Picture this: You have a full “to do” list for the day and show up at the plant ready to rock it. You do not make it 10 feet inside the gate before being told there was another fire in your rotary dryer last night. Luckily, the literal fires have been put out, but now you get to deal with the metaphorical ones. So much for tackling that “to do” list. The board will not be happy to hear you had to put off your reports again.
Because the plant is still down, you decide to open the dryer for an internal inspection. You cannot remember the last time that happened — until you realize it was probably the last time it caught fire. You usually try to stay out of the dryer operations altogether to work on more important things, but today you are going in. Your capacity has been down for several months, and you have getting uneven final product. Your customers are starting to complain, and now you had a fire. What is going on?
Your guys open the dryer for you. You crawl in carefully — wow, that hatch is small — and take a look around. You notice some bent flights and wonder if that is new, or if they have been that way. Did the last person to inspect the dryer just ignore them? Then, a glint catches your eye, but whatever it was, it disappears when you shine light on it.
On a whim, you turn off your flashlight. Something is wrong. You should not be able to see anything, but you can. A half-dozen pinpricks of visible light shine into the dryer. But with all of the dryer flighting, you know it is likely there are more, and they are just being blocked. Turning the flashlight back on, you carefully make your way back in the drum, trying not to rip your new shirt on the flights. Then, the board and the wife would be upset with you.
You are able to identify one of the pinpricks through the shell and get up close to inspect. It is definitely a wear spot. There is a groove that looks like it is going around the whole drum at this point. Worse, it looks like more spots are about to break through. You will need to get a welder to patch those before starting the dryer back up. The last thing you need is the drum breaking in half and falling on the floor. Looks like there will be some time spent with Google in your near future.
If you have a rotary biomass dryer, chances are you have had a day like this. Also, chances are you have dealt with the three most common problems in biomass rotary dryer systems:
- Fires and explosions.
- Nonuniform moisture content in final product.
This article will help you identify common problems and potential solutions for rotary biomass dryers.
Biomass Dryer Problem: Rocks
Rocks and sand can cause problems in biomass rotary dryer systems as well as any downstream equipment. Common problems associated with rocks are wear through the shell, bent or broken drum flighting, damaged conveyors and excessive wear on the crushing equipment.
What do you do about these problems? The three responses you may think of first are all reactive. You can weld the shell back together. You can repair or replace flighting or conveyors. You can replace components on the crushing equipment. Replace, replace, replace.
Alternatively, you can significantly reduce the amount of rocks that pass through the dryer in the first place. How can you get product to go one way and rocks to go the other?
Using basic physics, it is easy to understand that drying biomass will be carried by the gas stream in the dryer drum from inlet to outlet, in a concurrent system. But, rocks are heavy enough to fall through the gas stream. Unless your flights are designed to push material toward the exit, rocks can easily get stuck in an endless loop, wreaking havoc on the drum’s interior. Wherever there is a space enough for a rock, pebble or grains of sand to go around in circles — at the inlet, between rounds of flights, at drum seams for inboard track drums, or at the outlet — the rocks will start cutting a groove in the metal. Most biomass dryers are carbon steel, which is a relatively soft metal, so the rocks can cut a path like water in a canyon. Eventually, you will be able to see daylight through the shell during inspections.
Rock-Return System. Instead of allowing the rocks to get stuck in a rut (literally), modern rotary-drum technology can be used to direct them toward the inlet of the dryer. Many dryer companies have developed special designs to encourage forward movement of the rocks and other heavy objects. Using this technology throughout the drum will help ensure that most rocks travel toward the front of the dryer. This will help prevent rocks from continuing to circle inside the drum or exiting through the back to wreak havoc downstream. Typically, the drum technology will drive the rocks toward the drum’s inlet end, where they will be bounded into the gas inlet. These rocks can be removed during routine maintenance.
Rock-Removal Conveyor. It is likely that you will not know whether a rock-removal screw is a necessary component until after you have installed and monitored the effectiveness of your rock-return system. In many drums with a rock-return system, the rocks will pile up at the dryer inlet. If scheduled routine maintenance cannot keep up with the required rate of rock removal, it is time for a rock-removal screw.
A rock-removal screw is installed at the drum inlet. It is positioned to receive rocks being deflected toward the inlet. It will need a heavy-duty airlock to minimize the chance of air leaking into the system.
Stop Bars and Wear Plates. If you are not concerned about removing rocks but would like to stop them from wearing a groove, stop bars can be added at places where rocks tend to get stuck and travel around in circles indefinitely. A stop bar is placed parallel to the drum axis and perpendicular to the travel of the circling rocks. Stop bars interrupt the rocks’ path and bounce them back into the gas stream, where they will be able to continue through the drum. Wear plates also can be added to spots susceptible to wear by rocks. Wear plates can be welded or bolted into your drum.
