Exergy Flow Analysis

The coal exergy flow, calculated on an higher heating value (HHV) basis, is constant (525.7 MW) and clearly dominates the input stream. Most of the exergy is conserved in the gas phase, as can be seen from the respective values of gas chemical exergy flow, which reflect the cold gas efficiency to a certain extent. 

The oxygen consumption is the highest for slurry-feed GE gasification and the lowest for the fluid-bed systems. Dry-feed entrained-flow systems and the E-Gas gasifier achieve values between these extremes. The oxygen consumption level for South African (SAf) coal is elevated compared to Pittsburgh No. 8 (Pitt 8) coal. 

Exergy benefits Gas chemical Ihermomechanical l l Steam I l Exergy efforts Coal Oxygen I I Steam I I Auxiliaries I I 

All non-slurry systems require moderator steam to process Pitt 8 coal this applies to all new concepts. For SAf coal, mineral matter moderates the temperature to some extent. Consequently, only the fluid-bed systems require steam as a gasifying agent. Only for the standard Siemens, new Shell, and new GE-Q configurations is there any question as to whether the moderator steam demand can be covered by the internal steam production during gasification. 

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The analysis requires the calculation of three exergy flow rates, at 0°C, 50°C, and 100°C. As no heat or work is transferred between the considered system and its environment, the first law, Equation 6.1, yields that the overall enthalpy change is zero ... 

Consider lkg/s of coal that is combusted with an adequate amount of air (approximately zero exergy contribution). The rate at which exergy flows into the system is therefore 23,583 kW. The combustion releases heat, namely, at a rate of 21,860 kW at a temperature T. Since we have created a heat source at temperature T, it is straightforward to compute the work potential (exergy) of this heat source. All we need to do is multiply the heat release rate (21,860 kW) by the Carnot factor 1 - (T0/T). This means that if the combustion takes place at temperature T = 1200 K for a fluidized bed reactor (Table 9.1), the efficiency of the combustion alone is combustion = (21,860/23,583) [1 - (T0/T)] = 0.93 [1 - (T0/T)] = 0.93 [1 - (298.15/1200)] = 0.7 This means that already 30% of the maximum work has been lost We summarize this simplified analysis in Figure 9.15. 

In this paper, methods to estimate the heat of formation AHj0 and the absolute entropy S° for coal and coal-derived liquids are proposed based on the group contribution method. By applying these methods and the Structured Process Energy-Exergy-flow Diagram (SPEED, 11), an exergy analysis for the H-Coal process is performed. 

The purpose of an exergy analysis is generally to identify the location, the source, and the magnitude of true thermodynamic inefficiencies in power plants. Exergy flow... 

Exergy Analysis with a Flow Sheet Program... 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

As some intermediate thermodynamic data are missing or not well known (enthalpy of mixing of CuCl and CuCl2 in HC1/H20 mixtures for example) and some heat exchanges or separation are not linear (for instance, HC1/H20 mixture has an azeotrope which cannot be crossed), we decided to proceed to a global exergy analysis instead of a flow sheet analysis. 

Example 4.12 Exergy analysis of a power plant A steam power plant operates on a simple ideal Rankine cycle (see Figure 4.18). The turbine receives steam at 698.15 K and 4200 kPa, while the discharged steam is at 40 kPa. The mass flow rate of steam is 3.0 kg/s. In the boiler, heat is transferred into the steam from a source at 1500 K. In the condenser, heat is discharged to the surroundings at 298 K. Determine the energy dissipated at each state. 

Here, exm is the flow-exergy destruction, or irreversibility, and T0 the reference temperature. The system will be thermodynamically advantageous only if the Nx is less than unity. The exergy destruction number is widely used in second-law-based thermoeconomic analysis of thermal processes such as heat exchangers. 

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