DeNOx recovery

DeNOx Recovery is a process for NOx emissions treatment.

The main advantages are:

• >99% of NOx reduction.
• Salts recovery.
• Chemical wastes elimination.
• No hazardous compounds are released into the atmosphere.

The DeNOx® process, patented by Condorchem Envitech, was conceived for the treatment of the emissions from power generation solar thermal plants, by converting the (NOx) pollutants into products re-used in the solar thermal plant process.

In a solar thermal plant, sunlight is concentrated by mirrors in a collector that reaches temperatures of up to 1,000°C. This heat is used to heat a fluid and produce steam, which moves a turbine to generate electricity. Although the first power stations could only operate during the hours of solar radiation, nowadays it is possible to store the heat to produce at night.

The energy obtained from solar radiation is stored in salts, a mixture of sodium nitrate and potassium nitrate, which have an appropriate melting point. They are in a liquid state, for which a temperature of 280°C is necessary. The liquid salts are heated to a temperature of 565°C at which they are stored.

The innovation lies in the fact that the generation of electricity depends on the level of stored hot salts and not on solar radiation. To produce electricity, hot salts are used to boil water in a heat exchanger and produce steam at 540°C and 100 bar. The steam moves a turbine which produces electricity according to real time demand.

Despite the numerous advantages that this technique brings, it also poses a major environmental problem. The liquid salts release nitrogen oxides (NOx) during heating in the form of discontinuous emissions of variable concentration.

The acronym NOx refers to gaseous compounds of nitrogen and oxygen, although reference is mainly made to nitric oxide (NO) and nitrogen dioxide (NO2). A feature of all of them is how harmful they are to both the environment and health.

The WHO Air Quality Guidelines state that high concentrations of nitrogen dioxide, even for short periods, can have very harmful effects on health (e.g. irritation of the respiratory and ocular systems and the appearance of chronic respiratory and cerebrovascular diseases). In addition, NOx can give rise to secondary pollutants once emitted into the atmosphere.

NOX emissions must be properly treated before the gas stream can be released into the atmosphere to prevent any environmental impact. The DeNOx® solution is based on selective catalytic reduction (SCR), one of the most competitive and efficient techniques for the elimination of NOx in a gas stream.

The main advantages of the DeNOx® process over conventional alternatives are:

• NOx removal yields of over 99%
• Conversion of waste into reusable raw material in the solar thermal energy accumulation process
• No chemical waste produced
• No hazardous compounds in atmospheric emissions
• Copying the natural atmospheric self-regeneration mechanism
• Simple and reliable operation
• Fully automated and robust operating process

The DeNOx® process has been especially designed for the effective removal of NOx from gaseous streams from solar thermal power storage systems based on heating a mixture of sodium and potassium nitrate salts.

However, the conception of the process means there is no limit to the application of this innovative, efficient treatment in situations requiring the removal of NOX efficiently and competitively; as occurs in power plants, some industrial plants, cement plants, waste incineration plants, glass manufacturing plants and refineries.

The DeNOx® process emulates the natural, self-purification mechanisms of the atmosphere itself, through a controlled combination of NOx emissions with water vapor, ozone and ultraviolet radiation.

The process is based on 4 stages:

1. Photooxidation of the gaseous stream, producing ozone (O₃) and hydroxyl radicals (OH·):

   O₃ + H₂O → 2 OH· + O₂

2. Conversion of the nitrogen oxides (NO and NO₂) into nitric acid (HNO₃) through the high reactivity of the ozone and hydroxyl radicals:

   NO + O₃ → NO₂ + O₂
   NO₂ + OH· → HNO₃

3. Absorption of nitric acid in a gas scrubbing process and its neutralization by an alkaline solution of NaOH or KOH, obtaining the respective salts (NaNO₃ or KNO₃).
4. Recovery of the salts formed in a crystallization process and subsequent re-use in the energy storage system itself.

This elegantly simple process guarantees that the NOx treatment is highly effective, combining high efficiency and robust and reliable operation.