Since 2007, federal regulators have overseen the growth of the
biodiesel industry, which converts discarded animal fats, used
cooking oil and other materials into fuel that can power cars,
bulldozers and heavy-duty trucks.  The U.S. Environmental
Protection Agency required large fossil-fuel refiners and
importers to produce or purchase enough renewable fuels,
including biodiesel, to account for up to 9 percent of total
production. If the companies couldn't produce the alternative
fuels, they had to buy credits from others that did.  But in
recent months, the credit-trading market has been rattled by
fraud allegations, plummeting confidence and lawsuits. The EPA
has identified more than 140 million fraudulent credits, a
sizable figure considering that 1.6 billion credits were
generated last year.  The turmoil has raised concern about the
EPA's system for subsidizing production of biodiesel, which has
transformed into a complex web of trading in largely unregulated
financial instruments.  The EPA has taken acti