利用绝对反应速率理论研究生物质热解机制

文章采用热重分析仪,分别以10、20、40、50℃/min的升温速率对玉米秸秆和松木屑的不同粒径进行了热解实验。利用绝对反应速率理论,采用一级反应模型,并结合CoatsRedfern积分法及最小二乘拟合法对主反应区反应机理进行深入研究,利用实验数据求取了活化焓ΔH≠、特征点处的活化熵ΔS≠、活化自由能ΔG≠和反应速率常数k,并分析不同升温速率及不同粒径对反应的影响。结果表明：化学反应速率是由活化焓和活化熵共同决定的,利用这些参数可预测其难易程度。升温速率增加,活化焓增大,活化熵增大,活化自由能增加,所需反应能级增大,热解时间缩短;粒径减小,活化焓降低,热解越容易且热解越充分。     

Thermodynamic analysis of an isopropanol–acetone–hydrogen chemical heat pump

An isopropanol–acetone–hydrogen chemical heat pump is investigated in the ASPEN Plus shell, the influences of some important operation parameters on the six different evaluation criteria are presented, and the different evaluation criteria for the heat pump are also analyzed. The decrease of distillation to feed ratio improves the performance of the chemical heat pump, and the increase of endothermic reaction temperature improves the performance of heat pump based on first law of thermodynamics but weakens the performance of heat pump from the viewpoint of second law of thermodynamics. There exists an optimum reflux ratio in terms of enthalpy efficiency, entransy efficiency, and exergy efficiency, but the performance of heat pump deteriorates as the reflux ratio increases in terms of entropy generation number, revised entropy generation number, and ecological COP. The entransy efficiency tends to integrate the behaviors of enthalpy efficiency and exergy efficiency. Compared with entropy generation number, the behavior of revised entropy generation number is more consistent with the practice. Copyright © 2014 John Wiley & Sons, Ltd.     

燃煤电站锅炉受热面灰污监测研究

锅炉受热面积灰和结渣会导致锅炉运行的安全性和经济性下降。定义了新的受热面清洁因子,考虑了锅炉各受热面清洁时与灰污平衡时工质进出口焓升的变化,该清洁因子能够更显著地反映出受热面的灰污程度。应用BP神经网络预测不同工况下锅炉各受热面的工质焓升极大值与极小值,实时计算出锅炉各受热面的灰污状况,可指导和优化锅炉吹灰操作,并以屏式过热器为例进行了验证。     

Research into the origins of engineering thermodynamics

This paper draws attention to a series of misconceptions and misstatements regarding the origin and meaning of some of the most basic concepts of engineering thermodynamics. The six examples exhibited in the paper relate to the concepts of reversibility, entropy, mechanical equivalent of the calorie, the first law of thermodynamics for open systems, enthalpy and the Diesel cycle. A complete list of the pioneering references concludes the paper.     

Estimation of thermodynamic properties of the ternary molten salt system, LiF-NaF-BeF2, by the modified Peng-Robinson equation

The molten salt reactor (MSR), which is one of the generation IV reactors, can meet the demand of transmutation and breeding. The thermodynamic properties of the molten salt system like LiF-NaF-BeF₂ influence the design and construction of the fuel salt and coolant in the MSR for the new generation. In this paper, the equation of state of the ternary system 15%LiF-58%NaF-27%BeF₂, over the temperature range from 873.15 to 1 073.15 K at one atmosphere pressure, is described using a modified Peng-Robinson (PR) equation. The densities of the ternary system and its components are estimated by this equation directly, and compared with the experimental data. Based on the equation of state, the other thermodynamic properties such as the enthalpy, entropy and heat capacity at constant pressure are estimated by the residual function method and the fugacity coefficient method respectively. The densities calculated by PR equation are highly in agreement with the experimental data, and the enthalpy, entropy and heat capacity evaluated by the two different methods are consistent with each other. It can be concluded that the modified PR equation can be applied to evaluate the density of the molten salt system, and it is recommended that it be used as the basis to estimate the enthalpy, entropy and heat capacity of the molten salt system.     

