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Facing the challenge: part one

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World Cement,

Today’s regulatory climate has cement plants worldwide facing dual, if not conflicting challenges – how to handle tougher environmental regulations while at the same time being pressured to reduce production costs. One common strategy to decrease production costs is to reduce the cost of fuel. Over the years, this has led to a worldwide increase in petcoke usage. The advantages and disadvantages of this strategy have been thoroughly discussed within the industry. What has received less attention is the impact of this fuel change on the dust collector in the coal grinding circuit, generally referred to as the ‘coal mill dust collector’. This discussion provides helpful advice for maintenance and production personnel to keep the dust collector operating efficiently.

General design practices

The increased substitution of coal with petcoke has created challenges for the dust collector that vents the coal mill. Increased material fineness and the stickier nature of petcoke can make it more difficult to clean the filter bags. Since most dust collectors in today’s plants use pulse-jet cleaning systems, this article and the discussed design guidelines are limited to pulse-jet dust collectors. Compared to today’s standards, many older dust collectors were delivered with marginally designed cleaning systems. Many older collectors and even some newer ones operate at or above practical design limits because of a push for increased kiln production that has maximised airflow and material loading through the coal mill circuit. The relatively high sulfur content in petcoke also increases the likelihood of corrosion, especially in the clean air plenum of the collector and outlet ducts. The following information and design criteria will help you to evaluate if your coal mill dust collector is up to the challenge.

Airflow and filtration area

The relationship of Air Volume per Filter Area is called ‘Air-to-Cloth Ratio’ or ‘Filtration Velocity’. To provide reliable service and acceptable filter life, the Air-to-Cloth ratio for a typical dust collector should not exceed 3.5 ACFM per 1 ft2 filter area. The metric equivalent is 1.07m3/min/m2. A higher Air-to-Cloth ratio typically results in short filter life, increased differential pressure, higher cleaning and compressed air consumption, potentially higher emissions and generally higher operating costs.

Cleaning system

Pulse-jet cleaning systems have evolved over the past four decades. The newer dust collectors generally provide more cleaning energy to filter bags than the earlier designs. An undersized cleaning system causes excessive buildup of combustible, fine and potentially explosive material on the filter bags. The pulse-jet system shoots a burst of compressed air into the filter bag. This pressure wave travels down the inside of the filter bag, violently shaking the bags and blowing off the accumulated dust on the outside of the bag. Most dust collectors are cleaning online, meaning that there is always airflow through the filter bags. This airflow opposes the pressure wave being injected into the filter bags by the cleaning system. The higher the gas volume through the dust collector, the more contrary gas flow the pressure wave encounters and the less efficient the cleaning becomes.

As pressure drop increases across the filter fabric, more cleaning energy is required. Older dust collectors typically have smaller, less powerful pulse valves, therefore the cleaning systems become easily overwhelmed by increased airflows through the collector. If the dust collector uses either single diaphragm pulse valves or valves that are smaller than 1.5 in., the collector might be a candidate for an upgrade. A reliable source of clean and dry compressed air to the pulse valves is vital to ensure a continuous operation of the cleaning system.

Can velocity

In a pulse-jet dust collector, can velocity is defined as the theoretical upward velocity of dust laden gases between the filter bags. Excessively high can velocity could prevent dust from dropping into the hopper after being blown from the filter bags during bag cleaning. The suspended dust is simply sucked back onto the filter bags once the pressure burst from the cleaning system diminishes. Finer and lighter dusts require lower can velocities. Petcoke is typically ground finer than coal, so excessive can velocity can quickly reduce the efficiency of the cleaning system and result in high pressure drop across the filter bags and excessive dust accumulation. Increasing the ventilation volume through a dust collector increases can velocity. For a coal mill dust collector, the can velocity should not exceed 240 fpm or 1.22 m/s.

Filter bags

Polyester, acrylic and aramid are the typical fabric choices for this application. Without membrane, these fabrics require a dust cake for fine particle filtration. However, a dust cake increases the risk of combustion. Fabric with ePTFE membrane does not require a dust cake.

There are engineered ePTFE membranes with properties to handle specific dust challenges. The non-stick characteristics of the ePTFE membrane, combined with a specially formulated treatment, provides the characteristics necessary to help keep the ePTFE membrane from plugging and blinding.

BHA® Preveil® membrane technology has shown promising results and enabled efficient operation with little dust cake in several test applications. To reduce static electricity and sparks, all filter bags should be grounded.

Inlet design

A good inlet design will distribute the gas flow evenly throughout the hopper and eliminate abrasion problems. Abrasion causes holes in filter bags and subsequently emissions. If abrasion problems exist and the gas volume is as per design, one can either redesign the inlet duct, install baffles (Figure 2) to slow the dust particles and redistribute the gas stream or install filter bags with sacrificial ‘abrasion’ skirts or reinforced areas.

This is part one of a two-part article written for World Cement’s November issue and abridged for the website. Subscribers can read the full November issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the November 2016 issue here.

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