Oxyfuel process

The oxyfuel process (from Oxy for Oxygen ( oxygen ) and fuel for fuel ) is a combustion process can be achieved in the extremely high flame temperatures. It can be used for both gaseous and liquid and solid fuels. In contrast to conventional combustion with air , the fuel is burned with almost pure oxygen (i.e. with little or no nitrogen and argon ). In order to be able to influence the resulting flame temperature, a certain amount of exhaust gas or flue gas is internally or externally (cooled) recirculated (recirculation gas ), i. H. in the external case blown together with the fuel and the oxygen into the combustion chamber. The flame temperature depends on the temperature of the recirculated flue gas mass flow, its ratio to the fuel mass flow and the air ratio .

The oxyfuel process is also suitable as a basis for power plant processes that allow the separation and thus sequestration of the carbon dioxide (CO ₂ ) produced during combustion . These power plant processes are therefore currently being researched and developed intensively worldwide. In this case, both gas turbine power plants , which are usually fired with natural gas , and coal-fired steam power plants come into question as basic processes . Mineral oils are not used in Germany for large-scale electricity generation and also play a subordinate role internationally.

Historical development

The first concept study of an oxyfuel power plant by Degtiarev and Gribovski in 1967 aimed to produce CO ₂ for industrial applications while simultaneously generating electricity. First experimental investigations of the combustion in the oxyfuel process in the 1980s were again motivated by the efficient production of CO ₂ for "enhanced oil recovery " (EOR).

As of 2019, the new processes are hardly used industrially. In the EU, the Italian system manufacturer ITEA is a pioneer with a 5-megawatt system in southern Italy, and in Germany Vattenfall and Alstom commissioned a 30-MW system in Schwarze Pump in 2008. The US company ThermoEnergy operates a 15 megawatt pilot plant in Singapore that uses Oxy Combustion .

In the steel industry, over 100 heating furnaces have been converted to the oxyfuel process. The first were converted in the late 1980s. Most of the time it is about enabling increased heat capacity in the same furnace or the use of fewer furnaces for the same production. But there are also examples where the main goal was only lower NOx emissions or lower energy consumption. The fuel saving with the oxyfuel process is 25–75% in high-temperature processes. Depending on how high the exhaust gas temperature and the recovered energy is through air preheating in recuperators or regenerators. Fewer NOx emissions are achieved through flameless combustion. Flameless combustion is created by slowing down the reaction speed, which in turn is achieved by mixing in exhaust gases through recirculation (internal or external). As a result, the combustion takes longer and more volume, and thus the flame temperature drops.

Oxygen supply

The provision of oxygen for the oxyfuel process is associated with considerable technical effort. The state of the art for the large-scale production of oxygen is cryogenic air separation ( Linde process ). This process requires large amounts of electrical energy , which has a negative effect on the energy efficiency of oxyfuel processes and thus excludes the economic profitability for current as well as for applications to be found. In recent years, however, methods based on membranes that are permeable to oxygen but not to nitrogen have been explored. These processes have the potential to significantly reduce the effort involved in providing oxygen.

However, the membrane processes for generating oxygen and their possible uses in energy generation are in the experimental stage and commercial, large-scale use is out of the question in the medium term. In contrast, it has been shown that the specific energy requirement for oxygen generation by means of cryogenic air separation is significantly reduced if the highest oxygen concentration (> 99.5 percent) that has been customary in the industry is dispensed with and a concentration of z. B. 95 percent chooses. Since these purities are already used on an industrial scale in gasification processes, this savings potential of cryogenic oxygen generation can be used immediately.

Scientific studies have also shown that there are hardly any significant differences in the achievable efficiency even in the thermodynamic comparison of oxyfuel power plant processes with oxygen supply through cryogenic air separation plants and through high-temperature membranes.

Commercial application

The high flame temperatures that can be achieved with the oxyfuel process are used to advantage in the glass and steel industries. In addition, the oxyfuel process offers potential for saving energy in these processes. Compared to conventional combustion, the absence of nitrogen reduces the exhaust gas mass flow, so that with a constant exhaust gas temperature, the heat losses of the process are lower, which in turn leads to reduced fuel consumption. This is offset by the energy required to provide the oxygen.

Another possibility for process optimization is to operate the furnace with oxygen-enriched air, i.e. H. Air with an oxygen content of more than 21 parts by volume. For this purpose, it is also necessary to provide pure oxygen, which is mixed into the air before combustion. The result is a higher adiabatic combustion temperature and thus a higher flame temperature. The amount of oxygen to be provided is lower in this process management, and recirculation can be dispensed with.

