Clark Memorandum: Fall 1995

Clark Memorandum: Fall 1995 Soldiers of the Spirit The Profession Everyone Loves to Hate Selected Letters The Atonement—Infinite and Eternal Out of Kindergarten, Out of Law Schoolhttps://digitalcommons.law.byu.edu/clarkmemo_gallery/1025/thumbnail.jp

Photovoltaics : reviewing the European feed-in-tariffs and changing PV efficiencies and costs

Feed-in-Tariff (FiT) mechanisms have been important in boosting renewable energy, by providing a long-term guaranteed subsidy of the kWh-price, thus mitigating investment risks and enhancing the contribution of sustainable electricity. By ongoing PV development, the contribution of solar power increases exponentially. Within this significant potential, it is important for investors, operators, and scientists alike to provide answers to different questions related to subsidies, PV efficiencies and costs. The present paper therefore (i) briefly reviews the mechanisms, advantages, and evolution of FiT; (ii) describes the developments of PV, (iii) applies a comprehensive literature-based model for the solar irradiation to predict the PV solar energy potential in some target European countries, whilst comparing output predictions with the monthly measured electricity generation of a 57 m² photovoltaic system (Belgium); and finally (iv) predicts the levelized cost of energy (LCOE) in terms of investment and efficiency, providing LCOE values between 0.149 and 0.313 €/kWh, as function of the overall process efficiency and cost. The findings clearly demonstrate the potential of PV energy in Europe, where FiT can be considerably reduced or even be eliminated in the near future

Operation diagram of circulating fluidized beds (CFBs)

CFBs are widely used in the chemical, mineral, environmental and energy process industries. Several authors stressed the need for a clear identification of the different operation regimes in the riser of a CFB, to ensure a better comprehension of the hydrodynamic context, and thus better define the operation and design parameters. First approaches to develop a “work map” of the riser operation, were presented by e.g. Grace[1], Yerushalmi and Avidan[2], Bai et al.[3]. It was further developed by Chan et al.[4] and Mahmoudi et al.[5,6] for both Geldart A- and B-type powders, in terms of the operating gas velocity (U) and the solids circulation flux (G), which jointly delineate different regimes, called respectively Dilute Riser Flow (DRF), Core-Annulus Flow (CAF) (possibly with a bottom Turbulent Fluidized Bed, TFBB), and Dense Riser Upflow (DRU). For a given powder and its associated transport velocity, UTR, the combination of U and G will determine the flow regime encountered. Experiments in CFB risers of 0.05 (2.5 m high), 0.1 and 0.15 m I.D. (both 6.5 m high), have demonstrated that common riser operations can be hampered by a specific (U,G) range where choking occurs. Angular sand, rounded sand, and spent FCC (all A-type powders) were used as bed material. Gas velocities were varied between 2 and 10 m/s, for solids circulation fluxes between 10 and 260 kg/m2s. Choking is understood as the phenomenon where a small change in gas or solids flow rate prompts a large change in the pressure drop and/or solids holdup during the gas-solid flow: the stable riser upflow regime is no longer maintained when G-values exceed a certain limit for a given gas velocity. Experimental results were empirically correlated, and proved to be about 30% lower than predicted by the correlation of Bi and Fan[7], but largely exceeding other predictions. Introducing the findings into the available operation diagram [5,6], adds a region where stable riser operation is impossible. The adapted diagram enables CFB designers to better delineate the operating characteristics

