Laminar Entrained Flow Reactor Research Articles

Optimization of Biomass Pyrolysis Vapor Upgrading Using a Laminar Entrained-Flow Reactor System

A custom bench-scale continuous-flow catalytic fast-pyrolysis (CFP) reactor system was implemented for the optimization of ex situ CFP and screening of catalysts to gain insight into commercial sca...

Application of Bound-to-Bound Data Collaboration approach for development and uncertainty quantification of a reduced char combustion model

The development of efficient industrial oxy-coal boilers can be significantly aided by Computational Fluid Dynamics (CFD) tools, as far as fidelity in modeling coal combustion is also complemented by feasible computational costs. Reduced and predictive models are the most suitable for this application scale. Reduced models feature predictivity when they are validated against a broad range of experiments and targeted by Uncertainty Quantification (UQ) procedures. This work proposes a numerical procedure that uses Bound-to-Bound Data Collaboration (B2B-DC) to derive a reduced char combustion model describing transport phenomena and reactions between char carbon and O2, CO2 and H2O, in both conventional and oxy-conditions. The approach determines the consistency between a numerical model and an experimental dataset. The latter is made up of the experiments carried out in an optically accessible laminar entrained flow reactor, operated by Sandia National Laboratories. The procedure follows five steps towards predictive modeling capability, namely: quantification of the uncertainty in the experiments, via instrument verification and modeling; development of a physics model and continuous improvement of its fidelity, via model-form uncertainty; identification of the uncertain and most sensitive parameters and of their prior bounds; sampling of the initial uncertain parameter space and training of a surrogate model; validation of the physics model via inference from the data. The last step, also known as inverse problem, is performed by applying the Bound-to-Bound Data Collaboration approach. A char combustion model is found consistent with the experimental data and its validity stands for conventional and oxy-combustion conditions. It accounts for heterogeneous reactions at the particle surface, mass transport of species in the particle boundary layer, pore diffusion and surface area changes. The consistent reduced model overcomes the differences in mass transport and kinetics observed in the experimental campaign. A reduction of the initial degree of uncertainty in both model and experiments is achieved.

Collaboration of simulations and experiments for development and uncertainty quantification of a reduced char combustion model

Computational fluid dynamics (CFD) plays a decisive role in the development of cost-effective oxy-coal combustion technologies to improve process efficiency and to decrease pollutant emissions. The implementation of detailed physical models describing coal devolatilization, char oxidation, gas-phase reactions and pollutant formation ensures accurate CFD simulations of coal combustion but is still challenging for large-scale combustors due to the significant computational efforts required, especially in the framework of Large Eddy Simulation (LES). The development of reduced physics model with quantified model-form uncertainty is needed to overcome the challenges of performing LES of industrial coal-fired boilers. Reduced models must reproduce the main features of the detailed models and the capability of bridging scales and being predictive. A tight coupling of simulation and experiments is necessary to ensure predictivity with uncertainty quantification for a reduced model. This work proposes a combined experimental/numerical methodology that uses global sensitivity analysis to rank fundamental input parameters of a reduced char oxidation and gasification model describing reactions between char carbon and O2, CO2 and H2O reagents in both air and oxy-coal conditions. A careful evaluation of uncertainty in the data, in the model form and in the model parameters is performed. The reference dataset, consisting of the experiments carried out in a laminar entrained flow reactor operated by Sandia National Laboratories, has been exploited. The methodology is based on the use of so-called instrument models to include all the physical sub-models and the sources of uncertainty considered in the experiments and in the numerical simulations and affecting the main quantities of interest, e.g. reaction rates. The quantified uncertainty in the instrument models provides the range of uncertainty for the reduced char combustion model. Then, the reduced model with quantified uncertainty will be validated against the experimental data. The reduced model capability to address heterogeneous reaction at the particle surface, mass transport of species in particle boundary layer, pore diffusion and internal surface area changes will be assessed.

Estimating the Temperature Experienced by Biomass Particles during Fast Pyrolysis Using Microscopic Analysis of Biochars

Biomass particles can experience variable thermal conditions during fast pyrolysis due to differences in their size and morphology, and from local temperature variations within a reactor. These differences lead to increased heterogeneity of the chemical products obtained in the pyrolysis vapors and bio-oil. Here we present a simple, high-throughput method to investigate the thermal history experienced by large ensembles of particles during fast pyrolysis by imaging and quantitative image analysis. We present a correlation between the surface luminance (darkness) of the biochar particle and the highest temperature that it experienced during pyrolysis. Next, we apply this correlation to large, heterogeneous ensembles of char particles produced in a laminar entrained flow reactor (LEFR). The results are used to interpret the actual temperature distributions delivered by the reactor over a range of operating conditions.

