NZ202639A - Stable coal-water slurries and a method for their preparation - Google Patents

Abstract

A process for making fluid, stable slurries of finely divided coal in water and products thereof, which can be sufficiently highly loaded to serve as a fuel, comprises: a. Admixing: (i) ultrafine coal particles having a maximum size (as determined by a sedimentation technique based on Stoke's law) of 10 mu m MMD (Mass Median Diameter) in an amount comprising more than 30% and up to 50% by weight of the slurry, (ii) larger coal particles within the size range of from 20 to 200 mu m MMD in an amount sufficient to provide a desired total coal concentration in the slurry, (iii) water, and, (iv) a minor amount of dispersant consisting of an alkaline earth metal salt of organo-sulfonate in which the organic moiety is multi-functional, and b. subjecting the mixture to high shear at a rate of at least 100 sec.

Description

New Zealand Paient Spedficaiion for Paient Number £02639 202639 Prk-'ity Date(s): ; .3.'.??.

Complete Specification Filed: Class: CJ.Qkl/'3£ Publication Date: ..

P.O. Journal, No: ..

NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION PROCESS FOR MAKING FUEL SLURRIES t1 26 iiQ]f ,* •»>?* WE, ATLANTIC RESEARCH CORPORATION, a Company incorporated in Delaware, U.S.A. of 5390 Cherokee Avenue, Alexandria, Virginia 22314, United States of Vjt.

America, do hereby declare the invention for which-ir pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: J Ur ■ K A high fuel value coal-water slurry which can be injected directly into a furnace as a combustible fuel, can supplant large quantities of increasingly expensive fuel oil presently being used by utilities, factories, ships and other commercial enterprises. Since the inert water vehicle reduces fuel value in terms of BTU/lb, it is desirable to minimize its concentration and maximize coal concentration for efficient use of the slurry as a fuel. High coal content also improves the combustion characteristics of the slurry.

It is important, therefore, that the slurry be loadable with finely-divided coal in amounts as high, for example, as about 50% to 70% of the slurry. Despite such high solids loading, the slurry must be sufficiently fluid to be pumped and sprayed into the furnace. The coal particles must also be uniformly dispersed. The fluidity and dispersion must be stably maintained during storage.

Fluid, pourable slurries comprising up to about 70% or higher of coal stably dispersed in water and produced by admixing finely-divided coal having a critical distribution of particle sizes, water, and an organic dispersant in a high shear rate mixer. An inorganic buffer salt may also be added. The term "fluid" as employed in this specification and claims means a slurry which is fluid and pourable both at rest and in motion or a slurry which gels or floculates into a substantially non-pourable composition at rest and becomes pourably fluid with stirring or other application of relatively low shear stress.

Controlled distribution of coal particles sizes is essential for both fluidity and stability. The partial size mixture, necessary for fluidity of the highly loaded slurry, comprises ultrafine (UF) particles having a maximum size of up to about 10p MMD (mass median diameter), preferably about lju to 8jj MMD and larger particles (F/C) having a size range of about 20p to 200ja MMD, preferably about 20/i to 150/1 MMD. The UF particles may comprise 10 to 50% by wt. of the slurry. For stability of the slurry, the UF particles should comprise about 10 to 30% by wt. of the slurry, more preferably about 15 to 25%, and possibly but less preferably more than about 30% by wt.

The actual degree of coal loading is not critical and will vary with the given use and operating equipment.

The concentration of coal successfully incorporated into a given slurry varies with such factors as the relative amounts of UF and F/C particles, size of the F/C particles used within the effective range, and the like. In general, percentage loading increases with increasing F/C size. An organic dispersant is essential to maintain the coal particles in stable dispersion. It has been found that the highly-loaded slurries are very sensitive to the particular type of surfactant used, especially with respect to fluidity and storagea-bility. The dispersants which have proven to be effective in producing stable fluid mixes are high molecular weight alkaline earth metal (e.g. Ca, Mg) organosulfonates in which the organic moiety is polyfunctional. Molecular weight of the organosulfonate is desirably about 1,000 to about 25,000. The surfactant is used in minor amount, . 0.5 to pph of coal, preferably about 1 to 2 pph.

