| Coal is the corner stone for the energy supply of China.Burning coal has provided stable and economic power for the development of the society,but is also confronted with challenges of emitting huge amount of CO2 into the atmosphere.However,an efficiency penalty of at least 10 percentage points is expected when capturing the CO2.The supercritical carbon dioxide(sCO2)power cycle has received extensive attention and research in recent years for its potential of reaching high efficiency in nuclear,solar,waste heat recovery and fossil fuel power generation.The power generation system of the integrated gasification direct-fired sC02 power cycle is studied in this paper.This research aims to lower the efficiency penalty of the carbon capture while completing the energy conversion process.The main research contents are as follows.1.The model of the integrated gasification direct-fired sCO2 power cycle is built first,with the addition of a detailed zero dimension cooling model of the CO2 turbine.The needed modification of the coal gasification process for capturing CO2 in the direct-fired sC02 power cycle is discussed and the heat integration between the sCO2 power cycle and the syngas heat recovery is studied.Key cycle parameters,such as the turbine inlet temperature,turbine inlet pressure,turbine outlet pressure,the cycle low temper-ature,turbine coolant temperature and ASU specific energy,are studied to assess their effect on the efficiency.For the baseline cycle layout,when the turbine inlet tempera-ture is 1200℃,turbine inlet pressure is 30 MPa,turbine pressure ratio is 10,cycle low temperature is 25 ℃,the efficiency could reach 39.27%,while capturing almost 100%of the combustion derived CO2.This efficiency is 3.32 percentage points higher than that of the IGCC power plant that captures 90%of the CO2 with pre-combustion capture technology.2.A parameter optimization and heat integration study is conducted with the black-box heat exchange model method,in order to obtain the best thermodynamic efficiency without assuming specific heat exchange process.After parameter optimization,the efficiency without integrating the heat from the air compressor is 39.54%,while the efficiency increases to 41.72%when integrating the heat from the air compressor.The turbine inlet temperature and turbine pressure ratio that correspond to the highest effi-ciency are 1200 ℃and 10,respectively.The result from the parameter optimization in a wider parameter range shows that the turbine pressure ratio and the allowable hot end temperature of the regenerator are not the reason that limits the turbine inlet tempera-ture.From the exergy analysis and the analysis of the T-H diagram,higher turbine inlet temperatures are not recommended because that:(a)as the turbine inlet temperature in-creases,the efficiency penalty caused by the turbine cooling would gradually outweigh the efficiency increment by the higher turbine inlet temperatures;(b)at higher turbine inlet temperatures,the working fluid mass flow changes as a result of the increased tur-bine coolant mass flow so that there is abundant heat in the hot streams that the cold streams cannot recover efficiently as their mass flow reduces,which also penalizes the efficiency.3.A novel cycle layout with two CO2 turbines is proposed by modifying the heat integration of the baseline cycle layout.Apart from the adiabatic compression heat from the air compressor,the syngas heat is also integrated into the regeneration process after first utilized in the nested CO2 turbine.By modifying the oxygen compressor and the syngas compressor into adiabatic or partial adiabatic compression,the compression heat is also introduced into the regeneration process.These measures solve the poor thermal match problem of the regenerator caused by the specific heat cp variation of the sCO2.The T-H diagram of the regenerator shows that an ideal thermal match is reached,while the hot and cold end temperature differences both approach 10 ℃.Due to the huge heat duty of the regenerator,the thermal match of the regenerator becomes a key factor that determines the efficiency of the sCO2 power cycle.The efficiency of the novel cycle layout is 41.25%,which is 1.98%percentage points higher than that of the baseline cycle layout.If the cycle low temperature is lowered to 17 ℃,the efficiency could further increase to 43.67%.The efficiency of the conventional heat integration cycle layout in which the heat in the system is recovered by multi streams of CO2 is 42.26%.The efficiency of the novel cycle layout for CO2-coal slurry gasification is 41.86%,and 44.2%for the subcritical CO2 gasification case.4.To determine the best sCO2 cycle layout for waste heat recovery,different sCO2 cycle layouts are studied and compared.The dual rail cycle has the highest efficiency.The reason is that by various stream split and merge,the adverse effect of the specific heat cp variation on the thermal match is eliminated.An ideal thermal match is therefore reached.The calculation shows that,under the same flue gas parameters,the power generated by the dual rail cycle is 91 MW,which is slightly lower than the 99.8 MW power generation of the triple pressure steam Rankine cycle.The cycle layout and syngas heat recovery scheme have been designed for the system that uses the sCO2 power cycle as the bottom cycle in place of the steam Rankine cycle.The efficiency is 43.1%,which is slightly lower than that of the 43.7%efficiency of the IGCC power plant.5.A modified MATIANT cycle layout is proposed to solve the problem of the regenerator’s high hot end temperature and the poor thermal match of the regenerator of the MATIANT cycle.To reduce the hot end temperature of the regenerator,the re-heat process is eliminated to enlarge the pressure drop through the turbine and reduce the turbine exhaust temperature.To improve the thermal match of the regenerator,re-compression and stream split are added in the modified MATIANT cycle layout.The result indicates that the average hot end temperature of the regenerator is reduced from 806 ℃ to 625 ℃.The analysis of the T-H diagram shows that the thermal match of the regenerator is improved.With regard to the efficiency,although the regeneration process is improved,the efficiency of the modified MATIANT cycle is slightly lower than that of the original MATIANT cycle due to the elimination of the reheat process and lower average heat absorbing temperature,but with an efficiency decrement of less than 1 percentage point.This research has conducted innovative work in the cycle aspect of the integrated gasification sC02 power cycle.Novel cycle layout is proposed.New insight on key cycle parameters,such as the turbine inlet temperature,is obtained through this research work.Beneficial exploration has been conducted in the novel technology route for capturing the CO2 with the direct-fired sCO2 power cycle,which has important practical significance. |
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