Fuel cell power generation system

The fuel cell power generation system usually consists of a stack, a gas supply system, a water management system, a thermal management system, and a DC output adjustment system.

The fuel required by the battery is hydrogen, so natural gas, coal gas, methanol and other fuels must be reformed before they can be used. The reformed gas (about 70% H2, 10% CO) enters the fuel electrode, and the air (about 20% O2) is sent to the air electrode by the compressor or blower, and the battery reaction is carried out in the stack to generate water and generate direct current. The voltage of the battery system is determined by the number of cells that make up the stack. The output conditioning system converts DC to AC or outputs DC depending on the battery’s use.

During the battery reaction process, heat is also released (the calorific value is related to the battery operating temperature), and the thermal management system discharges the heat out of the stack and utilizes it. Usually, there are many ways to use high temperature heat removal, and the utilization technology of low temperature heat removal is under development and research.

The water generated by the reaction can be used as pure water after being treated by the water treatment device, or as a reactant required for the fuel reforming reaction.

1. Fuel processing device

The fuel processing device is shown in Figure 1 . Natural gas is required to add sulfide to make it a smelly gas. Therefore, the use of natural gas as fuel must first be taken out of the bowl. The commonly used desulfurization methods are activated carbon, zeolite and other adsorbents to adsorb desulfurization, or use hydrogen and sulfur to react to generate hydrogen sulfide for reaction desulfurization. The gas after passing through the desulfurizer enters the reformer, so that the natural gas (the main component is hydrocarbons such as methane) reacts with the catalyst at about 700 ℃, and the reformed gas (with hydrogen as the main component) is obtained. The steam reforming method is highly efficient and widely used. In the process of gas reforming, CO2 and CO are also produced in addition to hydrogen.

Fuel cell power generation system
Figure 1 – Fuel handling unit

The steam reforming reaction formula is:

CH4+H2O→H2+CO2    (1-1)

The CO in the reformed gas will poison catalysts such as phosphoric acid fuel cells and proton exchange membrane fuel cells during use, and must be removed in advance. The reformed gas containing CO is finally reduced to the range of 1×10-3%~1% through the conversion reaction (formula (1-2) and then enter the CO remover, through the selective oxidation reaction (formula (1-3)) in the CO converter.

The temperature in the converter is 300℃-400℃, and the CO conversion reaction will occur:

CO+H2O→H2+CO2    (1-2)

When the temperature in the remover is 150℃ ~ 200℃, CO selective oxidation reaction will occur:

 CH4+1/2O2→CO2    (1-3)

But for molten carbonate fuel cells and solid oxide fuel cells, CO is the fuel for the cell reaction, and no converter and remover are used.

2. Thermal management system

For fuel cells such as phosphoric acid fuel cells and proton exchange membrane fuel cells, the heat generated by the cell reaction can be recovered through the internal circulating water and gas. Sometimes special media such as pure water or antifreeze are used to prevent leakage and freezing of the medium.

The heat of reaction can be utilized not only through heat exchange, but also to generate hot water and steam. Phosphoric acid fuel cells with operating temperature higher than 200℃ can use the heat of reaction to obtain hot water and steam at 60℃ to 160℃; the proton exchange membrane fuel cell operating at a temperature of 80℃ to 90℃ can obtain hot water at a temperature of 60°C to 70℃.

At the same time, the heat in the reformate and exhaust gas can also be recycled together with the heat generated by the stack.

For high temperature fuel cells such as molten carbonate fuel cells and solid oxide fuel cells, the operating temperature is 650℃ ~ 1000℃, and air is generally used as the cooling medium for high temperature stacks. The exhausted heat can be used not only to obtain steam and hot water, but also to generate electricity jointly with heat utilization machinery (refrigerators) or gas turbines to improve the total power generation capacity and efficiency of the system.