INTEGRATED HEAT RECOVERY AND MATERIAL RECYCLING FROM HOT SLAGS: TOWARD ENERGY SAVING AND EMISSION
REDUCTION
Yongqi Sun1, Zuotai Zhang1,2, Seetharaman Sridhar3
1Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, P.R. China
2School of Environmental Science and Engineering, South University of Science and Technology of China, Shenzhen, P.R. China
3WMG, International Digital Laboratory, University of Warwick, Coventry CV4 7AL, UK Keywords: Heat recovery; Hot steel slags; Coal gasification; Syngas production; Kinetic
mechanism Abstract
Steel slags, untapped at 1450-1650 oC, represent a large potential of energy saving and material recycling in metallurgical industry. Conventionally, the methods used for the heat recovery could be categorized into two types and the chemical methods offered the advantage of combination of multiple industrial sectors and production of the high value syngas. Herein, a novel chemical method was investigated, i.e., gasification reaction in the atmosphere of CO2
where both raw coal and raw char from coal pyrolysis were employed. Not only the thermodynamics of the gasification process in terms of syngas yields but also the kinetic mechanisms of the char gasification process were analyzed. It was clarified that the slag additions showed a great catalytic effect of the char gasification. Furthermore, the transformation of the iron valence state during the gasification was clarified and the potential of energy saving and emission reduction were estimated.
Introduction
Nowadays with the continuous urbanization and industrialization in China, the emission reduction and resource shortage gradually become severe issues to be addressed in modern society. To deal with these problems, improving the energy efficiency in the traditional energy-intensive industries such as the iron and steel industry contributes to a significant strategy.
In the past decades numerous advanced technologies have been introduced into the steel industry such as the continuous casting [1] and it is generally believed that heat recovery from the hot slags represents one of the greatest potentials of energy efficiency improvement in the steel industry [2-3]. In 2014, the output of crude steel in China was ~823 million Mt [4], accounting for half of the global production, and consequently, ~123 Mt steel slags were discharged in the steel industry. These high temperature slags, untapped at 1450-1650 oC, carried substantial amounts of high-grade thermal energy, accounting for an important energy resource and material resource; while the recovery ratio of this energy was quite limited, i.e., less than 2% [5].
To recover the waste heat from the hot slags, numerous methods have been developed in the past decades, which could be generally divided into physical granulation methods and chemical methods [2-3,6]. The physical methods were mainly focused on the granulation of the hot slags into small particles and then the improvement of the heat transfer between the hot slags and the working medium. Thus lots of granulation methods have been exploited including rotary cup
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
atomizer (RCA) [7], rotary cylinder atomizer (RCLA) [8] and spinning disk atomizer (SDA) [9]
methods. Compared to physical methods, chemical methods have been more extensively investigated recently because of the apparent advantages such as the production of high-value syngas and the integration of multiple sectors [6]. Meanwhile coal gasification is a clean and highly efficient route utilizing the coal resource and conventionally the heat required for coal gasification is supplied by the part combustion of the coal materials. In this case, the thermal energy in the steel slags could be a potential energy carrier supplying the heat for coal gasification, which was investigated in this study.
Experimental and methods
Sample preparations
In this study, a low-rank coal sample was first collected from Pingshuo power generation plant in Shanxi Province, China. The proximate analysis results show that the primary coal was mainly composed of 3.8% moisture, 31.6% volatile, 21.5% coal ash and 45.9% fixed carbon (wt. %). Meanwhile, an industrial steel slag was collected from Shougang Corporation in Beijing, China. The X-ray fluorescence (XRF) results show that the chemical compositions of this steel slag mainly comprised 41.9% CaO, 22.6% SiO2, 7.3% MgO, 4.1% Al2O3 and 22.6% Fe2O3 with the Fe2+/TFe of 0.63.
It should be pointed out that there were two ways to treat the primary coal for the further gasification. One is to heat the raw coal in a fixed tube furnace before gasification in the N2
agent to obtain the coal char. The compositions of the coal char prepared were 0.4% moisture, 11.7% volatile, 18.0% coal ash and 70.3% fixed carbon (wt. %). Then the coal char obtained was thoroughly mixed with the steel slags with the mass ratio of 1:1 for the sequent gasification reactions. Another strategy was that the raw coal was thoroughly mixed with the steel slags before gasification and then the pyrolysis process was proceeding in advance during the heating process from the room temperature to the coal char gasification temperature designed. These two treatment approaches would not influence the final gasification results and thus herein only the coal char gasification integrated with heat recovery from steel slags was analyzed. In addition, before gasification all the materials used were dried for 10 hours and ground into small particles less than 200 meshes for the full contact of these materials.
