| During industrial processes,there are a large number of high-temperature bulk materials,such as high-temperature sinter(～800℃)and hot coke(～1000℃)in the iron and steel production.The efficient recovery of waste heat from these bulk materials is of great significance for industrial energy saving and consumption reduction.Depend on the temperature,particle size distribution and air permeability of the bulk particles,the recovery methods are different.For example,annular coolers with gas-solid cross-flow translational bed structures are employed for sinters with uneven particle size and poor permeability to recover the waste heat.Due to the limitations in the quality of heat recovery,comprehensive utilization efficiency of waste heat shows great room for improvement.On the other hand,coke with good permeability achieves efficient recovery through gas-solid countercurrent vertical quenching.Its combination with thermochemical recovery is the main way to further improve the comprehensive utilization effect.In this dissertation,combined with the particle characteristics of the two typical bulk particles,sinter and coke,the gas-solid heat transfer laws of different packing structures were studied;the difference and improvement direction of the waste heat recovery between the translational bed and the vertical bed were analyzed.The novel processes of waste heat recovery in multi-stage cascade translational bed and vertical coke wastewater gasification quenching were proposed respectively.The feasibility and optimization direction of the novel processes were discussed from the aspects of gas-solid heat transfer mechanism,energy efficiency and economy.Firstly,according to the characteristics of the bulk materials,the accumulation structures with different compactedness,particle size segregation and the permeability of particles are constructed.A gas-solid heat transfer model was established to study the effect of accumulation characteristics on gas-solid heat transfer.The results indicate that the volume heat transfer coefficient increases first and then becomes stable with the increase of Reynolds number in the uniformlypacked particle beds,while the bed pressure drop increases exponentially.Both the volumetric heat transfer coefficient and bed pressure drop are positively correlated with the compactness of the packed structures.For the particle size segregation packing structure,due to uneven bed resistance distribution,it is easy to cause gas flow bias,which makes gas-solid heat transfer and temperature distribution more uneven.However,if there are pores inside the particles,the effect of particle size segregation on the inhomogeneity of gas flow and gas-solid heat transfer in the bed is significantly reduced.Further analysis shows that for bulk materials,such as sinter with wide particle size distribution,vertical heat exchange devices can easily result in particle size segregation,making it difficult to guarantee the cooling and waste heat recovery.The translational bed structure is still a relatively reliable waste heat recovery method.For the bulk materials with good permeability(i.e.,coke),relatively good heat transfer uniformity can be guaranteed even if there is particle size segregation in the vertical device.Next,for the traditional translational and vertical waste heat recovery processes,one-dimensional unsteady and steady-state mathematical models describing gas-solid temperature distribution and bed pressure drop were established,respectively.The exergy recovery efficiency considering recovered exergy and power consumption was defined.Taking the waste heat recovery of high-temperature sinter as an example,the actual engineering equipment of the two processes was selected for numerical simulation.The energy efficiency differences between the translational and vertical processes was compared and analyzed.The results show that at the same cooling temperature(150℃),the total amount of waste heat recovery via the two processes is the same,but there are significant differences in the temperature and energy level of the recovered gas.The recovered gas exergy via the vertical process is 36%higher than that via the translational process.The thickness of the bed,gas average temperature,and gas flow rate per unit section of the vertical process are all relatively large,and the fan power consumption can reach 10.7 times that of the translational process.As the fan power consumption is much less than the exergy of recovered gas,the exergy efficiency of the vertical process is still 11%higher than that of the translational process.Based on the traditional translational bed waste heat recovery process for sinter,a novel multi-stage cascade translational bed waste heat recovery process was proposed.Through mathematical modeling and numerical simulation,the effects of different series and parallel and cascade modes on the heat transfer process and comprehensive energy efficiency of the novel process were studied.Based on the structure of an actual annular cooler,a multi-stage cascade transformation scheme was designed,and the impact of equipment air leakage rate on the effect of waste heat recovery and the economy was analyzed.The results indicate that,as the number of cascade increases,the recovered gas exergy significantly increases first and then levels off;while the fan power consumption continues to increase,making exergy efficiency increase first and then decrease.When the number of cascade reach four-stage or above,the exergy efficiency with cascade was lower than that traditional process without cascade.Under several selected conditions,the exergy efficiency reached the maximum(13-22%higher than that in no-cascade process)at the one-stage or two-stage cascade.Exergy efficiency of 420 m2 annular cooler in a sinter plant reached the maximum at one-stage cascade at different air leakage rates,with a maximum increase of 18%compared with the traditional process,and the incremental investment payback period of process reform is at least 0.67 years.On the basis of the traditional coke dry quenching(CDQ)process,a novel vertical coke wastewater gasification quenching(CWGQ)process using coking wastewater as the cooling medium was proposed considering the treatment requirements of coking wastewater and coke powder.A gas-solid heat transfer,phase change,and reaction model was established for the coke quenching furnace,and numerical simulation and parameter analysis were conducted on the thermal process.By comprehensively considering the bi-objective optimization of system benefits and costs,the optimal structure and operating parameters of the CWGQ process were obtained and the thermal economy was compared with the CDQ process.The results indicate that the optimal height-diameter ratio,volume ratio,amount of injected wastewater,and intermediate steam extraction rate of the CWGQ process under the same weight of net income exergy and total annual cost are 0.267,0.723,0.239 t-water/t-coke and 0.364,respectively.The net exergy income of the optimized CWGQ process is 38%higher than that of the CDQ,while the total annual cost is only 0.24 times that of the CDQ.Increasing the weight of cost objective is beneficial for improving the net annual profit of CWGQ process,which is 1.41 times that of CDQ.The multi-stage cascade translational bed process and the CWGQ process proposed in this dissertation as well as their energy efficiency and economy analysis provide new insights and theoretical data for efficient waste heat recovery of hightemperature bulk materials with different particle packing structures. |
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