Development of calcium based sorbent for flue gas cleaning applications

Abstract

In the past few decades, carbon dioxide emissions have increased rapidly. This rise in levels of CO2 originating from anthropogenic sources of which power plants operated by various fossil fuel are main culprit causes global warming. Carbon capture and storage (CCS) is a propitious approach to alleviating the rapid increase in CO2 in the atmosphere. Still, CCS is confronting with some challenges; such as large scale application requires high energy demands, leading to high cost. With the aim of making CCS suitable for application optimization of the capture process there is need to decrease the energy requirement and cost minimization. Calcium looping is a post-combustion capture technology that is currently being developed on a pilot-scale. It has many promising advantages such as use of a relatively inexpensive and abundant sorbent, ability to re-use high-grade heat, easily scalable technology for both power generation and cement manufacturing. Calcium looping works on the principle of reversible reaction to convert Calcium Carbonate (CaCO3) into Calcium Oxide (CaO) for CO2 capture. Major disadvantage of the process is the deactivation of CaO-sorbent during carbonation and calcination cycles through sintering. Different natural sources of CaO are present such as dolomite and limestone which are readily available (in form of waste) and cheap as compared to synthetic sources of CaO. Waste derived sources of CaO such as Mussel shells, Oyster shells, Eggshells, Shellfish shells and Cuttlefish bones were also used for capturing CO2 and reducing waste simultaneously. In this research study waste derived sources of CaCO3 such as eggshell and waste marbledust were prepared with improved conversion and sintering resistance for CO2 capture. Also marbledust was used for the very first time for CO2 capture and acid and dopant based modifications were performed to improve capacity and stability over the cycles. This research study is divided into three parts i.e. acid based modification, dopant based modification and fly ash based modification. Raw material characterization was performed using SEM/EDS, XRF, XRD, TGA and BET. The initial multi cyclic carbonation/calcination cycles were performed usin Thermogravimetric analyzer TGA (Q50) for determining the carbonation/calcination conditions and also for performing CO2 adsorption studies. In first part, both the raw materials i.e., eggshell and marbledust were modified using eight different organic acids (10vol%) which includes Formic Acid, Oxalic Acid, Acetic Acid, Propionic Acid, Citric Acid Monohydrate, Dihydrate, Lactic Acid, Tartaric Acid and Malic Acid. Three best eggshell and marbledust acid modified sorbents were selected based on CaO conversion and decay rate. In case of eggshell modify sorbent Formic Acid, Acetic Acid and Citric Acid modified sorbents achieved higher conversion of 55.5, 55% and 54.39% with lower decay rates, whereas in case of marbledust modify sorbents Formic Acid, Acetic Acid and Propionic Acid achieved higher conversion of 57%, 76% and 81.5% and lower decay rate. By performing these comparative analyses between these acids modified sorbent, marbledust achieved higher conversion and lowest decay as compared to eggshell modified sorbents. In the second part dopant based treatment was performed, three different methods were used to prepare that includes Dry, Wet and Sol Gel Combustion Method with dopant Aluminium Nitrate, Magnesium Nitrate and Lanthanum Nitrate. Of all these methods, Sol Gel Combustion method with Lanthanum Nitrate as dopant achieved highest conversion and stability over 20 cycles. These results shows that marbledust doped samples have higher conversion and stability as compared to eggshell. Also wet method is a cost effective and time saving method, was selected as compared to sol gel combustion method for further modifying the sorbents with 10 and 20 weight%. It was concluded from the results that 10wt% dopant with CaO from marbledust has highest conversion. Part three of the research study consist of utilization of fly ash as a waste derived from coal power plant was added into marbledust to improve conversion and stability. Also it will reduce the cost as compared to synthetic materials used. Four different methods were utilized i.e., dry, wet (acidic and basic), sonochemical and ball milling to prepare the sorbent. Out of all the methods, ball milling method achieved the highest conversion and showed higher stability. Thus fly ash modified sorbent through ball milling was designated as an efficient sorbent material for CO2 capture. The overall conclusion of this extensive research study and experimentation is that acid modified 50vol% waste marbledust, wet method with CaO: Dopant ration of 90:10 and Ball milling with CaO: FA 95:5 with 15vol% acetic acid modified sorbent was most efficient CO2 capture adsorbent and economical available.