AU2010277760B2

AU2010277760B2 - Process for removing harmful substances from liquid carbon dioxide and apparatus for performance thereof

Info

Publication number: AU2010277760B2
Application number: AU2010277760A
Authority: AU
Inventors: Manfred Baldauf; Carsten Graeber; Marc Hanebuth; Gerhard Zimmermann
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2009-07-30
Filing date: 2010-07-16
Publication date: 2015-02-19
Anticipated expiration: 2030-07-16
Also published as: CA2769495A1; RU2012107387A; CN102470287A; AU2010277760A1; RU2551510C2; EP2459293A1; KR20120055576A; CA2769495C; WO2011012470A1; US20120144860A1; BR112012001719A2; DE102009035389A1; CN102470287B

Abstract

A process for removing harmful substances from a gas stream which comprises essentially carbon dioxide CO

Description

e UT/t.eVUiU/U~UiiJ'- / zUU~lei5Uqbwu Description Process for removing harmful substances from carbon dioxide and apparatus for the performance thereof The invention relates to a process and to an apparatus for separating harmful substances from a carbon dioxide stream. Compounds of sulfur or mercury are examples of harmful substances. On account of the so-called greenhouse effect and the global temperature rise associated therewith, the emission of greenhouse gases into the atmosphere is subject to increasing criticism. A considerable proportion of the greenhouse effect is attributed to carbon dioxide C02 which is produced during the combustion of fossil fuels. There is currently a consensus across society that separating out carbon dioxide from power station waste gases may counteract an increase in the greenhouse effect. For this reason, corresponding further developments are directed at making possible plants having low levels of carbon dioxide emissions or even none at all. Accordingly, the latest concepts for power stations having low levels of carbon dioxide emissions or which are free of carbon dioxide emissions are at the development stage. All of these concepts pursue the common objective of separating the carbon dioxide as completely as possible and at a high level of purity with minimal energy consumption. Here the focus is on the separation of the carbon dioxide in the first instance. In addition, however, further undesirable substances are present which occur depending on the type of 2 fuel and which need to be removed. Sulfur, mercury or their compounds are the most commonly encountered substances in terms of quantity in this context. The latest power station concepts include general conditions which in each case provide the most advanced techniques for separating harmful substances according to the current prior art. According to the prior art, harmful substances which accumulate in power stations, such as sulfur or mercury compounds, are removed directly from the gas phase. Thus, hydrogen sulfide, for example, is separated by means of gas scrubbing. This entails routing the gas stream through an absorber in which a liquid medium absorbs the harmful substances. Liquid absorber media are for example aqueous alkanolamine solutions, specifically aqueous methyl diethanolamine, or for example cold methanol, employed in the Rectisol process. Such processes using liquid absorber media are known and established in various technologies. Disadvantages in the prior art consist primarily in the fact that gas scrubbing processes involve an increased energy requirement because they may be associated with process steps such as compression and/or cooling. The regeneration of absorber media is accomplished by means of distillation, with which a substantial degree of energy consumption is associated. Furthermore, processes of said type are relatively complicated in terms of infrastructure and setup, which has disadvantageous implications in relation to process 3 implementation and investment costs. The object of the invention is to describe an energy-saving option for removing harmful substances in power stations which are substantially fired by fossil fuels. It is furthermore intended to disclose an easy to-use apparatus for performing the process. According to a first aspect of the present invention, there is provided a process for separating off at least one harmful substance from a gas mixture, wherein the gas mixture substantially comprises carbon dioxide C02 and at least one valuable substance from the group of substances hydrogen H, carbon monoxide CO, nitrogen N 2 or noble gas, wherein a carbon dioxide condensation is carried out in order to generate and separate off liquid carbon dioxide, wherein - hydrogen sulfide H 2 S or carbonyl sulfide COS is treated as the at least one harmful substance, - hydrogen sulfide H 2 S or carbonyl sulfide COS is separated from the liquid carbon dioxide by means of an adsorptive process, and - a process temperature in the range from -30"C to -70C is set. According to a second aspect of the present invention, there is provided the use of the process of the first aspect, wherein the separation of at least one harmful substance from cryogenic carbon dioxide takes place in low-carbon dioxide or carbon dioxide-free power stations. According to a third aspect of the present invention, there is provided an apparatus for separating off at least one harmful substance from the group mercury, sulphur or their respective compounds, from a gas mixture which substantially comprises carbon dioxide and also at least one valuable substance from the group of substances hydrogen, carbon monoxide, nitrogen or a noble gas, wherein for generating liquid carbon dioxide, a carbon dioxide condensation unit is present, wherein - hydrogen sulfide H 2 S or carbonyl sulfide COS is treated as harmful substances, - a separation by adsorption of hydrogen sulfide H 2 S or carbonyl sulfide COS from the liquid carbon dioxide is carried out in at least one fixed-bed adsorber, and - an operating temperature of the apparatus lies in the range from -70 0 C to -30C.

