Post-combustion CO2 Capture Plant Research Articles

Relevancy of Emission Reduction Fund (ERF) policy towards large-scale deployment of carbon capture technology in black coal-fired power plant

This study implements a single objective multi-constrained optimisation technique to evaluate the relevance of large scale deployment of post-combustion CO2 capture (PCC) technology as an emissions reduction fund (ERF) ‘project’ for black coal power generation in Australia. We target maximum net operating revenue, by generating forecasts of power plant load and CO2 capture rate, and while subject to the dual operational and environmental constraints. Four different hypothetical Australian Carbon Credit Unit (ACCU) prices ($AU 5, 15, 25 and 50/tonne CO2) are evaluated, through a single objective multi-constrained optimisation algorithm, for a 7-year contract period between 2016 and 2020 and with a 7.1 MT CO2 of emission baseline. The results indicate that at ACCU price of $AU 25/tonne CO2 (Scenario 3), represents a feasible solution for future deployment of PCC technology under ERF project settings. Across the contract period, PCC plant captures 90% CO2 from the power plant emissions with total plant net operating revenue at approximately $AU 1,765 million. The gross revenue gains from ERF incentive and selling of electricity are at 25% and 75% respectively. These findings point to the value of this computational approach for power plant operators in Australia considering low emissions technologies (viz. PCC) as ERF projects, aiding in their short and medium-term planning. Such an approach is extendable to other countries and regions under varying emissions trading schemes.

Improving Flexibility and Energy Efficiency of Post-Combustion CO2 Capture Plants Using Economic Model Predictive Control

To reduce CO 2 emissions from power plants, electricity companies have diversified their generation sources. Fossil fuels, however, still remain an integral energy generation source as they are more reliable compared to the renewable energy sources. This diversification as well as changing electricity demand could hinder effective economical operation of an amine-based post-combustion CO 2 capture (PCC) plant attached to the power plant to reduce CO 2 emissions. This is as a result of large fluctuations in the flue gas flow rate and unavailability of steam from the power plant. To tackle this problem, efficient control algorithms are necessary. In this work, tracking and economic model predictive controllers are applied to a PCC plant and their economic performance is compared under different scenarios. The results show that economic model predictive control has a potential to improve the economic performance and energy efficiency of the amine-based PCC process up to 6% and 7%, respectively, over conventional model predictive control.

Using sodium thiosulphate for carbon steel corrosion protection against monoethanolamine and 2-amino-2-methyl-1-propanol

The corrosion performance of carbon steel (C1018) with inhibitor sodium thiosulphate (STS) has been compared to C1018 (without an inhibitor) and stainless steel (SS316 L) at 80 and 120 °C to determine the feasibility of use in a post-combustion CO2 capture plant (PCCC). The corrosivity of variable ratio monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP) aqueous solvent blends were assessed after seven days using a gravimetric method for mass change, Fe ion solution concentration (ICP–OES), surface imaging (SEM) and analytical techniques (EDX and XRD). At low concentrations of MEA (25%), the use of the corrosion inhibitor is ineffective as it prevents the formation of protective films naturally developed by AMP. The performance of the inhibitor with MEA–AMP blends was noteworthy at higher MEA concentrations of 50 and 75%. In these cases, reduced corrosion rates were observed through gravimetric and ICP analyses. Imaging by SEM showed reduced surface corrosion and adsorption of STS-derived species. Since these higher MEA concentration solutions offer better CO2 loadings, but still some exhibit corrosive effects when used alone on carbon steel, the use of STS can facilitate an economical usage of carbon steel for PCCC plants.

