When faced with excessive amounts of dust and spillage from conveyor systems, operators logically focus on cleanup, maintenance, and mitigating downtime to maintain production levels. Often these dust and spillage issues can be deemed “minor inconveniences” or “the cost of doing business,” until they raise the cost of operation to such a level that they can no longer be overlooked.
However, when a new plant begins production and these issues present themselves at the outset, it is best to address them early. This was the case for San Pedro BioEnergy (SPBE), a biomass power producer in the Dominican Republic. The conveyors carrying organic matter experienced so much fugitive dust and spillage that relatively new components were failing due to fouling. This caused excessive downtime and required excessive maintenance. The increased labor, expense of equipment replacement, and downtime for maintenance raised the cost of operation and rendered the situation unsustainable over the long term.
“We convert low-cost waste products sourced from local agricultural and recycling partners into affordable power for surrounding communities and industries,” explained Ing. Luis Alberto Pantín González, General Manager at SPBE. “Efficiency is a priority for us. That is why we engaged Pegran and Martin Engineering for a solution.”
Sustainable power production
Opened in 2017 in the province of San Pedro de Macorís, SPBE was the first renewable energy plant with biomass-based generation technology in the Dominican Republic. The power producer complies with the standards of the United Nations Global Compact. Its operations are aimed at significantly reducing carbon dioxide emissions and promoting the Sustainable Development Goals (SDGs) of the Dominican government and global community.
The biomass used by the plant consists of 90% sugarcane bagasse and 10% wood chips from construction and demolition (C&D) recycled material. SPBE’s cogeneration unit has a 30.5-megawatt turbine and a high-efficiency boiler running on a production cycle of 335 days per year with a 30-day annual maintenance shutdown. The traveling grate boiler has a production capacity of 140 tonnes (154 tons) of steam per hour at 32 bars (464 psi) of pressure.
Along with providing power to the CAEI Sugar Factory — the provider of sugarcane bagasse — the plant distributes power to the National Interconnected Electrical System (SENI for its Spanish acronym) which serves local communities, as well as several industrial customers.
“Providing clean power allows us to directly employ over a thousand people and indirectly bring income to thousands of other contractors, partners and servicers,” Ing. Pantín said. “With so many people depending on us, we take any outage in production and operations very seriously.”
Clean energy, dirty conveyors
Operating a total of 30 conveyors, the plant was mainly experiencing fugitive dust and spillage at three critical transfer points leading from the mulcher to the incinerator. Two of these run horizontally from towers high above the ground and one is inverted at a 45º angle. Transporting mulch, the 1,828-millimeter- (72-inch-) wide belts ran for 15 meters (50 feet) then dropped the material 6-9 meters (20-30 feet) through unobstructed chutes directly onto impact idlers.
Processed to between 1 and 100 millimeters (.04 and 4 inches) in size, the mulch became lighter and more prone to emit dust. The long unobstructed drops through the chutes caused impacts that created turbulent air within loading zones that were not equipped to control airflow. One of the transfer points was only enclosed by a grated structure, offering no dust or spillage control at all. Two of the loading zones had enclosures with skirting, but the impact idlers created large gaps where high volumes of fugitive dust and fines escaped.
“The dust was dense, and the spillage reached waist height in a matter of hours,” said Alfonso Granata Jr. of Pegran. “When we were invited to examine the issue, we pointed out some of the causes and offered solutions.”
In its report, the inspection team identified several system design issues that contributed to the dust and spillage. One was that the inlet chutes were dropping material behind the loading zones onto the transitions from the tail pulley to the first troughed idler instead of directly onto the impact idlers. This caused the belts to slump, opening large gaps under the skirting where material could escape. Without support, this also caused uncentered loading. Spillage collected underneath the loading zones, piled up past the return belt lines and fouled the tail pullies. Another issue related to induced and displaced air in the loading zones. Because of the unobstructed drop height, the impact created “splash” that resulted in dust and spillage being forcefully exhausted.
