The influence of surfactants to the stability of coal water suspension Saule

Ahstratl In this work a coal-watcr su.s/tension with containing o f 30% (пшяи) ічніі was invest lo tted Obtained coal-water suspension и as Mobilized by sothuni d/ніесуіsulphate (SIKS) and ox\rthylated alkyl phenol (ОГ-І0) A wettability isotherm o f coal sur­ face by surfactants, optimal regime o f coal dispersion, adsorp­ tion isotherm, viscosity fluidity and heat o f combustion o f 30% (mass) amiwater suspension were determined. Obtained results allow to rccommcnd 30% axil-water sus/nrnsion stabi­ lized by surf actants as a fuel.

1 In tro d u c tio n I1»c stable tendency o f a pnee growth o f oil product:, leads to an intensive rcscarch of a new technology o f a coal refin ing Iwcausc the explored coal reserves arc 20 limes more than nil reserves Coal-watci suspensions arc paid attention as a real alternative to liquid fUels from oil in the countries such as Japan.
People's Rqniblic o f China, Italy.USA.Sweden and Russian Federation This is connected with a high scientific and techni cal potential gathered by these countries in the area o f produc tion.transportation, burning o f coal-water fuels in boiler, cogen eration plants, combined-cycle plants and gas turbines [1-8].
However, despite o f these' advantages o f a coal-water fuel, there are several disadvantages, among which the most impor tant is the instability o f a liquid fuel.It is known that the viola tion o f stability, sedimentation o f particles and the lamination of the dispersed system arc due to the differences in specific gravity o f the dispersed phase and a liquid dispersion medium (6) Therefore for the preparation o f coal-water suspensions from a tlteoretical point o f view is advisable to use the fractions with the same (proportional) size o f coal particles 19 -12].
Another important way to improve the stability of coal-water suspension (CWS> is the addition o f stabilizers (polymers and surfactants) that reduce the coagulation (coalescence) o f coal particles and their sedimentation [13][14][15][16][17] In addition, in accordance to fundamental principles o f phys icochemical mechanics and the modern theory o f the stability of dispersed systems (theory of Dcrjaguin-Landau-Verwcy-Ovcrbeck) the stabilization o f the dispersion is determined by the potential curve o f the dispersed phase (DP) interaction In particular, the coagulation interaction in dispersions at the concentrations o f dispersed phase is higher than the critical micelle concentration depends on tlic nature o f the particles o f dispersed phase, temperature, ionic strength and the presence of low and high molecular surfactants It is also known that the stability o f dispersed systems is determined by the surface properties o f the dispersed phase It is known that the coal surface is hydrophobic so surfactants with different nature have higher affinity to the coal surface The purpose of this work is to study o f the stability of 30K*(mats ) watcr-coal suspension in the presence of anionic surfactant &odium dodecyl sulphate (SDS), nonionic surfactant oxycthylated alkylphcnol O P-10 and the determination o f thermophysical properties (heat of combustion) of the obtained composite system

