OCR Text |
Show 3 Coal-fired boilers are generally designed with a larger combustion space (see Figure 3) than oil-fired units. The increased combustion space permits the burn-out of the coal, which characteristically requires a longer residence time than oil for complete combustion. A larger radiant heat exchange surface reduces peak and exit gas temperatures, and thereby minimizes slagging, or the formation of hard bonded deposits, both in the combustion chamber and further downstream in the convective section of the boiler. As a consequence of these basic design differences for coal and oil combustion, conversion from oil to CVS firing in existing installations can be expected to require some degree of derating. The significantly lower quantities of ash associated with oil combustion, over that of coal, permit the design of convective sections with a tighter tube spacing, so as to increase the velocity of the combustion gases flowing past the tubes, and to increase the efficiency of heat transfer (see Table 1). Conversion to CVS may require removal of tubes from the convective section to prevent excessive fouling and/or erosion, and also, to compensate for the loss, may require the installation of additional heat transfer surface in the form of an economizer or air preheater. Other combustion engineering related problems entailed in the retrofit of oil-fired boilers for CVS include the necessity of installing soot blowers, provision for removal of ash from the bottom of the combustion chamber, and a more extensive stack gas clean-up system to handle higher particulate loadings. All of these technical problems - ignition and flame stability, carbon conversion efficiency, ash deposition, stack gas particulates clean up, and derating - can, to varying degrees, be ameliorated through an understanding of coal-water slurry combustion processes and through careful control of flame input variables and CVS fuel properties. The fineness or quality of atomization, which plays a major role in determining the effective surface area of the fuel available for heat transfer, can have a direct bearing on CVS ignition and flame stability. Coal particle burning times in the external diffusion-limited combustion regime are proportional to the square of the particle diameter. Atomization quality, therefore, can have a major impact on carbon conversion efficiency. The 'near-field' aerodynamics of the flame, as determined by the combustion air swirl degree, the air exit velocity and the angle of divergence of the burner throat, are of major importance in their effect on the degree of back-mixing of hot combustion products, or the convective energy feedback to the entering cold fuel. The intensity of fuel-air mixing as affected by these variables can have a strong affect on rates of fuel burnout. The flame temperature distribution as affected by intensity of fuel-air mixing, air preheat and heat extraction can be expected to have a strong bearing on the nature of the deposits formed on heat exchange surface, i.e., whether they will be bonded or friable. |