|By Robynn Andracsek, P.E., Burns & McDonnell and Contributing Editor, and Nick Bauer, P.E., Burns & McDonnell|
Time is money when starting a resource to meet load demand. Startup emission rates, however, can greatly exceed steady-state emission rates and they can pose a hurdle in the permitting, as well as the compliance, of a facility.
With the growth of intermittent resources such as wind and solar, gas turbine and heat recovery steam generator (HRSG) manufactures are designing for faster starts and improved operational flexibility. These improvements increase their viability for responding to energy fluctuations in the market and promote grid stability. Being online and selling energy or capacity has several economic advantages. Reducing startup emissions are a benefit as well; to the tune of 30% reduced greenhouse gas emissions compared to a traditionally designed combined cycle plant, according to some manufacturers.
Traditionally designed combined cycle facilities are limited by stresses imposed on steam generation equipment due to high thermal transients in the bottoming cycle. The gas turbine is ramped to a low-load hold point to allow the cycle to safely reach its ideal steam conditions before eventually making its way to full load capability. Recently, HRSG have been designed with thinner walled drums to reduce the time required to meet these conditions. While this results in much faster ramp rates, it is still slower than the capability of a once-through steam generator (OTSG).
OTSG designs either remove all drums from a traditional HRSG or replace the high pressure drum with a thin walled separator, allowing for maximized gas turbine ramping. To accommodate the fast start of the gas turbine, steam is initially bypassed to the condenser as the steam turbine and piping are safely warmed. To minimize startup time, these facilities also include an auxiliary boiler to keep the steam turbine seal system and attemperation system warm to avoid thermal shock. Taken together, all of these features result in fast energy to the grid and significantly reduced startup emissions.
Traditional combined cycle plants have various startup times depending on the duration of the shutdown. In other words, startup time is a function of the cooling which has occurred in the cycle. Depending on the configuration, a traditional startup can range anywhere from 90 minutes to four hours. The integration of fast start features can reduce startup times by up to 50 percent. It is commonplace in the industry for simple cycle gas turbines to achieve a start cycle in 10 minutes, regardless of the amount of time it has been shut down. Similarly, reciprocating engines are capable of reaching full load in as little as five minutes (but are sometimes permitted for startup times of 10 to 30 minutes).
Permitting of startup conditions requires a special balance between reality and operating margin. Emissions during startup and shutdown are not excluded from Best Available Control Technology (BACT) limits but are usually evaluated as a separate operating scenario from full load operation and given their own numeric limits.
Startup emissions must be included in dispersion modeling and can be problematic for short-term National Ambient Air Quality Standards (NAAQS) such as one-hour nitrogen dioxide (NO2). One modeling solution is to account for the fact that during a single hour of operation, startup emissions might occur for 20 minutes with the rest of the hour at the controlled emission rate. This will result in a lower modeled pound per hour than assuming startup lasts for a full hour. A longer startup time in the permit will increase operational flexibility but will make the modeling results higher.
A construction permit may set limits on the number of starts per year, but care should be taken to avoid limits on starts per day unless absolutely necessary. Annual potential emissions of CO2 will often represent a greater percentage of the Prevention of Significant Deterioration (PSD) major project thresholds than will be the case for other pollutants. If a source wanted to remain below major source thresholds, CO2 emissions would then decide the operating hour permit limit.
The key to successful permitting and flexible operation is upfront consultation with the permitting agency to ensure that they understand how the plant will be dispatched (base load, wind-following, seasonal). Permit limits should provide a level of margin above manufacturer guarantees regarding emission limits and startup durations. A good permit will keep your plant running smoothly.
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