Air recuperation

After the analysis of PCFB-1.0 plant design documentation, the circuit of new hybrid co-generation power plant with use of PCFB gasifier, solid oxide fuel cells, and gas turbine power plant with built-in air recuperator was proposed (see Fig. 7). Thermal capacity of power plant will be 1.14 MW at gasifier operation under pressure of 0.35 MPa and Ukrainian bituminous coal consumption of 222.6 kg/h. Electric capacity of solid oxide fuel cell module will be 375 kW and of electric capacity of high-speed gas turbine plant will be 125 kW. 

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. 

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. 

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. 

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... 

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... 

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. 

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

A3 Recuperative, CO2 removal SC/CBTX Recuperator Natural gas/air LP (chemical) Simple CO2 removal... 

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

Improvements also have been made to the gas turbine for naval applications. An intercooled recuperative (ICR) gas turbine has been designed to improve the fuel consumption of naval power plants. The engine has a recuperator to take the heat that would otherwise be wasted m the exhaust and transfers it to the air entering the combustor. The new engine is expected to save about 30 percent ot the fuel consumed, compared to the simple gas turbine. The ICR engine is, however, larger and more expensive than the simple gas turbine. 

Several of the gas turbine cycle options discussed m this section (intercooling, recuperation, and reheat) are illustrated in Figure 4. These cycle options can be applied singly or in various combinations with other cycles to improve thermal efficiency. Other possible cycle concepts that are discussed include thermochemical recuperation, partial oxidation, use of a humid air turbine, and use of fuel cells. 

Any process using a fossil fuel will involve the rejection of the products of combustion following heat transfer. These flue products will contain sensible heat that is lost and represents inefficiency in the process. Unless some form of recuperation is practiced, the flue products must be at a higher temperature than the process, and this cannot be reduced. The amount of excess air can, however, is controlled. 

Figure 19.1 shows the heat carried away in flue gases, particularly for high-temperature processes. For example, for a furnace operating at 800° C with no excess, air losses are 40 per cent. Recovery of a proportion of these losses is possible by means of recuperation. 

In many processes, load recuperation is not practicable, and combustion air is preheated in a heat exchanger by means of the outgoing flue products. Figure 19.3 gives an indication of the savings to be made for different operating temperatures. It is not normally considered economic to operate a recuperator at flue temperatures below about 750°C. 

Figure 26.4 Multi-plate air-to-air heat exchanger (Courtesy of Recuperator Ltd)...

Recuperators are heat exchangers which are mainly used for the recovery of heat from waste gases to preheat the gaseous fuel or the air used for combustion. The two streams - the hot fluid stream and the colder stream - are not allowed to mix with each other as... 

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. 

All the CO resulting from the pseudo solid-solid reaction is conducted, together with entrained char, from the top fluidized section through a constriction, in which the high-velocity gas flow prevents backflow, to a transport combustor, where the CO is burned to C02 with preheated air, along with as much of the char as is called for by heat balance to maintain the endothermic FeO-C reaction. The heated recycled char is separated from the off gas at the top of this transport combustor in a hot cyclone and is returned as a thermal carrier to the lower part of the lowest j igged section, while the hot flue gas from the transport combustor is used to preheat the incoming air in a recuperator. 

Figure 6.5 provides a detailed process flow diagram of the system. During normal operation air enters the compressor and is compressed to 3 atmospheres. This compressed air passes through the recuperator, where it is preheated and then enters the SOFC. Pressurized fuel from the fuel pump also enters the SOFC, and the electrochemical reactions take place along the cells. The hot pressurized exhaust leaves the SOFC and goes directly to the expander section of the gas... 

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. 

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