A feasibility study of municipal solid waste incineration fly ash utilisation in Estonia

Abstract

The purpose of this paper is to discuss the alternative environmental management options for the utilisation of municipal solid waste (MSW) incineration fly ash (FA), which is generated at Iru Power Plant where MSW is incinerated in Estonia. To determine sustainable and economically feasible environmental management options for MSW incineration FA in Estonia, CO₂ sequestration with a further carbonation process was examined. A partial Cost & Benefit Analysis has been conducted to compare the carbonation process to the current situation. Two carbonation options were developed. Option 1 is to use carbonated FA in any other processes based on the waste-to-product principle. Option 2 is to send carbonated FA to the non-hazardous landfill in Tallinn, Estonia. Important parameters, such as Net Present Value (NPV), Internal Rate of Return (IRR), Benefit–Cost Ratio (BCR) and Break Even Point (BEP), have been calculated for carbonation options and the current case. In addition, a sensitivity analysis has been conducted to examine its robustness. The results showed that the best option is carbonation Option 1 with NPV of 9,209,662 EUR, IRR of 43%, BCR of 2.63 and BEP between 2018 and 2019. Both Options 1 and 2 constitute more sustainable and environmentally friendly management options compared to the current situation. It can be concluded that this preliminary feasibility study showed that running a carbonation plant may be profitable and sustainable for Estonia. Currently, there is no treatment technology for MSW incineration FA in Estonia and FA is sent to a neighbouring country for further utilisation. This is the first study to demonstrate FA management options with economic and environmental benefits.

Keywords

Municipal solid waste incineration Cost & Benefit Analysis fly ash utilisation

Introduction

The increasing amount of municipal solid waste (MSW) is a worldwide environmental concern. Traditionally, landfilling had been the most preferred option for MSW management in Eastern Europe. In recent times, however, MSW incineration has been extensively used as an alternative to landfilling. Incineration can drastically reduce the mass and volume of the MSW with a remarkable energy recovery.

In Estonia, there is one MSW incineration plant, which is located in Iru village, Maardu. MSW is mainly treated in this incineration plant in addition to the remnants of landfill disposal after recycling and reuse. In 2013, Eesti Energia AS began to operate an up-to-date waste to energy (WtE) unit to produce heat and electricity by means of MSW incineration at Iru Power Plant. The Iru unit can convert 85% of the energy in waste into electricity and heat. Also, this new unit has an annual capacity of 220,000 tonnes of waste incineration. It allows Estonia to use up to 70 million m³ less natural gas every year (Eesti Energia AS, 2015). Due to the Iru incineration plant, landfilling on a large scale has ended in Estonia.

The final residues from waste incineration include boiler ash (particulate matter carried over from the furnace and removed from the flue gas without the injection of sorbents) and Air Pollution Control (APC) residues, aside from bottom ash (BA) (Alba et al., 2016). Boiler ash and APC residues are generally taken together as an output from MSW incineration plants. Therefore, the mix of boiler ash and APC residues is termed as fly ash (FA) in this study. However, FA as a solid residue after the MSW incineration process is classified as a hazardous waste under code 19 01 07 in the European Waste Catalogue (ECD, 2000) and needs to be appropriately treated. The ultimate difficulty of MSW incineration FA treatment is that it usually includes metal oxides (CaO, SiO₂ MgO, Al₂O₃, etc.), water-soluble salts (NaCl and KCl), heavy metals (Cd, Zn, Pb, Hg, Cu, Cr and Ni) and organic pollutants (polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs)). Anna Bogush et al. (2015) indicated that hazardous pollutants, especially Zn (0.098–0.73 wt.%) and Pb (0.05–0.2 wt.%), as well as As, Cd, Cu, Mo, Sb, Sn and Se, are enriched in all the United Kingdom WtE APC residues in addition to Ca-bearing phases, such as calcite (CaCO₃), anhydrite (CaSO₄), portlandite (Ca(OH)₂), lime (CaO) and soluble salts, such as NaCl and KCl. The presence of lime (CaO) gives FA a high alkalinity, which leads to high leachability feature in landfills. Le Forestier and Libourel (1998) pointed out that the high chlorine content of FA is primarily due to the incineration of plastics, for example, polyvinyl chloride (PVC). Wang et al. (2001) noted that the Cl content in FA should be less than 0.2 wt.% to comply with the requirement of the solidification/stabilisation (S/S) method, for example, carbonation. Chlorine salts are soluble without difficulty on a landfill site where they leaches out a material that leaves a sponge behind. In addition, the leach-out brine is a transport medium for pollutants when the pH is rather alkaline. Finally, heavy metals and organic pollutants need to be treated before spreading into the environment.

