| Traditional power generation based on thermal power conversion is limited by the highest parameters of the thermal cycle and cannot use high-temperature combustion energy effectively.Therefore,it is a key technical idea to achieve efficient energy utilization that building a combustion photo-thermal energy cascade conversion system based on energy-quailty matching principle.In addition,oxy-fuel combustion is not only a key technology for CO2 control,but also can generate valuable high-temperature radiative energy.Therefore,it is of great significance to study the radiative energy characteristics in oxy-fuel combustion and its corresponding photo-thermal energy cascade utilization system from the perspective of energy-quailty matching utilization.The contents of this thesis are organized as follows:（1）the evaluation theory of monochromatic radiation energy quality for oxy-fuel combustion;（2）development of a high-efficiency,high-precision and wide-range total radiation model in oxy-fuel atmosphere;（3）theoretical analysis of radiative energy characteristics in oxy-fuel combustion;（4）experimental analysis of radiative energy characteristics in oxy-fuel combustion;（5）development and analysis of two photo-thermal energy cascade conversion systems based on oxy-fuel combustion.In order to reasonably evaluate the quality of high-temperature radiative energy in oxy-fuel combustion,a radiative thermodynamic theory to characterize the available energy of spectral radiation is established first.Based on the discussion of several expressions of blackbody radiative exergy,a radiation machine model is established from the perspective that the radiation and thermal energy is different.This model proved the validity of Petela’s formula for blackbody radiative exergy.Based on the concept of radiative equivalent temperature,an expression in intergral form for the exergy of a monochromatic photon is proposed by establishing an infinite staged Carnot heat engine model.At the same time,an approximate relationship between the equivalent temperature and the radiation wavelength is also given.Finally,the entropy of monochromatic photon is discussed using the infinite staged Carnot heat engine model,and the expression of the photon entropy in integral form is given.Moreover,it is verified that the entropy and exergy of monochromatic photon satisfy the thermodynamic relationship,which can reflect the difference between radiative energy and thermal energy.The theories for the development of the weighted-sum-of-gray-gases（WSGG）model is summarized in detail,and the structure of the WSGG model is improved to make it compatible with the parameters in a wider pressure range.Based on the SNB model of the EM2C laboratory,the WSGG model coefficients for the oxy-fuel combustion under three typical pressure conditions are developed.The improved model is applied to both 1D and 2D cases to verify its accuracy.The calculation results of improved model are in good agreement with that of the benchmark model,which illustrates the rationality of the improved model.On this basis,an improved WSGG radiation model adapted to a wider range of H2O/CO2 mixed atmosphere is further developed.The new model can be applied in the pressure range of 0.1-3 Mpa;its temperature range is 500-2500 K,the path-length range is 0.001-60 m,and H2O/CO2 molar ratio range is 0.125-4.The new WSGG model is suitable for multiple fuels and most combustion equipment.In addition,the effect of pressure on the radiative heat transfer of mixed gases is also studied in depth based on the new model.It is found that the emissivity of mixed gas calculated by the new WSGG model is in good agreement with that of the benchmark model under high pressures.The error of average source term in one-dimensional cases at a path-length of 1 m is less than 4%,and the the error of average heat flux is less than 3%.It is very meaningful to establish a wide-range model since the traditional WSGG model can not achieve good results under high temperatures.This study also found that increasing the pressure can enhance the radiative heat transfer of H2O/CO2 mixed gases within a certain pressure range,and there is an optimal pressure to enhance the gases radiative heat transfer.At the same time,the pressure has a greater effect on the radiation intensity of mixed gases with lower H2O/CO2 molar ratio.Based on the second law of thermodynamics,the radiative transfer equation is combined with the exergy theory of monochromatic radiation to establish monochromatic radiative entropy transfer equation and radiant exergy transfer equation,which can be used to calculate and analyze the energy quality in the spectral radiation transfer process more reasonably and accurately.Through theoretical and numerical verification,it is found that the radiative entropy transfer equation and radiative exergy transfer equation is in accordance with thermodynamic laws.On this basis,the radiative energy characteristics of the combustion media are calculated by establishing a one-dimensional furnace case.The effects of temperature,gas molar ratio,pressure,path length,and particle number density on the spectral radiative energy and its proportional distribution are investigated.The results show that the characteristics of the spectral