Thermal efficiency, 4.4, 4.6, 4.7

Modem fired heaters operate at thermal efficiencies of between 80 to 90 per cent, depending on the fuel and the excess air requirement. In some applications additional excess air may be used to reduce the flame temperature, to avoid overheating of the tubes. 

Where the inlet temperature of the process fluid is such that the outlet temperature from the convection section would be excessive, giving low thermal efficiency, this excess heat can be used to preheat the air to the furnace. Tubes would be installed above the process fluid section in the convection section. Forced draft operation would be needed to drive the air flow through the preheat section. 

Heat losses from the heater casing are normally between 1.5 to 2.5 per cent of the heat input. 

Here is a graph that shows how thermal efficiency can be determined from excess air and stack gas temperature. 

GPSA Engineering Data Book, Gas Processors Suppliers Association, Vol. 1, 10th Ed. 

Exit air usually is maintained far from saturated with moisture and at a high temperature in order to prevent recondensation of moisture in parallel current operation, with a consequent lowering of thermal efficiency. With steam heating of air the overall efficiency is about 40%. Direct fired dryers may have efficiencies of 80-85% with inlet temperatures of 500-550 C and outlet of 65-70°C. Steam consumption of spray dryers may be 1.2-1.8 lb steam/lb evaporated, but the small unit of Table 9.19(b) is naturally less efficient. A 10% heat loss through the walls of the dryer often is taken for design purposes. Pressure drop in a dryer is 15-50in. of water, depending on duct sizes and the kind of separation equipment used. 

Proper atomization of feed is the key to successful spray drying. The three devices of commercial value are pressure nozzles, pneumatic nozzles, and rotating wheels of various designs. Usual pressures employed in nozzles range from 300 to 4000 psi, and 

The design of spray dryers is based on experience and pilot plant determinations of residence time, air conditions, and air flow rate. Example 9.10 utilizes such data for the sizing of a commercial scale spray dryer. 

Variable Increased Factors Increased Factors Decreased 

Chamber inlet temperature Feed rate and thus product rate, particle size b), product moisture content, chamber wall build-up (a) bulk density (b) 

TABLE 9.18. Effects of Variables on Operation of Spray Dryers 

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Biomass whether trees, plants, grasses, algae, or water plants, has a heating value of 15.1 X 10 J/dry t, and is converted in integrated biomass planting, harvesting, and conversion systems to SNG at an overall thermal efficiency of 50%. 

Another hydrogenation process utilizes internally generated hydrogen for hydroconversion in a single-stage, noncatalytic, fluidized-bed reactor (41). Biomass is converted in the reactor, which is operated at about 2.1 kPa, 800°C, and residence times of a few minutes with steam-oxygen injection. About 95% carbon conversion is anticipated to produce a medium heat value (MHV) gas which is subjected to the shift reaction, scmbbing, and methanation to form SNG. The cold gas thermal efficiencies are estimated to be about 60%. 

There has been increased interest in firing wood waste as a supplement to coal in either pulverized coal (PC) or cyclone boilers at 1—5% of heat input. This appHcation has been demonstrated by such electric utilities as Santee-Cooper, Tennessee Valley Authority, Georgia Power, Dehnarva, and Northern States Power. Cofiring wood waste with coal in higher percentages, eg, 10—15% of heat input, in PC and cyclone boilers is being carefully considered by the Electric Power Research Institute (EPRI) and Tennessee Valley Authority (TVA). This practice may have the potential to maximize the thermal efficiency of waste fuel combustion. If this practice becomes widespread, it will offer another avenue for use of fuels from waste. 

Because of their very low boiling points, helium, neon, and hydrogen are noncondensable under the conditions at the top of the nitrogen column, and they concentrate in the nitrogen gas there. Because they cut down on the rate of condensation of nitrogen and thereby reduce the thermal efficiency of the process, they must be withdrawn. The noncondensable stream withdrawn may have a neon, helium, or hydrogen content that varies from 1 to 12%... 

