Exit Gas Composition Research Articles

Experimental Study on the Operational Performance of an Advanced Two-Stage Entrained-Flow Coal Gasifier

In this paper, an advanced two-stage entrained-flow coal gasifier was built and its operational performance was examined. Detailed comparisons between exit gas compositions, consumptions of coal and oxygen, carbon conversion efficiencies, and cold gas efficiencies in single- and two-stage operational modes have been experimentally performed. The results showed that the coal and steam supplied in the second stage always contribute to more H2 and CH4, while the CO concentration varies insignificantly. The two-stage process demonstrated potential technical and economic advantages compared to the single-stage process. The introduction of coal and steam in the second stage improves gas quality, reduces the coal and oxygen consumptions, and therefore, lowers the investment. However, there is an observed small decline in coal carbon conversion in the two-stage process.

Parametric study for an immobilized amine sorbent in a regenerative carbon dioxide capture process

An amine-based solid sorbent process to remove CO2 from flue gas has been investigated. The sorbent consists of polyethylenimine (PEI) immobilized onto a silica (SiO2) support. Experiments were conducted in a packed-bed reactor and exit gas composition was monitored using mass spectrometry. The effects of feed gas composition (CO2 and H2O), temperature, and simulated steam regeneration were examined for both the silica support as well as the PEI-based sorbent. The gas analysis artifact of the empty reactor was also quantified. Sorbent CO2 capacity loading was compared to past thermogravimetric (TGA) results to further characterize adsorption isotherms and better define CO2 working capacity. Sorbent stability was monitored by periodically repeating baseline conditions throughout the parametric testing and replacing with fresh sorbent as needed. The concept of using this sorbent within a system where sorbent continuously flows between the absorber and regenerator was introduced. A basic tenet is to manipulate or control the level of moisture on the sorbent as it travels around the sorbent circulation path between absorption and regeneration stages to minimize its effect on regeneration heat duty.

Semiempirical Determination of Overall Volumetric Molar Transfer Coefficient for the Scrubbing of Gaseous Tars in Biodiesel

This work contains a description for the determination of the liquid phase molar transfer coefficient (KXa) by a method which is different from that by Onda correlation. The method is different because KXa is for the transfer of the tars which change their phase and size as the temperature and pressure change. In the method, the solubility of the tars in biodiesel was predicted and ultimately the equilibrium coefficient was calculated. The equilibrium coefficient and measured tars compositions in liquid and gas phases at steady state were used to determine KXa. In this way, the determination of KXa for the wet scrubbing of tars into biodiesel was achieved semiempirically. The predicted equilibrium coefficient of the 0.3818 mole ratio at liquid phase temperature of 300 K and atmospheric pressure was used in the calculation. Under these conditions, KXa depended on the liquid to gas flow rate ratio, the inlet and outlet liquid phase compositions, and the exit gas composition. As a result, the measured values of these parameters together with the predicted equilibrium coefficient were fitted into a semiempirical correlation defined as KXa = 0.0143L0.7704G0.0282. This fitting is reasonably satisfactory as it satisfies the conditions cited in the literature and can be effectively applied at liquid to gas flow rate ratios of 36 and 38.

Three dimensional model of transport and chemical late phenomena on a MSW incinerator

A mathematical model was developed of the drying, volatilization, and combustion phenomena on the packed bed of solid wastes on a moving grate. Gas–solid flow and the heterogeneous chemical reactions were considered. Since experimental work in this area of study is very difficult and costly, the development of such a model is very important. The gas phase turbulence was modeled using k–ε turbulent model and the particle phase was modeled using kinetic theory of granular flow. The reaction rates of heterogeneous reaction were determined by Arrhenius–eddy dissipation reaction rates and the Arrhenius-diffusion reaction rate. Flow patterns, gas velocities, particle velocities, composition profiles of gas product and distributions of reaction rates were obtained. Predicted values using the present code were compared with experimental data for validation. The results showed that the predicted exit gas compositions were in good agreement with the experiments. This indicates that the numerical model presented is valid and provides a promising way to simulate the combustion of solid waste on a moving grate, which is the dominant technology for waste incineration.