These aspects make MSW incineration FA management one of the most important environmental concerns regarding the incineration of MSW. In order to utilise MSW incineration FA and lessen its environmental impact, there are up-to-date treatment processes that include separation processes, S/S processes, thermal methods and some other methods (Quina et al., 2008; Reijnders, 2005). Some of the preferred FA management options seem to be both separation processes with CO₂ sequestration with a further carbonation process and the usage of FA as a raw material in any other industry as a S/S process. On the other hand, sintering or vitrification as a thermal treatment is rather costly. The residue after thermal treatment is even more hazardous than the thermally untreated FA (Ecke, 2003; Jiang et al., 2013; Xiaomin Li et al., 2007). The washing process as a separation process eliminates water-soluble salts and heavy metals (Chimenos et al., 2005; Francois and Criado, 2007; Nzihou and Sharrock, 2002; Zacco et al., 2014). Moreover, Nowak et al. (2012) pointed out that a significant amount of heavy metals and chloride could be removed by using MgCl₂ as a thermal treatment. Furthermore, Lima et al. (2012), Pedersen (2002) and Pedersen et al. (2005) showed that the electrodialytic remediation method could moderately eliminate Pb, Cu, Zn, Cd and Cr. Zacco et al. (2014) drew attention to the fact that heavy metal leachability could be lowered by the incorporation of FA into cementitious mixtures.

In Estonia, there is no current treatment technology for FA generated at Iru Power Plant. FA has been sent to a neighbouring country for further utilisation. However, there have been numerous researches on the utilisation of FA from the oil shale power plants in Estonia. It was reported that the main ash flows from pulverised firing (PF) and the circulating fluidised bed (CFB) could be utilised (Raado et al., 2014; Uibu et al., 2015a, 2015b; Velts et al., 2011, 2013). Apparently, the lack of treatment technology for MSW incineration FA comprises an environmental and economic burden for Iru Power Plant. Therefore, an innovative utilisation process is required as an alternative management option to sending it to a neighbouring country.

This paper studies CO₂ sequestration with a further carbonation process as an alternative management option for MSW incineration FA. In this paper, the main emphasis was on the economical assessment. A partial Cost & Benefit Analysis (CBA) has been conducted to compare the application of the carbonation process with the current situation. Moreover, the environmental benefits of the carbonation process are discussed.

Materials and methods

In this preliminary assessment, the analytical framework of CBA suggested by Boardman et al. (2006) was used. Originally, they proposed a nine-step analytical framework. However, this study used a six-step framework, as described below. This partial CBA was conducted at the discount rate of 0.04 for a 10-year period. For each scenario, the Net Present Value (NPV), Internal Rate of Return (IRR), Benefit–Cost Ratio (BCR) and Break Even Point (BEP) were calculated. As a last step, a sensitivity analysis was performed in order to examine the robustness:

specify the set of alternatives and the base case;

identify benefits and costs;

monetise and quantify costs and benefits;

find out the discount rate, NPV, IRR and BCR;

sensitivity analysis;

final recommendation.

Specification of an alternative and the base case scenario

In the CBA, it is crucial to specify a well-advised number of alternatives against the base case scenario. This is because it may be rather difficult to conduct a detailed evaluation within many courses of action.

In this paper, CO₂ sequestration with a further carbonation process has been chosen as an alternative against the base case scenario.

Identification of benefits and costs for monetisation

The base case scenario

In the WtE unit at Iru Power Plant, there are two types of residues other than BA: these are boiler ash and solid waste residues from APC.

Boiler ash is gathered from a stack placed in the boiler and subsequently delivered into a silo with the capacity of 75 m³. In this silo, it is kept for approximately four days. An approximate annual maximum quantity is 3700 tons. The main elemental analysis showed that the boiler ash includes 31.9% SiO₂, 29.8% CaO, 7.83% Al₂O₃ and 3.79% Fe₂O₃. Due to the existence of lime, it is highly alkaline and the pH was determined as 12.66. Moreover, the main heavy metals were as follows: 19 mg kg⁻¹ of Cd, 240 mg kg⁻¹ of Cr, 350 mg kg⁻¹ of Cu and 600 mg kg⁻¹ of Pb. Also, the chloride content was measured as 28,000 mg kg⁻¹.