Efficient energy management of CO2 capture plant using control-based optimization approach under plant and market uncertainties

This paper employs a control-based optimization algorithm encompassing of an intelligence model predictive control (MPC) scheme and mixed integer non-linear programming (MINLP) for coal-fired power plant retrofitted with flexible solvent-based post combustion CO2 capture (PCC) plant (integrated plant). The agility and robustness of the developed control algorithm (MPC) is demonstrated through the control response time and efficiency of energy requirement including the financial and operational benefits of the plant subjected to plant and market uncertainties. While, the MINLP is utilized to forecast plant operational modes by ensuring the operational fidelity of integrated plant. This involves utilization of historical (2011) and forecast (2020) market conditions (electricity tariff and carbon price) subject to maximum plant net operating revenue. The outcomes show that the future power plant will operate in mixed operation modes, for instance in unit turndown and load following modes, which contribute to a minimum capture energy penalty at 3.13 MJth/tonne CO2. Moreover, under the same year (2020), MPC exhibits superior control performance by satisfactorily obtain 94% actual CO2 capture from the ideal cumulative CO2 capture. Additionally, the integrated plant is capable to resume approximately 96% actual revenue from the ideal net operating revenue projected by the control-based optimization algorithm. The algorithm demonstrates that the installation of control system package (MPC) into the flexible PCC plant associated with coal-power generator could contribute to efficient energy management subjects to unprecedented uncertainties.

Nonlinearity Analysis and Multi-Model Modeling of an MEA-Based Post-Combustion CO2 Capture Process for Advanced Control Design

The monoethanolamine (MEA)-based post-combustion CO2 capture plant must operate flexibly under the variation of the power plant load and the desired CO2 capture rate. However, in the presence of process nonlinearity, conventional linear control strategy cannot achieve the best performance under a wide operation range. Considering this problem, this paper systematically studies the multi-model modeling of the MEA-based CO2 capture process for the purpose of (1) implementing well-developed linear control techniques to the design of an advanced controller and (2) achieving a wide-range flexible operation of the CO2 capture process. The local linear models of the CO2 capture process are firstly established at given operating points using the method of subspace identification. Then the nonlinearity distribution at different loads of an upstream power plant and different CO2 capture rates is investigated via the gap metric. Finally, based on the nonlinearity investigation results, the suitable linear models are selected and combined together to form the multi-model system. The proposed model is validated using the measurement data, which is generated from a post-combustion CO2 capture model developed in the go-carbon capture and storage (gCCS) simulation platform. As the proposed multi-linear model has a simple mathematical expression and high prediction accuracy, it can be directly employed as the control model of a practical advanced control strategy to achieve a wide operating range control of the CO2 capture process.

Nonlinear dynamic analysis and control design of a solvent-based post-combustion CO2 capture process

A flexible operation of the solvent-based post-combustion CO2 capture (PCC) process is of great importance to make the technology widely used in the power industry. However, in case of a wide range of operation, the presence of process nonlinearity may degrade the performance of the pre-designed linear controller. This paper gives a comprehensive analysis of the dynamic behavior and nonlinearity distribution of the PCC process. Three cases are taken into account during the investigation: 1) capture rate change; 2) flue gas flowrate change; and 3) re-boiler temperature change. The investigations show that the CO2 capture process does have strong nonlinearity; however, by selecting a suitable control target and operating range, a single linear controller is possible to control the capture system within this range. Based on the analysis results, a linear model predictive controller is designed for the CO2 capture process. Simulations of the designed controller on an MEA based PCC plant demonstrate the effectiveness of the proposed control approach.

Flexible operation of post-combustion solvent-based carbon capture for coal-fired power plants using multi-model predictive control: A simulation study

Solvent-based post-combustion CO2 capture plant has to be operated in a flexible manner because of its high energy consumption and the frequent load variation of upstream power plants. Such a flexible operation brings out two objectives for the control system: i) the system should be able to change the CO2 capture rate quickly and smoothly in a wide operating range; ii) the system should effectively remove the disturbances from power plant flue gas. To achieve these goals, this paper proposed a multi-model predictive control (MMPC) strategy for solvent-based post-combustion CO2 capture plant. Firstly, local models of the CO2 capture plant at different operating points are identified through subspace identification method. Nonlinearity analysis of the plant is then performed and according to the results, suitable local models are selected, on which the multi-model predictive controller is designed. To enhance the flue gas disturbance rejection property of the CO2 capture plant and attain a better adaption to the power plant load variation, the flue gas flow rate is considered in the local model identification as an additional measured disturbance, thus the predictive controller can calculate the optimal control input even in the case of flue gas flow rate variation. Simulation results on an MEA-based CO2 capture plant developed on gCCS show the effectiveness and advantages of the proposed MMPC controller over wide range capture rate variation and power plant flue gas variation.

