Synthesis Of Highly-efficient,Ca-based CO₂ Sorbents From Steel Slag And Application For Carbon Capture In The Iron And Steel Industry

As a pillar industry for the development of the national economy,iron and steel industry faces urgent demands of both steel-slag utilization and CO₂ abatement.Aimed at the value-added utilization of steel slag,this thesis systemically investigated the effect,influencing factors,and kinetics of CO₂ sequestration by direct gas-solid carbonation of steel slag,studied in detail the ways to separate calcium?Ca?and iron?Fe?from steel slag and their recovery via acid extraction.Importantly,we successfully developed highly efficient,CaO-based CO₂ sorbents using steel slag as a feedstock,proposed and experimentally demonstrated the feasibility of a novel auto-thermal CO₂ capture process by coupling calcium looping?CaL?and chemical looping combustion?CLC?cycles within the steel slag-derived,Ca-Fe bi-functional CO₂ sorbents.Thus,the simultaneous CO₂ abatement from iron and steel industry and element recovery from steel slag is realized,which provides a new way for value-added utilization of steel slag.The main conclusions drawn from this thesis are as follows:Due to a lack of knowledge on carbonation theory of steel slag and its reaction kinetics from current studies,we for the first time experimentally determined the theoretical CO₂ sequestration capacity of steel slag by using the X-ray diffraction coupled relative intensity ratio?XRD-RIR?method.The steel slag sample used in this thesis has a theoretical CO₂ sequestration capacity of 159.4 kg CO₂ t-1.Additionally,we adopted the first-order reaction kinetic equation and the Ginstling equation to effectively model the direct gas-solid carbonation kinetics of steel slag and determine key reaction parameters?eg.the activation energy and rate constant?,which helps develop the direct gas-solid carbonation theory for CO₂ sequestration of industrial wastes.Due to the restriction of a low content of Ca-containing phases available for carbonation to sequestrate CO₂ in the steel slag,we developed an acid extraction method with low dosages of acetic acid to realize an efficient separation of Ca and Fe from steel slag,which are recovered in the form of high-purity lime and high-quality iron ore,respectively.In this thesis,approximately 270 kg of lime with a Ca O purity as high as 90% and the Fe-rich minerals with an iron content of as high as 70.6% could be recovered from oneton of steel slag under the optimal operating conditions.We prepared Ca O-based CO₂ sorbents from Ca-rich leachate of steel slag using an acetate co-precipitation technique,which exhibited superior CO₂ capture characteristics over the commercial Ca O.The highest CO₂ uptake of the steel slag-derived CO₂ sorbents could achieve 0.62 g CO₂ gsorbent-1,1.5 times as much as that of the commercial Ca O.The cyclic CO₂ uptake of the steel slag-derived CO₂ sorbents could be almost twice as much as that of the commercial Ca O under realistic calcium looping conditions.Importantly,the cyclic stability for CO₂ capture of the steel slag-derived CO₂ sorbents was significantly improved when compared to the commercial Ca O.The economic superiority of the steel slag-derived CO₂ sorbents over naturally derived Ca O was proved based on a mass flow analysis.The steel slag-derived,Ca O-based CO₂ sorbent developed in this thesis is technically and economically very promising for practical applications in the Ca L-based CO₂ capture process.We originally proposed a new class of auto-thermal,combined Ca L-CLC CO₂ capture process,where the exothermic oxidation step of a reduced metal oxide is used to provide the heat required to calcine Ca CO3.To demonstrate the practical feasibility of the process proposed,novel Ca-Fe bi-functional CO₂ sorbents were developed using steel slag as a feedstock.Using the steel slag-derived,Ca-Fe bi-functional CO₂ sorbents with a proper molar ratio of Ca to Fe,the combined Ca L-CLC process can be operated auto-thermally for CO₂ capture.Additionally,we illuminated the mechanism of heat integration between Fe-based CLC and Ca-based Ca L cycles,and experimentally demonstrated the technical superiority of the combined Ca L-CLC process proposed,which helps develop the high-temperature solid looping CO₂ capture theory.