AU601983B2 - Coal treatment process - Google Patents

Description

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COMMONWEALTH OF AUSTR; ONE HUN Patents Act 1952 COMPLETE SPECIFICATION (Original) Class FES AMP TO VALUE OF AjACHED MAIL OFFICER.. Int. Class Application Number: Lodged: PH 06274 05.06.1986 Complete Specification Lodged: Accepted: Published: Priority: Related Art: This document contains the 'amendments made under| S;ection 49 and is correct 0o; Srinting.

Name of Applicant: SWINBURNE LIMITED and UNIVERSITY OF MELBOURNE Address of Applicant: John Street, Hawthorne, Victoria, Australia and Grattan Street, Parkville, Victoria, Australia respectively Actual Inventor: DAVID E. MAINWARING, GREGOR B. CHRISTIE, DAVID V. BOGER and YEE-KOWONG LEONG Address for Service: EDWD. WATERS SONS, QUEEN STREET, MELBOURNE, AUSTRALIA, 3000 *P

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Complete Specification for the invention entitled: COAL TREATMENT PROCESS LODGED AT SUB-OFFICE 4 JUN 187 Melbourne The following statement is a full description of this invention, including the best method of performing it known to us.

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;.i prescribed by its Articlex of Amoclatlcn. D. MISCHLEWSKI THE COMMISSIONER OF PATENTS. Registered Patent Attorney Edwd. WAttrs ons. 2 COAL TREATMENT PROCESS This invention relates to a process of treating low rank coals to improve their utility as fuels.

Low rank coals have not been able to replace high rank coals in some fuel applications such as coal water mixtures. Low rank coals have inherently high structural porosity and hence high moisture contents and low density when dried. Conventional drying practices lead to products of low volumetric specific energies due to remaining structural porosity. Such products are characterized by a general instability over a range of characteristics from abrasion to spontaneous combustion. These problems have lead to a range of densifying techniques that range from briquetting through alkali digestion to hot water drying.

C. Advances in drying processes for low rank coals have been largely aimed at improving energy efficiency beyond that of conventional evaporative thermal drying.

These range from non-evaporative processes such as high pressure Fleissner drying and hot water drying to low energy solar drying.

It is an object of this invention to provide an effective method of treating low rank coals to provide a hard, stable and water inert product. Water inertness is a property which means that the coal will not reabsorb water if immersed in water for an extended period. This is a preferred mode of conveying coal as fuel by forming a coal water slurry which is pumpable and easier to convey than dry product and avoids dust creation.

To date only high rank coals have been used in coal water mixtures because of the high porosity of treated low rank coals.

To this end the present invention provides a method of treating low rank coal comprising digesting a slurry of low rank coal with an hydroxide of ammonia, sodium, calcium and/or potassium in an amount sufficient to provide a pH of at least 10 and subsequently thermally modifying the coal T 0 L7 7 t I 9 9 99 09 r* 9 9 999 t 99I* 3 within the temperature range of 280 0 C to 360 0 C to produce a project having low porosity and low water take up. A preferred method of treating low rank coal comprises the steps of:forming a slurry of low rank coal having a solids content up to (ii) adding sufficient ammonium, potassium, sodium and/or calcium ion to provide a pH of 10 or above.

(iii) drying the slurry.

(iv) subjecting the dried slurry to a thermal treatment to complete drying and to release the ammonia and decompose oxygen functional groups in the coal.

recovering the ammonia for re use in the process.

(vi) recovering an improved coal product.

The uptake of ammonium ions by solids and their subsequent thermal decomposition to ammonia and protons on the surface is a well known technique for decationation of materials Mainwaring Proc. RACI Vol 40, 293, 1973).

The ammonium ion will undergo an acid base reaction with the carboxylic acid groups and displace the metal ion from the carboxylate groups by an ion-exchange reaction rendering them capable of removal. Such reactions are involved in alkali digestion separating the basic coal structural units. Upon removal of the majority of liquid these coal units are reorientated to for.m a lower porosity material. Subsequent thermal treatment decomposes the ammonium ions to ammonia accomplishing protonation of both the above types of groups and giving a product lower in ash forming metal ions. Although the use of sodium, potassium or calcium hydroxides in alkali digestion will give rise to metal ions remaining in the coal generally the metal ion level is reduced from that of the pretreated coal. Thermal treatment also accomplishes decomposition of these oxygen containing surface groups yielding a water inert product.

ii: i: consists (i) (ii) 000a0 So 0 0 °oos o or 0 00 A o o o rr.

