Pyrolysis of biomass in a rotary kiln finds application both as an intermediate step in multistage gasification as well as a process on its own for the production of biochar. In this work, a numerical model for pyrolysis of lignocellulosic biomass in a rotary kiln is developed. The model is based on a set of conservation equations for mass and energy, combined with independent submodels for the pyrolysis reaction, heat transfer, and granular flow inside the kiln. The pyrolysis reaction is described by a two-step mechanism where biomass decays into gas, char, and tar that subsequently undergo further reactions; the heat transfer model accounts for conduction, convection and radiation inside the kiln; and the granular flow model is described by the well known Saeman model. The model is compared to experimental data obtained from a pilot scale rotary kiln pyrolyzer. In total 9 pilot plant trials at different feed flow rate and different heat supply were run. For moderate heat supplies we found good agreement between the model and the experiments while deviations were seen at high heat supply. Using the model to simulate various operation conditions reveals a strong interplay between heat transfer and granular flow which both are controlled by the kiln rotation speed. Also, the model indicates the importance of heat losses and lays the foundation for scale up calculations and process optimization.

The effect of pyrolysis reactors on the properties of biochars (with a focus on flammability and mechanical characteristics) were investigated by keeping factors such as feedstock, carbonisation temperature, heating rate and residence time constant. The reactors employed were hydrothermal, fixed-bed batch vertical and fixed-bed batch horizontal-tube reactors. The vertical and tube reactors, at the same temperature, produced biochars having comparable elemental carbon content, surface functionalities, thermal degradation pattern and peak heat release rates. The hydrothermal reactor, although, a low-temperature process, produced biochar with high fire resistance because the formed tarry volatiles sealed water inside the pores, which hindered combustion. However, the biochar from hydrothermal reactor had the lowest nanoindentation properties whereas the tube reactor-produced biochar at 300 °C had the highest nanoindentation-hardness (290 Megapascal) and modulus (ca. 4 Gigapascal) amongst the other tested samples. Based on the inherent flammability and mechanical properties of biochars, polymeric composites’ properties can be predicted that can include them as constituents.

In most industrial applications, combustion and gasification of char progresses under regime II conditions. Unlike in other regimes, both particle size and density change simultaneously in regime II due to non-uniform consumption of carbon inside the particles. In this work, mathematical predictions of diameter changes in regime II were made by a one-dimensional simulation tool, where transient species balances are resolved locally inside the particle. This simulation is computationally expensive and usually not appropriate for the implementation in comprehensive CFD simulations of combustion or gasification processes. To overcome this restraint, an alternative shortcut method with affordable computation time has been developed and validated against the detailed model. This method allows the calculation of diameter changes during combustion and gasification from precalculated effectiveness factors. Additionally, the change of particle size has been investigated experimentally in a single particle converter setup. Therein, particles are fixed on a sample holder placed in the hot flue gas of a flat flame burner. Size and temperature trends are optically assessed by a 3CCD camera.

This research aims to provide a full view of knowledge in charcoal production for fossil coal replacement. Charcoal from biomass is a promising material to replace fossil coal, which is using as heating source or reactant in the industrial sector. Nowadays, charcoal with quality comparable to fossil coal is produced by high-temperature pyrolysis, but efficiency of the production is relatively low due to the trade-off between charcoal property and yield by pyrolysis temperature. Increasing charcoal yield by means of secondary char formation in pyrolysis of large wood particles is the primary method considering in this work. This research has explored increasing efficiency of charcoal production by bio-oil recycling and CO₂ purging. These proposed techniques significantly increase concentration and extend residence time of volatiles inside particle of woodchip resulting extra charcoal. Characterization of charcoals implies negligible effect of these methods on charcoal properties such as elemental composition, heating value, morphological structure, and chemical structure. Besides, reactivity of charcoal slightly increased when these methods were applied. A numerical model of pyrolysis in a rotary kiln reactor has been developed to study the effect of design parameters and conditions in reactor scale. The simulation results showed fair prediction of temperature profiles and products distribution along the reactor length. Nonetheless, to deliver full knowledge in charcoal production, further works are planned to be done at the end of this doctoral research.

