Henning Bockhorn

KINETIC MODELING OF SOOT FORMATION WITH DETAILED CHEMISTRY AND PHYSICS: LAMINAR PREMIXED FLAMES OF C2 HYDROCARBONS

Abstract The present study reports an updated detailed chemical kinetic model for soot formation. The model combines recent developments in gas-phase reactions, aromatic chemistry, soot particle coagulation, soot particle aggregation, and develops a new submodel for soot surface growth. The model was tested against experimental profiles of major and minor chemical species, aromatics, soot volume fractions, and soot particle diameters reported in the literature for nine laminar premixed flames of ethane, ethylene, and acetylene. The numerical agreement between the model predictions and experimental data is generally within a factor of 3. This level of agreement is very encouraging, considering the current uncertainties in the thermodynamics and kinetics of aromatics and soot chemistry. The principal accomplishment of the present study is that the demonstrated level of agreement all around—main flame environment, aromatics, and soot—can be attained with a single reaction model.

Kinetic study on the thermal degradation of polypropylene and polyethylene

Abstract In this work, a kinetic study on the thermal degradation of polyethylene and polypropylene is presented. The thermal degradation of these polymers is investigated under isothermal conditions using a gradient free reactor with on-line mass spectrometry. Apparent kinetic parameters for the overall degradation are determined. In the case of polyethylene degradation, a change of the apparent reaction order with temperature is observed, whereas in the case of polypropylene degradation, a constant apparent order of reaction within the investigated temperature range was found. Fractional pyrolysis under dynamic conditions of polypropylene performed at temperatures below the isothermal measurements reveal a decreasing amount of alkenes and alkanes and an increasing amount of dienes with temperature. On the basis of these data and corresponding literature, a detailed discussion of the reaction mechanisms is possible. Rate equations are formulated and kinetic models for polyethylene and polypropylene degradation are discussed, which are consistent with the measured rate coefficients.

A comparative kinetic study on the pyrolysis of three different wood species

The catalytic effect of pH-neutral inorganic salts on the pyrolysis temperature and on the product distribution was studied by fractionated pyrolysis followed by GC/MS and GC/FID and by thermogravimetric analysis (TGA) of cold-water-washed hornbeam wood. Sodium and potassium chloride have a remarkable effect on the pyrolysis temperature and on the product distribution, whereas calcium chloride only changes the low temperature degradation of hornbeam wood and the product distribution is nearly unchanged compared with water-washed hornbeam wood. All studied potassium salts (KCl, KHCO3, and K2SO4) decrease the amount of levoglucosan the order of magnitude being dependent on the anion: chloride has a more pronounced effect than sulphate, and sulphate a more pronounced effect than bicarbonate. The thermal degradation of three different wood species (hornbeam, walnut and scots pine) was investigated by analysis of thermogravimetric/mass spectrometric pyrolysis. Commonly used model substances for the main components of wood, like xylan, pure cellulose or filter pulp, were found to be unreliable for the evaluation of formal kinetic parameters that are able to describe the pyrolysis of wood. A method for the individual evaluation of formal kinetic parameters for the main components of wood was used, that uses specific ion fragments from lignin degradation products to study the lignin degradation. Coniferous lignin is thermally more stable than deciduous lignin, and the latter produces smaller char yields. The differences in wood species mainly result in different degradation rates for the lignin and for the early stages of the hemicellulose degradation.

Mechanisms and kinetics of thermal decomposition of plastics from isothermal and dynamic measurements

Abstract The kinetics of decomposition of plastics are of interest from different points of view, i.e. evolution of harmful substances during fires or waste incineration, recovering of chemical raw materials from plastic refuses and designing of recycling procedures. To measure the formal kinetic parameters of the degradation of polymers isothermal and dynamic methods are applied in this work. Dynamic measurements are performed by combined thermogravimetry mass spectrometry (TG-MS), the isothermal measurements are carried out with a new closed loop-type reactor. To evaluate consistent kinetic data from isothermal and dynamic measurements, the energy balance for the sample in dynamic measurements has to be considered to obtain the true sample temperature and heating rate. Subject of this investigation is the exploitation of dynamic and isothermal methods for measuring and interpreting the kinetics of thermal decomposition of plastics. Results for commodity plastics polyethylene and poly(vinyl chloride) (PVC) are presented. The combined application of TG–MS, isothermal experiments in the closed loop-type reactor and DSC leads to new results for the decomposition kinetics of PVC. The dehydrochlorination mechanism at moderate temperature can be distinguished in an endothermal and exothermal part. The benzene formation is identified as a second order reaction. A great advantage of the isothermal method is, that changes in the mechanisms are detectable, i.e. changes in the apparent order of the reaction and the apparent activation energy. From that, new mechanistic aspects of the decomposition kinetics of polyethylene were obtained.

