Energy Convers. Mgmt Vol. 33, No. 5-8, pp. 451-458, 1992 0196-8904/92 $5.00+0.00 Printed in Great Britain Pergamon Press Ltd

PARTIAL AND TOTAL REDUCTION OF CO2-EMISSIONS OF AUTOMOBILES USING CO2-TRAPS

W. SEIFRITZ*

Institute for Energy Economics and the Rational Use of Energy University of Stuttgart, HeBbrfihlstr. 49a, D-7000 Stuttgart 80

* On leave from Paul Scherrer Institute, CH-5232 Villigen/Switzerland

ABSTRACT

A new hybrid hydrogen/hydrocarbon-driven motor-car is proposed which is equipped with a CO2-tra p and which exhibits an energy absorption ability in its storage systems (with respect to their masses) much larger than that of a hydrogen- or battery-powered car. The principal idea is to use light metal hydrides (like MgH2) as energy storage facilities whose metallic atoms not only carry fuel atoms but, if is discharged, also the carbon dioxide from the burned hydrocarbons.

In the opinion of the author, a system is presented for the first time which would be able to bind and collect the CO2 of a large number of diffuse CO2-sources for the purpose of its central disposal.

INTRODUCTION

If fossil fuels are to be used further on a massive scale and the greenhouse effect avoided, carbon dioxide (CO2) emissions into the atmosphere must be greatly reduced [1].

It seems to be possible now that the CO 2 of large sources, like fossil fuelled power stations, can be sequestered and disposed of if an appropriate flow sheet for the necessary process steps (gasification, CO- shift and CO2-separation) was chosen [2]. But only about 30 % of all CO2-emissions worldwide stem from fossil-fuelled power stations. A major problem are still the large number of small Co2-emitters, e.g. hundreds of millions of motor cars, which are scattered all around the world and whose number is increasing steadily. Although the contribution of the transportation sector to the global CO2-emissions is still 25 % [7], sooner or later, this sector will be asked to reduce its CO2-emissions, too. More than half of transports' CO2-share is from passenger cars.

The transport sector contribution to non-CO 2 trace gases increase is estimated to be about 10 - 20% at present, mainly due to car air-conditioners (chlorofluorocarbons) and vehicle exhaust gases (CO, NO x and HC) which contribute to the increase in atmospheric concentrations of methane and tropospheric ozone [7]. Environmental policy may be expected to reduce the transport contribution of these non-CO 2 greenhouse gases by means of production bans for chlorofluorocarbons and national exhaust gas regulations for motor vehicles. Therefore, they are not the subject of this paper.

Until now it seemed to be impossible to prevent the CO 2 in the exhaust gases from escaping into the atmosphere. In contrary to fossil-fuelled power stations and central heating installations, automobiles are mobile and therefore diffuse CO2-emitters scattered on large areas.

Hitherto, there axe two proposals to realize C02-free motor cars: • the "hydrogen-car" with an OTrO-engine or fuel cell driven by hydrogen fuel and • the "electro-car" with one or several electrical engines driven by electricity that was

electro-chemical form, i.e., batteries. stored in

451

452 SEIFRITZ: REDUCTION OF CO2-EMISS1ONS FROM AUTOMOBILES

The condition in both cases for a CO2-fr~ operation is, however, that the fuel, i.e. the hydrogen and the electricity, respectively, is produced in a CO2-fre¢ manner, too.

It is a well known fact that the ratio of stored energy with respect to the mass of the energy storage facility is relatively low for both types of cars. The general requirement to drive at least 400 km in a day between two tank f'fllings can not be met in general. Therefore, particular emphasis has to be given to the weight or mass of any kind of CO2-retcntion facility on board of automobiles.

In the following we present a possible solution for a CO2-free automobile exhibiting a tolerable energy/mass ratio. At the sarr,¢ time, the system could be understood as a possibility to bind and to collect the CO 2 of a large number of diffuse CO2-sources for the purpose of its disposal in empty natural gas fields or in the deep ocean. Generally speaking, a spatial "CO2-compression system" is described reducing the entropy of the diffusively produced carbon dioxide.

THE WORKING PRINCIPLE

The Hybrid MgH~Gasoline Motor-Car

In the new hybrid motor-car, the OTTO-engine receives its fuel in roughly equal shares of hydrogen gas from a magnesium hydride storage, and gasoline from a normal gasoline tank.