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.

Comparison of CFD simulations to experiment under methane steam reforming reacting conditions

The highly endothermic commercial methane steam reforming (MSR) reaction was studied experimentally in a single-pellet-string fixed bed reactor. The temperatures inside the active catalyst particles, the temperatures on the outer surfaces of selected pellets and the exit gas composition were measured. The MSR reaction showed strong effects on the temperature profile along the reactor, causing it to decrease initially. A computational fluid dynamics (CFD) model was used to predict temperature and species profiles under the experimental MSR reaction conditions. Comparison of CFD and experimental data showed very good qualitative as well as quantitative agreement for temperature inside catalyst particles at different inlet gas temperatures, and for the temperature drop from outside to inside the pellets due to the reaction heat sink. Trends in methane conversion were also well-represented by the CFD simulation.

CFD simulation of entrained-flow coal gasification: Coal particle density/sizefraction effects

Computational Fluid Dynamics (CFD) simulation of commercial-scale two-stage upflow and single-stage downflow entrained-flow gasifiers was conducted to study effects of simulating both the coal particle density and size variations. A previously-developed gasification CFD model was modified to account for coal particle density and size distributions as produced from a typical rod mill. Postprocessing tools were developed for analysis of particle–wall impact properties. For the two-stage upflow gasifier, three different simulations are presented: two (Case 1 and Case 2) used the same devolatilization and char conversion models from the literature, while Case 3 used a different devolatilization model. The Case 1 and Case 3 solutions used average properties of a Pittsburgh #8 seam coal ( d = 108 μm, SG = 1.373), while Case 2 was obtained by injecting and tracking all of the series of 28 different coal particle density and size mass fractions obtained by colleagues at PSU as a part of the current work, for this same coal. Simulations using the two devolatilization models (Case 1 and Case 3) were generally in reasonable agreement. Differences were observed between the single-density solution and the density/size partitioned solution (Case 1 and Case 2). The density/size partitioned solution predicted nominally 10% less CO and over 5% more H 2 by volume in the product gas stream. Particle residence times and trajectories differed between these two solutions for the larger density/size fractions. Fixed carbon conversion was 4.3% higher for the partitioned solution. Particle–wall impact velocities did not vary greatly. Grid independence studies for the two-stage upflow gasifier geometry showed that the grid used in the comparison studies was adequate for predicting exit gas composition and wall impact velocities. Validation studies using experimental data for the Pittsburgh #8 coal from the SRI International pressurized coal flow reactor (PCFR) at 30 atmospheres indicated adequate agreement for gasification and combustion cases, but poor agreement for a pyrolysis case. Simulation of a single-stage downflow gasifier yielded an exit gas composition that was in reasonable agreement with published data.

Modelling of a downdraft biomass gasifier with finite rate kinetics in the reduction zone

A model of a downdraft gasifier has been developed based on chemical equilibrium in the pyro-oxidation zone and finite rate kinetic-controlled chemical reactions in the reduction zone. The char reactivity factor (CRF) in the reduction zone, representing the number of active sites on the char and its degree of burn out, has been optimized by comparing the model predictions against the experimental results from the literature. The model predictions agree well with the temperature distribution and exit gas composition obtained from the experiments at CRF=100. A detailed parametric study has been performed at different equivalence ratios (between 2 and 3.4) and moisture content (in the range of 0–40%) in the fuel to obtain the composition of the producer gas as well as its heating value. It is observed that the heating value of the producer gas increases with the increase in the equivalence ratio and decrease in the biomass moisture content. The effect of divergence angle of the reduction zone geometry (in the range of 30–150°) on the temperature and species concentration distributions in the gasifier has been studied. An optimum divergence angle, giving the best quality of the producer gas, has been identified for a particular height of the reduction zone. Copyright © 2009 John Wiley & Sons, Ltd.

