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Cite as: Alarifi, A. A., H. N. Phylaktou , G. E. Andrews, J. Dave and O. A. Aljumaiah (2015), Toxic
Gas Emissions from a Timber-Pallet-Stack Fire in a Full Scale Compartment, In Proceedings of the
10
thAsia-Oceania Symposium on Fire Science and Technology, Tuskuba, Japan.
Abdulaziz Alarifi () · Herodotos Phylaktou · Gordon Andrews
Energy Research Institute, University of Leeds, Energy Building, Leeds LS2 9DY, UK e-mail address: [email protected]
Jim Dave
States of Jersey FRS, PO Box 509, Rouge Bouillon, St. Helier, Jersey JE4 5TP, UK Omar Aljumaiah
Energy Research Institute, King Abdulaziz City for Science and Technology, Saudi Arabia.
Toxic Gas Emissions from a Timber-Pallet-Stack Fire in a Full Scale
Compartment
Abdulaziz A. Alarifi, Herodotos N. Phylaktou, Gordon E. Andrews, Jim Dave and Omar A. Aljumaiah
Abstract
A stack of wooden pallets was burnt in a 41m3 room with air supplied via the door connected to a 3m long corridor, the only air supply path to the room. Combustion emissions were sampled from within the fire room and analyzed using a heated Fourier Transform Infrared (FTIR) analyser, via a heated sampling system. Development of the fire was also captured photographically and presented here with associated measured fire characteristics. Detailed full-scale compartment fire characteristics measurements are provided including: source fuel composition, temperature gradient at different locations, mass loss rate, heat release rate, calculated equivalence ratio and measured toxic emission yields. Yield measurements are presented as a function of the equivalence ratio. These measurements are useful for fire investigation and fire modeling purposes. Carbon monoxide yields approaching 0.3 g/g were much higher than those currently recommended for fire modeling. Acrolein and total hydrocarbon yields were in good agreement with other tests in the literature.
Keywords
Full Scale Fire. Wood Fire. Toxic Yields. Carbon Monoxide. Total Hydrocarbon.
1
Introduction
Exposure to toxic products of incomplete combustion in fire compartments is the cause of 70% of fire fatalities [1, 2]. Purser [3] showed that the main toxic fire effluents are CO, HCN and irritant gases for the majority of fires. There are very limited number of toxic emissions yield measurements in wood compartment fires under limited ventilation conditions, and available data are rarely associated to the prevailing combustion conditions as characterised by the fire equivalence ratios.
Fire models are used for the purpose of establishing the fire hazard in buildings but such models, for example FDS [4], are not capable of predicting incomplete combustion products relevant to fire smoke hazard in terms of acute lethal toxicity and/or severe irritation. Such data are typically entered by the user as fixed yields usually based on published data from well ventilated bench scale tests. The area of fire toxicity modelling is in its infancy, and model developers are restricted by the lack of information on toxic products in general and in particular for under-ventilated compartment fires.
The lack of information on fire smoke toxic species, not only affects the fire modelling development, but also affects the fire safety engineering practice in a wider sense. For example, the British Standards Institution [5], in its latest guidance on initiating a fire within the enclosure of origin for fire safety engineering analysis, recommends the use of 0.13 g/g as the generic CO production yield from the design fire. It also reports a range of CO yields for different fuels ranging from 0.004 to 0.063 g/g based on Tewarson’s [6] data for well ventilated fires with samples usually taken from heavily diluted flows. The provided figures are not representative of the toxicity problem in present-day ventilation controlled fires.
More experienced engineers, who use CFD techniques, adopt what might be considered a better industry practice, which starts with specifying a fixed fire heat release rate fire, typically ranging from 1-5 MW, depending on the building occupancy, and assume a fixed production yield of carbon monoxide appropriate to fully developed compartment fires typically at yields of 0.2 g/g [7]. Based on this, engineers, more often than not, restrict smoke hazard analysis to visibility
and carbon monoxide levels (this is an under-estimate of the smoke hazard which is frequently reported by surviving fire victims as irritant and impairing escape). They use the output information for establishing the effectiveness of smoke ventilation systems proposed to provide clear and safe means of escape and/or validate the building compartmentation..
