Article pubs.acs.org/EF
Examination of Kinetics of Non-catalytic Steam Gasification of Biomass/Lignite Chars and Its Relationship with the Variation of the Pore Structure Shinji Kudo,*,† Yasuyo Hachiyama,† Hyun-Seok Kim,† Koyo Norinaga,† and Jun-ichiro Hayashi†,‡ †
Institute for Materials Chemistry and Engineering, and ‡Research and Education Center of Carbon Resources, Kyushu University, 6-1 Kasuga Koen, Kasuga 816-8580, Japan S Supporting Information *
ABSTRACT: There have been considerable studies on char gasification under catalysis of inherent or extraneous metallic species, while underlying non-catalytic gasification has not necessarily received attention, despite the importance to clarify the intrinsic kinetics and its correlation with the structural parameters, such as surface area. The present authors investigated the noncatalytic steam gasification of chars from the pyrolysis of seven types of biomass and lignite. The chars, which were nearly free from catalytic metals, showed reactivities similar to one another with the activation energy within a range from 228 to 241 kJ mol−1. The specific rates of reaction were steady over the entire range of the conversion. In contrast, the specific surface areas of the chars increased with the conversion from around 500 to even beyond 2000 m2 g−1. The results demonstrated that the chars underwent the gasification obeying the first-order kinetics, which was independent of the specific surface area.
1. INTRODUCTION In steam gasification of carbonaceous solid fuel, the gasification of char formed in situ from pyrolysis of the fuel is the ratedetermining step. The kinetics of gasification is therefore of essential importance in the design and operation of the gasifier. Biomass and lignite are attractive as feedstocks in light of the renewable nature and large reserves, respectively. The gasification of biomass and lignite chars often proceeds under catalysis of inherent metals, alkali and alkaline earth metallic (AAEM) species in particular. Their catalysis significantly increases the rate of gasification, which complicates its kinetics. The catalysis of AAEM species varies not only with the content and physicochemical nature but also with the char conversion as well as operating conditions.1,2 Kinetics of gasification needs to be established with a proper model that considers the behavior of catalytic metals. An approach for the comprehensive understanding of the catalysis is to distinguish catalytic gasification from non-catalytic gasification assuming progress of the two different types of gasification in parallel.3−5 The most reasonable way to quantify the contribution of the catalytic gasification to the overall gasification is to remove the catalytic species from the char and then measure the kinetics of non-catalytic gasification. The char gasification has typically been recognized as a heterogeneous gas−solid reaction, the rate of which changes in proportion to the physical or reactive surface area of the char. Theoretical models of gasification developed thus far, including a commonly used random pore model,6 are mostly on the basis of this idea. Nevertheless, there is still uncertainty whether the surface area is really a factor determining the rate of gasification.3,7,8 Employing the active surface area (ASA), which is generally measured by chemisorption of CO2 or O2, is an option to represent the reacting surface in the heterogeneous system. A previous study reported steady ratios of the surface area to the ASA.7 However, it seems that the above idea has not © XXXX American Chemical Society
necessarily been established with direct evidence but rather a priori. Simultaneous progress of the catalytic and non-catalytic reactions is a feature of gasification and makes it difficult to know the relationship between the kinetics and the surface area. Investigation of the non-catalytic gasification is therefore an effective way to know such a relationship. There have been a few studies on the non-catalytic gasification with different combinations of fuel and gasifying agent,2,4,5,7,9−23 although not exclusive to the ones with the above purpose. There are two methods of char preparation to investigate the non-catalytic gasification: (1) removal of catalytic species by acid washing of char from the pyrolysis and (2) pyrolysis of fuel, of which inherent catalytic species have been removed by acid washing or other methods. The former method enables us to know the kinetics of the noncatalytic gasification and to analyze the catalytic gasification by comparing the kinetics of the chars before and after the acid washing. A report24 mentioned that the kinetics of non-catalytic steam gasification of a char was described by assuming a first-order reaction with respect to carbon (X, char conversion to gas). dX = k(1 − X ) dt
(1)
However, this model was unsuccessful in many cases,10−15,17,21,22 especially when the chars originated from coal ranked higher than or equivalent to sub-bituminous. The rate of non-catalytic gasification for those chars with any reactant gas typically increases in an early stage of the conversion and then decreases.15,22 Similar rate profiles are reported for the Received: July 5, 2014 Revised: August 11, 2014
A
dx.doi.org/10.1021/ef501517n | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 1. C, H, N, Ash, and AAEM Species Contents of Char and A-Char (in Parentheses) MC
PC
0.3 (0.2)
0.5 (0.2)
C H N
89.4 (88.5) 2.1 (2.0) 0.1 (0.2)
89.3 (88.8) 2.2 (2.1) 0.1 (0.2)
K Mg Ca
