Environmental risk assessment of MEA and its degradation products from Post-Combustion CO2 Capture Pilot Plant:Drafting technical guidelines

Hajime Kimura^{a, *}, Toshiaki Kubo^a, Masatoshi Shimada^a, Hideo Kitamura^b Koshito Fujita^b, Kensuke Suzuki^b, Kenji Yamamoto^a, Makoto Akai^c

a:Mizuho Information & Research Institute, Inc.

b:Toshiba Corporation

c:National Institute of Advanced Industrial Science and Technology (AIST)

*Corresponding author

Abstract

To significantly reduce the amount of emission of carbon dioxide (CO2) to the atmosphere, which is a principal cause of climate change (global warming), and to realize a low-carbon society, the introduction of Carbon dioxide Capture and Storage (CCS) into coal-fired power plants is expected to be effective. Emissions from post-combustion CO2 capture plants using amine solution may affect the human health and environment. It is, therefore, important to evaluate the environmental impacts by conducting environmental risk assessment, and to, if necessary, employ emission reduction technologies, for effectively introducing CCS. Case studies on amine emissions from the CO2 capture process and the results of environmental risk assessment are reported. However, there are no guidelines on environmental risk assessment method for the CO2 capture process which enable plant owners to reduce the environmental impact of the process, to increase understanding of local residents (near CO2 capture plants) and public, and to promote the introduction of CCS.

In this study, as part of the project by Ministry of the Environment (Japan), Mizuho Information & Research Institute, Inc. (MHIR) and Toshiba Corporation drafted the guidelines presenting the basic principles (methods, basic points) of environmental risk assessment with regard to exhaust gas and other parameters involved in the CO2 capture process. Based on Toshiba's emission data from 10 ton-CO2/day scale pilot plant using MEA solution at Mikawa coal-fired thermal power plant, MHIR conducted a trial of environmental risk assessment for the CO2 capture process, examined the risk assessment procedure, and drafted technical guidelines. The guidelines are expected to enable plant owners to make reasonable judgement on "how far emission should be reduced", and thus to decide "to what extent they should employ emission reduction technologies". And, the guidelines include "whole-mixture approach", which not only helps to overcome the issues typical of the CO2 capture process but also is easy to understand and thus familiar to local residents and public.

Keywords: Post combustion; Environmental risk assessment; Technical guidelines; Component-based approach; Whole-mixture approach

1. Introduction

To significantly reduce the amount of emission of carbon dioxide (CO2) to the atmosphere, which is a principal cause of climate change (global warming), and to realize a low-carbon society, it is imperative to achieve reduction in energy consumption, low-carbon energy production, and change of energy use. For producing "low-carbon energy", in addition to the use of low-carbon power sources such as renewable energy, the introduction of Carbon dioxide Capture and Storage (CCS) into coal-fired power plants is expected to be effective. Regarding CO2 capture process among CCS processes, a chemical absorption method with the use of amine solvents is considered a powerful technology [1].

Emissions from post-combustion CO2 capture plants using amine solution may affect the human health and environment [2-4]. Amines themselves are known to have specific toxicity mechanisms against aquatic organisms [5]. And some degraded amines, such as nitrosamines, may also pose risks to the human health [6,7]. It is, therefore, important to evaluate the environmental impacts by conducting environmental risk assessment, and to, if necessary, employ emission reduction technologies, for effectively introducing CCS.

The cooperative research group of Norwegian Institute for Air Research (NILU) and Norwegian Institute for Water Research (NIVA) conducted environmental risk assessment for the CO2 capture process based on worst-case assumptions [8]. Azzi et al. from Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia reported the data collected at CSIRO's post-combustion CO2 capture pilot plant at the AGL Loy Yang brown coal-fired power plant and assessed the environmental risk by comparing the data with environmental guideline values [9-11].

