Challenges and Perspectives on Carbon Fixation and Utilization Technologies : An Overview

This paper provides an overview of state-of-the-art carbon fixation and utilization technologies. Several carbon capture processes, such as chemical absorption and chemical looping, are reviewed and illustrated. In addition, various types of chemicals and fuels that can be produced using concentrated CO2 (or other forms) through physical, chemical, or enhanced biological methods are presented. Among those carbon conversion methods, two promising approaches, i.e., microalgae ponds and accelerated carbonation using alkaline solid wastes, are reviewed in detail. Microalgae are fast-growing and ubiquitous photosynthetic organisms, which are rich in protein and can be converted to biodiesel fuel. They have been recognized as an alternative feedstock not only because they use CO2 from the atmosphere but also due to their high lipid content per biomass compared to other plants. In this study, for the microalgae technologies, the principles and applications of open pond systems are discussed in terms of both technological and economic considerations. The important operation parameters affecting productivity of microalgae, including light intensity, temperature, mixing, CO2 delivery, accumulation of dissolved oxygen, and salinity are summarized. On the other hand, accelerated carbonation technologies are an attractive and feasible approach to integrating alkaline solid waste treatment with CO2 fixation and utilization. In this study, the performance of various carbonation processes is critically reviewed from the perspectives of process design, energy consumption, and environmental benefits. The carbonated solid product can also be used as supplementary cementitious materials in a blended cement or concrete block. Accordingly, the performance of cement pastes with carbonated product, in terms of workability and strength development, are evaluated from the cement chemistry point of view. Cement manufacturing is an energy and material intensive process, with high annual production. It is noted that, through the accelerated carbonation process, significant indirect environmental benefits can be realized.


INTRODUCTION
Rapid economic growth in developing countries such as China and India is driving worldwide energy demand and usage.At the same time, it has been predicted that fossil fuels will remain the dominant energy source around the world for at least 20 years (Aresta, 2010).As CO 2 keeps accumulating in the atmosphere, concerns about serious and irreversible damage, such as rising water level and species extinction, are being raised regarding its influence on climate change.Consequently, it is clear that effective control of CO 2 emissions is required to achieve the goal of global CO 2 concentration below 550 ppm over next the 100 years (Fernandez Bertos et al., 2004).Deployment of carbon capture, storage, and utilization (CCSU) technologies worldwide from the CO 2 emission point source is a strategy that has been proposed to address the challenge of climate change and global warming.
Fig. 1 is a schematic diagram of the major CCUS technologies reported in the literature, including CO 2 capture, storage (sequestration), utilization (direct use), and conversion into chemicals and/or fuels.In general, carbon capture and storage (CCS) technologies can effectively capture CO 2 from emission sources, transport it, and then store it at suitable and permanent geological sites.However, global attention has recently focused on extending CCS to incorporate

CARBON FIXATION AND UTILIZATION TECHNOLOGIES
As a first step for CO 2 mitigation, dilute gaseous CO 2 in flue gas from conventional power plants or industries should be separated and concentrated to a high purity in a costeffective manner (i.e., low energy consumptions).After capture, the CO 2 can be sequestrated into geological or saline formations to ensure long-term storage of CO 2 , or it can be directly utilized or converted into carbon-based materials such as fuels and chemicals.Since CO 2 is a thermodynamically stable compound, conversion of CO 2 typically goes through a catalytic process with additional work input, e.g., renewable energy source.Extensive efforts have been underway to increase the CO 2 conversion efficiency under various novel processes.

