An now for something completly different. I've read over 100 papers going back to before WWI where veg. oils were used in diesels. I ran 100% cottonseed oil, esterified cottonseed oil and heated beef tallow (yes, right off the side of a steak) in IDI diesel engines for my thesis and my research found the following. The tri-glyceride molecule of veg-oils reduces max. cylinder pressure and requires 10-15% higher rack settings to produce similar power. So expect higher fuel consumption, but if the oils free who cares. The oils caused a cone shaped soot deposit to develop on the injector pintle within 25 hours of operation. This is typical of the results reported in several papers. Eventually this reduced spray angle, contributed to larger droplet diameter and reduced combustion efficiency. So, expect to inspect the injectors more frequently if you use 100% veg. oil. My tests did not run the engines long enough to find the piston ring lands filled with carbon, but this is a typical symptom in direct-injected diesels if larger droplets hit the cylinder walls and the rings collect them. You've seen some veg. oils dry out and become sticky? The same reaction happens in the cylinder only much faster because of the temp. Most of the papers on combining filtered veg. oil with diesel fuel showed very good long term results.
Onto the soot issue. Soot is a concern because it contains not only carbon but also poly-aromatic hydrocarbons (unburnt fuel). The typical diameter of an agglomerated soot particle is small enough that it enters the avoili in the lungs and deposits leaving a hydrocarbon compound in contact with cells for long periods of time. In some cases cells are damaged by the hydrocarbon and during celluar replacement they may deform (cancer). The key is to control exposure hence the enforcement of threshold limit values in the form of exhaust emissions.
I've got several MB diesels. I love them. They get great gas mileage and in my opinion they produce less amounts of harmful emissions because of their higher efficiency. But to say soot is not a concern is not correct. If you have been to Europe, particularly Paris in the last 10 years, you would see the tons of soot from diesel engines covering the outside of buildings and statues. Some of the cathedrals including Notre Dame were completely black on the inside and outside in 1999. France cleaned Notre Dame for FY 2000 celebration after it had been cleaned 10 years earlier. The reason: gasoline taxes doubled the price of gas in the early nineties so now diesel powered cars are everywhere. Nearly 50% of the cars, all the trucks and bus in Paris in the late nineties were diesel powered and the soot created a problem. By the way, how did we get onto the bio-fuels issue from this guys original problem?