Oxy-fuel combustion process
The oxyfuel process ( oxy for Oxygen (oxygen) and fuel for fuel) is a combustion process can be achieved in the very high flame temperatures. It is applicable for both gaseous as well as liquid and solid fuels. In contrast to the conventional combustion with air, the fuel with almost pure oxygen is burned (without or with only a small proportion of nitrogen and argon). In order to influence the resulting flame temperature is a certain amount of cooled exhaust gas or flue gas is recirculated ( recirculation gas ), ie injected with the fuel and oxygen together in the combustion chamber. The flame temperature is dependent on the temperature of the recirculated flue gas mass flow rate, the ratio of the fuel mass flow and the air ratio.
The oxyfuel process is also suitable as a basis for power plant processes that allow deposition and thus sequestration of carbon dioxide produced by combustion (CO2). This power plant processes are being currently researched and developed intensively worldwide. As basic processes occur in this case, both the gas turbine power plants that are fueled usually by natural gas, and coal-fired steam power plants in question. Mineral oils are not used in Germany for large-scale power generation and also play a minor role internationally.
- 4.2.1 heat radiation
- 4.2.2 kinetics
Historical development
Had the first concept study of the oxyfuel power plant Degtiarev and Gribovski of 1967, the production of CO2 for industrial applications with simultaneous power generation goal. First experimental studies of combustion in the oxyfuel process in the 1980s were in turn by the efficient production of CO2 to the "extended oil production " ( "Enhanced Oil Recovery ", EOR) motivated.
In 2012, the new procedures are not used industrially on a broad front. In the EU, the Italian system manufacturers ITEA forerunner with a 5 - megawatt plant in southern Italy. The U.S. company Thermo Energy operates a 15 -megawatt pilot plant in Singapore, use the Oxy Combustion.
Oxygen supply
The provision of oxygen for oxyfuel process is associated with considerable technical effort. State of the art for 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 the oxyfuel processes and thus the commercial attractiveness decreases both for current and for still to be explored applications. In recent years, however, the method based on membranes are investigated which are permeable to oxygen, but not nitrogen. These methods have the potential to reduce the burden on the oxygen supply significantly.
However, the membrane processes are for oxygen generation and their applications in power still in the experimental stage and to a commercial, large-scale use is not to think in the short and medium term. In contrast, it has been shown that significantly reduces the specific energy demand for oxygen production by cryogenic air separation, if you dispense with the highest oxygen concentration in the industry previously common (> 99.5 percent) and sufficient for the oxyfuel process, the concentration of such selects as 95 percent. Since these purities already a large scale are used in gasification processes, can this savings potential of cryogenic oxygen production use immediately.
Scientific studies have also shown that also arise in the thermodynamic comparison of oxyfuel power plant processes with oxygen supply by cryogenic air separation plants and by high-temperature membranes hardly any significant differences in the attainable efficiency.
Commercial Opportunity
The attainable in the oxyfuel process high flame temperatures advantageously be used in the glass and steel industries. In addition, the oxyfuel process offers potential for energy savings in these processes. Compared with the conventional combustion is reduced by the absence of nitrogen, the exhaust gas mass flow, so that at a constant exhaust-gas temperature, the heat losses of the process are low, which in turn leads to a reduced fuel consumption. This is offset by the need for the provision of oxygen consumption of energy.
Another way of process optimization is to operate the furnace with oxygen-enriched air, ie Air having an oxygen content of more than 21 parts by volume. For this purpose, the provision of pure oxygen is also required, which is admixed to the air prior to combustion. The result is a higher adiabatic combustion temperature, and thus a higher flame temperature. The amount of oxygen to be provided is less in this litigation, also may account for the recirculation.
Potential use for CO2 capture
Only a small proportion of foreign gases must be included for the sequestration of the greenhouse gas CO2. The oxyfuel process is well suited to produce CO2 with high purity, it was originally even developed for this purpose. ( above) The greenhouse effect at the time of the first studies indeed already known in principle, its impact on human living conditions, however, there was not yet the still existing scientific consensus, so that the present political conditions were not present, the research into the CO2 make sequestration attractive. Only since the late 1990s, the use of the method is explored for the development of power plant processes, which allow a separation of CO2 in the power plant and thus its sequestration worldwide.
