Efficiency enhancement in solar air heaters by modification of absorber plate-a review

This paper outlines a complete review on modifications made on the absorber plate of solar air heaters in order to improve the turbulence and heat transfer rate, thereby efficiency. Corrugated sheets, fins, extended surfaces, wire mesh, porous medium, etc., are a few of the modifications used. Most of such alterations in the absorber plate resulted with an increase in efficiency but associated with drawback of increased pumping power due to raising friction factor. Pumping power is considered here as a predominant comparison parameters of various solar air heaters with different absorber plate in terms of effective efficiency.     

Influence of flue gas cleaning system on the energetic efficiency and on the economic performance of a WTE plant

Gas cleaning systems of MSW (Municipal Solid Waste) incinerators are characterised by the process employed to remove acid gases. The commonly used technologies for acid gas removal are: (1) dry treatment with Ca(OH)₂ or (2) with NaHCO₃, (3) semi-dry process with Ca(OH)₂ and (4) wet scrubbing. In some recent plants beside a wet cleaning system, a dry neutralization with Ca(OH)₂ is used. The goal is to reduce the amount of acid to be removed in the wet treatment and the liquid effluents produced. The influence of these different technologies on the electrical efficiency was investigated by a detailed simulation of a WTE (Waste To Energy) plant with a capacity of about 100,000 t/y of MSW. The effects of the different gas cleaning systems on electrical efficiency were significant. The difference of efficiency between the most advantageous technology, which is dry treatment with NaHCO₃, and the least advantageous technology which is semi-dry treatment, is about 0.8%. A simple economic analysis showed that the few advantages of dry technologies can often be lost if the costs of chemicals and the disposal of products are considered.     

Modeling microbiological and chemical processes in municipal solid waste bioreactor,part I: Development of a three-phase numerical model BIOKEMOD-3P

The numerical computer models that simulate municipal solid waste (MSW) bioreactor landfills have mainly two components – a biodegradation process module and a multi-phase flow module. The biodegradation model describes the chemical and microbiological processes. The models available to date include predefined solid waste biodegradation reactions and participating species. Some of these models allow changing the basic composition of solid waste. In a bioreactor landfill several processes like anaerobic and aerobic solids biodegradation, nitrogen and sulfate related processes, precipitation and dissolution of metals, and adsorption and gasification of various anthropogenic organic compounds occur simultaneously. These processes may involve reactions of several species and the available biochemical models for solid waste biodegradation do not provide users with the flexibility to simulate these processes by choice. This paper presents the development of a generalized biochemical process model BIOKEMOD-3P which can accommodate a large number of species and process reactions. This model is able to simulate bioreactor landfill operation in a completely mixed condition, when coupled with a multi-phase model it will be able to simulate a full-scale bioreactor landfill. This generalized biochemical model can simulate laboratory and pilot-scale operations in order to determine biochemical parameters important for simulation of full-scale operations.     

Non-destructive quantification of water gradient in sludge composting with Magnetic Resonance Imaging

Sludge from a slaughter-house wastewater plant, and mixtures of bulking agent (crushed wood pallet) and sludge were studied by Nuclear Magnetic Resonance (NMR). The NMR spin–spin relaxation (T₂) and spin–lattice relaxation (T₁) signals for sludge, wet crushed wood pallet and mixtures of sludge and bulking agent were decomposed into three relaxation time components. Each relaxation time component was explained by a non-homogeneous water distribution on a microscopic length scale and by the porosity of the material. For all samples, the T₂ relaxation time value of each component was directly related to the dry matter content. The addition of wet crushed wood to sludge induced a decrease in the relaxation time, explained by water transfer between the sludge and the wood.Magnetic Resonance Imaging (MRI) and respirometric measurements were performed on sludge and wood mixtures. MR images of the mixtures were successfully obtained at different biodegradation states. Based on specific NMR measurements in an identified area located in the MRI cells, the results showed that grey levels of MR images reflected dry matter content. This preliminary study showed that MRI would be a powerful tool to measure water distribution in sludge and bulking agent mixtures and highlights the potential of this technique to increase the understanding of sludge composting.     

