TG- MS study on in-situ sulfur retention during the co-combustion of reclaimed asphalt binder and wood sawdust
Graphical abstract
Introduction
Reclaimed asphalt pavement (RAP) recycling is a common sustainable practice in the pavement industry with important economic and environmental impacts (Dony et al., 2013; Liu et al., 2016). Asphalt extracted from RAP materials is known as reclaimed asphalt binder (RAB) (Miró et al., 2011). RAB is widely utilized in the production of new asphalt mixtures, but addition of high percentages of RAB could bring about negative effects on pavement performance (Mogawer et al., 2012). Therefore, developing a new reclamation method that could consume large amounts of RAB is necessary (Zaumanis et al., 2014). Given the high carbon and hydrogen contents of RAB (Wang et al., 2017), reutilization of RAB for energy conversion may be an ideal way.
RAB consists of a complex mixture of hydrocarbons and non-metallic derivatives (Yang et al., 2020a). RAB combustion could release massive volumes of toxic gaseous products, which leads to serious environmental hazards (Xia et al., 2019a). Several studies on the gas emission behaviors of asphalt binders during mono-combustion have been conducted. Xia et al. (2019b) reported that the main gaseous products released from asphalt binder are hazardous to human health and the natural environments and that asphalt binder could be regarded as a hazardous material when exposed to high temperatures. Xu and Huang (2010a) found that the toxicity of gaseous pollutants during asphalt binder combustion could be closely related to the abundance of sulfur elements in these gaseous products. Restraining and eliminating the release of toxic gaseous products, especially sulfur-containing pollutants, during combustion is inevitable to realize the energy conversion of RAB. A few studies (Yang et al., 2020b; Sheng et al., 2020) have attempted to use inorganic additives to retard the release of hazardous sulfur-containing gases during asphalt combustion. However, these additives could delay the thermal transmission and, in turn, prevent the energy conversion of asphalt.
Biomass combustion is an attractive energy-generating option on account of its renewability, CO2 neutrality and low gaseous pollutant emissions (Guo and Zhong, 2018a; Tomsej et al., 2017; Namkung et al., 2018). As one of the major source of biomass, wood sawdust (WS) can be considered as an ideal alternative energy and has promising prospects for energy generation (Vassilev et al., 2013, 2014). Unfortunately, the development of WS mono-combustion has been restricted to a certain extent because of the low bulk density (Duan et al., 2013). Refuse-derived fuel (RDF) technology has been widely adopted by developed countries for the energy conversion of biomass to overcome the problem of bulk density and calorific value (Jiang et al., 2014; Guo and Zhong, 2018b). Asphalt binder possesses an excellent adhesive properties toward powders (Long et al., 2020), hence it can be regarded as a WS binder in the RDF pelletization process. On the other hand, WS shows prominent ability to retain sulfur-containing gases during combustion in-situ (Ren et al., 2017). Therefore, co-processing of RAB and WS via the RDF method may be a promising technology that could offset the shortcomings of each material. However, to the best of our knowledge, no research has yet studied the thermal degradation behavior and combustion kinetics of RDF blended with RAB-WS. The evolution of sulfur-containing gases and related influence mechanisms during the co-combustion of RAB and WS, which plays a critical role in energy conversion, also remain unknown.
In this work, thermogravimetry combined with mass spectrometry (TG-MS) method was employed to study the combustion characteristics, the evolution of gaseous sulfur-containing products and interactions of RAB and WS during combustion. The novel insights into the kinetic process and in-situ sulfur retention mechanism were also provided.
Section snippets
Materials
Reclaimed asphalt pavement (RAP) materials were obtained from FuYin highway, Hubei, China. Reclaimed asphalt binder (RAB) in the RAP materials was extracted in accordance with ASTM D2172 method and then recovered using ASTM D5404. Wood sawdust (WS) was collected from a local wood processing plant (Wuhan, China) and ground to powder with particle size below 200 μm. All of the raw materials were pre-dried at 105 °C and stored in a desiccator. The results of proximate and ultimate analysis of the
TG-DTG-DSC analysis
The TG-DTG curves of the pure materials (i.e., RAB and WS) at each of the heating rates tested are shown in Fig. 1. As the heating rate increased, the DTG peaks shifted toward higher temperatures because of hysteresis. Excessively high heating rates inhibited heat transfer from the surface of the sample to its interior (Xie et al., 2018). Overall, the TG-DTG curves of samples presented similar patterns at different heating rates (as shown in Fig. 1), therefore, the combustion process of
Conclusions
The combustion behavior of RAB-based RDF could be divided into four components reactions. WS addition inhibited the volatilization of saturates and aromatics, while promoted the degradation of resins and asphaltenes. The combustion characteristics of RAB were improved by WS introduction. Both E values calculated by the kinetics models (Starink and DAEM) indicated that WS addition accelerates the RAB combustion at all stages. Simultaneously, all sulfurous gases emissions (i.e., CH3SH, COS, SO2,
CRediT authorship contribution statement
Teng Wang: Data curation, Writing-Original draft preparation; Hao Rong: Conceptualization, Methodology, Software; Si Chen: Conceptualization, Methodology, Software; Yi Zhou: Visualization, Investigation; Jinping Li: Software, Validation; Yue Xiao: Supervision; Yongjie Xue: Writing-Reviewing and Editing.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgments
This research was supported by a grant from National Natural Science Foundation of China (No. 51908433) and Natural Science Foundation of Hubei province-China (No. 2019CFB236). The authors would also like to thank reviewers for commenting on this paper.
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