NOVELTY - A system for producing fuel from a solid carbon-containing feedstock, comprises: a hot box configured to be able to heat from an ambient temperature to an operating sublimation temperature; at least one substantially horizontal reaction chamber largely located within the hot box; a first powered transport mechanism configured to convey sublimation products of the solid carbon-containing feedstock; and a gas-tight element on both the input line and output line and configured to prevent hot product fuel gas from adversely escaping from the reaction chamber.    USE - The system and process are useful for producing fuel from a solid carbon-containing feedstock, and for converting a carbon-containing compound to product gas fuel and solid char fuel (all claimed).    ADVANTAGE - The system and process can efficiently convert renewable materials to fuels suitable for transportation, generation of electricity, and/or heating. Also, they can efficiently convert carbon-based feedstocks such as biomass into gaseous fuel with energy densities on the order at least that of natural gas.    DETAILED DESCRIPTION - A system for producing fuel from a solid carbon-containing feedstock, comprises: a hot box configured to be able to heat from an ambient temperature to an operating sublimation temperature, maintain an initial operating sublimation temperature and a final operating sublimation temperature that are stable within less than plus minus 10 degrees C, and cool from operating sublimation temperatures to an ambient temperature without leaking any oxygen into the hot box and having at least one heat source in communication with the interior of the hot box to supply heat as needed; at least one substantially horizontal reaction chamber largely located within the hot box, having a surface, configured to heat the solid carbon-containing feedstock without external catalyst or additional water to an operating sublimation temperature in a time frame that is short enough to sublime at least part of the solid carbon-containing feedstock without creating substantially any liquid, configured to heat from an ambient temperature to an operating sublimation temperature, operate at a sublimation temperature, and cool from an operating sublimation temperature to an ambient temperature without leaking any product gas fuel into the surrounding hot box, and comprising an input end outside the hot box and configured to receive compressed feedstock through an input line and an output end outside the hot box and configured to discharge product gas fuel gas through a discharge line and solid char fuel through an output line; a first powered transport mechanism that is located within the reaction chamber and is configured to convey sublimation products of the solid carbon-containing feedstock through the reaction chamber as the solid carbon-containing feedstock is transformed into product gas fuel and solid char fuel; and a gas-tight element on both the input line and output line and configured to prevent hot product fuel gas from adversely escaping from the reaction chamber. An INDEPENDENT CLAIM is included for a process for converting a carbon-containing compound to product gas fuel and solid char fuel, which involves: inputting solid carbon-containing feedstock into a substantially horizontal sublimating reaction chamber largely contained within a hot box and configured to be able to heat from an ambient temperature to an operating sublimation temperature, operate at a sublimation temperature, and cool from an operating sublimation temperature to an ambient temperature without leaking any hot product gas fuel from the reaction chamber into the hot box or atmosphere, or leaking any oxygen from outside the hot box into the hot box; heating solid carbon-containing feedstock to a sublimating temperature before it is able to form a liquid phase; maintaining the temperature at a sublimation temperature for a residence time that is as long a time as needed to convert the carbon-containing feedstock to product gas fuel and solid char fuel; and separating the product gas fuel from the solid char fuel.