Repowering coal fired power plants

image credit: Source: Siemens Energy
Hartmut Oehme's picture
Director Brownfield Transformation, Siemens Energy

I am with Power Generation Services since 30 years, starting as a field sales and implementation engineer for turbine maintenance activities in nuclear and fossil fired power plants. Since nearly...

  • Member since 2021
  • 3 items added with 1,373 views
  • Sep 7, 2021

There can be no doubt that we´re heading towards a carbon-free future. One of the key measures will be the decarbonization of power generation for utilities and industry. Obviously, it won´t come about by itself; it will take our complete technological toolkit. And since we cannot simply build the entire energy generation anew from scratch, assets seen as a burden today shouldn’t simply become stranded assets, but getting transformed or re-purposed

And it’s natural to start with coal-fired plants. Worldwide, they emit around 10 Gt of CO2 per year; that makes them one of the largest emitters of greenhouse gases.  And while the idea of repowering coal fired power plants is not entirely new and several large-scale projects during the past twenty years and more around the globe have proven it to be technically and commercially sound, for economic reason repowering never grew out of its niche. As legislations and consequentially markets transition towards clear decarbonization targets and concrete coal exit plans, repowering of legacy coal fired power plants should experience a strong renaissance.

From topping to full repowering

Repowering ranges from a simple boiler conversion to so-called ‘topping’, ‘boosting’, ‘parallel repowering’ and finally ‘full repowering’, in which not only the fuel is changed from coal to gas, but a power plant is converted into a combined cycle power plant. According to various market analyses, up to 25% of the world's installed coal-fired power plants alone have the potential for full repowering.

A boiler conversion involves a modification of the burner technology and a change of fuel to gas. While switching to gas as a fuel does not increase the efficiency of the steam plant, it does reduce CO2 emissions by up to 50%. ‘Topping’ requires the installation of a small gas turbine and results in a slight improvement in overall plant efficiency. The thermal energy contained in the flue gas of the ‘topping’ gas turbine is fed directly to the steam generator of the existing plant. Boosting requires the installation of an additional gas turbine. In contrast to topping, the heat energy contained in the flue gas of the gas turbine is used with the aid of a heat exchanger to generate high-pressure steam and to preheat the feed water of the existing process. Boosting essentially aims to make the steam power plant marginally more flexible, while at the same time slightly increasing efficiency and reducing CO2 emissions.

Reducing CO2 emissions by up to 70 %

Parallel repowering goes one step further and utilizes the thermal energy contained in the flue gas of the gas turbine, which also has to be newly installed, with the aid of a heat recovery steam generator for additional feedwater preheating and steam generation in the existing steam power plant process. This conversion also serves to increase the flexibility and efficiency of steam power plants, but to a greater degree than boosting.

Full repowering finally includes not only the fuel change from coal or oil to gas, but also the conversion of the existing steam power plant to a combined cycle power plant. However, the conversion of the existing steam power plant to a CCGT power plant can also be implemented for direct gas-fired steam power plants. In a full repowering project, one or more gas turbines with downstream heat recovery steam generators replace the steam generator previously fired with coal, oil, or gas. This way, the operational specific CO2 emissions of the existing site can be reduced by up to 70 %. It’s a result of the fuel switch from coal to natural gas in combination with a significant increase in efficiency from an average of 38% to up to 63% through the conversion to a high efficiency combined cycle power plant (CCPP).

Be it just with ‘topping’, ‘boosting’, ‘parallel repowering’ or ‘fully repowering’ of existing power plants – in many cases, it can help meeting national decarbonization goals, which in many countries are not just demanded by law, but also by the public. 


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Matt Chester's picture
Matt Chester on Sep 7, 2021

How do the costs of implementation vary between these different types of repowering? 

Dr. Amal Khashab's picture
Dr. Amal Khashab on Sep 8, 2021

Hi Matt

I think that sensitivity analysis has to be carried out to identify the specific cost of an additional MWH for these options.
- Switching the fuel from coal to natural gas with the existing steam power plant could be the lowest cost.
- While that fuel switching with a new Combine Cycle plus the demolition of existing steam power facilities could be the highest one, in spite of the highest efficiency of new CCPP.

Marcello Storrer's picture
Marcello Storrer on Sep 14, 2021

A good idea , considering it's not possible to shift all at once to renewables. Cheers!

on Sep 14, 2021

It’s an effective and proven solution to bridge the gap as the transition to renewables plays out. Emissions are reduced and grid reliability remains intact.

on Sep 15, 2021

A decade too late. Gas and coal must now both stay underground. My issue with shifting coal to gas are the following.


1. The latest IPCC report makes it clear that we have to address climate change more rapidly than previously thought, that means leaving most of the remaining fossil fuels in the ground. Even though the above ideas reduce carbon emissions they are still, relative to renewables, more intensive than we can now afford.


2. The cost of retrofitting existing coal power stations vs cost of renewables instead, is now in favour of renewables, with zero emissions in power generation.


3. The time needed to do make these changes is not a short, adding significant emissions over that period.


There may be some places where it may be economically viable and an acceptable environmental compromise but in most circumstances, it will be cheaper and more environmentally friendly to end the fosil fuels sooner, and not just re emmisions but other forms of pollution harmful to all forms of life.


10 years ago, I believe would have been a very different circumstance.  Renewables were way more expensive, the  economic life of many more coal stations would have made this more viable and we had the time to implement it.  But technology (in terms both of renewables and batteries) and economics today point to renewables as the ultimate solution.

Hartmut Oehme's picture
Hartmut Oehme on Sep 16, 2021

Let me share some more thoughts on this, which are not so much reflected in the original article, but are esssential to understand why repowering of existing coal fired power plants might be one of the viable decarbonization puzzle pieces.

(1) The switch from coal to gas should be seen as an intermediate step on the way to full decarbonization, to bridge the time gap until the green hydrogen economy is in full swing, and H2 can replace natural gas entirely to fire combustion turbines. Implementing CCUS solutions in parallel would help to lower the GHG impact even further.

(2) The reuse and/or repurposing of existing power plants and the related infrastructure has a huge economic, ecological and social sustainablity effect, compared to the shut-down and demolition of legacy plants and the greenfield installation of all newly required power generation facilities (of whatever nature).

I'll be happy to further discuss.

Hartmut Oehme's picture
Thank Hartmut for the Post!
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