Reducing emissions in nitric acid production with tertiary abatement systems 

By Stefano Cicchinelli, Product Manager of the Tertiary Abatement at Stamicarbon

Like most industries, the fertilizer industry strives to reduce greenhouse gas emissions. Apart from developing new technologies to achieve this, we are also witnessing an adaptation of traditional technologies to cope with climate change and reduce environmental impact while meeting the latest emission regulations. 

Existing greenhouse gas emissions regulations 

The number of systems and initiatives for emission regulations worldwide is increasing as countries work to achieve the climate goals set in the Paris Agreement. In the EU and several other regions of the world, Emission Trade Systems (ETS) are in place to regulate the amount of CO2 emitted by companies, but also other GHG emissions – specifically nitrous oxide and perfluorocarbons – in CO2 equivalents, where 1 kg of N2O is equal to 298 kg in CO2 equivalents1.

In countries with an ETS in place, carbon credits are awarded to companies, allowing them to emit a fixed amount of carbon dioxide or the equivalent amount of another greenhouse gas. Different kinds of ETS are already in place across the world, including the EU, the UK, Switzerland, Kazakhstan, China, New Zealand, South Korea, Mexico and several regions in the USA, Canada and Japan.    

With the need to reduce emissions, abatement systems are getting more important, as many existing plants do not have such systems and still need to comply with environmental regulations. A tertiary abatement reactor provides a solution by removing nitrous oxide (N2O) and nitrogen oxides (NOx) from the tail gas produced from nitric acid plants.

Why do we need emission control in nitric acid plants?

The tail gas (off-gas) from nitric acid production mainly contains nitrogen, water and impurities, such as NOx and N2O. NOx stands for nitrogen oxide and nitrogen dioxide, gases that contribute to air pollution, smog and acid rain. N2O – nitrous oxide, commonly known as laughing gas – is a powerful greenhouse gas, estimated to be 250-300 times more powerful than CO2

To comply with the environmental legislation, all the new and most of the existing nitric acid plants need to implement measures to reduce N2O emissions into the atmosphere. Abatement systems to reduce emissions are getting more important, as many existing plants do not have such systems and still need to comply with environmental regulations. 

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N2O is formed in the precious metal gauzes of the nitric acid plant as a by-product of ammonia oxidation. There are several locations in the plant where N2O could be eliminated, but to remove both N2O and NOx simultaneously a tertiary abatement system is the best solution. The process conditions at that location are suitable for the reduction of both components and therefore only one unit operation is required.

Stamicarbon collaborates with the most recognized catalyst suppliers in order to propose optimal and reliable tailor-made solutions for every type of nitric acid plant technology, process conditions and emission requirements.

Tertiary abatement system for grassroot Stamicarbon plants

The system consists of a reactor with two packed beds holding a Fe-zeolite catalyst. The first bed reduces the main part of the N2O, while the second bed eliminates the remainder of the N2O and the NOx. To achieve the latter, ammonia is supplied to the tail gas leaving the first catalyst bed. 

Tertiary abatement system

In the Stamicarbon nitric acid process, the tail gas temperature upstream the expander is above 480°C. This temperature is reachable by a proper heat integration in the system, with the last tail gas heater placed inside the burner and close to the catalytic gauzes. Such high temperature of the tail gas maximizes the energy efficiency of the process and provides several advantages to the abatement system. 

  • The high temperature increases the efficiency of the catalyst, reducing the catalyst volume needed 
  • N2O can be simply removed by thermal decomposition, which eliminates the use of natural gas in the first catalyst bed as a reducing agent
  • No possibility of ammonia slippage, since at 480°C, ammonia is decomposed into N2 and H2
  • Possibility of combining NOx and N2O removal in one bed since the temperature is suitable for both reactions
  • Application of standard stainless steel to be used as the material of construction for the tail-gas expander

In other words, the tail gas temperature of 480°C represents the optimal conditions in terms of CAPEX and OPEX: it reduces the amount of catalyst required, allows easy combined removal of NOx and N2O in a single device without the addition of methane and allows the use of standard stainless steel as the material of construction for the tail-gas expander. 

Flow diagram of the dual-pressure nitric acid process
Flow diagram of the dual-pressure nitric acid process

Upstream the abatement system, the tail gas typically has the following concentrations:

  • NOx: 400-600 ppmv
  • N2O: about 1000 ppmv

These figures are reduced to levels below 20 ppmv before being emitted into the atmosphere. 

The main features of the Stamicarbon abatement system are:

  • Simple and reliable design with guaranteed performance
  • One-stop-shop for design and delivery of equipment and catalyst
  • Retrofitting capability for every type of nitric acid plant
  • Adequate mixing between ammonia and tail gas, with no ammonia carry-over
  • Minimized pressure drop on the catalyst beds
  • Catalyst particles cannot reach the downstream tail gas expander
  • Flexibility in the catalyst selection depending on tail gas temperature

Collaboration to reduce emissions from nitric acid production 

To promote the installation of tertiary abatement systems, Stamicarbon is participating in the initiatives of the Nitric Acid Climate Action Group (NACAG)2, a global project initiated by the German Government. Through this initiative, nitric acid plants from partner countries can receive technical and financial support in implementing technology for N2O abatement. 

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Resources:

1) Statistics Netherlands, “CO2 equivalents.” Available at: https://www.cbs.nl/en-gb/news/2022/06/urgenda-reduction-target-for-ghg-emissions-achieved-in-2020/co2-equivalents
2) Nitric Acid Climate Action Group. Available at: https://www.nitricacidaction.org/