As mentioned above, case studies on amine emissions from the CO2 capture process and the results of environmental risk assessment are reported. And several technical guidelines on other aspects of CCS have been already published [12-16]. However, there are no guidelines on environmental risk assessment method for the CO2 capture process which enable plant owners to reduce the environmental impact of the process, to increase understanding of local residents (near CO2 capture plants) and public, and to promote the introduction of CCS. Communications between local residents and plant owners about environmental risk is important [17,18]. Also from the perspective of "environmental justice (especially procedural justice)", it is preferable that the results of risk assessment are easy to understand [19,20].

In this study, as part of the project by Ministry of the Environment (Japan), Mizuho Information & Research Institute, Inc. (MHIR) and Toshiba Corporation drafted the guidelines presenting the basic principles (methods, basic points) of environmental risk assessment with regard to exhaust gas and other parameters involved in the CO2 capture process. Based on Toshiba's emission data from 10 ton-CO2/day scale pilot plant using MEA solution at Mikawa coal-fired thermal power plant run by Sigma Power Ariake Co. Ltd. (Figure 1), MHIR conducted a trial of environmental risk assessment for the CO2 capture process, examined the risk assessment procedure, and drafted technical guidelines. The guidelines are expected to enable plant owners to make reasonable judgement on "how far emission should be reduced", and thus to decide "to what extent they should employ emission reduction technologies". And, the guidelines include "whole-mixture approach", which not only helps to overcome the issues typical of the CO2 capture process but also is easy to understand and thus familiar to local residents and public.

Figure 1. Toshiba Mikawa post combustion capture pilot plant (at Omuta, Fukuoka Prefecture)

2. Drafting guidelines on assessing environmental risk for the CO2 capture process

2.1. Purpose

The drafted guidelines present the basic principles (methods, basic points) of environmental risk assessment with regard to exhaust gas and other parameters involved in the CO2 capture process, to reduce the environmental impact of the process and increase local residents and public understanding, and to promote the introduction of CCS. Companies gradually introducing CCS (development, demonstration, initial phase of commercialization, etc.) can instill confidence in the public and local residents especially when they can ensure safety by conducting risk assessment of the CO2 capture process based on the guidelines the government established and, if required, by undertaking emission reduction measures for harmful chemical substances. Furthermore, on the basis of results of the risk assessment, the guidelines support "rational management" ensuring environmental safety (Figure 2).

Figure 2. Position of the Guidelines

2.2. Scope of application of the Guidelines

The guidelines are addressed to companies planning to implement the CO2 capture process in coal-fired power plants (Figure 3). We generally assume normal operating conditions. Thus, risks pertaining to accidents or disasters are outside the scope of the guidelines. In addition, we emphasize ensuring the safety of the general environment and not the working environment.

Figure 3. Conceptual scheme of carbon dioxide capture, transport and storage

2.3. Contents of the Guidelines

The provisional contents of the guidelines are shown in Figure 4. For "Procedure of environmental risk assessment and management in the CO2 capture process", see Section 4. The guidelines include the test result of environmental risk assessment using Toshiba's emission data from 10 ton-CO2/day scale pilot plant at Mikawa coal-fired thermal power plant (not shown in this paper). Emission reduction technology and proper disposal treatment are also included as reference materials (not shown in this paper).

Figure 4. Table of contents of the guidelines (provisional)

2.4. The benefit of conducting voluntary environmental risk assessment

Reduction of the environmental impact of the CO2 capture process will obviously promote the introduction of CCS. In addition, the implementation of environmental risk assessment will enable a reasonable judgement on "how far emission should be reduced". Thus, the development of CO2 absorbents based on environmental considerations could be expected. Furthermore, through publication of Environmental Reports by companies, their commitment to protecting the environment will highlight their corporate values.

3. The characteristics of the CO2 capture process using amine solvents

Acid gas separation and recovery using amine solvents based on the chemical absorption method is conventionally used in natural gas purification, the hydrogen–ammonia industry, and the likes. On the other hand, the CO2 separation and recovery process intended for the combustion exhaust gas of fossil fuels mainly differs from the existing processes in the following points:

• Because the exhaust gas from thermal power plants includes oxygen, oxidative decomposition of the amines is likely to occur in the CO2 capture system.
• In addition to oxygen, impurities contained in the exhaust gas from thermal power plants exert an impact on the degree of degradation of the amines (and thus on the amount of emission) and the type of degradation products.
• With the future commercial development, the scale of the CO2 capture system will increase.