A semi-dry management system of combustion gases is used at Iru Power Plant. The APC system consists of a reactor and one baghouse filter. Solid residues are collected from both the reactor and the baghouse filter and delivered to the residue silos with a capacity of 120 m³. The approximate annual maximum quantity is 7200 tons. The main elemental analysis indicated that the APC residue includes 8.52% SiO₂, 43.6% CaO, 2.83% Al₂O₃ and 0.967% Fe₂O₃. The pH of this residue was 12.39. Furthermore, the main heavy metals were as follows: 0.01 mg kg⁻¹ of Cd, 1.1 mg kg⁻¹ of Cr, 1.7 mg kg⁻¹ of Cu and 470 mg kg⁻¹ of Pb. The chloride content was 159,000 mg kg⁻¹.

In this paper, the mixture of boiler ash and solid residues from APC is termed as FA. The FA produced at Iru Power Plant is loaded into the trucks and delivered to a neighbouring country. The only costs for the base case scenario are transportation and treatment costs. In the base case scenario, no benefits could be monetised.

CO₂ sequestration with a further carbonation process

The carbonation process is one of the state-of-the-art stabilisation methods for FA. Several recent studies have investigated the carbonation process in detail (Cappai et al., 2012; Ecke, 2003; Xiaomin Li et al., 2007).

Naturally, the carbonation process occurs as a result of weathering processes where the substance makes contact with atmospheric CO₂. However, the natural carbonation process for FA stabilisation on a practical level is rather slow and is a long-term process. The accelerated carbonation process is a solution for reducing the required time for completion of the carbonation process. Jiang et al. (2013) pointed out that incineration flue gas could be used as CO₂-rich gas for the carbonation process. According to their research, SO₂ emissions inside the flue gas reduce the uptake and sequestration rate of CO₂; as a result, SO₂ blocks the pores of FA and decreases the ability of FA for the sequestration process. In addition, according to Xiaomin Li et al. (2007), the FA could combine with 7–10% w/w of carbon dioxide during the carbonation process. FA has a highly alkaline feature because it includes high concentrations of lime (CaO) and calcium hydroxide (Ca(OH)₂), resulting in a high potential for leaching. Xiaomin Li et al. (2007) showed that the original pH of raw FA reduced from 12–12.5 to around 7–10 thanks to the accelerated carbonation process.

The washing process takes place within the accelerated carbonation process to eliminate soluble chloride, sulphate and fluoride. Many authors have reported that the washing process with water can remove soluble salts and soluble heavy metals (Astrup, 2008; Wang et al., 2010; Xiaomin Li et al., 2007). In a detailed study, Wang et al. (2001) experienced 97% of chloride removal efficiency at the liquid/solid ratio of 10. In addition, Jianga et al. (2009) noted that up to 72.8% of Ca, Na, K and Cl was removed at the 10:1 liquid/solid ratio and approximately 12.3% removal was accomplished for Cr.

Figure 1 shows a block diagram of the carbonation plant. For this plant setup, the goal is to test the process at a technical scale with continuous operation at Iru Power Plant.

The mass balance for the carbonation process is presented in Figure 2. This figure is the base for the calculation of the operational cost in addition to the maintenance cost. In this Dansk Restprodukt Hantering (DRH) carbonation technology, firstly it is ensured that the water and FA are mixed properly. Also soluble salts, heavy metals and other soluble molecules are dissolved. Then, the carbonation reaction occurs in the second place where CaO reacts with CO₂. In this step, the stabilisation of FA occurs. Heavy metals are stuck in CaCO₃ and the alkalinity feature of FA decreases. After rinsing process, the stabilised FA is collected. Moreover, the sludge from the wastewater treatment process is pumped back to the process. The effluent from the wastewater treatment process could be discharged into the municipal sewerage system. The DRH process enables that leaching from treated MSW incineration FA is generally below the European Union (EU) acceptance criteria and could be landfilled in non-hazardous landfills (Astrup, 2008).

To implement this alternative at Iru Power Plant, a carbonation plant should be constructed, although this requires significant investment. During the operation of the plant, some additives and power will be combined with the operation costs. Furthermore, regular maintenance is needed throughout the operation. Also, three employees will be hired to run the plant. Upon implementation of this technology, no treatment or transportation costs will be paid to the neighbouring country. These reductions are a benefit to Iru Power Plant in monetary terms.

