Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials.

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Encapsulation of Silicon Nano Powders via Electrospinning as Lithium Ion Battery Anode Materials.

Silicon-containing polyester from tetramethoxysilane, ethylene glycol, and o-Phthalic anhydride were used as encapsulating materials for silicon nano powders (SiNP) via electrospinning, with Polyacrylonitrile (PAN) as spinning additives. In the correct quantities, SiNP could be well encapsulated in nano fibers (200-400 nm) using scanning electron microscopy (SEM). The encapsulating materials were then carbonized to a Si-O-C material at 755 °C (Si@C-SiNF-5 and Si@C-SiNF-10, with different SiNP content). Fiber structure and SiNP crystalline structure were reserved even after high-temperature treatment, as SEM and X-ray diffraction (XRD) verified. When used as lithium ion battery (LIB) anode materials, the cycling stability of SiNPs increased after encapsulation. The capacity of SiNPs decreased to ~10 mAh/g within 30 cycles, while those from Si@C-SiNF-5 and Si@C-SiNF-10 remained over 500 mAh/g at the 30th cycle. We also found that adequate SiNP content is necessary for good encapsulation and better cycling stability. In the anode from Si@C-SiNF-10 in which SiNPs were not well encapsulated, fibers were broken and pulverized as SEM confirmed; thus, its cycling stability is poorer than that from Si@C-SiNF-5.

Glucose hydrothermal encapsulation of carbonized silicone polyester to prepare anode materials for lithium batteries with improved cycle stability.

A silicon polyester (Si-PET) was synthesized with ethylene glycol and phthalic anhydride, and then it was carbonized and hydrothermally coated with glucose. The formed SiO x with layered graphene as the 3D network had an amorphous carbon layer. The graphene oxide (rGO) after carbothermal reduction was completely retained in SiO x , which improved the conductivity of the SiO x anode material. SiO x were encapsulated with a flexible amorphous carbon layer on the surface, which can not only improve the electrical performance, but also effectively relieve the huge volume changes of the compound. Further, the key point is that, the solid electrolyte interphase (SEI) film was mainly formed on the surface carbon layer. This would keep a stable SEI film during volume pulverization, and result in a good cycle stability. The SiO x /C-rGO material maintained a reversible capacity of 660 mA h g-1 at a current density of 100 mA g-1 for 100 cycles, a reversible capacity of 469.7 mA h g-1 at a current density of 200 mA g-1 for 300 cycles. The Coulomb efficiency was maintained at 98% except for the first cycle. After long cycling, the electrode expansion was 16%, which was much lower than those of silicon based materials. Therefore, this article provides a cheap, simple, and commercially valuable anode material for lithium batteries.

Status and development for detection and control of ammonium bisulfate as a by-product of SCR denitrification.

When denitrification technology using NH3 or urea as the reducing agent is applied to remove NOx from the flue gas, ammonium bisulfate (ABS) by-product will also be generated in the flue gas. ABS has an impact on catalyst life span, denitrification efficiency etc., air preheater and its downstream thermal equipment also have a significant negative impact due to its plugging and corrosion. The requirement for NOx removal efficiency is improved by ultra-low emissions in China. However, wide-load denitrification makes the flue gas composition and temperature changing more complicated. Increasing ammonia injection can improve the NOx removal effect, but too much ammonia injection will lead to the formation of ABS and the increase of deposition risk, the contradiction between these two aspects is amplified by ultra-low emissions and wide-load denitrification in many plants. Coordinating NOx control and reducing the impact of ABS on equipment are issues that the industry needs to solve urgently. In recent years, extensive research on ABS had been carried out deeply, consequently, there has been a relatively in-deepth knowledge foundation for ABS formation, formation temperature, deposition temperature, dew point temperature, decomposition behavior, etc., but the existing researches are insufficient to support the problem of ABS under full load denitrification completely resolved. Therefore, some analysis and detection methods related to ABS are reviewed in this paper, and the impact of ABS on SCR, air preheater and other equipment and the existing research results on reducing the impact of ABS are summarized also. It is hoped that this review will provide a reference for the industry to solve the problems of ABS that hinder wide-load denitrification and affect ultra-low emissions.

Scale-up experiments of SO2 removal and the promoting behavior of NO in moving beds at medium temperatures.

The dry flue gas desulfurization (FGD) method was studied, which is a part of the integrated removal of multi-pollutants at medium temperatures. Although dry flue gas treatment is a simple and effective method, it is still a highly empirical-led application technology. A superior desulfurization adsorbent, fine powder of NaHCO3 (hereinafter called fine NaHCO3), was selected by scale-up experiments. A deep understanding of the reaction process and mechanism is then explored, which helps the further optimization of dry desulfurization. Based on the multi-factor experiments for NaHCO3, the effect mechanism of NO on desulfurization using NaHCO3 is also proposed. The conversion of SO3 2- → SO4 2- is promoted by the existence of NO. Therefore, a slight decline can be found. According to the influences of the SO2 concentration and the residence time, it is concluded that the diffusion of SO2 into the channel of NaHCO3 is the rate-limiting step. Impressively, the reaction process of reactants was clearly studied by in situ FTIR spectroscopy to determine the whole process. Moreover, the recycling of NaHCO3 is the main direction for reducing adsorbent consumption in the next step. The predictable insights are beneficial for profoundly understanding the gas composition synergetic interaction for the SO2 removal by the dry treatment using NaHCO3.