Determination of Fire and Burning Properties of Spruce Wood Određivanje zapaljivosti i obilježja gorenja drva smreke

This paper deals with the determination of selected fi re properties of spruce wood. It describes the general characteristics of spruce wood, microscopic and macroscopic features. Broad application of this material requires the assessment of its properties regarding the fi re aspects, being a cause of fi re in forests or due to frequent occurrence of wildland fi res in the Slovak territory, and being a fl ammable material used in building industry, furniture industry, etc. This paper analyses the following fi re properties: fl ash-ignition temperature, spontaneousignition temperature, mass burning rate, ignitability of material exposed to a small open fl ame.


INTRODUCTION 1. UVOD
Wood is a natural organic material whose extraction, processing and application plays an important role nowadays.Therefore, it is necessary to accept the fact that wood in any form (raw material, semi-fi nished product, and end product) cannot be exposed to higher temperatures, and this is considered to be an important negative feature, especially when wood is used as a construction material.
Many authors, e.g.Kačíková (2006) and Osvald (2007), deal in their papers with both infl uence of wood properties on fl ammability and external conditions affecting thermal degradation of wood.
Based on the fl ash-ignition temperature and spontaneous-ignition temperature, according to the ISO 871:2006, the relative comparison of a material resistance against ignition can be determined.Karlsson and Quintere (2000) stated that for reaching the fl ash-ignition temperature, the temperature in 63 (3) 217-223 (2012) the fi re compartment should reach 500 to 600 °C or the radiant fl ux on the fl oor should be 15 to 20 kW•m -2 .The temperatures referred to above are signifi cantly higher than the spontaneous-ignition temperatures of most lignocellulose materials.

DRVNA INDUSTRIJA
The basic fi re performance properties of wood, from the point of construction, especially wooden constructions, were analysed by Delichatsios et al. (2003).They argued that by using the modifi ed conical calorimeter with the heat fl ux between 15 and 50 kW•m -2 , the other fi re characteristics could be derived such as conductivity, thermal capacity, spontaneous-ignition temperature, latent heat, gross heat of combustion, etc.The conclusion was that the heat fl ux of 18 kW•m -2 becomes critical for raw wood.
This paper introduced an assessment of wood properties related to fi re performance of spruce wood.It deals with the description of the general characteristics of spruce wood, microscopic and macroscopic features.Furthermore, it focuses on the description of the procedure for determining the selected fi re characteristics, especially the fl ash-ignition temperature, spon taneousignition temperature, mass burning rate, and ignitability of material exposed to a small open fl ame.
Spruce wood is the most widely used wood species in Slovakia (excellent technical properties, handling properties, and large domestic sources).It is the most important wood for processing; it is also used as a construction and auxiliary material in building industry and maintenance, in package making industry, in mining, as a transport construction material, in engineering, in furniture manufacture, plywood, fi brous boards, chipboards, etc. Spruce wood is also very valuable as a highly resonant wood (Korytárová and Osvald, 2000;Požgaj et al., 1997).
Taking into account the wide application of this material, it is important to assess it not only from its quality, functionality and utilization aspect, but also from the point of view of its fi re performance.
Spruce wood is also used in biomass production.During its combustion, it is important to know all the physical and chemical parameters of combusted materials as well as to study nitrogen oxides that represent the most important toxicological fi re products.Nowadays, the concentration can be estimated relatively accurately for the industrial combustion conditions.In industrial combustion, these conditions are set to reach maximum combustion effi ciency meeting the emission limits (Martinka et al., 2011).

Mikroskopska obilježja
The wood is without any core, it is fresh and with well a visible zone of ripe wood.Its colour is white or yellowish white, and it turns into yellow under light.
The annual rings are clearly visible, narrow summer wood gradually changes into broad spring wood.Resin channels are small and scattered, visible by the eye only on longitudinal well processed cutting profi les as gentle longitudinal darker ruptures.Fresh wood smells like resin.Rays can be visible by the eye.The wood is soft, well splittable and lightweight (ρ = 430 kg•m -3 ) (Korytárová and Osvald, 2000).

