Thermodynamic study of a two-stage vapour absorption refrigeration system using NH3 refrigerant with liquid/solid absorbents

A systematic investigation is made of the two-stage vapour absorption refrigeration system employing the refrigerant absorbent combinations of NH₃---H₂O and NH₃---LiNO₃. The system consists of coupling two conventional absorption cycles so that the first-stage evaporator produces cooling water to circulate in the absorber of the second stage. The effect of operating variables such as generator temperature, evaporator temperature, absorber temperature and condenser temperature on the coefficient of performance (COP), heat transfer rates and relative circulation have been studied for both single-stage and two-stage absorption refrigeration systems. It is found that the COP is higher for NH₃---LiNO₃ than for NH₃---H₂O, in both single-stage and two-stage absorption systems, especially at higher generator temperatures. Furthermore, the minimum evaporator temperature achieved is lower for NH₃---LiNO₃, and the system can be operated at lower generator temperatures.     

A high-efficiency, compound NH3/H2O-H2O/LiBr absorption-refrigeration system

A high-efficiency, compound absorption-refrigeration system is considered, which is composed of two cooperating absorption units using NH₃/H₂O and H₂O/LiBr solutions, respectively. The heat output from the NH₃/H₂O unit is employed to drive the H₂O/LiBr unit. The thermodynamics of the new system are simulated by using a procedure which showed that very high theoretical coefficients of performance may be obtained (up to 230%) compared to the corresponding theoretical values for the usual single absorption units, which do not exceed 100%.     

Comprehensive performance analysis and optimization of 1,3-dimethylimidazolylium dimethylphosphate-water binary mixture for a single effect absorption refrigeration system

The energy and exergy analyses of the absorption refrigeration system (ARS) using H₂O-[mmim][DMP] mixture were investigated for a wide range of temperature. The equilibrium Dühring (P-T-X_IL) and enthalpy (h-T-X_IL) of mixture were assessed using the excess Gibbs free non-random two liquid (NRTL) model for a temperature range of 20°C to 140°C and X_IL from 0.1 to 0.9. The performance validation of the ARS cycle showed a better coefficient of performance (COP) of 0.834 for H₂O-[mmim][DMP] in comparison to NH₃-H₂O, H₂O-LiBr, H₂O-[emim][DMP], and H₂O-[emim][BF4]. Further, ARS performances with various operating temperatures of the absorber (T_a), condenser (T_c), generator (T_g), and evaporator (T_e) were simulated and optimized for a maximum COP and exergetic COP (ECOP). The effects of T_g from 50°C to 150°C and T_a and T_c from 30°C to 50°C on COP and ECOP, the X_a, X_g, and circulation ratio (CR) of the ARS were evaluated and optimized for T_e from 5°C to 15°C. The optimization revealed that T_g needed to achieve a maximum COP which was more than that for a maximum ECOP. Therefore, this investigation provides criteria to select low grade heat source temperature. Most of the series flow of the cases of cooling water from the condenser to the absorber was found to be better than the absorber to the condenser.     

Nitrous oxide formation and destruction in lean, premixed combustion

Concentraion profiles of N₂O, NO, and N₂ from atmospheric pressure, flat-flame burner experiments are presented. Axial profiles of species and temperature are described for CH₄-air and H₂---O₂---Ar flames doped with either NH₃, NO, or N₂O.

Species concentrations were determined by microprobe sampling and direct analysis by gas chromatography or chemiluminescence analysis, for flames ranging in temperature from 1300 to 2000 K. Burner surface temperatures were also estimated for these flames, using heat transfer analysis and an optical method. Nitrogen-atom balances were achieved in each of the H₂---O₂---Ar flames to within experimental error and better than 6%.

Axial profiles of N₂O were similar for all flames. At sampling locations nearest to the burner (0.5–1.0 mm), N₂O concentrations were highest. Concentrations decreased monotonically in the downstream direction, with N₂O destruction essentially complete in the postflame region (height greater than 3 mm) for all flames except the one at the lowest temperature (1300 K). With NH₃ as dopant, early-flame and postflame N₂O concentration varied inversely with temperature. The lowest early-flame N₂O concentration was observed with NO as dopant, and the highest was observed with direct N₂O addition. Increased initial concentrations of NH₃ led to higher early-flame N₂O concentrations. With N₂O as dopant, product branching to NO and N₂ in the postflame region is approximately 8% and 92%, respectively. An NO removal process involving NH_i is active in lean, NO-doped flames.     

