Commercial Technologies for Coal Storage and Feed Preparation
Coal storage and feed preparation at a gasification facility consists of the following systems:
- coal handling and storage, which receives, unloads, conveys, and stores coal, and
- feed preparation, which crushes, screens, sizes and conveys the coal feed to the downstream pressurized gasifier vessel.
Most of the equipment/technologies in coal storage and feed preparation are typical of mechanical solids handling and size reduction equipment, and have been widely practiced in the coal industry. However, gasification requires special coal feeding that is gasifier technology-specific; in general, it falls under two separate categories of coal slurry feeding 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 to the right particle size and size distribution in order to form a stable, coal/water slurry at optimum 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.
Dry fed gasifiers of both the entrained-bed (Shell and Siemens) and fluidized-bed types (KBR) require drying of the coal to reduce its surface moisture to prevent the small coal particles from sticking to each other during transport and feeding.
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.