Pyrolysis and spontaneous ignition of wood under transient irradiation: Experiments and a-priori predictions

Wood is a material widely used in the built environment, but its flammability and response to fire are a disadvantage. Therefore, it is essential to have substantial knowledge of the behavior of wood undergoing external heating such as in a fire. The majority of studies in the literature use constant irradiation. Although this assumption simplifies both modelling and experimental endeavors, it is important to assess the behavior of materials under more comprehensive heating scenarios which might challenge the validity of solid-phase ignition criteria developed previously. These criteria are evaluated here for the spontaneous ignition under transient irradiation by combining experimental measurements and a-priori predictions from a model of heat transfer and pyrolysis. We have applied a two-step transient irradiation in the cone calorimeter in the form of a growth curve followed by constant irradiation. We use white spruce samples of size 100×100 mm and thickness of 38 mm . We measure the temperature at different depths and the mass loss. A one dimensional model written in the open source code Gpyro is used to predict the pyrolysis behavior. The model has a chemical scheme in which the components of wood (hemicellulose, cellulose, lignin) become active, then decompose in two competing reactions: one reaction to char and gas, and one reaction to tar. The kinetic parameters, as well as the thermal properties of the wood and char are taken from the literature, while ρ and moisture content are measured experimentally. A priori predictions of the temperature, made prior to the experiments, show excellent agreement with the measurements, being within the experimental uncertainty range. The mass loss rate (MLR) predictions are qualitatively similar to the measurements, but there is a large uncertainty in the measurements. For a-posteriori simulations, certain parameters are changed after having access to the measurements to improve the simulations. Also, we perform an evaluation of the solid phase ignition criteria used in the literature, and find that neither criteria is a consistent indicator of ignition. These results help understand the spontaneous ignition of wood subjected to transient irradiation and identify strengths and gaps in the topic.     

考虑水分影响的木材热解过程数学模拟

针对常见可燃物在典型火场情形中的热解行为提出了一种考虑水分影响的数学模型.模型采用了一阶阿仑尼乌斯热解动力学,考虑了固体内部瞬间传热过程、对流传热、热解过程的热效应,水分和挥发分流动等过程,模型揭示了热解木材中的温度、失重率和水分变化等重要特性.计算得到的结果和试验值吻合较好,通过对不同辐射,不同含水率下的木材可燃物热解过程的模拟计算,预测了木材在典型火灾环境中的热解行为.     

空气气氛下杯[4]芳烃的热分解动力学

研究杯[4]芳烃的热解过程与性能参数,计算表观活化能,求解主降解过程的热解动力学函数方程。研究表明,空气气氛下杯[4]芳烃发生氧化燃烧,稳定性下降;Kissinger方法和Flynn-Wall-Ozawa（FWO）方法计算表观活化能分别是89.72、101.33 kJ/mol;热解机理函数方程是g(█(α))= [-ln(1-█(α))]2/3,反应级数n=2/3,非等温热解机理为随机成核和随后生长反应。     

Investigation of thermal degradation of pine needles using multi-step reaction mechanisms

The objective of this study is to assess the relevance of several multi-step reaction mechanisms to describe the mass loss and the mass loss rate of pine needles in TGA at different heating rates in inert and oxidative atmospheres. The kinetic parameters of the different reactions were optimized using the Shuffled complex evolution (SCE) technique. Model results show that both mass loss and mass loss rate should be considered in order to evaluate properly the mechanism. The drying process is described accurately by a single reaction with a well-established set of kinetic parameters. The conversion of dry pine into char requires a five-step reaction mechanism that is combined of three reactions to describe the pyrolysis under inert atmosphere and another two reactions to describe the oxidative process. Less detailed mechanisms were found to be unable to reproduce the mass loss rate. In particular, the one-step reaction mechanism, widely used to model the pyrolysis process in wildland fire simulations, should be used with care. Finally, the char oxidation process can be described with a single step-reaction mechanism. The final complex mechanism is comprised of one reaction for drying, five reactions for the conversion of dry pine into char, and one reaction for the char oxidation, is promising. Further studies are required for its validation in large-scale experiments.     