A conservative fully adaptive multiresolution algorithm for parabolic PDEs

We present a new adaptive numerical scheme for solving parabolic PDEs in Cartesian geometry. Applying a finite volume discretization with explicit time integration, both of second order, we employ a fully adaptive multiresolution scheme to represent the solution on locally refined nested grids. The fluxes are evaluated on the adaptive grid. A dynamical adaption strategy to advance the grid in time and to follow the time evolution of the solution directly exploits the multiresolution representation. Applying this new method to several test problems in one, two and three space dimensions, like convection-diffusion, viscous Burgers and reaction-diffusion equations, we show its second-order accuracy and demonstrate its computational efficiency.

Combustion generated fine carbonaceous particles

Soot is of importance for its contribution to atmospheric particles with their adverse health impacts and for its contributions to heat transfer in furnaces and combustors, to luminosity from candles, and to smoke that hinders escape from buildings during fires and that impacts global warming or cooling. The different chapters of the book adress comprehensively the different aspects from fundamental approaches to applications in technical combustion devices.

Environmental engineering: Stepwise pyrolysis of plastic waste

Abstract Kinetic data obtained from micro thermogravimetry and gradient free reactor experiments confirm that different molecular structures of commodity plastics bring about different reaction mechanisms of thermal decomposition, different reaction rates, and different temperature dependencies of the decomposition rates. From that, stepwise pyrolysis of mixtures of plastics seems to be reasonable where the different components of the mixture are pyrolysed at different temperatures. To perform a stepwise pyrolysis in laboratory scale a cascade of well stirred reactors has been developed where mixing of the reactor contents occurs by circulating of stainless steel spheres. Examples for the separation of single plastics by stepwise thermal decomposition of mixtures of poly(vinyl chloride), polystyrene and polyethylene are presented. In the first step hydrogen chloride from poly(vinyl chloride) is released, in the second step styrene from polysytrene is formed and in the third step aliphatic compounds from polyethylene decompositon are trapped. Differences in the thermal degradation of single polymers and mixtures of polymers, e.g. in the apparent activation energies and preexponential factors, are investigated using mixtures and blends of polyethylene and polystyrene.

Inception and growth of soot particles in dependence on the surrounding gas phase

Abstract Two sets, each of four sooting laminar ethyne-oxygen-argon flames have been investigated at low pressure. Within each set the pressure, C/O ratio of the flames, and dilution with argon were varied. The flame conditions of the sets were chosen such that the final soot volume fraction was constant within each set and differed by a factor of two from set to set. Mean radii and number densities of soot particles were measured by laser light scattering and extinction. Temperatures are obtained from thermal radiation of soot particles and by applying the sodium-d-line reversal method. The flames were sampled and analysed with on-line mass-spectrometry for concentration of gaseous species. It is found that there is no simple dependence of the soot appearance rates on the ethyne partial pressure, the specific surface area of the soot particles, and the gas-phase temperature. One-dimensional calculations have been performed with a numerical code, that includes a detailed chemical soot model. Calculated profiles of the gaseous species, temperature, number density, and soot volume fraction are in good agreement with the experimental data. The experimentally observed phenomena can be explained in terms of a detailed hydrogen abstraction, carbon addition mechanism. A detailed one-dimensional sensitivity analysis shows that the rate limiting steps of soot formation are gas-phase reactions.

Investigation of the Kinetics of Pyrolysis of PVC by TG-MS-Analysis

ABSTRACT Pyrolysis or combustion of plastics are of interest from different points of view: on the one hand polymers from which harmful substances may arise are involved in fires or are fed to incinerators; on the other hand plastic refuses are an alternative resource of energy or chemical raw materials. For designing recycling procedures the behaviour of the decomposition of polymers with regard to the decomposition products and the kinetics of decomposition must be known. Plastic refuses encompass chlorine- and/or nitrogen-containing polymers which may form toxic or corrosive products under pyrolytic conditions or in combustion. These components must be separated before processing or the troublesome elements have to be eliminated by pretreatment. The aim of this work was to investigate the kinetics of pyrolysis of polyfvinyl chloride) and mixtures of poly(vinyl chloride) with polyethylene, polypropylene, polystyrene and polyamide 6. By means of simultaneous thermogravimetry-mass spectrometry the kinetic...

Size distributions of nanoscaled particles and gas temperatures from time-resolved laser-induced-incandescence measurements

Laser-induced-incandescence (LII) signal decays are measured in sooting premixed atmospheric and low-pressure flames. Soot particle temperatures are obtained from LII signals measured at two wavelengths. Soot particle size distributions P(r) and flame temperatures T are measured spatially resolved by independent techniques. Heat and mass transfer kinetics of the LII process are determined from measured soot particle temperatures, flame temperatures, and particle sizes. Uncertainties of current LII models are attributed to processes during the absorption of the laser pulse. Implications for LII experiments are made in order to obtain primary soot particle sizes. Soot particle size distributions and flame temperatures are assessed from measured particle temperature decays by use of multi-D nonlinear regression.