Gasification Kinetics in Continuous Supercritical Water Reactors

Supercritical water gasification (SCWG) is an emerging technology with synergistic applications in renewable energy and waste processing. Supercritical water (SCW) functions as a green reaction medium during the gasification process, serving to dissolve and decompose complex organic molecules via ionic, radical, hydrolysis, and pyrolysis reaction mechanisms. Researchers investigate the decomposition of model compounds in order to predict product yields and conversion efficiencies during the gasification of heterogeneous biomass waste, food waste, sewage sludge, and other available feedstocks. Continuous, laboratory-scale reactors are often employed to study reaction kinetics, pathways, and mechanisms. This chapter synthesizes previous work investigating model compound gasification in continuous supercritical water reactors (SCWRs). A summary of continuous reactor design strategies is presented for practical benefit, followed by a discussion on reaction chemistry in the supercritical water environment. Reaction pathways and mechanisms have been investigated for several model compounds, lending insight toward the conditions needed for the complete conversion of real-world feedstocks. Several studies assume first-order reaction kinetics and propose Arrhenius parameters for the decomposition reaction. The first-order rate assumption must be carefully evaluated, and the applicable temperature range must be specified. Opportunities for further research are discussed

Supercritical Water Gasification: Practical Design Strategies and Operational Challenges for Lab-Scale, Continuous Flow Reactors

Optimizing an industrial-scale supercritical water gasification process requires detailed knowledge of chemical reaction pathways, rates, and product yields. Laboratory-scale reactors are employed to develop this knowledge base. The rationale behind designs and component selection of continuous flow, laboratory-scale supercritical water gasification reactors is analyzed. Some design challenges have standard solutions, such as pressurization and preheating, but issues with solid precipitation and feedstock pretreatment still present open questions. Strategies for reactant mixing must be evaluated on a system-by-system basis, depending on feedstock and experimental goals, as mixing can affect product yields, char formation, and reaction pathways. In-situ Raman spectroscopic monitoring of reaction chemistry promises to further fundamental knowledge of gasification and decrease experimentation time. High-temperature, high-pressure spectroscopy in supercritical water conditions is performed, however, long-term operation flow cell operation is challenging. Comparison of Raman spectra for decomposition of formic acid in the supercritical region and cold section of the reactor demonstrates the difficulty in performing quantitative spectroscopy in the hot zone. Future designs and optimization of SCWG reactors should consider well-established solutions for pressurization, heating, and process monitoring, and effective strategies for mixing and solids handling for long-term reactor operation and data collection

EXPERIMENTAL AND FINITE ELEMENT ANALYSIS OF STAND-OFF LAYER DAMPING TREATMENTS FOR BEAMS

ABSTRACT Passive stand-off layer (PSOL

Balancing risks of injury and disturbance to marine mammals when pile driving at offshore windfarms

1. Offshore windfarms require construction procedures that minimize impacts on protected marine mammals. Uncertainty over the efficacy of existing guidelines for mitigating near-field injury when pile-driving recently resulted in the development of alternative measures, which integrated the routine deployment of acoustic deterrent devices (ADD) into engineering installation procedures without prior monitoring by marine mammal observers. 2. We conducted research around the installation of jacket foundations at the UK's first deep-water offshore windfarm to address data gaps identified by regulators when consenting this new approach. Specifically, we aimed to (a) measure the relationship between noise levels and hammer energy to inform assessments of near-field injury zones and (b) assess the efficacy of ADDs to disperse harbour porpoises from these zones. 3. Distance from piling vessel had the biggest influence on received noise levels but, unexpectedly, received levels at any given distance were highest at low hammer energies. Modelling highlighted that this was because noise from pin pile installations was dominated by the strong negative relationship with pile penetration depth with only a weak positive relationship with hammer energy. 4. Acoustic detections of porpoises along a gradient of ADD exposure decreased in the 3-h following a 15-min ADD playback, with a 50% probability of response within 21.7 km. The minimum time to the first porpoise detection after playbacks was > 2 h for sites within 1 km of the playback. 5. Our data suggest that the current regulatory focus on maximum hammer energies needs review, and future assessments of noise exposure should also consider foundation type. Despite higher piling noise levels than predicted, responses to ADD playback suggest mitigation was sufficiently conservative. Conversely, strong responses of porpoises to ADDs resulted in far-field disturbance beyond that required to mitigate injury. We recommend that risks to marine mammals can be further minimized by (1) optimizing ADD source signals and/or deployment schedules to minimize broad-scale disturbance; (2) minimizing initial hammer energies when received noise levels were highest; (3) extending the initial phase of soft start with minimum hammer energies and low blow rates

Underwater noise modelling for environmental impact assessment

Assessment of underwater noise is increasingly required by regulators of development projects in marine and freshwater habitats, and noise pollution can be a constraining factor in the consenting process. Noise levels arising from the proposed activity are modelled and the potential impact on species of interest within the affected area is then evaluated. Although there is considerable uncertainty in the relationship between noise levels and impacts on aquatic species, the science underlying noise modelling is well understood. Nevertheless, many environmental impact assessments (EIAs) do not reflect best practice, and stakeholders and decision makers in the EIA process are often unfamiliar with the concepts and terminology that are integral to interpreting noise exposure predictions. In this paper, we review the process of underwater noise modelling and explore the factors affecting predictions of noise exposure. Finally, we illustrate the consequences of errors and uncertainties in noise modelling, and discuss future research needs to reduce uncertainty in noise assessments

The Analysis of a Nonlinear Difference Equation Occurring in Dynamical Systems

A difference equation with a cubic nonlinearity is examined. Using a phase plane analysis, both quasi-periodic and chaotically behaving solutions are found. The chaotic behavior is investigated in relation to heteroclinic and homoclinic oscillations of stable and unstable solution manifolds emanating from unstable periodic points. Certain criteria are developed which govern the existence of the stochastic behavior. An approximate solution technique is developed giving expressions for the quasi-periodic solutions close to a stable periodic point and the accuracy of these expressions are investigated. The stability of the solutions is examined and approximate local stability criteria are obtained. Stochastic excitation of a nonlinear difference equation is also considered and an approximate value of the second moment of the solution is obtained.</p