Three-dimensional simulation of fluidized bed coal gasification

A comprehensive three-dimensional numerical model was developed to simulate the coal gasification in a fluidized bed gasifier. Both gas–solid flow and chemical reactions were considered. The gas phase was modeled with k − ɛ turbulent model and the particle phase was modeled with kinetic theory of granular flow. Coal pyrolysis, homogeneous reactions and heterogeneous reactions were considered. The reaction rates of homogeneous reaction and heterogeneous reaction were determined by Arrhenius–Eddy dissipation reaction rate and Arrhenius-diffusion reaction rate, respectively. Simulations were carried out in a fluidized bed coal gasifier with a height of 2.0 m and a diameter of 0.22 m. Flow patterns, gas velocities, particle velocities, composition profiles of gas product and distributions of reaction rates were obtained. Some calculated values were compared with experimental data. The results showed that the predicted exit gas compositions were in a good agreement with the experiments. This indicates that the current three-dimensional models and simulations are successful, which provide a promising way to simulate the coal gasification in fluidized beds.

Studies in an atmospheric bubbling fluidized-bed combustor of 10 MW power plant based on rice husk

In this paper an experience, environmental assessment, a model for exit gas composition, agglomeration problem and a model for solid population balance of 10 MW power plant at Jalkheri, Distt. Fatehgarh Sahib, Punjab, India based on rice husk has been discussed. Three phase multistage mathematical model for exit gas composition of rice husk in fluidized bed has been derived. The model is based on three-phase theory of fluidization and material balance for shrinking rice husk particles and it is similar to model developed by Kunii and Levenspiel. The burning of rice husk is assumed to take place according to single film theory. The model has been used to predict the exit gas composition particularly O 2, CO 2 and N 2. The agglomeration problem of above plant which is main reason for defluidization of bed has also been discussed. SEM of ash agglomerates has been done. Ash samples taken from the above 10 MW power plant at Jalkheri has been quantitatively analyzed. Finally solid population model has been formed to calculate bed carbon load and carbon utilization efficiency. Above two models are experimentally correlated with the data collected from the above 10 MW power plant at Jalkheri, Distt. Fatehgarh Sahib, Punjab, India which uses rice husk as a fuel input (at the time of study). All the results from the model for rice husk are coming with in permissible limits.

Model development and validation: Co-combustion of residual char, gases and volatile fuels in the fast fluidized combustion chamber of a dual fluidized bed biomass gasifier

A one-dimensional steady state model has been developed for the combustion reactor of a dual fluidized bed biomass steam gasification system. The combustion reactor is operated as fast fluidized bed (riser) with staged air introduction (bottom, primary and secondary air). The main fuel i.e., residual biomass char (from the gasifier), is introduced together with the circulating bed material at the bottom of the riser. The riser is divided into two zones: bottom zone (modelled according to modified two phase theory) and upper zone (modelled with core-annulus approach). The model consists of sub-model for bed hydrodynamic, conversion and conservation. Biomass char is assumed to be a homogeneous matrix of C, H and O and is modelled as partially volatile fuel. The exit gas composition and the temperature profile predicted by the model are in good agreement with the measured value.

Simulation of the performance for the direct internal reforming molten carbonate fuel cell. Part II: comparative distributions of reaction rates and gas compositions

This study examined the distributions of the three reaction rates and the compositions of the gases at each point of the unit cell in DIR-MCFC using a numerical simulation. The electrochemical reaction rates at the anode gas entering position were almost two times faster than those at the anode gas outlet position and most of the feeding CH4 reacted in the region from the position x=0 to the position x=0.3. In addition, the water–gas shift reaction became faster from near the half position of the unit cell to the gas outlet position. Therefore, in the rear position of the unit cell, the steam reforming reaction played an important role as a supplementary reaction for providing the H2 needed in the electrochemical reaction. The rates of the two catalytic reactions in the case without the electrochemical reaction were relatively slower than those in the DIR-MCFC. Unlike the distributions of temperature, the current density, gas compositions and the reactions rates at each point of the DIR-MCFC cell, the exit gas compositions from the simulation in particular could be comparative to those of the experimental results. Although there was an approximately 10% difference between both of them, the extent of the difference was considered to be reasonable for this simulation considering the experimental values that could be included in this simulation such as the lower conversion of the reactions, the lower current density and any other values. Copyright © 2005 John Wiley & Sons, Ltd.