After FA is stabilised by the DRH carbonation process, there are two options for managing the stabilised FA. Option 1 is to use it in any other process based on the waste-to-product principle. For instance, treated FA could be used in concrete, cement production, backfilling of oil shale mines, agriculture or asphalt production (Aubert et al., 2006; Saikia et al., 2007; Wang et al., 2010). Moreover, carbonated FA could be used in concrete blocks and as a horticultural growing medium in green roof systems (Bertos et al., 2004). Option 2 is to send the carbonated FA to a non-hazardous landfill near Tallinn city. The costs and benefits for Options 1 and 2 are shown in Table 1. In Option 1, no landfilling cost is imposed. However, further research is required to determine a process that would be economically and technically feasible for carbonated FA at Iru Power Plant. Option 2 involves transportation and disposal costs. Carbonated FA will be transported from Iru Power Plant to the landfill area near Tallinn.

Table 1.

Costs and benefits of Options 1 and 2.

Costs for Option 1	Costs for Option 2	Benefits for Options 1 and 2
Investment cost	Investment cost	Reduction in treatment cost
Operation and maintenance cost	Operation and maintenance Cost	Reduction in transportation cost
Job creation cost	Disposal cost
	Transportation cost (to landfill site)
	Job creation cost

Description of the discount rate, NPV, IRR and BCR

In a CBA, the criterion for decision-making is based on the NPV, IRR and BCR. Each value is compared to those of other courses of action to determine the best option.

The NPV is used to calculate the yearly benefit and cost components into a present value. This evaluation reveals whether the sum of the discounted gain is higher than that of the discounted losses. In a CBA, discounting is used to convert all future costs and benefits into present-day values, where the time value of money is taken into account. It is important to underline that discounting is not the same as inflation. Primarily, the alternative with the highest NPV is preferable. A general formula suggested by Boardman et al. (2006) for the calculation of the NPV is as follows

N P V = - K + \sum_{t = 0}^{n} \frac{B_{t} - C_{t}}{{(1 + p_{x})}^{t}}

(1)

In this equation, K refers to the initial capital investment for the first year; B and C are the benefits and costs in year t, respectively; and p_x denotes the discount rate.

The IRR is a discount rate at which a project’s NPV is equal to zero. If the IRR is greater than the discount rate, the NPV is positive, which demonstrates that the project is economically feasible. The IRR is used to assess the attractiveness of a project. The formula for the IRR, as suggested by Boardman et al. (2006), is given as follows

0 = - K + \sum_{t = 0}^{n} \frac{B_{t} - C_{t}}{{(1 + π)}^{t}}

(2)

Here, π represents the IRR and the other symbols are as described as in the NPV formula.

The BCR is a ratio that should be larger than or equal to zero, which fundamentally means that the NPV of benefits is superior to that of costs to gain a profit. The general formula, as suggested by Boardman et al. (2006), is as follows

B C R = \frac{\sum_{t = 0}^{n} \frac{B_{t}}{{(1 + p_{x})}^{t}}}{\sum_{t = 0}^{n} \frac{C_{t}}{{(1 + p_{x})}^{t}}}

(3)

Results and Discussion

Monetisation of costs and benefits with NPV, IRR and BCR

The base case scenario contains only one monetised cost composed of the treatment and transportation costs of FA. As a result, no monetised benefit is gained. The scenario involves no changes in the current situation. As was described earlier, annual FA generation at Iru Power Plant amounts to 10,900 tons (Eesti Energia AS, 2015). The treatment and transportation cost for FA is 2,000,000 EUR annually, based on market research. Figure 3(a) presents the discounted cash flows for the period between 2016 and 2025 for the base case scenario. As a result of the calculations, the NPV was found to be −16,870,663 EUR at the end of 2025 under the base case scenario. However, it is unreasonable to calculate either the IRR or the BCR because there is no monetised benefit in the base case scenario.

Firstly, an investment should be made to construct the desired carbonation plant. The investment cost was determined at 4,000,000 EUR. This consists of major equipment, land use, labour costs, transportation and civil infrastructure. The investment includes 1,500,000 EUR for major equipment, such as a mixing tank, stabilisation tank, vacuum bet filter, scrubber and flue gas compressor, and excludes pipelines and pumps. The other part of the investment cost is approximately 2,500,000 EUR, which covers land use, labour, transportation and construction costs.

