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Accepted Manuscript
Utilization of coal fly ash and bottom ash as solid sorbents for sulfur dioxide reduction
from coal fired power plant: Life cycle assessment and applications
Mahinsasa Rathnayake, Parnthep Julnipitawong, Somnuk Tangtermsirikul, Pisanu
Toochinda
PII:
S0959-6526(18)32547-2
DOI:
10.1016/j.jclepro.2018.08.204
Reference:
JCLP 13988
To appear in:
Journal of Cleaner Production
Received Date: 28 May 2018
Revised Date:
26 July 2018
Accepted Date: 20 August 2018
Please cite this article as: Rathnayake M, Julnipitawong P, Tangtermsirikul S, Toochinda P, Utilization
of coal fly ash and bottom ash as solid sorbents for sulfur dioxide reduction from coal fired power
plant: Life cycle assessment and applications, Journal of Cleaner Production (2018), doi: 10.1016/
j.jclepro.2018.08.204.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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Utilization of coal fly ash and bottom ash as solid sorbents for sulfur dioxide reduction
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from coal fired power plant: Life cycle assessment and applications
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Mahinsasa Rathnayakea, Parnthep Julnipitawongb, Somnuk Tangtermsirikulc,
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Pisanu Toochindaa,*
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a
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Technology, Thammasat University, Pathumthani 12121, Thailand.
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School of Bio-Chemical Engineering Technology, Sirindhorn International Institute of
Construction and Maintenance Technology Research Center, Sirindhorn International
Institute of Technology, Thammasat University, Pathumthani 12121, Thailand.
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c
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Thammasat University, Pathumthani 12121, Thailand.
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School of Civil Engineering Technology, Sirindhorn International Institute of Technology,
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*Corresponding author.
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E-mail: pisanu@siit.tu.ac.th
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Tel: +66-2-986-9009 ext. 2309
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Fax: +66-2-986-9112
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Abstract
Feasibility of sulfur dioxide (SO2) reduction from a coal fired power plant using fly
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ash and bottom ash as solid sorbents is evaluated. The study integrates inventory data from
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experiments, process simulation, published studies, and real plant operation for
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comprehensive life cycle assessment (LCA). The experimental results exhibit the capability
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of free lime contained fly ash and bottom ash to react with SO2 under the least efficient
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conditions. The scenario-based LCA results show that fly ash/bottom ash utilization can
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lower the SO2 load of untreated flue gas by 3.7-4.7% before an existing wet limestone flue
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gas desulfurization (WFGD) system. The combined desulfurization process with SO2
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reduction saves the annual energy consumption by 4.0-5.0%, the environmental impacts by
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3.0-5.0%, and the annual operating cost by 3.1-4.5%, compared to the existing WFGD
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process. The uncertainty analysis evaluates the probability of scenarios, and the sensitivity
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analysis recommends the parameter variations to further improve the combined
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desulfurization process and possible applications of the post SO2 capture fly ash. The
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approach can also be applied in other coal fired power plants for more environmentally-
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benign WFGD operation and cleaner electricity production.
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Keywords: Life cycle assessment, Fly ash, Bottom ash, Coal fired power plants,
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SO2 reduction, Flue gas desulfurization
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1. Introduction
Sulfur dioxide (SO2) is a toxic gas and a key airborne pollutant for acidification,
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photochemical smog formation, and various environmental/health problems (Thanh and
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Lefevre, 2001; Ward, 2009). Coal fired power plants are the major contributors for artificial
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SO2 emissions, causing high atmospheric SO2 levels. Strict regulations have been enforced on
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coal fired power plants to control SO2 emissions up to permissible levels (Liang et al., 2013).
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Therefore, every coal fired power plant must have a post combustion flue gas desulfurization
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(FGD) system for SO2 control. The conventional wet flue gas desulfurization (WFGD)
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technology consumes huge quantities of natural limestone (CaCO3) as a desulfurization
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material (Benko and Mizsey, 2007). Limestone particles react with SO2 in flue gas inside
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absorber towers that produce gypsum (CaSO4.2H2O) and direct CO2 (Reaction 1).
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SO2 (g) + CaCO3 (s) + ½O2 (g) + 2H2O (l) → CaSO4.2H2O (s) + CO2 (g)
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More than 80% of existing coal fired power plants in the world have installed WFGD
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systems due to high SO2 removal efficiencies (95-98%) at low temperatures (<150 ºC) and
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wide availability of natural limestone (Feng et al., 2014; Wu et al., 2017). However, the
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WFGD process suffers from excessive utility consumptions and operations with high
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environmental impacts, such as explosion of rock mines for limestone production,
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transportation of desulfurization materials, gypsum handling, and wastewater treatment (Feng
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et al., 2014). A case study has reported that a WFGD process for 1 tonne of SO2 removal
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from a coal power plant consumed 1,485 kWh of electricity, 1.8 tonne of limestone, 4.1 tonne
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of freshwater while discharging 3.2 tonne of gypsum, 2 m3 of wastewater, and 0.73 tonne of
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direct CO2 (Shi et al., 2017). The literature also reveals that a WFGD system can increase the
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environmental impacts, such as global warming, air pollution, and fossil depletion from an
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overall coal power generation process (Sampattagul et al., 2004; Sampattagul et al., 2005; Xu
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and Hou, 2010). Thus, a new approach that can lower the resources/energy consumptions,
(1)
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and environmental impacts of existing WFGD systems is desirable for smooth and cleaner
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electricity production.
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Fly ash and bottom ash are waste materials from coal fired power plants. Million
tonnes of fly ash/bottom ash are produced every year with increasing coal power generation
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(Kikuchi, 1999; Rashidi and Yusup, 2016). Thus, utilization of fly ash/bottom ash in
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environmentally-benign applications would be an attractive solution for waste management
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of coal fired power plants (Ahmaruzzaman and Gupta, 2012; Wang and Wu, 2006). Many
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research papers showed that fly ash and bottom ash can capture SO2 by both adsorption and
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reaction at a wide temperature range (30-600 ºC) (Lee et al., 2005a; Li et al., 2007). The SO2
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capture capacity by adsorption changes with the fly ash/bottom ash sorbent properties, such
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as surface area, pore volume, hydration period, etc. (Lee et al., 2005c; Li et al., 2001).
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Nevertheless, the SO2 capture capacity by reaction varies with the content of reactive
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components in flyash/bottom ash sorbents, reaction temperature, and flue gas conditions, i.e.,
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relative humidity (RH), gas flow rate, gas composition, etc. (Foo et al., 2011; Lee et al.,
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2005a; Mohamed, 2005). Experimental studies in the literature revealed that fly ash sorbents
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mixed with CaO/Ca(OH)2 (20-30 wt%) can achieve SO2 capture capacities of 54-768 mg
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SO2/g sorbent with calcium utilization rates (% mol SO2 reacted/ mol Ca in the sorbent) of
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20-75% by both adsorption and reaction at low temperatures (<150 ºC) in dry conditions (RH
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< 50%) (Lee et al., 2005a; Lee et al., 2008; Mohamed, 2005). Studies also reported that fly
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ash sorbents with CaO/CaCO3 can reach calcium utilization rates of 75-94% in humidified
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conditions (RH > 50%) (Liu et al., 2002; Liu et al., 2010; Zhai et al., 2017). Adding CaSO4
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into fly ash/bottom ash sorbents can promote the SO2 capture capacity by reaction (Fernandez
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et al., 2001; Lee et al., 2005a; Lee et al., 2005c). The literature confirms that free lime (CaO
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in the free form) in both fly ash and bottom ash sorbents is the key reactive component with
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SO2 (Lee et al., 2005c; Li et al., 2001; Lin et al., 2001). Hence, free lime contained fly
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ash/bottom ash can show a common reaction behavior (Reaction 2) with SO2.
