Concept and design of a 3.5 MW pilot plant for high temperature electrolysis of water vapor

0360-3199/86 $3.00 + 0.00 Pergamon Press Ltd. © 1986 International Association for Hydrogen Energy.

Int. J. Hydrogen Energy, Vol. 11, No. 5, pp. 309-315, 1986. Printed in Great Britain.

CONCEPT AND DESIGN OF A 3.5 MW PILOT PLANT FOR HIGH TEMPERATURE ELECTROLYSIS OF WATER VAPOR K. H. QUANDT and R. STREICHER Lurgi GmbH, Frankfurt am Main, F.R.G.

(Received 29 October 1985) Abstract--The high temperature electrolysis (HOT ELLY) of water vapor using zirconia as a solid oxide electrolyte has been demonstrated in laboratory experiments to be a very efficient method of hydrogen production. This program is a cooperation between Dornier Systems, Friedrichshafen and Lurgi, Frankfurt. In this paper, the thermodynamical advantages of electrolysis at very high temperatures, typically 900-1000°C, will be briefly reviewed first. After the principal technical realization has been explained the status of the advanced cell and module development will be illustrated. The process engineering for electrolysis plants will then be discussed and our concept for building up electrolysis units introduced. Finally a cost examination and comparison with conventional H2-production plants will be reviewed, INTRODUCTION Electrolysis of water to provide hydrogen for chemical processes plays only a minor role today. Less than 1% of worldwide hydrogen demand is produced electrolytically. The reason for this is the high energy requirement and cost of the electrical energy needed to split the water into hydrogen and oxygen. In comparison to hydrogen production from fossil feedstock, such as natural gas or oil fractions, electrolytically produced hydrogen will be only of interest if power demand for electroysis can be significantly reduced. Such improvement in the electrolytic process can be obtained by water vapor electrolysis at high temperatures because (1) the total energy demand for water splitting is lower in the vapor phase than in the liquid phase and the energy for vaporization can be provided thermally; (2) the minimum demand for electrical energy needed for electrolysis decreases with increasing temperature, i.e. one has the possibility of providing part of the splitting energy by thermal energy instead of electrical thus achieving higher total efficiency; and (3) the improved reaction kinetics at elevated temperatures lower overvoltages.

of about 80% hydrogen and 20% steam leave it at the other. Single cells and tubes containing up to 20 cells with an electrolyte wall thickness of 0.3 mm are being tested in the laboratory at present. More details on cell design and materials are given in [1]. Scaling up for future technical electrolyzers is a simple matter of arranging an appropriate number of electrolysis tubes on a support pipe to form registers. Several registers are built up to constitute a module as illustrated in Fig. 2. Steam is distributed to the electrolysis tubes via the

Fig. 1. Set-up and flow of current of series connected cells.

DESIGN OF ELECTROLYSIS M O D U L E Steam electrolysis operates on the principle of splitting steam using a solid electrolyte, namely yttriumstabilized zirconium (Y203 + ZrO2). A tubular cell has proved to be the most favorable geometry, particularly with respect to steam and gas routing as well as for separating the product gases. For practical solid state electrolyzers, tubular cells of solid electrolyte were developed by Dornier Systems. The tubular cells are coated with a cathode and an anode material inside and outside respectively. A considerable number of cells are connected electrically in series and form a long, gastight tube as shown in Fig. 1. The steam enters the tube at one end and a mixture 309

Fig. 2. Design of electrolysis module.

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K.H. QUANDT AND R. STREICHER

upper header and product hydrogen is collected in the lower header. Steam enters the cells through the upper pipe and is converted to hydrogen. The oxygen flows about the tubes. The hydrogen is passed via a control pipe in counter-current to the hydrogen header. Gastight jointing of the ceramic parts in the hot zone of an electrolysis unit is an important requirement. This has been successful on small test units. Further development work and module tests are currently being carried out.

