Thermodynamic Analysis of Hydrogen Production from Bio-Oil Steam Reforming Utilizing Waste Heat of Steel Slag

Abstract

(1) Background: The discharged temperature of steel slag is up to 1450 °C, representing heat having a high calorific value. (2) Motivation: A novel technology, integrating bio-oil steam reforming with waste heat recovery from steel slag for hydrogen production, is proposed, and it is demonstrated to be an outstanding method via thermodynamic calculation. (3) Methods: The equilibrium productions of bio-oil steam reforming in steel slag under different temperatures and S/C ratios (the mole ratio of steam to carbon) are obtained by the method of minimizing the Gibbs free energy using HSC 6.0. (4) Conclusions: The hydrogen yield increases first and then decreases with the increasing temperature, but it increases with the increasing S/C. Considering equilibrium calculation and actual application, the optimal temperature and S/C are 706 °C and 6, respectively. The hydrogen yield and hydrogen component are 109.13 mol/kg and 70.21%, respectively, and the carbon yield is only 0.08 mol/kg under optimal conditions. Compared with CaO in steel slag, iron oxides have less effect on hydrogen production from bio-oil steam reforming in steel slag. The higher the basicity of steel slag, the higher the obtained hydrogen yield and hydrogen component of bio-oil steam reforming in steel slag. It is demonstrated that appropriately decreasing iron oxides and increasing basicity could promote the hydrogen yield and hydrogen component of bio-oil steam reforming that utilizes steel slag as a heat carrier during the industrial application.

1. Introduction

The goal of a dual-carbon strategy was proposed via China in 2020, which is that the CO₂ emissions reach a peak in 2030 and are neutral in 2060. Steel enterprises are a national pillar of life and production, but have high energy consumption and high carbon dioxide emissions [1,2]. The development of green metallurgy technology could provide a solid foundation for the dual-carbon strategy. Statistics indicates that China is the largest crude steel producer in the world, with production of 1032.8 million tons [3]. The energy consumption of steel enterprises in China is about 23% of the total consumption [4]. However, 70% of the energy used in steel enterprises is discharged as waste heat and waste energy [5,6]. Reasonable recovery of waste heat and waste energy is an effective method for the green metallurgy process. Steelmaking is the primary process of steel enterprises and consumes a huge amount of energy. Steel slag is a by-product of the steelmaking process, and has a discharge temperature of up to 1450 °C, representing heat having a high calorific value [7,8,9]. However, steel slag is disposed of via the water quenching method in the actual production process. The high calorific value of steel slag is not recovered reasonably. Thus, the recovery of waste heat from steel slag could become a key technology to recover waste heat and waste energy during the steel production process.

The methods of recovering waste heat from slag include physical methods and chemical methods. The physical methods use the waste heat of steel slag to heat water or cool College of Metallurgy and Energy, air. The chemical methods use chemical reactions to replace the waste heat of steel slag. Li investigated the thermodynamic analysis of the physical and chemical methods used to recover waste heat from slag via the enthalpy–exergy compass [10]. The results implied that the heat efficiency and exergy efficiency of physical methods were 76.9% and 34.2%, respectively, but the heat efficiency and exergy efficiency of chemical methods increased to 92.2% and 60%, respectively. Compared to physical methods, chemical methods can recover waste heat from slag with a high reaction rate and high efficiency [11,12,13]. Using chemical reactions to recover waste heat from slag was proposed and has been investigated from the perspectives of thermodynamics, dynamics, product characterization, etc. Kasai first proposed using steam reforming of methane recovery of waste heat from blast furnace slag [14]. The results implied that steam reforming of methane could efficiently recover waste heat from blast furnace slag. The feasibilities of coal gasification [15,16], biomass gasification [8], and bio-oil steam reforming [17] to recover waste heat from blast furnace slag were demonstrated via thermodynamic calculations. These chemical reactions could efficiently replace the waste heat of blast furnace slag. The dynamic characterizations of coal gasification [18,19], biomass gasification [13,20], and sludge gasification [21,22] in blast furnace slag were investigated via dynamic calculations. The results implied that blast furnace slag could decrease the reaction activation energy, and was thus regarded as an outstanding heat carrier. The product characterizations of biogas [23], municipal solid waste gasification [24], coal gasification [25,26], and sludge pyrolysis [27,28] using blast furnace slag as a heat carrier were investigated. The results implied that blast furnace slag could weaken the C-C bond, which catalyzed the process of using chemical reactions to recover waste heat from blast furnace slag. The waste heat of blast furnace slag could be efficiently replaced via chemical reactions and these chemical reactions could provide energy for the steel production process. This was a foundation for green metallurgy development.

