Scholarly article on topic 'Life Cycle Assessment of Reusing Fly Ash from Municipal Solid Waste Incineration'

Life Cycle Assessment of Reusing Fly Ash from Municipal Solid Waste Incineration Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — T.Y. Huang, P.T. Chuieh

Abstract Many countries consider the fly ash produced from the incineration of municipal solid waste (MSW) as hazardous waste. In Taiwan, 24.5 thousand tons of MSW is incinerated and 554.7 tons of fly ash is produced daily. As landfill capacity in the country is limited, this vast amount of fly ash should ideally be reused. However, some treatment methods to enable the reuse of fly ash can have a worse impact on the environment than was previously believed. The aim of this research was therefore to compare the different processes for the reuse of fly ash by employing data collected from the incineration process in Taiwan in order to establish a life-cycle assessment (LCA) database. The database, containing information on the assessment of reuse treatments, is intended to inform decision making on the best practices for the reuse of fly ash. The study poses four scenarios for reuse treatment and disposal of one ton of fly ash: 1) landfilling after solidification, 2) reuse as cement after a washing process, 3) reuse as bricks after a washing process, 4) reuse as an alkali in the waelz process of steelmaking. The results from the LCA showed that the washing processes consumed more water but less electricity; however, more electricity was consumed and higher toxicity was caused in scenario 4, the reuse as an alkali in the waelz process. More water and chemical additives were used in scenario 2 because of the higher limits of chloride compounds and heavy metals used in construction materials. To conclude, the comprehensive LCA inventory database established by this study could assist in reducing the environmental impacts of reuse treatments for fly ash, and consequently could contribute to the safer management of fly ash worldwide.

Academic research paper on topic "Life Cycle Assessment of Reusing Fly Ash from Municipal Solid Waste Incineration"

Procedia Engineering

www.elsevier.com/locate/procedia

International Conference on Sustainable Design, Engineering and Construction

Life cycle assessment of reusing fly ash from municipal solid waste incineration

T.Y. Huanga, P.T. Chuieha' *

a Graduate Institute of Environmental Engineering, National Taiwan University, Taiwan

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Procedia Engineering 118 (2015) 984 - 991

Abstract

Many countries consider the fly ash produced from the incineration of municipal solid waste (MSW) as hazardous waste. In Taiwan, 24.5 thousand tons of MSW is incinerated and 554.7 tons of fly ash is produced daily. As landfill capacity in the country is limited, this vast amount of fly ash should ideally be reused. However, some treatment methods to enable the reuse of fly ash can have a worse impact on the environment than was previously believed. The aim of this research was therefore to compare the different processes for the reuse of fly ash by employing data collected from the incineration process in Taiwan in order to establish a life-cycle assessment (LCA) database. The database, containing information on the assessment of reuse treatments, is intended to inform decision making on the best practices for the reuse of fly ash. The study poses four scenarios for reuse treatment and disposal of one ton of fly ash: 1) landfilling after solidification, 2) reuse as cement after a washing process, 3) reuse as bricks after a washing process, 4) reuse as an alkali in the waelz process of steelmaking. The results from the LCA showed that the washing processes consumed more water but less electricity; however, more electricity was consumed and higher toxicity was caused in scenario 4, the reuse as an alkali in the waelz process. More water and chemical additives were used in scenario 2 because of the higher limits of chloride compounds and heavy metals used in construction materials. To conclude, the comprehensive LCA inventory database established by this study could assist in reducing the environmental impacts of reuse treatments for fly ash, and consequently could contribute to the safer management of fly ash worldwide.

© 2015 TheAuthors.Published byElsevierLtd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of organizing committee of the International Conference on Sustainable Design, Engineering and Construction 2015

Keywords: Fly ash, Reuse, Life cycle assessment, Municipal solid waste incineration

* Corresponding author. Tel.: 886-2-33662798 E-mail address: ptchueh@ntu.edu.tw

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of organizing committee of the International Conference on Sustainable Design, Engineering and Construction 2015 doi:10.1016/j.proeng.2015.08.539

1. Introduction

Awareness of the harmful residues from the incineration of municipal solid waste (MSW) has increased in recent years and many researchers have discussed the characteristic and reuse treatments for this residue, including fly ash and bottom ash [1-3]. In Taiwan, where 280 thousand tons of fly ash is produced per year, fly ash is regarded as hazardous waste as it contains toxic substances, such as heavy metal and dioxin/furans. On the other hand, fly ash contains large amounts of silicon oxide, calcium oxide, and aluminum oxide, which could be used in construction materials. Fly ash could be reused as a building material, cement, aggregate, bricks, or even as the alkaline substitute for lime in the treatment of waste-water, and in the manufacturing process [4].

