DE3501456A1 - Process for reducing the SO2 and NOx content of gases - Google Patents

Abstract

A process is described for reducing the SO2 and NOx content of gases, which are formed in the gasification of coal, and which, after desulphurisation, are burnt in a turbine. In order to achieve a reduction of the NOx emission with the flue gas and to achieve an increase of the efficiency of the power station as a result of lower compression energy for the combustion air, it is proposed to carry out the combustion of the desulphurised gas in the presence of a CO2/O2 mixture. As a result of the use of CO2, on the one hand the combustion temperature is reduced and on the other hand the NO2 problem is reduced, since a virtually N2-free flue gas is produced. In comparison with air, the CO2 amount required is less so that the energy requirement for the CO2 compression can be reduced.

Description

Process for reducing SO2 and NO

Content of gases The invention relates to a method for reducing the S02 and NOx content of gases produced during the gasification of coal, which burned in a turbine after desulfurization.

When generating electricity in coal-fired power plants, the all sulfur emitted as SO2 with the flue gas. To reduce the S02 emissions therefore the flue gas must be treated in a flue gas desulphurization system. A large number of processes have been developed for the desulfurization of flue gases and have been installed, which work according to the most varied of methods. The furthest Processes in which the SO2 is chemically added by adding milk of lime are widespread is converted into gypsum, but also the conversion of S02 with NH3 to ammonium sulfate or washing out the SO2 with physical and chemical detergents is possible.

To reduce sulfur emissions in power plants, a procedure has been developed developed in which the coal is first gasified with oxygen or air and the generated gas after desulfurization in a gas turbine to generate energy is burned. This process has become known under the name of combined cycle " consists of the following individual steps: - Partial oxidation of coal with air or pure O2 at increased pressure.

- Cooling of the gas obtained from reaction temperature to ambient temperature with simultaneous generation of steam.

- Desulfurization of the cooled gas with a scrubbing process. Normally physical washes are used for this, as H2S is simultaneously produced from the raw gas and COS must be removed. In contrast, the CO2 should largely be in the gas phase remain. This selective washing ensures that the sulfur compounds so concentrated that it can be worked up in elemental sulfur or others Usable sulfur compounds, e.g. sulfuric acid, without additional problems regarding Air pollution is possible. Typical washing processes for selective desulfurization of the gas are washes with polyethylene glycol ethers or cold washes with organic Solvents, e.g. toluene / methanol mixtures.

- Combustion of the desulphurized raw gas with air under pressure. To the Reducing the exit temperature from the combustion chamber is usually associated with great Excess air worked.

- Relaxation of the hot combustion gas in a turbine, which has a Generator to generate electricity and the compressor to compress the combustion air drives.

This process has the advantage of good sulfur and dust separation as a major disadvantage both a high energy requirement for the compaction of the Air as well as NOx problems in the flue gas.

The NO formation during combustion is directly related to air compression coupled. At a lower combustion temperature, the formation of NOX decreases accordingly the thermal equilibrium, but a larger amount of air must be compressed reducing the overall efficiency of the turbine.

It has also been suggested by adding water vapor before the Combustion to lower the outlet temperature of the pressurized flue gas. However, this steam addition is a complex and quite expensive process, because initially Steam must be generated at increased pressure.

In terms of energy, the expansion of steam in a steam turbine is more economical. A particular advantage is that the resulting condensate immediately can be used again for steam generation while adding the steam to the fuel gas the water vapor is contained in the flue gas and only with large additional Effort can be recovered. There is thus the need for provision of large amounts of fully demineralized boiler feed water, resulting in additional costs and chemicals means.

Lionel S. Galstaun and Robert F. Geosits were at a meeting of the American Institute for Chemical Engineers in Anaheim, California from May 20 to 24, 1984 proposed within the desulfurization of the fuel gas by coupling the power plant with a plant for the production of H2 or synthesis gas To deliver Cm2 from the converted H2-rich gas to the fuel gas.

By combining the two washes for desulfurization and for It is possible to separate CO2 according to the additional CO2 in the fuel gas the proportion of excess air that lowers the temperature of the combustion gas is needed to reduce and thus by saving the air compression energy to increase the energy output of the expansion turbine.

The main disadvantage of this method is the coupling of the so-called combined cycle "with an H2 or synthesis gas production. The process is only then interesting if a completely self-sufficient NH3 or methanol plant based on coal is built in which the own energy requirement through the so-called "combined cycle" is covered.

The present invention is therefore based on the object of a method to make available of the type mentioned, with which a reduction of the NOx emissions achieved with the flue gas and an increase in the output of the power plant can be achieved by lower compression energy for the combustion air.

This object is achieved according to the invention in that the combustion of the desulphurized gas carried out in the presence of pure O2 and the combustion temperature is reduced by adding CO2.

