OCR Text |
Show combustor. W h e n circumstances warrant (allowing flexibility for different plant usage), the upstream vertical combustor section can be isolated, and the boiler modified to operate with its o w n natural gas or oil burner to supply flue gas to the downstream section of the plant. Provisions have been made to inject water dissolved chemical reagents or moisture to condition the flue gases as they are cooled in the boiler or heat exchanger. In this instance, the gas path in the boiler also serves to provide the requisite residence time to evaporate the water droplets in the flue gas. Once cooled, the gases pass through a flue gas plenum from which they can be routed (depending on the pre-determined operating requirements), through a number of different types of gas cleaning devices. The first device of the gas cleaning train is a dry sorbent, flue gas desulphurisation ( F G D ) unit. In conjunction with upstream flue gas humidification in the boiler, the F G D unit can be operated either with conventional lime-based sorbents or narcholite (sodium bicarbonate) injection for S 0 2 capture. The design requirements of this reactor requires a minimum flue gas throughput for proper gas-solids contacting. In the event of reactor operation at low primary flue gas flow rates, a secondary fan automatically draws a flue gas stream from the outlet of the induced draft fan for recirculation through the F G D reactor. The second device in the flue gas path is an electrostatic precipitator (ESP) used for fine particulate removal. The precipitator has a corona charging section and two collection zones. The ash collected on the plates are removed on-line and discharged into a collecting hopper by periodically operating a sonic horn. The third device connected to the flue gas plenum is a bag or fabric filter unit for fine particulate control. The unit relies on the use of compressed air or C 0 2 for on-line reverse pulse-jet cleaning of individual bags. The ash filter cakes removed during cleaning are collected in an ash hopper prior to discharge from the bag house unit. In conjunction with the flue gas recycling mode of operation, it is anticipated that the bag filter will also be used for fine particulate removal in conjunction with, or in preference to the upstream electrostatic precipitator unit. Although currently not in place, the fourth device will be a catalytic reactor for NOx reduction in the flue gas. The specific type of reactor system has yet to be chosen, but it is expected to be of a design where catalysts developed by different manufacturers can be tested for their performance. The fifth and final gas cleaning device which also has yet to be designed and installed, is a condenser for the removal of moisture and residual acid gases. This condenser system will rely on the use of a refrigerant and will be utilised as an integral part of the dry flue gas recycle/oxygen combustion trials. During plant operation, the exit flue gas is monitored to determine the composition of CO, C02, N O x , S 0 2 and 0 2 using on-line gas analysers. The pilot plant ambient is sampled continuously to determine the oxygen concentration, mainly to monitor any C 0 2 leakages and to avoid any safety hazards for plant personnel. W h e n ambient leakage of C 0 2 is low, the combustor and the downstream flue gas plenum is operated under positive pressure so as to rninimise ambient nitrogen contaminant of the process gas. During normal operation with air as the combustion gas, 4 |