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CaO (s) + SO2 (g) + ½O2 (g) → CaSO4 (s)
(2)
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Fly ash and bottom ash can have other applications in addition to the utilization for
SO2 capture. Fly ash is commonly used as a mineral admixture in cement/concrete for
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construction applications (Siriruang et al., 2016; Tangtermsirikul, 2005). However,
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fulfillment of the specific fly ash properties, such as free lime content, SO3 content, moisture
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content, and particle size, etc. are required to serve in construction applications (Kaewmanee
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et al., 2013; Kaewmanee and Tangtermsirikul, 2014). The studies report that only 30% of the
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global fly ash generation is currently recycled in useful applications (Jayaranjan et al., 2014).
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In addition, above 85% of the bottom ash quantities remain in power plants as an onsite waste
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without useful applications (Rashidi and Yusup, 2016). Consequently, unused fly ash is
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disposed in landfills, and bottom ash is released to ash ponds in wet form, which can create
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many environmental problems. Therefore, this study investigates utilization of fly ash/bottom
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ash in order to lower the desulfurization load in flue gas from the same vicinity. The
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approach is not an entire replacement of existing desulfurization units. Nevertheless, it can
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provide waste reduction and value addition for SO2 control in a coal fired power plant.
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This work evaluates the feasibility of an industrial scale desulfurization process using
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fly ash and bottom ash as solid sorbents. The study covers a comprehensive assessment,
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including experiments to validate the capability of SO2 capture using fly ash/bottom ash,
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process simulation for a scaled-up SO2 capture unit based on inventory data from real
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industry operation, and detailed life cycle assessment (LCA) with uncertainty and sensitivity
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analyses. Moreover, no study in the literature has reported a LCA analysis on the utilization
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of fly ash/bottom ash for industrial scale SO2 reduction. In this study, the process simulation
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technique was used to predict the parameters of a pre-designed SO2 capture unit (Rathnayake
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et al., 2018). The novelty of this work is the comprehensive LCA based feasibility assessment
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on energy consumption, environmental impacts, and operating cost of a combined
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desulfurization process, i.e., WFGD system + SO2 capture unit by fly ash/bottom ash. In
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addition, potential applications of the post SO2 capture fly ash are also proposed in this paper.
In the industrial scale, flue gas contains water vapor and other gases (i.e., CO2, NOx,
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O2, N2, etc.) that can affect the SO2 capture capacity of solid sorbents. Many experimental
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studies in the literature have reported that the presence of water vapor (moisture), O2, and
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NOx in flue gas promotes the desulfurization activity of fly ash sorbents (Lee et al., 2005b;
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Lee et al., 2008; Tsuchiai et al., 1996). Liu et al. have experimentally confirmed that CO2 has
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an insignificant effect on SO2 capture in flue gas with SO2 > 1,000 ppm (Liu et al., 2010).
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Studies also exhibit that N2 in flue gas acts as an inert gas during SO2 capture (Lee et al.,
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2005c). Therefore, an overall positive effect by the presence of other gases can be considered
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for SO2 removal from fresh flue gas in the industrial scale using fly ash/bottom ash sorbents.
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In this study, the SO2 capture experiments were conducted for the least efficient case, i.e.,
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zero RH of gas, no presence of other gases except N2, and continuous gas flow with a low
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residence time. After experimental validation of the least efficient case, real flue gas
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conditions were considered for the scaled-up SO2 capture unit using process simulation. The
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existing WFGD system in the Mae Moh coal fired power plant of Electricity Generating
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Authority in Thailand (EGAT) is considered for the LCA analysis in this study. The existing
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desulfurization system handles the entire SO2 load in order to control the average SO2
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emissions from the power plant stacks below 320 ppm and the atmospheric SO2 level below
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0.15 ppm for 6 min, which are the limits set by the pollution control department of Thailand
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(PCD, 2018). Thus, reduction of the SO2 load using fly ash/bottom ash from the same power
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plant could support a method to improve the energy/environmental performance of the
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existing desulfurization process and more environmentally-benign power plant operation.
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2. Materials and methods
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2.1 Experimental sections
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2.1.1 Sample preparation
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Fly ash and bottom ash were collected at an interval time period of 6 months from the
Mae Moh coal fired power plant, Thailand. Two types of fly ash sorbent samples were
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prepared: (i) fresh fly ash (1.71 wt% free lime) and (ii) fly ash with 5 wt% free lime by
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adding free lime (free CaO) to fresh fly ash. Every fly ash sorbent sample was divided into
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two portions, and one portion was mixed with 1 wt% gypsum (CaSO4.2H2O) from the Mae
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Moh power plant. All four types of fly ash sorbent samples and fresh bottom ash samples
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were used for separate experiments of SO2 capture by adsorption and reaction. Sulfur dioxide
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gas (4.3 vol% of SO2 in N2, Linde, Thailand), and nitrogen gas (99.999 vol% purity of N2,
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Praxair, Thailand) were mixed to obtain the feed gas (4,300 ppm of SO2) at zero RH for the
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SO2 sorption experiments.
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2.1.2 Experiments for SO2 adsorption at 30ºC
Figure 1 shows the experimental setup for SO2 capture by adsorption and reaction.
For each experiment, 5g of a sorbent sample was placed inside the tubular reactor. Feed gas
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(4,300 ppm of SO2 in N2) was fed into the reactor, and enclosed for 1 h residence time (batch
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mode) at 30ºC, 1 atm. After 1 h, nitrogen gas was continuously fed at 30 ml/min for 1 h to
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remove the physically adsorbed SO2 over the solid sorbent sample. The SO2 desorption
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process was carried out by increasing the temperature to 150ºC. The gas samples were
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analyzed in a gas chromatograph (GC) to determine the SO2 adsorption capacity using fly ash
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sorbents.
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Figure 1 (Page 34)
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2.1.3 Experiments for effect of residence time for SO2 reaction at 30ºC
The same experimental procedure in section 2.1.2 was followed for SO2 capture by
reaction using the fresh fly ash and fresh bottom ash samples at residence times: 0.5 min, 1
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min, and 30 min, separately. The calcium utilization rate (mol SO2 reacted/mol free lime in
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the sorbent) was calculated for the experiments at different residence times.
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2.1.4 Temperature programmed desorption-mass spectrometry (TPD-MS)
A 1g sample of fly ash with 5 wt% free lime was placed inside the tubular reactor in
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Figure 1. The sorbent in the reactor was exposed to the SO2 gas in N2 at 30 ml/min (0.25 min
residence time) in continuous mode for 30 min at 50ºC, and 100ºC, separately. After SO2
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capture, the TPD-MS process was performed by purging 30 ml/min of nitrogen gas, and
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increasing the temperature (50-200ºC) at a rate of 10ºC/min. The adsorption capacities at
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50ºC, and 100ºC were calculated using the MS profiles of the TPD-MS processes. The SO2
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capture capacities by reaction for the sorbent samples were calculated using the EDTA
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titration method.
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2.1.5 Temperature programmed reaction-mass spectrometry (TPR-MS)
A 1g sample of fly ash with 5 wt% free lime was contacted with the SO2 gas in N2 at
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30 ml/min (0.25 min residence time) in continuous mode in the tubular reactor. After the
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mass spectrometry profile of the outlet gas flow was stabilized, the temperature was increased
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from 35ºC to 600ºC at a rate of 10ºC/min. The MS profiles were recorded for the outlet gas
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flow from the reactor.
