An energy saving opportunity exists in mining and smelting industries such as the Ferrochrome industry, in the form of waste heat extraction and recovery. The mining and smelting of metals requires the use of high-power, AC or DC electric-arc furnaces that melt raw feed material in order to separate and produce useful metals from the feed. The smelting of raw materials into metals inside arc furnaces produces large quantities of hot gas as by-products, according to specific process-controlled chemical reactions. These hot gases combined with small feed dust particles thrown up from the feed process must be extracted from the smelting process and therefore from the furnaces themselves. The furnaces operate at internal temperatures around 1 500 oC and therefore the extracted hot material, referred to as the smelting process off-gas, is extracted at extremely high temperatures and thermal energy levels.
The implementation of a bottoming cycle cogeneration system allows for the extraction and recovery of thermal energy from the furnace off-gases which can then be used to generate additional useful electrical energy via the heat exchanger and electrical energy generation components of the proposed cogeneration system. This additional energy improves the energy savings of the smelting process and the entire plant since thermal energy that was previously dissipated into the atmosphere and wasted is now used to generate additional and useful electricity. This additional electricity can either be used to power the furnace loads associated with the smelting of the raw materials into metals, thereby supplementing the utility grid electrical supply to the plant, or it can be fed back to or wheeled through the utility grid to be supplied to third party customers and consumers. This however creates a power dispatch and optimal power flow problem.
The power dispatch and optimal power flow problem is solved and controlled through the implementation of an Economic Power Dispatch (EPD) model. The model considers the operating conditions of the furnaces in order to determine how much additional electrical energy can be generated from the available thermal energy, recovered from the smelting process via the proposed cogeneration system. The model then utilises information regarding all costs associated with the consumption and generation of electrical energy as a result of the smelting and heat recovery cogeneration system operation. With these considerations the model then determines the optimal manner in which to dispatch the cogeneration generated electrical energy between the furnace loads and the utility grid so as to achieve the maximum possible overall system associated energy savings. The model is implemented as a half-hourly cogeneration-generated optimal power dispatch schedule.
The consideration of a bottoming cycle cogeneration system for heat recovery and additional electrical energy generation, and the development and implementation of an Economic Power Dispatch (EPD) model for the control of the dispatch of cogeneration generated power between the furnace loads and the utility grid, allow for considerable system associated energy savings. These savings are obtained due to the reduced net electrical energy required from the utility grid, and the financial incentives and rebates obtained due to the feed of electrical energy back to and through the utility grid to third party consumers.
Dissertation (MEng)--University of Pretoria, 2015.