帽儿山地区4种灌木热解特性研究

森林火灾对森林资源的破坏性极大,通过热解技术研究森林资源自身性质,有利于对不同树种采取不同防火优先级。以黑龙江省帽儿山地区的紫丁香、长白忍冬、榆叶梅、红瑞木4种灌木为实验材料,采用热重分析法研究其热解特性和动力学特性。选取粒径为40目的实验材料,环境的升温速率为20 ℃/min,在氧体积分数21%的空气氛围下进行实验,采用微分法和积分法相结合进行动力学研究,根据动力学方程求算活化能和指前因子。通过模糊聚类分析对主要失重阶段的最高失重速率点的温度、主要失重阶段的活化能和热解剩余质量3个因素综合分析得到4种灌木的易燃性排序。研究结果表明：4种灌木材料的热解过程分为失水阶段,半纤维素、纤维素和部分木质素热解阶段,部分木质素及热解产物热解阶段,灰分阶段4个阶段;4种灌木主要失重阶段是半纤维素、纤维素和部分木质素热解阶段,次要失重阶段是部分木质素及热解产物热解阶段;4种灌木主要失重阶段的活化能范围为260.435 1~326.171 3 kJ/mol,次要失重阶段活化能范围为315.836 7~425.920 1 kJ/mol。4种灌木最先发生化学反应的是主要失重阶段,根据模糊聚类分析对4种灌木的易燃性进行排序,结果为：紫丁香>榆叶梅>红瑞木>长白忍冬,表明4种灌木中紫丁香最易燃,在防火期需要重点关注,长白忍冬抗火性最好,可以作为抗火性树种。     

Multi-scale modeling of the thermal decomposition of fire retardant plywood

Due to the complexity and costs of full scale fire-test experiments, numerical simulations provide a useful alternative when investigating the fire behavior of new materials. The mass loss rate of the solid is one of the most important parameters in assessing fire behavior as it is directly linked with the pyrolysis gas flow rate and represents the initial factor of the combustion process. In this paper, fire retardant plywood is investigated with a focus on the solid mass loss rate modeling. A multi-scale approach is followed in order to establish the kinetic mechanism of thermal degradation. A combination of small scale and large scale tests were completed to fully develop and validate the proposed kinetic mechanism. For small scale testing, experiments are conducted by using thermo-gravimetric analysis coupled to gas analysis with FTIR technique under nitrogen and air atmospheres. These experiments were completed at several heating rates. Thermo-gravimetric results are used to propose a kinetic mechanism for the thermal decomposition of the solid and the kinetic parameters are calculated by using the genetic algorithms method. For larger-scale testing, experiments were carried out in a cone calorimeter coupled to a FTIR gas analyzer. The experiments were completed in air atmosphere in order to validate the kinetic mechanism developed from small-scale testing. The kinetic model developed is implemented into the general Gpyro model which takes into account both thermal and mass transfer phenomena inside the solid. The results showed good agreement between the model calculations and the experimental data.     

森林腐殖质热解动力学模型对比与分析

摘 要：林火是对森林资源破坏十分严重的一种自然灾害,一旦引发,会在森林中快速蔓延且不易扑灭,因此对林地内可燃物的热解特性进行研究分析有十分重要的现实意义。本文以帽儿山人工阔叶红松林内的腐殖质为研究对象,利用TG-DTG曲线分析腐殖质的热解过程。试验数据采用Coats-Redfern（CR）法、Doyle法、Flynn-Wall-Ozawa（F-W-O）法、分布活化能模型（DAEM）4种典型方程进行动力学模型分析,分别计算反应活化能、反应级数和频率因子。试验结果表明：腐殖质的热解主要分为4个阶段：脱水阶段、过渡阶段、主要失重阶段和炭化阶段。对主要失重阶段的动力学参数进行计算,结果显示,Doyle法和DAEM模型较其他2种方程更适合腐殖质的热解动力学分析。其中DAEM模型适合分析样品活化能的变化趋势,Doyle法适合求取不同失重率下的活化能。     

The application of a genetic algorithm to estimate material properties for fire modeling from bench-scale fire test data

A methodology based on an automated optimization technique that uses a genetic algorithm (GA) is developed to estimate the material properties needed for CFD-based fire growth modeling from bench-scale fire test data. The proposed methodology involves simulating a bench-scale fire test with a theoretical model, and using a GA to locate a set of model parameters (material properties) that provide optimal agreement between the model predictions and the experimental data. Specifically, a GA based on the processes of natural selection and mutation is developed and integrated with the NIST FDS v4.0 pyrolysis model for thick solid fuels. The combined GA/pyrolysis model is used with cone calorimeter data for surface temperature and mass loss rate histories to estimate the material properties of two charring materials (redwood and red oak) and one thermoplastic material (polypropylene). This is done by finding the parameter sets that provide near-optimal agreement between the model predictions and experimental data, given the constraints imposed by the underlying physical model and the accuracy with which the boundary and initial conditions can be specified. The methodology is demonstrated here with the FDS pyrolysis model and cone calorimeter data, but it is general and can be used with several existing fire tests and almost any pyrolysis model. Although the proposed methodology is intended for use in CFD-based prediction of large-scale fire development, such calculations are not performed here and are recommended for future work.     