The main idea in this hybrid system is that the empty hydrogen storage system serves at the same time as a "catcher" for CO 2 produced in the combustion process of the gasoline. Thus, the light metal atoms serve not only as a "bed" for the hydrogen atoms alone, as is the Case of a hydrogen car, but also as a "bed" for CO 2. In our case we have chosen the metal magnesium for this double purpose. In the beginning of the fuel cycle, we start with magnesium hydride, MgH2, and at the end of the fuel cycle we arrive with magnesium carbonate, MgCO 3.

In Fig. 1 the main reactions on board of the automobile arc summarized. Injecting water into a cell of the MgH 2 storage tank hydrogen is released that is fed together with gasoline (here represented for the sake of simplicity by n-bexane, CtHt4) and air into the OTTO-engine and burned. The exhaust gases arc partially condensed and two moles of condensed water ar~ fed back to the MgH2-cell while one mole of gaseous CO 2 is passed through the Mg(OH)2-tra p where the CO 2 is bound as insoluble magnesium carbonate, MgCOy

The production rate of hydrogen in the MgH2-cell is self-controlled because, if the motor is switched off, no condensed water is produced anymore and the hydrogen-production stops automatically. On the other hand, as the power of the engine increases, more water is produced in the exhaust gas and more hydrogen is released in the MgH2-cell.

As can be seen from the hydrogen production equation in Fig. 1 two moles of H 2 are produced if one mole of MgH 2 is converted into one mole of magnesium hydroxide Mg(OH) 2. This is twice as much than the discharging process of a classical rr~tallic hydritt¢ storage system in a conventional hydrogen car would yield.

SEIFRITZ: REDUCTION OF CO2-EMISSIONS FROM AUTOMOBILES 453

=o

=

-r-

i

O

Light Metal Hydride Tank :

Mg H 2 (Powder) . . . . ~ l ~ _ k ~ . . . .

H2-Production MgH2+2H20(I ) --~Mg(OH)=

+2H2 CO2-Absorption

Mg(OH)2+CO2(g)'-,'MgCO3 +1-~

(132.7kg) xl 7

H 20(g).~

Surroundings

er- E

8 q¢

T End of Driving Cycle

2H2

91-

Gasoline Tank :

1/6 Cs H14 (Liquid) (22.5kg)

C6 H14

coz(g)

2H20(I)

I Combustion (OTTO-Engine) : 12 02

2H2 + 02 ~ 2H20(g) -115.6 kcal p 1 19 -~C s H14+ ~ O 2 CO2+ ~- H20(g) - 149kcal

~ Exhaust :

1~ H2Olg)+CO2lg)

Condenser (Air cooled) :

3s H20(g)-* 2 H20(I)+ 7H20(g) T~ I CO2(g)'~ CO2 (g)

7 ~ .~o~Q) To Surroundings

Air

Fig. 1 The hybrid MgI-12/gasoline energy storage system for an automobile

The overall-reaction of the two energy storage systems of Fig. 1 including the CO2-absorption is given by

1 310 (from air)

-. MsCO 3 + ~ H20 (into a#)

(1)

If both the hydrogen and the gasoline are consumed, the mass of the produced magnesium carbonate reaches its maximum weight and the emissions consist only of water-steam ff the other chemical pollutants like NO x, CO and HC are eliminated in a first step by a catalyser which can be positioned at the exhaust of the OTrO-Motor. ECM 33-5/~--M

454 SEIFRITZ: REDUCTION OF CO2-EMISSIONS FROM AUTOMOBILES

The regeneration of the MgH 2 from MgCO 3 is carded out in an external fuel cycle. In a collecting center of MgCO 3, the first step is to decompose the carbonate into CO 2 and magnesium oxide, MgO. Magnesium has been chosen as the working metal because the decomposition temperature of MgCO 3 is rather low (~, 540 °C for a CO2-pressure of 1 bar) compared with limestone, CaCO 3, (~ 909 °C ) under corresponding conditions. In addition the reaction heat of CO 2 forming MgCO 3 is lower than that of CO 2 forming CaCO 3. Therefore, a high temperature nuclear process heat reactor (HI'R) with a helium outlet temperature of 950 °C and an intermediate heat exchanger for the hot helium gas could be used to accomplish this task without delivering a parasitical side-stream of CO 2. Thus, magnesium carbonate is decomposed in such a manner that a fluidized bed of dried MgCO3-dust is heated allothermically by a secondary helium circuit from an HTR.

