Different Coal Ranks

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	This figure shows the locations and type for all of the coal deposits in the U.S.2

Coal Types
The four main types, or ranks, of coal are:

• Lignite – Sometimes called “brown coal,” lignite is the youngest of the coal types, and has the lowest energy content, containing between 25 and 35 percent carbon. Lignite usually has not been subjected to the extreme temperatures and pressures typical of the higher energy content coal types. It is crumbly, has a high moisture content, and is typically burned in power plants for electrical production. About 20 lignite mines produce about 7 percent of total US produced coal, mostly in Texas and North Dakota.
• Subbituminous – Subbituminous coal has higher energy content than lignite, containing between 35-45 percent carbon. The subbituminous coal found in the US is normally at least 100 million years old and represents 44 percent of total US coal production, with Wyoming being the largest producer.
• Bituminous – Bituminous coal contains anywhere from 45 to 86 percent carbon, giving it an even higher heating value than subbitiuminous. High heat and pressure and between 100 and 300 million years are required to produce this type of coal. About half of the coal produced in the US falls in this category with West Virginia, Kentucky and Pennsylvania being the largest producers. Bituminous coal is used for electrical production and plays a large role in the steel and iron industries.
• Anthracite – Anthracite contains 86-97 percent carbon but has a slightly lower heating value than does bituminous coal. It is rare in the U.S. (all known anthracite deposits are located in Pennsylvania) and only represents less than half of one percent of total U.S. coal production.¹

Different Coal Types & Gasification

SOLID FUEL GASIFICATION EXPERIENCE³

The above table provides an overview of the level of experience for the various commercially available gasifiers by manufacturer for each coal type. The table highlights the flexibility of the gasifiers currently available for commercial operations. This experience will only continue to expand as more gasification facilities come online and more demonstrations are completed.

It is important to note that coal properties can vary widely, even within each of the ranks. The knowledge gained from gasification operation to date has strongly suggested the coal blending be a cooperative process between the gasifier operator and the coal mining operation.

Environmental Impact
Feedstock coal rank usually has little effect on the environmental impact of a gasification system. Sulfur, for example, can vary from 0.5 to 8 percent in coal, and while different technologies may be needed to handle the entire range, the resulting emissions would be quite similar and more based on environmental regulations. Coal rank also has very little impact on slag quality.³

Design Considerations
Variations in coal quality can have an impact on the heating value of the syngas produced by the gasification process. However, a desired throughput can be selected and then the size and number of gasifiers can be determined with the specific range of coal types in mind.³

The reactivity of coal generally decreases with a rise in rank or type, i.e. going from subbituminous to bituminous. Also, the smaller the particle size, the more contact area between the coal and the reaction gases causing faster reaction. For medium and low rank coals, reactivity increases with an increase in pore volume and surface area, but for coals having a carbon content greater than 85%, these factors have no effect on reactivity. In fact, in high rank coals, pore sizes are so small that the reaction is diffusion controlled.

The volatile matter content of coal varies widely for the four main coal ranks and is low for high rank coals such as anthracite and higher for increasingly low rank coals. The higher the volatile matter content the more reactive a coal is which means it can be more readily converted to gas while producing less char. Thus for high rank coals, the utilization of char within the gasifier is much more of a concern. For low rank coals, char is not a major concern; however, the ease with which they are gasified leads to high levels of tar in the syngas which makes syngas cleanup more difficult.

The ash content of the coal does not have much impact on the composition of the produced syngas. Gasifiers may be designed to remove the ash in solid or liquid (slag) form. In fluidized or fixed bed gasifiers, the ash is normally removed as a solid, which limits operational temperatures in the gasifier to well below the ash melting point. In other designs, particularly slagging gasifiers, the operational temperatures are designed to be above the ash melting temperature. The selection of which design to employ often is dependent on the ash melting/softening temperature of the feedstock which is to be used at the facility.

High moisture content of the feedstock lowers internal gasifier temperatures through evaporation and the endothermic reaction of steam and char. This means a limit must be set on the moisture content of coal supplied to the gasifier, which can be met by coal drying operations if necessary. For a typical fixed bed gasifier and moderate rank and ash content of the coal, this limit is about 35 percent. Fluidized and entrained bed gasifiers have a lower tolerance for moisture, limiting the moisture content to 5 to 10 percent for a similar coal feedstock.⁴ Oxygen supplied to the gasifiers must be increased with an increase in ash or moisture content in the coal.³

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