There are three broad gasification types: entrained flow, fluidised bed and moving bed (sometimes called, somewhat erroneously, fixed bed). It should be noted that these are types of gasification processes and are different from types of gasifier equipment. Gasifier equipment is generally classified as upward draft, downward draft and cross draft gasifiers, based on the direction of air/oxygen flow in the equipment. When selecting a specific gasification type for syn gas generation, different factors such as fuel type (coal or biomass etc.), plant size, reactivity of fuel and composition of air-oxygen oxidant mixture has to be taken into consideration. Some general guidelines for selection of a gasification process are available.
Entrained bed gasification
In entrained bed type gasification process, feed and oxygen enter in co-current flow. The feed is grounded to a size of 100 µm or less to promote mass transfer and allow transport of solids using gas flow. The key characteristics of entrained flow gasification is very high and uniform temperatures (usually more than1000°C) and the very short residence time of the fuel within the gasifier. Because of the high temperatures, very high conversions are possible. The high temperature operation requires high oxygen demand for these types of processes. Schematic of the operation of an entrained flow gasifier and variation of the temperature of coal and gas along the gasifier are shown in figure-1. In entrained flow gasification, ash is withdrawn in the molten form. Solids fed into the gasifier must be very finely ground and homogeneous, which in turn means that entrained flow gasifiers are not suitable for feed stocks such as biomass or wastes, which cannot be readily pulverised.
Figure 1 - Entrained flow gasifier and variation of the temperature of coal and gas along passage through gasifier
Fluidized bed gasification
Schematic of a fluidized bed gasifier and variation of the temperatures of coal and gas along the gasifier is shown in the figure-2 . Fluidized bed gasification offers good mixing of coal and air/oxygen and steam mixture, which promotes both heat and mass transfer. This ensures an even distribution of material in the bed and hence, a certain amount of partially reacted fuel is inevitably removed with the ash. This places limitation on overall carbon conversion in fluidized bed processes. Fluidized bed gasifiers generally operate below the ash softening temperature, because ash slagging can disturb the fluidization of the bed. Size of the particles is critical; material that is too fine will tend to entrained in the syn gas and leaves the bed over head. This is partially captured in cyclones and returned to bed. This type of gasifier is suitable for reactive feed stocks such as low rank coals and biomass.
Figure 2 - Fuidized bed gasifier and variation of the temperature of coal and gas along the gasifier
Companies Working in the field of fluidized bed Gasifier:
• KBR Transport
• Green Point Energy
Moving bed gasification
Schematic of a moving bed gasifier with various processes at different levels and variation of the coal and gas temperatures along the gasifier is shown in figure-3. Coal which enters at top and moves downward through gravity is gasified by the upcoming countercurrent air/oxygen and steam mixture. In this gasifier, the hot synthesis gas from gasification zone is used to preheat and pyrolyze the downward flowing coal. With this process the oxygen consumption is very low, but pyrolysis products are present in the product syn gas. This gasifier operates on lump coal and outlet temperature of syn gas is generally low.
Figure 3 - Moving bed gasifier and variation of the temperature of coal and gas along the gasifier
Companies working in the field of Moving Bed Gasifier:
• Lurgi Dry Ash
• British Gas Corporation