Kinetics of woodchips char gasification with steam and carbon dioxide

Kinetics of woodchips char gasification has been examined. Steam and CO₂ were used as the gasifying agents. Differences and similarities between kinetics of steam gasification and CO₂ gasification have been discussed. Comparison was conducted in terms of gasification duration, evolution of reaction rate with time and/or conversion, and effect of partial pressure on reaction rate. Reactor temperature was maintained at 900 °C. Partial pressure of gasifying agents varied from 1.5 bars to 0.6 bars in intervals of 0.3 bars. Steam and CO₂ flow rates were chosen so that both gasifying agents had equal amount of oxygen content. CO₂ gasification lasted for about 60 min while steam gasification lasted for about 22 min. The average reaction rate for steam gasification was almost twice that of CO₂. Both reaction rate curves showed a peak value at certain degree of conversion. For steam gasification, the reaction rate peak was found to be at a degree of conversion of about 0.3. However, for CO₂ gasification the reaction rate peak was found to be at a conversion degree of about 0.1. Reaction rates have been fitted using the random pore model (RPM). Average structural parameter, ψ for steam gasification and CO₂ gasification was determined to be 9 and 2.1, respectively. Average rate constant at 900 °C was 0.065 min⁻¹ for steam gasification and 0.031 min⁻¹ for CO₂ gasification. Change in partial pressure of gasifying agents did not affect the reaction rate for both steam and CO₂ gasification.     

Numerical studies on the combustion properties of char particle clusters

Particle clustering is an important phenomenon in dense particle–gas two-phase flow. One of the key problems worth studying is the reacting properties of particle clusters in coal particle combustion process in the dense particle region. In this paper, a two-dimensional mathematical model for the char cluster combustion in airflow field is established. This char cluster consists of several individual particles. The comprehensive model includes mass, momentum, and energy conservation equations for both gas and particle phases. Detailed results regarding velocity vector, mass component, and temperature distributions inside and around the cluster are obtained. The micro-scale mass and heat transfer occurred inside and around the char cluster are revealed. By contrastively studying the stable combustion of char particle clusters consisting of different particles, the combustion properties of char clusters in various particle concentrations are presented and discussed.     

Mutual effects of porosity and reactivity in char oxidation

The motivation for this review is the need to understand the interdependence of porous structure and reactivity of highly porous carbonaceous materials during oxidation. These materials can be oxidized in three regimes: regime I, kinetically controlled conditions; regime II, partial diffusion-controlled conditions; regime III, diffusion controlled conditions. Since the emphasis here is on the porous structure and its influence on reactivity, conditions where transport processes are dominant were not included for they mask the view of interest. Therefore, the review discusses only physicochemical processes occurring during oxidation of highly porous chars in regime I. Furthermore, reactivity is influenced by many factors, such as catalysis, volatile matter, and water content. To avoid the effect of these factors, highly porous synthetic chars with nothing but elemental carbon and residual hydrogen and oxygen was chosen. Mainly, we discuss a commercial product known as Spherocarb which consists of spherical particles with specific surface area of about 1000 m² g⁻¹ and porosity of about 0.6. These particles are well defined and reproducible in their properties. They serve well as model materials for various synthetic chars, coal chars, and other carbonaceous materials. The review presents in a systematic manner macroscopic properties and processes that shed light on different aspects of porosity and reactivity. These are presented both from experimental observation as well as modeling view. An attempt was made to present a porous structure model that can reconstruct all available experimental data on these particles during oxidation. In the review the following processes and properties are discussed: shrinkage, fragmentation, and porosity. All are directly connected to porous structure and reactivity.     

Combustion and gasification rates of lignocellulosic chars

This review critically examines the state of the art of rate laws and kinetic constants for the gasification, with carbon dioxide and steam, and the combustion of chars produced from lignocellulosic fuels, including a brief outline about yields and composition of pyrolysis products. The analysis also gives space to the role played by various factors, such as heating rate, temperature and pressure of the pyrolysis stage, feedstock and content/composition of the inorganic matter, on char reactivity. Finally, directions for future research are suggested.     

