Superheated steam, discussion

A further enhancement to the HRS process whereby the exhaust from a gas fired turbine is used to superheat steam from the HRS process is also possible (129). The superheated steam is then fed through a turbogenerator to produce additional electricity. This increases the efficiency of heat recovery of the turbine exhaust gas. With this arrangement, electric power generation of over 13.6 kW for 1 t/d (15 kW/STPD) is possible. Good general discussions on the sources of heat and the energy balance within a sulfuric acid plant are available (130,131). 

As discussed in Section 15.2.2, the gas turbine s main disadvantage is its low efficiency of around 25-35 per cent in open cycle. However, this can be significantly improved by the use of a heat-recovery boiler that converts a good proportion of the otherwise waste heat in the turbine exhaust gases to high-pressure superheated steam, which, in turn, drives a conventional steam turbogenerator for supplementary electrical power. This can increase the overall efficiency to 50 per cent for no further heat input as fuel. 

As was discussed in Section 3.3.1, the theoretical value of the index, y, is 1.67 for a monatomic gas such as helium or argon. 1.4 for diatomic gases such as hydrogen, oxygen and nitrogen and 1.33 for a polyatomic gas such as carbon dioxide or superheated steam. The true indices will deviate somewhat from these values in practice for instance the value of 1.3 is normally used as a better approximation for superheated steam. 

Biofuels and peat can be dried according to different thermodynamic principles. The most common method is the use of flue gases. Another is the use of superheated steam. Finally, double-effect drying will be discussed. 

Bond et al. [103] discuss the drying of paper by impinging jets of superheated steam. Svensson [105] discusses applications in the drying of wood pulp while Amoux et al. [106] consider the use of superheated steam in the drying of softwood biomass. 

The most desirable reference conditions for internal energy and enthalpy in processes where chemical reactions take place are 0 K or 25°C, zero pressure, and standard chemical elements, such as C (graphite), H2 (gas), O2 (gas), N2 (gas), CI2 (gas), and S (rhombic sulfur), rather than the chemical species themselves that are in the mixture. With this reference condition, internal energy and enthalpy changes automatically take into account heat of reaction. Felder and Rousseau (2000) discuss this reference condition. As an example, the enthalpy of 1 kg of superheated steam at 300°C and 1 MPa relative to the elements H2 (gas) and O2 (gas) at 0 K and 0 Pa is determined to be -12,209.3 kJ. Alternatively, from the steam tables in van Wylen et al. (1994), for a reference condition of saturated liquid water at 0°C, the enthalpy is 3,051.2 kJ/kg. 

Crotogino and Allenger, 1979). Mujumdar (1992) has discussed the superheated steam drying of paper in some detail including its history, current status, and potential. 

Pores were not uniform except after heating in steam. The change in pores by heating at 500 C in superheated steam was examined by Robinson and Ross (324), who observed that in air at 500 C, a gel of 660 m g area changed only to 600 m g" after 15 hr, but to 335 m g in steam. The pore volume does not change and as a result the pore diameter is increased from 25 to 45 A. However, there is evidence that steam treatment may introduce some pores smaller than 10 A (see discussion of micropores below). 

The basic fiindamentals of both conventional atmospheric-pressure Superheated steam drying and more novel low-pressure Superheated steam drying are presented in this chapter. The application of these processes to the drying of a wide variety of foods and biomaterials is illustrated and discussed, with emphasis placed on the use of LPSSD to preserve or even improve the properties of the drying materials. 

The paper focuses on the presentation and discussion of the results of the application of long term, continuous, AE structural monitoring to 2 large superheated (SH) steam outlet headers, belonging to 2 different full-size (600 MW, supercritical multifuel) ENEL power units. Continuous AE surveillance of the 2 SH headers started in October 1996 and is still ongoing. 

As indicated in Sec. IIB, ordinary nucleate boiling is a two-step process. First, nuclei must appear. Second, the nuclei must grow into bubbles large enough to move away from the nucleation sites. The rate of heat absorption by the liquid may be controlled by either one or both of these two processes. The growth of a nucleus (tiny bubble) into ordinary bubbles has received attention recently. The theoretical attack of Forster and Zuber was discussed in Sec. IIB2. Inasmuch as the theory of Zwick and Plesset (P3, P4, Zl, Z2) represents another attempt to obtain exact expressions for bubble growth, and since the theory fits well with the few data for steam bubbles in superheated water, their theoretical method is summarized below. 

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