In some cases, particularly at higher coal loadings, it has been found desirable to add an inorganic, alkali metal (e.g. Na, K) buffer salt to stabilize pH of the slurry in the range of about pH5 to 8, preferably about pH 6 to 7-5. The salt improves aging stability, pourability and handling characteristics of the slurry. It may be that the buffer counteracts potentially adverse effects of acid leachates from the coal. The salt, such as sodium or potassium phos-yphate or carbonate, including their acid salts is used in /■minor amounts sufficient to provide the desired pH, e.g. 7about 0.1 to 2% based on the water. The inorganic salts also e gaseous sulfur pollutants by deforming non-compounds. trafine and larger F/C coal particles, water, d inorganic salt components are mixed in a er mixing device which can deliver high shear hear .mixing, e.g. at shear rates of at least 1 -1 , preferably at least about 500 sec , is producing a stable slurry free from substan-tion. The use of high shear mixing and the ears to have a synergistic effect. Dispersant mixing results in an extremely viscous, non-y, while high shear mixing without dispersant serve to reduc g a &e<tu s sulfur The ul dispersant, an blender or oth rates. High s about 100 sec essential for tial sedimenta dispersant app with low shear pourable slurr WB 202639 produces a slurry which is unstable towards settling. With both dispersant and high shear mixing a fluid, pourable, stable slurry can be obtained.

The slurries are viscous, fluid dispersions which can generally be characterized as thixotropic or Bingham fluids having a yield point. In some cases, the slurries may gel or flocculate when at rest into a substantially non-pourable composition but are easily rendered fluid by stirring or other application of relatively low shear stress. They can be stored for considerable periods of time without excessive settling or sedimentation. The slurries can be employed as fuels by injection directly into a furnace previously brought up to ignition temperature of the slurry. The finely divided state of the coal particles improves combustion efficiency. Since the dispersants are organic compounds, they may be biodegraded with time. This can readily be prevented by addition of a small amount of biocides.

The ultrafine coal particles can be made in any suitable device, such as a ball mill or attritor, which is capable of very fine comminution. Preferably, though not essentially, the coal is milled with water so that the UF particles are in water slurry when introduced into the mixer. Some of the dispersant can be included, if desired, in the UF milling operation to improve a flow and dispersion characteristics of the UF slurry.

The required larger size coal particles (2Op to 200;j) can be made from crushed coal in a comminuting device such as a hammermill equipped with a grate having appropriately sized openings. Excessively sized coal residue can be used for making the UF particles.

The coal concentrations a;s used in the specification and in the following examples is on a dried coal basis which normally equals 98.5% by weight of bonedried coal.

The 3-6p MMD UF particles employed in Examples 3-8 were prepared in accordance with Example I and the UF particles were introduced in the form of the Example I aqueous slurry containing a portion of the dispersant. The total amount of dispersant given in the Examples includes the portion introduced in this way.

The 34u MMD and llOu MMD particles used in the Examples 3 were prepared in accordance with Example 2. 1Q7 Sedimentation measurement, which is based on Stoke's Law giving the relationship between particle size and settling velocity, was used experimentally in all cases to determine sub-sieve particle sizes. The particular sedimentation technique employed is one con-5 ventionally known as centrifugal sedimentation. The sedimentometer used was the MSA Particle Size Analyzer (C.F. Casello & Co. Regent House, Britania Walk, London NI). In centrifugal sedimentation, the local acceleration due to gravity, g, is multipled by w r/g where w is rotational velocity and r is radius of rotation. The "two layer" 10 method was used in the experimental procedures. All of the coal powder is initially concentrated in a thin layer floating on top of the suspending water fluid in a centrifuge tube. The fluid is centri-fuged at incrementally increasing rotational speeds. The amount of sedimenting powder is measured as a function of time at a specified 15 distance from the surface of the fluid. The cumulative size distri bution was determined by plotting the fractional weights settled out against the free-falling Stoke's diameter. Thus sub-sieve particle sizes disclosed and claimed herein were obtained by sedimentation measurement.

