I found the following on the Pros and Cons of biodiesel from the Consumer Energy
Council of America:

www.cecarf.org/Programs/F...0Facts.html

PROS: When blended with standard transportation diesel, biodiesel helps to extend
the energy capacity of the diesel. Biodiesel can also be used as a home heating
oil. Because biodiesel is produced from natural sources, it generally releases as
much carbon dioxide as it uses growing. A blend of 20 percent biodiesel will reduce
carbon dioxide (CO2) emissions by 15 percent, and adding biodiesel also reduces
the amount of particulates (PM), carbon monoxide (CO), and sulfur dioxide (SO2)
released as emissions. Biodiesel is less combustible than petroleum, making it safer
to store and transport. In addition, if biodiesel spills, it is biodegradable and
breaks down roughly four times faster than petroleum diesel.

CONS: Use of biodiesel results in increased levels of harmful nitrogen oxide (NOx)
emissions when used in diesel engines, although not usually in residential heating
equipment. Also, in and of itself biodiesel releases the same amount of hydrocarbon
(or soluble carbon) emissions when burned as regular diesel. Furthermore, pure biodiesel
has a high "clouding" point, meaning that liquid biodiesel begins to thicken
into a solid at low temperatures. Because of its
higher clouding point, biodiesel is more difficult to store and transport in cold
climates, which adds to its cost.