The Cleanest Cement Plant in the World: Part 2

DeCONOx facility

The DeCONOx process combines regenerative thermal oxidation (RTO or RNV) with a low-dust selective catalytic reduction (SCR): two proven technologies in a single system that fulfil two totally different tasks in exhaust gas purification. This combination makes it possible to reduce organic carbon compounds and carbon monoxide. The installation of catalysts also reduces the NOX concentration. The demand for energy in the afterburning is mainly or entirely covered by the fuels – CO, VOCs – contained in the flue gas, thus reducing the energy demand compared to pure low-dust SCR. The system will run autothermally with around 6500 mg/Nm³ CO in the flue gas.

The DeCONOx plant consists of five towers, two of which are pressurised with crude gas (before the reaction/combustion) and two of which are pressurised with clean gas (after the reaction/combustion). The fifth tower is purged with clean gas to avoid peaks of crude gas concentrations (during switchover cycles) and thus reduce the half-hourly mean values (emissions). Organic hydrocarbons and the carbon monoxide are converted in the combustion chamber at above 850°C. In order to guarantee complete oxidation, the combustion chamber is set to 860°C. During start-up (heat-up) and non-autothermal operation, the temperature in the combustion chamber is regulated by burners or gas lances.

The special burners only need natural gas during operation, but not during standstill. With a combustion chamber temperature above 750°C, the temperature can also be regulated with installed gas lances. Gas lances enable a fine-tuning of the temperature profile across the burning chamber and do not need any burner air, thus reducing the energy demand even further. The clean gas escaping DeCONOx is 25 – 35°C hotter than crude gas.

Before start-up, the chamber is heated with natural gas and fresh air. This takes about 6 – 12 hours. Energy is lead through the burner into the combustion chamber. The maximum heat-up rate of 6 k/min should not be exceeded because of material stress. As soon as the temperature on the catalyst reaches 250°C and the temperature in the combustion chamber is beyond 850°C, the facility can be pressurised with flue gas. During standstills and maintenance work, the plant must be rinsed with fresh air.

Regenerators serve the temperature transfer. During the cycles they are alternately heated up and cooled down by the flue gas. The catalysts are installed between the regenerators (into the optimum temperature frame). The geometric set-up of the catalysts corresponds with the set-up of the regenerators. Thus, the catalysts work as regenerators and replace parts of the regenerators. The catalyst must not be damaged in the course of permanent switching procedures and the temperature changes involved. The switchover is carried out every 50 –  120 sec., during which the gas absorbs heat with the upward flow and releases the heat with the downward flow. In the bottom regenerator, the crude gas reaches the necessary catalyst inlet temperature of at least 240°C. The ammonia injection and the catalyst layer is then followed by a second regenerator layer that then raises the flue gas to the combustion chamber temperature.

Selective catalytic reduction is the most effective method for controlling NO_X emissions from combustion sources. It is a commercially proven flue gas treatment technology that has been demonstrated to remove over 98% of the NO_X contained in combustion system exhaust gas. The catalyst is at the heart of the SCR process. It creates a surface for reacting the NO_X and ammonia, and allows for the reaction to occur within typical flue gas temperature ranges.

Conclusion

Kirchdorfer Cement plant seems to be so far removed from the norms seen in the majority of cement plants around the world. Even the immediate surroundings of the plant seem alien. Crisp green lawns meet quaint fountains that encircle the plant. Visitors could be forgiven for forgetting that they are on the doorstep of a cement plant. The plant chose to invest millions into environmental projects when the plant was already well below legal limits in terms of emissions and, as such, truly represents a cement plant of the future in its consciousness for the environment and its engagement with the community at large.