EP0239404A2

EP0239404A2 - Fluid compositions containing coal, water and flow improvers

Info

Publication number: EP0239404A2
Application number: EP87302639A
Authority: EP
Inventors: Francois Laurent Godouet; Gilbert Marie Georges Lecorbeiller
Original assignee: Exxon Chemical Patents Inc
Current assignee: ExxonMobil Chemical Patents Inc
Priority date: 1986-03-26
Filing date: 1987-03-26
Publication date: 1987-09-30
Also published as: EP0239404A3; GB8607546D0

Abstract

Coal/water slurries are given improved flow and stability by incorporating a synergistically cooperating combination of (i) a polyalkoxylated fluidifier of molecular weight greater than 4000 and (ii) a dispersant being an hydrophilic anionic salt of an alkyl aryl sulphonic acid of molecular weight not greater than 440.

Description

This invention relates to coal/water compositions which incorporate flow improvers for improving composition fluidity. Particularly but not exclusively the invention is concerned with the use of synergistic combinations of additives in coal/water compositions to improve fluidity.
With the increasing cost and anticipated eventual scarcity of crude oil and its refined products, renewed emphasis is being placed on the use of coal as a fuel. The physical form of coal, however, places severe limitations on its direct use if this is to be done in an efficient and environmentally acceptable manner, and consequently several techniques are currently being explored whereby these limitations may be overcome or minimised. Mention may be made, for example, of coal gasification processes and the reduction of the coal to finely divided form in suspension in oil, methanol or water. Coal-in-water slurries, in particular, offer a number of conceptual and practical advantages, notably low cost, ease of handling, safety and low transportation costs as compared with dry pulverised coal. The slurries are prepared by admixing the finely pulverised coal with water, usually in the presence of dispersing agents and suspension stabilisers. The role of the dispersing agent is to prevent flocculation or coagulation of the coal particles during the mixing process, whilst the suspension stabiliser serves to prevent settling out of the dispersed particles on storage of the slurry. Clearly, for economic reasons, it is desirable to maximise the coal content of the slurries, so as to minimise the absorption of heat due to evaporation of the water during combustion. High coal solids contents, however, generally lead to the slurries having extremely high viscosities, which in turn render very difficult or impossible the handling and transfer of the slurries, especially by pumping.
US-A-4470828 discloses coal/water slurries using a combination of two compounds, a polyether polyol compound and a surface active agent, which may be a salt of a naphthalene sulphonic acid. However, as demonstrated hereinafter, this combination is not efficient in reducing viscosity, particularly at low shear rates.
WO-A-81-02024 describes stabilising emulsions using a combination of a low molecular weight agent and a high molecular weight material. Among the materials described as low molecular weight agents is Toximul MP of Stepan Company which is said to be a combination of a calcium dodecyl benzene sulphonate and alkyl phenoxy polyoxyethylene ethanol. The,specification also requires the presence of a high molecular weight material, and one suitable material is modified carboxymethyl cellulose. As demonstrated hereinafter such compositions are paste-like materials and not slurries.
WO-A-83-04045 describes using a combination of an alkylene oxide adduct and a sulphonic acid where the sulphonic acid is a polyalkyleneoxy compound. EP-A-89766 describes a two step preparation of slurries using a dispersant alone, and such systems do not provide adequate viscosities in coal/water slurries.
According to the present invention there is provided a fluid composition of coal, water and flow improving additives comprising:

(a) from 40-90 parts by weight of pulverised coal;
(b) from 0.1-2 weight percent, based on the dry weight of the pulverised coal, of a fluidifier comprising a polyalkoxylated compound of molecular weight greater than 4000;
(c) from 0.02-1 weight percent, based on the dry weight of the pulverised coal, of a dispersant comprising an hydrophilic anionic salt of an alkyl aryl sulphonic acid, said acid having a molecular weight not greater than 440; and
(d) sufficient water to bring the total composition weight to 100 parts.

