Naian Liu

Kinetic modeling of thermal decomposition of natural cellulosic materials in air atmosphere

Abstract The wood and leaf samples of eight species are examined by non-isothermal means to determine the mass loss kinetics of the thermal decomposition with linear temperature programming in air atmosphere. A simple kinetic description, named in this work as ‘First Order Pseudo Bi-component Separate-stage Model (PBSM-O1)’, is developed based on the experimental results and integral analysis method. The model assumes that the mass loss process of any wood or leaf sample consists of three steps. The first step corresponds to the water evaporation, and the subsequent two mass loss steps are mainly due to two major pseudo components. The two pseudo components decompose respectively at two separate temperature regions, other than at the global temperature region as used in the previous developed models by other authors. The global mass loss rate of the sample is looked on as controlled respectively by the reactions of the two components respectively during the lower and higher temperature ranges. The kinetics of the two components are found to abide by the first order equation, which gives the best fits to the experimental data compared with other model functions. The advantages of PBSM-O1 are discussed by comparing it with other kinetic models. PBSM-O1 is additionally validated by the reasonable agreement between the experimental and calculated results.

Critical consideration on the Freeman and Carroll method for evaluating global mass loss kinetics of polymer thermal degradation

By means of regression analysis, an insight into the limitations of the widely used Freeman and Carroll method is offered, and it is indicated that the Freeman and Carroll method is essentially unstable for the determination of the reaction order. The reliabilities of the intercept and slope terms for any one regression equation are also investigated mathematically, suggesting that the slope term is in general easily to be determined accurately, whereas the intercept term has possibility to be obtained with great error. Based on these theoretical analyses, a new method to calculate the kinetic parameters for polymer thermal degradation is proposed, in which the three kinetic parameters are determined from the slopes of three regression equations described in this paper. The experimental data in the literature are used to verify the proposed method. By comparing the results from the Freeman and Carroll method and the proposed method respectively with those from the Doyle method, it is shown that the proposed method can be applied with a much better accuracy than the Freeman and Carroll method. # 1999 Elsevier Science B.V. All rights reserved.

Modelling the Thermal Decompositions of Wood and Leaves Under a Nitrogen Atmosphere

The thermal decomposition of six di⁄erent samples of wood and leaves in nitrogen has been studied by using dynamic thermogravimetry. In the experiments two main weight loss processes took place and the total weight loss at 500iC was over 95% in all six cases. By means of the Doyle method, the two processes were found to fit most closely the plot for the second-order equation in the form da/dt 5 k(1!a) 2 , and each of the weight loss processes was found to be controlled, respectively, by two dominant reactions as the temperature increases. It was inferred that competing reactions occur during the overall temperature interval for all the six samples. By comparing the activation energies using this model with those by the method of Moll et al., and by comparing the experimental and theoretical thermogravimetric curves, the ‘second-order’ model was tested to be able to predict the weight loss processes of the samples with very good accuracy. It can be concluded that the ‘second-order’ kinetic model acts much better than the conventionally adopted first-order model. ( 1998 John Wiley & Sons, Ltd.

Thermogravimetric analysis of peat decomposition under different oxygen concentrations

Smoldering combustion of peat is of global concern as a natural hazard to consume sequestered carbon and form wide-area haze. It is affected by thermal decomposition kinetics of peat and the diffusion and availability of oxygen. In this work, thermal decomposition behavior of peat was investigated using thermogravimetric analysis under the atmosphere with different oxygen concentrations. The results showed that thermal decomposition process of peat could be divided into three stages: dehydration, oxidative pyrolysis of organic matters into volatiles and char, and oxidation of the generated char. The apparent activation energies of peat decomposition under different oxygen concentrations were calculated by model-free methods of Kissinger, FWO, Starink, Gyulai, and Friedman. A two-step reaction model was proposed to describe thermal decomposition kinetics of peat (excluding dehydration stage) and the effect of oxygen concentration on the kinetic parameters was discussed. These results provide basic data for smoldering modeling of peat.

Interaction between flaming and smouldering in hot-particle ignition of forest fuels and effects of moisture and wind

Ignition of natural fuels by hot metal particles from powerlines, welding and mechanical processes may initiate wildfires. In this work, a hot steel spherical particle (6–14 mm and 600–1100°C) was dropped onto pine needles with a fuel moisture content (FMC) of 6–32% and wind speed of 0–4 m s–1. Several ignition phenomena including direct flaming, smouldering and smouldering-to-flaming transition were observed. The critical particle temperature for sustained ignition was found to decrease with the particle size (d) and increase with FMC as (°C), and the maximum heating efficiency of particle was found to be . As the particle size increases, the influence of FMC becomes weaker. The flaming ignition delay times for both direct flaming and smouldering-to-flaming transition were measured, and decreased with particle temperature and wind speed, but increased with FMC. The proposed heat-transfer analysis explains the ignition limit and delay time, and suggests that the hot particle acts as both heating and pilot sources like a small flame for direct flaming ignition, but only acts as a heating source for smouldering. This study deepens the fundamental understanding of hot-particle ignition, and may help provide a first step to understanding the mechanism behind firebrand ignition.

