Meeting the low-cost NOx reduction challenge

In anticipation of stricter future emissions regulations, Taiheiyo Engineering Corp has been developing various cement kiln NO_x reduction technologies. The most recent breakthrough combines the company’s new NO_x reduction technology with selective non-catalytic reduction to achieve significant reduction rates at low cost. By Toshiaki Hirose, Taiheiyo Engineering Corp, Japan

The regulated limits for NO_x emissions from cement kilns have become increasingly stricter and today NO_x reduction is one of the key sustainability challenges for the cement industry.

Conventionally, it was believed that selective catalytic reduction (SCR) was the only way to achieve these limits for NO_x emissions. However, since SCR investment and running costs are very high, the quest for a low-cost innovative NO_x reduction technology has been an important goal for the cement industry. 

Conventional NO_x reduction technology 

The first generation: Taiheiyo Two-Stage Combustion System 

Taiheiyo Engineering Corp (TEC) has been commercialising its NO_x reduction technology, the Taiheiyo Two-Stage Combustion System for NSP kilns, for over a decade.

This technology comprises one or two deNO_x burners installed at the low oxygen area between the riser duct and the calciner, where part of the calciner fuel is combusted. So, NO_x from the kiln is reduced and NO_x generation is suppressed in the calciner.

By combusting ~30 per cent of calciner fuel by deNO_x burners,  ~10-30 per cent of NO_x emissions can be reduced at the stack (depending on the number and position of the deNO_x burners).

Based on this result, it was expected that NO_x emissions could be reduced further by increasing the fuel to the deNO_x burners. However, taking into account coating issues at the riser duct, the fuel to the deNO_x burners was limited to up to ~30 per cent of calciner fuel.

The second generation: Taiheiyo DeNO_x System  

To increase the fuel to the deNO_x burners without causing coating issues, TEC developed the Taiheiyo DeNO_x System by combining the Taiheiyo Two-Stage Combustion System with the Taiheiyo Coating Solution (TCS) System.

However, even with the Taiheiyo DeNO_x System, the fuel to the deNO_x burners was limited to up to 50 per cent of calciner fuel (see Table 1).

 TEC’s latest NO_x reduction technology

In recent years, TEC has been developing and commercialising its TTR®-G technology to gasify plastic waste using hot meal for the efficient use of alternative fuels. The gas from the TTR®-G system contains a considerable level of hydrocarbons that are highly effective in reducing NO_x as shown in the following reaction formulas:

• NO + CO → N₂ + CO₂ 

• NO + CmHn → NH₃ + CO 

• NH₃ + O₂ → NH₂ + H₂O

• NH₂ + NO → N₂ + H₂O

Figure 2 provides an outline of the new NO_x reduction technology. 

Hot meal with a temperature of 740°C is extracted from the bottom of the second cyclone chute and enters the TTR-G system. The hot meal and plastic waste are mixed together in the system, and the mixed material (hot meal and residue of plastic waste) and flammable gas are discharged via separate routes. They are subsequently merged and fed into the riser duct. 

The outlet temperature from the TTR-G system is controlled at 520°C. As all hot meal in the second cyclone from the bottom would have originally entered the calciner, the flow of hot meal is the same even if a portion of it is extracted and enters through the TTR-G system. Therefore, kiln operations are not affected. 

Furthermore, the low temperature of the mixed material and flammable gas feeding to the riser duct prevent coating. Therefore, 100 per cent of the calciner fuel can be combusted by the deNO_x burners. TEC also improved the design of the deNO_x burners to enable the efficient mixing of the TTR-G gas and the NO_x reducing agent. As a result, NO_x emissions at the stack can be reduced by 40 per cent.

New NO_x reduction technology + SNCR

To further develop its NO_x reduction technology, TEC has combined the new NO_x reduction technology (TTR-G + deNO_x burners) with SNCR to achieve a NO_x reduction rate of ~70 per cent. Table 2 shows the NO_x reduction effect.

Lower heat demand

Table 3 shows a comparison between TTR-G and direct feeding of plastic waste to the calciner.

According to Table 3, the gasification process also improved combustibility and reduced CO concentration, specific gas volume, and gas temperature at the IDF outlet. Therefore, by reducing the heat loss of exhaust gas, including unburnt CO at the IDF outlet, the reduction in heat consumption was 190.6kJ/kg of clinker (coal equivalent: 1.64 tph).

Conclusion

High NO_x reduction effect

Taiheiyo Engineering Corp’s new NO_x reduction technology (TTR-G + deNO_x burners) and SNCR can achieve almost the same NO_x reduction efficiency as SCR (see Table 4).

Furthermore, the amount of NO_x-reducing agent by the new NO_x reduction technology is enormous compared with the amount of NO_x generated in the cement kiln. Therefore, further NO_x reduction is possible by improving the mixture of the NO_x-reducing agent and NO_x.

In addition, a reduction in CO concentration can be confirmed due to the improved combustibility by gasification and a reduction of total hydrocarbon emissions is expected.

Low investment cost

The investment cost of the new NO_x reduction technology is ~20 per cent of the investment cost of SCR. 

In addition, large-scale modification of the preheater, such as increasing the volume of the calciner, is not required. The system’s installation work can be completed within a month during the maintenance period.

Improvement of the clinker production process

The new NO_x reduction technology has several advantages, as it:

• improves combustibility
• reduces heat consumption compared with direct feeding to the calciner
• prevents coating issues
• contributes to stable operation.

The new NO_x reduction technology is just one of many efforts by Taiheiyo Engineering Corp to contribute to environmental protection. The company is committed to developing and supplying the most innovative technologies and services for a green and sustainable society. 

References

¹ YAMAMOTO, Y (2020) ‘Drying sewage sludge with preheated raw meal’ in: ICR, September p79-80.
² YAMAMOTO, Y (2021) ‘No time to waste’ in: WC, (6), p51-54.
³ YAMAMOTO, Y (2022) ‘Gasification technology for shredded solid waste’ in: ICR, November, p65-66.

This article was first published in International Cement Review in August 2024.