Estimation of heat generation rate in solid oxide fuel cell module from single cell performance and module performance based on impedance analysis

Heat generation rate in SOFC module was estimated under various thermal self-sustained conditions. SOFC module and system was designed to evaluate power generation property and temperature of module. Single cell was also evaluated the performance and electrode overpotential by impedance analysis under the similar condition to module power generation state. We estimated the heat generation rate with enthalpy calculation based on the actual module performance, and also with entropy calculation based on the impedance analysis of single cell. It was found that the heat generation rate calculated by enthalpy is approximately corresponded with that calculated by entropy. There still contains small error between heat generation rate calculated by enthalpy and that calculated by entropy. It was considered that these errors are originated from distribution in stack temperature and reforming gas temperature in the module. According to impedance analysis, it was found that the ohmic resistance is varied under operating condition and related with the current distribution which is calculated with the current path length in the cell. It was suggested that power generation state of module is affected by the current path length in the cell (in another word, distribution of power density) and distribution of overpotential; these phenomena is dominated by gas composition and thermal self-sustainable temperature.     

Nitric oxide emissions during the evolution of gasified char in the reduction layer during a grate-fired process

This work examined the effects of CO₂ gasification in the reduction layer on NO emissions during subsequent char combustion in the oxidation layer during a grate-fired process. In experimental trials, the pyrolysis char of Shenhua bituminous coal was initially gasified in CO₂, employing different conversion rates so as to vary the residence time of the char in the reduction layer. The pore structure and degree of graphitization of the char were monitored and the effects of CO₂ gasification on NO emissions were ascertained. In this manner, the relationship between the transformation of char nitrogen and the physicochemical characteristics of the char was clarified. The effects of CO₂ gasification on the properties of the char and NO emissions in the subsequent oxidation layer were determined. The results show that CO₂ gasification can improve the pore structure and degree of graphitization of the char, such that NO emissions are reduced in an inert atmosphere. However, the conversion rate of char nitrogen will increase in conjunction with oxidative combustion.     

On the thickness dependence of open circuit voltages of p-n junction solar cells

A reduction of the thickness of solar cells with low surface recombination is known to result in enhanced open circuit voltages, provided that the short circuit current can be maintained sufficiently high by light trapping schemes. Lower volume recombination is generally assumed to cause this effect. We offer another interpretation: The voltage increases because thinning the cell at constant short circuit current enhances the minority carrier generation rate per unit volume and hence the steady state carrier concentration. Thermodynamically, an increased carrier concentration is equivalent to a reduction of the entropy production per photon, thus leading to larger voltage.     

Entropy stabilization of deformed regions characterized by an excess volume for hydrogen storage applications

A general framework using the universal equation of state is given to quantify the excess entropy present in deformed regions of metals and metal hydrides characterized by an excess volume. The conditions under which the excess entropy leads to a stabilization of the deformed regions with respect to multiple hydriding/dehydriding cycles are determined. The impact of the energy barrier created by the excess entropy on the recrystallization rate of the deformed regions is quantified using a homogeneous nucleation and crystal growth model. At high temperature, due to the energy barrier, metal hydride systems containing a large excess volume can have a recrystallization rate that is several orders of magnitude smaller than the recrystallization rates of regions containing low excess volume. This entropy stabilization can create a regime where the reduced enthalpy of formation of the metal hydride could be maintained over multiple cycles because of the increased stability of the nanostructures responsible for the reduction in the enthalpy of formation.     

Effect of Al-doping on lithium nickel oxides

The electronic structures of LiNiO₂, LiAlO₂ and LiNi_0.5Al_0.5O₂ are investigated using density-functional theory (DFT) in the local-density approximation (LDA). The effect of lithium intercalation and the influence of aluminium doping on the structure and electrochemical properties of LiNiO₂ are discussed. An increase in the open circuit voltage is observed with Al doping in LiNi_0.5Al_0.5O₂ compound.     

Temperature dependence of the enthalpy of formation of metal hydrides characterized by an excess volume

A universal framework to calculate the temperature dependence of the excess enthalpy present in regions characterized by an excess volume is calculated for metals and metal hydrides. At high temperatures, the different contributions from the pressure–volume, heat capacity, entropy and work associated with the thermal expansion are studied separately and their magnitudes and signs are compared. It is found that the pressure–volume contribution opposes and dominates the other three contributions at both high temperature and excess volume, and it is thus found that this contribution becomes the leading temperature dependent contribution to the enthalpy of a material. The conditions under which a temperature change will reduce the enthalpy of formation of metal hydrides are also given and the Mg/MgH₂ system is studied as an example. Excluding the heat capacity contribution, an increase in temperature tends to offset the effect of the excess volume on the enthalpy of formation. It is also demonstrated that the impact of temperature will be more favorable to a reduction of the enthalpy of formation if a large fraction of the metal hydride is in a state of small excess volume compared to a small fraction of the hydride in a state of high excess volume.     

