Article pubs.acs.org/est
Ex Situ CO2 Capture by Carbonation of Steelmaking Slag Coupled with Metalworking Wastewater in a Rotating Packed Bed Shu-Yuan Pan,† Pen-Chi Chiang,† Yi-Hung Chen,‡ Chung-Sung Tan,§ and E-E Chang∥,* †
Graduate Institute of Environmental Engineering, National Taiwan University, Taiwan Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taiwan § Department of Chemical Engineering, National Tsing Hua University, Taiwan ∥ Department of Biochemistry, Taipei Medical University, 250 Wu-Hsing Street, Taipei City, Taiwan ‡
S Supporting Information *
ABSTRACT: Both basic oxygen furnace (BOF) slag and cold-rolling wastewater (CRW) exhibiting highly alkaline characteristics require stabilization and neutralization prior to utilization and/or final disposal. Using CO2 from flue gases as the stabilizing and neutralizing agents could also diminish CO2 emissions. In this investigation, ex situ hot stove gas containing 30 vol% CO2 in the steelmaking process was captured by accelerated carbonation of BOF slag coupled with CRW in a rotating packed bed (RPB). The developed RPB process exhibits superior results, with significant CO2 removal efficiency (η) of 96−99% in flue gas achieved within a short reaction time of 1 min at 25 °C and 1 atm. Calcite (CaCO3) was identified as the main product according to XRD and SEM-XEDS observations. In addition, the elimination of lime and Ca(OH)2 in the BOF slag during carbonation is beneficial to its further use as construction material. Consequently, the developed RPB process could capture the CO2 from the flue gas, neutralize the CRW, and demonstrate the utilization potential for BOF slag. It was also concluded that carbonation of BOF slag coupled with CRW in an RPB is a viable method for CO2 capture due to its higher mass transfer rate and CO2 removal efficiency in a short reaction time.
1. INTRODUCTION Accelerated carbonation is a feasible and safe method for carbon capture, utilization, and storage (CCUS) because CO2 reacts with metal-oxide bearing materials to form stable and insoluble carbonates that are environmentally benign. 1 Industrial residues such as steelmaking slags,2−8 combustion residues,9 fly ashes,10−12 and cement kiln dust13 are alkaline and appear to be potential raw materials for CO2 capture by accelerated carbonation due to the fact that these materials are generally rich in calcium and magnesium oxide, which are the most favorable metal oxides for reacting with CO2. Generally, the chemical formula of accelerated carbonation for steelmaking slag can be expressed as shown in eq 1:1−4
via carbonation of steelmaking slags could be attractive, as no mining or transportation is needed, and the consumption of raw materials is avoidable. In addition, the content of free-CaO in fresh BOF slag is typically 3−6 wt %, which is the main constraint for further utilization in concrete and asphalt aggregate, road base, and fill materials because it tends to have high water absorption and expansion properties. However, improvement in the chemical and physical characteristics of treated residues by accelerated carbonation can facilitate the reuse of steelmaking slag in a variety of applications such as construction material.14 Moreover, the metalworking wastewater, for example, coldrolling wastewater (CRW), is also highly alkaline (e.g., pH > 11) and produced in large amounts in the course of steel manufacturing, which is desirable for carbonation reaction. Both BOF slag and CRW exhibit high alkalinity, which requires stabilization and neutralization to ensure safe landfill and effluent. Using CO2 from flue gases as a stabilizing and neutralizing agent could also diminish CO2 emissions.
