WO1995028998A1

WO1995028998A1 - A method for purifying combustion-generated gases and flue gases

Info

Publication number: WO1995028998A1
Application number: PCT/SE1995/000440
Authority: WO
Inventors: Seshadri Seetharaman; Xingguo Xiao; Sichen Du
Original assignee: S SEETHARAMAN AB
Current assignee: S SEETHARAMAN AB
Priority date: 1994-04-22
Filing date: 1995-04-21
Publication date: 1995-11-02
Anticipated expiration: 1996-10-22
Also published as: SE9401375L; SE503097C2; SE9401375D0

Abstract

A method for extracting from gases of combustion and flue gases which have boiling points beneath -160 °C at least one contaminating other gas which has a boiling point within the range -140 to 25 °C. The combustion gas and the flue gas are cooled with the aid of at least one gas stream in a cryogenerator which functions in accordance with an expansion process. The gas stream has a temperature which is sufficiently low to condense the contaminating gas. The contaminating gas is then collected for possible further treatment. An arrangement for carrying out the method includes an air-seperation apparatus and at least one heat-exchanger (160A or 160B) for cooling the combustion gas and the flue gas in the apparatus with the aid of cold gas flows. The arrangement also includes a conduit (165) for delivering combustion gas and flue gas to the heat-exchanger or exchangers (160A, 160B), cooling gas delivery conductors (140, 170), a conductor (180) for discharging purified gas, and a conductor (185) for discharging condensed contaminated gases.

Description

A METHOD FOR PURIFYING COMBUSTION-GENERATED GASES AND FLUE GASES

The present invention relates to a method for extracting from combustion-generated gases and flue gases which have a boiling point below -160°C from at least one contaminating gaseous substance which has a boiling range of -140°C to 25°C. The invention relates particularly, but not exclu¬ sively, to a method for purifying coke gas or blast furnace gases from H₂S, S0₂, C0₂, COS or mixtures thereof. The invention also relates to a device for carrying out the method.

The toxic and ecotoxicological drawbacks associated with sulphur-containing gases and nitrous gases in particular, such as H₂S, S0_2/ COS and N0₂, are well known, as are also the ecological problems associated with C0₂ (the so-called green¬ house effect) . It is also recognized that these gases are difficult to avoid in industrial processes and are valuable commodities in many cases. It is therefore particularly important to find a method of handling these industrial gases, so as to keep emission values to the lowest possible level while making the best possible use of these gases in respective industries.

Traditionally, combustion gases and flue gases are purified, or cleansed, with the aid of adapted physical and/or chemical sorption processes or with the aid of a catalyst. Unfortu¬ nately, these processes will often fail to purify the gases to the extent desired and are also highly sensitive to kinetic factors, such as residence times and temperatures. Sorption processes result in large quantities of waste, such as calcium sulphite for instance, when CaO is used for the chemical sorption of S0₂, which constitutes an ecological and ecotoxicological problem. The contaminant gases extracted cannot be utilized readily after the purification process. Catalytic processes more often than not require the provision of a downstream sorption process which handles the reaction products obtained with the catalytic process. Large and complicated purification systems are generally required to handle the large volumes of gas produced in industrial contexts, and to regenerate sorption materials or catalysts.

The use of cryogenerators in this context is also known to the art. In principle and in an industrial context, such generators require the provision of apparatus which will generate temperatures below -70°C, so as to condense gases whose boiling points lie above -160°C. However, this technique is always applied in conjunction with processes that pertain to the production of the gases in question, for instance for converting the gas to a condensed state when the condensed gas is the final product, or for condensing a gaseous mixture with the intention of separating its differ¬ ent components by fractional distillation, or a combination of these processes. Combustion gases and flue gases are, in themselves, often contaminated with minor concentrations of other gases whose boiling points lie above -160°C and must therefore be purified or cleansed. However, the cryotechnical condensation process and/or the separation process is typically always preceded by one of the aforesaid sorption or catalytic processes, since this is considered to simplify the process (c.f., for instance, Vol. A12, page 269, 2nd column, 3rd paragraph, in Ullman's Encyclopedia of Industrial Chemistry, 5th edition, VCH Verlagsgesellschaft mbH, ein- heim, Germany, 1988) or indeed to be necessary in order to be able to carry out the cryotechnical process effectively (c.f., for instance, Vol. A13, page 379, section 5.3.6, in Ullman's Encyclopedia of Industrial Chemistry, 5th edition).