粒子群优化神经网络在高压断路器机械故障诊断中的应用

提出了一种以振动信号小波包特征熵为特征向量的高压断路器机械故障诊断的智能算法,该算法利用小波包分解原理将高压断路器振动信号分解到不同的频段中,计算各频段的能量熵值,并将其作为神经网络的输入向量,同时利用粒子群算法对神经网络进行优化,以提高故障诊断的精度。试验结果表明：该方法不仅能够取得良好的分类效果,而且诊断速度与精度均高于传统神经网络算法,适用于高压断路器机械故障诊断     

Thermodynamic Property Surfaces for Adsorption of R507A,R134a,and n-Butane on Pitch-Based Carbonaceous Porous Materials

The thermodynamic property surfaces of R507A, R134a, and n-butane on pitch-based carbonaceous porous material (Maxsorb III) are developed from rigorous classical thermodynamics and experimentally measured adsorption isotherm data. These property fields enable us to compute the entropy, enthalpy, internal energy, and heat of adsorption as a function of pressure, temperature, and the amount of adsorbate. The entropy and enthalpy maps are necessary for the analysis of adsorption cooling cycle and gas storage. We have shown here that it is possible to plot an adsorption cooling cycle on the temperature-entropy (T–s) and enthalpy-uptake (h–x) maps.     

Electrochemical investigation of sulfonated poly(ether ether ketone)/clay nanocomposite membranes for moderate temperature fuel cell applications

In the present study, polyelectrolyte membranes based on partially sulfonated poly(ether ether ketone) (sPEEK) with various degrees of sulfonation are prepared. The optimum degree of sulfonation is determined according to the transport properties and hydrolytic stability of the membranes. Subsequently, various amounts of the organically modified montmorillonite (MMT) are introduced into the sPEEK matrices via the solution intercalation technique. The proton conductivity and methanol permeability measurements of the fabricated composite membranes reveal a high proton to methanol selectivity, even at elevated temperatures. Membrane based on sPEEK and 1 wt% of MMT, as the optimum nanoclay composition, exhibits a high selectivity and power density at the concentrated methanol feed. Moreover, it is found that the optimum nanocomposite membrane not only provides higher performance compared to the neat sPEEK and Nafion^®117 membranes, but also exhibits a high open circuit voltage (OCV) at the elevated methanol concentration. Owing to the high proton conductivity, reduced methanol permeability, high power density, convenient processability and low cost, sPEEK/MMT nanocomposite membranes could be considered as the alternative membranes for moderate temperature direct methanol fuel cell applications.     

偏振度对均匀折射率介质内矢量辐射传输过程中辐射熵产的影响

在半透明均匀折射率介质内矢量辐射传输过程中辐射熵传递方程及其数值模拟方法的基础上,研究了偏振度对矢量辐射传输过程中辐射熵产的影响。均匀折射率介质内辐射光束的起偏和改偏通过相距阵实现。计算结果表明:由介质内吸收发射过程的不可逆性产生的光谱辐射熵产数随着偏振度增加而减小,而由介质散射过程的不可逆性产生的光谱辐射熵产数随着偏振度增加而增加;偏振度对介质内的光谱辐射熵强度的影响很大,若不考虑偏振,光谱辐射熵强度的相对误差最大可达到18.04%;在整个系统中,光谱辐射熵产数满足热力学第二定律。     

Calculation for the thermodynamic properties of an alternative refrigerant (R508b) using artificial neural network