Integrated assessment of a new Waste-to-Energy facility in Central Greece in the context of regional perspectives

The main aim of this study is the integrated assessment of a proposed Waste-to-Energy facility that could contribute in the Municipal Solid Waste Management system of the Region of Central Greece. In the context of this paper alternative transfer schemes for supplying the candidate facility were assessed considering local conditions and economical criteria. A mixed-integer linear programming model was applied for the determination of optimum locations of Transfer Stations for an efficient supplying chain between the waste producers and the Waste-to-Energy facility. Moreover different Regional Waste Management Scenarios were assessed against multiple criteria, via the Multi Criteria Decision Making method ELECTRE III. The chosen criteria were total cost, Biodegradable Municipal Waste diversion from landfill, energy recovery and Greenhouse Gas emissions and the analysis demonstrated that a Waste Management Scenario based on a Waste-to-Energy plant with an adjacent landfill for disposal of the residues would be the best performing option for the Region, depending however on the priorities of the decision makers. In addition the study demonstrated that efficient planning is necessary and the case of three sanitary landfills operating in parallel with the WtE plant in the study area should be avoided. Moreover alternative cases of energy recovery of the candidate Waste-to-Energy facility were evaluated against the requirements of the new European Commission Directive on waste in order for the facility to be recognized as recovery operation. The latter issue is of high significance and the decision makers in European Union countries should take it into account from now on, in order to plan and implement facilities that recover energy efficiently. Finally a sensitivity check was performed in order to evaluate the effects of increased recycling rate, on the calorific value of treated Municipal Solid Waste and the gate fee of the candidate plant and found that increased recycling efforts would not diminish the potential for incineration with energy recovery from waste and neither would have adverse impacts on the gate fee of the Waste-to-Energy plant. In general, the study highlighted the need for efficient planning in solid waste management, by taking into account multiple criteria and parameters and utilizing relevant tools and methodologies into this context.     

Briquetting of charcoal from sugar-cane bagasse fly ash (scbfa) as an alternative fuel

Brazil is the largest worldwide producer of alcohol and sugar from sugar-cane and has an extensive alternative program for car fuel which is unique. The objective of this work is to offer one management option of a solid residue produced by this industrial segment. The pressed sugar-cane bagasse is burned to produce steam and electricity by cogeneration. The combustion yields both bottom and fly ashes which contain high amounts of silicon oxide as a major component. Fly ash which contains a high volume (>30% by weight) of charcoal was used in this work. The ash was sieved to separate the thick charcoal from inorganic materials which are concentrated in the thinner fraction. The briquettes were hand pressed using charcoal mixed with a binder (starch) obtained from cassava flour (a tropical root). The results (density, mechanical resistance) obtained with 8% by weight of starch binder are presented here. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to characterize the ashes and the briquettes. The results show that sugar-cane bagasse fly ash (SCBFA) can be used to produce briquettes with an average density of 1.12 g cm^?3 and an average calorific value of 25,551 kJ/kg.     

Accelerated carbonation of different size fractions of bottom ash from RDF incineration

This paper investigates the effects of accelerated carbonation on the characteristics of bottom ash from refuse derived fuel (RDF) incineration, in terms of CO₂ uptake, heavy metal leaching and mineralogy of different particle size fractions. Accelerated aqueous carbonation batch experiments were performed to assess the influence of operating parameters (temperature, CO₂ pressure and L/S ratio) on reaction kinetics. Pressure was found to be the most relevant parameter affecting the carbonation yield. This was also found to be largely dependent on the specific BA fraction treated, with CO₂ uptakes ranging from ～4% for the coarse fractions to ～14% for the finest one. Carbonation affected both the mineralogical characteristics of bottom ash, with the appearance of neo-formation minerals, and the leaching behaviour of the material, which was found to be mainly related to the change upon carbonation in the natural pH of the ash.     

