Design of a process to generate power from natural gas, using catalyzed decomposition and fuel cells

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CPDNR

3326

Conceptual Process Design

Process Systems Engineering

DelftChemTech - Faculty of Applied Sciences Delft University of Technology

Subject

Design of a proeess to generate power from natura! gas,

using eata!ysed deeomposition and fue! eells

Authors

A.M. van de Linde!oof

V. Brouérius van Nidek

A.G. van der Neut

K. Kuijvenhoven

M.P. Sehmidt

Keywords

(Study nr.)

1025805

1056816

1061801

1046659

1014110

Telephone

06-18472220

06-24580766

06-24863334

06-54296358

06-44326458

Power generation, Methane deeomposition, Fue! eells,

DDM, SOFe, DCFC, Proeess intensifieation

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Summary

In this CPD project a design has been made for a process that generates 50 MW of power from natural gas, using catalyzed decomposition and fuel cells. The decomposition and the fuel cells are integrated into one unit, operated at 1100 K and 1 bara. The overall electrical efficiency of the process amounts to 70.9 %. The system has two product streams, one pure CO2 stream and one exhaust stream containing oxygen, nitrogen, water and a small amount of carbon dioxide. In addition, a negligible amount ofNOx is emitted. The system is operated continuously and has an

on-stream factor of 0.97. A very small amount of down time is needed for maintenance, but in essence there is no build up of contaminants since the only solid that can be formed is carbon which is electrochemically removed.

Building a power station that provides 50 MW of electricity, means that a market share of 0.44 % of the Dutch market is produced. On this market, the price of electricity is 4.3 cents per kWh.

The building of this process, which is considered to have an economicallifetime of 30 years, requires a total investment of 108 M€. The rate of return on this investment is -4.47 %, meaning that an increasing loss will be made. Therefore the pay out time and the DCFRR cannot be calculated for this design. The sensitivities on the annual cash flow for 10 % decrease are 99 % for the purchase costs of equipment, 72 % for the natural gas price, -19 % for the CO2 price and -150 % for the electricity price. The process is very efficient compared to other power generation methods. The CO

2-emmision rights can be traded and in the future also the NOx rights might weU be

tradable, providing additional income.

Since the process is very innovative, it is not proven to work. Hydrogen fuel cells are the only proven technology for this design. Therefore the feasibility is questionable, both technically as economically, as is shown in the previous paragraph. But with developments in the technology used, or perhaps even subsidized build of the process, the process could weU become economically viable. Subsidy could be achievable since the process is very sustainable.

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1 Introduction

At present, electricity production is largely based on the combustion of fossil fuels. However, thermodynamics limits the amount of electricity that can be generated from a certain amount of chemical energy in these traditional combustion power plants. Since the electricity production is indirect, via steam, which causes that the efficiency can never exceed the Rankine efficiency. Increasing awareness that fossil resources are finite is just one out of many driving forces for the search for higher efficiency electricity generation.

One way to improve the efficiency of electricity generation is to use fuel cells. Fuel cells are electrochemical devices that convert chemical energy directly to electricity and are thus not limited by the Rankine efficiency. Most fuel cell power plants operating on natural gas as a feedstock work in a two-step process. In the first step natural gas is converted to hydrogen and carbon monoxide by steam reforming. The second step is the conversion of this hydrogen and carbon monoxide with oxygen to yield electricity and heat in a fuel cello

An alternative would be to use catalytic decomposition of methane to produce hydrogen gas and solid carbon. Both products can be converted to electricity in different fuel cells, leading to an improved yield. Such a highly efficient process to convert natural gas to electricity is theoretically possible.

This observation leads to the following objective for this conceptual process design project:

Design of a process to generate power from natural gas, using catalyzed decomposition and fuel cells. The constraints that have to be met by the final design are:

• No major changes to the local ecosystem

• No toxic emissions to air, water and land at the location • Competitive price for manufactured productlproduced energy

• Process has to be accepted by the Dutch society and local communities The project principal is professor Kapteijn. He is mainly interested to know whether the proposed route from methane to electricity is really as promising as claimed by certain researchers [1]. Furtherrnore he would like to know what the main obstacles are that have to be overcome in order to use this route and how this technology should be implemented.

The proposed process would consist of three main steps:

1. A reactor for the Direct Decomposition of Methane (DDM), which produces hydrogen gas and solid carbon.

2. A fuel cell that produces electricity and heat from the conversion of hydrogen to water.

3. A Direct Carbon Fuel Cell (DCFC) that pro duces electricity and heat from the conversion of solid carbon to C02.

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types of hydrogen fuel cells exist, each with its specific advantages and applications, but they all rely on the same basic principle: hydrogen and oxygen are separated from each other by an electrolyte that allows ions to migrate from one side to the other but is impermeable to electrons. An extemal electric circuit allows electrons to 'bypass' the electrolyte and thus create a c10sed loop, which enables the reactants to react. The flow of electrons in the extemal circuit can be used to do work. Carbon fuel cells work according to the same principle, as was described in 1855 by Antoine César Becquerel [4].

The technology for DDM and DCFC is less mature than the technology for hydrogen fuel cells, but nevertheless a number of artic1es has been published on both topics. The carbon produced during DDM has a nano-fibrous structure and is formed on the surface of the catalyst partic1es [1]. A major challenge is to remove the carbon from the catalyst partic1es in order to maintain catalytic activity and allow the carbon to be transported to the DCFC. Most studies on this subject focus on either the regeneration of the catalyst (during which the carbon is lost) or the harvesting of the carbon (without regenerating the catalyst). Considerable attention will therefore be given to the design of this part of the process.

One of the main choices that had to be made during the design was the scale at which the process will operate. One option was to design a process suitable to be installed in individual households «5 kW). The other extreme is a full-scale power plant (>50 MW) suitable to replace (or complement) current power plants. Of course, intermediate scales are also possible. The advantages and disadvantages of the different scale possibilities will be reviewed in this report before the decision is made. During this project a design will be made, optimizing an electricity production process based on the principles and constraints mentioned above. In order to do this the available technologies will be evaluated and where necessary innovative and creative solutions for the problems that will emerge during the design will be developed. Attention will be given to the sustainability and safetyaspects as weIl as to the economics of the process. The design will especially be focused on the thermodynamic efficiency of the design and a comparison wiIl be made with competing alternative technologies such as catalytic partial oxidation, auto-thermal steam reforming, conventional gas fired power plants and natural gas fed solid oxide fuel cells.

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1.1 Market situation and competition

As stated above, the scale at which this technology should be implemented is of key importance for the design. As will be explained in chapter 2, the final design will consist of a combined unit for the decomposition of natural gas and the carbon fuel cell, in combination with a so called Solid Oxide Fuel Cell for the conversion of hydrogen. The installation will operate at an electricity production level of 50 MW. The reasoning behind this scale is mentioned in section 3.3.1.

