By Megan Parsons, Burns & McDonnell, and Robynn Andracsek, P.E., Burns & McDonnell and contributing editor
The battle for a good permit begins well before the application is submitted, with the initial Front End Engineering Design (FEED) and development of conceptual engineering information used as inputs to permit modeling and development.
A lack of communication between permitting and design engineers can lead to big problems for a facility, as each group has its own perspective, language, drivers, and needs. Ultimately however, alignment between permitting and design engineers will best serve the long term interests of the facility.
Particulate emission limits are a frequent source of permitting/design disconnect. A major contributor to condensable particulate matter (PM) is the amount of sulfur in the fuel gas and the amount of oxidation of sulfur dioxide (SO2) that occurs through the gas turbine combustion process. This occurs throughout the heat recovery steam generator (HRSG) in the selective catalytic reduction (SCR) system, carbon monoxide (CO) catalyst, and duct burner. The maximum amount of sulfur in the gas may not be easy to define over the life of the plant. Conservatively using the sulfur tariff for the gas pipeline is often too high an assumption and can lead to serious impacts during dispersion modeling, especially considering that the actual gas sulfur content is typically significantly lower than tariff value. However, owners are often hesitant to rely on past gas supply sulfur levels as a reliable prediction of long term levels, as several shales predict a potential for increasing sulfur content as production areas shift. The type and location of SCR and oxidation catalyst impacts the conversion of SO2 to sulfur trioxide (SO3) throughout the gas turbine/HRSG train, and the amount of ammonia injection and slip impacts the amount of SO3 in the exhaust gas that is converted to ammonium bisulfate. Because conversion of SO2 to SO3 is not widely understood, most owners are prudent to assume 100 percent conversion of sulfur to particulate when establishing their plant PM limit.
Start-up emissions are another area of concern. Actual hot, warm, and cold start-up emission rates are highly dependent on the gas turbine manufacturers (OEM) and starting package selection, the HRSG and steam turbine generator design, OEM selections, the overall steam cycle design, and balance of plant equipment design. “Conventional” start-up times are based on holding the gas turbine at select, low operating loads to allow the HRSG materials to gradually warm. These hold points also provide time for cycle water quality to be brought within specification before steam can be admitted to the steam turbine. This typically results in the gas turbine operating outside of emissions compliance load during start-up with NOx, CO, and VOC emissions at orders of magnitude higher than during normal steady state operation. An alternative is to remove the gas turbine low load hold points and reduce the overall startup emissions. It is also important to understand how to appropriately estimate start-up emissions for the final plant configuration. Calculation of start-up emissions is not easy. Regardless of major equipment selection, start-up emissions are highly dependent on, and influenced by, the overall cycle design.
Duct firing is another element of plant design that has tripped up many owners during permitting. Air permits include separate limits when operating with and without duct firing. Typically the maximum amount of duct firing is set by either the desired amount of peak plant output or the maximum practical design limit. Often, preliminary engineering is completed to estimate the amount of duct firing that is required to achieve one of these limits. Emissions produced during duct firing are calculated based on this heat input. However, the actual required amount of duct firing is determined by final major equipment OEM selection and thermal cycle design optimization. The finalization of these two decisions is often completed after air permit issuance. This may result in limitations on duct firing capability. In this case, it is important that the design engineers determining cycle design and the permitting engineers developing emissions estimates understand and consider the impact various major equipment OEMs and variations in cycle design may have on heat input and associated permit limitations.
Gas turbine technology is evolving at a rapid pace. In the past three years, most of the major gas turbine OEMs have released several performance improvements. Many owners, especially those with project delays or longer permit approvals, have been caught with air permit requirements restricting the ability to implement the latest gas turbine technology platform without revising the air permit. The issues described above can be mitigated or eliminated when the permitting and the design engineers communicate.
Coordination up front can save time and money in the end.