Saving energy is a very important topic and issue here in United States. So much so that President Bush has been quoted as saying, "Energy is a problem that requires action, not politics, not excuses, but action." So much of what we read and hear concerning "saving energy" is directed toward process and equipment such as steam traps and motors. I believe that the action President Bush spoke of should be directed toward the steam-generating boiler and specifically those boilers that have cyclone burners. Cyclone burners use more energy (oil, gas, and coal) than any other component or equipment found at an industrial or power generating facility.
A "Cyclone Burner" is sometimes called a "Cyclone Furnace." For now, we shall refer to them only as "cyclones," though they aren’t to be confused with the "cyclones" on a circulating fluidized bed boiler. To describe briefly, cyclones are a component of a cyclone fired steam-generating boiler that’s used to burn the coal to create the fire or heat in the furnace area (around 2,300 degrees fahrenheit [F]) so the boiler can meet its steam and heat requirements. They are an energy-consuming component that’s made up of three basic parts:
- The burner area at the back end, which ignites the coal by using oil or gas.
- The barrel area where the coal and air are mixed in a swirling or "cyclone" action and ignited by the oil or gas fired burners.
- The re-entrant throat where the fire from the ignited coal is forced into the furnace area of the boiler.
The cyclone barrel, re-entrant throat and burner throat areas are comprised of small tubes (1-1/8 inch to 1-1/4 inch in diameter) and are pin-or-flat studded. The refractory material is required on the studs and tubes for protection from the swirling (or cyclone) action of coal and from the slag created when the coal is burned.
Many things have changed over the years since the first cyclone boiler was introduced into the industry back in the early 1940s, but very little has changed to the application of the refractory. In the beginning, chrome plastic material was used and was referred to as plastic chrome ore. It was inexpensive and was rammed through the pin studs back to the tube surface. Kromight Gun (a brand name manufactured by the then Babcock & Wilcox Refractory Division) was another inexpensive gun-applied product.
These chrome-based products worked well in this hostile environment. Since the materials were less than $300 per ton, the total cost (per cyclone) to replace the refractory was relatively cheap. Unfortunately, chrome products are a potential health hazard and must be avoided. Today the products available are non-chrome based (high alumina or silicon carbide) and are almost six times the cost per ton (almost $1 per pound or $2,000 per ton) but the man-hours and quantity of material required remain almost the same.
Cyclones use oil or gas for the burners/igniters and coal to generate the furnace heat. The refractory that goes in the cyclone is a main contributor to its efficiency in burning coal. If the refractory fails to do its job then the cyclone will not operate efficiently-it will take more coal to meet the boilers’ steam and heat requirements. After the refractory fails it won’t be long before the cyclone itself will fail when the tube surface (and pin studs) is left exposed to the corrosive and abrasive nature of burning coal in a cyclone. Refractory is an energy saver because it has a direct affect on lower fuel costs (5 percent to 7 percent per year) and equipment and maintenance savings.
Lower fuel costs naturally coincide with lower equipment and maintenance costs. These savings are the result of properly installed refractory. Refractory can save energy and money at a rate that’s essential for efficient plant operation. The longer the refractory lining stays in place, the longer the boiler can remain in normal operation.
Longer Lasting Linings
There are several basic steps to follow for longer lasting linings. First, examine the existing refractory (or the lack thereof). Second, select the right materials. Next, properly mix and handle. Finally, follow the correct curing and dry out procedures.
Step 1: Examine the Existing Refractory
When replacing old refractory material, don’t automatically use the same material as the original. It’s better to examine the reasons for failure and adjust the selection accordingly. Ask yourself: Did the material spall due to thermal shock? Has it shrunk due to temperatures above its use limit? Does that gouge in the refractory lining indicate mechanical abuse? If the surface appears "glassy," is it due to operation at temperatures above the use limit? Does the lack of material indicate improper installation? The old refractory lining will offer several good clues.
Step 2: Select the Right Materials
Look at all service conditions (coal type, ash content and air temperature, to name several examples) before choosing a material. Refractory materials vary widely in temperature use limits, thermal shock resistance, and abrasion resistance. Pick a refractory material with the best combination of properties for your application and type of fuel you’re using. Refractory selection can now be considered using the data from Step 1 and knowing which application will be required or desired.
Step 3: Mix and Handle Properly
Most problems with refractory materials can be traced to improper handling or mixing. Care must always be taken in the storage of any type of refractory materials. They should always be stored in dry, well-ventilated conditions. This will ensure that the refractory will not lose any strength and should give the refractory material a shelf life of up to one year.