Makroskopska obilježja
The basic construction elements of spruce wood are tracheids -vessels that represent up to 94.5…95.3…96.5 % of the whole wood volume.They are 1.7…2.9…3.7 mm long.Rays are dominantly of one layer, consisting of parenchymatous cells; on their circumference, transverse tracheids are located with smooth or weakly waved walls with paired dots.The yield of rays is 4.4…4.7…5.5 %.Spruce wood is characterized by the presence of resin channels well visible on transverse cutting profi le -vertical channels.Horizontal channels can be seen on tangential cutting profi le.About 30 to 100 vertical resin channels can be found on 1 cm 2 of the transverse cutting profi le area of spruce wood.They are mostly present in summer wood and can be seen by the eye as gentle grooves or signifi cant lines on longitudinal cutting profi les.Spruce wood has very narrow resin channels (0.09 mm) and the yield share is 0.2 %.The resin has a protective function for the tree (when the tree is mechanically or biologically damaged) and an impregnation function -it increases the wood resistance against fungi (Korytárová and Osvald, 2000;Požgaj et al., 1997).
Physical, mechanical and fi re and burning properties of wood are described in many publications, for example by Rowell (2005); the characteristics of spruce wood are introduced in the catalogue of fi re and technical properties of materials (The catalogue of fi re and burning behaviour properties of materials, 1984).The physical, mechanical and fi re and burning properties of spruce wood are presented in Table 1.

Determination of fl ash-ignition temperature
and spontaneous-ignition temperature 2.2.Određivanje fl ash temperature paljenja i temperature spontanog zapaljenja Testing was carried out in accordance with the standard STN ISO 871: 1999.The basic terminology is as follows: fl ash-ignition temperature -(FIT) is the minimum temperature at which, under specifi ed test conditions, suffi cient fl ammable gases are emitted to ignite momentarily by application of a pilot fl ame; spontaneous-ignition temperature (SIT) is -the minimum temperature at which, under specifi ed test conditions, ignition is obtained by heating in the absence of any additional fl ame source.
The standard is based on the principle of heating a material in the heating chamber of a hot air furnace (see Figure 1) using various temperatures.

Flash-ignition temperature (FIT)
The electric power supplied to the heating coil is adjusted until the air temperature remains constant at the desired initial test temperature.The pan with the specimen is inserted into the furnace.The timer is activated, then the pilot fl ame is ignited and evidence of a fl ash or mild explosion of combustible gases, which may be followed by continuous burning of the specimen, is observed.Also fl ame-burning or glowing combustion can occur in case of sudden increasing of the temperature T 1 compared with the temperature T 2 .If in 10 minute interval, the fl ash ignition has occurred, the temperature T 2 is lowered or raised by 50 ºC and the test is repeated with a fresh specimen.If the range in which the fl ash-ignition temperature is reached has been determined, tests begin at the temperature lower by 10 ºC than the highest temperature and continue by lowering the temperature by another 10 ºC until the temperature at which no ignition occurs in a 10 min interval is reached.The lowest air temperature T 2 , at which a fl ash ignition was observed during the 10 min interval, is considered as the fl ashignition temperature (STN ISO 871:1999).

Spontaneous-ignition temperature (SIT)
The same procedure is carried out as for FIT but without the pilot fl ame.The lowest air temperature T 2 at which fl aming or glowing combustion of the specimen is observed during the 10 min interval is considered as the spontaneous-ignition temperature.
To determine the fl ash-ignition temperature and spontaneous-ignition temperature, the spruce wood specimens were prepared so as to meet the requirements for sample specimen mass of 3.0 g ± 0.2 g (according to the standard STN ISO 871), while for the comparative purposes these specimens were prepared from the tree trunk, branch and root (the branch and root thickness was lower than 5 mm).

