WE CLAIM:
1. A continuous slag processing system comprising:
a first rotating parallel disc pump comprising opposing discs coupled to a first shaft, a first outlet configured to continuously receive a fluid at a first pressure, and a first inlet configured to continuously discharge the fluid at a second pressure less than the first pressure, wherein the first rotating parallel disc pump is configurable in a reverse-acting pump mode and a letdown turbine mode;
a first motor coupled to the first rotating parallel disc pump, wherein the first motor is configured to drive the opposing discs of the first rotating parallel disc pump about the first shaft and against a flow of the fluid to control a difference between the first pressure and the second pressure in the reverse-acting pump mode; and
a first brake coupled to the first rotating parallel disc pump, wherein the first brake is configured to resist rotation of the opposing discs about the first shaft, wherein the first brake is configured to control the difference between the first pressure and the second pressure in the letdown turbine mode.
2. The system of claim 1, wherein the first brake comprises a friction brake, an eddy current brake, or any combination thereof.
3. The system of claim 1, wherein the first brake comprises an electric generator, and resisting rotation of the opposing discs about the first shaft is configured to generate electricity.
4. The system of claim 1, comprising a controller coupled to the first motor and to the first brake, wherein the controller is configured to control the first motor and the first brake based at least in part on a desired pressure of a downstream system that receives the fluid from the first rotating parallel disc pump, and the controller is configured to control the first motor and the first brake so that the second pressure is approximately equal to the desired pressure.

5. The system of claim 1, comprising a controller configured to control a spacing between the opposing discs of the rotating parallel disc pump.
6. The system of claim 1, comprising a controller configured to control a speed of the first motor and the opposing discs for the first rotating parallel disc pump in the reverse-acting pump mode based at least in part on a flow rate of the fluid at the first outlet, a temperature of the fluid at the first outlet, or any combination thereof.
7. The system of claim 1, comprising:
a second rotating parallel disc pump comprising opposing discs coupled to a second shaft, a second outlet configured to continuously receive the fluid at the second pressure, and a second inlet configured to continuously discharge the fluid at a third pressure less than the second pressure, wherein the second rotating parallel disc pump is configurable in the reverse-acting pump mode and the letdown turbine mode;
a second motor coupled to the second rotating parallel disc pump, wherein the second motor is configured to drive the opposing discs of the second rotating parallel disc pump about the second shaft and against a flow of the fluid to control a difference between the second pressure and the third pressure in the reverse-acting pump mode; and
a second brake coupled to the second rotating parallel disc pump, wherein the second brake is configured to resist rotation of the opposing discs about the second shaft, wherein the second brake is configured to control the difference between the second pressure and the third pressure in the letdown turbine mode, wherein the difference between the first pressure and the second pressure is approximately equal to the difference between the second pressure and the third pressure.

8. The system of claim 1, comprising a controller configured to increase a speed of the first motor in the reverse-acting pump mode when a quantity of the fluid upstream of the first rotating parallel disc pump increases; and the controller is configured to increase the resistance of the first brake in the letdown turbine mode when the quantity of the fluid upstream of the first rotating parallel disc pump increases.
9. The system of claim 1, comprising a controller configured to increase a speed of the first motor to a first speed for a time period in the reverse-acting pump mode of the first rotating parallel disc pump to reverse the flow of the fluid to be discharged from the first outlet when the fluid flow to the first outlet from an upstream system is at least partially obstructed, wherein the controller is configured to decrease the speed of the first motor to a second speed upon lapse of the time period, wherein the time period is less than approximately 15 seconds.
10. A gasification system comprising:
a gasifier configured to react a carbonaceous feedstock into a mixture of a gas and slag, wherein the gasifier comprises a quench chamber configured to discharge the slag via a slag outlet;
a slag crusher coupled to the gasifier, wherein the slag crusher is configured to continuously receive the slag and a quench liquid at a first pressure from the slag outlet, to reduce a size of particles of the received slag, and to form a slag slurry at a first pressure with the reduced particles of the slag and the quench liquid;
a rotating parallel disc pump comprising opposing discs coupled to a shaft, an outlet configured to continuously receive the slag slurry at the first pressure, and an inlet configured to continuously discharge the slag slurry at a

