Method for operating fuel gas manufacturing device

A method for operating a fuel gas manufacturing device, wherein the fuel gas manufacturing device comprises:

• a desulfurizing unit configured to desulfurize a heavy hydrocarbon source gas to produce a desulfurized source gas,
• a reforming unit configured to reform the desulfurized source gas by using water vapor to produce fuel gas, wherein the fuel gas comprises methane as a main component,
• a fuel gas return path configured to return a portion of the fuel gas supplied from the reforming unit to the desulfurizing unit,
• a moisture removing unit configured to remove moisture contained in the fuel gas supplied from the reforming unit to produce a dry fuel gas, and
• a circulation driving unit configured to route the dry fuel gas upstream of the desulfurization unit;
• the method further comprises performing a standby operation comprising:
• a) closing the source gas valve and the product gas valve;
• b) routing the dry fuel gas in its entirety to a point upstream of the desulfurizing unit and after the source gas valve as a circulated fuel gas,
• c) heating the circulated fuel gas by a heating unit set to a standby temperature equivalent to a non-standby operation reforming temperature, and
• d) continuing to supply water from a water vapor supply unit situated between the desulfurizing unit and the reforming unit in an amount sufficient to prevent thermal decomposition of the fuel gas and subsequent carbon deposition, and wherein the amount of supplied water vapor is less than the amount in non-standby reforming.