According to data from Eurostat [5], a 50 MW power plant equals 0.44% of the electricity production capacity in The Netherlands. The amount of natural gas required for this production level equals 0.11 % of the total consumption of the Netherlands. Because the proposed plant equals only a very small fraction of the total electricity production, as weIl as of the total natural gas consumption it can safely be assumed that the impact on both the feedstock and product markets is negligible. Currently, electricity production in the Netherlands is accomplished for over 94% with conventional thermal power plants, of which coal (25% of total electricity production) and natural gas (59% of total electricity production) have the largest market share. Renewable energy accounts for 5.5%1 of electricity production and nuclear energy accounts for 4.2%.

About 25 patents were found discussing decomposition of methane, most of which use nickel as a catalyst.

Two patents were found which combined decomposition of fossil fuel (including natural gas) with electricity production in a direct carbon fuel cell [6,7]. In both cases the decomposition takes place in an electric arc hydrogen plasma black reactor. Molten salt was circulated between the reactor and the carbon fuel cello One of the patents also applies a Solid Oxide Fuel Cell (SOFC) to convert the produced hydrogen and carbon monoxide to electricity. When a SOFC is applied, an overall· electrical efficiency of 70% to 80% and higher was reported [6]. It was emphasized that the wastes producedwere of high purity, what makes these streams commercially atlractive.

Up to 40 patents were found concerning electricity production from hydrogen using a SOFC. 17 Patents regarding tubular SOFC units were found. Siemens Westinghouse is known to produce this type of fuel cells and has filed 4 patents concerning a tubular design of a SOFC [8-11]. None of the SOFC's mentioned in the patents are used as carbon fuel cells. Also none of the patents mentions the possibility of integrating the decomposition of methane and the carbon fuel cell into one unit.

1 Ofwhich 4% consists ofbiomass fued power stations, which are also included in the figure for

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2 Process options and selection

In order to get to the eventual process design some choices have to be made. These choices are clarified in the following chapter.

2.1 Process options

A number of different concepts were evaluated before the final process option was selected. One of the key decisions that had to be made, was the choice of the catalyst to be used for the decomposition of methane, since this has a major influence on the whole process design. Two different catalysts that can be used for the direct decomposition of methane have been reported in literature. One catalyst is based on nickel; the other option is to use activated carbon. It is reported that the nickel-based catalyst yields a conversion of up to 95% on a molar basis [12,13]. Activated carbon on the other hand yields only 40% conversion [14-16]. Since the carbon produced during methane decomposition adheres to the catalyst, activated carbon has the advantage that there is no need to separate the two. Instead, the whole carbon complex can be fed to the carbon fuel cell (for a description of carbon fuel cells see appendix A.l), whereas the nickel option clearly requires the separation of catalyst and carbon in one way or another.

A brainstorm in the form of a design-dialogue with a moderator was used as a technique to develop the different process alternatives (see chapter 12). The brainstorm was organized in two separate parts:

• Development of process alternatives based on an activated carbon catalyst • Development of process alternatives based on a nickel catalyst

The concepts developed in this way are described in appendix A.3. These concepts have been ordered using a tree starting at the main issue to be addressed, the CatalystiC particle that is formed ( Figure 2.1).

Three concepts are thought to be the most promising for the decomposition reactor: the Alternating DecompositioniOxidation Reactor (ADO-reactor), the Contained Snooker BalI Reactor (CSBR) and the membrane separation ofhydrogen and methane in a C-catalysed DDM reactor. To make a choice between these three options, a multi-criteria analysis was made. In this analysis, each concept was ranked against the following five criteria:

• Innovatory • Feasibility

• Ease of separation

• Ease of mass and heat transport • Efficiency

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Separate C from Ni-cat. before DCFC

-Microwaves -SnookerbalI Reactor

-Gasification of Carbon (Boudouard) -Dissolve C

-Dissolve Ni (acid batb)

Do not separate C -ADO-reactor

fromNi-cat.before'" a Iati - - - + I

DCFC - reu ng

carbonate batb

Do not separate metbanelbydrogen

-Convert metbane in bydrogen Catalyst

and carbon formsolid particIe

- - . fnel eeD (lISe SOFCIMCFq

Separate

metbanelbydrogen

-Recycle metbane after bydrogen fuel eeD (Wie - - . PEMFq

-Use membrane separation for

V" C~,"",t m \ D. oot '~ft

.. ,

DDM

\,"""R'"

Increase conversion bydrogenlmetbane -Catalyst surface/loading -Residence Time -Process conditions (f,P) -Multiple reactors Figure 2.1: Option tree for the concept chosen

Table 2.1: Results ofthe multi-criteria analysis for process option selection

Criterion Range Membrane reactor CSBR ADO-reactor

Innovatory F easibility

Ease of separations

Ease of mass and heat transport Efficiency 1-5 1-5 1-5 1-5 1-7

2

4 4 3 2 4 1 3 3 5 Total: 15 16

To further facilitate the process selection, a table with advantages and disadvantages

of each concept was developed (Tabie 2.2). This table is based on appendix A.4.1.

Table 2.2: Pros and Cons of the th ree main concepts

Membrane reactor

Pro

Product stream contains only

H

2

Maximal energy use of methane

100% conversion Simple

Continuous operation possible No need for separation CatiC

Con

Higher pressure needed

Purge needed for N2

Capturing of fines needed

Massive flow through membrane

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Pro Coo

Snooker balI reactor - Separation Ni/C inside reactor - Liquid needed for

ADO-reactor

Continuous operation possible ultrasonic bath Innovative concept Production of fines Relatively high conversion Inert beads damage

reactor

No need for separation of Ni/C

No need for solids transport Innovative concept

Heat transport inside reactor All operations occur at the sametell1perature

High convers ion

Solid transport needed Questionable feasibility Continuous operation not possible

Difficult construction Questionable feasibility Low use of electrode area

Looking at Table 2.1 and Table 2.2 clearly shows that the ADO-reactor concept is superior to the other two options. The conversion is possibly not as high as in the ll1ell1brane reactor, but thought to be higher than in the snooker balI reactor. It is the only concept where solids transport is not needed, which is a serious advantage. Heat transport can be (partially) do ne inside the reactor, where in the other two all heat has to be delivered froll1 outside the reactor. It is an innovative concept, which cOll1bines endothernnic and exothernnic reactions in one unit, ll1aking it an excellent example of process intensification [17]. With all this, it is concluded that within the context of this CPD-project, the ADO-reactor is the best concept.

Obviously, with the decision to use the ADO-reactor concept, the process selection is not yet finished. The type of fuel cell for the produced hydrogen still needs to be chosen. Different hydrogen fuel cells are described in appendix A.2. Four ll1ain options are available, the SOFe, MCFC, P AFC and the PEMFC. In appendix AA.2 the pros and cons of these four fuel cens are surnnned up. Table 2.3 shows a SUll1ll1ary

of this appendix.

The advantages of the SOFC seell1 to outweigh its disadvantages and the advantages of the other fuel cells. F or the sake of cOll1pleteness it should be ll1entioned that there is also a fifth type of fuel cen: the Alkaline Fue! cen (AFC). This type of fuel cen is of no value for the process under consideration because it can only operate on extrell1ely pure hydrogen and oxygen. For this reason this technology is lill1ited to ll1ilitary and space applications only.

