20.2.1 Reforming of fossil feedstock

Gasification (see next Section 20.2.2 for further information) is used for heavy feedstocks like heavy fuel oil and coal while reforming is used for lighter feedstocks, namely natural gas, associated gas, and naphtha. Steam (and dry) reforming are catalytic processes where commercially nickel on alumina-based catalysts are used. To reach high enough conversions to synthesis gas, the temperature of reactor outlet is approximately 850-1000°C at operating pressures up to 30 bar. The inlet temperature can be significantly lower (~500°C). Excess steam, when compared to the stoichiometric reactions, is used to steer the equilibrium conversion of methane and avoid kinetic carbon deposition. Four essentially different steam reforming processes can be identified, namely:

(1) One-step or single-step steam reforming: An externally fired tubular reactor is being used over which a temperature gradient is applied to reach full conversion.

(2) Two-step reforming: Initial pre-reforming of the heavier components in the feedstock towards methane and carbon oxides is done around 350-550°C,7 which is then followed by a consecutive (higher temperature) reformer.

(3) Auto-thermal reforming: Next to steam, oxygen is supplied internally to generate heat for the strongly endothermic reforming reactions.

(4) Partial (catalytic) oxidation: Part of the feedstock is combusted without adding additional steam which generates very low hydrogen over carbon monoxide ratios (<2).

For biomass, the pre-reform reaction looks like:

C6H10O5 + H2O ^ 3CO2 + 3CH4 [20.7]

Since steam reforming catalysts and catalysts used in downstream processes (like for methanol and Fischer-Tropsch production) are very sensitive towards sulfur poisoning, the feedstocks are desulfurized. For a complete and detailed review on steam (and dry) reforming of fossil feedstocks, the reader is referred to Rostrup-Nielsen et al.8

20.2.2 Gasification of fossil feedstock

Already around 1850 there was a considerable coal gasification industry. The Siemens gasifier (1861) and the Winkler gasifier (1926) were successful low — temperature (<900°C) air blown systems producing fuel gas. In 1938, the Koppers — Totzek entrained flow gasifier came into commercial use. This gasifier produced synthesis gas (CO + H2) on continuous basis containing no tars and methane at approximately 1850°C and atmospheric pressure from oxygen-entrained coal. At the end of the 1940s and the early 1950s, Texaco and Shell developed technologies for the production of the synthesis gas by oil gasification. These were entrained — flow reactors with top-mounted burners (atomizers) in the down-flow. Operating pressures and temperatures were up to 80 bar and in the range of 1250-1500°C, respectively. Apart from Texaco and Shell, Lurgi also developed oil gasification technology, known as multi-purpose gasification. Nowadays, most oil gasifiers are part of a refinery and are used for poly-generation of power, H2, synthesis gas and steam. As a result of the oil crisis of the early 1970s coal gasification was taken up again. It was again Texaco and Shell (together with Krupp-Koppers) who developed entrained-flow high pressure (20 bar to 70 bar) and high temperature (>1300°C) coal gasification. A good and complete review on gasification is given by Higman and Van der Burgt.1