Aljumaiah et al. [8] performed a series of air starved wood fires in 1.6 m3 fire compartment with varied air supply rates from 3 to 37 air changes per hour (ACH). Only tests with 11, 21, and 37 ACH achieved fully developed flaming fires, while the 3 ACH test didn’t propagate and the 5 ACH self-extinguished after burning 15% of the initial weight. Valuable yield data of main fire effluents were presented as a function of time and equivalence ratio. They also showed that levels up to 0.25 g/g CO yields, 0.06 g/g total hydrocarbons yields, 0.008 g/g acrolein yields, and 0.019 g/g formaldehyde yields were present.
An earlier study by Gottuk et al. [9] investigated air starved wood fires in a 2.2m3 compartment where carbon monoxide yields were reported to reach 0.165 g/g with equivalence ratio near unity. Beyler [10] reported a peak of 0.142 g/g CO yield at 1.7 equivalence ratio for wood burnt in a 1.6 m3 hood at variable equivalence ratio.
The authors [11] have reported the present test with respect of the effects and consequences of fire-fighting on the characteristics of compartment fires. This paper does not discuss the fire-fighting stage but concentrates on the measured toxic product yields and comparison to the literature data
1.1 Objectives
The objective of this paper is to provide measured data on the most significant fire species yields in a full scale compartment fire under typical ventilation controlled conditions represented by an open door along a supply corridor. We are reporting these yields as a function of the fire equivalence ratio and we are comparing the findings to those from other experimental studies and to the empirical correlations by Tewarson [6]. Such data are essential for modelling fire scenarios more reliably in order to provide the required level of fire safety in building designs with greater confidence.
2
Experimental methodology
2.1 The compartment
The detailed experimental set up is described in [11]. The geometric set-up with full dimensions is shown in Fig. 1. The 4.15 x 4.25 m2 main room of a bungalow was used as a burn room for the test, with a ceiling height of 2.35m, making the total volume of the room at just over 41 m3. The main open door of the bungalow leads to a 3.5 m long corridor and through a second open door to the main burn room. There was a bathroom and a kitchen on either side of the corridor but the doors to both were closed during the experiment. The ceiling and the back wall of the burn room was double lined with 12.5mm plaster board and there were no other ventilation openings.
The need for fire fighting access by fire service personnel made the installation of instrumentation (such as flow meters) not possible in this pathway.
2.2 Fuel
A stack of 9 wooden pallets with a total weight of 143 kg was placed on the top of the mass balance located near the back wall opposite the room door. The fire was ignited using 400 ml of methanol in a tray located at the centre of the base pallet (as shown in Fig. 1). The wooden pallet material elemental composition was identified using Thermo Scientific Flash 2000 combustion based elemental analyser. The composition on mass basis was 45.2% carbon, 5.58% hydrogen and 49.22% oxygen giving a mass based stoichiometric air to fuel ratio of 5.0. The actual air to fuel ratio is based on the sampled combustion emissions [12] and from these measurements the equivalence ratio was determined (see Section 2.5)and plotted in Fig. 5. The net calorific value of the fuel was determined to be 15.4 MJ/kg, based on theoretical oxygen consumption requirements [13].
2.3 Thermal conditions monitoring
The spatial temperature within the compartment was monitored by 25 thermocouples distributed into two vertical trees and 8 thermocouples monitoring the ceiling layer as shown in Fig.1.
Fig. 1. Compartment dimensions with locations of fuel and instrumentations.