3a According to a fourth aspect of the present invention, there is provided the use of the apparatus of the third aspect wherein the separation of at least one harmful substance from cryogenic carbon dioxide takes place in low or zero-carbon dioxide power stations. The invention is based on the fact that in the case of a gas mixture which in large part consists of carbon dioxide C02 and includes proportions of valuable gases and harmful substances, particularly in the case of condensation of the carbon dioxide, the harmful substances preferably accumulate in the liquid carbon dioxide. This knowledge is exploited to the effect that the separation of the harmful substances from the liquid phase of the carbon dioxide takes place at low temperatures through the use of adsorber materials/adsorbents, preferably solid adsorber materials. In this case it is particularly advantageous that the energy balance associated with separating harmful substances at low temperatures turns out to be positive, in other words the overall energy requirement is less. The separation of the harmful substances from the liquid phase can take place at low temperatures very advantageously by means of adsorber materials because the materials employed here have large surface areas which are available for the absorption of the harmful substances. If the process for separating harmful substances from a gas stream consisting substantially of carbon dioxide is combined with a process in which carbon dioxide is already present in liquid form, synergistic effects in the overall energy balance P1C'1'/E&2UIU/UbUdJb / 2UU9P13U40WO 4 can thereby be achieved. It is particularly advantageous to set and maintain the process temperature at less than -30C. It is not beneficial to use temperatures below -70*C because carbon dioxide exists in the solid phase at such low temperatures. The process pressure must in principle lie above the triple point of carbon dioxide in accordance with the temperature/pressure diagram. It is thus at least 5 bar. A temperature range which can likewise be used for the process temperature begins at -5'C and extends toward lower temperatures. The use of the process for separating harmful substances from a gas mixture which substantially consists of carbon dioxide can be implemented particularly advantageously in so-called zero-carbon dioxide power stations. In said power stations the separation of the carbon dioxide can be carried out cryogenically. With this approach the carbon dioxide is brought to a low temperature, liquefied and separated out. The liquid phase is very well suited for the adsorptive separation of harmful substances because the latter by preference accumulate in the liquid carbon dioxide. Furthermore, low temperatures promote adsorption, which means for example that a gas scrubbing stage as provided in the prior art is no longer necessary. The use of fixed-bed adsorbers for the adsorption of harmful substances is advantageous because solids having a large surface area are employed here. These are in particular alumina (aluminum oxide), activated carbon, silica gel, zeolites or polymers having a large surface area. Since the surface area of the adsorbents becomes populated e (_'*J:/1!eUiU/UbU55 / Zuu )ei3U'bVU 5 with harmful substances in the course of the process, a periodic regeneration of individual fixed-bed adsorbers is advantageously provided. Exemplary embodiments will be described in the following with reference to schematic accompanying figures, in which specifically: Figure 1 schematically illustrates a fixed-bed adsorber through which liquid carbon dioxide containing harmful substances is fed, the harmful substances being adsorbed in the fixed-bed adsorber. Figure 2 illustrates adsorbers connected in parallel which can be switched over individually or in groups between the adsorption and regeneration operating states by means of corresponding valves. The adsorption of harmful substances is carried out in fixed bed adsorbers, such as are illustrated in Figures 1 and 2. After a certain period of use it is necessary to regenerate the adsorbents. This is effected by means of a reduction in pressure, an increase in temperature, or by feeding through a gas or vapor, or by a combination thereof. For this reason there exist a plurality of adsorbers which are in operation or can be regenerated on an alternating basis. An arrangement of three fixed-bed adsorbers is illustrated in Figure 2. Analogous configurations for two or more than three adsorbers are possible. During the carbon dioxide condensation a substantial accumulation of harmful substances contained in a gas mixture occurs automatically in the liquid carbon dioxide. It has been . PUT/EFZU±U/UbUi35 / ZUUh1j3U4IdWU 6 possible to demonstrate this with the aid of simulation calculations taking as example hydrogen sulfide H 2 S and also carbonyl sulfide COS. The harmful substances can subsequently be separated out from the liquid phase at low temperatures by means of adsorber materials. The energy-saving separation of the harmful substances at lower temperatures in the overall concept is advantageous. A process according to the invention can be excellently combined with a power station concept which already provides cryogenic carbon dioxide separation. The temperature range for operating the process can lie between -5*C and -70 0 C. The lower temperature limit lies in the region of the solidification of carbon dioxide from the liquid phase, which means that solids can block the process. As a general rule it is necessary to ensure that the fixed-bed adsorbers are kept open for the liquid reaction stream. However, the energy balance of the entire process reveals that temperatures of -30*C and lower are advantageous for the separation of harmful substances. The valuable gases such as hydrogen, carbon monoxide, nitrogen or noble gases which may be present in the gaseous or liquid stream in the process are not adsorbed in the course of the described process approach. Harmful substances such as mercury, sulfur or their compounds, which are to be found substantially in the liquid carbon dioxide stream, accumulate on the surface in the fixed-bed adsorbers, in other words are adsorbed by the adsorbents, and are thus retained in the fixed-bed adsorbers. A further harmful substance which can be separated out in this manner is carbonyl sulfide COS.

