Greenhouse gas emissions and fertilizer production

Greenhouse gases are produced as a result of numerous industrial processes. In the fertilizer industry, greenhouse gas emissions are primarily associated with three industrial processes: ammonia production, phosphoric acid production and nitric acid production.

Ammonia production

The industry’s main energy requirement is the fuel and feedstock used in manufacturing ammonia. Virtually all nitrogen fertilizers are derived from ammonia, and ammonia production accounts for 87% of the industry’s total energy consumption. The manufacture of all N fertilizers together accounts for about 94% of the sector’s energy use. About 20% of the ammonia manufactured is destined for industrial uses and is therefore not part of the agricultural life cycle.

On average, one-third of emissions from natural gas-based ammonia production are associated with the burning of fuel and two-thirds with the use of fossil fuel feedstock to obtain the hydrogen needed to synthesize ammonia. In coal-based production, the proportions are 25% and 75% respectively. CO₂ emissions related to the consumption of feedstock are subject to the laws of chemistry, and efficient producers today are very close to the theoretical minimum emissions. However, this “process” CO₂ is clean and can be captured fairly easily. In contrast, flue gas (resulting from fuel combustion) must be cleaned before that CO₂ can be captured and stored or re-used, which entails additional costs.

For ammonia production, natural gas is the dominant feedstock globally although a significant portion (27%) is coal-based. Natural gas has the highest hydrogen content of any existing energy source, which makes its highly desirable as a source of hydrogen for ammonia synthesis. Moreover, among current energy sources, natural gas has the lowest CO₂ carbon dioxide emissions per energy unit.

According to the most recent IFA report on Energy Efficiency and CO₂ Emissions in Ammonia Production (IFA 2008b), some 36% of the CO₂ produced in 2008 was recovered. About one-third of the CO₂ generated globally was captured for production of urea – which has the chemical formula CO(NH₂)₂ . The remaining CO₂ captured by the fertilizer industry (2.2%) was sold into other value chains, e.g. to the oil and gas industry (for injection into wells) or to the beverage industry. Enhanced oil and gas recovery is the only current downstream use of the ammonia sector’s CO₂ that qualifies under the term “Carbon Capture and Storage” as used in climate change policy discussions. CO₂ captured for urea production, or for use in beverages and other downstream products, is released to the atmosphere later in the value chain. When more CO₂ is produced than can be reused on-site or sold, the balance is vented to the atmosphere.

Nitric acid production

Nitric acid (HNO₃) is required as an intermediate in the production of nitrate fertilizers (and several other industrial chemicals). In an average medium-pressure plant, the nitric acid process creates 6-8 kg nitrous oxide (N₂O)/tonne HNO₃. A Non-Selective Catalytic Reduction (NSCR) process, which has existed for several decades to abate NO_x emissions, also reduces N₂O emissions (unlike Selective Catalytic Reduction). Tertiary systems have become available to destroy N₂O; although they are very effective, they affect energy efficiency and CO₂ emissions and are often expensive to retrofit in existing plants. A recently developed secondary catalyst has significantly improved the economics of nitrous oxide abatement.

Production of phosphate and potash fertilizers

Phosphate rock releases some CO₂ when reacted with sulphuric acid in the phosphate fertilizer manufacturing process. Additionally, some CO₂ is emitted from fuel combustion during this process. The production of phosphate fertilizers has become to a significant extent energy and greenhouse gas neutral, largely due to energy cogeneration during sulphuric acid production. There are nonetheless emissions related to the production of raw materials and intermediates, as well as to the transport of inputs and final products.

The energy requirement for potash mining and beneficiation varies according to mining processes and ore quality. In the context of overall fertilizer production, this sub-sector’s energy consumption and greenhouse gas emissions are low. Beyond the plant gate, emissions related to transport should not be overlooked.