The high-temperature gas-cooled reactor represents one type of the next generation of nuclear reactors for safe and reliable operation as well as for efficient and economic generation of energy. Present HTGR designs are based on about four decades of R&D including operating experience with six prototype reactors [37].
2.2.2.1. Features of HTGR-Based Chemical Processing
The concept of the high-temperature gas-cooled reactor has important features such as
• production of electricity,
• production of high-temperature heat up to 1000 °C,
• production of high-temperature steam of about 530 °C,
• realization of fundamental safety features, and
• potential for economic attractiveness.
In reference to the CHP mode, the industrial power plant with HTGR could be named "CHHP", because it provides three energetical connection points: high-temperature heat, low-temperature heat, and electricity. With respect to electricity production, the achievement of a 40 - 43 % efficiency is possible when using steam turbines. Gas turbines reach efficiencies up to 48 %. A combination of both could even reach the 50 % level.
Cogeneration applications are estimated to allow efficiencies in the range of 80 - 90 % [24].
Physical requirements for process heat HTGRs are
• gas outlet temperature of 950 -1100 °C,
which allows for sufficient reaction velocity of chemical process;
• gas inlet temperature of 350 - 500 °C,
which is necessary in order to use recuperation heat of product gas;
• lower system pressure,
which raises the efficiency of the chemical processes;
separation of the nuclear system from the chemical system,
which reduces the risk of radioactivity release after an accidental chemical explo-sion;
• separate components for high-temperature and low-temperature heat consumption, which allow for different constructions and materials;
• arrangement of primary and secondary circuit in a non-integrated design because of better accessibility.
Steam applications include the generation of electricity in the steam cycle, the production of process heat in cogeneration with electricity, and the production of district heat. Steam on a higher temperature level offers the chance to open up new markets for process steam applications in the chemical and petrochemical industries or for enhanced oil recovery. The ratio of electricity to steam production can be adopted over a wide range according to the needs of the consumer. In heat applications, heat is delivered from the helium coolant with an outlet temperature at the high end of 750 - 950 °C, for the conversion of solid and liquid fossil fuels into "clean" gaseous and liquid fuels synthesis gas, hydrogen, methanol. Chemical energy transmission systems could be most effectively utilized when integrated with a parallel electric transmission system, taking the high-end temperature range for driving the heat pipe and the low-end for generating electricity.
Chemical industrial plants could be optimized in terms of product spectrum and product volume such that the total energy output of an HTGR, whether as heat or steam or electricity, could be utilized on-site [7].
A direct-cycle reactor system strongly depends on the HTGR fuel quality. The available high quality TRISO (SiC) fuel4 has not been rigorously tested under gas turbine conditions, but appears satisfactory under moderate conditions, i.e., reactor outlet tempera-ture of 850 °C and time-averaged fuel temperatempera-ture under operating conditions of < 1200
°C. Options to improve fission product retention capability are to enlarge the thickness of the SiC layer or to replace the SiC coating layer by a ZrC coating layer. TRISO (ZrC) fuel, however, will require a long and expending development program. ZrC coatings are under study in Japan and the USA. The effect of a thicker SiC layer has been investigated within the German HHT project [16, 31].
2.2.2.2. Chemical Processes for Hydrogen Production
A widely applied method for the production of hydrogen is the decomposition and gasification of fossil fuels. Reaction temperatures are typically in the range > 500 °C, which could be ideally provided by an HTGR. The non-fossil generation of heat for the endothermal reforming reaction represents a means of reduction of COi emissions. In relation to the same amount of CO2 produced, the reforming of natural gas with an external heat source increases the hydrogen output by 40 - 50 %, in case of coal gasification even by
4 A TRISO coated fuel particle consists of a 0.5 mm diameter UOj kernel surrounded by subsequent layers of buffer, inner pyrocarbon, silicon carbide (SiC), and outer pyrocarbon. The 35 nm thick SiC layer provides an efficient barrier to all safety relevant radionuclides.
