DDGS storage and handling challenges
October 01, 2008
by World Grain Staff
When comparing two grain-based ethanol byproducts, wet distillers’ grains and dried distillers grains (DDGS), there are striking differences when it comes to storage issues.
Wet distillers’ grains typically have a moisture content of 65%. They don’t require drying, which means tremendous energy savings for the producer, and they are also a preferred feed for livestock.
However, wet distillers’ grains have a very short shelf life and are very corrosive, which has a detrimental effect on material handling equipment. Stainless steel is typically required for equipment that will convey the wet distillers’ grains.
If wet distillers’ grains are produced, they are commonly conveyed to a flat storage pad outside the drier building. Since they are typically transported by trucks, front end loaders are used to scoop the material from the pad to the open top trailer.
Meanwhile, DDGS, which has moisture content in the range of 10% to 11%, can be effectively stored and reclaimed, which provides transportation options and flexibility. DDGS is much easier to convey than wet distillers’ grains, maintains feed quality longer, and offers the potential of being produced in pellet form.
Perhaps the only disadvantage linked with the production of DDGS is the significant energy cost associated with the driers.
Many ethanol plant operators believe DDGS must sit in an open pile for a length of time to "cure" or "temper," a process that ranges from 24 to 48 hours. The idea behind curing DDGS is that the material can be conveyed, stored and transported more easily after this process. It is an attempt to address the issues of heat dissipation or any chemical reactions following the production and drying of DDGS. Plant owners who choose to include the curing process need to account for it in the layout and design of their facilities.
There are several facility and transportation issues to consider when designing the DDGS storage and handling system at an ethanol plant. One involves the size of the production capabilities of the ethanol plant, since DDGS production is directly linked to the amount of ethanol produced. Another consideration involves the percentage of wet distillers’ grains that will be produced versus DDGS. This decision is typically influenced by the availability of a local market for the wet product, which cannot be shipped long distances because of its short shelf life.
While most ethanol plants load trucks, many are now being built with substantial rail facilities, and there are also plants that can load product onto ships. The available forms of transportation can dictate loadout capacity for DDGS. If a plant cannot load out fast enough, it may be forced to pay demurrage charges for the extra time required.
Transportation issues may also affect the amount of DDGS storage that will be required at the plant. For example, if a plant intends to load 75- or 100-car trains, then adequate storage and fast reclaim systems must be included in the plant layout. The majority of ethanol plants have approximately 10 days of DDGS storage.
THREE BASIC CONCEPTS
There are three basic material storage and handling concepts utilized at ethanol plants in the United States. They include flat storage, automated vertical storage and a combination of both.
The flat storage concept simply requires a very large building. The flat storage buildings are most common at 50-to-60-million-gallon-per-year ethanol plants. The DDGS is pushed or carried to receiving hoppers in the building floor and then conveyed to loadout equipment. The main advantage of a flat storage building is that it adequately addresses the curing issue and may require less upfront investment than other options. A disadvantage is that flat storage buildings are very labor intensive, requiring front end loaders or other such equipment to move material, a process that can result in lower than desired loadout rates.
The method of DDGS storage that is most common at ethanol plants producing in excess of 100 million gallons (378.5 million liters) per year is automated vertical storage, where DDGS is stored in a specially designed silo and uses a heavy-duty reclaim auger.
Perhaps the biggest advantage of a vertical storage system is the capability of large automated loadout rates. The automated feature will reduce labor costs associated with front end loader operators. It also addresses the safety concerns of having those operators work in dusty building conditions.
One disadvantage of vertical storage systems is that they may involve more upfront capital cost. Also, a separate building is needed if a plant operator believes a curing process is required.
An issue often raised about the vertical storage concept involves the ability to send material directly to the silo. There are plants that send DDGS directly to the storage silo, skipping the curing portion. However, a small flat pad is commonly used at ethanol plants for certain conditions, such as during plant startups, when initial DDGS production may not be to specifications.
The third material storage and handling concept is a combination of both flat storage and automated vertical storage. This is most commonly seen at plant expansions where flat storage exists and advantages are seen in adding a silo versus adding another building. This method takes advantage of the fast, automated loadout rates of a silo and can accommodate the needs of those that believe in the curing process.
HANDLING DDGS AT FEED MILLS
While DDGS and soybean meal possess some similar flow characteristics when stored in bulk, there is evidence suggesting that DDGS should be stored and handled differently than soybean meal, and some caution should be exercised.
DDGS seems to have a higher cohesive strength than soybean meal, which means it is better able to stick together or gain strength over time.
An important part of this discussion involves recent trends being seen at feed mills. The challenges associated with handling DDGS may be aggravated by these trends, which include construction of much larger feed mills and production expansions at existing feed mills requiring more on-site material storage. Another recent trend is the concept of receiving multi-rail car loads of meal, which also leads to increased storage capacity requirements.
To explain how the characteristics unique to DDGS affect the ability to successfully handle it at feed mills, you need to look at what really happens in a storage silo. It is common to expect initial flow when you convey the majority of grainbased meals into a storage silo. However, there are silo conditions such as the passage of time, the introduction of moisture (either from leaks, humidity or material temperature), compaction, material densification, and cohesiveness that can lead to restricted flow in the silo.
The material can often become "stagnant" in the corner of silos, which is defined as the point where the cone meets the straight wall. As time passes, more material can become stagnant in the corners and create an environment where material does not flow freely. Eventually, a condition called "rat holing" can occur. A rat hole is where stagnant material creates a solid ring around the silo, reducing the active diameter of the silo. Ultimately, conditions can deteriorate to the point where material will create a "bridge" and material flow will stop. Two methods can be used to avoid this problem. The first is to design a storage system that will permit the DDGS to recirculate or move frequently. The second is to invest in an aggressive reclaim system that can actively keep material from becoming stagnant in areas of the silo.
DDGS is being effectively stored and reclaimed at many ethanol plants and feed mills. From a material handling perspective, DDGS can be utilized much like soybean meal. Careful consideration of storage requirements and discharge rates is important to ensure long-term success with the handling of DDGS. firstname.lastname@example.org.
Mike Schuster is vice-president of sales at Laidig Systems, Inc., Mishawaka, Indiana, U.S. He can be reached at