Meanwhile, with the depletion of fossil fuels, renewable energy has gradually been explored by researchers. Hydrogen is regarded as a green energy resource for metallurgy industrial applications [29,30], and it can be obtained via petroleum cracking, coal pyrolysis, natural gas reforming, oil reforming, biomass pyrolysis, biomass gasification, etc. [31]. Biomass pyrolysis and gasification applications are outstanding methods used to obtain hydrogen. However, during the process of biomass pyrolysis and gasification, bio-oil can be obtained, decreasing the utilization efficiency of biomass. Utilization of bio-oil reforming has been proven to be a feasible and efficient method to increase the utilization efficiency of biomass and obtain hydrogen [32,33,34]. The reaction rate and heat source of bio-oil steam reforming are the main factors that limit the technological progress. In order to obtain hydrogen efficiently, studies have investigated the characterizations of the bio-oil steam reforming process over catalysts and obtained the catalyst property of the reforming process [32,33]. In order to identify a proper heat source for bio-oil reforming, our team proposed using reforming of bio-oil recovery of waste heat from blast furnace slag [17,35,36]. The results implied that CaO in blast furnace slag was the basic component for catalyzing the steam reforming process.

In a review of recovering waste heat from slag, researchers have primarily focused on recovering waste heat from blast furnace slag. Results showed that the higher the ratio of CaO to SiO₂, the higher the rate of the obtained methane–steam reaction [37]. The ratio of CaO to SiO₂ in steel slag is higher than that in blast furnace slag [8]. However, the characterizations of steam reforming of bio-oil in steel slag are ambiguous, and are first proposed in this paper. Temperature and steam are the main factors affecting the process of steam reforming of bio-oil. In addition, the components of steel slag vary enormously during the production processes. The effects of temperature, S/C, and type of steel slag on the equilibrium productions of steam reforming of bio-oil are explored via thermodynamic calculations in this paper, and the optimal condition is obtained. This can provide a theoretical foundation for the recovery of waste heat from steel slag during the process of metallurgy industrial application in the future.

2. Methodology

2.1. Materials

To obtain the thermodynamic characterizations of bio-oil steam reforming using steel slag as a heat carrier, the chemical compositions of bio-oil and steel slag should be determined. The bio-oil is complicated, but it mainly contains alcohols, acids, ketones, and phenols. In addition, the equal mass ratio of acetic acid, ethanol, acetone, and phenol can be used as the bio-oil model compound during the process of bio-oil reforming, as demonstrated in our previous studies [17,35]. The equal mass ratio of acetic acid, ethanol, acetone, and phenol is also used to replace bio-oil in the paper. In addition, the mass ratio of steel slag to bio-oil is 1:1 and the bio-oil is also defined as 1 kg during the process of thermodynamic calculations The contents of FeO and Fe₂O₃ in steel slag and the basicity of steel slag may affect the equilibrium production of bio-oil steam reforming using steel slag as a heat carrier. Steel slag with different quantities of FeO and Fe₂O₃ and levels of basicity is used in the thermodynamic calculation and their characterizations are shown in

Table 1. The compositions of steel slag.

Type of Steel Slag	CaO	SiO2	Al2O3	MgO	Fe2O3	FeO	Fe2O3 + FeO	R
1	54.00	18.00	8.00	5.00	4.75	10.25	15.00	3
2	50.25	16.75	8.00	5.00	6.33	13.67	20.00	3
3	46.50	15.50	8.00	5.00	7.92	17.08	25.00	3
4	42.75	14.25	8.00	5.00	9.50	20.50	30.00	3
5	39.00	13.00	8.00	5.00	11.08	23.92	35.00	3
6	41.33	20.67	8.00	5.00	7.92	17.08	25.00	2
7	44.29	17.71	8.00	5.00	7.92	17.08	25.00	2.5
8	46.50	15.50	8.00	5.00	7.92	17.08	25.00	3
9	48.22	13.78	8.00	5.00	7.92	17.08	25.00	3.5
10	49.60	12.40	8.00	5.00	7.92	17.08	25.00	4

R is basicity, which is the mass ratio of CaO to SiO₂.

