 61. For the rest of the number of lofted embers present, the angle at which the embers experiment, however, the number of embers decayed (a mean of were launched, and the distance they covered (Fig. 12). To obtain around 10 embers per image) and the shooting angle rose sharply these outputs, we considered that embers followed a parabolic at certain instants due to local wind gusts. The maximum distance trajectory, with a mean initial point halfway along a radius of the calculated during the initial phase was 3.5 m. The impact on the pile, i.e. between the fan and the edge of the pile. A shooting angle polyester fabric at that distance is shown in Fig. 13b. The ﬁgure for each detected ember was obtained by computing the position shows a binarized image of a 0.86 m2 portion of the fabric located of the ember with respect to the initial point and the horizontal between 3.5 and 4 m of its total length. This is the segment of the (Fig. 13a). An arithmetic mean value of the shooting angle (y)was fabric that had the greatest percentage of its area riddled by holes then calculated for each frame. The distance covered by the (9.1%). This means that hot embers with a temperature equal to or embers upon landing (L) was theoretically determined by the higher than the fusion point of this material (260 1C) fell in that Author's personal copy ARTICLE IN PRESS

80 E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81 area. The embers were mainly lofted during the initial phase, 4. Conclusions although this distance could also have been reached at a later time. The residual temperature distribution of the piles over time The distances covered by the embers during the wind gusts was studied by IR and thermocouple monitoring. The results may have been underestimated using a constant value for V0. This shown in the IR histograms confirm that the piles that were implies that the maximum distances of around 6 m plotted in Fig. properly extinguished (i.e. using a high-pressure, homogeneously 13a may have been even larger – or more specifically, would have distributed water flow of 50 l/min for four minutes) had a uniform increased exponentially – with wind gusts. temperature profile and did not have significant hot spots. The At the end of the wind simulation, IR monitoring of the pile temperature reached equilibrium with the environment in less showed mean temperature values of around 500 1C and maximum than 10 h after extinction. In contrast, a smouldering front moved values of up to 750 1C—that is, a fully reactivated pile after three throughout the poorly extinguished piles, which had a wide days of experiments. temperature distribution and hot spots of up to 700 1C. The temperature data was crossed with wind measurements. Under low wind conditions (less than 1.5 m/s), the piles underwent a cooling process, but could easily be reactivated with higher 80 wind speeds even if there were only a few hot spots of moderate 70 temperature. In this study, 50 1C was shown to be enough to reach 60 higher temperatures (up to 500 1C) after 24 h. 50 The statistical analysis performed to determine the effect of 40 the extinction parameters (quality of extinction and elapsed time 30 until extinction) on the reactivation of the piles showed that 20

Number of embers quality of extinction was the only influential factor. The interval 10 between the two levels of elapsed time (50 min) was not 0 sufficient to observe any effect. However, an analysis of the self- 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 extinguished pile (pile 6) showed that after 24 h, even with 30 favourable wind conditions, hot spots inside the pile did not 25 undergo a temperature increase. ) ° A simulation of windy conditions after three days of experi- 20 ments on a poorly extinguished pile showed that the reactivation 15 of charcoal combustion was possible. It gave a high-risk scenario 10 to cause a wildfire, with hot smouldering embers being trans-

Shooting angle ( ported by wind flow. 5 In summary, we can conclude that pile burning protocols should 0 be carefully designed and performed. Such protocols must quantita- 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 tively establish parameters for how burning piles should be 6 extinguished, blacked out and patrolled. From this study we can 5 derive some provisional guidelines towards this objective; i.e. concerning the composition of the piles: piles no bigger than 2 m 4 of diameter and 1 m tall made up of fine fuel less than 15 mm and 3 fed with logs less than 10 cm are strongly recommended in order to 2 ensure safe and efficient burning and extinction. Concerning the extinction procedures; a high-pressure flow of 50 l/min during 4 Distance covered (m) 1 minutes has been sufficient for the experimental piles to reach 0 ambient temperatures in less than 10 h. Therefore, double extinction 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 time (8 min) would be preferable to guarantee a faster cooling Time (s) process in order to reduce the patrolling period. Concerning this last

Fig. 12. Top: number of lofted embers in each image; middle: mean launching phase, weather monitoring is advised, particularly local wind angle of the embers and bottom: theoretical distance covered during the wind velocity and direction, since eventual smouldering fronts are very simulation for pile 2. sensitivetogentleairﬂows.