2.4 Gas analysis
Fire effluent gases were sampled through a multi-hole sampling probe across the centre of the room ceiling (effectively extracting a mean ceiling gas sample) as shown in Fig. 1. This sample was transported through heated sampling lines, heated pump and heated filters to a heated Fourier Transform Infrared (FTIR) analyser which enabled identifying and quantifying more than 50 species simultaneously [14-16]. After the FTIR the sample was cooled and the water extracted, and then the sample was analysed for oxygen using a paramagnetic analyser (Servomex Series 1400) and for CO and CO2 using Non-Dispersive Infrared (NDIR) analysis (Hartmann and Braun). The NDIR measuring of CO and CO2 was used as a check of the accuracy of the FTIR measurements and very good agreement was obtained giving confidence in the reported FTIR data.
2.5 Fire emissions based equivalence ratio
Based on the online fire effluent gas analysis and by knowing the composition of the fuel, the amount of oxygen from air involved in the combustion, was determined and thus the actual air to fuel ratio (AFRactual) was calculated. This was effectively a species mass balance between reactants and products. Detailed description of the calculation is explained elsewhere [12, 17, 18]. Then by using the stoichiometric air to fuel ratio (AFRstoich) based on the fuel composition as described in Section 2.2, the transient ER (equivalence ratio) was determined and plotted at the top of Fig. 5 using the following relationship:
ER=AFRstoich/AFRactual = FARactual/FARstoich (1) FAR is the fuel to air ratio, inverse of AFR.
For ER <1 combustion is fuel lean and for ER>1 combustion is fuel rich.
2.6 Visual monitoring of the fire
Photographs were taken from outside the bungalow through the corridor sightline. Valuable visual insight to the fire development was achieved with this simple method enabling a better understanding of the fire development. By
compiling the photographic images with the measurements throughout the development of the fire the collage in Fig. 3 was produced. The smoke layer descended during the fire development, dropping below the 1 m level as shown in the picture taken after 3 minutes and 29 seconds from ignition.
Fig. 2. Total mass of the stack as a function of time (Right hand-side scale). Heat release rate (HRR) based on mass loss rate (Left hand-side scale). and a corrected curve for inefficiency (detailed discussion of inefficiency covered in [11]).
Fig. 3. Through the corridor and door photographs of the compartment fire development at progressive timings, with the corresponding measurements of; HRR, Equivalence Ratio (ER), oxygen level, carbon monoxide yield, temperature above the fire, and ceiling temperature. Also a reference height 1 m from floor level is drawn. Photographs times are shown at the bottom in reference to the ignition time.
3
Results and Discussion
3.1 Thermal conditions
The Heat Release Rate (HRR) profile for the fire was determined using the mass loss measurements and the calorific value of the fuel, as shown in Fig. 2. The fire reached a fully developed stage (ventilation control) after approximately 155 seconds.
As shown in Fig. 4, the vertical temperature profile provided by the central and sidewall thermocouple trees showed that the separation between the hot and cold layer was more clearly indicated by the sidewall tree. The highest temperature gradient was between 1.6 and 1.1 m level at 95s, then at 209s the slope between 0.9 and 1.1 m level started increasing, indicating a more defined boundary between the hot and cold zones is dropping to that level. This corresponds well with the photographic evidence in Fig. 3.
Fig. 4. Vertical temperature profile at different stage of the fire (corresponding to the images presented in Fig. 3) Left: near sidewall temperature profile) showing a clearer definition of the hot layer by temperature. Right: central temperature profile.
3.2 Main toxic species yields
The equivalence ratio, and concentrations of the most dominant species (acrolein, formaldehyde, CO, and total hydrocarbon (THC) as methane equivalent, were charted in Fig. 5 as a function of time. Measurements of fire effluent concentrations are important for evaluating tenability conditions for building occupants escaping and designing appropriate measures to dilute these effluents to safe levels.