The next step is to convert MgO into MgH 2. Normally, the chlorine produced is used together with coal to convert MgO into MgCI 2 and the carbon monoxide produced is shifted to CO 2 and H 2 by the CO-shift reaction. Finally, metallic magnesium from the MgC12-electrolysis using CO2-frec electricity will be hydrogenated to yield the desired MgH2-powder. This inflammable material will be filled pneumatically into containers for each motor car that can start now a new driving cycle if its tank is filled with the corresponding amount of gasoline at the same time.

The overall-reaction of the regeneration steps is given by

MgCO 3 + C + H20 --~ MgH 2 + 2 CO 2 (2)

and the net reaction over the total fuel cycle reads:

1 31 c 6 a , , + o 2 (from air) + C

-. 7_ H,o (o o caO ÷ 2 co= 6

(3)

Hence, all of the CO 2 produced in a large number of scattered cars is collected at one place. It can be disposed of by pumping it into empty natural gas fields or it can be condensed and disposed in the deep ocean [2]. This leads to exhaust gases from the motor-cars free of greenhouse gases since only water is released to the atmosphere. Although hydrocarbons are involved in their fuel cycle, these motor-cars can be called "climate-neutral". Furthermore, the rides to the gasoline stations are instrumental to the diffusively released CO 2 in the form of a solid. Therefore, the system works as an entropy reduction system with respect to the material concentration of CO 2. It is obvious, however, that a new infrastructure for the transportation sector is necessary.

Assuming that the o'FrO-engine of a conventional car requires 10 kg of gasoline per 100 km (= 25 mpg) and we want to drive the car 400 km between two tank fillings, the energy content of the required gasoline has to be roughly 4.16 x 105 keal.

According to_ the combustion equations in Fig. 1, our hybrid car requires a hydrogen and gasoline storage of 1.82 x 105 and 2.34 x 105 kcal, respectively. For the sake of simplicity we assume that the mechanical efficiencies of the OTtO-engine with respect to hydrogen and gasoline fuels are identical in a first approximation.

At the beginning of the cycle the weight of the MgH2-storage (without castings) can be calculated to be 41.4 kg and the gasoline tank has to be filled with 22.5 kg of gasoline.

At the end of the cycle, after a 400 km ride, the gasoline tank is empty and the weight of the CO2-tra p, i.e., the mass of MgCO 3, will be 132.7 kg since about 1.57 x 103 moles of CO 2 (= 69 kg CO 2) are produced during the whole driving cycle. Thus, the mean weight W of the storage system during the cycle, (weight at the beginning + weight at the end of cycle)/2 is therefore given by

SEIFRITZ: REDUCTION OF CO2-EMISSIONS FROM AUTOMOBILES 455

~ .= [41.4/~g (M&H2). 22.5 kg (gaso//ne)] + [132.7kg(MgC03) + 0/¢Ig (Sa.m//ne)] (4)

= 9 8 . 3 k g

Including the castings of both storage systems an effective weight of 120 - 150 kg may result - a value which seems to be tolerable.

The FeTi-H 2 hydride storage system [3], in which one mole of H 2 can be stored in one mole of FeTi-H 2, yielding an energy storage capacity of 57.8 kcal / 105.8g = 0.55 kcal/g, the theoretical weight of a hydride storage facility for a 400 km ride would be at least 4.16 x 105 kcal/0.55 kcal per g - 760 kg. For a battery-powered car the weight would be even much higher [4]. The weights of these storage systems do practically not change during a ride.

On the other hand, the mean weight of the storage medium in case of a classical gasoline powered car is given by

W = 4o/~ (be~,~ of cycle) - o /~ (e=z of~c~) 2

(s)

= 2 0 kg .

This means in practice that our CO2-free hybrid car exhibits a clear superiority over the classical hydrogen or battery powered cars. Its performance index, in terms of the energy/mass ratio, is by a factor of 5 worse than the classical gasoline car but by a factor of 7.5 better than the presently discussed versions of CO2-free motor ears in which the hydrogen is stored under ambient temperatures. Only a liquid hydrogen tank would have a slightly better performance index (= 60 kg, i.e., W -- 30 kg) than the MgI-I2/ gasoline hybrid car with a CO2-tra p but there are other severe problems when handling cryogenic fuels.

