Basic Study On A Nonflame Combustion Technique With Thermal Cyclic Carrier Of Molten Salt

In the foreseeing short and medium future, fossil fuels still will be the main energy resources to produce electricity and heat for human. Unfortunately, combustion of fossil fuels leads to emissions of CO2, NOX and SOX into the atmosphere, which are believed to contribute to air pollution and global warming. In order to reduce the pollutants derived from combustion of fossil fuel, such as CO2, NOX and SOX, the developed countries have made great efforts to develop novel combustion technologies with high efficiency and low pollution aiming to reduce pollution caused by combustion of fossil fuels. Theses technologies have achieved considerable progress to control the pollutants emission during combustion. However, it is impossible for all over the present combustion technologies to eliminate CO2 emission fundamentally in combustion process. A novel combustion system, nonflame combustion technique with a thermal cyclic carrier of molten salt (NFCT), has been developed based on the knowledge of energy chemistry, fuel cell, combustion technology and chemical-looping combustion in the present work. In this technique, oxygen is separated from air and then burned with fuels in a molten bath to produce a high concentration of CO2 which is favorable for subsequent capture and storage.At the beginning of this work, the concept, technological process and mechanism of NFCT were introduced in detail. In this technique, some kinds of transition metal oxides such as Fe2O3, CuO and NiO were selected as the active phase in oxygen carriers. The fuel and molten bath used in the research work were CH4 and 1:1 weight ratio of Na2CO3 and K2CO3 respectively. The thermodynamics parameters of several typical NFCT system have been calculated. It is clearly that the NFCT system selected in this work is favorable in thermodynamics in a wide temperature range. Based on the principle of a system free energy minimum, the equilibrium compositions of metal oxides-CH4 system at various temperatures were calculated using thermodynamics software and database. Also, the equilibrium compositions of metal oxides-air systems were calculated. The equilibrium compositions diagrams of these systems were created through the software. The calculation results show that the lattice oxygen in the transition metal oxides can be used as oxygen resources for CH4 total oxidation. The lattice oxygen of metal oxides lost in the reaction with CH4 can be recovered when the reduced metal oxides were exposed in air atmosphere.Three series of Fe2O3-, CuO- and NiO-based oxygen carriers were prepared with mechanicalmixing, coprecipitation and impregnation techniques. The oxygen carriers were characterized by means of XRD, SEM, BET, TG, O2-TPD, CH4-TPR, etc.. The redox quality and cyclic reactivity of the oxygen carriers were examined in a fixed-bed reactor and a thermal gravimetrical respectively. The possibility of using natural iron ore as oxygen carrier in NFCT was evaluated. The results show that iron oxide will undergo state changing step by step as follows: Fe2C>3—>Fe3O4—>FeO—>Fe during reacting with CH4. CuO will firstly convert into C112O, and then into Cu being exposed into CH4 atmosphere. NiO will directly convert in metal Ni when it reacts with CH.J, and Ni will return to NiO when it is oxidized in air. To some extent, the inert binder and its content can affect the reactivity of oxygen carriers. Generally speaking, NiO-type oxygen carriers show better reactivity than the CuO-type oxygen carriers. The reactivity of Fe2O3 is not as good as the two former oxygen carriers, but the reactivity of Fe2C>3-based O-carriers can be improved through some techniques. Furthermore, the results show that the three series oxygen carrier enjoy good redox quality and cyclic reactivity. The reactivity will become lightly better with the numbers of cyclic reactions. Perhaps, this is partially explained by the fact that the structure in the carriers particle has been changed by the thermal impact and chemical reactions.Outlet gas products of the fixed-bed reactor were analyzed by a gas analyzer. It is found that at the beginning reaction stage CH4 is mainly converted into CO2, which concentration was over 85% when CH4 was introduced into the Fe2O3/Al2C?3 and CUO/T1O2 bed. With reactions proceed, GH4 and CO were detected in the outlet gases. At the beginning of oxidation reaction, the outlet product is almost pure N2, no O2 being detected. This means that all over the oxygen in the air are reacted with oxygen carrier and changed into lattice oxygen. The effect of reaction temperature on the concentration of product gases was investigated. It is revealed that the higher reaction temperature is, the faster of CO2 concentration rise up, and the bigger of the CO2 peak concentration. In the NFCT system using CH4 as fuel feedstock, carbon deposition reaction, which should be suppressed, will take place and accompany the main combustion reaction. The experimental results indicated that a desired mole fraction of H2O addition to CH4 can dramatically suppress the carbon deposition reaction. The micrographs of the surface nd cross section of the oxygen carrier particles were characterized by means of SEM. It clearly can be seen that the surface of fresh particles show a smooth texture. However, when the particles were exposed to alternating oxidizing and reducing conditions the surface changed to a more coarse texture with the development of cracks and fissures in the particle. The crosssection of particles has been changed from nonporous to porous after being exposed to alternating CH^-air conditions.A stainless steel single-reactor was designed by the authors. The reaction process of CuO/TiO2 and Fe2O3/Al2O3 oxygen carrier reacting with alternating CH4 and air in the molten bath were studied, and the gases product were also analyzed. It was found that the CO2 yield reached 79.6-89.0% when CuO/TiO2 reacts with CH4 in molten bath. N2 yield of 91.2-93.9% was obtained during air being blast into the reactor reacting with the reduced oxygen carrier. CO2 yield was around 85% in the reaction between Fe2O3/Al2C>3 and CH4 in the molten bath. The CO2 yield increased slightly with the temperature growing up. The results show that CH4 was burned almost completely in flameless in the molten bath.Possibility and assumption of engineering application about NFCT were discussed. The scenario and thought of designing an engineering reactor of NFCT were put forward. Take a 10MW NFCT boiler as an example, the main operation parameters and design data of that scale NFCT boiler were calculated. The heat balance of a NFCT unit was analyzed based on a black box model. The exergy efficiency of a CH4 fired conventional combustion-GT (gas turbine) and a NFCT-GT system were calculated and compared. Moreover, the Grassmahn Diagram of exergy flow for the two systems were made. It is observed that the exergy loss was about 20.75% in the reaction reactors in NFCT-GT system. In contrast, the exergy loss was 32.10% in the combustor in conventional combustion-GT system, which is much bigger than the corresponding value in NFCT system. It is found that the largest exergy destruction in thermal power plants occurs in the combustor in conventional system. Comparing the two systems, the total exergy efficiency of the NFCT-GT system and conventional combustion-GT system were 43.52% and 37.47% respectively. If the techniques that can be used to separate CO2 from a flue gas were added to the conventional combustion-GT system, which means a relative reduction in the overall efficiency of a power plant of 10-20%.In conclusion, the NFCT system has a bright prospect in increasing the efficiency of energy conversion process, as well as in reducing green house gas emission.