Apparatus and method for coal gasification

Abstract

A process and apparatus for converting a coal into a substitute natural gas generates raw gas in a conventional coal gasification unit and passes at least a portion of the raw gas into a partial oxidation unit to convert the at least portion of the raw gas into a secondary raw synthesis gas substantially devoid of higher hydrocarbons. Optionally, the raw gas is quenched and only the resulting condensate is passed to the partial oxidation unit for conversion to the secondary raw synthesis gas.

Description

BACKGROUND OF THE INVENTION

[0001] The invention concerns an apparatus and process for converting coal into a substitute natural gas. More particularly, the invention concerns converting, via non-catalytic partial oxidation, liquid condensates from a primary gasification process into secondary synthesis gas, resulting in utilization of substantially all by-product streams for production of additional raw gas, thereby minimizing undesirable effluent produced by the gasification process.

[0002] Coal gasification technology is well known and has been in commercial use, for example, in South Africa for many years. The most commonly employed coal gasifier is that developed by Lurgi Kohle und Mineraloeltechnik GmbH. The Lurgi process utilizes a fixed bed gasifier into which coal of a selected particle size is fed countercurrently to a stream of steam and oxygen.

[0003] Coal gasification processes are accompanied by the generation of by-products essentially comprised of oil, tar and phenolics. Disposition of this by-product presents significant environmental and economic problems.

[0004] Therefore, there is seen to be a need in the art for an apparatus and process which will utilize essentially all of the by-product streams of a gasification process for production of further raw gas to maximize the synthesis gas produced by the gasification process.

SUMMARY OF THE INVENTION

[0005] Accordingly, a process for converting coal into a substitute natural gas begins by placing a charge of coal into a coal gasification unit and causing gasification of at least a portion of the charge by exposing the charge to a gasifying agent and heat. Primary raw gas is recovered at an outlet of the coal gasification unit and at least a portion of the primary raw gas is passed into a non-catalytic partial oxidation unit where a partial oxidation agent and the temperature is maintained to convert the at least portion of the primary raw gas into a secondary raw synthesis gas substantially devoid of higher hydrocarbons.

[0006] In another aspect of the invention, a process for converting coal into a substitute natural gas begins by placing a charge of coal into a coal gasification unit and causing gasification of at least a portion of the charge by exposing the charge to a gasifying agent and heat in the coal gasification unit. Primary raw gas is recovered at an outlet of the coal gasification unit and subjected to quenching to separate condensable hydrocarbon containing liquid therefrom. The liquid is then subjected to a non-catalytic partial oxidation in the presence of a partial oxidizing agent at a temperature sufficient to convert the liquid into a secondary raw synthesis gas substantially devoid of hydrocarbons other than carbon monoxide, carbon dioxide and methane.

[0007] In still another aspect of the invention, apparatus for converting coal into substitute natural gas includes a plurality of coal gasification units, each operable to cause gasification of at least a portion of a charge of coal fed thereto and to produce a primary raw gas at a gasification unit output. A quenching system having an input coupled to each of the gasification unit outputs receives the primary raw gas therefrom and is operative to separate condensable hydrocarbons in liquid form from the primary raw gas, to deliver the liquid to a quenching system liquid output and to deliver cooled raw gas as the substitute natural gas to a quenching system gas output. A partial oxidation unit having an input coupled to the quenching system liquid output is operative to subject received liquid hydrocarbons to partial oxidation at a temperature sufficient to convert the liquid hydrocarbons into a secondary raw synthesis gas substantially devoid of hydrocarbons at a gas output of the partial oxidation unit.

BRIEF DESCRIPTION OF THE DRAWING

[0008] The objects and features of the invention will become apparent from a reading of a detailed description, taken in conjunction with the drawing, in which:

[0009] FIG. 1 is a block diagram of a coal gasification system arranged in accordance with the principles of the invention;

[0010] FIG. 2 is a block diagram showing a primary gasification unit and a partial oxidation unit coupled via an optional quenching system; and

[0011] FIG. 3 is a block diagram of an exemplary gasification plant using a single partial oxidation unit with four primary coal gasification units and a multi-stage quenching system, arranged in accordance with the principles of the invention.

