ARTICLE pubs.acs.org/IECR
Carbon Capture and Mineralization in Singapore: Preliminary Environmental Impacts and Costs via LCA Hsien H. Khoo,*,† Paul N. Sharratt,† Jie Bu,† Tze Y. Yeo,† Armando Borgna,† James G. Highfield,† Thomas G. Bj€orkl€of,‡ and Ron Zevenhoven‡ † ‡
Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, 627833 Singapore Thermal and Flow Engineering Laboratory, Abo Akademi University, Piispankatu 8, 20500 Turku, Finland
bS Supporting Information ABSTRACT: The total energy and CO2 emissions of a mineral carbonation process are investigated using a life cycle assessment (LCA). The LCA investigation takes into account the energy and greenhouse gas emissions from mineral mining operations and shipment from Australia, the recovery of CO2 based on amine scrubbing technology (if required), and two possible options for mineral carbonation in Singapore where the final carbonate products have potential use in the construction industry and as land reclamation material. Four scenarios were investigated the first two with CO2 recovery via amine scrubbing prior to mineralization and the last two with direct mineralization of CO2 from the NGCC flue gas. The most promising options turned out to be scenarios 3 and 4 these cases result in 215 and 154 kg of CO2 avoided per 1 MWh, respectively. Scenario 1 results in 90 kg of CO2 avoided per 1 MWh. The life cycle costing results are 70.680.8 USD/tonne CO2 avoided for scenario 3 and 119.9159.1 USD/tonne CO2 avoided for scenario 4.
1. INTRODUCTION International concerns over global warming have identified the urgent need for large-scale sequestration, reduction, or utilization of CO2. Any aims to effectively capture and store large volumes of CO2 should take into account the life cycle of energy use and overall carbon footprint of the sequestration system itself.1 Various studies have been carried out for carbon capture and storage focusing on ocean, geological reservoirs, and biological fixation.2,3 For Singapore, mineral carbonation has been identified as the most suitable means of CO2 sequestration due to lack of land for geological storage and ocean territories.4 Moreover, carbon sequestration via mineralization has been suggested as the safest and most stable way of locking away large amounts of CO2.5,6 It is envisaged that the final carbonate products the potential use in Singapore’s construction industry as well as for applications in land reclamation. Mineral carbonation involves the fixation of CO2 as carbonate in alkaline and alkaline-earth oxides, such as magnesium or calcium oxide. Magnesium oxide or MgO is present in latent form in massive deposits of naturally occurring silicate rocks such as serpentine, Mg3Si2O5(OH)4, and olivine, Mg2SiO4. Huge deposits of alkalineearth (Mg-based) silicate minerals of the peridotite and serpentinite families exist in countries around Singapore. A few locations around Western Australia are found to have serpentinite deposits.7,8 In this article, the energy, emissions, and costs of mineralization of CO2 from an NGCC power plant is investigated using a Life Cycle Assessment approach. The mineral carbonation process is developed by Åbo Akademi University, Finland. 2. MINERALIZATION OF CO2 A process for extracting magnesium from serpentinite rock is being developed and optimized at Åbo Akademi University, r 2011 American Chemical Society
Finland and is described in detail in Nduagu et al.9,10 In this process magnesium is extracted as Mg(OH)2, which can subsequently be carbonated in a pressurized fluidized bed (PFB) under elevated pressures (>20 bar) and temperatures (>450 °C), to form stable magnesium carbonate MgCO3. The calculations here are based on serpentinite containing 84% serpentine, 13% iron oxides, and 3% impurities by weight. It is assumed here that we are able to source for mineral rocks from Australia that generally contains at least 84% serpentine or more. The complete process, together with the reaction steps, is depicted in Figure 1.11 The chemical reactions involved in the process steps (14) are as follows Step 1 : Mg3 Si2 O5 ðOHÞ4 þ 3ðNH4 Þ2 SO4 f 3MgSO4 þ 2SiO2 þ 5H2 O þ 6NH3 FeO þ ðNH4 Þ2 SO4 f FeSO4 þ 2NH3 þ 2H2 O
ð1Þ ð2Þ
Step 2 : FeSO4 þ 2NH4 OH f FeðOHÞ2 þ ðNH4 Þ2SO4 ð3Þ 1 2Feð2 þ Þ ðOHÞ2 þ O2 f 2Feð3 þ Þ OðOHÞ þ H2 O 2
ð4Þ
Step 3 : MgSO4 þ 2NH4 OH f MgðOHÞ2 þ ðNH4 Þ2 SO4
