Results and Findings
Key lessons learned from the testing of the prototype system are that:
Reduce reformer operating pressure; to allow for the required inlet pressure of 9.5bar at the fuel cell, the natural gas supply to the reformer was pressurised to give 12bar at entry into the reformer. By increasing the pressure of the reformate, the temperature required for the reaction to occur was increased, meaning the catalytic combustion tubes were required to operate hotter. This increase in temperature combined with the catalytic reforming environment caused significant corrosion in the combustion tubes which caused the annulus where catalytic combustion occurs to become partially blocked. For future designs it would be better to reduce the reformer pressure, allowing for standard materials to be used whilst avoiding the corrosion issues, then pressurise the reformate at a later point in the process after the main reactor.
Remove Pressure Swing Absorption (PSA) system; The PSA system produced high quality hydrogen suitable for use in the fuel cell, but due to it being a batch process large storage tanks, 1.8m3 for syngas and 3m3 for hydrogen, were required to keep the outlet pressures stable enough for operation. Without these tanks the pressure fluctuations would have caused the fuel cell system to shut down as pressure dropped below 9bar. In future systems, if a method for cleaning the reformate could be implemented that significantly reduced the pressure fluctuations and removed the need to store hydrogen and syngas, then the risk of explosion could be significantly reduced, simplifying the permitting and regulatory compliance requirements of the site. The risk of explosion would be reduced from removal of the tanks themselves and more importantly by significantly reducing the volume of gas stored in the system at any one time. This would also reduce the parasitic loads on the system by removing the loads associated with the PSA and the safety shutoff valves on each inlet and outlet pipework; as these shutoff valves are required to prevent the tanks emptying in the event of a leak.
Use Technically Tight Fittings to remove ATEX zone requirements; when the system was designed, ATEX zones were defined for each of the plant areas as required by regulation with standard explosive atmosphere fittings and fixings used for these environments. For the ATEX zones in the reformer and Fuel Cell containers it became apparent during the build and subsequent audits of the site that these could be removed if durably technically tight fittings had been specified. Durably technically tight fittings are leak tight if properly maintained and if these had been used the plant design would have had a reduced risk of explosion. It is recommended that for future designs gas storage should be kept to an absolute minimum and wherever possible durably leak tight fittings should be used. With natural ventilation and gas detection used as secondary layers of protection to meet the required safety standards.
Fuel Cell isolating transformer and noise susceptibility; when connecting the fuel cell to the grid through an inverter, great care was taken above normal installations to reduce electrical noise to a minimum. After installation, the noise on the incoming 380V DC supply was <0.001%. However, because the fuel cell system is not designed to see any noise of this type on the output connection, even at this level issues were seen. The issue was caused because the output connection cable runs parallel with the 12VDC safety interlock signal, therefore the noise from the 380VDC was being transferred and amplified on to the 12VDC interlock signal. To resolve this issue, parallel capacitances were placed in-line with the interlock signal to negate the noise.