Clean Coal Utilization Technology Research Articles

A study on the occurrence modes and distribution characteristics of sulfur and mercury in coal gasification slag

Although coal gasification is recognized as a core technology for clean coal utilization, S and Hg can migrate to gas, wastewater, and slag during the gasification process, causing environmental pollution. It is well known that S and Hg have a strong affinity. Therefore, the interaction between S and Hg during the gasification process warrants further investigation. In this study, the occurrence modes and distribution characteristics of S and Hg in gasification slag were systematically investigated using multiple treatments and various characterization techniques. The results showed that sulfur in the gasification slag exists predominantly as organic sulfur, while mercury mainly exists as HgS/silicate-bound Hg. Although the relative enrichment index of Hg (RE < 0.7) indicates its high volatility, a more pronounced enrichment effect for mercury of fine slag was indicated by its much higher RE than coarse slag. When exploring the relationship among the contents of C, S, and Hg and specific surface area in gasification slag, it was found that the correlation between S and Hg content in fine slag was the strongest, with a correlation coefficient (R2) of 0.96, indicating a significant influence of sulfur content in fine slag on mercury content. Additionally, it was observed that the S and Hg contents in gasification slag increased with the decrease of particle size and density. These results provide an important understanding of the distribution, migration, and transformation behavior of S and Hg during coal gasification, as well as a certain theoretical support and technical basis for the resource utilization of gasification slag.

Quantitative evaluation of radiative heat transfer from reactor surface to multiphase reaction medium in a supercritical water gasification reactor for coal

Supercritical water gasification (SCWG) of coal is a clean coal utilization technology, which converts organic matter in coal into hydrogen-rich gases through a series of thermo-chemical reactions. In this process, thermal radiation is an important heat transfer mechanism and significantly affects thermal efficiency and gasification performance. This paper aims to develop a comprehensive numerical model integrating with solving radiative transfer to precisely simulate the process of SCWG of coal, and quantitatively evaluate non-grey radiative heat exchange between reactor surface and multiphase reaction medium. The simulation results are validated against experimental data. It is shown that under practical operating conditions, thermal radiation accounts for 30 % – 42 % of total heat exchange in the reactor. More than 94 % of radiant energy lies in the spectral range of 1.99 – 50.0 μm, which just covers the three most important radiation bands of supercritical water. Because both coal particles and supercritical water participate in radiative heat transfer, enhancing radiative heat transfer can effectively improve reaction medium temperature and gasification rate. On average, for every 20 % increase in the proportion of radiative heat transfer in the reactor, fluid medium temperature increases by around 7.1 K, and the gasification reaction rate rises by 9.5 %. These findings would provide valuable guidance for reactor design and optimization in practice.

Energy, exergy and economic (3E) evaluations of a novel power generation system combining supercritical water gasification of coal with chemical heat recovery

Supercritical water gasification (SCWG) is a promising clean coal utilization technology. A proposed power system with integrated coal SCWG, supercritical turbine, gas turbine and chemical heat recovery is proposed in this paper. And the power system with integrated combined cycle and parallel heat recovery is re-proposed for structure simplification and efficiency enhancement. With a gasification condition of 660℃/250 bar, and a coal-water-slurry concentration of 11.3 %, the efficiencies of the proposed system and the system with parallel heat recovery are 49.06 % and 47.93 %, respectively. The power efficiency of the proposed system is greatly improved compared with the traditional systems. This is mainly due to the reasonable use of the pressure energy of the gasification product through the supercritical turbine, the release and use of high level chemical energy stored in the syngas in the gas turbine, and chemical heat recovery of the sensible heat of the flue gas from the gas turbine. Exergy and economic analyses are also performed. The levelized cost of electricity (LCOE) of the proposed power system is 0.2963 ¥/kWh. The power system proposed herein can be considered as one of the suitable options to achieve the carbon peak and carbon neutrality goals.

Study on supercritical water gasification reaction and kinetic of coal model compounds

Supercritical water gasification (SCWG) of coal is a clean coal utilization technology that has gradually developed in recent years. It has great application prospect because of low pollution, good economy and high energy efficiency. Due to the complexity of coal molecular structure, it is difficult to directly study SCWG mechanism of coal at molecular level. Based on the Wiser molecular structure model of coal, diphenyl ether and diphenylmethane were selected as model compounds to study the bond breaking characteristics and gasification law of characteristic bridge bonds. The effects of catalyst, temperature, concentration and other factors on model compounds gasification were discussed. The results showed that temperature had the most significant influence on gasification effect. The effect order of catalyst on promoting gasification from high to low was: K2CO3 > Na2CO3 > Ca(OH)2. The increase of catalyst amount and residence time had a certain promotion effect on gasification reaction. Under the same condition, the gasification effect of diphenyl ether was better than that of diphenylmethane. At the same time, the kinetic model of coal-like structure model compound gasification in SCW was established, and the key parameters were solved. The activation energy of diphenyl ether and diphenylmethane was 82.96 kJ/mol and 99.92 kJ/mol, respectively.