Determination of the mass burning rate 2.3. Određivanje brzine gorenja (smanjenja mase)
Testing apparatus schematically illustrated in Figure 2 is used for model burning tests.This apparatus consists of the electronic weight (4) with the accuracy of two decimal places, weight protection unit ( 3), e metal holder (6) for placing testing specimen ( 5), metal loading frame (2) for placing radiant heat source and infrared thermal heater (1) with the input of 1000 W, heater surface temperature of 652.7 °C, maximum wavelength of 3.11 μm, and radiation intensity of 3.055 W•cm -2 (Zaťko, 1993).
Testing procedure -the experiment is based on exposing the testing specimens to the thermal infrared heater (radiant heat source) in 10 minute intervals at the distance of 30 (35, 40, 45 and 50 mm) from the radiating heater surface.During the test, the mass loss is recorded in regular 10 second intervals.

Determination of ignitability 2.4. Određivanje zapaljivosti
The testing procedure is determined according to the standard STN EN 11925-2: 2004.This test determines ignitability of a product when exposed to a small fl ame source.The test is used with different classes of reaction-to-fi re B, C, D, E (construction products except fl oor coverings and thermal insulating products for linear ducts), B fl , C fl , D fl , E fl (fl oor coverings), and B L , C L , D L , E L (thermal insulating products for linear ducts).
The testing apparatus consists of a combustion chamber (see Figure 3) made of metal sheet and rustproof steel, equipped with thermal resistant glass doors, enabling access and observation from the front side and a lateral side.
The test specimen is clamped in the specimen holder so that one end and both sides are covered by the holder frames and the exposed end is 30 mm from the end of the frame.
The distance between the burner and the specimen is checked by the relevant spacer.For this purpose two kinds of distance spacers can be used.One is used to measure the distance between the burner and specimen on the specimen front principal surface and the other on the specimen lateral side surface.
The burner is lighted in the vertical position to stabilize the fl ame.A fl ame height of 20 mm is adjusted by the burner valve.The turner is tilted to 45° with respect to its vertical axis and moved horizontally until the fl ame reaches the pre-set contact point with the test specimen.
The timer is switched on at the moment when the fl ame contacts the test specimen.The fl ame is applied for 15 s or 30 s (according to the user`s requirement or depending on the reaction-to-fi re class) and then the burner is retracted in a smooth continuous manner.
If the fl ame application time is 15 s, the total test duration is 20 s, starting at the moment when the fl ame was fi rst applied.If the fl ame application time is 30 s, the total test duration is 60 s, starting at the moment when the fl ame was fi rst applied.In both cases the position of fl ame application is recorded and during the test interval (i.e. during 20 s or 60 s) it is observed whether the specimen ignition occurs and whether the fl ame spreads in the vertical direction more than 150 mm above the fl ame application point.At the same time the physical behaviour of the specimen is observed.

Determined values of the fl ash-ignition
temperature and spontaneous-ignition temperature 3.1.Vrijednosti fl ash temperature paljenja i temperature spontanog paljenja The experiment was carried out according to the procedure required by the standard STN ISO 871.
Determined values of the fl ash-ignition temperature and spontaneous-ignition temperature of the tested specimens as well as the fl ash-ignition and spontaneous-ignition times are presented in Table 2.
Table 2 clearly shows that by comparison of the fl ash-ignition temperature (FIT) and spontaneous-igni-  tion temperature (SIT) of the spruce tree trunk, branch and root, the lowest values (370 and 400 °C) were determined for specimens prepared from the trunk (those results are comparable with those referred to in Table 1), higher values (380 and 440 °C) of both temperatures were reached by specimens made from the root.The highest temperatures FIT (390 °C) and SIT (460 °C) were observed for the spruce branch specimens.
From the fi re prevention point of view, besides temperature, the time also plays a signifi cant role.Table 2 points out the higher variability of results due to reached time values for the fl ash-ignition that varies from 420 s for the branch specimens to 485 s for the root specimens and up to 560 s for the trunk specimens.
Similar variability can also be seen with the spontaneous-ignition.These values vary in the range from 370 s for the root specimens, 395 s for the branch specimens and 550 s for the trunk specimens.With the higher temperatures, at which the fl ash-ignition and spontaneous-ignition occur for the branch and root specimens, shorter times were observed of fl ash-ignition and spontaneous-ignition.
For the Australian pine tree, Delichatsios et al. (2003) recorded the spontaneous-ignition temperature of approximately 478 °C, which is comparable with the temperature reached by the spruce branch specimens (460 °C) in this experiment.Also Hagen et al. (2009) refer to the spontaneous-ignition temperature of 487.9 °C in the case of the Norway Spruce wood specimens not treated by fi re retardant, which is also comparable with our results for spruce branch specimens.

