Increasing the efficiency of high temperature furnaces through a topping cycle cogeneration-a case study

Based on the energy requirements of a set of continuous metallurgic heat treatment furnaces, this paper describes and compares several energy efficiency and cogeneration scenarios in a Portuguese metallurgic plant in terms of operating costs vs. savings. Such scenarios are analyzed in terms of hourly and annual savings. As part of improved heat integration, an energy cascade was designed and several hypotheses were studied to better match the decreasing thermal requirements of various heat treatment furnaces, each with different set point temperature and operating times. The scenario which presents the highest primary energy and economic saving adopts a special kind of gas turbine, one that due to its own characteristics requires a larger amount of excess air, hence being capable of providing enough O-2 for a post-combustion in the existing burners. This setting is essential so that the furnaces are able to reach the required set point temperatures. This solution, along with additional simple energy saving actions, accounted for primary energy savings of 7.9 %, a decrease in 5 % in carbon dioxide emissions and a reduction of the energy costs over 40 %.