Technology

Topsoe's SNOX™ technology is an advanced flue gas cleaning technology that simultaneously removes sulfur dioxide (SO₂), nitrogen oxides (NOx), particulates, carbon monoxide (CO), and other contaminants from industrial emissions. The technology handles refinery waste streams (H₂S, Claus tail gas). Unlike conventional methods, it recovers sulfur as commercial-grade sulfuric acid (H₂SO₄) and converts NOx to harmless nitrogen (N₂), eliminating waste, water consumption, and absorbent use (e.g., limestone). Developed by Topsoe, it is deployed in power plants, refineries, and carbon black facilities to meet stringent environmental standards while improving energy efficiency^[1][2][3][4]. 

Process Flow Diagram

A detailed process flow diagram of the SNOX™ technology is shown in Fig. 1:

Figure 1 - SNOX™ Process Schematic

Process Description

The SNOX process involves five catalytic and condensation steps:

1. Dust Removal:
   Flue gas passes through a high-efficiency fabric filter or electrostatic precipitator to remove particulates (>90% efficiency), protecting downstream catalysts^[3][5].
    
2. NOx Reduction (SCR Reactor):
   The gas is heated to 730°F (388°C), mixed with ammonia (NH₃), and routed to a Selective Catalytic Reduction (SCR) reactor. Using Topsoe’s DNX catalyst, NOx is reduced to N₂ and water:
   
   NO + NH₃ + 0.25 O₂ −> N₂ + 1.5 H₂O
   
   NOx removal exceeds 90% with minimal NH₃ slip due to downstream oxidation^[3][5][6].
    
3. SO₂ Oxidation:
   Gas is reheated to 780°F (415°C) and enters an SO₂ converter with Topsoe’s VK-WSA catalyst. SO₂ oxidizes to SO₃:
   
   SO₂ + 0.5 O₂ −> SO₃
   
   Conversion rates exceed 95%^[3][6].
    
4. Heat Recovery:
   Hot gas transfers heat to incoming flue gas via a gas/gas heat exchanger, improving energy efficiency^[4][5].
    
5. Acid Condensation (WSA Tower):
   Gas enters a Wet Sulfuric Acid (WSA) condenser, where SO₃ hydrates to H₂SO₄ vapor and condenses into 95–98% commercial-grade sulfuric acid:
   
   SO₃ + H₂O −> H₂SO₄ (g) −> H₂SO₄ (l)
   
   Residual contaminants (CO, mercury) are removed, and clean gas is released^[1][3][4].

Process Efficiency

• Yields:
  • Sulfur recovery: >95% as salable H₂SO₄.
  • NOx removal: >90%.
  • Particulates: <1 mg/Nm³ ^[3][4].
     
• Energy Efficiency:
  • Exothermic reactions generate surplus heat, reused to preheat boiler air or produce steam. This improves net energy efficiency by ~5%, reducing CO₂ emissions by a similar margin. For example:
    • Petrobras’ RNEST refinery gains 10.3 MWe net energy (Fig. 2)^[4].
    • A 310 MW power plant saves 0.8% in fuel consumption^[7].
       
• Environmental Performance:
  • Consistently achieves emissions below 10 ppm SOx/NOx and 1 mg/Nm³ particulates^[4][10].

Figure 2 - Petrobras RNEST SNOX™ configuration*

*Residual oil/petcoke fired steam and power plant. Two SNOX™ plants also treat Claus tail gas, H₂S gas, Sour Water Stripping (SWS) gas and other sulphur-containing waste streams.

Process Economics

• CAPEX:
  Independent of fuel sulfur content. Competitive with conventional FGD+SCR systems, especially at high sulfur loads^[5][8].
   
• OPEX:
  • Negligible reagent costs (only NH₃ for NOx reduction).
  • No water, limestone, or waste disposal expenses.
  • Revenue from sulfuric acid sales offsets operating costs.
  • OPEX decreases with higher SO₂ concentrations due to increased acid production^[4][8].
     
• Savings Example:
  SNOX reduces OPEX by $14.9 million over 10 years vs. conventional Claus + amine systems in refineries^[9].

Commercial Experience

• Global Deployments:
  • 35+ plants since 1991, including power plants (e.g., Ohio Edison, USA; Nordjyllandsværket, Denmark), refineries (e.g., Petrobras RNEST, Brazil; Agip/ENi, Italy), and carbon black facilities (e.g., Orion, USA)^[4][8][10].
     
• Recent Projects:
  • Essar Oil UK (Stanlow, 2028): SNOX integral to a £360M carbon capture project targeting 1M tonnes/year CO₂ reduction^[11].

References

1. topsoe.com, SNOX™ technology. (accessed 26^th Jun 2025)
2. topsoe.com, Flue gas treatment. (accessed 26^th Jun 2025)
3. Wikipedia, SNOX process. (accessed 26^th Jun 2025)
4. Rodrigo Lavich, Petrobras and Torben Christensen, Haldor Topsøe. COBRAS 2013, Brazil. Petrobras using Topsøe SNOXTM sulphuric acid technology for flue and sour gas treatment in the new RNEST oil refinery in Pernambuco, Brazil.
5. U.S. Department of Energy (DOE) National Energy Technology Laboratory, Jun 2000, DOE/NETL-2000/1125, SNOX^TM Flue Gas Cleaning Demonstration
   Project: A DOE Assessment.
6. Nikhilesh Mukherjee, 18^th Sep 2018, Linkedin article, Flue gas desulfurization: SNOX process with unique catalytic converter for NO_x and SO₂.
7. Christian Strøbech, ACCOAT, CO₂ case: Calculation of energy savings in the Topsøe SNOX™ flue gas cleaning plants. (Document date: 29^th Oct 2013)
8. U.S. Department of Energy - Office of Scientific and Technical Information, Conference, 1^st Jul 2002, The SNOX process for power plants using high sulfur fuels.
9. J. Lewis et al., Hydrocarbon Processing, Mar 2023, New state-of-the-art in sulfur recovery? Part 3: Economic Benefits.
10. topsoe.com, Most effective sulfur-removal solution. (accessed 26^th Jun 2025)
11. Digital Refining, 22^nd Dec 2023, Essar selects SNOX™ technology from Topsoe for carbon capture project.