A comprehensive CFD model for the biomass pyrolysis

The present work addresses the study of the pyrolysis of biomass particle, with the aim to improve the comprehensive mathematical model of the thermochemical processes involving solids decomposition. A new CFD model for the biomass pyrolysis was developed at the particle scale in order to properly describe the relative role of reaction kinetics and transport phenomena. The model is able to solve the Navier-Stokes equations for both the gas and solid porous phase. The code employs the open-source OpenFOAM® framework to effectively manage the computational meshes and the discretization of fundamental governing equations. The mathematical algorithm is based on the PIMPLE method for transient solver and exploit the operator-splitting technique that allows the separation of the transport and the reactive term in order to handle complex computational geometries minimizing the computational effort. The model was tested with experimental data for both reactive and non-reactive conditions. The code is able to provide correct information about temperature distribution within the particle, gas, tar and char formation rates

Detailed kinetics of pyrolysis and combustion of catechol and guaiacol, as reference components of bio-Oil from biomass

Fast biomass pyrolysis is an effective process to produce bio-oils thus allowing to partially replace nonrenewable fossil fuels. Bio-oils are complex mixtures with a great amount of large oxygenated organic species, such as substituted phenolic components. Although experimental and kinetic modeling studies of phenol and anisole pyrolysis and combustion are available in the literature, only a minor attention has been devoted to kinetic mechanisms of substituted phenolic species, such as catechol and guaiacol. Multiple substitutions on aromatic ring can originate proximity effects and thus significantly modify bond energies, consequently affecting reaction pathways. Careful evaluations of bond dissociation energies and reference kinetic parameters, based on theoretical computations, are first performed. Guaiacol and catechol pyrolysis and combustion reactions are then compared with the corresponding phenol and anisole mechanisms. This kinetic study allows to identify some preliminary rate rules useful to validate a detailed kinetic mechanism of bio-oil pyrolysis and combustion

Novel coal gasification process: improvement of syngas yield and reduction of emissions

This article is intended to propose and model an innovative process layout for coal gasification that improves the production of syngas and also reduces the sulfur and CO2 emissions. The typical coal gasification process uses Sulfur Recovery Units to convert H2S to sulfur, but these have some disadvantage, e.g low sulfur price, coal charge with low sulfur flow rate, use of Tail Gas Treatment unit. Compared to the Claus process, this solution converts H2S and CO2 into syngas (economically appealing), reduces emission of H2S and CO2 and allows the use of coal charge with high sulfur flow rate, e.g. 9.5% mol/mol. The novel process takes advantage of a double amine wash, a thermal regenerative furnace and considers the recycle of the acid gases coming from the catalytic reactor to further promote the H2S conversion. In particular, the double amine wash is useful to purify the H2S stream to be sent to the thermal furnace from the syngas and CO2, in order to reduce the reactor inlet flow rate. The regenerative furnace is simulated using a large detailed kinetic scheme to appropriately describe the minor species (among them, pollutants like CS2 and COS). As a result, the recycle appears to substantially reduce the pollutant emissions. In addition, the conversion of the Claus process into the novel process doesn't require any change in the main equipment, just needing for a variation in the layout and the operating conditions431483148812th International Conference on Chemical and Process Engineering (ICheaP)2015-05ItaliaMilan

Mathematical Modeling of Fast Biomass Pyrolysis and Bio-Oil Formation. Note II: Secondary Gas-Phase Reactions and Bio-Oil Formation

This paper summarizes the research activities done at Politecnico di Milano in the field of the detailed kinetic modeling of fast pyrolysis of biomass to produce bio-oil. Note I of this work already discussed biomass characterization and the multistep pyrolysis mechanisms of reference species. The model is able to provide a detailed composition of pyrolysis products and char residue. Different critical steps are involved in this multicomponent, multiphase and multiscale problem. The first complexity relies in biomass characterization. Then, fast pyrolysis process involves detailed kinetic mechanisms, first in the solid phase for the biomass pyrolysis, then in the gas-phase for the secondary reactions of released products. The complexity of these kinetic mechanisms requires strong simplifications, thus chemical lumping procedures are extensively applied. Successive or secondary gas phase reactions of gas and tar components released during the pyrolysis process complement the kinetic model, together with the heterogeneous reactions of residual char. The modeling of fast pyrolysis process requires a comprehensive description of the coupled transport and kinetic processes, both at the particle and the reactor scale. A few examples and comparisons with experimental data validate the reliability of the overall model. Finally, the composition and physical properties of the pyrolysis bio-oil are also discussed, with emphasis on combustion and pollutant emissions

Mechanism Comparison for PAH Formation in Pyrolysis and Laminar Premixed Flames

Polycyclic aromatic hydrocarbons (PAHs) are known precursors of harmful carbonaceous particles. Accurate predictions of soot formations strongly rely on accurate predictions of PAHs chemistry. This work addresses the detailed kinetic modeling of PAH formation using two models: CRECK [8] and ITV [12], aiming to compare the model predictions with experimental data in olefin pyrolysis and laminar premixed flames. The two kinetic mechanisms are validated and compared highlighting similarities and differences in PAHs formation pathways. The validation highlights the critical role of resonance-stabilized radicals leading to the PAH formation