Potential use for CO ₂ capture

For the sequestration of the greenhouse gas CO ₂ , only a small proportion of foreign gases may be contained. The oxyfuel process is well suited to produce CO ₂ with a high level of purity; it was originally developed for this purpose. (see above) At the time of the first investigations, the greenhouse effect was already known in principle, but the current scientific consensus did not yet exist about its effects on human living conditions, so that the economic framework conditions that support research into CO ₂ sequestration were also lacking. It was only since the late 1990s that research into the use of the process for the development of power plant processes has been carried out worldwide, which allows the CO _{2 to be} captured in the power plant and thus its sequestration.

CO ₂ purity

Production of pure CO ₂

If one assumes in a highly idealized scenario that a pure hydrocarbon is used as fuel, then with complete combustion with pure oxygen at an air ratio of 1 (“ stoichiometric combustion”), an exhaust gas consisting exclusively of carbon dioxide and water is obtained. In principle, the oxyfuel process can be implemented with moist or dry recirculation. In the first case, this exhaust gas is recirculated directly, in the latter case it is cooled down to the point where the water, which has a higher boiling point than carbon dioxide, condenses . Even in the case of moist recirculation, however, this condensation is carried out after part of the exhaust gas has been diverted so that pure carbon dioxide is obtained, which is then to be fed to the sequestration.

Factors influencing the degree of purity of CO ₂

Although an oxyfuel process can produce CO ₂ with high purity, the ideal case of pure carbon dioxide described above cannot be achieved. The liquefaction of the CO _{2 is} necessary for the transport from the power plant to the storage facility . Increase Impurities i. d. Usually the pressure required for liquefaction and thus the energy expenditure associated with compression, which in turn has a negative effect on the energy efficiency of the overall process. In addition, contamination increases the required storage volume and is also questionable with regard to safe, long-term storage. The previously described idealized scenario without any impurities deviates from reality in the following points:

• While natural gas can often be modeled as a pure hydrocarbon to a good approximation, this assumption is only permissible for very basic considerations in the case of coal. In principle, coal can contain all elements of the periodic table . In particular, exposure to nitrogen and sulfur is problematic here, since sulfur oxides and (with lower conversion rates ) nitrogen oxides are formed during combustion . Coal also contains non-combustible components ( ash ).
• Combustion is never complete. This can u. a. lead to contamination by carbon monoxide (CO) and unburned hydrocarbons (e.g. soot ).
• Coal is burned with the principle of atmospheric dust combustion, which is common for today's large steam generators, at air conditions of approx. 1.15. This leads to residual oxygen in the flue gas.
• With cryogenic oxygen supply using the Linde process, the oxygen contains 0.5–5.0 percent argon, depending on the number of distillation stages .
• Coal-fired steam generators with atmospheric dust firing are not airtight in today's design. This leads to the entry of ambient air ( false air ) and thus nitrogen in particular into the combustion chamber. In new systems with conventional combustion with air, this false air can account for up to 3 percent of the total combustion air. In the case of older systems, this value can rise to 10 percent.

Increasing the purity of CO ₂

• The ash particles produced during the combustion of coal can be removed from the flue gas in the way that is now used in conventional power plants, namely with electrostatic precipitators . In processes with membrane-based oxygen supply, the flue gas must be cleaned at high temperatures at which electrostatic precipitators can no longer be used, so ceramic filters should be used.
• Combustion at an air ratio of more than 1 inevitably leads to residual oxygen in the flue gas. However, a reduction in the air ratio leads to ever larger proportions of unburned fuel. (Both carbon in the ash particles and CO in the gas) So a compromise has to be found here. By using a different combustion principle that guarantees high burnout rates even with near-stoichiometric combustion (e.g. fluidized bed combustion ), the goals of CO and oxygen reduction could be better coordinated . An oxygen excess of 15 percent is currently considered realistic.
• As a noble gas, argon is very inert ( inert ). A cleaning based on chemical reactions as well as absorption or adsorption processes is therefore not possible.
• The contamination of the CO ₂ with nitrogen through the entry of false air into the steam generator can be reduced by sealing the steam generator. In the case of combustion with air, there is hardly any need for this one-off measure, so that this is not done with today's conventional steam generators.
• In the event that a purity of the CO ₂ of more than 90 percent is to be achieved, the possibility of cryogenic purification by means of partial condensation and subsequent distillation ( rectification ) is considered.