Bio-coal produced from biomass is a promising material to replace fossil coal in order to achieve net-zero greenhouse gas emission from the industrial sector. Bio-coal with quality comparable to that of fossil coal can be produced by high-temperature pyrolysis at ≥500 ºC, but the production efficiency is relatively low due to low bio-coal yield at high pyrolysis temperatures. This trade-off suffers the economic feasibility of bio-coal production. The overall objective of this doctoral thesis is to develop a pyrolysis process that can produce bio-coal for fossil coal replacement in the industrial sector, while maintaining a high process efficiency.   To increase bio-coal yield and process efficiency, secondary char formation during the pyrolysis of thick biomass, for example, woodchips, is the primary method considered in this work. Secondary char formation can be promoted by increasing volatile concentration during pyrolysis and/or extending residence time of volatiles inside the pore structure of wood particles. This study investigated how to increase secondary char formation using bio-oil recycling and CO2 purging. Bio-oil recycling increased bio-coal yield by not only increasing the reactants, but also through the synergetic effect between bio-oil and woodchips upon physical contact. Using CO2 as a purging gas reduced mass diffusion of volatiles inside the pore structure of woodchips, producing extra bio-coal. In addition, the effect of these techniques can be maximized by ensuring good contact between the volatiles and the solid surface using thick particles and slow heating. In parallel, a numerical model of pyrolysis in a rotary kiln reactor was developed to increase the understanding of parameter implementation in pyrolysis reactors. Two important parameters were studied: rotation speed and feeding rate. Rotation speed controlled the solid residence time, while the feeding rate influenced the heat capacity of holdup materials and product distribution.   Bio-coal is prone to self-heating and usually causes spontaneous ignition during production, storage, and transportation, which can lead to losses in the production and health of workers. In this study, self-heating at low temperatures was investigated by using numerical simulations describing the changes in local properties inside different bio-coal containers such as closed metal containers and woven plastic bags. The kinetic parameters of bio-coal were measured and implemented in the model. It was observed that the bio-coal temperature slowly increased from the initial temperature due to the heat released during O2 chemisorption. Thermal runaway occurred in some storage conditions, even at intial bio-coal temperatures of ca. 155 ºC. The simulation results suggest that self-heating can be mitigated by using small and wide particle distribution, limited storage volume, and low ambient temperature. This study also provides the criteria for estimating the cooling demands in bio-coal production processes.   Bio-coal properties are the main challenges for utilizing it as a substitute for fossil coal. Although the elemental composition and heating value of the bio-coal produced in this study are equivalent to those of fossil coal, the reactivity of bio-coal is relatively high. To replace fossil coal in existing industrial processes, bio-coal reactivity is preferred to be similar to that of fossil coal to avoid major process modifications. This thesis has concluded that pyrolysis temperature, heating rate, and biomass feedstock are the major parameters influencing the gasification rate under chemical reaction limitation. It was found that potassium in biomasses increased bio-coal reactivity even at low gasification temperatures such as 800 ºC, while calcium did not play a significant role at temperatures below 1600 ºC. Furthermore, bio-coal reactivity increased only slightly by promoting secondary char formation using the proposed methods. These findings suggest that we can achieve high bio-coal yield, both mass and energy, while maintaining similar fuel properties through pyrolysis with bio-oil recycling and CO2 purging.   In the most industrially relevant applications, the gasification rate is dominated by diffusion mass transfer. Therefore, it is necessary to reflect gasification behavior of bio-coal under these circumstances. At the particle scale, where intraparticle diffusion controls the overall reaction rate, bio-coal particle size was nearly constant until high conversion. This implies that particle size changes should be considered only at high conversion. Meanwhile, large particles exhibit low gasification rate at the particle scale following the Thiele modulus. The contrary result appears at the packed bed scale, where both intraparticle and interparticle diffusions play roles. Large particles increased the gasification rate in packed beds because of the large bed channel size, high void fraction, and low tortuosity. This observation led to an opportunity to minimize the apparent gasification rate in a packed bed by using polydisperse particles, which have a wide particle size distribution. Large particles maximize the intraparticle diffusivity of CO2, while small particles fill the gaps between large particles, thus increasing interparticle diffusivity, which reduces apparent reactivity. This outcome was confirmed experimentally.   By combining the knowledge obtained in this doctoral thesis, an efficient pyrolysis process is proposed to produce bio-coal for a sustainable industry.