Table 2.3: Pros and Cons of th ree different fuel ceUs

SOFC

Pro

Same tell1perature as ADO-reactor

Converts wide range of feedstocks (hydrocarbons) Same type of equipll1ent as for ADO-reactor

High efficiency

Con

High dell1ands on ll1aterial properties, due to high tell1perature.

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MCFC

PAFC

PEMFC

Pro

Medium demand on material properties, due to intermediate temperature

Converts wide range of feedstocks (hydrocarbons)

Mature technology Reasonable electrical efficiency

Low demands on material properties, due to low temperature

Well known technology Methane can be fed back to ADO

Con

C02 has to be mixed with cathode feed

Converting methane yields lower efficiency than hydrogen

Operating temperature lower than other units in the process Integration with other units impossible

Prospects for improvement are low

Sensitive to impurities in feedstock

Operates at temperature much lower than ADO-reactor Does not convert methane Low efficiency

Integration with other units impossible

Calculations (see appendix C.S) show that the methane decomposition reaction requires more heat than can be provided by the carbon oxidation reaction. Integration of a hydrogen fed SOFC inside the ADO-reactor will allow for the direct use of the heat produced there. It will not be difficult to do this since the ADO-reactor consists of tubes that can be used as hydrogen fuel cello In this way, a part of the tubes is continuously operated as hydrogen fuel cell, while the other part is used as decomposition reactor or as carbon fuel cell alternatively. The hot exhaust streams leaving the ADO-reactor can be used to preheat the entering streams. Excess heat can be converted to electricity in a Rankine cycle.

2.2 Process concept selected

The option fmally selected to design during this CPD project consists of a multi-tubular ADO-reactor in which part of the tubes alternate in function between methane decomposition and carbon fuel cell, while another set of tubes works as a hydrogen fuel cell (SOFC type). It is possible to combine these different types of tubes in one unit because they all operate at the same temperature and pressure. The advantage of combining the different functions in a single unit is that heat transfer from the fuel cells to the decomposition tubes is relatively easy.

There will be a large number of combined DDMlDCFC tubes. The system will be designed such, that at any given time, part ofthe tubes functions as DDM tube, while the remaining tubes function as DCFC tubes. In this way there will be no fluctuations in the electricity production, but rather a smooth and continuous operation is assured. The number of H2FC tubes will also be large, and these tubes will be distributed evenly throughout the reactor, to ensure the temperature in the reactor will not show large gradients.

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transfer medium, facilitating the heat transfer from the exothermal fuel cells to the endothermic decomposition reaction.

To prevent the occurrence of oxidation during the methane decomposition, the electrical circuit has to be open, since a closed circuit will allow electrochemical oxidation. A more extensive description of this ADO-reactor can be found in appendix A.3.2.

Because it is impossible to reach 100% convers ion of hydrogen (and unconverted methane) in a SOFC fuel cell, the ADO-reactor is followed by a fumace in which these compounds are further oxidized.

Finally, the hot exhaust from the ADO-reactor is sent to a Rankine cycle to convert some ofthe excess heat into electricity.

In the following paragraphs the reactions taking place in the different parts of the process will be explained.

2.2.1 Decomposition of methane

In the reactor for the decomposition of methane, carbon is deposited on a nickel catalyst, while hydrogen leaves the reactor. The reaction occurring is the following:

CH₄(g) ~ 2H₂(g) + C(s) (2.1)

The formation of 2 gas molecules for every molecule consumed means low pressure favours the forward reaction.

2.2.2 Carbon fuel cell

The carbon deposited on the nickel catalyst is electrochemically oxidized to carbon dioxide in the fuel cell mode of the reactor. The reaction occurring at the anode side, where this happens, is:

(2.2) The 02- ions are transported from the cathode si de through an yttrium-stabilized

zirconium (YSZ) electrolyte. These ions are formed on the cathode side:

(2.3)

2.2.3 Hydrogen fuel cell

Since the hydrogen fuel cell is also an SOFC, like the carbon fuel cell, oxide is transported through the YSZ. The formation of this oxide on the cathode side is exactly the same:

(2.4) On the anode si de the oxide reacts with the hydrogen present to form water:

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Obviously, the anode reaction has to occur twice for every one time the cathode reaction occurs.

The kinetics ofthe above reactions win be described in section 4.5.

2.3 Bloek schemes

Figure 2.2 shows the block scheme of the concept chosen. A detailed process flow scheme can be found in section 5.2. Note that the carbon stream is not an actual stream since the carbon is not transported from the DDM-reactor to the DCFC. The mass streams are taken from section 5.3 and the temperatures and pressures from sections 3.3.4 and 3.3.5. ! !

I

DCFC i 1100K

I

1 bara I I

i

Carbon I

I

: 3.251 tonlhr I _{: 1 lOOK} I

I

_DDM

I

<2> Natural gas ! _1100K 6.145 tonlhr

I

298K, 10 bara 1 bara ! <15> Hydrogen I

<1> Fresh air I unconverted natural gas

.

'

310.084 tonJhr I 2.894 tonlhr ! l100K,lbara 298K, 1 bara

I

<13> Air to furnace , SOFC l _{293.288 tonlhr} I

I

1100K 1100K, 1 bara

I

<14> Fuel to furnace 1 bara 11.030 tonlhr 1100K 1 bara 1rotalin:

I

316.229 tonlhr

L

ADO-reactor

Figure 2.2: Block scheme of concept chosen including mass flows

Fumace 1159K 1 bara <18> CO₂product 11. 911 tonlhr 332K, 10 bara <4> Exhaust air 304.318 tonJhr 434K, 1 bara 1rotalout: 316.229 tonlhr

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DCFC 1100K 1 bara : Heat : 4.0MW ;

+

DDM 1100K 1 bara SOFC 1100K 1

I

Electricity ---4---.