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2.1.6 Analysis Techniques
The chemical compositions of prepared fly ash and bottom ash samples were analyzed
using an X-ray fluorescence (XRF) spectrometer, PANalytical PW-2404, Netherlands. The
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fly ash samples after SO2 capture were characterized by the X-ray diffraction technique
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(TTRAX diffractometer, Rigaku, Tokyo, Japan) with Cu Kα1 radiation, at 50 kV, 300 mA,
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15°-65° 2-theta, 0.01° step size, scan speed 3°/min. The recorded XRD patterns were
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compared with the powder diffraction files-2003 (ICDD PDF) database for phase
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identification of the peaks.
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The gas chromatography was performed using a Perkin Elmer (Waltham, USA) auto
system XL gas chromatograph (GC), with Chromosil 330 packed column (Supelco,
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Bellefonte, PA, USA), coupled with a thermal conductivity detector (TCD). The mass
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spectrometry was conducted using the gas chromatography and mass spectrometry (GC/MS)
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analyzer, GCMS-2010 Ultra, Shimadzu Corporation, Japan. The SO2 capture capacities by
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adsorption were calculated and averaged from the GC and MS profiles based on the manual
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peak integration of peak intensities at m/z = 64 after calibrating the GC and MS profiles.
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The EDTA (Ethylene Diamine Tetra Acetic acid) titration method was used to
determine the amount of free lime reacted with SO2. The difference between the free lime
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available in the sorbent before and after SO2 capture was calculated to obtain the reacted free
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lime amount and the SO2 capture capacity by reaction. The titration process was performed
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with EDTA (≥99%, Ajax Finechem Pty Ltd.) in deionized water as the standard solution,
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ammonium chloride (≥99.5%, Sigma Aldrich) and ammonium hydroxide (28%, Sigma
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Aldrich) in deionized water as the buffer solution, and Eriochrome Black T (Panreac) as the
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indicator.
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2.2 LCA methodology
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2.2.1 Goal and scope definition
The ISO 14040/44 framework was adopted as the LCA methodology in this study.
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The goal of this study is to support decision making for introducing SO2 reduction using fly
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ash/bottom ash in order to lower the desulfurization load of an existing system. The scope of
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this study includes the desulfurization process in the Mae Moh power plant, Thailand. A
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scenario-based life cycle inventory is developed for the existing desulfurization process with
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and without SO2 reduction using fly ash/bottom ash. The LCA inventory includes the related
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resources/energy inputs, and environmental emissions: airborne pollutants (CO2, SO2, NOx,
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particulates, etc.), waterborne pollutants (suspended solids, heavy metals, etc.), and solid
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wastes.
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2.2.2 Functional unit
The Mae Moh power plant has an electricity generating capacity of 2,400 MW with
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ten operating units: unit no. 4-7 (4×150 MW), unit no. 8-11 (4×300 MW), and unit no. 12-13
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(2×300 MW). Every power generation unit connects with the existing WFGD system that
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includes three absorber towers. Annual operation (1 year) of the Mae Moh power plant is
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selected as the functional unit. This unit basis can correlate the amounts of fly ash/bottom ash
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generation with the mass/energy flows in the WFGD system. Table 1 lists the annual average
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operating data of the Mae Moh power plant. All inventory results in the FU basis are
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exported to the SimaPro LCA software for the LCA analysis. The ReCiPe midpoint (H)
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V1.12 method, which is a recent and well-established model is used for the environmental
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impact assessment. For comparison, the midpoint impact results are normalized using
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percentage of division-by-maximum.
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Table 1 (Page 41)
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2.2.3 System boundary
Figure 2 illustrates the system boundary for the LCA study. The study subdivides the
desulfurization system into three unit processes to link the mass/energy flows of FGD
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operation and other background processes as follows:
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1. Production of desulfurizing material (process 1)
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2. Transportation (process 2)
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3. Desulfurization system operation (process 3)
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The process 1 contains the limestone extraction process with integrated emissions,
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and fly ash/bottom ash generation. The process 2 considers the transportation processes of
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different materials with related emissions. The process 3 accounts for the WFGD system
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operation, including the resources/energy inputs, wastewater treatment, and integrated
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emissions.
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Figure 2 (Page 35)
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2.3 Inventory calculations and uncertainty
Inventory calculations of the individual unit processes consider few estimations based
on the WFGD plant data obtained from the Mae Moh power plant, and the published data
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sources in the literature (Table S1 in the supplementary material). For uncertainty analysis,
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previously specified uncertainty values are used for the published data whenever available.
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The uncertainty values for the real plant data and other inventory data were defined using the
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Pedigree matrix (Ciroth et al., 2016).
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2.3.1 Process 1: production of desulfurizing material
Required limestone for the WFGD system is supplied from the local mines in
Lampang province, northern Thailand. Average CaCO3 composition in local limestone is
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taken as 95 wt%. Heavy duty vehicles used in the mining process consume diesel (Dubsok
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and Kittipongvieses, 2016; Kittipongvises et al., 2016). Electricity for the limestone
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extraction machinery is supplied from the Thai national grid-mix (Krittayakasem et al.,
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2011). The average inventory data for the limestone production process in Thailand is
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obtained from (Kittipongvises, 2017). Ammonium nitrate/fuel oil (ANFO) is used as the
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blasting agent for limestone mining, which is considered as an imported material to Thailand.
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Thus, the inventory data for ANFO production/detonation are retrieved from the foreign
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databases (Ecoinvent, 2017). The factors to calculate the emissions from diesel consumption,
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grid-mix electricity generation, and blasting agent production/detonation in the limestone
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production process are listed in the supplementary material (Table S2).
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The total amount of bottom ash from the Mae Moh power plant is currently not used
in specific applications. Unused fly ash that does not fulfill the specifications for commercial
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applications are also available in the power plant. Thus, inventory allocations from the coal
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power generation process are not considered for unused fly ash/ bottom ash production
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because, they remain as waste materials and utilized for SO2 reduction in the same plant.
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2.3.2 Process 2: Transportation
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Fuel consumptions and related emissions are calculated for the transportation
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processes: limestone transportation to the desulfurization system, unused fly ash/bottom ash
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transfer between the ash disposal sites and the desulfurization system, gypsum transportation
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from the desulfurization system to the landfill, and ANFO transportation to the limestone
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mines. All road transportations use 10 tonne capacity diesel trucks with a unit diesel
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consumption of 0.03 L/tonne-km (Cheewaphongphan et al., 2017). The average fuel oil
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consumption rate by container ships is 0.08 MJ/tonne-km (Ecoinvent, 2017). The emission
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factors from fuel burning are classified in the supplementary material (Table S3). The
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following estimations are proposed for inventory calculations of the transportation processes.
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i. The average round-trip road distance from the limestone mines to the Mae Moh
power plant is 100 km.
ii. The average distance for maritime transportation of the imported materials to
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Thailand is 6,000 km. The average road distance from the seaport is 1,000 km.
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iii. The average road distance from the WFGD system to the gypsum landfills is 5 km.
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iv. The average road distance between the ash disposal sites and the desulfurization
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system is 2 km.
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2.3.3 Process 3: Desulfurization system operation
The average plant data for the recent five years (2013-2017), obtained from the Mae
Moh power plant are used for inventory calculations of the desulfurization system operation.