The pulverised coal for incorporation in the compositions of the invention may be obtained by techniques already well known such as by the use of tumbling mills or ball mills with recirculation of the ground material until the,desired particle size distribution is obtained. The particle size of the coal is not generally critical for the purposes of the invention, but for pulverisation systems which produce coal with a unimodal particle size distribution it is preferred that at least 80% of the particles, on a weight basis, have a particle size less than 80 microns, more preferably with from 2-30% of the particles being less than 30 microns in diameter. It is also known to pulverise coal to a multimodal, e.g. bimodal particle size distribution, and in this case it is preferred that from 20-60% of the particles, on a weight basis, should have a particle size of 0-30 microns and from 40-80% of the particles, on the weight basis, should have a particle size of from 40-200 microns.
The coal content of the compositions may vary between 40 parts and 90 parts by weight out of a total composition weight of 100 parts; more typically, it will lie in the range 50 parts to 80 parts by weight, especially 65-75 parts by weight.
The proportion of the dispersant and fluidifier used in the compositions of the invention depends on the chemical nature of the additives used, and on the characteristics of the coal component. In general, all types of coal are usefully converted to slurry form: such slurries are used in respect of highly volatile bitumenous coals both for enabling transportation through pipelines and associated pumping, and for delivery of the coal into furnaces, since the highly volatile coals are readily combusted. Slurries containing anthracite coals, on the other hand, are less readily combusted because of the inherent properties of the coal; however, such coal types can usefully be converted to slurry form to facilitate transportation. It is a feature of the present invention that the combination of additives incorporated in the coal/water slurry act synergistically to give properties to the composition which are not manifested when only one of the additives is present. The general effect of the combination has been found to be that the viscosity of the composition is considerably reduced under shear rate conditions which equate to pumping the composition through a pipeline, whilst this reduced viscosity level in somewhat increased under low shear rate conditions which equate to storage of the compositions. Thus the compositions of the invention have improved fluidity for transportation and good stability on storage.
With regard to the fluidifier, this should be present in an amount of 0.1-2 wt % based on the dry coal weight, it has been found that lower proportions have much reduced effect in combination with the dispersant, whilst higher proportions, because of the polymeric nature of the fluidifier, can actually lead to an. increase in composition viscosity. On technical and economic considerations, the fluidifier is preferably present in 0.5-1 wt % based on dry coal weight.
The amount of dispersant required in accordance with the invention is 0.02-1 wt % based on dry coal weight. Again, less than this amount shows a much reduced effect, whilst greater than 1% shows no outstanding technical benefit and, of course, an economic debit. Preferably, the dispersant is used in amounts of 0.1-0.2% based on dry coal weight. The optimum proportions for each of the fluidifier and dispersant depend on coal content, coal type, coal particle size, the amount and specific chemical nature of the respective other essential additive employed, and on the principal requirement of the slurry composition (storage, transportation). Such optimum proportions are readily assessable by laboratory scale experiments.
The percentages mentioned with regard to the additives relate to the amount of active material, not including any diluent which may optionally be used for ease of handling or mixing.
