Design And Thermodynamic Analysis Of Energy Recovery System By Hydrothermal Carbonization Coupled With Steam Gasification From Sludge

Sewage sludge contains a large amount of organic matter and toxic and harmful substances,and has the dual properties of resource and pollution.Energy utilization in the process of sludge disposal is an important issue.Energy utilization of sludge usually has problems such as high energy consumption for dewatering,high CO₂emission and pollution control,and the high ash content and low calorific value of the sludge itself also cause low efficiency and instability of the treatment system.Hydrothermal carbonization technology can directly treat sludge with high moisture content and improve its dewatering performance,reduce dewatering energy consumption,and reduce the N and S content in sludge to reduce NO_x and SO_xgeneration for its fuel utilization.Steam gasification technology can convert sludge into clean energy gas with high calorific value.Blending a certain percentage of biomass in sludge can improve the fuel quality of the blended material.Therefore,this paper designs a sludge energy recovery system with hydrothermal carbonization pretreatment and steam gasification energy conversion as the main components,and carries out a comprehensive analysis and optimization of the system based on Aspen Plus simulation and thermodynamic analysis methods.Firstly,the basic thermal parameters of the system were simulated and calculated based on Aspen Plus,the dewatering energy consumption level of sludge after hydrothermal carbonization pretreatment,the energy saving level of waste heat recovery and the steam gasification characteristics of hydrochar were discussed,and the influence of pine chips blended sludge on the system performance was analyzed.The results showed that:waste heat recovery by flashing saves about 30～35%of energy consumption in the hydrothermal carbonization unit;hydrothermal carbonization-mechanical dewatering-thermal drying process can save about 74%of dewatering energy consumption than direct thermal drying;blending pine chips can improve the syngas yield of hydrothermal carbon steam gasification and improve the efficiency of cold gas,when the blending rate of pine chips increased from 0 to 0.5,the syngas yield increased from 1.2 Nm³/kg to 1.38 Nm³/kg,and the cold gas efficiency was increased from 80.7%to 82.3%.The effect of gasification temperature from 600°C to 750°C on steam gasification performance was more significant.The steam/hydrochar ratio increased between 0.5 and 0.75 and decreased between 0.75 and2,taking a value of 85.07%at 0.75.Secondly,mass and energy balance analysis of the system based on the first law of thermodynamics showed that:for a sludge blend with a pine chip blending rate of0.5,the system can obtain 67.54 kg of dry syngas per 1 t of raw material processed,and about 86.9%of the total energy consumed was the heat consumption of hydrothermal carbonization and steam gasification;when the sludge blended with pine chip for blending rate of 0.5,the energy efficiency of the system was 40.3%with dry syngas as the target product,and the energy recovery ratio of the feedstock is about53.4%,while for the sludge feedstock without pine chips blending,the energy efficiency of the system was only 26.81%and the energy recovery ratio of the feedstock is only 43.75%.Finally,the system was analyzed and optimized based on the second law of thermodynamics,and the results showed that the exergy efficiency of the hydrothermal carbonization unit was about 90%with low temperature flash slurry as the target product,and the exergy efficiency of the unit was reduced by increasing the hydrothermal temperature and water content of the raw material;the exergy efficiency of the dewatering and drying unit was 64.64%with 10%dry hydrothermal carbon as the target product,which was higher than the exergy efficiency of direct thermal drying（56.87%）under the same conditions;the exergy efficiency of steam gasification unit increased rapidly and then decreased slowly with the increase of gasification temperature,and reached the highest value of 61.45%at 750℃;the blending of pine chips can greatly improve the exergy efficiency of gasification of blended hydrochar,and the highest exergy efficiency was 90.18%when the blending ratio of pine chips was 0.5;The exergy efficiency of the gasification unit was reduced by increasing the gasification steam/hydrothermal carbon ratio.For the sludge blended with pine chips for blending rate of 0.5,the exergy efficiency of the system was 39.7%with dry syngas as the target product,while the exergy efficiency of the system was only 26.64%for the pure sludge raw material without pine chips blending.The mechanical dewatering filtrate and coke from gasification accounted for the largest share of the external exergy loss of the system.After optimization of energy recovery by coke combustion,the total exergy loss of the system（with a blending rate of 0.5 for pine chips）was reduced by20.3%,and the exergy efficiency was increased by 5.5%to 45.2%.Energy recovery through filtrate anaerobic digestion can also further improve the exergy efficiency of the system.