Types of gasification process
Gasification is a process of converting carbonaceous fuel into gaseous product with a usable heating value. Carbonaceous fuel includes solids, liquids and gases such as coals, biomass, residual oils and natural gas. Similar to conventional solid-fuel boilers, gasifiers can be divided into three basic types - entrained flow, fluidised bed and moving bed (sometimes called, somewhat erroneously, fixed bed). Fluidised bed gasifiers are exactly analogous to fluidised bed combustors; entrained flow gasifiers are similar in concept to pf-firing; and moving bed gasifiers bear some resemblance to grate firing.
Selection of gasifier suited to the requirements depends on a number of different factors. Different types of gasification processes are compared in the following sections. It should be noted however, that these types are different from gasifier equipment types - which are classified based on flow direction of air/oxygen.
Entrained Bed Gasifiers
In these gasifiers, fuel and oxygen enter in co-current flow. The fuel to be gasified is ground to fine sized particles (around 100 micron), carried to the gasifier by gas flow and the fuel oxygen mixture is introduced from inlet at the top. These gasifiers typically operate at high temperatures (higher than 10000C) and have short residence time due to the high temperature to avoid combustion. For details refer to 'Types of Gasification Process'.
Fluidized Bed Gasifiers
In this gasifier fuel is fluidized by air/oxygen flow and hence the name. This gasifier offers good mixing of coal and air/oxygen and steam mixture, promoting heat and mass transfer. Typically overall carbon conversion achieved is limited in fluidized bed gasification. Syn gas is taken out from outlet at the top. For details of this process refer to 'Types of Gasification Processes'.
Moving Bed Gasifiers
In moving bed coal gasifiers, coal or other fuel is introduced from top inlet and air/oxygen/steam comes in from the bottom. Downward falling coal or fuel is gasified by counter current of gas and ash is taken out from bottom. Syn gas produced flows out from top. For details of the process refer to 'Types of gasification Processes'.
Comparison of gasifier types
|Entrained Flow||Fluidised Bed||Moving Bed|
|Fuel Types||Solid - Liquid||Solid||Solid|
|Fuel Residence time||1-10s||5-50s||15-30min|
|Gas outlet temperature||900-1400||700-900||400-500|
Selection of Gasifier Type
The technical issues that influence the choice of gasifier include the characteristics of the material to be gasified and the scale of the project.
Coal: All three main types of gasifier (i.e. entrained flow, fluidised bed and moving bed) can be used to gasify coal. Characteristics of the coal that can influence selection of gasifier, include ash content and melting point and coal reactivity. High ash content and viscositiy of coal are lowered using limestone. As fluidised bed gasifier doesn’t depend on ash melting so for low ash content coal entrained bed gasifier is used while for high ash content coal fluidsed bed gasifier is used.
Reactivity: Lower gasification temperature of fluidised bed gasifier makes them very attractive for reactive lignites but on the other hand, less reactive coal has very less conversion efficiency in fluidised bed gasifier.
Plant size: Entrained bed gasifier plant size can go upto 300 MW. In comparison, Moving bed and fluidised bed gasifier plants tend to be smaller and hence for a large capacity plant multiple gasifiers are required. In this way we can loose some economy of scale.
Oxidant : for the use of oxidant we have option of 'air' and 'oxygen'. In fluidised bed gasifier, both oxidants can be used while mostly oxygen is used in entrained and moving bed gasifier. The use of Air helps us to get rid of ASU (Air Separation Unit) which is a very expensive item in a plant but as a flipside, Nitrogen present in the air makes downstream processing equipment larger.