It has now been found that a purifying process of the kind defined in the introduction can be carried out technically in a simpler manner and economically in a more advantageous manner and with a much better effect than that achieved with known techniques. This can be achieved in accordance with the present invention by cooling the combustion gas and flue gas to be purified with at least one stream of gas, which may eventually be condensed, in a cryogenerator which operates in accordance with an expansion process, wherein the tempera¬ ture of the gas stream is sufficiently low to at least condense the contaminant gas, whereafter the contaminant gas is collected for possible further treatment. This process is technically simple and is highly effective in purifying or cleansing combustion gases and flue gases. The contaminant gas extracted can be readily utilized industrially, therewith making an economic contribution.

In accordance with the inventive method, the melting range of the gas contaminating substance will generally lie within a temperature range of -150°C to -5°C, although it is not limited to this range. The inventive method is preferably applied to extract contaminating gases in the form of H₂S, S0₂, C0₂, COS or N0₂, or combinations of these gases.

By "contaminating gas" is meant here any undesirable gas which is present in combustion gas and flue gas in a concen¬ tration of at most 15 percent by volume, preferably at most 10 percent by volume and particularly at most 5 percent by volume, calculated on the total gas volume.

When practicing the inventive method, the contaminating gas is condensed to a liquid or solid state prior to being collected for further treatment.

The cryogenerator used when practicing the inventive method is preferably a liquid air machine, preferably such a machine that includes an air-separating device, and then particularly a Heylandt air-separating system.

The meaning of the terms "expansion process" and "liquid air machines" (which is the same as "machines for manufacturing liquid air") will be found, inter alia, in "Energiteknik I" by Gδsta Rosenblad (Almq ist & Wiksell Fόrlag AB, Stockholm, 1968), pp. 141-145. "The Heylandt air-separation system" is described in Vol. B3, chapter 20, section 5.7, of Ullman's Encyclopedia of Industrial Chemistry, 5th edition.

The inventive method is mainly intended for purifying gases of combustion, by which is meant gases or gas mixtures that are burned when combusting with air or oxygen and used as fuels. The method is particularly intended for purifying coke oven gases and blast furnace gases and provides a highly successful solution to the problems encountered in the iron and steel industry with regard to contaminated industrial gases, while, at the same time, utilizing effectively the hitherto unused, and therewith wasted, cooling capacity generated in the production of oxygen gas within this industry.

The present invention also relates to an arrangement for carrying out the method, this arrangement being characterized by the combination of an air-separation apparatus and at least one heat-exchanger which functions to cool gases of combustion and flue gases with the aid of at least one gas stream in the apparatus, wherein the arrangement further includes at least one conduit for delivering combustion gas and flue gas to the heat-exchanger, at least one cooling gas delivery conduit, at least one conduit for carrying away purified combustion gas and flue gas, and at least one conduit for carrying away condensed extracted contaminant gases.

The invention will now be described in more detail with reference to exemplifying and operating installation thereof, and with reference to a working example, and also with reference to the accompanying drawings, in which

Fig. 1 is a principle diagram illustrating one installation of an inventive method, based on the use of a Heylandt air- separation system; and Fig. 2 is an enlarged view of the area marked with a broken line in Fig. 1.

