A novel slag carbon arrestor process for energy recovery in steelmaking industry
Introduction
Steel production is a major indicator for economy growth especially for developing countries. However, steel making is a highly energy intensive process accounting for nearly 5% of the world's total energy consumption and approximately 6.7% of total world CO2 emissions. Also, the heat recovery in a steel making process is typically low at only ~ 17% [1]. The rising cost of energy and high demand for greenhouse gas emission reductions represent major challenges for the steel industry.
Currently, approximately 17% of the operating cost of the steelmaking industry is energy. This energy originates from multiple sources, such as coal, electricity, natural gas, recycled coke oven gas (COG) and blast furnace gas (BFG) [2]. Recycling of waste heat and recovery of energy rich by-products from these energy sources are identified as key measures to improve energy efficiency and reduce costs and emissions of the modern-day steelmaking process. It is estimated that the energy recycled from COG supplies ~ 20% of the total energy consumed in the present steelmaking process, with potential of increasing to 40% if fully utilised. BFG, on the other hand, has the potential to supply up to another 40% of the total plant energy consumption despite its low energy density (~ 1/3 of that of COG) [3].
Motivated by this path, our research team at the University of Newcastle, Australia developed a novel Slag Carbon Arrestor Process (SCAP) to improve the energy recovery of the steelmaking industry. The work is also part of a major research theme on low-emission energy technology options [4], [5], [6], [7], [8], [9], [10] being developed at the University of Newcastle, Australia. The SCAP process is to be introduced in the following texts.
Fig. 1 shows the conventional steel production process which primarily consists of two integrated unit operations: coke production and iron ore reduction. The coke making process involves carbonization of coal at high temperatures (800–1200 °C) in an oxygen deficient atmosphere in order to concentrate the carbon. During the coke making process, hot COG along with unwanted aromatic hydrocarbon compounds (i.e. tar) are generated, which contain valuable carbon and energy [1]. The produced COG is considered to be a good fuel source [11]. However, to ensure effective utilisation of COG, tar must be removed as it can create operational problems such as condensation and pipe blockages [11], [12]. To achieve this, the hot COG, at temperatures between 800 and 850 °C, emitted from coke ovens is spray cooled with an aqueous ammonia solution in order to remove most of the higher hydrocarbons in the tar, such as benzene (C6H6), toluene (C7H8), and naphthalene (C10H8). In addition, complex processing plants are required for the conversion of tar into valuable chemical by-products [13]. These processes, as mentioned by Yue et al. [14], cause significant heat losses and serious secondary pollution due to tar losses in the waste water. Therefore, instead of physically and chemically separating tar from COG, it would be highly beneficial if the tar could be decomposed into light fuel gases in-situ with the assistance of a catalyst. The heat and chemical energy embodied in the hot COG can also be used in this process.
After the coke making process, iron ore reduction and steel production occur in the blast furnace (BF) and basic oxygen furnace (BOF), respectively. Coke, sinter, and limestone are added in these two furnaces. The purpose of the blast furnace is to chemically reduce iron oxides into liquid metal and physically separate the liquid metal from slag. The operation of the blast furnace and basic oxygen plant usually results in the production of a high amount of slag, containing high amounts of CaO, FeO, SiO2 and Al2O3[15]. The majority of slag produced is currently used in the cement industry or as a fertilizer, while a fraction of the slag is recycled in the sintering process and in the blast furnace for supplying limestone and iron [16]. Heat recovery from slag is generally low and difficult due to its low thermal conductivity between 0.1 and 3 W/mK [17]. In general, this contributes to the low heat recovery of the steelmaking industry.
To overcome the aforementioned issues such as low heat recovery, high heat losses, and possible secondary pollution, several options have been suggested in the literature such as hot-slag heat recovery and more efficient process design [18]. Nonetheless those options are not fully developed for commercial implementation. As a step change solution, a novel Slag Carbon Arrestor Process (SCAP) was developed.
Fig. 2 shows the proposed SCAP process in which a tar reformer is introduced to the conventional steelmaking process. It is essentially the integration of a conventional steelmaking process and a tar reforming/hydrogenation process. In the tar reformer, tar decomposes under a catalytic reaction with steelmaking slag (see reaction R1) while COG is converted into a hydrogen-enriched gas [19],Tar → H2 + CO + CO2 + CH4 + other light hydrocarbon + C.
Normally very high temperatures are required for tar decomposition; catalysts such as steelmaking slag can help to reduce such temperatures. Generally, calcium and iron are considered to be favourable catalyst materials for pyrolysis, reforming or decomposition of coal and/or tar as well as hydrogen-enriched gas production [20]. The innovativeness of the SCAP process is that the tar reforming process makes use of slag, which is rich in calcium and iron oxides, in the place of traditional calcium/iron oxide catalysts.
The slag entering into the tar reformer is best in the form of granulated slag (providing more surface area for catalytic reaction), which is a sand-like product produced using instant quenching of molten slag. Nevertheless, hot molten slag/rock type slag (produced by slowly cooling the molten slag) should not be excluded for future study. Also generated along with the tar reforming process is a possible soot formation/carbon deposition on the surface of slag, which can then be recycled back to the sinter machine and blast furnace. With such carbon recycling, the SCAP process is expected to not only reduce coke consumption in the steelmaking process, but also eliminate tar associated problems as well as the production of a hydrogen-enriched COG. Another advantage of SCAP process is that it can be readily retrofitted to existing steel production plants.