Investigation of Ammonia Formation during Gasification in an Air-Blown Spouted Bed: Reactor Design and Initial Tests

A laboratory scale, pressurized gasifier has been developed to simulate the operating conditions of air-blown spouted bed gasifiers. The new 28 mm i.d. reactor operates with a continuous coal feed of typically 3 g min-1, at temperatures up to 980 °C and pressures up to 20 bara. Fluidizing gases can be combinations of air, steam, and nitrogen. The gases and coal enter the gasifier through the inverted conical base of the reactor, as a high velocity jet, which induces the rapid mixing of the fresh coal with the bed char. At the pilot scale this mode of operation had been found to avoid the operating difficulties associated with the agglomeration of sticky coal particles. Gases within the reactor may be sampled from the spout jet (using a specially designed water-cooled probe) and from the exit of the reactor. In the present study, the reactor has been used to study NH3 formation under conditions relevant to those of the pilot-scale gasifier that was used during the process development stage of the Air-Blown Gasification Cycle. The pilot-scale work had found high, variable, and at times difficult-to-predict concentrations of NH3 in the fuel gas. The present stage of the work was aimed to help understand the pilot-scale data by focusing on the effect of operating conditions on NH3 formation reactions. The gasifier has been shown to run with continuous feed for periods of up to 30 min, before sinter builds up on the base and prevents normal operation. Within this time data are collected on the spout and exit gas compositions. In this paper we report preliminary findings, which suggest that the NH3 is produced during pyrolysis and by the effect of steam on the char-N. The latter is the most important reaction and the mechanism is at present unclear. High concentrations of NH3 appear to form through the latter mechanism and to vary with operating conditions, temperature, steam content, presence of sorbent, and coal:air ratio.

Numerical simulation of entrained flow coal gasifiers. Part I: modeling of coal gasification in an entrained flow gasifier

A comprehensive three-dimensional simulation model was developed for entrained flow coal gasifiers. In the model, the numerical methods and the submodels conventionally used for the pulverized coal combustion modeling were used. An extended coal gas mixture fraction model with the Multi Solids Progress Variables (MSPV) method was applied to simulate the gasification reaction and reactant mixing process. Four mixture fractions were employed to separately track the variable coal off-gas from the coal devolatilization, char–O 2, char–CO 2, and char–H 2O reactions. The influence of turbulence on the gas properties was taken into account by the pdf model with a clipped Gaussian distribution function. A series of numerical simulations were performed for a 200 td −1 two-stage air blown entrained flow gasifier recently developed for the IGCC process. The predicted gas temperature profile and the exit gas composition were in general agreement with the measurements. Model simulations illustrating the importance of accounting for varying coal off-gas and the effects of turbulence/reaction affecting the prediction capability were also presented.

Reduction of iron-silicon-oxysulfide by CO gas injection

The reduction of liquid oxysulfide in the Fe-Si-S-O system by CO gas injection has been studied by monitoring the exit gas composition. The reduction rate of oxygen was calculated from the volume of evolved CO2. Sulfur-bearing species such as COS were close to the detection limit of the mass spectrometer, which indicated that the reduction of sulfur was very limited. The volume of evolved CO2 reached steady values 1 minute after CO injection. The reduction reaction was controlled by a chemical reaction. The observed maximum reduction rate of oxygen at 1250 °C was 8.3×10−6 g-O/cm2 s, which was within the range of the reduction rates in other melts such as iron oxide and iron silicates.

