AU608988B2 - Pyrolysis - Google Patents

Description

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xl: ~88 FORM PATENTS ACT 1952 COMPLETE P E C I F I C A T I ON FOR OFFICE USE: Class Int.Class Application Number: Lodged: Complete Fjpecification Lodged: Accepted: Published: Priority: This document contains te amcndinents made under Sction, 49 and is correct for ing. 1Aelated Art: oe 0 00 Goo o Name of Applicant: d 4~ 0001* Go 00 Address of Applicant: *0 u Actual Inventor:

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Fc(1>D A.S c r P. LIMlLt7I) PINE ER Z N TERNAZNAL-LXED '0 -1-ltb--F:loo t7-55-Macqu ar:e-S t r-eet-Sydne, -New-South-Wa-lees -2 00n ,-Austxali a Le es( ilam coi Souflas Ban0y Charles$ Williagn. oulas Blandy Address for Service: SHELSTON WATERS, 55 Clarence Street, Sydney 0 0 1* a Complete specification for the Invention entitled:

"PYROLYSIS"

t 00( The following statement is a full description of this invention,including the best method of performing it, known to us:- (Complete of P18909 dated 22nd June, 1988) I 4, This invention relates to a process for manufacture of hydrocarbon aromatic chemicals and carbon products from coal.

The classic process for the production of liquid hydrocarbons from coal involves hydrogenation. Coal is first ground and washed to remove non-carbonaceous materials. The carbonaceous residue is then hydrogenated in the presence of catalysts and at pressures in excess of 200 atmospheres and typically around 700 atmospheres at approx. 48 0 0C to 490 0 C. The classic process is usually 0 conducted in stages, distillable oils being separated from S. tars in a first stage and being further hydrogenated in a 0 the vapour phase. As some oil is lost together with solid residues which are typically subsequently carbonized, it °C is important to use coals of ash content of about 20% w/w 000° in the classic process.

a 0* It has also been practiced to pyrolyse coal for the 0000 production of petro-chemicals tars and/or residual char, Thermal decomposition results in the formation of *eoo 66°°20 condensible organic liquids (tars and light oils), non-condensible gases, and solid residue ("char").

In most pyrolysis schemes the char which is a substantial portion of the products is envisaged as a replacement for coal in utility boilers and the like. The viability of the process therefore depends upon the tars and carbon products made from tar by further distillation and light oils being of sufficient quality and value to subsidize the char &nd enable the char to be sold at a price near to or below that of coal, 2 To provide a sufficient incentive for a coal burner to switch to char as a fuel the market price of the premium products (tars and oils) must cover at least capital and operating costs of pyrolysis, any product upgrade costs and any costs of substituting char for coal.

Extensive studies of the Lurgi process were conducted between 1984 and 1986 (Reported in March EPRI Journal 1986 at Page 36) using Utah coals. The resulting chars had volatile contents of from 4 8.4% and high ash contents (27 33%) and suffered fouling problems on firing. The studies concluded that coal pyrolysis was not a viable .0 route for electric utilities to pursue and that the t O process resulted "in an adequate fuel, coal, being turned

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into an unusable liquid and a marginally acceptable coal substitute".

S o The use of flash pyrolysis techniques for thermal decomposition of coals has been intensively investigated by the CSIRO but to date has not resulted in development beyond pilot scale levels. CSIRO reports envisage a system employing two pyrolysis chambers. Part of the r product char of the first chamber is burnt in the second chamber to heat fluidized sand which is circulated between the vessels to pyrolyse coal in the first. Pyrolysis conditions are optimized for the production of tar at around 600 0 C and of parrafinic hydrocarbons. Gases from the pyrolyser pass through a hot cyclone (to collect the bulk of the char without condensing tar), then to a quench scrubber where the tar is collected in a scrubbing oil.

The tar/oil mixtures are pumped to a continuous 3 9*00 0000 fee hydrogenator and gases are flared.

The flash pyrolysis processes so far proposed have had a coal to oil conversion ratio of less than about and have apparently not indicated an economically attractive product mix, or at any rate have not attracted capital.

Pyrolysis processes have been considered particularly unattractive in relation to feed coals having a high content of mineral matter. For example, MILLTAERRAN coal, as mined from the Commodore deposit of the Walloon type, has ,in ash content of up to 38%. As herein used, the term "ash contetat" refers to ash expressed as a percentage w/w of coal on an air dried basis. Proposals for flash pyrolysis of such coals have involved washing to reduce the mineral levels prior to pyrolysis. This involved crushing the coal to pass a 30mm screen, wet screening at 0.5mm, and processing of +0.5mm material in a dense medium cyclone circuit for further separation. However, those steps only reduced the ash content of the coal to about and have a relatively low efficiency (approximately to The high remaining mineral content of MILL~ME1RRAN c'oal results in product char having a very high ash content (around 35% About 50% w/w d~a.f. coal of the product of flash hydrolysis was char. Only a part is utilized for heat in the process. The balance is pyrophoric and must be disposed of. Such chars are of limited commercial value being of use only for fuel, and, in view of the high ash, being undesirable even for that 4purpose. This in turn adversely effects the economics of the pyrolysis process especially since the char may contain a significant quantity of tars. Furthermore, the tars recovered from the pyrolysis of MILLMERRAN coal by the CSIRO process also contain ash levels too high to make high quality products such as anode carbons and pitch.

