Comparative thermal efficiency

When the value of AS is positive, the potential of the cell is equivalent to less energy units than are obtained by direct combustion as heat. We can define a factor to compare the amounts of energy produced by these two processes. The comparative thermal efficiency of a cell reaction is... 

Synthetic fuels derived from shale or coal will have to supplement domestic suppHes from petroleum someday, and aircraft gas turbine fuels producible from these sources have been assessed. Shale-derived fuels can meet current specifications if steps are taken to reduce the nitrogen levels. However, extracting kerogen from shale rock and denitrogenating the jet fuel are energy-intensive steps compared with petroleum refining it has been estimated that shale jet fuel could be produced at about 70% thermal efficiency compared with 95% efficiency for petroleum (25). Such a difference represents much higher cost for a shale product. 

The thermal efficiency of steam-tube units will range from 70 to 90 percent, if a well-insiilated cylinder is assumed. This does not allow For boiler efficiency, however, and is therefore not direc tly comparable with direct-heat units such as the direct-heat rotaiy diyer or indirect-heat calciner. 

Employing wood chips, Cowan s drying studies indicated that the volumetric heat-transfer coefficient obtainable in a spouted bed is at least twice that in a direct-heat rotaiy diyer. By using 20- to 30-mesh Ottawa sand, fluidized and spouted beds were compared. The volumetric coefficients in the fluid bed were 4 times those obtained in a spouted bed. Mathur dried wheat continuously in a 12-in-diameter spouted bed, followed by a 9-in-diameter spouted-bed cooler. A diy-ing rate of roughly 100 Ib/h of water was obtained by using 450 K inlet air. Six hundred pounds per hour of wheat was reduced from 16 to 26 percent to 4 percent moisture. Evaporation occurred also in the cooler by using sensible heat present in the wheat. The maximum diy-ing-bed temperature was 118°F, and the overall thermal efficiency of the system was roughly 65 percent. Some aspec ts of the spouted-bed technique are covered by patent (U.S. Patent 2,786,280). 

The thermal efficiency of the process (QE) should be compared with a thermodynamically ideal Carnot cycle, which can be done by comparing the respective indicator diagrams. These show the variation of temperamre, volume and pressure in the combustion chamber during the operating cycle. In the Carnot cycle one mole of gas is subjected to alternate isothermal and adiabatic compression or expansion at two temperatures. By die first law of thermodynamics the isothermal work done on (compression) or by the gas (expansion) is accompanied by the absorption or evolution of heat (Figure 2.2). 

This cycle produces an increase of 30% in work output, but the overall efficiency is slightly decreased as seen in Figure 2-15. An intercooling regenerative cycle can increase the power output and the thermal efficiency. This combination provides an increase in efficiency of about 12% and an increase in power output of about 30%, as indicated in Figure 2-16. Maximum efficiency, however, occurs at lower pressure ratios, as compared with the simple or reheat cycles. 

Fig. 5.4 shows a carpet plot of overall efficiency against specific work for the cooled [CBTJici plant (single step) with pre.ssure ratio and combustion temperature as parameters. As shown earlier, by the preliminary air standard analysis and the subsequent calculations in Chapter 4, there are relatively minor changes of thermal efficiency compared with the uncooled plant [CBT]iuc, but there is a major effect in the reduction of specific work. 

However, the turbine work has been increased because of the extra water vapour flow through the turbine, while the compressor work is unchanged. Thus Eq. (6.17), which is still valid, with turbine work equal to the heat supplied, shows that the thermal efficiency increases compared with the dry cycle. It is important to realise that this efficiency is increased not because of a reduction in the heat rejected (Qa) but because of the increa.se in Wi- The heat rejected is still equal to the compressor work. 

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. 

Macchi et al. did not undertake parametric studies of the CHAT cycle and there appears to be no comparably thorough examination of this cycle in the literature but Nakhamkin describes a prototype plant giving a thermal efficiency of some 55% at a very high pressure ratio, i.e. about 70, compared with the dry CICBTBTX cycle optimum of about 40 shown in Fig. 6.17. 

