Research Article pubs.acs.org/journal/ascecg
Deployment of Accelerated Carbonation Using Alkaline Solid Wastes for Carbon Mineralization and Utilization Toward a Circular Economy Shu-Yuan Pan,†,‡ Kinjal J. Shah,†,‡ Yi-Hung Chen,§ Ming-Huang Wang,† and Pen-Chi Chiang*,†,‡ †
Carbon Cycle Research Center, National Taiwan University, 71 Fan-Lan Road, Da-an District, Taipei City, 10672 Taiwan (ROC) Graduate Institute of Environmental Engineering, National Taiwan University, 71 Chou-Shan Road, Da-an District, Taipei City, 10673 Taiwan (ROC) § Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3, Zhongxiao E. Rd., Taipei City, 10608 Taiwan (ROC) ‡
S Supporting Information *
ABSTRACT: This study suggests that the waste-to-resource supply chain can offer an approach to address simultaneously the issues of waste management and CO2 emissions toward a circular economy. Alkaline solid wastes can be used to mineralize CO2 through an accelerated carbonation reaction, especially if the wastes are generated near the point source of CO2, to achieve environmental and economic benefits. To enhance the performance of accelerated carbonation, a highgravity carbonation process using a rotating packed bed reactor was developed and deployed. Due to additional energy consumption in high-gravity carbonation, the environmental benefits and economic costs should be critically assessed from a life-cycle perspective. In this study, the resource potential of alkaline solid wastes in Taiwan was first determined for CO2 mineralization and utilization using the high-gravity carbonation process. Then, the performances of the process from engineering, environmental, and economic perspectives were evaluated and exemplified by a steelmaking plant. The results indicated that, with a CO2 removal ratio of 97−98%, the energy consumption of the high-gravity carbonation was estimated to be ∼345 kWh/t-CO2. From the perspective of environmental benefits, CO2 emission from the cement industry could be indirectly avoided by roughly one t-CO2-eq/t-slag due to the utilization of carbonated products. KEYWORDS: Rotating packed bed, Waste-to-resource, Resource potential, GREET model, Stabilization, Cement mortar, Life cycle assessment, Cost benefit analysis
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INTRODUCTION
of BOFS can be improved, and the contents of free-CaO and Ca(OH)2 in BOFS can be eliminated.4 Thus, this is beneficial to the subsequent utilization of carbonated BOFS as supplementary cementitious materials (SCM) in blended cement.5 However, from the reaction design point of view, improvement of the mass transfer among the gas, liquid, and solid phases is critical as molecular diffusion is considered the rate-limiting step for carbonation reaction.6,7 A rotating packed bed exhibits excellent micromixing efficiency via high centrifugal force to improve mass tranfer.4,8,9 Tiny droplets and thin liquid films can be formed under high gravity in a rotating packed bed to enhance the mass transfer rate between gas and liquid phases.10−12 Through the use of a rotating packed bed for accelerated carbonation, i.e., highgravity carbonation (HiGCarb), the carbonation conversion of
In response to global climate change and greenhouse gas (GHG) emissions, process innovation and integration for cleaner production is the key to achieving an environmentally sustainable carbon cycle.1,2 In the steelmaking industry, the CO2 emitted from the processes is generally not captured or fixed due to the regulation uncertainties, high project cost, and technology concerns such as cost-effective CO2 capture and utilization. During production, most of the alkaline solid wastes such as basic oxygen furnace slag (BOFS) are disposed at a landfill plant, while the alkaline cold-rolling mill wastewater (CRW) is normally neutralized by chemical agents at a wastewater treatment plant for discharge and/or further utilization in the process. To integrate multiwaste treatments in the steelmaking industry, accelerated carbonation is an attractive, viable, and environmentally friendly concept since the emitted CO2 can be directly used to neutralize alkaline wastewater and stabilize solid waste.3 After carbonation, the physicochemical properties © 2017 American Chemical Society
Received: January 27, 2017 Revised: June 27, 2017 Published: July 2, 2017 6429
DOI: 10.1021/acssuschemeng.7b00291 ACS Sustainable Chem. Eng. 2017, 5, 6429−6437
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. System boundary of HiGCarb process for CO2 capture, wastewater neutralization, and steelmaking slag stabilization and utilization. Materials and unit processes are symbolized in circles and squares, respectively. Green circles represent the inputs of a unit process such as feedstock, while the red circles are the outputs of a unit process such as pollutant emissions and products. Acronyms: BAU (business as usual), BOFS (basic oxygen furnace slag), CRW (cold-rolling mill wastewater).
