New South African legislation decreed that existing plants must comply with the Minimum Emission Standards (MES) in compliance to NOX (Nitrogen Oxides) levels of less than 350 mg/Nm3.

Catalytic decomposition of N2O and catalytic NOX reduction with ammonia over zeolite catalysts.

Catalytic decomposition of N2O and catalytic NOX reduction with ammonia over zeolite catalysts. All images supplied by thyssenkrupp Uhde

In chemistry, NOX is a generic term used for the nitrogen oxides that contribute to air pollution and the destruction of the ozone layer. 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 in the production of fertilisers, among others.

“Operators are under increasing pressure to produce more efficiently both from an operational cost and environmental point of view,” notes Rajend Govender CEO of thyssenkrupp Uhde South Africa. “To meet the challenge of managing emissions to minimum levels and still be up on production requires proven technologies that allow cost-effective set-up and operation and at the same time make a significant contribution to environmental protection.”

The EnviNOx® process from the company is capable of almost completely removing NOX – and, if required, the greenhouse gas N2O – turning it into harmless nitrogen and water. The technology has been proven on an industrial scale with around 55 plants using the EnviNOx process worldwide. Four EnviNOx systems have been sold in South Africa. The company is also the licensor of the nitric acid process for five of the six nitric acid plants in South Africa. The earliest EnviNOx unit has been in operation in a 1 000tpd (tons per day) nitric acid plant in Linz, Austria, since 2003. The first unit on the African continent came on stream three years later in a 1 830tpd nitric acid plant in Egypt.

The EnviNOx process removes NOX through a process not dissimilar to what is used to reduce emissions in modern vehicles, namely selective catalytic reduction (SCR), which uses a catalyst and a small amount of ammonia in the reaction process.

While SCR technology is already used by a number of suppliers, thyssenkrupp stands out as having successfully developed a reactor and catalysts specifically for the abatement of both N2O and NOX in nitric acid plant tail gas. “The N2O and NOX removal rates of our EnviNOx® technology, which was developed in Germany in the early 2000s, have been proven commercially to be very high, achieving N2O removal of up to 99%, and NOX emissions to nearly zero,” states Nithesh Mohun, Business Development Manager.

The central component of the EnviNOx system is the reactor. It contains a special catalyst developed and optimised by the company and its partners and is typically located between the final tail gas heater and the tail gas turbine.

thyssenkrupp offers 2 EnviNOx process variants. Variant 1 is especially applicable for tail gas temperatures higher than approximately 425°C while variant 2 is suitable for temperatures between around 300°C and 550°C. Flow schemes of both variants are depicted below: 

 Catalytic reduction with NOX with ammonia and N2O with hydrocarbons over zeolite catalysts.

Catalytic reduction with NOX with ammonia and N2O with hydrocarbons over zeolite catalysts.

The combination of N2O and NOX emissions reduction in a single reactor vessel renders EnviNOx extremely flexible as it provides end-users the option of initially investing in NOX reduction only leaving open the option to upgrade to N2O abatement at a later stage.

thyssenkrupp Uhde recently completed a contract for engineering and procurement to retrofit its EnviNOx technology at two of South Africa’s nitric acid plants. “As the customer initially only required NOX emission reduction, we supplied the EnviNOx ready solution to treat the tail gas stream from their nitric acid plant. When more stringent South African legislation on the abatement of N2O comes into effect, we can simply add a second catalyst to the customer’s reactor.”                          

Source: Supplied by thyssenkrupp Uhde