Transport of nuclear heat by means of chemical energy (nuclear long-distance energy)

NUCLEAR ENGINEERING AND DESIGN 34 (1975)65--72. © NORTH-HOLLAND PUBLISHING COMPANY

TRANSPORT

O F N U C L E A R H E A T BY M E A N S O F C H E M I C A L E N E R G Y (NUCLEAR LONG-DISTANCE ENERGY)

K. KUGELER, H.F. NIESSEN, M. ROTH-KAMAT lnstitut .for Reaktorentwicklung, Kernforschungsanlage J~lich GmbH, 51 7 Jidich, Federal Republic of Germany

D. BOCKER, B. RI]TER and K.A. THEIS Rheinische Braunkohlenwerke A G, 5 KOln 1, Federal Republic of Germany Received 20 June 1975 Nuclear long-distance energy, i.e. the transportation of chemically bound energy, represents a potential application for process heat plants in which the endothermic reaction takes place at the heat source (high temperature reactor) whereas the exothermic back reaction occurs at the region of heat utilization (consumer). Due to the following criteria, i.e. reversibility of the chemical reaction, sufficiently large reaction enthalpy, favourable temperature region for the forward and back reactions, and the available technology, a combination of the methods of endothermic steam reforming of methane and exothermic methanation is chosen. As well as supplying household and industrial consumers with heating, process steam and electrical energy, an interconnected system with synthesis gas consumers (e.g. methanol production and iron ore reduction plants) is possible. It is shown that the amount of reactor heat which is convertible into long-distance energy depends considerably on the helium temperatures in the high temperature reactor and lies between 60 and 73% of the reactor power. Conceivable circuit schemes for the nuclear steamreforming plants and the methanation plants are described. Finally, it is demonstrated, with the help of a simple model for cost estimations, that the nuclear long-distance energy system can make heating for households available in competition with oil heating and that due to the lower specific transport costs, for distances larger than 50 km it is also more economical than the hot water supply from the thermal power coupling of steam turbine plants using light water reactors (LWRs) or high temperature reactors (HTRs).

1. Fundamental principles In this paper the term 'nuclear long-distance energy' will be used to mean the transportation of nuclear heat by means of chemical energy. Nuclear plants, which aim at the production of long-distance energy, can be situated far away from the place of heat consumption, e.g. densely populated areas, if the nuclear energy is transported by means of 'cold' chemical energy. This can be achieved by the combination of an endothermic chemical reaction taking place at the heat source and an exothermic chemical reaction occurring at the region of heat utilization. Such systems can consist of liquid and gaseous reactants [1,2]. For such energy transportation systems, a plurality of chemical reactions is conceivable in which the following conditions should hold good with respect to their

possible application: (1) reversibility of the chemical reaction system, i.e. no loss of reactant through irreversible subsidiary reactions; (2) sufficiently large reaction enthalpy and as high a conversion as possible so that high energy densities result for the products to be transported; (3) favourable temperature region for the forward and the back reaction (i.e. for the endothermic reaction, up to 850°C and for the exothermic reaction, possibly higher than 300°C); (4) the required catalysts should be available in sufficient amount and at low costs; (5) use of strongly corrosive or toxic substances should be avoided; and (6) availability of the utilized substances in large amounts and at low costs.

66

K. Kugcler et al.. Transport o f nuclear heat

A technically and economically applicable method of fulfilling the above conditions is the process proposal of the Rheinische Braunkohlenwerke AG. This proposal consists of a combination of the endothermic chemical reaction of the steam reforming of methane to 'reformer gas' (H2, CO and CO2) and the exothermic chemical reaction of methanation [3]. Such a system can be conveniently accommodated into the existing technology and existing infra-structure.

2. Description of the system of nuclear long-distance energy supply In the conversion of methane in a steam-reforming plant heated by helium at 950°C from a high temperature reactor, the gases H2, CO and CO 2 are mainly formed according to the following reactions: CH4 + H20 -+ CO + 3H2, CH4 + 2 H 2 0 ~ C O 2 + 4 H 2 ,

AH = +205.2 kJ/mol, AH=+163.3kJ/mol.

