General Introduction:

Nitrous oxide (N2O) is an unwanted, invisible and previously neglected by-product of the manufacture of nitric acid. It is formed alongside the main, desired product nitric oxide (NO) during the catalytic oxidation of ammonia in air over noble metal gauzes. The production of nitric acid takes place in three main process steps as indicated by the following reactions:

1. Ammonia (NH3) combustion to form nitric oxide (NO)1:

4 NH3 + 5 O2 → 4 NO + 6 H2O (main reaction 1) (1) Simultaneously nitrous oxide (N2O), nitrogen (N) and water (H2O) are formed as well, in accordance with the following equations:

4 NH3 + 3 O2 → 2 N2 + 6 H2O (side reaction 1) (2)

4 NH3 + 4 O2 → 2 N2O + 6 H2O (side reaction 2) (3) NO yield depends mainly on pressure and temperature in the ammonia oxidation process and usually is in a range of 95% to 97%.

2. NO is oxidised to nitrogen dioxide (NO2):

       2 NO + O2 → 2 NO2                                      (main reaction 2)                    (4) 

1. (According to the technical process) Absorption of NO2 in water to form nitric acid (HNO3):

       3 NO2 + H2O → 2 HNO3 + NO                              (main reaction 3)                    (5)          (NO is oxidised to NO2 according to main reaction 2) 

Nitric acid plants are, in the vast majority of cases part of a chemical complex. They are built and operated to supply acid for consumption in downstream process units. The ammonia plant that feeds the nitric acid plant is also frequently a part of this chemical production facility. The most common use for nitric acid is for fertilisers, with smaller quantities going into the manufacture of organic compounds and mining explosives.

On leaving the ammonia oxidation reactor some of the N2O may be destroyed in the part of the plant upstream of the absorption tower by high temperature homogeneous gas phase decomposition and by catalytic decomposition on platinum deposits formed from metal lost from the ammonia oxidation catalyst. Since platinum dust carryover into the tail gas section of the plant is prevented by the absorption tower, which acts as a very efficient scrubber, and the temperatures encountered in the tail gas section of the plant are lower than those leading to homogeneous gas phase N2O decomposition there is no relevant loss of N2O in the tail gas section unless a N2O destruction facility is installed. N2O that has reached the tail gas section is thus discharged to atmosphere in the tail gas, and has no economic value.

Depending on technical parameters (e.g. tail gas temperature) the project applicant has to decide either to install a catalytic decomposition process or catalytic reduction process for the proposed project activity.

Description of catalytic decomposition process:

Catalytic decomposition of N2O occurs when the N2O is split into its constituent elements by contact with a catalyst. A catalyst is a material which accelerates the speed of the reaction without itself being transformed or consumed by the reaction.

Overall reaction:

       2 N2O → 2 N2 + O2                                                                            (6) 

The products of N2O decomposition are the substances that result from decomposition reaction (N2 and

2. Description of catalytic reduction process:

Although the term catalytic reduction nowadays has a more general definition in terms of the transfer of electrons, the following definition is sufficient for present purposes: Catalytic reduction of N2O occurs when reactions take place between N2O and other substances in contact with a catalyst, such that the oxygen is removed from the N2O molecule and forms one or more compounds with other species. The substance or substances that react with N2O to remove oxygen are termed reducing agent. A general reaction equation for the catalytic reduction of N2O can be given as:

n N2O + x RA → n N2 + y1 P(1)OA+y2 P(2)OB + .....+ z1 Q(1) + z2 Q(2) + ….. (7)

where RA is a molecule of the reducing agent, P(1)OA, P(2)OB are the compound formed by reaction with the oxygen of the N2O and Q(1), Q(2) represents further products of the oxidation reaction, n, x, y1, y2, z1, z2 are the appropriate stoichiometric coefficients.

Equations reduction N2O with hydrocarbons:

1. g.

Reducing agent is methane:

CH4 + 3 N2O → CO + 2 H2O + 3 N2 (8)

CH4 + 4 N2O → CO2 + 2 H2O + 4 N2 (9)

2. g.

Reducing agent is ethane, overall reaction is:

C2H6 + 5 N2O → 2 CO + 3 H2O + 5 N2 (10) or

C2H6 + 7 N2O → 2 CO2 + 3 H2O + 7 N2 (11) e.g.:

Reducing agent is propane, overall reaction is:

C3H8 + 7 N2O → 3 CO + 4 H2O + 7 N2 (12) or

C3H8 + 10 N2O → 3 CO2 + 4 H2O + 10 N2 (13) e.g.