E x p e rim e n ta l
As an object coal with a moisture W*-7,7%, ash content A '-13 67%, volatile matter content Y,4,r-43 3%, carbon content in a dry form Cdaf-73.89% was chosen All experiments were performed at t (22±0.I)°C Hie error of wetting was 10, the error o f the adsorption at the solid/fluid 0.1-0 2%, whereas the error for the sedimentation measurements was equaled to ±0.30 units in eyepiece Particle with the sizes in the range 0 2-0.5 microns were obtained by dispersing in the colloid mill By sifting through a sieve with a pore size of 0 25 mm a fraction up to size <0.25 mm was separated which was used in further studies The par ticle size o f coal and sedimentation analysis was determined by Figurovskyi method which was described elsewhere [17] First experiments according to method [17] were carried to determine the size of the dispersed phase, and then from the obtained data the path (N) of deposited particles after mixing was defined According to it the sedimentation rate and particle radius were determined where tt -the time which ncads to finish to sedimentate a given fraction, mm; H-distance o f the dispersed phase from the liq uid surface to the bottom of the cup.cm After the determination of the sedimentation rate the radius (r() o f particles in tins fraction was found where rij and nt -ocular division.The dispergation time mcrease up to 180 minutes did not show significant influence to the particles dispcrsity That is why for further investigations dispergation time was kept 120 minutes.
According to the physico-chemical mechanics laws o f the highly dispersed particles obtaining the dispergation would be better with the presence o f surfactants.Iliat is why as sur factants sodium dodecyl sulphate (SDS) and noniomc surfactant oxycthylated phenol (OP-IO) were used For the selection of surfactants optimal concentration the wetting isotherms of coal surface by the aqueous solutions of surfactants were obtained With increasing o f surfactant con centration the significant decrease of contact angles of coal sur face by SDS ( from 65° up to 15-28°) was observed (Figure 2) Note that the adsorbed surfactant molecules do not only reduce the coal durability but also increase the aggregate stability of its suspension in water According to the well-known lliermodynamic relation for the highly dispersed systems the reduce of tl»e surface tension leads to a decrease of'the free energy and stability increase where F-llelmholtz free energy, о -surface tension o f the solid, S -surface area of the solid Thus, on the basis of the experimental results the optimal regime o f coal dispergation -dispergation o f preliminary crushed coal in wet state in the presence of 0 8% OP-10 or 0 4% SDS for 120 minutes was established For the preparation o f suspensions the efTcct o f dispersed phuse concentration on the lifetime of a water-coal suspension was investigated It was found that the increase of the coal con centration from 10% to 50% leads to an increase of the stability (Figure 3) The To establish the effect o f hydrophih/ation and electrostatic coal surface charge on the stability of the coal-water suspen sion the kinetics of sedimentation o f 30% coal-water suspen sion m the presence o f O P-10 and SDS was investigated Small additions of these surfactants did not have a significant effect on the stability of the coal-water suspension (Figure 4), only when the concentration is close to the critical micelle con centration (0 2-0 4%) a stabilizing effect o f these surfactants on the coal-water suspension is started The experimental results are shown for the C'MC(OP-IO) C-0.4% and CM C(SDS)-0 2% The study of the effect of these surfactants to stubilizc the WCS at lower conccnuutions o f O P-10 0 1,0 15.0 2.0 25.0 3% and in the case of SDS 0 02, 0 05% showed complete destabilization o f watct-coal suspen sion in 15-50 minutes This is most likely explained by insuf ficient amount of the stabili/ei hi the system Free location of surfactant macrornoleculcs on the surface of coal particles ічотоіоя to the binding togcthci by hydrophobic interactions  To determine the mechanism of the stabilizing effect of sur factants their adsorption on the coal surface was studied (Figure 5) The adsorption isotherm OP-10 corresponds to the S-shaped multilayer adsorption isothenn As the calculations showed in the investigated concentration range (<1.0%) on the coal sur face the adsorption layers of surfactants which arc closed to the saturation are formed As the capacity of a saturated monomo-Iccular adsorption layer O P -10 is A /=7.0T0'1 mg/g (Figure 5) it is possible to calculate the degree o f the coating o f a coal sur face by surfactant molecules: for the coal concentrations 0.4%, 0.8% and 1.0% the concentration o f O P-10 solutions is 80%, 90% and 140%, respectively The similar results were obtained in the investigation o f SDS adsorption For more information on the surfactant adsorption on coal The spectra o f the SDS shows the absorption bands of defor mation valent vibrations of the C-H bond Although the intensity is not high, the band can 1« seen well in all cases (Figures 6,7).Thus, with increasing of the surfactant concentration in the hydrosuspcnsion on the coal surface the multilayer adsorption layer is formed.The formation of a such satiaatcd adsorption layer increases the strong stabilizing factor for dispersed systems (Rehbmder's struct ural-mccliamcal factor).
As the ultimate objective o f the study was to prepare a sus tainable coal-water fuel for the hydrotransportation and direct combustion in boilers, furnaces and other objects o f power plants important thermal characteristics of coal-water suspen sions such as combustion licnt (Q) was studied for using of coal-water suspensions in practice ( Table 1).
density of the solid dispersed phase, d, -the density of the fluid, // coefficient o f viscosity of die medium, g -acceleration of gravity The obtained aggregative unstable hydrosuspension was stabilized with surfactants Anionic surfactant sodium dodecyl sulphate (SDS) with molecular weight M^288, nonionic sur factant oxycthylatcd phenol OH-10 with molecular weight M '646 were used to stabilize the hydrosuspension Aqueous solutions o f polymers with a concentration in the range of 0 I -1 0 % o f monomer units o f polymer were used Contact angle was determined by goniometer and horizontal microscope with a movable tumbler [18] Contact angle was found on basic sizes (height h and radius o f a basis r) of liquid drops on the surface.Cos Ө values were calculated by formulas r * -h 2 h In case o f# < 9 0 c o s # --:-----of droplet basis deposited on the Gonio wafer, h the height o f drop o f fluid determined by the formula h = (n?-n,), 3 E x p e rim e n ta l re s u lts For the preparation o f a stable hydrosuspension highly dis persed coal particles were obtained Particle size distribution of dispersed coal particles is shown in figure I Coal dispergation was carried up to 120 minutes, in this case the polydispersed coal panicles were formed Despite the fact that maximum o f the panicle size distribution (26 9%) is in the range o f radii 8.0-8 4 pm , the other particles radii are in the range o f 0.3-50 pm stability of the coal-watcr suspension was controlled by reducing Uie sedimentation rate o f dispersed particles ITie growth of the kinetic (sedimentation) stability of concentrated (40% -50%) coal suspensions is probably due to the formation o f coagulation structures in them From this perspective the concentrated coal suspensrons nre useless to study the effects o f various liiclois on (lie stability and rhcological properties Iheiefore for further investigation the 10% of coal-water sus-|M*union was chosen which aio sensitive to the inlluencc of external factors (surliictani and |>olymct additives, the method o f suspension, etc )

