American Electric Power (AEP) is installing a $175 million selective catalytic reduction (SCR) system to reduce NOx emissions at both 1300 MWe units of its Gavin coal-fired plant, the largest generating station in Ohio. The project aims to reduce NOx levels to 0.15 lb per million BTUs – about 70 per cent below its current rate – to meet EPA standards.
In an SCR, ammonia is added to the exhaust gases, which are then channelled through a catalyst and broken down into elemental nitrogen and water. Normally, the ammonia needs of such an SCR system would entail on-site storage of large amounts of anhydrous ammonia. But this has been a particular concern for people living near Gavin. Therefore, in December 2000, AEP announced plans to switch to an on-demand ammonia delivery system. The switch will increase construction and operating costs over the plant’s remaining lifetime. But it will avoid problems associated with ammonia storage, which requires high pressure tanks or refrigeration to keep pressure low and subjects the plant owner to complying with complex regulations. There is also the risk of damaging relations with the local community.
The on-demand process produces the ammonia needed from urea, a non-toxic, dry, granular nitrogen fertiliser. The urea is converted to ammonia just prior to the point at which it is injected into the plant’s exhaust gases. At present, only two other power plants in the United States employ this AOD technology, Southern Energy’s Canal Plant in Massachusetts and another smaller plant in Huntington Beach, California. The Gavin project represents a major scaling up, from 600 lb/h for the Canal installation, which entered service in June 2000, to 7000-8000 lb/h. AEP’s ambitious plan is to have the urea-based system in operation in time for the 2001 ozone season (May-September). The scale-up effort will be spearheaded by AEP’s Pro Serv subsidiary.
AEP plans to use the urea-to-ammonia technology of Environmental Elements Corp (EEC), with whom it signed an agreement in December establishing EEC as the exclusive supplier to AEP’s generating fleet should the company decide to proceed with additional installations. AEP is also granted an exclusive agreement to market and sell urea-to-ammonia systems in competitive markets for turnkey engineering, procurement and construction projects, and in the build, own, operate or maintain marketplace. The licensing agreement applies throughout North America, including Mexico. AEP will also serve as EEC’s preferred supplier of engineering, design, project management, and construction management services for third-party turnkey air emission control projects. EEC has also recently signed a co-operative marketing alliance with SCR provider Peerless Manufacturing Company.
The AOD process
The AOD process begins with dry, granulated urea pneumatically conveyed to a vertical steel storage silo. From the silo it is fed through metering equipment into a mix tank, where it is dissolved in recycled weak urea solution. The freshly prepared urea solution is pumped through a heat exchanger to the hydrolyser where the dissolved urea reacts with water to form ammonia and carbon dioxide. Steam is added to maintain process temperature and to aid in the release of the ammonia. The gaseous products are conveyed to the dilution air blower and mixer through a control valve, and from there to ammonia injection grid and the SCR catalyst. The spent urea solution is returned to the mix tank to be recycled. The process is controlled by an integral PLC system and is completely automatic. Provision is made for connection to the plant DCS, so that power plant operators can monitor the status of various parameters in the AOD process.
Experience so far with the first commercial installation, at the Canal station, has shown the system to respond quickly to abrupt changes in demand, as called for by the SCR system. On a shutdown signal, ammonia delivery stops within minutes, safely and predictably.
