Producer Gas Implementation in Steel Reheating Furnaces from Lab to Industrial Scale : A Computational Fluid Dynamics and Thermodynamics Approach
Sammanfattning: The integrated steel-making plants in Sweden contributed with approximately 8 % of the total CO2 emissions in the country in 2011. A major contributor to these emissions is the combustion of fossil fuels in different process units. Therefore, it is essential to reduce emissions by limiting the fossil fuels consumption in the steel industry. A possible solution to reduce the emissions is to implement alternative fuels, which are produced from various combustion and gasification sectors in the iron and steel-making industry. Currently, the blast furnace gas (BFG) and coke oven gas (COG) are extensively used for district heating purposes. Depending on the availability of biomass in a region, gasified biomass (Syngas) can also be used as an alternative fuel source. In addition, the extracted energy from these producer gases can be used in other heat treatment processes such as reheating furnaces. However, these producer gases contain several impurities such as, alkali metals, halogens, particulate matter, sulfur compounds and other mineral contaminants, which can be problematic. For instance, in the steel reheating furnaces, these impurities can form sticky layers of solutions on the steel slab surfaces which are not easy to remove. The High Temperature Agent Combustion (HiTAC) technology has several advantages compared to the conventional methods. These include temperature uniformity, a flexibility of fuels, low pollutant emissions and a volumetric combustion. In this study, these factors have been investigated for the pulverized coal combustion, when the coal particles are assumed to follow a Rosin-Rammler distribution. Moreover, due to the mentioned superior properties of HiTAC technique, it has also been applied for the combustion of producer gases as alternative fuel for steel reheating furnaces. A coupled Computational Fluid Dynamics (CFD) and thermodynamics approach has been developed to analyze the combustion of producer gases and the behavior of impurities in these gases for the steel reheating furnaces. The obtained results prove the capability of HiTAC technique to be used for the combustion of producer gases by enhancing the temperature and by reducing the size of steel reheating furnaces. The findings also show that the Low Calorific Value (LCV) of BFG and the presence of 52 % nitrogen in the gas are responsible for a lower heat release in comparison to other producer gases. The impurities in steel reheating furnaces are considered as ash particles having a particle size distribution similar to the pulverized coal particles. The accumulation of the ash particles at the steel slab surface is predicted using the CFD simulations. Furthermore, the thermo-chemical calculations are used to understand the effect of all the involved chemical compositions in an equilibrium thermodynamics system of impurities and iron-oxides. This thermodynamics study of impurities is divided in two steps. In the first study, at the steel slab surface, the temperature gradients and the concentration of impurities are not considered. This investigation is carried out to identify the reactivity and phase transformation of different ash mineral components with respect to the temperature zones (preheating, heating and soaking) in steel reheating furnaces. Here, chloride compounds are the most reactive compounds in comparison to other impurities. It is also found that an increased temperature from the preheating zone up to the soaking zone leads to an increased iron-oxide formation. In the heating and soaking zones, an addition of mineral compounds like SiO2 and CaO is also found to accelerate the formation of the sticky solutions at the steel slab surface. Moreover, by increasing the steel slab temperature the formations of sticky layers are highly abated in the late heating zone and the entire soaking zones. In the second study the concentration of particles, density of particles and temperature gradients at the steel slab surface are taken into account. Thereby, the shortcomings of the first thermodynamics system are improved. It is found that for the considered furnace configuration, the particles received the same velocity as the injected fuel (70 m/s) and they are heated up to a temperature of 1600 °C. The most of the particles, with the average size of 50 µm, are evacuated through the exhaust ports due to the inertial dominant force. Only around 10 percent of these particles have a tendency to stick to the steel slab surface at the heating zone rather than at the soaking zone. These findings could be applied for improvements in the combustion systems and furnace designs to reduce unwanted accumulations and hot-spots of sticky layers on the steel slab surface. This information may also be useful for planning of new investments in gas cleaning systems, if producer gases are used as fuels.
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