Techno-economic Analysis And Energy System Integration Of Coal-based Polygeneration Process Of Synthetic Natural Gas,Methanol And Power

As China’s primary energy reserves are rich in coal and lack of natural gas and oil,coal has been the main primary energy in China for a long time.However,due to the inherent hydrogen-carbon mismatch between coal and chemical products,the utilization ratio of coal resources is low.The coal chemical industry generally suffers from high carbon emissions and high energy consumption.Therefore,to realize the scientific,reasonable,clean and efficient utilization of coal resource is an important issue to ensure the sustainable development of coal chemical industry.In this paper,the typical coal-based polygegeration process is studied,aiming to effectively solve the problems in the existing coal to synthetic natural gas process,such as natural gas excess in non-heating season,poor economic performance,high carbon emissions and low energy utilization ratio.A noval coal-based polygeneration process of synthetic natural gas,methanol and power is proposed,including two kinds of polygeneration schemes,which can flexibly adjust the outputs of synthetic natural gas and methanol,and better adapt to the demand change and price fluctuation of product market.In order to improve the utilization efficiency of resources and energy,a gas-steam combined cycle unit is adopted to utilize the by-product steam and exhaust gases for power.Besides,CO₂ captured by the Rectisol is stored in liquid phase after a multi-stage compression process to reduce carbon emissions.Aspen Plus is used to model and simulate the polygeneration system in detail.The technical and economic analysis methods are adopted to compare the proposed polygeneration system with the conventional coal to synthetic natural gas process.Results show that the carbon utilization ratio of the proposed two scheme is improved by 5.9%and 6.7%,respectively.As a result,CO₂ emissions can be reduced by 14.2%and 14.6%respectively.In addition,the exergic efficiency can be increased by 5.9%and 3.9%,respectively.Although the total investment costs are slightly higher,the product cost is reduced by 6.6%and 7.6%,respectively,and the internal rate of return is significantly increased from 6.82%to 13.32%and 15.52%.In summary,the proposed polygeneration system can effectively adapt to product market changes,achieve high value-added production and product diversification,help the coal to natural gas projects achieve better economic performance and ensure sustainable development.Coal chemical industry is energy-intensive process with many units and often insufficient integration of energy system.Heat exchanger network synthesis based on Mathematical Programming method is considered to be an important way to improve energy efficiency and obtain significant technical and economic performance.Due to the influence of the combinativity and non-convexity of the mathematical model,the heat exchanger network synthesis problem can easily fall into the local optimal solution.The utilities of traditional stage-wise heat exchanger network superstructure model can only be located at the end of the streams,which is difficult to meet the complex matching requirements between process streams and external utilities in the actual industrial process,and it is difficult to obtain the global optimal solution.In this paper,we propose an extended heat exchanger network superstructure model based on the built-in utility strategy,where heaters and coolers can be located on intermediate stages.This extended superstructure model will expand the search space of the algorithm to some extent and make the model more complex.Gnetic Agorithm is used as the solution approach,and local optimization strategy,adaptive scheme,elite strategy,and structure identification and structure control strategy are integrated to improve the search capability and solution efficiency.Finally,the proposed heat exchanger network optimization method is applied to four cases to verify its reliability and efficiency.Results show that this method is able to solve complex heat exchanger network synthesis problems within acceptable computational time and ensure that optimal solution can be obtained.Through the analysis of the proposed coal-based polygeneration system,it is found that the methanol synthesis,methanol distillation and methanation units need a lot of external utilities,so it is necessary to optimize the heat exchanger network of these units to improve the heat recovery and reduce the total investment costs.In this paper,the extended superstructure model is used to optimize the heat exchanger network.Methanol synthesis and methanol distillation are optimized as one system and results show that the methanol plant can save 33.7%of hot utility and 21.3%of cold utility.The methanation unit can significantly save 76.7%of hot utility and 17.9%of cold utility.In order to further reduce the energy consumption of the methanol plant,a five-column methanol distillation acheme is developed,and the results show that the methanol plant can further reduce 41.2%of hot utility and 38.3%of cold utility.After the optimization of heat exchanger network,there is still a large amount of low-temperature waste heat that cannot be recovered in the proposed polygeneration system.After analyzing waste heat of each unit,cascade waste heat collection scheme is selected to recover waste heat from the CO₂ compression process,the methanol synthesis unit and the methanation unit.The waste heat is recovered by heat exchangers or absorption heat pumps and used to heat boiler feed water,and the hot water produced is used for district heating.Finally,3942 m³/hr of hot water with temperature of 74.3°C can be obtained,and a total of 112.8 MW of waste heat can be recovered.The heat exchanger network optimization and waste heat recovery can reduce a total of 134,185 tons of carbon emissions per year,which is a significant effect of energy saving and carbon emission reduction.