ð5Þ Received: March 24, 2011 Accepted: August 25, 2011 Revised: August 21, 2011 Published: August 25, 2011 11350
dx.doi.org/10.1021/ie200592h | Ind. Eng. Chem. Res. 2011, 50, 11350–11357
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Summary of Process Parameters ratio of serpentine:
percentage
mineralization
CO2 carbonation
carbonation
process
(tonne/tonne)
Option I
energy demand
(reaction efficiency) (GJ/tonne CO2)
2.1:1
100%
5.8
3.1:1
90%
7.7
(best estimate) Option II
Figure 1. A schematic of the carbonation process with a Mechanical Vapor Recompression process step for ammonium sulfate recovery.9,10
Step 4 ðCarbonationÞ : MgðOHÞ2 þ CO2 f MgCO3 þ H2 O
ð6Þ The main features of the process are a reaction between grinded serpentine and ammonium sulfate powder which produces MgSO4, and the subsequent conversion of MgSO4 to Mg(OH)2, which is then carbonated in a gas/solid reaction. It is assumed that of the iron oxides only FeO reacts with the ammonium sulfate.12 Iron oxide-hydroxide FeO(OH) is also produced as a byproduct of this process.9,10,13,14 The energy demanding process steps are the extraction and recovery of the ammonium sulfate input. The carbonation step is exothermic but does not produce enough heat to provide for the above-mentioned energy demand.12 Zevenhoven et al.11 estimate the energy input, excluding ammonium sulfate recovery, as 4 GJ/t CO2. A mechanical vapor recovery process for the recovery would require an estimated additional 1.8 GJ/t CO2 or a total of 5.8 GJ/t CO2.15 With lower reaction extents the energy demand increases to some degree. According to Zevenhoven et al.11 75% Mg extraction and 90% carbonation demands 5.9 GJ/t CO2, which, assuming the ammonium sulfate recovery remains unchanged, equals 7.7 GJ/t CO2 in total. The two possible sets of process parameters for the mineral-to-CO2 ratio of carbonation, reaction rates, and efficiencies are labeled as options I and II in Table 1.
3. LIFE CYCLE ASSESSMENT OF CO2 MINERALIZATION The input-output flow of energy and CO2 of a potential mineral carbonation system for Singapore is investigated using a life cycle approach. The work is carried out according to ISO 14040 standards.16 3.1. LCA Study and Objective. The goal of the LCA is to evaluate the life cycle CO2 (or carbon footprint) and energy balance of mineral carbonization in Singapore. Other air emissions, such as oxides of sulfur and nitrogen (NOx, SOx) and particulates (PM), especially from mining will also be included. The mineral rocks considered are from the mining areas scattered around Kalgoorlie, Western Australia.8 This site was chosen due to the large amount of mineral deposits found in that area so that a large supply of mineral feedstock can be ensured. Moreover, the costs of freight from Australia to Singapore seem rather reasonable (as presented in section 5). The functional unit is of utmost importance since it will determine the flow of energy and material balances in the LCA
system. The functional unit selected for this case is 1 MWh electrical power generated from the NGCC plant to the user. A total of four scenarios are projected for the LCA from the following process options: I) Two cases of CO2 capture: 9 CO2 capture by amine scrubbing from an NGCC power plant in Singapore, followed by mineral carbonation; the recovery rate of CO2 from the power plant is 90% with an energy penalty of 16%. Amine scrubbing was selected over other CO2 recovery methods (membrane, pressure swing adsorption, etc.) because it is a commercially available and mature technology with high CO2 recovery rate.1 9 Carbonation of CO2 direct from the NGCC power plant flue gas (transport by pipeline within a distance of less than 0.1 km). This option is investigated based on the vision of having a power plant with direct CO2 sequestration as an integrated facility. II) Two cases of mineral carbonation (from Table 1). In the carbonator, the CO2 conversion gives H2O, i.e. steam; expanding this will recover (most of) the pressurization energy. A combination of the four scenarios considered is shown in Table 2. At the NGCC power plant, gas is combusted at 45 bar pressure. From there, a relatively small amount of energy is required to transfer the gas via a short distance pipeline of less than 0.1 km17 for scenarios 3 and 4. Assuming negligible gas losses, it is estimated that 26 kWh/tonne CO2 energy is required to purge the flue gas using low pressure pipeline (operating at