Ranking the Level of Openness in Blind Compartment Fire Modelling Studies

The paper considers what is meant by, and the purpose of, blind modelling in the context of fire engineering. A discussion is presented on the very important interaction between the model and the model users, and five generic groups of model users are defined. The paper goes on to provide details of previous blind modelling studies from the literature, as well as a recent blind modelling programme that was conducted in New Zealand. A so-called ‘Openness Assessment Framework’ is proposed as a way of scoring or ranking existing or planned blind modelling programmes, with the recent New Zealand programme forming the case study for the evaluation. This framework is then applied to a range of evaluation definitions that are given in the literature, the five generic model user groups, as well as the range of blind modelling exercises already identified. Finally, a seven-step methodology for designing blind modelling studies and experimental comparisons in the context of full-scale multi-item compartment fire experiments is proposed.     

Modeling the removal of dissolved organic carbon by ion exchange in a completely mixed flow reactor

A mathematical model was developed to describe removal of dissolved organic carbon (DOC) by a macroporous, strong-base anion exchange resin in a completely mixed flow reactor with resin recycle and partial resin regeneration. The two-scale model consisted of a microscale model describing the uptake of DOC by the resin coupled with a macroscale model describing the continuous-flow process. Equilibrium and kinetic parameters were estimated from batch laboratory experiments. The model was validated using continuous-flow data from two pilot plant studies. Model predictions were found to be in good agreement with the observed pilot plant data.     

沥青在贫氧气氛下的热解动力学实验研究

隧道内沥青热解燃烧通常在贫氧环境下进行。为研究沥青在真实火灾情况下，环境氧体积分数对其热解过程和动力学的影响，利用热重分析仪，对10#铺地基质沥青，在氧体积分数分别为0、7%、15%和21%的氧氮混合气氛下进行热解实验，研究沥青的热解过程，并进行动力学分析。结果表明：沥青在氧体积分数为0(纯N2)的气氛下热解过程只有一个主失重阶段，最终剩余量高；在有氧气氛下热解过程均呈现出3 个失重阶段，最终剩余量低于10%；随着氧体积分数的增加，沥青的起始分解温度升高、热解速度增加、高温段失重加剧，低温段活化能小幅降低，反应过程均属于扩散模型；高温段活化能在氧体积分数为21%时最高，反应模型由贫氧时的扩散模型转变为三级简单模型。     

Using suppression and detection devices to steer CFD fire forecast simulations

Firefighters would greatly benefit from a technology based on predictive fire simulations, able to assist their decision making process. For those simulations to be useful, they need a certain degree of precision and resolution that can only be provided by CFD type fire models. But CFD simulations typically take large periods of time to complete, and their results would thus not be available in time to be of use during an emergency. Due to the high complexity of fire spread dynamics that arises from the interaction between solid and gas phase and the corresponding physical-chemical processes (e.g. pyrolysis), the spread of the fire cannot be predicted from first principles in real-time using contemporary computers, and has to be given as parameters to the model. Data can be incorporated into the model to characterise the fire, but only a limited range of measurements are recorded in current buildings. While it might be possible that buildings of the future incorporate a higher density of sensors than contemporary buildings, it is likely that emergency response systems will have access only to conventional data such as smoke detectors and sprinkler activation time for the foreseeable future. In this study the use of conventional detection and suppression devices for the estimation of fire characteristics by means of an inverse modelling framework is explored. Additionally to the growth rate of the fire, the location of the fire origin is successfully estimated. Inverse CFD modelling and tangent linearisation is used to assimilate the data. The nature of the incoming data is consistent with current detection and suppression devices, in such that only a time of activation is recorded and fed into the model. It is shown that the growth rate of the fire and the location of its origin can be correctly and efficiently estimated using sprinkler and smoke detector activation time only. It is further shown that the estimated spread rate is not sensitive to fire origin location.     