The degree of degradation of the amines (and thus the amount of emission to the environment) or the type of the degradation products formed depends on the type of amine utilized as the main agent of the CO2 absorbent and on the operating conditions of the CO2 capture process, such as amine concentration in the CO2 absorbent, CO2 loading (ratio of amine molecules bonded to CO2),reaction temperature in the absorber, temperature of the stripper, amount of oxygen, SOx, NOx, and particulate matter in the gas emitted from the power plant, composition of the particulate matter (iron, nickel, vanadium, phosphorus, chromium, cobalt, etc.), and catalytic effect owing to the material constituting the CO2 capture system [21-23].

Since the types of degradation products generated in the CO2 capture process are extremely diverse, identification of "all" degradation products (component-based approach; see Section 4.2) is economically and technically inefficient. From the perspective of environmental risk assessment, there are concerns that highly toxic substances with very small amount of emissions might be emitted into environment. For this reason, the use of the whole-mixture approach is considered complementary, as described in Section 4.3.

There are also differences between the CO2 capture process and existing process, for example, in the case of natural gas purification, the product is a treated gas free from oxygen, after passing through the absorber (thus not discharged into the environment).

4. The framework of environmental risk assessment and management for the CO2 capture process

Environmental risk assessment involves, in general, various steps such as determining the chemicals substances emitted into the environment and the amount of emissions, toxicity assessment, exposure assessment, and risk characterization [24].

In the CO2 capture process, numerous substances are generated secondarily and it is economically and technically not feasible to identify "all" these degradation products, as described in Section 3. Thus, conventional methods of risk assessment and management may not sufficiently ensure safety. For example, highly toxic substances (e.g. nitrosamines) with very small amount of emissions might be emitted into environment without identification. In addition, it may not be possible to publicize the components of the CO2 absorbent owing to intellectual property rights.

We consider the procedure shown in Figure 5 to conduct environmental risk assessment and management for the CO2 capture process. Here, we combine the conventional "component-based approach" with the "whole-mixture approach" to deal with the challenges for environmental risk assessment for the CO2 capture process. The following cases are given as examples, some substances are unidentified due to the detection limit, substances without toxicity information are included, and the components of CO2 absorbent solutions cannot be disclosed owing to intellectual property protection. It should be noted that the whole-mixture approach requires further testing and application (for the elemental technology; see Section 5.3 and Section 5.4). In the guidelines, the description is focused on the conventional component-based approach and outlined the perspective for the use of a bioassay among the techniques of the whole-mixture approach.

Figure 5. The framework of environmental risk assessment and management for CO2 capture process (provisional)

4.1. Substances requiring risk assessment and management

With regard to the components of the CO2 absorbent (all purposely used substances including the main agent) and the degradation products generated in the CO2 capture process, appropriate management based on risk assessment is necessary. Furthermore, it is necessary to identify the degradation products within technically and economically appropriate limits and measure their amount of emission to the environment.

4.2. Component-based approach

A general risk assessment is conducted through the component-based approach [24]. In addition to the component(s) of the CO2 absorbent (all purposely used substances including the main agent), every chemical substance generated as degradation products in the CO2 capture process is identified one-by-one and individually subjected to toxicity assessment (collection of toxicity data, implementation of toxicity tests), exposure assessment (collection of data concerning physicochemical properties, estimation of environmental concentrations using a mathematical model, and if necessary, conducting environmental monitoring), and risk characterization.

In general, risk assessment involves certain uncertainties. Therefore, ideas and methods with regard to safety should be viewed from a conservative scope within reasonable ranges. And, environmental risk assessment could be discussed thoroughly in detail, or implemented in a cost-efficient manner within scientifically reasonable range. For example, in the guidelines, in toxicity assessment, regarding the assessment of substances with missing toxicity data, toxicity tests should be performed as a general rule. However, before the toxicity tests are complete, risk characterization may be conducted using the toxicity levels of the most harmful substances among those assumed in the emissions as a reference. In addition, when the safety is not ensured because of the various uncertainties, an assessment through "category approach" could be implemented [25,26]. Furthermore, in the guidelines, in exposure assessment, the concentration in the environment is estimated using a mathematical model such as the Plume Model near the emission source. When the safety is not ensured because of the various uncertainties, safety can be confirmed by implementation of environmental monitoring.

