Study on the Effects of the Pyrolysis Atmosphere on the Elemental Mercury Adsorption Characteristics and Mechanism of Biomass Char

Abstract

Biochars prepared in three pyrolysis atmospheres of N2, O2, and CO2 were investigated. The pyrolysis characteristics, pore structures, and functional groups of the biochars were characterized by thermogravimetric analysis (TGA), specific surface area and porosity analyses, and Fourier transform infrared (FTIR) spectroscopy. The adsorption mechanism was further explored by the temperature-programmed desorption (TPD) technique in combination with the adsorption kinetics. The results showed that the Hg0 adsorption capacities of the biochars produced in an O2 pyrolysis atmosphere were worse than those of the biochars obtained in a N2 atmosphere, whereas CO2 significantly promoted the adsorption capacity. The biochar prepared in 20% CO2 had the highest adsorption capacity. Three pyrolysis reaction pathways were possible in the O2 atmosphere. In addition, a critical concentration existed between O2 concentrations of 5 and 7%. Above 750 °C, CO2 could react directly with the biochar through the Boudouard reaction. Mercury adsorbed to the surface of the biochar in various ways in the mercury adsorption process and thus was attached to the surface of the biochar in several forms. The products of chemical adsorption were Hg-OM and HgO, and a “secondary adsorption layer” was proposed to form in the mechanism. In the process of mercury adsorption by the biochar, a small part of the Hg0 chemically adsorbed to the active sites through their functional groups, which were mainly carbonyl and carboxyl groups. At the same time, the formation of a monolayer or multilayer “secondary adsorption layer” occurred, and the remaining Hg0 could further adsorb to this “secondary adsorption layer”.

Recent Developments in Understanding Biochar’s Physical–Chemistry

“…The results from Guizani et al [117] have been confirmed by Fan et al [118], Valdés et al [119], and Luo et al [120], whereas only partially by Molenda [121]. In particular, the latter reported that "biocarbon obtained in an atmosphere of carbon dioxide is characterized by a higher proportion of oxygen compared to the product produced in a nitrogen atmosphere".…”

“…These characteristics have been related to the better adsorption properties for organic and inorganic contaminants as compared to the same materials obtained in an N 2 atmosphere [118]. The ameliorated adsorption properties of the biochars obtained in a CO 2 atmosphere may also suggest a better nutrient exchange ability when they are applied to soils as amendments.…”

“…The differences among these studies lay in the different pyrolysis modes used to perform the CO 2 -driven thermal degradations. Fast pyrolysis was applied in Guizani et al [117], Fan et al [118], Valdés et al [119], and Luo et al [120], whereas slow pyrolysis followed by slow biochar cooling to room temperature was used in Molenda [121]. It is conceivable that the slow pyrolysis conditions may have avoided removal of the oxygenated functional groups (revealed by means of the infrared spectroscopy) laying on the surface of the biomass feedstock.…”

Kinetic study on the CO2 gasification of biochar derived from Miscanthus at different processing conditions

“…The ordering degree of biochar obtained under the four atmospheres from high to low was N 2 , CO 2 , CO and H 2 . Fan et al [10] analysed the walnut shell derived char prepared in N 2 , O 2 and CO 2 atmospheres. It was found that CO 2 had an activation effect, which resulted in the product having very high specific area and adsorption capacity.…”

Study on Adsorption Mechanism and Failure Characteristics of CO2 Adsorption by Potassium-Based Adsorbents with Different Supports

“…The results are shown in Table 3. When studying the effect of the pore structure on the ability of the adsorbent to adsorb CO 2 , the specific surface area per unit volume Z is introduced to characterize its pore richness [25,26], which is represented by Formula (3) below. normalZ=trueS0V0where S 0 is the BET specific surface area of the adsorbent, m 2 /g.…”