The objective of this paper was to study the feasibility of the SCAP process by (i) investigating via simulations the extents of COG quality improvement, carbon deposition and tar decomposition (ii) conducting energy and mass balance analyses of the process, (iii) examining the effect of tar decomposition extent on carbon deposition, energy saving, and emission reduction potentials, and (iv) using preliminary experiments to confirm the concept and verify simulation results.
Section snippets
Process simulation
The process simulation was carried out using the process simulation package - Aspen Plus v8.4. The Aspen model solves all the equilibrium constant equations simultaneously and calculates the equilibrium conditions by minimizing the total Gibbs free energy of the system. Moreover, thermodynamic database - HSC chemistry (developed by Outotec) - was also used as an assisting tool. With the above tools, energy and mass balance analyses were performed for the SCAP process.
Fig. 3 shows the process
Results and discussion
The simulation and experimental results are presented in 3.1 Simulation, 3.2 TGA-FTIR evaluation of coal and slag mixtures, respectively.
Conclusion
A novel slag carbon arrestor process using steelmaking slag was proposed for energy recovery in the steelmaking industry. In the SCAP process energy recovery can be achieved by utilising the energy embedded in the hot raw COG while the tar reforming process is able to produce a carbon-rich slag and a higher quality COG product. The results indicate that with SCAP process the COG energy content can be increased from ~ 34.6 MJ/kg to ~ 37.7 MJ/kg (or by 9%). Also, recycling the carbon-rich slag in the
Acknowledgement
The authors wish to acknowledge the financial support they have received from Australian Research Council (ARC) and the University of Newcastle, Australia.
References (35)
Hydrogen enrichment of fuels using a novel miniaturised chemical looping steam reformer
Chem. Eng. Res. Des.
(2012)- et al.
Performance characteristics of a miniaturised chemical looping steam reformer for hydrogen enrichment of fuels
Int. J. Hydrog. Energy
(2012) - et al.
An in-depth assessment of hybrid solar–geothermal power generation
Energy Convers. Manag.
(2013) - et al.
Assessment of geothermal assisted coal-fired power generation using an Australian case study
Energy Convers. Manag.
(2014) - et al.
Equilibrium thermodynamic analyses of methanol production via a novel chemical looping carbon arrestor process
Energy Convers. Manag.
(2015) - et al.
Hydrogen production by the partial oxidation and steam reforming of tar from hot coke oven gas
Fuel
(2006) - et al.
Catalytic reforming of model tar compounds from hot coke oven gas with low steam/carbon ratio over Ni/MgO–Al2O3 catalysts
Fuel Process. Technol.
(2010) Energy recovery from molten slag and exploitation of the recovered energy
Energy
(1997)- et al.
Energy efficiency and carbon dioxide emissions reduction opportunities in the US iron and steel sector
Energy
(2001) - et al.
Integrated coal-pyrolysis tar reforming using steelmaking slag for carbon composite and hydrogen production
Fuel
(2013)
Correlating the effects of ash elements and their association in the fuel matrix with the ash release during pulverized fuel combustion
Fuel Process. Technol.
Utilising of the oiled rolling mills scale in iron ore sintering process
Resour. Conserv. Recycl.
Optimum temperatures for carbon deposition during integrated coal pyrolysis–tar decomposition over low-grade iron ore for ironmaking applications
Fuel Process. Technol.
The analysis of organic matter in coke oven emissions
Fuel
Production of FT transportation fuels from biomass; technical options, process analysis and optimisation, and development potential
Energy
Modified dolomites as catalysts for palm kernel oil transesterification
J. Mol. Catal. A Chem.
A TG–FTIR investigation to the catalytic effect of mineral matrix in oil shale on the pyrolysis and combustion of kerogen
Fuel
Cited by (4)
An Overview of Utilization of Blast Furnace and Steelmaking Slag in Various Applications
2019, Materials Today: ProceedingsA review on high-temperature thermochemical energy storage based on metal oxides redox cycle
2018, Energy Conversion and ManagementCitation Excerpt :In general, the pure system is easier to be produced, while the mixed one may demonstrate advantages in reversibility and cost. A wide range of pure metal oxides redox systems has been extensively investigated in the area of chemical looping technologies [51–55], where cyclic performances and stability of many redox materials were examined in details. However, past studies regarding the thermochemical energy storage performances of metal oxides redox systems are less frequently reported, and not all of the examined metal oxides are suitable for TCES applications.
Performance of pavements incorporating industrial byproducts: A state-of-the-art study
2017, Journal of Cleaner ProductionCitation Excerpt :The two primary lines of thoughts regarding the analysis of energy consumption and GHG emission of BFS are as follows: BFS is a waste and is a useless material that is already in steel manufacturing industries, which is one of the most energy-intensive industries contributing to almost 5% of the global total energy consumption and approximately 6.7% of total world CO2 emissions (Zhou et al., 2016). Therefore, the environmental impacts of BFS is already included in the LCA of steel products; for instance, BFS production accounts for almost 30% of the waste heat released by iron and steel manufacturing industries (Zhang et al., 2013; Zhang and Zhou, 2009).
Estimation of the carbonation reaction kinetic parameters for dilute methane and carbon dioxide conditions in a calcium looping process
2018, Environmental Progress and Sustainable Energy