AU2012101392A4 - System and method for syngas processing - Google Patents

Abstract

The invention provides a system and method for separating methane from product gas obtained from an underground coal gasification process. Co ,cCI

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT SYSTEM AND METHOD FOR SYNGAS PROCESSING The following statement is a full description of this invention, including the best method of performing it known to me/us: 1 SYSTEM AND METHOD FOR SYNGAS PROCESSING TECHNICAL FIELD [0001] The present invention relates to a system and method for separating methane (CH 4 ) from product gas obtained from an underground coal gasification process. BACKGROUND ART [0002] Underground coal gasification (UCG) is a process by which product gas is produced from a coal seam by gasifying the coal in situ in the presence of an oxidant. The product gas is typically referred to as synthesis gas or "syngas" and can be used, for example, as a feedstock for various applications, including clean fuels production, chemical production, and electricity generation. [0003] The product gas (raw syngas) generated by UCG typically comprises syngas as well as other components, and the constituency will depend on various factors including the type of oxidant used for UCG (air or other oxidant, such as oxygen or oxygen-enriched air), water presence (both ground water and exogenous water), coal quality, and UCG operating temperature and pressure. [0004] Typically, product gas will contain: (1) main syngas components (e.g., CO, H 2 , C0 2 , N 2 , and CH 4 ); (2) solid particles/particulates (e.g., soot, ash and coal particles); (3) water; (4) minor components such as C 2

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6 hydrocarbons, oxygen, argon, sulphur containing components (e.g., H 2 S, COS, CS 2 , mercaptans, and thiophenes), nitrogen based components (e.g., NH 3 and HCN), hydrocarbon components (e.g., coal condensate, BTEX (benzene, toluene, ethylbenzene and xylenes) and PAHs (polycyclic aromatic hydrocarbons)); and (5) trace components such as heavy metals (arsenic and mercury) and chlorides. [0005] An ongoing problem in UCG is separation of one or more components of the syngas stream. For example, it can be advantageous to separate one or more of the main syngas components from each other in order to provide various product streams for specific downstream applications. [0006] Thus, there is a need in the art for cost effective systems and methods for treating a 2 syngas stream generated by UCG at the site of production to separate one or more of the main syngas components from each other. SUMMARY OF INVENTION [0007] An object of the present invention is to provide a system and method for processing raw syngas obtained from UCG that minimises one or more of the problems of the prior art. [0008] According to an aspect of the present invention, there is provided a system for separating methane from a syngas stream generated by UCG, the system including a) a source of treated syngas generated by UCG, b) a physical separation unit for removal of any component in the syngas stream that could freeze during cryogenic distillation, c) a cooling unit for cooling the syngas stream using at least one product gas stream from the system, and d) a cryogenic distillation unit with reflux for separating methane in the syngas stream from other syngas components. [0009] In one embodiment, the physical separation unit for removal of any component in the syngas stream that could freeze during cryogenic distillation includes a molecular sieve. [0010] In another aspect, the present invention resides in a method for separating methane from a treated syngas stream generated by UCG, the method including the steps of a) removing components of the syngas stream that could freeze during cryogenic distillation to produce a dry syngas stream, b) cooling the dry syngas stream, c) introducing the dry, cooled syngas stream into a cryogenic distillation column, and d) withdrawing a methane-rich bottom product stream from the distillation column. [0011] In one embodiment, the treated syngas stream generated by UCG has been treated to remove at least one metal component from the syngas stream, hydrogenate at least one sulphur species present in the syngas to hydrogen sulphide, and remove hydrogen sulphide and carbon dioxide. [0012] In another embodiment, removing components of the syngas stream that could freeze during cryogenic distillation includes utilising a molecular sieve. [0013] The main process vessel of the system and method of the present invention is a cryogenic gas upgrader including one or more columns (e.g., one or more tray columns).

