Carbon: Hydrogen carrier or disappearing skeleton?

hr. J. Hyakogen Energy, Vol. 20,No. 6, pp. 493-499,1995

InternationalAssociationfor HydrogenEnergy Else& ScienceLtd Printedin Great Britain 036O-3199/95 $9.50+ 0.00

Pergamon 034&3199(94)ooo814

CARBON:

HYDROGEN

CARRIER

K. P. DE JONGt$

OR DISAPPEARING

SKELETON?*

and H. M. H. VAN WECHEM§

@hell Research BV (Koninklijke/Shell-Laboratorium, Amsterdam), Badhuisweg 3, 1031 CM Amsterdam, The Netherlands @hell Internationale Petroleum Maatschappij BV, Care1 van Bylandtlaan 30, 2596 HR Den Haag, The Netherlands (Received for publication 24 June 1994)

Abstract-The useof liquid hydrocarbonsasenergycarriersimpliesthe useof carbonas a carrier for hydrogento facilitate hydrogen transport and storage. The current trend for liquid energy carriers used in the transport sector is to maximize the load of hydrogen on the carbon carrier. The recently developed Shell Middle Distillate Hydrogenation process for the manufacture of high quality diesel from aromatic refinery streams fits this picture. In the future, the hydrogen required to raise the product’s H/C ratio will be increasingly produced via gasification of large amounts of heavy residues. In the light of the strong preference towards using liquid fuels in the transport

sector,the ShellMiddle Distillate Synthesisprocessto convertnatural gasinto dieselof very high quality is discussed. Finally, a few comments on the use of hydrogen without a carbon carrier are made. Long lead times and the ltkelihood of producing the “first” hydrogen from fossil fuel are highlighted.

1. INTRODUCTION

vaba(wJll) In the transport sector, the annual worldwide use of fuels ssc (gasoline, keroseneand diesel) amounts to more than one billion tonnes. In these fuels, some 100 million tonnes of 200 hydrogen are chemically bound to carbon, via which this hydrogen can be supplied and used in a “liquefied” form. Fossil fuels contain hydrogen to a variable extent and, accordingly, the state of aggregation for coal, oil and ,M) natural gas shifts from solid to liquid to gas (Table 1). Table 1. Energy carriers Type Coal Oil Natural gas Hydrogen

H/C ratio @t/at) 0.6 1.8 4.0

co

---.-__

50

State of aggregation

0

I

2

3

4

H/c ratie(aVat)

Solid Liquid Gas Gas

Fig. 1. Value of hydrocarbon fuels (FD = flashed distillate; LPG = liquefied petroleum gas).

The current preferenceof the transport sector for liquid fuels is reflected in their relatively high market value (without taxes) compared to solid and gaseous energy carriers. In Fig. 1 this is illustrated by plotting the indicative market values of a broad range of energy carriers as a function of their respective H/C ratios. The maximum value around H/C values of 2 relates, firstly, to the state of aggregation and, secondly, to their composition and quality (e.g. sulfur level). *Paper presented at the Nationale Themadag Waterstof, Energieonderaoek Centrum Nederland (ECN), Petten. t To whom correspondence should be addressed. 493

H/C ratios significantly above two lead us into gaseous energy carriers (liquefied petroleum gas or LPG and methane or natural gas).Although both LPG and natural gas are very clean, their states of aggregation complicate their transport and their use as automotive fuels. Interestingly, the market values of natural gas differ widely depending on the location of the source. In so-called remote locations (e.g.Alaska, Siberia), a value of natural gas of, say, 25-50 S/t is typical, whereas in the United States a value of 100-150 $/t may be noted. The difference in value between a remote and an industrialized site reflects roughly the cost of transport of the gasin liquefied form (LNG).

494

K. P. DE JONG and H. M. H. VAN WECHEM

ante of carbon as the “skeleton” of hydrogen in the transport sector.

H/C atomic ratio *.O” I

2. FROM LOW TO HIGH HYDROGEN-TO-CARBON RATIOS

(1990 estimates)

2.1. Hydrogenation of gas oil

1.80 F Crude supply 1.75 1 1980

1985

1990

1995

2000

I 2005

Year Fig. 2. Projected trends of H/C atomic ratio of products and crude oils.

