Bed Ignition Research Articles

Comparing Ignition of Fuel Beds to Firebrand Showers to Ignition of Fuel Beds by Non-Reacting Heaters

ABSTRACT The combustion of various fuels results in the release of firebrands during wildland-urban interface (WUI) fires. Firebrands represent a hazard, since the release of firebrands may lead to multiple ignitions at locations distant from the original combustion source. A newly developed experimental protocol using a non-reacting heater was used to simulate a firebrand. A direct comparison of this simplified non-reacting heater setup was undertaken to realistic fuel bed ignition to actual firebrand showers. Firebrand showers were generated using a custom experimental setup developed for this investigation. The novelty of this research is a detailed comparison for the first time to fuel bed ignition from non-reacting heaters to actual firebrand showers, for the same fuel bed type. The focus here is on smoldering ignition (SI) of the fuel beds, as this is seen as a complex and least understood aspect of firebrand ignition of actual WUI fires. Results indicate that the overall phenomenology of the ignition process is different between a non-reacting cartridge heater and a shower of firebrands. Complicated smoldering ignition phenomena of fuel beds by firebrand showers cannot be simulated by a simple non-reacting heater.

Experimental and Theoretical Approaches to Elucidate Fuel Bed Ignition Exposed to Firebrand Showers and Radiant Heat

Firebrand showers are known to result in massive destruction in large outdoor fires. A key missing piece is how these ignition scenarios may be influenced by firebrand showers in conjunction with external radiant heat that would be generated by nearly combustibles. The combined effect of firebrand shower exposure and radiant heat exposure on fuel bed ignition under different firebrand arrival flux was investigated using two different approaches. The first approach considered the ignition based upon a unit area methodology. The second approach considered investigating each localized ignition. The time to ignition and the number of firebrands associated with each ignition process was measured under different experimental conditions. A theoretical treatment was devised to explain the experimental results. Findings show that the higher wind speeds and longer preheating times led to shorter times to ignition. The firebrand arrival flux became an important parameter on fuel bed ignition for low arrival flux values, resulting in intermittent heating of the fuel bed. In this case, fuel beds may lose heat due to convective cooling from the wind. The heat contribution from a firebrand was also calculated. Firebrands under higher wind speed contributed more to heat the fuel beds than firebrands under lower wind speed. The heat contribution from a firebrand calculated by the unit area approach decreased as the firebrand arrival flux decreased due to the intermittent arrival of the firebrands. The study remains the first work to show the importance of continuous arrival of firebrands for fuel bed ignition.

Sensitivities of Porous Beds and Plates to Ignition by Firebrands

The increasing occurrence of severe wildfires, coupled with the expansion of the wildland urban interface has increased the number of structures in danger of being destroyed by wildfires. Ignition by firebrands is a significant avenue for fire spread and structure loss; thus, understanding processes and parameters that control the ignition of fuel beds by firebrands is important for reducing these losses. In this study the effect of fuel bed characteristics (i.e., particle size and porous or solid fuel bed) on ignition behavior was considered. Modelling and analysis was conducted to better understand parameters that are dominant in controlling ignition. The fuel beds, made from Douglas-fir shavings, Douglas-fir plates, or cardboard plates, were heated with a cartridge heater (i.e., surrogate firebrand) to observe ignition. Smaller particles were observed to ignite more readily in porous beds than larger particles when heat transfer from the heater is primarily through conduction. This occurs in large part due to differences in contact area between the fuel bed and the heater coupled with thermal properties of the fuel bed. As particle sizes increased, ignition was more likely to occur at extended times (>100 s) due to the increased importance of radiation heat transfer. Douglas-fir plates were primarily observed to ignite at times where conduction was the dominant mode of heat transfer (<10 s). Heat flux delivered to the fuel bed was observed to be a more accurate predictor of ignition likelihood and ignition time than heater temperatures. The characteristic ratio of transport and chemical timescales can be used, in conjunction with the measured heat flux and thermal diffusivity of the fuel beds, as a first approximation to predict ignition for the porous fuel beds. This suggests that future work focusing on these parameters may produce a general characterization of fuel bed ignition probability across fuel beds materials and morphologies.