The fluidifier is required to be, a polyalkoxylated compound of molecular weight greater than 4000, preferably in the range 5000-30000. Compounds below the minimum specified molecular weight have been found not to have the desired flow improving characteristics, whilst compounds of too high a molecular weight have been found actually to inhibit slurry flow. The fluidifier functions to fluidify a coal/water mixture which has the characteristics of a damp powder, into a flowable liquid mixture. Polyalkoxylated phenols and alcohols of the specified molecular weight range have been found suitable for the purposes of the invention, especially where the polyalkoxylation is performed using ethylene oxide or propylene oxide or both of these. However, it is preferred that a polyalkoxylated amine is employed. Such amine may be mono-, di- or multi-amine functional and again alkoxylation may be, for example, with ethylene oxide, propylene oxide or both of these. In the case of amine based fluidifiers, those with molecular weights in the lower end of the preferred range, for example, 6000-8000, and those with molecular weights in the higher end of the preferred range, for example, 25000-28000 have each been found to give good results when used with a dispersant in accordance with the invention. It is particularly preferred to employ as fluidifier an amine, such as ethylene diamine, which has been alkoxylated with both propylene oxide and ethylene oxide, preferably in that sequence. The ratio of ethylene oxide:propylene oxide used is preferably in the range 70:30 - 90:10. The preferred degree of polyalkoxylation in the fluidifier compound is from 75-720, and in the case of compounds which have been reacted with both ethylene oxide and propylene oxide it is preferred that the compound includes 60-600 ethoxide groups and 15-120 propoxide groups.
The dispersant used in the compositions of the invention must be hydrophilic to give the desired result in combination with the specified fluidifier, and is an anionic salt of an alkyl aryl sulphonic acid. It functions to reduce the viscosity of the composition by increasing the wettability of the coal, and hence permits the fluidifier component to function more efficiently. The salt is preferably an alkali metal salt such as lithium, sodium or potassium, although amine salts may be used. The molecular weight of the acid component of the salt must not exceed 440, and is preferably from 330-390. Higher molecular weight materials generally have a reduced dispersant effect.
The aryl group of the acid is preferably substituted with one or more relatively long chain (branched or linear) alkyl groups, to a maximum total chain length of C15. , More preferably the alkyl substitutent is C9 to C12. The aryl group may be, for example, based on benzene, xylene or toluene, i.e. in addition to the relatively long chain alkyl substituent it may carry one or more short chain, e.g. methyl, substituents. It is particularly preferred to employ as the dispersant a nonyl or dodecyl (preferably branched) orthoxylene sulphonic acid salt.
Various coal/water/dispersant/fluidifier compositions according to the invention have been found to perform even better when a stabilizer is additionally present in an amount of from 0.001-0.5 wt %, preferably 0.01-0.2 wt %, based on the dry coal weight. The stabilizer functions to increase the viscosity of the composition at low shear rates, thereby reducing or even eliminating the settling out of coal particles on storage. Viscosifying polymers such as carboxy methyl cellulose or hydroxyethylcellulose are preferably used, but others which may be mentioned by way of example include biopolymers of the polysaccharide type such as xanthan or scleroglucan polysaccharide.
Other components which may optionally be included in the compositions of the invention include, for example, antifoam agents and biocides. A further potential additive is a heavy hydrocarbon oil, for example, in amounts up to 10 wt % based on the total weight of the composition, preferably in amounts up to 5 wt %. The presence of such oils enables the other active additives to be employed with the same effect on the flow properties of the composition, but with lower treat rates.
The compositions of the invention may be prepared by the steps of dissolving or dispersing the active flow improving additives in the water, adding the pulverised coal and subjecting the mixture to the action of a mixer for a sufficient period of time to produce a dispersion. It is normally satisfactory to carry out these operations at or near room temperature. As mentioned, in certain cases it may be desirable to add an anti-foaming agent to prevent excessive foam formation during the mixing operation.
An alternative preparation technique which may be mentioned is to add the additives dissolved in water to a cake of pulverised coal particles containing up to about 20% wt water. The "solid" cake mixes readily with the aqueous additives to produce the fluid composition of the invention. A further preparation option is to incorporate the additive combinations, dissolved in water, into the milling stage of the technique where the coal is pulverised.
The following Examples illustrate the invention.

