CO2 is vented to the atmosphere today, but there are options for capturing it for subsequent sequestration.
Worldwide production of hydrogen is about 41 million tons per year (ORNL, 2003). Since over 80 percent of this production is accomplished by steam methane reforming (SMR), this method is discussed first.
Steam methane reforming involves four basic steps (see Figure G-1). Natural gas is first catalytically treated with hydrogen to remove sulfur compounds. It is then reformed by mixing it with steam and passing it over a nickel-on-alumina catalyst, making CO and hydrogen. This step is followed by catalytic water-gas shift to convert the CO to hydrogen and CO2. Finally, the hydrogen gas is purified with pressure swing adsorption (PSA). The reject stream from PSA forms a portion of the fuel that is burned in the reformer to supply the needed heat energy. Therefore, CO2 contained in the PSA reject gas is currently vented with the flue gas. If the CO2 were to be sequestered, a separations process would be added to capture it.
The reforming reactions are as follows:
CH4 + H2O → CO + 3H2
CO + H2O → CO2 + H2 (water-gas-shift reaction)
Overall: CH4 + 2 H2O → CO2 + 4H2
The reaction of natural gas with steam to form CO and H2 requires a large amount of heat (206 kJ/mol methane). In current commercial practice, this heat is added using fired furnaces containing tubular reactors filled with catalyst.
Partial oxidation (POX) of natural gas with oxygen is carried out in a high-pressure, refractory-lined reactor. The ratio of oxygen to carbon is carefully controlled to maximize the yield of CO and H2 while maintaining an acceptable level of CO2 and residual methane and minimizing the formation of soot. Downstream equipment is provided to remove the large amount of heat generated by the oxidation reaction, shift the CO to H2, remove CO2, which could be sequestered, and purify the hydrogen product. Of course, this process requires a source of oxygen, which is usually provided by including an air separation plant. Alternatively, air can be used instead of oxygen and product hydrogen recovered from nitrogen and other gases using palladium diffusion. POX can also be carried out in the presence of an oxidation catalyst, and in this case is called catalytic partial oxidation.
As already indicated, SMR is highly endothermic, and tubular reactors are used commercially to achieve the heat input required. When oxygen and steam are used in the conversion and are combined with SMR in autothemal reforming (ATR), the heat input required can be achieved by the partial combustion of methane. The reformer consists of a ceramic-lined reactor with a combustion zone and a subsequent fixed-bed catalytic SMR zone. Heat generated in the combustion zone is directly transferred to the catalytic zone by the flowing reaction gas mixture, thus providing the heat needed for the endothermic reforming reaction. As will be discussed, ATR is used today primarily for very large conversion units. There are several other design concepts that combine direct oxygen injection and catalytic conversion, including secondary reforming.
It has been suggested that methane conversion to hydrogen and elemental carbon might also be an attractive route, but the committee believes that this is unlikely. Such an approach would generate a large amount of carbon by-product,2 and less than 60 percent of the combined heats of combustion of the hydrogen and carbon products is associated with the hydrogen. For this approach to become a viable alternative, uses for large amounts of carbon must be found.
Steam methane reforming is widely used worldwide to generate both synthesis gas and hydrogen. The gas produced is