Two-Stage Coal Gasification for High Ash Fusion Temperatures
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Solution Overview
Problem
Existing coal gasification technologies are inefficient in processing high ash, high ash fusion temperature bituminous coals, leading to low carbon conversion and the generation of undesirable components like tar, due to limitations in operating temperature and reactivity, resulting in high energy penalties and operational challenges.
Innovation Solution
A two-stage gasification process involving a primary circulating fluidized bed transport gasifier operating at 900°C to 1100°C followed by a high temperature partial oxidation step in a fluidized bed, with an internally circulating fluidized bed syngas cooler to achieve over 98% carbon conversion and minimize tar and clinker formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high ash fusion temperature bituminous coals are gasified in conventional fluidized bed gasifiers, then the process operates at lower temperatures avoiding clinker formation, but carbon conversion is limited to below 90% and tar is generated
Solution Approach 1:
The gasification process is divided into two distinct stages: a first fluidized bed gasifier operating at lower temperature (850-1150°C) to avoid clinker formation, and a second entrained flow gasifier operating at higher temperature (1200-1500°C) to achieve complete carbon conversion. This segmentation allows each stage to operate within its optimal temperature range, resolving the contradiction between avoiding clinker and achieving high carbon conversion.
Solution Approach 2:
The first fluidized bed gasifier performs preliminary gasification of the coal, converting a portion of the carbon to syngas and reducing the coal's reactivity issues before the material enters the second gasifier. This preliminary action prepares the coal for more efficient processing in the second stage, enabling higher overall carbon conversion while maintaining reliability in the first stage.
2Productivity
If high ash bituminous coals are gasified in slagging entrained flow gasifiers at higher temperatures to improve carbon conversion, then carbon conversion improves, but the energy penalty to melt large amounts of ash with added fluxing agents makes the process economically unviable
Solution Approach 1:
The two-stage system segments the gasification process so that the first fluidized bed gasifier handles the bulk of the coal at lower temperature, converting approximately 60-80% of carbon without requiring fluxing agents. The second entrained flow gasifier then completes the conversion of remaining carbon with much lower ash content, significantly reducing the energy penalty for ash melting compared to conventional single-stage slagging gasifiers.
Solution Approach 2:
The system changes the operating parameters between stages: the first gasifier operates at lower temperature (850-1150°C) without fluxing agents, while the second gasifier operates at higher temperature (1200-1500°C) but with reduced ash load. This parameter change optimizes energy efficiency by minimizing the total amount of ash that requires high-temperature melting.
3Loss of energy
If high ash bituminous coals are gasified in conventional fluidized bed gasifiers, then the process avoids high energy penalties, but the low reactivity of the coals leads to lower carbon conversion and generation of undesirable tar components
Solution Approach 1:
The two-stage configuration segments the gasification process to address tar generation: the first fluidized bed gasifier operates at controlled temperature to minimize tar formation during initial gasification, while the second entrained flow gasifier operates at higher temperature to thermally crack and decompose any remaining tar components in the syngas, converting them to useful combustible gases.
Solution Approach 2:
The system employs parameter changes across stages to control tar: the first gasifier uses lower temperature (850-1150°C) to reduce thermal cracking that produces tar, while the second gasifier uses higher temperature (1200-1500°C) to decompose formed tar into smaller hydrocarbon molecules and hydrogen, thereby reducing harmful tar content in the final syngas.
4Ease of operation
If bituminous coals with high caking tendency are processed in moving bed gasifiers, then the gasifier can handle the coal feed, but carbon conversion is even lower than in fluidized bed gasifiers and large amounts of tar and phenol water are generated requiring expensive treatment
Solution Approach 1:
The system segments the gasification process with the first fluidized bed gasifier handling the caking coal feed at controlled temperature and residence time, preventing excessive caking and agglomeration. The second entrained flow gasifier then processes the partially gasified material at higher temperature with rapid heating rates that minimize caking while completing carbon conversion and reducing tar and phenol water through thermal decomposition.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process achieves high carbon conversion efficiency while reducing tar and clinker formation, extending equipment life and producing nearly dust-free syngas suitable for chemical synthesis or power generation.
Implementation Method 1
a gasifier combining bituminous coal and an oxidant to produce syngas, the syngas containing char carbon particles
Implementation Method 2
a partial oxidizer that receives the syngas and converts at least a portion of the unwanted species into syngas
Implementation Method 3
a syngas cooler to cool the syngas from the partial oxidizer
Implementation Method 4
an unwanted species removal system that removes at least a portion of the char carbon particles from the syngas from the syngas cooler
Data Source
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AI summary
This invention relates to gasification of high ash bituminous coals that have high ash fusion temperatures. The ash content can be in 15 to 45 weight percent range and ash fusion temperatures can be in 1150°C to 1500°C range as well as in excess of 1500°C. In a preferred embodiment, such coals are dealt with a two stage gasification process - a relatively low temperature primary gasification step in a circulating fluidized bed transport gasifier followed by a high temperature partial oxidation step of residual char carbon and small quantities of tar. The system to process such coals further includes an internally circulating fluidized bed to effectively cool the high temperature syngas with the aid of an inert media and without the syngas contacting the heat transfer surfaces.