I

32.7MW

I

Heat recovery _{____}_{_}_{______________}_{____}~}~_~1!!~i~ ____ •

O.2MW (Rankine cyc1e)

I

Electricity ---. I 17.1MW

I

1 bara

I

I._

~~~~__~

________

J

Total electricity out:

50MW

Figure 2.3: Block scheme with electricity and heat flows (or the concept chosen

2.4 Alternative processes

Besides the concept that is described above there are alternative technologies to produce electricity from natural gas. There are three alternative processes that can be distinguished. First there is the currently widely used gas fired power plant. This uses natural gas and combusts it to generate steam, which is used in a turbine to generate power.

The second is steam reforming followed by a hydrogen fuel cello With steam reforming, natural gas is converted into carbon dioxide, carbon monoxide and hydrogen using steam.

The third alternative is catalytic partial oxidation in a membrane reactor followed by a hydrogen fuel cell again. Here the natural gas is converted into carbon monoxide and hydrogen.

Methane can also be converted in a SOFC directly, without prior decomposition. This is possible because the high operating temperature allows for internal reforming. A small amount of water at the anode is required to be able to convert the carbon to CO2 . A drawback of this process is that it is not possible to produce pure CO2, since the produced CO2 is contained in the anode exhaust gas stream, which contains all the

impurities in natural gas (for instance nitrogen).

Figure 2.4 shows the electrical efficiencies of differently sized power plants. From

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The efficiency in a steam reforming process followed by a hydrogen fuel cell, is even lower, roughly 50% [18]. For the membrane reactor with partial oxidation the efficiency is at comparable levels, roughly 55-70% dependent on temperatures [19]. In all it is expected that the efficiency of the proposed system of a decomposition unit for methane followed by a hydrogen and a carbon fuel cell is higher than that of any alternative. This becomes clear in section 5.5, where the overall electrical efficiency ofthe proposed process is calculated to be 70.9%.

Efficiency

70 % 60

":1"

S1

11d Oxid Fuel Cen·

~

~ll!ijl!!

,

~U

,

!~

:,

... "., .. " ... , (;(.

!mtlin~~~~~:H.JIWlJ~

$chaltungen ~ .... 50 40 30 20 1S 0.1 0.5 1 5 10 50 100

"Natura! Gas Operation

Power plant output

Figure 2.4: Output and efficiency range of fossil fired power plants [20]

As far as emissions are concemed, the emissions from all systems are non.:.pure streams rich of carbon dioxide and often containing NOx • It is a clear advantage ofthe proposed system that it· produces a pure stream of carbon dioxide, which can be sequestrated or used as a feed stream for washing, solvent manufacturing or as a greenhouse feedstock, since C02 is taken up by plants during photosynthesis. In addition negligible amounts ofNOx are formed.

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3 Basis of design

3.1 Description of the design

The goal of this CPD-project is to design a process to generate electricity from natural gas, using catalyzed decomposition of methane and subsequent oxidation of the decomposition products in fuel cells.

The constraints that have to be met by the design are: • No major changes to the local ecosystem

• No toxic emissions to air, water and land at the location • Competitive price for manufactured productlproduced energy

• Process has to be accepted by the Dutch society and local communities

Special emphasis is given to the energy efficiency of the process, as weU as to the optimal scale at which the process should be designed.

Since part of the required technology is still in the experimental stage, the actual feasibility of the process wiU not be demonstrated. Furthermore, everything outside the battery limits ofthe process (see section 3.3.3) will not be taken into account. This implies for instance that it is assumed that the feedstock (natural gas) is available at the location where the plant will be built and that the infrastructure to discharge the product (C02) is also present at this site.

3.2 Process definition

3.2.1 Process concept selected

As stated in section 2.2, the process to be designed consists of a methane decomposition reactor and two separate fuel cells for the electrochemical oxidation of carbon and hydrogen respectively. From the different alternatives described in paragraph 2.1 the Altemating Decomposition/Oxidation reactor (ADO-reactor) in combination with a Solid Oxide Fuel Cell (SOFC) and Rankine cycle for heat recovery was selected for the final design. The reasoning for this selection is stated in section 2.2.

The ADO-reactor is a multi-tubular device in which each tube altemates between a DDM-reactor during which solid carbon is deposited on the nickel catalyst and a DCFC during which the carbon is electrochemically oxidized to CO2 gas. The same

unit also contains SOFC tubes in which the hydrogen is oxidized. In both fuel cells the cathode and anode are separated from each other by an yttrium stabilized zirconium electrolyte that allows oxide ions to pass through but is impermeable to electrons.

3.2.2 Stoichiometry

The stoichiometry of aU reactions taking place is shown below. Methane decomposition over nickel-based catalyst:

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Carbon fuel eell:

C(s) +202-~ CO₂(g) + 4e

-02 (g) + 4e- ~ 202

-Hydrogen fuel eell:

H₂(g) + 0 2-~ _{H 20(g) +} 2e-0 2(g)+4e- ~ 20 2-3.2.3 Kinetics (anode) (cathode) (anode) (cathode) (3.2) (3.3) (3.4) (3.5)

The reaction rate for the decomposition of methane decreases with increasing conversion due to inhibition by hydrogen. The reaction rate profile is shown in Figure 4.4 (section 4.5) and varies between 6.5 and 0 molC gcarl hol. The average reaction rate between 0 and 95% conversion is around 2.67 molC gcarl hol.

The usual way to calculate the reaction rate in a fuel cen is via the current density. The produced current of a fuel cen can be measured and divided by the electrode surf ace, which results in the current density. Current densities for carbon and hydrogen fuel cells are given in Table 3.1, as wen as the calculated reaction rates. Details about reaction kinetics are described in section 4.5.

Table 3.1: eurrent densities and reaction rates for bydrogen and carbon fuel celIs

Hydrogen fuel cell [23] Carbon fuel een [24]

2000 200

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3.2.4 Block scheme

A block scheme showing the total mass streams to and from the process (both absolute as wen as per kWh of electricity produced) is shown in Figure 3.1. More details about the block scheme can be found in section 2.3, the calculation of mass streams per kWh electricity is inc1uded in section 5.5.

i

I

DCFC I 1100K 1 bara I

I

i

Carbon

I

: 3.251 tonlhr : 1l00K (0.065) Î DDM î i <2> Natural gas

I

1100K 6.145 ton/hr (0.123)

I

298K, lObara 1 bara

I

< 15> Hydrogen

<1> Fresh air

I

unconverted natura! gas

~i _2.894_tonlhr_(0.058) 310.084 ton/hr (6.197)

I

298K, 1 bara I 1100K, 1 bara I

I

SOFC

I

1100K I ! _{1 bara}

I