上转换材料TiO2∶Er3+/Yb3+/Li+的制备及其在玻璃盖板中的应用

采用溶胶凝胶法和旋转镀膜法制备Er³⁺/Yb³⁺/Li⁺掺杂TiO₂胶体和薄膜,确定上转换材料最优制备方案为n(乙酰丙酮)∶n(C₁₆H₃₆O₄Ti∶H₂O)∶n(异丙醇)∶n(Er(NO₃)₃·5H₂O)∶n(Yb(NO₃)₃·5H₂O)∶n(LiNO₃)=1∶3∶9∶70∶0.12∶0.60∶0.15(物质的量之比),水的滴加速率为10 s/滴,溶液pH值为2~3,溶胶呈透明均匀淡黄色。吸收光谱在近红外区峰值明显。可见光透光率最高可达94.42%,较普通玻璃提高1%~2%。光伏组件通过光电转换效率测量系统进行检测,玻璃盖板镀膜后光伏组件的光电转换效率从16.5%升至17.2%,增加约0.7%。研究结果表明,该薄膜可提高玻璃盖板透光率,扩大光伏组件光谱吸收范围,增加其光电转换效率。     

Emissions of nitrous oxide from combustion sources

Nitrous oxide (N₂0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N₂O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N₂0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO₂, H₂O and NO in combustion gases react in a sampling vessel to produce N₂O, has revealed that, in fact, less than 5 ppm of N₂0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N₂O levels of ca. 50ppm in their off-gases.

The gas-phase reactions of N₂0 in flames are reviewed first. It is clear that in most cases N₂0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N₂0 are thermal decomposition to N₂ and O₂ and also radical attack in e.g. N₂O + H → N₂ + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH₃; the reactions are NCO + NO → N₂0 + CO and NH + NO → N₂0 + H. The levels of N₂O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH₃ at producing N₂0. The removal of N₂O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts.

When coal burns in a fluidized bed, most of the N₂O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH₃ are produced from nitrogenous material released during devolatilization; these two compounds give N₂0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O₂ on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N₂0, in addition to CO, CO₂ and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N₂O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N₂O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N₂O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N₂O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N₂O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N₂O emissions from fluidized bed combustors are identified.     

Simplified models for solar-powered absorption cooling systems

Ideal three-heat-reservoir cycles with constant internal irreversibilities and external heat transfer irrevesibilities are used to obtain the performance limits of solar operated absorption cooling systems. Analytical expressions are obtained for the coefficient of performance (COP) and the cooling capacity of the plant. The results for ideal cycles are compared with those obtained by detailed simulation of the absorption machine. The ideal cycle models are found to give realistic upper limits for the cooling capacity and the COP of solar powered absorption cooling systems.     

Phase change stability of sodium acetate trihydrate and its mixtures

Mixtures of NaCH₃COO·3H₂O with NaBr·2H₂O (10 wt%) or NaHCOO·3H₂O were found very stable on repeated phase change (over 1000 cycles) in a heat cycle test (60 30°C), using vertical glass tubes of 950 mm height. On the other hand, NaCH₃COO·3H₂O deteriorated rapidly under similar experimental conditions (70 40°C), settling anhydrous NaCH₃COO. Pressed mixture of anhydrous NaCH₃COO, Na₂HPO₄ and polyethylene powder was found effective as a nucleating agent. A brief discussion on the heat of fusion of a binary mixture is presented in the appendix.     