Apparatus and Procedure
In the present study, a series of isothermal experiments were carried out to clarify the coal char gasification mechanism and the role of the steel slags. A gasification system was used to perform these tests, as displayed in Figure 1, which was mainly composed of two parts, i.e., a TG analyzer part (S60/58341, Setaram) to perform the gasification experiments and a syngas analysis part (Testo pro350, Testo) to measure the contents of the syngas especially the transient CO content. From the viewpoint of carbon emission reduction, pure CO2 was chosen as the gasifying agent with a flow rate of 100 ml/min. In order to confirm the complete gasification of the coal char, the gasification temperatures were selected as 1000 oC, 1100 oC, and 1200 oC.
The whole gasification process was mainly composed of several steps. The raw materials prepared were first placed into a corundum crucible with the height of 5 mm and diameter of 8 mm and then put into the TG heating area. Then the materials were heated from room temperature to the set gasification temperature with a heating rate of 10 K/min in the agent of 100 ml/min N2 gas. After reaching the gasification temperature, it would be held for 10 minutes to stabilize the temperature and atmosphere. Then the N2 agent was replaced by the 100 ml/min
CO2 gas and the char/CO2 gasification took place. Meanwhile mass evolutions during the gasification was detected and recorded by the TG system and the syngas released was simultaneously measured by the syngas analysis system.
Figure 1. Schematic diagram of coal gasification system (a). This system could be mainly divided into two parts, i.e., (b) a part of gasification reaction and (c) a part of syngas
measurement; (d) temperature file of the experiments Results and discussions
General process of coal gasification
As aforementioned, generally the whole coal gasification process could be divided into two stages, i.e., a coal pyrolysis stage at low temperatures and a coal char/CO2 gasification stage at high temperatures. As an example, the mass evolutions of the samples during gasification at 1000 oC are displayed in Figure 2 in detail. As can be observed, the whole experimental process could be obviously divided into two steps. First, during the non-isothermal heating process, the raw coal samples were primarily pyrolyzed into coal char in the agent of N2, during which the chemical bonds in the organics were cracked and part of the syngas was thus released. After that the gasifying agent was transferred from N2 to CO2 during the isothermal reactions at high temperatures, the coal char prepared would react with the CO2 agent through the Boudouard reaction and the sample mass was further decreased.
2 r 1273
CO +C=2CO, HH θ=166.7 kJ/mol (1) From Figure 2(a), it can also be noted that during the second stage of coal char gasification, the slope of the TG curves was remarkably increased and the reaction rate was enhanced in presence of steel slags, which could indicate a catalytic effect. In order to further clarify this point, the DTG curves of the samples during the second stage of coal char gasification was further calculated, as displayed in Figure 2(b) in detail. As can be observed clearly, in presence of steel slags, the reaction rate was improved and the reaction time was greatly shortened, which undoubtedly proved that the steel slags could act as not only a good heat carrier but also an effective catalyst for the coal gasification reactions.
Figure 2. Mass evolutions of the samples during coal gasification at 1000 oC. (a) TG curves of raw coal gasification process and (b) DTG curves of coal char/CO2 gasification process.
Kinetic mechanism of the coal char gasification
As aforementioned, the general process of coal gasification in presence of steel slags could be first clarified based on the TG and DTG curves. More importantly, the kinetic mechanism of the coal gasification could be identified using the TG data and in this study the mechanism of second stage of coal gasification, namely coal char/CO2 gasification, was mainly analyzed. The analysis of the kinetic mechanism of the char gasification could be mainly divided into several steps. First, the conversion degree of the char gasification process x could be calculated based on the TG curves. Second the relationship between the conversion degree x and the time t could be described using the following equation:
F( )= 0 ( )
( )
x dx
x k T t
f x =
∫
(2) where x, t, k(T), T, f(x) and F(x) are the conversion degree of coal gasification, time, apparent gasification rate constant, absolute temperature, differential and integral mechanism function, respectively. Then the linear relations of the integral mechanism function F(x) versus time t could be analyzed using various mechanism models of gas-solid reactions developed in previous studies including Avrami-Erofeev models, shrinking core models and diffusion models [10-13].It should be pointed out here that the analysis of the kinetic mechanism of coal char gasification was performed mainly based on three principles, i.e., the general understanding (physical meanings) of the individual kinetic models, the results demonstrated by the previous studies and the correlation coefficients (R2) of all plots. Actually, a good linear kinetic model
could suggest none effective physical-chemical meanings despite of a good mathematic relationship [10]. All the plots were calculated and analyzed and the results for the gasification at 1000 oC are displayed in Figure 3 in detail. It can be observed that the char/CO2 gasification could be interpreted by an A2 model (Avrami-Erofeev) without slags; while in presence of steel slags, the mechanism model changed from an A2 model to an A4 model. This model variation was first in agreement with a previous study [11]. Furthermore, an Avrami-Erofeev was reasonable to describe the coal char gasification because the porosity of the samples gradually varied with the reaction proceeding with time [10-13].