If the material is stored improperly, where the bags or boxes can become wet or exposed to dampness for extended periods, a certain amount of moisture may get into the material and cause a partial setting-up of the refractory. The refractory materials used in cyclones are too expensive to waste. Make sure that you check the manufactured date on the bags. For your cyclones, always have your refractory made just prior to the outage or installation and never use any refractory material that was manufactured more than six months prior to the installation.
Paying attention to the four following points should produce a serviceable lining in your cyclones:
The proper amount of water is essential if you’re gunning a refractory material. True, a wetter mix handles more easily, but it robs the refractory material of its needed strength. Too dry a mix, on the other hand, is difficult to place and it may set to a weak, porous, "popcorn" structure. A proper mix will usually seem on the thick side compared with conventional concrete.
Many common industrial compounds can easily contaminate a refractory mix, and seriously affect its performance. Certain salts, for example, react with the binder to make it useless. So be sure to use clean water and clean mixing and handling equipment. Also, it’s best to use potable water because it is free of minerals normally found in tap water. Those minerals could affect the castable’s ability to reach its proper strength.
The Right Mixer
It’s vital to know that some gun-applied materials will require pre-wetting. This is important to know to make certain that your installing contractor isn’t using a continuous feed mixer. The continuous feed method means the dry material is dumped into a hopper and the water is only added at the nozzle. This could impact the ability for that refractory to meet its proper strength. For a refractory used in cyclones, reaching its proper strength is imperative.
In cold weather, the strength of a refractory (plastic or castable) will be adversely affected if the material is installed in freezing temperatures or if mixed with cold water. It’s recommended by almost all manufacturers of refractory that both the material and the water temperature are in a range of 50 degrees to 70 degrees F.
Step 4: Curing and Drying
Once you have properly installed the refractory material, it must then be cured and dried. It’s only after the refractory has been cured and dried will the refractory be capable of performing as it was designed to do. All refractory materials except those that are phosphate bonded must be cured prior to the dry-out. Unfortunately, a lot of people don’t cure the cyclone refractory after installation. This is the number one reason for cyclone refractory failure or lack of longevity, because curing allows the chemical action to take place inside the refractory and assures that the refractory can reach its maximum strengths.
The cyclone refractory is normally dried separately from the rest of the boiler refractory. This is done by closing off the re-entrant throat opening and drying the refractory inside each cyclone separately. It’s very important to understand the temperature "hold" points and to have thermocouples inside the cyclones to monitor temperatures. Drying allows the refractory material to reach is maximum strength.
Know Your Quantities
Cyclones come in four sizes based on their barrel diameter (7, 8, 9 and 10 feet). Understand that the refractory material, regardless of the type, should always follow the contour of the tubes, as opposed to being flat or straight across. This is also known as "filling in the valleys." The refractory should be calculated for ordering and then installed to the top of the pin studs. Any material 1/8 inch or more over the studs will be gone soon after you fire your boiler into operation. The refractory materials are too expensive to waste by installing it too thick.
I recommend that all plant operations personnel know the exact amounts of material that’s required for their cyclones. It’s not hard to calculate. Once you have determined the amount of materials, you can compare material costs (remember, each product may have a different density) and compare labor applications if you’re comparing a gun-applied material versus a rammed material.
It doesn’t take long when calculating with a Lotus or Excel program. A plant only has to calculate for the size it has. As an energy consultant, my computer has each area (cyclone side re-entrant throat, furnace side re-entrant throat, cyclone side burner throat, furnace side burner throat, cyclone front closure, and barrel) broken down separately. This allows for efficient, quick and exact calculations. Additionally, I have done the same for labor and material estimates for both a rammed and gunned product. This allows for a thorough review when choosing between different products, taking into consideration the difference of material density and application.
Savings Significant when Done Correctly
Materials and refractory installation costs are very small compared to most components found in either the industrial or power boiler industry that’s using cyclones. Power plants can spend as much as $75,000 per cyclone per year. Refractory materials and their installation can cost as much as 20 percent of this yearly expenditure due to improper material selection or installation.
Refractory failure is a major contributor in boiler shutdowns. Refractory, properly designed and installed in your cyclones will last longer, minimizing the amount of shutdowns required, and save 5 percent to 7 percent in your annual fuel cost (oil, gas and coal). So pay attention to all aspects of the refractory materials that go into your cyclones and remember that improperly designed, specified, stored, installed, cured or dried, refractory will have an adverse affect upon energy usage and boiler operation. That’s why experts say, "Refractory installed to save energy also saves money at a rate that is essential for efficient plant operation."
- ASTM C-64
- Refractories in the Generation of Steam Power – McGraw-Hill Book Company, F. H. Norton (1949)