By: Drew Robb, Robb Editorial
The debate over stainless steel versus plastic in coal handling equipment such as bunkers, silos, hoppers and railcars has persisted for decades. Plastic advocates cite impressive numbers about coal flow, the elimination of plugging, and improved coal handling economics. Opponents question its long-term durability and its need in areas where eastern bituminous coal has been successfully handled for years using traditional stainless steel hoppers.
Ahead of the curve compared to many utilities, American Electric Power (AEP) appears to be firmly behind plastics, having installed plastic liners in dozens of hoppers and railcars over the past few years. “We have been experimenting with polymer linings like TIVAR 88 for almost twenty years to resolve coal flow problems,” said Swadhin Chakraborti, a principal materials handling engineer for AEP, and an expert in coal flow. “Provided it is properly installed, TIVAR 88 has worked very well for us at many AEP facilities.” Manufactured by Poly-Hi Solidur of Fort Wayne, Ind. (a Menasha subsidiary), TIVAR 88 liners are a proprietary formulation of polyethylene designed specifically for bulk material handling.
Plastic liner made of TIVAR 88 employed on a coal hopper at an AEP facility.
As the largest coal consumer in the country, AEP has closely studied coal flow issues for many years. It began using half-inch thick TIVAR 88 in 28 silos (each 500 to 800 tons) at its Indiana-Michigan Rockport Plant. This facility has annual output capacity exceeding 9,000,000 kWh and burns about 10 million tons of Powder River Basin coal annually. While PRB has low sulfur content, which makes it less of an emission concern, its moisture content is high — between 30 and 40 percent — and it is often composed of a high percentage of fines.
“Characteristics in coal such as moisture content and fines mean that the existing steel structures designed for eastern bituminous coal can often encounter problems when the facility switches to PRB, classified as sub-bituminous coal,” said Chakraborti. “While there are cost and emissions benefits to PRB, the downside is that coal flow and other problems like spontaneous combustion must be dealt with.”
The Rockport silos, for example, are made of 3/8 inch high-strength low alloy steel with a 16 gauge 304 stainless steel 2B finish liner in the hopper cone. The 65-degree angle of the hopper cone and the 2-foot wide door, however, were found unsuitable for PRB. Several coal flow and plugging problems developed, lowering the electrical output of the generating units:
- Ratholing: Ratholing, which is quite common in coal handling silos, takes place when a channel forms above the outlet with stagnant coal on either side. If the coal being handled has sufficient cohesive strength, coal in the stagnant zone does not flow into the channel. Thus, when the rathole is emptied, no more coal flows out of the silo.
- Bridging: Also known as arching, bridging occurs when an obstruction in the shape of an arch or bridge forms above a hopper outlet and prevents further discharge. Particles can either interlock mechanically due to the presence of large particles above a relatively small outlet or they form into a cohesive arch when particles pack together.
Problems such as ratholing and bridging caused huge losses to AEP. During the 3 to 4 hours required to restore full flow to each of the 28 silos at Rockport, for instance, the utility estimates losses of 100 MW per silo. Plant personnel attempted to use sledgehammers, portable heating units and other solutions to alleviate the coal plugging problems, but with little success.
Flow experts from Westford, Mass.-based Jenike and Johanson Inc. evaluated AEP’s coal handling system and isolated the problem – the silo design was resulting in funnel flow rather than mass flow. “Mass flow is the recommended flow pattern when handling coal for the simple reasons that all material will discharge uniformly and consistently, and have more or less the same residence time,” said Rod Hossfeld, senior consultant at Jenike and Johanson. “With these features, the risk of inconsistent feeds and collapsing ratholes are avoided, and the potential for spontaneous combustion is drastically reduced.”
In funnel flow, an active flow channel forms above the outlet, with non-flowing material at the periphery. As the level of material in the silo decreases, layers of the non-flowing material may or may not slide into the flowing channel, which can result in the formation of stable ratholes. In addition, funnel flow provides a first-in-last-out flow sequence, and increases the potential for spontaneous combustion in stagnant regions.
In mass flow, on the other hand, all of the material is in motion whenever any is withdrawn from the hopper. Material from the center as well as the periphery moves toward the outlet. Mass flow hoppers provide a first-in-first-out flow sequence, reduce the potential for spontaneous combustion, reduce sifting segregation, and provide a steady discharge with a consistent bulk density and a flow that is uniform and well controlled. Requirements for achieving mass flow include sizing the outlet large enough to prevent arching and ensuring the hopper walls are sufficiently smooth and steep to promote flow at the walls.