It was found that by using CO2 alone for the lowering the combustion temperature reduces the NOx problem and the energy requirement at the same time for CO2 compression can be lowered.

While by lowering the N2 content and the O2 content, the NOx formation can be greatly reduced (according to the equilibrium for the System N2, 02, NO according to the formula kp = NO x (N2 x ° 2) 1/2 = constant at the same time, the required amount of CO2 compared to air decreases correspondingly to the significant greater specific heat of CO2 compared to air.

According to a particularly advantageous embodiment of the invention The CO2 required for lowering the temperature is taken from the process The flue gas leaving the turbine is separated, compressed and returned to the turbine.

In general, the extraction of CO2 from flue gas is quite complex, because the gas is unpressurized and the CO2 partial pressure due to the large air / N2 proportion is very low.

If, however, according to the invention, air is not used for the combustion of the gas, but pure O2, for example from an air separator, is used Flue gas only consists of CO2, water vapor and residual O2 as well as some inert gases such as Ar and N2. In this way, according to the invention, CO2 can be extracted from the flue gas by cooling and condensation of the water vapor contained therein can be carried out. Because only a partial amount of CO2 is returned to the burner, is also an accumulation of Ar and N2 not to be feared.

In summary, it can be said that with the return of CO2 practically N2-free using O2 instead of air for combustion Flue gas is produced, which greatly reduces the formation of NOX.

By using CO2 with a higher specific heat compared to air, the amount of gas circulated can be reduced, whereby the energy requirements for the Reduced CO2 compression and the released Power of the power plant is increased. After all of the CO2, which is in the Combustion produced by carbonaceous coal is obtained economically can be, according to the invention there is the possibility of additional CO2 from the flue gas to win and, for example, for EOR (enhanced oil recovery) or syntheses, such as the synthesis of urea from natural gas.

If the coal is used for gasification or partial oxidation is to be introduced into a reactor with liquid CO2 (e.g. according to DE-OS 2 631 185), according to a further embodiment of the inventive concept, there is the possibility of to gain additional CO2 from the flue gas and after liquefaction for mashing to use the feed coal upstream of the gasification plant. Depending on the pressure of the fuel gas A dryer for the C02 may be required in front of the combustion chamber Avoid water ice or hydrate formation.

In the following the invention is based on that shown in the figure Block diagram explained in more detail.

Coal is brought in via a line 1 and in a partial oxidation (PO) plant 2 is gasified in a manner known per se under increased pressure the PO system via line 3 pure O2, for example from a via line 4 can originate with air supplied air separator 5 and was compressed in 6, supplied. The gas obtained in line 7 is fed to a heat recovery system 8 and there from reaction temperature to about ambient temperature at the same time Cooled generation of steam.

The steam can be obtained via line 9.

The cooled gas is then passed through line 10 to a desulfurization device 11 supplied. Here is an example wise by means of a polyethylene glycol ether or a toluene / methanol mixture selectively removes H2S and COS by absorption.

When the loaded detergent is regenerated, these components are removed free again and can be withdrawn via line 12 and possibly a Claus plant be fed for further processing.

The desulphurized raw gas finally arrives at a line 13 Gas turbine 14, in which it burns under pressure in the presence of a CO2 / O2 mixture will. For this purpose, compressed pure O2 is supplied from the air separator 5 via line 20 and mixed with CO2 from line 21, which will be discussed in more detail below. That CO2 is used to reduce the outlet temperature from the combustion chamber.

The hot combustion gas is in a turbine, which is a generator drives to generate electricity and a compressor 15 for CO2 compression, relaxed and fed via line 16 to a system 17 for flue gas cooling. When cooling down The water vapor contained in the flue gas condenses out and is passed through a line 18 deducted. Thus, raw CO2 can be withdrawn via line 19, which is just that Contains small amounts of 02, N2 and Ar. The raw CO2 is in the compressor 15 on the pressure required for the combustion is compressed and partly via a line 21 mixed with the pure O2 used to burn the raw gas and the other part won for EOR via line 22, for example.

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Claims (5)

Claims 1.) Method for reducing the SO2 and NOx content of gases resulting from the gasification of coal, which after desulfurization in one Turbine to be burned, characterized in that the combustion of the desulphurized Gas carried out in the presence of pure O2 and the combustion temperature through CO2 addition is reduced. 2. The method according to claim 1, characterized in that the for the temperature reduction required CO2 from the flue gas leaving the turbine separated, compressed and returned to the turbine. 3. The method according to claim 2, characterized in that the CO2 extraction from the flue gas through cooling and condensation of the water vapor it contains is carried out. 4. The method according to claim 2 or 3, characterized in that in addition CO2 is obtained from the flue gas. 5. The method of claim 2 or 3, wherein the coal together with liquid CO2 is denied, characterized in that additional CO2 from the Flue gas obtained and after liquefaction for mashing the feed coal in the Gasification plant is used.