A modal analysis technique for the on-line particle size measurement of pneumatically conveyed pulverized coal

The knowledge of the particle size distribution of the pulverized coal used to fuel a fossil fuel power station is an important and useful parameter in the control and efficiency of power generation. However, to put this parameter to its most effective use, it is necessary to measure it on-line. An investigation conducted into techniques used for the on-line measurement of particle size distribution revealed a cost effective technique to achieve this goal. In short, it entails the measurement of the resonant vibrations or modes evoked in a structure when pneumatically conveyed particles collide with it. By analyzing the frequency spectra of these evoked vibrations or modes, variations in the size distribution of the particles colliding with the structure can be observed. The method is based on the principle that particles colliding with a structure cause that structure to start vibrating at its natural or resonant frequencies. Different sized particles cause different resonant frequencies of the structure to be exited or evoked. Thus, a variation in particle size distribution, will cause a variation in the amplitudes of the different resonant vibration peaks of the structure. By analyzing these peaks, variations in the particle size distribution can be monitored. Experimental testing and evaluation of the technique on a pilot plant proved it to be a very cost effective solution to the measurement problem. Extraction of the absolute particle size information from the complex vibration signals was accomplished using special parameter back-calculation techniques. A numerical/model-oriented approach using vector decomposition and a knowledge-based approach using neural networks allowed absolute particle size distribution to be determined.