Electrolysis Plant Research Articles

Study of influence of the charge granulometric composition on the quality of burned anodes

Many factors affect the quality of produced anodes: composition and grade of raw materials, recipe, technology, operating parameters, the granulometric composition of charge materials etc. The most significant influence is from the last factor. The article provides research results on the effect of fine fractions on the quality of anodes. The research object is charge of different granulometric composition used to produce anodes at the Kazakhstan Electrolysis plant in Pavlodar. Six types of anode mass with different granulometric composition were prepared for the experiment. The anode mass was made of petroleum coke, pitch, recycled anodes, and dust from filters and grinder produced during coke treatment. Several properties of the specimens were studied. In total, six specimens were made and, specimen № 4 drew particular attention. Its granulometric composition is as following: coarse fractions – 25 %, medium fractions – 20%, fine fractions (grinder dust) – 50 %, and filter dust – 5 %. The results showed that some specifications of anodes, like gas permeability and apparent density that affect anode consumption, are improved as the fineness of dust grinding. For instance, the addition of dust fraction in the charge increases anode density from 1.542 to 1.639 kilogram per cubic decimeters and decreases gas permeability from 1.01 to 0.78 nPm. In addition, the presence of dust fraction minimizes the porosity of the anode block. The results of the investigation help adjust the optimal recipe of anode mass to obtain high-quality anodes.

Optimal operation of the hydrogen-based energy management system with P2X demand response and ammonia plant

Hydrogen production is the key in utilizing an excess renewable energy. Many studies and projects looked at the energy management systems (EMSs) that allow to couple hydrogen production with renewable generation. In the majority of these studies, however, hydrogen demand is either produced for powering fuel cells or sold to the external hydrogen market. Hydrogen demand from actual industrial plants is rarely considered. In this paper, we propose an EMS based on the industrial cluster of GreenLab Skive (GLS) that can minimize the system’s operational cost or maximize its green hydrogen production. EMS utilizes a conventional and P2X demand response (DR) flexibility from electrolysis plant, hydrogen storage tank, electric battery, and hydrogen-consuming plants to design the optimal schedule with maximized benefits. A potential addition to the existing components at GLS - an ammonia plant is modelled to identify its P2X potential and assess the economic viability of its construction. The results show a potential reduction of 51.5–61.6% for the total operational cost of the system and an increase of the share of green hydrogen by 10.4–37.6% due to EMS operation.

Refining of Primary Aluminum from Vanadium

The paper investigates the technology of refining primary aluminum from vanadium impurities, based on flux treatment with boron-containing fluxes. In the Pavlodar region of the Republic of Kazakhstan, on the basis of local enterprises, the production of primary aluminum and products based on local raw materials is developing. The main problem in the production of primary aluminum on the basis of JSC “Kazakhstan Electrolysis Plant” is the presence of undesirable vanadium impurities, which pass into metal during electrolysis from baked anodes based on calcined coke (vanadium content up to 800 ppm) of the local enterprise LLP UPNK-PV (Pavlodar, Kazakhstan). The authors investigated the process of ladle refining of aluminum from vanadium using the Al-B (3% B) alloy. Laboratory and industrial tests have shown a decrease in the vanadium content by an average of 78% in the bulk of the metal, with an increase in its content in volume up to 5-10% of the ladle capacity. It was found that mixing leads to a certain averaging of the vanadium content in the ladle volume.

Carbon-neutral fuels and chemicals: Economic analysis of renewable syngas pathways via CO2 electrolysis

