A system includes a solid feed pump having a housing, a rotor disposed in the housing, a curved passage disposed between the rotor and the housing, a solid feed inlet coupled to the curved passage, and a solid feed outlet coupled to the curved passage. Also, a solids packing device is coupled to the solid feed inlet of the solid feed pump. The solids packing device includes a first channel configured to receive a solid feed with a first range of sizes, a second channel configured to receive transport assisting particles (TAP) with a second range of sizes. The first range is different from the second range. A third channel is configured to receive and mix the solid feed and the TAP to provide a solid feed-TAP mixture with the TAP filling interspatial spaces between the solid feed. The third channel is coupled to the solid feed inlet.

In a general aspect, a system can include a reactor for combusting fuel and producing high-temperature, high-pressure liquid as a byproduct, and at least one vessel defining a cavity to be partially filled with water, with an air pocket within the cavity above the water. The system can further include respective valves to control admission of liquid from the reactor into the air pocket when the air pocket has a pressure lower than an operating pressure of the reactor, and to control emission of the water from the at least one vessel through of the vessel after the water in the at least one vessel has been pressurized by the liquid from the reactor. The system can also include a hydroelectric drive system for receiving water emitted from the cavity, and for converting energy in the received water into electrical energy.

In a general aspect, a system can include a reactor for combusting fuel and producing high-temperature, high-pressure liquid as a byproduct, and at least one vessel defining a cavity to be partially filled with water, with an air pocket within the cavity above the water. The system can further include respective valves to control admission of liquid from the reactor into the air pocket when the air pocket has a pressure lower than an operating pressure of the reactor, and to control emission of the water from the at least one vessel through of the vessel after the water in the at least one vessel has been pressurized by the liquid from the reactor. The system can also include a hydroelectric drive system for receiving water emitted from the cavity, and for converting energy in the received water into electrical energy.

Fuel mixed in water is combusted in a reactor having an internal operating pressure and temperature greater than 3200 psi and greater than 374 C., where the combustion of the fuel is exothermic. Air and fuel are pressurized for introduction into the reactor to a pressure greater than the internal operating pressure using energy generated from the combustion of the fuel, and the pressurized air and the pressurized fuel are injected into the reactor. Pressurized water from the reactor is injected into a drive water column that is partially filled with water to increase a pressure of the drive water column, and water at a temperature less than 100 C. is injected into the reactor to replace water from the reactor that is injected into the drive water column. Pressurized water from the drive water column is used to drive a hydroelectric drive system to produce electrical power.