Optimization Of Power Cycles For Waste Heat Recovery From Medium-to High-temperature Flue Gas And Experimental Study On Thermal Stability Of Organic Working Fluids

Large quantities of medium-to high-temperature flue gas are generated during industrial production,and waste heat recovery is helpful for reduction of energy consumption,environmental pollution and CO₂ emission.This thesis focuses on waste heat recovery from flue gas in temperature range from 200 to 700 ?.Current power cycles usually have bad thermal matching with flue gas heat source,which is mainly caused by shortcoming of power cycle-working fluid combination or bad thermal stability of organic working fluid,and produce less power output.In view of the above problems,working fluid selection,parametric optimization and cycle design were performed to obtain the optimal combination of“cycle design-working fluid-cycle parameters”under different heat source temperature,taking the maximization of net power output per unit mass flow rate of flue gas as objective function.Experimental investigation on thermal stability of some organics were performed and corresponding thermal stability data were obtained.Comparison among optimization results of current power cycles,such as organic Rankine cycle?ORC?,organic transcritical cycle?OTC?,steam Rankine cycle and CO₂transcritical cycle?CO₂-TC?,shows that ORC or OTC produces much more power output than steam Rankine cycle under source temperature from 200 to 400?.In the condition of heat source higher than 500?,the maximum cycle temperature is restricted by thermal stability of organic working fluid,and cycle thermodynamic performance declines.Steam Rankine cycle produces the maximum power output under heat source of 700?.The variation of system thermodynamic performance with cycle configuration,working fluid and cycle parameter was obtained.Transcritical cycle outperforms Rankine cycle,and regenerative cycle outperforms basic cycle under the same heat source condition.The optimal working fluid depends on heat source temperature,and fluid with higher critical temperature usually more suitable for higher temperature heat source.The optimal cycle parameter is influenced by cycle configuration,working fluid and heat source.In view of the characteristic of medium-to high-temperature flue gas?high inlet temperature and large temperature drop during heat rejection?,some cycle improvements were proposed in order to improve cycle performance.Regenerative and over-expansion were considered for CO₂-TC.Five combined cycles in which air Brayton cycle or CO₂-TC works as high temperature sub-cycle and OTC works as low temperature sub-cycle were investigated.Results indicate that the over-expansion CO₂-TC increases expansion ratio while the main compression ratio and high pressure keep constant.Turbine exhaust temperature decreases and expansion work increases.The regenerative and over-expansion CO₂ transcritical cycle produces the maximum power output among stand-alone cycles in heat source temperature range of 500 to 600?.The combined cycle using working fluid with good thermal stability?like air and CO₂?for high temperature cycle,therefore a better matching between heat source and working fluid is obtained.Compared with stand-alone cycle,the combined cycle achieves higher thermodynamic perfection and power output.Among the combined cycles,the recompression CO₂-TC+OTC parallel-type combined cycle produces the maximum power output.The thermal stability testing system was designed.Thermal stability of n-butane,R134a and R245fa was investigated based on the static capsule test.Results indicate that n-butane shows obvious thermal degrade in temperature range of 325-343 ?.No obvious thermal degradation occurs for R134a and R245fa under 442? and 385?,respectively.