Capture
Although various CO 2 capture technologies are available, only a few processes have been deployed on a large scale because of significant mass transfer limitations in the processes and the need to treat a significant amount of flue gas (Yu et al., 2012).Successful development and deployment of CO 2 capture processes require breakthroughs in innovative reactor concepts and process schemes as well as advanced materials.Fig. 2 shows various approaches to capturing CO 2 from flue gas or air.Typically, CO 2 capture technologies can be classified into several categories: (1) absorption using alkaline solutions (Yu et al., 2012;Lin and Chu, 2015); (2) adsorption using zeolite (Lee et al., 2013), activated carbon (Chen et al., 2014), and metal organic frameworks (Ganesh et al., 2014); (3) mineral carbonation using natural ores and/or solid wastes (Olivares-Marín and Maroto-Valer, 2012); (4) selective membrane (Lin et al., 2013;Ramasubramanian et al., 2013); ( 5) cryogenic (Wang and Gan, 2014); (6) high-temperature solid looping processes such as calcium looping (Chang et al., 2013) and chemical looping (Chiu and Ku, 2012); (7) ionic liquid (Zhang et al., 2012); and (8) biological including microalgae and enzymebased processes (Klinthong et al., 2015).Some of the above capture technologies, such as mineral carbonation and biological methods, are directly related to utilization or conversion because the physicochemical property of CO 2 is changed after capture.In this case, no CO 2 storage site is required with the capture plant.However, most of the remaining capture technologies concentrate the dilute CO 2 in flue gas to nearly pure CO 2 , which should take the sequential storage (or utilization) into consideration.
Chemical absorption using aqueous alkanolamine solutions is considered the most applicable technology for CO 2 capture by 2030 (Rochelle, 2009).It can be simply accomplished in a two-stage process, namely, absorption of CO 2 using an absorbent (solvent) followed by desorption using pressure, temperature, or electric swing.Several technological issues should be critically addressed in using alkanolamine aqueous solutions as absorbents: (1) absorber volume, (2) equipment corrosion, and (2) energy consumption in regeneration.To enhance the mass transfer between gaseous CO 2 and solution, an innovative modification of the absorption process has been proposed, for instance, a rotating packed bed (Tan and Chen, 2006;Lin et al., 2010).Besides that, various absorbent genomes have been proposed and evaluated to obtain high capture efficiency and low regeneration energy, such as using piperazine with diethylenetriamine (Yu et al., 2014), piperazine with diethylene glycol (Yu et al., 2013), and NaOH solution (Lin and Chu, 2015).
A great amount of demonstration plants using amine absorption for CO 2 capture shows that the conventional amine absorption/stripping technology is an energyintensive process (Knudsen et al., 2014;Zhao et al., 2014).The overall cost of the absorption/stripping for CO 2 capture is approximately 52-77 USD t -1 CO 2 -1 (Rochelle, 2009).Therefore, several innovative amine regeneration processes were developed and implemented, such as electrochemicallymediated amine regeneration (EMAR).In the EMAR process, the CO 2 -saturated amine solution is regenerated through electroplating of the cupric ions onto a separate copper Fig. 2. Different approaches to CO 2 capture technologies.cathode, where a high CO 2 removal efficiency could be achieved with a reasonable cost (Stern and Hatton, 2014).
The chemical looping process is a new combustion process where CO 2 is inherently separated from the other flue gas components.Oxygen-carrier materials such as Fe-, Cu-, Ni-, Mn-and Co-based metal oxides were used in the transfer of oxygen from combustion air to the fuel.Therefore, direct contact between air and fuel is avoided, resulting in near 100% CO 2 in flue gas.However, the development of Ni-and Cu-based oxygen carriers is still limited due to their high fabrication cost, in spite of their great reactivity for chemical looping process (Chiu and Ku, 2012).Regarding the capture scale, the chemical looping process has been successfully demonstrated in actual operation in the size range of 0.3-1.0MW and should be ready to scale up to a size of 1-10 MW (Abanades et al., 2015).So far, several challenges in commercialization, including (1) development of a low-cost oxygen carrier with high reactivity and recyclability, and (2) optimization of reactor design and operating condition, still need to be addressed.Because of the avoidance of large energy penalties and gas separation process, the major costs of chemical looping process are accounted for the oxygen carrier and the fuel reactor systems.Typically, the price of ores, metals and oxides may vary from year to year, e.g., in the range of 220-275 USD ton -1 for manganese ore and 165-220 USD ton -1 for ilmenite.With the above evidence, the capture cost and energy penalty could be reduced to 22 USD t -1 CO 2 -1 and < 4%, respectively (Abanades et al., 2015).

Storage
The captured CO 2 can be pressurized and stored in geological formations underground such as deep ocean (Phelps et al., 2015), saline aquifers (Soong et al., 2014;Yang et al., 2014), unminable coal beds (Kieke et al., 2009), and depleted oil/gas reservoirs (Olajire, 2014).With widespread geographic distribution, CO 2 injection into confined geological formations offers a potentially large storage capacity (IPCC, 2007).However, from the technological perspective, the risks of geological CO 2 storage include long-term containment risks (potential leakage and induced seismicity) and site performance risks (improper analysis of wellbore, nearwellbore and reservoir factors) (Pawar et al., 2015).Successful CO 2 sequestration in geological formations requires a cross-disciplinary effort in mineralogical, geochemical, and petrophysical properties of reservoir and seal materials from the microscopic to the macroscopic scales.Prediction and monitoring of storage capacity, CO 2 migration (e.g., phase state), and long-term reservoir behaviour are of great importance to ensure the safety and security of CO 2 sequestration.
The four major mechanisms for CO 2 geological storage include dynamic fluid trapping, dissolution trapping, residual trapping, and mineral trapping.Numerous studies have been conducted to evaluate changes in host rock properties when exposed to CO 2 (Birkholzer et al., 2015).Recently, a nanofluid-based supercritical CO 2 technique has been attractive for geological storage because nanofluids can not only enhance homogeneous CO 2 transport in reservoir but also mitigate the adverse effects of stratigraphic heterogeneity on migration and accumulation of CO 2 plume (Yang et al., 2014).Moreover, large-scale commercial storage should be conducted with potential exploration of geothermal resource in deep-seated hot rocks.

Utilization (Direct)
CO 2 can be directly applied in various fields such as the food industry, soft drinks, fire extinguishers, solvents, and extractants.Direct use of CO 2 involves phase changes (i.e., states of matter) such as gas, liquid, and supercritical fluid.Supercritical CO 2 has been extensively used as a solvent for reactions, separation, synthesis, and modification of material (Huang and Tan, 2014).The economics of the CO 2 utilization process depend on the quality of CO 2 and the capture technologies involved.Although there is a large difference of predicted CO 2 price between the studies, it was estimated that the CO 2 price in 2050 might be in the range of 100-400 US $ per ton CO 2 (Hoel et al., 2009).However, the market scales of such applications are small, and can not contribute to a huge impact on the overall CO 2 emission mitigation.
In fact, the most important perspective in CO 2 utilization is not the amount of CO 2 used, because the fixed CO 2 will be reformed within a short time after a CO 2 -made chemical is used.Rather, it is the introduction of innovative technologies for cleaner production that may lead to a reduction in the use of materials and energy.Recently, direct use of CO 2 by microalgae has attracted great interest because microalgae can not only consume CO 2 but also be converted to biofuels or biochemicals.Due to its environmental benefits such as its carbon neutral property, the production of bio-based chemicals in biorefineries using biomass feedstocks is a key opportunity for significant green growth.Bioproducts can offer reductions ranging from 39 to 86% in greenhouse gas emissions, compared to their fossil counterparts, based on a cradle-to-grave life-cycle assessment (Adom et al., 2014).In addition, biotechnologies are expected to contribute to 2.7% of GDP in 2030 within the OECD region and make the largest economic contribution in industry and primary production (OECD, 2009).

