Interstitial gas

Powder Insulation A method of reahzing some of the benefits of multiple floating shields without incurring the difficulties of awkward structural complexities is to use evacuated powder insulation. The penalty incurred in the use of this type of insulation, however, is a tenfold reduction in the overall thermal effectiveness of the insulation system over that obtained for multilayer insulation. In applications where this is not a serious factor, such as LNG storage facihties, and investment cost is of major concern, even unevacuated powder-insulation systems have found useful apphcations. The variation in apparent mean thermal conductivity of several powders as a function of interstitial gas pressure is shown in the familiar S-shaped curves of Fig. 11-121. ... 

FIG. 11-121 Apparent mean thermal conductivities of several powder insulations as a function of interstitial gas pressure. 

Effect of Interstitial Gas Pressure on the Small-Scale Gap-Test... 

Fig 12 Effect of Interstitial Gas Pressure on the Small-Scale Gap-Test Sensitivity of High Specific Surface PETN (Ref 96)... 

The effect of pressure on the heat transfer coefficient is influenced primarily by hgc (Botterill and Desai, 1972 Xavier etal., 1980). This component of h transfers heat from the interstitial gas flow in the dense phase of the fluidized bed to the heat transfer surface. For Group A and small Group B particles, the interstitial gas flow in the dense phase can be assumed to be approximately equal to Um ed. 6/i s extremely small for... 

Li, H., Mechanics of Arching in a Moving-Bed Standpipe with Interstitial Gas Flow, Powder Tech., 78 179-18 (1994)... 

Group B 100-800 Bubbling occurs at velocity >umf. Most bubbles have velocities greater than interstitial gas velocity. No evidence of maximum bubble size. Sand. 

Group D 1000 All but largest bubbles rise at velocities less than interstitial gas velocity. Can be made to form spouted beds. Particles large and dense. Wheat Metal shot... 

First of all, the physical structure of the packed bed in the conversion system is defined. The fuel bed structure can be divided into three phases, namely the interstitial gas phase, the intraparticle solid phase, and the intraparticle gas phase. By means of this terminology it is easier to address certain mass and heat transport phenomena taking place on macro and micro scale inside the packed bed during the thermochemical conversion, see Figure 8. 

The maximum combustion heat flux (grate loadings) was 1510 kW/m and the maximum interstitial gas temperature obtained was 1300°C during the overfired batch mode. 

The objective of one of the papers, which was a part of Orlanders thesis [9], was to study the effects of the bed stoichiometric ratio (SRb) (air factor of conversion system) and fuel nitrogen content on the formation of various gaseous species (O2, CO, CO2, H2 and NO) in the interstitial gas, off-gas and flue gas. 

Figure 19 below shows that a fuel-bed system actually has three important structures. Starting from a macro perspective, the fuel-bed can be divided into an interstitial gas phase and a solid phase (particle phase). The particle phase can in turn be broken down into an intraparticle gas phase and an intraparticle solid phase [15,20,27]. These intraparticle phases can be seen in the partial char volume in Figure 19 [28]. 

In conclusion, the fuel-bed system comprises three structures, which are interstitial gas phase, intraparticle gas phase and intraparticle solid phase, see Figure 18. In a comprehensive partial differential theory of the conversion system these three structures need to be considered. The fuel-bed structure is independent of conversion concept (see definition below) applied that is, the three structures shown in Figure 18 will be the same for all categories of packed fuel-bed systems. 

This survey focuses on the so-called fixed beds, according to the Kuuni and Levenspiel classification, but these authors will refer to packed beds. Fluidized beds are outside the scope of this review. Furthermore, there exist no fixed beds in a physical sense if by fixed we mean the relative movement of both the interstitial gas... 

The fuel-bed system may also be characterised with respect to what is here called fuel-bed configuration. Fuel-bed configuration may be defined as the relative movement between air flow (interstitial gas phase) and solid phase flow [19,38], Three basic configurations arise as a consequence of the possible 90°-combinations and are commonly referred to as cocurrent, countercurrent and crosscurrent., see Figure 27. 

The preheating of solid fuel and the ash cooling are not included in the thermochemical conversion process. The basic criteria for these four thermochemical conversion reactions are that the solid-fuel convertibles (or moisture, char, volatiles) are converted from the solid phase into the interstitial gas phase and finally to the offgases (Figure 16 and Figure 19). The part of the solid-fuel convertibles that is converted into the interstitial gas phase is defined as the conversion gas [3]. The conversion gas is associated with two important physical properties, namely the empirical stoichiometry [CxHyOz] and the mass flux [kg/m s]. 

The water is transported from the intraparticle phase to the interstitial gas phase. It is then further transported through the porous system of the bed towards the bed surface by means of forced convection. 

The pyrolysis gas (VOC, tar, etc) evolved in the intraparticle phase enters the interstitial gas phase (see Figure 42) from which it is transported by forced convection to the bed surface and out to the over-bed section. The pyrolysis gases are burnt in the... 

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