From Fig. 1, the main economic driving force for conversion of either hydrogen-deficient (coal, oil) or hydrogen-surplus feedstocksto hydrogen-rich liquid hydrocarbons becomesapparent. A second driving force to shift the H/C ratio of the product relative to that of the feedstock is the quality of the products (Fig. 2). As a major trend we observe an increasing hydrogen content of liquid transportation fuels, which is associated with the extensive hydroprocessing of oil fractions. When carbon is considered as a hydrogen carrier a number of important technologies emerge. In the first place, oil fractions are hydrogenated to maximize the load of hydrogen on carbon in liquid hydrocarbons. In this context we will elaborate on the Shell Middle Distillate Hydrogenation (SMDH) process to upgrade diesel fractions. Secondly, the manufacture of hydrogen in a refinery by gasification of heavy residue will be touched upon, together with the hydrogen balance of a modern refinery. Thirdly, the conversion of gas to liquids will be introduced by a brief discussion on the Shell Middle Distillate Synthesis (SMDS) process. Finally, some comments will be made on the possible disappear-

The trend to increase the H/C ratio of products is driven by a multitude of factors. Here we restrict the discussion to Automotive Gas Oil (AGO) or diesel. In Fig. 3 we show data on the cetane number (a measure of the ignition quality) and the density of several potential components of the AGO pool in a refinery. Clearly, for most of the components the quality does not reach the level demanded in Europe. A considerable reduction of the density and an increase of the cetane quality is required. In terms of chemistry this can be achieved by hydrogenation of the aromatics present in these AGO components. The naphthenes thus produced display the required characteristics. Figure 4 illustrates that straight hydrogenation of aromatics at acceptable hydrogen pressuresis not possible to any great extent. The high temperatures required to hydrogenate the refractory compounds in an environment of sulfur and nitrogen conflict with the thermodynamic equilibria of aromatics and naphthenes. At elevated temperatures the equilibrium mentioned favors aromatics under the conditions shown. For the reasons indicated the SMDH processhas been developed. In this two-stage process (Fig. S), desulfurization of the oil fraction is carried out in a first stage followed by a rough separation and hydrogenation of the aromatics in a second stage.In the latter stage we make use of a proprietary zeolite containing noble metal catalyst. Deep desulfurization and hydrogenation can be realized at moderate pressures(Fig. 6). The first commercial plant in our Gothenburg refinery has now been in operation for more than 1 year. The product quality meetsthe Class 1 specifications in Sweden (Table 2). For further details on this new SMDH process, seeRef. [l].

0-

T- 40

T

T+40

WABT, “C

Fig. 3. Typical properties of diesel fuel blending components.

Fig. 4. Limitations of the single-stage concept: temperature operating window for aromatics saturation.

495

CARBON: HYDROGEN CARRIER OR DISAPPEARING SKELETON?

Fuel Gas Product

Fig. 5. Shell Middle Distillate Hydrogenation processscheme:the Gothenburg line-up (DEA = diethanolamine; MBU = membrane unit). Table 2. Class 1 and 2 AGO production for the Swedish market: the first commercial application Characteristics CCH, (wt %)’ Yield (wt %) Gas oil Naphtha Density (kg/m3) s @pm W N @pm wt) Total aromatics (vol”6y Cetane index3

Feedstock Class 2 Class 1 0.55

820 4500 10 30 50

97 3 806 15 18 55

0.95 93 I 801 5

specs (Class 1)

800-820
4.5 57

‘Chemical hydrogen consumption ‘Modified IP-391. 3Cetane index-ASTM D 4737.

<5 >50

2.2. Hydrogen production in a refinery The complexity of hydrogen management in a modern refinery is shown by the flow diagram of the Pernis refinery (Fig. 7) as envisaged [Z]. Conventionally, hydrogen is produced by reforming naphtha. A next step is to use Steam Methane Reforming (SMR) to produce hydro-

gen from natural gas. SMR was introduced in Pernis in 1988 to manufacture hydrogen for the hydroconversion of residue (HYCON). In view of the enhanced demand for hydrogen, with the construction of a large hydrocracking unit a next step at Pernis will be the use of the Shell Gasification Hydrogen Process (SGHP). SGHP converts heavy residues via partial oxidation into synthesis gas. Treatment of the synthesis gas followed by the water-gas shift reaction will produce hydrogen (Fig. 8).

Feed

1st Stage OS

Aromatics

2nd Stage

q cetaneIndex

Fig. 6. The integrated two-stage Shell Middle Distillate Hydrogenation process: light cycle oil upgrading.

