| OCR Text |
Show 4 The selection of a coal with a moderate to high volatiles content will significantly improve the ignition properties of the slurry fuel. CVS formulated from coals of high quality ash, combined with extensive benefi-ciation, can greatly ease problems associated with ash management and boiler derating. Ultra-fine grinding of the coal may hold further benefits in terms of improved coal-cleaning, promotion of carbon burn-out, and the formation of more friable deposits produced more predominantly by particle diffusion rather than by inertial impaction mechanisms. 2.0 Coal-Vater Mixture Combustion Studies at MIT 2.1 The Research Objectives A comprehensive research program is currently underway at MIT under the sponsorship of a number of utility and fuel companies, and of DOE, aimed at identifying and examining the technical problems underlying the combustion of coal-water slurries, as related to their use in boilers designed originally for oil. The specific objectives of the study are to characterize a number of CVS flames obtained under a variety of input conditions characteristic of utility practice with respect to: o ignition and flame stability, o carbon conversion efficiency, o radiative heat transfer from the flame, o gaseous pollutant emissions (NO ), and o the physical-chemical nature of the fly ash and of ash deposits formed in the combustion chamber. The information obtained from these studies is intended to lead to recommendations for combustion process improvements and modifications, and for CVS fuel property specifications, to ameliorate some of the combustion-related problems associated with the utilization of CVS. In addition, the research is being carried out with a long range goal of obtaining a better fundamental understanding of coal-water slurry combustion processes, and of developing models for their prediction and interpretation. 2.2 The Experimental Apparatus and Measurements The pilot plant scale 1.2 m x 1.2 m x 10 m MIT Combustion Research Facility (CRF) is being used for the experimental study. The combustion tunnel (Figure 4) is comprised of a number of 0.3 m wide calorimetrically water-cooled sections which are either refractory-lined or bare-metal on the fireside. The sections are interchangeable, and can be arranged to vary the heat extraction along the length of the flame. The CRF is equipped with a highly versatile burner, which permits a wide variety of flame aerodynamic types to be obtained. These include fast and slow mixing flames and those characterized by low or high degree of back-mixing of hot combustion products with the fresh fuel. Major features of the burner include an infinitely variable swirl generator (of IFRF 'moveable block' design), an interchangeable gas/liquid/slurry fuel gun which can be moved laterally along the axis of the jet to vary the position of fuel injection, and interchangeable convergent-divergent combustion air exit nozzles which allow variation of air exit velocities and the angle of divergence. |