CO2 purity
Production of pure CO2
If one goes into a highly idealized scenario assumes that a pure hydrocarbon is used as fuel, the result is complete combustion with pure oxygen at an air ratio of 1 (" stoichiometric combustion " ) an exhaust gas which is composed exclusively of carbon dioxide and water. Basically, the oxyfuel process can be realized with moist or dry recirculation. In the former case, this exhaust gas is recirculated directly, in the latter case, it is previously cooled down to such an extent that there is condensation of water, which has a higher boiling point than carbon dioxide. Even in the case of a moist recirculating this condensation is, however, carried out after a portion of the exhaust gas is branched off, so as to obtain pure carbon dioxide, which will be then supplied to the sequestration.
Factors influencing the purity of CO2
Although an oxyfuel process, CO2 can produce high purity, the ideal situation described above is pure carbon dioxide can not be reached. For transportation from the power plant to deposit the liquefaction of CO2 is necessary. Increase impurities i.d.R. necessary for the liquefaction pressure and thus affiliated with the compaction energy expenditure, which in turn has a negative effect on the energy efficiency of the overall process. In addition, impurities increase the required storage capacity and are also in for a safe permanent storage questionable. The idealized scenario described above without any impurities differs in the following points from reality:
- While natural gas can be modeled to a good approximation as a pure hydrocarbon often, this assumption is only valid for very basic considerations in coal. Coal can in principle contain all the elements of the periodic table. In particular, the exposure to nitrogen and sulfur this problem because sulfur oxides therefrom and formed ( with lower turnover rate ) of nitrogen oxides during combustion. Coal also contains non-combustible constituents ( ash).
- A combustion never runs to completion. This can lead, inter alia, to contamination by carbon monoxide (CO) and unburned hydrocarbons ( eg, carbon black ).
- Coal is burned in the usual for today's steam generators principle of atmospheric dust firing at air ratios of about 1.15. This results in residual oxygen in the flue gas.
- In the cryogenic oxygen supply by the Linde process comprising oxygen, depending on the number of distillation stages 0.5-5.0 percent argon.
- Coal-fired steam boiler with atmospheric dust firing are not airtight in today's execution. This leads to the entry of ambient air ( secondary air ) and thus in particular nitrogen in the combustion chamber. This air can with conventional combustion with air cover up to 3 percent of the total combustion air for new plants. On older systems this value can rise up to 10 percent.
Increase the degree of purity of CO2
- That are produced during the combustion of coal ash particles can access the today used in conventional power plants way, namely with electrostatic filters are removed from the flue gas. In processes with membrane-based oxygen supply, the flue gas must be cleaned at high temperatures at which electric filters are no longer used, so that ceramic filters are to be used.
- A combustion at an air ratio of greater than 1 necessarily leads to residual oxygen in the flue gas. However, a reduction of the air ratio leads to ever larger proportions of unburned fuel. ( Both carbon in the ash particles and CO in the gas) Here, a compromise must be found. Through the use of another Feuerungsprinzips that even at near-stoichiometric combustion high Ausbrandraten guaranteed ( eg fluidised bed combustion ) the objectives of the CO and oxygen reduction could be more compatible with each other. Currently, an oxygen excess of 15 percent is considered to be realistic.
- Argon behaves as an inert gas very unreactive ( inert). A based on chemical reactions as well as absorption or adsorption purification is not possible.
- The contamination of CO2 with nitrogen by the false air entry in the steam generators can be reduced by sealing the steam generator. The combustion air is little occasion for this one-time measure, so that they are lacking in today's conventional steam generators.
- For the case that the purity of the CO2 by more than 90 percent is to be achieved, the possibility of the cryogenic purification by partial condensation and subsequent distillation ( rectification) is contemplated.
Thermal and combustion technology problems
Research activities in the field of CO2 capture are investigating the heat transfer in the combustion chambers and the combustion.
Heat radiation
In furnaces, heat transfer plays a dominant role by radiation. The related research needs created by the changing composition of the flue gas whose radiative properties change due to the greatly increased amount of CO2. In contrast to the otherwise existing in large amounts of nitrogen absorbed CO2 radiation in the near infrared region of the spectrum, an area in which the majority of the radiation emitted at the usual temperatures in a furnace according to Wien's displacement law. In the region of the gas flame radiation beam is superimposed on the radiation of the particles at the same flame temperatures is not significantly different from that of the air combustion.
