Carbon-Hydrogen Conversion Law And Energy Utilization For Low Rank Coal-Based Chemicals Cascade Coproduction Systems

In a relatively long period of time, coal is the main energy source of China. With the exploitation of high rank coal, the problem of efficient conversion and clean utilization of mid-low rank coal becomes more and more serious. Meanwhile, the solvent of this problem is an important measure to guarantee the energy safety of our country.This paper is funded by the National Basic Research Program of China (973 Program 2011CB201306), and an innovation research is developed to study the carbon-hydrogen conversion principle and energy utilization rule of low rank coal based chemicals cascade coproduction system. The main research is as follow:Firstly, the material conversion and energy utilization laws of chemicals cascade coproduction system were illustrated. The material conversion principle of coal conversion process to produce carbon-hydrogen based chemicals was described from the points of thermophysic property, chemical equilibrium and carbon emission of chemicals. As to the characteristic of pure chemicals coproduction system, the new knowledge of chemicals cascade coproduction system was proposed. Besides, the evaluation method and core indexes were also developed to reveal the material conversion and energy utilization principles of chemicals cascade coproduction system. Meanwhile, the hierarchical heat integration was proposed to obtain the heat integration scheme with higher viability, which was directed at the structure complexity of coal coversion process.Many parameters were integrated to illustrate the energy utilization of coproduction process which could produce syngas, semicoke and light oil from low rank coal. The low-temperature pyrolysis process and atmospheric and vacuum distillation were combined to build low rank coal-based process system to coproduce syngas, semi-coke and light oil. The modelization study was also conducted. The calculation shows that the lignite, with the humidity of about 40%, is conversed to char, gas and light oil, whose yields are 42.30%, respectively,8.47% and 4.10%, respectively. Through integrating three parameters, product yields-energy consumption/exergy loss-process unit, the material conversion and energy utilization of coproduction system were analyzed. At the target of above products yield, the unit with the highest energy consumption and exergy loss in the process is the drying unit, followed by the pyrolysis unit. Meanwhile, as to the unit consuming heat utility, the higher the energy consumption is, the higher exergy loss of the process is. The energy coupling and conversion principle of coproduction system was illustrated by the integration of stream grade, utility temperature and process unit. Energy consumption and exergy loss for each unit gradually decrease in accordance with the progress of process, while those of some products increase step by step. In addition, the gradual increase of energy quality factors of dry coal and coal is at the cost of energy consumption and exergy loss of unit process. Different units require different energy qualities of heat or cold source, and those of streams outputted system are also different.Then, the hierarchical heat integration was applied to study the heat integration of pyrolysis coproduction system. Through the energy analysis of above system, it could be found that there is a large potential to save energy for the coproduction system. In accordance with the hierarchical heat integration method, four hierarchies of heat integration were performed for low rank coal pyrolysis coproduction system. According to the criteria of hierarchical heat integration, the third hierarchical heat integration was selected as the final heat integration scheme for the coproduction system. The total energy consumption of the system is 382.8 kW, and the energy-saving ratio is 39.8%. The case studies show that hierarchical heat integration is suitable for the process with complex structure. The method solves the problem of heat integration scheme with lower feasibility in practical production from the technical level.The chemical reaction mechanism of furnace off-gas steam reforming was studied to explore the influence of technical parameters on the target products. In the coprodution system, the oxy-thermal furnace off-gas and syngas are mixed to perform steam reforming. The reaction mechanism was revealed according to the thermodynamic characteristics of independent reaction equation. The equilibrium components of system were determined by the balance of four forward and reverse reactions. Based on the reaction mechanism, the effects of temperature, pressure and water steam ratio on the CO and CH4 conversion ratio, H2 selectivity and reaction energy were analyzed. Four two-dimensional maps were integrated to describe the relationship among the H2 production, steam to gas ratio, pressure, temperature, energy consumption and the H2/CO molar ratio in reaction products. The process parameters of steam reforming of furnace off-gas could be determined by this method according to the utilization of furnace off-gas in the later period.The better coproduction scheme of furnace off-gas was researched based on the construction principle of chemicals cascade coproduction. The target products were determined by comparing process characteristic and technology maturity of furnace off-gas based chemicals. After the consideration of process structures, four cascade coproduction schemes were also built to produce methanol, dimethyl ether and fuel oil. With the building principle and evaluation method of the pure chemicals cascade coproduction system, the characteristics of core indicators were compared and analyzed among four schemes. The preliminary conclusion is that the process structures of schemes 1 and 4 are better than those of the single production and other coproduction schemes.At last, the process of cascade coproduction system was constructed to produce low rank coal-based chemicals based on the previous work, and the furnace off-gas coproduction scheme was determined further. The principles of material conversion and energy utilization were focused on, which were included effective atom yield, energy consumption distribution, carbon emission and pollutants. The analysis shows that the furnace off-gas utilization scheme has a great influence on the carbon hydrogen conversion and energy utilization of the cascade coproduction system. The optimization of furnace off-gas coproduction system should be paid on more attentions.