Char Oxidation Research Articles

Recovery of Carbon Fibre from Waste Prepreg via Microwave Pyrolysis.

Management of waste from carbon fibre composites has become a significant societal issue as the application of composite grows across many industries. In this study, carbon fibres (CF) were successfully recovered from cured carbon fibre/epoxy (CF/EP) prepreg under microwave pyrolysis at 450, 550 and 650 °C followed by oxidation of any residual char. The recovered fibres were investigated for their tensile properties, surface morphologies and the elements/functional groups presented on the surface. The chemical compositions of gaseous and oil pyrolysis products were also analysed. The microwave pyrolysis effectively pyrolyzed the epoxy (EP) resin. Char residue remained on the fibre surface and the amount of char reduced as the pyrolysis temperature increased. Compared to virgin fibres, the recovered fibre suffered from a strength reduction by less than 20%, and this reduction could be mitigated by reducing the pyrolysis temperature. The surface of recovered fibre remained clean and smooth, while the profile of elements and functional groups at the surface were similar to those of virgin fibres. The main gaseous products were CO, H2, CO2 and CH4, whilst the liquid product stream included phenolic and aromatic compounds.

A TGA–DSC-based study on macroscopic behaviors of coal–oxygen reactions in context of underground coal fires

Underground coal fires (UCFs) cause remarkable loss of energy resources and significant environmental pollution. Due to the limited capacity of oxygen transport, the inception and development of UCFs represent a very unique mode of coal–oxygen reactions. Therefore, a high-volatile flammable coal sample is thermally analyzed with the combined TGA–DSC approach under four oxygen concentrations (21%, 15%, 9% and 3%) and three heating rates (1 °C min−1, 2 °C min−1 and 5 °C min−1). It is found that the oxygen concentration does not significantly influence the early (low-temperature) stage of coal–oxygen reactions. With the decrease in oxygen concentration, the intensity of the exothermic reactions is reduced and the duration of reactions is extended. Based on the experimental results, the apparent activation energy is calculated. The variation of the apparent activation energy reflects the different reaction stages: volatiles burning and char oxidation, which is verified by the TGA–DSC results. Under the extreme condition of 3% oxygen concentration, a very distinct macroscopic thermochemical behavior is observed, and the limited oxygen supply controls the reaction rate throughout the entire process, which qualitatively explains the persistency of the burning phenomena in most UCFs.

Derivation of the kinetics of devolatilisation and oxidation of pulverized biomass in a drop tube furnace: Sensitivity to volume evolution and drag-coefficient model

Combustion experiments of raw and torrefied pine and demolition wood particles (600–800 µm) are performed at 800 °C in a drop tube furnace. The results provide the oxygen and carbon monoxide profiles along the reactor axis. These data are then used in a numerical model, developed to determine the kinetic parameters of devolatilisation and oxidation of the pulverized biomasses. In order to simulate the gas phase reactions, the model also takes as input the composition of the volatiles of the tested fuels measured during pyrolysis experiments at 800 °C in the drop tube furnace. The model adopts different scenarios of particle volume evolution and different drag coefficient models in order to test their influence on the derived kinetic parameters. One of the volume evolution scenarios is a specific sub-model obtained by optical diagnostics of the combustion of the three biomasses in a previous study. Four other volume sub-models found in literature are also tested. For each of these scenarios, the model estimates close activation energy for devolatilization with a maximum variation of 2 kJ·mol−1 from one scenario to another, while the activation energy of char oxidation is more influenced, varying by 14 kJ·mol−1 with different scenarios. The five scenarios show similar gas concentrations and burnout versus the distance travelled by the particle. Nevertheless, this gives rise to a noticeable difference in the particle temperature along the furnace axis (±100 °C at some positions), in addition to different particle velocity and residence time (~ ±10%). The influence of the drag force is also studied using enhanced non-spherical model versus a spherical model. The non-spherical model leads to 10 to 14 kJ·mol−1 higher devolatilisation activation energies and 10 to 19 kJ·mol−1 higher char oxidation activation energies than the spherical model, along with a better prediction of the CO levels.

