OCR Text |
Show Stability of CWM A result of the stability test is shown in Figure-11. Excellent stability with no hard-packs and small differences of viscosity and particle size distribution between the samples of top and bottom, was obtained in storing after fifty days without mixing. For the stability it is assumed that not only the uniform absorption of additives to the coal particles but also the component of fine sizes accounts for its stability. roduction and Handling Test Production Production procedure is very important because the slurry properties made by this procedure greatly affect the combustion performance. So it is necessary to reduce the viscosity and the water content added to prepare CWM. And also chemical additives should be minimized for reasons of slurry preparation cost and boiler ash deposition. Figure-12 shows a result by our improvement to reduce these parameters. As an injecting procedure into the wet type mill, staged injection method is effective for improvement. As compared with the conventional one stage injection, effectiveness of 1% in solid content or 300 cP in viscosity had been successfully achieved under the condition of same additive injected. This means, reduced amount of additive from approximately 0.7 % to 0.4 % is achieved to get the same slurry properties of viscosity and solid content. For this reason, a schematic model shown in Figure-13 is suggested. In the conventional procedure of single stage injection, after absorption of all additive to the coal surface or to itself, grinding is performed. So there remain some surfaces of the coal particles which should absorb more additive. On the other hand, the improvement procedure of multi staged additive injection enables uniform absorption to the surface of coal particles because the new surfaces of finer coal particles ground can absorb enough additive which is injected after grinding. Transportation A series of pumping and piping test were performed to determine the characteristics of rheology at initial starting-up and normal continuous operation of supplying for the burner combustion system. Rheology results of two different CWM are shown in Figure-14. And Figure-15 shows the piping pressure loss when the CWM flow rate and velocity were changed. By these Figures, it is clarified that CWM rheological characteristics can be treated as nearly the same as Newtonian fluid in the range of higher shear rate or higher flow velocity which is selected as normal, economic range. However, it should be noticed that in the lower shear rate, which means static state or initial starting-up of the system, higher viscosity by high shear stress causes difficulty of starting-up of pumps and necessity of high starting energy required in certain types of CWM. 5 |