Before reusing fly ash, the first step would be to reduce the toxic substances and chloride, which could be released into the environment and could erode cement and construction materials. Three kinds of treatment exist to remove toxicity and reduce the possibility of releasing heavy metals: 1) extraction and separation, 2) thermal treatment, and 3) stabilization/solidification [5]. Extraction and separation is a simple method to separate the harmful substances by using water, acid leaching, or filtration, but it requires large amounts of resources (energy and materials). Thermal treatment uses heat and high temperature to destroy the toxic organic compounds and to volatilize the low boiling point metals. In addition, this method provides excellent stabilization to limit the release of heavy metals that are hard to volatilize at a high temperature. Although thermal treatment may seem the best way to limit the release of heavy metals, this method consumes substantial energy and it is expensive to maintain the machinery required. In contrast, stabilization/solidification is an effective and less expensive method, using materials, such as cement, glass and the like to encapsulate the fly ash. This method is normally used in Taiwan before disposing of the solidified fly ash to the landfills. However, the increasing population of the country has resulted in less land being available for landfills. Furthermore, over time the solidified fly ash could release the heavy metal and toxic organic compounds underground, which could have a harmful effect on the environment.

As there are numerous treatments available for the reuse of fly ash, stakeholders have to make a complete assessment by considering all the aspects, including the cost and the cost to the environment, to select the best process. This research therefore uses life cycle assessment (LCA) as one way of assessing the environmental impact of different reuse treatments, quantifies the energy and resources used, and the emission to the environment. The overall aim of this study was to find the best way to treat fly ash, with a minimum effect on the environment, by comparing four scenarios: 1) landfill after solidification, 2) reuse as cement after a washing process, 3) reuse as bricks after a washing process, 4) reuse as alkaline in the waelz process of steelmaking. In the future, we intend broadening the assessment of the treatment of fly ash by including additional factors, such as risk management and cost-benefit analyses, for further discussion and a more complete evaluation.

Nomenclature

AHP analytic hierarchy process EAFD electric arc furnace dust LCA life cycle assessment MSW municipal solid waste

2. Characteristics of fly ash from MSW incineration

MSW fly ash is composed of many toxic substances, with the majority metal salts, but the composition of fly ash does differ somewhat from country to country. In this research, we collected the data on the composition of fly ash from four countries, namely Taiwan, Japan, France, and China, where fly ash has been reused for years. The fly ash was collected from the incineration of MSW in northern Taiwan. It was found that the major components of this fly ash were 22.08% SiO2 and 12.44% CaO, which were also the main components of the fly ash from the other countries, except for the fly ash from Japan that contained 7.16% Na2O. The results indicated that as all the fly ash contained CaO and SiO2, it could be reused as construction material or as alkaline. The fly ash in this research was similar to that of China, and, not only was the ratio of the chemical compounds the same as the major component but

also the minor compounds did not exceed 10%. However, the traces of heavy metal and the chloride in the fly ash remain the main problem for reuse as the toxic substances could be released to the environment, causing great harm to humans and the ecosystem. With regard to heavy metals, the most abundant metal in fly ash is Ca at 337.66 mg/g, and Al at 214.25 mg/g, but these metals are not harmful to the environment. On the other hand, Zn, Fe, Pb, and Cu, which are present in lesser amounts in fly ash, do harm the environment and should be removed or encapsulated before the fly ash is reused. The fly ash from the different countries contained similar amounts of heavy metal, because heavy metals have a fixed boiling point.

Table 1. Comparison of chemical compounds of fly ash between countries

(%) Taiwan (This research) Japan Saikia (2006) France Aubert (2005) China Huang (2008)

CaO 12.44 13.86 25.2 24.36

Si02 22.08 12.01 20.7 22.04

A1A 1.55 8.1 10.0 7.79

Fe203 6.77 1.21 2.7 5.17

Na20 7.16 17.19 1.4 5.45

MgO 1.28 2.62 2.7 3.72

K20 N.D, 7.41 1.4 7.43

Table 2. Comparison of heavy metal of fly ash between countries

Metal (mg/g) Taiwan (This research) Japan Kim (2003) France Aubert (2005) China Huang (2008)

Ca 337.66 128 N.D. 111.62

Al 214.25 50 N.D. 39.79

Zn 9.62 8.1 24.05 5.22

Fe 4.16 7.0 N.D. 22.51

Pb 2.79 2.81 8.82 2.1

Cu 0.61 1.13 1.71 0.55

Cd 0,16 0.102 0.59 0.06

Cr 0.09 0.126 2.08 0.18

3. Modeling approach

3.1. Life cycle assessment framework

IMPACT 2002+ was used as a tool to quantify the environmental impact, categorized into four damage categories: 1) human health, 2) ecosystem quality, 3) climate change, and 4) resources. Each impact weight was based on the analytic hierarchy process (AHP), taking into account the suggestions from stakeholders. In this paper, the functional unit was the "treatment or disposal of one ton of fly ash, including the benefits of reusing the products." The boundary of the scenarios contains pretreatment of fly ash, reuse treatment, and the final product, or disposal. For improved assessment and localization to Taiwan, the study established a database on the electricity generated in Taiwan and the treatment of waste water that has been implemented in recent years.