Evaluation of Biochar Post\u2010Process Treatments to Produce Soil Enhancers and Phosphorus Fertilizers at a Single Plant

This study evaluates the potential to produce phosphorus (P)-rich fertilizer substrates with high plant availability as well as carbon (C)-rich biochar with soil enhancement properties in a single slow-pyrolysis plant. Campaign-based production or co-production of soil enhancers and fertilizer substrates may increase the potential societal value of slow pyrolysis plants. The assessment focus on conventional slow pyrolysis operated at 600 °C to produce biochar from various substrates as well as two options for post-process char treatments—char oxidation at 550 °C and char steam gasification at 800 °C, as a potential way to improve substrate fertilizer value. Four P-rich biomass residues including municipal sewage sludge (SS), biogas fiber (BF), cattle manure (CM), and poultry manure (PM) as well as two C-rich biomasses: wood chips (WC) and wheat straw (WS), were tested. Production yields of biochar and ash from char oxidation and steam gasification were compared and the materials were characterized to be used as soil enhancers and P-fertilizers through direct analysis and soil incubation studies with two different agricultural soils. All thermal treatments increased the concentration of the plant nutrients P, potassium and magnesium in the resulting biochar and ashes compared to the dry biomass. At the same time, concentrations of nitrogen and sulfur were reduced. The dry biomasses generally increased the amount of available P in the soils to a greater extent than biochar or ashes at an application rate of 80 mg P/kg soil. The P-rich biochar and ash made from BF, CM and PM had higher P fertilizer values than those made from SS. In terms of thermal processes, pyrolysis with subsequent char steam gasification was found to be the best option for high P availability in both soils, except for operation on SS where the oxidized char gave the best results. The C-rich biochars made from wood and wheat straw both showed potential for improving soil properties including soil organic matter (SOM) content, cation exchange capacity (CEC) and water holding capacity (WHC). The study shows that campaign operation of slow pyrolysis with the option for char steam gasification is a viable option for producing fertilizer substrates with high levels of plant available P as well as biochar with substantial soil enhancing properties on a single plant. In addition, results also indicate that direct co-pyrolysis of P-rich substrates—especially BF and CM, with any of the two tested C-rich substrates—without subsequent char treatment may be a sufficiently well integrated option for combined soil fertility and soil P fertilization management.Graphic

Multi‐scale simulations of the thermal degradation of thin wooden plates: Evaluation of two kinetic mechanisms

SummaryThis study evaluates two kinetic mechanisms to predict, on matter‐level and material‐level scales, the thermal degradation of thin plates of oak and eucalyptus as representing vegetative fuel involved in wildfires. The lumped mechanism considers four successive steps, whereas the simplified one includes two steps. The kinetic parameters of these mechanisms were identified with thermogravimetric analysis in air for heating rates between 2 and 30 °C min−1. On the matter‐level scale, kinetic mechanisms were tested for heating rates inside and outside the parameters optimization range. The simulations of the mass loss were close to the experimental data. The lumped mechanism gave better results than the simplified one. On the material‐level scale, the mass loss and temperature recorded during thermal degradation of thin wooden plates, with a heating cone, were compared to simulations performed with GPYRO. When only the gasification stage occurred, both mechanisms predicted results close to the experimental data. When char oxidation occurred, the beginning of the gasification stage was well predicted, whereas some differences appeared during the transition between the gasification and char oxidation stages. The performance of both mechanisms indicates that a two‐step mechanism is capable of modeling the thermal degradation of thin wooden plates on a material scale.

Investigation of Turbulent Pulverized Solid Fuel Combustion with Detailed Homogeneous and Heterogeneous Kinetics