In Fig. 5 the concentration measurements of acrolein, formaldehyde and CO are compared to the lethal concentration
(LC50) levels, which is the “concentration of a toxic gas or fire effluent, statistically calculated from concentration-response data, that causes death of 50% of a population of a given species within a specified exposure time and
post-exposure time” [19, 20] and the AEGL-210 min values which are particularly relevant to impairment of escape in fires.
“AEGL-2 is the airborne concentration of a substance above which the general population could experience an impaired
ability to escape” [21]. It is evident in Fig. 5 that these critical benchmark levels for these 3 species were exceeded by several times in this test.
Figures 6 to 9 show the yields obtained from this test compared with data from the literature for similar fuel (wood) at different scales.
Firstly, four sets of yield data for different fire species were reported by Aljumaiah et al. [8] from burning wood in a 1.6m3 fire compartment at different (metered) air supply rates. As discussed earlier, only tests with 11, 21, and 37 ACH achieved fully developed flaming fires, while the 5 ACH test self extinguished after burning 15% of the mass. CO yield data reported by Gottuk et al. [9] from burning wood in the 2.2m3 compartment.
Beyler’s 1.6m3 hood [10] where by controlling the supply of air, variation of equivalence ratios were achieved (no details of the rate of supplied air to the compartment were reported).
Finally, widely used Tewarson empirical correlations [6] were also used for the comparison. Tewarson’s correlations are based on yield measurements for wood, at different equivalence ratios using the bench-scale fire propagation apparatus.
Fig. 5. Equivalence ratio and volumetric concentrations of total hydrocarbons (THC), carbon monoxide (CO), formaldehyde, and acrolein as a function of time (ignition at 0 seconds). Also threshold limits (lethal concentration LC50 [20] and impairment of escape AEGL-210min[21] are
marked where available.
3.2.1 CO yields
Figure 6 shows good agreement between Tewarson’s correlation and Aljumaih’s et al data at the lower ventilation rate. However, for a significant number of data for apparently larger compartments and/or larger ventilation rates there is significant deviation from Tewarson’s correlation which starts at equivalence ration of 0.5 and higher where the correlation significantly underpredicts the data. There is good agreement between the present data and those of Gottuk and Beyler for equivalence ratios of 0.5 to 1.3. Their data does not extend beyond of 1.6 while the present data extend up to of approximately 2, where yields of CO approaching 0.3 were measured. Some of Aljumaiah’s data at the highest ventilation rate give comparable yields but at much higher of 2.5.
3.2.2 Total hydrocarbon yields
Figure 7 shows the total hydrocarbon yields and shows reasonable agreement of the Tewarsons correlation with the reported data only at of 1.6 to 1.8. However, at lower the correlation shows lower THC yields than the other experimental data.
The majority of the experimental data show no yields exceeding 0.05 even at high equivalence ratios while the Tewarson correlation exceeds this apparent threshold yield value.
3.2.3 Formaldehyde yields
Figure 8 shows that formaldehyde yields were higher than reported smaller scale wood tests. As an oxygenated hydrocarbon, formaldehyde usually forms at lower temperatures and with the excess of oxygen in the reaction. It can be
seen that when equivalence ratios were between 1.5 and 2.0 for the present data formaldehyde yields dropped from 0.013 to 0.003 g/g.
3.2.4 Acrolein yields
Figure 9 shows good agreement between the full scale acrolein yields data and the reduced 1.6 m3 compartment yields. Since acrolein would only form at low temperatures, the size of the compartment and its effect on the temperature was not a significant influence on the production of acrolein.
Fig. 6. Carbon monoxide yields as a function of equivalence ratio for the present experiment compared to other wood yield data from smaller compartment tests [8, 9] and to Tewarson’s correlation [6].
Fig. 7. Total hydrocarbon yields as a function of equivalence ratio for the present experiment compared to other wood yield data from smaller compartment tests [8, 9] and to Tewarson’s correlation [6].
Fig. 8. Formaldehyde yields as a function of equivalence ratio for the present experiment with other wood published yield data from smaller compartment tests [8].