.

2.2. Thermodynamic Method

The chemical reaction of steam reforming of the bio-oil recovery of waste heat from steel slag contains complex mechanisms. When using the minimization of Gibbs free energy method, mastering the possible reactant production can only be used to obtain equilibrium productions. The minimization of Gibbs free energy has been proven to efficiently yield the equilibrium production and was applied in our previous studies [17,38]. The HSC 6.0 software with the function of the minimization of Gibbs free energy is also used in this paper. The thermodynamic characterizations of bio-oil steam reforming in blast furnace slag were investigated in our previous study [17]. The oxide species in steel slag are consistent with those of blast furnace slag. Thus, the possible reactant productions of bio-oil steam reforming in steel slag are consistent with those in blast furnace slag. The principle of minimization of Gibbs free energy in HSC 6.0 software, and the possible reactant production of bio-oil steam reforming in steel slag, are described in our previous study [17]. The primary reactions in the process of the steam reforming of the bio-oil recovery of waste heat from steel slag are shown as follows:

Thermal cracking reaction (TCR):

$${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}{\mathrm{O}}_{\mathrm{k}}\to {\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{z}}+o{\mathrm{H}}_2+p\mathrm{CO}+q{\mathrm{CO}}_2+r{\mathrm{CH}}_4+s\mathrm{C},\Delta H_{800°\mathrm{C}}^{\theta }>0$$

(1)

Steam reforming reactions (SRRs):

$${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{C}{\mathrm{O}}_2+6{\mathrm{H}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=210.26{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(2)

$${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{C}{\mathrm{O}}_2+4{\mathrm{H}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=153.33{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(3)

$${\mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}+5{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}{\mathrm{O}}_2+8{\mathrm{H}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=295.90{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(4)

$${\mathrm{C}}_6{\mathrm{H}}_6\mathrm{O}+11{\mathrm{H}}_2\mathrm{O}\to 6\mathrm{C}{\mathrm{O}}_2+14{\mathrm{H}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=473.68{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(5)

$${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{z}}+(2x-z){\mathrm{H}}_2\mathrm{O}\to x\mathrm{C}{\mathrm{O}}_2+(2x+y/2-z){\mathrm{H}}_2,\Delta H_{800°\mathrm{C}}^{\theta }>0$$

(6)

$$\mathrm{C}{\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{O}}_2+4{\mathrm{H}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=191.10{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(7)

Methanation reactions (MRs):

$$\mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\to \mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=-225.22{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(8)

$$\mathrm{C}+2{\mathrm{H}}_2\to \mathrm{C}{\mathrm{H}}_4,\Delta H_{800°\mathrm{C}}^{\theta }=-89.45{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(9)

Water gas reaction (WGR):

$$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=135.77{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(10)

Water gas shift reaction (WGSR):

$$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=-34.12{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(11)

Bell reaction (BR):

$$2\mathrm{C}\mathrm{O}\to \mathrm{C}+\mathrm{C}{\mathrm{O}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=-169.88{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(12)

Reactions of mineral oxides with the generated gases (RMGs):

$$\mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\to \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3,\Delta H_{800°\mathrm{C}}^{\theta }=-167{.62\mathrm{kJ}\mathrm{mol}}^{-1}$$

(13)

$$\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2,\Delta H_{800°\mathrm{C}}^{\theta }=-61.88{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(14)

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\to 2\mathrm{F}\mathrm{e}\mathrm{O}+{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=28.89{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(15)

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{C}\mathrm{O}\to 2\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=-5.23{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(16)

$$4\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{C}{\mathrm{H}}_4\to 8\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=306.64{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(17)

$$1.132\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\to 2.395\mathrm{F}{\mathrm{e}}_{0.945}\mathrm{O}+{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=40.95{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(18)

$$1.132\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{C}\mathrm{O}\to 2.395\mathrm{F}{\mathrm{e}}_{0.945}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=6.84{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(19)

$$4.527\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{C}{\mathrm{H}}_4\to 9.581\mathrm{F}{\mathrm{e}}_{0.945}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=353.98{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(20)

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\to 2\mathrm{F}\mathrm{e}+3{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=63.356{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(21)

