2 Outline of operating flexibility of power plants without CCS
Most of the information available in the public domain refers to the combined cycles, especially in relation to the improvements made in the recent years for flexible operation. Much less information is available on operational flexibility of PC boiler plants, as well as IGCCs without CCS. This is because PC boiler and, moreover, IGCC plants have been generally designed to operate at base load, due to the lower weight of the variable costs (i.e. fuel) on the overall cost of electricity.
2.1 Natural Gas Combined Cycle (NGCC)
Depending on seasonal load and dispatch rank of the plant, driven by competition and fuel prices, the newly designed NGCC plants operate as cycling units over their lifetime, increasing load during the day or peak hours and reducing it to the minimum or shutting down during the night or when the electricity demand is low. In general, the operational flexibility of the combined cycle plants is characterized by the following main elements:
- Low technical minimum environmental load: this is the minimum load at which the Gas Turbine is able to operate while meeting the environmental limits, in particular NOX and CO emissions. It is generally from 30% to 50% of the production.
- Good efficiency at partial load: for newly designed plants the efficiency penalty corresponding to a load reduction down to 60% is only a few percentage points (2-3) lower than the base load operation, even if the expected impact on the cost of electricity is much higher (7-8%), as the cost for fuel consumption represents a significant portion of the economics of the plant.
- High cycling capability: recently built plants are generally characterized by fast start-up (45-55 min in hot conditions vs. 90 min of older plants) and shut down, fast load change and load ramps, low start-up emissions, high start-up reliability.
- Frequency control: it occurs whenever the electricity supply and demand are not in balance. Frequency control is generally made in three different steps: primary, secondary and tertiary. In many countries, the request for frequency control (at least the primary) is mandatory for NGCC power plants interconnected with the national grid, which are typically able to respond within a few seconds, restoring the nominal value of grid frequency.
- Low operating costs: this means high start-up efficiency or short start-up time.
In addition to the above, it is noted that a flexible plant opens up new business opportunities, like utilizing hourly and seasonal market arbitrage or participation in a peak load market. For this last opportunity, power production can be increased by Air chilling, Gas Turbine over-firing or HRSG post-firing.
For these plant types, the aero-derivative gas turbine technology has several features that provide further answers to the needs of the liberalized electricity market, in particular for their capability to participate in the peak load market and their possible use integrated with renewable energy sources.
2.2 Ultra Super Critical-Pulverized Coal (USC-PC) power plant
Nowadays, coal-fired plants are generally required to operate in the mid merit market, so a medium operating flexibility is also required for these plant types. In general, the operational flexibility of USC-PC boiler plants is characterized by the following main elements:
- Good cycling capability: Supercritical and ultra-supercritical PC boiler power plants show cycling capability much greater than conventional subcritical plants. In fact, subcritical plants use drum-type boilers that require a controlled heating, limiting the load change rates generally to 3% per minute. On the other hand, supercritical or ultra-supercritical facilities use once through steam generators that can achieve quick load changes, even up to 8%.
- Fast load response: 5% to 15% of the power output can be provided in few seconds by using the capacity of the steam/water. For limited time, following measures can be used: opening overload valve(s) or opening throttled turbine control valve(s), opening/closing a feed water supply valve to the LP feed water heaters, opening/closing of the steam supply valve to the final feed water heaters.
- Fast change rate: Typical ramp rates (%rated power/min) are: 2-3 from 30% to 50% load, 4-8 from 50% to 90% load, 3-5 from 90% to 100% load.
- Fast start-up: Typical start-up times are: <1 h (very hot start, <2h shutdown), 1.5-2.5h (hot start. 2-8h shutdown), 3-5 (warm start. 8-48h shutdown), 6-7 (cold start, >72h shutdown).
- Good efficiency at partial load: reduction of plant efficiency of supercritical units is about 2 percentage points at 75% load, compared to 4 percentage points reduction in efficiency for subcritical plants under comparable conditions.
2.3 IntegratedCombined Cycle (IGCC)
IGCC plants show dispatch flexibility lower than other power plants, due to the inertia related to the process units (gasification,cooling and conditioning line, etc.), as well as the Unit (ASU), to generate and prepare the fuel at the conditions required by the gas turbine. As a matter of fact, gasification and syngas cleaning processes are chemical processing plants, operating best at design point condition and at steady-state conditions over long period of time, minimizing shutdown, start-up and changes of process conditions, as it takes time to re-adjust after upset condition.
These features are generally in contrast with the common requirements of a flexible operation. Furthermore, IGCC requires significantly longer time for start up, because of pre-heating requirements related to the gasifier, particularly for refractory-lined and less for slag wall type gasifiers, downstream unit pressurization and because of the deep cool-down sequence of the ASU. In general, the operational flexibility of IGCC plants is characterized by the following main elements:
- Low cycling capability: although the load of the gas turbine can vary freely between 0 and 100% of base load, in practice the lower limit is around 50-60%. In fact, for syngas operation diffusion burners only are available. Below 60% of base load, the concentrations of NOx and CO in the flue gas increase drastically, potentially creating environmental issues. In addition, the minimum load achievable during night period is limited by the minimum turndown of the gasification and the Air Separation Unit and their inertia related to the syngas production. In order to increase plant flexibility some modification should be introduced in the plant design: syngas storage, oxygen/nitrogen storage, syngas/auxiliary fuel co-firing, chemicals and electricity co-production.
- Low change rate: load changes are generally conditioned by the gasification and the ASU: 3% per minute is the expected load change rate from the light off of coal to minimum capacity (generally 50%), while 5% is foreseen increasing the load from minimum to full capacity. Faster ramp rates can be achieved if the gas turbine co-fires syngas and natural gas, as the syngas generation plant can follow its own ramp rate while natural gas is added to the fuel mixture of the gas turbine.
- Long start-up: start-up time depends on the start-up of the single units or equipment, e.g. Gasification, Gas Turbine, ASU, as well as on the thermal integration of the various units, including the possible air integration between the Gas Turbine compressor and the ASU. A total time of about 80-90 hours is expected for the cold start-up of the entire IGCC, in case of no or partial air integration. An additional 10-20 hours will need to be added in case of full air integration.For a hot-start-up, the key factor is the ASU cold box temperature: start-up sequence lasts approximately 6 hours (instead of the 36-48 hours for “cold” start-up). Typical hot start-up and restart-up time after minor upsets for the gasification island is in the range from 6 to 8 hours, which is the minimum time required for de-pressurization and purging of the gasifier and downstream components.