Boiler Modelling of Simple Combustion Processes

Boiler Modelling of Simple Combustion Processes

Radovan Nosek (University of Zilina, Slovakia), Jozef Jandacka (University of Zilina, Slovakia) and Andrzej Szlek (Silesian University of Technology, Poland)
Copyright: © 2012 |Pages: 24
DOI: 10.4018/ijeoe.2012070105
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Abstract

The aim of the work is to investigate coal combustion in fixed bed reactor. The experimental results were worked out in the form of approximation functions describing gas composition at the exit of fixed bed reactor. Furthermore, developed functions were applied for defining the boundary conditions at the interface between the fixed bed and gas phase using FLUENT. The simulations of a domestic boiler have been done and the relative effects of different factors in CFD code were evaluated by sensitivity analysis. The validity of the model was verified by measurements which were done in a 25 kW domestic boiler. Model predictions were compared with the experimental gas temperature and species concentration measurements.
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1. Modelling Approaches Of Fixed Bed Combustion

Travelling grate boilers have been used and investigated for many years, but there are still some problems which should be eliminated. In these boilers a lot of improvements are needed in order to decrease pollution and improve emission controls, including the effectiveness of fuel bed combustion and mixing. Several researchers have successfully designed and optimized grate furnaces using CFD code as an efficient tool. A few publications concerning modelling of fixed bed combustion have been published and in the following section is extensive review of developed and applied fixed bed combustion models.

Lin, Ji, Luo, and Wang (2009) presented a mathematical model of combustion process in a coal-fired travelling grate boiler. The model consists of a ’bed-model’ part and a ‘furnace-model’ part. Within the description of the ‘bed-model’, a non-isothermal particle model is proposed in order to accommodate temperature gradients within combusting particles. The model is applied using UDF (user define functions) on a FLUENT platform. Validity of the model has been verified by experiments in a fixed-bed laboratory scale furnace. Measured concentrations of flue gas released from the bed have been measured to provide verification data.

The experimental fixed bed furnace was set up in order to simulate combustion in a real chain grate stoker. Concentrations of gas species at the bed surface were measured. The gas velocities, temperatures and species concentrations released from the bed are set as the inlet boundary conditions in CFD of the furnace.

Calculated results were compared with those from a fixed-bed laboratory scale furnace for verification of the model. Numerical predictions of gas species concentrations (O2, CO, CO2 and H2) at the bed surface are in good agreement with measured results. Results from the non-isothermal particle model matches the experimental data better than calculated data from the model using an isothermal particle model (Lin, Ji, Luo, & Wang, 2009).

Ji, Luo, and Hu (2008) introduced their grate boiler model which simulates the unsteady combustion mechanism in an industrial travelling grate boiler. In their study, a real travelling grate boiler in which volatile-related unsteady combustion occurs is chosen as the prototype for the experimental combustor. They used natural gas for simulation of volatile from coal.

First is the mixed-type air distribution, where the air and the natural gas are supplied to the chamber from the same rooms, which makes the air and the natural gas mix well. The second is the separated-type air distribution; the air and the natural gas enter the combustion chamber from different rooms, which results in their poor initial mixing.

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