Other issues included the lack of proper equipment to adequately control and contain material. For example, there were no wear liners on the enclosures, which left the polyurethane skirting to perform double duty containing material and taking the brunt of the transfer force. This resulted in premature failure of the skirting, contributing to fugitive dust and spillage. Missing consistently across the three transition points were stilling zones (also known as “settling zones”) and dust collectors to mitigate airflow and control emissions. After the loading zones, dust that did not escape the chute accumulated on the inside walls of enclosures rather than settling back onto the material flow, posing a risk of fire or explosion from accidental sparking.
Fugitive material was not just a problem at the transfer points, it was also an issue throughout the transit and discharge of material. Uneven loading made the material settle off center, causing the belts to drift. Once the belts reached the discharge zone, the existing belt cleaners did not adequately clear the material from the belts’ surface, allowing fines to cling to the belt and carry back, piling spillage along the belt path. Since these conveyors were located above other parts of the plant, the carryback would drop a long distance, spreading and collecting on all surfaces throughout the plant. This required workers to regularly be taken from other tasks and assigned to clean the spillage.
“Every other day we assigned a four-person crew to the job of cleaning up around these areas, which took an entire shift. This raised the cost of labor, but without it, material would pile up, encapsulate the conveyor and make things worse,” said Ing. Pantín. “We were also struggling with dust fouling the bearings of idlers, pulleys, and machinery, causing them to fail prematurely. Every time this happened, we had to shut down for maintenance, which affected power production.”
Modern transfer point design
Reimagining the system meant rethinking every aspect from tail to head pulley. This began with doubling the length of the enclosures from 6 meters (20 feet) to 12 meters (40 feet) and raising the height of each one a few centimeters. The chute enclosure is modular so that it can be modified in the future. The extra length and height allowed for more airflow control, dust control equipment, an external wear liner, and room for the active dust to settle back into the material flow.
Starting at the loading zones, the existing impact idlers created a bumpy belt path and gaps between the belt and skirting, allowing dust and fines to escape. Those were replaced by impact cradles and slider support cradles with smooth static bars down the length of the enclosure. The technicians installed 1,524-millimeter- (60-inch-) wide impact cradles with medium-duty impact absorbing bars. The bars are reinforced by a steel support structure with a base of 50-durometer styrene-butadiene rubber (SBR) and a top layer of slick UHMW plastic. The benefit of bars over idlers is that they provide an even belt plane for the skirting to properly seal the enclosure with no gaps. The placement of the units promoted centered loading to mitigate belt drift and reduced bumping through the settling zones to minimize dust production.
Following the impact cradles, the slider cradles were set at a 35º trough angle. Offering belt support with a smooth transition, they have 62-durometer (shore D) UHMW polyethylene side bars and 140-millimeter (5.5-inch) center support rollers set at the base between the bars, away from the belt edge to avoid fouling. The unique “box” design allows maintenance staff to turn the bars over for a second service life without disassembly of cradle components. The rugged steel assembly is attached to its frame on a track, allowing a single worker to simply pull the cradle away from the frame to perform any service on sidebars or idlers from outside the stringer.
“In between each of the cradles are specialized troughed idlers,” Granata Jr. said. “The purpose is to ensure there are no gaps between the cradles where fugitive material can escape. These are also set on a track and pull out for easy service.”
The modular enclosure chutes above the loading and settling zones are designed to offer operators options when making modifications to the design. The tail sections were capped with metal shields formed to the trough angle to prevent rollback. The enclosure exits were covered by flexible dust curtains made of thick 6-millimeter (0.25-inch) rubber strips to moderate air turbulence. Each section of the enclosure chutes can be economically swapped out for future changes or additions to the system. The enclosures lengthened and heightened the settling zones to control airflow. Above the transition between the loading zones and settling zones, holes were cut to accommodate dust bags that filter escaping air and capture airborne particles. Once the system is shut off, the collected dust falls from the bags back onto the conveyors.
