| Hydrogen is considered to be the ideal energy source in the 21st century.Under the background of global carbon emissions reduction and carbon neutrality,both academia and industry are exploring low-cost and large-scale hydrogen production routes.Due to the limitation of natural resources endowment,coal-to-hydrogen is the main way of large-scale hydrogen production in China.Underground coal gasification（UCG）-based hydrogen production（UCG-H2）coupled with carbon dioxide geological storage can convert traditional energy into hydrogen,and is expected to become a low-cost and large-scale hydrogen production route.China is rich in deep coal resources,and the coal resources buried deeper than 1000 meters are about 2.86 trillion tons.Under the situation that currently the feedstock for hydrogen production in China still relies on coal,UCG-H2 based on deep coal seams possesses good development prospect.Taking UCG-H2 as research objective,this thesis systematically carried out a quantitative study including process efficiency and energy utilization,carbon emissions and energy consumption level,and economic performance,so as to provide scientific basis and evaluation basis for the demonstration and application of underground coal gasification hydrogen production technology.Based on the world’s only deep underground coal gasification test scenario,and supplemented by necessary process units,the UCG-H2 route was formed and taken as research objective in this thesis.The simulation models were established for the main process units of UCG-H2 with annual hydrogen production of 1.2 billion Nm3.On the basis of simulation results,the following research work was carried out:Based on exergy analysis method,the exergy efficiency of each process unit of UCG-H2 from raw coal to hydrogen was calculated,and the exergy balance equation was used to analyze the distribution of process exergy loss.The cumulative exergy consumption（CEx C）considering equipment factor was introduced as the index to measure the process efficiency,so as to solve the problem of discontinuous energy input in UCG-H2 and exergy inequivalence in traditional exergy analysis;The Life cycle assessment model of UCG-H2 from raw coal to hydrogen output,covering process energy production and utilization was established.By introducing primary fossil consumption factor and carbon emission factor,the carbon emissions and external fossil energy consumption level were quantitatively evaluated;By establishing cost calculation model and using investment coefficient method,the economic performance of UCG-H2 represented by total investment and product cost was investigated;UCG-H2 with different CO2 capture rates were designed to solve the serious CO2 emissions problem,and the variation of its economic performance,as well as fossil energy consumption and carbon emissions in its life cycle with CO2 capture rate was analyzed.Also,the impact of introducing carbon tax on economic performance was investigated.According to the characteristics of UCG-H2,two carbon emissions reduction approaches of carbon dioxide storage in gasification cavity and hydrogen-urea co-production were proposed,and the corresponding emissions reduction potential was evaluated.In all research processes,the evaluation indexes of UCG-H2 were compared and analyzed with Lurgi surface coal gasification-based hydrogen production（SCG-H2）,coke-oven gas-based hydrogen production（CG-H2）and natural gas-based hydrogen production（NG-H2）routes.The main research conclusions are as follows:The analysis of process efficiency and energy utilization shows that when annual hydrogen production is 1.2 billion Nm3,the total exergy loss of UCG-H2 from raw coal to hydrogen output is 677.7 MW.The UCG unit contributes 207.67 MW,accounting for30.6%,and graphical exergy analysis shows that 79.8%of its exergy loss occurred in the process of liquid water vaporization and overheating.The methane reforming unit contributes 339.89 MW,accounting for 50.2%,mainly due to the irreversibility of fuel gas combustion reaction in the burner.The process efficiency is measured by the consumption amount of cumulative exergy.When 1 kg of hydrogen is produced,the CEx C of UCG-H2 is 376.1 MJ,which is only 83.6%of that of SCG-H2（449.7 MJ）,indicating that it is a more efficient hydrogen production route than traditional hydrogen production from surface coal gasification,but still higher than that of CG-H2（339.2 MJ）and NG-H2（237.9 