Gasification Regime Research Articles

Investigation of characteristics of gas and coke residue for the regime of quasi- and non-stationary steam gasification of coal in a fluidized bed: Part 1

The steam oxygen-free gasification allows deep processing of coal and obtaining gas with high contents of H2 and CO. The design of dual fluidized bed gasifiers (DFB) is considered promising. There are insufficient experimental data on the steam gasification process in DFB for more accurate mathematical modeling. The paper presents the experimental results in dynamics of the quasi-stationary and non-stationary gasification of two types of coals in a laboratory gasifier. It has been established that when using superheated steam for preliminary and main gasification process, it is possible to obtain a gas with a total content of H2 and CO of more than 75% vol, maximum cold gas efficiency 87%. The conditions for the expedient use of two chambers for gasification of coals of different degrees of metamorphism are identified. The gasification regimes that will increase H2 and CO formation in DFB are determined. This research substantiate the necessarily to use only steam in DFB for improving the synthesis gas quality.

Relevant aspects of propagating a combustion front in an annular reactor for out-of-bed heat recovery

This work investigates for the first time the classical aspects of smoldering front propagation in an annular reactor while presenting new concepts and making new contributions in methodologies of establishing the longitudinal temperature profile, calculating the front velocity, and determining the combustion front’s regime. Twenty-nine experiments using different porous beds supported the argument that annular reactor design is, at present, the best for recovering the combustion heat for concomitant processes. Three methods for calculating the front velocity were employed, of which the mass derivative and mass balance methods were specially developed for combustion in heterogeneous beds. The mass balance modeling constrained by the gas analysis results revealed the reactions’ stoichiometry and front velocity relationships with the product formation rate. As a result, the model returned the total oxygen mass, front velocity correction, actual fixed carbon amount oxidized by the front, and the Equivalent Ratio (ER) value, a concept borrowed from gasification studies. It was found that the smoldering front propagated under a gasification regime 0.21 ¡ ER ¡ 0.37. The front structure revealed that the oxidation reaction’s heat was mostly convected downstream of the reaction front resulting in an average bed temperature increase even when the front was close to its propagation’s limit. Finally, the Energy Availability (EA) assessment confirmed that the smoldering front propagation in an annular section is one exceptional design to drive the energy out-of-bed to feed a concomitant process — considering a pyrolysis heat of 1100 kJ/kg for the study.

Combustion of Flat Shaped Char Particles With Oxygen

Abstract Thin flat-shaped carbon black particles of 1.5 mm thickness by 22.5 mm diameter were combusted in pure oxygen at atmospheric pressures and temperatures in the range of 500–650 °C. One-film kinetic-diffusion model was derived to characterize the kinetic and energy parameters for particles arranged in the form of a thin flat-shaped configuration. The kinetic and energy parameters, and operating regimes of thin flat-shaped char particles were characterized during the nonisothermal combustion process. The gasification regimes during preheating were also analyzed. Steady-state energy processes were considered to derive an energy conservation equation used for calculating the evolution of char surface temperatures as well as released peak energy rates and the specific energy, which are considered key engineering design parameters. The one-film kinetic-diffusion model showed that combustion of such particles was purely kinetic controlled under these conditions. The activation energy obtained varied between 50 and 74 kJ/mol using discrete time and linear fits to the Arrhenius equation. The total energies released per weight of char converted varied between 32.8 and 40.6 kJ/g. The highest peak energy rate released was 134 J/s when combusting char in O2 at a reactor temperature of 504 °C.

Modeling and numerical simulations of lignite char gasification with CO2: The effect of gasification parameters on internal transport phenomena

The purpose of this study is to develop an experimental data-based char particle gasification model in order to assess the effects of particle size, gasification temperature and char generation heating rate on global gasification parameters. Also, the effect of initial porosity is observed by performing parametrical numerical simulations. A continuum-based model is used to solve the gasification inside a char particle and within the external boundary layer. The intrinsic rate of CO2 gasification reaction is computed according to Langmuir-Hinshelwood (LH) mechanism. External mass transfer is modeled by Stefan-Maxwell relations, and Cylindrical Pore Interpolation Model (CPIM) is used for intra-particle molecular diffusion. In the model, all the effects due to particle internal structure changes are represented by a global conversion function, f(X) which is computed from local reaction rate values. In this study, f(X) is deduced from experimental results instead of phenomenological models almost impossible to validate. The best reproduction of the experimental gasification results is obtained for the function f(X) postulated as a summation of two Gaussian functions which represent the char particle random pore structures and their dynamics during gasification. Comparative simulation results show that the Gaussian for low conversion interval is shifted to even lower conversion values for higher gasification temperature and higher initial porosity. Thereby, the Gaussian function for low conversion rates (large particle sizes) is interpreted as representative of the diffusion-limited gasification regime in conjunction with the network of macropores and molecular diffusion rates. The modification of the pore structure due to char generation heating rates causes a shift of the second Gaussian towards higher conversion rates. It is therefore postulated that the second Gaussian function corresponds to the boundary layer diffusion-controlled regime related to available outer surface area of the particle.