DETAILED DESCRIPTION

[0012] As used in this description, the term "higher hydrocarbons" refers to hydrocarbons having a composition C.sub.nH.sub.m, where n and m are integers and n is 2 or higher.

[0013] With reference to FIG. 1, a basic block diagram showing the optional arrangements of the invention is displayed. Primary gasifier 102 directs raw gas at its output 110 to either a quench and liquor separation unit 106 via path 110A or via path 110B to a non-catalytic partial oxidizer unit 104. If path 110B is utilized, then the input to partial oxidizer 104 is basically in gaseous form. If the quench and liquor separation unit 106 is used, then the input 116 to partial oxidizer unit 104 is in liquid form, being condensate generated by the cooling process taking place in unit 106.

[0014] It will be apparent to those skilled in the art, that only a portion of the raw gas 110 may be fed via optional path 110B to the partial oxidizer unit 104.

[0015] When the raw gas 110 is subjected to quenching via path 110A prior to being fed to partial oxidizer 104, the resultant generated secondary synthesis gas at output 114 would then be routed via path 114A back to an input of quenching system 106 for further cooling. Otherwise, if quenching is not performed prior to partial oxidation, the secondary raw synthesis gas at 114 may be directed to system output 112 via path 114B.

[0016] What differentiates the instant invention from known gasification processes is the inclusion of a non-catalytic partial oxidation unit 104. Unit 104 produces additional raw synthesis gas from the tars and oils present in raw gas stream 110 in gaseous form or at input 116 in liquid form. Hence, all by-product streams are utilized for the production of raw gas, minimizing the effluent produced from the process.

[0017] Partial oxidizer 104 converts higher hydrocarbons into carbon monoxide and hydrogen and some inadvertent carbon dioxide. This is accomplished at a very high temperature using direct contact with a hot flame burner in a substoichiometric oxygen atmosphere which prevents a vast majority of the generated carbon monoxide from converting to carbon dioxide.

[0018] Direct feed of raw gas 110 via path 110B makes sense in those applications where methane is not desired in the final substitute natural gas end product. However, when using the quench and liquor separation system 106, only the derived liquor is passed to the partial oxidizer and not the raw gas exiting at path 110. In this type of application, methane is usually a desirable component of the raw substitute gas at system output 112 and will be passed directly thereto via system 106 without going to partial oxidizer 104.

[0019] With reference to FIG. 2, a basic arrangement of the primary gasifier, non-catalytic partial oxidizer and an optional quenching system 212 are depicted.

[0020] A coal lock hopper 204 is a pressure vessel and allows the gasifier 206 to be fed in a batch operation. Coal lock 204 has a bottom and top closure which are operated hydraulically. Coal flows through a disposal chute 202 into the coal lock 204 when coal lock 204 is at atmospheric pressure (with the bottom cone enclosed and the top cone open). After coal lock 204 is full, the top cone is closed and coal lock 204 is pressurized with a raw gas taken downstream of the gas cooling unit. Final pressurizing is done through a direct line from the top section of the gasifier 206 to the coal lock 204.

[0021] When coal lock 204 is at the gasifier pressure, a bottom cone opens and coal begins flowing into gasifier 206 via a distributor 208, preferably comprised of a cyclone skirt. When coal lock 204 is empty the bottom cone closes and is ready for recycling.

[0022] Gasifier 206 is a double-walled pressure vessel. High pressure boiler feed water is kept in the jacket formed by the double walls so as to limit jacket and gasifier wall temperatures. High pressure boiler feed water is circulated through the jacket by downcomers. During operation, a considerable amount of heat is transferred from the fuel bed to the jacket. The jacket steam is added to the high pressure steam and the total steam is mixed with oxygen at a ratio of approximately 0.4 pounds of steam per SCF oxygen. This gasification agent is routed via a rotating grate 209 into the gasifier fuel bed. Consequently, grate 209 is cooled by the gasification agent. Grate 209 is powered by alternating current drives and serves to first, enable distribution of the gasification agent on the cross section of gasifier 206 via gasification agent ring slots. The distribution is completed in the ash bed.

[0023] Secondly, grate 206 carries ash towards the ash lock hopper 210, helps disintegrate ash aglomerates and grinds ash lumps to a maximum size to avoid blockages of the ash lock cones. Finally, grate 209 keeps the fuel bed in motion.