Two-step Consecutive Reaction Model of Biomass Thermal Decomposition by DSC

The thermal decomposition of one kind of biomass in air has been investigated by differential scanning calorimetry (DSC) analyzer. The results indicate that the heating process of samples from ambient temperature to 923 K at low heating rates shows two obvious exothermic peaks. According to the decomposition mechanism, the first exothermic step is attributed to oxidative degradation of hemicellulose and cellulose, and the second exothermic step is attributed to lignin degradation and char oxidation. The reaction model of the studied biomass thermal decomposition has been studied by iso-conversional methods and optimization computation. The results suggest that the two-step consecutive reaction model is suitable to describe the exotherm of biomass thermal decomposition in air.

Effect of slope on spread of a linear flame front over a pine needle fuel bed: experiments and modelling

This paper experimentally evaluates the effect of slope on spread of a linear flame front over a pine needle fuel bed in still air. The slope angle of the fuel bed varied from 0 to 32°. The fuel mass consumption in flaming fire spread, temperature over the fuel bed, velocities of the flow around the flame front and heat fluxes (total and radiant) near the end of the fuel bed were measured. The mass loss rate and rate of fire spread both increased with increasing slope, whereas the fuel consumption efficiency varied in the opposite way. It was shown that a weak reverse inflow and an upslope wind (induced by the flame itself) exist respectively ahead of and behind the flame front, and their significant difference in velocity (causing a pressure difference) plays an essential role in the forward tilting of the flame front. This mechanism promotes burning, especially on higher slopes. Natural convective cooling has a remarkable effect on the fuel pre-heating in the spread of linear flame fronts under slope conditions. A fire spread model for a linear flame front was developed to consider the natural convective cooling and the fuel consumption efficiency. The model agrees well with the experimental data on fire spread rate. Its reliability, especially for higher slopes, was verified by comparison with other models.

Experimental Research on Burning Rate, Vertical Velocity and Radiation of Medium-Scale Fire Whirls

Fire whirls are often reported in forest and urban fires with devastating damage to life and property. This work conducted experiments using a medium-scale facility to study the initiation mechanism, the vertical velocity and radiative heat transfer of fire whirls. Heptane was used as the fuel in the experiments. The variations of burning rates versus time indicate that the drag force on the root of the flame plays an important role in the initiation and decay of a fire whirl. Analyses show that the pressure difference due to whirling, characterized by the circulation 

Experiments and Analysis of Interaction Among Multiple Fires in Equidistant Fire Arrays

The study of dynamics of multiple fires is important to gain a physical insight of the conditions under which destructive phenomena could result in city fires such those caused by earthquakes. Particularly, heavy populated cities such as Tokyo are highly vulnerable. Unfortunately, previous studies on multiple fires and their dynamics are rather limited. An extensive experimental study has been carried out to examine the fire interactions among freestanding equidistant multiple fires in square arrays, to supplement the authors’ previous related studies. Four square arrays, namely, 5×5, 9×9, 15×15 and 17×17, with various inter-fuel pan distances were treated. The burnout time (BOT) from ignition at every fire in the array was experimentally recorded and expressed as multiples of the BOT of a single free-standing fire as a reference. Since the BOT at any fire location in an array is inversely proportional to an average burning rate (BR) at that location, the local BR can then be directly inferred, and their comparisons thus indications of the physical interactions as affected by the fire location, inter-fuel pan distance and size of the fire array. It is shown that all these parameters play remarkable roles in the interactions among multiple fires in square fire arrays.Copyright

Experimental Study on Cross-ventilation Compartment Fire in the Wind Environment

When fire occurs in the rooms of high-rise buildings, the strong ambient wind will play an important role in fire spread and smoke movement behavior. However, wind effect on compartment fire in cross ventilation condition has not been fully studied so far. In the present study, an effort has been made to study crossventilation compartment fire in the wind environment through experimental investigations. The experimental fire was generated by 250ml (10cm×10cm tray burner) or 500ml (20cm×20cm tray burner) nheptane on the floor of a cube enclosure with two opposite vents on the walls. The inside and outside gas temperature profiles at different vertical and horizontal locations were recorded by two thermocouple matrixes. The ambient wind velocity was set to zero, 1.5m/s and 3m/s. It is observed that the ambient wind has two contradictory effects on the compartment fire: promoting fire severity by more oxygen supplying and cooling the fire by heat removing and combustible gases diluting. The spilled-out flame/plume extends horizontally farther with the increase of wind speed. It is found that the compartment fire with 500ml fuel reaches post-flashover stage while that with 250ml doesn’t. The wind effect is obviously observed in larger fires while not significant in smaller fires.