K. P. DE JONG and H. M. H. VAN WECHEM

496

Fig. 7. RefinerySowschemeasprojectedfor Pemis[FCC = fluid catalyticcracker;HDS = hydrodesulfurization;LPG = liquefied Petroleumgas;LR = long residue(atmospheric);Mogas= motor gasoline;SMR = steammethanereforming;SR = short residue (vacuum);VGO = vacuumgasoil]. The hydrogen requirement of a refinery increases dramatically on progression from a complex (conversion of vacuum gas oil to distillates) to a no-fuel (full conversion to distillates) operation. Figure 9 shows the overall hydrogen balance for the refinery operations mentioned.

The hydrogen deficiency of the feed relative to the (total) products is assumedto be bridged by gasification of the heaviest aromatic components in the crude (represented by Conradson Carbon Residue,CCR). Depending on the crude and the product quality envisaged in the future, FrCill other trains

oil PmCesS return w I

I

Claus gas Fuel gas to to SRU

Co

-crYi-

,

“2’U

HCU

To other trains Bleed IO SWS

b sootqlmoh Fran other trains

I

Fig. 8. ShellGasificationHydrogenProcess(BFW = boiler feedwater; HCU = hydrocrackingunit; SRU = sulfur recovery unit; SWS= sour water stripper).

491

CARBON: HYDROGEN CARRIER OR DISAPPEARING SKELETON? Efficiency (%LHV) ““c

Arab ligh /

.____- --.,--._ 50 -

129 Hydrogen

13.7 13.3 content of products,

40 0.5

14.1 %m/m

--

--

~--

2

1 H/C ratio of b.kduct

Fig. 11. Maximum thermal efficiency of synthetic fuels: synthesis gas to hydrocarbons, Fig. 9. Hydrogen balance for different refineries and crudes. CCR = Conradson Carbon Residue. Complex: refinery including hydrocracking of vacuum gas oil. No-fuel: refinery including (hydro)conversion of vacuum residue. For further explanation see text.

for natural gas, the most efficient process should opt for

hydrogen-rich products. This is one of the reasons that Shell has aimed for paraffinic middle distillates rather than aromatic gasoline as the prime product. The SMDS process converts methane with oxygen to

between 50 and 100% of crude CCR will have to be gasified in a no-fuel operation, whereas the complex refinery hardly needs any hydrogen from gasification (becausehydrogen from platforming is almost sufficient).

synthesis gas by partial oxidation. Next, heavy paraffins are made from the synthesis gas using Fischer-Tropsch technology. For reasons of both yield and product quality, the heavy linear paraffins a!e cracked (back) to lighter branched ones with excellent Ignition quality. The

3. FROM HIGH TO LOW HYDROGEN-TO-CARBON RATIOS The conversion of natural gas to liquid transport fuels is driven by their price differential (Fig. l), as well as the growth of the proven gas reservesrelative to those of oil (Fig. 10). In considering the manufacture of synthetic fuels from natural gas (or coal), the maximum allowable thermal efficiency when synthesis gas is the intermediate is of relevance. The data summarized in Fig. 11 indicate that, Billion Barrels Oil Equivalent ‘OoOI 800 800 -

processlay-out is shown in Fig. 12 and the diesel quality is indicated in Fig. 13. For details on the SMDS process, please consult Ref. [3]. 4. HYDROGEN WITHOUT A CARRIER-SOME CONSIDERATIONS Worldwide, some 40 million tonnes of hydrogen are produced per annum (Mt/a), mainly for ammonia production and oil refining (Table 3). The 1990world energy consumption amounted to about 3000 Mt/a of hydrogen equivalent. In other words, the current hydrogen production capacity is just the equivalent of 1% of the total energy consumption. The estimated figure for the year 2050 for hydrogen production in the sectors indicated is around 100 Mt/a, mainly because of the upgrading of heavy residue to distillates, calling for huge amounts of hydrogen in the oil-refining industry (Table 3). The large

Mainly OPEC revisions-

investments involved will bring the hydrogen level to 2% of the energy consumed. These figures immediately indicate that, should we wish to move to a hydrogen-based economy, the lead time would be considerable.

/l

Table 3. World hydrogen production Use

1970

1975

1980

1985

1989

Source: Shell estimates

Fig. 10. Evolution of proven world oil and gas reserves.

Ammonia Refining Methanol Rest Total Total energy (hydrogen equiv.)