A comprehensive Euler–Lagrange framework for pulverized coal combustion using detailed multi-step heterogeneous kinetics is presented. The heterogeneous kinetics employ the POLIMI model that involves 37 species (22 solid species and 15 gas species) and 49 reactions to describe detailed pyrolysis as well as char oxidation, gasification, and annealing for a wide range of coals. The porous structure of the coal particles is considered, and the heterogeneous reactions are assumed to occur throughout the entire particle in a volume-based approach. The ordinary differential equations of the heterogeneous kinetics are integrated on each Lagrangian coal particle and predict the conversion of the raw coal components to light volatile hydrocarbons, heavy tar species, and char off-gases. Hence, the composition of the solid fuel components and the released gas changes dynamically in space and time, providing high-fidelity predictions of solid fuel combustion. The chemical conversion of the released species in the gas phase is described by a homogeneous kinetic mechanism with 76 species and 973 reactions that was reduced from the comprehensive CRECK-G-1407 kinetic mechanism. The new modeling framework is employed within carrier-phase direct numerical simulations (CP-DNS) of pulverized coal combustion in a three-dimensional turbulent mixing layer. This configuration includes the additional physics of turbulence and particle group combustion by mixing solid fuel particles suspended in a primary oxidizer stream with the products from lean volatile combustion in a secondary stream. The CP-DNS results are analyzed with and without the available set of 14 char conversion reactions, and a low degree of char conversion indicated by an increased rate of CO production is captured for particles with temperatures higher than 1800 K. The CP-DNS results from the detailed POLIMI approach feature a distinct bimodal shape of the volatile release curve and multi-regime combustion. The POLIMI data are used to evaluate the predictive capability of simpler pyrolysis models. The original competing two-step model (C2SM) by Kobayashi is investigated and shown to predict heavily delayed ignition. A new competing two-step devolatilization approach is proposed as an alternative model reduction suitable for fitting bimodal volatile release rates, such as that predicted by POLIMI. The CP-DNS using the alternative pyrolysis model faithfully captures the onset of ignition and multi-regime flame branches. Differences arise in the local tar species compositions in the gas phase as a result of the time-varying (POLIMI) and fixed (new C2SM) volatile compositions for the respective models. The flame structure is further analyzed by chemical explosive mode analysis (CEMA), and the occurrence of premixed and non-premixed flames zones is confirmed, whereas a simpler flame index analysis fails to correctly indicate the multi-regime nature of the flame. This recognition of multi-regime combustion serves as a guidance for selecting suitable conditioning variables for flamelet and other combustion submodels in large eddy simulation.

Nox formation in fixed-bed biomass combustion: Chemistry and modeling

The renewable and carbon neutral nature of biomass makes it an alternative clean energy resource. However, NOX emission during biomass combustion, a widely used approach for extracting energy from biomass, poses serious environmental concerns. A fundamental investigation of nitrogen species formation mechanism during biomass combustion is important to minimize NOX and nitrous oxide emissions. In this study, a comprehensive computational fluid dynamics (CFD) model that combines nitrogen chemistry with flow and combustion simulations is presented. We call this model BASIC: bulk accumulated solids incineration code. BASIC combined sub-models for drying, devolatilization, volatiles combustion, and char oxidation, nitrogen chemistry, and conservation equations. The model is first validated against data on fixed bed combustion of biomass from literature. Results show that particle size and the initial temperature have significant impact on NO and N2O formation, whereas pressure shows a less significant effect. NOX formation mechanism pathways show that the oxidation of ammonia has a significant influence on NO production, while the reduction of NO by surface dominated hydrogen play an important role in reducing its concentration in the gas phase. Net N2O formation is determined both by the reactions of precursors with NO and the process of N2O decomposition to N2. Unlike prior CFD models, BASIC can predict not only N2O formation for biomass combustion, an important greenhouse gas, but also optimal parameters, e.g., particle size and temperature, for the lowest NOX production.

Characteristics of two-stage air gasification of polystyrene with active carbon as a tar removal agent

Air gasification of polystyrene was conducted using a two-stage gasifier consisting of a fluidized bed and a tar-cracking reactor filled with active carbon. The aim was to obtain a hydrogen-rich producer gas with a low level of tar. In addition to the possibility of tar removal, the effects of the reaction temperature and equivalence ratio on the producer gas quality were investigated. In this study, it was found that the gasification of polystyrene had different characteristics to the gasification of other plastic, resulting in a high production of char. Active carbon played a crucial role, significantly decreasing the tar content in gas to 11 mg/Nm3. Furthermore, gasification with active carbon produced a gas having a high content of hydrogen (26 vol%). The change in fluidized bed gasifier temperature within the range of 700–900 °C exerted no significant effects on the gas quality. In contrast, a high tar-cracking reactor temperature clearly increased hydrogen and carbon monoxide contents. With an increasing equivalence ratio, oxidation of char was promoted, resulting in a significant increase in gas production and carbon oxides. The current study showed a good possibility for the recycling of polystyrene via gasification, producing a clean and hydrogen-rich gas.