Relevant model complexity for non-charring polymer pyrolysis

The choice of the heat, mass and chemical mechanisms included in a pyrolysis model is often subjective, and detailed justifications of the inclusion or exclusion of the different mechanisms are infrequent. The implicit assumption that models with a higher number of mechanisms reproduce more accurately the reality has led to the recent growth of complexity in pyrolysis modelling seen in the literature. However, as we show in this work, the comparison of several conceptual models predicting the same experimental results does not support this assumption, but reveals the presence of unnecessary complexity and multiple sources of uncertainty. Using a novel approach corresponding to a mechanism sensitivity, the influence of the heat, mass and chemical mechanisms on the transient predictions of surface temperature and mass loss rate (non-flaming conditions) for PolyMethylMethAcrylate (PMMA) samples is investigated. While a small change in the chemical degradation mechanism has a large effect on the predictions of the mass loss rate, the surface temperature is not affected. The heat transfer mechanisms appear to have however a significant effect on both quantities of interest. This study demonstrates that the use of complex chemical mechanisms (e.g. multi-step reaction scheme or oxidative reaction) is not justified if the mechanisms of the heat transfer are kept simple. It is therefore recommended to use consistent levels of crudeness dictated by the heat transfer.     

生物质热解研究的进展

通过分析生物质热解的物理特性和运行条件,采用动力学模型和传热、传质模型,对生物质热解进行描述。     

玻璃纤维网与XPS复合层火灾危险性分析

为探究玻璃纤维网对XPS材料火灾危险性的影响,利用热重仪测得材料XPS的热解特性参数,通过锥型量热仪获得XPS与常见3种孔径的玻纤网复合层的燃烧特性参数,采用层次分析法计算出与不同孔径玻纤网复合的保温层的火灾危险程度。实验结果表明：XPS材料从280 ℃开始热解,在425 ℃时达到最大热解速率;增加玻璃纤维网会促进XPS的燃烧反应,XPS复合的玻璃纤维网孔径越小,燃烧释放的热量与烟气量越高,火灾危险性越大。说明玻璃纤维网会加剧XPS保温材料的火灾危险,并且在建筑外墙保温设计时应减少或避免使用小孔径的玻璃纤维网。     

Forecasting fire growth using an inverse zone modelling approach

A new methodology to effectively forecast fire dynamics based on assimilation of sensor observations is presented and demonstrated. An inverse modelling approach with a two-zone model is used to forecast the growth of a compartment fire. Sensor observations are assimilated into the model in order to estimate invariant parameters and thus speed up simulations and recover information lost by modelling approximations. A series of cases of a compartment fire radially spreading at different growth rates (slow, medium and fast) are used to test the methodology. Spread rate, entrainment coefficient and smoke transport time are the invariant parameters estimated via a gradient-based optimization method with tangent linear differentiation. The parameters were estimated accurately within minutes after ignition and the heat release rate reproduced satisfactorily in all cases. Moreover, the temperature and the height of the hot layer are forecasted with a positive lead time between 50 and 80 s, depending on the fire growth rate. The results show that the simple mass and energy conservation equations and plume correlation of the zone model are suitable to forecast the main features of a growing fire. Positive lead times are reported here for the first time in fire dynamics. The results also suggest the existence of an optimal width for the assimilation window. The proposed methodology is subject to ongoing research and the results are an important step towards the forecast of fire dynamics to lead the emergency response.     

木材热重试验及动力学分析

选取文物建筑中 4 种常见的木材为研究对象,对其在 5 K/min 的升温速率下进行热重实验。将木材的热解过程分为 4 个阶段,木材热失重主要在第二和第三阶段进行。对不同温度阶段下的热解过程进行分析,并建立了木材热解的动力学方程,求出热解动力学参数。木材热稳定性从高到低依次为：杉木热稳定性最好,楠木次之,白松和红松稳定性相当。     

Sensor-steered fire simulation

A sensor-linked modelling tool for live prediction of uncontrolled compartment fires, K-CRISP, has been developed in order to facilitate emergency response via novel systems such as FireGrid. The modelling strategy is an extension of the Monte-Carlo fire model, CRISP, linking simulations to sensor inputs which controls evolution of the parametric space in which new scenarios are generated, thereby representing real-time “learning” about the fire. CRISP itself is based on a zone model representation of the fire, with linked capabilities for egress modelling and failure prediction for structural members, thus providing a major advantage over more detailed approaches in terms of flexibility and practicality, though with the conventional limitations of zone models. Large numbers of scenarios are required, but computational demands are mitigated to some extent by various procedures to limit the parameters which need to be varied. HPC (high performance computing) resources are exploited in “urgent computing” mode. The approach adopted for steering is shown to be effective in directing the evolution of the fire parameters, thereby driving the fire predictions towards the measurements. Moreover, the availability of probabilistic information in the output assists in providing potential end users with an indication of the likelihood of various hazard scenarios. The best forecasts are those for the immediate future, or for relatively simple fires, with progressively less confidence at longer lead times and in more complex scenarios. Given the uncertainties in real fire development the benefits of more detailed model representations may be marginal and the system developed thus far is considered to be an appropriate engineering approach to the problem, providing information of potential benefit in emergency response.