Mechanism of particle build-up on gas-solid fluidization column wall due to electrostatic charge generation

Particle build-up on gas-solid fluidization column wall due to electrostatic charging causes significant economic loss in industrial processes such as gas-phase ethylene polymerization to produce polyethylene. It is well acknowledged that in fluidization process electrostatic charges are generated as a result of continuous particle-particle and particle-vessel wall contacts. However, the mechanism of charged particles attraction and adhesion to the fluidization column wall is still under investigation. This work proposes a mechanism for particles coating the column wall by experimentally investigating the extent of particles wall fouling, the fouled particles net specific charge density with an online Faraday cup (1) as well as particles charge distribution with a charged particle separator apparatus (2). The experiments were carried out in a pilot plant gas-solid fluidization system consisting of a 0.15 m in diameter metallic column. Two types of linear low-density polyethylene resins (20-1500 µm) directly received from commercial reactors were fluidized in bubbling flow regime. Experimental results showed that the layer built-up on the column wall contained both positively and negatively charged particles. The wall coating mechanism proposed indicates that particles migration towards the metallic column wall is due to image and electrostatic forces. The image forces are generated by particle-wall contacts causing induction charging on the column wall, in turn attracting the oppositely charged particles towards the wall. Conversely, particle-particle contacts within the bed generate bipolarly charged particles, which depending on their polarity some will be attracted towards the oppositely charged particles fouled on the column wall due to electrostatic forces. Please click Additional Files below to see the full abstract

INVESTIGATION OF THE EFFECT OF FLUIDIZATION TIME ON ELECTROSTATIC CHARGE GENERATION IN GAS-SOLID FLUIDIZED BEDS

This work focused on determining the effect of the fluidization time on particle charging. Charge equilibrium was reached on the bed particles after 60 minutes. The wall particle layer was built with negative and positive charges. Similar sized particles migrated to each region of the bed regardless of fluidization time

CFD modelling of electrostatic charge generation in gas-solid Fluidized Bed-A preliminary work

Gas-solid fluidized beds have been developed for a large variety of industrial applications, which include polymerization, combustion, drying, etc. The solid particles in this flow system tend to generate electrostatic charges due to particle-particle and particle-wall interactions. Particularly in the case of polymerization fluidized beds, the electrostatic charge generation results in particles collecting on the reactor walls. This accumulation of particles might instigate wall fouling (known as “sheeting”) and consequently force a reactor shutdown for clean-up. Although the fluid bed electrification has been experimentally investigated, its computational fluid dynamic (CFD) modeling has received limited attention. Previously, in a work conducted by Rokkam et al. (1) an Euler-Euler multi-fluid and electrostatic model was used to simulate laboratory-scale experiments on electrostatics. In that work, the CFD model used experimentally measured particles charge-to-mass ratio (q/m) as an input for the simulation. In the present work, the electrostatic model is modified to simulate charge generation due to particle interactions. Single particle contact experiments are conducted to obtain charge generation values and used as an input to an Euler-Lagrange model accounting for electrostatics. The goal is to obtain simulated values of electrostatic charge of particles which are comparable to measurements from laboratory-scaled fluidized bed experiments. REFERENCES R. G. Rokkam, A. Sowinski, R.O. Fox, P. Mehrani, and M.E. Muhle, Computational and experimental study of electrostatics in gas-solid polymerization fluidized beds. Chemical Engineering Science, 92: 146–156, 2013

Comparison of multi-component kinetic relations on bubbling fluidized-bed woody biomass fast pyrolysis reactor model performance

Modelling of the thermochemical conversion process of biomass has been widely studied in the past. However, most of the work pertaining to fast pyrolysis is focused solely on the behavior of biomass particles. Only a few works have been devoted to modelling pyrolysis at the bubbling fluidized-bed reactor level. Different types of models have been developed, each varying with respect to the compromise between computational efforts and prediction accuracy and potential. They are primarily sorted according to the adopted fluid-dynamic simplifications, ranging from simple Black-Box Models (BBM) to Fluidization Models (FM) to Computational Fluid Dynamic Models (CFDM). The selection depends on the desired outputs and available experimental information. Undeniably the most significant shortfalls of these models lie within the implemented devolatilization schemes. This work in particular explores the improvements in multi-step and multi-component fast pyrolysis kinetic mechanisms and compares their impacts on reactor measurable outputs such as product (tar, gas and biochar) yields and distinctive compositional values. The model results are compared to the experimental results from the NRCan CanmetENERGY (Ottawa, Ontario) pilot-scale fluidized bed rapid pyrolysis (0.1 m in diameter) unit at a fixed operating temperature and gas residence time. Results generated from this research are aimed to further understanding of large-scale fast pyrolysis processes and assist in predicting reactor performance in a cost and time-effective fashion