The maintenance cost is determined as 2% of the investment cost. This component includes the replacement of worn parts, such as pH-probes, CO₂ injection nozzles and spear parts. Also, specialists for servicing special parts will be needed as an external cost. Altogether, maintenance costs will amount to 80,000 EUR per year.

The calculated operation cost for the carbonation plant is presented in Table 2. The annual operation and maintenance costs are 107,393 and 80,000 EUR, respectively. Altogether, the annual operation and maintenance costs are 187,393 EUR.

Table 2.

Operation cost of the carbonation process with components.

Component	Quantity [per ton FA]	Market price	Cost [per ton FA]	Annual cost
Water	4 [m³ t⁻¹]	0.026 [€ m^3–1]	0.106 [€ t⁻¹]	1,155 [€ y⁻¹]
Power	62.5 [kWh t⁻¹]	0.061 [€ kWh⁻¹]	3,812 [€ t⁻¹]	41,550 [€ y⁻¹]
Flue gas	2,500 [m³ t⁻¹]	–	–	–
NaOH	4.2 [kg t⁻¹]	0.417 [€ kg⁻¹]	1,751 [€ t⁻¹]	19,009 [€ y⁻¹]
HCl	8.4 [kg t⁻¹]	0.075 [€ kg⁻¹]	0.63 [€ t⁻¹]	6,867 [€ y⁻¹]
TMT15	0.5 [kg t⁻¹]	7,121 [€ kg⁻¹]	3,560 [€ t⁻¹]	38,812 [€ y⁻¹]

FA: fly ash.

The other cost component is job creation. It was decided to hire three employees during the operation. This number may change depending on the needs and ongoing situation. The relevant cost was taken as 36,000 EUR per year. Moreover, job creation would ensure social benefits, such as an increase of financial security, promotion of higher living standards and trigger of economic activity.

For the carbonation process, a benefit source is the reduction in the current treatment and transportation costs in the base case scenario. This comprises an unspent 2,000,000 EUR per year.

The discounted cash flows under the waste-to-product carbonation scenario (Option 1) are presented in Figure 3(b). From the investment in 2016, no benefit was derived since the FA is still sent to a neighbouring country. From 2017, the carbonation plant with full operation will be in use. The FA will be carbonated and used in different industrial areas as a stabilised product.

At the end of 2025, under Option 1, the NPV, IRR and BCR will be 9,209,662 EUR, 43% and 2.63, respectively. This amount fundamentally represents the NPV of the profit from the loss end of the 10 years compared to the base case scenario.

Under the landfilling carbonation scenario (Option 2), the treated FA will be delivered to Tallinn’s landfill, which is 10.8 km away from Iru Power Plant. The cost of transportation by trucks per ton of FA per km is 0.07 EUR. This comprises an annual transportation cost of 11,330 EUR at 14,987 tons of annual carbonated FA. The landfilling fee for stabilised waste is 78.41 EUR per ton at Estonian landfills. The annual disposal cost for carbonated FA is 1,175,131 EUR. Discounted cash flows under Option 2 are presented in Figure 3(c).

At the end of 2025, under Option 2, the NPV, IRR and BCR will be 387,931 EUR, 6% and 1.03, respectively. This indicates that Option 2 has better economical results than the base case scenario.

One of the most important parameters in CBA is the payback period. The payback periods for carbonation under Options 1 and 2 are shown in Figure 4. The figure indicates that the funds expended in the investment will be recouped in less than three years under Option 1 and less than 10 years under Option 2. This point is known as a BEP. Under Option 1, the BEP is prior to 2019. In fact, it may be even shorter in the case of revenue from sales of the treated FA to any other industrial process. In addition, Figure 4 illustrates that the profit from the loss under Options 1 and 2 will increase with time. Figure 4 also shows that it could recoup the funds expended in the investment under both Options 1 and 2 within 10 years.

Furthermore, CBA has been used by several authors in different fields related to environmental engineering. Wang et al. (2016) used CBA to investigate greenhouse gas (GHG) emission reduction in WtE projects in China. They also proposed two different scenarios and pointed out the better scenario with the benefits. In the energy field, Groth and Scholtens (2016) conducted a CBA to compare biomass and natural gas combined heat and power (CHP) projects in Denmark and the Netherlands. They assessed the same projects based on the guidelines in both countries. In addition, Clinch and Healy (2001) used a comprehensive CBA to assess domestic energy efficiency programmes.