Figure 3. Kinetic mechanisms of the coal char gasification at 1000 oC. (a) Coal char without steel slags and (b) coal char with steel slags.
After determining the kinetic mechanism of the char gasification process, the apparent gasification rate could be calculated by means of Eq. (2). The using these rate constants, the apparent activation energy for gasification could be further deduced using the Arrhenius equation.
As a result, it was found that the activation energy remarkably decreased from ~90 kJ/mol to ~ 15 kJ/mol due to the addition of steel slags. This indicated that the steel slags showed a great catalytic effect on the coal char gasification process. On the other hand, this visibly decreasing activation energy also indicated that the gasification would be greatly influenced by the mass diffusion step, which should be considered during an actual gasification process.
Syngas productions and the role of steel slags
As aforementioned, during the gasification process, not only the mass evolutions of the samples were detected but also the concentrations of the syngas was simultaneously measured, especially the transient CO content. Based on the transient curves of the CO content, the total CO yield during the gasification could be calculated, which was actually one of the main objectives in this study. Analyzing the CO yields during gasification reactions, it was found that the CO yield in presence of steel slags was higher than that without steel slags. The possible reason could be that the FeO in the slags was oxidized by the CO2 agent and thus part of CO was produced. In fact, there was high content of FeO in the steel slags, which could be oxidized by the pure CO2 gas. In order to confirm this point, the raw steel slags were heated in the pure CO2
agent at 1100 oC and the content of the syngas produced was measured. The CO content in the
syngas is shown in Figure 4. It can be proved that the raw steel slags could be oxidized, which enhanced the CO production during the char gasification process. In order to further clarify the reaction occurring, the slags after gasification were characterized by X Ray Diffraction (XRD) technique and it was found that the Fe3O4 phase in the slags remarkably increased. This indicated the existence of the following reaction during the coal gasification process:
2 3 4 r 1273
3FeO+CO =Fe O +CO, HH θ=16.1 kJ/mol (3)
Figure 4. Industrial concept integrated of sludge gasification and heat recovery from high temperature slags
Conceptual model summarized in the present study
In the end, a conceptual composed of multiple industrial sectors could be summarized herein including the steel industry, the coal industry and the chemical engineering industry, as displayed in Figure 5 in detail. The whole route of this strategy mainly consisted of several steps. First, the high temperature steel slags were produced during the steel making process in the steel industry.
Meanwhile the primary coal was produced in the coal industry and pyrolyzed into raw coal char.
Then there were two pathways to perform the char/CO2 gasification reactions in presence of steel slags. One was that the steel slags directly contacted with the coal char and the char gasification occurred; during this process, the full contact between these two materials was a significant factor to be controlled. The other was that the steel slags were first granulated and broken into small particles to increase the surface area through different granulation methods [7-9,14].
Consequently, the hot slags particles obtained were mixed with the coal char, the char gasification took place and then the syngas including CO, H2 and CH4 was produced. It should be pointed out that the syngas produced could indirectly be used in the chemical engineering industry for chemical product preparations or directly used in the steel industry after necessary gas separations.
Figure 5. Conceptual model of the integration of heat recovery from high temperature steel slags and coal gasification
Conclusions
In this study, an emerging chemical method, i.e., coal gasification was investigated for the purpose of recovering the waste heat from high temperature steel slags in the steel industry. A series of isothermal experiments were performed in the temperature range of 1000-1200 oC under the CO2 agent. The results proved that the coal gasification process could be overall divided into two stages, i.e., a low temperature pyrolysis stage and a high temperature char gasification stage. Furthermore, the steel slags greatly changed the kinetic mechanism of the coal gasification, i.e., the kinetic model of char gasification changed from an A2 model to an A4 model and the activation energy was greatly decreased indicating a catalytic effect of the steel slags. Moreover, from the thermodynamic respect, the CO yield was increased by the steel slags because of the reaction between the FeO in the steel slags and the CO2 agent.
Acknowledgement
Supports by the National High Technology Research and Development Program of China (863 Program, 2012AA06A114) and Key Projects in the National Science & Technology Pillar Program (2013BAC14B07) are acknowledged. The authors also acknowledge financial support by the Common Development Fund of Beijing and the National Natural Science Foundation of China (51472006, 51272005 and 51172001).
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