Experimental synthesis and performance comparison analysis of high-efficiency wetting enhancers for coal seam water injection

Abstract

To improve the wettability effect of coal seam water injection, and reduce dust production from the source, this paper proposes the preparation of three kinds of high-efficiency wetting enhancers. In this study, 2-acrylamide-2-methylpropionic sulfonic acid and itaconic acid were introduced into the polymer chain of sodium alginate in different combinations. Anionic surfactant sodium dodecyl sulfate and nonionic surfactant aliphatic alcohol polyoxyethylene ether were added in different proportions to obtain three kinds of enhancers. The three kinds of enhancers have good viscosity and fluidity, and the optimal emulsifier additive is determined by a surface tension test. The hydrophilic hydroxyl, carboxyl or sulfonic groups of enhancers were obtained by a Fourier-transform infrared experiment, which was beneficial to water absorption. The contact effect of different solutions on coal block was studied by a scanning electron microscopy experiment. An energy dispersive spectrometry experiment proved that the solution was well dispersed. Through reverse osmosis experiment and contact angle analysis, the wettability of different solutions was compared, and the best enhancers was obtained. The molecular dynamics simulation was used to verify the relationship between them, and finally the mechanism of action was deduced. It was proved that AMPS-IA/SA+SDS/AEO had better wettability. Thus, AMPS-IA/SA+SDS/AEO is an ideal enhancer for coal seam water injection.

Introduction

During coal mining, a great deal of dust is produced (Zhang et al., 2018; Li et al., 2020; Yu et al., 2018). The harm caused by dust is extremely serious (Han et al., 2020a; Zhou et al., 2020a; Hu et al., 2016a; Fan et al., 2018; Zhou et al., 2018a; Liu et al., 2021). Dust not only endangers human health and causes pneumoconiosis, but also damages mining equipment, reducing its service life, and ultimately causes the loss of manpower, material and financial resources (Zhou et al., 2017a; Liu et al., 2020a; Han et al., 2020b). According to the latest data, there are more than 1.2 million severe pneumoconiosis patients in China, accounting for more than half the world's pneumoconiosis patients, and more than 4000 people die each year, resulting in yearly direct losses of more than RMB 8 billion Yuan. Among all pneumoconiosis caused by various kinds of dust, coal miners account for the highest proportion (Sun et al., 2020). Therefore, reducing coal dust production is the top priority (Hu et al., 2015; Feng et al., 2018). Wang et al. (Wang et al., 2019a) studied a multi nozzle spray that is conducive to reducing dust. Ma et al. (Ma et al. (2018a)) prepared an agglomeration-cementing agent for dust suppression in open pit coal mining. In the study of the dust-proof effect of wall attached swirl ventilation, Wang et al. (Wang et al. (2019b)) found that wall attached swirl ventilation can effectively reduce dust content. The dust-control effect of wall swirling ventilation depended on not only the axial air curtain but also the radial air curtain. In addition, in all dust-control methods, coal seam water injection can reduce dust production from the source (Liu et al., 2018). Wang et al. (Wang et al. (2017)) built a complete coal model using five typical coal models to simulate coal seam wetting based on two-dimensional particle flow code. It is found that the infiltration of water can wet the coal seam and reduce dust at the source. According to the mining conditions at Panel 9801 of the Yangquan coalfield, Hu et al. (Hu et al. (2016b)) designed a water injection system through the inseam gas drainage boreholes to control dust resulting from the longwall face.

