Brad Pronschinske, Martin Engineering, explains how air cannon technologies can effectively prevent material build up in the preheater, whilst maintaining workplace safety and minimising unscheduled downtime.
In cement plant preheaters, intense heat courses upwards through the system, raising the temperature of the meal to ensure it does not cool the kiln when it arrives. Material fed into the preheater has a temperature as high as 200°F (93°C) but within seconds, it enters the kiln heated to 1500°F (815°C). This saves on energy consumption, reduces the calcining time and promotes plant efficiency. However, no matter how many stages the preheater has, as the material flows through each stage, it can quickly adhere to the coarse refractory, build up and clog. This slows or stops the flow and leads to expensive downtime.
Many operators mitigate clogs by implementing regular cleaning schedules, assigning workers with water lances extended through access holes. A worker ascends the tower and dons a suit of high-heat personal protective equipment (PPE). Lancing the material with high-pressure water clears the blockage, and the workers restore proper flow. Unfortunately, this procedure is always done while the preheater is still in operation, causing a tremendous amount of heat and some molten material to blow back. The PPE, the heat and the safety issues make preheater cleaning one of the most unpleasant jobs in a cement plant.
To improve safety and increase efficiency, virtually all cement plants in the USA have air cannons installed on the preheaters. Over the years, Martin Engineering has worked to improve the technology to the point where both installation and maintenance can be performed safely without a shutdown or exposure to intense heat.
Preheater towers, in some form, have been in operation since the 1920s. Today’s designs can have as many as six stages in towers up to 12 stories (120 ft/35 m) tall. The mix flows down a chute to a splash box, proceeds to the airflow of the riser duct where it gets heated, and progresses to the next stage, and the procedure happens all over again.
One of the biggest contributors to material buildup is high heat and material velocity. As material gets hotter, it gets stickier, clinging to the sides of the flow chutes and splash box, as well as in the riser duct. If left unchecked, clogs can form quickly, stopping the material flow, which leads to unscheduled downtime and lost production. Large buildups can even completely block the outlet of the cyclone.
Whenever operators open the access door, this lowers operating temperatures. Cold water also reduces temperature, and injecting water into the hot tower creates steam, which can result in a dangerous steam ‘explosion.’ A better alternative is a series of low-pressure air cannons – a technology originally developed and patented by Martin Engineering in the 1970s.
Early cannon designs were engineered to use very high pressures – some as high as 34 474 kPa (5000 PSI) – which were expensive to operate and introduced safety issues. But today’s low-pressure ‘air blasters’ are fed by compressed air (or some other inert gas), delivering a powerful surge through a specially designed high-heat nozzle to clear a specified area.
Air cannon nozzles are strategically positioned in the tower, riser duct or cyclone. As adhered material is dislodged, it returns to the flow, with the pressurised shot facilitating the flow and enhancing efficiency.
In the past, operators have been concerned with valve maintenance that required tank removal and nozzle replacement, which could only be performed during scheduled shutdowns that involved a system cooldown. Back then, most cement plants used their annual scheduled shutdown to maintain or clean nozzles. This often required the removal and replacement of refractory, where any disruption can lead to cracking. But over the last decade, these issues have been addressed by new technologies, including retractable nozzles that can be installed and serviced during production.
Many designers proactively include the mountings for air cannons in new designs, so that future retrofit can be done without vessel entry or extended downtime. New technology has even been developed for installing air cannons in high-temperature applications without a processing shutdown, allowing specially-trained technicians to mount the units on furnaces, preheaters, clinker coolers and in other high-temperature locations while production continues uninterrupted.
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