Infrared thermographic image processing for the operation and control of heterogeneous combustion chambers

Abstract

The measurement of temperature in a combustion chamber, using conventional devices such as thermocouples, can be misleading. An infrared thermographic camera can achieve a non-intrusive measure. Having acquired a still image of, for example, a klin, it can be used to produce a map of the effective temperature of the waste bed, walls, and combustion gases. This paper describes a technique for processing an infrared image to identify what really happens within such a combustion chamber. The use of a thermographic camera, to produce a temperature map of a combustion chamber, can be helpful, particularly for control purposes. This paper addresses the criteria adopted in the selection of the thermographic system, in terms of both wavelength sensitivity and geometric location within the chamber. Moreover, a detailed description of the zonal method is reported, together with the identification procedure adopted to infer the temperature map from an infrared image. As a matter of fact, the presence of soot and fly-ash within the combustion chamber does complicate the radiative model, because a gray gas analogy must be accounted for. Soot and fly-ash give rise to a foggy and diffusive effect on the image with a consequent apparent homogeneous temperature profile. To simulate the effective radiative energy flux entering the camera lens and impinging on the CCD photoelectric cell, a raytracing technique has been developed. Each discrete area and volume, within the combustion chamber, emits a pencil of radiation, which after passing through the hot gases, reaches the CCD device. Such a light pencil is generated by the energy emitted and reflected from the discrete surfaces, plus all the energies emitted by the volumes of gas distributed along the path, minus any attenuation. The total energy balance equations, coming from the zonal method, must be coupled with the temperature-energy maps acquired by the infrared camera to identify the unknown effective temperatures. Once the temperatures of the walls and bed are known, it is possible to use them to improve the control strategy by means of a set of new measures and combustion efficiency indexes, which are usually unavailable when conventional thermocouples are adopted. Finally, a validation of the proposed procedure is presented with an online application to an incinerator for industrial solid waste. The combustion dynamics within the primary kiln is analyzed and quantified, in terms of both absolute temperatures and characteristic times.