Biodiesel, NOx and the Value of Environmental “Prices”

Written by tom


This weekend I was checking out my friend Dave’s new VW Jetta, emblazoned with biodiesel stickers, as he told me how switching to biodiesel had lowered his carbon emissions 80%. (Biodiesel is based on biogenic carbon — the carbon emitted when you drive was absorbed by soybeans earlier in the season.)

I decided to push him a bit on the environmental bona fides of the car. “What about the NOx emissions? They’re a lot higher, you know.” Diesel, bio or not, burns at a higher temperature than gasoline and therefore causes more smog-forming oxides of nitrogen (NOx) emissions.

This is a common ding against diesel, and I immediately felt bad about repeating it. An 80% reduction in carbon emissions is nothing to scoff at. NOx and carbon emissions are pretty much impossible to compare, so wasn’t I just flinging mud?

Well, there is one way to compare carbon and NOx. Markets exist for credits in both pollutants, and theoretically we can use the respective price signals as an indication of their relative environmental costs. With this in mind, I sat down and calculated what Dave would have to pay for both his CO2 and NOx emissions (amazing what a new CEO will free you up to do in your spare time).

The results (Excel) show that given prices for carbon ($10/ton) and NOx ($2,500/ton), Dave’s use of biodiesel lowers his total environmental cost to $18.27/year, about 50% of a gasoline Jetta and just 4% more than a Prius. In short, compared to a gasoline Jetta, the savings in carbon emissions ($30.31) more than outweigh the $11.78 he suffers as a result of higher NOx emissions.

Did I mention my friend had to buy this car used and out-of-state to skirt the California restrictions on diesels? For car geeks, this situation does look to be getting better. VW plans to test its “Clean TDI” technology in 2008 that will meet Tier 2 Bin 5 standards. In English, this means the emissions will meet “middle of the road” car standards and be available for sale in all 50 states. If Dave trades up for the new Jetta with this technology, his total environmental impact would be $7.68 — just 44% of the Prius.

For policy wonks, this example is a microcosm of many policy issues related to auto emissions. For example, Europe has already decided to encourage diesel passenger cars. In 2005, diesel made up 50% of new car registrations in Europe. Clearly that decision is based on a similar weighing of environmental goods (carbon reductions, biofuel use, energy conservation) and bads (NOx). It’ll be interesting to see how traditionally diesel-hating states like California adapt to diesel cars that both meet lower emissions targets and are more amenable to a biofuel strategy that can reduce CO2 in the near term (compared to ethanol).

One question to TerraPass readers with biodiesel cars: would you buy a TerraPass with a blend of small amounts of carbon offsets and the appropriate NOx credits? You’d have to submit your own DOT based testing data on NOx, as no comprehensive data set exists for these emissions. We’d source and retire from the US NOx market. Let us know what you think in the comments.

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  1. Susanna

    Even Better! Why not convert a diesel car to Veggie? Check the Mercedes craze here in Naples, Florida.
    Yuuummmm, I smell fries instead of petrol!

  2. Karl

    Two questions:

    1. In the spreadsheet, the Prius and regular-fuel Jetta have different CO2 emissions per gallon of fuel. Is this a typo or are the fuels really different in some way?

    Ed Note: Duh! Fixed spreadsheet and numbers in blog. Thanks for the sharp eyes

    2. Does the 4 lbs-CO2/gallon account for the lifecycle CO2 emissions of producing biodiesel? I didn’t think the net gain was nearly as good as reflected in these figures.

    By the way, I’m supportive of biofuels as part of a long-term liquid fuels strategy, but my understanding is that the state of the art is still such that the net CO2 emissions from biofuels are only about 20% lower than for fossil fuels.

  3. D

    Please confirm. Your calculations are for biodiesel not petro-diesel. I think post #1 missed that, and would answer #2.1 question.

  4. Pete D

    Just a science correction Nitrous Oxide (N2O) is different from NOx (NO and NO2). Nitrous Oxide is a greenhouse gas, while NOx participates in Ozone forming chemical reactions.

    Ed Note: Corrected, thanks

    Out of curiosity what are the particulate emissions from this car? Diesels put out substantially more particulate matter than gasoline cars.