0,5x10
900 950 1000
l°cJ
900 950 1000
Fig. 2-7: Dependence of hydrogen and electricity production on helium temperature at steam generator inlet (T2) with Tp reactor outlet temperature (950 °C), and TS: reactor intlet temperature (250 °C), from [23]
Table 2-5: Influence of process heat temperature split-up in a 3000 MW(th) nuclear process heat plant with six loops, from [23]
Temperature split-up [°C]
Steam reformer
Power for process heat [MW(th)]
AT [°C]
Heat flux density [kJ/(m2 s K)]
Required heat transfer area [m2] Power density [MW/m3]
Required heat transfer area [m2] Power density [MW/m3]
Table 2-6: Mass balance for hydrogen production, from [23] 66 % compared with the conventional processes [35]. Cogenerated electricity is applicable in H2 production methods based on the decomposition of steam in the high-temperature electrolysis or in thermochemical cycles. The chemical processes of hydrogen generation are described in further detail in chapter 5 and appendix A, respectively.
2.2.2.2.1. Reforming Process
The HTGR allows a splitting of the nuclear reactor heat for methane reforming and for steam generation. According to the selected design, the shares of synthesis gas and electricity production are variable over a wide range adjusted by the steam generator inlet temperature, T2. The dependence is shown in Fig. 2-7.
For the industrial processes of ammonia synthesis, hydrocracking, hydrogenation, a suitable heat split-up would be made at a T2 value in the range of 625 - 670 °C. In the direct iron ore reduction process, a steam generator inlet temperature, T2, of 810 - 830
°C would be chosen. It is obvious that the heat split-up has a significant influence on the designs of both steam reformer and steam generator [23]. Some exemplary data are given in Table 2-5 for different T2 values. A mass balance for hydrogen production based on the medium T2 value (= 700 °C) is given in Table 2-6.
Unlike conventional fossil-fueled reformer tubes, the helium-heated reforming plant connected to an HTGR has to meet the much more stringent requirements of a "nuclear"
component in terms of construction, quality assurance, and scheduled re-testing. The reforming tube placed in the primary circuit has the function of forming a radioactivity barrier between the primary helium and the process gas. Another major difference is the manner of heat input, which is convective transport with the helium in the nuclear version.
Synthesis gas is the starting material for numerous chemical products and it can be processed to hydrogen if desired. It is the intermediate step in the synthesis of methanol to serve as a motor fuel.
2.2.2.2.2. Steam-Coal Gasification
Since heat input for the coal gasification process is a significant cost factor, nuclear process heat provided by an HTGR was considered as a substitute for conventional firing.
From the perspective of Germany, the most economic hydrogen may be produced from imported cheap coal in a steam coal gasification process using HTGR heat [2]. Another important advantage of nuclear coal gasification is its environmental effect since no coal is fired to provide the reaction heat. Only smaller amounts of pollutants such as SC»2, CC>2, dust are emitted into the atmosphere. The main problem, however, is the need to transfer the nuclear heat via two heat exchangers to the coal gasification process.
Fig. 2-8 presents a flow chart of the steam-coal gasification process with the numerical example based on a 3000 MW(th) HTGR. High-temperature heat of 950 °C is passed to a secondary helium circuit via a He/He heat exchanger where it enters with a 900 °C temperature a steam gasifier. Hot steam is routed into a coal bed where the coal is gasified in two steps to give synthesis gas. As a result of the carbon-steam reaction, the helium is cooled to approx. 810 °C. If desired, a subsequent methanation process could be added to provide synthetic natural gas (SNG) [9]. Compared with a conventionally fired coal gasification plant, the nuclear-driven process could increase the output by up to about 60 %, because no coal is spent on direct combustion [4]. As an example, with 680 MW(th) power from an HTGR plus 3470 t of hard coal, the yield is 2690 Nm3 of SNG [18].
For methanol synthesis, the purified coal gas is compressed to 7.5 MPa. The power of 680 MW(th) from an HTGR plus 2680 t of hard coal result in 2390 t of methanol [18].
A combined system of coal, steel, and nuclear energy has been discussed for the production of methanol and raw iron comprising the steps:
(i) partial (50 %) steam gasification of hard coal by means of HTGR process heat, (ii) Klockner-steel-gas process for iron ore reduction by means of fine coke,
(iii) methanol synthesis from the product gases.
The combination of all three steps appears economically attractive. A balance assessment assumes an input of 1.4 million t/yr of hard coal and 500 MW(th) delivered from two HTGR units to result in the production of 1.23 million t/yr of methanol and 0.8 million t/yr of raw iron [1].