The cooled product gas (reformer gas) is compressed to a pressure of 6 0 - 7 0 bar, usual nowadays for long-distance transportation, and is then transported as 'cold gas' to a consumer situated far away. At the end of this long-distance transport line, i.e. at the consumer centre, the reformer gas can be led via a district distribution network to different methanation plants, where a catalytic conversion occurs into methane and steam. Heat, which was utilized from the nuclear reactor for the steam reforming of methane, is thereby nearly completely recovered. According to today's technology, gas temperatures of 450°C can be attained by methanation. By means of the reaction heat set free, hot water can be produced which is supplied to consumers. Furthermore, the production of steam at low pressure is possible. In a subsequent developmerit step, it is aimed at attaining gas temperatures of above 600°C. This would render the following possibility. On the one hand, the production of high-pressure turbine steam for the generation of electricity with a favourable net efficiency, and on the other, the production of process steam at a high temperature for industrial consumers. The methane formed in methanation is fed back via a second long-distance transport gas line to the steam-reforming plant, after condensation and separation of the water formed.

Fig. 1 shows the basic flow scheme of this closed circuit. For a constant reactor power and helium outlet temperature, the nuclear thermal efficiency available for the steam-reforming plant depends considerably on the reactor inlet temperature of helium. At a reactor inlet temperature of 350°C in fig. 1, nearly 60% of the reactor power can be converted into long-distance energy. If, in the future, it would be possible to attain an inlet temperature of 450°C, then as much as about 75% of the reactor power could be utilized. In this case, additional subsequent aggregates would not be necessary except for the required steam generation for methane conversion. In the concept shown in fig. 1, the remaining part of the reactor power can be used exclusively for electricity production or for a combination of electricity production and so-called district heating. The principle underlying nuclear long-distance energy for an open circuit system is illustrated in fig. 2. A special line is not necessary for bringing back the methane produced in methanation as the methane can be fed into the natural gas network. On the other hand, the methane necessary for the steam-reforming process could be taken from such a network, if one exists in the vicinity of the reactor site. A combination of this system with coal gasification processes is also possible. Thus, energy as well as raw material could be transported by such pipe-line systems. The surplus methane in methanation plants could be supplied to the natural gas network. Furthermore, an interconnected operation of methanation stations with other synthesis gas consumers (e.g. methanol production or ore reduction plants) is possible. The size of the areas of supply could be reduced due to the manifold application possibilities. Apart from the above-mentioned extensive application possibilities of the system described and additionally of advantageously substituting fossil raw materials by nuclear heat with a view of providing for long-term energy planning, the nuclear long-distance energy displays the following additional advantageous features considering the limiting conditions known today: (a) known and surveyable technology; (b) considerable lessening of contamination emission in the production and consumption of energy; (c) the energy production costs are only slightly increased by an increment in raw material prices,

K. Kugeler et al., Transport of nuclear heat

67

district heo.t°current supply , w a s t e heat

950°C i

03

20°C 64 bar

D

CO,H2,CO2

1 '°°°r0350%

70kin

hot -water

®

©

'J (530°CO l, Obar)

20%

20 bor

H20

Fig. 1. Principle underlying nuclear long-distance energy (closed circuit system). Key: 1. high temperature reactor; 2. steam reformer; 3. preheater; 4. coolant fan; 5. waste heat recovery; 6. H2, CO, CO2 compressor; 7. methanation; 8. heat exchanger; and 9. CH4 compressor.

district heat.current supply • woste heat

synthesis gas pipe

950"C 40bar

CO,~2"C02

O)

,~

~

hot - w o t e r

3500C

®

H20

H20

~-

i CH4

CH4. v

househectting cooking

I methane pipe

CH4

Fig. 2. Principle underlying nuclear long-distance energy (open circuit system). Key: h high temperature reactor; 2. steam reformer; 3. preheater; 4. coolant fan; 5. waste heat recovery; 6. H2, CO, CO2 compressor; 7. coal gasification; 8. methanation; 9. heat exchanger; 10. direct reduction of iron ores; and 11. methanol synthesis.

