SBREFRA Presentation: Understanding Unit Design and its Implications in Power Generation

1. SBREFRA Presentation Wayne Penrod Executive Manager, Environmental Policy Sunflower Electric Power Corporation

2. Unit design is unchangeable Unit design is determined (fixed) by the owners engineer before any major components are ordered and before any construction is commenced. Conditions are influenced by the required size, the expected usage (peaking, intermediate, or base - often called the duty-cycle), and the need to minimize the initial cost of the generating facility. The most critical of these factors, steam pressure, steam temperature, and condenser back-pressure cannot be altered without deep reconstruction of the plant.

3. Sub-critical steam cycle Modern sub-critical pulverized coal units (>100 MW) were first constructed in the late 1940s, initially by the larger investor-owned utilities. These subcritical units operated at pressures of 1600 psi to 1800 psi and temperatures of 1000F. Larger, more efficient subcritical pulverized coal units, operating at increased pressures (2450 psi) and temperatures (1000-1020F) began to be constructed in the mid-1970s. These became typical design conditions for smaller investor-owned and cooperative generating facilities. Smaller subcritical units operate at pressures of 1200 to 1600 psi and 950 to 1000F. Older, subcritical units may operate at pressures of 400 to 1000 psi, and 800 to 900F.

4. Supercritical steam cycle Supercritical operating pressures are greater than 3600 psi. Throttle steam temperatures were initially at 1000F. Supercritical pulverized coal units were first constructed in the mid- 1960s by the larger investor-owned, and federal government- owned utilities. AEP, Duke, Southern, and TVA are the leading examples. Because of higher initial cost these units tend to be large, generally above 600 MW. These early units were much plagued by scale-up design problems and water chemistry. Further advancement occurred with foreign development beginning in the 1990s. Newer, more efficient units, such as AEPs John Turk, operate at higher throttle steam temperatures - approaching 1100F.

5. Factors affecting unit operating efficiency Best generating unit efficiency obtained when new, at design conditions defined in the specifics of the contract guarantees. Units almost never operate at those conditions. Operation below design, or optimum conditions necessarily introduces thermal inefficiencies, thereby increasing the determination of the units heat rate.

6. Ambient and maintenance factors affecting efficiency Higher ambient air temperature and humidity negatively impacts a unit equipped with cooling towers; higher surface water temperature negatively impacts the performance of a once-through cooled unit. Mechanical issues related to steam generator or turbine generator operation can further introduce thermal inefficiencies (i.e., turbine steam packing leakage, air heater leakage, furnace slagging, etc.). Components or sub-components of a generating unit will deteriorate with age and small incremental losses of efficiency will occur. (Condenser tube fouling and pluggage, and loss of air heater basket materials through corrosion are some examples.)

7. Operating factors affecting efficiency Peaking, or intermediate duty operation, by design, are undertaken at substantial penalties to the thermal efficiency of a steam turbine generator. Load (or frequency) regulation, or automatic governor control, is used to match an individual electric company energy supply to their instantaneous customer load; this necessary function requires the assignment of certain portions of a generating units capacity to reserve requirements established by the power pools. This decrease in unit load decreases the efficiency of each connected generator to the grid. Small systems, with one or two operating units, experience this impact more fully. Larger systems may reassign reserve requirements and limit the impact to a smaller percentage of their fleet.

8. New factors affecting operating efficiency Regulated versus unregulated markets may have an effect on dispatch of unit; a base load unit may become a load following unit. Integrating renewable resources into the supply portfolio causes increased cycling duty for existing resources. Required installation of 316(b) cooling towers on units designed for once-through cooling dramatically decreases the overall unit efficiency. Increased auxiliary loads due to the required installation of new tail-pipe control technologies for CATR and EGU MACT will decrease the overall unit efficiency.

11. Case studies Marion 123 Chamois Fort Dodge San Miguel 1 Genoa 3 Garden City Gas Steam

12. Take-home message Design efficiencies were fixed in the development phase of the plant, and any improvements are at best marginal and at worst hindered by the specter of NSR review. Regulatory influences either indirectly hinder the maintenance of original design efficiencies (NSR review) or directly require the addition of equipment which penalizes the thermal efficiency of a facility. A output-based GHG standard (lb-CO 2 /MWh) is not a plausible metric for NSPS purposes.