Conversion
Conversion of the captured CO 2 to useful products is of strategic importance toward a sustainable carbon cycle.The CO 2 molecule has the highest oxidation state, i.e., 4+, among the carbon-species compounds.For this reason, CO 2 conversion can be realized by either reduction reaction (i.e., to a negative-going oxidation state) (Wang et al., 2014) or mineralization (i.e., to a lower Gibbs free energy) (Duan et al., 2014).As shown in Fig. 3, numerous chemicals, such as methane and ethanol and polymers, can be produced from CO 2 reduction reaction.The CO 2 reduction reaction can be achieved by several different approaches, such as catalytic hydrogenation (Kiss et al., 2016), complex metal hybrids (Yamazaki et al., 2015), electrochemically catalytic (Angamuthu et al., 2010;Yan et al., 2014), photocatalytic (Wang et al., 2014), and biological (Klinthong et al., 2015) methods.The principles and challenges of the above CO 2 reduction methods are quite different from each other.For instance, the challenges in catalytic hydrogenation process for CO 2 reduction include (1) the limited affordable source of hydrogen, and (2) the efficient and effective materials of catalysts.However, in photocatalytic systems, semiconducting (e.g., TiO 2 and CdS) and/or metal-organic complex materials are commonly employed.In spite of the environmental benefits of photocatalytic reactions, they typically suffer from low efficiency of CO 2 reduction reaction because of several limiting factors, including fast electron-hole (e --h + ) recombination rates, complicated backward reactions, and low CO 2 affinity of the photocatalyst (Wang et al., 2014).
Another CO 2 reduction reaction is via the biological methods (or so-called enhanced biological fixation), which corresponds to the production of aquatic or terrestrial biomass under non-natural photosynthetic conditions.For instance, microalgal cells are sunlight-driven cell factories that can convert CO 2 into raw materials for producing biofuels (Mwangi et al., 2015), animal food chemical feedstocks, and high-value bioactive compounds (e.g., docosahexaenoic acid).It can be achieved by two separate reactions: lightdependent and light-independent sets.In the former reaction, solar (light) energy is used to excite electrons for reducing the coenzyme NADP+ to NADPH and creating the highenergy molecule ATP.After that, in the latter reaction, these reduced molecules are utilized to convert CO 2 to organic compounds that can be used as a source of energy by algae.
On the other hand, CO 2 mineralization can be achieved by accelerated carbonation reaction, which has been proven thermodynamically practical for enhancing the natural weathering process (Lackner et al., 1995).The typical feedstock of accelerated carbonation includes natural ores and/or alkaline solid wastes, in which the most reactive components reacting with CO 2 to form carbonate are calcium-and magnesium-oxides.In particular, industrial solid wastes, such as steelmaking slag, bottom and fly ashes, are suitable feedstock due to their low material cost.Moreover, the potential environmental impact of utilizing those solid wastes, including heavy metal leaching and highly alkaline properties, can be reduced and eliminated.Therefore, it can provide an integrated approach to capturing CO 2 in flue gas and stabilizing the alkaline solid wastes.
In the following sections, these two processes, i.e., biological methods using microalgae and CO 2 mineralization by accelerated carbonation, are reviewed and summarized.

MICRO-ALGAE POND FOR CO 2 FIXATION AND UTILIZATION
Algal technology is under consideration for its potential to combat the global energy and CO 2 crisis and malnutrition while also generating several value-added products.Microalgae (recognized as a third-generation source of biofuels) are fast-growing and ubiquitous photosynthetic organisms.They are rich in protein and chemical compounds, which can be converted to biodiesel fuel using a variety of different methods.Microalgae are superior in terms of biomass and biodiesel yield (50-70 Mt ha -1 yr -1 in open ponds and 150 Mt ha -1 yr -1 in photobioreactors) compared to terrestrial energy crops (~3 Mt ha -1 yr -1 for soybeans, ~9 Mt ha -1 yr -1 for corn, and ~13 Mt ha -1 yr -1 for switch grass) (Adesanya et al., 2014).Therefore, microalgae have been recognized as an alternative, so-called thirdgeneration feedstock not only because they use CO 2 from the atmosphere, but also due to their high lipid content per biomass compared to other plants (Trivedi et al., 2015).However, the productivity (or yields) of the product have certain stoichiometric and thermodynamic constraints.Fundamental principles of biochemistry show that the maximum theoretical energy conversion of the full sunlight spectrum into organic matter is around 10% (Trivedi et al., 2015).
Microalgae can be used as a feedstock for obtaining a number of products, i.e., bioenergy (such as biodiesel, biogas, bioethanol) and non-energy bioproducts (such as carbohydrates, pigments, protein, biomaterials).Algal cultivation is one of the technological thrust areas not only because it accounts for one third of the total cost involved in the algal biofuel production process but also due to its huge market potential for biodiesel as well as other valuable biochemicals (Kumar et al., 2015).Microalgae require the same basic element inputs as plants, including light, water, CO 2 , and other inorganic nutrients.Therefore, environmental factors such as pH, pCO 2 , PO 2 , light intensity, temperature, and salinity play an important role in microalgae productivity.During algal growth, the chlorophyll-α content, algae cell density, pH and DO of the solution can grow rapidly.