Technical evaluation of post-combustion CO2 capture and hydrogen production industrial symbiosis

The aim of this study is to develop an industrial ecosystem whereby wastes/products from a Post-combustion CO2 Capture (PCC) plant are utilised in a hydrogen biorefinery. Subsequently, five hydrogen biorefinery models are developed that use PCC's model amine i.e. monoethanolamine (MEA) as a nitrogen source during microbial hydrogen production and CO2 as a process chemical. Technical evaluations of the five case models are carried out to identify the ones that maximise value by multiproduct generation from biomass and fulfil total/partial parasitic energy demand. The case meeting these criteria, produces 3.1t of succinylated lignin adhesive, 4.9t of dry compost and 2744 kWh of electricity from 10t (dry) of sawdust feedstock, daily. Its daily power and heat duties stand at 3906 kWh and 52.1 GJ respectively. Simulations also demonstrate biohydrogen's potential as an energy storage vector for peak/backup power with an annual 1001.4 MWh of power storage capacity from 10t/d feedstock.

Solvent effects on design with operability considerations in post-combustion CO2 capture plants

This work presents an evaluation of the operation of alternative solvents for post-combustion CO2 capture under varying process conditions. The proposed methodology is part of a generalized design and operability framework and relies on the determination of the processes’ steady-state controllability as an inherent system feature, independent of the control structure selection. Assessment of such characteristics can shed light on the effect different capture solvents, process configurations and control structures have on the achieved dynamic responses and further associate them to the physico-chemical properties of the solvents. Three amine based solvents, namely monoethanolamine (MEA), diethanolamine (DEA) and 3-amino-1-propanol (MPA) are introduced to an advanced CO2 capture flowsheet previously designed for optimal steady-state operation. The dynamic characteristics of the investigated solvent-process systems under the influence of disturbances emulating real-time industrial conditions are then assessed and rank-ordered using an index that measures the departure of the process variables from the region of desired operation. As such, for a given disturbance, the drift from the acceptable steady-state operating point and the response of dynamic closed-loop simulations determine the suitability and applicability of each solvent in the employed CO2 capture process configuration. Results reveal the superiority of the dynamic behavior of MPA in terms of economic performance as well as expected process operability for variations in the inlet flue gas flowrate, greatly surpassing the performance of the benchmark solvent, MEA.

Dynamic modeling and advanced control of post-combustion CO2 capture plants

In this paper, model predictive control (MPC) strategies are implemented to address different stripper configurations for the CO2 capture process as part of supercritical pulverized coal-fired (SCPC) power plants. Dynamic models of the conventional and lean vapor compression (LVC) CO2 capture configurations are introduced. Linear and nonlinear MPC strategies are implemented to the CO2 capture processes for a power plant load-following scenario that simulates power plant cycling. Implementation of the MPC strategies for both the conventional and the LVC configurations are successful. Closed-loop responses for the carbon capture rate and total equivalent work are compared and analyzed. The results show that the proposed nonlinear MPC improves the performance by 95% in terms of the integral square error (ISE) results when compared to a PID controller. Also, the proposed LVC configuration with NLMPC implemented has a 5.5% economic improvement when compared to the traditional configuration with PID controllers.