3.2. Impact assessment: scenarios

Four scenarios are discussed here: 1) landfill after solidification, 2) reuse as cement, 3) reuse as bricks, and 4) reuse as alkaline in the waelz process (see Figure 1). Each scenario is described and discussed below.

1) Scenario 1: landfill after solidification

The most common treatment of fly ash in Asia is disposal to landfills after solidification. After the washing process, the fly ash is mixed with cement and a chelating agent for stabilization, after which the solidified product is transported to a sanitary landfill site.

2) Scenario 2: reuse as cement after the washing process

After the water wash and chelation, 10% fly ash was added to the raw cement materials that include limestone, clay, sands, and iron ore. Making cement consists of four processes: extracting raw materials from the mine, blending the raw material with fly ash, calcining the product in a rotary kiln, and milling the cement. For this study, one ton of fly ash was transported after incineration to a cement factory in Ilan, North-eastern Taiwan.

3) Scenario 3: reuse as bricks after the washing process

After the water wash and chelation, 20% of the fly ash was added to the raw material for making bricks. Making bricks also consists of four processes: clay preparation, molding, drying, and firing. For this study, one ton of fly ash was transported after incineration to a brickmaking factory in Miaoli, North-western Taiwan.

4) Scenario 4: reuse as alkaline in the waelz process of steelmaking

The electric arc furnace produces a great amount of air pollution during the steelmaking process, especially electric arc furnace dust (EAFD). The most common treatment for EAFD is the waelz process, which uses heat and rotation mixing in a waelz kiln to recycle the zinc volatilized from the EAFD, and to finally collect the zinc oxide for further purification. In scenario 4, the fly ash from the incineration of MSW was water washed and subsequently about 7% of the fly ash was added in the waelz process for the treatment of EAFD. Factors for consideration are the consumption of water, the substantial use of energy, and the air pollution produced, but also the benefit of reducing the environmental impact of making ZnO. In this scenario, the disposal of slag from the waelz process was not taken into consideration.

Pretreatment

Treatment

Product or disposal

Landfill after solidification

Landfill

Fly ash

Reuse as cement

^ Cement

Washing ^^

Reuse as bricks

i> Bricks

Reuse as alkaline in the waelz process

^^ Zinc oxide

Fig. 1. Four reuse treatment scenarios assessed in the LCA.

4. Results and discussion

4.1. Comparison of the environmental impact of all the scenarios

Based on the same functional unit of the treatment of one ton of fly ash, the environmental impact of the four different scenarios is indicated below (see Figure 2). The reuse as alkaline in the waelz process had the worst environmental impact of all four scenarios because of its higher consumption of energy and the significant air pollution caused by the emission from the waelz kiln. However, the ZnO product of the waelz process reduces the impact of some categories of terrestrial eco-toxicity and global warming. Landfills had the second largest environmental impact of the four scenarios, but the impact is far less than is that of the reuse as alkaline, only one-eighth. However, landfilling consumes more electricity than does the reuse as alkaline because the fly ash is solidified by mixing with cement that has a worse effect on global warming and on renewable resources. The other two scenarios, reuse as cement and bricks, were found to be more environmentally friendly, and provided the benefits of reusing the products for construction, especially the brickmaking process. Each impact of the scenario contains a majority ratio of respiratory inorganics owing to the emission of air pollution and the waste water treatment in the washing process. Therefore, the better option would be to reduce the impact of the pollution emission.

Fig. 2. Comparison of the environmental impact between the four scenarios.

4.2. Comparison of the environmental impact of the processes in the four scenarios

In scenario 1, landfill after solidification, a comparison of the impact of the processes is shown below (Figure 3). The greatest impact was the consumption of cement, which was equal to half the amount of fly ash. The washing process of the fly ash and the use of sodium sulfate for chelation also had a greater effect than had the other processes. To improve this treatment option, the consumption of cement and chelating agents must be reduced to moderate amounts.

Fig. 3. Scenarios 1: landfill after solidification, comparing the impact of the different processes.

In scenario 2, reuse as cement, the greatest environmental impact was from the air pollution that was caused by the reuse process itself. Global warming and respiratory inorganics are therefore the main categories of impact of this reuse process. The next greatest impact was the pretreatment of fly ash, because of the higher fraction of the total impact. The third greatest environmental impact was the use of electricity in the cement making process. To improve this treatment option, the emission of pollutants, including carbon dioxide and particularly nitrogen oxide, must be reduced, as well as the heavy metals released during the firing process. In addition, the release of heavy metals from the fly ash could increase the risks associated with the use of cement in construction.

