Discussion of Carbon Emissions for Charging Hot Metal in EAF Steelmaking Process

Conditionals

The hot metal is the product of blast furnace in ironmaking. Scrap is collected from the society. Composition of hot metal and scrap is all unstable. Therefore, for studying the reaction in the process of steelmaking, composition and temperature of hot metal, scrap, and molten steel need to be setting, which are given in Table 1.

The EAF is charged with hot metal and scrap that mainly need to complete decarburization and dephosphorization. In addition, silicon and manganese are oxidized. Some of the chemical reactions are shown in eqs. (1)–(5), as follows:

The calculation for hot metal

Table 2 has been prepared according to composition and temperature of hot metal and molten steel in Table 1.

According to the chemical reactions are shown in eqs. (1)–(5), oxygen consumption, oxidation product quantity, and heat output are calculated as shown in Table 3 on the basis of 100 kg of inlets hot metal in EAF steelmaking process. For decarburization reaction, 10 % C is oxidized to CO₂, 90 % C is oxidized to CO.

According to the material balance calculation, and considering the loss caused by dust, splash, and iron beads, 1,103.82 kg hot metal is consumed as shown in Figure 1 for 1,000 kg molten steel.

Figure 1:

Hot metal consumption for per ton steel in EAF steelmaking process.

The calculation for scrap

Table 4 has been prepared according to composition and temperature of scrap and molten steel in Table 1.

According to the chemical reactions are shown in eqs. (1)–(5), oxygen consumption, oxidation product quantity, and heat output are calculated as shown in Table 5 on the basis of 100 kg of inlets scrap in EAF steelmaking process.

According to the material balance calculation, and considering the loss caused by dust and splash, 1,044.69 kg scrap is consumed as shown in Figure 2 for 1,000 kg molten steel.

Figure 2:

Scrap consumption for per ton steel in EAF steelmaking process.

Calculation of energy balance

There is a lot of physical heat by charging hot metal in EAF steelmaking process. Meanwhile, oxidation reactions by C, Si, Mn, P elements in hot metal produce large amounts of chemical energy. According to the theoretical calculation and practical experience, hot metal has a large number of surplus energy in steelmaking. This part of the heat can be provided to the scrap and reduce the energy supplement (consumption of power) of EAF steelmaking process.

The solid specific heat of pig iron and liquid specific heat of hot metal is 0.745 and 0.837 kJ/kg °C, respectively, the latent heat of fusion of the hot metal is 218 kJ/kg. The solid specific heat of steel and liquid specific heat of steel is 0.699 and 0.837 kJ/kg °C, respectively, the latent heat of fusion of the steel is 272 kJ/kg. The heat of slag is 1.248 kJ/kg °C and the latent heat of fusion of the slag is 209 kJ/kg. Physical heat of the furnace gas in EAF is the energy taken away by the high-temperature gas. The specific heat of the furnace gas is 1.137 kJ/kg °C, for which the temperature is 1,200 °C [9, 10]. According to the study of EAF steelmaking, heat loss is 2.0–4.0 % of total energy income in EAF steelmaking process, this article set to 3.2 %.

According to Table 3 and the thermodynamic parameters, the energy balance during the EAF steelmaking process for hot metal is calculated as shown in Table 6. The physical heat of per ton hot metal is 329.1 kW h. The chemical heat of per ton hot metal is 263.0 kW h. The physical heat of 905.95 kg (Figure 1) molten steel is 360.8 kW h.

According to energy balance calculation, it is concluded that the surplus energy of hot metal is 96.1 kW h/t. Similarly, according to scrap energy balance calculation, it is concluded that endothermic value of scrap melting is 402.4 kW h/t, and exothermic value of scrap elements oxidation is 42.8 kW h/t. So the exothermic value of scrap is 359.6 kW h/t in EAF steelmaking.

As the surplus energy of hot metal is less than the endotherm heat of scrap in EAF steelmaking process, lack of energy need to be studied, as shown below in eq. (6).

According to the material and energy balance calculation (Figures 1 and 2 and Table 6), the quality of hot metal, the quality of scrap and lack of energy under the condition of different hot metal charging ratio are calculated, as shown in Table 7.

Analysis of power consumption

With the increase of hot metal charging ratio, physical and chemical heat supply greatly increased and lack of energy for EAF reduced. Power that is supplied to the furnace through the electrodes is the main mode of external power, and injection of carbon powder and natural gas can also supply part of energy. This article assumes that the lack of energy for EAF is all provided by power. Power consumption is affected by power efficiency, which is shown in eq. (7), as follows:

kPower is the energy use efficiency of power.

Energy use efficiency of power is different in different smelting conditions in EAF steelmaking process. In previous studies [9], in the condition that the hot metal charging ratio is 50 %, 170 kW h for per ton molten steel in average is supplied by the electric energy during the EAF steelmaking process. Meanwhile, according to the other research of the power consumption for charging hot metal in EAF, it is concluded that the power on average for per ton molten steel under different hot metal charging ratio is 420 kW h for 0 %, 275 kW h for 30 %, 170 kW h for 50 %, and 60 kW h for 70 %. That is shown in Table 8.

Analysis of carbon emissions in EAF process

Carbon emission is a collective name or abbreviation of greenhouse gas emissions. Carbon dioxide as a major greenhouse gas has been attached great importance to the whole society due to the huge emissions. Steel industry as one of the heaviest carbon dioxide emitters need to control carbon emissions. Therefore, carbon emissions for charging hot metal in EAF steelmaking also need to be analyzed based on the analysis of the material and energy balance calculation in this article.

