Repowering coal fired power plants

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.