The co-combustion of sludge and coal in the existing coal-fired power plant is a promising way for the disposal of sludge. However, slagging in boilers is one of the potential problems if the sludge was mixed improperly with coal during co-combustion. In this paper, the ash fusion characteristics and mineral transformation behavior of a bituminous coal, an industrial sludge, a municipal sludge, and their mixtures have been studied. The results showed that the ash fusion temperatures of the mixed samples show a nonlinear relationship with the proportion of sludge. It was found that the variation of the ash fusion temperature of blended samples of coal with different sludges can be effectively predicted by a ternary phase diagram. Thermal analysis indicated that the decreasing of ash fusion temperature of the blends was caused by the melting phenomenon between the original minerals in coal and sludge which formed new minerals with a low ash fusion temperature. Mineral analysis showed that the formation of diopside, anorthite, and iron eutectic can reduce the ash fusion temperature of coal mixed with sludge, and the formation of merwinite, spinel, and mullite can increase the ash fusion temperature of the mixture.

It is hard to use lignite directly because of the problems in low thermal efficiency and difficulties in transportation. Thermal upgrading is a potential way for the use of lignite with the increasing consumption of coal in China at present. In this study, the pyrolysis characteristics of Chinese Shengli (SL) lignite were investigated by thermogravimetric analysis (TGA), and the upgraded lignite obtained by isothermal (250–750 °C) and non-isothermal (5, 12, and 20 °C/min) upgrading from the tube furnace was used to characterize the gasification properties of it. The structural information of upgraded lignite was identified by a nitrogen adsorption isotherm and Fourier transform infrared spectroscopy (FTIR), and the gasification reactivity of the upgraded lignite was determined by TGA in a self-designed thermobalance reactor. The results indicated that the temperature was the primary factor affected on the property of upgraded lignite. Isothermal treatment at 350 °C could be an optimal condition for SL lignite considering both the quality of products and the economic issues. The chemical and physical structural characteristics of lignite changed significantly after thermal upgrading, which influenced the gasification characteristics of upgraded coal. The decomposition of surface functional groups and ordering of the crystalline carbon structure resulted in the decrease of active sites and further reduction of the reactivity of upgraded lignite. The total surface area (TSA) could not be used to explain the change of reactivity of upgraded lignite obtained above 550 °C because of the coal particle collapse and the loss of active sites. The higher heating rates enhanced the porosity of upgraded lignite and led to the increase of reactivity, and the residual macromolecules remaining in upgraded lignite pores at a higher heating rate could release again at the initial gasification stage, which had a negligible effect on the gasification reactivity. The isothermal upgrading within 450–550 °C should be the optimal operation condition for the use of SL lignite in the process of gasification after thermal upgrading.

Thermal upgrading is a promising way to use lignite efficiently and safely. However, moisture readsorption properties of the upgraded lignite would partially offset the upgrading effect. Asphalt is so repellent that it could be used as an additive in lignite thermal upgrading to prevent moisture readsorption of upgraded lignite. In this study, a Chinese lignite upgraded at various temperatures (200–500 °C) with various concentrations of asphalt addition (0–10 wt %) was thoroughly investigated. The changes in chemical structures of the upgraded lignites were investigated using Fourier transform infrared spectroscopy (FTIR). The changes in physical structures were analyzed by N2 adsorption isotherm and scanning electron microscopy (SEM). The moisture readsorption and combustion characteristics of the upgraded lignites were studied using a constant temperature/humidity chamber and a thermogravimetric analyer (TGA), respectively. The results indicate that the abundance of oxygen-containing groups (i.e., hydroxyl and carboxyl) decreased with the increasing upgrading temperature, while it was not significantly influenced by the addition of asphalt. The pore volume and surface area of the upgraded lignites increased with the increasing temperature, while the pore diameter decreased. SEM images revealed that some pores in upgraded lignites were covered by asphalt and the surface became much smoother with the increasing asphalt concentration, which resulted in the decrease of the pore volume and surface area. The moisture readsorption ratio and spontaneous combustion tendency to the upgraded lignites decreased with the increasing temperature, while the influence of asphalt on the moisture readsorption and combustion characteristics was temperature-dependent.

