The Mechanism Study Of A New Near Zero Emissions Coal Utilization System With Combined Gasification And Combustion

The first-phase mechanics research of a new zero emissions coal utilization systemwith combined gasification and combustion presented by Zhejiang University wasmainly carried out in this paper. Because the hydrogen production unit with combinedgasification and combustion of this system is the basis of mass and energy of thewhole system, and it is the core of achieving hydrogen production, electric powergeneration and zero emissions of this new zero emissions system, the hydrogenproduction unit is the main research object of this paper. It is also the priory and hotresearch of the other zero emissions systems based on CO₂ acceptor gasificationtechnology.To provide the basis for setting the subsequent experimental conditions and providea preliminary theoretical verification for the feasibility of this combined hydrogenproduction system, thermodynamic equilibrium calculation software FactSage 5.2 isused to predict the influences of operation pressure, temperature, the mole ratio of[H₂O]/[C] and [Ca]/[C] in the gasifier on the hydrogen production. With acomprehensive considering of the quantity and concentration of hydrogen, theoptimum operation parameters in the gasifier are obtained. To meet the demand ofCaCO₃ calcination in the combustor, two ways of combustion: pure oxygencombustion and air combustion are calculated and the minimum calcinationtemperatures under different pressures are obtained.In the research of the same kind zero emission systems, the studies on thegeneration and transportation rule of the pollutants elements in coal are infrequent,which is very important for the achievement of the pollutants zero emission. For thecurrent state of lack of reaction dynamic parameters of relevant pollutant elements,the studies on the aspect of thermodynamic equilibrium show to be very important.Therefore, FactSage 5.2 software was adopted to calculate the transportation andconversion rule of sulfur, nitrogen, fluorine, chlorine, mercury, arsenic, selenium et almajor, minor, trace quantity elements in coal. The results will provide theoreticalguidance for the repression of the pollutants generation of the zero emission coalutilization system and for the choice of appropriate coal gas purification technology.For the experimental studies of various reaction processes, because of involvingmany different types of reaction processes, the thermobalance utilized to do themechanical research is the best choice definitely. Thus, the TherMax 500 pressurethemobalance is chosen to do experimental studies. Meanwhile, the scanning electronmicroscope (SEM) and other analytical means are also adopted.Pure water steam gasification of coal coke under pressure is the main reaction ofanaerobic hydrogen production. Isothermal method is used to achieve the influencesof gasification temperature and pressure on the coal coke pressure gasification. Theresults show that the effect of temperature on coal coke gasification reaction activityis more obvious than pressure. Increasing temperature is better than increasingpressure for coal coke gasification reaction. Based on experiment results, hybrid model function is used to achieve the dynamic parameters of coal coke water steamgasification under pressure.For the characteristics of rich CO₂ in the combustor, the study of combustioncharacteristics under pressure of coal coke under O₂/CO₂ atmosphere was carried out.With the ignition temperature and the combined combustibility index S as judgingparameters, the effects of total pressure, oxygen concentration, particle diameter andthe rate of heating-up on the char combustion were studied systematically. Thecombustion characteristics of three kinds of typical bituminite chars were compared.On the above basis, the dynamic parameters of char combustion under variouspressure and O₂/CO₂ atmosphere are obtained.The carbonation performance of CaO in the gasifier has a direct influence on thecoal anaerobic hydrogen production. Isothermal method is used to achieve theinfluences of reaction temperature, CO₂ partial pressure and particle diameter on thecarbonation process of CaO. The experimental results show clearly that the CaOcarbonation process has the features of two steps which are fast step and slow step.The fast step (chemical reaction controlling stage) is the key step determing the CaOrate of conversion. The dynamic parameters of CaO carbonation reaction underpressure are obtained using the mechanical function of Avram-Erofeev when n equals2.The CaCO₃ calcination in the combustor is very important for the activity ofregenerated CaO and achieving high concentration of CO₂. Temperature-programmedmethod is used to get the influences of total pressure, CO₂ partial pressure and the rateof heating-up on the CaCO₃ calcination. The results show that the diffusion of CO₂generated by CaCO₃ calcination from interior to exterior is the main velocitycontrolling factor under both pressure and rich CO₂ atmosphere. For the solution ofdynamic parameters, the quality of several common mechanical functions arecompared and analyzed. The analysis shows that the liner correlation of the singlestep random nucleation mechanical function is best, and this function is appropriatefor the solution of the dynamics of CaCO₃ calcination in this paper.For CaO in the system is utilized repeatedly, studying the characteristics ofCaO-CaCO₃ multi-cycle process is very important for maintaining CaO activity. Forthe characteristics of operation in this system, the influences of common pressureregeneration-pressurized carbonation, pressurized regeneration-pressurizedcarbonation and pressurized carbonation- humidification activation-pressurizedcarbonation on CaO activity were sdudied in this paper. SEM reflects theconfiguration of surface of regenerated CaO under different cycle index, disclosingthe innor mechanics of CaO activity changing. The activity and endurance of CaOcarbonation can be increased effectively under water steam atmosphere in the gasifier.In order to consider the interrelationship of various reactions comprehensively, thedynamic simulation model of the combined coal gasification and combustion systemis established in this papar. The sub-processes such as coal pyrolysis, gasification,CaO carbonation, char combustion and CaCO₃ calcination are consideredsystematically in the model. This model can be used to study the influences ofreaction conditions, the proportion of the reactants on the gradients of the reaction products and the reaction rate. Utiliziing the mathematical model of the coalcombined gasification and combustion hydrogen production, the operationcharacteristics of the system under different operation conditions are simulated andanalyzed.A set of pressurized double circulating fluidized bed experimental system with themaximum operation pressure of 2.5MPa was designed and constructed. The systemdebugging and cold test were done.With the studies of the above segments, the reaction characteristics and the innermechanics of the coal anaerobic gasification hydrogen production processes will begrasped profoundly. The theoretical and experimental preparations have been madefor the further research and the early achievement of the whole zero emission system.