Total in: 316.229 ton/hr i; ADO-reactor I t < 13> Air to fumace Fumace 293.288 tonlhr (5.861) 1100K, 1 bara 1159K <14> Fuel to fumace 11.030 tonlhr (0.220) 1 bara 1100K, 1 bara < 18> CO₂product 11.911 ton/hr (0.238) 332K, 10 bara <4> Exhaust air 304.318 tonlhr (6.082) 434K, 1 bara Totalout: 316.229 tonIhr·

Figure 3.1: Block scbeme of tbe process, sbowing total mass streams to and from tbe system. Values in parenthesis represent mass (in kg) per kWb of electricity produced. .

3.2.5 Thermodynamic properties

Pure component properties, such as boiling points, melting points, heat capacities and heats of reaction are tabulated in chapter 4. A comparison was made between thermodynamic data from the literature and the models implemented in the flow sheeting application Aspen Plus, which was used to model the process. If the right thermodynamic model is selected in Aspen Plus, the output of the program corresponds very weU with the literature data. The only exception is the viscosity of methane at high temperature. Detailed information can be found in chapter 4.

3.3 Basic assumptions

3.3.1 Plant capacity

Three possible scales were considered: a household scale «5kW), a city district scale

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The decision matrix completed with advantages and disadvantages for each scale is shown in Table 3.2. The criteria and the reasoning behind the advantages and disadvantages in the decision-matrix on sealing are explained in appendix A.6.

Table 3.2: Deeision matrix for sealing

<5 kW -5MW 50MW

Transport losses + + 0

A vailability of raw materials 0 0 0

Risk/safety 0

Controllability/ease of operation 0

Costs Efficiency + ++ ++ Maintenance 0 Size 0 0 Acceptation 0 0 + Reliability + 0 0 Emissions + + ++ Feasibility A vailability of altematives + +

Based on these results it is decided to focus the design on a production capacity of 50 MW electricity. This requires a natural gas feed of6.145 tonlhr.

Besides natural gas, air is required at a feed rate of approximately 310 tonlhr. Pure carbon dioxide will be produced at a rate of 11.9 tonlhr and lean air will be emitted at a rate of 304 tonlhr. The lean air emitted to the atmosphere contains a small amount of carbon dioxide (less than 0.8 tonlhr) and approximately 10.2 tonlhr of water, which is one of the reaction produets from the process. Full details about all streams are tabulated in appendix C.4.

For the calculation of the economie plant life, it was assumed that the process will operate for 8,500 hours a year. It has been assumed that the installation will run for 30 years Cafter a4 year construction period). Half way during the economical plant life the ADO-reactor unit needs to be replaced because the technical lifetime of the reactor and fuel cell tubes is 15 years. For more information the reader is referred to chapter 11 and section 9.3.

3.3.2 Plant location

A specific site for the plant was not defined. An obvious choice would be to build the installation next to an existing gas fired power plant since both a high capacity electricity grid connection and a sufficient supply of natural gas are easy to realize at such a site. A requirement for the plant location is the presence of facilities to dis charge the product C02. Since no specific site was selected, it was simply assumed during the design that all these requirements of the site were met. For more information see also section 3.3.3.

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3.3.3 Battery limits

Limits have to be set on which units will be designed during the conceptual process design and which units are outside the battery limits. As is described before, the main units within the battery limit are the ADO-reactor (including hydrogen fuel cell), furnace and Rankine cycle. The turbines, compressors, pump and heat exchangers will also be designed during this conceptual process design. The battery limits are graphically represented in Figure 3.2. The details for every stream crossing the battery limits will be explained in the following sections.

r---

_I

DCFC ; I <_11.91118> co, _tonlhrproduct

I

_332K_,_{10 bara} 1100K I

I

1 bara

î

Carbon

I

i 3.251 tonlhr i llOOK DDM <2> N atural gas 1100K _I 6.145 tonJhr I

I

298K, 10 bara

I

1 bara •

I

<15> Hydrogen <I> Fresh air ... 1

unconverted natural gas

I

310.084 tonlhr

I

2.894 tonlhr j

2981(, I bara I lOOK, I bara i

I

_I

<\3> Air to fumace

I

SOFC ! 293.288 tonlhr ~ Fumace _<₄_>_{Exhaust air}

I

1l00K, 1 bara

1159K 304.3 18 tonJhr

! 1100K 1 1 _{434K, 1 bara}

i I _<₁₄_>_{Fuel to fumace} I

I

1 bara

I

11.030 tonlhr 1 bara

I I

i 1 lOOK, 1 bara I

I

(23)

3.3.4 Streams entering the battery limits

To be able to design the units within the battery limits, assumptions on the composition and conditions of the streams entering the battery limits have to be made. The streams entering the system are natural gas (Tabie 3.3) and air (Tabie 3.4). Natural gas is supplied to the site by pipeline. Air will be taken in from the atmosphere.

Table 3.3: Properties of natural gas entering the battery Iimits

Stream Name:

I

N atural gas <2>

Comp. Units Specification Note numbers

Available (1) Desiê! Notes

Methane mol% 81.3 84 (2) (1) As obtained in Slochteren [25] Ethane mol% 2.9 0 (2) (2) Methane and ethane behave in the Propane mol% 0.4 (3) same way in DDM. Therefore both

Butane mol% 0.2 (3) are taken together.

Pentane mol% < 0.1 (3) (3) Not harmful for process, not Hexane mol% < 0.1 (3) included in mass balances

Nitrogen mol% 14.3 15 (4) Only valid for this scale [26]

Oxygen mol% 0.01 (3)

Carbon dioxide mol% 0.9 1

Total 100.0

Process Conditions and Price

Temp. K 298

Press. Bara 10

Phase V/LIS V

Price €/GJ 3.75 (4)

Table 3.4: Properties of air entering the battery Iimits

Stream Name:

I

Air <1>

Comp. Units Specification N ote numbers

A vailable [27] Design Notes

Nitrogen mol% 78.08 79 (1) Not harmfill for process in these Oxygen mol% 20.95 21 concentrations, not inc1uded m

Argon mol% 0.93 (1) mass balances

Carbon dioxide mol% < 0.1 (1)

Neon mol% < 0.1 (1) Helium mol% < 0.1 (1) Krypton mol% < 0.1 (1) Hydrogen mol% < 0.1 (1) Xenon mol% < 0.1 (1) Total 100.0

Temp. K 298

Press. Bara 1

Phase V/LIS V

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3.3.5 Streams leaving the battery limits

The electricity produced is the main product. The price must be competitive with

electricity produced via existing technologies. The electricity will be distributed to the

customers through the existing electricity grid. The price of electricity at this scale is 0.043 €/kWh [26].

The other streams leaving the battery limits are carbon dioxide (Tabie 3.5) and lean

air (Tabie 3.6). Lean air will be discharged into the atmosphere via a stack. Carbon

dioxide will be transported away from the plant by a pipeline.

Table 3.5: Properties of carbon dioxide leaving the battery Iimits

Stream Name:

I

Carbon dioxide <18>

Comp. Units S pecification Note numbers

Available Design Notes

Carbon dioxide mol% 100 100 (1) Value of emission rights [[28]]

Total 100.0

Temp. K 332

Press.[29] Bara 10

Phase V/LIS V

Price €/ton 23 (1)

Table 3.6: Properties of air leaving the battery Iimits