Gasification is a key technology for the utilization of biomass as an energy carrier. The WoodRoll process developed by Cortus Energy is a multistage gasification process where drying, pyrolysis and gasification are conducted in separate units. A central role is thereby given to the pyrolysis step which provides the gas to heat the entire process. In the WoodRoll process pyrolysis is run in an indirectly heated rotary kiln. In this work we study pyrolysis in a rotary kiln by means of numerical simulations and by evaluating pilot plant data obtained from a 500 kW pilot. The simulations indicate the importance of the heat transfer to the solid bed and the exothermic pyrolysis reactions that occur in the late stage of the pyrolysis process. The latter can cause an overshoot of the solid bed temperature. Evaluation of the pilot plant data shows the robustness of the process, expressed in good reproducible and stable operation.

This study investigates the influences of intraparticle and interparticle diffusions on the reaction rates of char gasification in a packed bed without forced convective flows. The main objective is to elucidate how the dominant scales of mass diffusion resistance change based on particle size distributions (PSD). CO2 gasification rates were measured by thermogravimetric analyses (TGA) of spruce char produced from pilot-scale reactors. Experimental setups using two TGA devices highlighted the effects on different rate-limiting steps. Effects of intraparticle diffusion were investigated with a single layer of monodispersed particles between 75 µm and 6.3 mm using a commercial TGA. Effects of interparticle diffusion were investigated with a packed bed of monodispersed and polydispersed particles using a macro-TG. At the particle scale, gasification rate decreased with the increase of particle size when the reaction was controlled by intraparticle diffusion. This effect can be described by the effectiveness factor with Thiele modulus. At the bed scale, void fraction and tortuosity of the packed bed are influential parameters on diffusivity of CO2 through the bed channels. Due to its non-sphericity of the char particles, the bed of relatively large particles had high void fraction and the presence of smaller particles were essential to lower the bed void size. Consequently, smaller size fraction in the PSD had a major impact on the diffusion resistance at bed scale. It means that the diffusion resistances at particle and bed scales are sensitive to different size fractions in the PSD. It allows one to tweak the overall reaction rates in packed beds by manipulating the PSD if the dominant mass transport mechanism is diffusion.

Biochar is attracting attention as an alternative carbon/fuel source to coal in the process industry and energy sector. However, it is prone to self-heating and often leads to spontaneous ignition and thermal runaway during storage, resulting in production loss and health risks. This study investigates biochar self-heating upon its contact with O₂ at low temperatures, i.e., 50–300 °C. First, kinetic parameters of O₂ adsorption and CO₂ release were measured in a thermogravimetric analyzer using biochar produced from a pilot-scale pyrolysis process. Then, specific heat capacity and heat of reactions were measured in a differential scanning calorimeter. Finally, a one-dimensional transient model was developed to simulate self-heating in containers and gain insight into the influences of major parameters. The model showed a good agreement with experimental measurement in a closed metal container. It was observed that char temperature slowly increased from the initial temperature due to heat released during O₂ adsorption. Thermal runaway, i.e., self-ignition, was observed in some cases even at the initial biochar temperature of ca. 200 °C. However, if O₂ is not permeable through the container materials, the temperature starts decreasing after the consumption of O₂ in the container. The simulation model was also applied to examine important factors related to self-heating. The results suggested that self-heating can be somewhat mitigated by decreasing the void fraction, reducing storage volume, and lowering the initial char temperature. This study demonstrated a robust way to estimate the cooling demands required in the biochar production process.