“Carbon footprint” is the foreign widely used method for calculating carbon emission. For calculating carbon emissions, the total products are calculated based on the defined system boundaries. Then the total CO₂ emissions of the system are gotten [11, 12, 13]. The results are converted to values on the basis of tons of steel [14, 15].

Calorific value of standard coal is 7,000 kcal/kg (29.3076 MJ/kg). According to the IPCC database, carbon content of standard coal such as lignite CScoal is 25.8 kg/GJ.

According to the results of energy conservation condition in the Chapters 2 and 3, carbon emissions (for per ton molten steel) in different hot metal charging ratio are calculated as shown in eq. (8) [5, 9].

MEAFi is the carbon emissions of power, i is hot metal ratio, i=0, 30, 50, or 70; QElectrici is the power consumption of per ton molten steel.

1. Calculation for carbon emissions of power at hot metal ratio of 0 %.

   (9)MEAF0=4412×QElectric0×CScoal=4412×420.0×3.6×10−3GJ×25.8kg/kgGJGJ=143.0kg

2. Calculation for carbon emissions of power at hot metal ratio of 30 %.

   (10)MEAF30=4412×QElectric30×CScoal=4412×275.0×3.6×10−3GJ×25.8kg/kgGJGJ=93.7kg

3. Calculation for carbon emissions of power at hot metal ratio of 50 %.

   (11)MEAF50=4412×QElectric50×C= 4412×170.0×3.6×10−3GJ×25.8 kg/GJ=57.9 kg

4. Calculation for carbon emissions of power at hot metal ratio of 70 %.

   (12)MEAF70=4412×QElectric70×C= 4412×60.0×3.6×10−3GJ×25.8 kg/GJ = 20.4 kg

Carbon emissions of hot metal and scrap are calculated by decarburization reaction in EAF steelmaking process, which is shown in eq. (13), as follows:

MSumi is the total carbon emissions for per ton molten steel; MHotmetali is the carbon emissions of hot metal per ton molten steel; MScrapi is the carbon emissions of scrap for per ton molten steel.

According to the above calculation, carbon emissions (for per ton molten steel) under the condition of different hot metal ratio in EAF is summarized in Table 9.

Analysis of carbon emissions by waste heat utilization in EAF process

Actually, CO₂ emissions calculated in this paper is composed of two parts as carbon emissions oxidized from carbon in hot metal or scrap and indirect carbon emissions from the electricity consumption in the EAF. Addition, as different scrap ratio leads to different furnace gas and different physical heat away from the furnace gas has a greater impact on the results, the recycled waste heat of furnace gas should be included as a deductible item in the final greenhouse gas emissions. So, the system boundaries have been amplified, which contains waste heat recovery process.

A lot of energy, about 15 % of the total EAF energy and sometimes as high as 30 % or above, was taken away by the high temperature furnace gas, which has great potential for energy conservation and emissions reduction. In the research on the ways for EAF gas waste heat utilization (scrap preheating, electricity generation, steam production, and coal gas production), it is concluded that coal gas production is the most effective way for energy saving and emission reduction [9, 16, 17].

Coal gas production is the way that recycles carbon monoxide in gas in EAF steelmaking process. Based on data from Tables 3 and 5, 82.32 kg carbon monoxide is produced for per ton hot metal, and 2.10 kg carbon monoxide is produced for per ton scrap. Meanwhile, the calculation for carbon emissions by gas waste heat utilization (coal gas production) is shown in eq. (14), as follows:

MSum.Ai is the total carbon emissions for per ton molten steel by gas waste heat utilization (coal gas production), Mcgpi is the carbon emission reduction for per ton molten steel by gas waste heat utilization (coal gas production). MCOi is the quality of carbon monoxide produced for per ton molten steel in EAF steelmaking process.

Therefore, by using gas waste heat utilization (coal gas production), carbon emissions (for each per ton molten steel) under the condition of different hot metal ratio in EAF is summarized in Table 10 and Figures 3–6.

Figure 3:

Schematic for carbon emissions under the condition of 0 % hot metal charging ratio in EAF steelmaking process.

Figure 4:

Schematic for carbon emissions under the condition of 30 % hot metal charging ratio in EAF steelmaking process.

Figure 5:

Schematic for carbon emissions under the condition of 50 % hot metal charging ratio in EAF steelmaking process.

Figure 6:

Schematic for carbon emissions under the condition of 70 % hot metal charging ratio in EAF steelmaking process.

By using gas waste heat utilization (coal gas production), 143.4, 98.5, 65.8, and 31.5 kg/t of carbon emissions are produced at a hot metal ratio of 0 %, 30 %, 50 %, and 70 %, respectively.

Analysis of carbon emissions in BF–EAF process

With the hot metal ratio increased, carbon emissions are reduced in EAF steelmaking unit process. It is not concluded that charging hot metal in EAF is an energy conservation and emissions reduction process.

The blast furnace is the most energy-intensive step in the blast furnace/basic oxygen furnace (BF/BOF) steelmaking process, generating large quantities of CO₂. Energetics, Inc. gives a range of energy use of 13.0–14.1 GJ/t pig iron [18, 19], which the average carbon emissions is 349.6 kg/t (CScoalis 25.8 kg/GJ). Therefore, carbon emissions are increased by charging hot metal for the whole BF–EAF steelmaking process, as shown in Figure 7.

Figure 7:

Schematic for carbon emissions in BF–EAF process (hot metal ratio is 50 %).