Devolatilization kinetics of woody biomass at short residence times and high heating rates and peak temperatures

This work combines experimental and computational fluid dynamics (CFD) results to derive global kinetics for biomass (pine wood) devolatilization during heating rates on the order of 105Ks-1, bulk flow peak temperatures between 1405 and 1667K, and particle residence times below 0.1s. Experiments were conducted on a laboratory laminar entrained flow reactor (LFR) using solid fuel feed rates on the order of 10–20mgh-1. Employing a simple single step first order (SFOR) mechanism with an Arrhenius type rate expression, the best fit of the pyrolysis kinetics was found to be: A=18.9×103s-1,Ea=21305Jmol-1. The accuracy of the derived global kinetics was supported by comparing predictions to experimental results from a 15kW furnace. The work emphasizes the importance of characterizing the temperature history of the biomass particles when deriving pyrolysis kinetics. The present results indicate faster kinetics than found in the literature, leading to predicted residence times required for full conversion one order of magnitude lower than when compared to thermogravimetric analysis (TGA) derived kinetics.

Fuel Particle Conversion of Pulverized Biomass during Pyrolysis in an Entrained Flow Reactor

This study addresses the change of char morphology and fuel conversion during pyrolysis in a laminar entrained flow reactor by experiments and particle simulation. Three experimental parameters were examined: reaction temperature (1073 and 1273 K); particle size (125–250, 250–500, and 500–1000 μm); and the length of reaction zone (650 and 1885 mm). The scanning electron microscopic (SEM) images showed that biomass swelled during heating and shrank during initial stage of pyrolysis. Then, char morphology transformed to cenospheres after the plastic stage. The yields of solid residue from the experiments were reasonably predicted by particle simulation. To give a guideline for the design of laminar entrained flow pyrolysis reactors, the required reactor length for complete conversion of biomass was also calculated for the pyrolysis. High reaction temperature, small particles, and slower gas flow were favorable for high fuel conversion.

Flamelet modeling of coal particle ignition

This paper numerically investigates the ignition of a single coal particle during the devolatilization phase in a laminar entrained-flow reactor, for which experimental data are available from Molina and Shaddix [3]. Different numerical approaches are combined to evaluate the non-premixed flamelet approach for coal particle ignition. First, the particle trajectory and the particle heating are simulated with a Lagrangian–Eulerian approach using a detailed pyrolysis model. In a second step, these results are used as transient boundary conditions for a simulation fully resolving the flow, the mixing field and the chemical reactions around the particle. Finally, in combination with the boundary conditions the time-dependent scalar dissipation rate profiles from the resolved particle calculation are used in a flamelet calculation for the particle up- and downstream directions. Very good agreement is obtained in terms of ignition delay as well as temperature and chemical species distributions in the mixture fraction space when the resolved particle calculation and the unsteady flamelet calculation are compared in the downstream direction. Good agreement is obtained when the numerical results for the ignition time and the time-averaged OH distribution are compared with the available experimental data. The results show the capability of the laminar flamelet approach to correctly predict coal particle ignition during devolatilization using accurate scalar dissipation rate profiles.

Physical phenomena of char–slag transition in pulverized coal gasification

Performance of entrained-flow coal gasifiers is in large part dictated by the burnout behavior of coal particles. In particular, the transition from porous, reactive char to molten, low reactive slag affects overall coal conversion. In this work, the physical phenomena associated with char–slag transition were studied for three coals using a laminar entrained-flow reactor under simulated gasification conditions. Partially oxidized particles with various conversions were prepared at temperatures above the ash fluid temperatures. The physical properties of the char and slag particles were characterized, including the particle density, size, internal surface area and morphology. Results show that at a coal-dependent critical conversion, the particles undergo remarkable physical changes, such as density increase, size reduction and surface area decrease. These phenomena indicate the char–slag transition.