Types of Open Pond Systems
In general, open ponds have greater CO 2 storage capacity than tubular reactors because of their greater culture volume per square meter (100-300 L m -2 for open ponds vs. 8-40 L m -2 for 1-5 cm tubes) (Weissman et al., 1988).There are four major open pond systems that can be used for algal cultivation: (1) big shallow ponds, (2) tanks, (3) circular ponds, and (4) raceway ponds.Since each of them has its own characteristic features, the selection of an appropriate open cultivation system depends on the types of algal species, local climatic conditions, and cost of lands and water (Borowitzka, 1999).
It is reported that microalgal production in raceway ponds should be the most promising technology for algal cultivation, especially in the large scale (Kumar et al., 2015).The surfaceto-volume ratio of raceway ponds is typically 5-10 m -1 (Chiaramonti et al., 2013).The shallow configuration of raceway ponds is necessary to prevent light limitation inside the culture.However, this limits the areal productivity of the system, resulting in a linear land footprint when it comes to scale-up.Moreover, high risk of culture contamination, low final biomass concentrations incurring high harvesting costs, lack of temperature control, and poor gas/liquid mass transfer are typical process hurdles for raceway ponds (Posten, 2009).

Key Parameters Affecting Productivity
Table 1 presents the significant factors affecting algal growth, biomass accumulation, and production for most algae in open pond systems.Both external and internal factors will significantly affect algal growth and productivity in the open pond system, including environmental (e.g., location of the cultivation system, rainfall, solar radiation), engineering (e.g., pond depth, CO 2 delivery system, methods of mixing, power consumption), and biological (e.g., light, pH, oxygen accumulation, salinity, algal predators) parameters.Normally, the most challenging and important factors discussed in photobioreactor design and operation are in terms of mixing, carbon utilization, and the accumulation of photosynthetically produced oxygen (Weissman et al., 1988).Jimenez (2003) suggested that controlling pH, conductivity and O 2 concentration of the culture are important to obtain a high biomass concentration and productivity.In light of their principal component analysis, an excess of both pH and dissolved O 2 in ponds significantly reduced biomass concentration and productivity, especially in mid-summer, while no inhibition of growth by excess irradiance and temperature is expected (Jimenez, 2003).
In the following section, several important factors, including light, temperature, mixing, CO 2 delivery and cultural pH control, accumulation of dissolved oxygen, and salinity are briefly reviewed and discussed.

Light (Radiation)
Light is an important limiting factor for algal growth because the growth of all algal species was found highly dependent on solar radiation of the pond or water body.Maximum algal growth rate was obtained at the light saturation point.Beyond the light saturation point, algal growth was inhibited due to photo inhibition.In addition, the depth of open ponds is dependent on irradiance spectra, which can be determined by measuring the irradiance attenuation coefficient as a function of wavelength for each strain (Murphy et al., 2015).

Temperature
Maintenance of temperature in open raceway ponds is important but challenging work.Algal productivity increases with increasing pond temperature up to an optimum temperature, above which increasing algal respiration and photorespiration reduce overall productivity.According to a report by Singh and Singh (2015), for most of the green algae (such as Chlorella, Spirogyra, Chlamydomonas, Botryococcus, Scenedesmus, Neochloris, Haematococcus, Nannochloropsis, Ulva) species as well as a few species of Typical 15-20 cm (raceway) (Klinthong et al., 2015) Typical 20-30 cm (Greenwell et al., 2010) Productivity increased 134-200% in depth of 40 cm (compared to 20 cm) (Sutherland et al., 2014) Mixing light utilization efficiency, oxygen removal rate, etc. Kept between 5 and 30 cm s -1 (Kumar et al., 2015) CO 2 delivery system affected by diffusor At least 65 µmol L -1 (Weissman et al., 1988) Power consumption affected by water head and types of paddlewheel Temperature cellular chemical composition, uptake of nutrients and CO 2 , growth rates for every species of algae minimal temperature around 18°C (Richmond, 1986) different optimal temperature ranging from 24 to 42°C (Vonshak and Tomaselli, 2000) Nutrient/media (N, P, S) growth and composition of benthic algae.Formation of microalgae (CH 1.7 O 0.4 N 0.15 P 0.0094 ) N: 4-8% per dry weight basis algae P: 0.1% per dry weight basis algae S: 0.5% per dry weight basis algae (Greenwell et al., 2010) brown, red, blue-green algae, the optimum ranges of temperature and light irradiance for growth are 20-30°C and 33-400 µmol m -2 s -1 .Similarly, the algae raceway integrated design minimizes diurnal and seasonal temperature fluctuations and maintains temperature within the optimal range between 15 and 30°C.

Mixing
Mixing, especially vertical mixing, was considered the most significant factor affecting the performance of raceway ponds because it ensures better light utilization efficiency.Mixing also accounts for nearly 69% of total utility costs (Hreiz et al., 2014).The importance of sufficient mixing includes (1) periodic exposure of cells to sunlight, (2) keeping cells in suspension, (3) availability of the nutrient to algal cells, and (4) removal of photosynthetically generated dissolved oxygen to avoid photooxidation and photoinhibition by respiration, thereby enhancing light utilization efficiency (Chiaramonti et al., 2013;Prussi et al., 2014;Kumar et al., 2015).As a result, the mixing level should be adjusted according to the environmental conditions; for example, the mixing velocity should be reduced during the night and even in winter time to avoid excess biomass loss in the absence of light.

CO 2 Delivery and Effect of Culture pH Value Control
CO 2 delivery depends on culture pH and the factors affecting mass transfer coefficients such as type of mixing and sparger, liquid velocity, and gas-liquid contact time, etc.In open pond systems, the absorbed CO 2 after recarbonation tend to be desorbed to the atmosphere.The mass transfer coefficient, K L , for CO 2 release through the surface of a 100-m 2 pond was determined to be 0.10 m h -1 (Weissman et al., 1988).Therefore, open ponds must be operated at higher pH and lower alkalinity than tubular reactors.In high-rate ponds, a supply of concentrated CO 2 , i.e., at least 5 vol.%, is needed to sustain algal growth (Putt et al., 2011).
The rate of CO 2 absorption increases with an increase in the pH of the solution.For most algae, the optimal pH ranges between 7 and 8, where bicarbonate is the dominant species in the solution (Gonzalez-Lopez et al., 2012).Similarly, de Godos et al. (2014) reported that a sufficient CO 2 mass transfer rate for the actively growing algal cells is at pH 8.In some cases, however, maintaining a pH in the range of 9.5-10.5 (such as Spirulina sp.) is necessary to minimize the chances of contamination with other microalgae (Jimenez, 2003;Kumar et al., 2014).In addition, the typical C:N ratio in the algal cells ranges from 6 to 8 (Kumar et al., 2014).However, when wastewater is used for cultivation, the general C:N ratio of wastewater is 3, which is much lower than typical operation.In this situation, additional CO 2 delivery is imperative for maintaining a sufficient carbon source.