A new flame sheet model to reflect the influence of the oxidation of CO on the combustion of a carbon particle

A model with a moving flame front is proposed for the combustion of a carbon particle, taking into account the effect of CO oxidizing in the boundary layer around the particle. Using this model, the continuous transition of the effective combustion product from CO₂ under the ignition condition to CO under the condition of diffusion control has been successfully realized. Good agreement was obtained with the experimental measurements of Young and Niksa; such agreement could not be obtained using the customary single-film model.     

Syngas yield during pyrolysis and steam gasification of paper

Main characteristics of gaseous yield from steam gasification have been investigated experimentally. Results of steam gasification have been compared to that of pyrolysis. The temperature range investigated were 600–1000 °C in steps of 100 °C. Results have been obtained under pyrolysis conditions at same temperatures. For steam gasification runs, steam flow rate was kept constant at 8.0 g/min. Investigated characteristics were evolution of syngas flow rate with time, hydrogen flow rate and chemical composition of syngas, energy yield and apparent thermal efficiency. Residuals from both processes were quantified and compared as well. Material destruction, hydrogen yield and energy yield is better with gasification as compared to pyrolysis. This advantage of the gasification process is attributed mainly to char gasification process. Char gasification is found to be more sensitive to the reactor temperature than pyrolysis. Pyrolysis can start at low temperatures of 400 °C; however char gasification starts at 700 °C. A partial overlap between gasification and pyrolysis exists and is presented here. This partial overlap increases with increase in temperature. As an example, at reactor temperature 800 °C this overlap represents around 27% of the char gasification process and almost 95% at reactor temperature 1000 °C.     

Energy conversion performances during biomass air gasification process under microwave irradiation

Biomass gasification technology under microwave irradiation is a new and novel method, and the energy conversion performances during the process play a guiding role in improving the energy conversion efficiencies and developing the gasification simulation models. In order to improve the energy utilization efficiency of microwave biomass gasification system, this study investigated and presented the energy conversion performances during biomass gasification process under microwave irradiation, and these were materialized through detailing (a) the energy conversion performance in the microwave heating stage, and (b) the energy conversion performance in the microwave assisted biomass gasification stage. Different forms of energies in the biomass microwave gasification process were calculated by the method given in this study based on the experimental data. The results showed that the useful energy (energy in silicon carbide (SiC), 18.73 kJ) accounted for 31.22% of the total energy input (electrical energy, 60.00 kJ) in the heating stage, and the useful energy (energy in the products, 758.55 kJ) accounted for 63.41% of the total energy input (electrical and biomass energy, 1196.28 kJ) in the gasification stage. During the whole biomass gasification process under microwave irradiation, the useful energy output (energy in the products, 758.55 kJ) accounted for 60.38% of the total energy input (electrical and biomass energy, 1256.28 kJ), and the energy in the gas (523.40 kJ) product played a dominate role in product energy (758.55 kJ). The energy loss mainly included the heat loss in the gas flow (89.20 kJ), magnetron loss (191.80 kJ) and microwave dissipation loss (198.00 kJ), which accounted for 7.10%, 15.27% and 15.76% of the total energy, respectively. The contents detailed in this study not only presented the energy conversion performances during microwave assisted gasification process but also supplied important data for developing gasification simulation models.     

Analysis of turbulent combustion in inert porous media

The objective of this paper is to present an extension of a simplified reaction kinetics model that, combined with a thermo-mechanical closure, entails a full-generalized turbulent combustion model for flow in porous media. In this model, one explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. These mass fractions are double decomposed in time and space and, after applying simultaneous time-and-volume integration operations to them, distinct terms arise, which are here associated with the mechanisms of dispersion and turbulence. Modeling of these extra terms remains an open question and the derivations herein might motivate further development of models for turbulent combustion in porous media.     

Kinetics of devolatilization and oxidation of a pulverized biomass in an entrained flow reactor under realistic combustion conditions

In this paper the results of a complete set of devolatilization and combustion experiments performed with pulverized (∼500 μm) biomass in an entrained flow reactor under realistic combustion conditions are presented. The data obtained are used to derive the kinetic parameters that best fit the observed behaviors, according to a simple model of particle combustion (one-step devolatilization, apparent oxidation kinetics, thermally thin particles). The model is found to adequately reproduce the experimental trends regarding both volatile release and char oxidation rates for the range of particle sizes and combustion conditions explored. The experimental and numerical procedures, similar to those recently proposed for the combustion of pulverized coal [J. Ballester, S. Jiménez, Combust. Flame 142 (2005) 210-222], have been designed to derive the parameters required for the analysis of biomass combustion in practical pulverized fuel configurations and allow a reliable characterization of any finely pulverized biomass. Additionally, the results of a limited study on the release rate of nitrogen from the biomass particle along combustion are shown.     