BOFS and CO2 removal performance of the flue gas were found to be superior to conventional reactors, such as slurry and autoclave reactors.13 According to our previous study,14 the overall gas-phase mass transfer coefficient for HiGCarb was estimated to be 0.645 s−1. The average height of the transfer unit for a conventional fixed bed column was 50−100 cm,15 whereas the HiGCarb process can potentially reduce this value to 3−8 cm. Meanwhile, the process chemistry,8 reaction kinetics,9,16 and mass transfer14 of the HiGCarb process have been investigated in previous studies. On-site experiments were carried out at steelmaking17 and petrochemical18 industries to evaluate the effect of various operating parameters on the engineering performance of the HiGCarb process. Moreover, the use of the carbonated product as SCM in cement mortar was carried out, where the strength and durability of blended cement were improved in the cases of carbonated BOFS17 and carbonated fly ash.18 To determine the maximum achievable capture capacity of HiGCarb process at a single site, the establishment of a triangle model for 3E (engineering−environmental−economic) analysis was proposed.19 However, to achieve a circular economy system, several issues such as (1) resource availability and potentials of alkaline solid wastes from various industries, (2) effect of carbonated product transport and utilization on environmental impacts, and (3) cost benefit analysis still were not critically addressed. To assess the effectiveness of CO2 reduction by deployment of the HiGCarb process, the potential of CO2 fixation capacity by the HiGCarb process depending on the quantity of alkaline waste available in relevant industries should be evaluated. The objectives of the present study were (1) to assess the resource potential of alkaline solid wastes in Taiwan for establishing a waste-to-resource supply chain, (2) to evaluate the performance
of CO2 capture and utilization using the HiGCarb process exemplified by a steelmaking plant, (3) to quantify the environmental impacts and benefits of the HiGCarb process from the life-cycle perspective, and (4) to determine the costs and benefits of the HiGCarb process at an industrial-plant scale.
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EXPERIMENTAL SECTION
Geospatial Distribution of Resource Potentials in Taiwan. A geographic information system (ArcGIS 10.2, Esri Inc., USA) tool was used to map the spatial distribution of the alkaline solid wastes of Taiwan. Information on the quantities of various types of alkaline solid wastes, such as iron/steel slags, fly ash, and bottom ash, was gathered from the Environmental Protection Administration, Taiwan.20,21 The chemical compositions and maximum achievable carbonation conversion for various types of alkaline solid wastes were obtained from the literature.6,22 High-Gravity Carbonation (HiGCarb) Process. In this study, a bench-scale HiGCarb process was carried out using hot-stove gas (28−32% CO2) at a blast furnace plant in the China Steel Corporation (CSC, Kaohsiung, Taiwan) for CO2 mineralizationn and utilization. The fresh BOFS and alkaline CRW were gathered and sampled directly from the steelmaking manufacturing process. The major chemical composition of fresh BOFS was CaO (∼37.2%), where the Ca(OH)2 and free-CaO contents were 7.7% and 0.8%, respectively. Other components in the fresh BOFS included Fe2O3 (∼36.2%), SiO2 (∼10.7%), and MgO (∼8.2%). Moreover, the specific gravity of the fresh BOFS was 3.14 g/cm2, with a mean particle size of 8.7 μm. The alkaline CRW (pH in the range of 8.9−12.2) was used as a liquid phase for the accelerated carbonation because it could effectively enhance the leaching of Ca2+ ions from BOFS.23 The major compositions of CRW were Na+ (∼860 mg/L), K+ (∼90 mg/L), Cl− (∼2430 mg/L), and SO42− (∼240 mg/L) ions. The concentrations of Ca2+ and Mg2+ ions were 4.5 and 0.5 mg/L, respectively, of which their contribution to CO2 fixation could be neglected. Figure S1 (Supporting Information) shows a schematic diagram of the bench-scale HiGCarb process used in this study. The gas and 6430