3 [0014] The cryogenic gas upgrader of the invention (also referred to as "purifier") for upgrading treated syngas (including partially treated syngas) to a quality, which upon further processing, yields a super pure make gas suitable for clean fuels production by catalytic processes, such as Fischer-Tropsch (FT) synthesis, methanol synthesis, and other similar applications. In addition, the process configuration is so set that alternate feeds such as pipeline natural gas, sub-pipeline quality natural gas with excess nitrogen or C0 2 , gas mixtures containing rare gases (e.g., helium), associated gas from primary oil production, and CO2/N 2 floods can also be processed to required specifications without major changes to process or equipment. The elegance of this purifier application emanates from the fact that, while the purifier improves energy efficiency and operability of clean fuels production in normal operation, the overall process can still operate, though at lower loads, even when the purifier is off-line, leading to capital avoidance in spared or redundancy systems (e.g., pure methane compressor). [0015] The main performance metrics for the purifier are maximum rejection of methane from a CO/H 2 stream and minimum loss of CO (boiling point of -191.5 C) with the methane (boiling point of -161.7 C). As will be understood by one of ordinary skill in the art, these boiling points are at standard temperature and pressure (STP) and appropriate adjustment is required to account for non-standard conditions. [0016] The loss of CO with the methane is not only an economic issue. Increasing slip of CO with methane can cause metal dusting issues in downstream reformer units where methane is further processed to syngas. The performance metrics are preferably achieved through managing the methane heat pump and setting the reflux temperature at the column top. [0017] The use of cryogenic separation as described herein is developed for use in processing coal gasification product gas that contains from about 5% to about 30% methane through a simple cold box design that can include only one distillation column and a small heat pump. [0018] The present invention includes at least one physical separation unit/process in order to remove any component in the syngas that could freeze during cryogenic distillation (i.e., components of the syngas that have a higher melting point than methane). [0019] Typically, the physical separation unit is (or includes) a front-end molecular sieve bed that eliminates components that freeze at colder temperatures, such as water, C0 2 , and 4 traces of sulphur species and ammonia that may also be present before the syngas is cooled to cryogenic temperatures for gas separation. [0020] The molecular sieve beds are typically designed in multiple vessels, so that they can be changed over upon exhaustion for regeneration and reuse periodically (e.g., every 8 to 24 hours). [0021] As oxygen bearing compounds, such as nitrogen oxides, can cause explosive gum forming precursors, the molecular sieves can also be designed to eliminate nitrogen oxides. [0022] The present invention also includes at least one cooling unit to cool the syngas using at least one product gas stream from the system. To achieve the low distillation temperatures required, typically a plurality of cooling units are used. [0023] Further, the cold equipment in the system is preferably kept within an insulated enclosure (commonly called a "cold box"). [0024] The feed gas devoid of components that freeze at colder temperatures enters the purifier cold box and is typically first cooled against outgoing cold product gas streams and liquids in a multicore exchanger to liquefaction temperatures. [0025] The liquefaction temperature required will depend upon the components to be separated, and particularly the components to be drawn off or removed as a liquid. For the purposes of removing methane as a liquid, the liquefaction temperature will normally be at least below approximately -161.7 'C and preferably above approximately -191.5 'C. [0026] More than one cooling unit can be used as required to achieve the desired temperatures. [0027] One or more knock-out-drum or vapour-liquid separators can be used in the cooling portion of the process in order to separate phases. Preferably, at least two cooling units will be provided with a knock-out-drum or vapour-liquid separator located between the two cooling units. [0028] The resultant cooled syngas stream will normally be a two phase stream. This stream is then treated in the cryogenic distillation unit. [0029] The present invention also includes at least one cryogenic distillation unit with 5 reflux to separate methane in the syngas from other syngas components, forming one or more product gas streams. Essentially, the cryogenic distillation unit is a rectifier, typically a refluxed tray column, directed toward separating the higher boiling methane from other syngas components. [0030] As discussed herein, the key performance metrics (separating methane but minimising slip of CO into the methane stream) are achieved through managing the methane heat pump and setting the reflux temperature at the column top. An appropriately precise control system is therefore preferably provided. [0031] Further optimisation of the process can also be done by multi-level draw offs from the preferred tray column cryogenic distillation unit and/or use of multiple stage cryogenic distillation through use of more than one tray column cryogenic distillation unit to accommodate separation of more energy intensive components, such as nitrogen. [0032] The methane-rich bottom stream from the tray column cryogenic distillation unit after vaporization in feed effluent exchangers is preferably compressed in a methane compressor (preferably of the centrifugal type with dry gas seals) for delivery to downstream reformers. A part of this compressor capacity can be profitably used to serve as a heat pump that at least partially addresses the refrigeration needs of the purifier. [0033] More than one compressor can be included in a compression train in order to achieve the pressure required. Typically, one or more knock-out-drums or vapour-liquid separators can be used in the compression train of the process in order to separate phases that may be generated by compression or prior to entry into the compression train to minimise damage to components. [0034] Alternatively, the methane-rich bottom liquid stream can be pumped (using a liquid cryogenic pump) to the required pressure (to avoid the gas compressor) for the downstream reformer unit and vaporized in a feed effluent exchanger. A part of this stream can be profitably used to serve as a heat pump that addresses the refrigeration needs for the purifier operations. [0035] The refrigeration for the separation duty and heat loss can be provided by several means, such as a gas compressor as described herein or through external supply as liquid nitrogen from an on-site air separation unit, if available.

