Application of Hydrochar for Low-CO2 Emission Steel Production

Abstract

Steel is an indispensable material of the modern society and yet the production of steel is one of the largest anthropogenic CO₂ emission sources on the planet. The conventional blast-furnace-basic-oxygen-furnace (BOF) process is responsible for generating 85% of the steel industry’s total CO₂ emissions, which is the result of a high coal consumption rate for the reduction of iron ores and for providing the heat necessary for the high-temperature process. In order to meet the climate goal set by the Paris Agreement, the iron and steel industry must drastically decrease its CO₂ emissions and aim at achieving net-zero emissions by 2050. Bioenergy is a form of renewable energy, and if it is managed sustainably throughout its life cycle, it can be considered carbon-neutral. Replacing fossil fuels with biofuels consumed during the steelmaking processes is one way to decrease CO₂ emissions. However, this approach has not been widely adopted by steelmakers over the world due to the high price and the limited availability of wood-based biofuels. Hydrochar is a coal-like solid material that is produced from the hydrothermal carbonization (HTC) of biomass. It has attracted great interest from steelmakers due to its coal-like properties and the fact that it can be produced from a wide range of organic waste streams that can be found in almost every country. Previous studies focused on the use of hydrochar for the blast furnace process. This thesis therefore examines the potential use of hydrochar in the direct-reduction-electric-arc-furnace (DR-EAF) process, and particularly in two applications where the use of fossil coal is difficult to abate—the coal-based direct reduction of iron ore and the carburization of liquid steel in the EAF. This thesis begins with a characterization study of a hydrochar produced from lemon peel waste (LPH) and its comparison with a fossil reference material (anthracite) and two bio-reference materials (charcoal). The results reveal that LPH is a highly volatile material that is characterized by a low fixed carbon content and a medium calorific value. The volatile matter of LPH consists of gas, tar, and aqueous liquids, and contains approximately half of the total carbon and energy content of LPH. On the contrary, charcoal, anthracite, and the pyrolyzed char of LPH (PLPH) hardly emit any volatiles and are stable up to a high temperature (1200 °C). These materials are characterized by high fixed carbon contents and high calorific values, which makes them ideal fuel, carburizers, and reducing agents. On the other hand, LPH seems to be more efficient when it is applied in areas where its volatile matter content could be utilized to an advantage, such as to provide heating energy and to reduce metal oxides. Next, two hydrochars (produced from lemon peel and rice husk) were tested for coal-based direct reduction and their performance were compared to that of anthracite. Hematite-carbon mixtures prepared with varying fixed-carbon-to-oxygen ratios (C/O) were heated in nitrogen atmosphere up to 1100 °C for direct reduction. The hematite in briquettes with molar C/O ratios greater than 1.0 were completely reduced to metallic iron, whereas briquettes with C/O ratios equal to 0.4-0.5 were reduced by 63-86%. It was confirmed that the volatile matter released by the carbonaceous materials and the organic binder reduced hematite up to a maximum of 35% but the utilized fractions of the volatile matter were quite low (12-56%). As a result, the reduction of hematite was dominated by carbothermic reduction which involved fixed carbon. Thus, the efficiency of a carbonaceous material as a reducing agent for the coal-based direct reduction processes is still predominantly determined by its fixed carbon content. Then, LPH was tested for carburization of liquid iron in a laboratory setup under an inert atmosphere and its performance was compared with that of charcoal. Iron-carbon briquettes, which have higher apparent densities than the carbonaceous material itself, were utilized as carburizers with an aim to improve the carbon’s penetration depth in the liquid iron. The briquettes were experimented in two different ways to simulate the carbon addition practices in an EAF. With the first method, the briquettes were slowly heated from room temperature up to 1600 °C, which simulates the loading of carbon into an EAF at the beginning of a heat via a scrap bucket. With the second method, briquettes were directly charged into a pool of liquid iron. The results reveal that the carburization yield is predominantly determined by the fixed carbon content of the carbonaceous material, and when a more aggressive carbon addition method (e.g. direct charging) was used, there were additional carbon losses which lowered the yield.  In the final part of the thesis, two types of hydrochars (those produced from orange peel and green waste) and an anthracite were applied for carburization tests in a pilot-scale EAF. Carbonaceous materials were either top-charged into the EAF at the beginning of a heat, or injected as powder via a lance directly into liquid steel after scrap meltdown. The results show that hydrochar and anthracite has a similar carburization yield (based on fixed carbon) when the same carbon addition method was used, and the carburization yields achieved by top-charging were higher than that achieved by lance injection. Based on the results obtained in this thesis, three main conclusions are drawn. Firstly, hydrochar can completely replace fossil coal as a reducing agent for the direct reduction of iron ores and as a carburizing agent in the EAF process. However, it is more efficient to use pyrolyzed hydrochar than to use pristine hydrochar since the fixed carbon content of the material mostly determines its substitution ratio for anthracite. Secondly, some negative impact of the ash content of hydrochar has been identified in this study. For example, the reduction rate of hematite-carbon composite mixture is lowered by the hindering effect of ash on carbothermic reduction. Furthermore, ash increases the slag volume and decreases the slag’s basicity in the EAF. Hydrochars produced from fruit peel wastes (lemon peel, orange peel) have lower ash contents than hydrochars produced from plant wastes (rice husk, green waste) and are more suitable to be applied directly in steelmaking processes. Lastly, the substitution of anthracite with charcoal or hydrochar lowers the total amount of sulfur introduced into the EAF. The increase in the amount of phosphorous introduced into the EAF resulting from the addition of hydrochar can be resolved either by controlling the amount of hydrochar added, or by lowering the phosphorous content of hydrochar through additional impurity reduction treatment following the HTC process, which should be investigated in future studies. 

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