Theoretical Analysis on the Criteria of MILD Coal Combustion

MILD combustion is featured with a uniform heat flux and low NOx emission, and is thus a promising technology for clean coal utilization. Establishing the theoretical criteria of coal combustion mo...

Thermodynamic and environmental analysis of integrated supercritical water gasification of coal for power and hydrogen production

Coal combustion for thermal power generation causes serious environmental problems. Relatively efficient and clean coal utilization technologies need to be developed to alleviate the growing energy crisis and environmental pollution pressure. A novel integrated supercritical water gasification of coal system with 40 t/h throughput of coal and water was designed. A thermodynamic analysis and a life cycle environmental assessment under different conditions were performed using Aspen Plus 8.4 and SimaPro 9.0 based on the Ecoinvent 3 database, respectively. Results show that the maximum energy loss is induced by the waste heat of the effluent from the heat exchanger. By using organic Rankine cycle, the energy efficiency of the system could reach 53.3% when the temperature and coal concentration are 700 °C and 15 wt%, respectively. The global warming potential, acidification potential and nitrogen oxides are 0.058 kg carbon dioxide eq/(kW·h), 3.63 × 10−4 kg sulfur dioxide eq/(kW·h) and 7.04 × 10−5 nitrogen oxides eq/(kW·h), respectively, for the process combined with carbon dioxide capture and storage under the temperature of 700 °C and the coal concentration of 15 wt%. The comparison of the thermodynamic and environmental performance among different systems shows that the exergy and energy efficiencies of integrated supercritical water gasification of coal system combined with organic Rankine cycle are higher than those of integrated gasification combined cycle, and the environmental emissions of the integrated supercritical water gasification of coal system are less than those of integrated gasification combined cycle system. This study reveals that the integrated supercritical water gasification of coal is a relatively clean and efficient coal utilization technology with good industrialization prospects.

A clean coal utilization technology based on coal pyrolysis and chemical looping with oxygen uncoupling: Principle and experimental validation

The coal poly-generation system offers the possibility to produce value-added chemicals, synthetic gas fuels and electricity simultaneously, which can improve the coal utilization efficiency. To realize the multi-generation of gas, tar and heat/electricity with CO2 capture, a new clean coal utilization technology integrating coal pyrolysis and CLOU (chemical looping with oxygen uncoupling) process was proposed. Both thermodynamic process simulation and batch-scale fluidized bed CLOU experiments were conducted to verify its feasibility and obtain some fundamental features. From simulation results, the total exergy efficiency had been improved by 0.82 percent points compared to the typical coal poly-generation system. Based on CLOU experiments of two pyrolytic semicokes of SL lignite (SL-20 and SL-60), it was found that the reactor temperature and the semicoke characteristic were two key factors in determining the CLOU process, in which higher fuel reactor temperature and semicoke with shorter pyrolysis time were favorable to the conversion process. In addition, both the semicokes showed very high CO2 gas yield (higher than 99%) and relatively low unburned gas yields, whilst the combustion efficiencies reached to higher than 93%. The results suggested that SL-20 with CLOU temperature of 900 °C was more favorable for the operation of the new system.

Hydrogen production by catalytic gasification of coal in supercritical water with alkaline catalysts: Explore the way to complete gasification of coal

Supercritical water gasification (SCWG) of coal is a promising technology for clean coal utilization. In this paper, hydrogen production by catalytic gasification of coal in supercritical water (SCW) was carried out in a micro batch reactor with various alkaline catalysts: Na2CO3, K2CO3, Ca(OH)2, NaOH and KOH. H2 yield in relation to the alkaline catalyst was in the following order: K2CO3 ≈ KOH ≈ NaOH > Na2CO3 > Ca(OH)2. Then, hydrogen production by catalytic gasification of coal with K2CO3 was systematically investigated in supercritical water. The influences of the main operating parameters including feed concentration, catalyst loading and reaction temperature on the gasification characteristics of coal were investigated. The experimental results showed that carbon gasification efficiency (CE, mass of carbon in gaseous product/mass of carbon in coal × 100%) and H2 yield increased with increasing catalyst loading, increasing temperature, and decreasing coal concentration. In particular, coal was completely gasified at 700 °C when the weight ratio of K2CO3 to coal was 1, and it was encouraging that raw coal was converted into white residual. At last, a reaction mechanism based on oxygen transfer and intermediate hybrid mechanism was proposed to understand coal gasification in supercritical water.