Stream Name:

I

Exbaust air <4>

Comp. Units Specification N ote numbers

Notes

Nitrogen mol% 79.0

.Oxygen mol% 15.6

Carbon dioxide mol% 0.2

Water mol% 5.3

Total 100.1

Temp. K 434.4

Press. Bara 1

Phase V/LIS V

Price €/kg 0

3.4 Economic margin

The economic margin is defined as the total value of all outgoing strearns crossing the battery limit of the process minus the total value of all incoming streams crossing the battery limit. This margin is expressed in terms of money per kilowatt-hour of electricity produced (€/kWh). The streams entering the process are natural gas and air. Outgoing streams are carbon dioxide, lean air and electricity.

(25)

the amount of natural gas consurned is 0.094 kmol/s. With a methane mol fraction of 0.84 (see Table 3.3, section 3.3.4) and using a heat of combustion for methane of 891 kj/mol [30], one can calculate that this represents an energy flow of 70.5 MW. If it is assurned that the installation will operate for 8,500 hours per year, the consurnption of natural gas will be 2,157,000 GJ/yr. At this consurnption rate, the price ofnatural gas is 3.75 €/GJ [26].

The electricity output of 50MW equals a total annual electricity production of 425,000 MWh/yr (with 8500 operating hours per year). At this production level, the electricity price is 43 €/MWh [26].

Since January 2005, installations larger than 20 MW are subject to the European emission trading system. Because the direct carbon fuel cell used in this process produces pure CO₂at the anode it can relatively easily be captured instead of discharged to the air. This CO2 can then be sequestrated or used in greenhouses for example. When this is done there are no CO2 emissions, which means the emission rights allocated to this installation can be sold. The present value of C02 emission rights is about 23 euro per ton of CO2 [28]. To calculate the economic margin it was assurned that the CO2 will be captured and hence there is also a revenue stream from the sale of the emission rights. This income was set equal to the amount of carbon dioxide produced (101,000 ton/yr, see appendix C.4) times the value of23 €/ton. Because large-scale power plants are usually built relatively far from populated places it was assumed that the waste heat cannot be used to heat up buildings and is thus no valuable product. Of course some of the heat contained in the hot exhaust gases can be converted to electricity using a Rankine cycle and this has been taken into account. Table 3.7 summarizes the stream values mentioned above. This shows that the economic margin is a little less than 13 million euro per year, or 3.0 cents per kWh.

Table 3.7: Sizes and va lues of the streams crossing the battery limits of the design for a 50 MW installation Stream Natural gas Electricity Carbon dioxide Consumed 2,157,000 GJ/yr Produced 425,000 MWh/yr 101,000 tonlyr

Price Value ofstream (€/yr)

3.75 €/GJ -8,090,000 43 €/MWh 18,500,000 23 €/ton 2,323,000

Economic margin: 12,733,000 (0.030 €/kWh)

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Tab1e 3.8: Maximum aUowed investment using a DCFRR of 10% for different life times Life time (years)

10 20 30 Maximum investment (€) 86,000,000 119,000,000 132,000,000

The information presented above shows that the economie margin for a 50 MW e1ectricity produetion process based on catalytie decomposition of methane and subsequent conversion of the decomposition produets in fuel eells is a little less than 13 million euro per year, or 3.0 eurocents per kWh. The maximum allowed investment depends on the lifetime of the project, the longer the lifetime, the higher the allowed investment.

(27)

4 Thermodynamic properties and reaction kinetics

4.1 Pure component properties

In Table 4.1 the technological data of the pure components are given and in Table 4.2 the safety and medical data. Most components are in the gas phase at ambient temperature and at the operating temperatures of 1100 K all components are gaseous except carbon and nickel.

Table 4.1: Technological data of pure components [30],[31],[32]

Component Name Technological Data

Design Formula Mol. Phase Boiling Melting Liquid Vapour Notes

Weight Point Point Density Density

(1) (2) (2) (1) (1) /[g/mol] /[K] /[K] /[kg/m3] /[kg/m3] Activated C 12.000 S carbon - - - -Carbon black C 12.000 S - - -

-Carbon dioxide CO2 44.010 G 216.59 194.69 720 1.799 (3),(4),(5)

Carbon CO 28.010 G 81.65 68.13 791 1.145 (6) monoxide Hydrogen H2 2.016 G 20.39 13.95 - 0.082 Methane CH4 16.043 G 111.67 90.68 423 0.763 (7) Nickel Ni 58.693 S 3186.15 1728.15 8900 -Nitrogen N2 28.013 G 77.35 63.15 - 1.145 Oxygen O2 31.999 G 90.20 54.36 - 1.308 Water H20 18.015 L 373.12 273.15 997 0.598 (8) Notes:

(1) At 298 K and 101.3 kPa, unless specified otherwise

(2) At 101.3 kPa, unless specified otherwise (3) Liquid density at 298 K and elevated pressure (4) Boiling point is triple point

(5) Melting point is sublimation point at which vapour pressure reaches 101.3 kPa (6) Liquid density at 256 K and elevated pressure

(7) Liquid density at 111 K (8) Vapour density at 373 K

4.2 Temperature dependent properties

The specific heats, enthalpies of formation, enthalpies of evaporation, surface tensions, liquid densities and viscosities are calculated as function of temperature and can be found in appendix B.l.

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4.3 Heat of reaction

The heat of reaetion is given by equation (4.1) and the enthalpies of formation are

given in appendix B.1 (Table B.2). Since all reactions take place at 1100K, the

enthalpy of formation has to be evaluated at this temperature.

(4.1)

Table 4.2: Safety and medical data for pure components

Component Name Safety Data Medical Data

Auto- Lower Vpper

Design Formula ignition Explosion Explosion MAC LD50 LC50

Temper Limit Limit value Oral

ature (LEL) (VEL)

(1) (2) I[K] 1[%] 1[%] l[mg/m2] l[mgJkg] I[mg/I] Activated carbon C >573 - - 3.5 - -Carbon black C >558 - - 3.5 15400 -Carbon dioxide CO2

-

9000

-

-Carbon monoxide CO >878 10.9 76.0 29

-

2.1 Hydrogen H2 833 4.0 75.6

-

- -Methane CH4 810 3.0 15.5

-

-Nickel Ni

-

1.5

-

-Nitrogen N2 - - - -Oxygen O2 - - - -Water H20 - - - -Notes:

( 1) Oral in mgJkg for a rat

(2) Concentration in air or water for 4 hours exposure time

(3) Concentration which causes no irreparable health-risk within 30 minutes (4) MAC value: 5000 ppm

(5) IDLH in mg/m3

The heats of reaction are shown in Table 4.3. In the fuel eells this heat is partly

eonverted into e1eetrieity and partly into thermal energy, depending on the e1ectrical

efficiency, as depicted in Table 4.4.

Table 4.3: Heats of reaction at 1100 K

Heat of reaction /[kJ/mol] Deeomposition of methane

Carbon fuel eell Hydrogen fuel eell

Table 4.4: Electrical efficiencies of the fuel cells

90.28 -394.93 -211.64

Electrical efficiency /[%] Hydrogen fue1 eell [3]

Carbon fuel cell [33]

(29)

-4.4

Data validation

