chemistry

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Increasing demand for energy

Introduction

With the ever increasing demand for energy all over the world, the fuel cell, as a sustainable energy source, is considered as one of the most reliable source in electric power. A fuel cell system based on a proton exchange membrane fuel cell should be applied in the area of residential combined heat and power unit. They are not only characterized by higher efficiency, but also environmentally clean, because they have nearly zero emission of oxide of nitrogen and sulfur and have very low noise.

At present, the available fuel cell can be categorized by temperature: low-temperature, medium-temperature, and high-temperature[2].The low-temperature fuel cell contains alkaline fuel cell (AFC), and solid polymer fuel cell (SPFC) or proton exchange membrane fuel cell (PEMFC). The medium-temperature class has the phosphoric acid fuel cell (PAFC). The high-temperature class has the molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC).

Applications for residential combined heat and power system.

The fundamental requirement

At the moment, there are two methods to produce hydrogen using natural gas. Hydrogen is the ideal fuel for a PEM fuel cell because it simplifies the system integration[3]. Both has its advantage and disadvantage. The fundamental requirement is that convent methane to a hydrogen rich gas. The heat required during the reaction can be obtained from both external source (allothermal reforming) or internal (antothermal reforming), respectively.

In the case of autothermal reforming, a mixture of methane (natural gas), air, and water is fed into the reactor. A fraction of the methane reacts with oxygen and provides the heat. The allothermal reforming reactor is fed with methane and steam only. The endothermic energy is supplied by an external burner.

The chemical reactions are shown:

Steaming reforming:

Components

Due to the high hydrogen yield with the high reformer efficiency, there is a PEM fuel cell system assembled[4], the fuel processor can be classified into 5 main components:

  1. Burner,
  2. Reforming reactor,
  3. Heat exchanger,
  4. CO conversion shift reactor,
  5. CO methanisation reactor.

In the reforming process mentioned above, it is been powered by natural gas. The water is preheated with the process of methanisation and single-shift, which takes place at the inlet and the outlet of the CO cleaning unit. The waste air from the fuel cell is used to adjust to an optimum temperature range for the CO cleaning unit. However, the unconverted hydrogen in the stock is then flow back and burned in the reformer burner.

Furthermore, there is another component exist in the fuel, the heat, and the power management system as a compact control unit. Then, the case of application shown below:

First of all, as mentioned before, the reforming process takes place with the components of water and methane, to produce hydrogen, carbon dioxide, and carbon monoxide. The energy produced in this process can be used to preheat the water etc. Then, a second heat exchange transfer the rest of heat to heat around the house and the combustion gas leaves the system. After reformer, the gas is cleaned in a catalytic reactor and then fed back to the fuel cell. The unconverted hydrogen is fed back to the reformer burner. The energy can be also used to heat the drinking water. The PEM fuel cell not only provides heating and hot water, but also electricity. The grid integrated into the system to ensure constant fuel cell operation, and the storage tank is used to keep the unused thermal energy.

Different fuel cell based systems.

The cogeneration is widely considered as the major option to achieve energy, and utilized for residential and provide commercial application. During the research, proton exchange membrane (PEM) fuel cell has been detected as the suitable technology among other similar applications. This type of fuel cell can operate at a temperature of up to 90℃, and the material been used is also very flexible with no problem, for instance, plastic foil can be the electrolyte. There is such a lot of advantages, which make proponents of this technology expect a significant cost reduction. This is very important for the reformer technology that is much more complicated and expensive than for the solid oxide fuel cell (SOFC). Referring to the present application of 3 kW electrical power, its electric efficiency varying from 35 to 40 percent and with a total efficiency of 90 %[5].

In the SOFC process, due to their high operating temperature in-between 600℃ and 1000℃, a wide variety of fuel cell can be processed[6-8], and it is obviously requires special material, which may also means the material is fairly expensive. For example, the electrolyte is made of high technology ceramic. With an electric efficiency of 40% which perform better than PEM technology, nonetheless, it takes much longer in start-up and cooling phases which affects time and costs for installation, maintenance and repair. Commercial units of PEM and SOFC system need a boiler and a hot water tank to operate in domestic or commercial applications.

Limitations

The Proton Exchange Membrane (PEM) system has highly manufacturing coats and complex water management issues. The stack contains hydrogen, oxygen and water. If dry, the input resistance is quite high and water must be added to get the system keep on going, however, too much water leads flooding. Freezing can also damage the stack and the warm up is slow and the performance is poor when cold, the cooling systems are also expensive.

As mentioned before, the PEM fuel cell require some heavy accessories, such as operating compressor, pumps and other equipments consumes up to 30% of the energy generate. And the life span is short caused by intermittent operation. The PEM fuel cell requires pure hydrogen which means there is little tolerance for contaminates such as sulfur compounds or carbon monoxide. Carbon monoxide can poison the system. Finally, the complexity of repairing a fuel cell stack becomes apparent when considering that a typical 150V, 50 kW stack contains about 250 cells.

Conclusion

Most fuel cell systems nowadays would be required to operate on hydrogen produced by reforming hydrocarbon fossil fuel, the primary source of energy today and in the foreseeable future. This could result in a loss of performance due to the poisoning of PEM fuel cell anodes by the CO present in the fuel stream. Operating PEM fuel cell at a temperature greater than 120℃ would mitigate the effect of CO on the fuel cell performance by weakening the CO chemisorptions.

Increasing the operating temperature also provide advantages such as enhancements in the anode and cathode reaction kinetics, availability of high quality waste heat, less expensive cooling system, system start-up and shut-down without liquid water, and other system integration advantages.


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