Calibration and validation of a comprehensive kinetic model of coal conversion in inert, air and oxy-fuel conditions using data from multiple test rigs

This work presents detailed information on pyrolysis and char oxidation for a high-volatile Colombian bituminous coal. The investigation includes experiments at low and high particle heating rates, performed in a thermogravimetric analyzer (TGA), a drop-tube reactor (DTR), a flat-flame burner (FFB) and a fluidized-bed reactor (FBR). The TGA and DTR data were used when developing and calibrating the kinetic model for the conversion of coal in air and oxy-fuel atmospheres, while the FFB and FBR data were used to validate the resulting mechanism. The proposed model is an updated version of the CRECK-S-C model from the Politecnico di Milano (PoliMi), consisting of a fuel characterization step, coupled with a multi-step kinetic mechanism based on reference coals. Both the devolatilization and heterogeneous char reactions are accounted for and interconnected seamlessly. Key reactions were introduced and the existing reactions were calibrated to account for the particularities of this fuel and the effects of the abundant CO2 concentration in the reactors. The importance of successive gas-phase reactions was observed and a gas-phase kinetic model was coupled to properly simulate such conditions. The resulting model is applied to simulate and systematically evaluate the experimental findings, highlighting the model’s features and limitations.

The influence of temperature on the nature and stability of surface-oxides formed by oxidation of char

A coal char has been oxidized isothermally at temperatures comprised between 300 and 1073 K. The pre-oxidized chars have been subjected to Temperature Programmed Desorption (TPD) and to core-level high-resolution X-ray photoelectron spectroscopy (XPS) analysis using Synchrotron radiation to infer the nature of the carbon oxides that populate the surface and their evolution throughout thermochemical processing. For low oxygen coverages and mild oxidation temperatures the prevailing carbon-oxygen moieties are epoxy. Raising the oxidation temperature up to ~723K the edge carbon oxygen complexes (ether-hydroxyl and carbonyl-carboxyl) increase. The amounts of CO + CO2 desorbed during TPD also increase with temperature and duration of oxidation for relatively mild oxidative treatments (temperature below ~723K). Upon further increase of the oxidation temperature the amount of CO + CO2 decrease and the ratio of CO/CO2 increases remarkably. Altogether, results suggest the existence of a strong link between a remarkable shift of surface oxides from epoxy to ether/carbonyl and the desorption of CO and CO2. Moreover, the CO/CO2 ratio during desorption can be well correlated with the relative abundance and stability of epoxy moieties with respect to the “edge” oxides. Results are analyzed in the frame of a semi-lumped kinetic model of carbon oxidation with a focus on the role and nature of surface oxides as intermediates in carbon gasification reactions.

Role of CO2 and H2O on NO Formation during Biomass Char Oxidation

We have investigated how CO2 and H2O influence NO release rates during char oxidation of biomass. In addition, the role of catalytic elements in these processes has been studied. NO release rates w...

Numerical Simulation of the Burning Process in a King-Size Cigarette Based on Experimentally Derived Reaction Kinetics

Summary A comprehensive two-dimensional (2D) mathematical model has been proposed to simulate the burning process of a king-size cigarette. The characteristics of this model are including: 1) the use of kinetic models for the evaporation of water, the pyrolysis of tobacco and the oxidation of char, 2) the application of mathematical relationships between the release amounts of certain products (i.e., “tar” and CO) and different reaction variables (i.e., temperatures and oxygen concentrations), 3) the introduction of mass, heat and momentum transports, 4) the consideration of filtration effects of the cigarette filter on “tar”. These characteristics were expressed in a set of coupled equations that can be solved numerically by FLUENT. The information about the char density field, temperature field, flow velocity field, “tar” and CO density fields and the filtration efficiency could be obtained from the model. This model was validated by comparing the predictions with experimental data on puff number, the temperatures at specific locations, the filtration efficiency and the yields of “tar” and CO under different puff intensities. The calculated results show a good agreement with the experimental data. The predicted puff number was 7.3, and the experimental puff number was 6.8. The standard root mean square error (NRMSE) between the experimental and the predicted temperatures at specific locations is < 18%. The predicted filtration efficiency for “tar” was 46.1%, and the experimentally determined filtration efficiency for nicotine was 44.5%. The maximum relative deviations of the yields of “tar” and CO under different puff intensities were 8.9% and 10.6%, respectively.