DOI: 10.1021/acssuschemeng.7b00291 ACS Sustainable Chem. Eng. 2017, 5, 6429−6437
Research Article
ACS Sustainable Chemistry & Engineering
Table 1. Resource Potentials of Alkaline Solid Wastes from Key Industrial Plants in Taiwan for High-Gravity Carbonation category
alkaline solid wastes a
potentials (Mt/yr)d
CaO contents (%)e
η (%)f
2.37−2.96
39.0−41.0
70.0
647.0−849.5
32.3−36.9
potential CO2 capture amount (Kt/yr)
percentage (%)
iron slag
HBFS
steel slag
BOFSa DSSa EAFOSb EAFRSb
1.04−1.29 0.25−0.35 1.26−1.28 0.24−0.38
42.4−55.9 43.0−49.0 23.9−35.1 40.8−60.0
93.5 80.0 80.0 62.7
412.3−674.2 86.0−137.2 240.9−359.4 61.4−143.0
23.5−25.7 4.9−5.2 13.7−13.8 3.5−5.4
ash
EAF dustb APC dustb MSWI bottom ashc
0.32−0.40 0.18−0.26 1.28−1.35
24.8−29.7 36.4−53.0 16.3-21.1
80.0 80.0 90.7
63.5−95.0 52.4−110.2 189.2−258.4
3.6−3.7 3.0−4.2 10.8−9.8
total
−
−
−
−
1752.8−2627.0
100
a
Available data from China Steel Corporation from 2011−2013.38−40 Basic oxygen furnace slag (BOFS); Hydraulic blast furnace slag (HBFS;, Desulfurization slag (DSS). bAvailable data from Environmental Protection Administration (EPA), Taiwan, from 2011−2012.20 Electric arc furnace oxidizing slag (EAFOS); Electric arc furnace reducing slag (EAFRS); Electric arc furnace (EAF); Air pollution control equipment (APC). cAvailable data from EPA, Taiwan, from 2011−2013.21 dMt/yr: million tons per year. eCaO contents were taken from the literature.6 fη is carbonation efficiency reported in the literature.6,22 slurry flow rates were 0.33 m3/min and 0.33−0.56 m3/h, respectively. In this case, the capture scale of the HiGCarb process could be achieved at 75−170 kg CO2 per day. The maximal rotation speed of the packed bed is designed to be 900 rpm in order to provide a centrifugal acceleration of up to 2065 m/s2 (about 210 times greater than gravitational acceleration). The packing zone of the HiGCarb process is in horizontal rotation with an arithmetic mean diameter of 465 mm, and the axial height was 199 mm. The stainless steel wire, with a mesh size of 10 mm × 10 mm and a thickness of 0.3 mm, was packed in the packing zone. In addition, CO2 concentrations in the inlet and exhausted gas streams were measured by an NDIR CO2 analyzer (ZRJ CO2 Analyzer, Fuji, Japan), with the measurement range from 0% to 100% and a resolution of 0.1%. Life Cycle Assessment (LCA). To evaluate the environmental impacts and benefits of integrating the HiGCarb process in the steelmaking industry, the performance before and after the integration of HiGCarb process was evaluated through a cradle-to-gate approach using a life cycle assessment (LCA). The environmental impacts of the entire HiGCarb process were quantified by Umberto 5.6, using the ReCiPe methodology for midpoint and endpoint assessments. Figure 1 shows the system boundaries of the steelmaking industry with and without the HiGCarb process. In the Umberto program, materials and unit processes were symbolized in circles and squares, respectively. Then, the links between the materials and the unit processes were established with arrows. The green circles were designated as the inputs of a unit process such as feedstock, while the red circles represented the outputs of a unit process such as pollutant emissions and products. Without the integration of the HiGCarb process (yellow part in Figure 1), three separate existing waste sources were analyzed and treated: CO2 emission from stack without capture, wastewater treatment and discharge, and BOFS stabilization and/or disposal. In contrast, with the integration of the HiGCarb process (blue part in Figure 1), the CO2 emitted from the stack was directly reacted with the slurry of wastewater and BOFS. After the HiGCarb process, the reacted slurry was utilized as SCM (green part in Figure 