Fuel heat-exchanger applications

Atomic power production The necessity for avoiding contamination of operative liquids, together with other requirements which must be met in selecting constructional materials in this highly specialised field has resulted in the choice of austenitic steels for applications in heat exchangers, pressure vessels, pipelines and fuel processing. 

The results of the performance calculations for the fuel cell, Rankine cycle heat recovery system, summarized in Table 9-24, indicate that the efficiency of the overall system is increased from 57% for the fuel cell alone to 72% for the overall system. This Rankine cycle heat-fuel recovery arrangement is less complex but less efficient than the combined Brayton-Rankine cycle approach, and more complex and less efficient than the regenerative Brayton approach. It does, however, eliminate the requirement for a large, high temperature gas to gas heat exchanger. And in applications where cogeneration and the supply of heat is desired, it provides a source of steam. 

The critical technology development areas are advanced materials, manufacturing techniques, and other advancements that will lower costs, increase durability, and improve reliability and performance for all fuel cell systems and applications. These activities need to address not only core fuel cell stack issues but also balance of plant (BOP) subsystems such as fuel processors hydrogen production, delivery, and storage power electronics sensors and controls air handling equipment and heat exchangers. Research and development areas include ... 

Both gas turbine and process heat versions of the HTGR are based on the demonstrated high-temperature capability of the fuel and core structure. However, some development in the metallic components, such as the turbine, hot duels and intermediate heat exchanger is necessary, Present commercial alloys would have limited lifetime under service conditions at 1650°F (899°C) and above. However, currently envisioned advancements in ceramics and carbon-carbon composites indicate that high-temperature nonmctallic substitutes for metallic alloys will soon be available. These materials advances are the key to making future application of the IITGR a reality. 

The simplest case of a ROM would be lumped models or zero-dimensional models where the fuel cell is modeled as a single set of control volumes one for each component, e.g. air gas channel, fuel gas channel, PEN, interconnect, etc. (see for example Elizalde-Blancas et al., 2007a). This hides most of the details of what occurs inside the fuel cell but allows for fast simulation times. Lumped models are appropriate for use in system modeling applications where the fuel cell interacts with other devices such as heat exchangers, combustors, turbines, etc. This kind... 

Such heat exchangers have been developed for cryogenic and low-temperature applications (31) and for fuels cells (32). They are suitable for a large range of operating conditions, but there is very little information on their thermal and hydraulic behavior. Furthermore, as the heat is transferred by conduction in the plate, the temperature distribution is not homogeneous. 

Integrated reactors One type of integrated reactor is micro structured heat exchanger/reactor concepts, which may work as cross- or counter-flow reactors. Another type couples endothermic and exothermic reactions in two separate flow paths normally operated in the co-current mode. Both reactor types are designed as prototype components of future fuel processors for mobile applications. 

Micro structured heat exchanger reactors offer unique capabilities of improving fuel processor performance as described in the sections above. However, manufacturing costs are currently considerably higher than for conventional mature technology. This will be overcome in the future with the application of cheap manufacturing and coating techniques suitable for mass production (see Sections 2.9 and 2.10), a procedure which is currently underway. 

A fuel gas containing 95 mole% methane and the balance ethane is burned completely with 25% excess air. The stack gas leaves the furnace at 900 C and is cooled to 450°C in a waste heat boiler, a heat exchanger in which heat lost by cooling gases is used to produce steam from liquid water for heating, power generation, or process applications. 

In the following, relatively simple geometries will be considered. No attempt will be made to deal with complex situations such as multitubular heat exchangers, multirod nuclear fuel elements, and so on. Though such topics are important, the space available in this chapter does not permit them to be covered. Process applications of boiling are dealt with by Hewitt et al. [13] and nuclear applications by Tong and Tang [5],... 

Develop highly effective reactors, fuel and water vaporizers, recuperative heat exchangers, and condensers broadly applicable to fuel processing and fuel cell systems. 

---