O* 0 0 o 0 Ilt n ,o 00 0 (iii) (iv) 4 A preferred form of the process of this invention of a:conventional slurry preparation stage wherein the coal may be ground or homogenized to assist in process kinetics. Slurry concentration may be in the range 30% to 35% by weight solids dependent on the ease of mixing required.

mixing with the ammonium ion to carry out the alkali treatment. This may be incorporated in the slurry preparation stage. This may be carried out at ambient temperature. A slurry pH greater than 10 is required.

slurry concentration may be accomplished by weight thickening or centrifuging followed by conventional drying or most preferably by drying alone in accordance with any conventional drying process within a temperature range of 50 0 °C to Generally a relationship exists between product particle size and drying rate. Slower drying rates allow large coal lumps to be produced. If drying is near or above 100°C steam generation may result in porosity increase in the coal.

thermal treatment within the temperature range of 2800 to 360° 0 C accomplishes ammonia recovery for recycle decomposition of oxygen functional groups water stable coal particles low water readsorption and uptake A temperature above 350°C may be used but this could consume volatiles in the coal and reduce the i thermal value of the fuel.

ammonia recovery and recyle is accomplished by dissolution of the liberated ammonia in water to yield the alkali medium for re use.

5 A flow chart of a preferred embodiement of this invention is shown in the drawings.

Raw low rank coal 1 is fed into a digester 2 and formed into a slurry with sufficient ammonium hydroxide (NH40H) via line 4.

Further NH4OH(S) may be added following digestion.

A typical slurry digestion is described as below.

10 kg. of raw coal 12 kg. of water Aqueous ammonia solution (hydroxide) Slurry is ground or homogenized to assist kinetics, water is added to keep slurry pumpable but close to gel formation.

Add ammonium hydroxide (2.2 kg. solution) to maintain pH at 10.5 over period of digestion.

o Digest slurry for 8 to 24 hours.

Following digestion a thickening treatment may be used but generally this is not desirable because it removes the finer particles and tends to increase the macro porosity .20 of the coal.

The digested slurry from digester 2 is then passed to the predrier 6 where hot gasses 7 such as flue gas are o 0r passed over the slurry. The water removed is passed to an ammonia-heat recovery unit 9.

(ii) Experiments were carried out to determine the *o optimum time for short-time drying during densification.

Two uncentrifuged slurries were dried at 21 C anr at 70 C and the porosities were determined and compared. (See table 1) In table 2 bed thickness as a drying rate variable is examined.

o. 's 0.: go a 4 coo 000 0 0 a a a 0 -6 Drying Time 3 weeks 3 days -0.01lu 0.911 0.010 Table 1 Macro porosity (cc/g) 0.01 -0.lu 0.1 -lu 1 -lou 0.014 0.004 0.003 0.014 0.003 0.004 10 Ou 0.007 0.021 100+U 0.022 0.024 Total.

0.061 0.075 Bed thickness Drying Time 3 cm 3 weeks 3 nm 16 hrs.

Olum 0.011 0.01 Table 2 Macro porosity (cc/g) +0.01-0. lum +0.l-l.OuM 0.014 0.004 0.01 0.003 0-l0um 0.003 0.003 +10 100um 0.007 0.010 7lO0um 0.022 0.02 Total 0.08 1 7 After predrying, the dried slurry is passed to the heat station 10 for thermal modification of the coal with hot gases 7 such as flue gas. Once again, steam and liberated ammonia are recovered in another recovery unit 9 to which water is added via line 11. The regenerated NH 4

OH

is mixed with make-up NH 4 OH (12) for use in the digester 2.

Thermal modification has two consequences: it decationates the coal functional groups by converting the ammonium ions to ammonia gas plus the remaining proton on the coal.

increases the surface hydrophobicity by removing the functional groups.

Decationation was demonstrated by measuring the enhanced weight loss of the ammonium exchanged coal over raw i coal as the coals were increased in temperature to 350°C in an inert atmosphere. The extra weight loss was due to the loss of ammonia from the coal. The exit gas was sampled and shown to contain ammonia.

