EP0050412B1 - A process for making fuel slurries of coal in water and the product thereof - Google Patents

Abstract

A process for making fluid, stable slurries of finely divided coal in water, which can be sufficiently highly loaded to serve as fuel, comprises: a) Admixing: (i) ultrafine coal particles having a maximum size of 10 mu m MMD in an amount comprising from 10 to 30% 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 essentially of an alkaline earth metal salt of an organosulfonate in which the organic moiety is multifunctional, and b) subjecting the mixture to high shear at a rate of at least 100 sec<-><1>. Preferably, calcium lignosulphonate is used as the dispersant and the pH is maintained in the range of 5 to 8 using an inorganic alkali metal buffer salt. The slurry thus produced exhibits thixotropic or Bingham fluid-like properties enabling prolonged periods of storage with subsequent restoration of fluid characteristics to enable use as a fuel.

Description

• This invention relates to the production of fuel slurries of coal in water which can be injected directly into a furnace as a combustible fuel. A high fuel value coal slurry 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 (J/kg) it is desirable to minimise its concentration and maximise coal concentration for efficient use of the slurry as a fuel. A 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.
• An object of this invention is to provide an improved process for producing a slurry suitable for this purpose.
• According to the present invention there is provided a process for making substantially stable coal-water slurries comprising:
• a) Admixing:
  • i) ultrafine. coal particles having a maximum size of 10µm MMD in an amount comprising from 10 to 30% by weight of the slurry, •
  • (ii) larger coal particles within the size range of from 20 to 200µm 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 essentially of an alkaline earth metal salt of an organosulfonate in which the organic moiety is polyfunctional, and
• b) subjecting the mixture to high shear at a rate of at least 100 sec-¹.
• This invention further provides a coal-water slurry which comprises:
• a) ultrafine coal particles having a maximum size of 10µm MMD, in an amount comprising from 10 to 30% by weight of slurry;
• b) larger coal particles within the size range of from 20 to 200pm MMD in an amount sufficient to provide a desired total coal concentration in the slurry;
• 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.
• Thus fluid, pourable slurries comprising up to about 70% or higher of coal stabley dispersed in water are 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 used 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 flocculates 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 particle size mixture, necessary for fluidity of the highly loaded slurry comprises ultrafine (UF) particles having a maximum size of up to about 10_jum MMD (mass mean diameter), preferably 1µm to 8µm MMD and larger particles (F/C) having a size range of 20µm to 200µm MMD, preferably 20µm to 150µm MMD. For stability of the slurry, the UF particles should comprise 10 to 30% by weight of the slurry, preferably 15 to 25%.
• 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 slurried are very sensitive to the particular type of surfactant used, especially with respect to fluidity and storageability. Examples of 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 poly-functional. Molecular weight of the organosulfonate is desirably 1,000 to 25,000. The surfactant is used in minor amount, e.g. 0.5 to 5 pph of coal, preferably 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 from pH 5 to 8, preferably from pH 6 to 7.5. The salt improves ageing 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 phosphate or carbonate, including their acid salts is used in minor amounts sufficient to provide the desired pH, e.g. 0.1 to 2% based on the water. The inorganic salts also serve to reduce gaseous sulfur pollutants by forming non-gaseous sulfur compounds.
• The ultrafine and larger F/C coal particles, water, dispersant, and inorganic salt components are mixed in a blender or other mixing device which can deliver high shear rates. High shear mixing, e.g. at shear rates of at least about 100 sec^-1, preferably at least about 500 sec^-1, is essential for producing a stable slurry free from substantial sedimentation. The use of high shear mixing and the dispersant appears to have a synergistic effect. Dispersant with low shear mixing results in an extremely viscous, non-pourable slurry, while high shear mixing without dispersant produces a slurry which is unstable towards settling. With both dispersant and high shear mixing a fluid, pourable, stable slurry an 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 flow and dispersion characteristics of the UF slurry.
• The required larger size coal particles, (20µm to 200j_M) 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 as used in the specification and in the following examples is on a dried coal basis which normally equals 98.5% by weight of bone-dried coal.
• The 3.6µm MMD UF particles employed in Examples 3-8 were prepared in accordance with Example 1 and the UF particles were introduced in the form of the Example 1 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 34µm MMD and 110µm MMD particles used in the Examples were prepared in accordance with Example 2.
• 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 conventionally known as centrifugal sedimentation. The sedi- mentometer used was the MSA Particle Size. Analyzer (C.F. Casello & Co, Regent House, Britania Walk, London N1). In centrifugal sedimentation, the local acceleration due to gravity, g, is multiplied by w₂r/g where w is rotational velocity and r is radius of rotation. The "two layer" 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 centrifuged at incrementally increasing rotational speeds, The amount of sedimenting powder is measured as a function of time at a specified distance from the surface of the fluid. The cumulative size distribution was determined by plotting the fractional weight settled out against the free-falling Stoke's diameter. The sub-sieve particle sizes disclosed and claimed herein were obtained by sedi-^mentation measurement.
Example 1
• 50% by wt. crushed coal, 1 % calcium lignosulfonate (Marasperse C-21, Registered Trade Mark) and 49% water were ball milled for 2 hours. The size of the resulting UF coal particles was 3.6µm 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 110µm MMD.
• B. Crushed coal was comminuted in a hammermill at 13,800 RPM with a 0 HB grate. The particle size of the resulting product was 34µm MMD.
Example 3
• A. 65% by wt, of coal comprising 55% 110µm MMD coal and 45% 3.6µm MMD coal, 1.3% Marasperse C-21 (a Registered Trade Mark for calcium lignin sulfonate, Ca content as CaO 5.2%, Na content as Na₂0 6.1 %. Mg content as MgO 0.3%.) and 33.7% water were mixed in a blender at 6,000 RPM at a shear rate of 1,000 sec-¹. The resulting slurry was a paint- like gel that 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 -6.7P.
• B. A mix was made identical to A except that 34µm MMD particles were substituted for the UF particles. The mix, though initially fluid was unstable. Within 3 days it separated, forming a large supernatant and a highly packed subsidence. It could not be remixed into a uniform, pourable dispersion.
Example 4
• A. A 65% coal slurry comprising 15% 3.6µm MMD and 50% 34pm MMD particles by wt. of the slurry, 1.3% Marasperse C-21 (Registered Trade Mark) 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 4 days it had separated into liquid and aggregated sediment.
Example 5:
• A. A 65% coal slurry comprising 26% 3.6µm MMD particles and 39% 110µm MMD particles, 13% Marasperse C-21 (Registered Trade Mark) 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.
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% 110µm MMD particles and 24.5% 3.6µm MMD particles, 1.4% Marasperse C-21 (Registered Trade Mark), and 28.6% water solution buffered to pH 7 by 0.15% Na₂HP0₄ added in the blender was mixed at 6,000 RPM. The resulting slurry has an EOM (end of mix) 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 Na_zHP0₄ in amount providing buffered pH 7 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.
Example 9
• A. 30 wt.% of hammermilled coal fines (30µm MMD), 0.3% Marasperse C-21 (Registered Trade Mark) (1 pph coal), and 69.7% water were milled in an attritor for 30 minutes. The resulting slurry was very fluid. The UF coal particle size was 3.88µm MMD.
• B. A 65 wt.% coal slurry comprising 50 wt.% 34µm MMD coal particles, 15 wt.%, 3.88µm 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-dispersed, pourable slurry. After 56 days the slurry was a stable, soft, non-pourable 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. 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 pH 7 by the addition of Na₂HP0₄. The 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 supernatant. With slight stirring, it became very fluid and pourable. It was still.stable and pourable after 24 hours and, although somewhat more viscous, retained its stability and pourability 5 days after the initial stirring.
• 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. Example 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 temperatures and heating values compared very favorably with fuel oil, as is shown in the following Tables:

