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Understanding the Sulfation Pattern of CaO-based Sorbents in A Novel Process for Sequential CO2 and SO2 Capture Donglin He, yan shao, Changlei Qin, Ge Pu, Jingyu Ran, and Li Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02827 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016
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Industrial & Engineering Chemistry Research
Understanding the Sulfation Pattern of CaO-based Sorbents in A Novel Process for Sequential CO2 and SO2 Capture Donglin He, Yan Shao, Changlei Qin *, Ge Pu, Jingyu Ran, and Li Zhang
Key Laboratory of Low-grade Energy Utilization Technologies and Systems of Ministry of Education, College of Power Engineering, Chongqing University, Chongqing 400044, China
*Corresponding author: Tel.: +86-23-65103114. Email:
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ABSTRACT The CaO-based sorbent is considered to be a potential candidate for high-temperature CO2 capture. However, its application is limited by the loss in CO2 sorption activity with increasing the number of calcium looping (CaL) cycle, which leads to vast spent sorbents with a potential for pollution. To solve the problem, the optimized utilization of CaO-based sorbents through a novel process to achieve sequential CO2 capture and SO2 retention is studied in the work, and the sulfation pattern of spent CaO coming from CaL process under different variables is investigated. It is observed that spent CaO-based sorbents experiencing dozens of carbonation/calcination cycles under severe CaL conditions even have a better capacity for SO2 capture than the fresh CaO. Using a combination of testing approaches, the phenomenon is revealed to be related to the variation of the sulfation pattern of CaL-spent CaO, and pores with a diameter of approximately 60-100 nm in the material is thought to be the key factor determining the capacity of spent sorbents for SO2 retention. Based on the results, a novel sulfation model is proposed to understand the reaction behavior of the highly cycled CaO in SO2 capturing. Moreover, it is found that sulfation mode of CaL-spent CaO is sensitive to the variation of calcination in CaL process and particle size of the CaO-based sorbents.
Key words: Calcium looping; Spent CaO-based sorbents; CO2 and SO2 capture; Sulfation model.
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1. INTRODUCTION Global warming, attracting an increasing attention across the world, is believed to be mainly caused by the anthropogenic emission of greenhouse gases, especially CO2 according to the report of IPCC.1 Coal-fired power plants are the largest stationary sources for CO2 production with its concentration technically and economically suitable for CO2 capture. Therefore, the implementation of CO2 emission controlling on coal-fired power plants is of great significance in alleviating global warming and related climate change problems. In recent years, calcium looping (CaL) process characterized with the loop of a calcium-based sorbent in carbonation/calcination cycles, obtained rapid development as a promising CO2 capture technology due to its competitive cost and flexibility in the application of CO2 capturing from coal-fired power plants and hydrogen production via enhanced coal gasification.2-12 Though with significant advantages, calcium looping suffers a well-known problem of loss-in-capacity. The CO2 capture capacity of all natural materials-derived CaO-based sorbents decays with increasing carbonation/calcination cycles due to textural structure change as a result of sintering and sulfation, thus a significant amounts of spent sorbents will be produced.13-19 Besides, attrition of sorbents with increasing cycles
20-25
could also
contribute to the production of the spent sorbents. Abandonment of the large amount of CaO-based sorbents not only reduces the economic efficiency of the CaL process,26,
27
it is also potentially
contaminative to the environment, as the highly exothermic reaction with water could cause expansion in the landfill and damage to the ecosystem. Therefore, it is very necessary to give concern on the disposal of spent CaO-based sorbents from calcium looping.28, 29
Figure 1. Schematic diagram of a novel process for sequential SO2 and CO2 capture.
Although spent sorbents lose the majority of capacity in CO2 capture, they still possess the potentiality for SO2 capture, which is also a pollutant in fossil fuel combustion. To have a more comprehensive utilization of CaO-based materials, a novel system with SO2 and CO2 being captured ACS Paragon Plus Environment
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sequentially comes into being, as shown in Figure 1, and the reactions involved are given in equation 1 and 2. The flue gas produced in firing fossil fuels is firstly desulfurized in the combustor and then goes into the carbonator for CO2 capture by calcium looping. From the material flow perspective, the fresh CaO-based sorbents are firstly applied to capture CO2, and then the spent sorbents are fed for desulfurization. Thus, the purpose of high-level resources utilization could be achieved. CaO + CO ↔ CaCO ∆H298k ∓ 178 kJ/mol 1
CaO+SO2 + 2 O2 CaSO4 ∆H298k - 502 kJ/mol
(1) (2)
It is interesting to note that CaL was ever modified for the simultaneous capture of CO2 and SO2, and preliminary experiments were carried out. Iyer et al.30 studied the carbonation and sulfation of CaO simultaneously in a flue gas (10 % CO2, 3000 ppm of SO2, 4% O2 in N2) and observed that the formation of stable CaSO4 led to a significant decay in the capture capacity of CO2. Ryu et al.31 investigated the simultaneous CO2/SO2 capture characteristics in a pilot-scale fluidized-bed reactor and found that the CO2 capture capacity decreased as the increase of CaL cycle number and the SO2 concentration, while the SO2 retention capacity increased with the number of cycles and the SO2 concentration. This adverse effect of SO2 was also confirmed by Sun et al.32 when testing two commercial limestones in a thermo-gravimetric analyzer. The research of Lu and Smirniotis 33 reported a nearly 35 % carbonation conversion drop of limestone particles in the presence of SO2, despite SO2 concentration was only several orders of magnitude lower than CO2. These results clearly show that SO2 must be maximally avoided before CaO-based sorbents were used in CO2 capture. Till now, only limited work was reported on the utilization of spent CaO-based sorbents from calcium looping for SO2 capture. Sun et al.34 used limestone and dolomite as SO2-capturing sorbents in a fluidized-bed combustor following cyclic CO2 capture, and reported that the sulfation conversion of CaO after 15 CaL cycles was higher than that of CaO after either 2 cycles or 7 cycles, but lower than that of the fresh CaO. The similar effect was also confirmed by Grasa et al.35 and Manovic et al.17. Li et al.36, 37 investigated the sulfation behavior of CaO after long-term CaL cycles, but attentions were
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mainly paid to the various influence factors in SO2-capture process. Although there is a little inconsistence with the conclusions obtained by Manovic et al.17, a slight increase of spent CaO in capturing SO2 was still demonstrated at cycle 40. The similar sulfation trend of CaO with the increasing CO2-capturing cycles was also observed by Chen et al.38. After reviewing the literature, it is clear that sulfation behavior of spent CaO-based sorbents from calcium looping is still in controversy. Although some concerns have gone to the impact of CaL cycle number and sulfation conditions on sulfation behavior, the influence of CaL process, which determines the property of spent sorbents and should be firstly ascertained, is still lack of a good understanding. To get to the target, in this work, we firstly investigated the mutual effect of raw material type, particle size, cycle number and calcination temperature in CaL on the following sulfation behavior. Then, the sulfation pattern of CaO from the novel process was explained.
2. EXPERIMENTAL 2.1. Sample Preparation and Analysis Two limestones (denoted as LA and LB from Shandong and Hunan Province, respectively) and one dolomite (denoted as DA, from Hebei Province) were chosen as raw materials. They were firstly smashed and milled, and then sieved by filter screens to acquire 4 particle size fractions of