Thermal upgrading is a promising way to improve the quality and broaden the range of use of lignite. This paper investigated the characteristics of organic sulfur evolution in a Chinese lignite from Shengli coalfield in the process of thermal upgrading by a fluidized-bed reactor under different atmospheres. The results showed that 5 min of fluidization of feed coal within the temperature range from 200 to 500 °C was capable to obtain a preferable upgraded coal with the increase of the calorific value and the decrease of the moisture. The removal of total sulfur via thermal upgrading, reaching the top at about 60%, was attributed to the evolution of organic sulfur. X-ray photoelectron spectroscopy results showed that aliphatic, aromatic and thiophene, and sulfone sulfur were the major organic sulfur species in Shengli lignite (SL). Aliphatic sulfur was the most unstable organic sulfur and began to release at 200 °C. Aromatic sulfur was found to release at 300 °C. Thiophene sulfur was most stable and could not be removed even if the temperature reached 500 °C under different environments. Thermal decomposition was the primary approach for the evolution of organic sulfur during thermal upgrading. The addition of oxygen and ethanol in the environments showed different capabilities for selective decomposition of organic sulfur species in coal. Despite the increased combustible matter loss, a mild oxidizing environment provided a possible method for the lignite thermal upgrading using the flue gas from the mine mouth power plant. On the other hand, the organic environment could effectively increase the removal of organic sulfur in coal, which could be achieved at low temperatures.

Mercury and sulfur emissions from power plants is becoming increasingly an environmental concern. In this study, two Chinese coals from the Guizhou province and one Canadian coal from Alberta were selected to study the potential of thermal upgrading as mercury and sulfur emission control strategy prior to coal use. A low-temperature asher (LTA) and X-ray diffractometer (XRD) were used to characterize the occurrence of mercury and sulfur in coal. An experiment of mild thermal upgrading at different atmospheres was performed to explore the removal efficiencies of mercury before combustion. Our study shows that mercury is associated with different minerals in these coal samples. The correlation between mercury and sulfur is also investigated. Mercury was found to be removed effectively by mild thermal upgrading at 400 °C under a 4% O2−N2 atmosphere. An oxidizing atmosphere increased the removal of pyritic sulfur significantly at low temperatures. The mercury release rate was found to be higher under an oxidizing atmosphere than under a nitrogen atmosphere between 300 and 400 °C during mild thermal upgrading.

•Case-based reasoning was used for operation optimization of thermal power plant.•Five indexes were used to evaluate the coal blends combustion.•An evolutionary strategy was proposed in CBR algorithm.•The boiler performance was improved after the CBR system applied.Coal blending is becoming increasingly common as more and more off-specification coals are received in coal-fired power plants, given the present coal market in China. This situation requires optimization of the operating parameters for matching the varying coal properties. The motivation for such optimization includes confirming good performance of the units regarding the security, the economy and environmental protection. However, the current adjustments to operation of the plant rely mostly on human experience because of the imperfections of existing theoretical models for coal-blend combustion. In this paper, a Case-Based Reasoning (CBR) method providing online decision-making for optimization of coal-blend combustion was investigated using cases representing successful operation of the unit for specific coal blends and loads. A case base containing a wealth of knowledge about optimal operation modes was constructed from a large number of cases. The development process for the CBR system includes case design, case evaluation, case generation, case retrieval and case reasoning. Case evaluation focused mainly on heating surface security, output capability, slagging tendency, comprehensive fuel consumption and pollutant emissions. Five indexes were introduced to quantify the above characteristics based on actual combustion parameters. A case-generating algorithm employing an evolutionary strategy was proposed in which the case base evolves while retaining new cases. Two methods for measuring case similarity – termed entirely similarity and eigenvalue similarity – were used for case retrieval. Run-time optimization strategies were recommended by the case-reasoning model based on the current operating status. The CBR system using Browser/Server framework were successfully applied to a 600-MW power plant, which provided an opportunity for coal-blend combustion optimization.Graphical abstractA case-based reasoning (CBR) system providing online decision-making for optimization of coal blends combustion was developed in this paper as the following figure shows. The application results indicated that the boiler performance on efficiency and NOx emissions were improved. This system can be a good helper for operators in thermal power plants.Download full-size image

Two coals from different coal mines, Liuzhi (LZ) and Zunyi (ZY) in the Guizhou province, with high mercury (Hg) and sulfur (S) contents were studied in this article. Different methods were used to identify the occurrence and correlation of Hg and S in the two coals, such as, float-sink test, sequential leaching test, low temperature ashing (LTA), and also X-ray diffraction (XRD). The results show that the sulfur forms are different in the two coals. Sulfur in the LZ coal is mainly in the pyritic form and in the ZY coal in the organic form. Mercury is mainly associated with pyrite for LZ coal and with clay minerals for ZY coal. In LZ coal, mercury and sulfur have a good correlation, but in ZY coal the correlation between mercury and sulfur is not so good.