Waves are an important agent in the construction, shaping and destruction of river deltas. Notwithstanding the commonality of waves in oceans and seas, wave influence on deltas varies considerably ...depending on the coastal morphology and nearshore bathymetry. Although there have been advances in understanding the way waves approaching a delta shape its shoreline, much still remains to be known of the interactions between waves and river deltas. Deltas are built essentially from sediments supplied by rivers. Sand-sized and coarser sediments may also be derived from nearby abandoned delta lobes or from older relict nearshore deposits, transported by wave reworking and longshore currents. Alternatively, delta erosion by waves can also release sediment that is redistributed alongshore or that accumulates offshore. The extent to which bedload is supplied to and sequestered in, or lost by, deltas through waves and longshore transport strongly depends on interactions between waves and fluvial discharge at the river mouth. These interactions and the mutual adjustments they engender are not only important in the overall balance between delta retreat, progradation or aggradation but also in processes such as avulsion and channel switching, as well as in the eventual survival of a delta in the face of sea-level rise. Where waves are important, fluvial liquid discharge is high, and sediment supply is rich in bedload, two important aspects are the blocking of waves and longshore currents by strong river discharge and the formation of bars at the river mouth. Field studies of the complex interactive processes prevailing where river flows encounter waves are, however, non-existent and numerical modelling, though promising, hampered by scale constraints and the difficulty of replicating them and generating mouth bars in the presence of longshore currents. This interaction influences the seaward extent of the delta mouth protuberance and its stability; this protuberance then forming the regional shoreline template to which waves and longshore currents adjust. Longshore currents can redistribute wave-reworked mouth bar deposits emplaced during strong river flow. Transport may be either divergent from the mouth or may be regionally unidirectional but wherein the symmetry of some deltas, probably rare, may be maintained by a strong river blocking effect on transport from the updrift flank. The mouth protuberance may be such as to foster transport reversal (counter-drift) at the delta margins that contributes to sediment sequestering within the delta. These interactions largely contribute in shaping delta shorelines, and together with the abundance of sediment supply and grain size, determine the resultant wave-formed shoreline barrier types, which include spits, more or less closely-spaced beach ridges, and barrier islands and cheniers in situations of punctuated progradation or retreat. Where several distributary mouths occur, pronounced longshore variability in wave processes and wave-induced sediment transport may ensue, resulting in multiple drift cells that assure the retention of sand and coarse-grained sediments within the delta. Waves can also be an important agent in the reworking and retreat of mud-rich deltas that generally conform in morphology to the ‘river-dominated’ (such as the Mississippi) or ‘tide-dominated’ (such as the Ganges–Brahmaputra or Chao Phraya) types, resulting in the episodic formation of sandy cheniers and beach ridges.
Although sea-level rise is likely to lead to enhanced wave reworking of deltas, the possible prevalence of aggradation (in lieu of progradation), channel switching and avulsion, and washover processes, may contribute to the disorganization of waves and longshore transport, fostering deltaic sequestering of sand and coarser-sized sediment and delta survival. The weakening of river discharges resulting from human activities will invariably lead, however, to enhanced wave reworking of deltas and to deltaic sediment redistribution by longshore currents. The massive swing towards significant reductions in fluvial sediment supply today may signify the ultimate demise of many deltas in the coming decades through a process of delta shoreline straightening by waves, in addition to accelerated sinking. These various foregoing aspects of the relationship between waves and river deltas are reviewed here across a range of timescales, and new interaction concepts proposed, using numerous examples of deltas in the world and on the basis of case studies, conceptual studies and numerical modelling studies in the literature spanning more than forty years.
•Waves influence river delta shoreline plan shape and overall delta development.•Interaction between waves and river flow determines river-mouth bar formation.•River-mouth bars are the building blocks of many wave-constructed delta shorelines.•Longshore drift around the river mouth influences delta sediment loss or conservation.•Weakened river influence will lead to the ultimate demise of many deltas by waves.