2.2.2.2.3. Hydro-Gasification
For the hydro-gasification process (Fig. 2-9), an intermediate circuit is deemed unnecessary. An alternative to the above described coal gasification process was pursued by the German Rheinische Braunkohlenwerke AG, Cologne: the nuclear hydriding coal gasification. Nuclear power is here introduced in a steam reformer for methane splitting.
From the resulting synthesis gas, hydrogen is taken to be the input for the exothermal gasification reaction.
Coupling hydro-gasification of lignite with a methanol synthesis process is possible after cooling and purification of the coal gas and mixing it with the reformer gas. The crude methanol yielded is then separated from the residual gas mixture by condensation. For the
4 MPa
Fig. 2-8: Flow chart of the process of steam gasification of coal (hard coal), from [9]
Helium
Fig. 2-9: Flow chart of the process of hydro-gasification of coal (lignite), from [9]
example of 680 MW(th) power from an HTGR, the input of 2650 t of lignite would allow a production of 2530 t of methanol [18]. If synthetic natural gas production is desired, the coal gas passes a purification and a cryogenic gas separation step to obtain H2, CO, and CH4 fractions. Some of the CH4 is used for H2 production in the steam reformer to serve as a feed gas in the gasifier. The remaining methane is the product [18].
The process of hydrogen or methanol production by using a raw iron producing blast furnace, also for stack gas utilization, can be realized with a process heat HTGR, shown in [34] for a 250 MW(th) unit (see section
4.3.2.8.)-A concept for the "non-integrated" employment of an HTGR in the iron and steel industry has been proposed in [28] as described in section 7.1.3. The nuclear reactor is sited at a location favorable for the reduction gas generation from which the gas is delivered via a pipeline to the iron and steel industrial sites. A 3000 MW(th) nuclear power plant (HTGR) was chosen where 2167 MW(th) are taken for naphta splitting and an operation time of 8000 h/yr assumed would be able to provide 6.75* 109 Nm3/yr of hydrogen [28].
An HTGR is favorable, if an H2 - CO mixture as a reduction medium is used.
2.2.2.2.4. Biomass Conversion
A process for the conversion of biomass into methanol by means of an HTGR has been proposed [3]. Wood, represented by the chemical formula C6Hg 64.03.7, is gasified by superheated steam. Electrolytic hydrogen is added to the product gas and then converted to methanol or methane. Both heat and electricity are provided by an HTGR. The partial processes of steam gasification, water splitting and methanol / methane synthesis are summarized in the overall reaction [3]:
+ 2 H2O + 5.66 H2O -» 6 CH3OH + 2.68 O2 + 2 H2O + 5.66 H2O -» 6 CH4 + 5.68 O2
Additional requirements to an energy system as to be environmentally benign and application friendly, the energy carrier of choice would be liquid methanol rather than gaseous methane due to lower losses and easy and economically competitive storage and transportation.
2.2.2.2.5. Water Splitting
The high-temperature heat from HTGRs could be used to convert heat energy directly into chemical energy in the form of the (lower) heat of combustion of hydrogen. There are three processes of water splitting appropriate for process heat utilization from an HTGR:
• high-temperature electrolysis,
• thermochemical cycle,
• thermochemical-electric hybrid cycle.
There is a great economic potential of nuclear thermochemical cycles, i.e., the H2 production by means of water and nuclear heat compared with electrolytic H2 from nuclear electricity. The HTGR is typically considered the high temperature heat source of
Battery J Steam and electricity
generation
Battery A+E High-temperature reactor
Gas turbine Electricity
Battery G
Electrolysis
H2S04
SO2
Process heat
Battery H
H2SO4 decomposition
02
H2O SO
02
02
Battery I
SO2 / O2 separation
H2O 02
Fig. 2-10: Diagram of the Westinghouse sulfuric acid hybrid cycle, from [20]
choice, although other heat sources are, of course, also possible. Most of the promising thermochemical cycles are based on sulfuric acid processes [20] (see section 5.2.2. and appendix A).
In the Westinghouse sulfuric acid hybrid cycle shown in Fig. 2-10 (see also appendix A.3.2.), HTGR heat will be used for the H2SO4 decomposition step. Both high-temperature and electric steps have been experimentally investigated at the Research Center Julich. The above hybrid cycle has undergone a detailed balance and cost analysis already in a plant design based on nuclear power [8].
Materials corrosion problems arise by the fact that the system pressure of 4 MPa on the primary side of the HTGR needs to be also on the secondary side of the process gas.
2.3. COMPONENTS