Randy Rahm, CoalTech Consultants Inc., outlines how an in-depth coal handling assessment can help deal with the difficulties of handling sub-bituminous coal.
A mechanical failure ignited sub-bituminous coal dust, causing an explosion that resulted in a conveyor fire. The fire cost the utility US$ 11 million in repairs and over three months of lost generation during peak season.
Today, over 400 million tpa of coal is shipped out of the US Powder River Basin (PRB). The first sub-bituminous coal shipments left the PRB in 1974. The coal was shipped to power plants designed for bituminous coals. The engineers were mainly concerned about how these new coals would perform in the boiler and not about how they would be handled in the coal yards. This resulted in many fires and explosions in the coal handling systems due to the low ignition temperature of sub-bituminous coals, its propensity to spontaneously combust and the excessive dust associated with the handling of these coals. There were several fatalities and many severe injuries associated with the fires and explosions at some plants.
Learning from experience
Today, there is more experience in handling sub-bituminous coal and in sharing that experience through the PRB Coal Users’ Group in the US and the more recently formed Asian Sub-Bituminous Coal Users’ Group. Ultimately, however, plants must be taken on an individual basis. This should start with a coal handling assessment by a company or individual with experience in handling sub-bituminous coals. The assessment will provide a power plant with observations of the current deficiencies and recommendations for how to mitigate the problem areas with the best available technologies. A competent company will also provide training and recommendations on mitigating the hazards associated with combustible dusts, emergency preparedness, fire suppression and detection systems, dust collection, transfer chutes, load zones, belt cleaners, dust suppression systems, wash down systems, silo/bunker management and housekeeping methods.
There are few coal handling systems that have been correctly designed to minimise coal spillage in the load zones with engineered transfer chutes and load zones. Most of the dust collection systems currently in use do not meet any of the National Fire Protection Association (NFPA) standards. Some of these dust collectors are installed inside buildings with the explosion vents venting into the building. These systems were designed by companies that did not understand the type or quantity of dust their systems would collect.
It is common that the engineering, procurement and construction (EPC) contractor for a project installs the same technologies they have used for the past 40 years because they are bidding to get the project. Unless the owners specify the technologies in the bid specifications, they will only receive the bare minimum technology necessary to move the coal from point A to point B. An experienced consultant should be engaged during the development of the coal handling project specifications to ensure the new plant has the latest technologies in its coal handling systems. This will save the future expense of going back and replacing chutes and belt cleaners, while also needing to add inspection doors, new dust collection and suppression systems, wash down systems and fire protection systems, etc.
The first line of defence in fighting a fire in a coal handling system is to install correctly located carbon monoxide (CO) detectors and then the fixed fire detection and suppression systems. Over 90% of the fire suppression and detection systems in coal handling system are inadequate. Most coal handling systems have fire detection and suppression systems designed for a warehouse or hotel. The engineers who design these systems often do not understand the characteristics of a moving fire in a coal handling system. They do not understand where the fires typically occur or how to install the fire detection and spray nozzles on a conveyor to effectively detect and extinguish a conveyor fire before it reaches the power plant. There are many enclosed conveyors without properly installed draft barriers with explosion vents to slow down the fire or pressure wave from an explosion. The loss of the main conveyor fuelling a large power plant has a major financial impact on the plant’s generation revenue. The structural and equipment loss is minimal compared to the lost generation.
The main objective of a coal handling assessment is to improve the health and welfare of power plant employees and reduce the risk of a catastrophic event. The assessment should provide the plant with a detailed, itemised plan of actions to safely handle sub-bituminous coals. The plan begins with the lowest cost items, which will provide an immediate impact on reducing spillage and dust accumulations. The next items are those that require some engineering and finally those items that require engineering with a longer lead time. It is important that the plant is left with a plan they understand and can defend in the budget process.
If a plant has plans to test burn sub-bituminous coals, it often proves prudent to have a sub-bituminous coal readiness assessment performed on their coal handling system and provide combustible dust training for their employees. Employees are accustomed to handling bituminous coals with not many issues and now have to be educated on how to manage sub-bituminous coals.
Note: This article first appeared in the July 2014 issue of World Coal.
Read the article online at: https://www.worldcoal.com/special-reports/12082014/world-coal-a-vital-assessment-handling-sub-bituminous-coal-coal1192/