Counteracting Corrosion

Introduction
This article will discuss the effects of corrosion, not only on the metal surfaces of a pulse jet dust collector, but how it is effecting environmental compliance. The methods available for use to reduce, control, or minimise corrosion will also be discussed.
Corrosion is defined as a natural process, which converts a refined metal to a more chemically-stable form, such as an oxide, hydroxide, or sulfide. It is the gradual destruction of material (usually metal) by chemical and/or electrochemical reaction with their environment.
In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant, such as oxygen or sulfur. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxides or salts of the original metal, and results in a distinctive orange colouration. Corrosion degrades the useful properties of materials and structures, including strength, appearance, and permeability to liquids and gases.

Cement kiln baghouses
All of the ingredients are present in a cement kiln dust collector to corrode mild steel surfaces. Until recently, the major concern with corrosion was the damage to the metal surfaces, requiring corrective action. These actions ranged from patching damaged areas, to complete replacement of tube sheets, blow pipes, housing walls, roofs, hoppers, and access doors. With today’s stricter environmental particulate regulations, corrosion is causing many cement kiln dust collectors to be over their particulate limits. The rust flakes that are produced get into the air stream and are carried out of the stack by the emission monitors. Even when no filter bags are leaking, the rust flakes are enough to exceed most plants’ particulate limits. Emission monitors read particles going by; they do not analyse them to determine their origin. So, a kiln dust collector with 5000 filter bags all doing their job, with no leaks, could still be over the allowable limit for particulate emissions, if there is enough rust scale and flakes on the clean side of the dust collector.

Based on the above information, what are the options to eliminate the effects of the corrosion?
- From an emissions standpoint, the rust scale and dust could be removed from the clean side of the baghouse daily. This is not a realistic option. In addition, this will do nothing to slow down the damage to the metal surfaces.
- Limiting dew point exposure will slow the corrosion in some kiln baghouses, but not all. Some of the newer kiln systems operate with inlet temperatures very close to, if not in, the dew point range.
- Sandblasting and coating the metal surfaces in the clean air plenum will control corrosion for longer periods of time.
- Existing dust collectors, constructed of mild steel, can have the clean air plenum lined with a light gauge stainless steel. This is a longer-term solution.
- The dust collector can be constructed of stainless steel from the tube sheets up. This is the best option, with the lowest cost of ownership, but is the most expensive up front.

Examining the options
Daily maintenance
Most will agree this is not a viable option. It may not be required daily, but even weekly would not work. It would require each compartment to be isolated, the tube sheet to be covered, the walls and ceiling scraped, and all material removed from the tarp, allowing as little as possible to fall down the filter bags. This option has a high labour price and, in most cases, the plant will not be able to maintain the schedule. In addition, production will not be at optimum levels during this weekly procedure. Using this method will require major metal patching or the replacement of the entire clean air plenum every ten years or less.

Limit dew point exposure
During kiln start-up, the dust collector can operate for as long as 36 hours in or around the dew point. This means, at a minimum, you are manufacturing sulfuric acid and, even worse, if chlorides are present, hydrochloric acid. To minimise metal surfaces to prolonged dew point exposure, only start up with a minimum number of compartments online. All compartments are not needed to reduce air volume and temperature. Usually only two compartments are needed during initial start-up. Start with the back two compartments online. This reduces the area that will be exposed to the initial gas stream, allowing it to get above the dew point quicker, as well as heating the entire length of the inlet duct. Start the dust collector with the cleaning system off. As the inlet temperature increases to 300˚F (149˚C) and the overall differential pressure reaches 150 mm, bring the next compartment into service. Repeat this, going from side to side, until all compartments are online. Activate the cleaning system once feed is introduced to the kiln. This method will isolate the worst damage to the back compartments, or sacrificial compartments. Even the sacrificial compartments will not experience the dew point exposure that the entire dust collector would be subjected to, if the kiln was started with all compartments online. Again, this will not eliminate the corrosion; it will just slow it down. This method will extend the time that major metal replacement will need to take place by a couple of years.