Accumulation of Dissolved Oxygen (DO)
Oxygen as a byproduct is generated from the algal culture in the course of photosynthesis.A high concentration of dissolved oxygen (DO) during algal cultivation severely damages the algal cells by photooxidation and inhibits photosynthesis by respiration, thereby leading to a reduction in biomass productivity.The energy required for increasing mass transfer and reducing oxygen concentrations is more than compensated for by increased biomass and potential energy yields.Therefore, removal of accumulated DO from system is a more critical design criterion than carbon supply, which may be necessary to maintain mass transfer by sparging even when no carbon is required.
Stoichiometric analysis revealed that 1.9 gram of oxygen was generated per gram of the algal biomass synthesis (Kumar et al., 2014).At maximal rates of photosynthesis, a 1-cm tubular reactor would accumulate 8-10 mg O 2 L min -1 , which may easily result in the O 2 concentrations reaching 100 mg L -1 (Weissman et al., 1988).In contrast, the DO in open ponds rises much more slowly as a consequence of the much greater volume per unit surface area, where the maximum concentration of dissolved oxygen is typically 25-40 mg L -1 (Weissman et al., 1988;Kumar et al., 2014).However, it was observed that an increase in DO concentration greater than 25 mg L -1 had a negative impact on the biomass productivity (Jimenez, 2003).This suggests that the maximum DO levels for a 100-m 2 surface and a 20-cm depth pond should be 14.5 mg L -1 at a mixing velocity of 30 cm s -1 , and 19.0 mg L -1 at a mixing velocity of 3.7 cm s -1 (Weissman et al., 1988).

Salinity
Fluctuation in the salinity concentration due to evaporation, rain, and precipitation is a common problem in open ponds, especially with brackish or saline water (Kumar et al., 2015).It can affect the growth and cell composition of microalgae by osmotic changes, ion (salt) stress, and changes of the cellular ionic ratios due to the membrane selective ion permeability (Fon Sing et al., 2014).The simplest way to solve the problem is to add extra freshwater or salt as necessary.Daily refilling with freshwater can maintain the conductivity of the culture ranging between 22 and 28 mS cm -1 (Jimenez, 2003).Except for consecutive addition of freshwater in open ponds, appropriate water treatment/separation processes for the culture can also be applied to maintain the salinity of the culture.Several technologies are available for the culture desalting, such as membrane separation (Datta et al., 2013).However, the potential challenge is the precipitation of calcium salts, especially in calcium-laden water, thereby causing loss of alkalinity and other minerals such as iron and phosphorus (Shimamatsu, 2004).

Economic Considerations
It is noted that the most challenging problems in assessing the economics are the cost of the CO 2 supply and the uncertain nature of downstream processing (Williams and Laurens, 2010).From the perspective of economic and principles of green design, biofuels should be produced simultaneously with value-added co-products if algae-tofuel technology is to be successful (Trivedi et al., 2015).In some circumstance, however, the high cost of feedstock CO 2 supply is the major obstacle for algal biomass production (Chi et al., 2011).It is noted that the high cost of CO 2 capture from the flue gas is typically due to (1) the need of regenerating high purity and pressurized CO 2 for sequestration use, and (2) the huge size of the equipment for treating the entire flue gas.In the case of stripping process, the dilute CO 2 in the flue gas could be directly introduced into algal growth system in a compact zone.Moreover, converting CO 2 into a bicarbonate/carbonate aqueous solution is preferred because it can be easily transported in a water pipeline under normal pressure (Chi et al., 2011).Consequently, to address the issue, various innovative processes have been evaluated for efficient use of CO 2 from flue gases in aqueous bicarbonate/carbonate solution, such as a carbonate-bicarbonate buffer (Gonzalez-Lopez et al., 2012), CO 2 hydrate (Nakano et al., 2014) and an electrochemical membrane process (Datta et al., 2013).A comprehensive comparison among these processes is critical to achieve high engineering performance as well as low cost and environmental impacts.

