Combustion permeation

Analytical Characterization. The lignins were characterized analytically by the following methods 1H NMR spectra, gel permeation chromatography (5), gas chromatography (6), thermal measurements, elemental analysis, sugar content, extractions, solubility, and combustion properties. 

Favre, E. (2007) Carbon dioxide recovery from post-combustion processes can gas permeation membranes compete with absorption Journal of Membrane Science, 294, 50. 

The steady-state permeation model of in situ coal gasification is presented in an expanded formulation which includes the following reactions combustion, water-gas, water-gas shift, Boudouard, methanation and devolatilization. The model predicts that substantial quantities of unconsumed char will be left in the wake of the burn front under certain conditions, and this result is in qualitative agreement with postburn studies of the Hanna UCG tests. The problems encountered in the numerical solution of the system equations are discussed. 

The last ten years have witnessed a number of extensive field tests of underground coal gasification (UCG) in the United States and Europe. Model development is essential to the proper understanding of these test results and to the planning of future experiments. This report will focus upon the steady-state "permeation" or "packed bed" model of in situ gasification (forward combustion mode). In this useful but idealistic model the coal bed is assumed to be uniformly permeable to reactant and product gases. 

An expanded formulation of the steady-state permeation model has been presented. Two numerical problems - stiffness and an ill-conditioned boundary value problem - are encountered in solving the system equations. These problems can be circumvented by matching forward and reverse integrations at a point near the inlet (n = 0) but outside the combustion zone. The model predicts a... 

Alternatively it may take the form of a ceramic or metallic monolith, of which a variety of physical shapes is available monoliths are now widely used as supports for the active catalyst, which lines the channels which permeate the structure. They find particular application for the control of exhaust from vehicles powered by internal combustion or diesel engines. If the catalyst particles are small enough, a fast flow of reactants causes the bed to expand and the particles to move about like molecules in a liquid. We then have a fluidised bed reactor, which affords a more uniform temperature profile than is possible in fixed bed reactors, and is therefore more apposite to strongly exothermic reactions. 

Nanoparticles are not only a product of new nano-technologies, but are also present in the environment, and nanoscale phenomena permeate and often control natural processes.12 Humans have always experienced exposure to nanosized particles, but with the advent of the industrial revolution, this exposure from anthropogenic sources (e.g. internal combustion engines, power plant etc.) has increased dramatically. 

Figure 28. Cyclohexane dehydrogenation in different membrane reactors, adopted from ltoh [55], a) microporous inert membrane, b) H2 selective Pd membrane, c) H2-selective Pd membrane with combustion on the permeate side (from [56])...

For natural permeation combustion, the reactant gas may flow in either the same (cocurrent flow. Fig. 27a) or opposite (countercurrent flow. Fig. 27b) direction as that of combustion wave propagation, and in some cases even in both directions Fig. 27c). One-dimensional flow regimes can be achieved by placing the sample in a quartz tube, which in turn can be closed at one end (Pityulin et al, 1979). In the general case (see Fig. 26), the reactant gas also enters from the side surface, and the process can be described by two- or three-dimensional numerical simulations (cf Ivleva and Shkadinskii, 1981 Dandekar et al., 1990, 1993). In this section, results for the simpler one-dimensional case are analyzed and features of the higher dimensional studies are briefly discussed. 

It is interesting to note that the average gas pressure in the reaction zone (and consequently the combustion velocity) decreases with higher sample permeation coefficients, kf, and porosity, e, owing to higher reaction rates. In addition, the relationship between combustion temperature and initial pressure, for an excess of (j o>f ) and lack (vo>v) of reactant gas, can be represented as... 

Based on these results, an explanation for the combustion velocity dependence on green mixture density has recently been suggested (Vadchenko et ai, 1996). For relatively low sample densities (po<0.6), escape of desorbed gases in the combustion wave is not limited by permeation and occurs without destruction of the reactant medium structure. In this case, increasing the green density... 

Oxygen-enriched air will be produced on the low pressure permeate side of the air-separating membranes. The oxygen-enriched permeate stream is usually vented, but there is an increasing interest in using this gas for combustion. High-purity oxygen... 

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