Energy analysis of the basic oxygen furnace

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Physico-chemical characterization of the conversion

The reactions occurring inside the converter are the refining reactions on the pig iron. The refining reactions correspond with the oxidation reactions. They happen very quickly and are triggered by the contact between the oxygen and the liquid metal. Whatever the way of blowing the oxygen, as soon as they both interact the dissolved elements (C, Si, Mn, P) are oxidized until there is no oxygen left (iron is also consumed with the oxygen in this process because its initial amount is very high).
It is the FeO created that oxidizes the silicon, the manganese, the phosphorus and the carbon (Sollac, 1976). Due to its reduced solubility into the metallic bath, the FeO remaining goes to the surface, helping form the slag.
The silicon contained in the pig iron is irreversibly oxidized and after a few minutes its amount in the metal is already really low. The main part of the silica formed goes in the slag and the rest reacts with the lime to form Ca2SiO4.
The oxidations of manganese and silicon occur simultaneously. One part of the MnO produced goes into the slag and the rest is decomposed and the manganese returns back into the bath.
The formation of P2O5 is only possible in the presence of CaO and gives for example P2O5(CaO)3.
When almost all of Si is consumed the oxygen contained in FeO reacts with the carbon to give CO. This reaction is the one consuming the bigger part of the oxygen. When the lance used to blow the oxygen is submerged, some of the CO produced is oxidized into CO2. This reaction is called post combustion and only a part of the energy supplied is retrieved by the gas.
The sulfur is principally brought into the converter via the pig iron. At high temperatures (1600-1800°C) and when the slag is basic, it is possible to realize a desulfurization of the pig iron. The more basic the slag is, the better this operation works.
The reduction of the iron oxide Fe2O3 contained in the iron ore is supposedly managed by the carbon (Huber, 2005).
Usually the refining part is studied by simplifying the chemical reactions occurring inside the converter. Here are the reactions we are going to work on:
The oxygen converter (basic oxygen furnace) we chose to study is the one used in the steel factory of Avilés (Spain), where this internship took place.

Calculation of the energy of reaction

To calculate the energy associated with a reaction we used the following formula: With being the enthalpy of reaction associated with the reaction considered in J/kg and being the mass of reactant consumed in the reaction. This value represents the mass energy associated with a reaction. If this amount is positive it means that the reaction is endothermic and needs energy to occur but if it’s negative, it means that the reaction is exothermic and produced energy (heat).
It is pretty hard to determine precisely the mass of reactant consumed in each reaction because we are not really sure of what’s happening inside the converter. It is said that the iron contained in the hot metal and in the scrap is oxidized by the oxygen blown in the converter through a lance. The FeO produced then reacts with the other components of the hot metal (Si, Mn, P and C) to produced oxides going in the slag. This information was found in the reference:
– Chapitre V. Modélisation physico-chimique du convertisseur à oxygène et du laminoir à chaud pour l’analyse de l’ICV.

Here are the reactions considered:

To determine the enthalpy of reaction we use the formula: With being the enthalpy of reaction of one of the elements involved in the reaction, being the enthalpy of formation in J/kg of the component considered at the environment temperature and being the temperature inside the volume where the reaction occurs. In our study the temperature inside the converter is . We determine for every element involved in the refining reactions and then we obtain by subtracting of the reactants of the reactions to of the product of the reaction considered (not forgetting to take into account the stoichiometric factors of the reaction).