Fig. 3 .
Fig. 3.The sedimentation of partick's from Ня 10%(I ), 2 0 % (2 ), 30% (3).40% (4) mid 50% (5) of the ивреівюп die infrared spectra o f the coal particles with adsorbed mol ecules o f the O P-10 and SDS was studied.The intensity of the absorption bands o f the O-H valent fluctuations bond for the adsorbed molecules is more (3428 c m 1) than for free but a structured absorption band is higher The position o f dns band does not change in die spectra for the systems formed by these reagents This indicates the absence of die interaction In the spectra o f the O P-10 absorption bands which indicates die hydroxyl group are observed at 1620 and 1616 cm'1 The vis ible absorption band is related to the covalent fluctuations of C -C bond o f aromatic ring

F 10 F
Ifl. 5 .T h e a d so rp tio n iso th erm o f O P -W o n ih c su rfa c e o f c o a l p a itic lc s F ig .6 .IR -sp cc lra o f c o a l m o d ifie d b y 0 8 % O P -Ig .7 .IR -sp cc lra o f c o a l m odifie d b y 0 8 % S D S Table I shows that the fuel characteristics o f die coal-water suspension do not worse than similar characteristics o f pure coal In some cases, for example for 30% coal-water suspen sions stabilized by composition o f 0 8% O P -10 the heat of combustion values superior to those for dry coal This phenom enon has been observed for the first time It can be inteipretcd by the dissociation o f water molecules at 900°C for IH and OH ions which participate in the coal combustion and reduce the combustion tem|>erature (300-400"C) o f fuel Tab. 1. RrsulU ofIhc cabrimcuk detcrniiuaUon оГlumbusium heal of coftl and its hydrosuspennions n c l u s i o n s In this work Ihc coal-water suspension with containing o f 30е/* (mas*) of coul was investigated I he obtained coal water sii.ifKiilion was stabilized by sodium dodecyl sulphate (SDS) and oxyrthylaled alkyl рікіюі (OP* 10) Uascd on the experimental results it was established that the best wettability have 0.8% OP-10 and 04% Optimal regime o f cool ilispngntion ан dry сіичікчі coal dispergation m wet Male in the presence of lurfactants was determined It was found that the stabilizing efTect of OP-10 and SDS on tl»e coal-water suspensions appears at surfactant concentrations close to the criti cal micelle concentration (0 2-0.4%)The adsorption isotherms of surfactants corresponds to S-shaped multilayer adsorption isotherm A c k n o w le d g e m e n ts The work was done in the frame of Ministry of Education and Science o f Republic o f Kazakhstan project entitled "Develop ment o f new type of coal-water fuel on the basis of Kazakhstani coal" R e fe re n c e s 1 Makarov A .S., Kllshchenko R. E .. Zavgorodnll V. A., Makarova E .V., The Impact o f the Water Salt ( imtent an the erues Ii f ( oalAifueous Sitys nsiani Journal of Water С bcmisUy and Technology.33(6).357-362 (2011). 2 Ki)o Klec/kuwnka A < innbustion at соЫ-ьчЧег sus/iensitms Fuel.90(2).865*877 (20||), O O I Ш Ш О ис12іЖ аш Ш 3 Zhou М ., Pan В., Yang D., Lou H ., Qlu X., klteologtcal Behavior ln\Kstig,Hiim o f t 'ofuvntnitcii ( <tal-Water Sus/unMan Journal of Dispersion Sciencc Technology.31(6).H38-843 (2010) 4 Lntkow skl J .8 " ( <нгі flotation and hut* С на/ UMUattan Elsevier.Amsterdam (2002) 5 Hudlteh P. J .Jackaon W R " Larkina F P .Chafteo A. L" Krlchko A A , Grigor'ova E. A ., Shatov 8 N.. ( чт/югимп o f the structure and reactivity o f a Kansk-Achinsk basin/IfSSRj coal »tth those o f a latrohe Ш іеу (Australia/ coal Fnrrgy and Fuel*, 4(1), 28*33 (1990).