• Energy Research
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Abstract A new formulation for the evaporation, flashing, condensation processes taking place in the effects of thermal desalination systems which simulates the operation of both forward and parallel/cross configurations is coupled with an exergo-economic model based on the SPECO method. The thermo-economic model uses accurate properties for the seawater, brine, pure water and vapour and is solved with an equation solver which does not require the development of a specific solution algorithm as in most previous studies. This flexible model is used to analyze the influence of the number of effects N and the temperature difference ΔT e between effects on the technical and economic performance of multi-effect desalination systems with ejector vapour compression. In particular, it is shown that the performance calculated by an earlier black-box approach is not attainable by technically and economically realistic systems. It is also shown that for each feed configuration and a given number of effects there exists an optimum value of ΔT e which minimizes the cost of the produced potable water. This last result forms the basis of a procedure that combines black-box results with the optimum value of ΔT e and can be used to select the appropriate system for any specific application.

Abstract Food waste (FW), primary sludge (PS) and waste activated sludge (WAS) were characterized and found to be complementary in the concentrations of carbohydrates, total Kjeldahl nitrogen (TKN), PO4–P and some metal for biological hydrogen production. Moreover, FW was found to have low pH buffering capacity while the values for PS and WAS were relatively higher. An anaerobic toxicity analysis (ATA) derived from a methanogenic ATA protocol showed that these waste materials had no toxicity to hydrogen production. Adding phosphate buffer to the FW significantly improved hydrogen production while initial pH was 7.0. Co-digestion of FW and sewage sludge was studied using a batch respirometric cultivation system. All combinations of the feedstocks (FW+PS, FW+WAS and FW+PS+WAS) showed enhanced hydrogen production potential as compared with the individual wastes. A mixing ratio of 1:1 was found to be the best among the ratios tested for all three co-digestion groups. A hydrogen yield of 112 mL/g volatile solid (VS) added was obtained from a combination of FW, PS and WAS. This yield was equivalent to 250 mL/g VS added if only FW contributed to hydrogen production. The reason for the enhancement of hydrogen production was postulated to be multifold in which the increase in buffer capacity in the co-digestion mixture was verified.

Abstract A dynamic model of vapour compression refrigeration system is developed. The overall model consists of the following basic components: a compressor, a condenser, an expansion valve, an evaporator, an evaporative cooler and a cool storage. The integrated system is referred to as chilled water cooling system with storage (CWCS). The mathematical modelling of the CWC system undertaken in this study predicts the change in state of refrigerant in the system with respect to time. A computer program is developed to solve the dynamic equations along with empirical correlations describing refrigerant properties. Open-loop tests are carried out to study the performance characteristics of the system under varied cooling load and compressor speed. The model is intended to serve as an analytical design tool and to provide a basis for control analysis. Based on a heuristic method, ‘sub-optimal on–off control’ strategies for the chilled water cool storage system are developed using a reduced order model. The methodology of generating such control profiles is illustrated and the tests for optimality show that the control profiles are near optimal. The on–off control scheme is simulated on the full order CWC system. The operating performance of the system is described under several simulated cases. The results show that the control scheme is capable of maintaining the chilled water temperature in the chosen range.

The objective of the present investigation has been to combine tracer principles and a hydrolytic microbial activity assay using fluorescein diacetate to monitor changes in microbial biomass within subsurface flow wetland mesocosms. The mesocosm hydrolytic activity was referenced to activated sludge concentrations treating a typical domestic wastewater at full scale. Microbial biomass activity levels within four laboratory wetland mesocosms treating a synthetic domestic wastewater were routinely monitored over a 21-week period of plant growth and rhizosphere development. Although above ground plant mass and tracer dispersion numbers suggested progressive root zone development, plant growth did not result in any measurable enhancement in microbial activity when compared to a mesocosm operating without plants. Dispersion numbers also suggested a reduction in the mass transport kinetics in these planted mesocosms. In-situ biomass monitoring enabled the assessment of a characteristic response in terms of the steady-state food to microorganism (F/M) ratio that was observed in mesocosms receiving both low and high organic loading. Wetland treatment performance is sensitive to the degree to which bed volume is exploited in terms of wastewater flow to regions of bioactivity. The in-situ reactive tracer technique for mesocosm biomass monitoring provided an assessment of the collective substratum and rhizosphere microbial biomass in direct contact with wastewater contaminants. Thus, in-situ biomass monitoring has application in further understanding of plant function and strategies for plant implementation in wetland research and development.

In this study, an agroforestry ecosystem project (AEP) is developed for confronting the conflict between agricultural development and forest protection. A fuzzy stochastic programming with Laplace scenario analysis (FSL) is proposed for planning water resources in an AEP issue under uncertainties. FSL can not only deal with spatial and temporal variations of hydrologic elements and meteorological conditions; but also handle uncertainties that are expressed in terms of probability, possibility distributions and fuzzy sets; meanwhile, policy scenario analysis with Laplace’s criterion (PSL) is introduced to handle probability of each scenario occurrence under the supposition of no data available. The developed FSL can be applied to an AEP issue in Xixian county, located in north of China. The result of ecological effects, water allocation patterns, pollution mitigation schemes and system benefits under various scenarios are obtained, which can support policymakers adjusting current strategy to improve regional ecological function with cost-effective and sustainable manners. Meanwhile, it can support generating a robust water plan for regional sustainability in an AEP issue under uncertainties.