In another example Newby et al. [6] calculated a cycle with the reformer operating at comparable pressure and temperature but with a higher recycling rate of 1.7, leading to a conversion rate of a = 0.56 (this is closer to the conversion rate of Lloyd s steam/TCR cycle, a = 0.373, described in the last section). A thermal efficiency of 38.7% is claimed for this FG/TCR cycle, slightly greater than the simple CBT cycle efficiency of 35.7% but much less than the calculated efficiency for the steam/TCR cycle (48.7%) and a comparable STIG cycle (45.6%). 

A diesel-electric locomotive uses as its prime mover a large, self-igniting, internal combustion engine of the type invented by RudolfDiesel and first successfully demonstrated in 1897. Thermal efficiency of these engines exceeded 30 percent, compared... 

The use of plastics as an energy source was demonstrated on a commercial scale at ICl Materials plastics manufacturing site in Dumfries, UK. This paper covers the preparation and use of pre- and post-consumer plastics as supplementary fuels in a circulating fluidised bed boiler specially designed for co-combustion with coal. Full emissions data on the 15% mixtures of individual plastics with coal are given, together with calculations of thermal efficiencies. Measurements by an independent body (British Coal Research Establishment) confirmed that the co-combustion of coal and plastic reduces some emissions compared with coal alone. Thermal efficiencies of around 80% were achieved and this heat was used effectively during the production of plastics. 7 refs. 

Thermal drying can be used to remove moisture from solids into a gas stream (usually air) by heat. Many types of dryers are available and can be compared on the basis of their thermal efficiency. 

In this study we show that the Pd/C catalyzed Suzuki-Miyaura coupling reaction can be performed in a microwave oven. Overall the microwave synthesis is faster than comparable thermal methods and the combination of the ease of use of the microwave oven and the facile work-up with Pd/C makes this a very efficient method for performing coupling reactions. 

There are several features of IGCC power systems that contribute to their improved thermal efficiency and environmental superiority compared to a conventional pulveriied-coal fired power plant. First, the... 

The processor was operated at atmospheric pressure and at 117—130 °C or 200 °C. A methanol-water mixture (1 1.5 molar ratio) was fed at 0.1 cm /h using a syringe pump. The reactors loaded with powder and pellets had comparable results, but the researchers preferred the powder packed bed form for its smaller volume and mass. The best hydrogen production was obtained at low temperatures, providing, on a dry gas basis, 70% hydrogen, 0.5% carbon monoxide, and residual carbon dioxide. Methanol conversion or thermal efficiency was not reported. 

A Diesel cycle has a compression ratio of 18. Air-intake conditions (prior to compression) are 72°F and 14.7 psia, and the highest temperature in the cycle is limited to 2500° F to avoid damaging the engine block. Calculate (a) thermal efficiency, (b) net work, and (c) mean effective pressure (d) compare engine efficiency with that of a Carnot cycle engine operating between the same temperatures. 

Supply side efficiency measures. Here we mean primarily increasing the efficiency of electricity production. Natural gas combined cycle power plants (NGCC) emit less C02 than single-cycle coal-fired power plants first, because natural gas emits about one-half the amount of C02 per fuel heating value than coal, and secondly, because the thermal efficiency of combined-cycle power plants is in the 45-50% range compared with the 35-38% range of single-cycle plants. In the... 

In a later section, the characteristics and performances of the most widely used equipment will be described in some detail. Many types are shown in Figure 9.4. Here some comparisons are made. Evaporation rates and thermal efficiencies are compared in Table 9.2, while similar and other data appear in Table 9.3. The wide spreads of these numbers reflect the diversity of individual designs of the same general kind of equipment, differences in moisture contents, and differences in drying properties of various materials. 

Fig. 13. Thermal efficiency of fossil-fuel operated luel-cell power plants compares favorahly with cnnseniinnat means ni energy conversion The efficiency is reduced in small units mainly because of losses in Ihe fuel processing...

Small dry cooling systems are commonplace today all over the world for various light duties, such as localized air conditioning, while larger systems are used in arid areas, often for electricity generation. Dry cooling systems, by their definition, do not need water they are more flexible in their siting requirements than wet systems and are less expensive to maintain. However, they are not as thermally efficient, and therefore are more expensive to own and operate for a comparable heat load. 

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