Staged Slurry Gasification System for Tar Prevention
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Solution Overview
Problem
The existing two-stage entrained gasification systems face issues with tar formation during the pyrolysis of carbonaceous materials, leading to deposition problems in downstream heat exchange surfaces and filters, as the tar is not adequately destroyed in the lower temperature second stage.
Innovation Solution
A system and process that involves partial combustion of dry solids and pyrolysis of a first slurry stream in two separate reactor sections, with a second slurry stream being fed to a drying unit downstream of the heat recovery zone to vaporize water and separate char from syngas, ensuring the syngas is tar and particulate-free by adjusting its temperature to prevent heavy molecular-weight tar emission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If slurry is fed to the lower temperature second stage gasifier, then water evaporation is achieved using heat from the first stage, but tar is not adequately destroyed and condenses to foul heat exchange surfaces
Solution Approach 1:
The gasification process is divided into two distinct stages: a high-temperature first stage for tar destruction and a lower-temperature second stage for slurry gasification. This segmentation allows each stage to operate at optimal temperatures for its specific function, resolving the contradiction between heat utilization and tar destruction.
Solution Approach 2:
The slurry is pre-dried in a drying section before entering the second stage gasifier. This preliminary action removes excess moisture, allowing the slurry to be gasified at lower temperatures without producing excessive tar, while still utilizing heat from the first stage effectively.
2Productivity
If the second stage operates at lower temperature for slurry pyrolysis, then energy efficiency is improved, but tar destruction is insufficient leading to downstream fouling
Solution Approach 1:
The system segments tar destruction functions into the first stage high-temperature zone, while the second stage focuses on slurry conversion. This functional segmentation ensures tar is destroyed upstream before the syngas enters downstream equipment, maintaining reliability while preserving gasification efficiency.
Solution Approach 2:
A drying section is introduced as an intermediary between slurry injection and the second stage gasifier. This intermediary component pre-processes the slurry by removing moisture, enabling efficient gasification at lower temperatures without generating excessive tar that would compromise downstream system reliability.
3Productivity
If char is recycled to the first stage gasifier, then conversion efficiency is maximized, but the system complexity increases
Solution Approach 1:
The system recycles unconverted char from the second stage back to the first stage gasifier. This recovery process maximizes carbon conversion efficiency by giving unreacted carbon another opportunity to gasify at the higher temperatures of the first stage, while the cyclic nature of the process integrates smoothly into the existing two-stage configuration.
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 effectively prevents tar deposition by ensuring the syngas exiting the system is tar and particulate-free, enhancing conversion efficiency and allowing for easier further processing, such as acid gas removal and sulfur recovery.
Implementation Method 1
pyrolysing therein with a first slurry stream comprising a slurry of particulate carbonaceous material in a liquid carrier thereby forming mixture products comprising a gaseous product stream comprising synthesis gas and char
Implementation Method 2
introducing a second slurry stream comprising particulate carbonaceous material in a liquid carrier and the mixture products exiting from the heat recovery zone into a drying unit wherein the water in the second slurry stream is vaporized
Implementation Method 3
passing the mixture products in step b through a heat recovery zone
Implementation Method 4
The char particles formed in the process are separated from the syngas by a cyclone
Data Source
AI summary
A system and process for gasifying carbonaceous feedstock with staged slurry addition in order to prevent the formation of tar that causes deposition problems. Dry solid carbonaceous material is partially combusted, then pyrolysed along with a first slurry stream comprising carbonaceous material in two separate reactor sections, thereby producing mixture products comprising synthesis gas. The second slurry stream comprising particulate carbonaceous material is fed to a drying unit downstream of a heat recovery zone along with the mixture product exiting the heat recovery zone. The resulting final temperature of the second stage mixture products and dried particulate carbonaceous material is between 450° and 550°F, a temperature range that is typically not conducive to the emission of heavy molecular-weight tar species.