This paper was delivered at Power-Gen International, December 2015.
Power demand is often greatest at the extreme temperatures due to an inherent desire (or required need) to maintain a steady, comfortable condition. The additional energy required to offset extreme ambient conditions, whether running an air conditioner or a heater, creates additional power demand. Unfortunately, a combustion turbine performance is highly sensitive to ambient air conditions and thus extreme hot and cold temperatures negatively impacts a generating unit’s performance and operation. Coil-based inlet air-conditioning systems are designed and operated to counteract these challenging conditions and maintain a combustion turbine performance and reliability throughout the ambient temperature range.
COMBUSTION TURBINE BASICS
Combustion turbines are constant-volume machines, in that the low pressure compressor rotates at a constant RPM, thus pulling the same volume of air through the compressor with each rotation. However, mass flow, and not volumetric flow, is the primary property that dictates combustion turbine output and efficiency, as demonstrated by an energy balance. Even with a fixed volumetric flow, mass flow may vary greatly according to ambient inlet air conditions. The function relating mass and volume is density and as the ambient air temperature increases, its density decreases (less mass per given area). When the inlet air is less dense, a combustion turbine pulls through less air mass and results in lower output.
Ice formation can occur in two different locations: the inlet house or on inlet guide vanes. When cold ambient conditions near saturation (high humidity – rain, snow, fog, etc.) are present, there is a danger of ice formation. In the inlet house, each disturbance to air flow will cause a pressure drop and corresponding reduction in temperature, which can lead to icing (localized temperature below freezing and the dew point). Typically the threshold to form ice occurs at the pre-filters (if present), inlet conditioning system (if present), or final filters. Once ice has begun to form, the pressure drop increases, which accelerates additional ice formation. Icing restricts air flow to the turbine, resulting in reduced output and efficiency. This condition can cause the turbine to trip due to a high inlet pressure drop and also may prevent a turbine restart. While this condition reduces the reliability of the plant, the ice is unlikely to pass through the final filters so the risk of turbine damage is small.
After the final filters, air enters the volute and bell mouth of a combustion turbine. The combination of increased air velocity (due to a reduction in cross-sectional flow area) and the compressor inducing a pressure depression produce a temperature reduction. Since the amount of water in the air stays constant, saturation conditions are approached (relative humidity increases). When the air hits the next disturbance (pressure drop), the temperature is reduced further and conditions exist which are conducive to ice formation. This flow disturbance is commonly located in the compressor inlet guide vanes. Due to the multiple temperature reductions discussed above, icing conditions can be present at the inlet guide vanes while ambient temperature is above freezing, as demonstrated in Figure 1. Ice formation at the inlet guide vanes can locally reduce airflow, impacting blade balancing and potentially causing vibration issues in the turbine. Ice can also form on the guide vanes and then separate and impact the compressor blades, causing blade fragmentation or liberation.
Both aeroderivative and frame combustion turbines require an anti-icing system in low temperature conditions to protect the unit’s low pressure compressor blades. The established target to prevent ice formation is a turbine inlet air temperature greater than or equal to ten degrees above that of the ambient air to compensate for the temperature reduction in the volute/bell mouth. Frame turbines typically accomplish anti-icing by compressor bleed heat (inlet bleed heat), which results in a two-fold output reduction: Turbine output and efficiency are reduced when the compressor is required to compress the air, but is unable to combust the air. Additionally, as inlet temperature increases, output capability decreases, as shown in Figure 2.
Aeroderivative turbines typically accomplish anti-icing by one of three possible options: compressor bleed heat, turbine room exhaust recirculation, or inlet coils heated by an external system (e.g., waste heat recovery, electric heaters, gas-fired heaters, steam, etc.). The compressor bleed heat function results in an output loss because, similar to a frame unit, the efficiency of the turbine is reduced when the compressor works to compress air and that air is not combusted, while the two latter options typically result in a slight output gain, unlike a frame turbine. Most aeroderivative turbines experience a “tent” curve on an ambient temperature versus output basis, as shown in Figure 3. This results the recovery of some additional output as the inlet air is heated towards the top of the tent curve.
This icing condition creates a scenario where a coil-based augmentation system can be operated to enhance turbine output. Traditionally, performance augmentation is considered on only the higher side of peak temperature, i.e., evaporative cooling or mechanical chilling, but would then completely disregard the colder side of the peak temperature. This is most likely due to one of two reasons: Aeroderivative turbines are a relatively young technology and the frame predecessors don’t experience this same phenomenon; hence the information is somewhat esoteric. Alternatively, and perhaps more likely, accomplishing this duty would require an additional external system which would yield output gains relatively minor to those experienced on the hotter side of peak temperature (30+% gains from chilling versus 5+% gains from heating). Plants serving either winter peaking or extreme peaking (summer and winter) could see these additional megawatts carrying a premium value. A benefit of a coil-based conditioning system is that the ability to deliver this performance heating is inherent in the design. Charts depicting theoretical gains in a sample of aeroderivative turbines are shown in Figure 4.