[0024] Grate 209 is automatically speed controlled by the oxygen flow to adapt the ash turn out to the ash production. Manual corrections of the grate speed are also possible. The turn-out capacity of grate 209 is determined by the number of plows installed underneath the grate and the speed of the grate 209. Grate 209 runs continuously and is only stopped for short periods when the ash lock 210 cycle begins.

[0025] The gasification agent, for example, a mixture of oxygen at conduit 233 and steam at conduit 231, is passed through the following reaction zones in vessel 206.

[0026] In the ash bed 206A, the gasification agent is superheated by ash leaving combustion zone 206B at a temperature of approximately 2730.degree. F. Under the assumption that a sufficient ash bed 206A is established, the ash is cooled down to a temperature higher than that of the gasification agent while the gasification agent is heated up on entry into vessel 206.

[0027] In a combustion zone 206B, carbon and oxygen are converted to carbon dioxide and heat. The temperatures of the gaseous flow going upwards and the ash (which has a carbon content of approximately 2%) sinking downwards, increases to nearly 2700.degree. F.

[0028] The gas streaming upwards from the combustion zone 206B consists mainly of carbon dioxide and steam and reacts in the gasification zone 206C at an average temperature of approximately 1560.degree. F. The dominating reaction in gasification zone 206C is the conversion of carbon and water into carbon monoxide, hydrogen and heat. A methanation reaction has a minor influence on the composition of the gas leaving gasification zone 206C.

[0029] In a carbonization zone 206D, volatiles in the coal are expelled. The carbonization reaction is heat consuming. Consequently, the gasification raw gas streaming upwards from the gasification zone 206C must heat up the downflowing coal and deliver heat for carbonization. Additionally, in carbonization zone 206D, recycled dusty tar is cracked to oil and coke.

[0030] Ash lock 210 is a pressure vessel having hydraulically operated bottom and top closures. Ash lock 210 serves to remove ash from gasifier 206 and is operated in cycles, each having the following steps.

[0031] The continuously running grate 209 turns the ash out of gasifier 206 through hydraulically actuated top cone into ash lock 210 with its bottom cone closed at gasifier pressure. As soon as ash lock 210 is full, grate 209 is stopped. After the ash lock top cone is shut and sealed, grate 209 is restarted.

[0032] Ash lock 210 is then lowered to atmospheric pressure, the bottom cone of the ash lock 210 is opened and ash flows out of ash lock 210 into a sluice-way 253 where it is quenched and hydraulically carried away to an ash plant.

[0033] Raw gas generated by gasifier unit 206 exits at conduit 235 and is directed to optional quenching system 212 whose output 237 contains liquid condensates from the quenching process. Alternatively, in a direct feed system quenching system 212 is not utilized and raw gas is fed to partial oxidizer unit 266.

[0034] In the optional scrubbing/cooling apparatus 212, excess water, certain condensable hydrocarbons and a small quantity of solids are separated from the raw gas stream in conduit 235. The liquid stream in conduit 237 serves as a feedstock to the partial oxidation unit 216.

[0035] In the non-catalytic partial oxidation unit 216 the feedstock reacts with oxygen in the presence of steam as a moderator to raw synthesis gas. Hot raw gas is cooled by direct injection of water in quench pipe 218 and quench vessel 220. A separation vessel 222 carries away slag at its output 249.

[0036] Non-catalytic partial oxidation unit 216 partially oxidizes the heavy fractions at a temperature of approximately 2500.degree. F. and at a pressure of about 435 psig. Oxygen at conduit 239 and steam at conduit 241 are added to the incoming feedstock in conduit 237 via a partial oxidation burner unit 214. Burner 214 ensures intensive mixing of the feed, which is necessary for a high conversion into the desired raw synthesis gas.

[0037] In an injection zone of reactor 216, the feed is partly oxidized in the flame of burner 214. The gross reactions in unit 216 essentially convert higher hydrocarbons to carbon monoxide and hydrogen in two phases.

[0038] In a first heating and cracking phase, feed and oxygen leave burner 214 at respective preheating temperatures. Prior to actual combustion, the reactants are further heated by the heat reflected from the flame and glowing brickwork of vessel 216. The high hydrocarbons of the feed crack into radicals.

