1. Introduction
Coal consumption has been increased from 2830 million tons to 2934 million tons in China in 2021.1 The clean and efficient utilization of coal thus becomes significant for the country to achieve the strategic goal of “Emission peak and carbon neutralization”. Nevertheless, in the clean transformation of coal, coal gas, as an indispensable raw material and power source, corrodes equipment and causes catalyst poisoning in subsequent processes, for the presence of H2S in it.2 Therefore, removal of H2S is an important prerequisite for the clean utilization of coal.3
Among the existed techniques of H2S removal, dry desulfurization, compared with wet desulfurization, is widely employed in the situations at medium and high temperatures, which not only makes full use of the sensible heat of coal gas, but also reduces the consumption for raw coal in coal-fired system.4 High temperature coal gas desulfurization is realized through the reaction between single or composited metal oxides and H2S with metal sulfide as products. Theoretically, the more active components the better sulfur capacity of desulfurization sorbent.5,6However, previous studies reveal that the pure metal oxides own a poor sulfur capacity due to the aggregated oxide grains and undesirable plugged structures, resulting in insufficient utilization of active components.7,8 In order to solve this problem, porous supporters such as activated carbon, zeolites, γ-Al2O3 and molecular sieve have been introduced into the preparation of sorbent.9-11 Liu et al. synthesized desulfurization sorbent with highly dispersion of active components by combining porous MAS-9 and Ca-based oxides.12 It was found that the sorbent exhibited a sulfur capacity of 17.16 g S 100 g-1 sorbent. Mi et al. applied MCM-41 as porous supporter to promote the desulfurization performance of ZnO-based sorbent.13 They achieved a sulfur capacity of 5.7 g S 100 g-1 sorbent due to the uniform distribution of ZnO nanophase on MCM-41. However, the utilization rate of the ZnO/MCM-41 sorbent is low (57%), which differs greatly from the theoretical value. The key to removing hydrogen sulfide is the replacement process of O2- (radius 0.140 nm) in metal oxide and S2- (radius 0.184 nm) in hydrogen sulfide, but the greater molecular volume of metal sulfide than that of the metal oxide is bound to occupy a larger space.14While within the sorbent preparation with porous solid supporters, the active components are expected to scatter in the internal surface of channels, the induced volume expansion of desulfurization products is likely to fill and plug the inner channel of supporters, which happens in the “pore closing” process, finally limiting the mass transfer and ion diffusion of the reaction. Besides, though the rich channels of meso-supporter contribute high surface area, they are also easily to be plugged due to the limited pore size, which cannot meet the requirements for desulfurization applications that need massive reaction agent.
To overcome the above challenges, carbon nanofibers (CNFs) with large specific surface area and opening three-dimensional structure have been proposed into preparing desulfurization sorbent.15,16For example, Bajaj et al. investigated the synergetic effect of activated carbon nanofibers (ACNFs) and CuxO nanoparticles for coal gas desulfurization. They found that ACNFs as supporting matrix play an important role in preventing aggregation of nanoparticles.17 Similar results have also been obtained by Kim et al., where the CNFs-supported sorbent showed 3 times higher in ZnO utilization efficiency compared to pure ZnO nanopowders.18 However, the loading content and dispersion of active components are heavily influenced by the limited surface area of CNFs. Besides, the preparation strategy applied lowers the utilization of active components due to the fact that metal ions are encapsulated by CNFs. By learning from these strategies and the opening three-dimensional structure of CNFs, we hypothesize that an enlarged surface area for desulfurization sorbent by introducing the CNFs supporter with high specific surface area and porous structural features with ease gas diffusion performance, which are beneficial for the uniform distribution of massive active components and increase its loading content (Figure. 1a ). More important, CNFs are conducive to the avoidance of the negative effects arising from the volume expansion of the desulfurization products based on its opening structure and flexibility. Upon the hypothesis, the CNFs materials with enlarged surface are expected to provide a much wider supporter area for active components and thus achieve a highly dispersed distribution, which can promote the loading content and utilization of desulfurization sorbent.
In this work, a surface area enlarged structure of Zn@MnOx /porous CNFs (PCNFs) desulfurization sorbent with highly dispersed active components is rationally constructed by electrospinning, carbonization, etching, oxidation activation and hydrothermal treatments, which facilitates improving the loading content and utilization of active component. PCNFs were obtained by taking SiO2 as sacrifice phase after the co-electrospinning of polymers and SiO2. The PCNFs are effective in increasing specific surface area, which is beneficial for keeping active components distributed in small grain size. Manganese oxides derived from oxidation activation by KMnO4 not only serve as active components for desulfurization, but also as acts as seeds for the growth of ZnO on PCNFs. Benefited from our rational structure design, the desulfurization performances of the as-prepared sorbents tested with a fixed-bed reactor at 500 °C achieves 9.63 g S 100 g-1 sorbent, with an overall utilization rate of active components up to 73%. It is worth noted that the utilization rate of ZnO reached up to 117% at the breakthrough concentration of 900 ppm.