OCR Text |
Show of improvements in plant availabili ty associated wi th changes in coal preparation or operating conditions. INTRODUCTION The deposition of coal ash on convective heat exchangers is necessarily a rather slow process. When it is not, deposit growth may be difficult to control by sootblowing with reasonable frequency. Because the process under normal conditions is so slow, the study of deposit growth in pilot scale equipment can be very time consuming. One way to speed up the process is to increase the gas and target temperatures . A method for analyzing such measurements to obtain a quantitative description of fouling applicable under other conditions is proposed in the present paper. The situation under consideration is an isolated tube subject to deposition of iron-containing ash from bituminous coal. The temperature is limited to values below that at which the surface of the deposit would fuse completely, so that the surface presented to the incoming flow has a time- and space-dependent composition which depends on the compositions of the particles most recently incorporated in the deposit. Formation of sticky surfaces via condensation of alkali on particles, deposits, or tubes is not considered here. The deposition of ash by inertial impaction, and the growth of deposit on a tube are illustrated in Figure 1. The particles impacting the tube are in various states of fusion, depending on their composition, size, and temperature history. At the beginning of the process, when the tube is perfectly clean, only those particles which arrive at the surface in a sticky condition contribute to deposit formation. If the surface temperature is low enough, the particles which stick solidify on cooling. 2 |