Example 1 5035 by wt crushed coal, 1% calcium lignosulfonate (Marasperse C-21) and 49% water were ball milled for 2 hours. The size of the resulting UF coal particles was 3.6u MMD. The UF coal-water slurry was fluid and pourable.

Example 2 A. Crushed coal was comminuted in a hammermill at 3,450 RPM with a 27 HB grate. The particle size of the product was 110 u MMD.

B. Crushed coal was comminuted in a hammermill at 13,800 RPM with a 10 HB grate. The particle size of the resulting product was 34u MMD.

Example 3 A. 65% by wt of coal comprising 55% 110 u MMD coal and 45% 3.6u MMD coal, 1.3% Marasperse C-.21 (calcium lignin sulfonate, Ca content as CA0 5.2%, Na content as Na20 6.1% Mg content as MgO 0.3%) 35 and 33.7% water were mixed in a blender at 6,000 RPM at a shear rate of 1,000 sec The resulting slurry was paint-like and set into a soft gel which was easily stirred to a liquid. After 23 days, it exhibited no sedimentation and was easily restirrable to a uniform dispersion having relatively low viscosity of 6.7p.

B. A mix was made identical to A except that MMD particles were substituted for the UF particles. The mix, through initially fluid was ustable. Within 3 dajs it separated, forming a large supernatent and a highly packed subsidence. It could not be remixed into a uniform, pourable dispersion.

Example ^ A. A 65% coal slurry comprising 15% 3-6p MMD and 50% 3^p MMD particles by wt. of the slurry, 1.3% Marasperse C-21 and 33.7% water were mixed in a blender at 6000 RPM. The resulting product was an uniformly dispersed gel which after 12 days in storage exhibited no supernatant, subsidence or sedimentation. The gel was non-pourable at rest and became a pourable fluid with stirring.

B. A mix was made identical to A except that the blender was run at a low shear rate of 60 RPM (10 sec~1).

The resulting slurry was unstable. Within ^ days it had separated into liquid and aggregated sediment.

Example 5 A. A 65% coal slurry comprising 26% 3.6p MMD particles and 39% 110ji MMD particles, 1.3% Marasperse C-21 and 33.7% water were mixed in a blender at 6,000 RPM. The resulting product was a uniformly dispersed slurry which was fluid and pourable and after 10 days was still pourable and substantially free from subsidence or sedimentation.

B. A mix was made identical to A except that the blender was run at a low shear rate of 10 sec"1. The resulting slurry was unstable. Within'3 days, it had separated into supernatant and aggregated sediment. 202639 Example 6 A 65% coal slurry was made identical to Example 3A except that no dispersant was added. The resulting product had the consistency of a stiff grease.

Example 7 A. A 70% coal slurry comprising 45.5% 110p MMD particles and 24.5% 3-6y MMD particles, 1.4% Marasperse C-21, and 28.6% water solution buffered to pH7 by 0.15% ^2*^0^ added in the blender was mixed at 6,000 RPM. The resulting slurry has an EOM viscosity of 1.48 Kp, is fluid and pourable.

After 7 days in storage it exhibited no supernatant liquid, settling or aggregation.

B. A mix was made identical to A except that phosphate salt was not added. The resulting slurry set up into a stiff non-pourable mass within 3 days.

C. A mix identical to A, except that the buffer salt was added to the ballmill producing the UF particles, was run in a blender at the low shear rate of 60 RPM (10 sec"1). The slurry was unstable and within 5 days separated into supernatant and stiff aggregated sediment.

Example 8 A mix was made identical to Example 4A except that ^2^0^ in amount providing buffered pH7 was added in the blender. The resulting slurry was fluid and pourable. Its viscosity was EOM-T-bar 0.92 Kp. It retained its stability and pourability during storage and after 12 days was free from separation. 2026 3 9 Example 9 A. 30% wt.% of hammermill coal fines (30p MMD), 0.3% Marasperse C-21 (1 pph coal), and 69-7% water were milled in an attritor for 30 min. The resulting slurry was very fluid. The UF coal particle size was 3.88jj MMD.