  5. Andrew

    As a Jetta TDI owner with over 30,000 miles on B100, I would love to have a TerraPass option that covers biodiesel use and NOx emissions. I currently buy a Hybrid TerraPass but as you note, NOx is still hanging out there as an issue for all diesel owners.

  6. Jessica

    The new Terrapass option to “capture” NOx emissions is a great idea!!

  7. Tom Arnold

    Susanna: In NREL testing, there was no difference in NOx emissions from veggie oil vs soy based biodiesel, so the same issues apply.

    Karl: Thanks for the comment – I fixed the numbers.
    The 4lb figure for biodiesel is just 20% of the dino diesel equivalent. There is a ton of literature that shows these kind of figures. See this UNH study for a very friendly one:

    Ethanol, on the other hand is widely considered to really only reduce 10-12%.

    Pete: Particulate emissions on the new Jetta are .2g/km. The upcoming model will have to meet the same standard as gasoline based cars for all major emissions (except CO2!), so while it may never reach SULEV status, it should be in line with most gasoline based cars.

  8. K

    Our 2002 Jetta TDI used for commuting has over 40K miles on it so far w/ using B100, and we would love a Terra Pass option for it! We have a TerraPass for our Toyota Sienna (something has to carry the dogs and kids!), and always wished we could have one for the VW. Great idea!

  9. Jay

    I second Pete’s earlier question about Biodiesel cars and particulate emissions. Per the American Lung association, particulate matter pollution can reduce lifespan and is especially damaging to children. Diesel trucks, buses, construction vehicles, and container ships are the worst offenders. Reducing C02 is outstanding, but we need to know if we are trading one problem for another (and can diesel emissions be more effectively “scrubbed” to substantially reduce particle matter).

  10. BDB

    Does the increased NOx emmission from biodiesel arise from the higher temperature of combustion, or does biodiesel simply have more nitrogen than traditional hydrocarbon fuel (which aside from additives, are composed solele of hydrogen and carbon?

  11. Vince

    A terrapass for NOx emissions would be amazing… And the levels of particulate are of great concern… what good it it to lower carbon if NOx and particulates will still be off the chart? Je ne sais pas…. i am still waiting for battery technology to get good enough that electric cars can have the same range as conventional ones… i drool over the tesla roadster either way…..

  12. kari

    I don’t know that I agree conceptually with offsets for NOx. NOx is a localized ozone forming compound and impacts communities directly where the emissions are released. you’d have to work somehow to implement the offsets on a local level, and I think that’s going to take a lot of effort, tracking, transparency, etc. it could be done, but I don’t think the philosophy behind NOx impact reduction and CO2 impact reduction is the same. CO2 is a global problem, and doesn’t have the same immediate effect on actual people on the street.

  13. Tom Arnold

    BDB — In general diesel (both bio and otherwise) has higher NOx emissions than gasoline. The jury is still out on whether biodiesel increases NOx above regular diesel. Sevearl studies suggest it does, by 10% or so. Other’s dont.
    Here is a recent NREL Study if you want to read more:

  14. whit

    Is any car maker, or aftermarket party studying the possibility of combining all of these technologies: biodiesel, hybrid, and plug-in ? That would seem to be the holy grail (at present) in terms of collectively lowering our petroleum consumption, and our emmissions.

  15. Tom Arnold

    I am sure people are studying it, but the problem is one of cost. Diesel adds around $1500 in cost, so together with the engineering cost of hybrid/plug in your talking a big price premium.
    There’s also a question of focus. Making a small number of ultra-green cars is less valuable than making incremental improvements in large numbers of more mainstream vehicles.

  16. Jon

    Yes – I would definitely be willing to pay for a “blended” option combining carbon and NOx. One question is how the would the offset money be invested?

  17. Jason Burroughs

    This is great info. I wrote in awhile back to suggest a TerraPass for biodiesel, and I’m happy to see you guys looking into.

