• Energy Research

Abstract The purpose of this study is to investigate the output power and efficiency of a TEG system using waste heat from heat pipes, and then optimize its performance. The TEG material is Bi0.4Sb1.6Te3 and its figure-of-merit (ZT) is 1.18 at room temperature. The predictions indicate that a longer length of the elements has greater power output and efficiency based on a fixed heat flux on the hot side surface, whereas a shorter length has greater output power based on a fixed temperature difference. The geometry of the TEG is designed through a multi-objective genetic algorithm (MOGA) to maximize its efficiency. When the temperature difference is fixed at 40 °C, the output power and efficiency of the TEG with optimization is increased by about 51.9% and 5.4%, compared to the TEG without optimization. Once the impedance matching, namely, the internal resistance is equal to the external load resistance, is used, the output power can be further enhanced by about 3.85–4.40%. When the heat flux is fixed at 20,000 Wm−2 along with the temperature difference of 40 °C, the output power and efficiency of a pair of elements can be increased to 7.99 mW and 9.52%, respectively. These results are much higher than those reported in other studies. Accordingly, it is concluded that the MOGA is a powerful tool to design the geometry of a TEG for maximizing its performance and real applications in industry.

This study aims to investigate the thermo-kinetics of high-ash sewage sludge using thermogravimetric analysis. Sewage sludge was dried, pulverized and heated non-isothermally from 25 to 800 °C at different heating rates (5, 10 and 20 °C/min) in N2 atmosphere. TG and DTG results indicate that the sewage sludge pyrolysis may be divided into three stages. Coats-Redfern integral method was applied in the 2nd and 3rd stage to estimate the activation energy and pre-exponential factor from mass loss data using five major reaction mechanisms. The low-temperature stable components (LTSC) of the sewage sludge degraded in the temperature regime of 250–450 °C while high-temperature stable components (HTSC) decomposed in the temperature range of 450–700 °C. According to the results, first-order reaction model (F1) showed higher Ea with better R2 for all heating rates. D3, N1, and S1 produced higher Ea at higher heating rates for LTSC pyrolysis and lower Ea with the increase of heating rates for HTSC pyrolysis. All models showed positive ΔH except F1.5. Among all models, Diffusion (D1, D2, D3) and phase interfacial models (S1, S2) showed higher ΔG as compared to reaction, nucleation, and power-law models in section I and section II.

Abstract Pretreatment of lignocellulosic biomass is of the utmost importance for the development of bioethanol because of the abundance and low cost of lignocelluloses. To figure out the hydrolysis characteristics of sugarcane bagasse in a microwave irradiation environment, the biomass is pretreated by a dilute sulfuric acid solution at 180 °C for 30 min, with the concentration ranging from 0 to 0.02 M. A variety of analyses, including fiber analysis, TGA, XRD, FTIR and HPLC, are employed to aid in understanding the physical and chemical characteristics of residual solid particles and solutions. A higher concentration is conducive to destroying bagasse; however, the buffering capacity possessed by the biomass is also observed in the pretreatment. The experimental results indicate that around 40–44 wt% of bagasse is degraded from the pretreatment in which around 80–98% of hemicellulose is hydrolyzed. In contrast, crystalline cellulose and lignin are hardly affected by the pretreatment. The maximum yields of xylose and glucose as well as the minimum furfural selectivity occur at the acid concentration of 0.005 M. Consequently, the aforementioned concentration is recommended for bagasse pretreatment and bioethanol production.

Abstract The torrefaction performances of microalga Nannochloropsis Oceanica under oxidative and inert atmospheres are characterized and compared with each other based on several operating parameters. By conducting several comparative indexes, the results suggest that oxidative torrefaction is more capable of upgrading microalgae due to its relatively lower solid yield and energy input, as well as relatively higher enhancement factor and upgrading energy index. Compared to inert torrefaction, the indexes indicate that oxidative torrefaction at 250 °C for 30 min has higher energy yield (1.02 times) and energy efficiency (2.2 times) but whereas lower energy input (0.4 times). With increasing torrefaction severity, the pyrolysis curve gradually becomes smooth and shift to a high-temperature zone. The peak temperatures of torrefied microalgae present an increasing trend, especially in the oxidative atmosphere. After oxidative torrefaction, microalgal solid biofuel is upgraded as peat and lignite, from the viewpoint of elemental composition. Furthermore, oxidative torrefaction is more suitable than inert torrefaction for producing bio-oil which mainly contains dianhydromannitol, neophytadiene, and palmitoleic acid. The TG-FTIR-MS results uncover the pyrolysis characteristics and reactivity of torrefied microalgae, and elucidate that oxidative torrefied microalgae is more reactive.

In this work, a direct prediction method coupled with a consecutive reaction model is developed to estimate the biochar yield and elemental composition in a biomass torrefaction process. Norway forest residues were chosen as feedstock and torrefied at different temperatures under nitrogen atmosphere in a thermogravimetric analyzer. Obtained data were modeled to predict the mass loss during torrefaction. Distributions of initial, intermediate and final solid products as well as torrefaction kinetic parameters are reported. Thereafter, a direct method to predict the elemental composition of biochar is introduced. The results show that the decomposition of initial biomass to form an intermediate solid has higher conversion rate than the degradation of the intermediate. Moreover, the predictions reproduce well the experimental thermogravimetric curves and show composition trends similar to the literature data. This method is useful for the design and optimization of industrial torrefaction processes with predictable biochar yield and elemental composition.