68

K, Kugeler et al., Transport oJ nuclear heat

because these form only a small percentage of the total costs; (d) a smaller amount of foreign exchange requirements for uranium and thorium as compared to crude oil; also a smaller dependence on the fluctuations in exchange rates due to economically more favourable storage possibilities of nuclear fuel; (e) by means of a gas interconnecting system it is possible to ensure supplies and reserves; (f) possibility of covering demands of day-time peaks by pipelines; (g) already existing distribution systems (infrastructure of district heating systems, heating systems in buildings) can be utilized; (h) heat provision known and tested at the consumer; (i) system easily expandible; and (j) by utilization of the system for electricity production in the vicinity of consumers, no need of transformation at high voltage level. 2. l. Production o f long-distance energy As the described overall system is to be used for the transportation of energy, the choice of the most important reaction parameters like pressure, temperature and the initial feed ratio of water/methane should be made according to the requirements of maximum energy utilization, and not according to the product gas composition as is the case in conventional steamreforming plants. The known conception of the high temperature reactor with a helium outlet temperature of 950°C and a circuit gas pressure of 40 bar supports the design of a steam-reforming plant with an operation pressure of 40 bar and a temperature of approximately 825°C. In fig. 3 the ratio of the nuclear long-distance energy to the thermal reactor power is plotted against the feed ratio of water/methane for various operation pressures. The lower set of curves only considers the change in chemical energy (heat of reaction), whereas the upper set of curves also contains, next to the sensible heat, the heat of condensation of the water formed in methanation. Taking into account the maximum long-distance energy obtainable, an optimum feed ratio of water/methane seems to be 1.5-3 tool H20/ mol CH4 . As already mentioned, the temperature level of nuclear process heat has a considerable influence on the nuclear thermal power available for the steam-

g~

• ~

"\301L~

-3

~

wilh cond¢-nsation

c o

20bar

~- ,

1,0

,

,

,--

1.5

2,0

2.5 P

, ---,-

~o.or

condensation

,

3,0 3.5 4,0 Z.,5 Katlo Steom/Melhone

5.0

Fig. 3. Dependence of nuclear long-distance energy to reactor power on initial feed ratio (water/methane) and various operation pressures.

reforming plant. For a high temperature reactor with a thermal power of 3000 MJ/sec, the power data for electricity and long-distance energy production for four different design conditions are compiled in table 1. The parameters here are the reactor inlet and outlet temperature of helium as well as the steam-reforming temperature. The variant described in case 1 is shown in fig. 4. The energy required for the steam-reforming process is supplied to the gas mixture in the plant at a high temperature level. Helium is cooled from 950 to 600°C due to the reforming reaction and due to the superheating of the reactant gases from 450°C to the reforming temperature of 825°C. Additionally, heat is given up to the reactant gas in the reformer tubes from the pigtails (inner gas return lines). Thus the sensible heat of the reformer gases between 825 and 600°C is regained for the process in situ. The enthalpy of helium between 600 and 350°C is utilized for the production of steam (204 bar, 460°C). This steam is led to a back-pressure turbine for electricity production (approximately 187 MW, see fig. 4) and 70% of this back-pressure steam is led to the steam-reforming plant as process steam after being preheated to 450°C by utilizing a part of the waste heat of the reformer gases. The remaining 30% of the back-pressure steam is led to a condensation turbine for electricity production (approximately 232 MW) after an intermediate super-

K. Kugeler et aL, Transport of nuclear heat

69

Table 1. Power data for different design conditions of the reactor (3000 MJ/sec) and the reformer tube. Design and power data

Case 1

Case 2

Case 3

Case 4

Helium temp. at reactor outlet (°C) Helium temp. at reformer tube outlet (°C) Helium temp. at reactor inlet (°C) Temp. of CH 4 + H 2 0 at reformer tube inlet (°C) Reforming temp. (°C) Temp. at pigtail outlet (°C) Reaction pressure (bar) Initial ratio H20/CH 4 (mol/mol) Gross electrical power (MW) Internal consumption including gas compressors (MW) Net electrical power (MW) Gross long-distance energy (from reformer tube on) (MJ/sec) Net long-distance energy (from methanation on) (MJ/sec) Overall efficiency (%)

950 600 350 450 825 600 40 2 417 182 235 1846 1772 66.9

950 600 400 450 825 600 40 2 359 206 153 2014 1933 69.5

950 600 450 450 825 600 40 2 249 231 18 2215 2126 71.5

1000 600 442 450 850 600 40 2 241 220 21 2289 2192 73.8

heating to 490°C by utilization of the remaining part of the waste heat of the reformer gases. The sensible heat of the reformer gases in the lower temperature region of 3 5 0 - 1 4 0 ° C is used to preheat feed water and methane for the steam-reforming process. After

that the reformer gases are cooled to 40°C. The condensate is used as feed water for steam generation, whereas the gases which are freed from this water (CO, CO 2, H 2 and CH4) are compressed to 64 bar, recooled to 40°C and then led to the consumer. The

gas m i x t u r e :

CO

7,8 vo~- %

H2 44,3 v o ; - ' / , CO 2 5,2 vol - I/J CH 4 11 6 v o l - ' l ,

4 9 0 ° C , 46 b a r

methane

I--1 V

H20 31A1 vo I - alu gas a m o u n t :

20 bar

4 36 1 0 6 m ] t h ( i , N , )

950°C40bar

v

1,12.~06m3/h(i N )

£00°C

t3~-~

I

I

I

I

I

141-~

I

....

