F. Marias

Modelling of thermal removal of tars in a high temperature stage fed by a plasma torch.

The thermal degradation of tars in a chamber fed by a non-transferred plasma torch is theoretically examined in this study. The input of this reactor is a product gas coming from a gasification unit with a temperature of about 800 °C. According to literature, naphthalene and toluene are chosen as model compounds to represent the behaviour of their classes. According to this choice and to the data available in the literature, a reaction pathway for the thermal degradation of tars and its associated kinetics are proposed in this study. This mechanism is introduced in a CSTR model in order to check the influence of the operating parameters of the reactor on the degradation efficiency. These computations clearly show that a complete conversion of toluene (>99.9%) and an important conversion of naphthalene (96.7%) can be reached in the reactor, with concentration levels compatible with the further use of gas engines for electricity production. This theoretical study requires to be validated by comparison with experimental results.

A model of a rotary kiln incinerator including processes occurring within the solid and the gaseous phases

Abstract This paper presents a new development in the study of a rotary kiln incinerator. The modelling of the furnace has been divided into two parts. On the one hand, a model describing the physico-chemical processes which occur within the burning bed of municipal solid waste (assumed to be a mixture of wood, cardboard and PVC) has been set. This model mainly relies on the assumptions of plug flow and macroscopic pyrolysis kinetics of burning waste. On the other hand, C.F.D. has been used to describe the processes occurring within the gaseous phase of the kiln and of the post combustion chamber (turbulence, combustion, radiation). A data processing tool has been built to automate the data exchanges between the two parts of the model. Some results of the overall model are shown in two different situation (working with and without extra burner).

Effects of Freeboard Volatile Release During Fluidized Bed Incineration of a Model Waste

This paper describes a model for the fluidized bed combustion of volatile matter from waste. This waste is assumed to be composed of wood, cardboard and PVC. The model assumes that a fraction of the volatile content is released within the freeboard, as a result of both pyrolysis and transport of the devolatilizing particles. The model describes precisely the diffusion-controlled combustion of the volatile matter within the bed, on the basis of a hydrodynamic model of the bed, combined with energy and mass balance equations. The results of the model show that the release of volatiles in the freeboard may increase the freeboard temperature by 200 K and may multiply the emissions of NO 25 times.

Quality of CFD models for jet flow analysis for the design of burners and boilers

CFD models are increasingly used for the design and optimisation of boiler combustion chambers. Numerous commercial codes are available, and the user is confronted with making a proper choice for a particular application. In this paper, the accuracy and effectiveness of the popular code FLUENT™ is investigated in terms of the different turbulence models and numerical schemes that are bundled in the software. The tests are performed for different simple experiments, involving classical hydrodynamic conditions with no combustion. The conclusion of these tests involves also the additional criterion of the computational time required for achieving a reasonable accuracy.

Simulation numérique d'un brûleur industriel. Analyse qualitative des effets de swirl sur l'écoulement et sur la production de polluants

Numerical simulation of industrial burner. Qualitative analysis of swirl on flowfield and pollutant production. This work takes place in an engineering global study of incineration processes (rotary kiln, bubbling fluidised bed); evaluation of pollutants emission at the exit of the primary processing fire chamber is one of its objective. The example developed here focuses on the impact of the swirl number of an industrial burner on pollutants emission at the supply of the post combustion chamber. The burner works with a rotary kiln fed with liquid waste. Fluent® UNS is used to achieve simulations, both for inert and reactive flows, taking into account turbulence and combustion interaction with probability density function model and 11 chemical species. Thermal NO production is evaluated using Zeldovich mechanism.

Mathematical Modelling of Slow Pyrolysis of a Particle of Impregnated Wood

Low-temperature pyrolysis is a possible route for the disposal of chromated copper arsenic (CCA) impregnated wood waste. A mathematical model (heat and mass transfer) including chemical reactions of the thermal degradation of a particle of wood is presented. A spherical particle is heated by a convective nitrogen flow. The progress of the pyrolysis process is characterized by three main steps: 1) as initially it contains water, the moisture content goes down and water vapour is evacuated through the outlet surface; 2) the dried wood temperature increases, reaches a certain temperature and pyrolysis reactions start; 3) up to 270°C, pyrolysis starts and wood produces a solid called char and several volatiles. During each step, time and space evolution are observed. Our model predicts intra-particle profiles for several variables (temperature, moisture content, concentration of wood). Simulations are presented with a spherical particle of 1cm radius. The results shown are in accordance with what was expected. It clearly shows three main steps: heating + drying; only heating; and heating + thermal degradation.

Dynamic Simulation of the Energy Recovery in an Incineration Plant

The context of this paper is energy recovery in an incineration plant. A dynamic model for the description of a drum unit is presented. In the first part, the physical system in question is presented. This description is performed by following the cycle of water within the system. The different devices of the network are presented, as well as the control valves and their roles. A complete list of the state variables used to represent the process is given. Secondly, we focus on the mathematical description of this system. Indeed, the set of equations representative of the process is rendered. This is a differential algebraic system composed of 6 ordinary differential equations and 113 algebraic ones. A predictor-corrector method (Gear’s algorithm) is used to solve the system. The results of the model are shown in case of the turning off of the plant, when the turbine needs to be bypassed. Finally, an explanation of the observed yields of the model is given.