The voidage in a CFB riser as function of solids flux and gas velocity

Circulating Fluidised Beds (CFBs) are widely applied in the process industry, for mostly gas-solid and gas-catalytic reactions. The riser is the key component of the CFB being the process reactor. The important design parameters are the operating gas velocity (U) and the solids concentration flux (G). The CFB operation starts at moderate to high superficial gas velocities. Its voidage exceeds ∼ 0.9 and is a function of the solids circulation flux. Different flow modes have been presented in literature, and result in an operation diagram where G and U delineate specific operations, from dilute riser flow, through core-annulus flow, to dense riser upflow (mostly at any U, G exceeding 80 to 120 kg m-2s-1). Increasing G whilst maintaining the gas velocity will cause an increase in suspension concentration. The riser flow can hence be characterized by its apparent voidage, ɛ. In the core-annulus operation, clusters of particles reflux near the wall, thus influencing the local radial voidage in the cross section of the riser, and also extending over a given distance, δ, from the wall to the core . Through measurements in CFBs of 0.1 and 0.14 m I.D., the research has been able to determine the average axial and radial voidages of the dense phase within the different regimes, whilst also determining the thickness of the annulus (in CAF-mode). Experimental results will be illustrated and compared with previous empirical equations, shown to have a limited accuracy only both for ɛ, and for the thickness of the annulus in CAF operation. Within the operating conditions tested, results demonstrate that the annulus thickness is about 15 to 20% of the riser diameter in CAF, and that the voidage in the riser is a function of U and G, with riser diameter and distance along the riser length as secondary parameters

Principles and potential of the anaerobic digestion of waste-activated sludge

When treating municipal wastewater, the disposal of sludge is a problem of growing importance, representing up to 50% of the current operating costs of a wastewater treatment plant. Although different disposal routes are possible, anaerobic digestion plays an important role for its abilities to further transform organic matter into biogas (60-70vol% of methane, CH4), as thereby it also reduces the amount of final sludge solids for disposal whilst destroying most of the pathogens present in the sludge and limiting odour problems associated with residual putrescible matter. Anaerobic digestion thus optimises WWTP costs, its environmental footprint and is considered a major and essential part of a modern WWTP. The potential of using the biogas as energy source has long been widely recognised and current techniques are being developed to upgrade quality and to enhance energy use. The present paper extensively reviews the principles of anaerobic digestion, the process parameters and their interaction, the design methods, the biogas utilisation, the possible problems and potential pro-active cures, and the recent developments to reduce the impact of the problems. After having reviewed the basic principles and techniques of the anaerobic digestion process, modelling concepts will be assessed to delineate the dominant parameters. Hydrolysis is recognised as rate-limiting step in the complex digestion process. The microbiology of anaerobic digestion is complex and delicate, involving several bacterial groups, each of them having their own optimum working conditions. As will be shown, these groups are sensitive to and possibly inhibited by several process parameters such as pH, alkalinity, concentration of free ammonia, hydrogen, sodium, potassium, heavy metals, volatile fatty acids and others. To accelerate the digestion and enhance the production of biogas, various pre-treatments can be used to improve the rate-limiting hydrolysis. These treatments include mechanical, thermal, chemical and biological interventions to the feedstock. All pre-treatments result in a lysis or disintegration of sludge cells, thus releasing and solubilising intracellular material into the water phase and transforming refractory organic material into biodegradable species. Possible techniques to upgrade the biogas formed by removing CO2, H2S and excess moisture will be summarised. Special attention will be paid to the problems associated with siloxanes (SX) possibly present in the sludge and biogas, together with the techniques to either reduce their concentration in sludge by preventive actions such as peroxidation, or eliminate the SX from the biogas by adsorption or other techniques. The reader will finally be guided to extensive publications concerning the operation, control, maintenance and troubleshooting of anaerobic digestion plants. (C) 2008 Elsevier Ltd. All rights reserved.[*]status: publishe

