Five-kilowatt wood gasifier technology: Evolution and field experience

Various elements of an efficient and reliable 5k W wood gasifier system developed over the last ten years are described. The good performance obtained from the system is related to the careful design of its components and sub-systems. Results from extensive testing of gasifier prototypes at two national centres are discussed along with the experience gained in the field from their use at more than one hundred and fifty locations spread over five states in the country. Issues related to acceptance of the technology are also included. Improvements in design to extend the life, to reduce the cost, and to reduce the number of components are also discussed. A few variants of the design to meet the specific requirements of water pumping, power generation and to exploit specific site characteristics are presented.

Modelling of void initiation and breaking phenomena in a packed bed

Void breaking and formation in a packed bed are important phenomena in stabilising and optimising the performance of reactors such as the blast furnace, spouted bed and catalytic regenerator. These phenomena have been studied using a mathematical model. The model is based on a previously published force balance approach to predict the cavity size. Limited numbers of experiments, at room temperature, have been carried out in order to compare the experimental results with theory. A good agreement has been found between the experimental and theoretical results. In addition, the predictions have been compared with published data, which give reasonable agreement. The role of various forces (friction, pressure and bed weight) on void initiation and breaking has been investigated. The effect of bed height, particle diameter and density, void fraction, as well as gas flow rate on void formation and breaking has also been studied.

Optimum atomic spacing for AlAs etching in GaAs epitaxial lift-off technology

With respect to GaAs epitaxial lift-off technology, we report here the optimum atomic spacing (5-10 nm) needed to etch off the AlAs release layer that is sandwiched between two GaAs epitaxial layers. The AlAs etching rate in hydrofluoric acid based solutions was monitored as a function of release layer thickness. We found a sudden quenching in the etching rate, approximately 20 times that of the peak value, at lower dimensions (similar to2.5 nm) of the AlAs epitaxial layer. Since this cannot be explained on the basis of a previous theory (inverse square root of release layer thickness), we propose a diffusion-limited mechanism to explain this reaction process. With the diffusion constant being a mean-free-path-dependent parameter, a relation between the mean free path and the width of the channel is considered. This relation is in reasonable agreement with the experimental results and gives a good physical insight to the reaction kinetics.

Analysis of a thick plate with a circular hole resting on a smooth rigid bed and subjected to axisymmetric normal load

A solution for the stresses and displacements in an radially infinite thick plate having a circular hole, one face of which resting on a smooth rigid bed and the other face subjected to axisymmetric normal loading is given. The solution is obtained in terms of Fourier-Bessel series and integral for the Love's stress function. Numerical results are presented for one particular ratio of thickness of plate to the hole radius and loading. It is also shown that the Poisson's ratio has a predominant effect on certain stresses and displacements. The solution would be useful in the stress analysis of bolted joints.Eine Lösung für die Spannungen und Verschiebungen in einer radial, unendlich ausgedehnten, dicken Platte mit einem kreisförmigen Loch, wobei eine Seite auf einer ebenen, starren Unterlage aufliegt, die andere Seite durch eine achsensymmetrische Vertikallast belastet ist, wird angegeben. Die Lösung wird in Form von Fourier-Bessel-Reihen und Integralen der Loveschen Spannungsfunktion angegeben. Numerische Ergebnisse werden für ein bestimmtes Verhältnis der Plattendicke zum Lochradius sowie zur Belastung angegeben. Es wird auch gezeigt, daß das Poisssonsche Verhältnis einen besonderen Einfluß auf bestimmte Spannungen und Verschiebungen hat. Die Lösung ist anwendbar für die Spannungsermittlung von Bolzenverbindungen.

Performance comparison of dynamical decoupling sequences for a qubit in a rapidly fluctuating spin bath

Avoiding the loss of coherence of quantum mechanical states is an important prerequisite for quantum information processing. Dynamical decoupling (DD) is one of the most effective experimental methods for maintaining coherence, especially when one can access only the qubit system and not its environment (bath). It involves the application of pulses to the system whose net effect is a reversal of the system-environment interaction. In any real system, however, the environment is not static, and therefore the reversal of the system-environment interaction becomes imperfect if the spacing between refocusing pulses becomes comparable to or longer than the correlation time of the environment. The efficiency of the refocusing improves therefore if the spacing between the pulses is reduced. Here, we quantify the efficiency of different DD sequences in preserving different quantum states. We use C-13 nuclear spins as qubits and an environment of H-1 nuclear spins as the environment, which couples to the qubit via magnetic dipole-dipole couplings. Strong dipole-dipole couplings between the proton spins result in a rapidly fluctuating environment with a correlation time of the order of 100 mu s. Our experimental results show that short delays between the pulses yield better performance if they are compared with the bath correlation time. However, as the pulse spacing becomes shorter than the bath correlation time, an optimum is reached. For even shorter delays, the pulse imperfections dominate over the decoherence losses and cause the quantum state to decay.