Sensitivity analysis

In practice, substantial uncertainties are always likely to be found in calculations in the CBA analysis. The significance of monetised sources may change, which may affect the results in possibly both a positive and negative way.

Firstly, the change of the NPV was discussed depending on the different discount rates. In this study, the optimum discount rate was determined at 0.04. The NPV changes with the discount rates of 0.03 and 0.07 were investigated. In addition, García-Gusano et al. (2016) recommends using a reference value not higher than 4–5% in European countries. Table 3 gives the NPV changes with different discount rates for costs, benefits and the total under both carbonation options and the base case scenario. The total NPV with 7% discount rate under Option 1 still presented a rather high profit. It shows that the NPV would not move below zero in a worst-case scenario. However, the total NPV with a discount rate of 7% under Option 2 was nevertheless minus. This indicates that it cannot be larger than zero in a worst-case scenario.

Table 3.

Discount rates of 3% and 7% compared to 4% base and assumptions altered for the cost source for carbonation options and the base case.

Carbonation Option 1
Assumptions	NPV – costs [€]	NPV – benefits [€]	NPV – total [€]
P_x = 3%	5,739,362	15,572,218	9,832,848
P_x = 4%	5,661,001	14,870,663	9,209,662
P_x = 7%	5,455,457	13,030,464	7,575,001
20% Cost
Increase	5,993,201	14,870,663	8,877,462
(P_x = 4%)
20% Cost
Decrease	5,328,801	14,870,663	9,541,862
(P_x = 4%)
Carbonation Option 2
Assumptions	NPV – costs [€]	NPV – benefits [€]	NPV – total [€]
P_x = 3%	14,977,277	15,572,218	594,941
P_x = 4%	14,482,732	14,870,663	387,931
P_x = 7%	–13,185,526	13,030,464	–155,062
20% Cost
Increase	–16,579,278	14,870,663	–1,708,615
(P_x = 4%)
20% Cost
Decrease	–12,386,186	14,870,663	2,484,478
(P_x = 4%)
Base case
Assumptions	NPV – costs [€]	NPV – benefits [€]	NPV – total [€]
P_x = 3%	17,572,217	0	–17,572,217
P_x = 4%	16,870,663	0	–16,870,663
P_x = 7%	15,030,464	0	–15,030,464

NPV: Net Present Value.

Secondly, it is important to analyse the change in the sources of costs to see the variations of the total NPV after 10 years. For instance, operation and maintenance costs may vary in the future. Furthermore, transportation or disposal costs may also change in either direction. Therefore, the cost sources are increased and decreased by 20%. Moreover, how the total NPV responds to this change is analysed for the options. Also, the decrease in cost sources will end up with a higher NPV, as desired. Table 3 shows the results for all the scenarios. Since the cost is not high, the change in the total NPV is lowest under carbonation Option 1. As a result, carbonation Option 2 has higher costs and the response to this change was rather high. It was observed that Option 2 had a minus value for a 20% increase in the cost sources. In this case, Option 2 would not be a better economical option than the base case scenario.

An attempt has been made to analyse the alternatives for the changes that are likely to occur over 10 years. The results showed that the total NPVs of the carbonation waste-to-product scenario even showed positive results in a worst-case scenario. Moreover, the change in NPV over the years with different discount rates is not vital.

Environmental benefits of the chosen alternatives

MSW incineration flue gas includes approximately 10–15% CO₂ and it is accepted that it has a substantial contribution to the greenhouse effect (Jiang et al., 2013). Under Options 1 and 2, there will be a noteworthy reduction in CO₂ emissions, namely 169 kg per year. The soluble salts are eliminated from FA through the carbonation process and its hazardousness is reduced. In addition, as presented in Figure 1, the sludge, including heavy metals and sulphides, is recycled back into the process, which prevents the release of heavy metals. The heavy metals presented in untreated FA are stuck in the treated FA due to the carbonation process. In other words, a buffer capacity in the neutral or slightly alkaline range (calcium carbonate) is desired, since it prevents heavy metals from leaching out at a low pH. Moreover, after the carbonation process, the pH of FA is lowered significantly to a neutral or slightly alkaline range, which stabilises the FA. Under Option 1, the carbonated FA becomes a product and will no longer be landfilled, which minimises the environmental burden.

Both Options 1 and 2 are environmentally better FA management options compared to the current situation.