The design of cyclonic pre-heaters in suspension cement kilns

© 2014, © 2013 Taylor & Francis. Cement manufacturing consumes two main types of energy: fuel and electricity. On average, energy costs represent 40% of the total production costs per ton of cement. The challenge is to reduce the consumption of energy to about 3000 MJ/ton clinker without the consumption of massive additional amounts of electricity, which is normally associated with additional fuel-saving measures. This can only be achieved by implementing sound thermal energy optimization measures. Energy-efficient suspension cement kilns are now widely applied and use a cascade of cyclonic pre-heaters of moist particulate feedstock, with heat transfer from hot kiln exhaust gas to particles being a function of heat transfer coefficient, temperature difference and gas–solid contact mode and time. The gas–solid contact mode and time depend on particle movement in cyclones, which has previously been studied by positron emission particle tracking. Heat transfer is governed by Nusselt–Reynolds equations, with gas velocity and properties being a function of the temperature profile along the cascade of cyclones. A stepwise approach of the design thus combines changing hydrodynamics and heat transfer along the successive cyclones, with the overall thermal balance of the cascade as control. This approach leads to major design recommendations, as developed in the present paper.status: publishe

Choking affects the operation diagram of a CFB riser

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The convection heat transfer coefficient in a Circulating Fluidized Bed (CFB)

Circulating Fluidized Beds are increasingly used in gas–solid and gas–catalytic reactions. A recent development involves their use in physical gas–solid processes such as drying, VOC adsorption or solar energy capture and storage. The heat transfer from the wall of the CFB to the flowing gas–solid suspension is the major design parameter, and was studied for different powders at different operating conditions as determined by the gas velocity and solids circulation flux. Measured values of the heat transfer coefficients are discussed, and compared with empirical predictions of Molodtsof–Muzyka, and Gorliz–Grace. Whereas Gorliz–Grace predicts heat transfer coefficients correctly within a narrow range of operating conditions only, the Molodtsof–Muzyka approach can be simplified into a linear relationship.publisher: Elsevier articletitle: The convection heat transfer coefficient in a Circulating Fluidized Bed (CFB) journaltitle: Advanced Powder Technology articlelink: http://dx.doi.org/10.1016/j.apt.2013.10.018 content_type: article copyright: Copyright © 2013 Published by Elsevier B.V.status: publishe

Assessment of the ozonation and Fenton process as a pretreatment of a subsequent biological treatment based on respirometric measurements

During the last decades, the industrial sector has been confronted with important challenges concerning the water treatment. Stricter norms and a rising fresh water cost make a more profound effluent purification, possibly coupled with water reuse, more and more necessarily. In this paper, the potential contribution of the integration of chemical oxidation with biological treatmetn in realising these challenges, has been examined. In this study, the Fenton process and the ozonation process are evaluated based on both conventional analytical measurements and respirometry. During the past decades, respirometry has increasingly been employed to obtain biokinetic characteristics, and it is considered one of the most important information sources in activated sludge process modelling. From the investigated AOPs, the Fenton process leads to the best results based on both degradation of organic pollutants and enhancement of biodegradability. An overall COD degradation by the combined treatmetn between 69% and 86% can be achieved.status: publishe

Circulating fluidized bed heat recovery/storage and its potential to use coated phase-change-material (PCM) particles

Within the thermal energy capture and/or storage systems currently available or investigated, PCMs are the sole latent heat stores. Despite their low thermal conductivity, that limits charging and discharging times, the higher energy storage capacity per unit weight in comparison with sensible heat stores, makes them increasingly attractive for high temperature applications, resulting in reduced storage volumes and required circulation rates within the heat collector. The present paper introduces these PCMs, and their potential application in high temperature energy capture and storage, using a circulating fluidized bed (CFB) as transfer/storage mode. Thermal considerations determine the optimum size range for the applied particles (<400 μm). The heat transfer from the wall of the CFB to the flowing gas–solid suspension is a major design parameter of the collector, and studied for different operating conditions as determined by the gas velocity and solids circulation flux. Measured values of the heat transfer coefficients are discussed, and compared with empirical predictions of Molodtsof–Muzyka, and Gorliz–Grace. Fair agreement is obtained only when the empirical parameters are carefully predicted. The application of a packet renewal mechanism at the wall is also investigated, with a fair prediction of the heat transfer coefficient in terms of the expected solid contact time at the wall.status: publishe