Nitric acid (HNO3) production is mainly driven by fertilizer industry. Nitric acid is neutralized with ammonia to give ammonium nitrate. This application consumes nearly 80% of the global nitric acid production. Virtually most of the nitric acid is produced by a catalytic oxidation of ammonia. In this industrial process, ammonia (NH3) is oxidized to form nitric acid (HNO3) finally.
However, within this process also some N2O is produced, together with residual NO and NO2 it will leave the absorption tower as tail gas. Downstream of the absorption tower the tail gas is usually treated in a DeNOx reactor before it is released to atmosphere. Ammonia gas is added to the tail gas before entering the DeNOx reactor. Then NOx reduction will take place with help of a catalyst. Depending on the plant type also N2O reduction can be performed in a separate part of the DeNOx reactor.
To control the reduction process as well as the emissions to atmosphere usual measuring points include the inlet & outlet of the DeNOx reactor and at stack. The measurement at stack can include other measuring components e.g. water, carbon monoxide, carbon dioxide, oxygen and sometimes hydrocarbons like methane or propane. Which components are being measured at stack depends whether only NO and NO2 are being reduced or N2O as well.
Measurements at inlet and outlet of the reactor are demanding due to the operating pressure of 4 - 10 bar and gas temperature at sampling point of 230°C - 430°C. NOx measurement at the inlet of the DeNOx reactor is necessary to inject the correct amount of ammonia. NOx measurement at the outlet is required to control the efficiency of the DeNOx reactor. In addition, ammonia is measured to ensure that the ammonia concentration is kept below the emission limit.
Hot wet extractive analyzers are very well suited for the described application. The high sampling and measurement temperature of 185°C to 200°C prevents buildup of salts even under start-up conditions of the nitric acid plant. Analyzers are typically based on a multicomponent infrared photometer to monitor 8-12 components simultaneously. For measurement at inlet and outlet of DeNOx reactor a heated sample preconditioning system (SPCS) is installed to reduce the gas pressure. For measurement at stack furthermore often a certification according to EN15267 is required.
Different solutions for this environmentally important application are presented and discussed.