Annual Report Department

2.10Applied combustion

2.10.1Low temperature pyrolysis of CCA treated wood waste

Low-temperature pyrolysis is evaluated as a possible technique for the disposal of chromated copper arsenate (CCA) treated wood waste. Since arsenic has been identified as the problem compound with respect to metal release during pyrolysis, the aim is to identify the mechanism of arsenic release. All experimental observations, own and from other researchers, are compiled and conclusions are drawn with respect to the thermal behaviour of arsenic oxides, as pure oxides, as part of a mixture of model compounds, and in CCA treated wood.

The literature has been extensively reviewed with respect to all technologies suggested for the disposal of CCA treated wood waste. These technologies are identified as belonging to one of the following categories: reuse, refining for recycling, treatment and destruction, landfill disposal. Thermochemical processes, which are part of the category treatment and destruction, are analysed in detail. Pyrolysis (slow and flash), gasification, incineration and co-incineration are evaluated as possible technologies for the treatment or recuperation of CCA treated wood waste. The best available thermochemical conversion technology (BAT) is identified on the short and the long term. On the short term co-incineration is the BAT as long as CCA treated wood waste has not to be treated separately and dilution is allowed. On the long term a sustainable solution has to be found, which can be one of the following: (1) low-temperature pyrolysis in a moving bed, or (2) high temperature gasification in a metallurgic furnace. Both technologies aim at recuperating the metals and energy (as secondary fuels) contained in the CCA treated wood waste, but both technologies still have to be proven.

Related projects: Bilateral project with ARCH Timber Protection: Feasibility study of pyrolysis as a possible disposal method for impregnated wood waste.

Publications and reports: 2003P11, 2003P16, 2003P22, 2003P29, 2003P23

Scientific staff: L. Helsen, E. Van den Bulck

2.10.2Research on confined annular swirling jet combustion

A promising technique is the use of lean near-premixed swirling jet combustion in order to control the NO_x emission whilst retaining flame stability, turndown ratio and thermal efficiency. This particular combustion is studied experi­mentally at the research facilities of the Katholieke Universiteit Leuven (KUL, Belgium) and the Technische Uni­versiteit Delft (TUD, The Netherlands). Five different flame states are identified in a compact combustion chamber that is fired by a 30 kW swirl stabilized partially premixed natural gas burner. These flame states include a nozzle-attached tulip shaped flame, a non-attached lifted ring-shaped flame suitable for single digit ppm NO_x emission and a second ring-shaped Coanda flame that clings to the bottom wall of the burner. Flame state transition is generated by changing the swirl number and may be further modified by premixing the combustion air with 70% of the natural gas flow. Detailed data including major species, temperature and velocity of two ring-shaped flames are reported. The reactants flow out of the burner in a conical sheet and enter the spread-out flame. Bimodal probability distributions of the axial velocities reveal an oscillatory behaviour of the internal recirculation zone (IRZ) as well as the flame envelop boundary and this emphasizes that the flame brush is strongly time dependent. Low pressure regions in the corner of the tight combustion chamber and altered combustion kinetics account for the flame transition and stabili­zation process. Finally, the strength of the IRZ is compared with data of other researchers revealing that both ring shaped flames feature a large and much stronger IRZ than reported in literature although limited swirl numbers apply. Currently, work is in progress to link experimental data of the cold flow field with the reacting flow phe­nomena. The experimental results will be compared to numerical calculations using the commercial CFD code CFX. Moreover, e.g. data on bimodal structures are highly wanted in order to validate numerical data of LES simulations of other researchers.

Publications and reports: 2003P07, 2003P04, 2003P05, 2003P49

Scientific staff: E. Van den Bulck, K. Vanoverberghe

2.10.3Thermoacoustic analysis of combustion processes

Premixed burners in longitudinal combustion chambers are studied with regard to combustion instabilities. Thermoacoustic analysis using linearized wave equations as well as numerical solutions of the Euler equations are implemented in computer codes. Several results have now been obtained and submitted for presentation at selected international conferences.

Publications and reports: 2003P18

Scientific staff: E. Van den Bulck

2.11Safety engineering

2.11.1Study of the influence of temperature and pressure on the flammability limits of gases

Flammability limits of reactive gaseous mixtures are influenced by many parameters related to initial conditions (pressure and temperature), to the physical environment (shape and size of the vessel), and to the characteristics of the ignition source. At ambient temperature and atmospheric pressure these limits are well established and the influence of vessel geometry and ignition source are well documented. At elevated pressure and temperature, on the other hand, only a small amount of data is available.