Fig. 9. Acrolein yields as a function of equivalence ratio for the presented experiment (41m3) with other wood published yields data from
smaller compartment tests [8].
3.3 The ACH in the Present Test
There is a suggestion in the proceeding analysis that the ventilation rate in terms of the ACH may play a role in the differences in yields reported by the various data sets rather than solely on equivalence ratio. To investigate this, the ACH corresponding to the present test were evaluated. A first approximation of the maximum ACH may be obtained by considering air inflow during the fully developed stage which for compartment fires can be estimated by the simple relationship [22]
(2)
Where, Av is the area of the vent and Hv is the height of the vent. In the present test this results in a value of ACH of about 84, which is significantly higher ventilation rate than in the Aljumaiah tests. However, the above relationship is based on fundamental flow dynamics of a fully developed fire compartment driving flows in and out of the compartment through a sharply defined aperture. In the present test the presence of a corridor will reduce these flows due to the pressure losses induced by the corridor.
As the fire develops, progressively more air is drawn into the compartment which allows the fire to get larger and the compartment hotter producing more fuel which makes the equivalence ratio higher. Accordingly, a more accurate evaluation of the ACH was performed on the basis of the emission-based AFR calculation and the fuel mass loss rate measurements. This is shown in Fig. 10 as a function of time and the equivalence ratio. Based on this approach, The rate of ventilation increases to values up to 45 ACH, however for higher than 30 ACH the equivalence ratio remains fairly
constant between 1.5 and 2 suggesting that despite the increasing influx of air, all of it is used to burn more fuel at essentially constant combustion chemistry.
This would suggest that ventilation rates per second are not the reason for the variation of the yields between data sets. The comparatively good agreement over specific ranges of , between the present full scale test and the much smaller compartments, implies that the size of the compartment is not the primary cause of differences at higher values.
This would leave the temperature in the different set-ups as the potential cause of yield variations. However, there are insufficient data to allow meaningful correlations between temperature, equivalence ratio, and emission yields and further investigation is needed.
Fig. 10. Air changes per hour (ACH) as a function of time and of the equivalence ratio.
4
Conclusions
Valuable full scale toxic yield measurements are reported as a function of equivalence ratio for a wood compartment fire with one door open. CO, acrolein, formaldehyde and unburnt hydrocarbons were the main toxic species produced by this compartment wood fire. The concentrations of the first three emissions in the layer were several times higher than the threshold limits for incapacitation and death.
Good agreement was demonstrated between CO yields from the present tests and other data in the literature for equivalence ratios of 0.5 and 1.3 and these were higher than the widely accepted Tewarson correlation results for the same fuel. The present CO yields reached a maximum approaching 0.3 g/g which is significantly higher than those recommended in the standards and used in fire modelling.
For the THC yields the Tewarson correlation was in agreement over a narrow range of of 1.6 to 1.8, and was lower than the other data for lower , while it showed significantly higher yields for THC at values of higher than 2, where all the other data suggested that yields THC were limited to below 0.05 g/g.
Yields reported here are important for modelling fires in buildings for the purpose of assessing the hazard associated with fire effluents and its influence on the means of escape.
Acknowledgement The authors thank Mr. Mark James (Chief Fire Officer of State of Jersey FRS) for permission to conduct the tests and facilitating resources for the set-up and safe conduct of the tests, Mr. Ian K. Gallichan (Housing Chief) who postponed the demolition of the bungalows to allow time for the proper preparation of the tests, all four watches who helped convert the rooms and who provided fire cover in their own time, Mr. Lez Ballingall (Fire Service Maintenance technician), Mr. Andy Reed of Normans Builders Merchant who supplied all the material free of charge, Mr. Bob Boreham the Leeds University technician responsible for the transport and set-up of all the instrumentation and the Saudi Ministry of Higher Education for sponsoring Abdulaziz and omar’s PhDs.
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