Numerical models for thermochemical degradation of thermally thick woody biomass, and their application in domestic wood heating appliances and grate furnaces

This paper reviews the current state-of-the-art of numerical models used for thermochemical degradation and combustion of thermally thick woody biomass particles. The focus is on the theory of drying, devolatilization and char conversion with respect to their implementation in numerical simulation tools. An introduction to wood chemistry, as well as the physical characteristics of wood, is also given in order to facilitate the discussion of simplifying assumptions in current models. Current research on single, densified or non-compressed, wood particle modeling is presented, and modeling approaches are compared. The different modeling approaches are categorized by the dimensionality of the model (1D, 2D or 3D), and the one-dimensional models are separated into mesh-based and interface-based models. Additionally, the applicability of the models for wood stoves is discussed, and an overview of the existing literature on numerical simulations of small-scale wood stoves and domestic boilers is given. Furthermore, current bed modeling approaches in large-scale grate furnaces are presented and compared against single particle models.acceptedVersio

Hydrothermal liquefaction of organosolv lignin to bio-oil

Hydrothermal liquefaction (HTL), a thermochemical conversion process for the production of bio-oil from lignin, is described in this thesis. Bio-oil is considered to be a viable source of aromatic compounds as well as a general energy carrier. Nonetheless degradation of lignin during HTL is currently not fully understood due to the complexity and heterogeneity of lignin. This study aims to investigate HTL of lignin under subcritical water conditions (270 °C, 290 °C and 310 °C) and three time levels 10 min, 20 min and 30 min to identify the quantitative formation and qualitative composition of bio-oil. The isolated bio-oil fraction contained a mixture of low molar mass lignin degradation products. A general characterization of this fraction was accomplished by applying a set of analytical methods including Gel Permeation Chromatography, Photoacoustic Infrared spectra, the Folin Ciocalteu method, Karl Fischer titration and elemental analysis. The results from Gel Permeation Chromatography measurement indicated the formation of monomers, dimers and trimers (Mw from 260 to 310 g/mol). The carbon content of bio-oil was slightly higher (65.03%) and its oxygen content slightly lower (28.33%) than in the original lignin sample (C content 64.14% and O content 29.88%) as revealed by elemental analysis. Based on its elemental composition a higher heating value (27.98 kJ/g) for bio-oil than for organosolv lignin (26.33 kJ/g) was calculated, emphazing the potential of bio-oil for being a future energy carrier. The Folin Ciocalteu method indicated a coherency between increasing retention times of HTL and increasing phenolic contents in bio-oil (0.157 g GAE/ g bio-oil (10 min), 0.159 g GAE/g bio-oil (20 min) and 0.191 g GAE/ g bio-oil (30 min)), especially at moderate temperatures (290 °C), outlining bio-oil’s high potential as aromatic source for chemical industry. These achievements indicated a valorization of lignin occurring during hydrothermal liquefaction

Numerical simulation of transient behavior of wood log decomposition and combustion

Heat from wood combustion in domestic wood stoves is a main contributor to the bioenergy in Norway. However, wood combustion in such small-scale combustion appliances can cause significant emissions, e.g. fine particulate matter. Therefore, optimization of old technologies and the development of new designs are required in order to manufacture wood stoves with reduced emission levels, higher efficiency and greater ease of use. To perform the required improvements, the combustion process inside the wood stove must be well understood. In the present thesis, thermochemical degradation of wood and successive char conversion were studied numerically by means of one-dimensional (1D) and two-dimensional (2D) models. Common CFD platforms, e.g. Ansys Fluent, have well-established models for the gas phase, but lack detailed solid phase models. The solid phase model, developed as a standalone code as part of this Ph.D. work, has the potential to be coupled to the gas phase model via user-defined functions in the future. Since processes in the gas and solid phase influence each another, a dynamic coupling of the two models is required to obtain an accurate simulation tool. Fundamental studies on wood combustion can still be done by means of the standalone code. The wood combustion process, implemented in the 1D and the 2D model, included drying, devolatilization and char conversion of the solid fuel. Since all three stages are interrelated, the detailed modeling of all conversion stages with respect to a distinct location inside the wood log had to be done to derive an accurate solid phase combustion model. The solid conversion model was developed for thermally thick particles, such that wood logs used in fired stoves or boilers could be modeled. Fundamental studies could be performed with the developed one-dimensional (1D) model, while detailed studies on the anisotropy of wood must use the extended two-dimensional (2D) model. Model validation was completed against experimental data, available for thermally thick particles' drying and devolatilization, as well as the combustion of a thermally thick near-spherical particle. The 2D model was validated against experimental data for a large, dry, hanging wood log, due to limited experimental data available on thermochemical degradation and combustion of wood logs, as used in domestic wood stoves. Presented results include the studies on numerical efficiency of different models, as well as qualitative and quantitative results showing how wood logs degrade under combustion conditions. Furthermore, grid-independence studies were performed as part of the model development in this Ph.D. project

Combustion of Thermally Thick Wood Particles: A Study on the Influence of Wood Particle Size on the Combustion Behavior

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Combustion of Thermally Thick Wood Particles: A Study on the Influence of Wood Particle Size on the Combustion Behavior

A one-dimensional (1D) comprehensive combustion model for thermally thick wet wood particles, which is also applicable for studying large wood logs, is developed. The model describes drying, devolatilization, and char gasification as well as char oxidation. Furthermore, CO oxidation is modeled, in order to account for the fact that exiting gas products can be oxidized and therefore limit the oxygen transportation to the active sites. The challenges for model validation are outlined. Model validation was done against experimental data for combustion of near-spherical wood particles. Furthermore, the validated model was up-scaled and the effect of wood log diameter on the thermal conversion time, the extent as well as the position of drying, devolatilization, and char conversion zones were studied. The upscaling was done for cylindrical wood logs with an aspect ratio of 4. The thermal conversion time significantly increased with the size. It was also found that the relative extent of the drying, devolatilization, and char conversion zones decreased as wood log size increased. The paper concludes with recommendations for future work

Comparison of numerical efficiency of the thermal and the kinetic rate drying model applied to a thermally thick wood particle

In this work, the drying and devolatilization of a thermally thick wood particle were modeled. The work was validated against experiments and good agreement was found. The work compared the numerical efficiency and accuracy of the thermal drying model and the kinetic rate drying model. The thermal drying model was used with a fixed boiling temperature (373 K). The kinetic data for the kinetic rate drying model was taken from an earlier work by Di Blasi [1] and additionally one set of kinetic data that was also tested, was assumed by the authors, with the main purpose of reducing the stiffness of the evaporation equation. The numerical efficiency was compared by comparing the CPU times associated with the different drying models. It was found that the thermal drying model is the most efficient drying model at both high and low moisture contents. Soft drying kinetics resulted in intermediate CPU times, while very stiff kinetics yielded the lowest numerical efficiency. No trend was observed regarding how CPU times of the different drying models behave with respect to increasing or decreasing moisture contents

Numerical models for thermochemical degradation of thermally thick woody biomass, and their application in domestic wood heating appliances and grate furnaces

This paper reviews the current state-of-the-art of numerical models used for thermochemical degradation and combustion of thermally thick woody biomass particles. The focus is on the theory of drying, devolatilization and char conversion with respect to their implementation in numerical simulation tools. An introduction to wood chemistry, as well as the physical characteristics of wood, is also given in order to facilitate the discussion of simplifying assumptions in current models. Current research on single, densified or non-compressed, wood particle modeling is presented, and modeling approaches are compared. The different modeling approaches are categorized by the dimensionality of the model (1D, 2D or 3D), and the one-dimensional models are separated into mesh-based and interface-based models. Additionally, the applicability of the models for wood stoves is discussed, and an overview of the existing literature on numerical simulations of small-scale wood stoves and domestic boilers is given. Furthermore, current bed modeling approaches in large-scale grate furnaces are presented and compared against single particle models

Drying of Thermally Thick Wood Particles: A Study of the Numerical Efficiency, Accuracy, and Stability of Common Drying Models

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