In flow sheet calculation programs models are used to estimate the properties of mixtures. Which model is used is mainly dependant on the components in the mixture and the interactions between molecules and intermolecular forces. Most components in the system are in the gas phase, except for the carbon. Carbon is present in the ADO-reactor. In this unit a model should be used which includes solids, in Aspen Plus this is the 'solids'-method. In the hydrogen fuel cell and in the fumace water is present. Due to the high interaction of water an activity coefficient method should be used. Wilson, NRTL, RK-Soave, Uniquac and Van Laar are good options and available in Aspen Plus.

In Table 4.5 a comparison is made between literature data (Yaws, C.L. [34]) and data acquired with Aspen Plus with the solid model. The comparison is made for a temperature of 1100 K, because the ADO-reactor operates around that temperature. The pressure is ambient. The specific heat and the viscosity are compared, as both parameters are important thennodynamic and transport parameters. Most values given by Aspen Plus correspond well with the data retrieved from the literature. The specific heat of carbon however is very different. A different fonn of carbon, for example graphite or carbon black, used by Aspen Plus and the literature could cause this difference. In general, it can be said that the models used by Aspen Plus are consistent with literature values.

Table 4.5: Comparison of literature data with Aspen Plus solid model for T = 1100 K

Literature Aspen Plus

(solids model)

Gases Cp ₁₁ Cp ₁₁

/[J/(mol K)] /[~Pa s] /[J/(mol K)] /[~Pa sI

Carbon dioxide 55.20 42.87 55.38 42.60 Carbon monoxide 33.71 43.07 33.71 43.05 Hydrogen 30.56 22.13 30.61 21.78 Methane 77.00 27.20 76.82 29.92 Oxygen 35.36 52.57 35.30 52.34 Water 42.54 41.55 42.52 41.68 Solids Cp Cp /[J/(mol K)] /[J/(mol K)] Carbon 21.49 34.73

(30)

Table 4.6: Comparison of literature data witb Aspen Plus Wilson-RK model for T = 373 K Literature Aspen Plus (RK-Soave

model)

Gases Cp

"

Cp

"

I[J/(mol K)]

_Ir

_{I-lPa s}

₁

I[J/(mol K)]

_Ir

I-lPa sl

Carbon dioxide 55.20 42.87 55.38 42.60 Carbon monoxide 33.71 43.07 33.71 43.05 Hydrogen 30.56 22.l3 30.61 21.78 Methane 77.00 27.20 76.83 29.92 Oxygen 35.36 52.57 35.30 52.34 Water 42.54 41.55 42.55 41.68

In Figure 4.1 and Figure 4.2 the specific heat and viscosity of methane are shown for

a temperature range of 300 K to 1100 K. The specific heat values of methane are very

similar. The viscosity begins to differ at higher temperatures, where Aspen Plus gives higher values than the literature data.

90

80

~70

~

60 o

E

50 -

- 40

::2.

-

_c.

30

o

20

10

o

+---~---~---~---, 300

500

700

TI

[K]

900

1100

(31)

35.00 30.00 -.25.00 tn ~ 20.00

.5

15.00

-

_~

₁₀_.₀₀ 5.00

Aspen plus

0.00 +---~---~---~---~ 300 500 700

TI

[K]

900 1100

Figure 4.2: Viscosity of methane calculated with Aspen Plus (RK-Soave) and Iiterature data

4.5 Reaction kinetics

4.5.1 Methane decomposition

In the DDM reactor or operation mode the methane decomposes to hydrogen and carbon filaments. The decomposition of methane has two aspects: the surface

reactions and the carbon filament formation. In the surface reactions the methane is

absorbed and stepwise dehydrogenated. The absorbed carbon, which results, is then turned into carbon filaments (see Figure 4.3).

Selvedge due to segregation behaviour Diffusion of carbon through nickel

c

-

Adsorbed isolated I carbon atom C CN1,f Ni-particle

c

=

CSOI CNi,t ftl Support

(32)

The model for the surface reactions inc1udes the following steps [35]: Surface reactions: CH₄+ I

P

CH₄-1 CH₄-1 + I

P

CH)-1 + H-1 CH)-1 + I

P

CH2-1 + H-1 CH -I + 1 c l CH -I + H-1 2 - - - [ CH[-1 + I

P

C-1 + H-1 2 H-1

P

H₂

+

2 1 Dissolutionlsegregation: C-1

P

CNi,f

+

I

Diffusion of carbon through nickel:

PrecipitationIDissolution of carbon:

(4.2)

(4.3)

(4.4)

(4.5) After the stepwise dehydrogenation of methane, the carbon, which is adsorbed on the nickel, diffuses to the bulk nickel phase. The carbon diffuses through the nickel from the gas side to the support side of the nickel partic1e. At the support side the carbon forms filaments. At steady state, the rates of all consecutive steps; surf ace reaction, dissolution and diffusion are equal. The abstraction of the first hydrogen atom from methane is the rate-limiting step [35].

For the system described, a rate equation for the conversion of methane is derived by Snoeck et al. [35].

kM' + K CH 'PCH - -k;; " 'PH 2

4 4 K 2

(4.6)

(33)

K _i

₌

A _i_·exp_[

-fjJfadS'

_R.T

i]

Table 4.7: Parameter estimation for reaction rate and equilibrium constants [35] Constant KCH4 Parameters AM+ EM+ A M-' Eû' ACH4 & lCH/ Ar" 0" LJHr Estimate Unit 23444 mol/gcat h 59033 J/mol 4389 mol/gcat barl12 h 60522 J/mol 0.21 ba{1 1.43 J/mol 1.109108 bar3/2 137314 J/mol (4.7)

Also the equilibrium constant Kr" follows the relation given in equation (4.7). The parameters of these reaction rate and equilibrium constant are given in Table 4.7. The reaction rates are calculated as a function of the conversion of methane, with the composition of natural gas as given in Table 3.3 (section 3.3.4), to a level of 95 %. Hydrogen, which is generated by the conversion of methane, will inhibit the reaction and the reaction rate will become lower as the partial pressure of hydrogen increases and the partial pressure ofmethane decreases (see Figure 4.4).

7 · ~6

.c

1ii _u 5 C)

5'4

-

_o

₃

E

...

- 2

""

oe: ~u 1 O +---,---,---,---~----~~ 0.00 0.20 0.40 0.60 0.80 1.00

X I [-]

Figure 4.4: Reaction rate versus conversion for the DDM

(34)

4.5.2 Fuel eells

In a fuel cell, the electrode kinetics can be described by the Butler-Volmer equation [23]. Assuming that the net flux of reactant to the electrode is equal to the rate in which the reactant reacts, the reaction rate can be described by:

(4.8)

The forward reaction is defined as the reduction reaction. For the anode ofthe carbon fuel cell this would be:

(4.9)

The reaction rate constants for the forward and backward reaction are given by [23]:

k·T

[-I1G:

f ]

(-n.

p

.F'I1r/J)

(-n.p.F.n)

k f = _ B _ . exp ' . exp

rev .

exp . f

h R·T R·T R·T

(4.1 0)

kB·T

[~I1G;'b]

(n.[1-

P1

·F'I1r/Jrev]

(n'[1-

P1

·F'77]

kb =--·exp ·exp ·exp

h RoT R·T RoT

The first exponential part is the chemical component of the reaction rate constant. The last two parts are the electrical component of the reaction rate constant. The reversible potential used to calculate the reaction rate constant is equal to the standard potential of the half reaction at 298 K.

The practical calculation of reaction rates via the Butler-Volmer equation is difficult due to difficulties in determining values of the Gibbs energy of activation. Also the preciseness of the Butler-Volmer equation is questioned. The popular way to calculate the reaction rate in a fuel cen is via the current density. The relation between the current density and the reactant flux is given by equation (4.11) [23].

i=n·F·j

(4.11 )

The produced current of a fuel cen can be measured and divided by the electrode surface, which results in the current density. Current densities for carbon and hydrogen fuel cens are given in Table 4.8, as wen as the calculated reaction rates.

Table 4.8: Current densities and reaction rates for hydrogen and carbon fuel cell

Hydrogen fuel cen [23] Carbon fuel cen [24]

2000 2 200 4

(35)