INTEGRATED ALKALINE WASTES TREATMENT WITH CO 2 FIXATION AND UTILIZATION
Among the CCUS technologies, accelerated carbonation (also referred as mineralization) using natural minerals and/or industrial alkaline wastes is attractive because an integrated approach to combining CCU with industrial waste treatment can be achieved.Gaseous CO 2 is fixed as thermodynamically stable solid precipitates, which are rarely released after mineralization.Fig. 4 shows the common processes of integrated solid waste treatment with CO 2 capture and utilization, i.e., (1) indirect carbonation, (2) direct carbonation, and (3) CO 2 curing.In indirect carbonation, the purity of CaCO 3 precipitates can exceed 99%, which can be implemented as high-value industrial materials, such as coating pigments and filters (Said et al., 2013;Azdarpour et al., 2015;Pérez-Moreno et al., 2015).
Direct carbonation (i.e., mineralization) using natural ores or alkaline solid wastes is superior in providing high storage capacity and long storage time (Seifritz, 1990;Lackner, 2003;Bobicki et al., 2012).Although there is more than enough natural ore on Earth to sequester the CO 2 emissions from all fossil fuels, cost-effective methods for accelerating carbonation are still needed (Lackner, 2003).Since alkaline solid wastes are relatively cheaper feedstock than natural ores, the CO 2 fixation capacity of those solid wastes through carbonation has been extensively evaluated (Bonenfant et al., 2008;Teir, 2008;Pan et al., 2013b;Santos et al., 2013a).As shown in Fig. 5, in general, alkaline solid wastes are chemically unstable with high calcium oxide content.They can be readily hydrated in the presence of water and reacted with CO 2 in a high-pH solution (i.e., normally above 10) to form carbonates.The CaO and MgO contents can be related to CO 2 fixation capacity for carbonation or CO 2 curing for cement.Moreover, the Fe 2 O 3 content contributes to hardness (grindability) of a material; for example, basic oxygen furnace slag (BOFS) is a relatively hard material due to its high Fe 2 O 3 content.The CaO and SiO 2 content are primarily related to the hydraulic and pozzolanic properties, respectively, if the material is used in cement and concrete.One study indicates that electric arc furnace slag (EAFS) is neither hydraulic nor pozzolanic due to its lack of tri-calcium silicates and amorphous SiO 2 content (Muhmood et al., 2009).Conversely, ordinary portland cement (OPC) is a hydraulic product, while blast-furnace slag (BFS) and flyash (FA) are respectively latent-hydraulic and pozzolanic byproducts (Gruyaert et al., 2013).
Another method of integrating CO 2 with alkaline solid wastes treatment is carbonation curing for blended cement or concrete through the injection of CO 2 gas (normally 99% purity) into a sealed chamber.Several studies have been carried out to evaluate the effect of carbonation curing on the properties of blended cement (Mo et al., 2015) and/or Fig. 4. Overview of integrated alkaline solid waste treatment with CO 2 capture and utilization through carbonation process.(Teir et al., 2007;Reddy et al., 2010;Sanna et al., 2012;Abo-El-Enein et al., 2013;Dri et al., 2013;Hekal et al., 2013;Muriithi et al., 2013;Santos et al., 2013a;Dri et al., 2014;Jo et al., 2014;Reynolds et al., 2014;Salman et al., 2014;Santos et al., 2014;Fujii et al., 2015;Ukwattage et al., 2015).concrete masonry (El-Hassan and Shao, 2015).In the following section, a direct carbonation process using alkaline solid wastes is reviewed and discussed.In addition, the performance of carbonated solid waste as supplementary cementitious materials in blended cement is evaluated.

Direct Carbonation: Energy Consumption
Accelerated carbonation generally involves several energyintensive processes, such as grinding of solid wastes, heating, and pressurization.From the perspective of energy consumption, the extensive uses of electricity for unit processes in carbonation might easily diminish the credits from carbon fixation.Xiao et al. (2014) conducted a lifecycle assessment for six scenarios of different types of carbonation processes, indicating that energy consumption is responsible for the increase in additional CO 2 emission and offsets the carbon capture efficiency.Although the overall energy consumption of indirect carbonation is less than that of direct carbonation, the manufacturing of chemicals for the extraction step may generate additional CO 2 emission and lead to other environmental issues.This suggests that the recovery process of the extractants with low energy consumption should be included for implementing indirect carbonation (Azdarpour et al., 2014;Xiao et al., 2014).
Heat recovery not only improves carbonation performance but also reduces energy loss.The temperature of exhaust gas streams is usually above the dew point and is high enough for carbonation.In practice, heat can be directly obtained from the exhaust gas streams and/or heatregenerating systems.For example, Santos et al. (2013b) proposed an integrated autoclave reactor where the high pressure is obtained by pumping liquid in a long reaction chamber.The reactor is also able to recycle the heat generated by the carbonation reaction (i.e., exothermal).
Pan et al. (Pan et al., 2013a;Pan et al., 2014) introduced a rotating packed bed reactor for direct carbonation, the socalled high-gravity carbonation (HiGCarb) process.The slurry was first pumped into the center of the high-gravity carbonation reactor, after which it flowed outward motivated by centrifugation.In the meantime, the gas entered the reactor from the tangent direction and moved inward by the pressure gradient.High micro-mixing between the slurry and gas phases enhances the overall mass transfer, thereby improving the carbonation conversion and reducing the residence time.It was noted that energy consumption for the high-gravity carbonation process, including grinding, pumps, blowers, and rotation, was estimated to be 268.6 ± 57.9 kWh t-CO 2 -1 , with 90% CO 2 removal efficiency (Pan et al., 2015c), which met the goals set by the U.S. DOE (Matuszewski et al., 2011).