Rapid pyrolysis and CO2 gasification of anthracite at high temperature

Anthracite could be burnt efficiently at high temperature utilizing oxy-coal technology. To clarify the effects of temperature and atmosphere on char porosity characteristics, char morphology, fuel-N conversion, and reducing products release, rapid pyrolysis and CO₂ gasification of anthracite was carried out in a high temperature entrained-flow reactor to simulate the condition in a pulverized coal furnace. Developed pore structure was formed in the gasification chars, which could be contributed to charCO₂ reaction at high temperatures. More mesopores were formed in internal carbon skeleton and retained against collapse and coalescent for gasification chars than pyrolysis chars. Compared with pyrolysis char, smoother and denser surface was observed in gasification char with the irregular bulges disappeared due to the destruction of external carbon skeleton. Char-N could be oxidized to NO in CO₂ atmosphere and then reduced to N₂ by (CN) on the char surface. Char-N release was greatly promoted due to gasification reaction along with poly-condensation at high temperature; and the preact release of char-N would result in a larger portion of NO_x reduction in the following reduction zone with the oxygen-staging combustion technology compared with that in air-staging combustion. Complementally, homogeneous reduction in NO_x emission would play a minor effect for anthracite in oxy-coal combustion because of the deficiency of CH₄ and HCN, especially at high temperature.     

The combustion of large particles of char in bubbling fluidized beds: The dependence of Sherwood number and the rate of burning on particle diameter

Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of , burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O₂ to the burning solid and of the products CO and CO₂ away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, dchar (for ). This linear dependence of Sh on dchar is expected from the basic equation Sh=2εmf(1+dchar/2δdiff)/τ, provided the thickness of the boundary layer for mass transfer, δdiff, is constant in the region of interest (). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for ), the thickness of the boundary layer for mass transfer in a fluidized bed, δdiff, is less than that predicted by empirical correlations based on the Frössling expression. In fact, δdiff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation

     

Effects of particle sizes on transport phenomena in single char combustion

A one-dimensional char combustion model including pore structure effects is used to numerically investigate single char particle combustion for several different types of char samples. Previously, it is expected that small char particles have less combustion time. However, the present work shows that this is true only if the combustion time is defined as that needed for a char particle diameter diminished below a certain value. If the combustion time is defined as time needed for the carbon conversion ratio higher than a certain value, there are optimal particle sizes in a limited combustion period. Just reducing the char particle sizes may not get high carbon conversion ratios. It has also been found that, in general, the larger particles have higher temperatures at the exterior surfaces.     

Analysis and modeling of char particle combustion with heat and multicomponent mass transfer

A char combustion model suitable for a large-scale boiler/gasifier simulation, which considers the variation of physical quantities in the radial direction of char particles, is developed and examined. The structural evolution within particles is formulated using the basic concept of the random pore model while simultaneously considering particle shrinkage. To reduce the computational cost, a new approximate analytical boundary condition is applied to the particle surface, which is approximately derived from the Stefan–Maxwell equations. The boundary condition showed reasonably good agreement with direct numerical integration with a fine grid resolution by the finite difference method under arbitrary conditions. The model was applied to combustion in a drop tube furnace and showed qualitatively good agreement with experiments, including for the burnout behavior in the late stages. It is revealed that the profiles of the oxygen mole fraction, conversion, and combustion rate have considerably different characteristics in small and large particles. This means that a model that considers one total conversion for each particle is insufficient to describe the state of particles. Since our char combustion model requires only one fitting parameter, which is determined from information on the internal geometry of char particles, it is useful for performing numerical simulations.     