Producing syngas, a blend of CO and H2, is the starting point to the large-scale production of valuable chemicals including fuels and methanol. This study evaluates pathways for syngas production from air-captured and tail-gas captured CO2 in terms of net CO2 emissions, energy efficiency and associated costs. We analyzed three direct air capture (DAC) to syngas plants (H2/CO = 2.5, suitable for Fischer Tropsch or methanol synthesis) that perform the CO2-reduction step by either: i) thermocatalytic reverse water gas shift (rWGS); ii) gaseous CO2-electrolysis, and iii) direct (bi)carbonate electrolysis of the carbon capture solution. The gaseous electrolysis plant did not offer any advantage over the thermolytic rWGS because it implied similar net CO2 emissions (equivalent to 50% of the CO2 captured initially), even if renewable electricity is used. The carbon footprint of the gas CO2-electrolysis pathway originates in the calcination step for the CO2 recovery from the air and tail-gas capture solutions. Only a significant improvement in state-of-the-art single pass conversion of CO2 in gas CO2-electrolyzers (10–30%) would decrease the associated CO2 emissions. In contrast, the novel DAC-(bi)carbonate electrolysis offered the best pathway in terms of net CO2 emissions. However, it entailed higher syngas costs (1.90 $ kg −1) compared to gaseous electrolysis (1.30 $ kg−1) or the conventional rWGS (1.1 $ kg−1) pathways. The DAC-carbonate electrolysis plant may become cost-competitive with reasonable improvements in performance, mainly faradaic efficiency to CO and cell voltage, electricity price ≤ 35 $ MWh−1 and membrane price < 450 $ m−2. Even in the best case scenario all the CO2-based alternatives entailed a higher levelized cost of syngas, synfuel or methanol than current retail prices.

Life-cycle analysis of greenhouse gas emissions from hydrogen delivery: A cost-guided analysis

The cost of hydrogen delivery for transportation accounts for most of the current H2 selling price; delivery also requires substantial amounts of energy. We developed harmonized techno-economic and life-cycle emissions models of current and future H2 production and delivery pathways. Our techno-economic analysis of dispensed H2 costs guided our selection of pathways for the life-cycle analysis. In this paper, we present the results of market expansion scenarios using existing capabilities (for example, those that use H2 from steam methane reforming, chlor-alkali, and natural gas liquid cracker plants), as well as results for future electrolysis plants that use nuclear, solar, and hydroelectric power. Reductions in greenhouse gas emissions for fuel cell electric vehicles compared to conventional gasoline pathways vary from 40% reduction for fossil-derived H2 to 20-fold for clean H2. Supplemental tables with greenhouse gas emissions data for each step in the H2 pathways enable readers to evaluate additional scenarios.

Arsenic removal procedure for the electrolyte from a hydro-pyrometallurgical complex

Commercial copper (Cu) is obtained by a hydro-pyrometallurgical process, where the Cu anodes obtained in the furnaces (Cu > 99.5%) are enriched up to 99.99% in “cathodes” by electrorefining at an electrolysis plant. During this process, some impurities accumulate in the electrolyte, mainly arsenic (As), which decrease the quality of the Cu cathode. For this reason, the electrolyte is sent to an electrolyte cleaning plant (ECP) for its purification. Electrolyte sludge (ES) is produced in the last stage of purification and is recirculated back to the furnace due to the high Cu content. This recirculation involves a severe problem of As accumulation in the industrial process. The objective of this work was to develop a procedure to fully dissolve the ES, removing the As and recovering its Cu content. The ES dissolution process was optimised (dissolution efficiency > 99%) in H2SO4 (1.4 M)/HNO3 (1.8 M) medium using a 1:20 g mL−1 solid-to-liquid ratio. As was removed from the ES solution by its precipitation as iron (III) arsenate, with high efficiency (more than 70%). After As removal, the Cu can be precipitated as copper sulphate, which is used in several applications.

Green hydrogen: the crucial performance of electrolysers fed by variable and intermittent renewable electricity