Numerical analysis of optimal performance of planar electrode supported solid oxide fuel cell at various syngas flow rates

With the gradual scarcity of fossil fuels and the emergency demand of greenhouse gas emission reduction, coal, as the major energy source of China for a long time in the future, should be used efficiently and cleanly. The integrated gasification fuel cell (IGFC) hybrid power generation system is a promising clean coal utilization technology. In order to improve the performance of IGFC hybrid power generation system, the optimal performance of SOFC at different flow rate of syngas were numerically studied in this work. The performance curves of SOFC at various syngas flow rates were obtained. The results show that the temperature in the channel of SOFC cell is decreasing linearly with the increase of the syngas flow rate, when the average current density is constant. With the increase of syngas flow rate, the output voltage is reduced, but the optimal output power and the possible optional range of current density will increase. Under different mass flow rates, the fuel utilization at the optimal output power density decreases with increase of syngas flow rate. The polynomial function between the fuel utilization at optimal output power density and the mass flow rate of syngas was obtained.

Hydrogen production by non-catalytic partial oxidation of coal in supercritical water: Explore the way to complete gasification of lignite and bituminous coal

Supercritical water gasification (SCWG) of coal is a promising technology for clean coal utilization. In this paper, hydrogen production by non-catalytic partial oxidation of coal was systematically investigated in supercritical water (SCW) with quartz batch reactors for the first time. The influences of the main operating parameters including residence time, temperature, oxidant equivalent ratio (ER) and feed concentration on the gasification characteristics of coal were investigated. The experimental results showed that H2 yield and carbon gasification efficiency (CE) increased with increasing temperature and decreasing feed concentration. CE increased with increasing ER, and H2 yield peaked when ER equaled 0.1. CE increased quickly within 1 min and then tended to be stable between 2 and 3 min. In particular, complete gasification of lignite was obtained at 950 °C when ER equaled 0.1, as for bituminous coal, at a higher temperature of 980 °C when ER equaled 0.2.

System Study on Partial Gasification Combined Cycle With CO2 Recovery

A partial gasification combined cycle with CO2 recovery is proposed in this paper. Partial gasification adopts cascade conversion of the composition of coal. Active composition of coal is simply gasified, while inactive composition, that is char, is burnt in a boiler. Oxy-fuel combustion of syngas produces only CO2 and H2O, so the CO2 can be separated through cooling the working fluid. This decreases the amount of energy consumption to separate CO2 compared with conventional methods. The novel system integrates the above two key technologies by injecting steam from a steam turbine into the combustion chamber of a gas turbine to combine the Rankine cycle with the Brayton cycle. The thermal efficiency of this system will be higher based on the cascade utilization of energy level. Compared with the conventional integrated gasification combined cycle (IGCC), the compressor of the gas turbine, heat recovery steam generator (HRSG) and gasifier are substituted for a pump, reheater, and partial gasifier, so the system is simplified obviously. Furthermore, the novel system is investigated by means of energy-utilization diagram methodology and provides a simple analysis of their economic and environmental performance. As a result, the thermal efficiency of this system may be expected to be 45%, with CO2 recovery of 41.2%, which is 1.5–3.5% higher than that of an IGCC system. At the same time, the total investment cost of the new system is about 16% lower than that of an IGCC. The comparison between the partial gasification technology and the IGCC technology is based on the two representative cases to identify the specific feature of the proposed system. The promising results obtained here with higher thermal efficiency, lower cost, and less environmental impact provide an attractive option for clean-coal utilization technology.

A104 数値解析による石炭ガス化炉内の灰挙動予測

The Integrated coal Gasification Combined Cycle (IGCC) power generation system is expected as a clean coal utilization technology in next generation, so some projects if IGCC system have been organized in Europe and USA. In Japan, the experimental researches with the 2T/D test-scale gasifier and the 200T/D pilot-scale gasifier have been completed, and a project of demonstrative IGCC plant will be started in the near future. In this paper, a numerical simulation of the 200T/D pilot-scale coal gasifier is carried out. To feed-back the results of numerical simulation into the design and operation of the coal gasifier, it is important to develop some method which specifies the phenomena in the coal gasifier. This paper presents a new procedure which can be estimated the trap-rate of the ash particles in combustor with the results of numerical simulation. The influence of the change in the burner arrangement and/or shape of gasifier on the trap-rate of the ash particles is estimated by this procedure.