Evaluating the impact of feed location on the bubbling fluidized bed gasification of biomass

For fluidized bed gasifiers of biomass the selection of feeding location has been identified as a significant factor in determining gasifier performance, including carbon conversion, gas efficiency, and tar concentration in the producer gas. Over-bed feeding is a simpler arrangement where the biomass feed falls onto the surface of the fluidized bed from above. This can cause elutriation of fines without ever making contact with the bed, limiting carbon conversion or increasing tar loading in the gas. On the other hand, in-bed feeding inserts the biomass feedstock beneath the surface of the bed meaning that all the biomass particles, regardless of size, must contact the fluidized bed. In-bed feeding systems are generally more complex since the feed system must seal against the hydrostatic pressure of the bed and there may be issues with heat conduction or hot sand erosion of feed system components. This work reviews published experimental comparisons between over-bed and in-bed feeding locations, including analysis of impact of the different feeding strategies on mixing and fluidized bed hydrodynamics. The findings from the review are compared against experimental results from a pilot scale (200-250 kg/h biomass feed rate) gasification of two woody feedstocks each from an in-bed and an over-bed feed position. At similar equivalence ratios, the bed temperature was decreased with in-bed feeding relative to the over bed feeding. Although in-bed feeding appeared to have improved carbon conversion to gas, the tar concentration in the producer gas was not decreased with in-bed feeding relative to over-bed feeding

Combined calcium looping and chemical looping combustion for post‐combustion carbon dioxide capture: process simulation and sensitivity analysis

In this work, a combined calcium looping and chemical looping combustion (CaL--CLC) technology is simulated at thermodynamic equilibrium conditions and the results in terms of efficiency, power production, and solids circulation rates are compared with the case of using CaL alone. In addition, a new solids looping configuration in the CaL--CLC process is proposed with the purpose of mitigating the loss of calcium oxide conversion after high cycle numbers. Simulations show an improved process efficiency of the CaL--CLC method compared with CaL alone (34.2 vs. 31.2 % higher heat value) and an increased power output (136 vs. 110 MWe additional power) due to the higher energy requirement to preheat the reactants. A sensitivity analysis of the process operating parameters highlights the particular importance of the temperature difference between reactors, which has a strong impact on the required mass of solids circulating in the loops. Finally, partial carbon dioxide capture scenarios are considered and indicate that lower capture levels are suitable to match regulation targets

Effect of pressure and gas velocity on residence time of particles susceptible to entrainment in gas-solid fluidized beds

In relation to pressurized fluidization processes such as oxyfuel coal combustion, understanding the influence of pressure on bed hydrodynamics and in turn their effect on parameters including feed residence time and entrainment rate is essential. The main focus of the work presented here was to evaluate the impact of pressure and gas velocity on particle elutriation rates and residence times. Experiments were conducted under cold flow conditions in a pilot-scale pressurized fluidized bed with an inner diameter of 0.15 m. The bed material was relatively large glass beads (0.8 to 1.2 mm in diameter) while the feed material was simulated with smaller glass beads (37 to 106 micron in diameter), susceptible to entrainment. Operating pressures and fluidization velocities tested were between atmospheric and 1200 kPa(a) and 0.4 and 1.1 m/s, respectively. Preliminary experiments carried out in batch mode resulted in particle elutriation rates increasing with fluidization velocity in a power law relationship. To simulate coal combustors, experiments were then conducted in a continuous mode where the finer material was continuously fed to the fluidized bed of large particles over a desired period of time, without recycling of fines. This work thus presents particle entrainment results for both batch and continuous operations

Simulation of a calcium looping CO2 capture process for pressurized fluidized bed combustion