Simulation and Optimization of Wood Biomass Gasification Regimes in a Flow of Steam-Oxygen Blast

Plant biomass (such as wood, agricultural and logging waste) is considered a raw material with better environmental characteristics than fossil fuels. The main prospects for bioenergy are associated with combustion and gasification in small power systems, including as part of hybrid power plants. To increase the efficiency of gasification processes, an oxygen-enriched gasifying agent is often used, as well as fine grinded fuel (to intensify transport processes and chemical transformations). Thermodynamic estimates show the possibility of achieving the efficiency of the gasification at the level of 0.8–0.9, however, the experimentally achieved values rarely exceed 0.7. In this work, using mathematical modeling, we investigated the possibility of increasing the efficiency of the biomass gasification.

Experimental study of underground coal gasification (UCG) of a high-rank coal using atmospheric and high-pressure conditions in an ex-situ reactor

This paper presents the results of an extensive experimental analysis of underground coal gasification (UCG) using large bulk samples in an ex-situ reactor under atmospheric and high-pressure (30 bar) conditions. The high-rank coal obtained from the South Wales (UK) coalfield is employed for that purpose. The aim of this investigation is to define the gas production rates, gas composition, gas calorific value, process energy efficiency and temperature changes within the UCG reactor during the gasification process based on the pre-defined reactants and flow rates. Two UCG trials, each lasting 105 h, consisted of six stages where the influences of oxygen, water, air and oxygen enriched air (OEA) under different flow conditions on the gasification process were investigated. Based on the results of two multi-day experiments, it is demonstrated that the gasification under high pressure conditions produces syngas with higher average calorific value (8.49 MJ/Nm3) in comparison to syngas produced at atmospheric pressure conditions (6.92 MJ/Nm3). Hence, the overall energy efficiency of the high-pressure experiment is higher compared to the atmospheric pressure test, i.e. 57.67% compared to 51.72%. This is related to the fact that the high-pressure gasification produces more methane (11.97 vol%) than the atmospheric pressure gasification (2.30 vol%). Under elevated pressure, the temperatures recorded in the roof strata are about 100 °C higher compared to the UCG process under atmospheric pressure conditions. This work provides new insights into the gasification of carbon-rich coals subject to different gasification regimes and pressures.

Numerical study of coal and woody biomass co-gasification in oxygen-fed gasifier

The use of biomass in the energy sector is associated with two important tasks: reducing the dependence of energy systems on expensive high-quality fuel, and increasing the environmental efficiency of thermal power plants (by processing carbon-containing waste and, in some cases, reducing harmful emissions). In addition to co-combusiton, other methods of fuels co-processing are possible, including co-gasification. The process of gasification of low-grade fuels can be unstable because of their low calorific value, often accompanied by the formation of tar products, so the addition of coal improves the efficiency of co-conversion. On the other hand, the high reactivity of biomass can contribute to the stabilization of combustion and gasification regimes of fuels with low-reactivity, such as coals with high degree of metamorphism or petcoke. In this paper, the process of pulverized fuel gasification is considered, and limitations on the efficiency associated with the melting of ash are investigated.

Mass, energy, and exergy analysis of the microgasification process in a top-lit updraft reactor: Effects of firewood type and forced primary airflow

Four representative firewood species from some Colombian regions were evaluated in a top-lit-updraft (TLUD) reactor under gasification regimes. The effect of primary airflow on the process was studied at three levels (20, 30, and 40 l/min) using the firewood species with the highest fuel value index (FVI; 9.8 MJ/cm3). The thermodynamic assessment of the microgasification process was characterized by means of response variables such as flame front velocity (Vff), maximum reaction temperature (Tmax), fuel/air equivalence-ratio (Frel), mass yields (gas-Ygas, char-Ychar, and tar-Ytar), cold gas efficiency (CGE), chemical exergy efficiency (CEE), and irreversibility (I). With regard to the influence of the firewood type, the performance of the process was mainly affected by physical properties, such as bulk density and hardness, which had a positive effect on both Frel and Ytar, with a 95% confidence level. On the contrary, Vff, Ychar, Ygas, CGE, CEE, and I decreased when the bulk density and hardness increased. With regard to the effect of the airflow, Vff and Tmax increased from 7.9 to 14.0 mm/min and from 724.9 °C to 838.3 °C, respectively, because the process tended to lead to combustion; however, CGE and CEE tended to slightly increase with the airflow because the producer gas flow increased.

