How inefficient is coal power

Technology for clean reuse of old coal-fired power plants

The current discussion about shutting down coal-fired power plants to avoid CO2 is taking place on many levels: on a political, economic, ecological or emotional level. There is little discussion on a technical or even physical level. That would be the better level to discuss, for example, a conversion of coal-fired power plants into environmentally friendly power plants with no or at least greatly reduced CO2 emissions. An opportunity to do justice to all parties involved.

But how should a conversion of coal-fired power plants take place?

To do this, you have to delve deeper into the technology and physics of coal-fired power plants. It is well known that coal-fired power plants have a very poor efficiency of 30-40%. The “why so little?” Is hardly discussed. The poor efficiency becomes more understandable if one deals with the exergy flow (not to be confused with energy). The term exergy can be understood in a simplified way as “energy actually used”. The counterpart is the anergy, the "unused energy".

The exergy flow diagram shows that only 55% of the original energy content of the coal is converted into steam for the turbine. 20% of the coal's energy is lost during combustion and 25% disappears with the flue gases. "Why is that" is better understood by looking at the typical course of a simple steam generation process.

Simple steam power process in the T-S diagram

The heat source for the three phases are the smoke gases from the coal. The flue gases give off their heat to the water, heat it up and cool down in the process. This works very well in phase 1: - Hot smoke in, cold smoke out - Cold water in, warm water out In phases 2 and 3, however, you have the situation that the temperature of the flue gases must be significantly higher than the evaporation temperature of the water ( which is typically 550-650 ° C) or the current temperature of the steam in phase 3. That is, hot flue gases from the coal combustion of well over 1000 ° C cool down to 550-650 ° C and evaporate or overheat the water the steam.

And what happens afterwards with the still very hot smoke gases? A part is used for phase 1, the "rest" for coal drying, combustion air preheating, flue gas cleaning and other secondary processes that no longer contribute to the actual steam generation (and thus energy generation). This “remainder” is 25% and more of the original energy content of the coal. This means that if you want to reduce CO2 emissions from coal-fired power plants (or increase efficiency), you have to start at this point.

What is the solution now?

To do this, you have to look back into the history of the hydrogen economy. At the end of the 1980s, a so-called H2O2 burner was developed in Germany. This burner was planned as a fast steam generator for the immediate reserve of coal-fired power plants. The burner could be ignited in a few seconds and delivered steam at temperatures up to 3000 ° C. In addition, liquid water had to be injected (which then becomes usable steam) in order to cool the high temperature from 3000 ° C to the maximum permitted 550-650 ° C. However, this technique was never put to practical use. And later, the pure burning of hydrogen was considered "out", since you had the fuel cell.

If today the coal-fired power plants were to stop generating steam by burning coal, dismantling this part and replacing it with steam generation using powerful H2O2 burners that are operated with environmentally friendly, green hydrogen, the former CO2 centrifuges would become a CO2-neutral peak-load power plant. If the necessary H2 / O2 is also generated and stored on the site of the power plant (or, for example, a former coal mine), a former coal-fired power plant becomes a CO2-free energy store. The idea is not new and is being intensively researched outside of Germany, as the sources show.

And the efficiency?

With the H2O2 burner, the chemical energy of the hydrogen is completely, i.e. 100%, converted into usable steam. Strange but true. Another 12-15% of this is lost in the turbine and generator. That means the combination of H2O2 burner and steam turbine would have an overall efficiency of over 85%. A value that is in no way inferior to a fuel cell.

And the waste incineration?

Many coal-fired power plants run with so-called "co-firering" and burn garbage and other waste. But where do you put the garbage after the coal combustion has been switched off? The H2O2 burner is also useful here. The previous coal combustion remains as a pure waste incineration with hydrogen burners as auxiliary firing. The technology for bringing in the garbage and the flue gas cleaning are already in place. The thermal energy of the waste is used to preheat the liquid water in phase 1 and, if necessary, to generate wet steam, which is then converted into superheated dry steam by the subsequent H2O2 burner. This would secure waste disposal and at least achieve a substantial reduction in CO2.

The technology for a CO2-free power plant with an H2O2 burner

All technical core components (Siemens Silyzer electrolysers, Linde compressors and hydrogen tanks, Alstom Steam Gen H2O2 burners) are available on the market as standard or have already been developed for smaller capacities.

Conclusion

The economic advantages of this solution are obvious:

- Large parts of the existing power plant technology (turbine, condenser, electrical infrastructure such as lines, transformers, control technology, ...) will continue to be used.

- Jobs are retained, new jobs are created.

- The CO2 emissions decrease.

- The share of CO2-free energy is increasing.

- Urgently needed high-performance energy storage devices are being created.

Even the questions of “concerned citizens” where the electricity should come from when the coal-fired power plants are switched off “... and the sun and wind do not supply electricity ...” can easily be answered: the power plants are still there and supply electricity when it is needed becomes.

So instead of paying the electricity producers billions of euros in tax money for "lost profits" by 2038 and then dismantling still functioning power plants, this money would be more sensibly invested in converting existing coal-fired power plants into clean energy storage power plants. It's good for everyone! Newer power plants such as Datteln 4 could become cornerstones of a real energy transition after being converted into storage power plants and bring their owners profits from economic activity well beyond the year 2038. The only thing missing is the political will to bring about change. But, as we all know, this is nothing new in a country whose last major innovation was car fraud software.

The author of this text is Kristian Roßberg from Bremen, who has been a subscriber to ERNEUERBARE ENERGIEN since 1998