Urea production is not carbon sequestration

To make urea, fertilizer producers combine ammonia with carbon dioxide (CO2), but when farmers apply that urea to the soil, an equal amount of CO2 is emitted to the atmosphere. No CO2 is permanently stored or sequestered through the production of urea.

This is a statement of the obvious, I’m told, but it’s worth stating for three reasons. First, not everyone knows it. Second, there was zero data in the academic literature supporting the fact, until now (see below). And third, next generation ammonia-urea plants with “zero-emissions” are becoming a reality, despite some of these new technologies relying on fossil fuel feedstocks.

If an ammonia plant consumes fossil fuels but has no local emissions, where does the CO2 go? If it goes into urea, is there any meaningful reduction of emissions beyond the boundary of the plant site? Are these industrial CO2 emissions simply being outsourced to the farmer? We need to be able to quantify a project’s environmental friendliness.

The carbon contained in urea contributes only 14% of urea’s total carbon footprint

I’m making a small point here, relative to the vast contribution of nitrogen fertilizers to global greenhouse gas (GHG) emissions. For one thing, urea’s carbon content is a fraction of its carbon footprint. One ton of urea will emit about 0.73 tons of CO2, but its carbon footprint, derived through a full life-cycle analysis, will be closer to 5.15 tons CO2-equivalent (CO2e). So the carbon contained in urea represents only about 14% of its total carbon footprint – the bigger issues are energy, used in production and transport, and emissions of N2O, which is 296 times as potent as CO2. Every market / producer / time period will have a different carbon footprint; these numbers are from Fertilizers Europe’s “carbon footprint reference values for European mineral fertilizer production and use in 2011,” available as a PDF here. [I’ve corrected this paragraph for its original maths error in unit conversions, and added the reference link.]

For further context, although the carbon contained in urea becomes almost 150 million tons of global GHG emissions every year, this big number is only a quarter of one percent of our total global CO2e emissions of over 40 billion tons per year.

What happens to the carbon in urea?

I wanted to judge the environmental impact of a zero-emission ammonia plant that achieved its low emissions, in part, by putting CO2 into urea. Could a local reduction in emissions lead to a global reduction in emissions? Would, for instance, any of the carbon in urea be sequestered in the soil? I’m not an agronomist and didn’t know the universally acknowledged answer. The answer is No, but I couldn’t find any data supporting this fact.

There is a vast academic literature addressing nutrient uptake from urea – but the carbon in urea is not a plant nutrient and is not measured. There are many field studies measuring the nitrogen pollution from urea – but none of it quantifies the CO2 pollution. I contacted many authors of academic studies: none of them had measured the rate or extent of CO2 emissions from urea; many had searched the literature and, like me, found nothing on the subject. It is simply understood that all the carbon in urea is emitted to the atmosphere as CO2.

Within 8 days of application, 98% of the carbon in urea will be emitted to the atmosphere

Somehow, my requests for information were passed on to the R&D department at Yara. They did their own literature search and saw there was no data measuring the rate of release of CO2 from urea. They decided it was a question worth answering and commissioned a lab trial, with the following results.

CO2 emissions of urea-Yara

This data merely demonstrates a universally acknowledged truth (that all the carbon in urea is emitted as CO2), but it is the only data out there, so far as I can tell. Yara kindly gave me permission to publish this, and you can download Yara’s PDF here.

The rate of urea hydrolysis

So, essentially, all the CO2 in urea will be emitted to the atmosphere in one week.

Technically, we’re measuring hydrolysis, in which urea reacts with water in the soil, catalyzed by an enzyme called urease, to form ammonia and carbamic acid. Carbamic acid is unstable, however, and quickly breaks down to ammonia and CO2. Like this:

[NH2]2CO + H2O (with urease) → NH3 + H2NCOOH → 2NH3 + CO2

Both ammonia and carbon dioxide are gases, and both will float off into the atmosphere – ammonia volatilization – unless the ammonia (NH3) reacts with more water to form ammonium (NH4+). Only when the soil contains ammonium is the plant able to take up the nitrogen nutrient. The CO2 is purely an atmospheric emission.

Why does this matter?

I was struck by the language used by Grannus to describe its new ammonia synthesis technology. Although their pilot plant in California won’t produce urea, the technology is primed to do so. The Grannus model involves carbon capture and utilization (CCU), as opposed to sequestration (CCS).

The difference is that with utilization there’s no reduction in eventual CO2 emissions. If producing urea, the CO2 isn’t emitted at the plant, but it is emitted at the farm. In the specific case of Grannus’s pilot plant, which aims to sell its CO2 to the beverage industry, we’ll belch 100% of that CO2 into the atmosphere just as surely.

The Grannus process offers many advantages to traditional fertilizer manufacturing by utilizing 100% of plant emissions for the production of fertilizer combined with strong cogeneration capabilities thereby reducing overall power usage. This paradigm shift allows the design and construction of green, zero-emissions thermal power plants in an otherwise uncertain permitting and political atmosphere; while also competitively reducing the cost to make fertilizer locally.

As Grannus points out, eliminating emissions at the plant makes the permitting process much faster and easier. This reduces project risk and increases the chances of successful financing. But it doesn’t stop carbon from being extracted from fossil fuels and emitted to the atmosphere.