Численное исследование режимов газификации угля и биомассы в поточном газогенераторе с парокислородным дутьем

The biomass (wood, agricultural, etc.) gasification can be unstable due to low calorific value, as well as accompanied by a significant amount of tars. Therefore, the addition of coal enhances the processing of biomass. On the other hand, the high reactivity of biomass can contribute to the stabilization of combustion and gasification regimes of low-reaction fuels, such as high grade coals or residue petroleum coke. Co-gasification of the fuels changes optimal gasifier operating modes (by specific consumption of oxidizer and slag conditions), and influences the processes associated with the preparation of producer gas for use in a combustion chamber or catalytic reactor. The stationary modes of gasification of coal dust with the addition of biomass in a entrained-flow generator with steam-oxygen blowing have been numerically studied. The calculated dependencies of the process efficiency on the control parameters are obtained: the fraction of biomass in the mixture with coal and the specific consumption of oxygen.

Effect of Air Staging Ratios on the Burning Rate and Emissions in an Underfeed Fixed-Bed Biomass Combustor

This experimental work studies a small-scale biomass combustor (5–12 kW) with an underfed fixed bed using low air staging ratios (15%–30%). This document focuses on the influence of the operative parameters on the combustion process, so gaseous emissions and the distribution and concentration of particulate matter have also been recorded. The facility shows good stability and test repeatability. For the studied airflow ranges, the results show that increasing the total airflow rate does not increase the overall air excess ratio because the burning rate is proportionally enhanced (with some slight differences that depend on the air staging ratio). Consequently, the heterogeneous reactions at the bed remain in the so-called oxygen-limited region, and thus the entire bed operates under sub-stoichiometric conditions with regards of the char content of the biomass. In addition, tests using only primary air (no staging) may increase the fuel consumption, but in a highly incomplete way, approaching a gasification regime. Some measured burning rates are almost 40% higher than previous results obtained in batch combustors due to the fixed position of the ignition front. The recorded concentration of particulate matter varies between 15 and 75 mg/Nm3, with a main characteristic diameter between 50 and 100 nm.

Mathematical Modelling of the Fixed-Bed Biomass-Coal Co-Gasification Process

The paper considers mathematical modelling of downdraft fixed-bed gasification process of the mixtures of woody biomass and coal. Biomass/coal ratio, biomass moisture content and air equivalence ratio are varying parameters. Boundaries of the efficient gasification regimes are estimated.

Modeling of Coffee Husks Degradation Process under High Temperature Gasification (HTAG) Regime

The prevailing global warming resulting from climate change is exacerbated by increasing environmental pollution due to high consumption levels of fossil sources of energy including petroleum oils and coal. Effects of climate change would thus provide adverse conditions for possible multiplication of diseases, lowering agricultural productivity, reduction in the availability of safe and clean water, and multiplication of extreme climatic events. Biomass utilization is becoming a subject of increasing interest as one of the attractive alternative solutions to clean fuel production. In order to achieve a sustainable harnessing of the biomass energy potential and to increase its contribution to the world’s primary energy consumption, there is therefore a need to develop and sustain contemporary technologies that increase the biomass-to-energy conversion. This paper investigates a novel gasification process employing high temperature air/steam gasification (HTAG) of biomass that will result in the production of syngas having an improved range of heating values as a direct result of both the gasifier design used and the reactants chosen. In addition to syngas, other products including char and tars produced at varying levels are examined. The technique makes use of highly preheated air to provide additional energy into the gasification process that enhances the thermal decomposition of the gasified solid feedstock materials. The gasification studies conducted are based on the use of coffee husks as feedstock materials under three gasification temperature regimes of 700°C, 800°C, and 900°C, whicht shows an increase in syngas production under high temperature. The results show an increase of the amount of CO yield and N2 under 2% O2 concentrations. The recorded amounts of chars left while gasifying at 700°Cand 800°Care respectively 2.4 and 1.5 times than that for the case of 900°C. The estimated kinetic parameter values for the coffee husks derived from thermal degradation analysis exhibits a reaction mechanism of zero order with apparent activation energy of 161 kJ/mol and frequency factor of 3.89104/min. When steam is injected as the oxidation agent, CO evolution yielded is in the tune of 13.1 to 19.5 Vol. %, compared to 20.8 and 23.8 Vol. %,by comparisonwith the case when steam is injected,which gives negative effects on CO evolution obtained when air is used as an oxidation agent. On the other hand H2 evolution is also seen to be favored when steam is used as an oxidation agent.

