Ultimate Analysis
For the ultimate analysis the percentages of carbon, hydrogen, oxygen, sulfur, and nitrogen are determined. In the past, oxygen was sometimes reported as “by difference.” If at all possible this should not be accepted, as it makes it impossible to have any control over the quality of the analysis. Proper balances are the basis for a good process design and a good operation of plants, but a good balance is equally dependant on a good elemental analysis. (Carbon and hydrogen ASTM D3178, BS1016 part 106: Nitrogen ASTM D3179, BS1016: Sulfur ASTM D3177, BS 1016 part 106)
Table 4-4 lists proximate and ultimate analyses of various types of coal.
The relevance of sulfur in the coal for gasification is the same as for oil-derived heavy residual feedstocks, which generally contain more sulfur than most coals, and which are described in Section 4.2. Sulfur contents in coal range from 0.5-6 wt%. It
may be present in coal in three forms: pyritic sulfur, inorganic sulphates, and as sulfur in organic compounds. These three forms of sulfur can be determined using ASTM D 2492. In coals with a high sulfur content, most of the sulfur is generally present in the form of pyrite. Note that the quantity of pyritic sulfur is an indicator for the potential abrasiveness of the coal.
The nitrogen content in coals ranges from 0.5-2.5 wt%. Only part of the nitrogen in the coal is converted into ammonia and HCN upon gasification, whereas the remainder is converted into elemental nitrogen. The presence of the coal-derived nitrogen in the product gas is one reason why it is not always essential to gasify coal with very pure oxygen (>99 mol%), even when the gas is used for the production of syngas or hydrogen. The percentage of the nitrogen in the coal that is converted into elemental nitrogen upon gasification will depend on the type of nitrogen compounds in the coal.