Use of a low temperature shift converter in a PSA hydrogen plant is not needed it does, however, reduce the feed and fuel requirements for the same amount of hydrogen production. For large plants, the inclusion of a low temperature shift converter should be considered, as it increases the thermal efficiency by approximately 1% and reduces the unit cost of hydrogen production by approximately 0.70/1000 (20/1000 ft ) (140,141). 

The water—carbon slurry formed in the quench vessel is separated from the gas stream and flows to the carbon recovery system needed for environmental reasons and for better thermal efficiency. The recovered carbon is recycled to the reactor dispersed in the feedstock. If the fresh feed does not have too high an ash content, 100% of the carbon formed can be recycled to extinction. 

Fig. 5. NO formation in a hydrogen engine having spark at 17° before top-dead center (BTC) rpm, 2900 and compression ratio, 5.5 1, where A is nitric oxide B, backfire C, power and D, brake thermal efficiency, (a) Effect of equivalence ratio, ( ) and (b), effect of water induction at 0 = 0.625.

Operating parameters of this German plant, on the basis of one cubic meter of raw gas, iaclude 0.139 m O2, 0.9 kg briquettes, 1.15 kg steam, 1.10 kg feed water, 0.016 kWh, and 1.30 kg gas Hquor produced. Gasifier output is 1850 m /h and gas yield is 1465 m /t dry, ash-free coal. The coal briquettes have a 19% moisture content, 7.8% ash content (dry basis), and ash melting poiat of 1270°C. Thermal efficiency of the gas production process is about 60%, limited by the quaHty and ash melting characteristics of the coal. Overall efficiency from raw coal to finished products is less than 50%. 

Helping to propel capacities upward has been the advent of greatly improved preheaters, which partially calcine the stone and significantly improve thermal efficiency. Modem preheaters improve capacity by 15—20% and decrease fuel consumption a similar percentage. Other kiln appurtenances and accessories that enhance efficiency and lime quahty are the contact coolers, and such kiln internals as metal refractory trefoil systems that act as heat exchangers, dams, and lifters. 

Another thermally efficient kiln is the modem mixed-feed vertical kiln ia which coke is admixed with 8.5—20 cm lump limestone and charged into the top of the vertical kiln by a mobile, overhead charging system. However, use of this kiln is waning since the quaUty of mixed-feed kiln lime does not equal that of the other three kilns described above, owing to ash contamination from the coke and poorer reactivity and to the higher cost of coke in most areas. 

Miscellaneous Kilns. A U.S. kiln, the Fluo-Sohds, appears to be another vertical kiln type, but this is its only similarity. It operates on a different principle. It utilizes as kiln feed only a discrete granulation of 0.225—2.4 mm (65—8 mesh) sizes. DeHcately controlled by air and exhaust gas pressure, the kiln feed of granules is fluidized as a dense suspension. Because it is instmmented, this kiln can produce a very reactive lime at better than average thermal efficiency. The kiln, however, has limited utifity because the cost of obtaining the kiln feed with many hard, compact limestones is prohibitive. 

The Calcimatic is a patented kiln of Canadian origin that is radically different from other kiln types. It consists of a circular traveling hearth of variable speed, supported on two concentric tiers of rollers. Kiln feed of 12.7 mm is fed onto the hearth in a 2.5—10 cm bed from a preheater chamber. The kiln is usually fired with natural gas or fuel oil, although the option of using pulverized coal has also been developed. After great interest, resulting in sales of many units throughout the world, the popularity of the Calcimatic has ebbed because of disappointment in the unit s mediocre thermal efficiency. 

The key feature of the pressurized water reactor is that the reactor vessel is maintained above the saturation pressure for water and thus the coolant-moderator does not bod. At a vessel pressure of 15.5 MPa (2250 psia), high water temperatures averaging above 300°C can be achieved, leading to acceptable thermal efficiencies of approximately 0.33. 

Increasing fuel costs and sizes of industrial and utiUty installations have forced the emphasis in economical considerations to shift to high thermal efficiency, rehabiUty, and avadabihty. The investment, operating, maintenance, transmission, insurance, and other costs as well as depreciation must also be considered, but these are often less important. 

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