Nitric acid is a key ingredient in N-based fertilizers. As a raw material, it also is used for the production of adipic acid (another important N2O source - see next section) and explosives, metal etching and in the processing of ferrous metals.
Nitric acid production involves the oxidation of ammonia (NH3) using a platinum catalyst (Ostwald process). Nitrous oxide forms during the catalytic oxidation of ammonia over platinum/rhodium gauzes, the major product being NO. It is estimated that around 5g of nitrous oxide are produced in this way for every kilogram of nitric acid that is produced. Nitric acid plants now represent the single largest industrial process source of nitrous oxide and there has been a strong need for technologies to lower these emissions. The installation of N2O abatement technologies is obligatory for nitric acid plants in the European Union because of regulations entering into force in 2013.
In principle there are three different ways to achieve this goal, all of which are based on the use of various ceramic catalysts. These may be placed at different points in a nitric acid plant, leading to various advantages and drawbacks of the respective measure. In the US the nitric acid industry currently controls N2O emissions by using both non-selective catalytic reduction (NSCR) and SCR technologies to reduce N2O to molecular nitrogen. While NSCR is more effective than SCR at controlling N2O, these units are not generally preferred in today's plants because of high energy costs and associated high gas temperatures.
Recently much progress has been made in reducing nitrous oxide emissions from the ammonia oxidation process by using more efficient oxidation conditions, with lower reaction temperatures preventing so much of the ammonia ending up as nitrous oxide. The commercially tried and tested EnviNOx® process for the abatement of N2O and NOx emissions in nitric acid plants provides a highly viable solution to this problem, as it achieves almost complete removal of N2O and NOx. This is accomplished by using a special iron zeolite catalyst, which showed under laboratory conditions a diverse reactivity towards N2O. The catalyst either decomposes N2O into N2 and O2 -an effect that increases significantly in the presence of NOx in the tail gas - or by reducing N2O using various reducing agents, such as hydrocarbons. In addition, the iron zeolites were proven to be excellent DeNOx catalysts, allowing N2O abatement to be ideally combined with NOx reduction. Currently four different process variants are available, allowing either the catalytic reduction of NOx alone, or the combined catalytic decomposition of N2O and catalytic reduction of NOx, the catalytic reduction of N2O and NOx, or, finally, the catalytic decomposition of N2O alone.
Several nitric acid plants have been equipped with these systems, which achieve N2O removal rates of 98-99 per cent, with NOx emissions being reduced to as low as 1ppmv. The longest-operating EnviNOx® unit has been running since 2003 to the complete satisfaction of its owner and still with the first charge of catalyst.
Basing their judgement on this successful track record and the exceptionally high rates of N2O and NOx abatement, the responsible European Union body has declared the EnviNOx® process to be the best available technique (BAT) for N2O and NOx abatement in nitric acid plants.
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