EP0089766B1

EP0089766B1 - A process for making coal-water slurries and product thereof

Info

Publication number: EP0089766B1
Application number: EP83301195A
Authority: EP
Inventors: Robert Stephen Scheffee
Original assignee: Atlantic Research Corp
Current assignee: Atlantic Research Corp
Priority date: 1982-03-22
Filing date: 1983-03-07
Publication date: 1986-06-04
Also published as: JPH0330638B2; AU556291B2; DE3363876D1; EP0089766A1; ATE20248T1; JPS58173193A; AU1183183A; ZA831302B; NZ202639A

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

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. Such a high fuel value coal-water 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/Ib (J/kg), it is desirable to minimize its concentration for efficient use of the slurry as a fuel. High coal content also improves the combustion characteristics of 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.
In EP-A-0050412 (Atlantic Research Corporation), which is a document falling within the terms of Art 54(3) EPC, a stable coal slurry and a process for making same are disclosed. The process comprises:

a) admixing:
- (i) ultrafine coal particles having a maximum size (as determined by a sedimentation technique based on Stoke's Law) of 10 µm 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 200 µm 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.

An object of the invention is to provide a further process for producing a slurry suitable for the purpose.
According to the present invention there is provided a process for making substantially stable coal-water slurries which comprises:

a. admixing:
- (i) ultrafine coal particles having a maximum size (as determined by a sedimentation technique based on Stoke's law) of 10 µm 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 um MMD in an amount sufficient to provide a desired total coal concentration in the slurry,
- (iii) water, and,
- (iv) a minor amount of disperant consisting of an alkaline earth metal salt of 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.

This invention therefore also provides a stable coal-water slurry in the form of a substantially thixotropic or Bingham fluid which comprises:

a. ultrafine particles having a maximum size (as determined by a sedimentation technique based on Stoke's law of 10 µm MMD, in an amount comprising more than 30% and up to 50% by weight of slurry;
b. larger coal particles within the size range of from 20 to 200 pm MMD (Mass Median Diameter) 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 of an alkaline earth metal organo-sulfonate in which the organic moiety is poly-functional and which dispersant provides a slurry which has long term stability. Thus, fluid pourable slurries comprising up to about 70% or higher of coal stably 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 miximum size of up to 10 pm MMD (Mass Median Diameter), preferably 1 um to 8 pm MMD and larger particles hereafter defined as (F/C), having a size range of from 20 µm to 200 µm MMD, preferably 20 µm to 150 µm MMD. For stability of the slurry, the UF particles should comprise more than 30 and up to 50% by wt. of the slurry.
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 increased with increasing F/C size. An organic dispersant is essential to maintain the coal particles in stable dispersion. It has been found that highly-loaded slurries 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 polyfunctional. 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 pH 5 to 8, preferably about 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 100 sec-¹, preferably at least 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 can be obtained.
The slurries are viscous, fluid dispersions which can generally be characterized as thioxotropic or Bingham fluids having a yield point. In some cases, the slurries may gel or flocculate when at rest into substantially non-pourable compositions but are easily rendered fluid by stirring or other application of relatively low shear stress. They can be stored for a considerable period 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 (20 um to 200 µm MMD) can be made from crushed coal in a comminuting device such as a hammer mill equipped with a grate having appropriately sized openings. Excessively sized coal residues 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.
3.6 pm MMD UF particles employed in Examples 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.
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 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 multiplied by w² r/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 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 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

50% by wt, crushed coal, 1% calcium lignosulfonate (Marasperse C-21, Registered Trade Mark), and 49% water were ball milled for 2 hours to give ultrafine 3.6 pm MMD coal particles. Crushed coal was comminuted in a hammer mill at 3,450 rpm with a 27 HB grate to give a 110 ^pm MMD coal particulate product.
A 65% coal slurry comprising 32.5% 3.6 pm MMD and 32.5% 110 um MMD coal particles by wt. of the slurry, 0.65% Marasperse C-21 (Resgistered Trade Mark), 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/cm^z (4.9 N/m²). With light stirring to overcome the yield point, the slurry was fluid and pourable. It had a Brookfield viscosity of 1,440 cP (1.44 kgm^-1s^-1) at 60 RPM. After 14 days the slurry was still substantially uniformly dispersed. It had a slight supernatant, was free of hard-packed sediment, and could easily be stirred to uniformity and pourability.

Example 2

The 3.6 pm MMD ultrafine coal component was made in accordance with Example 1, except that 1% Lomar UDG (Registered Trade Mark), a calcium napthalene sulfonate containing 11.5% Ca as CaS0₄, was substituted for the Marasperse C-21 (Registered Trade Mark). A 110 µm MMD coal component was prepared as in Example 2.
A 65% coal slurry, comprising 32.5% 3.6 µm MMD and 32.5% 110 µm 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/cm² (3.0 N/m²). With light stirring to overcome the yield point, the slurry was fluid and pourable. It had a Brookfield viscosity of 1,915 cP (1.915 kg m^-1s^-1) at 60 RPM. After 14 days, the slurry was free of hardpacked sediment, and could easily be stirred to uniformity and pourability.

Example 3

The ultrafine 3.6 pm MMD coal component was prepared by mixing 60 wt% coal with 0.6% Marasperse C-21 (Registered Trade Mark), 0.28% Na₂HP0₄, and 39.12% water and ball milling for 2 hours as in Example 1. The phosphate buffer salt was included to facilitate the grinding. A 110 µm MMD coal fraction was prepared by hammer- milling as in Example 1.
A 65% coal slurry comprising 50% 3.6 pm MMD and 15% 110 pm MMD coal particles by wt. of the slurry, Marasperse C-21 (Registered Trade Mark) 0.65%, 0.23% Na₂HP0₄, 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.
The stable, fluid coal-water slurries are efficient and considerably lower cost alternatives to fuel oil. Their flame temperatures and heating values compare very favourably with fuel oil, as is shown in the following tables:
The cost of the coal-water slurries including processing is about 1/2 that of No. 6 fuel oil at present prices.

Claims

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 10 pm 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 pm 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 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 buffer compound is added to maintain pH in the range of from 5 to 8.

3. A process according to claim 1 or 2 in which:

a. the ultrafine particles are within the size range of from 1 to 8 pm MMD; and

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

4. A process according to any one of claims 1 to 3 in which the dispersant is calcium lignosulfonate.

5. A process according to any one of claims 1 to 4 in which the minimum shear rate is 500 sec^-1.

6. A process according to any one of claims 1 to 5 in which the ultrafine particles are produced in the presence of water and at least a portion of the dispersant.

7. A stable coal-water slurry in the form of a substantially thixotropic or Bingham fluid which comprises:

a. ultrafine particles having a miximum size (as determined by a sedimentation technique based on Stoke's law) of 10 µm MMD, in an amount comprising more than 30% and up to 50% by weight of the slurry;

b. larger coal particles within the size range of from 20 to 200 pm MMD (Mass Median Diameter) 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 of an alkaline earth metal organo-sulfonate in which the organic moiety is poly-functional and which dispersant provides a slurry which has long term stability.

8. A slurry according to claim 7 in which:

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

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

9. A slurry according to claim 7 or 8 in which the dispersant is calcium lignosulfonate.

10. A slurry according to any one of claims 7 to 9 which is buffered to a pH of from 5 to 8 by means of an inorganic buffer compound.

11. A slurry according to claim 10 wherein the inorganic buffer compound is an alkali metal phosphate.