Increased hydrophobicity was demonstrated by measuring the water absorption isotherms of the coals.

Coals that had been ammonium exchanged then thermal modified showed isotherms of a substantially increased hydrophobic nature than the untreated coals. Coals that had been thermally modified and not ammonium exchanged also showed a substantial increase in hydrophobicity.

Hydrophobic coals have isotherms that are closer to the x-axis i.e. reduced water vapour uptake and show less hystorisus indicating less swelling of the coal structure in the presence of water.

Following thermal modification it is possible to further increase the hydrophobicity by altering functional groups on the coal surface and further decreasing the affinity of the surface to take up water. This can be achieved with a steam drying unit 13 from which an improved product 14 is obtained.

f I r a; 0 -8 This step was shown to further increase the hydrophobicity of the coal.

Moisture at 70% RH Raw coal dried at 105 0

C

Ammonium exchanged thermally mod.

Hot water dried 11 14% 5 8% 2 4% Table 3 illustrates the properties of low rank brown coal treated according to a range of conventional processes (2 to 5) with raw coal and the process of this invention 9 4 A4 1 @4,4 4*i Is 4 o II

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I ti 142~ C A~ 4.4r 11 iF i 1 ii -i B !F p Ii

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i: 9 TABLE 3 Victorian Brown Coal Types Moisture Content atmospheric equilibrium d.b) Pore Content (cc/g) Moisture Content after immersion in water for 2 days d.b) Size of 9mm Particles after 7 days of immersion in water Strength 1) Raw Coal 2) Air Dried 3) Briquette 4) Solar Dried: Physical comminution

NH

4 OH alkali digested with conventional drying 6) NH OH Alkali digested combined with thermal recovery of NH (invention) 20% 18% 15% 15% 1.50 0.60 0.12 40-50% 30% 1-7mm 9 mm crumbly hard soft 0.10-0.60 hard 0.06 90% 8% 0.5-7mm 9 mm hard hard 0.12 ~C I Benefits (i) (ii) (iii) (iv) (v) (vi) ~i 10 of the process of this invention are: the process allows alkali digestion without excessive consumption associated with once-through alkali digestion processes.

the process has the alkali cation removed and converted to a proton, thus removing the ash fouling and slagging characteristics of once-through alkali digestion systems that empty metal cations.

there is provision to remove, through ion-exchange reactions, ash forming metal cations present within the original coal.

thermal decomposition also removes hydrophilic functional groups which leads to low water uptakes, low swelling and coal stability in water.

the whole process combining digestion and thermal modification yields a product that is uniquely low in porosity and water stable.

the process can be operated to yield particles that are suitable for conventional lump coal handling.

Such characteristics are high lump strength, low abrasion, low moisture uptake, good weathering properties, and accepted specific energies. Such products also show improved spontaneous combustion characteristics compared to other brown coal products.

the products of the process are suitable for use in coal water fuel mixtures because flowable slurries of up to seventy percent solids are possible with coal treated by this invention.

(vii)

Claims (3)

1. 2. A method as claimed in claim 1 or 2 wherein prior S' to the thermal modification step the digested slurry is predried at a temperature between 50 0 °C and 90 0 C. 0 00 0 0
2. 3. A method of forming a low porosity coal product 0, with a low water take up from a low rank coal comprising olo forming a slurry of low rank coal having a solids content of up tc 35% by weight. (ii) adding sufficient ammonium ion to provide a pH above 10 and digesting the slurry. (iii) drying the digested product and subjecting it to a thermal treatment within the range of 280°C to d 360 0 C to remove water release ammonia and decompose oxygen functional groups in the coal. (iv) recovering the ammonia for re use in the process. recovering the coal product.
3. 4. A process as claimed in claim 3 wherein the thermally treated coal product is subjected to a steam drying step to increase the hydrophobicity of the coal surface. At ~i I; "i 12 A low porosity coal product having low water take up formed from a low rank coal by a process as defined in claim 1 or claim 3. DATED THIS 13th day of June, 1990. SWINBURNE LIMITED and THE UNIVERSITY OF MELBOURNE O 4O 000 09 00 go 0 o a ,-O WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN 3122 VICTORIA AUSTRALIA 5/10:DBM:va/JZ JTC i 1