372 ~j..~ I

MJ 3000~-

waste

heat:

070tlh

29s~

s

waste

hea~eat~

~3~_2

I~'

•

Internal pigtait s

3S0 ° J L . . . t 4600C, 2 0 4 b a r

•

gasmixture:

'I 8?MW

C0

11,3 v o l - "I.

H2 CO 2

6t.,2 vat- =I. 7,6 voJ-*le

CH4

t£,9 val- lie

3.03.406re]/h( i. N. I

A 2 5 5 0 t l h , 3 0 3 ° C 224 bar

output r a t e s : n u c l e a r tong d i s t o n c e e n e r g y

1772 - ~

gross c u r r e n t

419 MW

net current w a s t e heGt

Fig. 4. Production of long-distance energy and electricity.

supply

supply

255 PAW

883 MJ s

70

K. Kugeler et al., Transport of nuclear heat

~

waste heat of 883 MJ/see at a low temperature level caused by the electricity production in a condensation turbine and by the cooling of the reformer gas, leads to irreversible losses of 33.1% so that the overall efficiency of the process illustrated in fig. 4 is 66.9%.

duct

•~

T._ Pr~oduct

:

!

2.2. Utilization o f long-distance energy

The cold reformer gases are transported to the methanation plants where they are catalytically converted to methane and steam according to the following reactions: CO + 3H 2 -+ CH 4 + H 2 0 ,

AH = - 2 0 5 . 2 kJ/mol,

CO 2 + 4H 2 --, CH 4 + 2 H 2 0 , AH = - 1 6 3 . 3 kJ/mol. The reaction efficiency, which is defined as the ratio of moles CH 4 in the product gas of methanation to the moles C in the reformer gas, is shown as a function of reaction pressure and temperature in fig. 5. From this figure it is seen that at the already technically tested methanation temperatures of 450°C and at a pressure of 40 bar nearly 95% of the reformer gas is converted to CH 4. The remaining unconverted 5% can either be 'after-methanated' in a second step or be circulated unutilized in the circuit; this, however, leads to an increase in the specific transportation costs. Figure 6 shows various schemes of methanation re-

Conversion

1

,

0

o.~t

]

~

~

\, \\\..~

\.'\ \.~ '~

j / o,~ l

0:1

\ "~.__

p=160bar

"\ " \ \.\ \ "\

'Np:80bar ".,,, \. N Np:~o~ N \\

'\,

\\

"N, N N,,~ =20her

T/oC

1000

Fig. 5. Dependence of reaction efficiency of methanation on reaction pressure and temperature.

tl

,

F ed

Feed

1

2

3

Fig. 6. Various circuit schemes of methanation reactors. actors. In circuit 1, the reformer gas is made to undergo an adiabatic methanation and the product gas is cooled in a heat exchanger. To control the temperature, part of the product gas is circulated and mixed with the reformer gas before methanation. Many methanation steps are connected in series to limit the feedback o f product gas and hence the power of the circuit compressor. In the methanation shown in circuit 2, many adiabatically operating reactors with respective intermediate cooling, by adding cold reformer gas as well as product gas, are also connected successively. The actual heat utilization occurs after the last methanation reactor. The disadvantages of this circuit scheme, compared with circuit 1, are the large amounts of gas circulating and a relatively low temperature level. In the methanation according to circuit 3, the heat removal occurs directly in the catalyst bed. The circuit operation can be thus prevented or at least strongly reduced. Problems occur as a result of the migration of the temperature in the methanation reactor due to the aging of the catalyst. Figure 7 shows a flow scheme of a methanation plant with a thermal power of 177.2 MJ/sec. The power data shown result when the total long-distance power of 1772 MJ/sec (see table 1, case 1) is divided up in the consumer centre via a district distribution system into ten equally large methanation plants. The transported cold reformer gas is preheated to 80°C by cooling of th~ product gas and is then led to the first methanation reactor. The product gas of the first step leaves the reactor at a temperature of 450°C and then gives up its heat to hot water or steam as