7 Figure 1 shows a fixed-bed adsorber 1 into which a liquid carbon dioxide stream 2 containing harmful substances is introduced. A liquid carbon dioxide stream 3 free of harmful substances emerges at the outlet of the fixed-bed adsorber 1. Figure 2 shows three fixed-bed adsorbers 1 connected in parallel. The liquid carbon dioxide stream 2 is supplied at the top, harmful substances are captured in the fixed-bed adsorbers 1, and the liquid carbon dioxide stream 3 free of harmful substances can be removed at the bottom. Individual fixed-bed adsorbers 1 can be switched over in each case for regeneration purposes 4. While a direct catalytic conversion may be associated with a favorable state of equilibrium on account of the low temperatures, it does however exhibit very unfavorable kinetics. Very large quantities of catalysts would be required, which would probably become very rapidly deactivated due to the presence of sulfur.

Claims

1. A process for separating off at least one harmful substance from a gas mixture, wherein the gas mixture substantially comprises carbon dioxide C02 and at least one valuable substance from the group of substances hydrogen H, carbon monoxide CO, nitrogen N 2 or noble gas, wherein a carbon dioxide condensation is carried out in order to generate and separate off liquid carbon dioxide, wherein - hydrogen sulfide H 2 S or carbonyl sulfide COS is treated as the at least one harmful substance, - hydrogen sulfide H 2 S or carbonyl sulfide COS is separated from the liquid carbon dioxide by means of an adsorptive process, and - a process temperature in the range from -30*C to -70'C is set.

2. The process as claimed in claim 1, wherein a harmful substance additionally present in the gas mixture is separated off from the liquid carbon dioxide by means of an adsorptive process, wherein the additional harmful substance is selected from the group of substances sulfur, mercury, sulfur compounds and/or mercury compounds.

3. The process as claimed in claim 1 or 2, wherein as absorbent for separating off by adsorption the at least one harmful substance, at least one substance is selected from the group of substances activated carbon, zeolite, silica gel, alumina or at least one polymer having a large surface area.

4. The use of the process as claimed in any one of claims 1 - 3, wherein the separation of at least one harmful substance from cryogenic carbon dioxide takes place in low or zero-carbon dioxide power stations.

5. An apparatus for separating off at least one harmful substance from the group mercury, sulphur or their respective compounds, from a gas mixture which substantially comprises carbon dioxide and also at least one valuable substance from the group of substances hydrogen, carbon monoxide, nitrogen or a noble gas, wherein for generating liquid carbon dioxide, a carbon dioxide condensation unit is present, wherein - hydrogen sulfide H 2 S or carbonyl sulfide COS is treated as harmful substances, - a separation by adsorption of hydrogen sulfide H 2 S or carbonyl sulfide COS from the liquid carbon dioxide is carried out in at least one fixed-bed adsorber, and - an operating temperature of the apparatus lies in the range from -70'C to -300C.

6. The apparatus as claimed in claim 5, wherein the at least one fixed-bed adsorber for the separation by adsorption comprises at least one adsorbent from the group activated carbon, zeolite, alumina or at least one polymer having a large surface area. 9

7. The apparatus as claimed in claim 5 or 6, wherein the at least one fixed-bed adsorber is present more than once.

8. The apparatus according to claim 7 wherein the at least one fixed-bed adsorbers are connected in parallel in groups.

9. The apparatus as claimed in claim 8 wherein the fixed-bed adsorbers are connected in parallel in groups such that adsorbers or groups of adsorbers are switched over for regenerating the adsorbents.

10. The use of the apparatus as claimed in any one of claims 5 to 9, wherein the separation of at least one harmful substance from cryogenic carbon dioxide takes place in low-carbon or zero-carbon dioxide power stations.

11. A process for separating off at least one harmful substance from a gas mixture, said process as claimed in claim 1 and substantially as hereinbefore described with reference to any one of the accompanying drawings.

12. The use of the process as claimed in claim 11, wherein the separation of at least one harmful substance from cryogenic carbon dioxide takes place in low or zero-carbon dioxide power stations.

13. An apparatus for separating off at least one harmful substance from the group mercury, sulfur or their respective compounds, said apparatus as claimed in claim 5 and substantially as hereinbefore described with reference to any one of the accompanying drawings.

14. The use of the apparatus as claimed in claim 13, wherein the separation of at least one harmful substance from cryogenic carbon dioxide takes place in low-carbon or zero-carbon dioxide power stations. Siemens Aktiengesellschaft Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON