Reformer convective heat exchange

Another type of steam-reforming reactor that is attracting increasing attention is known as gas heated reformers or heat exchange reformers. In such reformers, heat is transferred by convection and the heat source is a hot process gas from another reformer or a partial oxidation reactor. A number of different installations of heat exchange reformers can be envisaged. In Fig. 5, the installation of a heat exchange reformer either in series or in parallel to an auto-thermal reformer (ATR) is illustrated. 

The tubular constraint (the last equation) can be made less restrictive by convective heat exchange reformers. This may involve the use of... 

Two concepts of a He - He intermediate heat exchanger for a heat rating of 125 - 170 MW have been selected. For both, a 10 MW test plant has been operated in the KVK loop verifying the operation of reformers with convective helium. A 10 MW decay heat removal system cooler, hot gas ducts including insulation and liner, hot gas valves, and a steam generator were other components of the KVK loop. Furthermore, a helium purification system was operated in a bypass of the main system. Starting in 1982, the KVK facility was operated for 18,400 h with approx. 7000 h above 900 C [28]. Hot gas duct with internal insulation was operated at temperatures up to 950 °C. The KVK experimental loop has demonstrated reliability and availability even of newly developed components. 

Convective reformers result in less waste heat. The flue gas as well as the product gas is cooled by heat exchange with the process gas flowing through the catalyst beds, so that they leave the reformer at about 600°C. The amount of waste heat is reduced from 50% in the conventional design to about 20% of the fired duty in the heat exchange reformer. This means that the steam generated from the remaining waste heat just matches the steam needed for the process, so that export of steam can be eliminated. 

However, the heat exchange is primarily by convection which generally leads to lower average heat fluxes (and hence bigger heat transfer surfaces) than in tubular reformers with radiant heat transfer... 

In convective reformers, the flow in the heat exchange channel(s) may be co-current or counter-current to the flow in the catalyst bed. In the counter-current case, iteration is performed on the outlet temperature and pressure of the channel until balance at the outlet of the catalyst bed. 

The convective reformer is compact, but has larger heat transfer areas compared with a tubular reformer due to the smaller heat transfer coefficients in the cold end. A fair comparison should thus include the heat exchange areas in the waste heat section in the tubular reformer, but still the fired tubular reformer remains the most economical solution for large-scale reforming. The economy of scale is more advantageous for a... 

Figure 3.12 A convective reformer concept. Udengaard ef a/. [502]. A burner in the centre tube and flue gas are cooled by two concentric catalyst beds with internal heat exchange to cool the product gas.

The exothermic reactions for decomposition of carbon monoxide, (Reactions R7 and R8 in Table 5.2) means that for a given gas composition and pressure, there is a temperature below which there is a thermodynamic potential for carbon formation. Likewise for the endothermic decomposition of methane, (Reaction R6 in Table 5.2), there is a temperature above which there is a thermodynamic potential for carbon formation. These carbon limits assume that there is no reaction during cooling or heating the gas. This is the situation when no catalyst is present such as in heat exchangers, boilers and convective reformers. Carbon formation may lead to fouling of the equipment or to metal dusting corrosion [211],... 

At fuel manifold inlets, gaseous species concentrations are specified as equilibrium compositions of the town gas reformate at 650°C. Steam-to-carbon ratio is kept as 3.06 for this particular steady-state analysis. Both fuel and air gas manifold inlet conditions are summarized in Table 9.5. Mixed convective and radiative heat transfer boundary conditions are applied to the side surfaces of the stack to accurately model the heat exchange with the balance of plant components. Top and bottom surfaces, on the other hand, are assigned with... 

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