Analysis of the effect of steam-to-biomass ratio in fluidized bed gasification with multiphase particle-in-cell CFD Simulation

Biomass has been identified as a key renewable energy source to cope with upcoming environmental challenges. Gasification of biomass is becoming interested in large scale operation, especially in synthesis of liquid fuels. Bubbling and circulating fluidized bed gasification technology has overrun the interest over fixed bed systems. CFD studies of such reactor systems have become realistic and reliable with the modern computer power. Gasifying agent, temperature and steam or air to biomass ratio are the key parameters, which are responsible for the synthesis gas composition. Therefore, multiphase particle-in-cell CFD modeling was used in this study to analyze the steam to biomass, S/B, ratio in fluidized bed gasification. Due to the complexity of the full loop simulation of dual circulating fluidized bed reactor system, only the gasification reactor was considered in this study. Predicted boundary conditions were implemented for the particle flow from the combustion reactor. The fluidization model was validated against experimental data in beforehand where Wen-Yu-Ergun drag model was found to be the best. The effect of the S/B ratio was analyzed at a constant steam temperature of 1073K and a steam velocity of 0.47 m/s. Four different S/B of 0.45, 0.38, 0.28 and 0.20 were analyzed. The biomass was considered to be in complete dry condition where single step pyrolysis reaction kinetics was used. Each gasification simulation was carried out for 100 seconds. 8% reduction of hydrogen content from 57% to 49% and 17% increment of carbon monoxide from 13% to 30% were observed when the S/B was reduced from 0.45 to 0.20. Countable amounts of methane were observed at S/B of 0.28 and 0.20. The lower heating value of the product gas increased from 10.1 MJ/kg to 12.37 MJ/kg and the cold gas efficiency decreased from 73.2% to 64.6% when the S/B was changed from 0.45 to 0.20. The specific gas production rate varied between 1.64 and 1.04 Nm3/kg of biomass

Analyzing the effects of particle density, size, size distribution and shape for minimum fluidization velocity with Eulerian-Lagrangian CFD simulation

Fluidized bed reactor systems are widely used due to excellent heat and mass transfer characteristics followed by uniform temperature distribution throughout the reactor volume. The importance of fluidized beds is further demonstrated in high exothermic reactions such as combustion and gasification where fluidization avoids the hot spot and cold spot generation. A bed material, such as sand or catalyst, is normally involved in fluidized bed combustion and gasification of biomass. Therefore, it is vital to analyze the hydrodynamics of bed material, especially the minimum fluidization velocity, as it governs the fluid flowrate into the reactor system. There are limitations in experimental investigations of fluidized beds such as observing the bed interior hydrodynamics, where CFD simulations has become a meaningful way with the high computer power. However, due to the large differences in scales from the particle to the reactor geometry, complex interface momentum transfer and particle collisions, CFD modeling and simulation of particle systems are rather difficult. Multiphase particle-in-cell method is an efficient version of Eulerian-Lagrangian modeling and Barracuda VR commercial package was used in this work to analyze the minimum fluidization velocity of particles depending on size, density and size distribution. Wen-YU-Ergun drag model was used to model the interface momentum transfer where default equations and constants were used for other models. The effect of the particle size was analyzed using monodispersed Silica particles with diameters from 400 to 800 microns. Minimum fluidization velocity was increased with particle diameter, where it was 0.225 m/s for the 600 microns particles. The density effect was analyzed for 600 microns particles with seven different density values and the minimum fluidization velocity again showed proportionality to the density. The effect of the particle size distribution was analyzed using Silica. Particles with different diameters were mixed together according to pre-determined proportions as the final mixture gives a mean diameter of 600 microns. The 600 microns monodispersed particle bed showed the highest minimum fluidization velocity. However, some particle mixtures were composed with larger particles up to 1000 micron, but with a fraction of smaller particles down to 200 microns at the same time. This shows the effect of strong drag from early fluidizing smaller particles. The only variability for pressure drop during packed bed is the particle size and it was clearly observed in all three cases

Image Processing and Measurement of the Bubble Properties in a Bubbling Fluidized Bed Reactor

