Nitrogen oxides in nitric acid plants

Paul Boughton

The measurement of nitrogen oxides is a critical and challenging task. Matthias F Kutter reports.

Industrial plants for production of nitric acid are serving many industries with base materials. A huge demand originates from agriculture: nitrogen fertiliser supply biological material all nutrients essential to the growth of plants. They are essential for high-yield harvest. Beyond the basic nitric acid, these plants produce other substances such as ammonium nitrate and urea.

Nitrogen oxide (NO) is the important gas in the multistage process of nitric acid production. In the oxidised form of nitrogen dioxide (NO2) it is absorbed in water to form nitric acid. This chemical compound is a highly corrosive and toxic strong acid, normally colourless but tends to acquire a yellow cast due to the accumulation of oxides of nitrogen.

The production of nitric acid comes with emissions of greenhouse gases. The control techniques and measures to minimise such emissions is a high priority task of the operations management of such a plant.

Emissions from nitric acid manufacture consist primarily of NO, NO2 (which account for visible emissions), trace amounts of HNO3 mist, and ammonia (NH3). By far, the major source of nitrogen oxides (NOx) is the tailgas from the acid absorption tower.

In general, the quantity of NOx emissions is directly related to the kinetics of the nitric acid formation reaction and absorption tower design. NOx emissions can increase when there is insufficient air supply to the oxidiser and absorber, low pressure, especially in the absorber, high temperatures in the cooler-condenser and absorber, and faulty equipment such as compressors or pumps that lead to lower pressures and leaks.

The measurement of nitrogen oxides is therefore a critical and challenging task. The technique chosen is Chemiluminescence Detection (CLD) due to its specifity and linearity over a broad range. It uses the specific reaction of nitrogen monoxide with ozone, which is produced inside the instrument. The resulting activated molecule of NO2 decays immediately by emitting a light quant. It is measured by a photomultiplier and this signal is proportional to the concentration of NOx. The inlet for the sample gas needs to be heated, the mainstream diverted to vent and the remaining small sample transported into a vacuum to minimise any damage to the instrument.

In a recent installation, four CLD analysers went into operation. At about 15 metres height of the stacks there are sample probes installed. The gas samples are transmitted by heated lines.

On the first production line pressure regulators were installed close to the probe, which proved to be troublesome for servicing. For the other production lines the new design with regulators next to analyser cabinet allowed comfortable access to the devices for service and maintenance.

State regulation require the plant operator to report continuous data on the NOx emissions, particularly on the efficiency of the catalytic reduction units. Typical measurements are shown in Table 1.

Despite the harsh conditions at the nitric acid plant the analytical equipment performed over one year of continuous measurement without failure up to the expectation and full satisfaction of the plant operator.

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Matthias F Kutter is with Eco Physics AG, Dürnten, Switzerland.