The present invention consists in a process for pyrolysis of coal comprising the steps of: pulverising a feed coal; separating minerals from the pulverised coal so as to o reduce the ash content of the coal to below flash pyrolysis of the coal from step 2; 00O a°°o recovering the volatile pyrolysis products from step, o0 0 9000 oo 3 and 3900 o O recovering the char remaining from the pyrolysis.

For preference a Walloon coal, or sub-bituminious coal of similar type, is selected and the ash content is ooj reduced to below 6% as hereinafter described prior to 000 o 0 pyrolysis with greater than 80% of inherent carbon in the 20 rw coal being recovered,, a o It has not previously been suggested to deash coal to below 10% in combination with flash pyrolysis. The a resulting low ash tar Is suitable for manufacture of toao 1 aluminium anode carbon, pitch and other products which have not previously tbeen obtainable from flash pyrolysis of high ash coals because of the high ash ca:ry over.

In prferaed embodiments of the invention the flash pyrolysis step is optimized for the production of olefins. Thus while previous proposed flash pyrolysis tv t3 processes were optimized for production of paraffinic hydrocarbons and produced char which was at best an inferior (high ash) fuel, preferred embodiments of the present invention maximize production of high value olefins and result in a higher value char and tar products than previously proposed processes.

An embodiment of the invention will now be described by way of example only.

For use in the present process it it' lesirable to ,aoQ select a coal which contains a high percei 'ge of 0 a o volatiles and a high ratio of hydrogen to carbon.

0oo a 0 0a Desirably, a Walloon type coal is used, for example 09 0 0 MILLMERRAN coal from the Comodore deposit. This coal has w/w d.a.f. coal of volatile matter and a high ratio o of hydrogen to carbon (close to A typical analysis 90 a of the MILLMERRAN coal is given in Table 1.

TABLE I ANALYSIS MILLMERRAN COAL 0a0o*0 WHOLE SEAM INCLUDING DIRT BANDS Air-dried basis w/w) Moisture 3.9 Ash 33.0 Volatile matter 35.3 Fixed Carbon 27.8 Specific energy (MJ/kg) 20.2 Dry ash-free basis w/w) Volatile Matter 55.9 Fixed carbon 44.1 Specific energy (MJ/kg) 32.0 0 0000 0 00 t 000 000 OqOO 00 00 I3b a 0 I*0 0 0 00 0 0,6 0 o*2 Importantly, when treated according to the invention selected coals produce a high yield of olefinic compounds. Also desirably the selected coals exhibit low oxidation levels.

The MILLMERRAN Coal as mined typically has an ash content of 30 38%. Conventional commercial coal washing does not reduce the ash content much below According to the present invention the coal is first beneficated to reduce the ash content to below 10% and preferetbly to below 6% while recovering around 95% w/w d.a.f. of the combustible coal.

This may be achieved for example by the following steps: The coal is first pulverized by conventional means so that 90% or more is smaller than 200 microns.

The coal particles are then slurried with water.

The slurry is then treated with sufficient intensity to cause ash to break away from the coal particles for example by ultrasonic techniques.

A small quantitiy (less than 10% and typically about of a suitable mineral oil (for example a #2 light oil) is then added together with selected chemicals to make the coal particles "oleophilic" and the ash particles "hydrophilic" to cause agglomeration of the oil and coal portions of the mixture.

The agglomerates of oil and coal are then separated from the water and ash for example by a screening process. If necessary, water may be removed by centrifuging or by heat or a combination thereof.

-7i i ir C CC C 1 9C C 4St 0 tU go S gOet 0000 00 0 60 a 0 St 00 V 004 00 0 0 0O 0 0~

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05 551065 S c i The beneficiated coal so obtained has an ash content preferably in the range of from 1% to 6%.

A suitable method for reducing the ash content of the coal to below 6% is described in more detail in Australian Patent 538, 626 but any other method of beneficiation able to reduce ash content to below 10% may be used.

The beneficiated coal is next subjected to flash pyrolysis for example in apparatus of the kind described in Australian Patent 531,008. The ash reduced coal may be fed to the flash pyrolysis unit as an aqueous slux:y, or as an emulsion in which oil coated coal powder is in the aqueous phase or in any other suitable form.

