The Coffeyville Resources nitrogen fertilizer plant in Coffeyville, Kan., produced 814,700 tons of ammonia and 1,268,400 tons of urea ammonium nitrate (UAN) in 2017, enough to supply approximately 5% of total U.S. demand. It is one of just two such plants in North America that doesn’t rely on natural gas as an energy source — and the only one that uses petroleum coke (petcoke) gasification to provide the hydrogen essential for its operations. The facility’s location adjacent to a refinery gives it ready access to the large volumes of petcoke produced there. About 80% of the petcoke fuel required for production comes from the neighboring refinery.
Historically, petcoke has been significantly less expensive than natural gas on a per-ton-of-fertilizer-produced basis, and coke prices have been more stable than those of natural gas. By using petcoke as its primary energy source, the plant traditionally has been the lowest-cost producer of ammonia and UAN fertilizers in North America.
The Coffeyville operation moves most of the petcoke via conveyor; the plant maintains completely redundant systems to protect against unplanned shutdowns. The dual conveyor systems have run for many years without major failures but were beginning to show their age. In addition, the site wanted to improve material control and overall efficiency by taking advantage of advancements in conveyor technology since those systems were installed.
One of the primary issues to address was material containment — reducing the amount of dust and spillage that escaped from the conveyors. After walking the entire length of two sets of twin conveyors, we identified conveyors 19A and 19B as being most in need of attention; each has a 24-in. wide belt, 300 ft long. Travelling at 400 ft/min, the belts carry an estimated 1,400 t/d of petcoke to an entrained flow gasifier. The twin belts have two load zones each; these were among the main points of concern.
The spillage was significant. Waist-deep piles were accumulating each day. An average of about 90 man-hours per week was necessary to safely clean the affected areas and haul the material away. In addition, cleaning and adjusting the belts, addressing worn components and keeping the system running required another 16 hours of maintenance time each week. Belts, rollers and other components were wearing out prematurely. Given the tab for replacement parts and labor, the opportunity for cost reductions looked good.
Based on site visits, technicians determined that the containment issues were primarily a result of carryback from insufficient belt cleaning and misalignment. The skirts and tail boxes were also allowing fugitive material to escape.
Stabilizing And Sealing
So, the plant launched a modernization project in 4th quarter of 2015. It included upgrading the transfer points to eliminate belt sag, provide effective sealing and improve belt cleaning, as well as alignment systems to deliver continuous adjustment and maintain a consistent belt path (Figure 1). Components used were specifically designed for durability under the heavy load and impact at the transfer points.
Technicians first installed three impact cradles on both 19A and 19B, located under the belt in the loading zones (Figure 2). The cradle design features a bed of steel angles lined by energy-absorbing impact bars with a top layer of low-friction ultra-high-molecular-weight plastic. The impact beds absorb the energy, so the belt doesn’t have to. They also create a flat edge for the edge seal, to prevent spillage and fugitive dust. The cradles feature wing supports that adjust to match Conveyor Equipment Manufacturers Association standard trough angles, as well as provide a 5% fine tuning. This allows the cradles to accommodate the idler profiles of different manufacturers and ensure a tight belt seal.
After the impact zone, a series of 16 slider cradles were installed on each conveyor to stabilize the belt line and eliminate bounce. Transfer points can be prone to spillage as the load lands on the receiving conveyor. Once the belt leaves the impact cradle, it also can sag while the material is still settling. This compromises the skirt seal, allowing dust and fines to escape while creating entrapment points where material can get caught and gouge the belt.