Contribution de l’analyse d’images à la modélisation de la résistance d’un lit de plaquettes de bois au passage d’un flux d’air

Contribution of image analysis to the modelling of the resistance of a wood chips bed when an air flow goes through

Context

The drying of biomass makes its combustion easier and improves the yield of the boiler while decreasing gazeous emissions. Furthermore, in order to optimize the yield of a combustion system, the fuel moisture content must be as constant as possible. Fuels with a variable moisture content require more complex combustion technologies and a checking system of combustion and emissions, which considerably increases investments. Considering the poor selling price of solid biofuels (mainly dictated by the price of fossil fuels), it is necessary to use simple drying techniques having a low investment and utilization cost. In order to decrease utilization costs, it is planned to take profit of the heat available in the ambiant air and to ventilate the matter to be dried.
The counter-pressure exerted by a layer of matter is linked to its granulometry. The granulometry is usually determined by means of a sieve, technique which only classes according to one of the sizes of the particles (the width). The recent evolutions of the image analysis make it possible to apply this technique to the granulometric classification of biofuels and to carry out quickly their analysis.

Objectives

• Characterization of the influence of the size and distribution of wood chips on the resistance when an air flow goes through.
• Setting up of image analysis and sampling adapted to biofuels.
• Granulometric characterization of different biofuels by image analysis.
• Comparison of the granulometric classifications coming from the image analysis, the horizontal sieving and the rotating sieve.
• Utilization of the granulometric classification by image analysis to assess the resistance of a wood chips bed when an air flow goes through.
• Determination of the evolution of the resistance of a wood chips bed when an air flow goes through during the drying.

Description of tasks

• Setting up of a standard procedure of granulometric classifcation by image analysis ;
• Characterization by image analysis of a ground material coming from a disc chipper according to the type of raw material ;
• Comparison of the granulometric classification by image analysis and sieving, according to the caracterictics of the material ;
• Modelling of the resistance of a bed of material when an airflow goes through, according to the caracteristics of the particles ;
• Modelling of the evolution when drying, of the resistance of a wood pellets bed when an airflow goes through.

Results obtained

At the present state of our researches, the influence of the size and distribution of wood pellets on the resistance when an airflow goes through was confirmed.
Image analyses parameters, specific to the material, showed strong links with the resistance when an airflow goes through.