PROCESS E N G I N E E R I N G Investigations were always made with regard to process engineering the overall process, i.e. for electrolysis operation, steam production and heat recovery from the product gases for various variations of the electrolysis operation. The electrolysis requires high temperature steam at 900°C, some of which is generated utilizing heat recovered from the product gases. It is possible to generate the overall steam requirement in the electrolysis unit itself. The appropriate water treatment units, feedwater preheaters, heaters and evaporators are to be provided. A process calculation has shown that the waste heat from product gases is not sufficient to heat the total feedwater to 900°C. An additional source of heat would be necessary. This process route requires a high quantity of equipment which leads to relatively high investment costs. A cost estimate showed that it is more advisable to procure slightly superheated feed steam from an available steam source. Here, the steam temperature is above 150°C, dependent upon the pressure in the electrolysis unit. The saturated steam can be brought up fully to the electrolysis temperature necessary utilizing the product gas stream in two heat exchangers (El, E2, see Fig. 4). The remaining heat from the product gases is used to again evaporate the condensate from the hydrogen stream. The process design for the overall process (arrangement and number of heat exchangers, high temperature heat requirement, etc.) is dependent upon the mode at which the electrolysis cell operates. There

are three means of operating high temperature electrolysis which can best be illustrated by the currentvoltage characteristic in Fig. 3.

Thermoneutral operation At a cell voltage of 1.3V (just corresponding to the splitting energy of 3.12 kWh m -3 H2) the process is thermoneutral; this means the electrolysis would generate enough heat to compensate endothermic losses. The product gases, hydrogen and oxygen, leave the cells at the temperature of the incoming steam. Thus, the feed steam cannot be brought to 900°C by the product gases (due to the necessary AT in the heat exchanger of about 50°C). An additional high temperature source is necessary. Initial deliberations indicate that the heat required from a high temperature source is relatively small for this process variation. In view of the considerable quantity of equipment required for peak superheating (possibly coupling to a high temperature reactor) and the low saving in electrical energy as against the exothermic case, this operation variant does not appear practical.

Exothermic operation In this process variant the overall feed steam is brought up to electrolysis temperature by the product gas. A high temperature source is not required. Therefore, we favor exothermic operation of the cells at a voltage slightly above 1.3 V. With a voltage of 1.32 V on average, excess heat (I2R-losses) is produced which superheats the product gases by approx. 70°C above the temperature of the steam at the cell inlet. Here only low temperature (e.g. 200°C) steam is used as feed for the electrolyzer. It is heated up to the operating temperature of 900°C in a heat exchanger by the hot product gases and subsequently electrolyzed. This process is characterized by a slightly reduced overall efficiency and its investment costs are lower. A further advantage of this process version is that it only requires low temperature steam and electricity. A schematic

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process flow sheet of this autothermic high temperature steam electrolysis is shown in Fig. 4.

Endotherrnic operation On dropping the cell voltage below the thermoneutral point, the current density and thus the quantity of hydrogen produced is reduced. At a current density of 0.3 A cm-Zand a voltage of 1.07 V, the electrical power required for electrolysis is reduced to 2.6 kWh mN-3 H 2. At the same time, in this case, it is necessary to couple in 0.6 kWh high temperature heat (Table 1). This leads to an increase in the overall efficiency. The disadvantage is the increased investment costs for this process.

PILOT PLANT CONCEPT Hydrogen at a certain pressure is required for most applications. The hydrogen ought to be produced at elevated pressure from the very beginning to facilitate subsequent compression or dispense with it altogether, Concepts were developed for various pressure levels for this reason and to permit the affects of the operating pressure upon the design to be assessed. The investment costs for a 25 bar and a 3 bar steam electrolysis plant were assessed. The results show that the investment costs for the 3 bar version (including compression of the H2 from 3 to 25 bar) are lower than for the version operating at higher pressure. The reasons are the higher costs for the vertical cylindrical vessel

Table 1. Energy requirements and overall efficienciesof hydrogen production per ms 3 of hydrogen High temperature steam electrolysis Endothermic Exothermic operation operation Electrical energy Additional heat Steam Current density Overall efficiency at n = 38% for power generation Conversion factor (HHV hydrogen per power)

2.6 kWh 0.6 kWh 0.6 kWh 0.3 Acm -2

3.2 kWh -0.6 kWh 0.6 Acm -2

Water electrolysis

4.3 kWh --0.2 A cm-2

44.7%

39.3%

31.4%

1.36

1.11

0.83

Compared with water electrolysis the electrical energy requirement for steam electrolysis is reduced to approx. 75% for authothermic operation and to approx. 60% for endothermic operation. The overall efficieneies are higher for both operating modes of steam electrolysis than for power generation.