6 [0036] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention. [0037] In order that the invention may be more readily understood and put into practice, one or more preferred embodiments thereof will now be described, by way of example only, with reference to the accompanying figures. BRIEF DESCRIPTION OF DRAWINGS [0038] Figure 1 is a simplified schematic process flow diagram of a system for separating methane from a syngas stream generated by UCG according to an embodiment of the present invention. [0039] Figure 2 is a flow diagram of a method for separating methane from a syngas stream generated by UCG according to an embodiment of the present invention. DESCRIPTION OF EMBODIMENTS [0040] According to an embodiment of the present invention, a system and method for separation of methane from a syngas stream is provided. [0041] The system and method of the present invention can be used to further treat a syngas stream that has already been treated to remove at least one metal component from the syngas stream (e.g., utilising a metal removal unit), hydrogenate at least one sulphur species present in the syngas to hydrogen sulphide (e.g., utilising a hydrodesulphurization unit), and remove hydrogen sulphide and carbon dioxide (e.g., utilising an absorber unit in which an amine solution absorbs the hydrogen sulphide and carbon dioxide). [0042] As will be understood by one of ordinary skill in the art, treatment of a raw syngas stream as described herein produces a polished make gas product stream, which is preferably the input for the system of the present invention. The system of the present invention will normally form a part of downstream processing for such polished make gas in order to produce economically viable products from the raw syngas from a UCG process. [0043] One preferred form of the system of the present invention is illustrated in Figure 1 and includes a front-end molecular sieve bed 13 in order to remove any component in the syngas 14 that could freeze during cryogenic distillation and a pair of cooling units 17 and 18, 7 provided with a knock-out-drum vapour-liquid separator 19 located between the two cooling units 17 and 18 in order to separate any phases that may be generated during cooling of the feed gas 16. The resultant cooled syngas stream will normally be a two phase stream. This stream is then treated in the cryogenic distillation unit 15. The cooling units 17 and 18 use product gas streams produced later in the process to cool the feed gas 16. [0044] The main process vessel in the system and method of the present invention is a cryogenic distillation unit 15 with reflux system (including one or more colunms, for example, one or more tray colunms) to separate methane in the feed gas 16 from other syngas components into one or more product gas streams. Essentially, the cryogenic distillation unit 15 is a rectifier, typically a refluxed tray colunm, directed toward separating the higher boiling methane from other syngas components. [0045] The main performance metrics for the purifier 15 are maximum rejection of methane from a CO/H 2 stream and minimum loss of CO (boiling point of -191.5 C) with the methane (boiling point of -161.7 C) achieved through managing the methane heat pump and setting the reflux temperature at the column top. [0046] An appropriately precise control system is therefore preferably provided. In the preferred embodiment of the invention illustrated in Figure 1, the reflux temperature at the top of the cryogenic distillation unit 15 is controlled using a pair of overhead condensers 20 and 21 in combination with a reflux drum 22 and a pump/compressor 23. A temperature control loop 24 is provided between the valve 25 on the methane-rich cross flow stream 26 into the second overhead condenser 21 and the temperature of the gas stream 27 exiting the cryogenic distillation unit 15. The reflux drum 22 is provided with a level control loop 28 which is in turn linked to a flow controller 29 and valve 30 located downstream of the pump/compressor 23 on the reflux side of the reflux drum 22 for return of a methane-rich reflux stream 31 to the cryogenic distillation unit 15. [0047] The methane-rich bottom stream 32 from the cryogenic distillation unit 15 proceeds through a subcooler 40 and the first overhead condenser 20 before the cooling units 17 and 18. [0048] The methane-rich bottom stream 32 from the cryogenic distillation unit 15, after vaporization in the cooling units 17 and 18, is preferably compressed in a compression train (which is not shown) including multiple compressor stages, preferably of the centrifugal type 8 with dry gas seals, for delivery to downstream reformers. A part of this compressor capacity is profitably used to serve as a heat pump that at least partially addresses the refrigeration needs of the purifier 15. Heat exchangers are provided after the compression