Article pubs.acs.org/EF
Influence of Chemical Pretreatment on the Internal Structure and Reactivity of Pyrolysis Chars Produced from Sugar Cane Bagasse Marion Carrier,
†
Hein Wjp Neomagus,‡ Johann Görgens,
†
and Johannes H. Knoetze*,
†
†
Department of Process Engineering, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa School of Chemical and Minerals Engineering, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa
‡
ABSTRACT: The characterization of char obtained from the vacuum pyrolysis (8 kPaabs, 460 °C, 18 °C/min) of sugar cane bagasse followed by steam gasification (60 wt % of steam, 700−900 °C) was studied. The influence of different chemical pretreatments prior to pyrolysis (acetone, ethanol, HNO3 and HF) on the ash content, the pore structure, and chemical composition of resulting chars and activated carbons were investigated. Changes in the pore structure were observed during the pyrolysis process and the steam gasification, namely, the development of a channel network and the increase of meso- and macroporosity distributions, respectively. These porous changes affect the pyrolytic yields. Clear influences of the surface area and ash content on the pyrolytic water yields and the inorganic devolatilization effect on the meso- and macropore distributions of chars were shown. Furthermore, the structural reorganization of chars obtained during the vacuum pyrolysis of HF and solvent-leached bagasse was beneficial to their reactivity toward steam gasification.
1. INTRODUCTION The reactivity of char is a major parameter in steam gasification, as it affects its conversion, yield and nature of gaseous products consisting mainly of CO2, H2, CH4, CO and H2O. The gasification can generally be described by different processes, such as intrinsic chemical reaction kinetics between H2O and the carbon, internal diffusion of H2O in the char pores, and external diffusion of H2O to the particle.1 In the case of internal diffusion limitation, the distinction between the effects of ash content and the porosity of the char is a critical aspect.2 Indeed if the porosity does not allow the diffusion of the oxidizing agent (H2O) through the particle, the oxidant (H2O) cannot enter into contact with active sites (C). This development of pores can be affected by the presence of ash, which melts and blocks the entrance of the reactant. Also, the ash by its heterogeneous nature can play a catalytic role affecting the reaction regime.2 Some authors selected a series of alkali and alkaline earth metallic (AAEM) species (mainly K, Na, Mg, and Ca) and investigated their behavior and effect during pyrolysis and gasification processes. They concluded that their presence influenced the quality of pyrolysis and gasification products.3−8 These AAEM species tend to volatilize during pyrolysis and gasification/combustion and are an important consideration in all aspects of biomass thermochemical conversion.9,10 During the pyrolysis process, the inorganic K and Ca species can catalyze biomass conversion and char forming reactions.11,12 Again, K was responsible for additional water and gas formation, leading to a decrease in organic liquid yields.13 During the gasification process, the volatilized AAEM species may also act as catalysts for the steam reforming of volatiles in the gas phase or water−gas shift reaction.14,15 Dupont et al.16 pointed out the catalytic effect of inherent K and the inhibition effect of inherent Si on the steam gasification of biomass chars. The volatilization and catalytic effects of AAEM species on the pyrolysis and gasification of coal have been studied extensively,17,18 while the effects of the presence of the inherent © 2012 American Chemical Society
metals on the porous structure of solid materials from vacuum pyrolysis and steam gasification of plant biomass have received less attention.1,19 The ash can be removed from feedstocks by the application of different chemical pretreatments. For this study, different pretreatments prior to vacuum pyrolysis have been chosen with the first goal to control and determine the influence of the presence of inorganics on the yields of products and quality of chars produced from sugar cane bagasse, without significantly changing the chemical composition of the lignocellulosic feedstock and, at a later stage, to quantify the influence of these chemical pretreatments on the char reactivity toward steam gasification. The selection of the pretreatment processes was based on those reported in the literature, wherein leaching of biomass9,19,20 and coal/carbon21,22 deashing processes have been investigated. A HF pretreatment should be interesting because it is effective in reducing the ash content of lignocellulose material to a negligible amount, although it could result in slight changes in the chemical composition.19 Indeed, wet chemical methods are used to fractionate the lignocellulosic structure. Concentrated and diluted acid treatments aim to hydrolyze sugars, reducing the amount of cellulose.23 Ethanol and acetone extractions are known to remove extractives and to rearrange the orientation of cellulose fibers, thereby improving the yield of pyrolysis products.20 Both HNO3 and HF are known to dissolve metals.24 Vacuum pyrolysis is an effective process to prepare char with a high reactivity toward steam gasification.2 It produces a feedstock that is more suitable to activation with steam, compared to char produced at atmospheric conditions. The application of vacuum limits the presence of carbonaceous Received: March 23, 2012 Revised: June 15, 2012 Published: June 22, 2012 4497
dx.doi.org/10.1021/ef300500k | Energy Fuels 2012, 26, 4497−4506
Energy & Fuels
Article
Table 1. Inorganic Contents in Virgin Solvents and Acid Solutionsa,b
a
element (μg/L)
acetone
ethanol
HNO3 0.5N
HF 3%
distilled water
Ca K Na Mg Si Cu Fe Mn Co Ni Zn Al Cr
10 ± 1 na bd bd bd bd bd bd bd bd