Studies in biogas technology. Part IV. A novel biogas plant incorporating a solar water-heater and solar still

A reduction in the heat losses from the top of the gas holder of a biogas plant has been achieved by the simple device of a transparent cover. The heat losses thus prevented have been deployed to heat a water pond formed on the roof of the gas holder. This solar-heated water is mixed with the organic input for ‘ hot-charging ’ of the biogas plant. A thermal analysis of such a solar water-heater ‘ piggy-backing ’ on the gas holder of a biogas plant has been carried out.To test whether the advantages indicated by the thermal analysis can be realised in practice, a biogas plant of the ASTRA design was modified to incorporate a roof-top solar water-heater. The operation of such a modified plant, even under ‘ worst case ’ onditions, shows a significant improvement in the gas yield compared to the unmodified plant. Hence, the innovation reported here may lead to drastic reductions in the sizes and therefore costs of biogas plants. By making the transparent cover assume a tent-shape, the roof-top solar heater can serve the additional function of a solar still to yield distilled water. The biogas plant-cum-solar water-heater-cum-solar still described here is an example of a spatially integrated hybrid device which is extremely cost-effective.

Studies in biogas technology. Part III. Thermal analysis of biogas plants

A thermal model for a conventional biogas plant has been developed in order to understand the heat transfer from the slurry and the gas holder to the surrounding earth and air respectively. The computations have been performed for two conditions : (i) when the slurry is at an ambient temperature of 20°C, and (ii) when it is at 35°C, the optimum temperature for anaerobic fermentation. Under both these conditions, the gas holder is the major “culprit” with regard to heat losses from the biogas plant. The calculations provide an estimate for the heat which has to be supplied by external means to compensate for the net heat losses which occur if the slurry is to be maintained at 35°C. Even if this external supply of heat is realised through (the calorific value of) biogas, there is a net increase in the biogas output, and therefore a net benefit, by operating the plant at 35°C. At this elevated temperature, the cooling effect of adding the influent at ambient temperature is not insignificant. In conclusion, the results of the thermal analysis are used to define a strategy for operating biogas plants at optimum temperatures, or at higher temperatures than the ambient.

Studies in biogas technology. Part II. Optimisation of plant dimensions

In this paper, the design basis of the conventional Khadi and Village Industries Commission biogas plants has been elucidated. It has been shown that minimisation of the cost of the gas holder alone leads to the narrow and deep digesters of conventional plants. If instead, the total capital cost of the gas holder plus digester is minimised, the optimisation leads to wide and shallow digesters, which are less expensive. To test this alternative, two prototype plants have been designed, constructed and operated. These plants are not only 25–40% cheaper, but their performance is actually slightly better than the conventional plants.

Studies in biogas technology. Part I. Performance of a conventional biogas plant

This paper gives an account of a conventional 5.66 m3/day (200 cubic ft/day) biogas plant which has been instrumented, operated and monitored for 2 1/2 years. The observations regarding input to the plant, sludge and biogas outputs, and conditions inside the digester, have been described. Three salient features stand out. First, the observed average daily gas yield is much less than the rated capacity of the plant. Secondly, the plants show ease of operation and a very slow response to reductions and cessations of dung supply. Thirdly, the unexpectedly marked uniformity of density and temperature inside the digester indicates the almost complete absence of the stratification which is widely believed to take place; hence, biogas plants may be treated as isothermal, ‘ uniform ’ density, most probably imperfectly mixed, fed-batch reactors operating at the mean ambient temperature and the density of water.

Incipient motion design of sand bed channels affected by bed suction

Seepage through sand bed channels in a downward direction (suction) reduces the stability of particles and initiates the sand movement. Incipient motion of sand bed channel with seepage cannot be designed by using the conventional approach. Metamodeling techniques, which employ a non-linear pattern analysis between input and output parameters and solely based on the experimental observations, can be used to model such phenomena. Traditional approach to find non-dimensional parameters has not been used in the present work. Parameters, which can influence the incipient motion with seepage, have been identified and non-dimensionalized in the present work. Non-dimensional stream power concept has been used to describe the process. By using these non-dimensional parameters; present work describes a radial basis function (RBF) metamodel for prediction of incipient motion condition affected by seepage. The coefficient of determination, R-2 of the model is 0.99. Thus, it can be said that model predicts the phenomena very well. With the help of the metamodel, design curves have been presented for designing the sand bed channel when it is affected by seepage. (C) 2010 Elsevier B.V. All rights reserved.