Although water injection can wet the coal seam and reduce dust production, the conventional water injection method has the shortcomings of difficult water injection and low wetting efficiency, so the research and preparation of enhancers for coal seam water injection have grown in importance (Ni et al., 2019; Lu et al., 2019). A new dust prevention technology of surfactant magnetized was designed and developed by Qin et al. The technology utilized the synergy between magnetization and surfactants to markedly improve the wettability of water (Zhou et al., 2017b, Zhou et al., 2018b). In addition, Qin et al. also found that the coupling magnetic field formed by the pulsing magnetic field and spiral vortex could better improve the solution wettability (Zhou et al., 2019). Xie et al. studied the effect of surfactant synergistic compound acidification on coal wetting characteristics. The results showed that the surfactant sodium dodecyl sulfate (SDS) intensified the substitution reaction of the benzene ring structure of coal by composite acidification, which was more conducive to the wetting and trapping of coal dust (Xie et al., 2020). Xi et al. investigated the effect of ionic liquids on coal structure and wettability and concluded that ionic liquid [HoEtMIm][NTF₂] showed the best wetting effect on coal as evidenced by the smallest contact angle for the treated coal with [HoEtMIm][NTF₂] (Xi et al., 2019). This indicated that ionic liquid can be used as a good wetting agent. Wang et al. used inorganic salt additives to improve the wettability performance of surfactants. It was found that inorganic salt additive can improve the wettability performance of the surfactant to a certain extent and improved the reverse osmosis moisture absorption of coal dust (Wang et al., 2020). To improve wetting characteristics of the mixture, Wang et al. introduced an ionic liquid as an auxiliary agent and synthesized wetting agent A. They proved that wetting agent A is more effective than other wetting agents by the capillary rise method (Wang et al., 2019c). In the research of Li et al., it was also mentioned that the application of surfactant can effectively enhance the water wettability of a coal matrix (Li et al., 2018). Wang et al. compared the wettability of various surfactants. The experimental results show that surfactant can improve the wetting ability of water to coal dust with different metamorphic degrees, especially anionic surfactant (Wang et al., 2019d). Wang et al. prepared a lubricant, which is a mixture of alkanes, cycloalkanes, aromatics and other hydrocarbons. The experimental results showed that the lubricant has high wettability on coal, and the average contact angle is 20 times smaller than that of water (Wang et al., 2019e). In conclusion, wetting enhancer has broad application prospects in wetting coal and reducing dust content. Therefore, based on the above research, this paper is committed to improving the wettability enhancer. Based on the synergistic effect of surfactants, chemical graft copolymerization technology is introduced to the preparation of enhancer (Gebru and Das, 2018; Zhou et al., 2017c, 2020b).

Graft copolymerization is a common method for modifying molecular structures in the field of chemistry (Li et al., 2019). Because of its simple and effective advantages in improving the properties of polymer materials, it is widely used in various fields. Das et al. (Das et al. (2019)) used graft copolymerization technology to modify starch to obtain a flocculant. Jalababu et al. (Jalababu et al. (2019)) grafted amino acids onto guar gum. Krishnappa et al. (Krishnappa and Badalamoole, 2019) prepared an efficient adsorbent for removal of ionic dyes from water by grafting with poly(2-(two methylamine) ethyl methacrylate) gel and Karaya gum-graft-poly(2-(dimethylamino) ethyl methacrylate) gel. Therefore, chemical graft copolymerization technology has matured and can be applied to the modification of the permeability enhancer.

In this study, sodium alginate (SA), which has a high polymer chain, was selected after a comparison of material properties, as shown in Fig. 1. SA is a natural polysaccharide compound with a linear macromolecule structure and exists widely in marine algae. In addition, due to its properties of safety and nontoxicity, SA has been widely used in the fields of food (Yang et al., 2019; Ruan et al., 2019; Yuan et al., 2019), medicine (Neamat‐Allah et al., 2019) and so on (Zhu et al., 2019). Its hydrophilicity, stickiness and stability are very good, because SA’s molecular structure has abundant hydroxyl −OH and carboxyl −COOH. Thus, SA was selected as the raw material.

Therefore, in this study, SA polymer chains were modified by graft copolymerization technology to improve their water absorption based on controlling SA viscosity. Sodium dodecyl sulfate and primary alcobol ethoxylate were added to explore the synergistic effect of the two surfactants. In addition, through contact angle and reverse osmosis experiments, the mechanism of interaction between surfactant, polymer chain and water molecules and the method of permeating the coal seam were analyzed (Xie et al., 2019; Wu et al., 2019). Finally, the molecular dynamics simulation was used to verify the interaction relationship of various substances. And action mechanism of enhancer was deduced.