The present geomorphology of the Mediterranean's coasts is largely a product of an intricate long-term relationship between Nature and human societies. A cradle of ancient civilisations, the ...Mediterranean has seen its shores occupied by Humans since Prehistory, and is, therefore, a particularly pertinent unit of analysis. The morphotectonic context and other forcing agents (e.g., climate) shaped out a highly diversified coastal morphology and generated a sediment-supply regime potentially favourable to the formation of numerous open-coast deltas and bay-head deltas in infilled rias as sea level stabilised during the mid-Holocene. This supply of riverine sediment has also been the key agent in mediating human occupation of the Mediterranean's clastic coasts. Expressions of this relationship have been extensively archived in clastic coastal deposits, including base-level deltaic and estuarine sedimentary sinks, which comprise records to explore the interactions between geosystems and the human environment. The stratigraphic sequences in these coastal sedimentary archives comprise, in many places, a clearly identified anthropogenic signature, notably in ancient harbours, some of which underwent extremely rapid silting up due to massive sediment sourcing generated by new agricultural practices from the Neolithic onwards. Increasing human influence, especially over the last 3000 years, has been, in turn, an important driver of changes in sediment supply, strongly modulating deltaic development. Pulses of sediment supply from catchments rendered vulnerable by human perturbations during the Roman period resulted in a new cycle of inception of many other deltas and in rapid delta growth (e.g. the Ebro, the Po, the Arno and the Ombrone). Another progradation dynamic during the Little Ice Age, at a time of strong rural population growth, river discharge increases, technological developments, and urbanisation, further consolidated delta growth. Understanding the life cycle of these deltas since their initial formation is, in turn, key to unravelling the relative role of natural and anthropogenic forcing agents. Rapid climate changes are deemed to have contributed through both the stripping of landscapes rendered fragile by human activities and active fluvial sediment transport to the coast, but disentangling climate change effects from human impacts in the Mediterranean remains a challenge. The patterns of subsequent deltaic growth and delta morphodynamics reflect adaptations to pulsed sediment supply, river discharge variations, the microtidal, fetch-limited context of the Mediterranean, and direct engineering interventions. The progradation dynamic of the Roman period and Little Ice Age contrasts markedly with the situation of common coastal destabilisation over the last two centuries, particularly well documented for the last 50 years. This period has been characterised by reduced sediment flux to base-level geosystems due to catchment reforestation, retenion within reservoirs, fluvial regulation and dredging, resulting in the erosion of deltas and barrier-lagoon and beach-dune systems. Large stretches of shoreline and narrow coastal plains have been massively engineered for coastal defence and protection against erosion, but also for the construction of marinas, leisure harbours and artificial beaches, resulting in the emergence of veritable artificial seafronts. These interventions have, collectively and progressively, raised societies to a pervasive and overarching position in the geomorphic stability-instability of the Mediterranean's coasts, a situation that will be exacerbated by pressures from sea-level rise, paving the way for rampant coastal erosion and delta destruction.
Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, ...more recently, its ability to facilitate the net removal of CO
2
from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO
2
capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.
Carbon capture and storage (CCS) is vital to climate change mitigation, and has application across the economy, in addition to facilitating atmospheric carbon dioxide removal resulting in emissions offsets and net negative emissions. This contribution reviews the state-of-the-art and identifies key challenges which must be overcome in order to pave the way for its large-scale deployment.