Effect of the Steam‐Methane Ratio on Reactions Occurring on Ni/Yttria‐Stabilized Zirconia Cermet Anodes Used in Solid‐Oxide Fuel Cells

Power‐generation experiments of solid‐oxide fuel cells with Ni/yttria‐stabilized zirconia cermet anodes were carried out by changing the water‐to‐methane ratio in the fuel. To separate the electrochemical reactions occuring on the three‐phase boundary of the anode from the steam‐reforming reactions, premixed gases corresponding to the composition at thermal equilibrium were used as the fuel. The reactions on the anode were quantitatively clarified at different steam‐to‐methane ratios by making a mass balance of the elements and electrons and by determining the anode exit gas composition. For dry (i.e., no water) methane fuel, the carbon deposited on the three‐phase boundary from the methane mainly reacted with oxide ions . With increasing steam‐carbon (S/C) ratio from S/C = 0.5 to 1, hydrogen became the dominant reactant. The anode potential vs. current density for S/C = 1 agreed with that for hydrogen diluted with argon. At S/C ratios higher than 1.5, CO in the inlet gas reacted with oxide ions. © 1999 The Electrochemical Society. All rights reserved.

99/01974 Modeling freeboard gasification in fluidized bed gasifier (1). Model set-up: Tang, Z. et al. Ranshao Kexue Yu Jishu, 1998, 4, (2), 150–156. (In Chinese)

A two-region model of a spouted bed gasifier devised by the present authors has been further developed to include an enthalpy balance. As before, the present model assumes first-order reaction kinetics for the gasification reactions and the spout is treated as a plug flow reactor of fixed diameter with cross flow of gas into the annulus. The annulus region is considered to be a single plug flow reactor. Solids move in plug flow in both annulus and spout, independent of temperature and reaction. The model allows predictions of axial profiles of temperature and gas in the spout and annulus as well as exit gas compositions and overall carbon conversion. The exit gas compositions were compared with those of the previous two-region and streamtube isothermal models. The effect of hydrodynamic parameters on model predictions are illustrated and a comparison made with experimental temperature profiles obtained in a 0.30 m diameter gasifier.

A nonisothermal model of a spouted bed gasifier

A two-region model of a spouted bed gasifier devised by the present authors has been further developed to include an enthalpy balance. As before, the present model assumes first-order reaction kinetics for the gasification reactions and the spout is treated as a plug flow reactor of fixed diameter with cross flow of gas into the annulus. The annulus region is considered to be a single plug flow reactor. Solids move in plug flow in both annulus and spout, independent of temperature and reaction. The model allows predictions of axial profiles of temperature and gas in the spout and annulus as well as exit gas compositions and overall carbon conversion. The exit gas compositions were compared with those of the previous two-region and streamtube isothermal models. The effect of hydrodynamic parameters on model predictions are illustrated and a comparison made with experimental temperature profiles obtained in a 0.30 m diameter gasifier.

Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO, and CaO−MgO for Hot Raw Gas Cleaning

The upgrading of the raw hot gas from a bubbling fluidized bed biomass gasifier is studied using cheap calcined minerals or rocks downstream from the gasifier. Biomass gasification is made with steam (not air) at 750−780 °C and about 0.5−1.0 kg of biomass/h. Calcined solids used are dolomite (MgO−CaO), pure calcite (CaO), and pure magnesite (MgO). Variables studied have been temperature of the secondary bed (780−910 °C), time of contact or space-time of the gas (0.08−0.32 kg·h/m3n), and particle diameter (1−4 mm) and type of mineral. Their effects on tar conversion, tar amount in the exit gas, product distribution, and gas composition are presented. Using a macrokinetic model for the tar disappearance network, the activities of the stones are expressed by their apparent kinetic constant. Apparent energies of activation for tar elimination (42−47 kJ/mol) and preexponential and effectiveness factors are given for all tested solids of which the most active is the calcined dolomite.

95/05016 A mathematical model for exit gas composition in a 10 MW fluidized bed coal combustion power plant

95/05016 A mathematical model for exit gas composition in a 10 MW fluidized bed coal combustion power plant