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In each WFGD unit, the received limestone is first sent through a grinding mill to prepare
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limestone powder. Limestone powder is mixed with water and pumped through a hydro-
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cyclone to separate over-sized limestone particles (recycled to the grinding mill) from the
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liquid slurry. The liquid limestone slurry is stored in a reagent feed tank, where a water
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recirculation/agitator system operates to avoid the limestone slurry clogging in the pipes. This
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limestone slurry is flowed to the sump of an absorber tower and pumped through the spray
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injectors at the top. The SO2 rich flue gas at 163ºC flows through a heat exchanger before
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entering the absorber tower at 145ºC. The falling limestone stream mixes with the flue gas
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and reacts with SO2. Compressed air is fed to the absorber tower to provide oxidation in the
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desulfurization reaction. After the desulfurization process at 62ºC, the SO2 removed flue gas
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is heated to 80ºC, and released to the atmosphere from chimney stacks. Gypsum is separated
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from the absorber sump using a vacuum system and discharged as the solid byproduct. All
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energy requirements for the desulfurization system are supplied by the electricity and steam
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generated in the Mae Moh power plant (1 kWh = 3.6 MJ). The major emissions from
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electricity/heat production in the plant (Table S4), and the emission factors for wastewater
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from the WFGD operation (Table S5) are listed in the supplementary material. Few
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estimations are set for the inventory calculations of the desulfurization operation as follows.
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ii. For the operating life cycle, the construction/installation processes of the
desulfurization system are excluded in this study.
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i. Desulfurization system operation only considers SO2 removal in the flue gas.
iii. The gypsum utilization rate for useful applications is 10 wt%, and the remaining 90
wt% is disposed in landfills (Chindaprasirt et al., 2011).
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iv. Wastewater from the desulfurization system is treated inside the power plant using a
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biological treatment process, and the waterborne concentrations in the Mae Moh
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reservoir are considered as waterborne emissions (Junshum et al., 2007).
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v. In calculations, both fly ash and bottom ash were considered having the same average
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free lime content (EGAT, 2018).
vi. The calcium utilization rate (% mol SO2 reacted/mol free lime in the sorbent) of fly
17
ash/bottom ash at real flue gas conditions reported in the literature is considered for
18
the LCA analysis. The literature findings from experimental studies are summarized
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in the supplementary material (Table S6).
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vii. A reactor for SO2 capture using fly ash/bottom ash is installed before every absorber
21
tower in the existing WFGD system. It is considered that the reactors are constructed
22
only using available materials in the power plant. Thus, the inventory data for
23
construction/installation of the reactors are negligible over the system life.
24
25
viii. Energy consumption for the SO2 capture process using fly ash/bottom ash is estimated
using the Aspen Plus (2016) process simulation software. The energy intensive
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operations include feeding ash to the reactor, heating/cooling duties in the reactor for
2
the reaction with SO2 in flue gas, and discharging ash after SO2 capture.
3
ix. The equivalent SO3 content in the post SO2 capture fly ash was calculated using
4
Equation 3.
5
SO content = Xwt% + 6
Where, X = SO3 content in fresh fly ash (wt%); Y = CaSO4 amount (tonne) produced
7
in fly ash after SO2 capture; Z = fly ash quantity (tonne) used for SO2 capture.
8
10
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2.4 Scenario description
$
× 100wt%
(3)
Three scenarios are developed to compare the effect of SO2 reduction using fly
ash/bottom ash on the existing WFGD system as follows:
12
(a) Base case (existing WFGD system)
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The existing WFGD process in the Mae Moh power plant is considered as the base
15
case. The other scenarios present two combinations of integrating SO2 reduction using fly
16
ash/bottom ash into the existing WFGD system. The average operating parameters of the
17
existing WFGD system are listed in the supplementary material (Table S7).
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(b) Scenario 1 (WFGD + SO2 reduction using 100% bottom ash)
In scenario 1, SO2 reduction using the entire annual bottom ash quantity in the Mae
21
Moh power plant is conducted before WFGD system. Scenario 1 also corresponds to the case
22
of total fly ash used in commercial applications.
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2
(c) Scenario 2 (WFGD + SO2 reduction using 100% bottom ash + 20% fly ash)
In scenario 2, a maximum portion of 20% from the annual fly ash generation in the
3
Mae Moh power plant is considered as unused fly ash with no commercial value. The unused
4
fly ash is also utilized for SO2 reduction in addition to bottom ash.
3. Results and discussion
7
3.1 Experimental results
8
3.1.1 Chemical composition of fly ash and bottom ash
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Table 2 lists the results for average compositions of fly ash and bottom ash from the
units 8-13 in the Mae Moh power plant from 01st-30th April, 2017. Results show that both fly
11
ash and bottom ash have similar composition of the major components, i.e., SiO2, Al2O3,
12
CaO, Fe2O3, etc. These chemical compounds in fly ash and bottom ash can provide the
13
thermal stability and sorbent properties (surface area, pore volume, etc.) in a wide
14
temperature range. Hence, both fly ash and bottom ash can be used as a solid sorbent to
15
capture SO2 by adsorption and reaction in different conditions. Fly ash and bottom ash from
16
Mae Moh power plant also have considerable free lime contents, which are close to each
17
other. Thus, free lime in fly ash and bottom ash can react with a fair amount of SO2 in the
18
flue gas. The average values, including the variation of composition were calculated from the
19
annual (01st January-31st December, 2017) composition data of fly ash and bottom ash from
20
all units in the Mae Moh power plant (EGAT, 2018). The annual average values and
21
variations (Table S8 in the supplementary material) were used in the simulation of the SO2
22
capture process and inventory calculations for the LCA analysis.
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Table 2 (Page 42)
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3.1.2 Effect of free lime/gypsum in fly ash for SO2 adsorption
Table 3 shows the SO2 capture capacities by adsorption at 30ºC, zero RH using fly
ash sorbents mixed with different ratios of free lime and gypsum. The average adsorption
4
capacities were evaluated using the GC analysis. The results indicate that SO2 captured by
5
adsorption slightly increases by adding 1 wt% gypsum in the sorbent, compared to the fresh
6
fly ash (1.71 wt% free lime). In the industrial situation, FGD gypsum can be mixed with fly
7
ash in the disposal sites of coal power plants (Jayaranjan et al., 2014). The experimental
8
results show that mixing fly ash with gypsum have a positive effect on SO2 capture by
9
adsorption. Many studies also confirm that the presence of CaSO4/gypsum promotes the SO2
10
reaction of fly ash/bottom ash sorbents (Fernandez et al., 2001; Lee et al., 2005c). Addition of
11
both free lime and CaSO4 also improves the SO2 adsorption capacity from 7.37 to 8.02 µmol
12
SO2/g sorbent. Nevertheless, the results imply that the fly ash sorbents have inconsiderable
13
SO2 adsorption capacities even at 30ºC. Thus, the calcium utilization rates, i.e., % free lime
14
reacted with SO2, using fresh fly ash/bottom ash sorbents are evaluated from the experiments
15
at different residence times in the reactor: 0.5 min, 1 min, and 30 min.
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Table 3 (Page 43)
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3.1.3 Effect of residence time for SO2 capture by reaction
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Table 4 shows the calcium utilization rates using fresh fly ash and bottom ash at 30ºC,
20
zero RH. The results reveal that almost 99.9% of the free lime in both fly ash and bottom ash
21
can react with SO2 within a long residence time of 30 min. However, a calcium utilization
22
rate of 62-76% (in batch mode) even in dry gas conditions can be achieved within a practical
23
residence time of 0.5-1 min. The results are comparable with the calcium utilization rates of
24
fly ash sorbents reported in other studies (Table S6). Thus, a reactor with 1 min residence
25
time in real flue gas conditions (RH > 50%) was considered for the industrial scale SO2
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capture unit using fly ash/bottom ash. In process simulations, the reactor was attached before
2
the existing wet limestone absorption towers as shown in the supplementary material (Figure
3
S1). Further experiments were conducted to study the temperature effect on the SO2 capture
4
using fly ash under the least efficient conditions, i.e., zero RH of gas, no presence of other
5
gases, and continuous gas flow with a low residence time of 0.25 min.