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3\mathrm{C}\mathrm{O}\to 2\mathrm{F}\mathrm{e}+3\mathrm{C}{\mathrm{O}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=-38.99{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(22)

$$1.333\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{C}{\mathrm{H}}_4\to 2.667\mathrm{F}\mathrm{e}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=275.35{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(23)

$$\mathrm{F}\mathrm{e}\mathrm{O}+{\mathrm{H}}_2\to \mathrm{F}\mathrm{e}+{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=17.24{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(24)

$$\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{C}\mathrm{O}\to \mathrm{F}\mathrm{e}+\mathrm{C}{\mathrm{O}}_2,\Delta H_{800°\mathrm{C}}^{\theta }=-16.88{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(25)

$$4\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{C}{\mathrm{H}}_4\to 4\mathrm{F}\mathrm{e}+\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O},\Delta H_{800°\mathrm{C}}^{\theta }=260.04{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(26)

2.3. Performance Evaluation

Gas yield and dry gas concentration are used to evaluate the performance of the process of steam reforming of the bio-oil recovery of waste heat from steel slag. The gas yield encompasses the H₂ yield, CO yield, and CH₄ yield. The dry gas concentration includes the H₂ concentration, CO concentration, and CH₄ concentration. The explanations of gas yield and dry gas concentration are shown as follows:

$$\mathrm{Gas}\mathrm{yield}=\frac{\mathrm{The}\mathrm{mole}\mathrm{of}\mathrm{production}\mathrm{of}\mathrm{bio}-\mathrm{oil}\mathrm{steam}\mathrm{reforming}}{\mathrm{The}\mathrm{quality}\mathrm{of}\mathrm{bio}-\mathrm{oil}}(\mathrm{mol}/\mathrm{kg})$$

(27)

$$\mathrm{Dry}\mathrm{gas}\mathrm{concentration}=\frac{\mathrm{The}\mathrm{mole}\mathrm{of}\mathrm{production}\mathrm{in}\mathrm{dry}\mathrm{reforming}\mathrm{gas}}{\mathrm{The}\mathrm{totalmole}\mathrm{of}\mathrm{dry}\mathrm{reforming}\mathrm{gas}}(\%)$$

(28)

The effects of conditions (temperature, S/C (the mole ratio of steam to the carbon in bio-oil), basicity, and iron oxide) on the process of steam reforming of the bio-oil recovery of waste heat from steel slag were obtained via PY and DGC. Subsequently, the mechanism of steam reforming of the bio-oil recovery of waste heat from steel slag was obtained.

3. Results and Discussion

3.1. Effect of Temperature

The process of steam reforming of the bio-oil recovery of waste heat from steel slag contains endothermic reactions and exothermic reactions. The temperature can affect the coupling endothermic reactions and exothermic reactions, further affecting the equilibrium product distribution [39,40]. The effects of temperature on the PY and DGC of steam reforming of the bio-oil recovery of waste heat from steel slag are illustrated in this section.

Taking the characterizations of steel slag-3 as an example, the hydrogen yields under different temperatures and S/C ratios are shown in Figure 1. Figure 1 shows that hydrogen production from bio-oil steam reforming in steel slag is feasible, and the hydrogen yield first increases then decreases with increasing temperature. Taking S/C of 6 as an example, the hydrogen yield increases from 0.90 mol/kg to 90.42 mol/kg as the temperature increases from 200 °C to 739 °C. Then, hydrogen yield decreases from 90.42 mol/kg to 84.94 mol/kg as the temperature increases from 739 °C to 1000 °C. At low temperatures, the endothermic reactions of TCR (Equation (1)), SRR (Equations (2)–(7)), and WGR (Equation (10)), and the exothermal reactions of MR (Equations (8) and (9)), are the main factors that affect the hydrogen yield. With the increase in temperature, TCR (Equation (1)), SRR (Equations (2)–(7)), and WGR (Equation (10)) shift to the right, and MR (Equation (8)) shifts to the left, and the hydrogen yield increases. However, at high temperatures, the exothermal reaction of WGSR (Equation (11)) is the main factor that affects the hydrogen yield. With the increase in temperature, WGSR (Equation (11)) shifts to the left, decreasing the hydrogen yield. Xie [40] and Yao [17] also investigated the characterizations of hydrogen production from the process of bio-oil steam reforming in blast furnace slag, and a similar variation in hydrogen yield was obtained. They provide further evidence that the results of this paper are accurate.