The efficiency of a fluidized bed reactor depends on the bed fluid dynamic behavior, which is significantly influenced by the bubble properties. This work investigates the bubble properties of a bubbling fluidized bed reactor using computational particle fluid dynamic (CPFD) simulations and electrical capacitance tomography (ECT) measurements. The two-dimensional images (along the reactor horizontal and vertical planes) of the fluidized bed are obtained from the CPFD simulations at different operating conditions. The CPFD model was developed in a commercial CPFD software Barracuda Virtual Reactor 20.0.1. The bubble behavior and bed fluidization behavior are characterized form the bubble properties: average bubble diameter, bubble rise velocity, and bubble frequency. The bubble properties were determined by processing the extracted images with script developed in MATLAB. The CPFD simulation results are compared with experimental data (obtained from the ECT sensors) and correlations in the literature. The results from the CPFD model and experimental measurement depicted that the average bubble diameter increased with an increase in superficial gas velocities up to 4.2 Umf and decreased with a further increase in gas velocities due to the onset of large bubbles (potential slugging regime). The bubble rise velocity increased as it moved from the lower region to the bed surface. The Fourier transform of the transient solid volume fraction illustrated that multiple bubbles pass the plane with varying amplitude and frequency in the range of 1–6 Hz. Further, the bubble frequency increased with an increase in superficial gas velocity up to 2.5Umf and decreased with a further increase in gas velocity. The CPFD model and method employed in this work can be useful for studying the influence of bubble properties on conversion efficiency of a gasification reactor operating at high temperatures.publishedVersio

Sensitivity Analysis and Effect of Simulation parameters of CPFD Simulation in Fluidized Beds

Fluidized bed technology is broadly applied in industry due to its distinct advantages. CFD simulation of fluidized beds is still challenging compared to single-phase systems and needs extensive validation. Multiphase particle-in-cell is a recently developed lagrangian modeling technique and this work is devoted to analyze the sensitivity of grid size, time step, and model parameters, which are the essences of accurate results. Barracuda VR 17.1.0 commercial CFD package was used in this study. 500µm sand particles and air was used as the bed material and fluidization gas respectively. Five different grids, having 27378, 22176, 16819, 9000 and 6656 computational cells were analysed, where five different time steps of 0.05, 0.01, 0.005, 0.001 and 0.0005 were used for each grid. One velocity step was maintained for 8 seconds. The bed pressure drop at packed bed operation was high for simulations with reduced time steps while equal pressure drops were observed during fluidization for all time steps. Time steps of 0.0005s and 0.001s and 0.005s produced equal result of 0.15 m/s for minimum fluidization velocity, irrespective of the grid size. The results from time steps of 0.05 and 0.01 are converged to the results from time steps of 0.005 and 0.001 by increasing simulation time per one velocity step.Sensitivity Analysis and Effect of Simulation parameters of CPFD Simulation in Fluidized BedsacceptedVersionpublishedVersionNivå

Sintering Behaviors of Synthetic Biomass Ash

Entrained flow gasification of biomass provides the opportunity to convert low-grade biogenic feedstocks to high-grade synthetic fuels. For a top-fired entrained flow slagging biomass gasifier, the thermophysical properties of the ash and slag limit process operation and affect process energy efficiency. The biomass ash has to be molten and slag viscosity has to be in a certain range for it to flow out of the gasifier. However, direct sampling, analysis, and evaluation of slag formation and behaviors are often challenging as entrained flow biomass gasification operates at high temperatures (i.e., 1200-1500°C) continuously. One alternative is to study synthetic ash's melting and sintering behaviors at elevated temperatures, which represent the major inorganic constituents in biomass ash. For thermochemical conversion of biomass, K, Ca and Si are typically the most common ash-forming elements. In this work, the synthetic ashes were prepared by mixing model compounds K2O, CaO and SiO2 in different mole ratios, which were pressed to form pellets. The selection of mole ratios was based on thermodynamic calculations that indicate that the tested model compound mixtures melt and flow with desired viscosity at certain temperature ranges. The pressed synthetic ashes were preheated at 900 °C for 8 hours to thermally homogenize them. Then the premelted synthetic ashes were heated at 1000 and 1400 °C in a muffle furnace with a residence time of 1 and 8 hours in air to study fusion behaviors and slag formation tendency, and were cooled down to room temperature gradually after the sintering test. The sintered residues were collected and analyzed by SEM/EDX to study the interactions of the model compounds and identify chemical compositions. The results showed that the mole ratios of model compounds have recognizable impacts on the composition, formation and transformation of mineral phases in residues from sintering tests. A strong correlation was also found between the sintering intensity of the synthetic ash and the mole ratios of model compounds.publishedVersio

Near-well simulation of oil production from a horizontal well with ICD and AICD completions in the Johan Sverdrup field using OLGA/ROCX

