NOVELTY - The sulfur breakthrough monitoring assembly comprises a heater (6a), a sulfur breakthrough detector (6b), a controller (501), a water flow controller, and a fuel flow controller. The heater is designed for heating desulfurized fuel that is humidified to a predetermined temperature of 450-600 degrees C, and heats water to the predetermined temperature. The sulfur breakthrough detector is adapted to receive heated fuel from the heater, and includes a reforming catalyst bed, first, second and third temperature sensors and a deoxidizing catalyst bed.    USE - The sulfur breakthrough monitoring assembly is useful for a fuel utilization system such as a fuel cell system for detecting sulfur-containing compounds in desulfurized fuel (claimed) and for sensing/monitoring species of sulfur-containing compounds including dimethyl sulfide, carbonyl sulfide, ethyl methyl sulfide, tetrahydrothiophene, carbon disulfide and hydrogen sulfide, where the fuel includes natural gas and anaerobic digester gas.    ADVANTAGE - The compact, cost effective and stable monitoring assembly continuously and accurately monitors the low level sulfur breakthrough in the fuel with high sensitivity and safety, and can be easily integrated with the fuel cell system and the fuel processing system.    DETAILED DESCRIPTION - The sulfur breakthrough monitoring assembly comprises a heater (6a), a sulfur breakthrough detector (6b), a controller (501), a water flow controller, and a fuel flow controller. The heater is designed for heating desulfurized fuel that is humidified to a predetermined temperature of 450-600 degrees C, and heats water to the predetermined temperature. The sulfur breakthrough detector is adapted to receive heated fuel from the heater, and includes a reforming catalyst bed, first, second and third temperature sensors and a deoxidizing catalyst bed. The reforming catalyst bed is designed for reforming the heated fuel. The first temperature sensor is designed for sensing the temperature of the heated fuel before the fuel is conveyed through the reforming catalyst bed, and senses the temperature of the fuel in the deoxidizing catalyst bed. The second temperature sensor is designed for sensing the temperature in the reforming catalyst bed. The controller: is configured for determining whether concentration of sulfur-containing compounds in the fuel exceeds a first predetermined concentration based on temperature outputs from the first and second temperature sensors; receives temperature outputs from the first and second temperature sensors; determines the difference between temperature outputs of the first and second temperature sensors; performs activates of an alarm (130); controls the fuel utilization system to perform actions; further receives temperature outputs from the second and third temperature sensors; and further determines whether the difference between the temperature outputs from the second and third temperature sensors exceeds a first predetermined amount (80 degrees C) and whether the difference between the temperature outputs from the second and third temperature sensors less than a second predetermined amount (4 degrees C/hour). The sulfur breakthrough monitoring assembly is adapted to receive water from a water supply (22), and receives the humidified desulfiirized fuel. The water flow controller is configured for controlling the flow of water to the sulfur breakthrough monitoring assembly. The fuel flow controller is configured for controlling the flow of the desulfiirized fuel to the sulfur breakthrough monitoring assembly. The concentration of the sulfur-containing compounds in the fuel exceeds the first predetermined concentration if the difference between outputs of the first and second temperature sensors decreases at a first predetermined rate of 1.1 degrees C per day. The predetermined concentration of the sulfur-containing compounds is 200 ppb. The predetermined actions include: controlling the flow of fuel to components of the fuel utilization system to reduce or inhibit the fuel flow; controlling the flow of fuel to the fuel utilization system to reduce or inhibit the fuel flow; controlling a desulfurizer assembly of the fuel utilization system to redirect the flow of fuel to be desulfurized from an operating desulfurizer to another desulfurizer of the desulfurizer assembly; controlling the desulfurizer assembly of the fuel utilization system to regenerate or replace the operating desulfurizer; and controlling a switch from an operating fuel supply to another fuel supply.    DESCRIPTION OF DRAWING(S) - The diagram shows schematic view of a sulfur breakthrough monitoring assembly.    Heater (6a)    Sulfur breakthrough detector (6b)    Water supply (22)    Alarm (130)    Controller. (501)