1). The functional equivalent (unit) was one ton of fresh BOFS delivered to the HiGCarb process for carbonation. The life span of the HiGCarb facilities was assumed to be 20 years. As shown in the system boundary, the stages of raw material extraction (i.e., reactor manufacturing), capture and mineralization, transportation of feedstock and product, and carbonated product use (i.e., as SCM in CEM I/42.5 Portland cement) were included for LCA. The main unit of the HiGCarb process consists of a slag grinding component, a slurry stirring apparatus, blowers, air compressors, pumps, an accelerated carbonation reactor (i.e., rotating packed bed), and liquid−solid separators. The power consumption of the stirring, blowers, air compressors, pumps, reactors, and separators was
determined by multiplying the operating voltage by the operating amplitude of the existing equipment. The grinding power of BOFS was estimated by Bond’s Equation.24 For electricity generation, the airpollutant emission coefficients of Taiwan in 2015 were used in the LCA, i.e., 528 g CO2-eq, 315 g SOx, 307 g NOx, and 26 g PM per kWh, which were specified as CO2, SO2, NOx, and PM10 in the Umberto model. In addition, the inventory data of wastewater treatment and BOFS disposal used in the BAU scenario was gathered from the ReCiPe database, as summarized in Tables S1 and S2 (Supporting Information), respectively. On the other hand, to account for the environmental impact of the cement substitution stage (i.e., product use), the avoided burden method25,26 was applied. In addition, the GREET (Greenhouse gases, Regulated Emissions, Energy use in Transportation) model, developed at Argonne National Laboratory, was deployed with the database of average fuel consumption and loading capacity of a dump truck in Taiwan. GREET is a publicly available LCA tool for transportation systems from wells to wheels, which is separated into the well-to-pump (WTP) and pump-to-wheels (PTW) stages. The WTP stage includes fuel production (such as feedstock recovery/production, processing, transportation, and storage), transportation, distribution, and storage, while the PTW stage models vehicle operations. In this study, the PTW stage models in GREET were modified to meet Taiwan’s use of a dump truck for the purpose of quantifying the environmental impacts due to the transportation of carbonated BOFS from the steelmaking plant to the cement company. The uses of fossil, petroleum, and total energy, as well as the emissions of GHG (e.g., CO2, CH4, and N2O) and criteria pollutants were calculated in the GREET model and then compiled in a new Umberto HiGCarb module. Cost Benefit Analysis. Cost benefit analysis is a policy instrument for decision making that assesses the costs and benefits of one or several specific activities or projects. Several indicators including net present value, payback period, and benefit-cost ratio were selected for economic evaluation. The formula for net present value is shown as eq 1 n
Net present value =
∑ t=0
Bt − C t (1 + i)t
(1)
where t is the year; Bt is the total benefit of the t year; Ct is the total cost of the t year; n is the calculated duration of the project in years (i.e., 20 years in this study); i is the discount rate, suggested to be 8% for environmental facility construction projects.27 A net present value >0 means that the project will result in a positive benefit compared with current expectations (social discount rate); a net present value = 0 means that the project will just meet expectations; a net present 6431
DOI: 10.1021/acssuschemeng.7b00291 ACS Sustainable Chem. Eng. 2017, 5, 6429−6437
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. Geospatial distribution of main alkaline solid wastes by quantity in Taiwan. Acronyms: EAFS (electric arc furnace slag), MSWI-BA (municipal solid waste incinerator bottom ash), FA (fly ash), BA (bottom ash), BF (blast furnace), and BOFS (basic oxygen furnace slag). value