This is not to say that Grannus’s zero-emission project is not environmentally friendly: if the carbon in urea is only about 14% of its carbon footprint, there is still a significant opportunity to reduce urea’s carbon footprint in other ways. The Grannus project promises an obvious improvement over existing ammonia plants for the following reasons.

First, CO2 is only one pollutant. The Grannus team tells me that their technology virtually eliminates emissions of NOx and SOx, which is significant, and which is “why we’re able to build the plant in the most stringent air pollution control districts in the world.”

Second, I’m told their process will be significantly more efficient, producing roughly 10% less CO2 per ton of ammonia, and allowing them to “reduce natural gas consumption by up to 30%.” The Grannus technology achieves this improvement in efficiency because it extracts its hydrogen from the natural gas feedstock using a partial oxidation process instead of the traditional steam methane reformation.

Third, the small size of the plant: it will produce 80,000 metric tons per year, one tenth the output of a new world-scale plant. This makes it attractive for a local production and consumption model, not global trade. If their model is scaled up, the potential emissions reduction achieved by avoiding ocean freight and long-distance distribution could be significant, if as yet unquantified.

Nonetheless, I’d suggest that a mere improvement over a highly polluting technology may not be enough. Non-fossil hydrocarbon feedstocks, like biomass or biogas from municipal waste, are allowing ammonia producers to recycle atmospheric carbon instead of extracting it from fossil fuels, although each alternative feedstock poses its own technical and environmental challenges. And other technologies that require no carbon whatsoever are now in development by major players, like Siemens, with their new project in the UK, or OCI Nitrogen, AkzoNobel, Proton Ventures, and others, with their joint project in the Netherlands.

Up to now, the urea industry has served the environment, in one sense, by commercializing carbon capture technology through creating large-scale CO2 demand adjacent to CO2-emitting ammonia plants.

In the future, I’m curious to see what role the urea industry will play in commercializing the nascent carbon removal industry, when the adjacent ammonia plants no longer use carbon feedstocks.


  1. Mark Brouwer says:

    It is true CO2 from urea is emitted when crop takes the nitrogen. But the nitrogen stimulates the growth of the crop, which again stimulates the CO2 absorption by the crop.
    Did you find any literature about the net result (or a life cycle analysis on crop) ?

    • Trevor Brown says:

      Hi Mark, always good to hear from you.

      Yes, absolutely. The net result of CO2 absorption from fertilized crops v unfertilized crops is zero.

      While fertilizer-boosted growth does absorb more CO2, that carbon is just part of the carbon cycle and, when it’s burned or eaten, it returns to the atmosphere.

      I defer to Yara’s carbon footprint analysis, available online here (PDF).

      “Plants capture large amounts of CO2 during growth. Optimum fertilization can increase biomass production, and thus CO2 uptake, by a factor of 4-5 compared to fields that remain long-term unfertilized. For example, at a yield of 8 t / ha achieved with 170 kg N / ha, the grain fixes 12 800 kg / ha of CO2. This corresponds to 75 kg of CO2 fixed per kg of N applied.”

      However …

      “Most of the biomass produced is consumed as food or feed. CO2 fixation is therefore only short term and cannot be considered a saving on a global scale. The balance is different for bio-energy since it avoids the burning of fossil fuels. For example, using biomass instead of mineral oil for heating purposes reduces the CO2 emission by as much as 70-80%.”

      My comments about alternative feedstocks illustrate the point that we actually have a choice between adding new, extracted carbon from fossil fuels/feedstocks, or recycling existing carbon from other sources. Apart from extraction and sequestration, everything else is temporary.

      All the best,

  2. Rune Ingels says:

    …And the ammonia lost in volatilization from the field, is oxidized and raining down on unproductive forest and marshland producing 0,3-3% N2O which is 300 times stronger than CO2 as a GHG.

  3. Hammer says:

    This is really interesting. Here in Australia, there seems to be a lot of gas companies proposing new Urea plants. Is it a way for them to simply push CO22 accountability down the chain to the farmer? One gas player is saying the cO2 will partly be sequestered by farmers. Based on what you are saying this is false. It doesn’t seem like the CO2 from the Urea is in current farmer calculations either over here.

    • Jon says:

      Hi Hammer, see my question below. After looking at Yarra’s study it says it measured urea ON bare soil. Not IN soil. I agree it would be great to sequester some carbon via this process so perhaps Yarra could broaden this study before writing off the statement?

  4. Jon says:

    A question regarding Yarra’s study: is the measurement of the CO2 based on Urea being spread on the surface of the soil in atmospheric conditions? Under some modern farming practices the Urea is placed alongside the seed in the soil. Does this change the outcome of the Yarra study? Thanks in advance for your time to respond.

    • Trevor Brown says:

      Hi Jon, as I understand it, there is no microbial or chemical mechanism that would reduce the CO2 emissions from urea, so (from a CO2-specific perspective) it doesn’t matter whether the fertilizer is placed on or in the soil.

      However, CO2 is only one GHG. Other GHGs, like N20, have a much higher global warming potential than CO2. These emissions can be reduced by using better methods (eg, more accurate placement or timing) or technologies (eg, slow release product formulation) that increase the nutrient use efficiency, and thus reduce the N20, NOx, and NH3 emissions from the field. None of these emission reduction pathways would reduce CO2 emissions.

      While there may be other physical transformations within the soil, like CO2 being absorbed in water, these are temporary and therefore – in any application – I don’t see any meaningful changes to the outcomes demonstrated in the Yara study referenced above.

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