Vrijednosti brzine gorenja
The variation of mass burning rate depends on changed distance of the tested specimen from the heat source.The results are shown in Figure 4, where the maximum burning rate values as well as the time required for reaching these maximum burning rate values are referred to.
Based on the variation of mass burning rate curves shown in Figure 3, it may be stated that maximum burning rates can be observed within the whole time interval, during which the specimens were exposed to  Based on the test results, it could be stated that the thermal degradation of material is defi nitely affected by the distance of the heat source.
The burning speed of spruce wood samples was also introduced in the study by Chrebet et al. (2011), where they used a substantially lower speed of burning.The values varied in the range from 0.0054 to 0.0072 g/s.Comparing those results with the results of our experiment, it can be concluded that the reason for lower speed values was the difference in the initial phase of sample burning, where it took 26 s for the samples at a distance of 30 mm from the source, to start burning.

Determined values of ignitability Dobivene vrijednosti zapaljivosti
The experiment was carried out with spruce specimens having dimensions of 250 x 90 x 10 mm carved in a longitudinal direction.The specimens were tested at 30 s intervals according to the procedure described in the Methods.The fl ame source was applied in two ways: fl ame exposure of the front principal area and fl ame exposure of the lateral side surface (edge) of the specimen.The results are presented in Table 4.
Table 4 shows the differences in ignitability depending on the way of fl ame exposure to specimens.When the specimens were tested on the surface fl ame application point (on the front principal area), the ignition occurred in one case only.However, by testing the specimen edge fl ame exposure (edge fl ame application point), the ignition occurred in all three cases.These results confi rmed the higher ignitability of edges.
Based on this testing, it can be concluded that in no case (neither at the surface fl ame exposure nor edge fl ame exposure) the fl ame height in the vertical direction was higher than 150 mm during 60 s (test duration).It means that the criterion F s ≤ 150 mm during 60 s (according to Table 1 in the standard STN EN 13501-1 + A1) for the reaction-to-fi re classifi cation into the relevant reaction-to-fi re class was fulfi lled.

ZAKLJUČAK
This paper briefl y describes three ways for evaluating fi re and burning properties of spruce wood.
It can be stated that every experiment (through its evaluation method) and the results obtained contribute to knowledge in the area of material assessment as regards fi re protection, for example: The results of the experiment related to the evalua-tion of the fl ash-ignition temperature and spontaneous-ignition temperature as well as the times of fl ash-ignition and spontaneous-ignition of spruce wood showed differences depending on the part of the spruce tree (trunk, branch, root) of which the specimens were made; By the evaluation of the mass burning rate, it was established that the value of the maximum burning rate depends on the distance between the tested sample and the initiating heat source; The ignitability test showed the difference of igni-tability depending on the way of fl ame exposure to specimens -exposure of the lateral side surface (edge) is more sensitive to ignition than the exposure of the main surface.

Figure 2
Figure 2 Testing apparatus for the determination of mass burning rate Slika 2. Uređaj za određivanje brzine gorenja

Table 1
Physical, mechanical and fi re and burning properties of spruce wood (Source: The catalogue of fi re and burning behaviour properties of materials.Coniferous wood X ) Tablica 1. Fizikalna i mehanička svojstva te svojstva zapaljivosti i gorenja drva smreke (Izvor: The catalogue of fi re and burning behaviour properties of materials.Coniferous wood X )

Table 2
Determined values of the fl ash-ignition temperature (FIT) and spontaneous-ignition temperature (SIT) and times of fl ash-ignition and spontaneous-ignition of the spruce wood specimens Tablica 2. Vrijednosti fl ash temperature paljenja (FIT) i temperature spontanog zapaljenja (SIT) te vremena fl ash paljenja i spontanog zapaljenja smrekova drva

Table 3
Values representing burning rate and burning time of the spruce specimens Tablica 3. Vrijednosti brzine gorenja i vremena gorenja uzoraka od smrekovine