The Investigations On Chemical Looping Combustion Of Solid Fuel Based On Fuel Staging Oxidation Principle

The global warming causes severe natural disasters.One of the important reasons is the greenhouse effect,which is aggravated by increasing amounts of CO₂ emission from the fossil fuel burning.Chemical-looping combustion?CLC?is a promising technology with inherent CO₂ separation and lower carbon capture cost.And this technology has received increasing attention under the growing environmental pressure.The process of CLC takes place in the reactor,which is mainly composed of air reactor?AR?and fuel reactor?FR?.Therefore,the design and optimization of the reactor are key to ensuring combustion efficiency of CLC.At present,most of the fuel reactors use single fluidized bed,and there still exist some limitations in improving fuel conversion efficiency and carbon capture efficiency.In this study,a new kind of interconnected fluidized bed reactor was built based on the principle of fuel staging oxidation.The cold and hot state experiments were carried out to study the design's effect on CLC of solid fuel.The gas-solid flow characteristics of the reaction system were studied on the cold-flow model by changing volumetric flow rate at the inlet of each reactor.It is found that the superficial gas velocity of air reactor?AR?and fuel reactor?FR?have a greater influence on pressure distribution and solid circulation rate of the system,and the superficial gas velocity at the inlet of syphon?SP?is a decisive factor to particle circulation rate.A majority of gases of SP flow into FR,and the improvement of flow rate at the inlet of AR and SP will lead to reduction of the gas distribution proportion from SP to FR.Almost the whole gases of air loop seal?ALS?flow into FR due to the higher pressure of riser over ALS.There are very few gases flowing into AR from FR,indicating that SP and ALS can play a good barrier function to prevent gas leakage between AR and FR.On the basic of cold state experiments,the hot interconnected fluidized bed reactor was built.And hot state experiments were performed with hematite as oxygen carrier and sewage sludge as fuel on this CLC unit.The total continuous operation time was more than 8h under stable condition.The effects of fuel reactor temperature?800⁹00??and fuel feed rate?300⁶00g/h?on chemical looping combustion of sewage sludge were investigated.The results show that FR-I has a larger oxygen carrier inventory and the pressure fluctuation in the downcomer is extremely stable during the steady operation period.The design of two-stage fuel reactor is beneficial to improve the carbon conversion efficiency of the whole system.There is an increase in the carbon conversion efficiency and carbon capture efficiency with the rise of fuel reactor temperature.And the oxygen demand is always less than 10%.Higher sewage sludge feed rate results in the decrease of carbon conversion efficiency and lower fraction of CO₂.XRD analysis of oxygen carrier extracted from the both fuel reactors shows that oxygen carrier in FR-II had more Fe₂O₃ compared with that in FR-I,thus leading to higher reactivity.Based on the principle of fuel staging oxidation,the CLC characteristics of coal were studied with hematite as oxygen carrier.The effects of FR temperature,water-fuel ratio and fuel feed rate on CLC of fuel were investigated.The results show that the carbon conversion and carbonaceous gas conversion efficiency are gradually improved with the increase of FR temperature.And the values for two parameters of FR-II are higher than those for FR-I at the same temperature.The carbon capture efficiency is less affected by FR temperature and is always higher than 90%.The increase of steam flow has a positive effect on the coal conversion.The optimum value of S/C is 0.45 for the lignite coal used in the experiment.The concentrations of CO increase with fuel feed rate for FR-I and FR-II,and the value in FR-I is higher than that for FR-II.After periodic operation,the iron oxide of oxygen carrier in FR-I is reduced to Fe₃O₄and FeO according to the X-Ray Diffraction and Energy Dispersive X-ray Detector analysis results.In addition,the reacted oxygen carrier possess similar reactivity to the fresh oxygen carrier.