In the flash pyrolysis unit the ash reduced coal, suspended in an inert carrier (for example nitrogen or process tail gas), is fed to a fluidized sand bed in a reactor maintained at elevated temperature. The volatiles are very rapidly removed from the reactor to an exhaust system and product recovering plant. Product recovery includes a cyclone whereby char is recovered; coolers; either an electrostatic precipitator or an oil quench and scrubbing system for recovering heavy oils and tars; a water quench and scrubbing system for recovering light oils and the BTX aromatics; and compressors for collecting and pressurising the valuable gas components.

The compressed gases are scrubbed to remove sulphur compounds and carbon dioxide, and water is removed by cooling and drying. The remaining components of the gas are then separated by further cooling and compression 8 combined with fractionation to ensure the desired purity of the products.

The yield of products from flash pyrolysis is influenced by factors such as heating rate, temperature and residence times of the liberated volatiles and coal particles in the pyrolysis zone. The requirements for flash pyrolysis are very high heating rates for coal particles (typically 10 4 0C per second or more) and low residence time of volatiles in the pyrolysis zone (i.e.

rapid removal and quenching).

a o Previous pyrolysis of MILLMERRAN coal has been 0 0 conducted at around 600°C under conditions chosen to a maximize the yield of tar and light parrafinic hydrocarbons and to prevent excessive thermal cracking of those products.

00". According to the present invention the temperature, residence time and other pyrolysis conditions are ago* selected so as to maximize the production of olefine and/or aromatic products. This is achieved by conducting 0i 0oo*26 the pyrolysis at temperatures above 8000C, preferably 8200C to 8700C and typically 8200C to 850°C.

For feed coals relatively rich in light volatile products pyrolysis temperatures in the vicinity of 870°C are preferred while for feeds relatively rich in heavy volatile products pyrolysis temperatures in the vicinity of 8200C are preferred.

Generally speaking a very short residence time at the high temperatures is desirable and in preferred 9 embodiments of the invention the products are rapidly cooled to 600 0 C or lower for example by means of quench boiler heat exchangers.

The residence time in the reactor is less than seconds and preferably below about 0.5 seconds. The short residence time in the reactor followed by rapid initial quenching is selected to ensure a maximum possible yield of olefins and aromatics.

The presence of moisture in the feed coal also BoQ increases yields and reduces coking on surfaces.

0 So Table 2 shows products obtained at a reactor 00 9 9 "0 temperature of 810 0 C. The products are expressed as 0Q 0 0 ODO percentage by weight of weight of feed coal on a dried ash-free coal basis.

oo, TABLE 2 o0 9 0 0 0 0 Reactor Temperature 810 0

C

aD* Gas flow rate in Pyrolyser (m 3 /h at NTP) Wet gas 32.1 Dry gas 27.5 Total Tar Approx. 23% Total Volatile Matter Approx. 54% 000o CH4 6.10% o2 C2H4 6.10% C2H6 1.00% C3H6 2.60% C3H8 0.20% Total Cl C3 16.00 The Walloon type coals when pyrolized at temperatures higher than 810°C e.g. at 825 8500C give a higher yield of olefins (about 10%) than shown in Table 2. When pyrolysis conditions are adjusted to optimize 10 olefin yield, sale of liquefied gases without further hydrogenation is possible.

In order to ensure that the gas remaining after separation of the C2 C4 olefins and parrafins is of high quality it is desirable that the fluidized bed may be recirculated between two reactors, the sand being raised to pyrolysis temperature in one reactor, preferably by combustion of the least valuable combustable pyrolysis product or products. The pyrolysis process is thus able to be carried out in the other Soo reactor and so the pyrolysis product is not contaminated oo t So with combustion products.

ac qt i: In the present example, it is proposed to use surplus gases after liquefaction of the hydrocarbons for power and/or steam genert.tion. Such surplus gases S"Oo include methane, hydrogen, carbon monoxide and ammonia, 0 The tar obtainable from the process are free of ash to a degree not previously contemplated and are therefore useful for the production of anode carbon b20 pitch, for aluminium production and other products by further distillation and other procedures which could not previously be obtained from flash pyrolysis processes.

This is because the ash carry over of previous processes has been nearly eliminated.

VThe low ash char (ie 2-15% ash in char) can be pelletized using tar from the pyrolysis as a binder and to prevent auto-ignition of the reactive char. The pelletized char may be marketed as such, or for 11

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00 0 009C Go 00 01 0 0 0 a0of 00 preference is recycled being used as fuel for the same or another pyrolysis unit.