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Table 4 (Page 44)
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3.1.4 SO2 capture using fly ash at 50ºC-100ºC
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Table 5 reports the SO2 capture capacities of fly ash (with 5 wt% free lime) by
adsorption and reaction at 50ºC-100ºC, zero RH from the TPD-MS and EDTA titration
11
experiments, respectively. The results indicate that the temperature affects both SO2
12
adsorption and reaction. The total capture capacity at 100ºC is higher than that at 50ºC.
13
Adsorption at 50ºC-100ºC is insignificant, and it is not a desirable way to capture a toxic gas
14
like SO2. However, the temperature increase from 50ºC to 100ºC promotes the reaction of
15
SO2 with free lime in fly ash, which converts SO2 into an irreversible form. Thus, reaction is
16
the preferable mode to capture SO2 in flue gas at above 100 ºC using free lime enriched fly
17
ash. In the industrial scale, most of the free lime can easily turn into hydrated lime, i.e.,
18
Ca(OH)2, with the excess moisture in fly ash/bottom ash and flue gas. The literature also
19
reports that the reaction between Ca(OH)2 in fly ash and SO2 can promote the calcium
20
utilization rate to form CaSO4 at temperatures below 150ºC (Reaction 4-5) (Li et al., 2007;
21
Liu et al., 2010).
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CaO (g) + H2O (g) ↔ Ca(OH)2 (s)
(4)
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Ca(OH)2 (s) + SO2 (g) + ½O2 (g) → CaSO4 (s) + H2O (g)
(5)
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Table 5 (Page 45)
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3.1.5 SO2 capture of fly ash by reaction at 100ºC-600ºC
Figure 3 indicates the mass spectrometry profiles for TPR-MS experiments over fly
ash (with 5 wt% free lime). The SO2 (m/z = 64) MS profile suddenly drops at 285ºC and
4
returns to the original position at around 435ºC. The drop-off in the SO2 MS profile indicates
5
that SO2 can be effectively captured by reaction using fly ash at 285ºC-435ºC. The average
6
SO2 capture capacity by reaction during this temperature range is equivalent to 572.8 ± 11.2
7
µmol/g sorbent. The highest calcium utilization rate of 67.3% occurs approximately at 400ºC
8
with a continuous flow of dry SO2 gas. The MS profile of H2O vapour (m/z = 18) indicates
9
two peaks at 100ºC and 400ºC. Initially, the moisture in fly ash evaporates at 100ºC.
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Secondly, H2O vapour releases corresponding to the SO2 capture at around 400ºC. The water
11
vapour emission at 400ºC corresponds to the H2O vapour release by Ca(OH)2 dehydration
12
(Reaction 4) by forming free lime. Thus, reaction with SO2 (reaction 5) boosts up at 400ºC
13
and form CaSO4 as the product.
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Figure 3 (Page 36)
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Figure 4 exhibits the XRD results of three fly ash samples after SO2 capture by
reaction: at 30ºC, at 330ºC, and after TPR-MS. The diffractograms of fly ash samples are
17
compared. The CaSO4 phase (JCPDS No: 37-1496) and Ca(OH)2 phase (JCPDS No: 44-
18
1481) were observed in the XRD patterns of the three fly ash samples. The XRD results show
19
that CaSO4 (anhydrite) is the only sulfation product detected in the post-capture fly ash
20
samples. The XRD peaks of CaSO4 increased after the TPR-MS process, which also confirms
21
the significant SO2 capture by reaction at 285-435ºC. The peaks of Ca(OH)2 disappear in the
22
XRD pattern of fly ash after the TPR-MS process, which elucidates consumption of all
23
Ca(OH)2 at 285-435ºC corresponding to the observation of water vapour release. Previous
24
studies have also reported that Ca(OH)2 in fly ash boosts up the reaction between free lime
25
and SO2 at around 400ºC in dry gas conditions and below 150ºC in humidified gas conditions
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(Lee et al., 2005b; Li et al., 2007). Thus, the XRD results confirm that fly ash from the Mae
2
Moh coal fired power plant can capture SO2 by sulfation reaction, forming CaSO4 (anhydrite)
3
as the product, even under the least efficient conditions. The industrial feasibility of using
4
both fly ash and bottom ash to lower the load of the existing desulfurization system in the
5
Mae Moh power plant is discussed through the LCA results.
6
Figure 4 (Page 37)
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3.2 Resources utilization
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Table 6 summarizes the major material/resource flows of the three FGD scenarios.
The results show that the process 3: desulfurization operation consumes a higher amount of
11
material/resources, compared to the process 1: production of desulfurizer and process 2:
12
transportation in all scenarios. In the base case, the existing WFGD system handles an
13
average SO2 load of 758 ktonne/year with 97% desulfurization efficiency. The results
14
indicate that introducing total bottom ash (1,375 ktonne) for SO2 reduction can lower the SO2
15
load by approximately 3.7% in scenario 1. Addition of unused fly ash (405 ktonne) in
16
scenario 2 can further reduce the SO2 load by 4.7%. As a result, the net desulfurization
17
efficiency of the desulfurization system can be improved by 0.1-0.2% with a lower SO2 outlet
18
by 3.7-4.7% into the atmosphere, compared to the base case. Hence, the use of fly ash/bottom
19
ash for SO2 reduction can lower all the resources inputs, including limestone, electricity, heat,
20
and fresh water for the existing WFGD system. Integrating a SO2 capture unit before the
21
existing WFGD system also brings down the by-product/waste generation, such as gypsum,
22
wastewater, and direct CO2. The comprehensive LCA analysis is conducted on energy
23
consumption (3.3), environmental impacts (3.4), operating cost (3.5), and sensitivity analysis
24
(3.6) to understand the effects of material/resources savings in scenarios 1 and 2, relative to
25
the base case. Table 6 (Page 46)
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3.3 Energy consumption
Figure 5 shows the annual energy consumptions (GJ) of the three FGD scenarios. The
3
shares of energy consumption by the individual fuels/sub-processes were calculated using the
4
heating values and energy factors in the supplementary material (Table S9). The annual
5
energy consumption of 2,171,707 GJ/year in the base case comprises 83.9% from electricity
6
for WFGD operation, 11.2% from diesel for limestone mining/transportation, 2.5% from heat
7
for WFGD operation, and 0.7% each from grid electricity and ANFO production for
8
limestone mining. The results show that the annual energy consumption can be lowered to
9
2,088,871 GJ/year in scenario 1 and 2,066,718 GJ/year in scenario 2. The annual energy
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savings are 82,836 GJ/year in scenario 1 and 104,989 GJ/year in scenario 2, compared to the
11
base case. The unit energy consumption is 129.3 GJ/GWh in the existing WFGD operation,
12
and 123.0-124.3 GJ/GWh in the proposed two scenarios (average electricity production:
13
16,800 GWh/year). Thus, introducing fly ash/bottom ash for SO2 reduction could improve the
14
unit energy efficiency of the existing WFGD system approximately by 3.8-4.8%.
