Coal Storage and Feed Preparation
The coal storage and feed preparation portion of a gasification facility consists of various solid handling systems to receive, unload, store, reclaim and prepare the coal to be fed to the gasification plant. Most of the equipment/technologies are mechanical in nature and have been widely practiced in the coal industry. Coal preparation involves crushing, screening, sizing and conveying the coal feed to the downstream pressurized gasifier vessel. The final coal feeding step is gasifier technology specific; and, in general, it falls under two separate categories slurry or dry coal feeding.
Slurry Coal Feed
GE (formerly Texaco) and ConocoPhillips’ E-Gas™ gasifiers are examples that use a coal slurry feed system. Coal slurry is prepared by wet grinding the coal in a rod mill. Coal is delivered by conveyor into the rod mill feed hopper. The coal has to be ground into the right particle size and size distribution in order to produce the desired slurry solids concentration, which is typically about 60 to 65% by weight. Recycled water is used when possible, to minimize usage. Prepared slurry is stored in an agitated tank, and pumped into the gasifier at high pressure. Pumping coal slurry is a proven process and can be readily designed to be operated at relatively high pressure. It has a drawback in that as a slurry, it requires a certain amount of water to accompany the coal feeding to the gasifier. This water then absorbs a significant amount of the heat inside of the gasifier, thus lowering the gasifier’s operating temperature. Higher oxygen consumption (see Air Separation Unit [ASU] for more information on oxygen supply) is required due to this lowered operating temperature, resulting in a lower overall gasification process efficiency.
Dry Coal Feed
Shell and Siemens gasifiers are examples that use a dry coal feed system. Dry feed systems use a lock hopper operating in a batch mode, intermittently charging coal fines into the pressurized gasifier via staged opening and closing of valves on the top and bottom of the pressure vessel. During operation, the top valve is opened to receive coal fines into the lock hopper, while the bottom valve is maintained in a closed position. After the top valve is closed, the lock hopper is brought to or above the gasifier system pressure, typically with nitrogen from the ASU. Following pressurization, the bottom valve is opened, and the coal fines are allowed to discharge into the gasifier. After emptying the lock hopper, the bottom valve is closed and the vessel is depressurized to allow another cycle. A parallel lock hopper system is normally employed to allow one unit to be online, i.e., discharging coal at pressure to the gasifier, while allowing the other unit to be in the filling and pressurizing mode.
The dry feed lock hopper system is reliable, but is limited in the operating pressure (up to 600 or 700 psi) it can achieve. Therefore it may not be suitable for some higher-pressure operations. The Department of Energy (DOE) is currently developing technology for high pressure dry coal feeding. More information can be found in the Gasification Research & Development section.