Gasification

The Texaco Process

The Texaco gasification process was developed in the late 1940s. Although the main focus at that time was on utilization of natural gas reserves, some work on coal gasification was also performed (Schlinger 1984). The process achieved commer­cialization initially with gas feed (1950) and later with liquids (1956). This techno­logy is discussed in more detail in Section 5.4. Against the background of a perceived medium-term oil shortage at the beginning of the 1970s, the previous work on coal gasification was taken up again. Both its background from the previous work on coal as well as its decision to maintain many of the concepts already proven in commercial oil gasification service allowed Texaco to develop its coal-gasification technology in a relatively short space of time, despite the many differences in detail between coal and oil gasifiers.

Two demonstration scale projects were operated in the late 1970s (RAG, Holten in Germany and Cool Water, CA), and three commercial-scale facilities were started up between 1983 and 1985 (two for Ube in Japan, and the Eastman plant at Kingsport, Tennessee) (Curran and Tyree 1998).

Since 1990 nine commercial coal-based plants have been brought into service, five in China and four in the United Sates. These have been predominantly for ammo­nia and electricity production, although two of the Chinese plants are for methanol and town gas production. Three of the United States plants (Coffeyville, El Dorado, and Delaware) use petroleum coke as feed. The fourth, Polk Power Station in Florida, is a 250 MWe IGCC unit, which went on stream in 1996.

Process Description

The Texaco process for coal gasification uses a slurry-feed downflow entrained - flow gasifier. The reactor shell is an uncooled refractory-lined vessel. As with their oil and gas gasification processes, Texaco maintains flexibility in syngas cooling concepts, offering both a radiant boiler and a total quench. The selection between these two alternatives is a matter of economics for the specific application.

The coal or petcoke feedstock is wet milled to a particle size of about 100 pm and slurried in essentially conventional equipment. The slurry is charged to the reactor with a membrane pump. The reactor pressure is typically about 30 bar for IGCC applications, where no gas expander is included in the scheme. For chemical appli­cations it may be as much as 70 to 80 bar. The slurry feed is introduced into the reactor with the oxidant (usually oxygen) through the feed-injector (burner), which is located centrally on the top of the gasifier. The gasification takes place at slagging temperatures, typically about 1500°C, depending on the ash quality of the feed (Figure 5-19).

In the quench configuration, the hot syngas leaves the reactor at the bottom together with the liquid ash and enters the quench chamber. Texaco’s quench system provides a total quench, so that the gas leaves the quench chamber fully water - saturated at a temperature of between 200 and 300°C. For chemical applications such as hydrogen or ammonia manufacture, these are suitable conditions for direct CO shift conversion. Particulates and hydrogen chloride are removed from the gas in a hot scrubber before it enters the catalyst bed.

The ash solidifies to a slag in the quench vessel and leaves it via a lock hopper. The water leaving the lock hopper is separated from the slag and recycled for slurry preparation.

In the radiant cooler configuration (Figure 5-20), which was used in the Cool Water and Polk IGCC plants, full use is made of the potential for heat recovery for maximum efficiency. The feed preparation and gasifier are identical to the quench configuration.

The hot syngas leaves the gasifier at the bottom and enters the radiant cooler where it is cooled to about 760°C. The molten slag falls to the quench bath at the bottom of the cooler where it solidifies. As with quench configuration, the slag is removed through a lock hopper arrangement. The gas leaving the radiant cooler is then cooled further in a horizontal fire-tube convection cooler to a temperature of about 425°C. Both coolers are used to raise high pressure steam. In the Polk plant the steam pressure is 115 bar.

As in the quench configuration, there is a final hot-gas scrubber to remove hydro­gen chloride and particulates.

Equipment Issues

The Texaco quench gasifier is definitely the most inexpensive design on the market. On the other hand, it is maintenance-intensive. To achieve the greater than 97% availability quoted by Eastman (Moock and Trapp 2002), it is necessary to have an installed standby reactor, which negates the low capital expenditure to a large extent.