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Figure 5 (Page 38)
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A comprehensive environmental impact assessment was performed using the ReCiPe
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midpoint (H) V1.12 method that includes uncertainty analysis, environmental hotspots
20
analysis, and scenario based impact comparison. The environmental impact scores of the
21
selected environmental impact categories with corresponding uncertainty values (GSD2) and
22
the probability comparison among the three FGD scenarios were evaluated using the Monte
23
Carlo simulations (Table S10 and Table S11 in the supplementary material). The results at
24
95% confidence interval show very low GSD2 values around 1.1 for all environmental impact
25
results, except a value around 1.3 for marine eutrophication and fossil depletion. The major
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reason for the low uncertainty values is that this study used the real plant data with
2
fluctuations from the same site measured for the most recent five years. The low uncertainty
3
results also indicate that the inventory data used in this study from published literature and
4
foreign databases have an insignificant effect on the overall LCA results. The probabilities
5
for the base case to have higher environmental impacts than the scenario 1 are within a range
6
of 75-85%, except 63% for fossil depletion only (Table S11). The corresponding probabilities
7
for base case ≥ Scenario 2 increase up to 80-95% and 69% for fossil depletion. Thus,
8
introducing fly ash for SO2 reduction in addition to bottom ash in this study have a higher
9
probability to lower the environmental impacts of the existing WFGD system (base case).
10
However, the probability range for Scenario 1 ≥ Scenario 2 is 53-73%, which implies the
11
extent of fly ash utilization in scenario 2, i.e., 20 wt% of total fly ash, is required to be further
12
increased in order to have probability values close to 100%.
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The shares of environmental impacts from the major sub-processes were evaluated for
hotspots identification in the individual scenarios (Figure S2 in the supplementary material).
15
For the base case, direct emissions, such as CO2 from the reaction with limestone and the
16
unreacted SO2 after the WFGD operation are the main contributors for climate change,
17
terrestrial acidification, and particulate matter formation impacts. Process energy
18
consumption (electricity/steam from lignite coal burning) leads the contributions for marine
19
eutrophication and fossil depletion impacts. For photochemical oxidant formation, both direct
20
emissions and process energy consumption share approximately equal contributions. LCA
21
studies on other WFGD process have also reported electricity consumption and direct
22
emissions as the key environmental hotspots (Feng et al., 2014; Shi et al., 2017). Wastewater
23
emissions from the WFGD operation and the limestone mining process are the major hotspots
24
for human toxicity and water depletion impacts, respectively. The results show similar
25
percentage contributions from the environmental hotspots in the two scenarios, compared to
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the base case. The major reason for similar patterns of hotspots in the scenario 1 and 2 is that
2
electricity/steam consumption and direct emissions from the WFGD operation dominates the
3
contribution to environmental hotspots, compared to the SO2 capture process using fly
4
ash/bottom ash. The environmental impact variations among the three FGD scenarios can
5
also be correlated to the related environmental emissions (Table S12 in the supplementary
6
material). The SO2 capture process using fly ash/bottom ash includes very low amounts of
7
environmental emissions. As a result, the environmental emissions are nearly proportional to
8
the SO2 load in each scenario. Hence, the extents of SO2 reduction using fly ash/bottom ash
9
in this study can lower the environmental impact scores rather than alter the key
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environmental hotspots.
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Figure 6 shows the relative environmental impacts for the three FGD scenarios, i.e.,
percentages of division-by-maximum from the absolute impact scores. The results exhibit
13
that the existing WFGD system in the base case contributes the highest (100%)
14
environmental impacts in all categories. Utilization of bottom ash in scenario 1 and bottom
15
ash/fly ash in scenario 2 for SO2 capture can lower all the environmental impacts. Climate
16
change, terrestrial acidification, human toxicity, and particulate matter formation are reduced
17
approximately by 3.6% and 4.7% as well as marine eutrophication and photochemical
18
oxidant formation are decreased by 3.5% and 4.6% in scenario 1 and 2, respectively. Water
19
depletion is lowered by 3.7% in scenario 1 and by 5.7% in scenario 2, compared to the base
20
case. In addition, fossil depletion is reduced by 3.1% and 4.1% in scenario 1 and 2,
21
respectively. The overall environmental impact assessment shows that integration of SO2
22
reduction using fly ash/bottom ash can make the existing WFGD system more
23
environmentally-benign.
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Figure 6 (Page 39)
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3.5 Operating cost analysis
The operating costs of the three FGD scenarios are calculated to compare the
3
economic perspective of integrating SO2 reduction using fly ash/bottom ash with the existing
4
WFGD system. The total operating cost comprises the limestone purchasing cost (200
5
THB/tonne), diesel cost for transportation (30 THB/L), and cost for utilities in FGD
6
operation: electricity (1.5 THB/kWh), fresh water (10 THB/m3), and heat (350 THB/tonne).
7
The unit costs were estimated, based on average prices in Thailand. Figure 7 shows the
8
annual operating costs in million US dollars (1 USD = 32 THB) for the three FGD scenarios.
9
The base case incurs the greatest annual operating cost of approximately 37.33 million
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USD/year. Electricity generating cost for the FGD operation is the main contributor, with
11
about 63.6% of the total operating cost in the base case. The limestone cost, and diesel cost
12
for transportations shares approximately 22.1% and 10.3% of the total operating cost,
13
respectively. The SO2 reduction using fly ash/bottom ash can save the annual operating cost
14
of the existing WFGD system approximately by 1.28 million USD/year in scenario 1 and
15
1.66 million USD/year in scenario 2, excluding the initial investment cost of the SO2 capture
16
units. The percentage cost savings are 3.1% and 4.5% in scenario 1 and 2, respectively.
17
Therefore, integration of SO2 reduction using fly ash/bottom ash into the existing WFGD
18
system is cost-effective in addition to the resources/energy and environmental impacts
19
savings.
20
Figure 7 (Page 40)
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3.6 Sensitivity analysis
Variations of parameter settings can influence the inventory data and LCA results of
24
the FGD scenarios. The annual variation ranges of the sensitivity parameters in this study
25
were evaluated based on the real plant data from the existing desulfurization process. Five
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parameters with high variations were selected to perform the sensitivity analysis. The
2
selected parameters are, the sulfur content in coal (1.2-4.0 wt%), the annual SO2 inlet in flue
3
gas (586.3-838.9 ktonne/year), annual electricity consumption of the existing WFGD system
4
(480.0-532.4 GWh/year), calcium utilization rate for SO2 capture by reaction using fly
5
ash/bottom ash (62-94% mol SO2/mol free lime), and free lime content in fly ash/bottom ash
6
(1.2-3.0 wt%). The sensitivity parameter ranges in each FGD scenario were defined in the
7
SimaPro LCA software and decreased/increased for the sensitivity analysis, accordingly.
8
Table 7 lists the sensitivity results of net energy consumption (GJ/GWh), operating cost
9
(USD/GWh), and major air pollution impacts, such as climate change (kg CO2 eq/GWh),
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terrestrial acidification (kg SO2 eq/GWh), photochemical oxidant formation (kg
11
NMVOC/GWh), particulate matter formation (kg PM10 eq/GWh), and fossil depletion (kg
12
oil eq/GWh). All sensitivity results were normalized with respect to a unit basis of 1 GWh
13
average electricity generation from the Mae Moh power plant to compare with related studies
14
in the literature.
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Table 7 (Page 47)
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The results in Table 7 show that the sulfur content in coal has the highest sensitivity.
The sulfur content in coal is affected by the type of coal used, coal pretreatment practices,
18
and geological/climate conditions of coal mining. At a sulfur content of 1.2 wt% in coal, the
19
improvements in terms of the energy consumption, environmental impacts, and operating
20
cost in the combined FGD process (scenario 1 and 2) increases up to 8-11%, compared with
21
the existing WFGD process. The degree of improvements in scenario 1 and 2 approximately
22
drops to 3% when the sulfur content in coal is 4.0 wt%. A similar relationship can be
23
observed in the sensitivity results by the variation in the annual SO2 inlet in flue gas, which
24
has the second highest sensitivity. In this study, the annual SO2 inlet can vary due to changes
25
in the annual operating hours, coal consumption, and combustion efficiencies in the power
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plant. Decreasing the annual SO2 inlet to 586.3 ktonne/year improves the savings on the
2
energy consumption, environmental impacts, and operating cost by around 5.0% in scenario 1
3
and by 6.0% in scenario 2, compared to the base case. Thus, the benefits of SO2 reduction
4
using fly ash/bottom ash improve depending on the sulfur content in coal and the annual SO2
5
amount in the fresh flue gas from the Mae Moh power plant. Consequently, the approach can
6
be more desirable for a coal fired power plant that burns coal with a low sulfur content or pre-
7
combustion sulfur removal.