Heat and combustion problems

The main focus of the research work in the field of CO ₂ separation is the investigation of the heat transfer in the combustion chambers and the combustion.

Thermal radiation

In furnaces, heat transfer through radiation plays a dominant role. The need for research in this regard arises from the changed composition of the flue gas, the radiation properties of which change due to the greatly increased CO ₂ content. In contrast to nitrogen, which is otherwise present in large quantities, CO ₂ absorbs radiation in the near infrared range of the spectrum, a range in which the majority of the radiation is emitted at the usual temperatures in a furnace according to Wien's displacement law . In the area of ​​flame radiation , the gas radiation is superimposed by radiation from the particles, which does not differ significantly from air combustion at the same flame temperatures.

Reaction kinetics

Further innovations are necessary due to the effects of the Boudouard equilibrium on the reaction kinetics when burning coal in a CO ₂ / O ₂ atmosphere. Is by the oxidation of the bound in the coal grain carbon with gaseous oxygen to gaseous CO ₂ release heat, then the resulting flue gas heats up:

${\displaystyle \mathrm {C_{fest}+O_{2}\ \rightleftharpoons CO_{2};\quad \Delta H=-393\ {\frac {kJ}{mol}}} }$

At a certain temperature , however , the CO ₂ reacts in turn with more carbon from the surface of the coal grain to form carbon monoxide (CO):

${\displaystyle \mathrm {CO_{2}+C_{fest}\ \rightleftharpoons \ 2\ CO\ ;\quad \Delta H=+172\ {\frac {kJ}{mol}}} }$

This reduction in CO ₂ is negligible in the case of combustion with air due to the low CO ₂ concentrations in the flue gas. In the case of oxyfuel combustion, however, it has a strong influence, since the CO ₂ proportion under these circumstances is in the range of 60 to 80 percent by volume . The reaction is endothermic . It absorbs heat and thus counteracts the heating of the flue gas according to the principle of the slightest pressure . In addition, the number of molecules in the gas phase increases in the course of the reaction (one CO ₂ becomes two CO). This has an influence on the aerodynamics on the burner and thus on the further progress of the reaction.

CO ₂ capture in gas turbine power plants

The AZEP (Advanced Zero Emissions Power Plant) project, completed in 2001, dealt with the research of an oxyfuel process based on a natural gas- fired combined cycle power plant and also included the development of membranes for providing oxygen.

CO ₂ separation in steam power plants

The main focus of research both in Germany and internationally, however, is CO ₂ capture from coal-fired steam power plants. The reasons for this are both technical and political in nature:

• The elements carbon and hydrogen, the oxidation of which leads to the generation of heat during combustion , prevail in coal in different proportions than in gaseous fuels. By the higher ratio of carbon to hydrogen in the coal will become apparent based on the heat generated higher CO ₂ - emissions .
• When generating electricity with coal in steam power plants , lower electrical efficiencies are achieved than in gas-fired combined cycle power plants.
• If one also takes into account the structure of the power plant park in Germany, it becomes clear that the CO ₂ emissions caused by coal combustion currently significantly exceed those caused by gas combustion. The absolute savings potential is therefore significantly higher in the case of coal than in the case of gas. This is often the case in other countries as well.
• Against the demands of environmental protection associations and other political actors to discontinue the generation of electricity from coal in view of the high CO ₂ emissions and instead to use natural gas to a greater extent, politicians and electricity producers often consider the issue of security of supply or dependence on imports Field led. While the global coal reserves are distributed over many countries (and also occur in the form of lignite in Germany), natural gas reserves are concentrated in significantly fewer countries. In addition, the calorific value -related price of coal is lower and its static range is higher.

After successful tests in the technical center, Vattenfall Europe AG set up a pilot system for CO ₂ capture between 2006 and 2008 , which officially went into trial operation on September 9, 2008.

"CO ₂ -free"?

In connection with power plant processes based on the oxyfuel process, the term “CO ₂ -free” or “zero emission” in English is sometimes used. It should be noted here that an oxyfuel process cannot literally be “CO ₂ -free”, since CO ₂ is inevitably produced when fossil fuels are burned . The English term “zero emission” or its literal translation “CO ₂ emission-free” do better justice to the real situation.

The background to these designations is the fact that in the oxyfuel process, the flue gas flow itself should become the purest , landfillable CO ₂ flow possible by removing other components . In the two other processes that can be used for separation, namely coal gasification and flue gas scrubbing , on the other hand, CO _{2 is} removed from the smoke or waste gas produced using a chemical or physical solvent. Parts of the CO ₂ inevitably remain in the gas. It is assumed that these processes allow 85 to 90 percent of the CO ₂ to be separated off, since a further increase in the technical effort and thus the costs increases sharply.

