Coal exergy flow

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. 

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. 

Another special situation arises when one resource has its origin in two different processes. For instance, when a process uses the electricity provided by an energy company, it is possible that this electricity is generated partly by burning coal and partly by burning natural gas. In this case, the process should be considered to use two different types of electricity electricity from coal and electricity from natural gas. Both types of electricity then have their own derived depletion time and, based on how much they contribute to the total amount of electricity supplied, their own exergy flow. 

Now, assume that there are other exergy flows into the system, which are required to make the process work. These amount to 410kj/mol of electricity, and comes from the combustion of coal (C is 410 kj/mol). How do the answers to a and b change What if the efficiency of electricity generation is 25% ... 

How the tools are organized into a methodology for process evaluation via exergy is illustrated in Reference 13 with a coal-fired boiler. It will be used to demonstrate the calculation of exergy flows, losses and consumptions. 

Figure 3. Exergy flow diagram for coal-fired boiler.

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. 

Thermodynamic analyses have been conducted for the various process steps in the H-Coal process system for producing synthetic fuels from bituminous coal. A Structured Process Energy-Exergy-flow Diagram (SPEED) for the H-Coal process is presented, which depicts the transformation of energy and exergy among the processes and the hierarchical structure of the process system with a compact format of the SPEED. 

Hemmes (2003) and coworkers Au (1999) and Peelen (2000) have examined the exergy flow in systems using a carbon fuel cell to oxidize coal only to CO at low current densities. The product gas is then used for its thermal value in combustion or shifted to H2. This approach takes advantage of the large entropy... 

Figure 6. Flow of exergy in the U.S. (All values reported are 1015 Btu [1.055(1018)J] (20).) Footnote a all values in ( ) are energy, other values reported are exergy. Footnote b the energy value for hydropower is reported in the usual manner of coal equivalent actual energy is 1.12 x 1015 Btu.

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