Oxidative prereforming

Based on experimental studies of the oxidation of methane mixtures with C2—C4 hydrocarbons and the demonstrated possibility of selectively reducing the content of C2—C4 hydrocarbons in such mixtures (Chapter 10), the authors of [303] proposed a two-step process of conversion of mixtures of methane with alkanes C24- to syngas. Proceeding from material balance of the process, a conceptual flow diagram of oxidative prereforming was proposed (Fig. 11.27). 

FIGURE 11.27 Conceptual diagram of the oxidative prereforming of C2—C4 hydrocarbons in natural gas. (For colour version of this figure, the reader is referred to the online version of this book.)... 

Like M( F(7s, S()F(7s can integrate fuel reforming within the fuel cell stack, A prereformer converts a substantial amount of the natural gas using waste heat from the fuel cell, (iornpoiinds containing sulfur (e,g, thiophene, which is cornrnonlv added to natural gas as an odorant) must be removed before the reformer. Typically, a hvdrodesiilfii-rizer combined with a zinc oxide absorber is used. 

A zinc-oxide mass containing copper may take care of traces of sulphur passing through the zinc oxide by establishing the chemisorption equilibrium over copper, which is independent of the presence of water (Table 1.18). The prereformer catalyst establishes the H2S/Ni chemisorption equilibrium at a much lower value than H2S/CU (refer to Section 5.4). However, it is expensive to use the prereformer catalyst as a desulphurisation mass. 

The dynamics of sulphur uptake in a prereformer is like a fixed-bed absorption as seen in a zinc-oxide bed (refer to Chapter 1). However, in a tubular reformer the pore diffusion restrictions in the sulphur adsorption in a single pellet has a complex influence on the transient sulphur profiles in the reactor and a mathematical model [112] [387] [389] is required to evaluate more exactly the time for fiill saturation and the breakthrough curves of sulphur. 

Figure 9.31 Wartsila WFC20 solid oxide fuel cell system with prereformer and catalytic afterburner [508].

Nevertheless, direct oxidation can be important under certain conditions, such as at the entrance of a cell. The degree to which an anode supports direct oxidation will then impact the degree of prereforming of the fuel that is required, which in turn typically impacts balance of plant complexity and cost. This is why there remains strong interest in the development of direct oxidation anodes. 

FIGURE 12.22. Westinghouse seal-less generator concept with integrated prereformer. (From Takeuchi, S., Kusunoki, A., Matsubara, H., Kikuoka, J., Ohtsuki, J., Satomi, T, and Shinosaki, K., Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, Singhal, S.C. and Iwahara, H., Eds., The Electrochemical Society, Pennington, NJ, 1993, 678-683. With permission.)... 

---