Experiments and analysis of propagation front under gasification regimes in a packed bed

The paper analyses the results of experiments on the propagation rate in a fuel bed under gasification conditions in a co-current reactor configuration. Experiments using wood chips with different values of moisture content have been conducted under gasification conditions. The influence of air mass flux on the propagation rate, peak temperature and gas quality is investigated. It is observed from the experiments that the flame front propagation rate initially increases as the air mass flux increased, reaching a peak propagation rate, and further increase in the air mass flux results in a decrease in the propagation rate. However, the bed movement increases with the increase in air mass flux. The experimental results provide an understanding on influence of the fuel properties on propagation front. The surface area per unit volume of the particles in the packed bed plays an important role in the propagation rate. It has been argued that the flaming pyrolysis contributes towards the flame propagation as opposed to the overall combustion process in a packed bed. The calorific value of the producer gas generated is nearly the same over the entire range of air mass flux for bone-dry and 10% moist wood.

Calculation of the fixed bed coal gasification regimes by the use of thermodynamic model with macrokinetic constraints

We discuss an equilibrium model of a fixed-bed solid fuel gasification process that takes into account macrokinetic constraints imposed on the rates of heterophase processes and allows the pyrolysis and gasification processes to be described in a fairly simple manner with the use of thermal analysis data. The results of calculations are compared with the measured parameters characterizing the Azeisk coal steam-air gasification process that were obtained in experiments on a laboratory setup.

Universal Flame Propagation Behavior in Packed Bed of Biomass

This article aims at seeking the universal behavior of propagation rate variation with air superficial velocity (V s ) in a packed bed of a range of biomass particles in reverse downdraft mode while also resolving the differing and conflicting explanations in the literature. Toward this, measurements are made of exit gas composition, gas phase and condensed phase surface temperature (T g and T s ), and reaction zone thickness for a number of biomass with a range of properties. Based on these data, two regimes are identified: gasification—volatile oxidation accompanied by char reduction reactions up to 16 ± 1 cm/s of V s and above this, and char oxidation—simultaneous char oxidation and gas phase combustion. In the gasification regime, the measured T s is less than T g ; a surface heat balance incorporating a diffusion controlled model for flaming combustion gives and matches with the experimental results to within 5%. In the char oxidation regime, T g and T s are nearly equal and match with the equilibrium temperature at that equivalence ratio. Drawing from a recent study of the authors, the ash layer over the oxidizing char particle is shown to play a critical role in regulating the radiation heat transfer to fresh biomass in this regime and is shown to be crucial in explaining the observed propagation behavior. A simple model based on radiation–convection balance that tracks the temperature–time evolution of a fresh biomass particle is shown to support the universal behavior of the experimental data on reaction front propagation rate from earlier literature and the present work for biomass with ash content up to 10% and moisture fraction up to 10%. Upstream radiant heat transfer from the ash-laden hot char modulated by the air flow is shown to be the dominant feature of this model.

Modeling Performance of High-Temperature Biomass Gasification Process

Biomass utilization is becoming a subject of increasing interest as an alternative to clean fuel. A novel gasification process using highly preheated air gasifier using agricultural residue such as sugar bagasse, rice husks, and palm stem widely available in Tanzania is presented. The study examines, irreversibilities making the gasifier the least efficient unit in the gasification process employing a thermodynamic equilibrium model allowing predicting the main product gas composition CO, CO2, H2, and CH4. The derived model equations are computed using the MAPLE process simulation code in MATLAB. The gasification regime is investigated at temperatures ranging from 800 K to 1400 K and at equivalence ratio (ER) values between 0.3 and 0.4. The results obtained conform to the second law efficiency based on chemical exergy yielding maximum values for the types of biomass materials used. These results indicate that the application of preheated air has an effect on the increase of the chemical exergy efficiency of the product gas, hence reducing the level of irreversibility. Similarly, these results show that the combined efficiency based on physical and chemical exergy is low, suggesting that higher irreversibilities are encountered, since the exergy present in the form of physical exergy is utilized to heat the reactants. Such exergy losses can be minimized by altering the ratio of physical and chemical exergy in the syngas production.