Model study on the influence of plant design, photochemistry and meteorology on atmospheric concentrations of nitrosamines and nitramines in vicinity of an amine-based CO2 capture facility

Post combustion using amine solvents is the most mature technology for the capture of CO2 from coal-fired power plants. Yet, amines atmospheric fate and their environmental and human health impacts, after their release by evaporation and solvent degradation from post combustion CO2 capture (PCCC) plants to the surrounding air, remains a key area to address in view of the worldwide installation of large-scale carbon capture and storage facilities. It is therefore crucial to test potential industrial applications with respect to plant design parameters influencing the fate of the amine degradation products. In this work, a method is developed to propose ideal PCCC plant parameters with respect to its design and operation. A risk limiting scenario was modelled on the UK’s largest CO2 capture pilot plant (CCPilot100+) in Ferrybridge using the air dispersion model ADMS, such that the total nitrosamines and nitramines from the amines realised did not exceed the reported atmospheric safety limits of 0.3ng/m3 with respect to human health in the local environment. A parametric sensitivity study was performed to determine the plant design parameters that most influenced the fate of the nitrosamines and nitramines. It was found that stack height and diameter, exit flue gas temperature and velocity are parameters of the PCCC plant can influence the total sum of nitrosamine (NS) and nitramine (NA) from +58% to −42% compared to the baseline parameterization. Environmental parameters, such as the site specific background meteorology and the background photochemistry were also inspected. Meteorology, i.e. mainly wind speed and wind flow field, affected the total NS and NA baseline concentrations from +28% to −10%, and background photochemistry from +165% −65%. In conclusion, if a PCCC plant is constructed with advantageous exhaust geometry, environmentally optimised operation conditions, and placed in an area with low background pollution, the risk associated with employing amines for capturing CO2 emissions might be reduced to a minimum.

Conventional and advanced exergy analysis of post-combustion CO2 capture based on chemical absorption integrated with supercritical coal-fired power plant

Post-combustion CO2 capture (PCC) based on chemical absorption is one of the strategic technologies identified to reduce emission of greenhouse gases from various power plants. However, PCC based on chemical absorption incurs serious energy penalty due to the use of energy for solvent regeneration. Reducing the energy/exergy use in this process can reduce energy penalties. It is also important to increase the efficiency of the CO2 capture system. This study focuses on: steady state simulation of a closed-loop PCC plant integrated with supercritical coal-fired power plant (SCPP); conventional and advanced exergy analyses of the PCC; and case studies on strategies to reduce exergy destruction in the system components. The conventional exergy analysis evaluates the amount and location of exergy destruction within the whole system. The advanced exergetic analysis estimates the sources of the exergy destruction in individual component or the whole system and the potential for reducing it. The results show that the energy consumption and the efficiency of the PCC process can be improved by recovering the avoidable exergy destroyed in the system components. This is important because for every 1% reduction in the energy required for capture, costs can be lowered to between 0.7–1%.

Effect of Amine Volatility on Aerosol Droplet Development in Absorption Columns

In absorption processes aerosols are generated by spontaneous condensation or desublimation processes in supersaturated gas phases. Amine volatility is a vital screening parameter for amines to be used in CO2 capture. Higher volatility will result in higher losses through the gas phase and may also result in undesired aerosol formation and thereby to higher amine emissions and environmental impact. These amine emissions are one of the challenges in the realization of full scale absorption based post combustion CO2 capture plants. It is crucial to understand the mechanisms governing in particular the aerosol formation and development through a column as this is not well understood.Rigorous modelling of aerosol dynamics leads to a system of partial differential equations. In order to understand mechanics of a particle entering an absorber an implementation of a droplet model is created in Matlab. The model predicts the droplet size, the droplet internal variable profiles and the mass transfer fluxes as function of position in the absorber. The focus of this paper is to study the effect of amine volatility and how this influences the droplet composition, growth rate, final size and the resulting amine loss.

Environmental Risk Assessment of MEA and its Degradation Products from Post-combustion CO2 Capture Pilot Plant: Drafting Technical Guidelines

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 conquer the issues typical of the CO2 capture process but also is easy to understand and thus familiar to local residents and public.