Amount (Mt/a) 1990

2050

2.5

40

10

50

3

6

2 40

4 100

3000

5000

498

K. P. DE JONG and H. M. H. VAN WECHEM Syngas manufacture

Synthesis

Conversion

Heavy

Nat.

paraffin gas

DisMates

conversion

CH, + ; O,--,

CO + 2H,

+(-CH,-)

+ H,O

heavy paraffins Fig. 12. Overall Shell Middle Distillate Synthesis process configuration (HMU = hydrogen manufacturing unit; SGP = Shell Gasification Process).

The introduction of liquid hydrogen in aviation [4] may serve as an example to further illustrate the latter point (Table 4). Table 4. Introduction of liquid hydrogen for aviation (source: R. Volkhausen, Deutsche Aerospace, 1993) Activity

Period

Technology development First planes/airports All large airports All aircraft

199&2010 201&2030 203&2050 205&

The data from Ref. [S], summarized in Table 5, strongly suggest that, should hydrogen be produced on a large scale, production from fossil fuels (e.g. reforming of natural gas) would be preferred if economic arguments were to predominate.

Cetane Number

Table 5. Hydrogen production via water electrolysis: cost in relation to source of electricity Cost ww Hydro-power Solar cells Power-coalfired Power-IGCC’ Nuclear

5 90 5 5 4

Reforming (direct)

1.5

‘IGCC = Integrated Gasification Combined Cycle.

Although the costs of the production of hydrogen from natural gas might be acceptable, the energy losses are considerable. In Table 6 we show thermal efficiency data typical for conversions of natural gas to hydrogen, methanol and middle distillates (via SMDS). In these conversions, 2@40% of the primary energy available in the methane is lost. The (environmental) benefits of these conversions should outweigh this energy loss in one way or another. A careful consideration of the pros and cons of all aspectswould seemto be mandatory. Table 6. Thermal efficiencies(% LHV) of natural gas conversion in actual processes’ Product Hydrogen Methanol Middle distillates

I 0

I

* 20

*,

. 40

. 60

8

' 80

1

I 100

SMDS Gas Oil Blend, %

Fig. 13. Gas oil produced by Shell Middle Distillate Synthesis.

Efficiency (%) 75 67 63

‘For comparison, the thermal efficiency of an oil refinerydepending on its complexity-ranges from 90 to 95 %.

Finally, in view of the long lead times mentioned, there is a considerable chance of a breakthrough emerging in

CARBON: HYDROGEN CARRIER OR DISAPPEARING

some as yet unforeseeable area of energy technology, changing the scenecompletely. In such a scenario energy carriers other than hydrogen could well take over the dominant role. 5. CONCLUSIONS In the transport sector today liquid energy carriers are preferred. A tendency to maximize the hydrogen load on carbon in liquid fuels is apparent. The hydrogenation of diesel components can be conveniently carried out using the Shell Middle Distillate Hydrogenation process. In the refinery of the future, gasification of heavy residues will increasingly take place to produce hydrogen. The hydrogen balance in a refinery shows that, in the future, a large amount of the Conradson Carbon Residue in the crude will have to be gasified to meet the demand. The conversion of natural gas to middle distillates in the SMDS process decreasesthe load of hydrogen on carbon required to obtain liquid transport fuels.

SKELETON?

499

. The current world hydrogen production is equivalent to just over 1% of the total energy consumption. . Should a hydrogen economy develop the lead times will be long. . If economic arguments are dominant, the “first” hydrogen will be produced from fossil fuels. In this case,however, energy losses will be considerable. REFERENCES 1. J. P. van den Berg, J. P. Lucien, G. Germaineand G. L. B. Thielemans, Deep desulphurisation and aromatics hydrogenation for automotive gasoil manufacturing. Fuels Processing Technd. 35, 119-136 (1993). 2. P. Ladeur and H. Bijwaard, Shell plans $2.2-billion renovation of Dutch refinery. Oil &Gas J. pp. 64-67 (26 April 1993). 3. S. T. Sie, M. M. G. Senden and H. M. H. van Wechem, Conversion of natural gas to transportation fuels via the Shell Middle Distillate Synthesis process(SMDS). Catalysis Today 8,371-394(1991). . . 4. R. Volkhausen, Liquid hydrogen could be aviation fuel of the future. The Clean Fuels Report, pp. 144-145 (September 1993). 5. K. Hassmann and H.-M. Kuehne, Primary energy sources for hydrogen production. Int. J. Hydrogen Energy l&635440 (1993).