New South African legislation will be in place by April 1, 2020, requiring existing plants to comply with minimum emission standards, or MES, for new plants to meet nitrogen oxides, or NOX, levels of less than 350 mg/Nm³. New technology is now available to assist nitric acid plants to become compliant with the 2020 NOX MES standards.

In chemistry, NOX is a generic term for the nitrogen oxides that are major contributors to air pollution. Nitrous oxide (N₂O), commonly known as laughing gas, is produced as a by-product in the manufacture of nitric acid, an important raw material used, among other things, to make fertilizers. Nitrous oxide is a potent greenhouse gas implicated in the destruction of the ozone layer.

“Operators are under increasing pressure to produce more and more efficiently both from an operational cost and environmental point of view,” notes thyssenkrupp industrial solutions GM Neville Eve.

The EnviNOx process is aimed at almost completely removing NOX and, if required, the greenhouse gas N₂O and turning it into nitrogen and water. The technology has been used on an industrial scale, with about 30 plants using the process worldwide, including two plants in South Africa. thyssenkrupp is the licensor of the nitric acid process for five of the six nitric acid plants in the country. The earliest unit has been in operation in a 1 000-tons-a-day nitric acid plant in Linz, Austria, since 2003.  The first unit in Africa came into operation three years later, in a 1 830-tons-a-day nitric acid plant in Egypt.

The reactor removes NOX through a process similar to that used to reduce emissions in modern vehicles, namely selective catalytic reduction, or SCR, which uses a catalyst and a small amount of ammonia in the reaction process.

Although SCR technology is already used by a number of suppliers, thyssenkrupp has developed a reactor and catalysts specifically for the reduction of both N₂O and NOX in nitric acid plant tail gas, achieving N₂O removal of up to 99 %, while reducing NOX emissions to nearly zero, states senior manager process industries Bruce Bassett.

The central component of the system is the reactor, which contains a special catalyst and is typically located between the final tail gas heater and the tail gas turbine.

The combination of N₂O and NOX emission reduction in a single reactor vessel makes the system flexible, allowing for an initial investment only in NOX reduction, with the option of including N₂O reduction at a later stage.

thyssenkrupp has recently secured an engineering and procurement contract to retrofit the technology at one of South Africa‘s nitric acid plants.