It 's interesting to note that nearly 5 m 3 of organic solvents on the basis of monoethanol amine (e.g. gas/Spec FS-1L of Dow Chemical, Ref. 2) would be necessary to bind the 69 kg of CO 2 released during the 400 km ride. It is not imaginable to carry such a large amount of solvent on board an automobile. It is therefore not wrong to think of a classical CO2-absorbant, like a light-metal carbonate, particularly if the metal atom is simultaneously used as a carrier of fuel atoms.

Entropy-Reduction by CO2-Traps

Aside from the practical aspect of chemically trapping carbon dioxide in light metal hydroxides on board of vehicles, the system proposed should also be considered from a more theoretical point of view. Trapping CO 2 in the above sense means that it cannot escape into the atmosphere where it will be diluted to a concentration of about C a = 350 ppm, while the CO2-concentration in the exhaust gases was in the realm of C o ,= I0 % = I0 5 ppm.

The entropy production due to this dramatic change in concentrations is given by Boltzmann's theorem being

A S 1 = S a - S o = . k Z In C a + k Z In C O

= kZ1n CO -- • 0 (6)

co

where C O >> C a. Z is the number of CO2-molecules considered and k is Boltzmann's constant = 1.38 x 10 -23 J/K.

456 SEIFRITZ: REDUCTION OF CO2-EMISSIONS FROM AUTOMOBILES

On the other hand, our CO2-trapping system increases the CO2-concentration if we compare the pure CO 2- atmosphere at 1 bar after the calcination process being C c, with the CO2-concentradon in the exhaus: gases being C o- 10 % only. Thus, the concentration process reduces the entropy by

A S I = S © - S o = - k z l n C c + k Z I n C O

~ - k Z l n C c < o co

(7)

and is negative because C¢ >> C o.

The overall effect of our CO2-trapping system is, however, that we have collected the CO 2 as a pure gas (at about 1 bar) instead of diluting it in the atmosphere. The total entropy reduction capability is therefore expressed by

A Sto t = S c - S . = A S 2 - A S 1

ffi - k Z l n C~ . co

With Cc/C a = 106 ppm/350 ppm = 2857 we obtain

AStot = - 1.1 x 10 "~ • Z, in J /K

(8)

(9)

Let us consider a model society, such as the Western part of Germany (without the five new Bundesl~der). There are 31.3 million cars consuming 55 billion of liters of gasoline and diesel per year [8]. This figure corresponds to about 5.4 x 1011 moles of gasoline (C6H14) and to a mean thermal power of = 64 GW(th). One mole of gasoline burnt produces six moles of CO 2. Therefore, the annual burning of 47 mio t of gasoline and diesel produces roughly Z = 3.3 x 1012 moles of CO 2 per year corresponding to Z = 2 x 1036released CO2-molecules per year. Introducing this figure into eq. (9), one obtains for the rate of entropy reduction

LLS = - 2.2 x 1014 J /K per year = - 6.8 M W / K . (10)

The bounded power at a mean ambient temperature of T = 300 K is therefore given by

TAS - - 2.1 GW. (11)

The same value is obtained if one uses the expression for the free mixing energy, given by

AFffi -Z'krh cc (12) co

where R = gas constant = 8.31 J/mole K and Z' = number of moles. Introducing Z' = 3.3 x 1012 moles of CO2/year into eq. (12) one obtains the same value for AF as given in eq. (II) . AF is the theoretical minimum of free power necessary to take the CO 2 out of the atmosphere, i.e., to reverse the dilution process. In practice, however, much more energy is necessary. In Ref. 2, for instance, it was shown that about 75 kcal(el) per mole of CO 2 are necessary, if it is chemically washed out of the atmosphere by means of sodium hydroxide in the form of sodium carbonate (soda). This means that a power of = 188 GW(el) would be necessary every year to extract the above mentioned 3.3 x 1012 moles of CO2/Yr out of the atmosphere - about three times more than the thermal power of the burnt gasoline.