The LCA results revealed that electricity consumption for WFGD operation is one of
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the major energy/environmental hotspots. Nonetheless, the corresponding sensitivity results
10
indicate considerable variations only in the net energy consumption, climate change, fossil
11
depletion, and operating cost. The level of SO2 reduction can also vary in scenario 1 and 2
12
due to the free lime content in fly ash/bottom ash. The results show that the variations in the
13
free lime content also has a significant sensitivity. Increasing the free lime content to 3.0 wt%
14
can improve the degree of lowering the net energy consumption (4.7-6.2%), environmental
15
impacts (5.3-6.8%), and operating cost (5.1-6.6%) in scenarios 1 and 2, compared to the base
16
case. The variations in the calcium utilization rate of SO2 capture by reaction occur due to
17
many conditions, i.e., reaction temperature, relative humidity of the flue gas, residence time,
18
etc. However, the results show that the variations in the calcium utilization rate has an
19
insignificant sensitivity for LCA results.
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The overall LCA results denote that the possible degree of improvements by SO2
21
reduction using fly ash/bottom ash can fluctuate within 3-11% subject to the sensitivity
22
parameter variations, and the average improvement is around 5%. The key reason for these
23
results is that this study considered only the estimated unused fly ash/bottom ash quantities
24
remaining as a waste in the Mae Moh power plant with a natural free lime content of 1.2-3.0
25
wt%. If the total fly ash quantity that goes for commercial applications is utilized for SO2
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reduction, an allocation rule would be required for fly ash generation from the coal
2
combustion process, and it would contribute significant energy consumption/emissions. Even
3
though it can lead to a greater SO2 reduction, mixing of commercial CaO with fly ash/bottom
4
ash is also not desirable due to additional CO2 emissions and energy consumption by CaO
5
production from limestone. Hence, the extent of SO2 reduction is limited by the availability
6
of unused fly ash/bottom ash and their natural free lime content. Hence, mixing fly
7
ash/bottom ash with other free lime enriched waste materials can be recommended to
8
improve the performance of SO2 reduction. Just to verify the limitation by the availability of
9
unused fly ash/bottom ash, an additional scenario was evaluated with 100% utilization of fly
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ash/bottom ash. For this scenario, the total fly ash quantity was considered as unused in
11
commercial applications (Table S13 and Figure S3 in the supplementary material). The
12
results show that the utilization of total fly ash/bottom ash quantities can increase the average
13
improvements by SO2 reduction from 5% to 10-11%. The probabilities for base case ≥
14
scenario also reach almost 100% that confirm the possibility of significant SO2 reduction.
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The major LCA results in this study were converted into the unit bases of 1 GWh
electricity generation, 1 tonne SO2 removal, and 1 Nm3 flue gas treatment in order to
17
compare with other related studies (Table S14 in the supplementary material). The results are
18
comparable with some studies. The net energy consumption of the existing WFGD system in
19
the base case is close to that in (Feng et al., 2014), lower than that in (Shi et al., 2017), and
20
higher than that in (Wu et al., 2017). The two scenarios in this study with SO2 reduction using
21
fly ash/bottom ash lowers the net energy consumption, compared to the WFGD systems in
22
(Feng et al., 2014) and (Shi et al., 2017). However, the environmental impacts, such as
23
climate change and terrestrial acidification for all scenarios in this study are higher than those
24
in other studies. The main reason for higher environmental impacts is the use of high sulfur
25
lignite coal and high SO2 concentration in the flue gas from the Mae Moh power plant. In
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addition, other LCA studies on WFGD systems have followed different impact assessment
2
methods, functional units, and parameter settings that create limitations to have an in-depth
3
comparison with the LCA results in this study.
4
The free lime contained in fly ash is converted to CaSO4 after SO2 capture by
reaction. The amount of CaSO4 available in post SO2 capture fly ash in scenario 2 is indicated
6
in Table 6. The equivalent total SO3 content in post SO2 capture fly ash was calculated
7
(Equation 3) considering the annual variations of the initial SO3 content in fresh fly ash (3.02
8
± 1.09 wt%), free lime content (2.07 ± 0.92 wt%), and the calcium utilization rate (62-94
9
wt%). The results show that post SO2 capture fly ash has an average SO3 content of 5.5 ± 2.6
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5
wt%. The post SO2 capture fly ash is not suitable for the use in common construction
11
applications when the average SO3 content > 5.0 wt% according to the ASTM C618 and TIS
12
2135 standards. However, the post SO2 capture fly ash with high SO3 content > 5.0 wt%
13
could be used to improve expansion/shrinkage of expansive concrete and reduce expansive
14
additive usage as tested in (Chatchawan et al., 2017). Therefore, the approach of high free
15
lime fly ash utilization for SO2 reduction can create future research on further applications of
16
post SO2 capture fly ash.
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Conclusion
This study shows that the utilization of fly ash/bottom ash for SO2 reduction provides
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an environmentally-benign approach to lower the SO2 load of an existing WFGD system in a
4
coal fired power plant. The experimental results confirm that fresh fly ash/bottom ash with
5
free lime have a fair capability of SO2 capture by reaction. The comprehensive LCA analysis
6
recommends SO2 reduction in the untreated flue gas using waste fly ash/bottom ash to save
7
the resources/energy, environmental impacts, and operating cost of an existing WFGD
8
system. The variation of the sulfur content in coal and annual SO2 inlet due to the operating
9
factors hold the highest sensitivity for the improvements of an existing WFGD system. The
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extent of SO2 reduction is limited by the sulfur content in coal and the quantity of fly
11
ash/bottom ash available for SO2 capture. The approach is more desirable for coal fired
12
power plants using low sulfur coal and having a low annual SO2 inlet with a high availability
13
of free lime contained fly ash/bottom ash or the ashes mixed with free lime enriched waste
14
materials. Post SO2 capture fly ash could be further used in specific construction applications
15
depending on the total SO3 content. Thus, the study provides a sustainable method to utilize
16
the accumulated fly ash/bottom ash in coal fired power plants for the improvement of their
17
existing WFGD units.
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Acknowledgement
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This study was supported by the Electricity Generating Authority of Thailand (EGAT),
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Center of Excellence in Material Science, Construction, and Maintenance Technology, and
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the Scholarship for Excellent Foreign Students (EFS) in Sirindhorn International Institute of
23
Technology, Thammasat University.