312

K.H. QUANDT AND R. STREICHER

Fig. 5. Model of a higher temperature electrolysis vessel (capacity: 1000mt~3 H2 h -1, pressure: 3 bar).

and the special structure to support the modules inside the vessel. Therefore, the present plant concept is based on an operating pressure of 3-5 bar. Figure 5 shows the model of a typical low-pressure steam electrolysis vessel to produce 1000 mN3 H2 h -1 and a power demand of 3.2 kWh mN3 (autothermic operation). A box-type electrolysis vessel can be employed as the operating pressure does not exceed 3-5 bar. However, the interior of this vessel has to be insulated to withstand the high operating temperatures. The vessel walls subjected to pressure comprise stainless steel plates with metal ribs welded to them vertically for reinforcement purposes. The shell is provided with four ports for inspection and installation purposes. The space between the modules inside the shell is sufficient to remove one electrolysis module and permit entry for installation and repair. The electrolysis modules (Fig. 2) are arranged horizontally on the same level. Electric power is fed to the unit through feeders in the side wall. The heat exchangers for steam superheating (El, E2, Fig. 4) are arranged horizontally on the opposite side and stacked to save space. The combination of large numbers of modules enables approximately 1000 mNJ H2 h- 1 hydrogen to be produced in one vessel. If a higher hydrogen output is required, several electrolysis vessels must be connected in parallel. The space requirement for such an electrolysis vessel is approx. 3.4 x 14.5 m. The height is about 2 m. A complete electrolysis unit with the pertinent equipment such as heat exchangers, pump and piping is the basis for the following feasibility study.

COST ESTIMATE A N D E C O N O M I C COMPARISON Covering the increasing hydrogen requirement in the short or middle term using electrolytic hydrogen depends almost exclusively on its production costs compared with the classic hydrogen production processes using fossil fuels. The economic data for the H O T ELLY process were determined for a production unit of 50,000 mN3 H 2 h - 1 (3.2 kWh mN-3 H2). To achieve this, 50 basic units with a production capacity each of 1000 m 3 h-1 (Fig. 5) are arranged in parallel. This was viewed in comparison with the conventional electrolysis process [2] as well as processes producing hydrogen from natural gas and coal [3]. The majority of the production costs for electrolytic hydrogen are power costs which can amount to 80% and capital costs of about 15%. The specific power requirement has been accurately determined by laboratory tests and therefore the percentage of the power costs in relation to the overall costs can be determined exactly. The investment costs are more difficult to assess. The equipment (vessels, heat exchangers etc.) can be calculated relatively accurately but the costs for the cell modules have to be estimated. The overall investment costs of about 1200U.S.$ mN-3 H2 h -1 are the basis for the feasibility study. The investment costs cover material, machinery, electrics etc. as well as those for engineering, transport, erection

