Transient combustion of municipal solid waste in a grate furnace : modelling and experiments. Dipl.-Ing. Y. Menard1, Dr.-Ing. F. Patisson1, Dr.-Ing. Ph.
Transient combustion of municipal solid waste in a grate furnace : modelling and experiments Dipl.-Ing. Y. Menard1, Dr.-Ing. F. Patisson1, Dr.-Ing. Ph. Sessiecq1, Prof. Dr.-Ing. D. Ablitzer1, Dr.-Ing. A. Merz2, Prof. Dr.-Ing. H. Seifert2 1 : LSG2M, Ecole des Mines de Nancy, France, 2 : ITC-TAB, Forschungszentrum Karlsruhe, Germany. To simulate the behaviour of a burning municipal solid waste (MSW) bed in a batch pilot reactor, a two-dimensional, axisymmetrical, transient mathematical model has been developed. It describes most of the physico-chemical and thermal phenomena occurring during waste combustion: gas flow, heat and mass transfer, drying, pyrolysis, combustion of pyrolysis gases, combustion and gasification of char, and bed shrinkage. To validate the model and increase the understanding of municipal waste combustion, batch pilot experiments were carried out in a cylindrical fixed bed reactor. The temperature was measured at different locations inside the burning bed and outlet gas analyses were performed for O2, CO, CO2, H2O and CxHy. The numerical simulations are compared with the experiments.
1. INTRODUCTION This study is part of a larger research program aiming at both a complete simulation of the MSW incineration process and the understanding of the behaviour of heavy metals during incineration. Work reported here focuses on the combustion phenomena at the scale of a waste bed. We investigated these phenomena using both modelling and experiments. A new computational model, able to describe the complex reactions involved, as well as heat and mass transfer inside the burning bed, is presented first. The waste bed, deposited on a grate, is crossed by an upstream air flow and submitted to radiative heating on its upper surface. A flame front formed at the top surface slowly progresses downwards, against gas flow. Wood chips as well as MSW samples were used as solid fuels. Experiment and model results are compared and discussed. Finally, the current continuation of this work is evoked.
2. MATHEMATICAL MODEL The mathematical model we developed, is based on the following assumptions: - the fixed bed is viewed as a porous medium consisting of spherical particles and crossflowed by the gas; - two temperatures are considered: one for the solid, one for the gas; - the waste materials consists of moisture, organic material and inert material; - the pyrolysis of organic material leaves residual carbon and produces several gases such as CO2, H2O, H2, CO, CH4. The last three gases can react with O2 inside the bed, leading to the formation of a burning front; - the residual carbon left after pyrolysis can either be oxidized by O2 or gazified by CO2 or H2O; - conduction, convection and radiation heat transfer modes are considered inside the fixed bed.
This two-dimensional, axisymmetrical, transient combustion model takes into account the typical sequence of all combustion processes of solid fuel (see Fig. 1): heating-up, evaporation of moisture, devolatilization, char oxidation and gasification, combustible gases oxidation and cooling of ash. As the combustion in the solid fuel proceeds, the spherical particles burn and the bed shrinks. The apparent density of the bed also changes. Heterogeneous reactions enthalpies are included in the solid heat balance equation, and homogeneous ones are taken into account in the gas heat balance equation. Organic material pyrolysis is described either by simple Arrhenius law for wood chips [1, 2] or by a two-equation devolatilisation model similar to [3]. Data for the latter were derived from thermogravimetric experiments we performed with MSW dried samples of MSW. Radiation heat transfer inside the burning bed is represented through an effective conductivity of the solid [4]. char oxidation CO CO2 O2
Porous Medium
char gazification
CO Char
H2 CO2, H2O
pyrolysis CO, CO2, CH4, H 2, H 2O
Ts
Tg
Ts
H2O
O2, N2
drying
Air Flow
Fig. 1: The combustion processes modelled
3. EXPERIMENTAL SET-UP The experimental set-up is represented in Fig. 2. The combustion chamber consists of the fixed bed reactor (25-cm in diameter, 30-cm high) and a preheated upper chamber. Primary air can be either preheated or not. It is injected under the grate of the reactor. Combustion gases then flow through the post-combustion chamber, the heat exchanger and the cleaning gas system. The temperature evolution inside the burning bed can be followed thanks to 13 thermocouples, and combustion gases such as CO, CO2 or O2 are in-line analysed before and after flowing through the post-combustion chamber. Combustion chamber
Solid fuel
Heat exchanger
Secondary air O2 CO CO2 H 2O Org. C O2 CO CO2 H 2O Org. C
Weighing system Primary air