Measurements of Final Volatiles Yield of Biomass Devolatization at Flash Heating Rate

Final volatiles yield of biomass devolatization at flash heating rate was determined via experiments. Samples of wheat straw and corn stalk powder were dispersed in the feeder and were carried by argon into the laminar entrained flow reactor. Flash devolatization of tiny biomass particles was accomplished in the moderate temperature reactor. Residue was collected and the ash content of it was detected in a thermo-grametric analyzer. Final volatiles yield was calculated using ash trace method. Altogether, 96 experiments (including repeat ones) of different reaction temperature (500, 550, 600, 650 °C), different collecting distance (550, 600, 650, 700 mm away from inlet). Results showed that final volatiles yields of wheat straw and corn stalk are 79%, 85 % separately. High reproducibility of the experiments showed it is a reliable method to determine final volatiles yield of biomass devolatization at flash heating rate. Kinetic parameters for wheat straw and corn stalk were modified as A=1009/s, E=3782kJ/mol and A=596/s, E=3716kJ/mol, respectively.

Pulverized coal stream ignition delay under conventional and oxy-fuel combustion conditions

The coal stream ignition process is critical to the performance of modern pulverized coal burners, particularly when operating under novel conditions such as experienced in oxy-fuel combustion. However, experimental studies of coal stream ignition are lacking, and recent modeling efforts have had to rely on comparisons with a single set of experiments in vitiated air. To begin to address this shortfall, we have conducted experiments on the ignition properties of two U.S. and two Chinese coals in a laminar entrained flow reactor. Most of the measurements focused on varying the coal feed rate for furnace temperatures of 1230–1320 K and for 12–20 vol.% O 2 in nitrogen. The influence of coal feed rate on ignition with a carbon dioxide diluent was also measured for 20 vol.% O 2 at 1280 K. A second set of measurements was performed for ignition of a fixed coal feed rate in N 2 and CO 2 environments at identical furnace temperatures of 1200 K, 1340 K, and 1670 K. A scientific CCD camera equipped with a 431 nm imaging filter was used to interrogate the ignition process. Under most conditions, the ignition delay decreased with increasing coal feed rate until a minimum was reached at a feed rate corresponding to a particle number density of approximately 4 × 10 9 m −3 in the coal feed pipe. This ignition minimum corresponds to a cold flow group number, G, of ∼0.3. At higher coal feed rates the ignition delay increased. The ignition delay time was shown to be very sensitive to (a) the temperature of the hot coflow into which the coal stream is introduced, and (b) the coal particle size. The three high volatile bituminous coals showed nearly identical ignition delay as a function of coal feed rate, whereas the subbituminous coal showed slightly greater apparent ignition delay. Bath gas CO 2 content was found to have a minor impact on ignition delay.

Elucidation of Biomass Pyrolysis Products Using a Laminar Entrained Flow Reactor and Char Particle Imaging

We have mapped the formation of tars during white oak thermochemical conversion using a bench scale laminar entrained flow reactor (LEFR). White oak particles (80 mesh, <180 μm) were pyrolyzed under conditions not limited by heat transfer. Measurements were made with residence times of 0.2, 0.4, 0.6, and 0.8 s, between 500 and 900 at 100 °C increments, and with residence times of 1 s at temperature from 450 to 950 °C at 25 °C increments. Products were monitored with a molecular beam mass spectrometer (MBMS), and the mass spectra were analyzed using model-free multivariate analysis (multivariate curve resolution). Six groups of correlated masses were identified that suggest the mechanisms of pyrolysis and gasification. The first group of masses (lowest temperature) is associated with primary species from lignin and hemicellulose, followed by cellulose products. The next two groups (increasing temperature) are composed of secondary products resulting from the cracking of carbohydrate vapors and the cracking of lignin in the gas or solid phase. Molecular weight growth products are seen in the next two groups including substituted aromatic compounds in the fifth group and polycyclic aromatic hydrocarbons (PAHs) in the sixth group. The results of this study show that as the temperature of pyrolysis is increased, the molecular weight of the tars decreases up to 750 °C, because the pyrolysis vapors are cracked. As the temperature increases beyond 750 °C, molecular weight growth is seen with increasing temperature. The analysis also shows that as the temperature increases from 450 to 950 °C, oxygen is lost from the tars and converted into CO and CO2. The char samples were collected and analyzed with light and electron microscopy. This analysis revealed that micropores develop in the cell wall around 550 °C and increase in size and coalesce into a cenosphere morphology with increasing temperature. Above 850 °C, these cenospheres appear to rupture, releasing their contents into the gas phase. This rupture event correlates with increased MBMS signals from PAH-associated masses.

