• Energy Research

A new method has been developed to predict the horizontal jet penetration of gas-liquid sprays injected into gas-solid fluidized beds. The technique involves combining a theoretical model to predict the momentum flux of two-phase sprays with the Benjelloun et al. (1995) correlation for gas jets. Following this treatment, a generalized version of the jet penetration correlation has been developed, which includes the effect of nozzle geometry. The correlation predictions are in very good agreement with the experimental data for a wide range of nozzle geometries, nozzle scales, and jet fluids.

Abstract A hydrodynamic model describing the flow structure of fluid catalytic cracking particles in laboratory scale downflow reactors is proposed. A correlation, incorporating the operating conditions of solids circulation rate and superfacial gas velocity has been developed which adequately correlates published data of solids hold-up for fully developed flow. From this, the radial gas and particle velocity profiles are determined. Mechanisms for the densification near the wall in downflow reactors are proposed in order to describe changes in the radial solids hold-up profiles with changing operating conditions. The operating conditions of superficial gas velocity and solids flux are related to the magnitude of the densification near the wall in terms of an energy minimization concept. The flatness of the solids hold-up profile can be described by the model parameter α. A low α indicates a flat solids hold-up profile, more homogeneous flow and better solids-gas contact efficiency. A slip velocity is calculated using the model which confirms that segregation increases with decreasing superficial gas velocity and increasing solids flux. Thus, the model is able to describe the radial flow structure and suspension homogeneity according to operating conditions and the importance of friction between the gas-solids suspension and the wall for laboratory scale downflow reactors. The study also highlights problems in using the hydrodynamic data obtained in laboratory scale downflow reactors for scale-up, since wall effects are likely less important in industrial scale downers. Different reaction schemes (both series and parallel) have been employed to study the effect of the operating conditions on conversions, yields and selectivities in the presence of fast deactivating catalysts. The study highlights the importance of using realistic hydrodynamic models to interpret kinetic data obtained using laboratory scale downflow reactors.

Abstract Mixing characteristics of a novel Mechanically Fluidized Reactor (MFR), a continuous, cylindrical, mechanically mixed reactor developed for biomass pyrolysis in the ICFAR laboratory, have been investigated with the specific objective of selecting the optimal stirrer geometry for fast pyrolysis of biomass. The MFR does not use any fluidization gas and its stirrer provides the required mixing between the injected biomass and the bed material while effectively breaking any possible agglomerate. In addition, good mixing is crucial to achieve effective heat transfer characteristics between the heaters and the bed, and between the bed and the biomass particles. In conventional fluidized beds, the torque required to mix the bed decreases as the superficial velocity of its fluidization gas increases, becoming constant above a critical superficial velocity which is a function of the stirrer characteristics. For the MFR, a method has been developed to monitor the torque and the power required to mechanically mix a bed of low density particles with the natural bed aeration resulting from the formation of gases and vapors during pyrolysis. In this study, three different shaped stirrers were first compared at various rotational speeds by artificially aerating the bed with nitrogen at different superficial velocities to simulate the generation of vapors and gases during pyrolysis. Furthermore, different fluidization gases were used in order to simulate the different characteristics of vapors produced during pyrolysis and, specifically, to evaluate the effects of their density and viscosity. The critical aeration rate at which the stirrer torque becomes constant is similar for all the stirrers. The second part of this study focused on actual wood pyrolysis tests. The reactor was first supplied with nitrogen above the critical aeration rate. Then, the nitrogen was shut off and wood pellets were fed into the reactor. The formation of pyrolysis gases and vapors greatly decreased the power required to mix the bed and this reduction was dependent on the type of stirrer. The stirrers were ranked in terms of their performance in minimizing the pyrolysis time. The vertical blade stirrer resulted in the smallest pyrolysis time and power consumption. Additionally no segregation of the biomass pellets was observed. Hence, gases and vapors were formed at the optimum location for effective aeration. The findings of this project have provided a better understanding of the MFR technology when used for the pyrolytic processing of biomass materials.

Abstract A predictive mathematical model, able to characterize and quantify all facets of the time-averaged gas and solids flow structure and properties within circulating fluidized bed (CFB) risers, is proposed. The model can be used as a tool to assess a-priori the operation of a riser and can be easily coupled to kinetic models for process simulation. The input parameters to the model include the riser operating conditions (that is, solids circulation flux and gas superficial velocity), riser geometry and gas and solids physical properties. The proposed model assumes the CFB riser to be axially composed of two regions: an acceleration zone at the riser base, where solids re-injected from a standpipe are accelerated to a constant upward velocity, and a fully-developed region, where the flow characteristics are invariant with height, extending from the end of the acceleration region to the riser exit. The model postulates the existence of a core—annulus type of flow structure and is based on both fundamental principles and empirical relationships. The model is successfully verified against experimental data from CFB units of various sizes and operating under different regimes of fluidization. The model outputs, consisting of axial pressure drop profiles, axial and radial voidage profiles, radial solids velocity and mass flux profiles, average gas velocity and core radius, are compared to existing data and are assessed critically.

