Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 33.2.5 Tail gas properties and emission reductionThe tail gas composition depends on the applied process conditions. Table 3.3 gives anoverview of tail gas properties.Parameter Level UnitNO x as NO 2 200 – 4000 mg/Nm 3NO/NO 2 ratio about 1/1 molar ratioN 2 O 600 – 3000 mg/Nm 3O 2 1 – 4 % v/vH 2 O 0.3 – 0.7 % v/vPressure 3 – 12 barTemperature after absorption 20 – 30 ºCTemperature after reheating 200 – 500 ºCVolume flowTable 3.3: Tail gas properties after the absorption stage[94, Austrian UBA, 2001], x [112, Gry, 2001]20000 – 100000 Nm 3 /hour3100 – 3300 x Nm 3 /tonne 100 % HNO 3The most common treatment techniques for tail gases from nitric acid plants are:• SCR (for NO x abatement, see Section 3.4.9)• NSCR (for NO x and N 2 O abatement, see Section 3.4.8.More recent strategies to reduce NO x or N 2 O emissions include:• optimisation of the oxidation stage (see Sections 3.4.1, 3.4.2, 3.4.3 and 3.4.5)• catalytic decomposition of N 2 O, applied already in the oxidation reactor, directly after theoxidation catalyst (see Section 3.4.6)• optimisation of the absorption stage (see Section 3.4.4)• combined catalytic NO x /N 2 O abatement with intermediate injection of NH 3 , applied to thetail gas in front of the expander turbine (see Section 3.4.7).3.2.6 Energy exportThe nitric acid process has undergone significant energy improvements during the last decadesfrom atmospheric plants, via M/M units to M/H plants. The reaction from NH 3 to HNO 3theoretically releases 6.3 GJ/tonne 100 % HNO 3 . However, energy losses in gas compressorsand cooling (water) reduce the net steam export. If all thermal energy is converted to electricalpower in a steam turbine, the net energy export will be reduced by approximately 65 %.GJ/tonne 100 % HNO 3RemarkModern M/H plant 2.4 As HP steamAverage net export of European plants 1.6Best plants 30 years ago 1.1Table 3.4: Overview of energy export from HNO 3 production[94, Austrian UBA, 2001, 107, Kongshaug, 1998]Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 99