Taking the characterizations of steel slag-3 as an example, the carbon monoxide yields under different temperatures and S/C ratios are illustrated in Figure 2. In Figure 2, the carbon monoxide yields increase with the increase in temperature. Taking S/C of 5 as an example, the carbon monoxide yield increases from 0.00 mol/kg to 22.49 mol/kg when the temperature increases from 200 °C to 1000 °C. The endothermic reactions of TCR (Equation (1)) and WGR (Equation (10)), and the exothermal reactions of MR (Equation (8)), WGSR (Equation (11)), BR (Equation (12)), and RMG (Equations (16), (22) and (25)), are the main factors that affect the carbon monoxide yield. With the increasing temperature, the endothermic reactions of TCR (Equation (1)) and WGR (Equation (10)) shift to the right, and the exothermal reactions of MR (Equation (8)), WGSR (Equation (11)), BR (Equation (12)), and RMG (Equations (16), (22) and (25)) shift to the right, decreasing the carbon monoxide yield. The results of increasing the carbon monoxide yield as the temperature increases are also demonstrated via fixed-bed experiments of bio-oil reforming in blast furnace slag [35].

Taking the characterizations of steel slag-3 as an example, the methane yields under different temperatures and S/C ratios are illustrated in Figure 3. The methane yield decreases with the increase in temperature, as shown in Figure 3. Taking S/C of 6 as an example, methane yield decreases from 24.40 mol/kg to 0.00 mol/kg as the temperature increases from 200 °C to 1000 °C. The endothermic reactions of SRR (Equation (7)) and RMG (Equations (17), (20), (23) and (26)), and the exothermal reactions of MR (Equations (8) and (9)) are the main factors that affect the methane yield. With the increasing temperature, the endothermic reactions of SRR (Equation (7)), RMG (Equations (17), (20), (23) and (26)) shift to the right, and the exothermal reactions of MR (Equations (8) and (9)) shift to the left, decreasing the methane yield. The characterizations of hydrogen production from the steam reforming of bio-oil over a Ce-Ni/Co catalyst were investigated [41]. The results showed that the higher temperature, the lower the methane yield. These results are consistent with this study.

The process of bio-oil steam reforming using steel slag as a heat carrier generates carbon, which might decrease the reactive activation and have no benefit for the steam reaction. The carbon yield is regarded as a significant factor in the research on the steam reforming process. Taking the characterizations of steel slag-3 as an example, the carbon yields under different temperatures and S/C ratios are shown in Figure 4. As shown in Figure 4, the carbon yield decreases with the increasing temperature. Taking S/C of 5 as an example, the carbon yield decreases from 11.58 mol/kg to 0.03 mol/kg with the increase in temperature from 200 °C to 1000 °C. The endothermic reaction of WGR (Equation (10)) and the exothermal reaction of BR (Equation (12)) are the main factors that affect the carbon yield. With the increase in temperature, the endothermic reaction of WGR (Equation (10)) shifts to the right and the exothermal reaction of BR (Equation (12)) shifts to the left, decreasing the carbon yield.

Taking the characterizations of steel slag-3 as an example, the hydrogen component, carbon monoxide component, and methane component under different temperatures and S/C ratios are illustrated in Figure 5, Figure 6, and Figure 7, respectively. In Figure 5, the hydrogen component increases at the beginning and then decreases as the temperature increases. Taking S/C of 5 as an example, the hydrogen component increases from 4.95% to 70.93% with the increase in temperature from 200 °C to 608 °C, but the hydrogen component decreases from 70.93% to 67.04% with the increase in temperature from 608 °C to 1000 °C. In Figure 6, the carbon monoxide component increases with the increase in temperature. Taking S/C of 5 as an example, the carbon monoxide component increases from 0.00% to 15.42% with the increase in temperature from 200 °C to 1000 °C. In Figure 7, the methane component decreases with the increase in temperature. Taking S/C of 5 as an example, the methane component decreases from 72.24% to 0.00% with the increase in temperature from 200 °C to 1000 °C. The variations in the hydrogen component, carbon monoxide component, and methane component are attributed to their yields. Thus, the variation trends of the hydrogen component, carbon monoxide component, and methane component are similar to those of their yields.

3.2. Effect of S/C

Steam functions as the reactant during the process of steam reforming of the bio-oil recovery of waste heat from steel slag. The S/C impacts the partial pressure of steam, further affecting the equilibrium product distribution. The effects of S/C on the PY and DGC of the steam reforming of the bio-oil of recovery waste heat from steel slag are illustrated in this section.