The pelletized char being of low ash may be used in steel making by injection directly to a blast furnace, may be used in the COREX process or other prc'cesses requiring ash free coke. It may also be burnt in power stations having the advantage of low environmental impact.

Char pelletized with tar is also suitable for combustion in a cement kiln, any residual mneral going into the cement product.

Because the product is low ash, the ash free tar can be recycled as feed to the pyrolysis.

As will be apparent from the above description, the process is applicable to other Walloon Valley coalsa and indeed to other coals, lignites and carbonaceous material, but has most advantage when mineral content of the feed is high.

The pyrolysis can be conducted in any shaped vessel or tube, pipe, cyclone or reactor, Materials other than sand can be utilized to provide a fluidized bed fot rapid pyrolysis of feed coal. For example, minerals separated from the coal may be used or the char product itself may be substituted for sand, or an alumina or other refractory may be employed.

The use of finely cominuted coal feed results in very rapid pyrolysiz and facilitate$ rapid removal of products, from the reactor. This has been found to promote high BTX aromatics and olefin yields. Further 1.2 increases in yields of aromatics and olefins is envisaged by separating and steam cracking product C2 C4 Parrafins, and recycling and flash pyrolysis of h and light oils. This would not be possible with tar products derived from high ash coals.

A typical flow chart is given in Figure 1 by way of example only. Coal feed 2 is first selected. The coal is pulverized 2 and treated 3 to remove ash 4 leaving a oa*, beneficiated coal 5 having an ash content of below 0 00 OAQ preferably below The beneficiated coal is subjected 0 0000 to flash pyrolysis 6 at from 810 8700C. The pyrolysed

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product 7 is first directed to a char separating stage 8, for example a cyclone, wherein char 9 is separated to char storage, The remainder 10 is fed to an oil quench 1ii wherein pitch oils and tars 12 are separated to storage and remainder 13 is fed to a water quench stage 14.

0 f In the water quench stage, pyrolysis gasolene and light distillates 15 are removed to storage while the remainder 16 is compressed and fed to a caustic washer 17 wherein sulphur 18 (together with other polymers formed) and carbon dioxide are removed. The remainder 19 is dried 20 to remove water 21 by cooling and separation and/or by molecular sieves. The dried stream 22 is compressed and cooled to liquefy and subsequently separate the various olefinic and parrafinic hydrocarbons which may be stored as fuel gas or may be separated to a required degree by fractionation. Ethane and Propane Way 13 recovering a low ash char remaining from the pyrolysis.

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be crack~ed to produce ethylene in addition to that obtained from the pyrolysis.

As will be apparent to those skilled in the art from the teachijIg hereof j process conditions may be varied in a manner readily determinable by routine trial without departing from the t8cope of the invention herein described.

*fee C, a 4 99 14

Claims (5)

1. A process for pyrolysis of coal comprising the steps of: pulverising a feed coal; separating minerals from the pulverised coal so as to reduce the ash content of the coal to below flash pyrolysis of the coal from step 2; recovering the volatile pyrolysis products from step 3 and recovering a low ash char remaining from the goof 10 pyrolysis.

2. A process according to Claim 1 further including the ogC S: C step of recovery of a tar with negligible ash content o ai from the volatile pyrolysis products. 0 9o 0000 o Oo 3. A process according to Claim 1 or 2 further comprising the step of selecting the conditions for flash pyrolysis so as to optimize the yield of olefinic and/or y aromatic compounds. S4. A process according to any one of the preceding 0 a claims further comprising the step of selecting as the, feed coal a coal of the Walloon type. S, 5. A process according to any one of claims 1 to 3 comprising the step of selecting as the feed coal a ,0 4 MILLMERRAN coal.

6. A pruocess according to any one of the preceding claims wherein the step of flash pyrolysis is conducted at a temperature of from O10°C to 870OC. 7, A process according to any one of the preceding 15 ~C i i i: C 4 claims wherein the step of flash pyrolysis is conducted at a temperature of from 825°C and 850°C.

8. A process according-to any one of the preceding claims wherein the char is pelletized using tar from the pyrolysis as a binder. 9, A process according to any one of the preceding claims wherein olefines Zre separated from said recovered volatile pyrolysis products. A process according to any one of the preceding claims wherein parrafins from the volatile pyrolysis O00S products are separated from the product stream and a oo0e cracked to produce olefins. 00 S 11. A process according to any one of the preceding 8000 0 0 aclaims wherein the ash content of the coal from step 2 is 00 o0 ap ,o below 6%.

12. A process subtantially as herein described with reference to Figure 1. 0 qa DATED this 20th day of June, 1989 re PIONEER INTERNATIONAL LIMITED 8,!t Attorney: IAN T. ERNST Fellow Institute of Patent Attorneys of Australia of SHELSTON WATERS a t t ii 16