Example 1

A medium volatility bitumenous coal from the Smoky River coal- field, Canada, having a Swelling Index of 1 (measured on temperature increase to 800°C according to Standard French Test NF M 11-001 of November 1968); 25.15 wt % volatile matter content; 10.1 wt % ash content; and 1.3 wt % moisture content was pulverised in a mill to a particle size distribution (measured by sieving) as shown in Table 1. ¹
The distribution was unimodal, with 87% of the particles, on a weight basis, being less than 80 microns in diameter.
The pulverised coal was incorporated into a nominally 70:30 coal/water slurry by mixing 70 parts by weight of the coal with 29.3 parts by weight of water and 0.6 parts by weight of a fluidifier, which amount of fluidifier corresponds to 0.86 wt % based on the coal weight. The fluidifier employed was a polyalkoxylated ethylene diamine having a molecular weight of 27000 and melting point 60⁰C, and comprising ethylene diamine which had been first propoxylated with propylene oxide, then ethoxylated with ethylene oxide, using propylene oxide:ethylene oxide in a weight ratio of 20:80, to the designated molecular weight. The coal/water mixture was powdery in nature and exhibited no flow characteristics at all, whereas the coal/water/fluidifier mixture was an unstable fluid from which coal particles settled very quickly on storage.
This fluid was divided into two aliquots (A) and (B), and to (A) was added a small amount of water to a final weight composition (A) of 70% coal/29.4% water/0.6%'fluidifier. To (B) was added an amount of dispersant to yield a final weight composition (B) of 70% coal/29.3% water/0.6% fluidifier/0.1% dispersant. As mentioned, composition (A) was unstable at room temperature on storage, whereas composition (B) according to the invention showed no tendency to settling out. The dispersant employed was dodecyl orthoxylene sulphonic acid sodium salt.
A further composition (C) was prepared using the same pulverised coal and the same dispersant as employed in composition (B), but this time no fluidifier was incorporated. The proportions of the components of composition (C) were adjusted to final weight values of 70% coal/29.9% water/0.1% dispersant.
The flow properties of compositions (A), (B) and (C) were evalu_- ated by measuring viscosity characteristics of each at ambient temperature (20°C) over a range of shear rates. However, in the case of composition (C) no viscosity values could be obtained since the composition was pasty in nature and exhibited no flow characteristics. The results are shown in Table 2 and plotted in Figure 1 of the accompanying drawings.
From these results it is seen that the use in coal water slurries of the fluidifier alone (at a treat rate of 0.86% based on coal weight) and of the dispersant alone (at a treat rate of 0.14% based on coal weight) each gives a poorer performance than in the case where the fluidifier and the dispersant are employed in combination; that is, the fluidifier and the dispersant have a synergistic effect on overall composition characteristics. The composition (B) of the invention generally has a viscosity of about half that measured for composition (A) under equivalent shear conditions, a major advantage when considering the suitability of (B) for transportation through pipelines and pumpability. Moreover, the viscosity is reduced at the high shear rates applicable to transportation and also at the higher shear rates associated with delivery of the fluid to furnace burners.
It is further noted that the characteristics of (A) are mirrored by (B) at low shear rates, namely the viscosity increases as shear rate approaches zero. This is a useful characteristic of coal/water slurries, since it means that at storage conditions (zero shear rate) the fluid will remain stable and, as observed with (B), there will be no or minimal settling of the coal particles. As mentioned, composition (C) was pasty in character and not pumpable.

Example 2

A highly volatile bituminous Polish coal having Swelling Index of 3.0 (French test NF M 11-001 of November 1968); 32.4 wt % volatile matter content; and 2.6 wt % ash content, was pulverised in a mill to a particle size distribution (measured by sieving) as shown in Table 3. A major proportion of the particles were below 80 microns in size.
In a manner corresponding to that used in Example 1, the coal was formed into a nominally 70:30 coal/water composition by mixing 70 parts by weight (pbw) coal with 29.3 pbw water, 0.5 pbw fluidifer and 0.08 pbw stabiliser. -The fluidifier was a polyalkoxylated ethylene diamine having a molecular weight of 7500, the amine having been alkoxylated with both propylene oxide and ethylene oxide in a propylene oxide: ethylene oxide weight ratio of 15:85. The stabilizer was carboxymethyl cellulose. The mixture was divided into two aliquots (D) and (E), and to (D) was added a small amount of water to give final weight composition (D) of 70% coal/29.5% water/0.5% fluidifier and, based on that composition, 0.08 wt % stabilizer. To (E) was added an amount of dispersant to yield a final weight composition (E) of 70% coal/29.3% water/ 0.5% fluidifier/0.2% dispersant and, based on that composition, 0.08 wt % stabilizer. The dispersant was nonyl orthoxylene sulphonic acid sodium salt.
A further composition (F) was prepared using the same pulverised coal, stabilizer and dispersant as employed in composition (E), but this time no fluidifier was incorporated. The proportions of the components of composition (F) were adjusted to final weight values of 70% coal/29.8% water/0.2% dispersant and, based on that composition, 0.08 wt % stabilizer.
The viscosities of (D), (E) and (F) were measured at ambient temperature (20°C) over a range of shear rates, although composition (F) was pasty and exhibited no flow characteristics. The results are shown in Table 4 and plotted in Figure 2 of the accompanying drawings.
Reference to these viscosity measurements shows that, as with Example 1, composition (E) of the invention has about half the viscosity of composition (D) which contains only fluidifier and stabilizer (in addition to coal and water) without the synergistically acting dispersant. The stabilizer/fluidifier pairing acid and stabilizer/dispersant pairing clearly do not function in the synergistic manner that is demonstrated for the defined fluidifier/dispersant system for coal/water slurries.

