| Coal gangue is one of the largest sources of industrial solid waste in China. Meanwhile, coal gangue is a kind of low calorific value fuel. So coal gangue has the dual attributes of waste and resources. The rational utilization of coal gangue in the power plants firing low heating value fuels is an important topic in the field of energy and environment. However, coal gangue has some problems of poor ignition and poor burnout in the process of combustion. This is mainly related with feedstock properties of coal gangue itself. To improve the combustion performance of coal gangue, optimizing the atmosphere or blending the fuel of good ignition performance is one of the alternative measures. From two aspects of combustion characteristics of coal gangue and improvement of combustion performance of coal gangue, this thesis studied the effect of feedstock property and particle size on coal gangue combustion, the effect of blending weathered coal and pine sawdust on coal gangue combustion, respectively. Moreover, the influence of feedstock property and atmosphere on combustion process, the change of mineral matter in coal gangue combustion and the interaction mechanism of mixture in co-combustion process were revealed. The main conclusions were as follows:(1) In the aspect of the effect of feedstock property on combustion process of coal gangue, the results showed that the weight loss of coal gangue of different feedstock properties included three stages. Among these stages, the first stage was the weight gain stage, owing to the chemical adsorption of oxygen molecule on the surface of coal gangue to generate the carbon oxygen complex. The degree of weight gain was related to stability of carbon oxygen complex. The second stage was the main combustion stage of volatile matter and fixed carbon and the peak characteristic of weight loss was related to the feedstock property. The ignition temperature of coal gangue was affected by volatile content and oxygen adsorption behavior jointly. The ignition performance of coal gangue was determined not only by ignition temperature but also by ash content. The burnout performance was influenced by the ignition performance. Moreover, there was the relationship between the component of coal gangue and the comprehensive combustion index. (Vad+FCad)/Aad exhibited positive correlation with integration parameters S and negative correlation with integration parameters Hf. The combustion kinetics was analyzed using the distributed activation energy model. The results showed coal gangue of different feedstock properties presented two types of activation energy change as conversion. The content of combustible substance and the species of mineral matter played a role on in the distribution of avtivation energy. There was the compensation effect between activation energy and frequency factor. The isokinetic relationship lines indicated coal gangue combustion was a multi-stage process. The theoretical analysis confirmed the distributed activation energy model could be used for the analysis of coal gangue combustion.(2) In the aspect of the effect of particle size on combustion process of coal gangue, the results showed that the increase of particle size (45-106 ?m) had little effect on the combustion of pulverized coal gangue. This was attributed to the combustion process of coal gangue was chemical kinetics control. As the increase of temperature, the mass transfer rate of coal gangue had obvious decrease and the apparent chemical reaction rate of coal gangue started increase significantly from 450?. However, the mass transfer rate was always more than the apparent chemical reaction rate. When the particle size further increase (53 ?m-2mm), the effect of particle size on the char conversion of coal gangue was obvious. When the char conversion of 53-75?m,180-250 ?m and 500-600 ?m coal gangue was 100%, the char conversion of 1-2 mm coal gangue was 25%. And the char conversion of 1-2 mm coal gangue was closed to 100% until the reaction time was up to 18 min. This was related to the combuston process of coal gangue of large particle size changed from chemical kinetics control to diffusion control. As the increase of temperature, the mass transfer rate of coal gangue could change from more than the apparent chemical reaction rate to less than the apparent chemical reaction rate. The change of temperature (700-900?) had no significant effect on coal gangue combustion under chemical kinetic control and had obvious effect on the char conversion of 1-2mm coal gangue as time. This was attributed to the effect of temperature on oxygen diffusion and heat transfer.(3) In the aspect of the effect of atmosphere on combustion process of coal gangue, the results showed that the thermal behavior of coal gangue contained alumino silicate was accompanied by the mineral decomposition process from kaolinite to metastable metakaolin through dehydroxylation. And the changes of atmosphere (N2, N2/O2, CO2/O2, CO2/N2) did not have significant influence on the decomposition behavior of minerals. Unlike coal gangue contained aluminosilicate, the decomposition behavior of dolomite was included in the thermal process of coal gangue contained carbonate. And the decomposition reaction of dolomite was affected by CO2 in atmosphere and showed the two reaction process. The increase of oxygen concentrations (2% ?21%) could promote the traditional combustion of coal gangue significantly. However, the further increased oxygen concentrations (20%?80%) could not improve the enriched oxy-fuel combustion performance of coal gangue markedly. The concentration of atmosphere did not have influence on the decomposition behavior of kaolinite in coal gangue contained aluminosilicate. But the decomposition reaction of dolomite in coal gangue contained carbonate was affected by the concentration of atmosphere. In the low CO2 atmosphere, dolomite having one step reaction was decomposed into MgO, CaO and CO2. In the high CO2 atmosphere, the decomposition reaction of dolomite was divided into two steps, corresponding to the decomposition of dolomite and calcite. Moreover, the low oxygen concentration (2%) would cause the partial overlap of the combustion reaction and dolomite decomposition reaction in the traditional combustion, which resulted in the inhibiting effect of CO2 product on the decomposition of dolomite.(4) In the aspect of the effect of co-combustion on combustion process of coal gangue, the results showed that adding weathered coal could improve the ignition performance of coal gangue. And the enriched oxy-fuel combustion (40% O2/60% CO2) could reduce the characteristic temperature of coal gangue/weathered coal blend in comparison with the traditional combustion (21% N2/79% O2). There was linear superposition for thermal behavior of coal gangue/weathered coal blend, not only in the process of co-pyrolysis and co-combustion, but also in the process of co-oxy-fuel combustion and co-gasification. There was no interaction between the components of the mixed fuel. However, the interaction between coal gangue and weathered coal existed in the enriched oxy-fuel combustion and the interaction increased observably with increasing blending ratio of weathered coal and oxygen concentration. The addition of pine sawdust could also improve the combustion performance of coal gangue. The peak temperature and burnout temperature in the combustion stage of coal gangue/pine sawdust mixtures decreased with increased pine sawdust blending ratio. There was no interaction between coal gangue and pine sawdust in the devolatilization stage. But the obvious interaction could be found in the combustion stage. And the decrease of oxygen concentration and heating rate would reduce the interaction. The interaction mechanism was related to the thermal effect between coal gangue and pine sawdust.Through the deep discussion on the combustion characteristic and influence mechanism of coal gangue, this thesis could provide the theoretical guidance for the traditional combustion of coal gangue in circulating fluidized bed firing low heating value coal and the application of enriched oxy-fuel combustion technology and mixture combustion technology in circulating fluidized bed firing low heating value coal. Moreover, this thesis had the practical significance for improving the comprehensive utilization of coal gangue, promoting the power generation of low heating value coal, and reducing the environmental pollution caused by coal based solid waste in China. |
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