Biofuels from biomass gasification are reviewed here, and demonstrated to be an attractive option. Recent progress in gasification techniques and key generation pathways for biofuels production, ...process design and integration and socio-environmental impacts of biofuel generation are discussed, with the goal of investigating gasification-to-biofuels’ credentials as a sustainable and eco-friendly technology. The synthesis of important biofuels such as bio-methanol, bio-ethanol and higher alcohols, bio-dimethyl ether, Fischer Tropsch fuels, bio-methane, bio-hydrogen and algae-based fuels is reviewed, together with recent technologies, catalysts and reactors. Significant thermodynamic studies for each biofuel are also examined. Syngas cleaning is demonstrated to be a critical issue for biofuel production, and innovative pathways such as those employed by Choren Industrietechnik, Germany, and BioMCN, the Netherlands, are shown to allow efficient methanol generation. The conversion of syngas to FT transportation fuels such as gasoline and diesel over Co or Fe catalysts is reviewed and demonstrated to be a promising option for the future of biofuels. Bio-methane has emerged as a lucrative alternative for conventional transportation fuel with all the advantages of natural gas including a dense distribution, trade and supply network. Routes to produce H2 are discussed, though critical issues such as storage, expensive production routes with low efficiencies remain. Algae-based fuels are in the research and development stage, but are shown to have immense potential to become commercially important because of their capability to fix large amounts of CO2, to rapidly grow in many environments and versatile end uses. However, suitable process configurations resulting in optimal plant designs are crucial, so detailed process integration is a powerful tool to optimize current and develop new processes. LCA and ethical issues are also discussed in brief. It is clear that the use of food crops, as opposed to food wastes represents an area fraught with challenges, which must be resolved on a case by case basis.
Energy storage is one of the most critical factors for maximising the availability of renewable energy systems while delivering firm capacity on an as- and when-required basis, thus improving the ...balance of grid energy. Chemical and calcium looping are two technologies, which are promising from both the point of view of minimising greenhouse gas emissions and because of their suitability for integrating with energy storage. A particularly promising route is to combine these technologies with solar heating, thus minimising the use of fossil fuels during the materials regeneration steps. For chemical looping, the development of mixed oxide carrier systems remains the highest impact research and development goal, and for calcium looping, minimising the decay in CO2 carrying capacity with natural sorbents appears to be the most economical option. In particular, sorbent stabilisers such as those based on Mg are particularly promising. In both cases, energy can be stored thermally as hot solids or chemically as unreacted materials, but there is a need to build suitable pilot plant demonstration units if the technology is to advance.
Display omitted
•Calcium and chemical looping can potentially be deployed at commercial scale within the next two decades.•Both technologies have the potential for energy storage, either by means of sensible heat or as chemical energy.•A critical requirement for such development is construction and testing of suitable pilot and demonstration plants.•Both technologies can be combined with solar power to provide thermochemical energy storage.
•The extensive literature on Ca looping sorbents and their properties has been reviewed.•Currently, there is a lack of experiments on doped sorbents in realistic systems.•Most complex methods of ...sorbent modification appear to be prohibitively expensive for CCS applications.•A major challenge of all sorbent modification processes is their scalability.
Calcium looping (CaL) is a promising technology for the decarbonation of power generation and carbon-intensive (cement, lime and steel) industries. Although CaL has been extensively researched, some issues need to be addressed before deployment of this technology at commercial scale. One of the important challenges for CaL is decay of sorbent reactivity during capture/regeneration cycles. Numerous techniques have been explored to enhance natural sorbent performance, to create new synthetic sorbents, and to re-activate and re-use deactivated material. This review provides a critical analysis of natural and synthetic sorbents developed for use in CaL. Special attention is given to the suitability of modified materials for utilisation in fluidised-bed systems. Namely, besides requirements for a practical adsorption capacity, a mechanically strong material, resistant to attrition, is required for the fluidised bed CaL operating conditions. However, the main advantage of CaL is that it employs a widely available and inexpensive sorbent. Hence, a compromise must be made between improving the sorbent performance and increasing its cost, which means a relatively practical, scalable, and inexpensive method to enhance sorbent performance, should be found. This is often neglected when developing new materials focusing only on very high adsorption capacity.
Biomass gasification is a widely used thermochemical process for obtaining products with more value and potential applications than the raw material itself. Cutting-edge, innovative and economical ...gasification techniques with high efficiencies are a prerequisite for the development of this technology. This paper delivers an assessment on the fundamentals such as feedstock types, the impact of different operating parameters, tar formation and cracking, and modelling approaches for biomass gasification. Furthermore, the authors comparatively discuss various conventional mechanisms for gasification as well as recent advances in biomass gasification. Unique gasifiers along with multi-generation strategies are discussed as a means to promote this technology into alternative applications, which require higher flexibility and greater efficiency. A strategy to improve the feasibility and sustainability of biomass gasification is
via
technological advancement and the minimization of socio-environmental effects. This paper sheds light on diverse areas of biomass gasification as a potentially sustainable and environmentally friendly technology.