Reducing agent is butane, overall reaction is:

C4H10 + 9 N2O → 4 CO + 5 H2O + 9 N2 (14) or

C4H10 + 13 N2O → 4 CO2 + 5 H2O + 13 N2 (15)

The definition does not exclude the possibility of side reactions resulting in consumption of reducing agent without any reduction of N2O, for example with propane:

2 C3H8 + 7 O2 → 6 CO + 8 H2O (16) or

       C3H8 + 5 O2 → 3 CO2 + 4 H2O                                (17) 

The world’s nitric acid plants represent the single greatest industrial process source of N2O emissions. Currently, approx. 700 nitric acid plants are operated globally with an estimated amount of N2O emissions of 400,000 t N2O p.a. (corresponding to 125 Mio t CO2e p.a.). In response to this, UHDE GmbH (see section A.3.), a leading company in the field of nitric acid technology, has undertaken the task of developing processes for removing N2O from nitric acid plant tail gas streams based on the catalytic decomposition or catalytic reduction of N2O. Efforts have been concentrated on treating the tail gas, as this end-of-pipe approach offers the general advantage, compared with other possible measures (called primary and secondary measures, see section A.4.3.), that minimum interference with the nitric acid production process is caused. In particular, any possibility of nitric acid product contamination, or loss of NO that could otherwise influence nitric acid production, is eliminated.

Project Specific description:

Principles of the EnviNOx® process

The EnviNOx® process used in the Abu Qir II nitric acid plant is based on the catalytic reduction of NOx (NO and NO2) with ammonia (NH3) and of nitrous oxide (N2O) with a hydrocarbon. The hydrocarbon used is natural gas of which the main constituent is methane (CH4). The reactions take place over two iron zeolite catalyst beds.

The first bed contains an iron zeolite that is especially effective in catalysing the reduction of NOx with ammonia according to such reactions as:

       6 NO2 + 8 NH3               → 7 N2 + 12 H2O                      (18)

       6 NO + 4 NH3                → 5 N2 + 6 H2O                       (19)

       NO + NO2 + 2 NH3            → 2 N2 + 3 H2O                       (20)

       4 NO + O2 +4 NH3            → 4 N2 + 6 H2O                       (21) 

Effectively all the NOx is removed. Some destruction of N2O also occurs.

The second and main bed contains an iron zeolite that is particularly efficient in catalysing the reduction of nitrous oxide with hydrocarbons. The proposed project activity will use about 52 kgCH4/h.

       3 N2O + CH4       → 3 N2 + 2 H2O + CO                            (22)

       4 N2O + CH4       → 4 N2 + 2 H2O + CO2                           (23) Similar reactions take place between nitrous oxide and the small quantities of higher hydrocarbons such as ethane C2H6, propane C3H8 and butane C4H10 that are present in natural gas. N2O reduction by these reactions is much more effective when NOx is absent. A large proportion of the carbon monoxide that is formed is further oxidised to carbon dioxide:

       2 CO + O2        → 2 CO2                                 (24) All the above reactions are exothermic which leads to a temperature rise over the EnviNOx® reactor. A small quantity of methane leaves the reactor unreacted. Compared with the reduction in greenhouse gas emission achieved by the destruction of N2O the additional greenhouse gas emissions (CO2 and CH4) caused by the process are insignificant. The proposed project activity will reduce the N2O emissions from the Nitric Acid Plant of Abu Qir Fertilizer Co. by up to 99% by installing the EnviNOx® process. The project will use CH4 as reducing agent.

It is important to emphasise that the purpose of the hydrocarbon and ammonia is not that of a fuel, increasing the temperature of the tail gas to a level at which high rates of N2O decomposition can take place, but that they are used as genuine chemical reagents that take part in reactions with N2O and NOX respectively on specific sites on the surface of catalysts specially developed for the purpose by Uhde. Thus the consumption of hydrocarbons corresponds to the stoichiometric ratio given in the reaction equations above.

The proposed project activity will consume about 52 kg methane per hour. The consumption will be fine- tuned during commissioning of the EnviNOx® reactor.

CARBON Egypt Ltd. (see section A.3.) will invest in the most efficient catalytic destruction technology for N2O emissions reduction in the tail gas of nitric acid plants (furthermore called “EnviNOx®-System”) provided by the technology provider UHDE GmbH (see section A.3.). CARBON will have one EnviNOx®-System installed at all the nitric acid plant of ABU QIR II (see section A.3.) that are in full commercial operation. AFC will operate the EnviNOx®-System at its nitric acid plant ABU QIR II, which was constructed by UHDE in 1991.

The project’s aim is to reduce (almost eliminate) N2O emissions at the nitric acid plant ABU QIR II, with potential additional environmental and secure social benefits. The project activity will not result in any revenues except the income from the sale of CERs. The catalytic N2O destruction project activity is expected to reduce 98% of the N2O emissions that would be emitted without the project activity. Under related project circumstances at the nitric acid of AMI in Austria, UHDE’s EnviNOx®-System reduces more than 98% of all N2O emissions of the nitric acid plant. The project applicant and project operator will voluntarily invest a share of the income from the sale of the CERs in a “Social Fund” to support social projects in the area of Abu Qir (additional social benefit).