Conclusion

This paper investigates CO₂ sequestration with a further carbonation process as an alternative to sending FA to a neighbouring country. This comprises an economical burden for Iru Power Plant in Estonia. DRH carbonation technology can be a profitable and sustainable solution for this problem.

This paper concludes that the best option for decision-making was Option 1 (the waste-to-product carbonation scenario). The NPV, IRR and BCR values were found to be 9,209,662 EUR, 43% and 2.63 at the end of 2025, respectively. In addition, the BEP was indicated before 2019. In sensitivity analysis, the NPV was still positive under a 20% cost increase or 7% discount rate.

Option 2 (landfilling carbonation scenario) had the NPV, IRR and BCR values as −387,931 EUR, 6% and 1.03, respectively. Over 10 years, it could reach the BEP just after 2024. In sensitivity analysis, a 7% discount rate could not succeed in making the NPV larger than zero. Fortunately, the NPV was calculated at 2,484,478 EUR under a 20% cost decrease.

The results showed that running a carbonation plant under Options 1 and 2 at Iru Power Plant in Estonia may be an economically and environmentally friendly choice compared to the ongoing situation.

It should be emphasised for future work that a comprehensive CBA should be conducted where all elements are taken into consideration. For instance, the price fluctuations of the cost and benefit sources, the layout and capacity of the plant and the depreciation and life time of the plant should be included. Furthermore, CO₂ reduction from the flue gas should be monetised as a benefit source under carbonation options. In addition, the risk and uncertainty associated with the project outcomes should be investigated.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

References

Alba

Gassó

Lacorte

et al (2016) Characterization of municipal solid waste incineration residues from facilities with different air pollution control systems. Journal of the Air & Waste Management Association 47: 1170–1179.

Astrup

(2008) Management of APC residues from W-t-E Plants. ISWA-WG Thermal Treatment of Waste.

Aubert

Husson

Sarramone

(2006) Utilization of municipal solid waste incineration (MSWI) fly ash in blended cement. Part 1: processing and characterization of MSWI fly ash. Journal of Hazardous Materials 136: 624–631.

Bertos

Simons

SJR

Hills

et al (2004) A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO₂. 112: 193–205.

Boardman

Greenberg

Vining

et al (2006) Cost benefit analysis – concepts and practice. 3rd ed. New Jersey, NJ: Pearson Education Inc.

Bogush

Stegemann

Wood

et al (2015) Element composition and mineralogical characterisation of air pollution control residue from UK Energy-from-Waste facilities. Waste Management 36: 119–129.

Cappai

Cara

Muntoni

et al (2012) Application of accelerated carbonation on MSW combustion APC residues for metal immobilization and CO₂ sequestration. Journal of Hazardous Materials 207–208: 159–164.

Chimenos

Fernandez

Cervantes

et al (2005) Optimizing the APC residue washing process to minimize the release of chloride and heavy metals. Waste Management 25: 686–693.

Clinch

Healy

(2001) Cost-benefit analysis of domestic energy efficiency. Energy Policy 29: 113–124.

10.

Ecke

(2003) Sequestration of metals in carbonated municipal solid waste incineration (MSWI) fly ash. Waste Management 23: 631–640.

11.

Eesti Energia

(2015) Technical specification of Iru Power Plant fly ash residues. Unpublished internal document.

12.

European Commission Decision (2000) 2000/532/EC of 3 May 2000 replacing Decision 94/3/EC establishing a list of wastes pursuant to Article 1(a) of Council Directive 75/442/EEC on waste and Council Decision 94/904/EC establishing a list of hazardous waste pursuant to Article 1(4) of Council Directive 91/689/EEC on hazardous waste.

13.

Francois

Criado

(2007) Monitoring of leachate at a test road using treated fly ash from municipal solid waste incinerator. Journal of Hazardous Materials 139: 543–549.

14.

García-Gusano

Espegren

Lind

et al (2016) The role of the discount rates in energy systems optimisation models. Renewable and Sustainable Energy Reviews 59: 56–72.

15.

Groth

Scholtens

(2016) A Comparison of Cost-Benefit Analysis of biomass and natural gas CHP projects in Denmark and the Netherlands. Renewable Energy 86: 1095–1102.

16.

Jiang

Tian

Zhang

(2013) Influence of SO2 in incineration flue gas on the sequestration of CO₂ by municipal solid waste incinerator fly ash. Journal of Environmental Sciences 25: 735–740.

