Experimental and modeling studies on CO2 gasification of biomass chars
Introduction
Fossil fuels (i.e., coal, natural gas, and petroleum, etc.) have played an important role in the past in transportation fuel supplies and will continue to do so. However, fossil fuels are not renewable and the declining reserves and the increasing demand for fossil fuels would cause great trouble in the future. The emissions of the greenhouse gases by burning fossil fuels have also brought a major environmental challenge [1]. Consequently, it is urgent to develop the alternative and sustainable energy technologies. Biomass is a kind of wide spread and renewable resource with high yield and carbon neutral property. In addition, during the utilization process, the emissions of sulfur oxide and nitrogen oxide are less than that of fossil fuel. All these properties make biomass resource have more advantages in environmental protection and social benefits. If biomass resource could be utilized in industry circle, it would contribute to the energy conservation and emission reduction [2], [3]. However, biomass resource also has some disadvantages like bad grindability, low density of energy and high content of moisture, which restrict its direct application in industrial field. Thereby, normally before utilization, the conversion process of biomass resource would be carried out, such as, combustion, gasification, carbonization, high temperature pyrolysis as well as alcoholic fermentation. etc. Among them, the gasification technology is the most promising one [4], [5], [6].
In general, biomass gasification process in the gasifier is very complex and includes water evaporation, volatiles pyrolysis, combustion, volatiles gasification and solid residue (char) with gasification agents. Generally, the rate of char gasification process is much lower than other processes, so it is usually considered as the rate-determining step in the overall conversion process [7]. Water has been widely used as gasification agent, but in recent years, due to the development of new energy technology, usage of water as gasification agent has reached its limit. However, as gasification agent, carbon dioxide has attracted more and more attention [8]. The advantage of using CO2 as a gasifying agent is to recycle the CO2 that is produced during the reduction processes and to convert it into a useful form of gas [9], [10]. As a result, investigation on the reaction behavior and kinetic parameters of CO2 gasification of biomass char under elevated temperature has an important impact on reactor design, control and efficiency [5], [11], [12].
CO2 gasification behaviors of Biomass char have been widely investigated in the past. Zeineb et al. [3] studied that the influences of textural, structural and chemical properties of biomass chars on the CO2 gasification rate. The results suggested that the gasification rate was shown to depend on the char external surface and the potassium content when the conversion was below 70%. At a higher conversion ratio, a satisfactory correlation between the Catalytic Index and the average gasification rate was identified. Okumura et al. [13] using Raman spectroscopy to investigate the influence of pyrolysis conditions on woody biomass char reactivity. It was shown that there was a correlation between the biomass char characteristics and the gasification rate. In particular, the relationship between the gasification reactivity and the carbonaceous uniformity structure have been noted. Wang et al. [14] studied the gasification reactivity of biomass chars and anthracite char with CO2 through the TGA method and various kinetic models such as the volumetric model (VM) [15], [16], the grain model (GM) [17], [18] and the random pore mode (RPM) [19]. The results show that RPM and VM models were the best for describing the reactivity of biomass char and the gasification process of anthracite char, respectively. Seo et al. [20] studied the gasification of biomass chars with carbon dioxide at different temperatures. Compared with the calculated results by VM, GM and RPM models, it was found that the experimental data agreed well with the RPM model. Fermoso et al. [21] studied the effect of the gasification temperature, pressure and CO2 concentration during gasification of biomass chars. It was found that among VM, GM and RPM models only the RPM model accurately predicted the conversion of different char except SPH14 which can be well fitted by the Langmuir-Hinshelwood model. Normally, RPM could well describe the gasification curve of char under CO2 atmosphere, but when peak reaction rate appears at high conversion range over 0.393, the discrepancy between experiment data and model calculation is significant. Therefore, based on RPM, many further investigations were carried out. Zhang et al. [22] evaluated a semi-empirical kinetic model to reconcile with gasification reactivity profiles of biomass chars. It was found that the fitting parameters introduced in the modified random pore model well predicted the amount of the active potassium (K) in biomass char. Zhang et al. [23] have developed a modified random pore model (MRPM), which could be reduced to a traditional VM model, GM model, hybrid model and RPM model by varying the model parameters.
In the previous studies on biomass char gasification, the analyses about relation between microstructure and macroscopic property, as well as reaction rate appears in high conversion range are not sufficient. In this study, the char gasification properties and kinetic behaviors of peanut shell (PS), maize cob (MC), wheat straw (WS), rice lemma (RL), pine sawdust (PSD) and bamboo sawdust (BS) were investigated by using the method of isothermal thermogravimetric analysis (TGA). Various factors including elementary composition, alkali index, surface area and carbonaceous structure were also analyzed systematically. VM, GM, RPM and MRPM models were used to describe the kinetics of CO2 gasification of biomass char at different temperatures.
Section snippets
Samples
Four types of wooden raw materials including peanut shell (PS), maize cob (MC), wheat straw (WS), rice lemma (RL), as well as two herbaceous raw materials including pine sawdust (PSD) and bamboo sawdust (BS) were collected from Yuzhou city, Henan province, China. The samples were cut to the sizes of 0.5–1 mm. The char was prepared by devolatilizing the raw materials in a fixed bed reactor under a flowing nitrogen atmosphere (4L/min) at 1373 K for 90 min to ensure the pyrolysis process was fully
Results and discussion
To assess the validations of selected kinetic models and predict the kinetic behaviors of the studied samples, the experimental data were fitted by various models. By plotting −ln(1−X), 3[1−(1−X)1/3], (2/ψ)[(1−ψln(1−X))1/2−1] as a function of time t, the slopes of curves at different reaction temperatures could be obtained to represent the reaction rate constants (kVM, kGM and kRPM) of the biomass gasified reaction process corresponding to the models of VM, GM and RPM, respectively. In the RPM
Conclusions
In this work, six different biomass chars were gasified in a thermobalance at atmospheric pressure with CO2. It was established that the char gasification reactions were carried out under chemical reaction control at all the studied temperatures. The best model for describing the gasification reaction of biomass char was found to be the MRPM. The activation energies derived from the MRPM for the biomass chars lie in the range of 132.84–163.75 kJ/mol, and dynamics compensation effect during the
Acknowledgements
The present work was supported by National Basic Research Program of China (973 Program) (No. 2012CB720401); Fundamental Research Funds for the Central Universities (FRF-TP-15-063A1); National Science Foundation of China & Baosteel under Grant (51134008).
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