  

  
• The cost of the coal-water slurries including processing is about t that of No. 6 fuel oil at present prices.

  

Claims (13)

1. a process for making substantially stable coal-water slurries comprising:

a) Admixing:

(i) ultrafine coal particles having a maximum size (as determined by a sedimentation technique based on Stoke's Law) of 10µm MMD (Mass Median Diameter) in an amount comprising from 10 to 30% by weight of the slurry,

(ii) larger coal particles within the size range of from 20 to 200µm 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 essentially of an alkaline earth metal salt of an organo-sulfonate in which the organic moiety is poly-functional, and

b) subjecting the mixture to high shear at a rate of at least 100 sec-¹.

2. A process according to claim 1, in which an inorganic alkali metal buffer salt is added to maintain pH in the range of from 5 to 8.

3. A process according to claim 2, in which the buffer salt if an alkali metal phosphate.

4. A process according to any one of claims 1, 2 or 3 in which:

a) The ultrafine particles are within the size range of from 1 to 8µm MMD and comprise from 15 to 25% by wt. of the slurry; and

b) the larger coal particles are within a size range of from 20 to 150µm MMD.

5. A process according to any one of claims 1 to 4 in which the dispersant is calcium lignosulfonate.

6. A process according to any one of claims 1 to 5 in which the minimum shear rate is 500 sec^-1.

7. A process according to 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.

8. A coal-water slurry which comprises:

a) ultrafine coal particles having a maximum size (as determined by a sedimentation technique based on Stoke's Law) of 10µm MMD (Mass-Median Diameter), in an amount comprising from 10 to 30% by weight of slurry;

b) larger coal particles within the size range of from 20 to 200µm MMD in an amount sufficient to provide a desired total coal concen- trution in the slurry;

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.

9. A slurry according to claim 8 in which:

a) the ultrafine particles are within a size range of from 1 to 8µm MMD, and,

b) the larger particles are within the size range of from 20 to 150µm MMD.

10. A slurry according to claim 8 or 9 in which the dispersant is calcium lignosulfonate.

11. A slurry according to any one of claims 8 to 10 which is buffered to a pH of from 5 to 8 by means of an inorganic alkali metal buffer salt.

12. A slurry according to claim 11 in which the buffer salt is an alkali metal phosphate.

13. A slurry according to any one of claims 1 to 12 in which the slurry is a substantially thixotropic or Bingham fluid.