Metallic recuperators

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. 

Metallic recuperators are 10 to 20 times lighter compared to an equivalent ceramic recuperator. 

Metallic recuperators occupy less space than ceramic ones. 

Metallic recuperators are practically leak-proof and so are also capable ofhandling toxic fuel gases. The ceramic recuperators leak to the extent of about 50% of the volume gases and air handled. 

Metallic recuperators are favorably disposed economically when applied for preheating air below 650 °C. Ceramic recuperators are only economic when applied for preheating air above 650 °C. 

Metallic recuperators can operate with higher pressure differentials between flue gas and air side than ceramic recuperators. 

Metallic recuperators are easier to maintain and install than ceramic ones, they also involve less initial cost. 

The same principle applies to blast furnace stoves and to the multiple-tower heat recovery units positioned around the periphery of vertical cylindrical incinerators for waste gases or liquids. For furnaces with lower temperature waste gases, such as boilers or steam generators, a Ljungstrom all-metal recuperator, rotating on a vertical shaft, is used. 

The steel industry has been using soaking pits for at least 125 years. Originally, they were simply refractory boxes in the earth with no combustion systems. From these simple units, the industry graduated to regenerative pits which had no instrumentation to the bottom-fired pits with ceramic recuperators to one-way top-fired pits with or without metallic recuperators. With the one-way top-fired pits, more pit area is under the crane per unit of real estate, so they became the universally accepted standard. Typical size 22 ft (6.7 m) long, 8.5 to 10 ft (2.6 to 3.0 m) wide, and 10 to 17 ft (3.0 to 5.2 m) deep. The combustion system has one or two burners located high on one end of the pit with the flue directly beneath them. 

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. 

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. )... 

Table 7.4 Comparison between metallic and ceramic recuperators.

Note Catalyst = platinum nanopartides supported on ACC (Pt/ACC, 5 wt-metal%) 3.0 g. Feed rate of methylcyclohexane = 3.5 mmol/min (superheated liquid-film state). a [Ratio of heat recuperation] = [reaction heat]/([reaction heat] + [evaporation heat]). 

A series of experimental results on the ratio of heat recuperation is shown in Table 13.4. Here temperatures of the catalyst layer, composed of a sheet of ACC dispersed with platinum nanoparticles (5 wt-metal%, 3.0 g), were varied at the constant feed ratio (3.5 mmol/min) of liquid methylcyclohexane (MCH) [39]. At the catalyst-layer temperature of 285°C, the heat recuperation ratio of 63.3% was attained, being much larger than the magnitudes of 15.6 and 47.5% for the catalyst-layer temperatures of 210°C and 245°C, respectively. 

The combination of the fuel cell and turbine operates by using the rejected thermal energy and residual fuel from a fuel cell to drive the gas turbine. The fuel cell exhaust gases are mixed and burned, raising the turbine inlet temperature while replacing the conventional combustor of the gas turbine. Use of a recuperator, a metallic gas-to-gas heat exchanger, transfers heat from the gas turbine exhaust to the fuel and air used in the fuel cell. 

The subsequent steam reforming section is operated at very high temperatures 850-900 °C. The SMR catalysts themselves are already active below 400 °C, but high temperatures are necessary to drive the strongly endothermic reaction forward [8]. In industry, nickel catalysts are used in high-alloy reaction tubes, which are heated by external burners. This design is expensive and leads to heat losses, although much of the heat is recuperated. Noble metal catalysts such as sup-... 

Use of the ceramic honeycomb packing structure in the recuperator keeps fuel and air substantially isolated as they travel through the recuperator. Various ceramic materials such as cordierite, mullite, alumina and silicon carbide can be used to fabricate honeycomb beds. While metallic materials have the potential to be used in honeycomb bed, corrosion resistance is a major issue... 

In special tank furnaces, e.g. for the manufacture of textile glass fibers, metal regenerators have been successful for heat recuperation. The regular reversal of the gas direction is then not necessary, simplifying temperature control over that with regenerative heat recuperation. 

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