Methodological issues

Nitric acid production involves three distinct chemical reactions that can be summarised as follows:

Nitrous oxide generation during the production of nitric acid is not well documented. Nitrogen oxidation steps under overall reducing conditions are considered to be potential sources of N2O. Nitric oxide (NO), an intermediate in the production of nitric acid, is also documented to readily decompose to N2O and nitrogen dioxide (NO2) at high pressures for a temperature range of 30 to 50°C (Cotton and Wilkinson, 1988).

Perez-Ramirez et al. (2003; p.123) specify three intermediate reactions during the oxidation of ammonia that might result in the formation of N2O:

NH3 + O2 ^ 0.5N20 + 1.5H2O NH3 + 4NO ^ 2.5N2O + 1.5H2O NH3 + NO + 0.7502 ^ N2O + 1.5H2O

Reactions that lead to the formation of N2O or N2 are undesirable in that they decrease the conversion efficiency of NH3 and reduce the yield of the desired product, NO (Perez-Ramirez et al, 2003; p.124). It is not possible to define a precise relationship between NH3 input and N2O formation because in general, 'the amount of N2O formed depends on combustion conditions, catalyst composition and state (age), and burner design' (Perez-Ramirez et al, 2003; p.123). Emissions of N2O depend on the amount generated in the production process and the amount destroyed in any subsequent abatement process. Abatement of N2O can be intentional, through installation of equipment designed to destroy N2O, or unintentional in systems designed to abate other emissions such as nitrogen oxides (NOx).

Perez-Ramirez et al. (2003; p.126) classify abatement approaches as follows and abatement measures associated with each approach are outlined in Table 3.2:

• Primary abatement measures aim at preventing N2O being formed in the ammonia burner. This involves modification of the ammonia oxidation process and/or catalyst.

• Secondary abatement measures remove N2O from the valuable intermediate stream, i.e. from the NOx gases between the ammonia converter and the absorption column. Usually this will mean intervening at the highest temperature, immediately downstream of the ammonia oxidation catalyst.

• Tertiary abatement measures involve treating the tail-gas leaving the absorption column to destroy N2O. The most promising position for N2O abatement is upstream of the tail-gas expansion turbine.

• Quaternary abatement measures are the pure end-of-pipe solution, where the tail-gas is treated downstream of the expander on its way to the stack.

Table 3.2

N2O abatement approaches and abatement measures

Abatement approaches

Abatement measures

Primary abatement

• Optimal oxidation process

• Modification of platinum-rhodium gauzes

• Oxide-based combustion catalysts

Secondary abatement

• Homogeneous decomposition in the burner

• Catalytic decomposition in the burner (process gas catalytic decomposition)

• Catalytic decomposition downstream of the burner (before the absorption column)

Tertiary abatement

• Thermal decomposition

• Non-selective catalytic reduction (NSCR)

• Tail-gas catalytic decomposition

• Selective catalytic reduction (SCR)

Quaternary abatement

• Non-selective catalytic reduction (NSCR)

• Catalytic decomposition

• Selective catalytic reduction (SCR)

Source: Adapted from Perez-Ramirez et al. (2003).

The abatement achieved will depend on the technology implemented with tertiary measures stated as, 'enabling the achievement of high levels of N2O removal (>99 percent)' (Perez-Ramirez et al., 2003; p.136). Additionally, it is noted that although NSCR is a proven technology for N2O reduction, the replacement of NSCR systems by SCR systems for NOx reduction has a negative side-effect on its application for N2O reduction. Further, 'NSCR is most likely not a viable option anymore, due to the high fuel consumption levels and high secondary emissions' (Perez-Ramirez et al., 2003; p.137).

Future adoption of technologies will depend on cost-effectiveness and the stringency of any emissions regulation. More cost-effective options are available for new plants than for existing plants. Tail-gas options are appealing since they do not interfere with the process. Direct N2O decomposition is a very attractive and cost effective option in plants with tail-gas temperatures greater than 723 K. However, two-thirds of the nitric acid plants in Europe have low-temperature tail-gases. To this end, preheating or using reductants (light hydrocarbons or ammonia) is required, making the after-treatment prohibitive. The most elegant and cost-effective option is the in process-gas catalytic decomposition, located in the heart of the plant (the ammonia burner). Concerns with this abatement option are chemical and mechanical stability of the catalyst as well as the possible NO loss. Several catalyst manufacturers and nitric acid producers have addressed this problem and catalysts are in the early stages of commercialisation. Advantageously, and contrary to the tail-gas option, this technology can be retrospectively applied to all existing plants. Further discussion of options is provided in Perez-Ramirez et al. (2003).

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