A CFD model for biomass flame-combustion analysis

The present work addresses the study of the combustion of individual biomass particle surrounded by a gas stream of N2/O2 under the operating conditions encountered in a drop tube reactor. The aim of this analysis is to give a better insight into the chemical and physical processes that occur both at particle and reactor scale where the volatiles, generated by the biomass pyrolysis, burn in a fuel particle enveloped flame. A comprehensive CFD model was developed within the open-source OpenFOAM® framework in order to properly handle the computational mesh and the discretization of the characteristic governing equations. At the reactor scale, the reactive flow was described by the equations for continuous, multicomponent, compressible and thermally-perfect mixtures of gases. At the particle scale, instead, the solid particle was considered as a porous media with isotropic and uniform morphological properties

Experimental and kinetic modeling study of pyrolysis and combustion of anisole

open7siFast biomass pyrolysis is an effective process with high yields of bio-oil, and is a promising technology to partially replace non-renewable fossil fuels. Bio-oils are complex mixtures with a large amount of oxygenated organic species, such as esters, ethers, aldehydes, ketones, carboxylic acids, alcohols, and substituted aromatic components. Anisole is a simple surrogate of primary tar from lignin pyrolysis and it is very useful to investigate gas-phase reactions of methoxy-phenol species, expected precursors of poly-cyclic aromatic hydrocarbons (PAH) and soot during biomass pyrolysis and bio-oil combustion. This work first presents new pyrolysis data obtained in the Ghent flow reactor, and then it discusses a detailed kinetic mechanism of anisole pyrolysis and oxidation. This scheme is further validated and compared, not only with these pyrolysis data, but also with recently published data of anisole oxidation in jet stirred reactors. Ignition delay time and laminar flame speed computations complement these detailed comparisons. This kinetic mechanism is a first step and places the basis towards a successive model extension to catechol, guaiacol, and vanillin, as representative phenolic components of bio-oil from biomass.openPelucchi, Matteo*; Faravelli, Tiziano; Frassoldati, Alessio; Ranzi, Eliseo; SriBala, Gorugantu; Marin, Guy B.; Van Geem, Kevin M.Pelucchi, Matteo; Faravelli, Tiziano; Frassoldati, Alessio; Ranzi, Eliseo; Sribala, Gorugantu; Marin, Guy B.; Van Geem, Kevin M

Numerical investigation of soot formation from microgravity droplet combustion using heterogeneous chemistry

The use of isolated droplets as idealized systems is an established practice to get an insight on the physics of combustion, and an optimal test field to verify physical submodels. In this context, this work examines the dynamics of soot formation from the combustion of hydrocarbon liquid fuels in such conditions. A detailed, heterogeneous kinetic mechanism, describing aerosol and particle behavior through a discrete sectional approach is incorporated. The developed 1-dimensional model accounts for (i) non-luminous and luminous radiative heat losses, and (ii) incomplete thermal accommodation in the calculation of the thermophoretic flux. The combustion of droplets of n-heptane, i.e., the simplest representative species of real fuels, was investigated as test case; an upstream skeletal reduction of the kinetic mechanism was carried out to limit calculation times. After checking the performance of the reduced mechanism against gas-phase experimental data, the transient evolution of the system was analyzed through a comprehensive study, including fiber-suspended (D0 1 mm) droplets. The different steps of soot evolution were quantified, and localized in the region between the flame front and the soot shell, where particle velocity is directed inwards because of thermophoresis, and residence times are much higher than what usually found in diffusion flames. As a result, growth, coalescence, and aggregation steps are significantly enhanced, and soot accumulates in the inner shell, with an evident modification of the particle size distribution, if compared to what observed in conventional combustion conditions. The model exhibits a satisfactory agreement with experimental data on flame temperature and position around the droplet, while for larger droplets an increasing sensitivity to the radiation model was observed. It is found that the latter has a significant impact on the production of soot, while scarcely affecting the location of the soot shell. On the other side, the inclusion of incomplete thermal accommodation in the thermophoretic law brought about more accurate predictions of both volume fractions and shell location, and highlighted the primary role of thermophoresis in these conditions, as already found in literature through more simplified approaches

GASDS: A kinetic-based package for biomass and coal gasification

In this paper, a simulation package called GASDS is introduced. It is particularly suited to evaluate the pyrolysis, gasification and combustion of biomass and coal feedstocks. The aim of this work is to describe the package from a numerical point of view and its interface. Additionally, experimental results for a countercurrent fixed-bed biomass gasification reactor are reproduced. The influence of reactor and particle discretizations are investigated with respect to accuracy and computational time. Some differences are present between experimental and simulation results. In order to improve the agreement between simulation and experimental results it is suggested to improve the kinetic scheme of the solid phase and gas-solid reactions. The negligible differences in terms of predictions, instead, do not justify the adoption of finer discretizations for the particle and reactor, which imply longer computational times