Article pubs.acs.org/EF
Gasification Reactivity and Pore Structure Development: Effect of Intermittent Addition of Steam on Increasing Reactivity of PKS Biochar with CO2 Guozhang Chang, Jianjun Xie,* Yanqin Huang, Huacai Liu, Xiuli Yin, and Chuangzhi Wu Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China S Supporting Information *
ABSTRACT: Intermittent addition of steam was employed to increase the gasification reactivity of palm kernel shell biochar with CO2. The reactivity and variations in pore structures were initially assessed during CO2 and H2O-assisted gasification of biochars in a tube furnace, followed by characterization using thermogravimetric and surface area analysis. A quadratic orthogonal rotation regression combination design was used to investigate the effects of intermittent H2O addition on the total reaction time (t100%) of CO2 gasification. The achieving results showed that the formation of micropores with sizes of 0.3 to 1.5 nm was favored by CO2 gasification, while the reactivity of biochar was highly correlated with the surface area of micropores of 0.93 to 1.47 nm. A pore expansion effect was the primary phenomenon observed during H2O gasification, while the reactivity of biochar with H2O was closely related to the surface area of pores with specific sizes. CO2 and H2O react with the biochar on separate active sites, and micropores of 0.93−1.54 nm are produced in the early stage of H2O gasification, which enhances CO2 gasification. The intermittent addition of H2O increases the reactivity of biochar with CO2, such that the t100% value during CO2/intermittent H2O gasification is 31.89 and 15.80% lower than the values associated with using only CO2 or a simple mixture of CO2 and H2O. interactions during CO2/H2O gasification.1,15−23 Zhang et al.1 showed that the reactivity of char with CO2/H2O increased after a decreased stage as the increasing CO2 concentration using a pressurized thermogravimetric analyzer system. They believed that the reaction of char with H2O was habited by the reaction of char with CO2. But there was no CO2 inhibition effect to be observed from the results of Jayaraman et al. and Guizani et al.15,16 Other studies revealed that H2O gasification of coal char occurred independently of CO2 gasification based on data from thermogravimetric analysis (TGA) and a fluidized bed gasifier. It was observed that the gasification reactivity of char with CO2/H2O was obviously lower than the sum of that of each single agent.17 Results of Bai et al.18 suggested that char reactivity under a CO2/H2O atmosphere is higher than the sum of the individual reactivities below 900 °C, representing a synergistic effect between the two gasification agents. Butterman et al.19 demonstrated a carbon conversion of 100% under a 25% CO2/75% H2O atmosphere, and found that the conversion dropped to 90% under a 100% H2O atmosphere over the same gasification time. In total, it is uncertain about the mechanism of CO2/H2O gasification, and whether CO2 and H2O gasification occur at the same active sites or at separate sites in biochar or coal char at present.20−23 Efforts have been made by researchers to explain this argument. Feng et al.24 reported that all pores of coal chars during CO2 gasification increased in specific surface area and pore volume as the increasing carbon conversion, and the
1. INTRODUCTION Biochar gasification, which proceeds at a relatively slow rate relative to devolatilization, determines the carbon conversion ratio of biomass.1 Compared with more commonly used gasification agents such as H2O and air, CO2 tends to generate the lowest rate with biochar.2 In addition, excess CO2 generated in the oxidation zone tends to overflow into the reduction zone of a gasifier in practice. For these reasons, studying the CO2based gasification of biochar is a significant step in advancing the theory and application of biomass gasification. There have been many reports of the gasification reactivities of biochars or coal chars with CO2, and the gasification reaction rates have been shown to be affected by various factors, such as the gasification temperature, the CO2 partial pressure, and the composition and carbonaceous geometry of the char.3,4 With regard to pyrolysis conditions, it has been shown that the CO2 gasification reactivity is enhanced with increases in the heating rate, pyrolysis temperature, residence time, and pressure.5−7 Decreasing reactivity is primarily attributed to the formation of a crystal lattice on the biochar surface.8 The pore structure (including pore volume and surface area) has been considered as a significant factor affecting biochar reactivity during CO2 gasification,9 and the gasification rate is closely related to the rate of pore diffusion at high temperatures when using larger particle sizes.10 The effects of catalysts on biochar gasification have also been assessed,3,11 while other researchers have attempted to increase CO2 gasification rates using microwave heating and torrefaction.12,13 The addition of steam is effective at promoting the gasification reactivity of biochar in conjunction with CO2,14 and numerous studies have investigated the mechanism and © XXXX American Chemical Society
Received: November 2, 2016 Revised: January 27, 2017 Published: February 1, 2017 A
DOI: 10.1021/acs.energyfuels.6b02859 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
surface area (SSA) values were calculated using the BET equation, and the external surface areas, micropore areas, and micropore volumes were determined by the t-plot method, using t values ranging from 0.2 to 0.5 of the relative pressure. Pore size distributions and pore volumes were calculated according to the quenched solid density function theory (QSDFT) method.28 2.2.2. Gasification Reactivity. Biochar gasification experiments were conducted with a thermogravimetric analyzer (STA449 F3, Netzsch, Germany). In each test, approximately 8 mg of biochar was loaded into an alumina crucible and subsequently heated from 25 to 850 °C at 5 °C/min under a N2 atmosphere (99.999%, 60 mL/min). At 850 °C, the N2 was switched to high purity CO2 (120 mL/min) or H2O (0.083 g/min), and these conditions were maintained for 1.5 h. After gasification, N2 was again supplied to protect the instrument. The carbon conversion ratio (x) of each biochar was calculated as follows:29
reaction rate in the earlier stage (