Experimental Investigation into Combustion Characteristics of Two Sub-bituminous Coals in O2/N2and O2/CO2Environments

There are still arguments on the coal combustion reactivity under conventional and oxy-coal combustion atmospheres. The selection of the experimental device in such a study is crucial. A laminar entrained flow reactor system that is capable of producing a similar combustion environment of a utility furnace in terms of the high heating rate and temperature was adopted in this study. This study investigated the characteristics of sub-bituminous coal combustion in its processes of devolatilization and char oxidation by examining the structure and length of the flame. The effects of the particle size and oxygen molar fraction in N2 and CO2 diluent gases were studied. The O2 mole fraction was varied from 0 to 50%. The flame became shorter as the O2 concentration increased. It means that reactivity in both the devolatilization and char oxidation processes increased in a higher O2 concentration environment. It was barely seen that the volatile burning realm shrunk, O2 diffusion accelerated, and luminosity concentrated near the burning particle as the O2 concentration increased. The larger size coals showed thicker and longer flames, and there is a small difference in the burning time between two different sub-bituminous coals. When the O2/CO2 and O2/N2 environment effects are compared, the flame was shorter in the oxy-fuel condition than in the O2/N2 condition.

Ash Deposition Behavior during Char−Slag Transition under Simulated Gasification Conditions

Ash deposition experiments at various conversions of a bituminous coal were performed under gasification conditions using a laminar entrained-flow reactor and a deposition probe. Results showed that the particle capture efficiency (a measure of ash deposition propensity) was a function of coal conversion. In particular, the capture efficiency increased dramatically at a critical conversion, which is ascribed to the increase in particle stickiness. To clarify this phenomenon, ash formation experiments were conducted to collect char and ash particles under experimental conditions identical to those in the ash deposition experiments. Collected particles were presumed to have the same properties as the particles approaching the deposition probe in ash deposition experiments. Properties of the particles including internal surface area and morphology were determined by isothermal gas adsorption and scanning electron microscopy, respectively. The internal surface area of the particles dropped sharply at the crit...

Investigation of Coal Char−Slag Transition during Oxidation: Effect of Temperature and Residual Carbon

The transition of coal char to molten slag at high conversion was studied for a bituminous coal using a laminar entrained-flow reactor under oxidizing conditions. Post-oxidized char particles were analyzed by various techniques including loss-on-ignition, gas adsorption analysis, and scanning electron microscopy to determine carbon content, internal surface area and pore size distribution, and char morphology, respectively. These analyses provide information concerning the effect of temperature and residual carbon on the transition from porous char to molten slag. Results showed that, at temperatures above the ash flow temperature, the transition from porous char to molten slag occurred at about 90% conversion for the coal used in this study. No transition occurred at temperatures below the ash flow temperature. This finding explains previous observations that there is a coal-dependent critical carbon conversion at which the ash stickiness increases dramatically. This result also indicates that surface area can be used as a criterion for determining the critical conversion of the transition. In addition, it was found that the randomly overlapping pore model cannot be directly applied to predict the surface area evolution of char particles during the transition without considering the reopening of closed micropores during the initial reaction and the ash fusion effect.

Pyrolysis kinetics of washed precipitated lignin

This article describes the pyrolysis behavior of precipitated washed lignin in a Laminar Entrained Flow Reactor between 700 and 1000°C and at different residence times. Lignin was precipitated by acidification of softwood black liquor using CO2. After acid washing, the solid material was dried and sieved (80-100 μm). This material was then fed into the reactor at a rate of about 0.1 g/min. The formed gases were analyzed with respect to CO, CO2, and CH4, and char was collected and weighed. A traditional first order Arrhenius kinetic expression, based on the temperature of the particles with respect to residence time, was adapted to the experimental results. The activation energy was found to be 32.1 kJ/mol. The low ash content in the washed lignin gave a very low solid material residue after the reactor.

Gasification of mixtures of black liquor and secondary sludge

The effects of secondary sludge addition on black liquor gasification were studied in a laminar entrained flow reactor. Additions of 0%, 5%, 10%, and 15% secondary sludge solids to hardwood black liquor solids were studied at gasification temperatures of 700oC and 900oC with residence times of 0.2, 0.7, and 1.2 s. Although both the amount and the rate of organic carbon conversion increased with temperature and residence time, the percentage of sludge addition did not affect the percentage of feed carbon recovered as organic carbon. Recoveries were expressed on a per-centage-of-feed-element basis. Recoveries of H2S were higher for purer (less sludge added) feed stocks for all experiments except those performed at 900oC and 1.2 s residence time. Recoveries of methyl mercaptan, n-propyl mercaptan, carbonyl sulfide, and ammonia were relatively independent of percentage sludge addition. Amount of ammonia recovered, however, increased at higher sludge addition rates.