Decomposition of nitrous oxide at medium temperatures

Flow reactor experiments were done to study the decomposition of N₂O at atmospheric pressure and in a temperature range of 600–1000°C. Dilute mixtures of N₂O with H₂, CH₄, CO with and without oxygen with N₂ as carrier gas were studied. To see directly the relative importance of the thermal decomposition versus the destruction by free radicals (i.e.: H, O, OH) iodine was added to the reactant mixture suppressing the radicals’ concentrations towards their equilibrium concentrations. The experimental results were discussed using a detailed chemistry model. This work shows that there are still some uncertainties regarding the kinetics of the thermal decomposition and the reaction between N₂O and the O radical. Using the recommendations applied in this work for the reaction N₂O + M ↔ N₂ + O + M and for N₂O + O ↔ products, a good agreement with the experimental data can be obtained over a wide range of experimental conditions. The reaction between N₂O and OH is of minor importance under present conditions as stated in latest literature. The results show that N₂O + H ↔ N₂ + OH is the most important reaction in the destruction of N₂O. In the presence of oxygen it competes with H + O₂ + M ↔ HO₂ + M and H + O₂ ↔ O + OH, respectively. The importance of the thermal decomposition (N₂O + M ↔ N₂ + O + M) increases with residence time. Reducing conditions and a long residence time lead to a high potential in N₂O reduction. Especially mixtures of H₂/N₂O and CO/H₂O/N₂O in nitrogen lead to a chain reaction mechanism causing a strong N₂O reduction.     

Kinetics of hydrogen absorption and desorption by ternary LaNi5-type intermetallic compounds

Kinetic rules of formation and decomposition of the hydrides derived from LaNi₅-type compounds are determined. Comparison between LaNi₅H₆ and more stable hydrides such as LaNi_5-χM_χ (with M = Al, Cu, Ti) is made in terms of pressure, number and time of cycles. Measurements are performed by a differential volumetric device functioning under constant pressure, during hydrogen absorption and desorption. The influence on the kinetics and hydrogen capacities of the gaseous impurities contained in industrial hydrogen (H₂O, CO₂, CH₄) is studied as a function of the number of cycles. It shows a good resistance to poisoning of the above mentioned compounds.     

下吸式固定床的生物质H2O/CO2气化数值模拟研究

利用Aspen Plus 软件建立干桦木屑在下吸式固定床气化炉中的气化模型,模拟值与文献实验值吻合良好。利用Aspen Plus的灵敏度分析模块模拟分别以水蒸气（H₂O）和二氧化碳（CO₂）为气化剂时气化剂/生物质碳比（GC值）对气化结果的影响,并结合H₂O、CO₂各自的特点研究其复合气化。结果表明,H₂O气化时可获得富氢煤气,但其净CO₂排放量较高;CO₂气化时碳转化率及冷煤气效率较低,但净CO₂排放量较低;H₂O、CO₂复合气化使碳转化率及冷煤气效率略有降低,但可有效减少气化系统中的净CO₂排放量。     

Ignition of CO/H2/N2 versus heated air in counterflow: experimental and modeling results

Nonpremixed ignition in counterflowing CO/H₂ vs. heated air jets is experimentally and computationally investigated. The experiments confirm the numerical modeling observation of the existence of three ignition regimes as a function of the hydrogen concentration. In all three regimes, we first detect experimentally the onset of chemiluminescent glow due to excited CO₂ followed by flame ignition, as the temperature of the air jet is raised gradually. The temperature extent of the glow regime, however, is progressively reduced with increasing hydrogen addition; no glow is detected for H₂ concentrations in excess of 73%. The temperatures for glow onset and flame ignition are represented by the boundary air temperatures for each threshold. The variation of these temperatures with system pressure and flow strain rate is explored, for pressures between 0.16 and 5 atm, and strain rates of 100 to 600 s⁻¹. The pressure variation is found to result in three p-T ignition limits, similar to the ignition limits observed in the H₂/O₂ system. This similarity is also observed on the effects of aerodynamic transport on ignition: within the second limit the ignition temperatures are found to be essentially insensitive to flow strain rate, whereas the other two limits are significantly affected by strain. The transport insensitivity is maintained even in the limit of very low H₂ concentrations, where an analogous H₂/N₂ mixture would fail to ignite. This behavior is explained computationally by the replacement of the shift reaction OH + H₂ → H₂O + H with the reaction CO + OH → CO₂ + H, thereby minimizing the effect of diminishing H₂ concentration. The experimental data are found to agree well with the calculated results, although discrepancies are noted in modeling the onset of chemiluminescence and its response to pressure variations.     