313

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and building. The first comparison in Fig, 6 shows the hydrogen costs using various electrolytic processes. The advantages of steam electrolysis are clearly recognizable, especially in the case of high power prices. In comparing the costs of hydrogen produced by steam reforming natural gas or gasifying coal, the prices of the primary energy are treated as parameters. Figure 7 shows the energy price pair (natural gas price vs power price). H O T ELLY hydrogen or natural hydrogen yields the best price. However, a statement can only be made with respect to the individual case due to the differing conditions prevailing in individual countries. In 1981, the production of electrolytic hydrogen in Germany was almost competitive economically with production from natural gas [Point (1) in Fig. 7]. Due to the drop in natural gas prices in 1982 the situation changed again in favor of natural gas. In countries with cheap natural gas (4-5 U.S. $ GJ-~), the natural gas price would have to increase by at least 50% to permit economic electrolytic hydrogen production. Figure 8 shows the ranges of power/coal price pairs within which it is more economic to produce hydrogen from coal rather than steam electrolysis. The costs for hydrogen production using steam electrolysis are almost the same in Germany as production from domestic coal [status 1982:3.5 U.S.$ GJ -1, Fig. 8, Point (1)]. Hydrogen production from German coal is however completely uneconomic compared with cheap imported coal (from U.S.A., Canada: 2.0-2.3 U.S.$ GJ -t) also available, and hydrogen production from natural gas which will be the cheapest process for a long time.

FUTURE DEVELOPMENT FOR REDUCING PRODUCTION COSTS The greatest percentage of the production costs for hydrogen is the power costs. Therefore, in the near future, the endothermal mode of operation will be more closely investigated. In this mode, steam is split with less electrical energy (see the chapter "Process Engineering"). At, for example, a specific power consumption of 2.6 kWh mR- 3 H2, high temperature heat of 0.6 kWh must be coupled in. This mode of operation permits an approx. 19% saving on power costs. However, at the same time it increases the investment costs due to the greater ar0ount of equipment required for coupling in the high temperature heat and higher investment costs for modules in view of the lower electrical density. Appropriate design investigations followed by cost estimates are still to be carried out. A further means of reducing costs is in improving the electrolysis cells by (1) reduction of polarization losses by improving materials and morphology of the electrodes, (2) covering of ohmic losses in the electrolyte by reducing the electrolyte thickness (thin film technique) and (3) optimization of cell geometry including new interconnection concepts. Investigations are to be carried out into this at Dornier Systems. A reduction in the investment costs for the HOT ELLY vessel itself could well be attained by utilizing a

314

K. H. QUANDT AND R, STREICHER

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OI PLANT SIZE 50000 Nm3H2/h Fig. 7. Cost comparison of hydrogen by steam electrolysis and natural gas conversion.

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315

HIGH TEMPERATURE ELECTROLYSIS circular vessel. Ribs are not required to stiffen a circular vessel and this reduces the cost considerably. At the same time, a second layer of electrolysis modules can be accommodated and, thus, the specific vessel cost is reduced by a further 50% mN -3 H 2. Dependent upon the location of the plant, a credit for the oxygen produced up to 0.05 U.S.$ mr~-3 02 can be obtained. This means an effective cost reduction for hydrogen of up to 15%. These examples indicate that there are still several means of reducing production costs in respect of high temperature steam electrolysis. Thus, in the long term, the electrolytic process may well be able to compete with hydrogen production from fossil fuels, particularly in view of the expected raw material shortage after the year 2000, especially of natural gas. CONCLUSION Hydrogen produced by electrolysis is currently not economical enough to compete with hydrogen produced from fossil fuels. Electrolytic hydrogen can play an important role, especially in those countries where the

primary energy mix for generating electricity is dominated by hydro and nuclear power instead of thermal power plants fired with fossil fuels. In view of the expected long-term shortage of oil and natural gas, electrolytic hydrogen will be an important factor in future energy investment decisions. Therefore, further development of hydrogen production using the high temperature steam process, one of the most promising processes, appears necessary. In view of the successful laboratory tests using individual electrolysis tubes, a demonstration plant will be built by the end of 1986 by Lurgi in cooperation with Dornier Systems. This plant is intended to produce I mr~3 H2 h -1 at a pressure of 3 bar making it possible to test all important parameters. REFERENCES 1. W. Doenitz and E. Erdle, High temperature electrolysis of water vapour. Status of development and perspectives for application, Int. J. Hydrogen Energy 10, 291 (1985). 2. Lurgi Express Information, Hydrogen from Water (1980). 3. Lurgi Presentation, Hydrogen Productionfrom Coal (1980).