After a short review of past and present EU green energy programmes, the article deals with the great surging interest on green hydrogen worldwide and the consequent programme of the EU. Considering the trend in the increasing percentage of variable wind and solar plants in the EU, a preliminary evidence on their variability is reported and based on their capacity factors during the different months of last year in France, Spain and Italy. The basic characteristics and ‘colours’ of hydrogen are underlined, and emphasis is placed on the current and future global demand in 2030 and 2050. The expected cost of the energy content of green hydrogen just at the exit of electrolysers (power to gas—P2G) fed by renewable electricity as derived by key international well-recognised organisations is presented. The high cost of hydrogen content of energy at the exit of electrolysers is underlined; even with the 2050 prospective low cost around €1.2/kg of hydrogen, it corresponds to €36/MWh to be compared with the EU present gas market price of €13/MWh that with a penalisation of around €130/t CO2 will reach €36/MWh. Even if the compression, transport and distribution of hydrogen pose important challenges, the article is limited to its production subject to the effects of variability of wind and solar on the performance of large electrolysis plants characterised by a complex balance of plant composed of various subsystems. Based on almost instantaneous data logging from some Italian wind and solar plants, questions are raised about the effects of variability and intermittency on efficiency, flexibility, reduction of the load factor of the electrolyser for its inherent limits. This is relevant due to its minimum load for operation, frequent cold starts, time to ramp up to the operating power. This leads to concerns on the feasibility of operation of an electrolyser plant if it is fed only by a variable and intermittent renewable dedicated plant not connected to the network. Owing to the specific variation versus time of the renewable electricity feeding the electrolyser and of the constraints of hydrogen demand, the cost of hydrogen provided in the literature is only valid for general scenarios. A tailor-made approach case by case for the optimisation and design of the plant appears to be necessary. In contrast the possible smooth behaviour of an electrolyser connected to the grid with power purchasing agreements for electricity from renewable plants spread in the system is analysed; the existing regulations foresee increments in the price of this electricity to remunerate its transport and for ancillary, thus increasing the hydrogen production cost. On the other hand, a better capacity factor could ameliorate the situation but also in this case detailed optimisations are needed. In the conclusions, the still great technological, legal, regulatory and socio-economic challenges for hydrogen production, transport, distribution and final uses are pointed out with the necessity of rigorous analyses and prototypal experiments for a better definition of its market share and relevant timing; specifically it is suggested to concentrate efforts also on a possible new way of operation of electrolysers not as up to now connected to a strong electricity network but fed by RES plants isolated from the grid and characterised by daily, monthly and seasonal variations with time intervals of total lack of electricity at all and sharp up-and-down ramps.

Plug Power, partners invest in green H2

The fuel-cell maker Plug Power will build a green hydrogen plant in south-central Pennsylvania with the power provider Brookfield Renewable Partners. Plug says the electrolysis plant will produce 15 metric tons (t) of H 2 per day when it starts up in late 2022, drawing power from the Susquehanna River’s Holtwood Dam. Plug is also planning a 45 t green H 2 plant near Rochester, New York. Separately, Plug will put $200 million into an H 2 infrastructure investment fund that it is forming with the cryogenic equipment company Chart Industries and the oil-field services firm Baker Hughes, each of which is contributing $60 million.

Enabling the use of seawater for hydrogen gas production in water electrolyzers

Hydrogen gas will play an increasingly critical role in developing a carbon-neutral energy infrastructure, but it will need to be produced by water splitting using renewable electricity sources. A recent study by Veroneau and Nocera in Proceedings of the National Academy of Sciences (PNAS) suggests an electrolyzer that produces desalinated water from abundant seawater by using forward osmosis.

Enrichment of Integrated Steel Plant Process Gases with Implementation of Renewable Energy

The steel industry is one of the most important industry sectors, but also one of the largest greenhouse gas emitters. The process gases produced in an integrated steel plant, blast furnace gas (BFG), basic oxygen furnace gas (BOFG) and coke oven gas (COG), are due to high shares of inert gas (nitrogen) in large part energy poor but also providing a potential carbon source (carbon monoxide and carbon dioxide) for the catalytic hydrogenation to methane by integration of a power-to-gas (P2G) plant. Furthermore, by interconnecting a biomass gasification, an additional biogenic hydrogen source is provided. Three possible implementation scenarios for a P2G and a biomass gasification plant, including mass and energy balances were analysed. The scenarios stipulate a direct conversion of BFG and BOFG resulting in high shares of nitrogen in the feed gas of the methanation. Laboratory experimental tests have shown that the methanation of BFG and BOFG is technically possible without prior separation of CO2. The methane-rich product gas can be utilised in the steel plant and substitutes for natural gas (NG). The implementation of these renewable energy sources results in a significant reduction of CO2 emissions between 0.81 million tonnes CO2eq and 4.6 million tonnes CO2eq per year. However, the scenarios are significantly limited in terms of available electrolysis plant size, renewable electricity and biomass.