ABSTRACT: This study aims at increasing char yield by recycling bio-oil without negative impact on char qualities, i.e. carbon content and heating value. Pyrolysis experiments on spruce and birch chips were carried in a macro-thermogravimetric analyzer. To examine the effect of bio-oil recycle, dried raw woodchips, pure bio-oil, and woodchips impregnated with bio-oil (10, 20 and 25% on mass basis) were compared. The experiments were carried out by introducing sample into the reaction zone with the flow of N₂ and at the temperature range of 300 to 600 ˚C. Pyrolysis of the bio-oil impregnated woodchip gave higher char yield than the pyrolysis of raw woodchip. By the 20% (m/m) bio-oil impregnation, char yield increased by 18.9% (spruce) and 19.1% (birch) on average from the raw woodchip pyrolysis. In addition, the char yield from bio-oil impregnated woodchips was higher than the interpolated char yield of raw woodchips and bio-oil, indicating that synergy effect exists by bio-oil impregnation compared with mere recycling of bio-oil. However, high heating rate corresponded to high temperature pyrolysis, i.e. above 400 ˚C, created cavities and breakages on woodchips, which minimized the secondary reaction. Neither carbon content nor heating value of char was influenced by bio-oil impregnation. Energy yield also showed improvement by increasing bio-oil recycling ratio. For example, energy yield of char from woodchips at the temperature of 340 ˚C increased from 48.4% with raw woodchips to 64.5% by woodchips with 25% of bio-oil impregnation.

Biomass char produced from pyrolysis processes is of great interest to be utilized as renewable solid fuels or materials. Forest byproducts and agricultural wastes are low-cost and sustainable biomass feedstocks. These biomasses generally contain high amounts of ash-forming elements, generally leading to high char reactivity. This study elaborates in detail how chemical and physical properties affect CO₂ gasification rates of high-ash biomass char, and it also targets the interactions between these properties. Char produced from pine bark, forest residue, and corncobs (particle size 4–30 mm) were included, and all contained different relative compositions of ash-forming elements. Acid leaching was applied to further investigate the influence of inorganic elements in these biomasses. The char properties relevant to the gasification rate were analyzed, that is, elemental composition, specific surface area, and carbon structure. Gasification rates were measured at an isothermal condition of 800 °C with 20% (vol.) of CO₂ in N₂. The results showed that the inorganic content, particularly K, had a stronger effect on gasification reactivity than specific surface area and aromatic cluster size of the char. At the gasification condition utilized in this study, K could volatilize and mobilize through the char surface, resulting in high gasification reactivity. Meanwhile, the mobilization of Ca did not occur at the low temperature applied, thus resulting in its low catalytic effect. This implies that the dispersion of these inorganic elements through char particles is an important reason behind their catalytic activity. Upon leaching by diluted acetic acid, the K content of these biomasses substantially decreased, while most of the Ca remained in the biomasses. With a low K content in leached biomass char, char reactivity was determined by the active carbon surface area.

Pyrolysis conditions in charcoal production affect yields, properties, and further use of charcoal. Reactivity is a critical property when using charcoal as an alternative to fossil coal and coke, as fuel or reductant, in different industrial processes. This work aimed to obtain a holistic understanding of the effects of pyrolysis conditions on the reactivity of charcoal. Notably, this study focuses on the complex effects that appear when producing charcoal from large biomass particles in comparison with the literature on pulverized biomass. Charcoals were produced from woodchips under a variety of pyrolysis conditions (heating rate, temperature, reaction gas, type of biomass, and bio-oil embedding). Gasification reactivity of produced charcoal was determined through thermogravimetric analysis under isothermal conditions of 850 degrees C and 20% of CO2. The charcoals were characterized for the elemental composition, specific surface area, pore volume and distribution, and carbon structure. The analysis results were used to elucidate the relationship between the pyrolysis conditions and the reactivity. Heating rate and temperature were the most influential pyrolysis parameters affecting charcoal reactivity, followed by the reaction gas and bio-oil embedding. The effects of these pyrolysis conditions on charcoal reactivity could primarily be explained by the difference in the meso- and macropore volume and the size and structural order of aromatic clusters. The lower reactivity of slow pyrolysis charcoals also coincided with their lower catalytic inorganic content. The reactivity difference between spruce and birch charcoals appears to be mainly caused by the difference in catalytically active inorganic elements. Contrary to pyrolysis of pulverized biomass, a low heating rate produced a higher specific surface area compared with a high heating rate. Furthermore, the porous structure and the reactivity of charcoal produced from woodchips were influenced when the secondary char formation was promoted, which cannot be observed in pyrolysis of pulverized biomass.