One of the main principles of improving oil recovery is maximizing the contact area between the well and the reservoir. To achieve this purpose especially in reservoirs with a thin oil column, long horizontal wells are widely used today. However, there are some challenges related to horizontal wells like water coning towards the heel due to the heel-toe effect as well as early water breakthrough owing to heterogeneity along the well. In order to tackle these issues, passive inflow control devices (ICDs) and autonomous inflow control devices (AICDs) can be used. ICDs are able to balance the drawdown pressure along the horizontal well and as a result, postpone the early water breakthrough. By applying AICDs, in addition to postponing the early water breakthrough, water can be partially choked back autonomously, and the negative impacts of early water breakthrough will be attenuated. The Johan Sverdrup field (JSF) is a giant oil field located in the North Sea and production from this field has been started recently. Since there is a plan for developing this oil field in the near future, and a few studies have been done on this field so far, further studies are needed to obtain more cost-effective oil recovery in this field. The main objective of this paper is the near-well simulation of oil production from the well 16/2-D-12 in the JSF by considering ICD and AICD completions. The simulation has been conducted based on the characteristics of the reservoir near this well for 750 days of oil production. OLGA in combination with ROCX has been used as a simulation tool. The simulation results showed that by applying both ICDs and AICDs the heel-toe effect, and heterogeneity along the well can be effectively handled and the water breakthrough time can be delayed by 255 days. Moreover, it was observed that by completion of the well 16/2-D-12 with AICDs, the accumulated water production can be reduced by 11.9% compared to using ICDs. In the same way, by using AICDs the flow rate of water production is reduced by 13.4% after 750 days. Furthermore, the results showed that using AICDs has a negligible impact on both the accumulated and the flow rate of oil production compared to using ICDs. Therefore, by completion of the well 16/2-D-12 with AICDs more cost-effective oil production can be achieved

Computational study of CO2 injection for enhanced oil recovery and storage

Injection of supercritical carbon dioxide (CO2) for enhanced oil recovery (EOR), plays a vital role to minimize the impact of CO2 emissions. CO2-EOR refers to the oil recovery technique where supercritical CO2 is injected in the reservoir to stimulate oil production from depleted oil fields. CO2-EOR can be used in combination with CO2 storage to mitigate the emissions levels to the atmosphere. The objective of this paper is to perform a computational study of CO2-EOR and storage at the Johan Sverdrup field. The study includes simulations of oil production using the commercial software Rocx in combination with OLGA. Production with inflow control devices (ICD) and autonomous inflow control valves (AICV) shows that AICVs have an oil-to-water ratio of 0.92 compared to 0.39 for ICDs. CO2-EOR in combination with well completion with AICVs, shows improved oil recovery, low water production, and low CO2 reproduction. The simulations and calculations performed in this paper indicate that the Johan Sverdrup field is highly capable for CO2-EOR and storage

Distribution of Solids in a Fluidized Bed Operated without a Gas Distributor

Despite several advantages of a gas distributor, there are also some challenges in its application. Using a gas distributor increases the fluidizing gas compression cost. Due to deposition of fine particles and product of chemical corrosion, the gas flow area across a distributor may also be reduced. This paper investigates the distribution of solids in a fluidized bed operated without a gas distributor. The gas supply is through two opposite points on the column wall. In this bed configuration, the fluidized bed behaviour simulated using the CPFD (Computational Particle-Fluid dynamics) Barracuda code is compared with an equivalent system where a uniform gas distribution is assumed. For different powders with mean size 480 – 710 µm, the results show that the axial distribution of pressure fluctuation in both types of bed configurations are similar, particularly near the bottom of the bed. The relative solid fraction fluctuation is lower than 0.2 around the central axis and it spans over 21 – 83% and 41 – 65% of the bed diameter, respectively. These results therefore show that stable bubbling behaviour and good distribution of solids can also be achieved in a bed operated without any distributor

Effect of Superficial Gas Velocity on Bubbling Fluidized Bed Behaviour in a Biomass Gasifier

This study investigates the behaviour of bubbling fluidized beds in biomass gasification processes based on the variation of superficial gas velocity at different temperatures and air flowrates. In the paper, the operating window is defined as the gas velocity between the minimum fluidization and slugging velocities, which are computed using the correlations in the literature. The analysis shows that the operating gas velocity depends on the amount of char accumulated in the bed. An increase in the char accumulation results in higher minimum fluidization and slugging velocities of the bed mixture. This therefore suggests that the gas velocity ratio required to achieve the desired operating fluidized bed regime is higher when the biomass accumulation is considered

Simulation of a purification plant for pyrolysis gas based on plastic waste

This study focuses on exctracting ethylene and propylene from other componenents in the pyrolysis gas mixture of plastic waste. Selective absorption of ethylene and propylene from the gas mixture by using a silver nitrate solution, was selected as a promising technology. A lab scale set-up was built and experimental tests were performed using a model gas mixture. Aspen HYSYSV10 was used to model and simulate the absorption process. The model was validated against experimental data. The validated model was further used to identify improvements for the separation process and increase the recovery of ethylene and propylene. The results from the simulated improved process show that the amount of ethylene and propylene in the product gas could be significantly increased. In the experimental study, only 25% of ethylene and propylene in the feed was captured, whereas the simulation of the improved process indicates that the recovery of the monomers could be almost quantitative (99%). However, the product gas from the separation system contained an undesirable high amount of CO and CO2. These gases act as pollutants of the polyolefin reaction and further studies are needed to obtain pure olefin gases