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Show Unmixed Combustion for Efficient Heat and American Flame Research Committee Mass Transfer in Chemical Processing Systems 1997 Fall International Symposium without any indication of failure or performance deterioration. Data collected by EER while conducting this process at the pilot scale show product hydrogen concentrations around 75 percent (dry basis), which is comparable to large-scale industrial hydrogen manufacture. However, E E R has taken this process further and has improved it in a way that will lead to a significant reduction in the overall cost of hydrogen production at small scales. This was done by introducing a carbon dioxide acceptor (calcium oxide) into the packed bed of nickel catalyst. This acceptor material serves three functions in the process. First it provides additional "thermal mass" to transfer sensible heat from the oxidation step to the reforming step of the process. Secondly, the absorption of carbon dioxide by the calcium oxide produces calcium carbonate in an exothermic (-168 kJ/mol) chemical reaction and delivers energy to the reforming process in situ during reforming. This is a very important aspect of U M R . During the regeneration cycle, the heat released by oxidation of nickel is largely absorbed by the process of decomposing the calcium carbonate, which substantially reduces the temperature rise. The enthalpy of nickel oxidation is thereby stored as potential chemical energy in the calcium oxide, and that energy is subsequently released during the reforming process when calcium carbonate is produced. This is a far more efficient means of for transferring energy to the reforming process than relying solely on the sensible heat stored in the ceramic catalyst support matrix. Finally, by removing carbon dioxide from the process of Reaction 1, equilibrium is shifted toward greater hydrogen production and reduced carbon monoxide concentrations. In this embodiment of U M C , pilot plant tests have demonstrated product gas hydrogen concentrations typically about 85 percent and as high as 93 percent with less than 3 percent C O and C 0 2 , and the balance primarily methane. The process is illustrated schematically in Figure 8. This figure attempts to portray the progress of the reaction starting at the left of the figure and moving toward the right. O n the far left is depicted a packed bed reactor containing a mixture of a nickel catalyst on a ceramic support, and calcium oxide. A gas-phase mixture of methane and steam is introduced through the top of the reactor and reacts to produce carbon monoxide, hydrogen, and carbon dioxide. The endothermic reforming reactions lower the reactor temperature until the carbon dioxide begins to react with calcium oxide to form calcium carbonate. Gas phase products from this reaction exit through the bottom of the reactor as shown, while changes in the solid phase composition are indicated by moving to the right. Moving one step to the right on Figure 8 the solid phase now consists of the nickel catalyst and calcium carbonate. At this point air is introduced to the reactor. The oxygen in the air reacts with the nickel to produce nickel oxide. Consequently, the temperature rises to about 850 °C and the calcium carbonate decomposes. The gaseous effluent from the reactor at this step consist of nitrogen, carbon dioxide and perhaps some residual unreacted oxygen. Fairmont Hotel Chicago, Illinois September 21 -24 1997 Page 9 |
