Digging Deeper Into Dust Control

Over the last decade, the cement industry has been working to change its image in terms of energy efficiency and environmental protection, with significant improvements in not only emissions levels but also in the appearance of the plant and the working environment.

In terms of dust emission regulations, a few countries are still maintaining the old standards, for example, kiln dust emissions <100 mg/m³. In contrast, major regions such as the US have made significant changes. Table 1 shows the standards in China, the United States, Japan, and the EU. China is the largest cement producing country in the world, with production levels of 2.1 billion t in 2022, accounting for approximately 55% of global cement production. The ‘Emission standard of air pollutants for the cement industry – GB4915’ was first published in 1985 but was revised in 1996, 2004, and 2013. In the last 5 years, the ‘super low emission standard’ has been applied in critical regions (mostly covering large cities).

Design

20 years ago, ESP was the legacy design for cement dust control. Dust emissions were typically 50 – 100 mg/m³ but with improvements, plants could reach 30 mg/m³.

Fabric filtration has been playing an increasingly important role over the last two decades for kiln and cooler dust control and has limited dust emissions to 1 – 30 mg/m³.

The key design criteria for fabric filtration is the filtering velocity, which is usually 0.8 – 1.2 m/s. The lower the velocity, the lower the dust emission concentration, the lower the pressure drop, and hence the lower the specific power consumption of the kiln exhaust fan.

In recent years, China has implemented the ‘super low emission limit for cement industry’, which states that the filtering velocity must be reduced below 0.8 m/s, even to 0.6 m/s. This implies an increased bag surface area and higher investment in baghouses.

The KPIs to evaluate the performance of a baghouse are:

• Dust concentration (in-house database: 1 – 30 mg/m³).
• Pressure drop (ΔP, in-house database: 220 – 2700 Pa).

The World Cement Association (WCA) has organised the Pegasus Project, which in phase 1 focused on energy efficiency, where one of the common issues was pressure drop. Figure 1 shows bag house pressure drop and kiln exhaust fan specific power consumption (SPC). In the plants visited by the WCA, the highest pressure drop was 2700 Pa, and the related kiln exhaust fan SPC was 7 kWh/t of clinker. The best plants are Evergreen (pressure drop: 224 Pa, SPC: 1.0 kWh/t of clinker) and Tai An CUCC of CNBM (pressure drop: 300 Pa, SPC: 1.3 kWh/t of clinker). Of course, it must be taken into consideration that the SPC also depends on a combination of factors including the fan's air leakage and mechanical efficiency.

In some plants, the pressure drop of the baghouse is higher than 1000 Pa, and in such cases, it is worth redesigning the baghouse to reduce the pressure drop.

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Read the article online at: https://www.worldcement.com/special-reports/23062023/digging-deeper-into-dust-control/