4.3. Whole mixture approach

Although a general risk assessment is conducted through the component-based approach, such as in the case of the CO2 capture process using amine solvents, when some substances are unidentified due to the detection limit, or substances without toxicity information are included, complementation with the whole-mixture approach is effective (Table 1).

In the whole-mixture approach [27,28], inspection is based on the bioassay (biological response test), which is conducted "directly" on wastewater. In the United States, this test is known as Whole Effluent Toxicity (WET) test [29]. With this approach, the risk assessment can be conducted even if the unidentified substances are included. This method is therefore efficient to evaluate the impact of small amounts of degradation products as well as their additive effect and/or synergistic effect.

This approach is already introduced in the United States and other countries as a wastewater management technique. In the guidelines formulated for the implementation of "the International Convention for the Control and Management of Ship's Ballast Water and Sediments", adopted by the International Maritime Organization (IMO), in addition to the identification and toxicity testing of all active substances and by-products used in the processing of ballast water, a WET test is prescribed on the ballast wastewater itself: that is, the component-based approach and the whole-mixture approach are combined [30].

Also in the context of environmental risk assessment (and management) for the CO2 capture process, whole-mixture approach is effective. In the future, bioassays targeting "exhaust gases" (risk assessment of human health effects) and bioassays targeting "exhaust gas solutions" (risk assessment of the effect to an aquatic organism) are expected to be implemented (see Section 5.3, Section 5.4 and Figure 6). Since the whole-mixture approach enables assessment even when the presence of unidentified substances, it is suitable not only when substances without toxicity information are present, but also when the components of the CO2 absorbent solution cannot be disclosed owing to intellectual property protection. In addition, in performing a toxicity assessment using the exhaust gas or exhaust gas solution itself, the method is easy to understand and persuasive. Furthermore, by utilizing whole-mixture approach, the development of next-generation CO2 absorbents based on environmental considerations could be expected. On the other hand, if impacts are detected as a result of bioassays, it is needed to be supplemented by the component-based approach to employ emissions reduction measures targeting specific causative substances. In the same way as they are used in combination in the management of ballast water, these two approaches are complementary also in the context of environmental risk assessment for the CO2 capture process (Table 1).

Table 1. Comparison of two approaches in risk assessment and management

	Component-based approach	Whole-mixture approach
Assessment feasible without substance identification?	No	Yes
Ease of "publishing" the results	Inconclusive	Positive
Ease of "understanding" the results	Inconclusive	Positive
Ease of establishing emission reduction measures targeted to specific causative substances	Positive	Inconclusive

4.4. About cases with possibility of risk concerns

When performing a step-wise evaluation, in the case of possibility of risk concerns despite reduction in uncertainty associated with the evaluation, additional emission control measures and/or analysis of the operating conditions such as reaction temperature of the absorber are performed, so as to reduce the environmental burden to a sufficiently safe level. It should be noted that the investigations are conducted in view of the characteristics of each CO2 capture plant, such as CO2 absorbent or operating conditions, and economic rationality. In the guidelines, in order to assist companies to select the optimal emission reduction technology in accordance with the characteristics of each plant, as reference information, various emission reduction technologies are introduced (not shown in this paper; see [31-33]).

5. Prospects of environmental risk assessment and management method for the CO2 capture process

5.1. Toxicity assessment for substances without toxicity information

An issue is that there is poor hazard information for substances emitted from the CO2 capture process. For example, in the test result of environmental risk assessment using Toshiba's emission data from 10 ton-CO2/day scale pilot plant using MEA solution at Mikawa coal-fired thermal power plant (Figure 1), 13 substances were identified as emitted substances. Of the 13 substances, there are hazard information for only 6 substances on human health and only 3 substances on aquatic organisms, respectively (Table 2).

