Gasification

New Developments

Despite various improvements over the years, it is generally recognized that hand­ling of the soot produced by partial oxidation of heavy residues places a consider­able financial burden on the overall process. This has caused operating companies and others to investigate alternatives. In the 1970s one operating company was already using a toluene extraction process to recover the soot as saleable carbon black. A number of other companies made similar attempts, but the economics of these processes, together with the variable product quality depending on feed quality to the gasifier, have prevented commercialization beyond single demonstration plants. Nonetheless, two process, both based on filtration of soot slurry and subsequent treatment of the filter cake, have been reported on in recent years and may form the basis for further development.

Norsk Hydro Vanadium Recovery Process

Norsk Hydro developed its own process for its 65t/h feed heavy oil gasification plant in Brunbiittel. This plant has been in operation now for several years and has definitely provided operating benefits compared with the original 1975-designed pelletizing plant (Maule and Kohnke 1999).

The Norsk Hydro VR (vanadium recovery) process (see Figure 5-33) is based on filtration of soot slurry and combustion of the filter cake. In this process the filter cake is first dried and pulverized before being burned in a special cyclone combus­tor in which part of the vanadium is combusted to a liquid V205 that is then scraped from the combustion chamber floor. The heat of combustion is used to generate steam, which in general is sufficient to provide the necessary heat for the drying stage. Fly ash from the combustion stage, which also contains V205, is collected in a bag filter and combined with that collected from the combustion chamber.

In the published flow diagram, which is more complex than what is shown in Figure 5-33, there is no specific provision for sulfur recovery from the flue gas. Depending on the circumstances, this may have to be included. A fuller description of the process is available in the literature.

In the meantime, Texaco has acquired the rights to this process, and it remains to be seen just what further development the process will experience in the hands of

a licensor. It is worth noting that Krupp Uhde reported a similar development under the name of CASH (Keller etal. 1997). No commercial application is known, however.

Soot Gasification

A totally new approach to handling filter cake is under development at the Engler - Bunte-Institut (EBI) of Universitat Karlsruhe (Figure 5-34) (Higman 2002). The development of the filtration-based processes was driven by the recognition that the behavior of the vanadium in the ash is crucial to the oxidizing treatment of the filter cake. In particular, the MHF concepts operate at a low temperature specifically to prevent exceeding the melting temperature of the V205 formed, which is about 700°C. This low operating temperature, for all its benefits, has the disadvantage of a low reaction rate, and thus high residence times and large equipment.

Soot gasification retains the vanadium in the trioxide state and, as with the main gasifier, can operate at high temperatures without creating liquid vanadium pentox - ide. The gasification is optimized to achieve maximum carbon conversion, whereby a higher level of C02 is tolerated than in most gasification processes, that is, mini­mizing the residual carbon in the ash is more important than H2+CO yield. The process exploits the existing gasification infrastructure by using oxygen and can thus produce a low-pressure synthesis gas with a fuel value. This is in contrast to the large waste gas flow of a multiple hearth furnace, which contains carbon monoxide and requires incineration.

The process line-up includes a typical filtration step followed by fluid-bed drying and milling of the filter cake to <500 pm. Gasification takes place with oxygen and steam in an atmospheric entrained-flow reactor. The ash is removed from the product gas in a dry candle filter and meets the requirements of the metallurgical industry.

This process, which is still under development, has the potential to reduce the costs of carbon management significantly. Further possibilities include the development of a pressurized version, that could handle soot filtered dry directly out of the main

syngas stream. This would decrease equipment size further, reduce the cost of the wash water circuit, and offer an economic possibility to recycle the fuel gas into the main stream, thus increasing the syngas yield. Clearly, developments in the field of oil gasification are by no means at an end.