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Show Unmixed Combustion for Efficient Heat and Mass Transfer in Chemical Processing Systems American Flame Research Committee 1997 Fall International Symposium destroyed to better than four nines (99.99 percent destruction). Application of Unmixed Combustion to Steam Reforming The majority of commercial hydrogen used in the world is produced by the steam reforming of fossil fuels-principally natural gas. Using methane as an example, the stoichiometric equation for this process (including the "shift" reaction) is as follows: CH4 + 2H20 = C02 + 4H2 AH™ = 179 kJ/mol 1) This reaction is endothermic and requires 179 kJ for each four moles of hydrogen produced. In conventional methane steam reforming this energy will be supplied by the combustion of additional methane in air. The energy is released through the reaction: CH4 + 2 02 = C02 + 2 H20 AHrxn = - 779 kJ/mol 2) Based on the heats of reaction for these two processes we see that one mole of methane must be burned in order to convert 4.35 moles of methane into 17.4 moles of hydrogen; or more succinctly, the maximum amount of hydrogen that can be produced for each mole of methane consumed is 3.25 moles. This sets one lower bound on the cost of hydrogen: i.e. the cost of the natural gas from which it is produced. The process shown in reaction 1 is typically carried out at temperatures between 750 - 850 °C. There are two reasons for this. Firstly, the rate of the reaction shown is very slow, and even at these high temperatures a catalyst-usually based on metallic nickel deposited on a high surface area porous ceramic support-is required to ensure an adequate reaction rate. Secondly, at lower temperatures chemical equilibrium favors methane over hydrogen, and at higher temperatures carbon monoxide is favored over carbon dioxide. Even at the optimal temperature both methane and carbon monoxide will be present in appreciable amounts. In order to heat the reactants in reaction 1 up to the required temperature, they are passed through a long tube packed with catalyst, and additional fuel is burned (reaction 2) to heat up the tube from the outside. The transfer of heat from the combustion products to the reactants is an inherently inefficient process, and in any practical system it is not possible to transfer all of the chemical energy released by combustion into the process being heated. In order to maximize the conversion of methane into hydrogen, the reforming process is usually performed in two or three steps, rather than the single step suggested by reaction 1. In the first step, the reactants are over heated to a temperature where virtually all of the methane is converted into reaction products. This results in significant amounts of carbon monoxide. At a somewhat lower temperature of 400 - 500 °C the carbon monoxide subsequently reacts with steam to produce hydrogen by a slower "shift" reaction using a different catalyst that inhibits the reformation of methane. This shift reaction may take place using either one reactor or two Fairmont Hotel Chicago, Illinois September 21 -241997 Page 7 |