After reviewing the arching and ratholing issues at Rockport, Hossfeld and his associates tested the PRB coal and wall friction characteristics of various liners. Samples of wall materials tested included stainless steel with a 2B finish and TIVAR 88.
“Based on those tests, we recommended that AEP line the existing conical hopper with the TIVAR liner,” said Hossfeld. TIVAR 88, an industrial polymer, offers a low coefficient of friction and high abrasion/corrosion resistance that make it well suited for industrial applications where material hang-up and accelerated wear is a concern. TIVAR 88, in fact, has proven to outlast mild and hardened steel by as much as 10 to 1 in this kind of environment.
“Using a polymer liner like TIVAR 88 reduces the coefficient of friction between the material and the substrate and that improves the flow rate,” said Chakraborti. He reports that at Rockport, plugging problems were greatly reduced after installation of the new liners.
Since that early success, AEP has installed TIVAR 88 liners on dozens of applications. According to Hossfeld, AEP plants at Tanners Creek, Kammer, Muskingham River and the Glyn Lyn Rail Unloading Hopper have adopted a similar approach to coal flow issues.
At the Kammer plant, for example, Units 1 through 3 (210 MW each) experienced coal flow problems in bunkers lined with 304-2B stainless steel, as well as in concrete reclaim hoppers partially lined with stainless steel in the lower portion near the outlet. This occasionally led to load curtailment due to serious flow problems resulting from wet coal. Patches placed on worn portions of the stainless steel added to the problem. Further, a couple of instances of self-ignition in stagnant bituminous coal led to the damage of some feeder belts.
The plant instituted quarterly bunker trims, whereby plant personnel ran the bunker empty and then began knocking out the stagnant coal with air lances. This provided only limited success, and as soon as completed, coal immediately began to re-adhere to the sides of the bunker. As with Rockport, 1/2 inch TIVAR 88 liners over the hopper walls eliminated most of the stagnation and flow problems. According to plant staff, there have been few coal flow problems since installing the liners.
The Case for Steel
Chakraborti notes that if the angle of the hoppers, silos and outlet openings is sufficient, a stainless steel lining is less prone to ratholing, bridging or sticking. But he admits that most coal plants were built with eastern bituminous coal in mind. With the prevalence of PRB and some bituminous coal with higher concentration of fines, the side angles in many facilities are inadequate for good coal flow.
“Most silos were designed for eastern bituminous coal at a time when no one even imagined the use of western PRB coal,” he said.
He cautions, though, that polymer liners are not the sole solution to coal flowability under certain conditions. “If the coal has been stagnant for an extended duration in silos/bunkers, or it has been sitting for a while, polymer liners alone won’t help you due to the cohesiveness of wet coal,” said Chakraborti. “You need a flow aid like air blasters to restart the flow, and sometimes even vibrators. Once the flow is started, though, TIVAR 88 makes it easy for the coal to continue moving.”
Hossfeld agrees. His recommendation is to determine the choice of liner via wall friction tests using the Jenike Shear tester. “Sometimes stainless steel is the right choice, sometimes the TIVAR 88 liner is better,” he said. “Without testing, you are dealing in guess work.”
Tests run using this method, however, appear to come out in favor of TIVAR 88 over steel in most instances. The hopper angle (degrees from the horizontal) required to achieve mass flow with PRB coal in a conical shaped hopper, for example, is 65 degrees for TIVAR 88 and 81 degrees for stainless steel. Interestingly, when tests were run on various coals that had been at rest for specific periods, eastern bituminous coal required an 86 degree angle in stainless steel after being at rest for only one hour compared to 65 degrees with a TIVAR 88 polymer liner (Table 1).
“Designing the handling system components based on measured flow properties can avoid problems in coal handling such as pluggages, flow stoppages from bunkers and silos, and spontaneous combustion of stagnant material,” said Hossfeld. “The typical cost/benefit analysis shows modifications such as a liner pay for themselves in a relatively short period of time.”
Drew Robb is the president of Robb Editorial, a company specializing in the writing of technical articles in the engineering and high tech fields. He graduated from the University of Strathclyde in Scotland, majoring in geology. Over the past five years, he has published more than 100 articles on a variety of engineering and power related subjects.