Optimisation et atténuation de la pollution atmosphérique et des nuisances sonores dans les centrales thermiques du Cameroun par la méthode des algorithmes génétiques

Abstract

Air and noise pollution due to the infrastructure of industrial sites is one of the main environmental concerns at the beginning of the 21st century. One way to assess, control and reduce this air pollution simultaneously with noise pollution is the bidirectional search for two optimums. The concentrations of pollutants in the atmosphere and the equivalent sound pressure levels in the receiving environment. The aim of this work is firstly to propose threshold levels of pollutants discharged around industrial sites, in order to establish a map that can provide a view of the dispersion and level of pollutant concentrations. In a second step, after having mapped the sound pressure level around the sites, to propose innovative acoustic protections using natural materials and to study their acoustic performance using numerical and experimental approaches such as genetic algorithms. The numerical approach can be further coupled with an optimization tool, developed in this thesis, to search for improved forms of such innovative noise protection devices. After a presentation of the main phenomena involved in the combustion of heavy fuel oil, turbulent atmospheric dispersion and the propagation of acoustic waves in complex outdoor environments, a state of the art of pollutant sources, the receiving environment and the main acoustic screens dedicated to industrial sites such as thermal power plants was established, allowing the choice of the study framework, boundary conditions, innovative noise protection insulation to study the limit concentrations of pollutants and the optimization of acoustic performance. An analysis of the main methods of numerical simulation, measurement and optimization of noise protection has enabled us to choose the methods adapted to our problem using natural means. The chosen methods were used in this work to evaluate the concentration of pollutants in the atmosphere, the sound power and sound pressure levels and the acoustic performance of these innovative screens through their attenuation indices. For air traffic control the main pollutants with high percentage released into the atmosphere are in decreasing order; Nitrogen Oxides NOX, Sulphur Oxides SOX, Particulate Matter PM of 10μm diameter, Organic Compounds (CO), Particulate Matter PM of 2,5μm and 1μm diameter. In addition, following the wind direction rosettes, dispersion orientations were obtained respectively for the MTC, CTD and CTO in the major directions W, SW and W and in the minor directions WSW, SSW and SSW. Stack exit concentrations of 0.012kg.m-3, 0.025 kg.m-3 and 0.016 kg.m-3, respectively, were coupled at selected in situ velocities in order to be able to represent a plume dispersion in a well-chosen tunnel, thus giving the local representation of the pollutant dispersion. As for the level of power and sound pressure, we studied these sound emissions at a receiving point located at a distance (S-R) of 117.6m for the CTO, 117.6m for the CTD and 105m for the CTM. We were able to evaluate these levels in a range of 90-150 dB for sound power and 45-110 dB for sound pressure. The sound pressure levels at the receiver are very high compared to the level recommended by the WCO and WHO. The direction of propagation of the iso contours was able to give a general orientation of the direction of noise nuisance around industrial sites respectively of the major NE-SW direction of the MTC, major NE-SW and minor NNE-SSW direction of the CT-D and major SSE-NNW direction of the CTO. Within the framework of the innovative acoustic protection related to the eight insulators tested, the following results were obtained a genetic algorithm well detailed for each site studied, a test was carried out at different sound frequencies (50-4000 Hz), for the CTD, the optimal individual or quality attenuation index is 38.1308 dB at a sound emission frequency of equipment and auxiliaries of 2256.54 Hz, corresponding to wet cellulose wadding for double walls. For the ceiling it is aluminum with an index of 15,305 dB, for the walls constituting the retention tanks the rock wool has been chosen with an index of attenuation of about 49,1877 dB and for the base as insulating materials the cellular concrete and bitumen respectively 76,2352 dB and 93,1352 dB of sound insulation. For the CTO, the optimal individual at a frequency of 1124.548 Hz, the dry cellulose wadding with 18.7569 dB, that of the aluminum ceiling with an index of 31.5917 dB, for the walls constituting the retention tanks, rock wool was chosen with an order of 15.1231 dB and the base as insulating materials cellular concrete and bitumen respectively of 57.0692 dB and 74.7235 dB. The results of the calculations have shown the interest of such devices for evaluation, control and search for optimums in order to mitigate air and noise pollution using natural means. We recommend the establishment of an Environmental Management Plan (EMP) that will take into account the limit values and optimums that we have established in order to prognosticate in the short, medium and long term a diagnosis of the personnel and equipment on site in the first instance, and of the local residents, fauna and flora for their well-being in the second instance.