The pressure and temperature dependence of flammability limits of methane/air, ethane/air, propane/air, n-bu­tane/air, ethylene/air and propylene/air mixtures has been determined experimentally in a 4.2 l spherical explosion vessel. In the pressure-temperature range tested (100 - 3000 kPa, 20-250 °C) a non-linear pressure dependence and a linear temperature dependence of the upper flammability limits were found.

Scientific staff: J. Berghmans, F. Verplaetsen, F. Van den Schoor

2.11.2Study of the influence of pressure on the auto-ignition temperature of gases

The aim of the present study is to improve the knowledge of certain parameters influencing the auto-ignition tem­perature (AIT) of gas mixtures and to determine real AIT values for industrial processes, both in an experimental and a theoretical way. The experimental study focuses on the influence of the pressure on the AIT. The auto-ignition behaviour of methane/air and ammonia/air mixtures is investigated in a closed vessel apparatus for pressures up to 5000 kPa and for a broad concentration range. It is found that the most ignitable mixture compositions are situated in the rich fuel range and that auto-ignition can occur even at concentrations far beyond the expected flammability limits. Cool flames are observed for methane/air mixtures in the rich fuel range, from 30 up to 80 mol%. For am­monia/air and methane/air mixtures, the AIT is strongly pressure dependent and can be correlated by a Semenov correlation within the experimental range. The data are used both to estimate realistic AIT values for industrial instal­lations, e.g. for urea plants, and to validate a theoretical model.

The development of a theoretical model to compute AIT values under realistic conditions is still in progress. The model has to include detailed chemical kinetics together with fluid dynamics. The present results of the kinetic modelling study are quite promising. The detailed oxidation chemistry is successfully reduced and will be incor­porated in a fluid dynamic model describing the influence of the flow conditions on the AIT.

Scientific staff: J. Berghmans, F. Verplaetsen, L. Vandebroek

2.11.3Study of the explosion and detonation hazards of gas mixtures in nitrous oxide

The influence of initial pressure and temperature on the explosion characteristics of mixtures of methane, hydrogen and nitrous oxide and of toluene and nitrous oxide is investigated. The flammability limits and explosion pressure are measured in an 8 litre spherical explosion vessel. Because nitrous oxide is being used as oxidiser instead of air the mixtures could give rise to detonations. Therefore, all experiments are carried out in a closed vessel apparatus with a design pressure of 3500 bar. The experiments will be compared with measurements in a 4.2 litre spherical vessel and with measurements using air as oxidiser.

Related projects: Bilateral project with DSM Fibre Intermediates: Experimental and theoretical study of the explosion and detonation hazards during the production of caprolactam.

Scientific staff: J. Berghmans, F. Verplaetsen, L. Vandebroek, F. Van den Schoor

2.11.4Temperature dependence of the lower explosion limit of n-alcohols in air

In this study the influence of temperature on the lower flammability limit of evaporated liquids in air is investigated. At this moment, experiments with mixtures of n-alcohols and air are executed in a glass tube according to the prEN1839 standard at the PTB in Braunschweig (Germany). Later, experiments will be performed in a glass tube according to the DIN 51649 standard and in a closed vessel according to the ASTM E918-3 standard at the labo­ratories of TME. The results of these experiments will be used to develop theoretical models for predicting the lower flammability limit as a function of temperature, geometry of the test vessel and test criterion being used. Other explosion characteristics such as the auto-ignition temperature and the lower explosion point will be addressed in a later phase.

Related projects: Bilateral project with Physikalisch-Technische Bundesanstalt (PTB): Temperature and pressure dependences of explosion characteristics.

Scientific staff: J. Berghmans, F. Verplaetsen

2.11.5Determination of the explosion characteristics of dusts and gases

The division routinely uses its dust- and gas-explosion test equipment to determine the explosion characteristics of industrial powders and gases.

The explosion potential of a dust is characterised by the maximum explosion pressure and the maximum rate of pressure rise. These values are measured in a 20-l explosion vessel. The minimum ignition energy of a dust is meas­ured in a MIKE 3 apparatus. Other characteristics which can be measured in accordance with international standards are the BAM ignition temperature of a dust cloud on a hot surface, the smouldering temperature of a dust layer and the limiting oxygen concentration.

For gases test equipment is available to determine the flammability limits at ambient and elevated temperatures and pressures.

In 2003 several tests have been performed with the equipment mentioned.

Scientific staff: J. Berghmans, F. Verplaetsen, F. Van den Schoor