Functionalization of sewage sludge char by partial oxidation with molecular oxygen to enhance its adsorptive properties

Abstract This study investigates the activation of sewage sludge char through partial oxidation with molecular oxygen with the goal to increase the concentration of oxygen-containing surface functional groups and to enhance its ammonium adsorption capacity. The effects of oxidation temperature (200–350 °C), oxygen concentration (5–21 vol %) and oxidation time (30–120 min) on the degree of char oxidation are analyzed. The results obtained by thermogravimetric and elemental analysis, and Fourier Transform Infrared and X-ray Photoelectron spectroscopy corroborate the incorporation of oxygenated functional groups on the surface of sewage sludge char either at low temperature (200 °C) using air or at higher temperature (350 °C) using 5% O2 for 30 min. The oxidation of sewage sludge char hardly increases its specific surface area. The operating conditions during oxidation must be carefully controlled to avoid runaway reactions that lead to total oxidation of char. The evolution of CO and CO2 during oxidation is a good indicator of the advance of the oxidation/burn-off reactions. The ammonium adsorption capacity of sewage sludge char increases after oxidation under mild operating conditions, providing support to the idea of onsite utilization of sewage sludge as ammonium adsorbent in waste water treatment plants. This would provide a step forward towards the implementation of a circular economy.

Catalytic influence of mineral compounds on the reactivity of cellulose-derived char in O2-, CO2-, and H2O-containing atmospheres

The catalytic effects of mineral compounds on the conversion of a biomass-derived char in air- and oxyfuel-related atmospheres were investigated by thermogravimetric analysis at atmospheric pressure. The applied char originated from the hydrothermal carbonization (HTC) of cellulose followed by pyrolysis at 1073 K and subsequent mixing with 20 wt% of minerals by grinding to achieve tight contact. The reactivities of the mineral-loaded HTC chars were evaluated based on isothermal experiments in O2-, CO2-, and H2O-containing atmospheres as a function of their composition applying a magnetic suspension balance. The reactivity sequence K2CO3 > Na2CO3 ≫ Fe2O3 > CaO > MgO ≥ mineral-free was derived consistently for char oxidation in O2/inert as well as for char gasification in diluted H2O and CO2 mixtures. In addition to this qualitative assessment, the kinetic experiments were first modelled based on a simple global nth-order power-law rate expression. Then, the more complex Carbon Burnout Kinetics (CBK/G) model and the PoliMi model were applied. All three modeling approaches enabled a systematic quantification of the catalytic effects and led to a comparable lowering in the apparent activation energy. In combination with the kinetic parameters determined for the mineral-free char, the lowered apparent activation energies specific for the applied mineral and atmosphere facilitate the implementation of catalytic effects on the conversion of biomass-derived char into combustion models.