17.

Jianga

Xib

Lia

et al (2009) Effect of water-extraction on characteristics of melting and solidification of fly ash from municipal solid waste incinerator. Journal of Hazardous Materials 161: 871–877.

18.

Le Forestier

Libourel

(1998) Characterization of flue gas residues from municipal solid waste combustors. Environmental Science & Technology 32: 2250–2256.

19.

Lima

Ottosen

Ribeiro

(2012) Assessing fly ash treatment: remediation and stabilization of heavy metals. Journal of Environmental Management 95: 110–115.

20.

Bertos

Hills

et al . (2007) Accelerated carbonation of municipal solid waste incineration fly ashes. Waste Management 27: 1200–1206.

21.

Nowak

Frias Rocha

Aschenbrenner

et al (2012) Heavy metal removal from MSW fly ash by means of chlorination and thermal treatment: influence of the chloride type. Chemical Engineering Journal 179: 178–185.

22.

Nzihou

Sharrock

(2002) Calcium phosphate stabilization of fly ash with chloride extraction. Waste Management 22: 235–239.

23.

Pedersen

(2002) Evaluation of assisting agents for electrodialytic removal of Cd, Pb, Zn, Cu and Cr from MSWI fly ash. Journal of Hazardous Materials 95: 185–198.

24.

Pedersen

Ottosen

Villumsen

(2005) Electrodialytic removal of heavy metals from municipal solid waste incineration fly ash using ammonium citrate as assisting agent. Journal of Hazardous Materials 122: 103–109.

25.

Quina

Bordado

Quinta-Ferreira

(2008) Treatment and use of air pollution control Residues from MSW incineration: an overview. Waste Management 28: 2097–2121.

26.

Raado

Hain

Liisma

et al (2014) Composition and properties of oil shale ash concrete. Oil Shale 31: 147–160.

27.

Reijnders

(2005) Disposal, uses and treatments of combustion ashes: a review. Resources, Conservation and Recycling 43: 313–336.

28.

Saikia

Kato

Kojima

(2007) Production of cement clinkers from municipal solid waste incineration (MSWI) fly ash. Waste Management 27: 1178–1189.

29.

Uibu

Somelar

Raado

et al (2015a) Utilization of oil shale ash as a concrete constituent and carbon sink. In: 5th International Conference on Accelerated Carbonation for Environmental and Materials Engineering, 21–24 June, New York, USA.

30.

Uibu

Somelar

Raado

et al (2015b) Oil shale ash based backfilling concrete – strength development, mineral transformations and leachability. Construction and Building Materials 102: 620–630.

31.

Velts

Uibu

Kallas

et al (2011) Waste oil shale ash as a novel source of calcium for precipitated calcium carbonate: carbonation mechanism, modelling, and product characterization. Journal of Hazardous Materials 195: 139–146.

32.

Velts

Uibu

Kallas

et al (2013) CO2 mineralisation: Concept for co-utilization of oil shale energetics waste streams in CaCO3 production. Energy Procedia 37: 5921–5928.

33.

Wang

Chiang

Lina

et al (2001) Effects of a water-extraction process on heavy metal behaviour in municipal solid waste incinerator fly ash. Hydrometallurgy 62: 73–81.

34.

Wang

Jin

Nieb

et al (2010) Recycling of municipal solid waste incineration fly ash for ordinary Portland cement production: a real-scale test. Resources, Conservation and Recycling 54: 1428–1435.

35.

Wang

Geng

Zhao

et al . (2016) Cost-benefit analysis of GHG emission reduction in waste to energy projects of China under clean development mechanism. Resources, Conservation and Recycling 109: 90–95.

36.

Zacco

Borgese

Gianoncelli

et al (2014) Review of fly ash inertisation treatments and recycling. Environmental Chemistry Letters 12: 153–175.

A feasibility study of municipal solid waste incineration fly ash utilisation in Estonia

Abstract

Keywords

Introduction

Materials and methods

Specification of an alternative and the base case scenario

Identification of benefits and costs for monetisation

The base case scenario

CO2 sequestration with a further carbonation process

Description of the discount rate, NPV, IRR and BCR

Results and Discussion

Monetisation of costs and benefits with NPV, IRR and BCR

Sensitivity analysis

Environmental benefits of the chosen alternatives

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References

CO₂ sequestration with a further carbonation process