Current Redistribution in a Superconducting Multi-Strand 35 kA DC Cable Demonstrator

High temperature superconductors (HTS) are discussed as energy-efficient solutions for industrial high-current applications beyond 10 kA e.g., bus bar systems in industrial electrolysis plants. In this contribution, the experimental test of a 3.6-meter-long 35 kA DC demonstrator, made from twelve high-current HTS CrossConductor (HTS CroCo) strands in an liquid nitrogen bath at T = 77 K is presented. In this work, a common connector concept of the twelve HTS CroCo strands is proposed. Compared to earlier results without common connector, lead resistances were effectively reduced and current distribution among the individual strands was significantly facilitated. This is confirmed by the observation of increased critical cable current of 37.6 kA compared to 33 kA in previous work without low-resistive common connector. Additionally, the current range, in which all twelve strands reached their critical electric field, was found to be reduced from >7 kA to 2 kA. Results are discussed and assessed with the help of an electric circuit model, from which the solder resistances at the connections could be obtained by fitting. Particular focus was given to the investigation of current redistribution in the demonstrator. Therefore, a heater was installed on one HTS CroCo strand, and activated to raise the temperature on this strand and quench a single strand locally. It is observed that current is redistributed through the common connectors to the other strands.

Development of high pressure membraneless alkaline electrolyzer

A technology for cyclic generation of hydrogen and oxygen using electrodes made of variable valency material that does not need the use of separating ion-exchange membranes is presented. The technological solution enables to fabricate electrolyzers for uninterrupted producing high-pressure hydrogen with reduced energy intensity of the production. The total work for compressing 1 m3 of hydrogen and 0.5 m3 of oxygen has been estimated. Results of investigation of influence of discrete supply of DC current to the electrolysis cell, in order to improve the processes of gas evolution and to simplify the power systems of the electrolysis plant, have been considered. There is also considered an electrolysis installation equipped with a thermosorption compressor in which LaNi5 is used as a hydride-forming compound. The comparative characteristics of the developed electrolyzer and the currently used hydrogen generators are given.

Short Circuit Fault Detection against High Thermal Background Using a Two-Level Scheme Based on DoG Filter

Short circuit is a key factor which drastically affects the efficiency of metal electrorefining. Infrared image of the intercell busbar region is used to perform short circuit detection. To cope with the high thermal background, a two-level short circuit detection method is designed. Firstly, with background subtraction, high intensity short circuit electrodes, as well as the background, are removed, and normal working electrodes are preserved. In the second stage, suspicious short circuit areas are sifted out by normal electrode detecting and texture period estimation. Gaussian difference filter (DoG) which is based on the human visual system is improved to match the target gray distribution. A comparative experiment indicates that the proposed orthogonal DoG outperforms the original DoG and top-hat in the accuracy of normal electrode detection. The two-level detection method in this paper is applied in a copper electrolysis plant and exhibits superiority in locating short circuits and avoiding miss detection.

Application of highly efficient hydrogen generation and storage systems for autonomous energy supply

Purpose. Development of scientific and engineering solutions to improve the reliability of power supply of stand-alone systems and mitigate the environmental burden by using hydrogen technologies for energy storage. Methodology. The calculation method provides a set of optimal technical solutions for determining the effective operating modes of a stand-alone power supply system for supplying hydrogen to a fuel cell based on the electric load schedules of a particular consumer by using a computational experiment. Findings. Based on the study, a technological scheme of a stand-alone power supply system based on fuel cells was developed, and an approach to the creation of a metal hydride system for accumulating and supplying hydrogen to fuel cells was substantiated. A calculation algorithm was developed that allows calculating the annual energy balance of a specific consumer and selecting the necessary equipment to implement the scheme based on the annual heat and electric load schedule. Originality. An alternative scheme of guaranteed electric power and heat supply for a stand-alone house without using imported fuel is proposed. The advantage of such a scheme is that it is closed because hydrogen is produced on site to power the fuel cell, while the metal hydride hydrogen storage system is capable of performing hydrogen absorption and its release due to the hot and cold water resources available in the system. Practical value. The technology for converting the energy of primary sources by creating a wind-driven energy technological complex using an electrolysis plant and a metal hydride hydrogen storage system will solve the problem of smoothing the irregular electric power supply from renewable sources.