Homogeneous formation of NO and N2O from the oxidation of HCN and NH3 at 600–1000°C

The oxidation of HCN and NH₃ with CO, CH₄, or H₂ addition has been studied in the temperature range between 600 to 1000°C. In most of the tests 10% oxygen was used. The experiments were carried out under well-defined conditions in a flow tube reactor made of quartz glass. The effects of NO addition and oxygen level have been tested. To study the importance of O/H radicals in the reaction mechanism and to confirm previous studies, iodine was added in some tests. A detailed chemical kinetic model was used to analyze the experimental data. In general, the model and experimental results are in good agreement. The results show that under the conditions tested CO significantly promotes NO and N₂O formation during HCN oxidation. During NH₃ oxidation carbon-containing gaseous species such as CO and CH₄ are important to promote homogeneous NO formation. In the system with CH₄ addition, the conversion of HCN to N₂O is lower compared to the other systems. In the HCN/NO/CO/O₂ system NO reduction starts at 700°C and the maximum reduction of approx. 40% is obtained at 800°C. For the NH₃/NO/CO/O₂ system the reduction starts at 750°C and the maximum reduction is 50% at 800°C. Iodine addition shifts the oxidation of HCN, NO, and N₂O formation as well as NO reduction to higher temperatures. Under the conditions tested, it was found that iodine mainly enhances the recombination of the O-radicals. No effect on NO formation was found in the HCN/CH₄/O₂ system when oxygen was increased from 6% to 10%, but when oxygen was increased from 2% to 6% NO formation decreased. The role of hydrocarbon radicals in the destruction of NO is likely to become important at low oxygen concentrations (2%) and at high temperatures (1000°C).     

Probing the reaction pathway of dehydrogenation of the LiNH2 + LiH mixture using in situ H NMR spectroscopy

Using variable temperature in situ ¹H NMR spectroscopy on a mixture of LiNH₂ + LiH that was mechanically activated using high-energy ball milling, the dehydrogenation of the LiNH₂ + LiH to Li₂NH + H₂ was investigated. The analysis indicates NH₃ release at a temperature as low as 30 °C and rapid reaction between NH₃ and LiH at 150 °C. The transition from NH₃ release to H₂ appearance accompanied by disappearance of NH₃ confirms unambiguously the two-step elementary reaction pathway proposed by other workers.     

生物质气化工艺废水特征及预处理实验研究

对生物质气化中试现场产生的废水进行了水质及水量特征分析,针对生物质气化工艺废水固体颗粒含量高、有机物浓度高、难生化降解、废水增量少的特点,采取减压蒸馏及芬顿氧化对生物质气化废水进行预处理。实验结果表明,在85 ~ 90℃、真空度 -0.07 ~ -0.095 MPa减压蒸馏条件下,废水COD、NH₄-N脱除率分别为74.38%、94.46%;在Fe²⁺-H₂O₂体系中,考察了H₂O₂与废水质量比、H₂O₂与Fe²⁺摩尔比、反应时间、H₂O₂浓度对COD、NH₄-N、TOC、TN等的影响,当H₂O₂与废水质量比为8.40%时,可将减压蒸馏蒸出液COD从2.05 × 10⁴ mg/L降至4.11 × 10³ mg/L,NH₄⁺-N从143 mg/L降至11.1 mg/L。     

High temperature rate coefficient for the reaction of O(P) with H2 obtained by the resonance absorption of O and H atoms

The reaction of O(³P) with H₂ has been studied behind reflected shock waves in the temperature range of 1713–3532K at total pressures of about 1.4–2.0 bar by Atomic Resonance Absorption Spectroscopy using mixtures of N₂O and H₂ highly diluted in Ar. The O atoms were generated by the fast thermal decomposition of N₂O and the reaction with H₂ was followed by monitoring the time dependent O and H atom concentrations in the postshock reaction zone. For the experimental conditions chosen, the measured O and H atom concentrations were primarily sensitive to the well-known N₂O dissociation and to the studied reaction and hence its rate coefficient could be deduced. The measured rate coefficient data are fitted by the least-squares method to obtain the following three parameter expression: K₄=3.72×10⁶(T/K)^2.17exp(−4080K/T)cm³ mol⁻¹8⁻, which is in excellent agreement with the recent ab initio calculations for the rate coefficient of this reaction in the overlapping temperature range. The present result is also compared to the experimental results reported by earlier investigators.     