Assessment of Amine Solvent Flexibility in the OCTAVIUS Project

This paper presents results of a study performed in the EU FP7 Project OCTAVIUS.The objective of this work is to assess what possibilities exist for commercial scale, post-combustion amine-based CO2 capture plants to switch between solvents. The influence of solvent properties on process design and operation is highlighted. Process simulations are then performed in order to determine equipment size for an improved flowsheet including Lean Vapour Compression, Improved Economizer Arrangement and Rich Solvent Split with MEA 30% wt and AMP 30% wt as solvents. These results are then used to design a solvent-flexible process that combines the largest pieces of equipment in order to improve flexibility.Process performances and operating parameters are found in line with the numbers obtained when each unit operation was tailored for a single solvent. On the other hand, it is found that operating an MEA-designed plant with AMP yields an important performance penalty since only a 60% capture rate can be achieved. However, the solvent flexibility allowed by the proposed approach implies additional investment costs associated with larger equipment.

Process Integration of Post-combustion CO2 Capture with Li4SiO4/Li2CO3 Looping in a NGCC Plant

Lithium-based sorbents require lower energy requirements for regeneration than other more studied and well known materials such as those used in the calcium looping process; however, thermodynamic assessments of these sorbents in power plants are currently missing. Accordingly, in this work, a thermal integration study of a post-combustion CO2 capture plant using regenerable lithium-based high temperature solid sorbents into a natural gas combined cycle (NGCC) plant has been conducted. A process simulation study has been completed for integration of a Li4SiO4/Li2CO3 looping cycle downstream of the gas turbine in a NGCC plant. A comparison between this technology and a chemical absorption capture plant with two different solvents, namely conventional monoethanolamine (MEA) and a second generation solvent (CESAR-1), has been established based on the European Benchmarking Taskforce (EBTF) methodology. The integration of post-combustion CO2 capture in the power plant results in a decrease of its net electric efficiency. However, capture with Li4SiO4 sorbents decreases the efficiency by 6.9 percentage points, which is lower than the 8.4 and 7.5 percentage points decrease obtained when CO2 capture is based on chemical absorption with MEA and CESAR-1 solvents, respectively. Hence, Li4SiO4/Li2CO3 looping is a promising alternative to amine-based systems when integrated into NGCC plants. Furthermore, additional improvements could be achieved through improved capture process heat integration and optimization.

Demonstrating load-change transient performance of a commercial-scale natural gas combined cycle power plant with post-combustion CO2 capture

The present work aims to study the transient performance of a commercial-scale natural gas combined cycle (NGCC) power plant with post-combustion CO2 capture (PCC) system via linked dynamic process simulation models. The simulations represent real-like operation of the integrated plant during load change transient events with closed-loop controllers. The focus of the study was the dynamic interaction between the power plant and the PCC unit, and the performance evaluation of decentralized control structures. A 613MW three-pressure reheat NGCC with PCC using aqueous MEA was designed, including PCC process scale-up. Detailed dynamic process models of the power plant and the post-combustion unit were developed, and their validity was deemed sufficient for the purpose of application.Dynamic simulations of three gas turbine load-change ramp rates (2%/min, 5%/min and 10%/min) showed that the total stabilization times of the power plant’s main process variables are shorter (10–30min) than for the PCC unit (1–4h). A dynamic interaction between the NGCC and the PCC unit is found in the steam extraction to feed the reboiler duty of the PCC unit. The transient performance of five decentralized PCC plant control structures under load change was analyzed. When controlling the CO2 capture rate, the power plant performs in a more efficient manner at steady-state part load; however, the PCC unit experiences longer stabilization times of the main process variables during load changes, compared with control structures without CO2 capture rate being controlled. Control of L/G ratio of the absorber columns leads to similar part load steady-state performance and significantly faster stabilization times of the power plant and PCC unit’s main process variables. It is concluded that adding the PCC unit to the NGCC does not significantly affect the practical load-following capability of the integrated plant in a day-ahead power market, but selection of a suitable control structure is required for efficient operation of the process under steady-state and transient conditions.