Modelling and experimental investigations on gasification of coarse sized coal char particle with steam

The steam gasification characteristics of coal char produced two sub-bituminous coals of different origin have been investigated through modelling and experiments. The gasification experiments are carried out in an Isothermal mass loss apparatus over the temperature range of 800–900 °C using a gas mixture of 65% steam and 35% N₂. A fully transient single particle gasification model, based on the random pore model, is developed incorporating reaction kinetics, heat and mass transport inside the porous char particle and the gas film. Stefan-Maxwell equation and Knudson diffusion are incorporated in the multi-component diffusion of species and pore diffusion. The model is validated with the experimental data of the present authors as well as that reported in the literature. The particle centre temperature is found to increase, then decrease and increase again to reach the reactor temperature finally, and the trend is more prominent for the larger particles. The pore opening phenomenon is more evident in SBC2 char, leading to a final char porosity of 0.65 vis-à-vis 0.52 in SBC1 and making it more reactive. Temporal evolution of contours of carbon conversion and concentration of other gaseous species like steam, H₂O, H₂, CO and CO₂ in the particle are computed to investigate the gasification process. A higher temperature is found to favour both the rate peak and the total production of H₂ for both the chars. The total H₂ production from SBC2 char is found to be 0.0189 mol and 0.0236 mol at 800 and 850 °C, while the same for SBC1 char is0.0232 mol and 0.0290 mol respectively. The reaction follows the shrinking core model at the outset, shifting to the shrinking reactive core model subsequently.     

On the burning behavior of pulverized coal chars

A model that predicts the physical changes that pulverized coal char particles undergo during combustion has been developed. In the model, a burning particle is divided into a number of concentric annular volume elements. The mass loss rate, specific surface area, and apparent density in each volume element depend upon the local particle conditions, which vary as a consequence of the adsorbed oxygen and gas-phase oxygen concentration gradients inside the particle. The model predicts the particle's burning rate, temperature, diameter, apparent density, and specific surface area as combustion proceeds, given ambient conditions and initial char properties. A six-step heterogeneous reaction mechanism is used to describe carbon reactivity to oxygen. A distributed activation energy approach is used to account for the variation in desorption energies of adsorbed O-atoms on the carbonaceous surface. Model calculations support the three burning zones established for the oxidation of pulverized coal chars. The model indicates two types of zone II behavior, however. Under weak zone II burning conditions, constant-diameter burning occurs up to 30% to 50% conversion before burning commences with reductions in both size and apparent density. Under strong zone II conditions, particles burn with reductions in both size and apparent density after an initial short period (<2% conversion) of constant-diameter burning. Model predictions reveal that early in the oxidation process, there is mass loss at constant diameter under all zone II burning conditions. Such weak and strong burning behavior cannot be predicted with the commonly used power-law model for the mode of burning employing a single value for the burning mode parameter. Model calculations also reveal how specific surface area evolves when oxidation occurs in the zone II burning regime. Based on the calculated results, a surface area submodel that accounts for the effects of pore growth and coalescence during combustion under zone I conditions was modified to permit the characterization of the variations in specific surface area that occur during char conversion under zones II conditions. The modified surface area model is applicable to all burning regimes. Calculations also indicate that the particle's effectiveness factor varies during conversion under zone II burning conditions. With the adsorption/desorption mechanism employed, a near first-order Thiele modulus-effectiveness factor relationship is obeyed over the particle's lifetime.     

Numerical study of the effects of material properties on flame stabilization in a porous burner

Development of porous burners has been encouraged by lower emission standards as well as the advantages these burners offer; such as fuel flexibility, the ability to operate at low equivalence ratios, and effective flame speeds greater than the laminar flame speed. Although a burner may be constructed from a single section of porous media, a burner consisting of two sections with different characteristics has received significant attention in the last decade. Through proper selection of the properties of the two sections, the interface between the two sections serves as a flame holder preventing flashback for a range of conditions. In this paper, we present the results from a one-dimensional computational study on flame stabilization in a two section porous burner. The stable operating limits are predicted for a range of equivalence ratios and are compared to experimental values. A parametric study, in which the properties of the two sections are varied independently, is presented. The results indicate that matrix properties significantly affect the stable operating range. In addition, the upstream section acts primarily as a flashback arrestor and for the widest operating range, it should have a low conductivity, low volumetric heat transfer coefficient, and high radiative extinction coefficient. The downstream section acts primarily to recirculate heat through the matrix; it should have a high conductivity, high volumetric heat transfer coefficient, and an intermediate radiative extinction coefficient.     

