Micromechanism And Combustion Characteristics Of Low-rank Coal Water Slurry Upgraded By Hot Water Treatments

There is a large reserve of low rank coals(LRC), such as brown coal and subbituminous coal in China, however, LRC is not so widely used because of the high water content and low heating value. If LRC is dried with conventional method, it maintains the ability of water reabsorption and the spontaneous combustion tendency. So, LRC is not fit to transport for long distances, and often combusted directly at power plants near the coal mines.Hot water treatments can decrease the inherent water in LRC effectively, also the spontaneous combustion tendency can be prohibited due to the decrease of oxygen content. After hot water treatments, the LRC products reabsorb water no more, with a higher heating value they can be utilized more widely. The LRC products also can be made into coal water slurry(CWS), which is able to partially replace oil as a new fuel in industry utilization.A series of work were performed under the subjects of hot water treatments, coal water slurry preparation from LRC, and the combustion characteristics of LRC water slurries.Using thermobalance, the isothermal drying mechanism of water in brown coals was investigated. It can be concluded that the drying process can be divided into two stages, which are controlled by capillary force and absorption/desorption mechanism respectively.Then, low rank coals were dewatered and upgraded by non-evaporative hot water treatments. Among the different conditions, temperatures and the initial vessel pressure play more important roles in the LRC upgrading process. After hot water treatments, the volatile decreases, while the fixed carbon and heating value increase, additionally, the mole ratio of oxygen and carbon elements goes down, which means an elevation of coal rank. The existing of H2S in product gases illustrates that hot water treatments can remove a little sulfur from the raw coals, which benefits to the environment.The coal water slurry prepared from LRC products shows pseudoplastic fluid, namely, shear-thinning. The solid concentration, fluidity and stability can be improved by elevating the final reaction temperature and the initial vessel pressure. The prolong of residence time shows no significant influnce on the CWS concentration increase, but can impove the slurry stability. The sample loading and the ratio between dry coal and water have little effect on improving the CWS performance. After hot water treatments, the maximum solid concentration of Xiaolongtan CWS can increase from 44.6% to 64.55%, while that of other low rank coals can also reach about 60%.After hot water treatments, some physical and chemical properties of LRC have changed, which may bring about the performance improvements of low rank coal water slurry. It can be concluded as follows:hot water treatments have changed the pore structure of LRC, which means volume decrease of pores with radius smaller than 1000 nano meters, also a decrease of specific surface area. Both changes can increase the solid concentration of LRC water slurry. The oxygen-containing groups such as carboxyl and phenolic hydroxyl were reduced, leading a decrease of hydrophilicity and an increase of hydrophobicity of coal surface, the contact angle between coal and water increases accordingly, as a result, the water holding capacity of coal decreases, which reduces the inherent water in LRCs and increases the solid concentration of low rank coal water slurries. Hot water treatments also make a change in coal molecular structure, as be seen from infrared spectrum, the increase of aromaticity or carbonization degree and the decrease of hydrophilic oxygen-containing groups all help to improve the CWS performance of LRCs.The combustion characteristics of LRC before and after hot water treatments were examined by thermobalance. It can be concluded that ignition temperatures of LRC products increase, which inhibits the spontaneous combustion tendency; and the integrated performance of combustion improves after hot water treatments, which is also better than Yanzhou bituminous coal. The results of kinetic analysis show that the prophase reaction rates of LRC products are lower than raw coals, while the anaphase reaction rates are higher.Two kinds of low rank coal water slurry were combusted in a pilot-scale cyclone furnace that is a horizontal-cylinder type. As a comparison, a bituminous coal water slurry prepared from Datong coal was also tested. Results show that temperature levels of all three CWS samples are close to each other. The axial distributions of temperature were bimodal, and the first temperature peak is due to the combuston of volatiles released from coal particles, while the other is due to the coal char combustion. Coal water slurry samples prepared from LRC products have a better burnout performance than Datong CWS. Particles in furnace at different combustion stages were analysed by a scanning electrical microscope(SEM), it can be found that because of high volatile contents and low caking property, the LRC particles exhibit a cracking tendency at the initial combustion stage, in contrast, Datong coal has a higher thermoplasticity, and the particles show a cenosphere structure with large holes at the early combustion stage. As combustion proceeds, the activation energy of residual char combustion increases, which means a decrease in burnout reactivity, and this is one of reasons why residual chars always exist in flying ash from furnace exit.The combustion process of low rank coal water slurry in the furnace mentioned above was then simulated using a CFD software, Fluent. Results show the simulated temperature field and atmosphere field are approximate to that of the real testing condition. The cold air condition and 70% load condition were also predicted and simulated. Results show that the ignition distance and high temperature zone are delayed by the cold air that is 30℃, and CO molar fraction at the furnace exit is much higher than the normal condition. It can be predicted that the ignition should be successful in cold air, but the combustion efficiency will be lowered. With the load reduced to 70%, the LRC water slurry still combusts well, while the furnace temperature level is lower, the flame length extends slightly and the CO molar fraction at the furnace exit increases a little.