Reaction kinetics
Other innovations are needed by the effects of Boudouard equilibrium on the reaction kinetics in the combustion of coal in a CO2/O2-Atmosphäre. If heat is released by the oxidation of carbon bound in grain carbon with gaseous oxygen to form gaseous CO2, then the resulting flue gas heats up:
Above a certain temperature, the CO2 but in turn reacts with further carbon from the surface of the carbon grains to carbon monoxide (CO):
This reduction in CO2 is negligible in combustion with air because of the low CO2 concentration in the flue gas. However, in the oxyfuel combustion it has a major influence, as the amount of CO2 in these circumstances is in the range of 60-80 percent by volume. The reaction is endothermic. It absorbs heat and thus operates according to the principle of least constraint against the heating of the flue gas. In addition, increases in the course of the reaction, the number of molecules in the gas phase ( from a CO2 two CO). This will affect the aerodynamics of the burner and thus to the further progress of the reaction.
CO2 capture in gas turbine power plants
The completed in 2001 AZEP project (Advanced Zero Emissions Power Plant ) dealt with the exploration of the oxyfuel process based on a natural gas-fired combined cycle power plant and also included the development of membranes for oxygen supply.
CO2 capture in steam power plants
However, focus of research both in Germany and internationally is the CO2 capture from coal-fired steam power plants. The reasons for this are both technical and political:
- The elements carbon and hydrogen whose oxidation leads during combustion to produce heat, predominate in coal in ratios other than in gaseous fuels. Due to the higher ratio of carbon to hydrogen in coal arise in relation to the heat generated higher CO2 emissions.
- When electricity is generated with coal in steam power plants lower electrical efficiencies are achieved than in gas-fired combined-cycle power plants.
- If one also considers the structure of the power plant in Germany, it is clear that at present the problems caused by coal combustion CO2 emissions significantly exceed by gas combustion. The absolute savings potential is thus in the case of coal significantly higher than in the case of gas. This is also often the case in other countries.
- Against the requirement of environmental organizations and other political actors to adjust the power generation from coal in view of the high CO2 emissions and instead rely to a greater extent on natural gas, the security of supply and import dependency is often by politicians and the generator into the field performed. While the world's coal reserves are spread over many countries ( and occur in the form of brown coal in Germany ), natural gas reserves are concentrated in far fewer countries. In addition, the calorific value -based price of coal is lower and their static range is higher.
After successful tests in the pilot plant, Vattenfall Europe AG has built a pilot plant for CO2 capture from 2006 to 2008, which officially went into trial operation on September 9, 2008.
"CO2 - free"?
In the context of power plant processes based on oxyfuel process is sometimes the term "CO2 - free" or " zero emission " is used in English usage. It should be noted that an oxyfuel process can not be literally " CO2-free " because inevitably CO2 produced during the combustion of fossil fuels. The English term "zero emission " or its literal translation " CO2- free" are better suited to the actual conditions.
Background of these terms is the fact that should the oxyfuel process the flue gas stream itself by removing other components for possible pure landfillable CO2 stream. In the two other processes that come for a deposition in question, including coal gasification and flue gas scrubbing, however, CO2 is removed from the resulting fumes or exhaust using a chemical or physical solvent. In this case, parts of the CO2 remain in the gas. It is assumed that these processes allow a separation of 85-90 percent of the CO2 as rise sharply with a further increase of the technical complexity and hence the cost.
Should a purification by distillation of CO2 from the oxyfuel process goes, depending on the design of distillation, a non-negligible proportion of the CO2 with the separated non-condensable gases O2, N2 and Ar lost. The price of the purification is such that a maximum degree of separation of about 98 percent is possible.
It should also be noted that is also removed in the oxyfuel process in the condensation of water on a small scale CO2 from the exhaust. After the condensation, it is present as carbonic acid in water and can not be sequestered so. If it is not removed from the water or the water is sequestered himself in some form, this carbonic acid must be regarded as CO2 emissions. However, given the small quantities of CO2, such a further treatment is unlikely to be worthwhile.
According to a preliminary injunction from the Landgericht Berlin of 4 December 2007, it is the Vattenfall Europe AG forbidden, than to describe the pilot plant located in Schwarze Pumpe industrial park "CO2 - free".