Thermal-to-electric energy conversion of a nanoporous carbon

As two nanoporous carbon electrodes are soaked by an electrolyte solution and placed at different temperatures, a significant transient potential was measured, providing a novel mechanism for thermal-to-electric energy conversion.     

Experimental and numerical study of syngas production during premixed and ultra-rich partial oxidation of methane in a porous reactor

The synthesis gas (syngas) production from the ultra-rich methane/oxygen mixtures via the thermal partial oxidation in an inert porous reactor was investigated numerically and experimentally. Thermodynamic analysis was firstly conducted based on Gibbs free energy minimization method to find the possible optimum routes of operation. Then, the experiments were performed on the constructed test-rig with a non-catalytic porous based reformer. The flame is stabilized within zirconia (ZrO₂) sponge, which has shown very high mechanical strength and thermal resistance. The main influencing parameters such as the equivalence ratio and thermal load have been investigated during different experiments. For this purpose, the reactor axial temperature profile and product compositions were determined experimentally. The obtained results reveal that the heat loss abatement; approaching to the adiabatic condition could effectively improve the amounts of syngas (H₂+CO) production. The maximum syngas production was obtained 69.5% of the exhaust gas at the equivalence ratio of 2.5 and thermal load of 8 kW. Moreover, the H₂/CO ratio was reported above 1.5, which can be suitable for feeding into other chemical processes. Finally, numerical simulation of the process was performed using the premixed and reactor network models. The contribution of heat loss from the reactor was also considered in the model due to its pivotal role observed in the experimental work. The average relative error of the reactor network model with respect to syngas generated from the reformer was found to be 6.72%. Therefore, the predictions obtained from this model are in fairly good agreement with the experimental data.     

Fuel conversion characteristics of black liquor and pyrolysis oil mixtures: Efficient gasification with inherent catalyst

Alkali metals inherent in black liquor (BL) have strong catalytic activity during gasification. A catalytic co-gasification process based on BL with pyrolysis oil (PO) has the potential to be a part of efficient and fuel-flexible biofuel production systems. The objective of the paper is to investigate how adding PO into BL alters fuel conversion under gasification conditions. First, the conversion times of single fuel droplet were observed in a flat flame burner under different conditions. Fuel conversion times of PO/BL mixtures were significantly lower than PO and comparable to BL. Initial droplet size (300–1500 μm) was the main variable affecting devolatilization, indicating control by external heat transfer. Char oxidation was affected by droplet size and the surrounding gas composition. Then, the intrinsic reactivity of char gasification was measured in an isothermal thermogravimetric analyser at T = 993–1133 K under the flow of CO₂–N₂ mixtures. All the BL-based samples (100% BL, 20% PO/80% BL, and 30% PO/70% BL on mass basis) showed very high char conversion. Conversion rate of char gasification for PO/BL mixtures was comparable to that of pure BL although the fraction of alkali metal in char decreased because of mixing. The reactivities of BL and BL/PO chars were higher than the literature values for solid biomass and coal chars by several orders of magnitude. The combined results suggest that fuel mixtures containing up to 30% of PO on mass basis may be feasible in existing BL gasification technology.     

A simplified numerical approach to hydrogen and hydrocarbon combustion in single and double-layer porous burners

Motivated by the fuel hydrogen applications in porous combustors, as well as hydrogen production in syngas porous devices, this work shows a simplified one-dimensional, steady state heat and mass transfer model for stabilized premixed flames in porous inert media. Single-layer and double-layer porous burner are studied. The model has three conservation equations, describing the heat transfer in the solid and fluid phases and the mass transfer in the reacting flow. The model considers a plug flow and is solved numerically by using the finite volume method. The results are compared with benchmark data, depicting the superadiabatic flames and the heat recirculation process. A parametric analysis of the model reveals the effects of the porous media properties and the Lewis and Peclet numbers on the heat and mass transfer processes. Furthermore, the effects of the flame stand-off parameter in double layer porous burner are also analyzed. The results have considered the values of the dimensionless parameters based on reference data for hydrogen/air and methane/air combustion in porous burners built with SiC and Al₂O₃.