The effects of S/C on the hydrogen yield are also shown in Figure 1. Figure 1 shows that the hydrogen yield increases with the increase in S/C. Taking the temperature of 706 °C as an example, the hydrogen yield increases from 36.08 mol/kg to 113.67 mol/kg as the S/C increases from 0 to 10. With the increase in S/C, SRR (Equations (2)–(7)), WGR (Equation (10)), and WGSR (Equation (11)) shift to the right, and MR (Equation (8)) and RMG (Equations (15), (18), (21) and (24)) shift to the left, increasing the hydrogen yield. The characterizations of bio-oil steam reforming for hydrogen production have been investigated via fixed-bed experiments [38]. The results implied that the hydrogen yield increased with the increase in S/C. These results are consistent with the findings in this study.

The effects of S/C on the carbon monoxide yield are shown in the Figure 2. Figure 2 shows that, at low temperatures (lower than 412 °C), the carbon monoxide yield increases with the increase in S/C. At low temperatures, MR (Equation (8)) and WGR (Equation (10)) are the main factors that affect the carbon monoxide yield and they shift to the left, increasing the carbon monoxide yield. However, at high temperatures (higher than 412 °C), the carbon monoxide yield increases first and then decreases with the increase in S/C. At high temperatures and low S/C, MR (Equation (8)) and WGR (Equation (10)) are the main factors that affect the carbon monoxide yield and they shift to the left, increasing the carbon monoxide yield. However, at high temperatures and high S/C, WGSR (Equation (11)) is the main factor affecting the carbon monoxide yield and it shifts to the right, decreasing the carbon monoxide yield.

The effects of S/C on the methane yield are shown in Figure 3. As shown in Figure 3, at low temperatures (lower than 216 °C), the methane yield increases with the increase in S/C. At low temperatures, RMG (Equations (17), (20), (23) and (26)) is the main factor that affects the methane yield and shifts to the left, increasing the methane yield. At high temperatures (higher than 216 °C), the methane yield increases first and then decreases with the increase in S/C. At high temperatures and low S/C, RMG (Equations (17), (20), (23) and (26)) is the main factor that affects the methane yield and shifts to the left, increasing the methane yield. However, at high temperatures and high S/C, SRR (Equation (7)) and MR (Equation (8)) are the main factors that affect the methane yield. With the increase in S/C, SRR (Equation (7)) shifts to the right and MR (Equation (8)) shifts to the left, decreasing the methane yield.

The effects of S/C on the carbon yield are also shown in Figure 4. As shown in Figure 4, the carbon yield decreases with the increase in S/C. Taking the temperature of 690 °C as an example, the hydrogen yield increases from 32.41 mol/kg to 0.02 mol/kg with the increase in S/C from 0 to 10. WGR (Equation (7)) is the main factor that affects the carbon yield and shifts to the right with the increase in S/C, decreasing the carbon yield.

Combining the effects of temperature and S/C, when S/C is 6, the maximal hydrogen yield is 109.13 mol/kg at 706 °C. In addition, the hydrogen component and carbon yield are 70.21% and 0.08 mol/kg at 706 °C, respectively. When S/C increases to 10, the maximal hydrogen yield and hydrogen component are 115.68 mol/kg and 72.34%, respectively. The purpose of the novel technology is to utilize hydrogen to recover the waste heat of steel slag. In addition, carbon may go against the process of steam reforming of bio-oil [38,42]. The hydrogen yield and carbon yield via thermodynamic calculations are regarded as the primary indexes for evaluating the process. When S/C increases from 6 to 10, the hydrogen yield increases, although not obviously. However, the energy consumption of the process increases on account of the increasing steam [5]. The optimal temperature and S/C are 706 °C and 6, respectively, where the hydrogen yield and hydrogen component are 109.13 mol/kg and 70.21%, respectively.

