First, burning the rice plant puts exactly 0 net gain air pollution into the atmosphere. Why? Because the rice plant is made of materials that were synthesized out of atmospheric CO2 in the first place. Rice takes CO2 out of the air, uses sunlight, nitrogen and water to turn it into various proteins, carbohydrates, etc. In other words, the whole rice plant is just air, water and sunlight. Burning it adds nothing to the atmosphere that didn't come out of the atmosphere in the first place.

Second, bio-fuel production requires more fuel than is produced by it. Modern farming techniques require about a 10:1 ratio of energy input versus energy output. (Note, that's a vast improvement over prior centuries, but still entropic.) That means you have to burn approximately 10 gallons of bio-fuel to produce 1 gallon of bio-fuel. Or 10 gallons of petroleum diesel to produce 1 gallon of bio-diesel. So long as the ratio is entropic instead of negentropic, that will still apply. All in all, bio-fuel actually produces more pollution than petroleum fuels.

Third, there just aren't enough McDonald's, etc., in the world to produce enough waste grease to power even a small city of cars. (Add in some teenage faces, and you might get closer, but even that isn't enough.:) )

All of this has been in recent headlines and scientific articles. Well ... except the part about teenage faces. I think that comes from Mad Magazine.