Fig. 13. Detection of embers (a) in an IR image and (b) on the polyester fabric. Author's personal copy ARTICLE IN PRESS

E. Pastor et al. / Fire Safety Journal 45 (2010) 69–81 81

Further points to be studied are the influence of the charcoal size [5] M., Galante, Forest fire causes in Portugal, in: Proceedings of the C-Studies and the influence of the elapsed time during the fuel consumption Conference, European Commission Forest Focus Programmes 2003–2004, Brussels, 22 October 2007, Belgium, 2007. phase (i.e. from the end of fuel supply until opening-out the embers) [6] New South Wales Rural Fire Service, Standards for pile burning, Public Report, on the smouldering front propagation within the piles. NSW Fire Brigades, Australia, 2006. [7] Catalunya, Decret 64/1995, de 7 de marc-, pel qual s’estableixen mesures de prevencio´ d’incendis forestals, Diari Oficial de la Generalitat de Catalunya, 2022, 10/3/1995, 1995. Acknowledgements [8] G. Rein, Smouldering combustion phenomena in science and technology, International Review of Chemical Engineering 1 (2009) 3–18. This study is supported by the Spanish Ministry of Education [9] W.H. Frandsen, The influence of moisture and mineral soil on the combustion limits of smouldering forest duff, Canadian Journal of Forest Research 17 (12) and Science under the project AGL2005-07269. It is also (1987) 1540–1544. supported by the Department of Universities, Research and [10] W.H. Frandsen, Ignition probability of organic soils, Canadian Journal of Forest Information Society (DURSI) of the Catalan Government, the Research 27 (1997) 1471–1477. [11] K. Miyanishi, E.A. Johnson, Process and patterns of duff consumption in the European Social Fund and the Technical University of Catalonia mixedwood boreal forest, Canadian Journal of Forest Research 32 (2002) (UPC). The authors want to acknowledge the Fire Brigades of the 1285–1295. Catalan Government (General Office of Fire Prevention and [12] G. Rein, N. Cleaver, C. Ashton, P. Pironi, J.L. Torero, The severity of smouldering peat fires and damage to the forest soil, Catena 74 (3) (2008) Extinction, DGPEIS) for their kind support during the experiments. 304–309. We also acknowledge the comments made by the reviewers, [13] M.G. Hille, S.L. Stephens, Mixed conifer forest duff consumption which contribute to improve the paper. during prescribed fires: tree crown impact, Forest Science 51 (5) (2005) 417–424. [14] E.R.C. Rabelo, C.A.G. Veras, J.A. Carvalho Jr., E.C. Alvarado, D.V. Sandberg, J.C. References Santos, Log smoldering after an Amazonian deforestation fire, Atmospheric Environment 38 (2004) 203–211. [15] C. Meesri, B. Moghtaderi, Experimental and numerical analysis of sawdust- [1] A.P. Dimitrakopoulos and I.D. Mitsopoulos, Global Forest Resources Assessment char combustion reactivity in a drop tube reactor, Combustion Science and 2005—Report on fires in the Mediterranean Region, Forest Resources Develop- Technology 175 (2003) 793–823. ment Service, Working Paper FM/8/E, FAO, Rome, Italy, 2006. [16] B. Moghtaderi, T. Poespowati, E.M. Kennedy, B.Z. Dlugogorski, The role of [2] D. Drysdale, in: An Introduction to Fire Dynamics, John Wiley and Sons, New extinction on the re-ignition potential of wood-based embers in bushfires, York, 1997. International Journal of Wildland Fire 16 (2007) 547–555. [3] Generalitat de Catalunya, Estadı´stica de la campanya d’estiu 2005, Public Report of [17] Generalitat de Catalunya, Protocol de crema de restes, Internal Report of the the Catalan Department of the Environment and Housing, Barcelona, Spain, 2005 Catalan Department of the Interior, Barcelona, Spain, 2007. /http://mediambient.gencat.net/Images/eng/104_78054.pdfS. [18] F. Pasquill, The estimation of the dispersion of windborne material, The [4] Ministerio de Medio Ambiente, Base de datos sobre incendios forestales, Meteorological Magazine 90 (1063) (1961) 33–49. Internal Report of the General Office for Biodiversity, Spanish Ministry of [19] R.V. Lenth, Quick and easy analysis of unreplicated factorials, Technometrics Environment, Madrid, Spain, 2004. 31 (1989) 469–473.