If the CO ₂ from the oxyfuel process is purified by distillation, depending on the design of the distillation, a non-negligible proportion of the CO _{2 is lost} with the separated, non-condensable gases O ₂ , N ₂ and Ar. The price of purification is that a maximum degree of separation of around 98 percent is possible.

It should also be noted that in the oxyfuel process, CO _{2 is also} removed from the exhaust gas to a small extent during the condensation of the water . After condensation, it is present as carbonic acid in the water and can therefore not be sequestered. Unless it is removed from the water or the water itself is sequestered in some form, this carbonic acid must be viewed as CO ₂ emissions. In view of the small amounts of CO ₂ , such further treatment is hardly worthwhile.

According to an injunction by the Berlin Regional Court of December 4, 2007, Vattenfall Europe AG is prohibited from designating the pilot system located in the Schwarze Pumpe industrial park as “CO ₂ -free”.

Individual evidence

1. ↑ Degtiarev, VL; Gribovski, VP: “Carbon dioxide semi-closed power plant”, Author sertif., USSR No. 295 897 of July 28, 1967, published in Bull. Inventions No. 8, F01k13 / 00, Co1b 3/00, Nov. 12, 1971
2. ↑ Vattenfall Black Pump ( Memento of the original from July 28, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
3. ↑ Coal-fired power plants should become cleaner , heise news from June 26, 2012.
4. ↑ ^{a b} AZEP - Development of an Integrated Air Separation Membrane - Gas Turbine (PDF; 37 kB); Sundkvist et al .; Second Nordic Minisymposium on Carbon Dioxide Capture and Storage, Göteborg, October 26, 2001
5. ^ I. Pfaff, Prof. A. Kather, J. Siek, Technical University Hamburg-Harburg; 40th KWTK Dresden 2008
6. ↑ Linde AG [1]  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
7. ↑ Svante Arrhenius : On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground . In: Philosophical Magazine and Journal of Science, Fifth Series, April 1896, London, Edinburgh and Dublin Archived copy ( Memento of the original dated October 6 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 4.5 MB) 
8. ↑ “Climate Change 2007”; IPCC Fourth Assessment Report, Summary for Policymakers [2] (PDF; 2.0 MB)
9. ↑ IPCC Special Report on Carbon Dioxide Capture and Storage Archived copy ( Memento of the original from February 10, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 23.9 MB) 
10. ↑ Kneer, R., Abel, D., Niehuis, R., Maier, HR, Modigell, M., Peters, N .: Development of a CO2-emission-free coal combustion process for power generation, VDI reports No. 1888 PL03, 2006
11. ↑ Koepke, D., Mieske, K., Kather, A., Eggers, R .: Phase Equillibria Measurements and their Application for the CO2 Separation from CO2 Rich Gases, 3rd Meeting of the Oxycombustion Network, Yokohama, Japan, March 2008; Archived copy ( memento of the original from January 5, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. 
12. ↑ Anderson, K .: Radiation modeling of gas and coal-fired oxyfuel-flames, 2nd Young Researchers Forum, September 18 and 19, 2008, Chalmers University, Göteborg, Sweden, http://www.co2captureandstorage.info/networks/ Oxyfuelyoung% 20researchersforum2.htm  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.  
13. ↑ Toporov, D., Förster, M., Kneer, R .: Combustion of Pulverized Fuel under Oxycoal Conditions at Low Oxygen Concentrations, 3rd Int. Conf. on Clean Coal Technologies for our Future, May 15-17, 2007, Cagliari, Sardinia, Italy
14. ↑ Vattenfall's CCS project - commissioning of the world's first pilot plant for a coal-fired power plant with CO2 capture , Vattenfall press release on September 9, 2008. Accessed on December 5, 2009.
15. ↑ Use of “CO2-free power plant” prohibited , Energy Agency NRW, December 11, 2007;

Web links

• Intergovernmental Panel on Climate Change (IPCC) ; Intergovernmental Panel on Climate Change
• COORETEC ; Initiative of the Federal Ministry of Economics and Technology in the field of CO ₂ reduction technologies
• OXYCOAL-AC ; RWTH Aachen research project to develop an oxyfuel coal-fired power plant with membrane-based oxygen supply
• ADECOS ; Research project of the TU Dresden , TU Hamburg-Harburg and FH Zittau / Görlitz for the development of a cryogenically operated oxyfuel coal power plant