Determine appropriate NOx and SO2 emission factors for McIntosh CAES plant (7063)

[grgmiller]

Plant 7063 is a compressed air energy storage project that operates on natural gas turbines. Because the prime mover is listed as CE for compressed air storage, there is not a matching emission factor for NOx or SO2, even though natural gas fuel consumption is reported for this prime mover. Thus, currently this natural gas is assigned a NOx and SO2 emission factor of zero. This appears to be a niche case (see https://github.com/USEPA/camd-eia-crosswalk/issues/20) but we should still fix this.

However, based on information about this specific plant in this research article,

The working process of a conventional large-scale CAES plant can be considered to have the similarities to that of a gas turbine based power plant except that the process of CAES decouples the compression and expansion cycles of a gas turbine into two separate processes occurring at different time [11]. This functional separation of the compression cycle from the combustion cycle allows a CAES plant to generate three times more energy with the same quantity of fuel compared to simple cycle natural gas power plant [2]. Both the Huntorf and the McIntosh plants were implemented through the conventional CAES technology. The lower-cost energy from off-peak electricity, or excess RE, is used to compress air to 44-70 bar [6]. The compressed air stream is cooled to near ambient via intercoolers and stored in an underground caverns. During peak periods (generation phase), or to support intermittent RE, the pre-compressed air from the storage cavern is preheated trough a heat recuperator, then mixed with natural gas or oil and burned in a combustion chamber (~550°C) [9] and then expanded through a multistage coupled turbine–generator.

Based on this, it seems that it would be appropriate to use the NOx and SO2 emission factors associated with combusting natural gas in a combustion turbine.

However, this dissertation notes that

Newer CAES designs feature higher inlet temperatures at the lp turbine. The added heat generated at this stage facilitates the removal of the hp combustor from the design altogether (as for the CAES unit shown in Figure 3). In addition to further reducing fuel consumption, these systems also offer significant NOx emissions benefits relative to prior designs [63]

This implies that perhaps there could be a different rate used for CAES systems than CT systems.

For now, however, we will move forward with the assumption that the CE prime mover should use the same NOx and SO2 emission factors as a combustion turbine.