Coal-water mixtures

The development of CWM does not have the long history of COM development. In the 1960s, the Germans and Russians also conducted several major CWM combustion tests. However, despite the success of these tests, no further major efforts were undertaken in this country until the major test at 1 million Btu/h input (Argonne, 1990c). 

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Heat Exchangers Using Non-Newtonian Fluids. Most fluids used in the chemical, pharmaceutical, food, and biomedical industries can be classified as non-Newtonian, ie, the viscosity varies with shear rate at a given temperature. In contrast, Newtonian fluids such as water, air, and glycerin have constant viscosities at a given temperature. Examples of non-Newtonian fluids include molten polymer, aqueous polymer solutions, slurries, coal—water mixture, tomato ketchup, soup, mayonnaise, purees, suspension of small particles, blood, etc. Because non-Newtonian fluids ate nonlinear in nature, these ate seldom amenable to analysis by classical mathematical techniques. 

Flocculation. The interaction of the cationic PEIs with anionic substrates leads to substrate flocculation. AppHcations of this property include the coagulation of latex (434), commercial appHcation in effluent treatments (435—437), and stabiHzation of highly loaded coal—water mixtures in mining (438). 

The coal-water mixture fed to the boilers has to be formulated to a certain specification. This is done with the proper inclusion of additives to facilitate combustion. The recommended additives include a petroleum-based surfactant (polysulfones or polycar-boxylates), a stabilizing (e.g. xanthan gum), a biocide and an anti-foaming agent (such as a silicone base). 

Based on the above mentioned, the programme of theoretical and experimental investigation of the main parameters of coal-methanol (or its water solution) mixture pipeline transport should be opened. As the first step of the programme the comparison of power consumption (dependency of hydraulic gradient I on slurry flow velocity V and solid concentration Cs) for the pipeline transport of coal-water mixture and coal-methanol solution mixture was realised. The special laboratory measurements were made to define unknown input data of semi-empirical relationships, i.e. the limit volumetric concentration Cm and the coefficient of mechanical friction of coal in the water or water-methanol solution ka. The resultant comparison of the hydraulic gradient I of the coal-water and coal-methanol solution mixture flow is presented in Figure 2, where density of coal was pc = 1480 kg/m3, diameter of the pipe was D = 0.103 mm, the maximal grain size of coal dmax was less than 0.25 mm, volumetric concentration - C = 20 %. 

The power consumption of the coal-methanol mixture is for about 20 % less than that of coal-water mixture for the same value of transport velocity (except the regime close to the critical velocity Vcr) The reason of it is probably lower viscosity of methanol than that of the water. Similar results were also confirmed experimentally by [17],... 

However, the fundamental economical criterion of any transport system is not the power consumption, but the total transport cost per unit mass and distance. Goedde [18] made the comparison of the total unit transport cost of the coal-methanol mixture pipelining with coal-water mixture pipelining, railway and barges transport. He pointed out that the transport of coal in methanol could be significantly profitable and competitive compared with the rest of the above-mentioned kinds of transport. Therefore, the attention... 

Figure 2. Comparison of hydraulic gradient I of coal-water mixture (1) and coal-methanol mixture (2)...

Slurry pipeline pipeline that can transport a coal-water mixture for long distances. 

The use of coal slurries, either coal-oil mixtures (COM) or coal-water mixtures (CWM), has been the subject of considerable research because of the potential to replace fuel oil in boilers. Recent work has shifted in favor of CWM over COM, as the cost savings for the latter were not enough to offset conversion costs. CWM is typically composed of 60-75 percent coal, 24-39 percent water, and 1 percent chemical additives. The advantages of CWM compared with dry pulverized fuel are the ease and safety of handling,... 

Takao, S. Ozaki, H. The effects of agitation on the rheology of a coal water mixture. Coal Prep. 1997, 18, 215-225. 

The increased emphasis on coal usage in this country and, indeed, the significant effort underway to consider coal water mixtures as possible oil substitutes in oil-designed boilers underscores the need to improve the prediction of mineral matter behavior in a boiler environment. 

The effect of temperature and surfactants on the viscosity of coal-water mixtures is shown in [281]. It has been shown that the volume content of coal in suspension can be increased while preserving its flowability at a certain surfactant content. Stable and pseudo-plastic suspensions were obtained both with anionic surfactants containing electrolyte, and with ethoxylated alkylphenols with a high content of oxyethylene groups. But the rate of reduction of the apparent viscosity begins to decrease with increasing temperature which seems to be connected with the change of the surfactant structure in solution and adsorption layers. 

Several investigations were carried out to remove toxic heavy metal ions from waste water by biosorption. Microbial cells loaded with heavy metals were recovered by flotation, e.g. Streptomyces griseus and S clavuUgerus loaded with Pb [108] and Streptomyces pilosus loaded with Cd [109]. In these flotation processes the microbial cells were dead therefore, they are not considered here. The removal of pyritic sulfur from coal slurries such as coal/water mixtures by Thiobacillus ferrooxidans and recovery of this iron-oxidizing bacterium by flotation is a special technique in the presence of high concentrations of solid particles (see e.g. [110]). The flotation of colloid gas aphrons was used for the recovery of yeast in continuous operation [ 111 ] for the recovery of micro algae, and in the presence of flocculants in batch operation [112]. These special techniques are not discussed here. 

At this point, it is appropriate to consider another form in which coal can be used as a fuel, and this is as COM or coal-water mixture (CWM). A coal-liquid mixture (CLM) consists of finely crushed coal suspended in varions liquids and typically small amounts of chemical additives that improve... 

Coal Properties to be Considered in the Preparation of Coal-Water Mixtures... 

Is there an explanation for this strong deviation In the model (2.1)-(2.3) the fuel feed is considered within the boundary condition of the carbon mass balance (2.1). Technically this corresponds to the feed of dry coal into the reactor through the sidewall. On the contrary in the existing PFBC plants coal is fed into the reactor as a coal water mixture (CWM) with a water... 

Special chemicals such as xanthan gum in levels of 200-600 ppm can be used as a stabilizer to prevent settling of coal slurries and to prevent the formation of hard-packed beds during hydrotransport. Miller and Hoyt (1988) recommended Pfizer Flocon 4800C as a very economical additive to coal-water mixtures. 

Morway (1965) obtained a patent for using a hydrocarbon oil with a small percentage of an imidazoline surfactant to coat coal particles uniformly. After adding this mixture, the coated coal can be mixed with water. The water weight concentration can be reduced to 20%. This slurry with low overall moisture is easier to heat at the final discharge point prior to combustion than plain coal-water mixtures. 

Typical coal water mixtures (CWMs) for direct combustion consist of a weight concentration of 70% coal and 30% water. Prior to combustion, it is important to degrade the coal slurry mixture by applying hot air to accelerate the evaporation of water (Garbett and Yiu. 1988). 

Garbett, E. S. and S. M. Yiu. 1988. The Effed of Convedlve Heat on the Disintegration of a Coal-Water Mixture in Pneumatic Atomization. In 13th International Conference on Coal Technology, Denver, CO. Washington, D.C. Coal and Slurry Technology Assrx iation. 

Figure 1.32 Rajendran Nair data of shear stress versus normal stress for coal-water mixtures (Rajendran Naireta/. 1990).

Vitolo, S., R. Belli, M. Mazzanti, and G, Quattroni. 1996. Rheology of coal-water mixtures containing petroleum coke. Fuel. 75(3) 259-261. 

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