Biomass Dryer Problem: Air Leaks
Why don’t you want daylight showing through the shell? Among other reasons, you want as few air leaks as possible into your dryer.
Chances are, your dryer is a direct-fired system. This means that nothing separates the burner and flame from the gases circulating in the dryer. Most wood dryers use solid-fuel burners while some will use natural gas or propane heaters. Direct solid-fuel burner dryers typically will have hot embers going through the system at all times, just waiting to ignite something. Dry product is kindling. What does a fire need? Just three things: fuel, ignition (or heat) and oxygen.
By definition, direct-fired dryers have fuel and ignition. Oxygen must be limited. The burner uses oxygen, so oxygen content is already lower than atmosphere. By employing a recycle loop, oxygen levels can be driven down even lower. However, if there are breaches in the dryer — due to poor drum seals, holes in the drum, holes in the ductwork, poor airlocks, etc. — air can be sucked into the system. If these breaches are large enough, or if there are enough small ones to admit an adequate amount of oxygen, you can get a fire or explosion in your dryer system.
Depending on the source, there are several ways to deal with air leaks.
- Seals. The biggest source of transient air in a rotary-dryer system typically is the seals. I find that high temperature rubber belting works best. The surfaces to which the seals are applied (and directly contact) also play an important role in how effectively the seal blocks out air ingress. Rounder and smoother surfaces help ensure that an airtight seal can be achieved.
- Drum Shell Breaches. Perform an internal inspection and make sure there are no breaches through the drum shell.
- Pressure-Relief Systems. Occasionally, pressure-relief systems can be the source of air leaks.
- Manways and Cleanouts. Check manways and cleanouts to ensure that they are sealed properly and not allowing in air.
- Airlocks. The blades on airlocks need to be replaced or refurbished every two years or more often in harsh conditions. Worn airlock blades can be a significant source of transient air.
Biomass Dryer Problem: Moisture Issues
Generally, it is accepted that rotary-drum dryers are one of the best options for high capacity drying of bulk solid materials. Rotary-drum dryers can effectively process materials with uniform shapes and sizes as well as those with nonuniform shapes and sizes.
Flighting should be designed for the specific material, product sizes, moisture variations, gas velocity and capacity to be dried. If the rotary-drum dryer is struggling to achieve uniform moisture content, and it is being operated inside the design criteria but the product mix has changed, it may be time to consider a new flighting design.
By contrast, if the product characteristics, process conditions and production rate have not changed, yet the dryer that used to process the material effectively is no longer able to, you may have flight damage. This damage can occur due to fire, rocks or even a slug of heavy material. Bent and damaged flighting cause changes such as overloaded flights, underloaded flights and wrong product-drop points. Any of these conditions will cause the material to no longer shower evenly. When this happens, the product is not exposed to the correct amount of contact with the drying gases, leaving you with an inconsistent end product.
There are several ways to deal with uneven moisture.
Repair or Replace Current Flights. This is self-explanatory and should be easy enough.
Rock-Return System. Adding a rock-return system potentially can extend the drying time of larger pieces of biomass. Depending on the design of the rock-return system, the heavier pieces can bounce back toward the inlet, allowing them more time to dry and giving you a more uniform final product.
Flighting Design Changes. The purpose of drum flighting is to re-entrain the product into the hot drying gases after it falls out of the gas stream. Signs that flights are overloaded include:
- Uneven outfeed product flow (surging).
- VOC creation and high emissions.
- Uneven final product-moisture content.
- Excessively high inlet temperature.
- Low outlet temperature.
- Buildup in ductwork.
If the flights are overloaded, the material will shower prematurely. This causes most of the material to fall on one side of the drum, which can cause drying gases to flow through the drum unevenly. When the gas flow is no longer even, the drying rate is no longer even. This may cause some of the particles to overdry while others underdry.
In conclusion, industrial dryers are more complex than most people think. Because almost everyone has a clothes dryer at home, they tend to think of all dryers as simple, mundane objects. However, an industrial-size rotary dryer has many design considerations that your clothes dryer does not. At home, if you hear something banging around in your dryer, you go remove it, and the problem solved. If your clothes are not dry enough, you put them back in.
Whether you seem to have problems that originate from rocks, air leakage or flighting design, there is always room for improvement. Inspections are the number one defense against unplanned shutdowns. Inspections are the only way to know what is going on inside and outside the dryer system; that knowledge lets you address the issues found with appropriate solutions.
Once you have inspected and repaired your rotary biomass dryer, fast-forward several months. Imagine again that you are walking into your plant with a long “to do” list for the day. Instead of being met with an emergency, you are met with a report that your dryer production is up, the final product has uniform moisture content and there have been no unplanned shutdowns for weeks. Now, you can get to that “to do” list and keep your board happy. And you won’t have to risk ruining another new shirt.