The article reviews diverse areas of conventional and advanced biomass gasification discussing their feasibility and sustainability
vis-à-vis
technological and socio-environmental impacts.
As international concern for the survival of deltas grows, the Mekong River delta, the world's third largest delta, densely populated, considered as Southeast Asia's most important food basket, and ...rich in biodiversity at the world scale, is also increasingly affected by human activities and exposed to subsidence and coastal erosion. Several dams have been constructed upstream of the delta and many more are now planned. We quantify from high-resolution SPOT 5 satellite images large-scale shoreline erosion and land loss between 2003 and 2012 that now affect over 50% of the once strongly advancing >600 km-long delta shoreline. Erosion, with no identified change in the river's discharge and in wave and wind conditions over this recent period, is consistent with: (1) a reported significant decrease in coastal surface suspended sediment from the Mekong that may be linked to dam retention of its sediment, (2) large-scale commercial sand mining in the river and delta channels, and (3) subsidence due to groundwater extraction. Shoreline erosion is already responsible for displacement of coastal populations. It is an additional hazard to the integrity of this Asian mega delta now considered particularly vulnerable to accelerated subsidence and sea-level rise, and will be exacerbated by future hydropower dams.
Growing public awareness of the environmental impact of coal combustion has raised serious concerns about the various hazardous trace elements produced by coal firing. Arsenic deserves special ...attention due to its toxicity, volatility, bioaccumulation in the environment, and potential carcinogenic properties. As the main anthropogenic source of arsenic is coal combustion, its behavior in power plants is of concern. Unlike mercury, arsenic behavior in coal combustion has not been subjected to systematic, in-depth research. Different researchers have reached opposing conclusions about the behavior of arsenic in combustion systems and, as yet, there is relatively little research on arsenic removal technologies.
In this paper, the volatilization, transformation, and emission behavior of arsenic and its removal technologies are discussed in depth. Factors affecting the volatilization characteristics of arsenic are summarized, including temperature, pressure, mode of occurrence of arsenic, coal rank, mineral matter, and the sulfur and chlorine content of the fuel. The behavior of arsenic during oxy-fuel combustion and the effect of combustion atmosphere (O2, CO2, SO2 and H2O(g)) are also reviewed in detail.
In order to better understand the pathways of arsenic in a power plant environment, a particular focus in this work is the transformation mechanism of ultra-fine ash particles and the partitioning behavior of arsenic. Finally, the effects of air pollution control devices (APCDs) on arsenic emissions are examined, along with the effectiveness of flue gas arsenic removal technologies with different kinds of adsorbents, including calcium-based adsorbents, metal oxides, activated carbon, and fly ash.
In this study, the thermal activation of different types of CaO-based sorbents was examined. Pretreatments were performed at different temperatures (800−1300 °C) and different durations (6−48 h) ...using four Canadian limestones. Sieved fractions of the limestones, powders obtained by grinding, and hydroxides produced following multiple carbonation/calcination cycles achieved in a tube furnace were examined. Pretreated samples were evaluated using two types of thermogravimetric reactors/analyzers. The most important result was that thermal pretreatment could improve sorbent performance. In comparison to the original, pretreated sorbents showed better conversions over a longer series of CO2 cycles. Moreover, in some cases, sorbent activity actually increased with cycle number, and this effect was especially pronounced for powdered samples preheated at 1000 °C. In these experiments, the increase of conversion with cycle number (designated as self-reactivation) after 30 cycles produced samples that were ∼50% carbonated for the four sorbents examined here, and there appeared to be the potential for additional increase. These results were explained with the newly proposed pore−skeleton model. This model suggests, in addition to changes in the porous structure of the sorbent, that changes in the pore−skeleton produced during pretreatment strongly influence subsequent carbonation/calcination cycles.
PDF