Example 3

In a manner corresponding to Example 1, coal/water compositions comprising 70 wt % coal were prepared by mixing Smoky River coal, water, 0.6 wt % fluidifier of Example 1 and 0.1 wt % of each of the dispersant of Example 1, ATLOX 4862 of ICI and HUMIFEN NBL 85 of GAF (which are each sodium salts of condensates of naphthalene sulphonic acid with formaldehyde). The test results comparing these compositions are given below:
The apparent viscosities measured for the above compositions are :
The above results show that the composition of the invention is much more efficient (especially at lower shear rates) making it possible to obtain a low viscosity slurry.

Example 4 (Comparative)

An attempt was made to prepare coal water slurry using TOXIMUL MP of Stepan company as an additive in place of the fluidifier and dispersant of the invention.
Although the invention provides fluid coal water slurries, coal water compositions with TOXIMUL MP were paste-like materials when using either Smoky River coal, or Scotia liquid coal (known to be good at forming slurries). The addition of carboxymethyl cellulose did not improve the fluidity of these compositions.

Claims

(1) A fluid composition of coal, water and flow improving additives comprising:
(a) from 40-90 parts by weight of pulverised coal;

(b) from 0.1-2 weight percent, based on the dry weight of the pulverised coal, of a fluidifier comprising a polyalkoxylated compound of molecular weight greater than 4000;

(c) from 0.02-1 weight percent, based on the dry weight of the pulverised coal, of a dispersant comprising an hydrophilic anionic salt of an alkyl aryl sulphonic acid, said acid having a molecular weight not greater than 440; and

(d) sufficient water to bring the total composition weight to 100 parts.
(2) A composition according to claim 1 wherein the pulverised coal has a unimodal particle size distribution and at least 80% of the particles, on a weight basis, have a particle size less than 80 microns.
(3) A composition according to claim 1 wherein the pulverised coal has a multimodal particle size distribution, from 20-60% of the particles, on a weight basis, having a particle size up to 30 microns and from 40-80% of the particles, on a weight basis, having a particle size of from 40-200 microns.
(4) A composition according to claim 1, 2 or 3 which comprises from 50-80 parts by weight of pulverised coal.
(5) A composition according to claim 4 which comprises from 65-75 parts by weight of pulverised coal.
(6) A composition according to any one of the preceding claims which comprises from 0.1-0.2 wt % of said dispersant.
(7) A composition according to any one of the preceding claims wherein the dispersant is nonyl or dodecyl orthoxylene sulphonic acid salt.
(8) A composition according to any one of the preceding claims which comprises from 0.5-1 wt % of said fluidifier.
(9) A composition according to any one of the preceding claims which additionally includes from 0.001-0.5 weight percent, based on the dry weight of the pulverised coal, of a stabilizer capable of increasing the viscosity of the composition at low shear rates and reducing settling out of coal particles.
(10) A composition according to claim 9, wherein the stabilizer comprises carboxymethyl cellulose or hydroxyethyl cellulose.
(11) A composition according to any one of the preceding claims which includes up to 10 weight percent, based on the dry weight of the pulverised coal, of a heavy hydrocarbon oil.
(12) A fluid composition of coal, water and flow improving additives comprising:
(i) from 50-80 parts by weight of pulverised coal;

(ii) from 0.5-1 weight percent, based on the dry weight of pulverised coal, of a fluidifier comprising a sequentially propoxylated then ethoxylated ethylene diamine of molecular weight in the range 6000-8000 or 25000-28000;

(iii) from 0.1-0.2 weight percent, based on the dry weight of pulverised coal, of a dispersant comprising an hydrophilic anionic alkali metal salt of nonyl or dodecyl orthoxylene sulphonic acid; and

(iv) sufficient water to bring the total composition weight to 100 parts.