Full Length ArticleDevelopment of a detailed kinetic model for the combustion of biomass
Graphical abstract
Introduction
The current global warming issues, especially the concerns about the alarming rise of atmospheric CO2 [1], have increased the importance of the use of fuels from renewable resources, primarily biomass. The use of wood and biomass for domestic small combustion installations (fireplaces, stoves and boilers) is widely widespread in countries where heating is needed in winter [2]. Residential wood burning significantly affects the air quality because it is one of the largest sources of fine particles (PM2.5, particulate matter of aerodynamic diameter below 2.5 μm). In addition, it is responsible for the emissions of a number of other pollutants (carbon and nitrogen monoxides, oxygenated hydrocarbons such as aldehydes and ketones, alcohols, furans and acids, PAH…) [3]. Environmentally sustainable wood heating will require a better understanding of the chemistry of the pollutant formation in such devices. Especially the development of kinetic models involving the chemical reactions responsible for pollutant formation is needed.
Lignocellulosic biomass pyrolysis and combustion are multi-scale, multi-phase, and multi-component processes [4], since biomass has a complex structure consisting mainly of a mixture of biopolymers: cellulose, hemicellulose, lignin with a small amount of extractives [5].
Biomass combustion includes four stages: drying, pyrolysis, homogeneous combustion of volatile products and heterogeneous combustion of char. Pyrolysis is the first step in any energy recovery processes such as combustion and gasification [6]. Modeling biomass pyrolysis is therefore essential for understanding combustion [7], which justifies the abundance of work in this field within the literature [8], [9], [10], [11].
In the literature, two simplified approaches are generally proposed to describe biomass thermal degradation: the homogeneous approach, where biomass is considered as a homogeneous solid and the heterogeneous approach which takes into account the contribution of each biomass constituents (cellulose, hemicellulose and lignin) to the overall mechanism. According to the first approach, one or some global chemical reactions may be sufficient to describe wood devolatilization. A one-step global mechanism is the most simplified kinetic model used by many works in the literature [12], [13], [14], [15], [16], [17]. Biomass decomposes into volatile compounds and a solid residue. This type of mechanism is generally used to model mass loss either by pyrolysis or thermogravimetric analysis (TGA) [8]. It is also used to model coupled chemical kinetics and physical phenomena, using for example a CFD (Computational Fluid Dynamics) approach [15], [17]. The second common type of kinetic models in the homogeneous approach is independent competitive reaction scheme. According to this model, biomass produces competitively tar, gases, and char [2]. By using this reaction scheme, yields of tar, char and permanent gases can be predicted, which is not the case with one-step global mechanism. Generally, this second type of model is coupled to secondary reactions of tars formed during pyrolysis [18], [19], [20], [21], [22]. Multistep mechanisms are also discussed. In those schemes, an intermediate solid is formed by the first step and continues to decompose into secondary products [23].
According to the heterogeneous approach, the pyrolytic behavior of biomass is deduced from that of its major constituents: independent parallel reaction mechanisms, representing respectively cellulose, hemicellulose and lignin degradation, have been widely used to model biomass pyrolysis [24], [25], [26]. This type of mechanism allows a better prediction of product yields. It can be applied to a variety of biomass since the content of cellulose, hemicellulose and lignin differs from one type to another. Interactions between biomass constituents is neglected. A number of studies uses the Broido-Shafizadeh’s scheme (Fig. 1) to model biomass pyrolysis. This scheme, which was developed for the first time for cellulose degradation, has been applied to lignin, hemicellulose and wood pyrolysis. Biomass is converted to intermediate solid called “active biomass” (reaction 1) which decomposes by competitive reactions (reactions 2 and 3) into gaseous, liquid and solid products [8].
The most complex reaction scheme in the literature to simulate biomass pyrolysis following the heterogeneous approach is that of Ranzi who developed in 2008 a first multi-step semi-detailed mechanism [28] extending the Broido-Shafizadeh’s approach. Dussan et al. proposed a new semi-detailed kinetic mechanism based on a recent Ranzi scheme [29] respectively for hemicellulose [30] and lignin pyrolysis [31]. More advanced approaches would involve micro-kinetic mechanistic models based on theoretical calculations as proposed by the team of Broadbelt for cellulose [32], [33], [34] and hemicellulose [35] and by Horton et al. [36] for biomass pyrolysis and gasification.
Compared to pyrolysis, significantly fewer studies were published about the kinetic modelling of biomass combustion. Thermal degradation under oxidative atmosphere is indeed more complex. The presence of an oxidizing agent (air, oxygen, etc.) generates homogenous gas-phase reactions between oxygen and volatiles compounds released during devolatilization and heterogeneous reactions between oxygen and char [37].
A two-stage reaction kinetic scheme was proposed by Gil et al. [37] and Shen et al. [38] to represent biomass and char combustion. Pérez et al. [39] proposed a scheme including three independent reactions to represent respectively, cellulose, hemicellulose and lignin combustion coupled with a fourth reaction representing char oxidation. Wang et al. [40] consider two simultaneous parallel reactions: an overall reaction describing biomass combustion and two individual reactions representing respectively volatiles and carbon residue oxidation. Navarrete Cereijo et al. [41] modeled biomass combustion as three sequential stages, which are drying, pyrolysis and char oxidation. In this work [41], as is shown in Fig. 2, five reactions were used to represent biomass decomposition into tars (reaction 1), volatiles (reaction 2), and char (reaction 3), as well as tar degradation in volatiles (reaction 4) and char (reaction 5). The char combustion stage was modeled using three reactions (reactions 6, 7 and 8): direct combustion with oxygen and two reactions respectively with CO2 and H2O.
A similar approach was used by Mätzing et al. [42] to model biomass combustion in fixed bed reactor with a slightly more detailed pyrolysis mechanism considering cellulose, hemicellulose and lignin devolatilization. However, approaches using global reactions cannot be an appropriate method to predict in details pollutant formation during biomass combustion in stoves or industrial systems for instance.
The main aim of this paper is to develop and validate a new detailed kinetic model of wood pyrolysis and combustion in order to predict pollutant emissions. This kinetic mechanism is tested against new TGA results obtained in parallel of this work, as well as a wide range of experimental results published in literature. The used literature results (29 datasets) and the comparisons with the present model predictions have been collected and organized as a database given as two spreadsheets in Supplementary Material (SM).
Section snippets
Description and validations of the chemical model
A new detailed kinetic mechanism, BioPOx, has been developed in Chemkin format. It includes two parts, (1) a semi-detailed mechanism to describe biomass pyrolysis, (2) a detailed mechanism of the gas-phase combustion of the volatiles species.
Conclusion
In the present work, we have developed a new kinetic model of wood combustion. To the best of our knowledge, this is the most detailed mechanism for biomass degradation since it considers in details the gas-phase reactions of the tars produced by the biomass devolatilization. Contrary to the existing lumped kinetic models, our model involves 710 species and includes 5035 reactions, amongst which 5006 gas-phase elementary reactions. This paper shows that BioPOx (Biomass Pyrolysis and Oxidation)
Acknowledgments
The authors gratefully acknowledges the Agence De l’Environnement et de la Maitrise de l’Energie (ADEME, France) for the financial support of this work, as well as the team of Prof. E. Ranzi for providing help for first using their biomass pyrolysis mechanism.
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