Hydrogen
A typical oil gasifier may have a pressure of 60 to 80 bar and a hydrogen content of, say, 30% (wet basis). In other words, a hydrogen partial pressure of 20 bar or more may prevail. The equivalent figures for a coal gasifier will probably be lower. For partial oxidation of natural gas there may be over 50% hydrogen, although for most applications the absolute pressures may be somewhat lower than for oil. The shell of a refractory-lined gasifier will have a normal operating temperature of around 250°C, but it will probably have a design temperature of 350°C to allow for a small amount of bypass gas streams through cracks in the refractory, or in the case of coal gasifiers, simple refractory wear. Materials in the equipment immediately downstream the gasifier will be subjected to similar conditions, which are well into the range where hydrogen embrittlement can occur. Attention must be paid to the selection of hydrogen tolerant alloys, which typically may be chrome-molybdenum steels or higher alloys, depending on actual hydrogen partial pressure and temperature. The best guide to material selection for this service is API 941 (API 1990), which contains the well-known Nelson curves.
6.11.1 Chlorides
In the reducing environment of a gasifier, a significant portion of any chlorine in the feed is converted to HC1. The most important corrosion aspect to consider is that at temperatures below the water dewpoint the condensate can cause pitting or stress corrosion cracking of the commoner austenitic steels. Good plant design can take care of exchangers and piping containing process condensate. It is an important matter for operations and maintenance procedures to avoid cold bridges through the insulation of warmer equipment (e. g., at pressure gauges), which can cause local condensation. Another point where operational care is required is during shutdown, when it would be necessary to avoid condensate formation.