Utilization of Carbonated Product as Supplementary Cementitious Materials
Fresh alkaline solid wastes, such as steelmaking slag (Crossin, 2015), fly ash (Mo et al., 2015), and bagasse ash (Chusilp et al., 2009), have been extensively studied for use as supplementary cementitious materials (SCMs) in a blended cement or concrete block.However, in the case of, for instance, fresh BOFS, several challenges exist in terms of its utilization as a concrete product or a road base material: (1) it is hard, so grinding to a certain fineness as SCMs is energy-intensive and costly; (2) the strength of the cement mortar is low, especially early stage; and (3) free-CaO and -MgO may lead to fatal expansion of hardened cement-BOFS paste (Monkman et al., 2009;Wu et al., 2009;Zhang et al., 2011).According to a report from the Portland Cement Association (Caldarone et al., 2005), the use of SCMs in blended cement may reduce the early-age strength and increase the later-age strength of the concrete, as compared with the use of only Portland cement.β-C 2 S and C 3 S are known as the primary strengthcontributing hydraulic phases in Portland cement.Although BOFS contains large amounts of β-C 2 S and C 3 S, the cementitious activity of BOFS was still low due to their large crystal size in BOFS.In addition, the predominant minerals in BOFS, e.g., γ-C 2 S, generally have no cementitious activity.
To overcome the above barriers, an attractive treatment for steelmaking slag is through carbonation with flue gas CO 2 before utilization as SCMs.The use of carbonated steelmaking slag as SCMs offers several benefits (Monkman et al., 2009;Pan et al., 2015a): (1) keep globally available industrial alkaline solid wastes out of landfills; (2) provide an economic approach to sequester CO 2 at the same time for construction use; (3) create an alternative source thereby reducing the need to transport suitable natural sands or the energy required to produce manufactured aggregates; (4) reduce the amount of leachable metals such as chromium after carbonation; and (5) reduce the amount of free CaO and its associated hydration expansion in service.Several studies have investigated the utilization of carbonated steel slag as SCMs in blended cement (Pan et al., 2015c) and a fine aggregate in concrete (Monkman et al., 2009).After carbonation, the physicochemical properties of steelmaking slag change significantly, thereby affecting the performance of blended cement.
The carbonation product, e.g., CaCO 3 , is superior to the original CaO or Ca(OH) 2 in the fresh steelmaking slag, in terms of physical properties.Haecker et al. (2005) suggested that CaCO 3 is a kind of highly elasticity-resistant material, which can improve early strength when used as SCMs in blended cement.The volume of CaCO 3 product is 11.8% greater than that of Ca(OH) 2 (Fernandez Bertos et al., 2004).Moreover, after carbonation, free-CaO content in steelmaking slag can be depleted, even entirely eradicated, which reduces the risk of fatal fracture from volume expansion (Chi et al., 2002).During the carbonation process, the product of micron-sized CaCO 3 formed gradually in the C-S-H matrix, building up a denser and more compacted structure, which is beneficial to the strength development of blended cement (Mahoutian et al., 2014).In addition, since CaCO 3 takes possession of vacuum within the mineral matrix, liquid can not intrude into the structure to induce corrosion and further damage to the structure (Chi et al., 2002).Similarly, Pang et al. (2015) reported that the number of pores smaller than 1 µm in steelmaking slag decreased by 24.4% after carbonation, resulting in a decrease in water absorption and an increase in impermeability.
The use of carbonated solid wastes as SCMs was also beneficial to the strength development of blended cement, especially the early-age strength.Pang et al. (2015) demonstrated that replacing non-carbonated slags with carbonated slags can result in a 20% increase in compressive strength in 28 days, as well as reduce environmental problems such as the leaching of heavy metals.A similar observation was made by Pan et al. (2015c).This might be attributed to the enhanced hydration of C 3 A by CaCO 3 to form stable calcium carboaluminate (C 3 A•CaCO 3 •11H), as described in Eq. ( 1), which helps to develop a higher mechanical strength in the early stage (Hawkins et al., 2003).On the other hand, the formed unstable compound (C 3 A•0.5CaCO 3 • 0.5Ca(OH) 2 •11.5H) will continuously convert to calcium carboaluminate after 1 d, as shown in Eq. ( 2). (2) In some circumstances, high gypsum (CaSO 4 ) content can be found in the fly ash due to the wet desulfurization of flue gas using CaO.If fly ash is utilized as SCMs, gypsum is related primarily to the C 3 A phase content in blended cement.It is noted that the hydration of C 3 A has a decisive effect on the rheological properties of cement paste, i.e., the high rate of hydration leads to saturation of the solution with aluminate and calcium ions and as a consequence to the crystallization of C 4 AH x , resulting in the quick stiffening of paste (Kurdowski, 2014).In general, gypsum is added to reduce the rate of C 3 A hydration because C 3 A hydration can be hindered by the ettringite layer covering the aluminate crystals and forming an impermeable barrier, as shown in Eq. ( 3).The direct reaction of C 3 A with ettringite is also possible, as shown in Eq. (4).
However, this means that the amount of ettringite formed at the early stage of hardening will be reduced with the increase of gypsum addition.As a result, disturbing the hydration process at early age will also affect the strength development at later age and may even reduce the strength.This suggests that the optimum gypsum content in blended cement is of crucial importance to achieve maximum strength and minimum drying shrinkage.

Carbonation Curing Using CO 2 for Blended Cement with Alkaline Solid Wastes
The use of SCMs is a sustainable practice to make the cement and concrete industries more environmentallyfriendly.Instead of using CO 2 in direct carbonation of solid wastes for the production of SCMs before cement preparation, the CO 2 can be used in the sequential curing process for cement preparation, so-called carbonation curing.This early-age carbonation in curing is the reaction between calcium silicates or early hydration products with carbon dioxide producing a hybrid binder structure of calcium silicate hydrates and calcium carbonates (El-Hassan and Shao, 2015).It has been shown that the carbonation curing of cementitious materials can improve their mechanical properties and durability.
Table 2 presents the performance of carbonation curing techniques for various types of wastes as SCMs in cement or concrete.Mo et al. (2015) found that curing using higher pressure CO 2 can lead to faster strength development due to the rapid penetration of CO 2 and carbonation of blended cement.With carbonation curing, early hydration products were converted to a crystalline microstructure, and subsequent hydration transformed amorphous carbonates into more crystalline calcite (El-Hassan and Shao, 2015).Ghouleh et al. (2015) suggests that the early strength gain can be attributed to carbonation of the γ-C 2 S component in steel slag, as shown in Eq. ( 5).It was noted that the nonhydraulic γ-C 2 S compact developed a compressive strength of 50 MPa after carbonation at 10 bar gas pressure for 15 min (Bukowski and Berger, 1979).