This study aims at increasing char yield by recycling bio-oil without negative impact on char qualities, i.e. carbon content and heating value. Pyrolysis experiments on spruce and birch chips were carried in a macro-thermogravimetric analyzer. To examine the effect of bio-oil recycle, dried raw woodchips, pure bio-oil, and woodchips impregnated with bio-oil (10, 20 and 25% on mass basis) were compared. The experiments were carried out by introducing sample into the reaction zone with the flow of N₂ and at the temperature range of 300 to 600 ˚C. Pyrolysis of the bio-oil impregnated woodchip gave higher char yield than the pyrolysis of raw woodchip. By the 20% (m/m) bio-oil impregnation, char yield increased by 18.9% (spruce) and 19.1% (birch) on average from the raw woodchip pyrolysis. In addition, the char yield from bio-oil impregnated woodchips was higher than the interpolated char yield of raw woodchips and bio-oil, indicating that synergy effect exists by bio-oil impregnation compared with mere recycling of bio-oil. However, high heating rate corresponded to high temperature pyrolysis, i.e. above 400 ˚C, created cavities and breakages on woodchips, which minimized the secondary reaction. Neither carbon content nor heating value of char was influenced by bio-oil impregnation. Energy yield also showed improvement by increasing bio-oil recycling ratio. For example, energy yield of char from woodchips at the temperature of 340 ˚C increased from 48.4% with raw woodchips to 64.5% by woodchips with 25% of bio-oil impregnation.

Charcoal from biomass is a promising alternative for fossil coal. Although its quality increases at high pyrolysis temperature, charcoal yield decreases, meaning lower economic performances of charcoal production processes. This work aims at demonstrating potential methods to increase charcoal yield while keeping its quality at satisfying levels. We suggested the recycling of bio-oil from pyrolysis process as a primary measure. In addition, we also investigated in detail the consequence of utilizing CO2 instead of N2 as reaction media under practical conditions (i.e. thick particles). An experimental investigation was carried out in a macro-thermogravimetric (macro-TG) reactor. Sample (woodchips, bio-oil, and woodchips embedded with bio-oil) was exposed to the reaction temperature either instantaneously (isothermal condition) or by slow heating (slow pyrolysis) in controlled gas flows of N2 and CO2. The results showed that char yield increases with the bio-oil recycling on wood chips at all pyrolysis temperatures (300–700 °C). By 20% of bio-oil embedding on wood chips, charcoal yield increased by 18.3% on average. The increase of charcoal yield was not only because of the increase in reactants, but also due to the synergetic effect between bio-oil and wood chips upon physical contact. Bio-oil recycling had negligible effects on the property of charcoal, such as carbon content and heating value. Although CO2 did not affect primary pyrolysis, it had effects on mass transfer processes. As a result, significantly higher char yield was obtained from pyrolysis in CO2 than in N2 by ensuring a good contact of volatiles and solid surface (i.e. usage of thick particles and slow heating). This study suggests that we can achieve high charcoal yield while maintaining the similar charcoal property by bio-oil recycling, CO2 purging, use of thick particles, and slow heating.