The installation illustrated in Fig. 1 is an example of how the present invention can be applied in order to utilize the existing, but earlier unused cooling capacity in an air- separation system, in the illustrated case a Heylandt system, for purifying coke furnace gas from primarily C0₂ and H₂S, and to produce pure sulphur. Similar to the case in a conventional Heylandt system, the air is first compressed in a piston compressor 10 to the pressure T₁ in several stages, not described in detail, while water vapour, carbon dioxide and hydrocarbons are extracted from the air in a known manner. The air is then cooled in a cooler 20 which comes in contact with ammonia, whereafter part of the compressed air, up to about 50 percent by volume, is passed to the expansion turbine 30, while the remainder of the air is passed to a main cooler 40. This part of the air is then passed from the cooler 40 to the high-pressure part 70 of a distillation column 60 while passing through a throttle valve 50, which lowers the air pressure isoentalpically to T₂. A flow of liquid air rich in oxygen gas (an oxygen content of about 36- 38 percent by volume) is passed continuously through a conduit 80 from the bottom of the high-pressure part 70 to the low pressure part 100 of the column 60 via an expansion valve 90, therewith to lower the pressure to T₃. Liquid nitrogen is passed through a conduit 110 from the high- pressure part 70 of the column 60 to the low-pressure part 100 of said column, first via a supercooler 120, in which the nitrogen is cooled, and thereafter through an expansion valve 130 which lowers the pressure to T₃. That volume of air which is able to expand in the turbine 30 has the pressure T₃. Distinct from a conventional Heylandt system, not all of the air is led into the low-pressure part 100. Instead, a subflow is led from the conduit 150 between the turbine 30 and the low-pressure part 100, through a conduit 140. In turn, this subflow is divided into two further subflows of generally equal volumes, these subflows being delivered to two heat- exchangers 160A, 160B. The air passed to the heat-exchangers 160A, 160B cools coke furnace gas originating from a conven¬ tional coking plant (not shown) and delivered to the heat- exchangers through conduits 165. Nitrogen gas is led from the top of the column 60 to the main cooler 40 and to the precooler 20 through a conduit 170, similar to the manner in which nitrogen gas is conducted through the supercooler 120 as a coolant in a conventional Heylandt system, although in the case of the inventive method, the nitrogen gas is conducted to the heat-exchangers 160A, 160B in order to cool incoming coke oven gas. The temperature of the air in the conduit 140 and the temperature of the nitrogen in the conduit 170, and dimensioning of the heat-exchanger surfaces in the heat-exchangers 160A, 160B is so adapted that H₂S and C0₂ will condense and settle in an essentially solid state on the walls of the coke oven gas passageways in the heat- exchangers 160A, 160B. The purified and extensively cooled coke furnace gas is then passed as a coolant through the supercooler 120, the main cooler 40 and the precooler 20, through conduits 180. The heat-exchangers 160A, 160B are regenerative, which in this context means that the heat- exchanger passageways can be used alternately to convey air and coke oven gas respectively, ie can be switched intermit¬ tently. This enables H₂S and C0₂ to be re-gasified and discharged through the conduits 185 for further treatment, in this case desulphurization with the aid of a desul- phurizing apparatus 190 in accordance with Claus (Claus processors and other desulphurizing methods appropriate to the present invention are described on page 95ff, section 2.7, Vol. A17, of Ullman's Encyclopedia of Industrial Chemistry, 5th edition) . Fig. 2 illustrates the subsystem which includes the regenerative heat-exchangers 160A, 160B, the desulphurizing apparatus 190 and associated conduits in more detail.

Example

An example of one method of carrying out the present inven¬ tion is described below with reference to Figs. l and 2.