second pressure less than the first pressure, wherein the rotating parallel disc pump is configurable in a reverse-acting pump mode and a letdown turbine mode;
a motor coupled to the rotating parallel disc pump, wherein the motor is configured to drive the opposing discs of the rotating parallel disc pump about the shaft and against a flow of the slag slurry to control a difference between the first pressure and the second pressure in the reverse-acting pump mode; and
a brake coupled to the rotating parallel disc pump, wherein the brake is configured to resist rotation of the opposing discs about the shaft, wherein the brake is configured to control the difference between the first pressure and the second pressure in the letdown turbine mode.
11. The system of claim 10, wherein the first pressure is greater than approximately 4136 kPa, and the second pressure is approximately 345 kPa.
12. The system of claim 10, wherein the first brake comprises a friction brake, an eddy current brake, an electric generator, or any combination thereof.
13. The system of claim 10, comprising a controller configured to increase a speed of the motor to a first speed for a time period in the reverse-acting pump mode of the rotating parallel disc pump to reverse the flow of the slag slurry to be discharged from the outlet when the slag particles at least partially obstruct the slag outlet or the slag crusher, wherein the controller is configured to decrease the speed of the motor to a second speed upon lapse of the time period, wherein the time period is less than approximately 15 seconds.
14. The system of claim 10, comprising a controller configured to control at least one of a speed of the motor and the opposing discs and a spacing between the opposing discs for the rotating parallel disc pump in the reverse-acting pump mode based at least in part on the first pressure, a flow rate of the

slag slurry at the outlet, a temperature of the slag slurry at the outlet, or any combination thereof.
15. A method comprising:
continuously receiving a slag slurry flow at a first outlet of a first rotating parallel disc pump; and
depressurizing the received slag slurry from a first pressure to a second pressure, wherein depressurizing comprises resisting the flow of the slag slurry from the first outlet to a first inlet of the first rotating parallel disc pump, and resisting the flow of the slag slurry comprises:
driving the first rotating parallel disc pump against the slag slurry flow via a first motor when the first rotating parallel disc pump is in a reverse-acting pump mode; and
engaging a first brake coupled to a first shaft of the first rotating parallel disc pump when the first rotating parallel disc pump is in a letdown turbine mode.
16. The method of claim 15, wherein resisting the flow of the slag slurry comprises adjusting a spacing between opposing discs of the first rotating parallel disc pump.
17. The method of claim 15, wherein resisting the flow of the slag slurry comprises coupling the first shaft to an electric generator when the first rotating parallel disc pump is in the letdown turbine mode, wherein the electric generator is configured to generate electricity by resisting the flow of the slag slurry.
18. The method of claim 15, comprising:
increasing a speed of the first motor for a time period in the reverse-acting pump mode of the first rotating parallel disc pump to reverse the flow of the slag

slurry to be discharged from the first outlet when the slag slurry flow to the first outlet from an upstream system is at least partially obstructed; and
decreasing the speed of the first motor to a second speed upon lapse of the time period, wherein the time period is less than approximately 15 seconds.
19. The method of claim 15, comprising:
continuously receiving the slag slurry flow at a second outlet of a second rotating parallel disc pump from the first inlet of the first rotating parallel disc pump; and
depressurizing the received slag slurry from the second pressure to a third pressure, wherein depressurizing comprises resisting the flow of the slag slurry from the second outlet to a second inlet of the second rotating parallel disc pump, and resisting the flow of the slag slurry comprises:
driving the second rotating parallel disc pump against the slag slurry flow via a second motor when the second rotating parallel disc pump is in a reverse-acting pump mode; and
engaging a second brake coupled to a second shaft of the second rotating parallel disc pump when the second rotating parallel disc pump is in a letdown turbine mode.
20. The method of claim 19, comprising controlling the depressurizing
of the received slag slurry from the first pressure to the second pressure so that the
difference between the first pressure and the second pressure is approximately
equal to a difference between the second pressure and the third pressure.