溴化锂吸收式制冷技术研究进展

回顾了近十年来有关溴化锂吸收式制冷技术的发展及主要研究成果。H₂O/LiBr作为一种广泛应用的吸收式制冷工质对,具有优良的热力学性能与环保特性,但存在结晶、腐蚀和循环性能低等问题。文章简述了醇类、盐混合物、离子液体及纳米颗粒等添加剂对H₂O/LiBr溶液传热传质、防结晶及防腐蚀等性能的提升;介绍了关键部件吸收器和发生器的理论及实验研究现状;回顾了吸收式制冷系统循环优化的研究成果。通过归纳分析,总结溴化锂吸收式制冷技术存在的一些问题及未来发展趋势,为后续的研究提供参考。     

A comprehensive study of hydrogen production from ammonia borane via PdCoAg/AC nanoparticles and anodic current in alkaline medium: experimental design with response surface methodology

In this paper, the optimization of hydrogen (H₂) production by ammonia borane (NH₃BH₃) over PdCoAg/AC was investigated using the response surface methodology. Besides, the electro-oxidation of NH₃BH₃ was determined and optimized using the same method to measure its potential use in the direct ammonium boran fuel cells. Moreover, the ternary alloyed catalyst was synthesized using the chemical reduction method. The synergistic effect between Pd, Co and Ag plays an important role in enhancement of NH₃BH₃ hydrolysis. In addition, the support effect could also efficiently improve the catalytic performance. Furthermore, the effects of NH₃BH₃ concentration (0.1–50 mmol/5 mL), catalyst amount (1–30 mg) and temperature (20°C–50°C) on the rate of H₂ production and the effects of temperature (20°C–50°C), NH₃BH₃ concentration (0.05–1 mol/L) and catalyst amount (0.5–5 µL) on the electro-oxidation reaction of NH₃BH₃ were investigated using the central composite design experimental design. The implementation of the response surface methodology resulted in the formulation of four models out of which the quadratic model was adjudged to efficiently appropriate the experimental data. A further statistical analysis of the quadratic model demonstrated the significance of the model with a p-value far less than 0.05 for each model and coefficient of determination (R²) of 0.85 and 0.95 for H₂ production rate and NH₃BH₃ electrroxidation peak current, respectively.     

Theoretical study of a solar powered absorption/MED combined system

In this article, a theoretical study is presented for a solar powered combined system comprising a LiBr---H₂O absorption cooling machine and a multiple-effect distillator (MED). The MED has 8 VTE of the falling film type, and it replaces the condenser in conventional absorption machines. Steam released at the generator pressure is supplied to the effect which matches its conditions, and the condensate follows its usual route towards the evaporator of an A/C unit. Thus, the MED is powered by the waste heat of the absorption machine which improves the overall gain and the thermodynamic characteristics significantly.

Governing equations for the combined system are given and are numerically solved. Medium parabolic concentrators are used to power the system, and a transient simulation for the combined arrangement is presented.

Results are given for a typical design summer day in Jeddah, Saudi Arabia, for a range of firing temperatures 150–190°C with a storage temperature amplitude of 10–20°C over a daily working period of 12 h. For a given cooling load of 100 ton refrigeration, the system can produce up to 40m³ of fresh water at a specific collector area of 12.41. H₂O plus 0.03 TR/m². The overall COP_o reaches 1.44, which is more than twice that of a conventional absorption machine at the same temperature levels.     

Nongray radiation from gas and soot mixtures in planar plates based on statistical narrow-band spectral model

The nongray behavior of combustion products plays an important role in various areas of engineering. Based on the statistical narrow-band (SNB) spectral model with an exponential-tailed inverse intensity distribution and the ray-tracing method, a comprehensive investigation of the influence of soot on nongray radiation from mixtures containing H₂O/N₂+soot, CO₂/N₂+soot, or H₂O/CO₂/N₂+soot was conducted in this paper. In combustion applications, radiation transfer is significantly enhanced by soot due to its spectrally continuous emission. The effect of soot volume fraction up to 1×10^-6 on the source term, the narrow-band radiation intensities along a line-of-sight, and the net wall heat fluxes were investigated for a wide range of temperature. The effect of soot was significant and became increasingly drastic with the increase of soot loading.