radiative energy ratio and the spectral radiative exergy ratio are consistent under various cases;and thus,the spectral distribution characteristics of radiative energy can be used to predict that of radiative exergy.The main parameter that affects the spectral distribution of radiaivet energy is temperature.Based on the modified tube furnace platform,the experimental research on the radiative energy flux of pulverized coal oxy-fuel combustion is carried out.The effects of temperature,oxygen concentration,atmosphere,and coal ranks on the characteristics of radiative energy flux are investigated.Based on the radiative thermodynamics theory developed in this thesis,the radiative exergy characteristics are also discussed.It is found that the increase of temperature and oxygen concentration enhances the radiative power of pulverized coal combustion,and the proportion of short-wavelength radiation below 4.1μm increased,where the effect of temperature is more obvious.The variation trend of radiative exergy power is basically consistent with that of radiative energy.The effect of coal ranks and oxygen concentration on the exergy-to-energy ratio is little,where the main factor is temperature.The proportion of spectral energy can also be predicted through establishing a one-dimensional calculation case.Based on the self-built Hencken burner setup,the spectral radiative energy characteristics of the semi-coke jet flame are experimentally studied,and the spectral radiative exergy characteristics are studied based on radiative thermodynamics.It is found that high temperature and high oxygen concentration directly enhance the radiation intensity.With other conditions unchanged,the radiation intensity is lower in the O2/CO2 atmosphere.The radiative energy proportion distribution under different cases is basically in accordance with the gray-body distribution,which can be predicted by constructing a one-dimensional calculation case.The radiation in the1.1-3μm band accounts for 60%at 1400°C.The spectral radiative exergy distribution in different cases is consistent with that of spectral radiative energy.The experimental results based on the pilot test facility show that the pure-oxygen combustion of solid fuel generates a spectrum similar to gray body.The calculated distribution characteristics of spectral radiative exergy are similar to that of the spectral radiative energy.Pure-oxygen combustion gives a high temperature of more than 2000 K and has extremely high energy quality.At this temperature,due to the limitation of the highest parameters of the traditional thermal cycle,a large loss of energy quality is caused.The idea of photo-thermal energy cascade utilization is more critical for pure-oxygen high-temperature combustion.Finally,after summarizing the theory and principles of combustion photo-thermal energy cascade utilization,an oxy-fuel combustion photo-thermal energy cascade conversion system that directly uses flame energy and an oxy-fuel combustion thermophotovoltaic-Brayton cycle-ORC combined photo-thermal energy cascade conversion system（TBRC）are proposed.For the oxy-fuel combustion photo-thermal energy cascade conversion system that directly uses flame energy,the system performance is analyzed by establishing a thermodynamic analysis model and based on radiation experimental data.Simulation results show that the system efficiency can be theoretically improved by about 13percentage points as the proportion of photovoltaics increases when compared with the basic Rankine cycle.In the new system,the boiler exegy loss（about 60%）is much larger than that of Rankine cycle（about 7%）.The addition of photovoltaic mainly reduces the exergy loss in boiler heat transfer,thereby reducing system exergy loss and improving system efficiency.In the oxy-fuel combustion photo-thermal energy cascade utilization system that directly uses flame energy,anthracite and bituminous coal are more efficient,while lignite is the worst.In the oxy-fuel combustion photo-thermal energy cascade conversion system characterized with the spectral radiation tuning（TBRC）,a thermophotovoltaic device is used to perform spectrum tuning for combustion radiation and further perform photovoltaic conversion.In this thesis,the system of thermophotovoltaic,Brayton cycle and Rankine cycle is constructed respectively to simulate and analyze a 150 kW small system.The results show that the efficiency of the system can be increased by20 percentage points compared with the thermal power cycle having same capacity.The optimal power of the system under 21%O2/N2 combustion condition is close to that under about 30%O2/CO2 condition.The effect of oxygen concentration on the system power is greater when the combustion atmosphere is O2/CO2 or the fuel is fuel oil.Furthermore,based on a high-parameter and high-efficiency gas-steam combined cycle system with the capacity of 480 MW,the theoretical maximum efficiency of corresponding photo-thermal energy cascade conversion system with oxy-fuel combustion can reach 86%,which is higher than that of air-fired combined cycle units by 26 percentage points.This reflects the development potential of the oxy-fuel combustion photo-thermal energy cascade conversion system. |
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