RAS LAFFAN INDUSTRIAL CITY, Qatar: The rat's nest of pipes and columns snaking across the desert harbors a secret process that will use cobalt to turn natural gas into a powerful, clean-burning diesel fuel. By next year, rulers of this tiny desert sheikdom hope, these gas-to-liquids (GTL) reactors under construction will bring in billions of dollars while clearing big city smog belched by trucks and buses. Petroleum experts who have sniffed vials of gin-clear GTL diesel speak of it with reverence. "It's a beautiful product," says Jim Jensen, a Massachusetts-based energy economist. "The kerosene smells like perfume." In all, some...