High Temperature Syngas Cooling and Heat Recovery

Synthesis gas (syngas) leaving the reactor is at high temperature; typically 2,500°F to 2,800°F for an entrained-flow gasifier. It is desirable to recover some of the sensible heat from the syngas.  Heat recovery systems can reclaim 5-25% of the energy in the feed, depending on the technology employed.  The actual design of a syngas cooling and heat recovery system has to consider the characteristics of the coal feed, syngas produced, and the overall gasification process application.

The raw syngas leaving the gasifier can be cooled by a radiant and/or convective heat exchanger and/or by a direct quench system, wherein water or cool recycled gas is injected into the hot raw syngas.  Then, the syngas typically passes through a series of heat exchangers for heat recovery at a lower temperature.  In all cases, steam is produced for in-plant power generation or process heating. The steam and water systems are integrated and optimized within the overall gasification facility.

Typical Syngas Cooling and Heat Recovery Arrangement

CLICK ON GRAPHIC TO ENLARGE   Tampa Electric Diagram

Radiant Syngas Cooler
A significant quantity of the sensible heat can be recovered via a radiant syngas cooler (RSC) to improve the overall gasification plant thermal efficiency. An RSC is a large, expensive piece of equipment. It can be prone to fouling and difficult to clean. The slag entrained in the syngas can stick to the RSC and causes deterioration in heat transfer. Despite that, the Tampa Electric plant seems to achieve satisfactory operational reliability with its RSC.

The Tampa Electric plant GE gasifier uses a radiant cooler, which is designed as a ring of tubes connected together in a configuration called a waterwall.  The hot syngas flows through the center of the waterwall, and high pressure (~1,650 psi) steam is generated inside the tubes using circulating boiler feedwater. At gasifier operating temperatures, heat is transferred primarily by radiation. The syngas exiting the center of the waterwall passes over the surface of a pool of water located at the bottom of the RSC, before exiting the gasifier/RSC vessel. The water pool collects and removes the slag and fly-ash from the reactor system before the syngas is sent downstream.

The Shell and Siemens gasifiers use a different design for radiant syngas cooling and have also demonstrated operability. These gasifiers have a built-in membrane wall inside the reactor vessel, into which cooling water is circulated to recover the reaction heat, and keep the wall cooled.  The inside of the gasifier is lined with a layer of refractory material to protect the wall.  During gasification, the ash in the coal melts and runs down the refractory-lined wall and when cooled, vitrifies and forms a protective slag layer inside the reactor vessel. The refractory also serves the purpose of allowing the molten slag to wet the gasifier wall. Circulating water through the membrane wall generates steam, while the slag layer on the inside of the membrane wall provides insulation.

Convective Syngas Cooler
The syngas leaving the RSC, typically at 1,100°F, is routed to a convective syngas cooler (CSC) for additional heat recovery. For the Tampa plant, the CSC consists of a double pipe heat exchanger with gas flowing in the inner pipe. The annulus contains the circulating boiler feedwater so that either high and/or medium pressure steam can be generated.  The gas leaves the CSC at a temperature between 700°F and 750°F.  It is then sent onto the syngas scrubbers in the syngas cleanup section.

Transporting Tampa Gasifier (source: Tampa Electric)	Tampa Radiant Syngas Cooler
Shell Gasifier with Membrane Wall

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