Abstract Membrane separation is a promising method to separate CO2 and H2 from hydrogen-rich gases. This simultaneously achieves H2 recovery and CO2 enrichment. The latter is conducive to subsequent carbon capture and storage, and thereby the development of negative emission technologies. In this study, single-, double-, triple-, and quadruple-tube systems with palladium (Pd) membranes and cross-flow configuration are considered, while the Reynolds number (Re) is in the range of 1–50. To maximize H2 recovery and CO2 enrichment in the systems, the systems are designed using a two-stage optimization in which the parametric sweep technology followed by the evolutionary computation of the Nelder-Mead simplex method is applied to find the best configuration and the exit H2 concentration is chosen as the objective function. On account of the scavenging waves stemming from the upstream tubes, the goals of the optimization is to diminish the concentration polarization effect of the upstream tubes upon the downstream ones. The predictions indicate that an increase in the number of tubes raises the optimization efficiency. Compared to the tubes in tandem, the optimized configuration at Re = 10 can improve the hydrogen recovery up to 12.2%, while the CO2 enrichment can be intensified by up to 7%.

This study aims to optimize the flow channel design for a proton exchange membrane electrolyzer cell (PEMEC) to minimize the pressure drop across the cell. The pattern of parallel flow channels is considered with a dual-porous layer structure sandwiched between the flow channel plate and the catalyst layer. Four geometric factors are considered in the optimization analysis, including the width of the flow channel, the depth of the flow channel, the particle diameter of the large-pore porous layer, and the particle diameter of the small-pore porous layer. Computational fluid dynamics (CFD) is used to simulate the flow field, and based on the results of the CFD simulation, the Taguchi method is employed to analyze the optimal flow channel design. The importance of the factors is further analyzed by the analysis of variance (ANOVA) method. Three inlet velocities are assigned in the Taguchi analysis, which are 0.01, 0.1332, and 0.532 m/s, and then an orthogonal array is constructed and analyzed for each inlet flow condition. It is found that the optimal combination of the factors is the depth of the flow channel 1 mm, the width of the flow channel 3 mm, the particle diameter of the large-pore porous layer 0.212 mm, and the particle diameter of the small-pore porous layer 0.002 mm. The pressure drop across the PEMEC is minimized at the condition with the optimal combination of the factors. The ANOVA analysis shows that the depth of the flow channel exhibits the most significant impact on the pressure drop, while the other factors play minor roles only.

The performance of a plate heat exchanger (PHE) using water as the working fluid with zigzag flow channels was optimized in the present study. The optimal operating conditions of the PHE are explored experimentally by the Taguchi method, with effectiveness as the objective function. The results are further verified by the analysis of variance (ANOVA). In addition, the zigzag flow channel geometry is optimized by the non-dominated sorting genetic algorithm-II (NSGA-II), in which the effectiveness and pressure drop of the PHE are considered the two objective functions in the multi-objective optimization process. The experimental results show that the ratio of flow rates is the most important factor affecting the effectiveness of the PHE. The optimal operating conditions are the temperatures of 95 °C and 10 °C at the inlets of hot and cold water flows, respectively, with a cold/hot flow rate ratio of 0.25. The resultant effectiveness is 0.945. Three geometric parameters of the zigzag flow channel are considered, including the entrance length, the bending angle, and the fillet radius. The sensitivity analysis of the parameters reveals that a conflict exists between the two objective functions, and multi-objective optimization is necessary for the zigzag flow channel geometry. The numerical simulations successfully obtain the Pareto optimal front for the two objective functions, which benefits the determination of the geometric design for the zigzag flow channel.

Abstract In recent years, torrefaction, a mild pyrolysis process carried out at the temperature range of 200–300 °C, has been considered as an effective route for improving the properties of biomass. Hemicellulose, cellulose, lignin and xylan are the basic constituents in biomass and their thermal behavior is highly related to biomass degradation in a high-temperature environment. In order to provide a useful insight into biomass torrefaction, this study develops the isothermal kinetics to predict the thermal decompositions of hemicellulose, cellulose, lignin and xylan. A thermogravimetry is used to perform torrefaction and five torrefaction temperatures of 200, 225, 250, 275 and 300 °C with 1 h heating duration are taken into account. From the analyses, the recommended values of the order of reaction of hemicellulose, cellulose, lignin and xylan are 3, 1, 1 and 9, respectively, whereas their activation energies are 187.06, 124.42, 37.58 and 67.83 kJ mol−1, respectively. A comparison between the predictions and the experiments suggests that the developed model can provide a good evaluation on the thermal degradations of the constituents, expect for cellulose at 300 °C and hemicellulose at 275 °C. Eventually, co-torrefaction of hemicellulose, cellulose and lignin based on the model is predicted and compared to the thermogravimetric analysis.