3.3. Effect of Type of Steel Slag

The compositions of steel slag mainly depend on the different kinds of steelmaking processes and steel types. The iron oxides and basicity of steel slag can affect the equilibrium production of the process of the bio-oil steam reforming recovery of waste heat from steel slag. Using bio-oil steam reforming recovery of waste heat from steel slag aims to obtain hydrogen efficiently. The effects of the type of steel slag on the hydrogen yield, carbon yield, and hydrogen component are illustrated in this section. Taking S/C of 2 as an example, the hydrogen yields and hydrogen components under different types of steel slag are shown in Figure 8 and Figure 9, respectively. As shown in Figure 8 and Figure 9, the hydrogen yield and hydrogen component decrease with the increase in iron oxides in steel slag. Taking the temperature of 592 °C as an example, the hydrogen yield and hydrogen component decrease from 72.36 mol/kg and 65.96% to 71.79 mol/kg and 65.32%, respectively, with the iron oxides increasing from 15% to 35%. However, the hydrogen yield and hydrogen component increase with the increasing basicity of steel slag. Taking the temperature of 706 °C as an example, the hydrogen yield and hydrogen component increase from 89.85 mol/kg and 66.12% to 90.36 mol/kg and 67.53%, respectively, with the basicity increasing from 2 to 4. The basicity of steel slag is calculated via the mole ratio of CaO to SiO₂. With the increase in the basicity of steel slag, CaO in the steel slag increases accordingly. CaO absorbs carbon dioxide via RMG (Equation (11)), decreasing the carbon dioxide yield. The decreasing CO₂ yield shifts TCR (Equation (1)), SRR (Equations (2)–(7)), and WGSR (Equation (11)) to the right, increasing the hydrogen yield and hydrogen component. However, with the increase in iron oxides, CaO in the steel slag decreases accordingly, which has adverse effects on hydrogen production. In addition, with the increase in iron oxides, RMG (Equations (15), (18), (21) and (24)) is the main factor that affects the methane yield and shifts to the right, decreasing the hydrogen yield and hydrogen component. Compared with CaO in steel slag, iron oxides have less effect on hydrogen production from bio-oil steam reforming in steel slag. The appropriate decrease in iron oxides and increase in basicity can be beneficial for the process of bio-oil steam reforming recovery of waste heat from steel slag in the actual application.

3.4. The Mechanism of Recovering Waste Heat from Steel Slag

Based on the equilibrium productions and our previous studies of the chemical reactions in the recovery of waste heat from slag [2,35,36], the mechanism of bio-oil steam reforming in steel slag was obtained, as shown in Figure 10. In the practical process, steel slag is discharged via the process of steelmaking and it is shown in the slag storage pot. Bio-oil and steam in a certain proportion are fed into the slag storage pot via pipelines. The reactions of bio-oil steam reforming in steel slag take place. The syngas (containing H₂, CO, CO₂, and CH₄) is obtained and discharged from the slag storage pot. The transfer of heat due to the chemical reaction of bio-oil steam reforming and steel slag is accomplished. As shown in Figure 10, the ions of Ca²⁺, Fe²⁺, and Fe³⁺ in steel slag weaken the bonds of C–C and H–O [2,35,43,44], catalyzing the process of bio-oil steam reforming recovery of waste heat from steel slag. Utilization of bio-oil steam reforming efficiently recovers waste heat from steel slag.

4. Conclusions

The novel technology of the bio-oil steam reforming recovery of waste heat from steel slag was investigated via thermodynamic calculations. The primary remarks are summarized as follows:

• The novel method of utilizing bio-oil steam reforming to recover waste heat from steel slag is proposed in this study. It is proven that bio-oil steam reforming can be used to obtain hydrogen and utilize the waste heat of steel slag, and can provide an opportunity for green metallurgy technology.
• At low temperatures, the hydrogen yield of bio-oil steam reforming in steel slag increases with the increase in temperature. However, at high temperatures, the water gas shift reaction is the main reaction affecting the hydrogen yield, resulting in the decrease in the hydrogen yield with the increase in temperature. The carbon yield decreases with the increase in temperature. The hydrogen component increases first and then decreases with the increase in temperature.
• The higher the S/C, the higher the obtained hydrogen yield and the lower the obtained carbon yield. By combining the hydrogen yield and energy consumption, the optimal S/C is 6 and the optimal temperature is 706 °C. At the optimal condition, the hydrogen yield, hydrogen component, and carbon yield are 109.13 mol/kg, 70.21%, and 0.08 mol/kg, respectively.
• Compared with CaO in steel slag, iron oxides have less effect on hydrogen production from bio-oil steam reforming in steel slag. The basicity of steel slag promotes hydrogen production. During industrial application, the hydrogen yield and hydrogen component of bio-oil steam reforming in steel slag can be promoted via an appropriate decrease in iron oxides and increase in basicity.