Biogas is one of the most attractive renewable resources due to its ability to convert waste into energy. Biogas is produced during an anaerobic digestion process from different organic waste resources with a combination of mainly CH4 (~50 mol/mol), CO2 (~15 mol/mol), and some trace gasses. The percentage of these trace gases is related to operating conditions and feedstocks. Due to the impurities of the trace gases, raw biogas has to be cleaned before use for many applications. Therefore, the cleaning, upgrading, and utilization of biogas has become an important topic that has been widely studied in recent years. In this review, raw biogas components are investigated in relation to feedstock resources. Then, using recent developments, it describes the cleaning methods that have been used to eliminate unwanted components in biogas. Additionally, the upgrading processes are systematically reviewed according to their technology, recovery range, and state of the art methods in this area, regarding obtaining biomethane from biogas. Furthermore, these upgrading methods have been comprehensively reviewed and compared with each other in terms of electricity consumption and methane losses. This comparison revealed that amine scrubbing is one the most promising methods in terms of methane losses and the energy demand of the system. In the section on biogas utilization, raw biogas and biomethane have been assessed with recently available data from the literature according to their usage areas and methods. It seems that biogas can be used as a biofuel to produce energy via CHP and fuel cells with high efficiency. Moreover, it is able to be utilized in an internal combustion engine which reduces exhaust emissions by using biofuels. Lastly, chemical production such as biomethanol, bioethanol, and higher alcohols are in the development stage for utilization of biogas and are discussed in depth. This review reveals that most biogas utilization approaches are in their early stages. The gaps that require further investigations in the field have been identified and highlighted for future research.

Abstract The catalytic activity of char (fixed carbon + metal oxides) recovered from a commercial waste gasification plant and char-derived ash (metal oxides only) from the oxidation of the char (i.e. the controlled oxidation of the amorphous carbon in the char) was investigated for the cracking and steam reforming of tar. The latter was obtained following the soxhet extraction of sludge collected during a cleaning process (water scrubbing) on primary syngas. The tar was evaporated along with water in an evaporator maintained at 650 °C and entrained by a nitrogen flow. Tar in the gas phase was sent to a reformer where cracking + reforming tests were carried-out at 900 °C and 1 bar using char and char-derived ash pellets as catalysts. These catalysts were characterized using TGA, AE, XRD, SEM, EDX, XRF, ICP-MS, BET and Matersizer. Char-derived ash and char both hold similar metals and mineral structures. Following a 2 h conditioning at 900 °C, the fixed carbon in the char exhibited a larger surface area and pore volume while the char-derived ash revealed larger particle from the merging of vicinal minerals. No metal evaporation was observed after 24 h at 900 °C. During cracking and reforming, the initial 2–6 rings tar mixture extracted from the gasification residues sludge, loaded in the stream of nitrogen + steam at 65 g/Nm3, was reduced to 173.3 and 90.2 mg/Nm3, when using char-derived ash and char respectively. Pure syngas (CO + H2) was produced as the only permanent gases while carbon deposition was noticed in the solid beds. Even though the extent and type of tar were essentially similar when char and char-derived ash were used, the kinetics to reach steady state were slower for the char-derived ash. Steam content did not seem to impact on the tar conversion. However WG and WGS reactions most probably occurred, which might contribute to modulate the H2/CO molar ratio in the produced gas according to its targeted downstream utilization.

Abstract Ultra heavy oil and bitumen contain high molecular weight compounds that resulted in more viscous fluids than most conventional crude oils. For the production and transportation of such heavy fluids, it is necessary to reduce their viscosity. The dilution of bitumen with liquid solvent is one of the practical methods to reduce the bitumen viscosity to a desired level. In this manuscript, the physical properties (density and dynamic viscosity) of a bitumen sample from Athabasca field and its diluted mixtures with n -decane have accurately been measured. The measurements have been taken under conditions applicable for both in situ recovery methods and pipeline transportation. The experiments have been conducted in the temperatures varying from ambient temperature up to 344 K and at pressures up to 10 MPa and on mixtures with different weight fractions of n -decane (0.05, 0.1, 0.2, 0.3, 0.4, and 0.5). The generated experimental density and dynamic viscosity data for raw bitumen and its mixtures with n -decane were evaluated with predictive schemes as well as with correlation models. The influence of pressure, temperature, and solvent weight fraction on the density and dynamic viscosity of the mixtures was considered in the models and evaluated from the experimental results. The results indicated that power law model could correlate the dynamic viscosity of Athabasca bitumen and n -decane mixtures well over the studied conditions. The density data were also well predicted with an equation that assumes no volume change upon mixing occurs.