B. A 65 wt.% coal slurry comprising 50 wt% 3^p MMD coal particles, 15 wt%, 3,88p MMD (using 50% wt% of slurry from 9A supra), 2 pph on coal of Marasperse C-21, and the remainder water, was mixed in a blender at a shear rate of 6,000 RPM (1000 sec 1). The product was a uniformly-disper-10 sed, pourable slurry. After 56 days the slurry was a stable, soft, non-pourabie gel free from settling or sedimentation. There was a very slight supernatant. Probably caused by water evaporation and condensation on the surface. The thixotropic gel became easily pourable with slight stirring. 15 At rest it returned to a stable non-pourable state within a short time. After 61 days it retained its stable characteristics after several stirrings to pourability.

C. A slurry similar to 9B was prepared except that the mix was buffered to pH7 by the addition of Na2HP0^. The 20 product was a uniformly-dispersed fluid slurry of relatively low viscosity. After 55 days the slurry was a weak, non-pourable gel free from settling or sedimentation. As in 9B there was a very slight fiber supernatant. With slight stirring, it became verj) fluid and pourable it was still 25 stable and pourable after 24 hours and, although some what more viscous, retained its stability and pourability 5 days after the initial stirring.

Example 10 The ultrafine 3.6u MMD coal component was made in accordance with Example 1. A HOu MMD coal component was prepared as in Example 2.

A 65% coal slurry comprising 32.5% 3.6u MMD and 32.5% HOu MMD coal particles by wt of the slurry, 0.65% Marasperse C-21, and 34.35% water, was prepared in a high speed blender at 6000 rpm (shear rate approximately 1000 sec"1). The resulting slurry was a soft thixotropic gel with a yield point of 49 dynes/cm2* With light stirring 2026 3 to overcome the yield point, the slurry was fluid and pourable. It had a Brookfield viscosity of 1,440 cp at 60 rpm. After 14 days the slurry was still substantially uniformly dispersed. It had a slight supernatent, was free of hardpacked sediment, and could easily be 5 stirred to uniformity and pourability.

Example 11 The 3.6u MMD ultrafine coal component was made in accordance with Example 1, except that 1% Lomar UDG, a calcium naphtalene sulfonate containing 11.5% Ca as CaSO^, was substituted for the Marasperse C-21. A 110 u MMD coal component was prepared as in Example 2.

A 65% coal slurry,comprising 32.5% 3.6u MMD and 32.5% HOu MMD coal particles by wt of the slurry, 0.65% Lomar UDG, and 34.35% water, was prepared in a high speed blender at 6000 rpm. The resulting slurry was a soft thixotropic gel with a yield point of 30 dynes/ 2 cm . With light stirring to overcome the yield point, the slurry was fluid and pourable. It had a Brookfield viscosity of 1,915 cp at 60 rpm. After 14 days, the slurry was still substantially uniformly dispersed. It had a slight supernatent, was free of hard-packed sediment, and could easily be stirred to uniformity and pourability.

Example 12 The ultrafine 3.6u MMD coal component was prepared by mixing 60 wt% coal with 0.6% Marasperse C-21, 0.28% Na^HPO^, and 39.12% water and ball milling for 2 hours as in Example 1. The phosphate buffer salt was included to vacilitate the grinding. A HOu MMD coal fraction was prepared by hammermilling as in Example 2.

A 65% coal slurry comprising 50% 3.6u MMD and 15% HOu MMD coal particles by wt of the slurry, Marasperse C-21 0.65%, 0.23% Na2HP0^, and 34.12% water was prepared in a high speed blender at 6000 rpm. The resulting slurry was a uniformly dispersed thixotropic gel after 5 days which became fluid and pourable with light stirring. 202639 Example 3 demonstrates the need for the UF particles in controlled size distribution to impart stability. Examples 4 and 5 show the need for high shear rate mixing. Examples 6 shows the importance of the dispersant. Example 7 illustrates the improvement made in a highly-loaded 70% slurry by use of an inorganic buffer salt and the adverse effect of low shear mixing. Example 8 shows that the use of the pH buffer salt maintained the slurry in a stable fluid condition. Example 9 shows that the buffer salt improved aging and its user and handling characteristics.