KINETICS OF FAST PYROLYSIS OF RICE HUSKS USING AN EXTERNALLY PLASMA-HEATED LAMINAR ENTRAINED FLOW REACTOR

A plasma-heated laminar entrained flow reactor (LEFR) was developed for studying the kinetics of fast pyrolysis of biomass particles at rapid heating rates. In order to achieve a controllable, stable, and uniform high-temperature gas flow in the reactor, a plasma jet was used as the primary heating source for the LEFR, and an electric heater was used as an auxiliary heating source. Reynolds number was used as the principle of similitude to determine the flow distribution in the reactor through a series of cold flow tests. Comprehensive temperature measurements were taken, and corresponding temperature profiles were obtained. Experiments using rice husk were conducted in the LEFR. The results showed that biomass particles below 100 .m could be pyrolyzed sufficiently, and thus the pyrolysis kinetics of biomass could be studied in this reactor. Based on the experimental data, a kinetic model for rice husk was proposed, and the kinetic parameters (i.e., activation energy and frequency factor) were determined.

Devolatilization characteristics of biomass at flash heating rate

The devolatilization characteristics of biomass (wheat straw, coconut shell, rice husk and cotton stalk) during flash pyrolysis has been investigated on a plasma heated laminar entrained flow reactor (PHLEFR) with average heating rates of 10 4 K/s. These experiments were conducted with steady temperatures between 750 and 900 K, and the particle residence time varied from about 0.115 to 0.240 s. The ash tracer method was introduced to calculate the yield of volatile products at a set temperature and the residence time. This experimental study showed that the yield of volatile products depends both on the final pyrolysis temperature and the residence time. From the results, a comparative analysis was done for the biomasses, and a one-step global model was used to simulate the flash pyrolytic process and predict the yield of volatile products during pyrolysis. The corresponding kinetic parameters of the biomasses were also analyzed and determined. These results were essential for designing a suitable pyrolysis reactor.

Flash pyrolysis of agricultural residues using a plasma heated laminar entrained flow reactor

Abstract In order to study the volatilization characteristics of biomass particles at flash heating rates, a plasma heated laminar entrained flow reactor (PHLEFR) was designed and built in our lab. Two agricultural residues, wheat straw and corn stalk, were chosen as feedstock for pyrolysis which were conducted on the PHLEFR with the aim of determining the extent of thermal decomposition at high heating rate (more than 104 °C s−1). Based on the experimental data, a first order kinetic model was introduced and the relevant kinetic parameters (apparent active energy and apparent frequency factor) were determined for the two straws: E = 31.51 kJ mol - 1 , A = 1028 s - 1 (wheat straw) and E = 33.74 kJ mol - 1 , A = 1013 s - 1 (corn stalk). The predicted conversion of the fitted model to the experimental data provided general agreements when one considered the experimental errors.

Carbon distribution in char residue from gasification of kraft black liquor

The char residue yields and the total carbon and carbonate content were measured for dry black liquor solids after pyrolysis or gasification in a laminar entrained-flow reactor. The experimental conditions were 700–1000°C in N2,CO2/N2 or water vapor/N2 at 1bar total pressure, for residence times from 0.3 to 1.7s.Fixed carbon yields, when measured at the same particle residence time, decreased with increasing reactor temperature. CO2 and water vapor diminished the char carbon significantly at temperatures above 800°C, compared with pyrolysis in N2. Water vapor oxidized the char carbon more rapidly than did CO2. At 1000°C, the reactions of carbon with sulfate and carbonate became faster, resulting in a smaller difference between carbon conversion rates in the different gas environments.By the end of devolatilization, the amount of carbonate in the char had changed very little at 700–800°C. After devolatilization, carbonate was formed more rapidly at higher temperatures. The presence of CO2 or water vapor increased the formation of carbonate. In the presence of these gases, more carbonate was measured at all temperatures and residence times. The maximum carbonate measured in the char was 16% of the carbon in the black liquor solids, as compared to 4.4% in the original dry liquor solids. Under most conditions, the carbonate, as a fraction of carbon input, first increased to a constant, temperature-independent value and then decreased.