Post-combustion chamber
Ash filter
Activated carbon adsorber
High frequency pressure regulator
Fig. 2: Experimental set-up used to study the transient combustion of a solid fuel (KLEAA facility, ITC-TAB Forschungszentrum Karlsruhe)
4. EXPERIMENTAL AND COMPUTATIONAL RESULTS COMPARISON An example of a typical evolution of temperature inside the burning bed and of the gaseous species evolution above the bed are shown in Fig. 3. When the solid fuel is introduced into the preheated furnace, whose temperature is about 850°C, the upper layers of the bed are exposed to radiation from the hot walls. As a first step, the top of the bed is vigorously heated and moisture is evaporated. After a time delay of about 250 s, the devolatilisation of the particles enriches the gas flow in combustible products such as CH4, CO and H2, which can react with available oxygen. As well, fixed carbon, left after pyrolysis, can be oxidized by O2, or gasified by CO2 or H2O. The local temperature in the upper layers of the bed rises quickly to around 1000°C, whereas the lower layers remain cool. This is because the solid bed has a low thermal conductivity, and the air supplied, continuously cools the bottom of the bed. The flame front thus propagates downwards, as it can be observed with the rise in temperature measured by the 13 thermocouples. The flame front propagation speed is relatively constant during the entire experiment. CxHy T1
H 2O
O2
CO2
CO
T13
(a)
(b) H2O T1 O2 CO CO2
T13 H2
(c)
CH4
(d)
Fig. 3: Comparison of experiments (temperature history recorded by each thermocouple (a), gas concentrations after the primary combustion chamber (b)) with model results (temperature evolution (c), gas concentration above the burning bed (d)). Solid fuel: wood chips (LHV : -1 3 -1 9200 kJ kg ), primary air flow rate: 15 Nm h , Tprimary air = 30°C, solid fuel moisture: 24 wt%.
Due to model results, the final rise in temperature can be attributed to late oxidation of the residual carbon formed during pyrolysis. The oxygen, which formerly was consumed by homogeneous reactions inside the burning bed, becomes available again to burn the residual carbon, after the end of organic material devolatilisation. This leads to the rise in temperature during the last minutes of the experiments (see Fig. 3 (a) and (c)) and to an increase of CO2 and CO in the fumes (see Fig. 3 (b) and (d)). The computed maximum temperature, rise in temperature, and flame front propagation speed agree satisfactorily with the experimental results. O2 concentration evolution above the fixed bed is also well depicted by the model.
The evolution of the other gaseous products, which strongly depends on the composition of the devolatilized mixture, is much more difficult to reproduce. We presently are working to better determine from specific experiments this composition of the pyrolysis gases.
5. CONCLUSION We investigated the transient combustion of wood particles and MSW charges in a fixed bed reactor to better understand the combustion in an industrial incinerator. A 2D, transient combustion model has been developed and was employed to simulate the complex phenomena of coupled heat and mass transfer in the burning bed. This model takes into account all of the transformations of a solid fuel during combustion (drying, pyrolysis, char combustion). It employs kinetic laws for the pyrolysis step, homogeneous and heterogeneous reactions. Bed shrinkage during combustion is also taken into account. Combustion experiments with wooden particles as well as MSW were carried out in a fixed bed reactor. Effects of different primary air staging, as well as primary air temperature or moisture content of the solid fuel, were also studied although not reported here. Experimental and computational results have been compared. Good agreement can be observed for the flame propagation speed inside the solid fuel, as well as for the maximum Fig. 4: Temperature field in an temperature reached. Some efforts still have to be industrial incinerator, K done to better reproduce the evolution of gas concentrations above the burning bed. This work is part of a larger research program aiming at a complete simulation of the MSW incineration process. For this purpose, we have developed a different version of our model that simulates the combustion of a MSW travelling bed on the grate of an incinerator. The results of this model are used as inlet boundary conditions to perform 3D simulations of the post-combustion zone and of the heater (see Fig. 4). These calculations are performed with the CFD code FLUENT 5. ACKNOWLEDGEMENTS
The authors thank ADEME and CNRS for the financial support of this study. They are grateful to Ing. R. Walter for his kind assistance during the pilot experiments at KLEAA. REFERENCES
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