Studies on CO2 Capture Using Adsorption from Simulated Refinery Flue Gas

Abstract

Carbon dioxide (CO2) capture is a major concern around the world because of its detrimental impacts on the Earth. As a mitigation measure, solid-based adsorbents have been extensively investigated to capture CO2 from dilute flue gas streams. It has been observed, in general, that CO2 adsorption-regeneration studies have been carried out on different types of adsorbents based on alkali metal carbonates as active precursor. So far, a great amount of work has been progressed with K2CO3-based adsorbents, still there is huge scope to improve the adsorption-regeneration characteristics of CO2 adsorbents; particularly to increase the capture capacity, effective dispersion of active phase (like K2CO3) inside the porous structure and stability after multiple tests. Moreover, activity of these K-based adsorbents in continuous CO2 removal is merely reported so far. The development of low cost, energy efficient CO2 capture process is also needed to minimize CO2 emissions into the atmosphere. The present work intends to bridge the gaps related with the improvement in both adsorbent and process development. CO2 adsorption studies were performed using a series of adsorbents prepared by incipient wet impregnation method. Several adsorbents using alumina-clay as support and sodium carbonate (Na2CO3) as active component were prepared and characterized for textural properties with the objective to correlate with CO2 adsorption and desorption. The effects of adsorption temperature, Na2CO3 loading on the support material and feed CO2 concentration were evaluated using simulated flue gas with 3 – 9 vol% CO2, 2.5 vol% H2O and balance N2 at 55 °C in a fixed bed reactor. 20-wt% Na2CO3 based adsorbent showed maximum CO2 adsorption capacity of 0.39 mmol/g of adsorbent at flue gas temperature of 55 °C and CO2 content of about 8 v/v %. At increased adsorption temperature, CO2 adsorption capacity of this adsorbent decreases. The best fitted parameters, Vm of 0.875 mmol/g and k of 0.148 bar-1 using the Langmuir equation are estimated. The estimated activation energy of this adsorbent system is 42 kJ/mol. The lower CO2 adsorption capacity with these adsorbent systems is due to the effect of water content in flue gas streams. That is why the role of water content in feed simulated flue gas needs to be examined. Similarly, textural properties of the support material play an important role to achieve higher CO2 adsorption capacity. Homogeneous dispersion of active phase needs to be assured to produce available active sites for CO2 adsorption. This work particularly focuses on the effect of adsorbent preparation by single- and multi-step impregnation of K2CO3 on alumina support and their adsorption/regeneration performance in a fixed bed reactor system. It also highlights the role of physico-chemical properties of adsorbents prepared by both methods on adsorption and regeneration characteristics. The multi-step impregnation (MI) method enables uniform dispersion of active species (K2CO3) in the broad macro-pores without blocking narrower meso-pores. This facilitates higher loading of accessible K2CO3 for CO2 adsorption and hence, higher adsorption capacity. The single-step impregnation (SI) method suffers from blockage of narrower meso-pores by excessive growth of K2CO3. This limits the CO2 accessibility towards active species in the porous structure due to formation of larger active species aggregates. For 50-wt% K2CO3/Al2O3 prepared by MI and SI method, the maximum CO2 adsorption capacity at 8 vol% CO2 is found to be 3.12 and 2.1 mmol/g respectively. The regeneration efficiency of 50MI and 50SI are observed to be nearly 65% and 56% respectively, at 130 oC in multi-cycle testing. From the results, it is concluded that adsorbent prepared by MI method shows better performance due to its tunable textural and morphological properties to achieve higher CO2 adsorption capacity. Another objective of this study is to develop new K2CO3-based adsorbents having improved regeneration properties with stable adsorption capacity during multiple cycle studies. The acidity of alumina support is presumed to be responsible for preferential formation of KAl(CO3)2(OH)2, which in turn requires high temperature for regeneration (>300 oC) as reported in literatures. Keeping this in view, the alumina support has been modified by (i) applying heat treatment, (ii) treatment with alkali hydroxide followed by calcination, etc. so as to reduce the surface hydroxyl concentration/ acid sites. The modified adsorbents were evaluated in a fixed bed reactor system over a temperature range of 55-75 oC with 8 vol% of CO2 in a simulated flue gas mixture. Various physico-chemical properties were studied to explain the adsorption as well as regeneration of such adsorbents. The effects of operating parameters such as adsorption temperature, thermal dehydration of support material, gas-hourly space velocity (GHSV) during regeneration have also been studied. The CO2 adsorption capacities are found to be in the range of 2.3 - 2.6 mmol CO2/g of adsorbent, which also shows good stability after multi-cycle tests. There is a significant reduction of regeneration temperature (from 350 oC to 130 oC) of these K2CO3/Al2O3 adsorbents. The developed adsorbents also show high attrition resistance and thus can be effectively used in commercial application for CO2 capture. This study also focused on carbonation-regeneration characteristics of 35-wt% K2CO3/ Al2O3 adsorbents in both non-circulating fluidization and continuous circulation between two inter-connected fluidized-bed reactor systems. The key interest of this study is to highlight the effect of process parameters such as adsorption temperature, water content in inlet flue gas, GHSV and nature of sweep gas used for desorption. The work attempts to bring out the important results of pilot scale trials using an existing FCC pilot unit. In particular, it presents results which are critical for transitioning from lab scale to the most efficient industrial scale in the development of low cost CO2 capture process with lower regeneration temperature and multi-cycle stability. The adsorption capacity was found to be 1.42 mol CO2/kg of adsorbent in non-circulating fluid bed at Pilot scale, which is 71% of the adsorption capacity achieved in fixed-bed adsorption at Lab scale. Multiple adsorption-regeneration cycles were done in two separate fluidized beds, with continuous circulation of the adsorbent between the two beds, in order to investigate the effect of adsorption and regeneration temperature, water vapor content in simulated inlet flue gas stream, gas-hourly space velocity (GHSV) and mode of adsorbent regeneration. More than 80% CO2 removal was achieved from a simulated flue gas stream having 8.3 vol% CO2, 15.8 vol% H2O and rest N2. The adsorbent showed excellent structural stability after 144 hours of continuous operation. It was found that the performance of the CO2 removal was very sensitive to the water vapour content in the inlet simulated flue gas and the gas velocity in the adsorber bed (gas-solid contact time in the adsorber). The performance evaluation results from the CO2 capturing adsorbent in fluidized bed system at pilot scale were very promising and encouraging to scale up the trials further to demonstration level. There is an increased interest in developing less expensive and/or energy integrated processes for capturing CO2 as the present capture cost using conventional amine absorption process is high (nearly 100 $/Ton CO2). The adsorption processes, generally employing solid adsorptive material that fall under the post combustion category, serve as an alternative to the absorption based process. This is because replacing water by solid support greatly reduces the energy required for CO2 capture due to the lower heat capacity of solid supports as compared to water. The proposed energy-integrated process for CO2 capture from flue gas stream is very cost-effective, energy-intensive process. The estimated cost per ton of CO2 captured is found to be one third of the cost associated with conventional amine absorption process. Significant improvements on utility and power requirement helps to make heat integrated based CO2 capture more economically competitive. An important feature of the study is analysis of key performance parameters that influence the cost economics. Understanding the nature of these impact, and the potential for reducing them, is crucial to projecting future costs and capabilities of new technologies for carbon capture.