Environmental Benefits and Impacts
The leaching behavior of heavy metals from alkaline solid wastes is of great concerns in terms of environmental impact.Therefore, extensive research has been performed on the effect of carbonation, pH, and mineral structure on the leaching behavior of solid wastes (Baciocchi et al., 2011;van Zomeren et al., 2011;Salman et al., 2014).Calciumcontaining minerals are alkalinity contributors in those alkaline solid wastes, which results in the pH ranging from 11-13 (steelmaking slag as example).Transformation of CaO into CaCO 3 with CO 2 gas can decrease the pH of the solution to about 6-9.In parallel with the decrease of pH, the leaching of most heavy metals from wastes, such as Cu, Zn, Cd, Cr, V, Pb, and Ba, was properly restricted due to the formation of insoluble carbonates (Baciocchi et al., 2011;van Zomeren et al., 2011).In addition, different oxidant states of heavy metals in steel slag resulted in different leaching behaviors of Cr species.Baciocchi et al. (2010) found that the leaching of Cr increased in the cases of electric arc furnace slag and argon oxygen decarburization slag, while it decreased for the other two kinds of blended slags.
However, it was also observed in a few reports that, during carbonation, the dissolution of calcium silicate minerals (e.g., C 2 S), as shown in Eq. ( 6), might break down the mineral structure, thereby potentially releasing heavy metals (e.g., vanadium), chlorine and fluoride ions into liquid (van Zomeren et al., 2011).
By taking the use of carbonated solid wastes as SCMs into account, accelerated carbonation can attain huge environmental benefits (Pan et al., 2016).Cement production is energy and material intensive, accounting for 4-5% annual CO 2 emission around the world (Gibbs et al., 2001).The major contributors to CO 2 emission from cement production include (1) the production of raw cement material lime (CaO) from limestone (CaCO 3 ) and (2) the intensive heat required.The former, called the clinker process, produces ~0.5 ton CO 2 per ton cement (Gibbs et al., 2001).China, where the CO 2 emission factor in China in 2011 was 0.545 t-CO 2 t-cement -1 , accounts for more than 60% of global cement production (Shen et al., 2015).In addition, Kumar et al. (2006) reported that 1.5-1.7 tons of natural resources (i.e., 1.3-1.5 tons of limestone and 0.2-0.4tons of clay) and 0.11-0.13tons of coal are used per ton of cement clinker production.

Conclusions
The large-scale separation of CO 2 from flue gases in power and industrial plants will make a huge volume of CO 2 available on site.The subsequent processes would be either storage in natural geological structures or direct utilization and conversion.In practice, however, the suitable storage sites are few in number, and the procedures still involve high energy and economic costs.Conversely, large-scale CO 2 utilization and conversion can be well integrated with the associated capture unit.In fact, rather than the amount of CO 2 used, the most important consideration in CO 2 utilization should be the development of innovative cleaner production technologies, thereby leading to a reduction in the use of materials and energy.
CO 2 fixation via fast-growing biomass, such as microalgae, (i.e., biological method) not only reduces CO 2 accumulations in the atmosphere but also uses CO 2 for furnishing chemicals and energy products.These chemicals, such as pigments, may have a high added value, providing an opportunity to cover the costs of algal production.Consequently, the biological process would be feasible from an economic standpoint and acceptable from an energy perspective.
Integrated alkaline waste treatment with CO 2 capture and utilization is an attractive approach to achieving direct and indirect reduction of greenhouse gas emissions from industries.The accelerated carbonation process not only fixes CO 2 from flue gas in industries as a "safe" carbonate precipitate but also stabilizes alkaline wastes.Therefore, the potential environmental impacts caused by utilization of those untreated wastes, such as highly alkaline and active properties and heavy metal leaching, can be eliminated.

Recommendations
To accelerate the implementation of CCUS technologies, a balanced performance in the 3Es (Engineering, Economic, and Environmental) is important.Any CCUS technologies implemented must be less energy and material intensive as well as economically viable.For microalgae technology, the optimal control of pH (also related to CO 2 concentration), conductivity and O 2 concentration of the culture is critical to obtain a high biomass concentration and productivity.Integration of an energy-efficient ex-situ water treatment process for the culture solution with existing open ponds is an alternative, by which provision of sufficient CO 2 concentration and removal of excess O 2 , (in)organic acid, and salinity from the culture can simultaneously achieved.Likewise, for CO 2 mineralization by carbonation using alkaline solid wastes, development of the process achieving high CO 2 fixation capacity with low energy consumption should be important in future research.This means that mass transfer among gas-liquid-solid phases needs to be enhanced within an achievable plant size.Moreover, the chemistry and mechanism of utilizing the carbonated product as supplementary cementitious materials in blended cement should be systematically determined in terms of the physicochemical properties of the product.

Fig. 3 .
Fig. 3. Plentifully potential uses of CO 2 as chemicals through various conversion technologies.

Table 1 .
Significant factors affecting algal growth, biomass accumulation, and production for most of algae in open ponds system.intensity) growth and cell composition of microalgae Minimum of 4.65 kWh m -2 d -1 (Benemann et al., 1982) Rainfall chance of culture dilution Not more than 1 m per year (USDOE, 2010)Land slope earth moving costs during pond constructionNot more than 5%(Bennett et al., 2014) Contiguous area ensure a commercial scale (i.e., 3.8 utilization efficiency, mixing, power consumption of mixing, etc.

Table 2 .
Utilization of carbonation curing techniques for various types of wastes as construction cements in cement or concrete.
a SS: steel slag; AODS: argon oxygen decarburization slag; BFS: blast furnace slag; FA: fly ash.b RH: relative humidity.c n.r.: not report.d f c = compressive strength of mortar; f t = tensile strength of mortar; f f = flexural strength of mortar.