Article pubs.acs.org/EF
Kinetic Study on Non-isothermal Pyrolysis of Sucrose Biomass Chao Wang,† Binlin Dou,*,† Yongchen Song,† Haisheng Chen,*,‡ Mingjun Yang,† and Yujie Xu‡ †
School of Energy and Power Engineering, Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116023, People’s Republic of China ‡ Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ABSTRACT: In this study, the pyrolysis characteristics of sucrose biomass were investigated with a thermogravimetric analysis coupled with gas chromatography (TGA−GC) system at different temperatures and various heating rates. The gas products from sucrose pyrolysis were measured, and the results showed that the gas product consisted of a major amount of CO, CO2, H2, and CH4. The temperature and addition of CaO could greatly affect the gaseous product yields. The distinct phases in the pyrolysis process of sucrose biomass were discussed. An improved iterative Coats−Redfern method was used to evaluate non-isothermal kinetic parameters for sucrose pyrolysis with/without the addition of CaO in different phases, and the activation energies and pre-exponential factors were calculated by means of linear regressions. The calculated results showed that the model with an Avrami−Erofeev mechanism could accurately be used to predict the main phase as well as the other phases for the pyrolysis processes.
1. INTRODUCTION Biomass is known as a carrier of energy and a widely distributed form of renewable energy. To reduce the strong dependence of fossil-based fuels, various forms of biomass energy sources should be developed. Three major pathways turning biomass into energy should be direct combustion, biochemical method (fermentation and anaerobic digestion), and thermochemical method (gasification, liquefaction, and pyrolysis). One of the most commonly used conversion methods to transform biomass into energy is direct combustion, and this method could make use of the biomass heating value. Biochemical conversion can be classified as biochemical liquefaction and microbial gasification based on converting biomass into alcohols or oxygenated products by biological activity. When considering thermochemical technology, two basic approaches can be achieved: one is gasification of biomass and conversion to hydrocarbons, and another approach is to liquefy biomass. Among these conversion methods, pyrolysis classified to the thermochemical process is one of the most promising technically, which is known as a useful method for outputting high-consistency energy.1,2 The process of pyrolysis is composed of a series of complex reactions and pyrolysis characteristics depending upon the experimental conditions, such as the heating rate in non-isothermal operations and the final temperature in isothermal operations. Moreover, high temperature and long gas residence time are usually recommended to be the process conditions for maximizing the gas product yield in the pyrolysis process.3 Figure 1 shows a conceptual illustration of the sucrose pyrolysis process. In the case of actual reactions, it is difficult to distinguish secondary reactions from primary reactions because of the close coupling of these multiphase processes. Sucrose, as a common sweetener, is not only used in the food industry but also studied as a renewable material for energy. Johnson et al. have given the analysis for sucrose pyrolysis products in 1969.4 In their study, many compounds from sucrose pyrolysis have been identified, and they also pointed out that volatile products from sucrose pyrolysis and an aqueous acid− © XXXX American Chemical Society
Figure 1. Conceptual illustration of the sucrose pyrolysis process.
stannous chloride degradation of glucose were similar in composition. Tanksale et al. have investigated the pyrolysis of sucrose derived from sugar cane to produce hydrogen.5 The results showed that liquid-phase reforming of sucrose can effectively produce hydrogen at a low temperature and moderate pressure over Pt-, Pd-, and Ni-based catalysts. Saleh et al. have studied the syngas from sugar cane pyrolysis, and they reported that the addition of water to the biomass slowed the pyrolysis process and the produced gas amount decrease; this was caused by the existence of gaseous compound solution in the liquid phase.6 Dou et al. studied pyrolysis characteristics of sucrose biomass in a tubular reactor at different temperatures.7 As they reported, the temperature could greatly affect the product yields and the presence of CaO can increase the conversion of sucrose by capturing the CO2 produced. The protential advantages of the gasification-enhanced process by a CO2 adsorbent (CaO) additive are the possibility to reduce tars in the syngas, absorbing released CO2 from the pyrolysis to enhance the hydrogen production, and providing a certain amount of energy for the endothermic pyrolysis by the carbonation reaction.8 CaO is also recognized as a tar-cracking Received: January 5, 2014 Revised: May 22, 2014
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dx.doi.org/10.1021/ef500940q | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
catalyst in the biomass gasification process. Tar reduction in sucrose pyrolysis would be able to significantly prevent the decrease of system efficiency as well as overcome the setbacks of the blocking problem for downstream devices. The usage of lowcost natural minerals, such as limestone and dolomite, has been of interest.9−11 One of the objectives of the study is to observe the catalysis effect of CaO on sucrose pyrolysis. In previous studies, the distributed activation energy model (DAEM) was proposed to analyze the pyrolysis reactions for estimating kinetic parameters, such as the activation energy and pre-exponential factor.12−14 Different from those, an improved version of the Coats−Redfern method was applied in this study. The kinetic parameters were evaluated iteratively by linear regression for different phases in the non-isothermal pyrolysis process. In this study, sucrose was chosen as the biomass material and CaO was chosen as the sorbent/catalyst. The pyrolysis of sucrose with and without CaO was investigated by thermogravimetric analysis coupled with gas chromatography (TGA−GC). The weight loss of the sample with the temperature was recorded automatically, and the main gaseous products of sucrose pyrolysis were swept out of thermogravimetry (TG) by carrier gas and then detected by GC. An improved iterative Coats−Redfern method was used for non-isothermal kinetic parameter evaluation of sucrose pyrolysis in different phases.
2. EXPERIMENTAL SECTION 2.1. Materials. The sucrose biomass purchased from Shengyang Xinxing Chemicals Company, China, was extracted as ordinary sugar mainly from sugar cane and sugar beets. The particle size of each sucrose sample in this study was about 0.5−1.2 mm. Its calorific value was calculated using the Dulongs equation, shown as follows:7,15 LHV = HHV − 600(9H + W )
Figure 2. Schematic diagram of the experimental TGA−GC apparatus: (1) N2 cylinder, (2) pressure gauge, (3) rotameter, (4) thermocouple, (5) weighing chamber with a microbalance, (6) furnace, (7) analyzer, (8) condenser and silica gel, and (9) gas chromatography.
(1)
temperature to a desired temperature at which the weight loss of the sample does not occur, and then the system was cooled to room temperature after each run. For different heating rate experiments, other experimental conditions, except heating rates, are strictly controlled to achieve the conditions for each run as consistent as possible. The gaseous components in the pyrolysis products were analyzed for different temperatures. For liquid and solid components, the reaction products were analyzed by weighing. The gas yield was obtained by gas yield = 1 − total liquid yield − total solid yield.
where
⎛ O⎞ HHV = 8100C + 34000⎜H − ⎟ + 2500S ⎝ 8⎠
(2) 7
The composition analysis was published in the previous work, and those could be briefly described as follows: sucrose accounts for more than 98.0 wt %; sulfate is 0.002 wt %; chloride is 0.005 wt %; calcium is 0.001 wt %; and others are 0.003 wt %, with the calculated lower heating value (LHV) of 5718.2 kJ/mol. CaO was prepared by pelletizing the CaO substance from the catalyst industry, and it mainly consisted of a higher than 96 wt % CaO compound based on the data provided by the manufacturer. Other compounds listed were CaCO3 (