Ash Fusion During Combustion of Single Corn Straw Pellets

AbstractThis investigation identified conditions at which corn straw ash melts and examined how this phenomenon affects the combustion of char residues. Corn straw was pelletized in cylinders and was burned at elevated temperatures in the range of 1200–1400 °C, and at different air flow velocities. The pellets were inserted in a preheated furnace, where they were subjected to moderately high heating rates. Their combustion behavior was observed with cinematography, thermometry, and thermogravimetry. Upon insertion in the furnace, the pellets devolatilized and formed volatile envelope flames, upon extinction of which, the chars experienced concurrent heterogeneous combustion and ash fusion. Residues were assessed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), coupled to energy dispersive spectroscopy (EDS). The burnout times of the volatiles and the chars decreased drastically with increasing temperature. At 1300 °C and 1400 °C, the remaining ash underwent complete melting and the final structures of the pellets collapsed to molten pools. At 1400 °C, all of the chlorine and most of the potassium were released into the gas phase. The straw ash was identified as a high-density silicate melt. Although surface ash melted completely, it flowed to the base of the pellet. Therefore, it did not significantly hinder the oxidation of the carbonaceous char. Hence, to increase the likelihood of complete corn straw carbon burnout and of ash melting and flowing to the bottom of the furnace, operating temperatures higher than 1300 °C, in conjunction with mild air flow rates, are recommended.

Nano-carriers effects on the viability and efficiency of Pseudomonas strains as phosphate solubilizing bacteria

In order to develop nanotechnology application in the agricultural systems achieving more sustainability in the environment, we have used different nano-carriers for phosphate solubilizing bacteria. The viability and efficacy of two bacterial species; Pseudomonas putida (PP20) and Pseudomonas kilonensis (PK11) in solubilizing phosphate sources (i.e., tricalcium phosphate and hydroxyapatite) with different nano-carriers including nanoclay, natural char micro-particles (NCMPs), nanoclay + alginate, NCMPs + alginate, and natural char nano-particles (NCNPs)+alginate were investigated. Clay, talc powder, and natural char (NC) were included for comparison. The synthesized NCNPs and NCMPs were characterized using FTIR, SEM and Boehm titration analyses. The results confirmed that the chemical oxidation of pristine char made many oxygenated functional groups on the surface of tiny and spherical NCNPs (14.8 nm) which caused their effective incorporation in the matrix of alginate beads. Results of phosphate solubilizing study showed that P. kilonensis was the superior species for viability and stability of its performance on solubilizing phosphorus. The six months evaluation showed that NCNPs + alginate and nanoclay + alginate carriers at both temperatures (4 °C and 28 °C), were the proficient carriers for preserving both bacteria. The results of solubilizing phosphorus sources revealed that both bacteria solubilized tricalcium phosphate more than hydroxyapatite and PK11 showed more privilege in this regard. In addition, the solubilizing index determined after storage for 6 months at 4 °C was higher for all the carriers. Analysis of variance for phosphatase activity revealed that embedding both bacteria in nanoclay + alginate carrier guaranteed the highest phosphatase activity, even though differences between this carrier and NCNPs + alginate and NCMPs + alginate were not significant for the PK11.

Particle temperature and potassium release during combustion of single pulverized biomass char particles

This work investigated the combustion characteristics of single pulverized biomass-derived char particles. The char particles, in the size range 224–250 µm, were prepared in a drop tube furnace at pyrolysis temperatures of 1273 or 1473 K from four types of biomass particles – wheat straw, grape pomace, kiwi branches and rice husk. Subsequently, the char particles were injected upward into a confined region of hot combustion products produced by flat flames stabilized on a McKenna burner, with mean temperatures of 1460, 1580 and 1670 K and mean O2 concentrations of 4.5, 6.5 and 8.5 vol%. The data reported include particle temperature, obtained using a two-color pyrometry technique, and potassium release rate, measured using a laser-induced photofragmentation fluorescence imaging technique. In addition, particle ignition delay time and burning time, obtained from the temporal evolution of the thermal radiation intensity of the burning char particles, are also reported. The results indicated that ignition of the char particles occurs simultaneously with the starting of the potassium release, then the particle burning intensity increases rapidly until it reaches a maximum, after which both the particle temperature and the potassium release rate remain approximately constant until the end of the char oxidation process. The char ignition process is temperature controlled, and the char oxidation process is oxygen diffusion controlled, with the total potassium release being independent of the oxygen concentration and the temperature of the combustion products. The combustion behavior of the chars studied is more affected by the char type than by the conditions used to prepare them.

