(95c) Air Pollution Aftertreatment Control for Natural Gas Applications

Natural gas is an important part of the American infrastructure and economy, growing in significance with the rise of domestic shale gas. In 2013, 31% of the natural gas consumed in the United States was for electric power generation in 2,154 power plants. Regulated emissions from natural gas combustion include NOx, CO and VOC’s (volatile organic compounds). These pollutants can be controlled by the use of a post-combustion catalyst installed downstream of the engine prior to the stack. For the power industry to provide electricity economically while maintaining low pollutant emissions, plant operators and equipment vendors face a variety of technological challenges which require innovative solutions.

NOx emissions are controlled by a Selective Catalytic Reduction (SCR) system. In a typical SCR system, gaseous ammonia (NH₃) is injected into the exhaust stream upstream of the SCR catalyst. In the presence of oxygen, NO and NO₂ react with NH₃ over the SCR catalyst to form N₂ and H₂O. Liquid ammonia or aqueous ammonia must be heated and injected into the unit in a precise amount in order to limit unreacted ammonia, also known as ammonia slip.

There are a number of issues currently facing the NOx control market for natural gas-fired applications. In order to improve air quality, regulators periodically reduce the emissions limits for new units, increasing the performance requirements of the technology, while users are trying to limit the cost and risks associated with operating these systems. Stricter limits require more catalyst and/or new designs from suppliers. As emissions limits become lower, the ammonia efficiency of the SCR system must be improved so ammonia injection is optimized and ammonia slip is minimized. Issues with ammonia quality are also common and can further increase operating costs by poisoning the catalyst and reducing the catalyst life cycle. As the emissions limits have been reduced, the emissions control system places more and more restrictions on the power producers and limits their operating flexibility to meet demand. To counter this, emissions control system designers are required to provide systems that operate outside the preferred operating window for the catalysts. Some large units require the catalyst to operate at higher temperatures. While on the other hand, faster start-up times and stricter start-up limits require the catalyst to operate at lower temperatures. Both of these issues are hardware and catalyst issues that require the catalyst supplier and system suppliers to work together to develop the right product.

Oxidation catalysts are used to oxidize CO and VOC’s. The issues facing the oxidation catalyst are similar to the NOx control catalyst. The limits for CO and VOC’s emissions are being reduced and operators are seeking more flexibility in terms of operating parameters. The big issue in terms of operating temperature has to do with poisoning of the catalyst by sulfur. At lower temperatures sulfur blocks the active sites of the oxidation catalyst limiting activity. At higher temperatures the sulfur desorbs, which is why it is a temperature-dependent issue. There are methods to remove the sulfur from the catalyst, but as long as it is present in the fuel the issue will return. Finding formulations that operate at lower temperatures is an important area of development currently.