[0039] Next, in a reaction phase, on reaching ignition temperature, a portion of the hydrocarbons react with the oxygen in an exothermic reaction forming carbon dioxide and water. Practically, all of the oxygen available is consumed in this phase. The non-oxidized portion of the hydrocarbons reacts with steam and the reaction products are mainly carbon monoxide and hydrogen.

[0040] As mentioned previously, feedstock, oxygen and steam enter reactor 216 via burner 214, which is mounted at a top portion of reactor 216. Burner 214 preferably has a four-nozzle design with a central dummy tube which takes up the start-up burner. At start-up, the central tube bears the ignition and start-up burner, which is equipped with a flame control sensor. For heating up reactor 216, plant air and fuel gas are fed via the start-up burner at conduit 243. At higher temperatures, when higher heating duties are required, air and fuel gas are also fed via four lances of the burner. At a reactor temperature well above the self-ignition temperature of the fuel gas, normally at about 1470.degree. F., the start-up burner is removed and replaced by a steam-purged dummy. During the final heating up to approximately 2200.degree. F., fuel gas and air are fed through the four burner nozzles.

[0041] To maximize the volumetric capacity of partial oxidation reactor 216, fine coal may optionally be added to the liquid feedstock via lance burner 214.

[0042] Burner 214 is cooled with cooling water and by the media passing therethrough. The reactor outlet to quench pipe 218 is also cooled in order to minimize refractory wear at this point.

[0043] The conversion of hydrocarbons by partial oxidation occurs in refractory lined reactor 216. The refractory material is selected according to ash load and ash properties of the feed stock. The ash must melt at the reactor operating temperature to guarantee free flow of molten ash from reactor 216 to the quench vessel 220 and to avoid blockage of reactor 216 and quench pipe 218.

[0044] Hot raw secondary synthesis gas from reactor 206 is routed via quench pipe 218 to quench vessel 220. The secondary synthesis gas is instantaneously cooled from about 2470.degree. F. to an equilibrium temperature of approximately 430.degree. F. by water injection. Liquid slag flowing with the gas solidifies into particles. The particles could be leached in either an acid or alkali medium. Gas is separated from surplus quench water and slag particles below quench pipe 218 and the quench vessel 220. Gas is withdrawn through a separate nozzle and the slag water collected is routed via level control to slag separator 222. The slag is separated from the soot water leaving the quench vessel via conduit 251 in a slag-like system having slag separation vessel 222. The heavy slag particles settle from the soot water in the slag separation vessel 222 and are collected in a bottom cone for discharge via conduit 249.

[0045] Collected soot and ash may be mixed into a soot slurry and sent to a metal ash recovery system where the soot slurry is flashed to atmospheric pressure in a slurry tank. The slurry is filtered resulting in a filter cake and clear water usable for quenching and scrubbing operations.

[0046] FIG. 3 presents a block diagram of a gasification plant using a single non-catalytic partial oxidation unit with four primary coal gasification units. This type of arrangement takes advantage of the fact that only the pyrolysis products from the raw gas generated from the primary gasification units are being partially oxidized. Hence, one needs only the capacity of a single partial oxidation unit for several (up to five) primary fixed bed degasifier units.

[0047] As seen from FIG. 3, sources of coal to be gasified 301a-d are respectively fed to a disposal chute 350 of each of four fixed bed gasifier units 302a-302d. From input chute 350 the coal is passed into input lock hoper 352 which is coupled to the processing vessel. Near the top at the inlet to the processing vessel a cyclone skirt 354 assists in distributing the coal charge which flows downwardly through the vessel countercurrent to the flow of the gasification agent supplied to respective inputs of vessels 302a-d from oxygen source 305 and steam source 307. A rotating grate 356 distributes the gasification agent and processes ash within the system as described above with respect to FIG. 2.

[0048] Raw gas from the process is collected at outputs 303a-d which are coupled together at conduit 313 as a gaseous input to quenching system 306. As mentioned above, the gasification agent is a mixture of oxygen and steam.

[0049] Quenching system 306 is comprised of a serial connection of five heat exchange units 308, 310, 312, 314 and 316. The initial stages 308 and 310 receive a relatively high temperature gas input and generate high pressure steam from the heat exchange process. Condensates from 308 and 310 exit the units at liquid outputs 309a and 309b and are comprised primarily of thicker tars and ash.