Performance and costs of CCS in the pulp and paper industry part 1: Performance of amine-based post-combustion CO2 capture

The performance of an amine-based post-combustion CO2 capture and storage (CCS) process in an existing Kraft pulp mill and an existing pulp and board mill was assessed. The pulp and paper industry is an energy-intensive industry, with significant amounts of CO2 emitted onsite. The majority of this CO2 originates from the combustion of biomass, which renders it carbon neutral if the biomass used by the industry is grown and harvested in a sustainable manner. If the CO2 emissions from the pulp and paper industry were to be captured and permanently stored, then this could make the industry a potential carbon sink. In this evaluation, different configurations of capturing CO2 from the flue gases of the recovery boiler, the multi-fuel boiler and the lime kiln were assessed. For a stand-alone Kraft pulp mill, the excess steam available is sufficient to cover the demand from the CO2 capture plant. For an integrated pulp and board mill, there is less excess steam available for the CO2 capture plant and an auxiliary boiler may be required. The retrofit of a post-combustion CO2 capture plant into an existing pulp mill increases the steam demand by 1–8GJ/air dried tonne (adt) pulp, depending on the volume of the flue gas treated. This translates to a reduction in the amount of electricity exported to the grid by 0.1–1.0MWh/adt pulp for a stand-alone Kraft pulp mill, and by 0.1–0.5MWh/adt pulp for an integrated pulp and board mill. The total potential for negative CO2 emissions amounts to just under 2.0 Mt CO2/a both for the market pulp mill and for the integrated pulp and board mill.

Mass transfer studies on catalyst-aided CO2 desorption from CO2-loaded amine solution in a post-combustion CO2 capture plant

Two solid acid catalysts of contrasting characteristics were studied on their effects on the liquid-side mass transfer performance for the desorption of CO2 from CO2-loaded 5M monoethanolamine (MEA) solvent. The characteristics of the catalysts were both physical (surface area of catalyst which increases the interfacial area for mass transfer) and chemical (the catalyst acid sites) to catalyze CO2 desorption as illustrated by the use of protonated zeolite socony mobile-5 (HZSM-5) representing a higher Brφnsted/Lewis acid site ratio in a catalyst and γ-Al2O3 representing lower Brφnsted/Lewis acid site ratio. The experiments were conducted in a 1.07m×0.047m2 packed column in full cycle operation. The packing structure consisted of a 0.51m bed height of 6mm inert marble randomly mixed with a specific mass of catalyst and a 50.8mm Sulzer LDX structured packing acting as top and bottom support beds. The effects of amine flow rate and catalyst bed temperature on desorption performance were also analyzed. The catalyst physical activity in terms of the added interfacial area showed higher count for HZSM-5 as compared to γ-Al2O3. The mass transfer coefficient increased by an average of 16% and 22% using γ-Al2O3 and HZSM-5 catalysts, respectively at a CO2 desorption temperature of 85°C. These results are consistent with the greater ability of HZSM-5 to act as a proton donor to facilitate MEACOO- breakdown throughout the loading range of desorption as compared with γ-Al2O3, an amphoteric oxide, which plays the role of HCO3- to release CO2 with lower energy penalties at leaner CO2 loadings. The high percentage increase implies a corresponding reduction in the size of the desorber column which further translates to lower capital costs.

Selection and design of post-combustion CO2 capture process for 600 MW natural gas fueled thermal power plant based on operability

Post-combustion CO2 capture (PCC) plant for a typical 600 MW natural gas fueled thermal power plant was designed as a trade-off between operability and mitigation of the efficiency penalty. Two modified PCC plant configurations with low efficiency penalty were selected. The methodology for designing PCC plants was adapted by incorporating design constraints based on operability and the construction of absorbers. This was applied in sizing the equipment of the plants. Two configurations of absorbers were analyzed based on flue gas flow rate at full-load condition and at time-average of an assumed load variation of a power plant operating flexibly. It was found that the absorber designed at time-average load provided a reduction of approximately 4% in the purchased cost of absorbers. The performance of the designed plants under power plant load variation, flow maldistribution and variable capture ratio was analyzed using off-design condition simulations. The absorber designed at full-load condition was found to lead to lower reboiler duty in order to maintain a similar capture rate to that of the other absorber during part-load operation. Dynamic simulations of the plants with the existing control structure were performed under similar power plant load variations to confirm their operability, and suggestions for selecting one of them were presented.