CO2 - reduction techniques in conventional power plants

Author: Dr. Klaus Hassmann, Energy Technology Cluster (As of July 2017) There are different measures how to reduce CO2 emissions in fossil fuel fired power plants. The options are described and evaluated. CO2 storage is briefly discussed. Research and development work in this area has been carried out for many years by manufacturers, operators and research institutions with financial support from the federal and state governments. In the course of the energy transition, it has become very quiet around this work in Germany whether with respect to new construction or retrofitting - since economically unattractive.

1. Efficiency increase

The first logical step to reduce CO2 release in the exhaust gas of conventional power plants is to increase the electrical efficiency. The limits for dust, sulfur compounds and nitrogen oxides must always be complied with; this is usually done by filters in the air supply, by optimized combustion and by gas cleaning measures before the exhaust gas is released into the environment. Depending on the fuel, efficiency and combustion, the exhaust gas has different proportions of CO2, NOx.

This first logical step was very successful - after several decades of innovative development, modern lignite-fired power plants can now be built with over 40% electrical efficiency, coal-fired power plants with around 50% and combined gas and steam turbine power plants with over 60%. With heat extraction, the overall efficiency rises to about 85%. The federal government is pouring a lot of water into an excellent wine. This new generation of power plants is still being completed in isolated cases; the decision to build these projects was made almost exclusively before the Energiewende . These plants are not in the money, because not them, but depreciated less efficient power plants of whatever kind get the train due to the "variable cost rule" at the power exchange, even if the new generation is able to deliver electricity plus heat.

2. CO2 separation

The next logical step after the logical step of efficiency increase was the development of CO2 free power plants. There are several variants for this purpose, as described below.

2.1 CO2 scrubbing

The CO2 is separated from the exhaust gas at the cold end of the process after combustion and gas cleaning; the nitrogen contained in large quantities must be passed through, which has a negative effect on the mass flow to be treated and on the volume of the components required for this purpose. Chemical scrubbing processes are used for CO2 separation, which proceed in two stages. First, the exhaust gas is treated in an absorber, where the CO2 is brought into contact with a scrubbing liquid. This scrubbing liquid can be an alkaline solution to which the CO2 attaches. In the subsequent desorption stage, the CO2 is released from its bond with the scrubbing agent by means of elevated temperature and can be extracted. The detergent is reused. Various washes have been tested using very similar processes; CO2 filters are also an option.

The CO2 wash is the most promising of the various approaches; however, the separation processes generally have a relatively large energy consumption, which has a negative impact of about 10 percentage points on electrical efficiency. This means that the primary energy demand of the power plant also increases significantly; the power plants would be even less of a draw on the power exchange than the modern plants without CO2 scrubbing.

2.2 Combustion with Oxygen (Oxyfuel)

The oxygen required in this type of power plant is usually produced in an air separation unit, which can feed one or more power plants or also provide O2 for the chemical process. In this process, the amount of exhaust gas is significantly reduced; in the absence of nitrogen, it consists mainly of CO2; the remainder is water vapor, which is condensed; the CO2 can be transported in a compressed or liquefied state by pipeline to the storage site  . As an alternative to air separation, separation of O2 and N2 from air can be done by membranes. Combustion of coal or gas with O2 is not possible in the combustion chamber of conventional coal-fired plants or in the combustion chamber of gas turbines. This requires new burner systems. The investigation of individual issues was carried out in laboratories, but also in the "bypass" on the periphery in existing conventional plants. Again, the prediction is allowed - there is no market for such plants in the energy transition.

2.3 Gasification before combustion

This type is also known as power plant with integrated coal gasification. This process differs from the above two processes in that it is preceded by a coal gasifier, which is fueled by coal, oxygen  and steam. An H2 - rich synthesis gas is produced. With gasification, the amount of CO2 capture required before the syngas is burned to produce electricity is significantly reduced compared to capture at the cold end of the power plant. The electrical efficiency of coal gasification without CO2 capture combined with gas and steam turbine is well above 50%; plants with capture were assumed to be slightly above 40%; a modern coal-fired steam power plant with capture at the cold end would end up with a much lower value. Coal gasification power plants without CO2 capture have been built, one each in Spain and Holland, but subsequent plants have failed to materialize. There is a market for coal gasifiers in the chemical and process engineering industries. This component would be available immediately if CO2 capture were implemented. From today's point of view, this perspective is missing.

There is one more note in passing - the author dealt with coal gasification in the iron bath in the mid-1980s. An intelligent coupling of steel and power generation seemed achievable and was to be demonstrated in a pilot plant in Bavaria. The steel plant was closed, the demonstration project wound up.

3. Geological storage

According to geological findings, very deep formations lend themselves as CO2 storage sites in Germany to "bury" the CO2 forever; gas-tight cover layers must ensure this. Safety aspects play a major role; for example, drinking water reservoirs must be prevented from being affected by CO2 leakage from the reservoirs; also, earthquake-like events must not occur. These formations are located primarily in northern Germany, but also under the North Sea. Abandoned natural gas deposits would be suitable; however, it is hard to imagine that the required large volumes will be available in the next few decades. The price of CO2 is low and will remain low, so there is unlikely to be any incentive to capture and store it.

Research program Cooretec

For work on the CO2 reduction technologies described above, the federal government established the research and development (R&D) program Cooretec (short for "CO2 reduction technologies") in 2004, well before the decision to phase out nuclear power and launch the more than ambitious Energiewende project. Between 2004 and December 2015, 657 projects were carried out under the initiative; €324 million in funding was expended.

The following five working groups were set up by Project Management Jülich with the task of evaluating the R&D applications, proposing them for funding if necessary, and monitoring the progress of the work from a technical perspective:

• WG 1: Natural gas-fired combined cycle power plants with the highest efficiencies and natural gas-fired combined cycle power plants with downstream CO2 capture.
• AG 2: Coal-fired steam power plants with highest efficiencies and with/without downstream CO2 capture.
• AG 3: Combined cycle power plants with coal gasification and CO2 capture.
• AG 4: Steam power plants with combustion or gasification with oxygen.
• AG 5: Geological storage of CO2. The author was a member of WG1 for many years.

Cooretec has recently been replaced by a new program dedicated, among other things, to the new requirements for power plants arising in the context of the energy transition. These include, for example, increasing flexibility, their fast controllability to compensate for fluctuating power generation from renewables (PV, wind), increasing efficiency at full and partial load, or operational reliability during frequent load changes, to name just a few important features.

For more information, see http://www.cooretec.de

Conclusion

Implementation of CO2 capture and storage technologies is not going to happen anytime soon. As is well known, restructuring or cost-saving programs are well underway at electric/heat utilities as well as power plant manufacturers. The new program offers attractive new fields of activity for the scientists working in Cooretec. It is hoped that this will also prevent a rapid loss of the technological expertise developed within Cooretec.