[0050] Subsequent stages of the quenching system 306 result in generation of medium pressure steam and the condensation of lighter oils. As seen from FIG. 3, the gas output of each stage is connected to the gas input of a succeeding stage up until the final stage 316 whose output 311 forms the primary system output carrying cooled raw substitute natural gas.

[0051] Hence, gaseous output 317 is coupled to an input of heat exchanger 310 whose gaseous output 319 is in turn coupled to the input of heat exchanger 312. Gaseous output 321 of unit 312 is connected to the input of unit 313 and output 323 of unit 314 is coupled to an input of the final stage heat exchanger unit 316.

[0052] The liquid outputs 309a-e are coupled together at one primary quenching system liquid output 309.

[0053] Liquid hydrocarbons in conduit 309 are coupled to an input of non-catalytic partial oxidation unit 304. Oxygen from source 305 and steam from source 307 are additionally coupled to the input of unit 304 and the secondary raw synthesis gas comprised mostly of hydrogen and carbon monoxide exits unit 304 via conduit 315 and directed back to an input 313 of the quenching system 306 for further cooling.

[0054] Plant output 311 therefore contains quenched raw substitute natural gas at approximately 392.degree. F. which is essentially free of the pyrolysis products generated in the primary gasifiers 302a-d.

EXAMPLE

[0055] Unreactive coal (i.e., coal containing more than about 30% non-combustible contaminants) having ash content up to about 50 wt./% is fed to a primary gasifier arranged as shown in FIGS. 2 or 3 and a primary raw gas is produced at the output of the primary gasifier having a composition by volume percent of 28.2% carbon dioxide, 0.05% hydrogen sulfide, 0.69% higher hydrocarbons, 22.66% carbon monoxide, 38.51% hydrogen, 9.5% methane and 0.39% nitrogen. Raw gas as produced above is then directed to a quenching system wherein the products of pyrolysis and other liquids are condensed out of the gas stream and passed to the input of a non-catalytic partial oxidation unit running at a reaction temperature of about 2578.degree. F. The raw gas from the gasifier is then reformed in the partial oxidation unit, cracked and hydrolysed, and the established process in gasification reaction conditions result in the following typical gas components at the output of the partial oxidation unit expressed by volume percent: 1.9% carbon dioxide, 0.08% hydrogen sulfide, 52% carbon monoxide, 45.1% hydrogen, 0.3% methane and 0.62% nitrogen.

[0056] The application has been described with respect to a specific embodiment for the sake of example only. The scope and spirit of the invention are to be determined from appropriately interpreted claims.

Claims

1. A process for converting coal into a substitute natural gas comprising: placing a charge of coal into a coal gasification unit; causing gasification of at least a portion of the charge by exposing the charge to a gasifying agent and heat in the coal gasification unit; recovering primary raw gas at an outlet of the coal gasification unit; and passing at least a portion of the primary raw gas into a non-catalytic partial oxidation unit, adding a partial oxidation agent, and maintaining a temperature effective to convert the at least a portion of the primary raw gas into a secondary raw synthesis gas substantially devoid of higher hydrocarbons.

2. The process of claim 1 further comprising adding the secondary raw synthesis gas to the primary raw gas.

3. The process of claim 1 wherein the at least a portion of the primary raw gas is subjected to quenching to separate condensable hydrocarbons therefrom for transmittal to a non-catalytic partial oxidation unit to convert the condensable hydrocarbons into a secondary raw synthesis gas substantially devoid of higher hydrocarbons.

4. The process of claim 1 wherein the partial oxidation agent comprises a mixture of oxygen-containing gas and steam.

5. The process of claim 1 wherein substantially all higher hydrocarbons present in the at least a portion of the primary raw gas are cracked and hydrolized in the non-catalytic partial oxidation unit.

6. The process of claim 3 wherein substantially all higher hydrocarbons present in the condensable liquids separated from the primary raw gas are cracked and hydrolized in the non-catalytic partial oxidation unit.

7. The process of claim 1 wherein substantially all of the primary raw gas is passed into the non-catalytic partial oxidation unit.

