OCR Text |
Show oxygen. Among various types of fuel cells, PAFC, or the phosphoric acid fuel cell, is currently regarded as being viable as a result of energetic efforts continuing mainly in the U.S.A., Japan and Europe.[1,2] A typical PAFC consists of a power generation system having fuel cell stacks, a DC/AC converter system, an air preparation system and a fuel processing system which comprises a reformer as the main reactor. Reformers for fuel cell power plants produce hydrogen-enriched gas from process feed, such as natural gas. Feed gas is mixed with steam and heated to c.a.1050K in a Ni-based catalyst bed, where steam reforming reaction takes place together with shift reaction. The overall reaction is endothermic, thus requiring external heat supply by combustion. Although most current reformers are of the multitube type, the authors hav~ designed a compact IMW class reformer of the single tube type. [3] This reformer is unique due to its large diameter annular reformer tube of which the axis coincides with the axis of the cylindrical combustion chamber. The burners considered here are designed so as to meet the design requirement for this reformer. 1.2 Reformer Burner Requirement Fuels: In PAFC plants, the reformed gas from the fuel processing system is fed to the fuel cells where 80% of the inflow hydrogen is consumed to generate electric power. Thus, the exhaust gas from the fuel electrode contains unburnt hydrogen as well as methane and carbon monoxide that are not involved in the electrode processes. In most current fuel cell systems, water vapor is removed from the gas, although the degree of water removal differs from system to system. The resultant gas is called spent fuel, and it has a high heating value of 5 ~ 7.5 MJ/Nm3. The burner is required to handle both natural gas and spent fuel and to achieve stable transition from one fuel to the other. Natural gas is used only for heating up the reformer system, during which period no spent fuel is available. Spent fuel can be used when the plant is in load operation. Combustion Air. Spent air exhausted from the air electrode of the fuel cells is commonly used as part of the combustion air in order to achieve further reduction of NOx emission. The spent air normally contains oxygen of c.a.l0% by volume. In some early designs of PAFC plants, however, fresh air was used. Pressure Level: Upon cold start of a typical pressurized fuel cell plant, the air preparation system is first activated to ensure stable supply of pressurized air. The reformer is then pressurized with nitrogen, and advances to the burner ignition process. The pressure level in the pressurized reformers is normally in the range of 0.5 ~ 0.9 MFa. 2 |