Process for Making Coal-Water Fuel Slurries and Product Thereof

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 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 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 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 .mu.MMD (mass median diameter), preferably about 1.mu. to 8 .mu.MMD and larger particles hereafter defined as (F/C), having a size range of about 20.mu. to 200 .mu.MMD, preferably about 20.mu. to 150 .mu.MMD. For stability of the slurry, the UF particles should comprise about 10 to 50% by wt of the slurry, preferably about 10 to 30% and more particularly 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 successfuly 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 storageability. 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 25,000. The surfactant is used in minor amount, e.g. about 0.5 to 5 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 pH 5 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 phosphate or carbonate, including their acid salts, is used in minor amounts sufficient to provide the desired pH, e.g. abut 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.sup.-1, preferably at least about 500 sec.sup.-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 thixotropic 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 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 (20.mu. to 200 .mu.MMD) 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.

3.6 .mu.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.

34 .mu.MMD and 110 .mu.MMD particles used in Examples 3-9 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 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 .omega..sup.2 r/g where .omega. 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 cummulative 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.

50% 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 3.6 .mu.MMD. The UF coal-water slurry was fluid and pourable.

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 .mu.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 34 .mu.MMD.

A. 65% by wt of coal comprising 55% 110 .mu.MMD coal and 45% 3.6 .mu.MMD coal, 1.3% Marasperse C-21 (calcium lignin sulfonate, Ca content as CaO 5.2%, Na content as Na.sub.2 O 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.sup.-1. 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 havig relatively low viscosity of 6.7 p.

B. A mix was made identical to A except that 34 .mu.MMD particles were substituted for the UF particles. The mix, though initially fluid was unstable. Within 3 days it separated, forming a large supernatent and a highly packed subsidence. It could not be remixed into a uniform, pourable dispersion.

A. A 65% coal slurry comprising 15% 3.6 .mu.MMD and 50% 34 .mu.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 a 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.sup.-1). The resulting slurry was unstable. Within 4 days it had separated into liquid and aggregated sediment.

A. A 65% coal slurry comprising 26% 3.6 .mu.MMD particles and 39% 110 .mu.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 uniformily 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.sup.-1. The resulting slurry was unstable. Within 3 days it had separated into supernatant and aggregated sediment.

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.

A. A 70% coal slurry comprising 45.5% 110 .mu.MMD particles and 24.5% 3.6 .mu.MMD particles. 1.4% Marasperse C-21, and 28.6% water solution buffered to pH 7 by 0.15% Na.sub.2 HPO.sub.4 added in the blender was mixed at 6,000 RPM. The resulting slurry has a 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 ball mill producing the UF particles and was run in a blender at the low shear rate of 60 RPM (10 sec.sup.-1). The slurry was unstable and within 5 days separated into supernatant and stiff aggregated sediment.

A mix was made identical to Example 4A except that Na.sub.2 HPo.sub.4 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.

A. 30 wt% of hammermilled coal fines (30 .mu.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 3.88 .mu.MMD.

B. A 65 wt% coal slurry comprising 50 wt% 34 MMD coal particles, 15 wt%, 3.88 .mu.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.sup.-1). The product was uniformly-dispersed, pourable slurry. After 56 days the slurry was a stable, 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 Na.sub.2 HP.sub.4. 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 some what more viscous, retained its stability and pourability 5 days after the initial stirring.

The ultrafine 3.6 .mu.MMD coal component was made in accordance with Example 1. A 110 .mu.MMD coal component was prepared as in Example 2.

A 65% coal slurry comprising 32.5% 3.6 .mu.MMD and 32.5% 110 .mu.MMD coal particles by wt of the slurry, 0.65% Marasperse C-21, and 34.35% water, was prepared in a high speed bender at 6000 rpm (shear rate approximately 1000 sec.sup.-1). The resulting slurry was a soft thixotropic gel with a yield point of 49 dynes/cm.sup.2. With light stirring 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 hard-packed sediment, and could easily be stirred to uniformity and pourability.

The 3.6 .mu.MMD ultrafine coal component was made in accordance with Example 1, except that 1% Lomar UDG, a calcium naphthalene sulfonate containing 11.5% Ca as CaSO.sub.4, was substituted for the Marasperse C-21. A 110 .mu.MMD coal component was prepared as in Example 2.

A 65% coal slurry, comprising 32.5% 3.6 .mu.MMD and 32.5% 110 .mu.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/cm.sup.2. 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.

The ultrafine 3.6 .mu.MMD coal component was prepared by mixing 60 wt% coal with 0.6% Marasperse C-21, 0.28% Na.sub.2 HPO.sub.4, 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 .mu.MMD coal fraction was prepared by hammermilling as in Example 2.

A 65% coal slurry comprising 50% 3.6 .mu.MMD and 15% 110 .mu.MMD coal particles by wt of the slurry, Marasperse C-21 0.65%, 0.23% Na.sub.2 HPO.sub.4, 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.

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 compare very favorably with fuel oil, as is shown in the following tables: