Thermal-destruction products of coal in the blast-furnace gas-purification system

Abstract

Given the shortage and high price of coking coal, efforts are underway to reduce their consumption in their primary uses, including blast-furnace production and the smelting of copper and nickel ore in shaft furnaces. A promising approach is the replacement of some or all of the coke by less expensive coal [1] or by dust‐coal mixture injected in the blast furnace [2]. The lean, poorly clinkering coal and anthracite used to replace coke in blast furnaces has a considerable content of volatile components (low-molecular thermaldestruction products), which enter the water and sludge of the blast-furnace gas-purification system as petroleum products. Therefore, it is important to study the influence of coal on the petroleum-product content in the water and sludge within this system. The liberation of primary thermal-destruction products is investigated for anthracite with around 4 wt % volatiles, using a STA 449C Jupiter thermoanalyzer equipped with a QMC 230 mass spectrometer. The thermoanalyzer determines small changes in mass and thermal effects with high accuracy (weighing accuracy 10 ‐8 g; error in measuring thermal effects 1 mV). This permits experiments with single layers of coal particles, eliminating secondary reactions of its thermal-destruction products. Samples are prepared from natural anthracite in powder form. Particles of size class 0.5‐2 mm (first series of experiments) or <0.5 mm (second series) form a single layer; in the third series, (0.5‐2)-mm particles form several layers. The experiments are conducted in a current of argon. The results are shown in Figs. 1 and 2 in the form of curves of the mass (TG) and the rate of mass change (DTG) and differential scanning-calorimetry (DSC) plots with standard leucosapphire samples, along with mass-spectrometric data. According to experimental data, the evaporation of free water ends at 200‐220 ° C, and the formation of low-molecular thermal-destruction products of coal begins at 250 ° C. The liberation rate of volatiles from a single layer of (0.5‐2)-mm particles, like the evaporation rate of water (Fig. 1), is considerably higher than for the other series of experiments. In the particle layer, more favorable conditions for the reaction of intermediate products released from a single particle with other particles or thermal-destruction products are created. When the coal is ground to powder, the structure of the macromolecules is destroyed, and the resulting fragments of different molecular mass and chemical activity react at once with each other and with the partially disintegrated coal grain. This necessarily affects the decomposition rate. These aspects of the thermal destruction of anthracite particles must be taken into account in selecting the fraction of coal-dust fuel for injection in steel-smelting furnaces. According to mass-spectrometric data (Fig. 2), the basic components of the gas phase in the thermal destruction of anthracite are CH 4 , CO 2 , H 2 , and H 2 O. No carbon monoxide is observed, in contrast to [3]. The content of C n H m and N 2 is no more than 1 vol %, according to [3]; this is probably why it is not detected mass-spectrometrically. The composition of the gas phases varies considerably as a function of the temperature. Intense methane liberation occurs in the range 440‐500 ° C, while the CO 2 content gradually declines. At 580 ° C, hydrogen begins to predominate in the gas. Above 580 ° C, the hydrogen content increases, and the methane content declines; this is associated with displacement of the equilibrium in the reaction CH 4 = C + 2H 2 toward the products.