Recuperators

This section is a brief introduction to some of the important issues concerning the heat load in a furnace or combustor. In petrochemical production processes. 

Elevation view of infrared burner heating a moving web. (From Baukal, C. E., Heat Transfer in Industrial Combustion, Boca Raton, FL CRC Press, 2000 courtesy of CRC Press.) 

Convection dryers are also used to heat and dry substrates. Typically, high velocity heated air is blown at the substrate from both sides so that the substrate is elevated between the nozzles. In many cases, the heated air is used for both heat and mass transfer, to volatilize any liquids on or in the substrate such as water, and then carry the vapor away from the substrate. 

This type of load encompasses a wide range of materials including granular solids like limestone and liquids like molten metal. For this type of load, the heat transfers to the surface of the load and must conduct down into the material. This process can be enhanced by proper 

The bends and welded joints should be easily accessible for maintenance hence preferably should be located outside the gas enclosure. 

Total weight when empty and when full. 

Standard fittings and mountings (similar to those on a boiler) for example, safety valves, level indicator, vent valve, blow down valve and low level alarms. Gas inlet and exit boxes and internal fire/add resistant linings. 

Drain nozzles on the gas inlet and outlet boxes for draining out any (acidic) condensate. 

3 Air Preheater/Heat Recuperators/Other Devices for Heat Recovery 

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Continuous recuperative furnaces employing metallic recuperators (heat exchangers) have been in use since the 1940s. Operation of these furnaces is simplified and the combustion process is more precisely controlled no reversal of air flow causes temperature variations. The recuperator metal must be caretiiUy selected because of chemical attack at high temperature. Recuperative furnaces are often used in the production of textile fiber glass because they maintain a constant temperature. 

Typical heat-recuperation devices are finned gas exchangers, ceramic heat wheels, and Ljungstrom air preheaters. 

MIDREX Process. The primary components of a MID REX process plant include the shaft furnace, reformer, and heat recuperator. These components are supported by ancillary systems for handling iron ore, gas, water, and direct reduced iron. A flow sheet is shown in Figure 1. 

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

High Temperature. The low coefficient of thermal expansion and high thermal conductivity of sihcon carbide bestow it with excellent thermal shock resistance. Combined with its outstanding corrosion resistance, it is used in heat-transfer components such as recuperator tubes, and furnace components such as thermocouple protection tubes, cmcibles, and burner components. Sihcon carbide is being used for prototype automotive gas turbine engine components such as transition ducts, combustor baffles, and pilot combustor support (145). It is also being used in the fabrication of rotors, vanes, vortex, and combustor. 

Smelting. The fuel suppHed to the reverberatory furnace is in the range of 5—6 GJ/t (4.7-5.7 x 10 Btu/t) concentrate. Steam produced in the waste heat boiler is equal to ca 60% of the energy suppHed by the fuel. The additional heat recovered from the exit gases in the recuperator to preheat the combustion air is equal to ca 10% of the energy from the fuel. Hence, the heat recovered from the furnace is equal to ca 70% of the heat from the fuels. 

Regenerators are by nature intermittent or cycling devices, although, as set forth previously, the Ljunstrom design avoids interruption of the fluid stream by cychng the heat-retrieval reservoir between the hot and cold fluid streams. Truly continuous counterparts of regenerators exist, however, and they are called recuperators. 

The simplest configuration for a recuperative heat exchanger is the metallic radiation recuperator (Fig. 27-57). The inner tube carries the hot exhaust gases and the outer tube carries the combustion air. The bulk of the heat transfer from the hot gases to the surface of the inner tube is by radiation, whereas that from the inner tube to the cold combustion air is predominantly by convection. 

FIG. 27-57 Diagram of a metallic radiation recuperator. (From Goldstick Waste Heat Recovery, Faiimont Press, Atlanta, 1986. )... 

For standardization of validation procedure we suggested normalized coordinate system (NCS) X. = 100-C/C", Y. = 100-A/A", where C is a concentration, A - analytical response (absorbance, peak ai ea etc.), index st indicates reference solution, i - number of solution. In this coordinate system recuperation coefficient (findings in per cent to entry) is found as Z = IQQ-Y/X. As a result, coordinates of all methods ai e in the unified... 