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Figure 1: Experimental setup for SO2 capture
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Figure 2: System boundary for LCA analysis
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Figure 3: Mass spectrometry profiles of SO2 (m/z = 64.0) and H2O (m/z = 18.0) for (TPR-
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Figure 4: XRD patterns of fly ash samples after SO2 capture by reaction:
a. at 30ºC, b. at 330ºC, c. After TPR-MS
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▼: CaSO4 (Anhydrite), □: Ca(OH)2 (Portlandite)
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Figure 5: Annual energy consumption of FGD scenarios
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Figure 6: Relative environmental impacts of FGD scenarios
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Figure 7: Annual operating cost of FGD scenarios
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Table 1: Average operating data of Mae Moh power plant
Description
Unit
Average value
Variation range
Mae Moh coal fired power plant
15,120-18,480
GWh/year
16,800
Mtonne/year
16
15-17
wt%
2.8
1.2-4.0
ppm
4,657
ppm
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Annual fly ash generation
ktonne/year
2,025
Annual bottom ash generation
ktonne/year
1,375
Annual lignite coal consumption
Sulfur content in lignite coal
SO2 concentration in flue gas (before FGD)
SO2 concentration in flue gas (After FGD)
a
1,996-6,653
69-255
1,875-2,175
1,279-1,471
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Flue gas concentration: Standard state (101.3 kPa, 25 ºC), dry basis, 7 vol% O2
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Annual power generation
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Table 2: Major chemical composition of fly ash and bottom ash from Mae Moh power plant
Fly asha
Bottom asha
SiO2
34.22 ± 1.34
34.49 ± 1.43
Al2O3
17.90 ± 0.88
17.74 ± 0.40
Fe2O3
13.90 ± 0.81
14.24 ± 0.29
CaO
22.59 ± 1.69
21.97 ± 0.18
MgO
2.77 ± 0.16
1.90 ± 0.17
SO3
2.18 ± 0.71
1.71 ± 0.43
Free lime
1.91 ± 0.58
1.42 ± 0.92
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Test data from units 8-13 (6×300 MW) in Mae Moh power plant on 01st-30th April 2017 (EGAT, 2018)
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Table 3: Effect of free lime/gypsum for SO2 adsorption using fly ash at 30ºC
Composition of fly ash sorbent (%wt)
Average SO2 adsorption Capacity
CaSO4.2H2O
(µmol SO2/g sorbent)
1.71
-
7.23 ± 0.25
1.71
1
7.37 ± 0.34
5
-
7.55 ± 0.31
5
1
8.02 ± 0.28
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Free lime
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Table 4: Effect of residence time for SO2 capture by reaction at 30ºC
Calcium utilization rate
Sorbent
Residence time (min)
0.5
62.0 ± 1.8
1
76.0 ± 0.9
30
99.7 ± 0.2
1
73.2 ± 1.3
30
99.5 ± 0.3
Fresh fly ash
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Fresh bottom ash
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(% mol SO2 reacted/ mol free lime in sorbent)
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Table 5: Average SO2 capture capacities of fly ash at 50ºC-100ºC
SO2 capture capacity
Total
by adsorption
by reaction
SO2 capture capacity
(µmol SO2/g sorbent)
(µmol SO2/g sorbent)
(µmol SO2/g sorbent)
50
4.1 ± 0.7
45.2 ± 10.2
49.3 ± 10.9
100
3.5 ± 0.7
65.7 ± 17.4
Temperature (ºC)
69.2 ± 18.1
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*Residence time: 0.25 min
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SO2 capture capacity
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Table 6: Annual resource utilization of FGD scenarios
Material/resource
Unit
Base case
Scenario 1
Scenario 2
1. Production of desulfurizer
1.41
1.36
1.34
GWh
3.93
3.79
3.75
ktonne
0.37
0.35
0.35
Diesel
ML
4.08
Fuel oil
kL
3.80
ktonne
757.74
Grid electricity
Blasting agent (ANFO)
2. Transportation
3. FGD operation
SO2 inlet (WFGD system)
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4.01
3.99
3.66
3.62
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Diesel
730.07
721.91
ktonne
0.00
Limestone
ktonne
1,318.23
27.68
35.83
1,270.08
1,255.90
Unused fly ash (FA)
ktonne
0.00
0.00
405.00
Bottom ash (BA)
ktonne
0.00
1,375.00
1,375.00
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SO2 captured by fly ash/bottom ash
3
Mm
3.98
3.83
3.79
ktonne
27.04
26.05
25.76
Electricity (WFGD system)
GWh
506.18
487.69
482.36
Electricity (SO2 capture process)
MWh
0.00
92.28
119.46
ktonne
2,154.25
2,075.57
2,052.40
3
0.66
0.64
0.63
ktonne
550.54
530.43
524.51
Fresh water
Steam
Gypsum
Wastewater
Direct CO2
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Byproducts/wastes
Mm
ktonne
0.00
1,375.00
1,780.00
CaSO4 in post capture fly ash
ktonne
0.00
0.00
17.32
22.73
21.90
21.66
ktonne
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SO2 outlet
EP
(FA + BA) after SO2 capture
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Table 7: Sensitivity analysis of FGD scenarios
Varied parameter
kg CO2 eq/GWh
Terrestrial
acidification
kg SO2 eq/GWh
Photochemical
oxidant formation
kg NMVOC/GWh
Particulate matter
formation
kg PM10 eq/GWh
Fossil depletion
kg oil eq/GWh
Operating cost
million USD/GWh
Free lime
content
(wt%)
1.2-4.0
586.3-838.9
480.0-532.4
62-94
1.2-3.0
124-135
119-130
118-129
62,139-65,199
59,882-62,830
59,218-62,133
1,412-1,418
1,360-1,366
1,345-1,351
217-227
209-219
207-216
296-298
285-287
282-284
129
124-126
123-125
63,669
61,110-61,986
60,356-61,490
1,415
1,358-1377
1,341-1,366
222
213-216
210-214
297
285-289
281-287
129
123-127
121-126
63,669
60,323-62,390
59,337-62,013
1,415
1,340-1,386
1,318-1,378
222
210-217
207-216
297
281-291
276-289
3,865-4,121
3,746-3,992
3,710-3,954
2,149-2,295
2,075-2,216
2,053-2,193
3,993
3,853-3,908
3,812-3,883
2,222
2,137-2,168
2,112-2,152
3,993
3,804-3,933
3,748-3,916
2,222
2,110-2,179
2,076-2,170
129
125
124
63,669
61,357
60,675
1,415
1,363
1,348
222
214
212
297
286
283
55-185
51-180
50-179
27,287-90,956
24,974-88,643
24,293-87,962
606-2,021
555-1,969
540-1,954
95-317
87-309
85-307
127-424
116-413
113-410
97-143
92-139
91-137
47,752-70,491
45,439-68,178
44,758-67,497
1,061-1,566
1,009-1,515
994-1,500
166-246
158-238
156-235
222-328
212-318
209-314
Base case
Scenario 1
Scenario 2
Base case
Scenario 1
Scenario 2
3,993
3,869
3,832
2,222
2,145
2,123
1,711-5,704
1,587-5,580
1,550-5,543
952-3,175
876-3,098
853-3,075
2,995-4,421
2,870-4,296
2,834-4,260
1,667-2,460
1,590-2,384
1,568-2,361
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Base case
Scenario 1
Scenario 2
Base case
Scenario 1
Scenario 2
Base case
Scenario 1
Scenario 2
Base case
Scenario 1
Scenario 2
Base case
Scenario 1
Scenario 2
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Calcium utilization rate
(% mol SO2/mol free lime)
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Climate change
Electricity
consumption
(GWh/year)
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GJ/GWh
Scenario
SO2 inlet in flue gas
(ktonne/year)
EP
Net energy
consumption
Units
Sulfur content
in coal
(wt%)
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Indicator
Initial
value
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Highlights of the manuscript:
Coal fly ash and bottom ash utilization for SO2 reduction from coal power plants.
•
Comprehensive LCA integrates experiments, process simulation, and real plant data.
•
Case of an existing WFGD system for a scenario-based feasibility assessment.
•
Reduction of energy consumption by 4.0-5.0% and environmental impacts by 3.0-5.0%.
•
Possible applications for post SO2 capture fly ash based on the SO3 content.
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