Lurgi, a process development company, developed a pressurized dry-ash updraft gasifier. It is called dry ash because the ash produced is not molten. One that produces molten ash is called a slagging gasifier.

Though the peak temperature (in the combustion zone) is 1200 °C, the maximum gasification temperature is 700 to 900 °C. The reactor pressure is in the neighborhood of 3 MPa, and the residence time of coal in the gasifier is between 30 and 60 minutes (Ebasco, 1981). The gasification medium is a mixture of steam and oxygen, steam and air, or steam and oxygen-enriched air. It uses a relatively high steam/fuel carbon ratio (~1.5).

The coal is first screened to between 3 and 40 mm (Probstein and Hicks, 2006, p. 162) and then fed into a lock hopper. The gasifying agent moves upward in the gasifier while the solids descend. The reactor is a double-walled pressure vessel. Between the two walls lies water that quickly boils into steam

under pressure, utilizing the heat loss from the reactor. As the coal travels down the reactor, it undergoes drying, devolatilization, gasification, and combustion. Typical residence time in the gasifier is about an hour (Probstein and Hicks, 2006, p. 162). In a dry-ash gasifier, the temperature is lower than the melting point of ash, so the solid residue dries and is removed from the reactor by a rotating grate.

The dry-ash technology has been used at SASOL in South Africa, the world’s biggest gasification complex. SASOL produces 55 million Nm3/day of syngas, which is used to produce 170,000 bbl/day of Fischer-Tropsch liquid fuel.

Slagging Gasifier

The British Gas/Lurgi consortium developed a moving-bed gasifier that works on the same principle as the dry-ash gasifier, except a much higher tempera­ture (1500-1800 °C) is used in the combustion zone to melt the ash (hence its name, slagging gasifier). Such a high temperature requires a lower steam-to — fuel ratio (~0.58) than that used in dry-ash units (Probstein and Hicks, 2006, p. 169).

Coal crushed to 5 to 80 mm is fed into the gasifier through a lock hopper system (Minchener, 2005). The gasifier’s tolerance for coal fines is limited, so briquetting is used in places where the coal carries too many of them. Gasifica­tion agents, oxygen and steam, are introduced into the pressurized (~3 MPa) gasifier vessel through sidewall-mounted tuyers (lances) at the elevation where combustion and slag formation occur.

The coal introduced at the top gradually descends through several process zones. The feed is first dried in the top zone and then devolatilized. The descending coal is transformed into char and then passes into the gasification (reaction) zone. Below this zone, any remaining carbon is oxidized, and the ash content is liquefied, forming slag. Slag is withdrawn from the slag pool through an opening in the hearth plate at the bottom of the gasifier vessel. The product gas leaves from the top, typically at 400 to 500 °C (Minchener, 2005).