stages to cool the methane stream of heat generated by compression. A control system is provided to prevent surging in the compression train and regulate the flow of product methane 33 returned to the system. [0049] A portion of the product methane 33 after compression can be returned to the purifier 15. Off gas 34 can be sent to a fuel header (which is not shown). H 2 -rich make gas 35 can be used for further downstream processing (e.g., FT synthesis). [0050] The inclusion of purifier 15 at the chosen location in system leads to several benefits to the overall process for conversion of underground coal and residue gasification gases to clean fuels, such as Fischer-Tropsch synthesis products. [0051] For example, the FT make gas processed through purifier 15 is of very high purity, with most catalyst poisons removed to "not detectable" levels. The expected increase in catalyst life is a major cost savings. [0052] Additionally, the reduction of the level of inert substances in the FT make gas limits the purge rate required from the loop. Reduction of purge gas limits downgrading of the reactants (CO+H 2 ) and useful hydrocarbon components to fuel, allowing increased recycle to produce liquid fuels. It also provides the ability to make the best use of this super pure gas, instead of value downgrade through co-product power. Maximum utilisation of the "tail gas" reduces the requirement of fresh feed, allowing considerable capacity reduction (25% to 45%) in the upstream units, thereby reducing overall capital and operating costs. It also obviates the need for distressed sale/use of fuel gas for other purposes, and improves energy efficiency by improving the FT yields. [0053] The separation of methane from the syngas stream also improves front-end operating efficiency by reducing the oxygen consumption by 10% to 35% and reducing the required reforming capacity, as the reformer process only the methane stream (however, in order to allow flexibility in processing alternate feeds, such as natural gas, the reforming capacity could still be kept sufficiently high, by the addition of a pre-reformer). [0054] The raw syngas may carry nitrogen oxides that have a tendency to form explosive 9 gums in cryogenic systems. The continuous removal of the liquid inventory from the cold box makes the system inherently safe by not allowing the gums to accumulate. [0055] Turning to Figure 2, a flow diagram depiction of a method for separating methane from a treated syngas stream generated by UCG, in accordance with an illustrative embodiment of the present invention, is provided. Components of a treated syngas stream 110 that could freeze during cryogenic distillation are removed 120 to produce a dry feed gas stream 130. The dry feed gas stream 130 is cooled 140 and cryogenically distilled 150 to produce a methane-rich product stream 160. [0056] Throughout this specification, unless the context requires otherwise, the words "'comprise", "comprises" and "comprising" will be understood to mean the inclusion of a stated integer, group of integers, step, or steps, but not the exclusion of any other integer, group of integers, step, or steps. [0057] Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. It will therefore be appreciated by those of skill in the art that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention.

Claims (5)

1. A system for separating methane from a syngas stream generated by underground coal gasification (UCG), comprising: a) a source of treated syngas generated by UCG; b) a physical separation unit for removal of any component in the syngas stream that could freeze during cryogenic distillation; c) a cooling unit for cooling the syngas stream using at least one product gas stream from the system; and d) a cryogenic distillation unit with reflux for separating methane in the syngas stream from other syngas components.

2. The system of claim 1, wherein the physical separation unit comprises a molecular sieve.

3. A method for separating methane from a treated syngas stream generated by underground coal gasification (UCG), including the steps of: a) removing components of the syngas stream that could freeze during cryogenic distillation to produce a dry syngas stream; b) cooling the dry syngas stream; c) introducing the dry, cooled syngas stream into a cryogenic distillation column; and d) withdrawing a methane-rich bottom product stream from the distillation column.

4. The method of claim 3, wherein the treated syngas stream generated by UCG has been treated to remove at least one metal component from the syngas stream, hydrogenate at least one sulphur species present in the syngas to hydrogen sulphide, and remove hydrogen sulphide and carbon dioxide.

5. The method of claim 3 or claim 4, wherein removing components of the syngas stream that could freeze during cryogenic distillation comprises utilising a molecular sieve.