EP0339853A1

EP0339853A1 - Coal slurry composition and treatment

Info

Publication number: EP0339853A1
Application number: EP89303832A
Authority: EP
Inventors: Rodney M. Webb; Edward A. Benson; Michael J. Dawkins
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1988-04-29
Filing date: 1989-04-18
Publication date: 1989-11-02
Also published as: KR900700580A; DK259890A; DK259890D0; AU3422189A; JPH03504024A; NZ228867A; WO1989010393A1; CN1037533A

Abstract

Coal slurry compositions are stabilized against settling by a cellulose ether, especially methyl-cellulose.

Description

This invention relates to coal slurry compositions and to a method for their preparation and treatment. Particularly, this invention relates to stabilization of coal slurries and the treatment of coal slurries to prepare more uniformly consistent particle size containing slurries.
Consideration of coal for gasification to provide alternate gaseous fuel for energy production in more economical combined cycle gas turbine generating plants has opened the way for use of large reserves of domestically available coal supplies. However, one disadvantage is that the coal reserves are often located far from the source of demand for electrical energy. This requires large trains of railroad cars or barges which are expensive to transport. Further, the handling and transport of dry solids has its own problems. For this reason it has been suggested that the coal be transported by pipeline as a slurry of finely divided coal particles. However, the use of coal slurries also has the problem of settling and packing which causes equipment plugging, e.g., in pumps, piping and storage tanks.
Several solutions to this problem have been suggested. In U.S. 4,722,740, to Donnelly, it is suggested that a dispersion of coal in aqueous solution include a multi-component additive which is a water soluble, nonionic wetting agent; a water soluble, low molecular weight polymer; and a water soluble, medium-to-high molecular weight polymer. A further limitation is on the particle size distribution requiring a major portion of the particles to have an average particle size of from 75 to 35 µm and a minor portion of particles to have an average particle size of from 6 to 15 µm. Another carbon slurry stability additive mixture is described in U.S. 4,729,795 to Fillipo, in which the additives include sodium carboxymethyl-cellulose and hectorite clay added in specified amounts to powdered activated carbon prepared by the destructive distillation of a wood precursor.
Further, even if long distance pipeline transport of coal slurry is not required, e.g., the generating plant is located adjacent the opened coal mine or the coal material is transported by bulk transport other than pipeline, the handling of coal slurries is still preferable, especially for feeding to pressurized gasification reactors. Therefore, a stable slurry which does not interfere with the gasification process nor add complexity in the components required to stabilize the slurry and which does not require a multicomponent stabilizer package is desirable and an object of the present invention.
The present invention which provides a solution to the problems of stabilizing coal slurries and satisfaction in use includes a coal slurry composition having a major portion of finely divided coal, a minor portion of water, and a stabilizing amount of a cellulosic compound. Preferably, the cellulosic compound is a cellulose ether and is selected from methyl cellulose ethers, hydroxypropyl methylcellulose ethers, hydroxybutyl methylcellulose ethers and mixtures of these. More preferably, the cellulose ether is present in an amount of from 0.0025 to 0.02, preferably 0.01 to 0.015, percent by weight of the total slurry composition.
As indicated above, the invention includes a coal in water slurry. As used herein, the term "coal" is employed to mean several forms of carbonaceous material which can be employed by various processes for gasification. Thus, any form of carbonaceous material which is a solid or can be formed into a solid and can be finely divided to form a particulate suspension or dispersion in water is contemplated. Typical coals include but are not limited to anthracite, bituminous, subbituminous, lignite, coke, coal char, coal liquefaction residues, petroleum coke, particulate carbon soot in solids derived from oil shale, tar sands and pitch.