5 Process structure and description

In chapter 2 the three main units of the process have been described: a methane decomposition reactor, a carbon fuel cell and a hydrogen fuel cello The design dialogue group has produced three ideas for setups of these units. A reactor that combines the function of methane decomposition and both carbon and hydrogen fuel cells was chosen as the most suitable solution.

The reactor contains two types of tubes: DDMlDCFC tubes and H2FC tubes. The

DDMlDCFC tubes will altemate between different functions: first the tubes are "charged" with carbon during decomposition. When sufficient carbon is deposited,

they switch function and "unload" by oxidizing the carbon to carbon dioxide. Unconverted methane and the hydrogen produced by the decomposition reaction are led through the hydrogen fuel cell tubes. Hydrogen and methane left over from the fuel cell are completely oxidized in a furnace. Methane decomposition is an endothermic reaction, whereas the reactions in the fuel cells are exothermic. By combining these operations in one unit (the so-called ADO-reactor), it is possible to eliminate a lot of heat transfer steps, thus improving the overall efficiency.

The process operates at high temperatures and is overall an exothermic process. After heat exchanging with cold streams, some hot streams are left. Some of this heat can be converted into electricity in a Rankine cyc1e (see appendix C.2).

5.1 Criteria and selections

The main layout of the process was already known at the start, leaving little choice for the overall process flow scheme. The final process differed somewhat from the original, through the combination of the decomposition reactor and the fuel cells in one unit. This unit has several advantages over separate units, for instance it eliminates the need for catalystlcarbon separation, solids transport is avoided and it allows for intemal heat integration. More details on the selection criteria can be viewed in chapter 2.

The DDM reactor works with 95% conversion, the unconverted methane and the hydrogen are led to a SOFC, which converts both compounds with 85% conversion. The carbon fuel cell can reach 100% conversion.

Because the hydrogen fuel cell operates at 85% conversion, it leaves some hydrogen and methane unconverted. Higher conversion would lead to unacceptable equipment size and therefore construction costs would be too high. In addition to that, the cell potential also drops with increasing conversion, which imp lies that higher conversions would not be desirabie. The unconverted fuel is fed to a furnace in which it is completely oxidized. The additional heat generated in this fumace is used to preheat the fresh air before it enters the process.

Because the decomposition reaction is endothermic and reactions in both fuel cells are exothermic, it is chosen to integrate all functions in one unit. As described, the DCFC and DDM are already integrated in the same type of tubes. The hydrogen fuel cell is also designed as a tubular SOFC, such that these tubes can be distributed between the DDMlDCFC tubes for better heat integration.

(36)

5.2 Process flow scheme

The process flow scheme (PFS) is shown in appendix C.3. Before this scheme could be developed, it had to be decided how heat integration could be implemented to minimize energy usage. In appendix C.l the reasoning behind the heat integration is explained.

In order to generate as much electricity as possible, a Rankine cycle is included in the process. This cycle converts some of the waste heat to e!ectricity, thereby maximizing the efficiency. Appendix C.2 describes the general features of a Rankine cycle. It

should be noted that the Rankine cycle included in this process is not fully optimized, thus leaving room for additional heat recovery. The decision not to put a lot of effort in optimizing the Rankine cycle was made because this is well-known technology (as opposed to the ADO-reactor concept) and therefore the other process units deserve more attention.

A step-by-step description ofthe PFS follows below:

Natural gas <2> enters the system at lObara. The pressure is lowered to 1.1 bara <5> in a turbine (TGO 1). The energy released in this operation is used to generate power. The natural gas leaving the turbine <5> is preheated in a heat exchanger (E02) by using the compressed CO2 <10> produced in the ADO-reactor (R03).

The preheated natural gas <11> is fed to part of the tubes in the ADO-reactor (R02), in which 95% of the natural gas decomposes in carbon and hydrogen. The carbon stays on the catalytic surface ofthe reactor, while the hydrogen leaves the reactor with the remaining methane and inerts <15>.

When the conversion of methane drops below 95%, the tubes used for decomposition are switched to fue! cell mode (R03). The side of the tubes where carbon is present becomes the anode ofthe fuel cell, while air is fed to the cathode. This air <1> enters the system at ambient conditions. The air is compressed (KO 1) and then preheated in a heat exchanger (EOl) to 1100 K <9>, using the hot exhaust gas <6> of the furnace (FOl). Before entering the heat exchanger, the gas is compressed (K02) to 1.1 bara <7>. This is done to make sure the gases leave the system at ambient pressure.

The carbon is converted into carbon dioxide of 100% purity <12>. Because pure C02 must leave the fuel cell, the anode must be purged to remove all hydrogen and natural gas present. This is done by using a small part ofthe CO2 produced <21>. The CO2 is compressed (K03) to a pressure of 10.2 bara <10> in order to be able to transport it to other locations via a pipeline.

The hydrogen leaving the reactor <15> is fed to the anode of the hydrogen fuel cell (RO 1), which is integrated in the ADO-reactor for better heat integration. This stream also contains unconverted natural gas. The air for the hydrogen fuel cell <9> is the same as the air of the carbon fue! cello In the hydrogen fuel cell, both hydrogen and methane leaving the ADO-reactor are converted for 85%.

Lean air leaving the ADO-reactor <13> and unconverted hydrogen and methane <14> are fed to a fumace (FOl). This results in a hot stream <15> that can be used to preheat ingoing strearns.

Design of a process to generate power from natural gas, using catalyzed decomposition and fuel cells

CPDNR

3326

Conceptual Process Design

Subject

Design of a proeess to generate power from natura! gas,

using eata!ysed deeomposition and fue! eells

Authors

A.M. van de Linde!oof

V. Brouérius van Nidek

A.G. van der Neut

K. Kuijvenhoven

M.P. Sehmidt

Keywords

(Study nr.)

1025805

1056816

1061801

1046659

1014110

Telephone

06-18472220

06-24580766

06-24863334

06-54296358

06-44326458

Power generation, Methane deeomposition, Fue! eells,

DDM, SOFe, DCFC, Proeess intensifieation

Summary

Contents

1

Introduction

1.1

Market situation and competition

2

Process options and selection

2.1

Process options

.. ,

\,"""R'"

2

H

2.2

Process concept selected

2.3

Bloek schemes

I

I

i

I

I

I

I

.

'

I

I

I

I

L

+

I

I

I

I

I

I

I

I._

~~~~~~_~~_~

________

J

2.4

Alternative processes

Efficiency

":1"

S1

~

~ll!ijl!!

,

~~~~__~

_I