Behdad Moghtaderi

Lack of synergetic effects in the pyrolytic characteristics of woody biomass/coal blends under low and high heating rate regimes

Pyrolytic behaviours of woody biomass/coal mixtures were investigated under both low and high heating rate conditions over a range of temperatures between 200°C and 1400°C. Results obtained from this comprehensive investigation indicated that the pyrolytic characteristics of the mixtures follow those of the parent materials in an additive manner. Therefore, under inert (non-oxidising) conditions the two fuels undergo independent thermal conversion without any chemical interactions. As such, the yield of the major pyrolysis products (e.g. volatiles and char) is proportional to the percentage of woody biomass and coal in the mixture. This confirms the hypothesis made by a number of researchers about the lack of synergistic effects in the yield of pyrolysis products from blended coal and woody biomass. However, in this study, we show that even the compositions of the gaseous products from blended samples are linearly proportional to those of their parent fuels (lack of synergistic effects). These findings can potentially help to understand and predict the behaviour of woody biomass/coal blends in practical combustion systems.

An integral model for the transient pyrolysis of solid materials

The modelling of the spread of fire and its extinguishment still represents a significant challenge. As part of a combined experimental and computational study of fires we have developed a general Computational Fluid Dynamics (CFD) model of fire spread and extinguishment. The primary objective was to produce a flexible computational tool which can be used by engineers and scientists for design or research purposes. The present paper deals with the description and validation of a solid pyrolysis model which has been applied, as a sub-model, in this general computer fire code. The pyrolysis model has been formulated using the heat-balance integral method. The model can be applied to slabs of char forming solids, such as wood, as well as non-charring thermoplastic materials, such as PMMA. Results are compared with analytical solutions, numerical simulations and experimental data. In all cases the integral model performs well. ( 1997 by John Wiley & Sons, Ltd.

Thermal study of decomposition of selected biomass samples

Fears of climate change and increasing concern over the global warming have prompted a search for new, cleaner methods for electricity power generation. Technologies based on utilising biomass are attracting much attention because biomass is considered to be CO2 neutral. Co-firing of biomass fuels with coal, for example, is presently being considered as a mean for reducing the global CO2 emissions. Biomass is also applied in thermal conversion processes to produce fuels with higher calorific values and adsorbents. In any case, thermal decomposition is essential stage where volatiles and tars are evolved followed by consequent heats of reactions. In this work sawdust biomass samples were selected in order to study their thermal conversion behaviour. Heats of decomposition for each sample were measured during continuous heating at a prescribed heating rate under inert atmospheric conditions. The decomposition generally commenced in all samples at 150°C and was completed at 460°C in a series of endo and exothermic reactions influenced by its lignin and cellulosic content. Single biomass sample was subjected to heating rates ranging from 10 to 1000°C min-1 and the effect of heating rate on decomposition was studied. The origin of reactions for each thermal sequence is herein discussed in detail.

A new correlation for bench-scale piloted ignition data of wood

This paper presents the results of a combined experimental and theoretical study of piloted ignition of cellulosic materials. The main objective is to present an engineering solution to the piloted ignition problem for wood exposed to radiant heat in a bench-scale piloted ignition test. This has been motivated by the need to have simple models of ignition for use in a computational fluid dynamics (CFD) model of fire spread and extinguishment in building fires. The experiments were conducted on oven-dry and moisture conditioned samples of three wood species using a cone calorimeter. As expected, the experimental data revealed that the effect of moisture content on the piloted ignition process is significant. It was also found that the ignition temperature depends on the external heat flux, which supports other recent studies. Based on the experimental observations, an approximate analytical equation was derived and then used for correlating the ignition data, as well as extracting the piloted ignition properties. The chief distinguishing feature of the present equation over other similar equations is that it takes into account the variation of the ignition temperature with external heat flux.

BIOMASS GASIFICATION KINETICS: INFLUENCES OF PRESSURE AND CHAR STRUCTURE

ABSTRACT In this study, the gasification kinetics of chars from different biomass species have been investigated within the temperature range 800–950°C, CO2 concentrations of 10 to 100% v/v, and pressures between 1 and 20 bar using thermogravimetric analysis (TGA). The quartz wool matrix (QWM) method was used to simulate circulating fluidized-bed (CFB) conditions. The reactivity results obtained by the QWM method were compared with those extracted using TGA data. Kinetic data from TGA analyses was found to be compatible for predicting CFB gasification reactivity. Effects of pyrolysis pressure on the chemical structure of char and char conversion reactivities were also investigated. Pressure has been found to have no effect on reactivity during char conversion while having a dramatic effect on chemical and physical structure during the pyrolysis process. The Langmuir–Hinshelwood rate equation represents the pressurized radiata pine char gasification kinetics well. The Langmuir–Hinshelwood expression for studied conditions can be described as Kinetic data found in this study are in good agreement with the literature.