Simultaneous in-situ measurements of gas temperature and pyrolysis of biomass smoldering via X-ray computed tomography

In-situ X-ray computed tomography (XCT) imaging is employed to investigate the smoldering dynamics of biomass at the sub-millimeter scale. This technique provides simultaneous and spatially-resolved information about the gas temperature and the biomass density, thereby enabling tracking of the pyrolysis and char oxidation fronts. To achieve well-controlled heating and flow conditioning, oak biomass samples are instrumented above a diffusion flame inside a tube, with total oxygen concentrations of 6% and 11% per volume. Experiments are performed on a laboratory XCT system. The flow is diluted with Kr to increase X-ray attenuation in the gas phase thus allowing for simultaneous 3D measurements of sample density and surrounding temperature. XCT scans are acquired every 90 s at a spatial resolution of 135 µm. The high spatial resolution enables the volumetric visualization of the smoldering process that is associated with pyrolysis and char oxidation. These measurements show how the grain structure affects flame stabilization and induces fingering of the pyrolysis front, while crack formation accelerates the char oxidation locally. Evaluations of the sample mass via XCT are compared with load cell measurements, showing good agreement. A low-order model is developed to evaluate the propagation speeds of pyrolysis and oxidation fronts from the X-ray data over time, and comparisons are made with the surface recess speed.

CFD modelling of air staged combustion in a wood pellet boiler using the coupled modelling approach

Optimal geometric and process parameters related to air staging, which ultimately lead to lower pollutant formation and a higher thermal efficiency, can be found by utilisation of the CFD simulation method. Simplified and computationally efficient methods for the simulation of complex phenomena which occur in a biomass boiler can, therefore, be of great use, provided they offer reliable and reasonably accurate predictions. A previously developed and validated Computational Fluid Dynamics combustion simulation methodology for large-scale waste to energy grate-fired boilers is modified and implemented for the case of a commercial 32 kW wood pellet hot water boiler. An experimental investigation is performed in order to determine process parameters used in the fuel conversion modelling, case setup and, finally, to enable the validation of the numerical modelling approach. The solid fuel conversion is modelled by a coupled empirical 1D bed model which predicts the composition of the combustible gas mixture released from the fuel bed into the freeboard. Compared to the originally proposed fuel bed conversion model, the modified model includes partial char and volatile oxidation, and relies on gas temperatures measured above the fuel bed. Using the modified fuel bed conversion model, several cases were simulated in order to find the most appropriate parameters of the combined finite rate – eddy dissipation combustion model. Results show that by implementing the modified fuel bed conversion model that relies on measurements, reasonably accurate predictions can be achieved of the gas temperature in the combustion chamber and CO concentration at the flue gas outlet.

Effect of phosphorus (P) on the structure and reactivity of biochars produced from the pyrolysis of acid-washed biomass loaded with P of various forms

This paper investigates the effect of phosphorus (P) on char structure and reactivity of char prepared from the fast pyrolysis of purposely-prepared P-loaded biomass samples at 1000 °C in absence of other inorganic species. Biomass was first acid-washed then loaded with P of three different occurrence forms (one organic P i.e. phytic acid, and two inorganic P i.e. orthophosphoric acid and polyphosphoric acid) at the same P content of 0.8 wt%. Experimental results show that both organic and inorganic P substantially increase char yields during pyrolysis from 6.2% for the biomass sample without P to 23.0–26.0% for P-loaded samples due to the enhanced crosslinking by P-containing structures in char, leading to increases in the char C and H contents and decrease in O content. The presence of P in biochars from fast pyrolysis of various P-loaded biomass samples plays important role in the evolution of char structure and intrinsic reactivity measured during low-temperature oxidation at 500 °C in air under chemical-reaction-controlled regime. After pyrolysis and subsequent char oxidation, all P in biomass either as organic or inorganic P are found to be present in forms of acid-insoluble organic structures. For char prepared from acid-washed wood, char reactivity increases with char conversion due to the increasing pore surface area at higher conversion. Comparatively, for char prepared from acid-washed wood loaded with various P at char conversion below 60%, the presence of P increases char intrinsic reactivity due to the enhanced crosslinking of reactive carbon structures and reduced condensation of char structures. However, at conversions above 60%, P-containing species in char lead to a significant decrease in char reactivity, due to the formation of abundant CO-P bonds, that is highly resistant to the oxidation in air, in the reacting chars.