Entrained Flow Gasifiers
|source: Electric Power Research Institute
In entrained-flow gasifiers, fine coal feed and the oxidant (air or oxygen) and/or steam are fed co-currently to the gasifier. This results in the oxidant and steam surrounding or entraining the coal particles as they flow through the gasifier in a dense cloud. Entrained-flow gasifiers operate at high temperature and pressure—and extremely turbulent flow—which causes rapid feed conversion and allows high throughput. The gasification reactions occur at a very high rate (typical residence time is on the order of few seconds), with high carbon conversion efficiencies (98-99.5%). The tar, oil, phenols, and other liquids produced from devolatization of coal inside the gasifier are decomposed into hydrogen (H2), carbon monoxide (CO) and small amounts of light hydrocarbon gases. Entrained-flow gasifiers have the ability to handle practically any coal feedstock and produce a clean, tar-free syngas. Given the high operating temperatures, gasifiers of this type melt the coal ash into vitreous inert slag.
The fine coal feed can be fed to the gasifier in either a dry or slurry form. The former uses a lock hopper system, while the latter relies on the use of high-pressure slurry pumps. The slurry feed is a simpler operation, but it introduces water into the reactor which needs to be evaporated. The result of this additional water is a product syngas with higher H2 to CO ratio, but with a lower gasifier thermal efficiency. The feed preparation system needs to be evaluated along with other process design alternatives for a particular application.
The high temperatures involved in this type of gasification tend to shorten the life of system components, including gasifier vessel refractory. Also, it may be necessary to add fluxes or blend feedstock parameters to achieve good slagging characteristics.
Entrained-flow gasifiers typically exhibit the following characteristics:
- Fuel flexibility; can accept a variety of solid feedstocks
- Large oxidant requirements
- Can either be oxygen or air blown, but most commercial plants are oxygen blown
- Uniform temperature within the reactor
- Slagging operation
- Short reactor residence time
- High carbon conversion, but low cold gas efficiency
- High level of sensible heat in product gas, heat recovery is required to improve efficiency
- Environmentally most benign; produced syngas consists of mainly H2, CO and carbon dioxide (CO2) with trace amount of other contaminants which can be removed downstream of the reactor; glassy slag is inert and easily disposed