Investigation Into Application Of Calcium Based Oxygen Carrier In Chemical Looping Combustion System

Nowadays it is generally accepted that the increasing greenhouse gas emissions, the typical one being carbon dioxide, is one of the serious problems we must face. To solve the problem, chemical-looping combustion (CLC) technology has been proposed as a new technology which would satisfy the capture of CO₂ with few energy losses. It has the inherited characteristics of CO₂ separation. High pure CO₂ can be obtained and captured without any additional CO₂ separation units. In the CLC system, the fuel does not react with the air directly. Both the oxygen from the air to the fuel and the heat from the air reactor to the fuel reactor are transferred by the oxygen carrier to realize the flameless combustion between the fuel and air. Therefore, the formation of NOx gases in the system is efficiently inhibited. In addition, the CLC systems coupled with the gas turbine or some other integrated power systems would be more potentially efficient than the systems with conventional combustion technology.The choice of oxygen carrier is a key for the performance of the CLC system. The current oxygen carriers are metal oxides, such as the oxides of nickel, iron, copper and cobalt, are characterized by high reactivity and good regeneration stability. However, the costs of these metal-oxide oxygen carriers are high but their oxygen carrying ability are low per unit mass. In addition, some leakage of the metal oxide particles becomes second pollution sources to the environment. Therefore, it is necessary to find a new kind of oxygen carrier. Recently, calcium sulfate(CaSO₄) is considered as a novel oxygen carrier because it has some obvious advatanges. Firstly, CaSO₄ is cheaper due to vast gypsum resources all over the world. Secondly, compared with the metal oxides, CaSO₄ has a relatively high oxygen carrying capacity. Thirdly, as a nonmetal sulfate, it is much more friendly to the environment. Therefore, it is very suitable for the fluidized bed reactor of chemical-looping combustion system.Based on the minimization of the total Gibbs free energy for all species, the effects of many factors on the conversion of CaSO₄ to CaS, the amount of the released sulfurous gases and the deposited solid carbonaceous products are discussed. It is indicated from the simulated results that higher reacting temperature inhibits the deposition of carbon but promotes the conversion of H2S to SO2. However, the operating pressure contradicts the temperature for the effects on the amounts of the deposited carbon. The occurrence of deposited carbon is promoted at higher operating pressure. In addition, the higher operating pressure results in the more amounts of the released sulfurous gases. Therefore, the high operating pressure in the fuel reactor should be avoided when possible. Moreover, the effects of the oxygen excess ratio on the carbon deposition and sulfur release are not neglected. When the oxygen excess ratio increases from 0.2 to 0.8, the released amount of the sulfurous gases greatly grows. However, when the oxygen excess ratio rises from 0.9 to 1.4, the effects of reducing oxygen excess ratio on the released amount of the sulfurous gases is quite small, especially when the reacting temperature is less than 700℃. In the air reactor, the higher reacting temperature reduces the conversion of CaS to CaSO₄ especially when the oxygen excess ratio is smaller than 0.75. The oxygen in the reactor should remain a little excessive because the bigger oxygen excess ratio is helpful to the conversion of CaS to CaSO₄.Two kinds of compound oxygen carrier samples have been prepared by the incipient wet impregnation method with the saturated solution of nickel nitrate and iron nitrate respectively as the active-phase precursor on the surfaces of calcium sulfate particles. The reactivity of oxygen carrier with both the gaseous and solid fuels is much better than before impregnation. The higher impregnation amount improves obviously the reactivity of CaSO₄ oxygen carrier with gaseous and solid fuels. The reactivity of compound oxygen carrier with sludge char or corn straw char is much better than with coal char. This indicates that sludge char and corn straw char are more suitable in CLC system using solid fuels. In addition, it is found that after the pre-treatment of CaSO₄ oxygen carrier with strong acids, the addition of CaCO3 nanoparticles greatly improved the recycle ability of calcium based oxygen carrier.The thermal decomposition behavior of calcium based oxygen carrier in nitrogen atmosphere is also studied. CaSO₄ particles begin to decompose slowly when the reacting temperature achieves 1250℃. The decomposition rate obviously accelerates once the reacting temperature increases to 1300℃. In the air reactor, the decomposition of CaSO₄ can be avoided at the temperature lower than 1300℃. In the decomposition, the growth activated energy of produced CaO is lower than the nucleation activated energy of CaO. Therefore, once the nucleation of CaO occurs in the surface of the oxygen carrier, it can grow rapidly. The activated energy reduces monotonically with the increasing of conversion of CaSO₄. Through the double-extrapolated method and Popescu method, the activated energy of the decomposition of CaSO₄ without any disturbance of side reactions is calculated to be 992.15 kJ/mol. The most possible decomposition mechanism of CaSO₄ is nucleation and nuclei growth mechanism. The most likely mechanism function is characterized by [ ? ln(1 ? X)]2.It is observed that the partial pressure of the reductive gases has a significant effect on the amount of the released sulfurous gases. The high partial pressure of the reductive gases is beneficial to reduce the released amount of sulfurous gases. When the reacting temperature increase to 1000℃or higher, the release of sulfurous gases can be inhibited fully if the partial pressure of CO or H2 maintains above 50 kPa. In particular, the conversion of CaSO₄ to CaS can reach 100 %.The flow rate of circulating CaSO₄ oxygen carrier is important to the system heat efficiency, the concentration of CO₂ and H2O in the gases emitted from the fuel reactor and the heat integration of the system. The great flow rate of circulating CaSO₄ oxygen carrier increases the concentration of CO₂ and H2O in the gases from the fuel reactor. Similarly, the heat efficiency of the system is improved monotonically with the increasing of the flow rate of circulating CaSO₄. However, the increasing rate of the heat efficiency slows down when the flow rate of circulating CaSO₄ increases. When the heat efficiency reaches 40 %, its increasing range is quite slight.