Wet Lock Hoppers
In principle it is possible to pressurize coal slurries making use of lock hoppers. It offers certain advantages over piston pumps for pressurizing coal-water slurries. Moving the coal-water slurry in a piston device is not simple. Valves for this service that are required in both lock hoppers and piston pumps are much less of a problem. Moreover, thicker slurries can be transported in lock hoppers. Before reaching the burners this thick slurry (paste) will become more dilute due to the expansion of the water.
The slurry is fed to the lock hopper with an open top valve and closed bottom valve (see Figure 6-3). Subsequently, the top valve is closed and the lock hopper is pressurized by opening a valve in a gas line between the high-pressure space and the top of the lock hopper. Then the bottom valve of the lock hopper is opened and the slurry leaves the lock hopper, flowing into the high-pressure system. The bottom valve of the lock hopper is then closed, and a second valve in the bottom of the lock hopper is opened through which water is admitted that pushes the gas in the lock hopper back into the high-pressure space. All valves are closed, and the top valve of the lock hopper is opened, thereby depressurizing the lock hopper. The water is then drained from the lock hopper through a valve in the bottom of the hopper, after which the cycle is repeated.
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In principle, it is possible to pressurize the slurry to 200 bar, which enables subsequent preheating to about 350°C without steam formation. This requires a high-pressure lock hopper, but the temperatures in this hopper are below 100°C, so low-alloy steels can be used.
Almost the same system can be used for pressurizing dry lump coal. After the lock hopper has been filled with water to push out the syngas in the hopper, all valves are closed and the valve in the water drain is opened, thereby depressurizing the lock hopper. After draining the water, the valve in the drain is closed and the cycle is repeated.
The only place where in some gasifiers wet lock hoppering is currently practiced is in the discharge of the slag-water slurry from the gasifier.