Computational fluid dynamics modelling of wood combustion

The modelling of the extinguishment of fires still represents a significant challenge. As part of an effort to predict fire spread and extinguishment using water sprays we are developing a computational fluid dynamics (CFD) model of fire spread. This paper deals with the description of the CFD model, the solid pyrolysis model (wood in this case) and the coupling of these models. Results are compared with experimental data from cone calorimeter tests and the model is shown to give good agreement. The sensitivity of the calculated results to uncertain parameters in the pyrolysis modelling is also considered.

Experimental and numerical analysis of sawdust-char combustion reactivity in a drop tube reactor

This article discusses the results of a combined experimental and numerical investigation of the combustion reactivity of sawdust char in a drop tube furnace. The work presented here was motivated by the lack of reliable data in the open literature on the combustion reactivity of woody biomass char. This study involved the experimental determination of the global kinetic parameters of sawdust-char oxidation and the subsequent application of these parameters in a computational fluid dynamics (CFD) code. The modified code was employed for mathematical modeling of the sawdust-char combustion in the drop tube furnace. The experiments were conducted at furnace temperatures of 1473 and 1673 K in a 10 mol% O 2 environment and 1473 K in air. Experimental results indicated that the bulk of the char oxidation reaction for sawdust occurred in Regime II where chemical reaction and pore diffusion rates are comparable. In addition, the sawdust char exhibited distinctive near-extinction combustion behavior at the early stage of the reaction. The combustion reactivity of the sawdust char, with an apparent reaction order ( n ) of 0.4, was comparable to that of some high-volatile bituminous coal chars. Good agreement was achieved between predicted and experimental results.

Solid fire extinguishment by a water spray

Heat and mass transfer between a water spray and a burning solid surface is considered in application to the fire extinguishment problem. The study combines analytical and computational fluid dynamics (CFD) approaches. In this study extinguishment is considered as a nonexistence of a steady-state burning regime. An analytical one-dimensional burning model of the solid phase is employed, which connects the temperature gradient in the solid with the burning rate. In order to get the critical boundary between the burning and extinction regimes the dependence of flame-to-surface heat feedback on the burning rate is determined using CFD fire simulations. The results are presented as a critical water flow rate required for extinguishment. Different types of burning materials are considered and the results are compared with the available experimental data.

Molecular Dynamics Simulation of the Low-Temperature Partial Oxidation of CH4

Low-temperature partial oxidation of methane was investigated using reactive molecular dynamics (MD) and quantum mechanical (QM) methods. In particular, the ReaxFF hydrocarbon force field [Chenoweth, K.; et al. J. Phys. Chem. A 2008, 112, 1040] was employed to simulate a [20 CH(4) + 10 O(2)] model system at 500 degrees C. The chemical mechanism of the partial oxidation of methane was proposed on the basis of analysis of the computed trajectory of this model system. The partial oxidation of methane was observed to be initiated by the abstraction of hydrogen from CH(4) by O(2) and the atomization of CH(4) itself. Subsequent radical recombination between hydrogen atoms and the dehydrogenation of CH(4) were the primary pathways by which H(2) was formed. In agreement with current models of low-temperature combustion, radicals including H(3)C-OO and H(2)C-OO were also observed during the MD simulation. The observed reaction mechanism was subsequently analyzed using QM methods. For instance, structural features of prominent radical species observed during the MD simulation were analyzed using density functional theory (DFT) and coupled-cluster (CCSD(T)) methods. Enthalpies of reaction of all observed chemical processes were calculated using DFT and the W1 composite method. Where possible, comparisons with experimental data were made.

Effect of Water Spray on Re-ignition Characteristics of Solid Fuels

A set of small-scale experiments was carried out to study the effect of a water spray on the re-ignition characteristics of solid fuels. The influence of other key parameters, such as the incident heat flux and pre-bum, was also investigated. The experiments were conducted on specimens of wood and PMMA using a cone calorimeter. A water spray was produced by a small commercial nozzle. As expected, the effect of water on the re-ignition time was found to be significant. It was also found that the re-ignition characteristics of charring materials, such as wood, are quite different from non-charring materials (e.g. PMMA) mainly due to the structural differences. Based on the experimental observations a set of empirical correlations was obtained for both wood and PMMA samples. Predictions of the re-ignition time made by these correlations agree well with the measurements. Simulations of the re-ignition times were also performed using a detailed mathematical model. Comparisons with the experimental data are provided.