Coal Plant Flexibility to Enable Renewable Energy Integration Economically

To accelerate renewable energy penetration in grid, one popular method is phasing out coal fired steam power plant. Nevertheless, phasing out coal power plant in developing countries need higher consideration than in developed country. Financial capacity, expert availability, and grid readiness need further examination.

One features needed by grid as more and more intermittent generation is a flexibility by existing power plant.
By flexibility are means :
1. Increasing ramp rates
2. Lowering minimum load
3. Increasing maximum load beyond rated load
4. Reducing start-up time

1. Increasing ramp rates
Increasing ramp rates will improve load-following capabilities, especially when other generation also have harsh swing. This characteristic will help to avoid load shedding in case of changes in demand, changes in production from other plants like in renewable plant, and when other plant trip. Higher ramp rates also make room for possibilities to include higher number renewable energy mix in grid, up to 50%.

However, attempt to increasing ramp rates in coal power plant are usually limited by boiler dynamics such as coal mill operation, drum temperature gradients and steam temperature control. Besides boiler dynamics, limitations could be anywhere so this work required profound plant knowledge from their engineers. This endeavour can be done without hardware investments, nevertheless some equipment upgrade can contribute more offset in ramping rates. Best practices coal power plant ramping rates flexibility is around 4% per minute.

2. Lowering minimum load
Lowering mminium load of coal power plant will keep plant on the grid without stopping to avoid start up penalty. When power plant can hold on a grid, it will responded faster to load changes without start-up preparation that usually made hours to getting back on grid.

Some typical limitation in lowering minimum load effort are safety margin of single mill operation, flame stability, flue gas temperature (below 380 degC) and conservatism. However, many can be done in terms of distributed control modifications and engineering man hours. Best achievable of minimum load in coal power plant are about 10% to 20% rated capacity.

3. Increasing maximum load beyond rated load
Increasing maximum load beyond rated load can be done by "overload mode". This mode will increase electrical output about 5%, but sacrificing efficiency. From grid stability view, this efficiency penalty generally acceptable.

Overload mode can be done by bypassing high pressure heaters. The absence of extraction steam resulting no reduction in steam turbine that allowed to expand through turbine and increase power. However, by shutdown HP heaters will reduce feedwater temperature, so firing rate should be increase to get same live steam properties. This fuel consumption escalation will push plant efficiency, generally about 2%.

4. Reducing start-up time
Reducing start-up time in coal fired steam powerplant have two main limitation that determined by boiler warm-up and turbine stress.

Different boiler type and manufacturer has different limitation, so the optimisation that can be done will be different case per case basis. Turbine stress also should be highly considered and can be observe to fasten the startup process with precautionary principle. Reducing startup time, in my opinion, is the hardest part of flexibility because its requires profound knowledge of process and components.
 
Enhancement of Coal Fired Steam Power Plant flexibility can be a solution for developing countries to gradually embedding renewable energy into their grid as demand growth, while waiting
financial support obligation to be realized from developed countries.