We claim:
1. A system comprising:
a first pump comprising a first outlet and a first inlet, wherein the first pump is configured to continuously receive a flow of a slurry into the first outlet at a first pressure and to continuously discharge the flow of the slurry from the first inlet at a second pressure less than the first pressure; and
a controller configured to control a first speed of the first pump against the flow of the slurry based at least in part on the first pressure, wherein the first speed of the first pump is configured to resist a backflow of slurry through the first pump from the first outlet to the first inlet.
2. The system as claimed in claim 1, wherein the first pump comprises a pair of opposing discs coupled to a shaft and configured to rotate in a first direction against the flow of the slurry, the first outlet is tangentially aligned opposite to the first direction, the first inlet is axially aligned with the shaft, the pair of opposing discs is configured to drive a portion of the slurry in a first radial direction from the shaft towards the first outlet, and the portion of the slurry is configured to recirculate in a second radial direction opposite to the first radial direction towards the first inlet based at least in part on a differential pressure between the first pressure and the second pressure.
3. The system as claimed in claim 2, wherein the controller is configured to adjust a distance between the pair of opposing discs based at least in part on a particle size of the slurry.
4. The system as claimed in claim 1, wherein the controller is configured to increase the first speed of the first pump to increase a differential pressure between the first pressure and the second pressure, the controller is configured to decrease the first speed of the first pump to decrease the differential pressure, and

the flow of the slurry through the first pump is based at least in part on the differential pressure.
5. The system as claimed in claim 4, wherein the controller is configured to control the first speed of the first pump to maintain the flow of the slurry through the first pump within a threshold range.
6. The system as claimed in claim 1, comprising one or more sensors configured to sense at least one of the first pressure and the second pressure.
7. The system as claimed in claim 1, comprising an isolation valve coupled to the outlet, wherein the controller is configured to close the valve in response to a rapid depressurization condition of the slurry through the first pump.
8. The system as claimed in claim 1, comprising a flow sensor coupled to the controller and to the inlet, wherein the controller is configured to control the first speed of the first pump to maintain the flow of the slurry through the first pump within a threshold range based at least in part on feedback from the flow sensor.
9. The system as claimed in claim 1, comprising:
a second pump coupled in series with the first pump, wherein the second pump comprises a second outlet and a second inlet, wherein the second outlet is configured to continuously receive the flow of the slurry from the first inlet at the second pressure, the second inlet is configured to continuously discharge the flow of the slurry at a third pressure less than the second pressure, and the controller is configured to control a second speed of the second pump against the flow of the slurry based at least in part on the first pressure.
10. A system comprising:
a reverse-acting pump comprising an outlet and an inlet, wherein the outlet is configured to continuously receive a flow of a slurry at a first pressure and the

inlet is configured to continuously discharge the flow of the slurry at a second pressure less than the first pressure;
an isolation valve coupled to the outlet of the reverse-acting pump; and a controller coupled to the reverse-acting pump and the isolation valve, wherein the controller is configured to control the flow of the slurry through the reverse-acting pump via control of a speed of the reverse-acting pump, to close the isolation valve in response to a sudden stoppage of the reverse-acting pump, or any combination thereof.
11. The system as claimed in claim 10, wherein the reverse-acting pump comprises a variable-speed reverse-acting pump, and the controller is configured to control the speed of the variable-speed reverse-acting pump based at least in part on the first pressure.
12. The system as claimed in claim 10, comprising a gasifier configured to supply the flow of the slurry to the isolation valve, wherein the slurry comprises a slag slurry.
13. The system as claimed in claim 10, comprising a pressure sensor coupled to the controller, wherein the pressure sensor is configured to sense the second pressure, and the controller is configured to control the speed of the reverse-acting pump to maintain the second pressure above a threshold pressure.
14. The system as claimed in claim 13, wherein the first pressure is greater than approximately 1,000 kPa, the second pressure is greater than the threshold pressure, and the threshold pressure is based at least in part on a downstream slag processing system configured to receive the slurry.
15. The system as claimed in claim 10, comprising a pressure sensor coupled to the controller, wherein the pressure sensor is configured to sense the first

pressure, and the controller is configured to control the flow of the slurry based at least in part on the first pressure.
16. A method comprising:
receiving a flow of a slurry at a first pressure through an outlet of a pump;
driving the pump at a speed configured to resist a backflow of the slurry from the outlet to an inlet;
controlling the speed of the pump;
discharging the flow of the slurry at a second pressure less than the first pressure from an inlet of the pump; and
controlling a rate of the flow of the slurry through the pump via controlling the speed of the pump.
17. The method as claimed in claim 16, wherein increasing the speed of the
pump decreases the rate of the flow of the slurry, and decreasing the speed of the
pump increases the rate of the flow of the slurry.
18. The method as claimed in claim 16, comprising sensing the first pressure of
the flow of the slurry and controlling the rate of the flow through the pump based
at least in part on the first pressure.
19. The method as claimed in claim 16, comprising closing an isolation valve coupled to the outlet based at least in part on a rapid depressurization condition of the slurry through the pump.
20. The method as claimed in claim 16, comprising controlling a distance between a pair of opposing discs of the pump based at least in part on a particle size of the slurry.