5.2.5 Gasifiers and Impact of Coal Rank and Coal Properties

The available commercial gasification technologies are often optimized for a particular rank of coal or coal properties, and in some cases, certain ranks of coal might be unsuitable for utilization in a given gasification technology. On the other hand, there is considerable flexibility in most of the common gasifiers; this is highlighted by the following table, which provides an overview of the level of experience for the various commercially available gasifiers by manufacturer for each coal type. This experience will only continue to expand as more gasification facilities come online and more demonstrations are completed.

SOLID FUEL GASIFICATION EXPERIENCE¹

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.¹

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.¹

The reactivity of coal generally decreases with a rise in rank or type, i.e. going from sub-bituminous 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: in this respect ash content of coal has a major impact on gasifier type and configuration.

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 bed and entrained flow gasifiers have a lower tolerance for moisture, limiting the moisture content to 5 to 10 percent for a similar coal feedstock.² Oxygen supplied to the gasifiers must be increased with an increase in ash or moisture content in the coal.¹

Gasifier

• Gasification Introduction
• Commercial Gasifiers
  • Fixed (Moving) Bed Gasifiers
  • Entrained Flow Gasifiers
  • Fluidized Bed Gasifiers
  • Gasifiers & Gasification Technology for Special Apps & Alternative Feedstocks
  • Gasifiers and Impact of Coal Rank and Coal Properties
• R&D for Gasifier Optimization/Plant Supporting Systems
• DOE Gasification Systems Program and Benefits