Erosive burning. Modeling problems

Existing models for the erosive burning of homogeneous energetic materials ignore the fact that the burning regime can be changed radically by exposure to a hot gas blowing over the burning surface. In other words, transition can occur from the gasification regime at very high blowing rates (where the burning rate is determined primarily by the heat transfer from the flow core) to the self-heating regime of the condensed phase at low or zero blowing if the heat release in the condensed phase is sufficient to heat it to the surface temperature. A possible method for solving this problem is proposed. The approach proposed provides a plausible explanation for the experimentally observed singular (kinked) dependence of the magnitude of the negative erosion effect on the initial temperature.

Linear acoustic characteristics of supercritical droplet vaporization

Our previously developed numerical model of supercritical droplet vaporization is applied to examine the linear acoustic characteristics of liquid-fuel droplets vaporizing in various supercritical environments. The model assumes the hold of phase equilibrium at the droplet surface and incorporates the property of vanishing diffusion coefficient at the critical mixing surface so that the transition to continuous phase change might be simulated. The rate of work conducted by a vaporizing droplet on its surrounding gas provides a fundamental quantity (volume source strength) to examine the way of acoustic interaction between droplet vaporization and ambient gas state fluctuations. In a dilute spray confined in a chamber, the natural pressure oscillation may be excited or attenuated when the amplification rate, which is derived by differentiating the volume source strength with respect to pressure, takes positive or negative values, respectively. The computed amplification rate exhibits distinct behavior in the three regimes of supercritical droplet gasifications, namely: (1) subcritical gasification regime in which the droplet has the surface throughout its lifetime, (2) transitional gasification regime in which the transition to continuous phase change takes place at a finite radius of droplet surface and (3) supercritical gasification regime in which continuous phase change occurs from the beginning. In the subcritical gasification regime, the value of amplification rate changes from positive to negative in a period of time comparable to the droplet temperature relaxation time, and its magnitude is relatively small. The transition to continuous phase change causes the amplification rate to increase abruptly, whereas in the supercritical regime, the amplification rate takes a vanishingly small value. Thus, unusual pressure oscillations may occur for a spray in the transitional gasification regime. The underlying physics are also explained.

A review of the kinetics of the nitric oxide-carbon reaction

The literature on the kinetics of the NO-carbon reaction is reviewed. Data are examined both from studies in which catalysis played no role and from studies in which catalysts were present as a result of addition or as natural inclusions (i.e. in coals). It is concluded that there generally exist three distinct reaction regimes, defined in terms of reaction temperatures. There is a near-ambient-temperature chemisorption regime, in which steady gasification is not possible. In this regime, both reversible and irreversible chemisorption processes are observed. A low-temperature gasification regime is observed at temperatures above ∼ 500 K, but below ∼ 1000 K. A high-temperature gasification regime is observed above this transition temperature. The low-temperature gasification regime is characterized by low activation energies, and the high-temperature regime by high activation energies. These two distinct gasification regimes however are not observed in all cases, and there is as yet no clear understanding of what determines this. In the reaction of pure NO with carbons, the order of reaction with respect to NO appears to be near unity in the gasification regimes and two in the low-temperature chemisorption regime. The apparent order in gasification is strongly influenced by the presence of CO in the reactant gas stream, and in certain reaction configurations the CO may be self-generated. In the presence of CO, the apparent order with respect to NO decreases, and there are several reports of apparent fractional order. Other oxidizing gases (e.g. O 2, H 2O) can also influence the kinetics and course of the NO-carbon reaction. The thermal history of the carbon can have a significant effect on its reactivity in NO. It appears however that the annealing of carbons towards NO cannot be reliably predicted from the annealing behaviour of the carbons towards other oxidizing gases (i.e. O 2). Reaction rates of NO with carbon can also be influenced by the presence of mineral matter. There is no general agreement on the quantitative importance of mineral catalysis in the reaction of carbons with NO.

Gasification Characteristics of a Liquid Fuel Droplet Exposed to Otherwise Quiescent, Supercritical Ambient.

Numerical simulations were conducted for a liquid fuel droplet gasifying in an otherwise quiescent gas with temperature and pressure exceeding the critical point of the fuel. The pressure dependencies of gasification lifetime were examined in terms of the ambient gas temperature and the initial droplet temperature as parameters. Three gasification regimes are identified on the plots of pressure and ambient gas temperature, which show the ambient condition in which the transition to continuous phase change takes place. For a fixed gas temperature, the gasification lifetime takes a minimum value at a pressure such that the droplet surface becomes critical immediately after its exposure to the hot gas. The transitional regime shrinks as the initial droplet temperature increases. The underlying physics is also explored.