Types of Gasifiers
Entrained flow gasifier.
Entrained flow is the most aggressive form of gasification, with the pulverized coal and oxidizing gas flowing concurrently. High reaction intensity is provided by a high pressure (20-30 atm), high temperature (>1400°C) environment. Extremely turbulent flow sees the coal particles experience significant back-mixing, and residence times are measured in seconds.
Entrained Flow Gasification (EFG) is specifically designed for low reactivity coals and high coal throughput. Single pass carbon conversions are in the range of 95-99%.
To experience smooth operation, the gasifier temperature must lie above the coal AFT or fluxes, which lower the melting temperature of the coal mineral matter, must be used. A number of system constraints impose an economic limit on gasification temperature at 1400–1500°C.
Fixed Bed Gasifiers.
Fixed bed gasifier operates at 26 bar and closely resemble a blast furnace. Coal and fluxes are placed on the top of a descending bed in a refractory lined vessel. On moving downwards, the coal is gradually heated and contacted with an oxygen enriched gas flowing upwards counter currently. Pyrolysis, char gasification, combustion and ash melting occur sequentially.
The temperature at the top of the bed is typically 450°C, and at the bottom approximately 2000°C. All coal mineral matter melts and is tapped as an inert slag. Ash melt characteristics influence bed permeability, and fluxes may need to be added to modify slag flow characteristics.
Fixed bed gasifier off gas contains tars – which must be condensed and recycled. The production of tars makes downstream gas cleaning more complicated than with other IGCC processes.
The residence time of fixed bed gasifiers is between 30 minutes and one hour, which places stringent restrictions on the coal’s physical and chemical properties. Coal size distribution must be controlled to ensure good bed permeability. Excess fines can be injected with oxygen at the tuyeres or briquetted to maintain stable operation.
Long residence times mean that fixed bed gasifiers have a low throughput and hence have limited application in large scale IGCC plants.
To ensure stable fluid bed operation, gasification temperatures are kept below the AFT of the coal. Above this temperature, particles become sticky and excessive levels of agglomeration will occur, resulting in bed defluidization.
Fluidized bed gasifier
A fluidized bed reactor is a vessel in which fine solids are kept in suspension by a gas such that the whole bed exhibits a fluid like behavior. This type of reacting system is characterized by high heat and mass transfer rates (increased reaction rates) between the solid and gas.
In fluid bed gasification, rising oxygen enriched gas reacts with suspended coal at a temperature of 950–1100°C and pressure of 20-30 bar. High levels of back mixing result in fluid bed gasifiers having a uniform temperature distribution.
The unusual characteristic of fluid bed gasifiers is that the majority of the bed material is not coal but accumulated mineral matter and sorbent (for in-situ desulphurization). Operating with a high inventory of inert bed material has a number of advantages:
To reduce the size of the fines recycle stream, it has been proposed that the gasifier is linked with a fluid bed combustor (Air Blown Gasification Cycle). In this process, the coal is first gasified to 70-80% carbon conversion. The unreacted char is then fed to the combustor where generated heat is used for steam production. The gasifier-combustor combination enables the use of low reactivity, high Ash Fusion Temperature (AFT) coals in an IGCC system.
Entrained flow is the most aggressive form of gasification, with the pulverized coal and oxidizing gas flowing concurrently. High reaction intensity is provided by a high pressure (20-30 atm), high temperature (>1400°C) environment. Extremely turbulent flow sees the coal particles experience significant back-mixing, and residence times are measured in seconds.
Entrained Flow Gasification (EFG) is specifically designed for low reactivity coals and high coal throughput. Single pass carbon conversions are in the range of 95-99%.
To experience smooth operation, the gasifier temperature must lie above the coal AFT or fluxes, which lower the melting temperature of the coal mineral matter, must be used. A number of system constraints impose an economic limit on gasification temperature at 1400–1500°C.
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Extremely high temperatures shorten refractory life.
There is a limit to the heat that can be recovered from the raw fuel gas, without water quench becoming necessary.
A cost is incurred in providing oxygen and coal to sustain the gasifier temperature.
Fixed Bed Gasifiers.
Fixed bed gasifier operates at 26 bar and closely resemble a blast furnace. Coal and fluxes are placed on the top of a descending bed in a refractory lined vessel. On moving downwards, the coal is gradually heated and contacted with an oxygen enriched gas flowing upwards counter currently. Pyrolysis, char gasification, combustion and ash melting occur sequentially.
The temperature at the top of the bed is typically 450°C, and at the bottom approximately 2000°C. All coal mineral matter melts and is tapped as an inert slag. Ash melt characteristics influence bed permeability, and fluxes may need to be added to modify slag flow characteristics.
Fixed bed gasifier off gas contains tars – which must be condensed and recycled. The production of tars makes downstream gas cleaning more complicated than with other IGCC processes.
The residence time of fixed bed gasifiers is between 30 minutes and one hour, which places stringent restrictions on the coal’s physical and chemical properties. Coal size distribution must be controlled to ensure good bed permeability. Excess fines can be injected with oxygen at the tuyeres or briquetted to maintain stable operation.
Long residence times mean that fixed bed gasifiers have a low throughput and hence have limited application in large scale IGCC plants.
To ensure stable fluid bed operation, gasification temperatures are kept below the AFT of the coal. Above this temperature, particles become sticky and excessive levels of agglomeration will occur, resulting in bed defluidization.
Fluidized bed gasifier
A fluidized bed reactor is a vessel in which fine solids are kept in suspension by a gas such that the whole bed exhibits a fluid like behavior. This type of reacting system is characterized by high heat and mass transfer rates (increased reaction rates) between the solid and gas.
In fluid bed gasification, rising oxygen enriched gas reacts with suspended coal at a temperature of 950–1100°C and pressure of 20-30 bar. High levels of back mixing result in fluid bed gasifiers having a uniform temperature distribution.
The unusual characteristic of fluid bed gasifiers is that the majority of the bed material is not coal but accumulated mineral matter and sorbent (for in-situ desulphurization). Operating with a high inventory of inert bed material has a number of advantages:
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The coal experiences high heat transfer rates on entry.
The gasifier can operate at variable load (high turndown flexibility).
To reduce the size of the fines recycle stream, it has been proposed that the gasifier is linked with a fluid bed combustor (Air Blown Gasification Cycle). In this process, the coal is first gasified to 70-80% carbon conversion. The unreacted char is then fed to the combustor where generated heat is used for steam production. The gasifier-combustor combination enables the use of low reactivity, high Ash Fusion Temperature (AFT) coals in an IGCC system.