Presentation of the exergy concept

The first and second laws of thermodynamics are incontrovertible ―laws‖ in the resolution of problems concerning energetic. The first law enunciates the equability of diverse forms of energy (thermal, mechanical, electrical…) and allows us to examine the energy flows to which the diverse systems are submitted.
However, we remark that even if there is quantitative equality of the diverse forms of energy, the quality of these forms changes from one to the other, even inside one given form, and also changes with the situations considered. Thereby, one MJ of thermal energy at 1000°C doesn’t represent the same energetic ―potential‖ that one MJ of the same thermal energy at 20°C. Similarly, the potential of use of one MJ of mechanical energy quickly appears to be different from the one of one MJ of thermal energy. Indeed, if mechanical energy can be spontaneously transformed into thermal energy (through ―deterioration‖ for example), the reversed transformation, non spontaneous, needs to proceed according a very specific scheme.
All these elements, linked with the energy quality and with the transformation of energy constitute the second law of thermodynamics, also considered as an evolution law.
The physical quantity associated with this evolution is entropy whose creation we observe for processes occurring outside of strict equilibrium, that is to say for all the industrial operations which necessarily have to present a certain kinetic to occur in a limited time. Thereby, the bigger the unbalance is in a process (heat transfer in a heat exchanger for example) the bigger the power necessary is. But there is a drawback to this observation: an important kinetic of transfer can be obtained by an important deterioration of the energy (spontaneous and irreversible transformation of an energy known to be ―noble‖ in heat) and an important creation of entropy.
Thereby, since a long time, entropy creation was used by scientists to measure the deterioration of energy caused by the irreversibilities of the energetic transfers and transformations. However, for the engineer, who is used to think in energetic terms in J, MJ or kW.h, or in terms of power in watts, kW or MW, this measure isn’t practical. Indeed, entropy, or its evolution in time, is measured in energy unit, or power unit, per Kelvin (J.K−1; W.K−1).
This fact is at least one of the reasons why it is interesting to use the notion of exergy to treat these problems of energy deterioration.
The exergy (unit: J), is the part of the energy amount for a specific transformation that can be potentially retrieved in the form of work or electrical energy (these are equivalently directly usable in a process). It is thus the maximum theoretical useful work obtainable as the system interacts with the ambient. The rest of the energy amount is called anergy and it’s the part of energy that cannot be retrieved.

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General calculation of exergy

Let’s consider the system of the figure bellow which is initially in any state. This system is defined by its energy , its entropy , its volume , its temperature , its pressure and its chemical potential . This system is not in balance with the ambient environment (thermally, mechanically and chemically) so heat, work and matter exchanges will occur.
We want to evaluate the maximum work that could be developed by the combined system. The boundary of the combined system allows only energy transfers by work ensuring that the work developed is not affected by heat transfers to or from the combined system. We are thus going to express the work (or mechanical energy) that this supersystem (system with the ambient environment) can produce when the system changes from its initial state to a dead state (non forced balance state with the ambient environment where the thermal, mechanical and chemical balance conditions are satisfied).
With being the variation of energy of the combined system (or supersystem) and being the heat developed by the combined system. We know that because only work interactions are allowed concerning the combined system (see figure). is the internal energy change of the supersystem and with being the variation of energy of the system and the variation of energy of the environment. Let’s also define as being the initial internal energy of the system and its final internal energy (internal energy of the environment).

Table of contents :

Introduction
The company ArcelorMittal
I) Profile of ArcelorMittal
II) History of ArcelorMittal
Part 1: The process of steel making
I) General description of steel making
II) The sintering plant
III) The coke oven
IV) The blast furnace
V) The basic oxygen furnace
1) General description
2) Physico-chemical characterization of the conversion
VI) The hot rolling
Part 2: Energy analysis of the basic oxygen furnace
I) Calculation of the thermal energy
II) Calculation of the energy of reaction
III) Results of the energy analysis
Part 3: Exergy analysis of the basic oxygen furnace
I) Presentation of the exergy concept
II) General calculation of exergy
1) Exergy of a heat reservoir at a fixed temperature T
2) Exergy of a heat reservoir with a declining temperature T
IV) Results of the exergy analysis
Presentation and interpretation of the results
Conclusions and perspectives
Appendix
I) Cp formulas of the elements involved in steel making processes
II) Explanation of the EXCEL file
1) The energy balance
2) The exergy balance
References

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