NOx reduction at Vidal Ramos

To lower NOx emissions Votorantim Cimentos decided to install SNCR equipment as well as implement alternative fuels (AF) use at its Vidal Ramos plant, Brazil. This required the optimisation of the urea injection system to ensure the lowest-possible consumption of the reducing agent before and after the introduction of the AFs. By Mario Henrique Interlenghi & Carlos Roberto Moreira da Cunha, Votorantim Cimentos, Brazil.

Because of the high temperatures required by the clinker production process, NOx emissions are difficult to manage. In addition, cement producers try to use low-cost and sustainable fuels, which can require high flame temperatures and therefore increase NOx generation.

Following the global trend, Brazil’s limits for gaseous emissions, including NOx, are increasingly restrictive, with further prospects for lower values envisaged. The current legislation sets maximum values of NOx emissions (expressed as NO2 and corrected to a dry gas basis at 10 per cent oxygen), as shown in Table 1.

According to the European Union BAT-Reference document, reducing emissions from a pyroprocessing system can be obtained by:
• maintaining the existing process while reducing the input of emission precursors to the system
• modifiying the existing process (primary or process integrated measures)
• maintaining the existing process while adding a separate gas-cleaning unit for the exhaust gas (secondary measures).

However, the application of these measures in a new plant compared to an existing works involves completely different issues, because of the costs incurred when implementing the different available solutions for each situation.

The need for NOx emission reduction and the use of the best-suited measures and technologies to meet the required environmental limits led Votorantim Cimentos to opt for a SNCR installation at its greenfield Vidal Ramos plant.

Furthermore, the advantages of using alternative fuels (which include GHG reduction, less land requirements for waste disposal, the appropriate disposal of harmful wastes and conservation of valuable resources) encouraged the company to further reduce NOx emissions by co-processing industrial waste and shredded tyres.

Vidal Ramos plant

The Vidal Ramos plant, located in Santa Catarina state, southern Brazil, launched operations in July 2011. FLSmidth supplied the 3000tpd kiln (which includes a five-stage preheater with an ILC calciner), a 285tph Atox 42.5 raw mill and a OK 30-4 cement mill. The clinker cooler was provided by IKN. Two types of cement are produced: CPII Z 32 and CPV ARI RS (pozzolanic and sulphate resisting types, respectively).

The main fuel is petcoke, and a mix of shredded tyres and industrial waste is introduced at the smoke chamber. Due to uncertainties at the time of the project regarding the supply of an ammonia solution in Brazil, the company chose a urea solution (40 per cent) as a reducing agent.

The plant seeks to operate with the lowest possible NOx emissions and, at the same time, the lowest consumption of reducing agent. To meet this challenge, operations had to be optimised, ie the primary process integrated measures had to be fulfilled.

Process integrated measures – main steps

After start-up and stabilisation of the kiln line, the process of adjusting the urea injection began. Temperature mapping was carried out in the region of the injection nozzles. The values found were below the range recommended for good reduction efficiency. Adjustments of the meal split from the fourth preheater stage were carried out, but the desired temperature level was still not achieved.

The decision was taken to relocate the injection nozzles to a lower region, where the temperature levels are higher. A process of adjustment and optimisation began, targeting the reduction of NOx levels. This mainly involved the adjustment of the oxygen level at the kiln inlet, a new meal split from the third and fourth stages, the distribution of fuel between the kiln burner and precalciner, the intensity of burning in the kiln and secondary air optimisation.

Effects of the meal split on NOx

The meal split from the fourth-stage cyclone, between the lower and the upper ducts, directly affects NOx emissions because by feeding a greater or lesser amount of material at the bottom of the riser duct, the temperature can be respectively lower or higher. Figure 1 shows the different meal splits and the related NOx emissions.

The best result in terms of NOx reduction at the stack (39.3 per cent) were obtained with the valve opened to 80 per cent at the top (above the entrance of the tertiary air in the calciner). However, build-up formation in the smoke chamber and in the riser duct was so strong, particularly in the region above the entry of the calciner fuel, that it made the operation of the kiln very difficult. With the valve opened at 100 per cent at the top, the lower material duct from cyclone four became blocked. Opening the valve at 75 per cent at the top resulted in a 26 per cent reduction of NOx emissions at the stack with more controlled build-up formation in the smoke box. This was the meal split that was selected for plant operation.

Relocation of the injection nozzles

The increase in temperature achieved by adjusting the distribution of meal from the fourth stage was not enough to achieve the ideal working range for NOx reduction. Temperature mapping of the smoke chamber and riser duct was carried out, and new injection points identified. The new location is just below the tertiary air intake, as seen in Figure 2.

Temperature stratification

In addition to the problem of low temperatures, stratification of the temperature along both the smoke chamber box and the riser duct was also identified by MI CFD modelling (see Figure 3). Temperature stratification negatively affects the reducing agent’s efficiency.

Operation adjustments

Oxygen at kiln inlet

Thermal NOx is formed from air nitrogen at temperatures higher than 1200°C, and its rate of formation is affected by temperature and oxygen levels. Kilns with high heat load and high oxygen levels at the front end mainly produce thermal NOx in considerable quantities. Tests were carried out with different oxygen contents at the kiln inlet. The results on the emission of NOx at the stack can be seen in Figure 4.

Kiln heat load and meal split from the third-stage cyclone

Initially, all material from the third stage was directed to the fourth-stage cyclone. The splitter was then adjusted so that 40 per cent of the material was directed to the calciner. This less-prepared material entering the kiln would increase the size of the calcining zone, reducing the length of the burning zone. For a constant flow of fuel in the main burner, this would cause cooling of the kiln. After the material split adjustment, the free lime increased (for a constant LSF) as can be seen in Figure 5.

With the kiln cooler, thermal NOx generation was lower and it was possible to reduce the urea injection rate. During the test, the urea injection control loop was enabled, and therefore the NOx signal stayed at a constant value (see Figure 5).

After adjustments the desired temperature levels were achieved. However, there was still stratification, the origin of which is linked to the geometry of the smoke chamber, of the tertiary air duct arrival and calciner. Figure 6 shows the temperature stratification before and after the modifications.

Alternative fuels and NOx

The plant began co-processing a blend of AFR and shredded tyres in July 2013. The current average thermal substitution rate is 27 per cent. Figure 7 shows the effect of a blend of 2tph of tyres and 5tph of AFR on the NOx. The average NOx reduction is 30 per cent, without injection of urea.

Figure 8 shows the effect of AFR use on NOx with urea injection. For a constant urea injection rate, the increase of AFR flow from 4tph to 8tph resulted in reduction of NOx emission from 770mg/Nm3 to 640mg/Nm3 at the stack.

Conclusion

The actions for NOx reduction in operating cement kilns must focus first on optimising the process and operation. This will enable reduction or even elimination of reducing agent use in a SNCR installation, with important savings in plant variable cost.

The use of alternative fuels is an efficient way to combine NOx reduction effect with savings in the fuel bill. Substituting fossil fuels with other sources of energy is a very good example of eco-efficiency, simultaneously reducing costs and emissions. By using waste instead of fossil fuels, Votorantim Cimentos saves resources for future generations, avoids GHG emissions and provides correct disposal solutions for a significant amount of harmful wastes.

Article first published in International Cement Review, November 2014.