Coke furnace gas containing in total about 2.2 percent by volume H₂S and C0₂, calculated on the total gas volume, and cooled to a temperature of about 30°C was delivered through conduits 165 to the regenerative heat-exchangers 160A, 160B at a flow rate of about 24,800 Nm³/h. The gas was cooled in the heat-exchangers with air and nitrogen having temperatures of between -189°C and -193°C, these gases being delivered to the heat-exchangers through respective conduits 140 and 170 at respective flow rates of about 1,230 Nm³/h and about 26,650 Nm³/h. The heat-exchangers used were plate-fin heat- exchangers of the kind described in more detail in Vol. B3, chapter 2, section 1.1.3.1, of Ullman's Encyclopedia of Industrial Chemistry, 5th edition) . The heat-exchangers had a heat transfer of about 74 W/m²K and a total heat transfer area of about 6,650 m². The coke oven gas leaving the heat- exchangers had a temperature of about -186°C, whereas the temperature of the air and the nitrogen was raised to about 26°C. About 99.98% of the sulphur content of the coke furnace gas was extracted from the gas, by virtue of H₂S and C0₂ condensing in the heat-exchangers to a solid state. The heat- exchanger passages through which the coke oven gas and the air were passed were switched intermittently to gasify condensed H₂S and C0₂, these gases then being discharged through the conduits 185. H₂S was made to react with part of the air-containing oxygen in the desulphurizing apparatus 190, therewith to form elementary sulphur and water. About 245 kg of sulphur were obtained each hour.

It will be understood that the invention is not restricted to the aforedescribed and illustrated installation thereof, nor yet to the Example. Many variants of these installation are embraced by the inventive concept, as those skilled in this art will be aware. For instance, the heat-exchangers 160A, 160B can be replaced with a single heat-exchanger, which need not be a regenerative heat-exchanger or a plate- fin heat-exchanger. Of course, it is also possible, and in many cases suitable, to utilize the cooling capacity avail¬ able in other flows in a cryogenerator, for instance the cooling capacity afforded by nitrogen gas and oxygen gas produced in an air-separation apparatus.

Claims

1. A method for extracting from combustion-generated gases and flue gases whose boiling points lie below -160°C at least one contaminating other gas whose boiling point lies within the range of -140°C to 25°C, characterized by cooling the combustion gas and the flue gas with at least one gas stream, said gas possibly being condensed to a liquid state, in a cryogenerator which functions in accordance with an expansion process, wherein the gas stream is given a temperature which is sufficiently low to condense the contaminating gas, whereafter the contaminating gas is collected for possible further treatment.

2. A method according to Claim 1, characterized in that the gas contaminating substance has a melting point which lies within the range -150°C to -5°C.

3. A method according to Claim l, characterized in that the contaminating gas consists of one or more of the group H₂S,

S0₂, C0₂, COS and N0₂.

4. A method according to any one of the preceding Claims, characterized in that the contaminating gas is present in the combustion gas and the flue gas in an amount corresponding to 15 percent by volume of the total gas volume.

5. A method according to any one of the preceding Claims, characterized by giving the gas stream a temperature which is sufficiently low to convert the contaminating gas to a solid or liquid state prior to collecting said gas for further treatment.

6. A method according to any one of the preceding Claims, characterized in that the cryogenerator is a liquid air machine.

7. A method according to Claim 6, characterized in that the liquid air machine includes an air-separation arrangement.

8. A method according to Claim 7, characterized in that the arrangement includes an air-separation Heylandt system.

9. A method according to any one of Claims 1-8, character- ized in that the gas to be purified is a combustion-generated gas.

10. A method according to any one of Claims 1-9, character¬ ized in that the gas to be purified is coke oven gas or blast furnace gas.

11. An arrangement for carrying out the method according to any one of Claims 1-10, characterized by the combination of an air-separation apparatus and at least one heat-exchanger (160A, 160B) which functions to cool the combustion gas and the flue gas with the aid of at least one gas stream, wherein the arrangement further includes at least one conduit (165) for delivering combustion gas and flue gas to said at least one heat-exchanger (160A, 160B) , at least one cooling gas delivery conduit (140, 170) , at least one conduit (180) for discharging purified combustion gas and flue gas, and at least one conduit (185) for discharging condensed contaminat¬ ing gases.