This paper presents thermal deactivation of beech wood chars during secondary pyrolysis in a drop-tube reactor. Pyrolysis temperature was varied between 1000 °C and 1600 °C at a constant residence time of 200 ms. The effect of pyrolysis conditions on initial conversion rate R₀ during gasification, graphitization of the carbon matrix and ash morphology was investigated. Gasification experiments for the determination of R₀ were conducted in a thermogravimetric analyzer using pure CO₂ at 750 °C and isothermal conditions. A linear decrease in initial conversion rate R₀ was observed between 1000 °C and 1400 °C. However, a strong increase of R₀ at 1600 °C was encountered. Micropore surface area of the secondary chars showed no correlation with the initial conversion rate R₀ during gasification with CO₂. Graphitization of the carbon matrix was determined using X-ray diffraction and Raman spectroscopy suggesting the growth of aromatic clusters and graphite-like structures for increasing pyrolysis temperatures up to 1600 °C. Furthermore, CaO dispersion was analyzed quantitatively and qualitatively using temperature-programmed reaction at 300 °C as well as SEM/TEM. CaO dispersion D_CaO decreases steadily between 1000 °C and 1400 °C whereas a strong increase can be observed at 1600 °C, which is in good accordance with the development of the initial conversion rate R₀ as a function of pyrolysis temperature. SEM/TEM images indicate the formation of a thin CaO layer at 1600 °C that is presumably responsible for the strong increase in initial conversion rate R₀ at this temperature. When excluding the catalytic activity of CaO via formation of the ratio R₀ D_CaO⁻¹, increasing graphitization degree has a linear negative influence on char reactivity at pyrolysis temperatures between 1000 °C and 1400 °C.

This paper provides a fundamental and critical review of biomass application as a reducing agent and fuel in integrated steelmaking. The basis for the review is derived from the current process and product quality requirements that also biomass-derived fuels should fulfill. The availability and characteristics of different sources of biomass are discussed and suitable pretreatment technologies for their upgrading are evaluated. The existing literature concerning biomass application in bio-coke making, blast furnace injection, iron ore sintering and production of carbon composite agglomerates is reviewed and research gaps filled by providing insights and recommendations to the unresolved challenges. Several possibilities to integrate the production of biomass-based reducing agents with existing industrial infrastructures to lower the cost and increase the total efficiency are given. A comparison of technical challenges and CO₂ emission reduction potential between biomass-based steelmaking and other emerging technologies to produce low-CO₂ steel is made.

Slow pyrolysis as a method of producing a high-quality energy carrier from lignin recovered from wood-based ethanol production has not been studied for co-firing or blast furnace (BF) applications up to now. This paper investigates fuel characteristics, grindability, moisture uptake and the flow properties of lignin chars derived from the slow pyrolysis of lignin at temperatures of 300, 500 and 650 °C (L300, L500 and L650 samples respectively) at a heating rate of 5 °C min^-1. The lignin chars revealed a high mass and energy yield in the range of 39-73% and 53-89% respectively. Pyrolysis at 500 °C or higher, yielded lignin chars with low H/C and O/C ratios suitable for BF injection. Furthermore, the hydrophobicity of lignin was improved tremendously after pyrolysis. Pyrolysis at high temperatures increased the sphericity of the lignin char particles and caused some agglomeration in L650. Large and less spherical particles were found to be a reason for high permeability, compressibility and cohesion of L300 in contrast to L500 and L650. L300 and L500 chars demonstrated high combustibility with low ignition and burnout temperatures. Also, rheometric analysis showed that L500 has the best flow properties including low aeration energy and high flow function.

Biochar is attracting attention as an alternative carbon/fuel source to coal in the process industry and energy sector. However, it is prone to self-heating and often leads to spontaneous ignition and thermal runaway during storage, resulting in production loss and health risks. This study investigates biochar self-heating upon its contact with O₂ at low temperatures, i.e., 50–300 °C. First, kinetic parameters of O₂ adsorption and CO₂ release were measured in a thermogravimetric analyzer using biochar produced from a pilot-scale pyrolysis process. Then, specific heat capacity and heat of reactions were measured in a differential scanning calorimeter. Finally, a one-dimensional transient model was developed to simulate self-heating in containers and gain insight into the influences of major parameters. The model showed a good agreement with experimental measurement in a closed metal container. It was observed that char temperature slowly increased from the initial temperature due to heat released during O₂ adsorption. Thermal runaway, i.e., self-ignition, was observed in some cases even at the initial biochar temperature of ca. 200 °C. However, if O₂ is not permeable through the container materials, the temperature starts decreasing after the consumption of O₂ in the container. The simulation model was also applied to examine important factors related to self-heating. The results suggested that self-heating can be somewhat mitigated by decreasing the void fraction, reducing storage volume, and lowering the initial char temperature. This study demonstrated a robust way to estimate the cooling demands required in the biochar production process.