Today, cement manufacturing plants have to contend with a double-edged sword: How do they reduce production costs and still meet environmental regulations? How do they accomplish their goals and directives while maintaining their equipment in a manner that prevents unscheduled downtime and hazardous conditions? Reducing the cost of fuel is a common strategy employed within the industry; in turn, this has led to an increase in the substitution of coal with petcoke.
Petcoke and its impact on the coal mill dust collector
One area that has been impacted by the use of petcoke is the coal mill dust collector. Coal mill dust collectors have a substantial effect on productivity, yet have received relatively little attention as companies implement cost-saving programs, like the use of petcoke as a fuel reduction measure. Petcoke is typically ground finer than coal, and it has a particularly sticky nature that makes it more difficult to clean from filter bags. Its high sulfur content increases the likelihood of corrosion. All of these things bring into question whether the dust collector in use is properly designed for the conditions and if the operation and maintenance plan is effective in preventing system upsets and safety concerns.
Here, we'll discuss effective design, operation and maintenance tips for this critical component of the coal grinding circuit and what you can do to evaluate the readiness of your coal mill dust collector.
How to keep your dust collector operating at peak performance
Dust collector operators must take into consideration these items as they evaluate their current equipment design:
Airflow and filtration area
An air-to-cloth ratio that does not exceed 3.5 ACFM per 1 square foot is recommended to prevent short filter life, high differential pressure, higher emissions and operating costs.
An undersized system can cause an excessive buildup of combustible material on the filter bags. The use of single diaphragm pulse valves or valves that are smaller than 1.5 inches often requires an upgrade.
Defined as the upward velocity of dust-laden gases between filter bags, excessive can velocity can result in higher pressure drop and excessive dust accumulation. For a coal mill dust collector, velocity should not exceed 240 fpm or 1.22 m/s.
Filter bag selection
Polyester, acrylic and aramid filter bag fabric materials require a dust cake for fine particle filtration. This can lead to an increased risk of combustion. Opting for an ePTFE membrane material, like the BHA Preveil filter from Parker Hannifin does not require a dust cake, reducing the combustion risk.
BHA Preveil filters are made from PTFE resin, a substance with inherent non-stick properties that can be used to help your dust collector operate at maximum efficiency by providing:
- 99.99% efficiency
- Higher airflow
- Better cleaning
- Optimum emissions control
Proper inlet design distributes gas flow evenly and eliminates abrasion issues, which cause holes in the bags and increased emissions.
Hopper and housing design
A steel, air-tight housing and hopper, free of shelves or other particle accumulation areas, equipped with level and temperature measurement devices is recommended to prevent hazardous conditions. To avoid buildup of combustible materials, hopper walls should be designed with a minimum angle of 70 degrees from horizontal.
A lack of insulation can result in moisture and accumulation of combustible dust. When petcoke is used this can also result in the increased formation of sulfuric acid and eventual corrosion of the dust collector.
Material handling system
It is essential that material handling equipment can withstand surges. A good practice is to size the equipment to the capacity of conveying equipment, even if the actual requirement is substantially less. Equipment should be equipped with motion sensors.
Operation and maintenance
With a properly designed dust collector, good operational practices can result in a reduction of downtime, emissions and hazardous conditions. The following practices are recommended:
- Startup and shutdown procedures: Establish specific, written protocols.
- Regular dust collector inspection: Checking pressure drop, visible emissions, the cleaning system, material withdrawal equipment, damper settings and fan operation once per shift is a recommended practice.
- A staggered sequence of actuation of the cleaning system pulse valves: This will reduce dust recirculation and pressure drop across the filter bags.
- Continuous operation of the cleaning system and material removal equipment: This will help to prevent the excessive dust accumulation on the filter bags and in the hopper.
Contributed by the Filtration Team.