To conduct a toxicity assessment for such substances without toxicity information, "category approach" is effective. However, the category approach requires further examination and application [25,26]. In such a situation, whole-mixture approach is also effective (see Section 5.3, Section 5.4 and Figure 6).

Table 2. Hazard information availability of MEA and its degradation products

Name of substance	Abbreviation	Hazard information available?
		Human health	Aquatic organism
Monoethanolamine	MEA	Yes	Yes
Diethanolamine	DEA	Yes	Yes
N-(2-hydroxyethyl)formamide	HEF	No	No
N-Nitrosodiethanolamine	NDELA	Yes	No
Pyrazine	PY	Yes	No
Methylpyrazine	MePY	Yes	No
N-(2-hydroxyethyl)imidazole	HEI	No	No
2-Oxazolidinone	OZD	No	No
N-(2-hydroxyethyl)acetamide	HEA	No	No
N-(2-hydroxyethyl)lactamide	HELA	No	No
N-(2-hydroxyethyl)glycine	HEGly	No	No
1-hydroxyethyl-2-piperazinone	HEPO	No	No
N-(2-hydroxyethyl)imidazolidone	HEIA	Yes	Yes

5.2. Mathematical model used for estimation of environmental concentration

The detailed mechanism of dynamics of treated flue gas composition, which would be driven by deviation of various operational conditions, is still unknown. Thus, it is difficult to determine the sufficiently representative numerical values which are treated as input data for mathematical models for estimating the environmental concentration. And, considering the increase of the scale of the CO2 capture system along with the future commercial development, it would be preferable to reduce the uncertainties derived from mathematical models by implementation of environmental monitoring, comparing the data with the estimated environmental concentrations, and improving mathematical model.

5.3. Bioassay targeting "exhaust gas" (assessment of impact on human health)

In future, to conduct a risk assessment of the impact on human health from the viewpoint of the whole-mixture approach, it is necessary to develop a method for comprehensive assessment of the toxicity of the exhaust gas (long-term inhalation exposure toxicity test). However, there are also issues relating to the technique of introducing the sample into the toxicity test equipment, while preserving its composition, as a first step. Therefore, genotoxicity tests, such as in vitro tests (ames test, and chromosomal aberration test) or comparatively simple in vivo tests with the use of exhaust gas (or exhaust gas solution) are necessary to evaluate the methods to be implemented. In addition, it is also necessary to consider a correlation between genotoxicity and carcinogenicity in risk assessment.

5.4. Bioassay targeting "exhaust gas solution" (assessment of the effect to an aquatic organism)

In future, to perform a risk assessment of the effect to an aquatic organism from the viewpoint of the whole-mixture approach (Figure 6), it is necessary to correlate the minimum dilution level of an exhaust gas solution, which is required to eliminate its adverse effects on aquatic organisms, with the substances' dilution rates in the environment. In particular, estimations of environmental concentrations using mathematical models, and environment measurements of concentration in the waters and deposition velocity are considered to be effective.

Figure 6. The concept of bioassay targeting "exhaust gas solution" (assessment of the effect to an aquatic organism)

6. Conclusion

We drafted guidelines on environmental risk assessment method for the CO2 capture process, which enable plant owners to reduce the environmental impact of the process, to increase understanding of local residents (near CO2 capture plants) and public, and to promote the introduction of CCS. Now, in order to complete the framework of environmental risk assessment and management (Figure 5), we are trying to overcome the issues as described in Section 5, through the demonstration using post-combustion CO2 capture pilot plant and etc.

From the perspective of "environmental justice (especially procedural justice)", it is preferable that the risk assessment method is easy to understand [19,20]. Whole-mixture approach seems to not only overcome the issues typical of the CO2 capture process but also to be easy to understand and thus familiar to local residents and public.

Acknowledgements

This study was carried out as a part of the project "Feasibility study for the introduction of sustainable CCS technology" funded by Ministry of the Environment, Government of Japan. The authors would like to acknowledge the subcommittee for environmental impact of CO2 separation and capture absorbent.

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