Techno-economic modeling of an integrated biomethane-biomethanol production process via biomass gasification, electrolysis, biomethanation, and catalytic methanol synthesis

Biological methanation (biomethanation) of syngas obtained from biomass gasification offers the opportunity to employ a low-pressure, low-temperature process to produce storable bio-derived substitute natural gas (bSNG), although its economic viability is limited by high energy and biomass costs. Research on syngas biomethanation techno-economic performance is limited and novel biomass-to-biomethane process configurations are required in order to assess opportunities for the enhancement of its efficiency and economic feasibility. In this study, we carried out the techno-economic modeling of two processes comprising integrated biomass gasification, electrolysis, and syngas biomethanation with combined heat and power recovery in order to assess and compare their fuel yields, energy efficiency, carbon efficiency, and bSNG minimum selling price (MSP). The first process operates standalone biomethanation (SAB) of syngas and can produce approximately 38,000 Nm3 of bSNG per day, with a total plant efficiency of 50.6%. The second process (integrated biomethane-biomethanol, IBB) exploits the unconverted carbon stream from the biomethanation process to recover energy and synthesize methanol via direct catalytic CO2 hydrogenation. In addition to the same bSNG output, the IBB process can produce 10 t/day of biomethanol, at a 99% purity. The IBB process shows little global energy efficiency gains in comparison with SAB (51.7%) due to the large increase in electrolytic hydrogen demand, but it shows a substantial improvement in biomass-to-fuel carbon efficiency (33 vs. 26%). The SAB and IBB processes generate a bSNG MSP of 2.38 €/Nm3 and 3.68 €/Nm3, respectively. Hydrogenation of unconverted carbon in biomass-to-biomethane processes comes with high additional capital and operating costs due to the large-scale electrolysis plants required. Consequently, in both processes, the market price gap of the bSNG produced is 0.13 €/kWhbSNG (SAB) and 0.25 €/kWhbSNG (IBB) even under the most optimistic cost scenarios considered, and it is primarily influenced by the cost of surplus electricity utilized in electrolysis, while the selling price of biomethanol exerts a very limited influence on process economics. Intensive subsidization would be required in order to sustain the decentralized production of bSNG through both processes. Despite their limited economic competitiveness, both processes have a size comparable with existing renewable gas production plants in terms of bSNG production capacity and the IBB process is of a size adequate for the supply of biomethanol to a decentralized biorenewable supply chain.

Conceptual Design Development of Coal-to-Methanol Process with Carbon Capture and Utilization

Methanol is a clean fuel and an important feedstock for the petrochemical industry. Conventionally, the coal-to-methanol process generates a substantial amount of CO2 emissions with a low yield of methanol. In this study, we propose the conceptual design development of coal-to-methanol process using captured CO2 from the gasification plant by implying process intensification. The base case and three alternative designs have been developed using the Aspen Plus to analyze the process performance. The four designs have been compared in terms of their energy consumption, economics, methanol production rate, and carbon emissions while maintaining the gasifier operation conditions, sulfur content in the syngas, and stoichiometric number at the methanol synthesis reactor. Among the developed designs, the most feasible design requires an energy requirement and product cost of 21.9 GJ and $142.5 per ton of methanol product. The study also showed that the coal-to-methanol process assisted with external hydrogen from electrolysis plant for achieving higher production rates and low CO2 emissions is currently not economical due to the high H2 cost.

Techno-Economic Analysis of the Sabatier Electrolyzer: CO2-to-Methane Using Proton-Conducting Ceramics

