Natural Gas
Compared with the variety of aspects needing evaluation when dealing with liquid feeds, natural gas is relatively simple, and the principle issues to be considered are more of an economic rather than technical nature. For the production of hydrogen-rich synthesis gas it is generally more economic to employ steam reforming rather than partial oxidation. Partial oxidation of natural gas is only likely to demonstrate favorable economics for hydrogen production where no purpose-built oxygen plant is required, or where the hydrogen is a by-product of carbon monoxide production. The advantage of partial oxidation—namely, that as a noncatalytic process, no large amounts of steam are required to prevent carbon laydown on the catalyst—only comes into its own when a СО-rich syngas is required. Further details are discussed in Section 7.1.4.
Since most applications for partial oxidation of natural gas aim at a СО-rich synthesis gas, quenching hot gas with water—advantageous if a CO shift is desired—is economically unattractive, and most such plants employ a syngas cooler.
There is no specific requirement on the hydrocarbon content of the natural gas. Clearly, heavier gases with high ethane or propane content will produce a synthesis gas richer in CO than pure methane. When looking at the still heavier components of natural gas, however, it is really only necessary that they be gaseous at the burner.
Nitrogen is a component in natural gas, which passes through the reactor largely as an inert. The amount of nitrogen (or argon) allowable in the feedstock is governed purely by the synthesis gas specification. This is different from the C02 case. C02 is a partner in the partial oxidation reactions and will increase the CO yield from the gas. This will be favorable in many instances, but must be reviewed on a case-by-case basis.
In contrast to catalytic processes such as steam reforming or autothermal reforming, partial oxidation is tolerant of sulfur. In fact there are good reasons to accept sulfur into the partial oxidation reactor. Firstly the synthesis gas has a high partial pressure of carbon monoxide so that in the absence of large quantities of steam there is considerable potential for metal dusting corrosion (Posthuma, Vlaswinkel, and Zuideveld 1997), more so than with the equivalent steam reformer (see Section 6.11 for details). The most effective form of protection against metal dusting is sulfur in the gas (Gommans and Huurdeman 1994).
The second advantage of leaving the sulfur in the gas is to prevent a spontaneous methanation reaction in the synthesis gas.