MJ）.Sensitivity analysis shows that when annual hydrogen production increases from 0.4 billion Nm3 to 1.6 billion Nm3,the CEx C of 1 kg hydrogen produced by UCG-H2 and SCG-H2 decreases from 471.4 MJ and 537.9 MJ to 368.1 MJ and 443.8MJ,and the CEx C gap between UCG-H2 and SCG-H2 widens further with the increase of scale,indicating that UCG-H2 can better show its advantage in efficiency on a large production scale.The life cycle impact analysis shows that,based on the production of 1 kg of hydrogen,UCG-H2 needs to consume 349 MJ of external fossil energy and produces 30.4kg CO2-eq of carbon emissions in its life cycle,which are 25%and 16.4%lower than that of SCG-H2（465.3 MJ and 36.4 kg CO2-eq）.The distribution of fossil energy consumption contribution shows that UCG-H2 can not only reduce coal consumption by 19.7%,but also reduce oil consumption caused by obtaining process energy by 83.1%;The stage distribution of carbon emissions shows that UCG-H2 can not only reduce carbon emissions of 3.6 kg CO2-eq in hydrogen production stage,but also reduce carbon emissions of 2.4 kg CO2-eq in the upstream coal mining,washing and transportation stage.Further analysis of the sources of carbon emissions contribution in hydrogen production stage shows that due to the high methane content characteristic of UCG-H2,when producing 1 kg of hydrogen,the CO2 emissions contributed by exhaust gas of acid gas removal unit and flue gas of methane reforming unit are 9.8 kg and 8.8 kg respectively.The carbon emissions of CG-H2 under any allocation principles are lower than that of UCG-H2,while except for under no allocation,the fossil energy consumption level is lower than that of UCG-H2 only under value allocation principle with coke-oven gas price lower than 0.35 RMB/Nm3,or coke price higher than 1232 RMB/t.Scenario analysis shows that under the premise that the annual hydrogen output of China is 21 million tons and the raw material structure remains unchanged,if UCG-H2 is used to completely replace SCG-H2,the consumption of fossil energy equivalent to 50 million tons of standard coal can be reduced every year,and 75.39 million tons of equivalent carbon dioxide emissions reduction can be achieved at the same time.The economic analysis shows that when annual hydrogen production is 1.2 billion Nm3,the investment per ton of hydrogen is 1292 RMB for UCG-H2,which is only 68.7%of that of SCG-H2（1879 RMB）,but is 2.1 times and 2.7 times of that of NG-H2 and CG-H2;The hydrogen production cost of UCG-H2 is only 0.54 RMB/Nm3,which possesses the advantage of low cost.In the initial stage of hydrogen energy industry,i.e.,hydrogen market price less than 1.5 RMB/Nm3,the internal rate of return of UCG-H2 can reach25.5%,higher than 18.9%of NG-H2,while the internal rate of return of SCG-H2 is only1.9%.By analyzing the impact of production scale on economic performance,according to the scale effect,it is determined that the annual hydrogen production of 0.46 billion Nm3 and 1.2 billion Nm3 is the lower and upper limit of production scale for UCG-H2,and if the scale continues to expand,the degree of investment and cost reduction will not be significant.In addition,the sensitivity analysis on coal seam thickness shows that the economic feasibility will not be limited by coal seam thickness within the variation range of 2～10 m.For UCG-H2 coupled with carbon capture,the capture rate of 80%can be regarded as the boundary between high capture rate and low capture rate.After the capture rate exceeds 80%,life cycle energy consumption and carbon mitigation cost will increase significantly,while the decline rate of carbon emissions will decrease significantly;Under the capture rate of 80%,despite the cost of UCG-H2 rises to 0.64 RMB/Nm3,the internal rate of return in the initial and mid-term stage of hydrogen energy industry still can reach21.8%and 34.9%,far higher than the standard of 12%.If the carbon tax reaches 132RMB/t CO2,the cost of UCG-H2 with 80%capture rate will be the same as that of without coupled carbon capture,and if the carbon price continues to increase in the future,the development of UCG-H2 with carbon capture will become more profitable.Under the capture rate of 80%,the cavity formed by gasification can store 61.8%of the CO2captured during the operation period,alleviating the problem of carbon dioxide source-sink mismatching that may be faced by subsequent treatment. |
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