High intensity combustion of coal for application to a blast furnace

Abstract

A 1.2 MW tunnel furnace has been used to evaluate combustibility of pulverized coals under the same firing conditions as that of a blast furnace. The coals used contain a proximate volatile matter of 20 to 40%. The combustion air velocity is 250 m/sec at 1200°C and the excess air ratio ranged from 1.6 to 2.5. The heating rates attained were approximately 1.0–1.5×10 6 K/sec. The maximum flame temperature was measured at 2020°C by a two-color pyrometer. Ash or titanium was used as a tracer and gas analysis was used to evaluate the combustibility taking account of distributions of velocity and CO 2 concentration in the furnace. The fractions of coal burned on the centerline of the furnace in about 6 msec ranged from 69 to 83% (d.a.f.). The combustion efficiencies defined by CO 2 production under usual blast furnace conditions were 62 to 93% at the position where the combustion of oil was completed. Combustibility in the initial region which corresponds to a raceway in the blast furnace, was strongly affected by the devolatilization process. The residence time in the raceway is insufficient to completely oxidize the residual char. Combustibility improved with increased volatile matter content in the raw coal, residence time, excess air ratio and combustion air temperature. The injection position had a large effect on the combustibility for the 30–35% volatile coal. Ignition and devolatilization were more rapid with an increase in the combustion air temperature. It was found that the firing conditions with rapid heating, high temperature and strong turbulent flow significantly enhanced the combustibility of pulverized coal compared with the previous result in a jet-stirred reactor in which combustion efficiencies of 60 to 70% were obtained in 10 to 20 msec.