Design of a primary off-gas scrubber for a ferro-manganese electric arc furnace process

Abstract

An electric arc furnace makes use of electrical energy in the form of an arc to heat charged material. In the ferromanganese smelting process the ferric oxide (Fe2O3) and manganese oxide (MnO2) are reduced with coke. The basic reaction that takes place is described in the following equation Fe203 + 2Mn02+ 7C = 2FeMn + 7C0 The high quantity of carbon monoxide (CO) produced in the process, which has a significantly high calorific value, can be used to generate energy to supplement certain areas of the process. Due to the moisture in the charged material, electrolysis takes place within the furnace, generating hydrogen (H2) and oxygen (O2). The high temperature allows a percentage of the carbon monoxide to combust instantly with the available oxygen, which forms carbon dioxide (CO2). The balance of gas in the process is primarily nitrogen (N2), which comes from the air drawn into the furnace, as it is impossible to seal the furnace off perfectly. The oxygen from the air also combusts with the carbon monoxide, however there is always a small percentage of oxygen that does not combust. The following table indicates the percentiles of the different gas compositions within the furnace Gas Composition Percentage (Typical) Percentage (Range) Carbon Monoxide 51.0 % 50.0 - 65.0% Carbon Dioxide 13.0% 10.0 - 20.0% Nitrogen 25.0% 20.0 - 28.0% Hydrogen 8.30% 7.50 - 12.0% Oxygen 2.00 % 0.50 - 3.50 % Methane 0.70% 0.40 - 0.80 % Table 1: Typical gas composition percentages. The power input into the furnace process to induce the reduction of the ferric- and manganese oxide, determines the rate at which the reaction takes place. The power input is most commonly measured in MVA and then multiplied by the furnace power factor, which is a function of the electrode characteristics as an inductor, to convert to MW. The rate at which off-gas is generated does not change significantly with the change in power input, however the dust load in the off-gas stream changes exponentially. Larger particulate is generated with the increase in power, as well as the total mass of dust per cubic meter of gas. The dust loading of the off-gas plays a critical role in the design of an off-gas scrubbing system. The following table indicates the increase in dust load with the increase of power input into the furnace. Power Input [MVA] Dust Emission Rate [μg/s] 30 1082.877 40 2793.574 50 7206.776 60 18591.82 70 47962.61 Table 2: Dust emission rate as a function of furnace power Another critical factor of the scrubbing system design is particle size distribution (PSD). The maximum emission of a plant is dictated by environmental legislation, and needs to be adhered to. The greater the dust load in the gas stream, the more efficient the scrubbing system needs to be, because small particulate, which are particles with a sub-micron aerodynamic diameter, is more difficult to remove from a gas stream. The greater the dust load per cubic meter, the greater the quantity of the sub-micron particulate, which significantly influences the design of the scrubber. The required increase in efficiency exponentially increases the power consumption of the scrubbing system, which greatly increases supply costs and service requirements of the plant. The following table indicates the particle size distributions Particle Size [μm] Percentage [Typical] < 1.00 20.0% 1.00 - 5.00 40.0% 5.00 - 10.0 20.0% 10.0 - 100.0 15.0% 50.0 - 100.0 4.00% 100.0 - 500.0 1.00 % Table 3: Particle size distribution at 40MW furnace load These parameters are paramount when conducting the front-end engineering of a scrubbing system for this application. Not only are there financial and commercial implications when failing to adhere to acceptable emissions, but the impact on the surrounding environment can detrimental. Diligent and accurate engineering benefits the customer, supplier and the environment, and satisfies environmental legislative requirements.