A Predictive Physico-chemical Model of Biochar Oxidation

Pyrolysis of solid fuels forms a solid carbon-rich fuel, also called char, whose physico-chemical description is rather complex. Heterogeneous oxidation reactions take place during thermochemical conversion of char. The present work proposes a predictive detailed kinetic model, opening a new path for a deeper understanding of the char conversion process. This model considers porosity, surface area, density of surface sites, and their evolution along the conversion process. The chemical aspects of char oxidation are modeled assuming a carbonaceous bulk structure, surrounded by a variety of surface sites which represent the chemical functionalities typically present in such materials. The heterogeneous chemical reactions and their kinetic parameters are defined based on previous studies in the literature and by analogy to homogeneous gas-phase reactions of aromatic species. A mathematical framework is proposed to couple physical and chemical descriptions of the oxidation process. Although the proposed model benefits from experimental information, it is able to comprehensively describe the conversion rate of a broad range of carbonaceous materials such as carbon nanotubes, graphite, and chars only on the basis of their elemental composition. The proposed model represents a first step in exploring the explicit and coupled treatment given to the physical and chemical evolution of the fuel throughout its conversion, allowing us to consistently describe the particle evolution, opening a path for reliable models to manage the chemistry of char conversion

Mathematical modeling of fast biomass pyrolysis and bio-oil formation. Note I: Kinetic mechanism of biomass pyrolysis

This paper discusses the research activities done at Politecnico di Milano in the field of the detailed kinetic modeling of pyrolysis and combustion of biomass and bio-oil formation. Different critical steps are involved in this multicomponent, multiphase and multiscale problem. The first complexity relies on biomass characterization with the selection of reference species: cellulose, hemicellulose, lignins, and extractives. Fast pyrolysis involves kinetic mechanisms, first in the solid phase for biomass pyrolysis, then in gas-phase for secondary reactions of released products. These mechanisms involve large number of species and reactions, which make computations expensive. They need to be simplified, while still maintaining their description capability. Lumping procedures are extensively applied to allow the development of the overall model. Multistep pyrolysis mechanisms of reference species are discussed in this Note, with several comparisons with experimental data. A peculiarity of the model is its ability to provide detailed compositions of pyrolysis products and solid residue. Catalytic effect of ash on pyrolysis products is also discussed. A companion paper will discuss the successive or secondary gas phase reactions of pyrolysis products, together with the heterogeneous reactions of residual char. Finally, the modeling of bio-oil formation requires a comprehensive description of the coupling of kinetic and transport processes, both at the particle and the reactor scale

Flamelet tabulation methods for SOx formation in pulverized solid fuel combustion

In this paper, four different flamelet tabulation methods are evaluated for predicting SOx formation, including SO, SO2 and SO3, in pulverized coal flames with fuel-bound sulfur. The flamelet tabulation methods are evaluated in laminar counterflow flames under different operating conditions, and compared to the detailed chemistry solutions used for reference. The SOx species mass fractions are obtained by either extracting them directly from the flamelet library or solving the corresponding transport equations using source terms from the flamelet library. The results show that different from the other major species, O2 is sensitive to the local combustion mode, and large discrepancies are obtained if the local combustion mode is incorrectly represented. The predicted O2 has a direct effect on H2S oxidation. It is found that SOx species mass fractions can be accurately predicted by all flamelet tabulation methods in most regions of the computational domain, although there are non-negligible discrepancies in relevant regions where the premixed combustion mode is dominant. The differences between the flamelet predictions and the detailed chemistry solutions are quantified by introducing a newly defined parameter, which is formulated based on the difference between the flamelet predictions and the detailed chemistry solutions. The suitability of the flamelet model in predicting the SOx formation in pulverized coal flames with fuel-bound sulfur is justified through a chemical timescale analysis. The chemical timescale analysis is consistent with the findings for the flamelet predictions

Advanced modeling approaches for CFD simulations of coal combustion and gasification

Coal is the most abundant fossil fuel and is widely used as an energy source for combustion and gasification. Both experimental methods and computational tools are required for the development of new advanced, innovative clean coal technologies and systems. In particular, 3D computational fluid dynamics (CFD) simulations can provide detailed local and global information on the interaction of fluid dynamics, mixing, and heterogeneous and homogeneous chemical reactions even for complex systems such as combustors, gasifiers, or chemical reactors. The predictive capabilities of CFD simulations depend directly on appropriate models and their mutual interactions. The current state of modeling is reviewed in this paper and the need for further improvements of both individual models and their respective coupling is addressed. In addition, to evaluate and validate the models and their interactions, systems with increasing complexity and well-defined boundary and operating conditions are required that can provide suitable experimental data. A number of reference burners and combustors, developed especially at universities and research institutions, are also presented and recent simulation data for these systems is reviewed