Abstract Fast pyrolysis of kraft lignin with partial (air) oxidation was studied in a bubbling fluidized bed reactor at reaction temperatures of 773 and 823 K. The bio-oil vapors were fractionated using a series of three condensers maintained at desired temperatures, providing a dry bio-oil with less than 1% water and over 96% of the total bio-oil energy. Oxygen feed was varied to study its effect on yield, composition, and energy recovery in the gas, char and oil products. The addition of oxygen to the pyrolysis process increased the production of gases such as CO and CO2. It also changed the dry bio-oil properties, reducing its heating value, increasing its oxygen content, reducing its average molecular weight and tar concentration, while increasing its phenolics concentration. The lower reaction temperature of 773 K was preferred for both dry bio-oil yield and quality. Autothermal operation of the pyrolysis process was achieved with an oxygen feed of 72 or 54 g per kg of biomass at the reaction temperatures of 773 and 823 K, respectively. Autothermal operation reduced both yield and total energy content of the dry bio-oil, with relative reductions of 24 and 20% for the yield, 28 and 23% for the energy content, at 773 and 823 K.

Abstract In order to model the gas–solid hydrodynamic flow structure in circulating fluidized bed (CFB) riser reactors, there is a need to predict the average and local suspension density and flow direction as functions of both axial and radial coordinates, gas and solids physical properties, and operating conditions (gas superficial velocity and imposed solids circulation rate). Existing hydrodynamic models fail to properly and reliably describe the flow characteristics typical of CFB catalytic reactors operating at high superficial gas velocities and solids mass fluxes. In the present work, we have compiled an extensive experimental database to encompass published laboratory, pilot, and available industrial scale hydrodynamic data in a concise and useable form. Thorough analysis of the database followed by an in-depth multiple regression analysis yielded a new correlation for calculating the “slip factor”, defined as the ratio of the interstitial gas velocity to the particle velocity, to describe Geldart Group A powder systems in the fully developed flow (FDF) region of the riser. The correlation allows the evaluation of the gas–solid suspension density in the riser in both the fast fluidization (FF) and pneumatic transport (PTR) flow regimes. In addition, a comparative study has been carried out on the database to investigate the effects of the geometrical configuration of the top exit of the riser on the suspension density and flow structure. Two reflection coefficient correlations are proposed to account for exit effects for both Groups A and B solids systems. The slip factor and reflection coefficient correlations have been further compared to existing correlations available in the literature for discussion and validation and shown to successfully characterize the large majority of data available in the literature.

AbstractSyngas produced by biomass and waste gasification processes must be adequately clean of tar compounds before being utilized in value‐added applications. Syngas cleaning by tar cracking at high temperatures is a promising technique that can utilize different kinds of catalysts. However, their use is limited by the deposition of coke layers, which induces a masking phenomenon on the active surface, and, consequently, the rapid deactivation of the catalyst. This study addresses how the temperature (750 and 800°C) and the steam concentration (0% and 7.5%) can affect the extent of water–gas and reforming reactions between steam and coke deposits. Two catalysts were used: a market‐available activated carbon and an iron‐based alumina catalyst. The tests showed better performance of the Fe/γ‐Al2O3 catalyst. A mass increase of the bed was measured in tests with both the catalysts, which confirms the deposition of the coke layer produced by tar dehydrogenation and carbonization. Scanning electronic microscopy‐energy‐dispersive X‐ray analysis (SEM‐EDX) and Raman spectroscopy were utilized to investigate the nature of coke layers over the catalyst surface, with the aim of acquiring information about their reactivity towards the water gas reaction. SEM‐EDX observations indicate that the thickness of these carbon layers is less than 2 μm. Raman spectra suggest a negligible effect of the reaction temperature in the tested range and, in particular, that the amorphous nature of coke layers deposited in the presence of steam is relatively more graphitic than that obtained without steam.

Abstract Fast pyrolysis of birch bark sawdust with partial (air) oxidation was studied in a bubbling fluidized bed reactor at reaction temperatures of 500 and 550 °C. The bio-oil vapors were fractionated using a series of three condensers maintained at desired temperatures, providing a dry bio-oil with less than 1 wt.% water and over 93% of the total bio-oil energy. Oxygen feed was varied to study its effect on yield, composition, and energy recovery in the gas, char and oil products. The addition of oxygen to the pyrolysis process increased the production of gases such as CO and CO2. It also changed the dry bio-oil properties, reducing its heating value, increasing its oxygen content, reducing its average molecular weight and its concentration of heavy bio-oil compounds, while increasing its viscosity and its phenolics concentration. The lower reaction temperature of 500 °C was preferred for both dry bio-oil yield and quality. Autothermal operation of the pyrolysis process was achieved with an oxygen feed of 0.08 g per g of biomass at the reaction temperatures of 500 and 550 °C. At these two temperatures, when compared with the standard pyrolysis with pure nitrogen, the yield of dry bio-oil was reduced by 22% and 31%, whereas the total energy content of the dry bio-oil was reduced by 25% and 34%, respectively.

Abstract Hydrotransport of solids through a pipe is a cost and energy efficient method to transport granular solid materials over long distances. A problem in the hydrotransport of fine particle slurries is the possible presence in the pipe of undesirable large materials, such as rocks or metal fragments broken off of shovels, which may enter the slurry pipe through breaks in screens. This large material can damage booster pumps and downstream equipment resulting in costly repairs and loss of production. Acoustic sensors along with signal analysis techniques can be used for online detection of oversized material in a hydrotransport system. Acoustic detection methods are ideal, since they are non-invasive and any probe located within the pipe would be unlikely to survive the harsh conditions present within the line. The objective of this study was to model the motion behaviour of large materials, such as rocks, travelling through a horizontal hydrotransport pipe. This information can then be used to determine optimum locations for the acoustic sensors to ensure that rocks are detected quickly and effectively.