Combustion characterization of a CH4/O2 rapid mixed swirl torch igniter for hybrid rocket motors

Combustion characteristics of a rapid mixed swirl torch igniter of CH4/O2 for hybrid rocket motors has been experimentally investigated. The igniter torch consists of four tangential slits circumferentially equispaced on the internal micro-combustion chamber allows tangential injection of O2 gas, meanwhile the CH4 flows into the main channel and interacts with four tangential O2 injections flow to generate a swirl mixing of CH4 and O2. The ignitability of this swirl torch igniter was verified by large number of experiments in variation of equivalence ratio and total flowrate of CH4/O2. The flame structure of the igniter and effect of main oxidizer injection acting as a cross flow on the flame has been investigated by using OH* and CH* chemiluminescences techniques. It is found that the torch igniter can reliably ignite instantaneously in the range of equivalence ratio 0.2∼1.4 and the effect of main oxidizer injection on the flame is tiny and can be neglected. In addition, the performance of this rapid mixed swirl flame has been investigated by using a lab scaled hybrid rocket motor and compared with the same hybrid rocket motor ignited by catalytic bed. The performance of this igniter and its comparison with catalytic bed ignition will be discussed and analyzed in detail.

Simulation of fuel bed ignition by wildland firebrands

A 3-D mathematical model of fuel bed (FB) ignition initiated by glowing firebrands originating during wildland fires is proposed. In order to test and verify the model, a series of experiments was conducted to determine the FB ignition time by a single pine bark and twig firebrand (Pinus sylvestris). Irrespective of the pine bark sample sizes and experimental conditions, the ignition of the FB was not observed. Conversely, pine twigs, under certain parameters, ignited the FB in the range of densities (60–105 kg m−3) and with the airflow velocity of ≥2 m s−1. The results of the mathematical modelling have shown that a single pine bark firebrand ≤5 cm long with a temperature of T ≤ 1073 K does not ignite in the flaming mode the FB, and only the thermal energy of larger particles is sufficient for flaming ignition of the adjacent layers of the FB. The analysis of the results has shown that the firebrand length is a major factor in the initiation of ignition. Comparison of the calculated and observed FB ignition times by a single firebrand have shown that our modelling accords well with the experimental results.

Spot fires: fuel bed flammability and capability of firebrands to ignite fuel beds

A series of tests were conducted under laboratory conditions to assess, first, the capacity of several fuel beds to be ignited by firebrands and to sustain a fire and, second, the capability of different types of firebrands to ignite fuel beds. Fuel beds and firebrands were selected among the most common in southern Europe. Regarding fuel bed flammability, results show that grasses are more flammable than litter and, among litters, Pinus species are the most flammable. The increase in bulk density and fuel moisture content involves an increase in the time to ignition, and a decrease in the other flammability parameters. The capability of firebrands to ignite fuel beds is higher when the firebrands drop in the flaming phase and with no air flow than in glowing phase with air flow. Logistic regression models to predict fuel bed ignition probability were developed. As a whole, results show a relationship between ignition probability of fuel bed and type or weight of firebrands. Pinus pinaster cone scale, P. halepensis cone scale, and Eucalyptus globulus leaf and bark can have ignition probabilities at least twice higher than pine bark when falling while in flaming combustion.

Mathematical modelling of particle mixing effect on the combustion of municipal solid wastes in a packed-bed furnace

Packed bed combustion is still the most common way to burn municipal solid wastes. In this paper, a dispersion model for particle mixing, mainly caused by the movement of the grate in a moving-burning bed, has been proposed and transport equations for the continuity, momentum, species, and energy conservation are described. Particle-mixing coefficients obtained from model tests range from 2.0x10(-6) to 3.0x10(-5)m2/s. A numerical solution is sought to simulate the combustion behaviour of a full-scale 12-tonne-per-h waste incineration furnace at different levels of bed mixing. It is found that an increase in mixing causes a slight delay in the bed ignition but greatly enhances the combustion processes during the main combustion period in the bed. A medium-level mixing produces a combustion profile that is positioned more at the central part of the combustion chamber, and any leftover combustible gases (mainly CO) enter directly into the most intensive turbulence area created by the opposing secondary-air jets and thus are consumed quickly. Generally, the specific arrangement of the impinging secondary-air jets dumps most of the non-uniformity in temperature and CO into the gas flow coming from the bed-top, while medium-level mixing results in the lowest CO emission at the furnace exit and the highest combustion efficiency in the bed.

Study of coal and coke ignition in fluidized beds

Ignition tests were conducted with delayed and fluid petroleum cokes, a high volatile bituminous coal and their blends in bench- and pilot-scale fluidized bed combustors. In the bench-scale FBC tests, a visual inspection ignition criterion was developed based on the ‘CO–CO 2’ profiles obtained as a function of time and bed temperature. In the pilot-scale unit, the rapid increase in SO 2 levels was used as the indicator of fuel ignition. In addition to the fluidized bed ignition tests, thermogravimetric analysis (TGA) measurements were made on all fuels and their chars. The results showed that, while the fluid coke (which has the lowest volatile content) was the most difficult to ignite, ignition was not a simple function of volatile content. Further, different test methods and ignition criteria demonstrated significantly different ignition temperatures, with the largest variation arising between bench- and pilot-scale equipment. Finally, tests on mixtures of petroleum coke and coal showed that there was no interaction between these two fuels, and that the coke ignited only when it achieved its own ignition temperature.

Quantifying the Resistance to Airflow during Iron Ore Sintering

Sintering tests, involving airflow measurements before and after bed ignition, were carried out on three optimized iron ore blends-containing only hematite (HAEM), hematite with pisolite (PISO), and hematite with pisolite and hematite-goethite (PIMM)-to assess the resistance of the flame front and associated regions. Results showed that post-ignition airflow rates for the three blends were comparable for the same pre-ignition airflow rate, indicating that reactive ores i.e. pisolite and hematite-goethite ores, did not have an adverse effect on flame front resistance. For the PIMM blend, increasing the level of fines (minus 0.15 mm fraction) also did not have any effect on the flame front resistance. Reducing the coke level in all the three blends and increasing basicity decreased the value of the resistance. The resistance was shown to be a strong function of green bed airflow velocity and semi-empirical equations, with single variables, were derived to characterize the ‘sinterability’ of iron ore mixes.

Mass Transfer and Warming during Adsorption of High Concentrations of VOCs on an Activated Carbon Bed: Experimental and Theoretical Analysis

An experimental and theoretical study was carried out to predict the warming and the mass-transfer rate during adsorption of high concentrations of volatile organic compounds (VOCs) in an activated carbon bed. A linear driving force (LDF) model is found to provide an acceptable fit to the measured data. An empirical correlation of the mass transfer rate is proposed as a function of the strength of adsorbent−adsorbate interactions and the gas velocity to design the adsorption process without using any adjustable parameter. The model was validated for an adsorption unit with high loadings (up to 100 g.m-3) of seven kinds of VOC and within the velocity range 0.139−0.556 m.s-1. The prediction of the temperature rise inside the adsorber is improved by the use of the differential heat of adsorption instead of the integral heat. A theoretical parameter sensitivity test indicates that the temperature rise is strongly dependent on the molar VOC concentration, the adsorption heat, and the volumetric heat capacity of the carrier gas. The warming of the activated carbon bed can be well predicted from these variables. The relation obtained appears to be a practical means for designing the safety of an adsorber and preventing carbon bed ignition.

The ignition temperature of lignite char in a fluidized bed combustor

AbstractThis paper summarises the experimental and modelling work carried out for the variation of bed ignition temperature of a fluidized bed combustor with the char particle diameter and the fluidizing velocity. A lignite char was used and its reactivity was represented using data from Field (1967) and Turnbull and Davidson (1984). The modelling involved solving the steady state heat balance around the fluidized bed combustor at the ignition temperature. A correlation of the total area of char ignited per unit bed mass was determined as a function of the char particle diameter and the fluidizing velocity. This correlation was used to determine the ignition temperature of the fluidized bed combustor operating at different conditions. The fluidized bed combustor heat balance was then solved for the bed ignition temperature which was influenced by both the rate of heat loss from the bed and the reactivity of the char. A sensitivity analysis suggests that the chemical rate reaction coefficient is the most prominent variable when determining the ignition temperature of a fluidized bed combustor.

Prevention of activated carbon bed ignition and degradation during the recovery of cyclohexanone.

Oxidation of cyclohexanone adsorbed on activated carbon was investigated in relation to solvent recovery. It became clear that the respective reactions through which 1,2-cyclohexanedione and adipic acid formed were largely responsible for carbon bed ignition and carbon degradation. Results were explained assuming that the former reaction is of pseduo-first-order and proceeds much faster than the latter. Further, it was concluded that the concentration of carbonyl group on the carbon surface and that of oxygen in the gas phase affected both reactions strongly.

Experimental study of ignition and extinction waves and oscillatory behavior of a tubular nonadiabatic fixed bed reactor for the oxidation of carbon monoxide

An experimental study of axial temperature profiles in a nonadiabatic tubular fixed bed reactor has been made under the transient operation. The catalytic carbon monoxide oxidation occurring on a Pt/alumina catalyst has been used. Ignition and extinction waves have been studied both in a short and a long catalyst bed. It is shown that in a short bed ignition and extinction occur at the reactor outlet while for a long bed the ignition and extinction process takes place inside the reactor and is not influenced by the outlet part of the reactor. A systematic measurement indicates that in a narrow range of inlet parameters the integral nonadiabatic nonisothermal reactor may operate in an oscillatory state. These oscillations can be periodic or aperiodic.