Sand blasting and coating
Sand blasting and coating the metal surfaces in the clean air plenum can be an effective method for controlling corrosion. But several things have to happen in order for this method to perform for extended periods of time. First, the metal surfaces have to be clean and sandblasted to a white metal finish. Next, the coating must be applied to the correct thickness and cured at the proper temperature. Selecting a coating is another important step in the process. There are numerous coatings available. It is important to sufficiently research the coating during selection. Gas stream chemistry, now and in the future, and operating temperature must be considered. References from other plants that have already gone through the process are advised. Under the best conditions in the field, the application of high-temperature coatings is a challenge. Coating in the shop, as a dust collector is being fabricated, is ideal. In most cases, the coating will have to be touched up yearly because of cracking, blistering, or flaking. The entire job will most likely have to be repeated every five to ten years.

Lining with stainless steel
This is a more permanent fix compared to the above options. Several plants in the US have used this option with great success. The tube sheets cannot use this method; they will have to be replaced with new stainless steel tube sheets or coated. The walls, ceilings, and the inside of the access doors are lined with thin sheets of stainless steel, usually 17-7 stainless steel. All seams are seal welded to prevent gases from passing through and reaching the mild steel surfaces. The current market price of stainless steel will have a big impact on the price of the project. The labour cost is another factor, considering almost all of it will be field labour. Some of the stainless steel sheets can be precut ahead of time, saving time in the field. With over two years run time, the oldest installation looks like it did the day it was completed. If done correctly, there is no annual maintenance other than inspection.

Stainless steel construction
The best option is to fabricate the tube sheet and the clean air plenum out of stainless steel from the beginning. If this option had been used in the US in all kiln dust collectors installed in the last 15 years, the savings would be in the millions of dollars. Cost savings in a full replacement of clean air plenums, tube sheet replacement, repeated metal repair, sand blasting and coating, short filter bag life, and unplanned outages would have been realised. Not to mention the annoyance of being over particulate emissions limits, because of rust scale going out the stack, would have been avoided. As stated earlier, the initial cost is higher, but the cost of ownership is the lowest. There have been several new dust collectors supplied in the last fifteen years that use the stainless steel option, and the plants where they are installed are enjoying low emissions and low maintenance costs.

Conclusion
With the ever changing gas stream chemistry in kiln dust collectors and the lower particulate emission limits, corrosion has to be controlled for financial and environmental reasons. It is up to each plant to decide the option to take, but addressing corrosion early will save a considerable amount of money. Obviously the first two options are just delaying the inevitable and will result in expensive projects in the future. Sand blasting, and coating and lining the clean air plenum in stainless steel, both have merits and doing research up front is advisable. A dust collector with a stainless steel tube sheet and clean air plenum should be seriously considered.

This article dealt strictly with pulse jet dust collectors, but all the information discussed would hold true for reverse air kiln dust collectors too.
 

About BWF Envirotec:
BWF Envirotec is the international market leader in the field of filter media for industrial filtration. The product portfolio ranges from needlona® brand filter media, including the PM-Tec® membrane product line to Pyrotex® KE ceramic filter elements. BWF Envirotec filter media find applications in the aluminium, cement and steel industry, in power plants and waste incineration plants, among others, or also in the food industry.

BWF Envirotec was the first manufacturer in the world to introduce needle felts as a filter medium in 1968. With its head office in Offingen/Bavaria and production plants in Germany, China, the USA, South Africa, Italy, Turkey, Russia, India and Austria, in addition to a sales network in more than 50 other countries, BWF Envirotec guarantees economically viable solution concepts and technical services in line with the market.

BWF Envirotec is part of the BWF Group. With more than 1400 employees throughout the world, the company operates in the field of industrial dedusting, production of technical felt materials, wool felts and in plastics technology.