Sensible heat recovery

Fig. 6.8-12 Details of two shaft furnace designs a) long shaft furnace with internal cooling, b) medium shaft furnace with external cooling and two alternative methods of sensible heat recovery [B.18]...

Exhaust Discharge Sensible Heat Recovery Heat Recovery below Dew Possible Total Heat... 

The crude raw gas leaving the gasifier at 2,300-2,700°F contains a small quantify of unburned carbon and a significant portion of molten ash. Depending on the end use, this gas stream would either be directly quenched in water (to cool the gas and remove solidified ash particles) or would be cooled in radiant/convection boilers for sensible heat recovery (via high-pressure saturated steam generation) prior to water scrubbing. 

The regenerative nature of the Wulff operation permits the recovery of most of the sensible heat in the cracked gas. The gases leave the furnace at temperatures below 425°C, thus obviating the need for special high temperature alloys in the switch valve and piping system. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

Heat recovery section of an AHU The part of an AHU in which a sensible or latent heat gain or loss takes place by means of a heat-transfer medium. 

The flue gas exits the cyclones to a plenum chamber in the top of the regenerator. The hot flue gas holds an appreciable amount of energy. Various heat recovery schemes are used to recover this energy. In some units, the flue gas is sent to a CO boiler where both the sensible and combustible heat are used to generate high-pressure steam. In other units, the flue gas is exchanged with boiler feed water to produce steam via the use of a shell/tube or box heat exchanger. 

Bjurstrom, H., and B. Carlsson, 1985. An exergy analysis of sensible and latent heat storage, Heat Recovery Syst., 5, 233-250. 

The air product gas ratio was 0.73, and energy recovery reached about 70 percent (or just over 80 percent If the sensible heat of the gas Is Included). The heat values recorded by the calorimeter are about 8 percent higher than those computed from gas compositions and showing peaks not detected by the gas chromatograph. 

C02 is recoverable by physical adsorptions of active carbons or zeolites, or a chemical sorption by carbonation of calcium oxide (CaO) at the C02 recovery and separation process. CaO is absorbable chemically C02 at temperatures of 500-800°C [Eq. (5)]. CaO can remove C02 from hydrocarbon reaction system at the reaction temperature for C02 production with small sensible heat loss, and also enhance reaction rate and yield of the C02 production reaction (Kato, 2003). 

The area of the tubes immersed in the bed is then determined from energy balances on the FBC (Figure 7). An energy input with the coal of 111 MW is needed to provide the desired thermal output of 100 MW and the losses of 9.8 MW with the stack gases (for an exhaust temperature of 450 K) and 1.3 MW with the spent bed solids (assuming no recovery of the sensible heat). Given a recycle of 13 MW to the bed as air preheat, 73 MW must be extracted by coils immersed in the bed in order to maintain the bed temperature at 1116 K. The tube area needed to achieve this is 515 m, provided for example by six rows of 50 mm O.D. tubes with a horizontal pitch of 150 mm and a vertical pitch of 125 mm. 

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