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.