Molten carbonate fuel cells fed with biogas: Combating H2S

The use of biomass and waste to produce alternative fuels, due to environmental and energy security reasons, is a high-quality solution especially when integrated with high efficiency fuel cell applications. In this article we look into the coupling of an anaerobic digestion process of organic residues to electrochemical conversion to electricity and heat through a molten carbonate fuel cell (MCFC). In particular the pathway of the exceedingly harmful compound hydrogen sulphide (H₂S) in these phases is analysed. Hydrogen sulphide production in the biogas is strongly interrelated with methane and/or hydrogen yield, as well as with operating conditions like temperature and pH. When present in the produced biogas, this compound has multiple negative effects on the performance and durability of an MCFC. Therefore, there are important issues of integration to be solved.Three general approaches to solve the sulphur problem in the MCFC are possible. The first is to prevent the formation of hydrogen sulphide at the source: favouring conditions that inhibit its production during fermentation. Secondly, to identify the sulphur tolerance levels of the fuel cell components currently in use and develop sulphur-tolerant components that show long-term electrochemical performance and corrosion stability. The third approach is to remove the generated sulphur species to very low levels before the gas enters the fuel cell.     

A lysimeter experimental study and numerical characterisation of the leaching of incinerator bottom ash waste

Incinerator bottom ash (IBA) is a residual produced from incinerating municipal solid waste. In the past, IBA presented a big waste disposal problem; however, various recycling approaches have been adopted in recent years to mitigate this problem, as well as to provide a useful alternative to using primary aggregate resources. The use of IBA as an alternative to conventional aggregates in different civil engineering construction applications helps to conserve premium grade aggregate supplies; however, when IBA is in contact with water in the field, as a consequence of precipitation events or changes in water table, elements, such as salts and heavy metals, may be released to the soil and ground water. In this work, IBA waste was mixed with limestone aggregate to produce a blend with acceptable mechanical properties and minimum environmental risks for use as road foundation. The study focused on evaluating potential environmental impacts of some constituents, including sulphate, chloride, sodium, copper, zinc and lead in IBA blends using a lysimeter as a large scale leaching tool. Moreover, a specific scenario simulating field conditions was adopted in the lysimeter to assess the potential impact of changing conditions, such as IBA content in the blend, liquid to solid ratio (L/S) and pH value, on long-term release of heavy metals and salts. Then, numerical modelling was used to predict the release of the aforementioned constituents from IBA based on initial measurement of intrinsic material properties and the kinetic desorption process concept. Experimental results showed that zinc and lead were released in very low concentrations but sodium and sulphate were in high concentrations. The control limestone only blend also demonstrated low release concentrations of constituents in comparison to IBA blends, where constituent concentrations increased with increase in IBA content. Experimental results were compared with numerical results obtained using a non-equilibrium desorption model. Good agreement was found between the two sets of data.     

Understanding the chemical and mineralogical properties of the inorganic portion of MSWI bottom ash

This paper investigates the changes of mineralogical composition of bottom ash in the environment. The chemical and mineralogical bulk composition was determined by X-ray fluorescence (XRF) and X-ray powder diffraction (XRPD) Rietveld method. Single bottom ash particles were investigated by optical microscopy, scanning electron microscopy with quantitative energy-dispersive X-ray microanalysis (SEM/EDX) and electron probe micro analysis (EPMA). SEM/EDX and EPMA are valuable complement to bulk analysis and provide means for rapid and sensitive multi-elemental analysis of ash particles. The fresh bottom ash consists of amorphous (>30 wt.%) and major crystalline phases (>1 wt.%) such as silicates, oxides and carbonates. The mineral assemblage of the fresh bottom ash is clearly unstable and an aging process occurs by reaction towards an equilibrium mineral phase composition in the environmental conditions. The significant decrease of anhydrite and amorphous contents was observed in the aged bottom ash, leading to the formation of ettringite, hydrocalumite and rosenhahnite under atmospheric conditions. In the water-treated sample, the calcite contents increased significantly, but ettringite was altered by the dissolution and precipitation processes in part, to produce gypsum, while the remaining part reacted with chloride to form hydrocalumite. Gypsum and other Ca based minerals may take up substantial amounts of heavy metals and subsequently control leaching behaviour of bottom ash.