The stable, fluid coal-water slurries are efficient and considerably lower cost alternatives to fuel oil. Their flame temperature and heating values compare very favorably with fuel oil,"as is shown in the following tables: Table I ADIABATIC FLAME TEMPERATURE AT 20% EXCESS AIR*;if 6 Fuel Oil;3095°F;70% coal-water slurry;3089°F;65% coal-water slurry;3028°F;* in a typical furance Table II HEATING VALUE IN BTU/lb OF COMBUSTION PRODUCTS if 6 Fuel Oil 991.0 70% coal-water slurry 983-3 65% coal-water slurry 975.5 The cost of the coal-water slurries including proces sing is about 1/2 that of #6 fuel oil at present prices.

Table III $4 .94 $2.24 $2.34 COST PER MILLION BTU if 6 Fuel Oil 70% coal-water slurry 65% coal-water slurry 202639

Claims (19)

WHAT WE CLAIM IS;

1. Process for making substantially stable coal-water slurries comprising: a. Admixing: (i) ultrafine coal particles having a maximum size of 10jnn MMD in an amount comprising 10 to 50% by weight of the slurry, larger coal particles within the size range of 20 to 200/im MMD in an amount sufficient to provide a desired total coal concentration in the slurry, wherein sub-sieve particle sizes in (i) and (ii) are defined in terms of those obtainable by sedimentation measurement employing Stoke's Law, water, and, a minor amount of dispersant consisting essentially of alkaline earth metal salt of organo-sulfonate in which the organic moiety is poly-functional, and subjecting the mixture to high shear at a rate of at least 100 sec-^-

2. Process of Claim 1 in which an inorganic alkali metal buffer salt is added to maintain pH in the range of 5 to 8.

3. Process of Claim 2 in which the buffer salt is an alkali metal phosphate.

4. Process of any one of Claims 1 to 3 in which: a. The ultrafine particles are within the size range of 1 to 8jim MMD, b. the larger coal particles are within a size range of 20 to 150/im MMD. - 11 - LATENT Of . 0 C JAM !9c6 (ii) (iii) (iv) b. R8Q£PV£P 202639

The process of any one of Claims 1 to 4 in which the dispersant is calcium lignosulfonate.

The process of any one of Claims 1 to 5 in which the minimum shear rate is 500 sec"-'-.

The process of any one of Claims 1 to 6 in which the ultrafine -particles are produced in the presence of water and at least a portion of the dispersant.

A coal-water slurry which comprises: a. ultrafine coal particles having a maximum size of 10pm MMD, in an amount comprising 10 to 50% by weight of slurry, b. larger coal particles within the size range of 20 to 200/um MMD in an amount sufficient to provide a desired total coal concentration in the slurry; wherein sub-sieve particle sizes in (a) & (b) are defined in terms of those obtainable by sedimentation measurement employing Stoke's Law, c. water; and d. a minor amount of a dispersant consisting essentially of an alkaline earth metal organo-sulfonate in which the organic moiety is poly-functional.

The .slurry of Claim 8 in which: a. the ultrafine particles are within a size range of 1 to 8/am MMD, and b. the larger particles are within the size range of 20 to 150pm MMD.

The slurry of either of Claims 8 and 9 in which the dispersant is calcium lignosulfonate.

The slurry of any one of Claims 8 to 10 which is buffered to a pH of 5 to 8 by means of an inorganic alkali metal buffer salt. 202639

12. The slurry of Claim 11 in which the buffer salt is an alkali metal phosphate.

13. The slurry of any one of Claims 8 - 12 in which the slurry is a substantially thixotropic or Bingham fluid.

14. The process of any one of Claims 1-7 wherein the ultrafine particles comprise 10 to 30% by wt. of the slurry.

15. The process of any one of Claims 1-7 wherein the ultrafine particles comprise more than 30% by wt. of the slurry.

16. The slurry of any one of Claims 8-13 wherein the ultrafine particles comprise 10 to 30% by wt. of the slurry.

17. The slurry of any one of Claims 8-13 wherein the ultrafine particles comprise more than 30% by weight of the slurry.

18. The process of Claim 1, substantially as herein described in any one of the examples.

19. The slurry of claim 8, substantially as herein described in any one of the examples. ATLANTIC RESEARCH CORPORATION By its Attorneys JAMES W PIPER & GO. - 13 - i