Using an Expander-Generator Unit in the Steel Mill CHP Plant for Producing Electricity and Improving the Efficiency of the Compressor Plant

Abstract

The possibility of energy saving in one of the most energy-consuming compressed air production plants, in which a significant part of the consumed energy gets lost, is considered. The proposed technical solution is based on the joint use of two energy-saving technologies: the first one is the use of a technological pressure drop of the transported natural gas, which is irretrievably lost when it is throttled at gas-pressure regulatory stations, and the second one is the inlet compressor air cooling to reduce the compressor work. An installation circuit design is proposed for a metallurgical enterprise combined steam blower and electric station, in which a two-stage expander-generator unit (EGU) is used to generate electricity and cold in addition to a power turbine producing electricity and heat and a steam-turbine two-stage compressor producing compressed air. The thermodynamics of gas expansion processes is considered, and the choice of a two-stage scheme is justified. The cold produced by the EGU is used to lower the inlet air temperature at the first and second compressor sections, reducing fuel consumption for air compression. Application of the proposed scheme makes it possible to reduce fuel consumption for the compressor drive. It seems reasonable to use the compressed air heat for heating the transported gas at the expander inlet and generating additional electricity. At the same time, the fuel is not consumed to produce electricity, and the heat of the cooling air is not discharged into the environment, i.e., the installation operation is characterized by high environmental performance. The fuel economy calculation procedure for the proposed scheme is presented. The assessment showed that using this scheme allows for a reduction in fuel consumption at the CHP steam blower by 11.2 thousand TOE/year for a gas consumption of about 200 thousand nm3/h, gas pressure at the gas-pressure control station of 0.7 MPa and required air pressure of 0.5 MPa, which is 0.84%. The energy output of the EGU in case will be 5.3 MW.