On the other hand, one needs only 28.1 kcal (th) process heat at 540 °C to calcinate the MgCO 3, in o r ~ r to get the CO 2 in pure form and under normal pressure. Since in a hybrid MgH2/gasoline system only about half as much gasoline is burnt, the mean thermal power of the high temperature reactor capacity,

SEIFRITZ: REDUCTION OF CO2-EMISS1ONS FROM AUTOMOBILES 457

serving the same 31.3 millions of cars, would be ,- 6.2 GW(th) corresponding to about 20 process heat reactors, each possessing a thexmally installed power of 500 MW(th).

This example shows that it would be energetically much "cheaper" to capture the CO 2 before it is released into the atmosphere. The entropy reduction scheme by CO2-traps, as shown, is a measure for the allocation of external environmental costs being not yet taken into account so far.

CONCLUSIONS

In conclusion it can be said that the use of light metal hydrides in combination with gasoline or with other light metal carbides can reduce completely or partially the emission of carbon dioxide into the atmosphere. The basic idea hereby is to use the fight metal atom not only as a bed for the "H" and "C"-fuel atoms of the automobile but also as a bed for the produced "COz-waste-molecules". Or in other words, instead of burning the hydrocarbons in the classical way via

n n H2 o C.H, +(m + ~)O2 - mCO2 +~ , (13)

where there is no possibility to bind the m moles of CO 2, we substituted in CmH n the C-atom by an M- atom, yielding the hydride.

The price we have to pay is an external fuel cycle for the regeneration of the hydrides. Since, however, the energy/mass ratio of these fuels is still very good, we believe that this important feature, allowing a good mobility, justifies the external fuel cycle.

Beyond its use in the transportation sector, this kind of "CO2-concentration system", reducing the CO T mixing entropy in the atmosphere, may be beneficial in collecting the CO2-emissions of other scattered and diffuse CO2-emitters, too.

ACKNOWLEDGEMENT

This work was performed in the frame of the project "Instrumente f'tir die Entwicklung yon Strategien zur Reduktion energiebedingter Klimagasemissionen in Deutschland", Teilprojekt 4 "Umwandlungssektor", Z/A-78, sponsored by the Bundesminister f@r Forschung und Technologic (BMFT), Bonn. Furthermore, I thank Dipl.-Ing. P. Schaumann for reading the English text.

LITERATURE

1. Schutz der Erde, Eine Bestandsaufnahme mit VorschHigen zu einer neuen Energiepolitik, Hrsg.: Deutscher Bundestag, Bonn, Bericht der Enquete Kommission des 11. Deutschen Bundestags " Vorsorge zum Schutz der Erdatmosphlire", ISBN 3-924521-61-1 (1990).

2. Seifritz, W., Der Treibhauseffekt - Technische M6glichkeiten der CO2-Entsorgung, C. Hanser-Verlag, Munich, ISBN 3-446-15842-1 (1991).

3. Petkov, T., T.N. Veziroglu and J.W. Sheffield, An Outlook of Hydrogen as an Automotive Fuel, Int. J. Hydrogen Energy 14, p. 449 - 474 (1989).

4. Heitland, H. et al., M@glichkeiten und Potentiale neuer Kraftstoffe und Antriebe im Verkehr und P. Pischinger, Verbrennungsmotoren, Vorlesungsvordruck, Aachen (1982) in: Energie und Klima; Bd. 6, Economica Verlag Bonn, ISBN 3-926831-93-6 (1990), sowie: Kommt das Oko-Autu, der Spiegel 28, S. 86-87 vom 8. Juli (1991).

458 SEIFRITZ: REDUCTION OF CO~-EMISSIONS FROM AUTOMOBILES

5. Seifritz, W., A New Hybrid Hydrogen/Gasoline Driven Motor Car with a CO2-Trap, and: Parti~ Reduction of CO2-Emissions of Automobiles Using Metallic Carbides as Fuel+ ~: ~ L HTdroger. Energy, Vol. 15, No. 10, p. 757-762 and p. 763-767 (1990).

6. Hollman-Wiberg, Lehrbuch der anorganischen Chemie, 71-80. Auflage, pp. 470, de Gruyter, ~efiin (1971).

7. Okken. P.A., A Case for Alternative Transport Fuels, Energy Policy, pp. 400-405, May (1991).

8. "Mit Vollgas ins Klima-Chaos", Der Spiegel, Nr. 37 vom 9. Sept. (1991).