Beyond air circulation, another reason the enclosure chute height was raised was to allow for wear liners and dual-efficiency skirting. Along the enclosure, brackets were placed and thick wear liner plates were installed to protect the chute wall from the punishing environment inside the loading and settling zones. Commonly, wear liners are welded to the inside of the chute, requiring chute entry and hot work to remove. The external wear liners are easily swapped out from the outside of the system, reducing labor and improving safety. The skirt seals are specially designed one-piece polyurethane strips that stretch down the length of the chute and seal the enclosures to the belts, preventing the escape of fines and dust. Mounted on the outside of the chute walls for simple external maintenance, they float on the belt surface and self-adjust to maintain an effective seal without maintenance. When the bottom side is worn, the skirting strips can be flipped over, extending their operational life and reducing equipment costs.
On the return side of the belt beneath the loading and settling zones, technicians installed V-plows and belt tracking systems. The plows are mounted with self-adjusting arms that hold the units in place as they glide lightly on the return side of the belts and deflect any fugitive material that could foul the tail pulleys. The belt tracking systems feature rollers set slightly above the return belt line. Two arms sense slight horizontal fluctuations in the belt line and adjust the rollers to compensate, ensuring that the belts are centered on the tail pulleys.
One of the biggest causes of spillage was carryback. Heavy-duty primary belt cleaners with metal tip blades were installed at the head pulleys. Due to periods of high humidity, installers fitted them with low-adhesion urethane blades specifically designed for sticky and tacky material.
“The curved design conforms to the belt, and the tensioning system ensures continuous contact across the belt profile,” said Granata Jr. “The blades do an excellent job of drastically reducing the volume of carryback and the thick dust.”
Retrofitting conveyor efficiency
The installation at SPBE was conducted during a scheduled outage by a handful of local contractors under the supervision of experienced Pegran and Martin Engineering technicians. When the system was run and tested, it was immediately apparent that the new equipment drastically reduced the volume of fugitive spillage and dust around the transfer points and throughout the plant.
“With only a few adjustments, the full-belt tests clearly showed that the system was much improved,” Granata Jr. observed. “Not only did this project reduce the spillage and dust, but it improved efficiency across several critical sections of the power plant’s throughput.”
Two years following installation, the equipment runs as projected with no degradation in efficiency. Unscheduled downtime from dust getting into rolling components and fouling machinery has not been a problem, which has lowered the cost of operation and made production more sustainable.
Managers at SPBE have noticed a significant decrease in expenditures for the replacement of idlers and other components. The spillage of fines no longer builds to excessive levels on equipment throughout the plant, so the cleanup schedule has been scaled back to once per week. These factors have reduced labor costs and maintenance schedules, resulting in more efficient and cost-effective operations.
“Less dust and spillage have made the plant a more pleasant and safer place to work,” said Ing. Pantín. “We’re impressed with the quality of the equipment and the service we received from Pegran and Martin. It’s a lasting solution that’s increased uptime with significantly less maintenance. Greater production meant a faster return on investment, which has freed up the budget to make improvements to other parts of the plant.”
Shane Tighe is territory manager at Martin Engineering. He has more than 35 years of experience in the industry, and his material knowledge ranges from flow-aid products that prevent plugging and increase production to conveyor products that reduce spillage and dust on a wide range of different materials. Martin Engineering has been a global leader in bulk material handling for more than 80 years, continuously developing new solutions to make high-volume conveyors cleaner, safer and more productive. The company’s series of Foundations books is an internationally-recognized resource for safety, maintenance and operations training — with more than 22,000 print copies in circulation around the world. Martin Engineering products, sales, service and training are available from 18 factory-owned facilities worldwide, with wholly-owned business units in Australia, Brazil, China, Colombia, France, Germany, India, Indonesia, Italy, Malaysia, Mexico, Peru, Spain, South Africa, Turkey, Japan, the US and UK.
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