The coal can be finely divided, usually by grinding in a ball, hammer or rod mill. The grinding or other type of comminution should be sufficient to permit the coal to be classified as boiler grind, i.e., 70 weight percent of the finely divided coal should pass a 200 mesh (75 µm) screen. This is not a limitation, but a convenient measure of general use in the industry. Both larger and smaller particles can be useful in the present invention such as 30 weight percent or less passing a 200 mesh (75 µm) screen or 100 weight percent passing. More preferably, the composition of this invention is most advantageous with coarser grinds because they have a larger stability problem, such as "pipeline" grind used for slurry transport which is 25 weight percent passing 200 mesh (75 µm) and 95 weight percent passing 16 mesh 1.1 mm) screens.
The coal is mixed with water, preferably just before or during the comminution, in order to have the suspension or dispersion formed for easy transport of the particulate coal. Although the least water employed in the slurry the more economical the operation, from 30 to 49 percent by weight of the slurry can be water. Preferably, the water content is from 40 to 49 percent by weight. It is only necessary that the slurry be of pumpable consistency.
The cellulose ethers which are used in the present invention are known and commercially available. However, cellulose compounds previously used have not been able by themselves to sufficiently stabilize the slurries. The cellulose ethers of the present invention are produced commercially by replacing the hydroxy groups of beta-anhydroglucose rings in the linear chains of cellulosic compounds or polymers with one or more organic substituents. The chemical nature, quantity, and distribution of the substituent groups govern such properties as solubility and surface activity. Generally, cellulose ethers can be divided into water-soluble and organic-soluble cellulose ethers. Water-soluble cellulose ethers are useful in the present invention because the slurrying liquid is water. Although a water-soluble cellulose ether is preferred, organic-soluble cellulose ethers can be employed in this invention, provided that an organic solvent is employed as the slurrying agent.
Typical water-soluble cellulose ethers include sodium carboxymethyl 2-hydroxyethyl cellulose, 2-hydroxyethyl cellulose, methyl cellulose, 2-hydroxypropyl methyl cellulose, 2-hydroxyethyl methyl cellulose, 2-hydroxybutyl methyl cellulose, 2-hydroxyethyl ethyl cellulose, and 2-hydroxypropyl cellulose which are commercially produced. Typical organic-soluble cellulose ethers which have been commercially produced include ethyl cellulose, ethyl 2-hydroxyethyl cellulose, and 2-cyanoethyl cellulose. More preferred are the water-soluble cellulose ethers, and most preferred are methyl cellulose, 2-hydroxypropyl methyl cellulose, 2-hydroxyethyl methyl cellulose, 2-hydroxybutyl methyl cellulose and mixtures of these; of which, methyl cellulose is most highly preferred.
Generally, the cellulosic compounds employed in the invention are available in powdered and granular forms, having a white or slightly off-white color and are essentially odorless and tasteless. The cellulosics have a degree of substitution (DS) from 1.3 to 2.0 and a molar substitution (MS) from 0.15 to 0.86. Generally, the particle size of the powder and the amount of the cellulosic compound employed determines the degree of stability of the coal slurry.
The coal, water, and cellulosic compound can be mixed in any, convenient manner. It is only necessary to contact the three components together. As a convenience, it has been found efficient to add the water and cellulosic compound to the coal grinding mill. In this manner the cellulosic compound can additionally serve as a grinding aid. Addition to the inlet of a rod mill ensures good mixing and flow through the mill, producing a more consistent grind size. Unstable slurries exhibit sloshing and erratic flow through the mill which results in washing coarse material through the mill outlet.
The stability of the slurry depends on the coal type, solids concentration, quantity of bound moisture, and particle size distribution. Stability can be determined by actual use in coal slurry handling equipment, with problems of plugging in transport piping, decreased usable tank volume, discharge nozzle plugging, pump plugging, and frequent rod mill oversize discharge indicating that the slurry is not stable. Before reaching commercial operations, a laboratory method of determining stability can be employed to investigate a particular type of coal. For example, the viscosity and shear stress of various mixtures of coal slurries at different shear rates can be measured using a Fann® viscometer to determine the level of stabilizer required without unduly increasing viscosity and the yield stress estimated by extrapolating the shear stress vs. shear rate curves to zero shear rate. The yield stress indicates the degree of stability; the higher the yield stress number the more stable the slurry.