8. The process of claim 3 wherein substantially all of the primary raw gas is subjected to quenching prior to transmittal of separated condensable hydrocarbons to the non-catalytic partial oxidation unit.

9. A process for converting coal into a substitute natural gas comprising: placing a charge of coal into a coal gasification unit; causing gasification of at least a portion of the charge by exposing the charge to a gasifying agent and heat in the coal gasification unit; recovering primary raw gas at an outlet of the coal gasification unit; subjecting the primary raw gas to quenching to separate condensable hydrocarbon containing liquid therefrom; and subjecting the liquid to non-catalytic partial oxidation in the presence of a partial oxidizing agent at a temperature sufficient to convert the liquid into a secondary raw synthesis gas substantially devoid of hydrocarbons other than carbon monoxide, carbon dioxide and methane.

10. The process of claim 9 further comprising adding the secondary raw synthesis gas to the primary raw gas.

11. The process of claim 9 wherein the partial oxidizing agent comprises oxygen and steam.

12. The process of claim 9 wherein the temperature to which the liquid is subjected is sufficient to crack and hydrolize substantially all higher hydrocarbons present in the liquid.

13. The process of claim 12 wherein the liquid is subjected to partial oxidation at a temperature of from about 2372.degree. F. to about 2732.degree. F.

14. The process of claim 12 wherein the liquid is subjected to partial oxidation at a temperature of about 2578.degree. F. and at a pressure of about 400 psig.

15. The process of claim 9 wherein the charge is comprised of coal having at least about 30% by weight non-combustible contaminants.

16. The process of claim 15 wherein the primary raw gas comprises about 28% by volume carbon dioxide less than about 1% by volume hydrocarbons, about 23% by volume carbon monoxide, about 38.5% by volume hydrogen, and about 9.5% by volume methane.

17. The process of claim 16 wherein the secondary raw synthesis gas comprises less than about 2% by volume carbon dioxide, greater than about 50% by volume carbon monoxide and greater than about 45% by volume hydrogen.

18. The process of claim 9 wherein the charge is comprised of coal having ash and non-combustible contaminants of up to about 50% by weight.

19. The process of claim 9 wherein the charge is comprised of coal having an oxygen content of up to about 3% by weight.

20. Apparatus for converting coal into substitute natural gas comprising: a plurality of coal gasification units, each operable to cause gasification of at least a portion of a charge of coal fed thereto and to produce a primary raw gas at a gasification unit output; a quenching system having an input coupled to each of the gasification unit outputs for receipt of primary raw gas and operative to separate condensable hydrocarbons in liquid form from the primary raw gas, to deliver the liquid to a quenching system liquid output and to deliver cooled raw gas as the substitute natural gas to a quenching system gas output; and a partial oxidation unit having an input coupled to the quenching system liquid output and operative to subject received liquid hydrocarbons to partial oxidation and a temperature sufficient to convert the liquid hydrocarbons into a secondary raw synthesis gas substantially devoid of higher hydrocarbons at a gas output of the partial oxidation unit.

21. The apparatus of claim 20 wherein the gas output of the partial oxidation unit is coupled to the quenching system input.

22. The apparatus of claim 20 wherein each gasification unit comprises a fixed bed gasifier having an input lock hopper having an input coupled for receipt of a coal charge and an output; a pressure vessel having a coal input coupled for receipt of coal from the input lock hopper output, a coal distributing cyclone skirt in the vessel coupled to the coal input, and a rotating grate positioned in a combustion zone of the vessel for combusting a portion of the coal and distributing ash towards a solids output of the vessel, and a gasification input for receipt of a gasifying agent coupled to the rotating grate; and an ash lock hopper having an input coupled to the solids output of the vessel.

23. The apparatus of claim 20 wherein the quenching system further comprises: a plurality of serially connected heat exchanger units, each having an input for receiving an input gas, a condensate output for presenting liquid condensed from the input gas, and a gas output presenting gas cooled by the heat exchanger unit, wherein the condensate outputs of the plurality of heat exchanger units are coupled together to form the quenching system liquid output, and the gas output of each heat exchanger unit is coupled to the input for receiving gas of a succeeding heat exchanger unit, except for the last heat exchanger unit in the serial connection, whose gas output comprises the quenching system gas output.