K. Kugeler et al., Transport of nuclear heat

71

in one of which long-distance energy and electricity are produced and in the other, long-distance energy, district heating in the form of hot water, and electricity are produced. The produced !ong-distance energy (1772 MJ/sec) is conveyed t~ a densely populated area 70 km away and then led via ten district distribution lines having an average length of 3 km to the individual methanation stations. Here there are two possibilities of application depending on the circuit: either heating alone or heating, process steam and electricity close to the consumer. The load time of the nuclear plant was assumed to be 4320 hr/yr. This high load is attained by constructing, parallel to each methanation plant, an equally large fossil-fired plant (for the production of heat); however, this is only applied to cover the peak loads (480 hr/yr). In the overall system under these conditions, 90% of the heat is obtained from nuclear plants and only 10% by the burning of fossil fuels. In the case of simultaneous application of heat, process steam and electricity production, the load time of the overall system can be assumed to be 8000 hr/yr. A comparison of the thermal heat coupling, using

250°C

51~08kg/h

9O

q

house -heating (rue[ o H , o n e t a m i t y )

Fig. 7. Flow scheme of a methanation plant.

,o

coolant. After adding the reformer gas, the product gas of the step is led to the second methanation step where it is reheated to 450°C by the heat of reaction. After renewed heat utilization and adding of the reformer gas, the gas is heated to 350ac in the third methanation reactor. The product gas of this third reactor is partly led to the first reactor and partly recooled to 40Oc by heating up of the reformer gas and is then fed into the methane duct.

I L i

A /

house,-heating (fuel oil,six families )

~6istrict (fuet o

u

5¢

~.~.2~/[

heating station _ _ ) /

/

~ ~

~

"ouaeor long --

I

disto . . . .

nergy

(4 320 h l a )

=o u /

3. Costs o f nuclear long-distance energy

To estimate the economy of the described system of nuclear long-distance energy supply, cost calculations were made in the scope of a study [4] for different variants of the closed overall system on the basis of investment values known today. The investigations are based on a high temperature reactor with a thermal power of 3000 MJ/sec and two conceivable circuits,

/

T zo

~- d i s t r i c t h e a t from nucteQr reactors (4 320 h / o )

50

Ioo •

suppty

d i s t a n c e ( km )

Fig. 8. Dependence of heating costs for consumers for heat produced from various sources on the supply distance.

"~-~

72

K. Kugeler et aL, Transport of nuclear heat

light water or high temperature reactors according to fig. 8 shows that up to a distance of approximately 50 km the transportation of hot water is more economical; for larger distances, however, the long-distance energy transport is the most economical possibility of heat transportation due to the considerably lower specific transport costs. The specific transport costs for pure heat-supplying systems of nuclear long-distance energy amount to 2.55 Dpf/GJ km, compared with 10.5 Dpf/GJ km for the transportation of hot water from light water or high temperature reactors. Thus in addition, a most flexible interconnecting system can be built up by the possibility of combined heat, electricity, steam and, in case of the open system, of raw material supply. As a result of the possible high load time, the specific transportation costs can be lowered to 1.91 Dpf/GJ km, which als6 results in a decrease of the heat costs to about 9 DM/Gcal (2.15 DM/ G J) as compared with the costs shown in fig. 8. As well as the advantages of manifold application possibilities, such systems also represent an especially economical utilization of the application of nuclear long-distance energy.

4. Conclusions The transportation of nuclear heat by means of chemical energy is very interesting from the point of view of economics when compared with the cost of energy from fossil fuels..The energy supply without the consumption of fossil fuel is possible. The reactor plants can be situated far away from the consumers. The technology is known and surveyable.

References [1] E. Lindberg, USA Patent, 3075361, Jan. (1963). [2] H.W. Niirnberg and G. Wolff, Nukleare Prozessw/irme, Jahresbericht 1967 der Kernforschungsanlage Jtilich GmbH. [3] F. Hilberath and H. Teggers, Deutsches Bundespatent, 1298233, Jan. (1968). [4] Feasibility-Studie, Transport yon Hochtemperatur-Kernreaktor-W/irme mittels chemisch gebundener Energie, Report of the Kernforschungsanlage Jtilich GmbH and the Rheinische Brannkohlenwerke AG, K61n Dec. (1974).