    A little more data about ‘clean diesel’ – in model year 2009, the diesel world moves out of the stone age, with respect to emissions technology. Much like the 1970’s brought us unleaded gas and therefore catalytic converters (which were impossible with the lead plugging them up), diesel engines are getting a particulate matter filter and NOx scrubber. This is due to pulling the sulfur out of the petroleum diesel fuel, which would otherwise plug up the PM filter. These new emissions equipment are standard on all MY 2009 and later diesels in the US and DRASTICALLY (90%) lower the emissions of a diesel using petro diesel. With biodiesel, it gets even better – using just B20 (20% biodiesel + 80% diesel), the PM filters work 60% better…that’s at least in part due to the fact that biodiesel contains no sulfur.

    So, a biodiesel terrapass is great – but we ne need to re-evaluate everything related to diesel starting next year – that is, MY09 and later vehicles will need a much smaller terrapass than those built before.

    Speaking of VegOil, check out – they have data showing EPA equipment testing a diesel burning Jetta TDI, but running on straight vegetable oil – the numbers will surprise you!

    Jason Burroughs
    DieselGreen Fuels, Austin TX

  18. Pamela

    While the lower levels of CO2 are great, some cities are in nonattainment for certain pollutants, like ozone, and need the NOx reduction to reach attainment and prevent federal sanctions. Currently, certain funding is only available if NOx is reduced.

    Regarding the elevated levels of NOx with regards to biodiesel – recent studies in Texas have shown that biodiesel may only increase NOx emissions in older diesel engines and LOWER it in newer models.

  19. Krystal

    Perhaps an answer from some posters above, #17?
    We have a lot of conversions to biodeisel in my farming/ag town of Eastern Washington–it’s great to see a community, where most citizens fear the “environmentalists” choosing to convert, though some of the reasons include local fuel stations price gouging for diesel.
    However, we are seeing problems with biodeisel in the cold (sometimes sub-zero) winter months. Does anyone know if any techonolgy in the pipe that would sustain a veggie diesel through the very very cold winter months? Currently, these farmers and orchardists cannot use these converted cars Nov-mid-March. Although it is always great to get less cars on the road *period*, our community thrives off the local agriculture in the valley. Are there any technologies coming down the line that may allow these farmers to work on their converted engines in the winter months?

  20. Jonathan Katz

    California is not a diesel hating state-the regs have created a barrier for diesel cars becuase NOX emissions cannot be controlled via technologoes like catalytic converters in exhaust systems due to the prescence of sulfur in the lower grade diesel fuel that has been permitted in the US. New standrds, in effect this year, for diesel (closer to european standards) will mandate sulfur free fuel. This means diesel vehicles can be sold in California and if they are euipped with catalytic conversters will have much lower NOX emissions along with the other advantages of biodiesel.

  21. Ethan

    I run biodiesel and would love to buy NOx offsets for it.
    A calculator would be particularly awesome, as I don’t want to have to do the research and calculations myself if possible. It would be ideal to offer offsets for both this and the 20% of CO2 as well, packaged into one product.

  22. Dan Kalafatas

    I’ve been trying to noodle up a way to buy NOx allowances “at retail”. Blend them, baby.

  23. brian

    it’s rewarding to see debate at this level on fuel alternatives. i’ve been wary of biodiesel from the beginning. in fact, i’m still not convinced. i’d like to offer three reasons why:

    (1) as a bicycle rider, particulate bursts from diesel vehicles can stop you in your tracks;

    (2) who is counting the heat produced from biodiesel vehicles? so many of these conversions are still on large SUV-type machines and emit lots of heat, a major component of the smog recipe, (try standing next to an idling truck and see what i mean);

    (3) the carbon trading idea appears likely to be the way we are headed. but who assumes the position to grant the right to despoil air we must all breathe? this is simply a sale on indulgences — with deadly consequences.

    I’d like to see these points come into the conversation as well. thank you.

  24. Adam Stein

    Hi brian,
    I’ll let others respond to the good questions you raise about diesel, but your question (3) has the situation exactly backwards. Please remember that in the situation we are in right now, everyone pollutes freely. There are no restrictions on how much gas we can put in our cars, how much electricity we can use, or how many plane flights we can take. This “right” isn’t something that carbon trading creates — it already exists.
    Carbon trading is a restriction on that right. Carbon trading puts a cap on the total amount of carbon that we as a society can create, and it leaves us to sort out amongst ourselves how we meet that cap. If some company X wants to pollute more than it is allowed under the cap, it can do so, but only if it pays company Y that pollutes less than allowed under the cap. This is not a “permit” to pollute — it is a punishment, which is a much better situation than we have now.

