Investigation On Oxygen-carrier-aided Combustion For Solid Waste Incineration And Pollutants Emission In Rotary Kiln

Solid waste（SW）refers to a type of solid or semi-solid waste generated from human activities such as production,consumption,daily life,etc.With the social and economic development,its output increases progressively,and its accumulation and toxicity have considerably impacted the ecological environment and human health.Incineration is a harmless treatment option that maximizes the reduction and elimination of SW.At the same time,energy recovery in the incineration process is cost-effective and in line with the concept of―sustainable development‖and the requirements of―resources efficient and clean production‖.Rotary kiln（RK）incineration technology has become an important way to achieve harmless,reduced,and resourceful SW due to its wide range of fuel adaptability and high treatment efficiency.However,due to the inclined-long-channel structure and rotating operation characteristic of the RK,the fuel and oxygen do not mix well,and the furnace often suffers from pressure surges and transient flue gas leaks accompanied by significant flue gas fluctuations,so as to affect the control and cleaning efficiency of the flue gas.Although the combustion and fuel conversion can be improved to some extent by adding burners,modifying the structure,and optimizing the operational parameters of RK,the RK still has the problem of unstable combustion,flue gas fluctuation as well as difficult-to-control abnormal emissions.Therefore,it is necessary to design more effective combustion and control method,to realize stable combustion and efficient pollutants control,so that RK incineration technology will be developed continuously.In this work,a method for SW combustion through oxygen carrier（OC）aided combustion in RK is proposed.OC,as an active oxygen-carrying material,was introduced into the RK to improve the mixing of fuel and oxygen,which can promote fuel conversion,achieve stable combustion and efficiently control pollutants.A systematic and in-depth mechanistic study was conducted on the impact of introducing OCs on SW combustion and the pollutants emission characteristics of SW combustion through a combination of experimental research and theoretical analysis.The main research contents and results are as follows.（1）Natural manganese and ilmenite ores were used as OCs.A low-cost method was proposed for synthesizing ilmenite-based OCs with O₂ uncoupling behaviour.The O₂uncoupling performance,the reactivity with gaseous fuels（CO and CH₄）,and the cycle stability of the OCs were evaluated using a TGA and a tube furnace experimental platform.The physical and chemical properties of the OCs were demonstrated by XRF,XRD,and SEM-EDS.The results show that the selected natural manganese ore,which is mainly composed of Mn oxides（Mn₂O₃/Mn₃O₄）,has good O₂ uncoupling behavior,and promote fuel combustion by releasing O₂ in a reducing or inert atmosphere.In ilmenite ore,the major components Fe₂Ti O₅/Fe Ti O₃ or Fe₂O₃/Fe₃O₄ oxide pairs contribute the lattice oxygen to fuel.Manganese ore exhibits weak O₂ uncoupling-oxidation and reduction-oxidation cycle stability,while ilmenite ore has good reduction-oxidation cycle stability and reactivity with CO.By mixing and then calcining ilmenite,calcined ilmenite,pre-activated ilmenite particles with manganese ore or calcined manganese ore powder in pairs,the Mn species in manganese ore can effectively migrate into the lattice of ilmenite ore particles,forming abundant(Fe_1-xMn_x)₂O₃/(Fe_1-xMn_x3)O₄crystal phases,thus endowing the ilmenite ore with significant O₂uncoupling behaviour.Moreover,the higher the temperature is,the more O₂ is released by ilmenite-based OCs,and the faster O₂ is uncoupled.（2）An experimental platform for RK combustion was designed and set up to conduct the experiments on SW combustion assisted by OCs.The effects of different OCs,oxygen concentrations,temperatures,and rotational speeds on the characteristics of SW combustion assisted by OCs were investigated.The effects of OCs on the conversion path of SW combustion were studied.The results show that due to the oxygen-carrying properties of OCs,OCs can continuously contribute oxygen to fuel in the furnace to promote the conversion of CO to CO₂,thus reducing the concentration and fluctuation of CO in the flue gas.At high temperatures and low oxygen concentrations,O₂ uncoupled by manganese ore and lattice oxygen from ilmenite ore can react with volatiles and char,respectively.This promotes the increase of carbon conversion.When OCs as solid oxygen-carrying materials are distributed in the RK,they can continuously transfer oxygen from the active layer（oxygen-rich zone）to the passive layer（oxygen-poor zone）of the bed with the rolling of the bed.This improves the uniformity of oxygen distribution and fuel combustion in the furnace.（3）The effects of different OCs,oxygen concentrations,and temperatures on the emission characteristic of pollutants（NO,SO₂,and HCl）during SW combustion assisted by OCs were investigated by using typical biomass SW（bamboo sticks）and hazardous SW（oil sludge）as fuel on the RK combustion experimental platform.The results show that the introduction of OCs can significantly improve the uniformity of oxygen distribution in the furnace,and promotes the conversion of fuel N to NO.Furthermore,at high temperatures,when OCs are distributed in the bed of the RK,the fuel in the oxygen-poor zone can obtain O₂ or lattice oxygen from the OCs,which promotes the decomposition and oxidation of S-containing compounds in fuels,and increases the SO₂concentration.In addition,OCs in bed provides more reaction sites for fuel conversion and promotes the release of Cl as HCl at high temperatures.（4）The effects of introducing OCs into the RK on the characteristics of the slag were experimentally studied.The adsorption characteristics of OCs for alkali metals K,Na,and alkali earth metal Ca were explored and the leaching characteristics of heavy metals in the slag were investigated.The testing results of the slag show that the porous structure and quantitative changes in the main elements Mn,Fe,O,and Ti on the surface of manganese and ilmenite ores particles in the slag further confirm that the OCs experienced the cycle reaction of being oxidized by air and reduced by fuel during the cycle movement from the active layer to the passive layer with the rolling of the bed in the RK.After multiple reuse cycles of manganese ore in the RK,the reactivity of manganese ore is gradually weakened due to the loss of O during the transformation from Mn₂O₃ to Mn₃O₄ in the particles and the abrasion of particles.However,ilmenite ore still has good reactivity after multiple reuse cycles in the RK due to the enrichment of elements Fe and O on the surface of particles.The OCs particles with more porous structure provide more adsorption sites for the deposition of alkali metal in the gaseous phase,and the element K deposited on the surface of OCs particles migrates into the particles,so that OCs are endowed with the adsorption capacity for alkali metal K.The introduction of OCs enhanced the fuel-oxygen mixing to facilitate the interaction between heavy metals such as As,Cr,Pb and Zn with oxygen to form stable and less volatile components more prone to retention in the bottom ash.The leaching toxicity of heavy metals from manganese and ilmenite ores in the slag is much lower than the Chinese standard limits.