As intermittent renewable-electricity generation achieves higher market penetration, the need for quick and efficient storage and dispatch of this energy becomes more pronounced. In this report, we present our techno-economic analysis of a Sabatier Electrolyzer based on proton-conducting ceramic materials. The Sabatier Electrolyzer technology is illustrated in the figure, and integrates steam electrolysis and hydrogen production with CO2 hydrogenation into a single reactor based on protonic-ceramic electrolysis cells (PCECs) and Sabatier catalysts. CO2 and H2O are fed to fuel and steam electrodes, respectively, of a protonic-ceramic electrolysis cell. An external power source drives H2O splitting at the steam electrode. The product O2 is exhausted from the cell. The product protons are driven across the protonic-ceramic membrane to the fuel electrode, where they react with CO2 to form CH4 and H2O. The combination of processes can match the exothermicity of CO2 hydrogenation with the endothermicity of H2O electrolysis to facilitate thermal balance and high efficiency.When powered by solar and / or wind energy, the Sabatier Electrolyzer has the potential to store intermittent renewable electricity in the form of commodity chemicals while simultaneously reducing carbon dioxide emissions, thereby addressing two principle societal concerns in a single device. Methane presents an attractive energy-storage method; it is stable, integrates well with our existing infrastructure, and can be dispatched quickly for energy generation. In addition to our experimental demonstrations presented in another report, we explore the market potential of the Sabatier electrolyzer through techno-economic analysis (TEA) of a 1.3-MW PCEC electrolysis plant that methanates CO2 produced from a Greenfield coal-fired power plant with a three-ton-per-day coal feedstock. Preliminary models establish a levelized-cost of storage (LCOS) for synthesized methane produced by the PCEC plant. This LCOS represents the required sale price of 1 MWh of synthesized methane to enable “break-even” investment of the SOEC plant over a 30-year plant life. We estimate LCOS of just under 400 $/ MWhe of synthesized methane, less than that of lithium-ion batteries, lead-acid batteries, and advanced lead-acid batteries for commercial and industrial standalone applications.The techno-economic model includes extensive cost estimates for PCEC balance of plant components taken from the literature, the Energy Information Administration, and the U.S. Department of Energy. Our sensitivity analyses show that PCEC Faradaic efficiency and methane selectivity have the largest impact on the Levelized Cost of Storage. A 20% reduction in methane selectivity raises LCOS of the plant by 258 $/MWh, or more than 50%. Further, a 20% decrease in Faradaic efficiency increases LCOS by nearly the same amount. Of the many model inputs, these two parameters prove most critical in estimating the Levelized Cost of Storage.Building on this result, we are now developing a Life Cycle Assessment using the well-established CO2U analysis from the DOE National Energy Technology Laboratory. LCA results will also be presented as part of this talk. Figure 1

Multi-state techno-economic model for optimal dispatch of grid connected hydrogen electrolysis systems operating under dynamic conditions

The production of hydrogen through water electrolysis is a promising pathway to decarbonize the energy sector. This paper presents a techno-economic model of electrolysis plants based on multiple states of operation: production, hot standby and idle. The model enables the calculation of the optimal hourly dispatch of electrolyzers to produce hydrogen for different end uses. This model has been tested with real data from an existing installation and compared with a simpler electrolyzer model that is based on two states. The results indicate that an operational strategy that considers the multi-state model leads to a decrease in final hydrogen production costs. These reduced costs will benefit businesses, especially while electrolysis plants grow in size to accommodate further increases in demand.

Optimal Sizing of a Real Remote Japanese Microgrid with Sea Water Electrolysis Plant Under Time-Based Demand Response Programs

Optimal sizing of power systems has a tremendous effective role in reducing the total system cost by preventing unneeded investment in installing unnecessary generating units. This paper presents an optimal sizing and planning strategy for a completely hybrid renewable energy power system in a remote Japanese island, which is composed of photovoltaic (PV), wind generators (WG), battery energy storage system (BESS), fuel cell (FC), seawater electrolysis plant, and hydrogen tank. Demand response programs are applied to overcome the performance variance of renewable energy systems (RESs) as they offer an efficient solution for many problems such as generation cost, high demand peak to average ratios, and assist grid reliability during peak load periods. Real-Time Pricing (RTP), which is deployed in this work, is one of the main price-based demand response groups used to regulate electricity consumption of consumers. Four case studies are considered to confirm the robustness and effectiveness of the proposed schemes. Mixed-Integer Linear Programming (MILP) is utilized to optimize the size of the system’s components to decrease the total system cost and maximize the profits at the same time.

HTS CroCo - a Strand for High Direct Current Applications

High temperature superconductors (HTS) are discussed as energy-efficient solutions for applications needing high direct currents beyond 10 kA e.g. for large high-field magnets or bus bar systems in industrial electrolysis plants. A number of high-current cable concepts based on REBCO tapes were developed such as the Roebel cable, co-axially wound tapes and several stacked-tape arrangements, among them the HTS CrossConductor (HTS CroCo), a stacked-tape conductor with high current density developed at KIT. In this manuscript, the experimental test of a high DC demonstrator, termed Supra-DC-Cable, made from twelve HTS CroCo strands is presented. The demonstrator was tested successfully at T = 77 K, reaching the expected critical current of 33 kA at 77 K and even for a constant-current operation at 36 kA for more than 30 minutes limited by the copper connections, not the superconducting cable. Currents and voltages were measured in all twelve strands individually during the parallel operation in the cable. These measured data allow the experimental validation of the modelled current distribution, based on the individual characterization of the twelve strands.