Scenario 3, reuse as bricks, had the smallest impact of the four scenarios. The greatest environmental impact was from the reuse process itself and the washing of the fly ash. The results for scenario 3, reuse as bricks, was found to be similar to the results for scenario 2, the reuse as cement. However, the impact of air pollution was much less than it was for the use in cement because of the lower total impact of reusing fly ash as bricks. To improve this treatment option, the treatment of waste water from the washing process has to be improved to reduce the pollution from the outflow water. Additionally, the consumption of resources and energy by the waste water treatment must be reduced and the air pollution eliminated.

80 —

\ m -m—I—

S 40 —

= - J = i ■ _

0--■■-T--T---T--T--T-

Total impact Washing fly ash Transportation Fuel for kiln Air pollution Leaching

-20 from kiln

Fig. 4. Scenarios 2: reuse as cement, comparing the impact of the different processes.

Fig. 5. Scenarios 3: reuse as bricks, comparing the impact of the different processes.

In scenario 4, reuse as alkaline in the waelz process, the most important environmental impact was the air pollution caused by the reuse process itself. The emission from the waelz kiln and the high concentration of pollutants in the emission contributed directly to air pollution. The second greatest impact was from the hard coal coke that was used to provide energy and improve combustion in the waelz kiln. For every ton of fly ash that is treated in this way, 380 kilogram of zinc oxide could be produced, with the benefit of recycling the ZnO, in addition to reducing the impact on non-renewable energy, global warming, and terrestrial eco-toxicity. Reducing the pollution emission from the waelz kiln would therefore be an instant way to improve this treatment option and eliminate the impact of the hot spot. Reducing the use of hard coal coke, the main factor in energy consumption, and increasing the production of zinc oxide would provide even more benefits.

Fig. 6. Scenarios 4: reuse as alkaline in the waelz prcess, comparing the impact of the different processes.

4.3. Best treatment of fly ash

In this study, we compared the different kinds of reuse treatment for fly ash, and, based on the environmental impact results, we selected scenario 3, reuse as bricks, as the best and most environmental friendly treatment for fly ash. There are two reasons for this choice, the first and most important being that this scenario had the least impact on the environment. In contrast, scenario 4, reuse as alkaline, had the most impact. The second reason for our choice

is that reusing fly ash as bricks provides construction materials, rather than simply disposing of the compacted fly ash to landfills without enjoying any benefit of the product. However, it is not sufficient to consider only the environmental impact in evaluating the best treatment for fly ash; other factors have to be considered as well. For instance, with regard to the cost benefit, reusing fly ash as bricks might be an environmentally friendly option, but the demand for bricks is waning and cement has been the customary construction material for decades. With regard to risk management, by reusing fly ash as alkaline in the waelz process, heat is used to stabilize the heavy metals and destroy dioxins/furans; therefore, this option could be safer than the other reuse treatments. Based on the environmental impact assessment, we considered reusing fly ash as bricks as the best treatment option. Nonetheless, other important aspects, such as cost-benefit, risk management, and product trends still have to be considered in evaluating the best treatment for fly ash. In the future, we intend including these important aspects in the study for an improved assessment of the reuse treatments for fly ash.

5. Conclusion

Fly ash from the incineration of MSW contains large amounts of heavy metals and toxic organics; therefore, before it can be reused, fly ash has to be washed to reduce the concentration of toxic substances, which, additionally, results in better quality cement and bricks from the reused fly ash. A comparison of the composition of fly ash from four different countries indicated that although the municipal solid wastes did differ from country to country, the fly ash compositions were mostly similar. The most abundant composition was CaO and SiO2, which is the most widely used material for construction. However, the possible release of heavy metals from fly ash is a matter of concern, especially Pb, Zn, and Cr.

Based on the environmental impact, this study assessed four scenarios to find the best treatment for fly ash. We concluded that of the four options assessed, reusing fly ash as bricks was the most environmentally friendly treatment. In scenario 3, reuse as bricks, the hot spot of impact was in the washing process and pollution emission of the reuse process. Therefore, reducing the air pollution emission and finding a more efficient treatment for wastewater would be crucial to reduce the impact of this fly ash reuse method.

However, this study cannot be considered as a complete assessment, as only one aspect of reuse treatment was evaluated. In future, we will also consider the cost-benefit and risk management aspects to select the best treatment for the reuse of fly ash.

Acknowledgements

The authors gratefully acknowledge the scientific and economic support of the Ministry of Science and Technology of Taiwan to the research.

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