  25. Tad

    Almost all of our vehicles are diesels, my wife’s Golf, my vintage Land Rover and the company truck. I don’t burn bio in the Land Rover because of the old components that are made of rubber but I am converting to SVO. (The other vehicles are human powered)
    My question has always been that isn’t output of particulates/CO2/NOx per gallon of fuel burned? So if the Golf is getting an average of 47 MPG isn’t it actually putting out less emissions then a comparable car that is getting 27 MPG? (I am asking, not stating)
    Plus, even if it cost more to produce bio-diesel in the long run most diesel vehicles get better fuel mileage thus using less fuel thus the need to produce less. When I burn even as low as B5 in the company truck (a Dodge/Mercedes Sprinter) I get better fuel mileage (about 10 to 15%) plus I think it runs smoother.

  26. Anonymous
    Fine particulate matter is a major health concern for urban areas. Evidence for increased death rates, especially among sensitive populations has been published and is available on the EPA website. All combustion based auto emissions and diesel to a higher degree because of the increased heat, are responsible for increased concentrations of fine PM. The way we regulate these emissions is not effective because it is a mass based regulation and that omits the smallest particles which are the most dangerous for longterm health. In my opinion, putting efforts into modifying combustion vehicles does not address this major concern that had been downplayed because big industry profits from keeping us driving.
    Take a look at the presentations from the ultrafine particulate conference for the current science perspective on this issue. Even the filters are not preventing particulate formation.. it just happens further from the source.. (closer to the people living around the freeway/combustion source..)
    We need to change our mind set (carpool, public transport, bike, walk).. not our fuel.

  27. Edward Mangold

    About the problems associated with diesel particulate emissions.
    There should be higher volatility hydrocarbons that can be added to biodiesel to make a vehicle able to start in cold temperatures. The old standby is to squirt starter fluid (either) into the air intake.
    The particulate emissions from diesel engines are strongly affected by engine adjustments and driving conditions. Particulate emissions are highest when the engine is operating under full load. By adjusting the fuel feed the highest emissions are reduced, at the expense of power. Since most diesels are operated on trucks, the owners are reluctant to operate this way, and they have enough political power to keep government regs from having any effect.
    The other factor is the horsepower to weight ratio of the vehicle and the way it is driven. If full throttle, maximum power conditions are avoided by going easy on the accelerator, the fully loaded, high particulate emission, max-power engine condition can be avoided.
    Ed Mangold

  28. Manuel

    Couple of comments from European bike commuter:
    -“Modern diesel” engines’ burst of particulate come out when accelerating hard (including from a stop in city traffic). Looking at the way Lexus/Honda Accords use the electric motor to give power burst in their hybrids, and considering what Ed Mangold just mentions above, this diesel/electric powerplant might be a solution to these short burst of particulate matter while keepig the benefits of low-charge diesel efficiency. However, diesel engines tend to be heavier per power output than gas engines (they need to be built sturdier), and this weight penalty would be combined with the extra weight of the electric engine (+ batteries+ braking converters, etc.) of hybrids, so it’s not clear that the greater efficiency of the diesel and hybrid offset the higher weight.
    -New/well tuned diesels emit much less particulates than poorly tuned diesel engines. An issue in Europe is ensuring that all the aging “modern diesels” are kept well tuned, so that all cars have to pass emmission testings about every 2 years, failing which they cannot circulate. Unless mandatory regular check-ups are instated in the US as in European countries, this is bound to become a problem.

  29. Wade

    First, I would definitely buy terrapass that addressed NOX. We burn B100 in summer and B20 in winter in Denver. Since ozone is local, I would like to buy Denver area specific terrapass. What methods would terrapass invest in to reduce NOX? Are there opportunities for NOX reduction in Denver?
    Second, does anyone know of an aftermarket catalytic converter/urea injection system that I might purchase and install on my ’83 Mercedes?

  30. Ros

    Okay this is going to sound very ignorant. I read the FAQ but I still don’t quite understand what TerraPass is.
    Is it a service that you pay for which lends to you the benefits stated in the FAQs (like the calculators and such)?