This study proposes a alternative approach based on artificial neural networks (ANNs) to determine the thermodynamic properties – specific volume, enthalpy and entropy – of an alternative refrigerant (R508b) for both saturated liquid–vapor region (wet vapor) and superheated vapor region. In the ANN, the back-propagation learning algorithm with two different variants, namely scaled conjugate gradient (SCG) and Levenberg–Marquardt (LM), and Logistic Sigmoid transfer function were used to determine the best approach. The most suitable algorithm and with appropriate number of neurons (i.e. 7) in the hidden layer is found to be the LM algorithm which has provided the minimum error. For wet vapor region, R² values – which are errors known as absolute fraction of variance – are 0.983495, 0.969027, 0.999984, 0.999963, 0.999981, and 0.999975, for specific volume, enthalpy and entropy for training and testing, respectively. Similarly, for superheated vapor, they are: 0.995346, 0.996947, 0.999996, 0.999997, 0.999974, and 0.999975, for training and testing, respectively. According to the regression analysis results, R² values are 0.9312, 0.9708, 0.9428, 0.9343, 0.967 and 0.9546 for specific volume, enthalpy and entropy for wet vapor region and superheated vapor, respectively. The comparisons of the results suggest that, ANN provided results comfortably within the acceptable range. This study, deals with the potential application of the ANNs to represent PVTx (pressure–specific volume–temperature–vapor quality) data. Therefore, reducing the risk of experimental uncertainties and also removing the need for complex analytic equations requiring long computational time and efforts.     

A novel interval-based approach for quantifying practical parameter identifiability of a lithium-ion battery model

Practical identifiability of battery model parameters, on which both modeling accuracy and robustness rely, is considered as a very important prerequisite for advanced onboard monitoring and control of Lithium-ion batteries. In this paper, a novel confidence-interval-based approach is proposed for the quantification and assessment of the practical identifiability of a widely used second order battery equivalent circuit model (ECM). This method utilizes profile likelihood and likelihood ratio subset statistic to calculate each parameter's confidence interval, based on which a normalized index is further derived for facilitating quantification and fast comparison of the identifiability degree among different parameters. Using this approach, the practical identifiability of the second order ECM under lab-collected experimental data is successfully evaluated, and the influences of several real-world factors are systematically examined through extensive simulations. The results show that the open circuit voltage and ohmic internal resistance have a much larger degree of identifiability in all the investigated conditions. Some practically useful insights on performing battery parameter identification are also provided.     

Thermodynamics and crystal structure anomalies in lithium-intercalated graphite

We found anomalous behaviors during lithium electrochemical intercalation into graphite in the entropy curve and the crystal structure, especially, during the stages from 2 to 1 transition at x = 0.5 in Li_xC₆. The entropy first decreases monotonously, then re-increased sharply at the transition. The average interlayer spacing displays a hysteresis during intercalation and de-intercalation and departs from linearity. The results are discussed in relation with a contribution of the vibrational entropy and possible occurrence of intermediary phase(s) between the two lithium-rich intercalation stages.     

Transient processes in different types of solar cell panels. Experimental investigation

The transient processes in six different photovoltaic panels based on single-crystal, polycrystal and amorphous solar cells are analyzed. Two types of transients, arisen because of instantaneous changes in load, are investigated. The transients analyzed are: (a) an instantaneous short-circuiting from an open circuit condition and (b) an instantaneous open-circuiting from a short circuit condition. Different characters of transients are observed. The character of the instantaneous short circuit processes for all panels is damped oscillations. These oscillations are characteristic of a second order circuit, whereas the instantaneous open circuit process takes for all panels the form of an exponential growth that is characteristic of a first order circuit. The similarities and differences of results obtained for the different panels are compared, analyzed and discussed.     

The influence of carbon nanofiber support properties on the oxygen reduction behavior in proton conducting electrolyte-based direct methanol fuel cells

Platinum nanoparticles supported on fishbone carbon nanofibers (CNFs) were synthesized and studied for the oxygen reduction reaction (ORR). The crystalline and textural properties of the CNFs were modified by synthesizing them at different temperatures, allowing the comparison of supports with either improved graphitization degree or improved porosity. A carbon black (Vulcan XC-72R) was used for comparison. Half-cell studies determined that the ORR activity is enhanced when using a CNF with improved graphitization, in contrast with CNFs with better textural properties such as surface area or pore volume. The catalysts were tested at the cathode of a direct methanol fuel cell corroborating the suitability of using highly graphitic CNFs, and a similar behavior was found in comparison with the state of the art carbon black used in this field.