The Canadian regulations on carbon dioxide emissions from power plants aim to lower the emissions from coal‐fired units down to those of natural gas combined cycle (NGCC) units. Since coal is significantly more carbon intensive than natural gas, coal‐fired plants must operate at higher net efficiencies and implement carbon capture to meet the new regulations. Calcium looping (CaL) is a promising post‐combustion carbon capture (PCC) technology that, unlike other capture processes, generates additional power. By capturing carbon dioxide at elevated temperatures, the energy penalty that carbon capture technologies inherently impose on power plant efficiencies is significantly reduced. In this work, the CO2 capture performance of a calcium‐based sorbent is determined via thermogravimetric analysis under relatively high carbonation and low calcination temperatures. The results are used in an aspenONE™ simulation of a CaL process applied to a pressurized fluidized bed combustion (PFBC) system at thermodynamic equilibrium. The combustion of both natural gas and coal are considered for sorbent calcination in the CaL process. A sensitivity analysis on several process parameters, including sorbent feed rate and carbonator operating pressure, is undertaken. The energy penalty associated with the capture process ranges from 6.8 –11.8 percentage points depending on fuel selection and operating conditions. The use of natural gas results in lower energy penalties and solids circulation rates, while operating the carbonator at 202 kPa(a) results in the lowest penalties and drops the solids circulations rates to below 1000 kg/s

Characterization of electrostatic charges in gas-solid fluidized beds

A novel on-line measurement technique was developed in this work based on the Faraday cup method by constructing a copper fluidization column of diameter 0.1 m as the inner cup and a second surrounding copper column as the outer cup to gain better understanding of charge generation inside gas-solid fluidized beds. Net charges generated inside fluidized beds were investigated for relatively large glass beads (566 p.m mean diameter) fluidized by extra dry air. It was concluded that particle-gas contacting had negligible effect on the particle charging mechanism for the conditions studied. Also, air ionization is expected to have played a negligible role with respect to dissipation of charges on the particles. Free bubbling fluidization of mono-sized and binary mixtures of particles consisting of relatively large glass beads (566 urn mean diameter) and fine glass beads (30 urn mean diameter) showed that the net charges generated inside the fluidized bed were caused by entrained charged fine particles from the fluidization column. The effect of adding different varieties of fine (<45 urn) particles (Larostat 519, glass beads, silver-coated glass beads, a catalyst and silica) on charge generation/dissipation inside beds of the relatively large glass beads and 558 um polyethylene particles was studied by investigating the change of the electrostatic behaviour of fines after their addition to the fluidized bed. It was found that fine Larostat 519, two types of glass beads and two types of silver-coated glass beads carried positive charges out of the fluidized bed of relatively coarse glass beads at different relative humidities of the fluidizing air (0, 15, 35 and 60%). Comparison of charge-to-mass ratios of different fines showed that the finer the particles, the higher the charges carried per unit mass. The Larostat fines helped to dissipate the initial bed charges by attaching themselves to the large glass beads. It was found that the higher the surface conductivity of the fines, the easier it was for them to lose their charges to the column walls, thereby dissipating the initial bed charges. As the relative humidity of the fluidizing gas increased, the charge-to-mass ratios decreased, as expected. Free bubbling fluidization of binary mixtures of fines (Larostat 519, catalyst, silica and silver-coated glass beads) with relatively large polyethylene particles showed that the polarity of the charges transported out of the fluidized bed depended on the relative humidity of the fluidizing gas. It was concluded that the relative humidity of the fluidizing gas can affect the bed material (polyethylene particles) and/or the electrical behaviour of added fines. Fine catalyst and silver-coated glass beads behaved similarly, probably due to having high surface electrical conductivities. Charge-to-mass ratios were higher for the catalyst and silica particles than for the other fines. Observations after fluidizing the binary particles mixtures confirmed that there were fewer polyethylene particles clinging to the column walls when Larostat 519 and silver-coated glass bead fines were present. Bi-polar charging was also investigated. For both the coarse glass beads and polyethylene particles tested, smaller particles were charged positively and larger particles negatively. Different fines charging mechanisms, charge transfer and charge separation between the fines and the coarse particles, as well as the column wall, and their significance were investigated. For different added fines, different leading charging mechanisms were determined. The fines charging mechanisms considered in this study included particleparticle, as well as particle-wall, interactions. The latter were important here because the fluidization column in this study was of laboratory scale, so that particle-wall contacts were significant. In industrial-scale units, particle-particle interactions are likely to be dominant. Such factors as the material, physical and chemical surface properties of the solid phases, as well as the moisture content of the fluidizing gas are also important. Overall fines added to an initially charged fluidized bed carry significant charges from the column. This is a significant finding since fines are always elutriated in fluidized bed processes. It also suggests that since electrostatic forces play a role in determining the flux of entrained fines from a fluidized bed, they should be incorporated into models developed to predict entrainment flux and, perhaps also, transport disengagement height.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat