More efficient compressed-air generation at steel plants

Abstract

At large steelworks, compressed air is produced by cogeneration systems, which generate electrical power, heat, and compressed air. The power and heat are produced in steam turbines. One- or two-stage compressors driven by steam turbines are generally used to compress the air. In the case of two-stage compressors, air cooling is only used ahead of the second stage, for the sake of energy efficiency. Heat exchangers and cooling units based on water are used for this purpose. The water temperature is reduced beyond the heat exchangers in ordinary cooling towers. Air at the ambient temperature is sent to the first stage of the compressor. When using this system, the temperature of the cooled air depends primarily on the ambient temperature. In many cases, that prevents cooling of the air to the required temperatures ahead of the compressor stages, especially in summer. In the present work, a new configuration is proposed for the cogeneration system at a metallurgical enterprise. Specifically, this involves the introduction of a sorptional thermal transformer operating as a refrigerator. The steam from the power turbine drives the sorptional thermal transformer, which lowers the air temperature at the input to the first and second stages of the compressor. The thermodynamic efficiency of the new system is assessed on the basis of its total fuel consumption, other conditions being equal. Formulas are proposed for the change in total fuel consumption in the generation of power, heat, and compressed air when using the sorptional thermal transformer. Estimates show that the sorptional thermal transformer reduces the total fuel consumption per hour by 0.15 t of conventional fuel when the air temperature at the input to the first and second stages of the compressor is reduced by 10°C.