These gas turbines have in many cases regenerators or recuperators to enhance the efficiency of these turbines. Figure 1-12 shows such a new recuperated gas turbine design, which has an efficiency of 38%. 

The power train consists of an HP and LP expander arranged in series that drives the motor/generator, which in this mode is declutched from the compressor train and is connected by clutch to the HP and LP expander train. The HP expander receives air from the cavern that is regeneratively heated in a recuperator utilizing exhaust gas from the LP expander, and then further combusted in combustors before entering the HP expander. The... 

Chodkiewicz, R. A Recuperated Gas Turbine Incorporating External Heat Sources in the Combined Gas-Steam Cycle, ASME Paper No. 2000-GT-0593. 

He did not put on the fire gear and proceeded to open the vessel and hose it out with a fire hose. An explosion resulted when water f dislodged crusted-over sodium aluminum hydride trapped in a nozzle. The worker was burned, requiring a two-week hospital stay and several months of recuperation. 

Recuperator A wall separating the flowing fluids is the most commonly encountered problem. 

Fig. 1.10 shows the processes of heat exchange (or recuperation), reheat and intercooling as additions to a JB cycle. Heat exchange alone, from the turbine exhaust to the compressed air before external heating, increases and lowers so that the overall... 

Fig. 1.10. Temperature-entropy diagram. showing reheat, intercooling and recuperation.

Fig. 5.11. Calculated exergy losses as fractions of fuel exergy for the Westinghou.se/Rolls-Royce WR21 recuperated (CICBTX) plant, for varying combustion temperatures (K) (after Ref. (13)).

Consider next a recuperative STIG plant (Fig. 6.5, again after Lloyd [2]). Heat is again recovered from the gas turbine exhaust but firstly in a recuperator to heat the compressed air, to state 2A before combustion and secondly in an HRSG, to raise steam S for injection into the combustion chamber. 

However in practice, for the same states 1-5 the steam raised S will be less hence there is no advantage in operating a STIG plant in this variation of the basic CBTX recuperative gas turbine plant. Nonetheless, this form of analysis as developed by Lloyd will prove to be useful in the discussion of the chemical recuperation plant in Chapter 8. 

The thermal efficiencies (ij) of these five cycles, all with perfect recuperation, are plotted in Fig. 6.7 against the isentropic temperature ratio a-, for = 0-8 and Tj/Ti = 5... 

Similar calculations (Fig. 6.10) were made for intercooled cycles, without and with water injection, i.e. comparing the efficiency of the dry CICBTX cycle with an elementary recuperated water injection plant, now a simple version of the. so-called RWl plant (see Section 6.4.2.1). Again there is an increase in thermal efficiency with water injection, but it is not as great as for the simple EGT plant compared with the dry CBTX plant the optimum pressure ratio, about 8 for the dry intercooled plant, appears to change little with water injection. 

There have been a larger number of proposals for recuperated cycles with water injection and evaporation, but all these can be interpreted as modifications of the EGT plant, which is essentially a wet CBTX cycle, as explained above. 

Fig. 6.14. Recuperated water injection (RWl) plant and humidified air turbine (HAT) plant compared (after...

To further understand the thermodynamic philosophy of the improvements on the EGT cycle we recall the cycle calculations of Chapter 3 for ordinary dry gas turbine cycles—including the simple cycle, the recuperated cycle and the intercooled and reheated cycles. 

Fig. 3.16 showed carpet plots of efficiency and specific work for several dry cycles, including the recuperative [CBTX] cycle, the intercooled [CICBTX] cycle, the reheated [CBTBTX] cycle and the intercooled reheated [CICBTBTX] cycle. These are replotted in Fig. 6.17. The ratio of maximum to minimum temperature is 5 1 (i.e. T nx 1500 K) the polytropic efficiencies are 0.90 (compressor), 0.88 (turbine) the recuperator effectiveness is 0.75. The fuel assumed was methane and real gas effects were included, but no allowance was made for turbine cooling.

Lloyd. A. (1991), Thermodynamics of chemically recuperated gas turbines. CEE.S Report 256, Centre For Energy and Environmental. Studies, University Archives, Department of Rare Books and Special Collections, Princeton University Library. 

El-Ma.sri, M.A. (1988). A modified high efficiency recuperated gas turbine cycle, i, Engng Gas Turbines Power 110, 233-242. 

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