Nitric acid: Production route
How plant pressure sets energy, platinum loss and N2O
What this tab covers How it is made: the main production routes or, for product families, the main product types and technologies, compared on share of supply, inputs per tonne, plant scale, yield, co-products and unit cost.Almost all nitric acid is made by the Ostwald process, but plants differ in operating pressure, which sets catalyst losses, energy recovery and N2O. Mono-pressure plants run oxidation and absorption at the same medium or high pressure; dual-pressure plants oxidise at medium pressure and absorb at high pressure for better efficiency. Weak acid (55-68%) is the main product; strong acid above the 68% azeotrope needs an extra concentration step. N2O abatement is added as secondary catalyst under the gauze or tertiary catalyst in the tail gas.
Routes compared
| Route | Share of supply | Inputs per tonne | Typical scale | Yield | Co-products | Unit cost |
|---|---|---|---|---|---|---|
| Dual-pressure Ostwald plant | a large share of modern capacity (indicative; not published) | 0.28-0.29 t NH3; Pt losses about 0.1-0.2 g per t (indicative); cooling water | single trains of 600-2,000 t/day; KBR references of 624, 835 and 1,200 t/day | about 94-96% ammonia to acid (indicative) | exported steam and power; N2O 5.6-8.4 kg/t if unabated (IPCC medium-pressure default range) | ammonia dominates: at $500/t NH3 about $143/t of feed; conversion about $20-40/t (indicative) |
| Mono-pressure (medium or high pressure) plant | common in North America and older plants elsewhere (indicative) | 0.28-0.29 t NH3; higher Pt losses at high pressure (indicative) | 300-1,500 t/day (indicative) | about 93-95% (indicative) | N2O 5.4-12.6 kg/t at high pressure if unabated (IPCC default range) | lower capex, similar feed cost (indicative) |
| Concentrated nitric acid (CNA) production | small, about 5-10% of output (indicative) | weak acid plus steam and recirculated dehydrating agent | 100-500 t/day (indicative) | high | none material | a premium of $100-200/t over weak acid (indicative) |
| N2O abatement (secondary and tertiary catalysts, NSCR) | about half of the world's roughly 500 plants (Clariant 2022); under 50% of plants in developing countries outside China (Calyx Global) | catalyst baskets; for NSCR, natural gas | any plant size | removes 80-98% of N2O (Calyx; KBR 95-98.3%) | nitrogen and oxygen | about $1-5 per t CO2e abated (NACAG) |
How each route works
Dual-pressure Ostwald plantAmmonia is burnt at 4-6 bar over Pt-Rh gauze and the gas is compressed to 10-12 bar for absorption. This gives high acid strength and low NOx, and recovers energy through a tail gas expander and steam turbine. It is the standard for large modern plants in Europe and with licensors such as thyssenkrupp Uhde, Casale and KBR.
Efficient and low NOx, but higher capital cost.
KBR technical paper; IPCC 2006 defaults via KBR Mono-pressure (medium or high pressure) plantOxidation and absorption run at one pressure, 4-8 atm (medium) or 8-14 atm (high). High-pressure plants are compact and cheap to build, common in the US and older designs, but lose more platinum and form more N2O.
Cheap to build; highest N2O and catalyst losses.
KBR technical paper Concentrated nitric acid (CNA) productionWeak acid is dehydrated by extractive distillation with 93% sulphuric acid or magnesium nitrate, or strong acid is made directly by reacting liquid N2O4 with oxygen and water. CNA above 98% serves nitration of aromatics, explosives and propellants.
Needed for nitrations; energy intensive.
Indicative N2O abatement (secondary and tertiary catalysts, NSCR)Secondary catalysts sit under the Pt gauze and decompose N2O as it forms; tertiary catalysts (iron-zeolite) treat tail gas before the expander. Non-selective catalytic reduction with fuel gas also cuts NOx and N2O. These are add-ons to any Ostwald plant.
Very cheap per tonne of CO2e; needs a carbon price, rule or credit to pay for it.
NACAG/IKI (2021); KBR; Calyx Global Unit economics are indicative ranges from the sources named; costs move with feedstock and energy prices, plant size and region.