(CaO, MgO)x (SiO2 )y (s) + xCO2 (g) → x(Ca, Mg)CO3(s) + ySiO2 (s)
(1)
Taiwan has a large steel industry, for instance, with annual production of basic oxygen furnace (BOF) slag averaging 1.1− 1.3 million tons in the China Steel Corporation (Kaohsiung, Taiwan). Reuse of the steelmaking slag as the source of calcium or magnesium oxide is beneficial due to its availability and low cost as well as the fact that it is often produced near large CO2 emission sources. Therefore, CO2 capture from the steel plant © 2013 American Chemical Society
Received: Revised: Accepted: Published: 3308
December 5, 2012 February 27, 2013 March 4, 2013 March 4, 2013 dx.doi.org/10.1021/es304975y | Environ. Sci. Technol. 2013, 47, 3308−3315
Environmental Science & Technology
Article
CO2 concentrations in the inlet and exhausted gas streams were measured using an NDIR CO2 analyzer (Fuji, ZRJ CO2 analyzer), with the measurement range from 0% to 100% and a resolution of 0.1%. The CO2 removal efficiency (η) for the developed RPB process was calculated as follows:
The use of a rotating packed bed (RPB) reactor can lead to the formation of thin liquid films and tiny liquid droplets via high centrifugal acceleration, thus enhancing the micromixing abilities and gas−liquid mass transfer.15−18 A previous study reported that accelerated carbonation of BOF slag in an RPB is a viable method for CO2 capture because its mass transfer rate and carbonation conversion are higher than those in traditional reactors such as slurry reactor and autoclave reactor.7,17,19 However, the experimental setup designed in the previous study was not practical for industrial processes; therefore, a continuous operation for carbonation of BOF slag in an RPB should be researched and developed for implementation in an industrial situation. In this investigation, alkaline wastes produced from steelmaking processes, that is, BOF slag and CRW, were selected as the solid adsorbent and liquid agent for CO2 capture, respectively. The ex situ CO2 capture from a hot stove gas in the steelmaking process of Company A in Taiwan was accomplished by carbonation of BOF slag coupled with CRW in an RPB. CO2 mass balance between the reduction amount in flue gas analyzed by CO2 concentration analyzer and the increasing amount of CO2 adsorbed on the BOF slag measured by TGA was studied. In addition, the potential to use the reacted BOF slags in construction materials was assessed via analysis of the changes of free-CaO and calcium hydroxide (Ca(OH)2) contents in the BOF slag before and after carbonation.
η=
(ρCO ,i Q g,iCg,i − ρCO ,o Q g,oCg,o) 2
2
ρCO ,i Q g,iCg, i 2
×% (2)
where ρco2,i and ρco2,o (g/L) are the CO2 mass density at the temperature of inflow and exhaust gas streams, respectively; Qg,i (L/min) and Qg,o (L/min) were the volumetric flow rate of the inlet gas and exhaust gas, respectively, and Cg,i (%) and Cg,o (%) were the CO2 concentration in the inlet gas and exhaust gas, respectively. Reacted slurry samples of 5 mL were taken from the slurrystorage tank and filtered immediately through PTFE membrane filters (Millipore, 45 μm pore size and 47 mm diameter) to separate suspended solids from the solution. The filter cake, that is, carbonated BOF slag, was dried at 105 °C in an oven for 3 h. The solid products were determined quantitatively by thermogravimetric analysis (TGA) and qualitatively by X-ray diffraction (XRD) and scanning electron microscopy with X-ray energy dispersive spectrometry (SEM-XEDS). The concentration of major cations in the liquid phase was determined by ICP-AES (Perkin AA800). The concentration of Cl− and SO42− in the liquid phase was measured by precipitation titrimetry (i.e., Mohr method) and the Nephelometer method, respectively. In addition, the concentration of the total inorganic carbon (TIC) was analyzed by Aurora 1030W TIC Analyzer (O.I. Analytical). 2.3. CO2 Mass Balance. The systematic diagram among gas, liquid, and solid phases is illustrated in SI Figure S2, and the CO2 mass balance equation was developed as shown in eq 3:
2. MATERIALS AND METHODS 2.1. Materials. The ex situ CO2 capture from hot-stove gas by carbonation of BOF slag in cooperation with CRW was carried out in an RPB. The ground BOF slag, CRW, and hotstove gas were supplied by Company A in Taiwan. The ground BOF slag was sieved to yield powders with a size of less than 62 μm and dried in an oven at 105 °C overnight to eliminate moisture. The dried BOF slags were stored at room temperature in a capped container. On the other hand, the hot-stove gas in Company A typically contains CO2 (∼30 vol %), N2 (∼68 vol%), O2 (∼1.5 vol%), SOx (