Load-following plants, which often run at less than full load, magnify another weakness of combustion turbines. Studies show there is a benefit to heating the inlet air during part load conditions. Consider a GE LM6000 – at full power, which has an optimum inlet temperature of 48°F (yielding the highest efficiency and lowest heat rate). With part load operation (i.e., as output decreases) the optimal (highest efficiency / lowest heat rate) temperature gradually increases, and peaks at 80°F at 30 MW (60% load). Figure 5 summarizes the efficiency improvement and associated heat rate reductions that can be attained by following this control scheme.
The GE Frame 7FA.05 exhibits similar behavior, in that as load changes so does the optimal compressor inlet temperature. Figure 6 summarizes the efficiency improvement and associated heat rate reductions that can be attained by following this control scheme.
While these heat rate improvements may not appear dramatic, it should be noted that these load following plants will incur substantial run hours at these partial load conditions and as a result, the fuel savings will accumulate over the life of a plant. In addition, these improvements in heat rate contribute to savings in emissions and may allow a plant to operate for more hours per year when considered against annual emissions limits.
Inlet cooling systems are typically broken down into two categories; passive cooling and active chilling. Passive cooling methods have a limitation that can’t be overcome by design, equipment, capital, etc. That limitation is typically psychrometric. The systems described below are evaporative systems and are limited by wet bulb temperature.
- Wetted media – Water runs down a honeycomb-esque material. As the air moves through the material, the water evaporates and the phase change from liquid to vapor absorbs heat from the air. These systems are typically installed upstream of final filters, and can result in up to 90% effectiveness (90% approach to the wet bulb temperature). Water is typically a blend of demineralized and potable.
- Fogging – Water is pumped through high pressure pumps (typically 1000-3000 psi) and then atomized in fogging nozzles. The atomized water evaporates and changes phase from liquid to vapor. These systems are typically installed downstream of final filters, and can exceed 90% effectiveness. Water is demineralized.
- Wet compression (over fogging) – Same operation as fogging system, but more water is injected into the air stream than can evaporate prior to entering the low pressure compressor. This water is carried over into the compressor and evaporates as the heat of compression is generated. Similar to SPRINT system.
Active chilling systems may be designed to deliver a desired compressor inlet temperature irrespective of ambient conditions. Reducing the inlet temperature results in an increase in density and an increased mass flow. This increased mass flow will counteract the effect of high ambient temperatures and ISO (or better) output can be recognized throughout the summer, provided the inlet conditioning system is designed adequately. The systems described below require an inlet conditioning coil to be present as the heat transfer surface for the inlet air. As opposed to passive cooling discussed earlier, these systems can actually generate water from the inlet house, in the form of condensate, under adequate ambient conditions.
- Vapor Compression –A gas is distributed to a condenser to remove the heat of compression and liquefy the refrigerant. This refrigerant is then sent through an expansion valve where a small percentage of the liquid is vaporized (flash gas), reducing the temperature of the refrigerant. The colder (mostly liquid) refrigerant is routed through an evaporator (inlet coils or a water/glycol loop) where the refrigerant undergoes a phase change (boils) and absorbs heat. This vapor is finally moved back to the compressor to complete the cycle.
- Lithium Bromide (LiBr) – Similar to the compression cycle, but the compressor, which adds heat and pressure, is replaced by a waste heat recovery system and pumps, resulting in lower parasitic load. With a LiBr system, water is used as the refrigerant so the cold approach to the evaporator is limited. The system is also hermetically sealed and works in a vacuum so any leaks pull air into the system.
- Ammonia (NH3) – Similar to the LiBr cycle, except NH3 is used as the refrigerant so there isn’t a practical limit (for combustion turbines) on the cold approach. Additionally, the system exists at a positive pressure so leaks push ammonia out, rather than pull air in.
Coil-based conditioning systems provide a combustion turbine generator a high degree of operational flexibility and the ability to maximize the benefits of both chilling and heating however conditions and pro forma dictate. Full chilling in the summer will generate maximum output, while heating in cold conditions will prevent ice formation and also maximize output. Furthermore, adjusting the inlet temperature based on ambient conditions and plant equipment will minimize heat rate and thus maximize efficiency.
The ability to manipulate inlet air temperature (down or up) provides a power plant the ability to reliably, and more importantly, predictably perform regardless of ambient air conditions. Figure 7 demonstrates an LMS100 able to achieve a near-constant output throughout the year by employing a coil-based conditioning system.