Example

To a slurry of 51.1 percent by weight of subbituminous coal from the Rochelle Mine of the Peabody Coal Company, ground so that 20.2 weight percent passes a 325 mesh (45 µm) screen, in water was added various amounts of METHOCEL F4M brand methyl cellulose ether, produced by The Dow Chemical Company, Midland, Michigan. The viscosity and shear stress was obtained using a Fann® viscometer and the yield stress calculated. The viscosities at various dial settings on the viscometer are given in the table below

Table 1

Viscosity of Stabilized Coal Slurries at Different Shear Rates
	VISCOSITY*, in centipoise (Pa·s)
RPM SHEAR RATE	600	300	200	100	6	3
(SEC-1)	(226)	(113)	(75)	(38)	(2.3)	(1.1)
WT. % STABILIZER ADDED
0	178	223	254	267	446	446
	(0.178)	(0.223)	(0.254)	(0.267)	(0.446)	(0.446)
0.0025	192	259	308	321	446	446
	(0.192)	(0.259)	(0.308)	(0.321)	(0.446)	(0.446)
0.005	241	357	401	428	891	891
	(0.241)	(0.357)	(0.401)	(0.428)	(0.891)	(0.891)
0.01	299	410	481	508	1114	1337
	(0.299)	(0.410)	(0.481)	(0.508)	(1.114)	(1.337)
0.015	579	847	936	1097	2674	3566
	(0.579)	(0.847)	(0.936)	(1.097)	(2.674)	(3.566)
0.02	722	1177	1471	1899	5795	8023
	(0.722)	(1.177)	(1.471)	(1.899)	(5.795)	(8.023)

*Measure at 73-75°F (23-24°C)

From the above data the yield stress is given as follows: Shear stress = 0.01 x viscosity x shear rate (Dynes/cm² (Pascals))
Yield stress = 1.9167 x (shear stress @ 3 RPM) - 0.9167 x (shear stress @ 6 RPM) (Dynes/cm² (Pascals)). If yield stress < 0 then yield stress = 0.

Yield Stress

Amt. of Stabilizer, in Weight % Dynes/cm² (Pascals)

0.0 0 (0)

0.0025 0 (0)

0.005 0 (0)

0.01 4.7 (0.47)

0.015 18.8 (1.88)

0.02 46.97 (4.697)

The above yield stress data indicates that only above 0.015 weight percent does the viscosity begin to increase greatly compared to the shear rate. Thus, below 0.02 weight percent, and preferably below 0.015 weight percent does the stabilizer add stability without greatly increasing the viscosity.

Claims

1. A coal slurry composition which is stable to settling comprising a major portion of a finely divided coal, a minor portion of slurrying agent, and a stabilizing amount of a cellulose ether.

2. A composition as claimed in Claim 1, wherein the slurrying agent is water.

3. A composition a claimed in Claim 2, wherein the water content is 40 to 49 percent by weight.

4. A composition as claimed in Claim 2 or Claim 3, wherein said cellulose ether is a methylcellulose, a hydroxyethyl methylcellulose, a hydroxypropyl methylcellulose, a hydroxybutyl methylcellulose or a mixture of two or more thereof.

5. A composition as claimed in Claim 4, wherein said cellulose ether is a methylcellulose.

6. A composition as claimed in any one of the preceding claims, wherein said coal is coal classified as bituminous, sub-bituminous or lignite.

7. A composition as claimed in any one of the preceding claims, wherein said coal is finely divided to a particulate size distrubution wherein 70 percent by weight passes a 75 µm (200 mesh) sieve.

8. A composition as claimed in any one of the preceding claims, wherein said cellulose ether is present in an amount of from 0.0025 to 0.02 percent by weight of the total composition.

9. A composition as claimed in Claim 8, wherein said cellulose ether is present in an amount of from 0.01 to 0.015 percent by weight of the total composition.

10. The use of a cellulose ether to stabilize a coal slurry composition.