The CWM (Coal-Water Mixture) fuel that comes to the front recently as one of important coal utilization technologies enables coal to be burned in a liquid-state to settle a lot of inconveniences and disadvantages that coal has as a solid-state fuel. It is assessed as a technology by which high economics and short-term commercialization might be realized in comparison with the gasification or liquefaction technologies of coal.
In order to enhance CWM combustion efficiency and to prevent sedimentation phenomenon during long-term storage, it is essential to prepare CWM with high concentration and stability but low viscosity. Hence, it becomes a very significant step in CWM preparation, storage, transportation, and atomization technology to investigate the rheological behavior of CWM fuel.
This study was carried out mainly for the following two purposes:
One is to find out various factors affecting the rheological properties of CWM, togetherwith their correlations, in order to develop highly-loaded CWM with low viscosity and high stability, considering the preparation, transportation, and atomization aspects of CWM.
The other is to investigate the discharge and atomization characteristics of CWM fuel, using one-nozzle internal-mixing atomizer, as a primary stage to secure the design data for atomizer by which fine CWM droplets can be produced.
Study for the Rheological Behavior of CWM
In order to investigate the rheological behaviors of CWM principal to the preparation, transportation, atomization and combustion technologies of CWM fuel, the influence of various parameters such as coal properties, coal loading, type and amount of additive, temperature, and size and its distribution of coal particles was studied on the apparent viscosity and non-Newtonian behaviors of CWM. Additionally, CWM viscosity, yield stress, and non-Newtonian behavior in the binary and ternary mixture composed of the coal particles with different mean size were investigated for 30 coal particle samples ranging from 13.7 to 97.4 m in MMD size, using anionic surfactant as additive. The apparent viscosity of CWM was measured using a Haake viscometer and the non-Newtonian properties of CWM were analyzed by the power-law model.
All the slurries exhibited non-Newtonian properties. CWM exhibited non-Newtonian properties more strongly with increasing solid volume fraction and hydrogen ion concentration (pH), and the decrease in temperature and mean particle size. Additionally, the variation pattern of apparent viscosity with many parameters was found to be very similar to that of the values of the flow consistency coefficient, K. The relationship between solid volume fraction and relative viscosity of CWM could be relatively well expressed with the equations proposed by Dougherty and Kreiger ; Chong et al.
The coal content in CWM could be enhanced significantly by the addition of anionic surfactant and electrolyte. With increasing amount of additive, the non-Newtonian property of shear-thickening CWM increased and then leveled off, but the degree of pseudoplasticity of shear-thinning CWM decreased and then leveled off. It was confirmed that the hydrogen ion concentration considerably affects the rheological behaviors of CWM. With decreasing pH values, the non-Newtonian property of CWM was reduced and the yield stress and apparent viscosity were increased.
All the CWMs prepared with the coal particles having the MMD over approx. 40 ㎛ revealed the shear-thinning behavior, whereas the CWMs prepared using the coal particles of MMD below about 30 ㎛ showed shear-thickening phenomenon.
The blending of two types of coal particles of different fineness is very useful for formulating highly loaded CWMs with low viscosity, and the CWM viscosity is the lowest when the fines content is 35±5 wt%. Additionally, the rod penetration test showed that CWM exhibits the highest sedimentation stability at the same mixing ratio of coal particles as gives the lowest viscosity. It was also verified that the distribution modulus, q and the solid volume fraction at maximum packing, m have minimum and maximum value, respectively, at the fine particle content of 35±5 wt% which coincides with the fine particle content showing minimum viscosity.
All the CWMs without additive showed non-Newtonian property with yield stress at the coal loadings over a specific value. The yield stress increased with increasing coal loading and decreasing coal particle size and additive amount.
When CWM was prepared by blending three different-size coal particles, the variation of yield stress and apparent viscosity with the fraction of each component particles could be interpreted by the mean size and PSD of coal particles, similarly to that in the binary system.
Study for the Atomization Characteristics of CWM
The combustibility of CWM varies with the properties of CWM, the performance of atomizer and burner, operating conditions, boiler structure, etc.. In particular, it was affirmed by many investigators that the atomizing performance of atomizer plays a key role in increasing the combustion efficiency of CWM fuel and maintaining the stabilization of CWM flame.
Therefore, in this study, the atomizing characteristics of CWM were intended to be understood through literature survey and experiments, and the atomizer structure and operating conditions were intended to be suggested to enhance atomization quality, because CWM atomization area is relatively in the lack of study in spite of its principal role in CWM combustion.
The atomization and discharge characteristics of CWM fuel at the internal-mixing type atomizer with one exit hole were investigated using Malvern droplet size analyzer based upon laser diffraction principle. Test results showed that over the atomizing pressures more than 2.0 $Kg_f$/㎠, a critical flow phenomenon occurs, and that, consequently, a linearly proportional relationship is established between air mass velocity and solute pressure in the mixing chamber, and further that the discharge coefficient of air is decreased with the increase of nozzle diameter and CWM mass velocity. The CWM droplet size in the range of 50 - 120㎛ MMD was sharply decreased with the increase of air/fuel ratio but was maintained at a fixed value over a certain level, regardless of the CWM feed rate. On the other hand, its SMD was in proportion to the 0.48 power of the term, (1 + fuel/air mass ratio). Also, along with the axial distance from the atomizer, CWM droplet size was linearly increased and the obscuration was decreased. Since the MMD of CWM droplets is shown to be larger than that of coal particles in CWM fuel by more than about 20㎛, it was clearly verified that a CWM droplet consists of several coal particles and water, not just one coal particle.